Displays

Organic and inorganic whole system metabolism for two Irish coastal areas were compared to evaluate carbonate system resilience to acidification. The two systems are characterized by contrasting watershed input types and composition. Kinvara Bay is fed by Submarine Groundwater Discharge (SGD) derived from a karstic catchment while Killary Harbour is fed by river discharge draining a siliciclastic catchment. Freshwater sources to sea have distinct Total Alkalinity (TA) and Dissolved Inorganic Carbon (DIC) concentrations, higher and lower than the open ocean, respectively, but both evidence seasonally variable low pH, ranging from 6.20 to 7.50. Retention of TA and DIC was calculated for the two areas using LOICZ methodology. In Kinvara bay, annually averaged retention of DIC was greater than for TA (5 × 10⁴ and 1.5 × 10⁵ mol d^-1), suggesting the system is acidifying further. Conversely, Killary Harbour shows negative TA and DIC retention, with DIC:TA <1, suggesting an internal buffer against ocean acidification is operating.

Net Community Production (NCP) was calculated for both systems using Dissolved Oxygen data. Subsequently, we estimated Net Community Calcification (NCC) from the ratio between TA and DIC. NCP was always positive in Killary Harbour with an average of 318 mmol O₂ m^-2 d^-1 (equivalent to 89 mol C m^-2 y^-1). However, Kinvara Bay shows relatively lower positive NCP in spring and summer (average of 46 mmol O₂ m^-2 d^-1), but negative NCP in autumn and winter. Therefore, Kinvara Bay’s Total Organic Carbon (TOC) production was low, at ~21 g m^-2 y^-1 and not enough to overcome acidification driven by the SGD source composition. These results emphasize the complexity of interactions between the drivers of coastal acidification rate, affecting our ability to accurately assess the resilience of the carbonate system in these areas to ocean acidification pressure in the future.

How to cite: Guerra, M. T. and Rocha, C.: Organic and inorganic whole system metabolism in two acidified coastal systems in Ireland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17499, https://doi.org/10.5194/egusphere-egu2020-17499, 2020.

Drawdown of atmospheric CO₂ over geologic timescales is largely controlled by imbalances in the carbonate-silicate cycles and the preservation of Organic Carbon (OC) in marine sediments. Up to 85% of this OC is buried in continental shelf sediments of which ~20% is associated with reactive iron (Fe) (hydr)oxides. Association with Fe (hydr)oxides may enhance OC preservation yet despite the importance of this, little is known about which Fe (hydr)oxide phase(s) is involved in OC uptake or the binding mechanism of OC to these reactive iron minerals.  

To estimate the importance of this OC-Fe association, a citrate-dithionite-bicarbonate (CDB) extraction method is commonly used to dissolve an operationally defined ‘easily reducible iron oxide’ fraction and release the associated OC from the sediment. However, natural samples often contain a range of Fe (hydr)oxide phases extractable by CDB, and the Fe phases extracted are defined entirely on the susceptibility of their pure forms to chemical reduction. This suggests that factors affecting mineral stability, including association with OC, could lead to incomplete or excessive phase extraction, which would affect estimates of OC bound to these Fe phases.

To address these issues, we simplified the geochemical system by synthesising OC-iron (hydr)oxide composites with known Fe (hydr)oxide phases and OC moieties with differing chemical structures, added them to OC-free sediment, and then applied the CDB extraction method to determine i) the precise Fe phases extracted; ii) the impact of OC moiety on Fe release and iii) the optimal experimental conditions for the extraction.

We show that reduction of our composites by CDB results in only partial dissolution of the most easily reduced Fe phase (ferrihydrite) and a recovery of only ~20% of total Fe. We also find that the recovery is likely controlled by the functional groups present in the OC and the handling/storage/preparation of samples prior to analysis. These factors could lead to misidentification of the mineral phases extracted and an underestimation of the amount of OC associated with Fe. A change in the estimates for OC associated with Fe would have widespread implications for our understanding of the role of OC-Fe interactions in global carbon cycling.

How to cite: Fisher, B., März, C., Faust, J., Moore, O., and Peacock, C.: What's af(Fe)cting OC-Fe interactions? An experimental approach to understanding iron bound organic carbon in sediments., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-855, https://doi.org/10.5194/egusphere-egu2020-855, 2020.

Advective flows of sea- and fresh groundwater through coastal aquifers form a unique ecohydrological interface, the subterranean estuary. Here, freshly produced marine organic matter and oxygen mix with groundwater, which is low in oxygen and contains aged organic carbon from terrestrial sources. Along the underground flow paths, dissolved organic matter (DOM) is degraded and inorganic electron acceptors are successively used up. Because of the different DOM sources and ages, exact degradation pathways are often difficult to delineate, especially in high-energy environments with dynamic changes in beach morphology, source composition, and hydraulic gradients. From a case study site on a barrier island in the German North Sea, we present detailed biogeochemical data from pore water samples collected in the shallow layer of the subterranean estuary. The samples were taken along the major flow paths of recirculating sea water and discharging fresh, meteoric groundwater, and analyzed for physico-chemistry, electron acceptors, and dissolved organic carbon (DOC). DOM was isolated and measured with soft-ionization ultra-high-resolution mass spectrometry, and chemical DOM characteristics were derived by assigning exact molecular formulae to the thousands of intact masses found in each sample. Using geographic and physico-(geo)chemical parameters (longitude, salinity, dissolved silicate, dissolved iron) as indicators of water origin and residence time, we evaluated the behavior of chemical DOM characteristics (H/C and O/C ratios, aromaticity) along the underground flow paths. Overall, DOC concentrations and an H/C-based molecular lability boundary index (MLB) decreased with decreasing oxygen concentrations and parallel increases of dissolved (reduced) iron and dissolved silicate concentrations, in line with the assumption that high H/C ratios are a trait of labile DOM which is continuously degraded. On the other hand, aromaticity indices and relative abundances of a “humic-like” fluorescent DOM fraction increased along the flow paths, likely through accumulation of compounds less susceptible to microbial attack. Our data indicates that even in a highly complex advective flow system like the subterranean estuary, molecular properties of DOM can be harnessed to identify key, perhaps even site- and season-specific biogeochemical processes.

How to cite: Waska, H., Simon, H., Ahrens, J., Beck, M., Schwalfenberg, K., Zielinski, O., and Dittmar, T.: Molecular properties of dissolved organic matter (DOM) in the subterranean estuary of a high-energy beach: Finding proxies for reactive transport, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3322, https://doi.org/10.5194/egusphere-egu2020-3322, 2020.

South-East Asian peatlands are a globally significant carbon store. Rivers draining these peatlands have some of the highest dissolved organic carbon (DOC) concentrations in the world and account for up to 10% of the global land-to-ocean carbon flux, thus representing an important input to the marine carbon cycle. The release of DOC from peatlands is a natural process, yet the rapid and extensive transformation of these peatlands for agriculture over the past two decades is thought to have increased fluvial carbon losses significantly. However, not only do we lack a firm understanding of the fate of this terrigenous DOC in tropical seas, the distribution and long-term variability in DOC have never been studied at large scales in SE Asia. We will present the seasonal climatology (2002-2018) of spatial distribution patterns of DOC concentrations and optical properties (absorption coefficients, spectral slope) of colored dissolved organic matter (CDOM) for coastal waters of Sarawak, Malaysian Borneo derived using a regionally tailored semi-analytical inversion model from MODIS Aqua. Our results reveal substantial inputs of DOC from Sarawak rivers DOC close to shore exceeds 125 µM, and CDOM across the study region shows predominantly terrigenous spectral signatures. DOC concentrations were higher during the rainier northeast monsoon than during the drier south-west monsoon. Our data suggest that long-term increases in DOC concentration have occurred across parts of our study region from 2002–2018, which has implications for the aquatic light regime and coastal biogeochemistry[PM5]. These results will be discussed in the context of past anthropogenic disturbance to coastal peatlands.

How to cite: Sanwlani, N., Martin, P., Cherukuru, N., Muller, M., and Evans, C.: Climatology and trends of Dissolved Organic Carbon in coastal waters off Sarawak, Borneo, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4393, https://doi.org/10.5194/egusphere-egu2020-4393, 2020.

Submarine groundwater discharge (SGD) is considered as an important route for water and dissolved material exchange between land and coastal seas. Both freshwater and (recirculated) seawater are referred to as SGD and may impact the composition and biogeochemical processes in coastal waters. The present study focuses on the identification and the spatial variability of SGD into the Wismar Bay, in the southern Baltic Sea. On across-shore transects covering Wismar Bay waters were sampled for analysis of Radium (Ra) isotopes, stable isotopes (H, O, C, S), dissolved inorganic carbon (DIC), nutrients and major cations. In addition, sediment cores were retrieved from several stations. The detection of short-living radium isotopes (²²³Ra and²²⁴Ra) in surface waters of the bay indicate benthic-pelagic coupling via pore water exchange with the water column that may be an indication for SGD. Moreover, enhanced concentration of dissolved manganese and barium, resulted from anoxic pore waters, were found in areas with higher Ra activity. Pore water profiles of salinity and major ions highlight the presence of freshwater about 50 cmbsf in sediments in the central part of the bay, probably related to the presence of a coastal aquifer. In contrast, other sediments demonstrate relatively constant pore water salinity distribution with increasing depth. Slight salinity maxima in almost all core at around 6 to 12 cmbsf seems to be relict from changing bottom water salinity in the past. The water isotope composition (δ¹⁸O, δ²H) of the low saline pore water is plot close to the local meteoric water line established for Warnemünde. Saline pore waters, in contrast, have water isotope composition aligned with southern Baltic Sea surface waters. The DIC concentrations increased with depths suggesting the mineralization of organic matter within the 50 cm sediments depth at all sides. Moreover, the values of DIC even exceeding the concentration found on the percolating fresh ground water. Thus, the overall contribution of elements to the coastal ecosystem is a function of the transport processes regulating element flux across the sediment-water interface.

The investigation is supported by the DFG research training school Baltic TRANSCOAST, DAAD, and Leibniz IOW.

How to cite: Ehlert von Ahn, C. M., Scholten, J., Schmiedinger, I., Liu, B., and Böttcher, M.: A multi-tracer study of submarine groundwater discharge into Wismar Bay, southern Baltic Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-822, https://doi.org/10.5194/egusphere-egu2020-822, 2020.

Atmospheric aerosol particles are of great importance for cloud formation in the atmosphere because they are needed to act as cloud condensation nuclei (CCN) in liquid-water clouds and as ice nucleating particles (INP) in ice-containing clouds. Changes in aerosol concentration affect the albedo, development, phase, lifetime and rain rate of clouds. These aerosol-cloud interactions (ACI) and the resulting climate effects still cause the largest uncertainty in assessing climate change as they are understood only with medium confidence.

The PACIFIC project, which is embedded in the French-German Make Our Planet Great Again (MOPGA) initiative, aims to improve our understanding of ACI by enhancing the representation of those aerosols that are relevant for cloud processes and by quantifying temporal changes in cloud properties throughout the cloud life cycle. PACIFIC uses a three-fold approach for studying ACI based on spaceborne observations by (i) using spaceborne lidar data to obtain unprecedented insight in CCN and INP concentrations at cloud level opposed to using column-integrated parameters, (ii) characterizing the development of clouds by tracking them in time-resolved geostationary observations opposed to resorting to the snap-shot view of polar-orbiting sensors, and (iii) combining the detailed observations from polar-orbiting sensors with the time-resolved observations of geostationary sensors – for clouds observed by both – to study the effects of CCN and INP on the albedo, liquid and ice water content, droplet and crystal size, development, phase and rain rate of clouds within different regimes carefully accounting for the meteorological background.

This contribution will present the scope of the MOPGA-GRI project PACIFIC and illustrate the first findings.

How to cite: Tesche, M., Seelig, T., Alexandri, F., Bräuer, P., Choudhury, G., Hu, Y., and Quaas, J.: Aerosol-cloud interactions from combined observations with geostationary and polar-orbiting sensors, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2598, https://doi.org/10.5194/egusphere-egu2020-2598, 2020.

Ocean warming and ocean acidification (OA) are increasingly influencing marine life. Parts of the increasing amount of CO₂ in the atmosphere will eventually get absorbed by the ocean, which changes the oceans carbonate chemistry and threatens the ecological competitiveness of calcareous marine organisms. Currently,  the global coverage of studies on the development of pH since preindustrial times is sparse. An important region to study environmental and climate variations is the northwestern coastal part of Cuba where the Loop Current (LC) joins the Florida Current and contributes to the Gulf Stream. The tropical Atlantic is a primary region for the formation of warm surface water of the thermohaline ocean circulation and the Caribbean in particular as a habitat for coral reefs in the Atlantic making them susceptible to changes in water temperatures and carbonate chemistry. This provides a unique chance to study multiple aspects of the implications of anthropogenic activities such as changes in SST, ocean pH, and carbonate chemistry using the coral skeletal geochemistry as an archive of climate and environmental changes. Here we present results from a multi-proxy approach for the reconstruction of environmental change and natural climate variability from a North Cuban Siderastrea siderea coral. The sub-seasonally resolved records indicate interannual to decadal changes in SST and seawater carbonate chemistry since 1830 CE. The comparison with pH will provide clues on whether the regional climate variability has been directly affected by atmospheric CO₂ forcing.

How to cite: Harbott, M., Wu, H. C., Kuhnert, H., Kasemann, S. A., Jimenez, C., Gonzales Diaz, P., and Rixen, T.: Reconstruction of anthropogenic environmental changes from a Cuban coral over the last 175 years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19578, https://doi.org/10.5194/egusphere-egu2020-19578, 2020.

Dimethyl sulphide (DMS), as a volatile organic sulfur compound, plays an important role among the reduced sulphur gases in the atmosphere. DMS emitted from seawater constitutes a significant component of the global sulphur cycle and may affect climate by forming atmospheric aerosols which could form cloud condensation nuclei and thus modify cloud properties. DMS is produced from its major precursor dimethylsulphoniopropionate (DMSP) by complex interactions of phytoplankton and bacterial processes. Dimethyl sulphoxide (DMSO) is the major non-volatile dimethyl sulphur pool in the ocean and plays an important role in the biogeochemical cycle of DMS, although its formation and consumption pathways are poorly understood compared to DMSP.

Although the Baltic Sea is the largest brackish water system of the world, observations of sulphur compounds from the Baltic Sea are limited. The variations of seawater DMS, DMSP and DMSO as well as various phytoplankton marker pigments were investigated at the Boknis Eck Time-Series Station (BE, located in Eckernförde Bay, southwest Baltic Sea) during the period 2009 - 2016.  Average DMS (1.8 nmol L⁻¹), dissolved DMSP (DMSP_d, 3.3 nmol L⁻¹) and particulate DMSP (DMSP_p, 10.5 nmol L⁻¹) concentrations were generally low, while dissolved DMSO (DMSO_d, 14.6 nmol L⁻¹) and particulate DMSO (DMSO_p, 13.1 nmol L⁻¹) concentrations were comparably enhanced in the water column during the study. Strong seasonal variations in the concentrations of the sulphur compounds have been linked to the phytoplankton succession over the entire investigation period. Depth profiles of sulphur compounds were generally related to Chlorophyll a concentrations. The averaged DMS flux was 16.3 µmol m^-2 day^-1 suggesting that BE is a net source of atmospheric DMS. Monthly averaged air-to-sea DMS fluxes at BE varied considerably and they were well-correlated with surface DMS concentrations as well as the relative abundance of haptophytes instead of the wind speed. This time-series study illustrates the importance of phytoplankton community in shaping the distribution of the sulphur compounds and fluxes to the atmosphere in the Baltic Sea.

How to cite: Zhao, Y., Booge, D., Schlundt, C., and Bange, H.: Variability of sulphur compounds at the Boknis Eck Time-Series Station in the Baltic Sea during 2009-2016, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17306, https://doi.org/10.5194/egusphere-egu2020-17306, 2020.

The influence of abiotic and biotic drivers on the concentration of dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO) was investigated and compared during two annual cycles in 2016 and 2018 within the Belgian Coastal Zone (BCZ, North Sea) at five fixed stations chosen to cover both the near-offshore gradient and a longitudinal gradient from the stations close to the Scheldt estuary to the most marine stations. Due to differences in light and temperature, significant differences of Chlorophyll a (Chl a) concentrations were observed between the two years with higher values in spring– and, to a lesser extent, in summer 2018 compared to 2016. The higher springtime phytoplankton biomass in 2018 compared to 2016 seemed to be related to better light conditions in early spring coupling with colder winter. Nevertheless, the seasonal and spatial DMS(P,O) patterns were nearly identical in 2016 and 2018. We then tested if the phytoplankton diversity based on genomic data and Chl a concentration could be used to predict the DMS(P,O)p concentration and better understand the observed variability in the field. The phytoplankton composition was characterized with high DMS(P,O) producers (mainly Dinophyceae such as Gymnodinium clade and Prymnesiophyceae with Phaeocystis sp.), occurring in spring, and low DMSP producers (various diatom species), occurring in early spring and in autumn, that influenced the most the DMS(P,O) concentrations observed in our field samples. We were able to estimate the DMSP concentrations with DMSP:Chl a ratio (mmol:g) for the main observed classes but the DMSO concentration was not properly assessed. The ratio used was not enough accurate to reproduce faithfully the interactions between the sulfur compound and the environmental stress.

How to cite: Royer, C., Borges, A. V., Lapeyra Martin, J., and Gypens, N.: Drivers of the variability of dimethylsulfonioproprionate (DMSP) and dimethylsulfoxide (DMSO) in the Southern North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6807, https://doi.org/10.5194/egusphere-egu2020-6807, 2020.

The potential for vertical mixing to support new production in the upper layers of the northeastern portion of the North Sea was analysed from observations obtained during the stratified period in July 2016. Five transects across the shelf edge between the relatively shallow central North Sea and the deep Norwegian trench showed a clear frontal structure in hydrography, turbulent mixing, nutrients and chlorophyll a across the shelf edge. Relatively large (up to >0.5 mmol N m⁻² d⁻¹) nitrate fluxes due to turbulent vertical mixing into the euphotic zone were found at some stations over the shelf edge, while low values (< 0.1 mmol N m⁻² d⁻¹) were found in the deeper open area north of the shelf edge. The low vertical mixing rates implied f ratios less than 0.02 in the open waters north of the shelf edge. In the shallow (<50 m) southern and central part of the study area, inorganic nutrients were low and nitrate undetectable, suggesting negligible new production here, despite relatively high concentrations of chlorophyll a being found in the bottom layer. Thus, high rates of new production seem to be concentrated around the shelf-edge zone and in association with localized features exhibiting enhanced vertical mixing. We find that the nutricline depth is significantly deeper at the shelf edge and interference with increased mixing in this deeper depth range can explain the increased diapycnal nitrate fluxes. Overall, this suggests that the shelf-edge zone may be the major nutrient supplier to the euphotic zone in this area during the period of summer stratification. Potential impacts on plankton ecosystem structure are discussed.

Reference:

Bendtsen, J. and Richardson, K.: Turbulence measurements suggest high rates of new production over the shelf edge in the northeastern North Sea during summer, Biogeosciences, 15, 7315–7332, https://doi.org/10.5194/bg-15-7315-2018, 2018.

How to cite: Bendtsen, J. and Richardson, K.: Turbulence measurements suggest high rates of new production over the shelf edge in the northeastern North Sea during summer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11427, https://doi.org/10.5194/egusphere-egu2020-11427, 2020.

The global ocean dissolved oxygen (DO) inventory is decreasing and the areal extent of DO deficiency is increasing. In the shelf sea BML, net DO removal can occur as a result of restricted ventilation due to seasonal thermal stratification, oxygen consumption via pelagic and benthic respiration of organic matter, and nitrification. DO decline is becoming evident in several shelf seas, with recent model studies estimating that large regions of the Northwest European continental shelf seas (325,000 to 400,000 km²) have the potential to become seasonally deficient in DO in late summer. It is therefore of vital importance that DO is monitored accurately and effectively in shelf seas.

Here we present results from AlterECO project, which aimed to provide an alternative, novel framework for the monitoring of shelf sea ecosystem health indicators, including DO, via the deployment of 20 gliders in the North Sea (NW European shelf). Between November 2017 and May 2019 the gliders provided 18 month continuous measurements of T, S, chlorophyll fluorescence, and DO in the seasonally stratified study area, capturing the onset and breakdown of two spring blooms. In both years the gliders captured a weakly stratified, deep (>60m) thermocline in late autumn which was responsible for oxygen deplete (75%)  ‘pools’ in the North Sea. Our results show that preconditioning of pre-bloom transitional periods as well as episodic mixing events drive inter-annual differences in BML DO concentrations. Large inter-annual variability between pre-bloom physical conditions was observed, with the occurrence of anticyclone Hartmut in February 2018 resulting in a much colder water column (and therefore higher solubility of DO) in spring 2018 than 2019. Additionally we will demonstrate that the erosion of mini-blooms during the onset of stratification results in mixing of supersaturated DO surface water into the BML, helping to prevent DO deficiency in the BML in late summer. Comparisons of our high resolution glider data with the latest state of the art biogeochemical models (AMM15-ERSEM) will also be presented. We postulate that understanding the drivers of inter-annual variability in pre-bloom physical conditions is crucial in terms of understanding and predicting DO depletion in shelf seas.

How to cite: Williams, C., Mahaffey, C., Palmer, M., and Greenwood, N.: Physical preconditioning of oxygen depletion in shelf seas , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20219, https://doi.org/10.5194/egusphere-egu2020-20219, 2020.

The high-latitude regions are experiencing some of the most rapid environmental changes observed anywhere on Earth, especially in recent years. The Greenland Ice Sheet, for example, is experiencing significant mass loss largely through surface melting, but also via ice discharge at glacier fronts. As well as changing freshwater budgets and ocean stratification and mixing, there has been increasing focus on the role of glaciers and ice sheets in supplying particulate and dissolved organic material and inorganic nutrients to marine systems. Here, we explore how a combination of ship-board and high-resolution ocean glider observations in shelf waters off SW Greenland inform on how these nutrients reach the coastal oceans and, eventually, mix off the shelf and into the open ocean. We find that the proportion of meltwater calculated using salinity and oxygen isotope mass balance agrees well with estimates from glider sensors. These meltwaters contain low dissolved macronutrients, but are characterised by high particulate and high dissolved organic content. Bio-optic sensors on the gliders reveal strong meltwater signals in fluorescing dissolved organic matter (FDOM), and a detectable signal in optical backscatter; these signals can be now observed extending further out into the open ocean in compiled biogeochemical (BGC) argo float data. The mixing of both dissolved and particulate macronutrients and organic matter off the shelf is likely driven by advection in geostrophic currents, tidal and buoyancy forcing, and is also impacted by storm events via wind-driven changes in mixed layer depth and resuspension.

How to cite: Hendry, K., Briggs, N., Opher, J., Brearley, J. A., Meredith, M., Leng, M., Woodward, E. M., and Henson, S.: Impact of glacial meltwater on biogeochemical cycling in coastal and shelf waters off South West Greenland: Insights from ship-board and glider observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7528, https://doi.org/10.5194/egusphere-egu2020-7528, 2020.

Total alkalinity (TA) production in vegetated coastal systems is considered a putative sink for atmospheric CO₂, due to the increase in the seawater buffer capacity when TA is produced in excess of DIC. Much of the TA generated in these habitats is derived from the reduction of NO₃ and Fe, but in oligotrophic tropical waters dominated by carbonate sediments, these sources of TA may be minimal. To address this uncertainty, we measured a suite of sediment-water fluxes (SO₄, N₂, TA, DIC, DOC, etc) in a tropical and calcifying seagrass meadow, allowing us to identify the biogeochemical processes responsible for TA generation and consumption. We placed this information into the context of water-air CO₂ exchange, which was measured by atmospheric eddy covariance. Net N₂ fluxes indicated that denitrification was a negligible TA source in this oligotrophic seagrass meadow, which at times was net N₂-fixing. Instead, sediment-water TA fluxes were dominated by the balance between SO₂ reduction, H₂S oxidation, and carbonate dissolution/precipitation. Air-water CO₂ exchange was small and variable, reflecting the highly-buffered seawater chemistry and oligotrophic nature of this seagrass meadow.

How to cite: Van Dam, B., Fourqurean, J., and Smyth, A.: Alkalinity and CO2 fluxes in a tropical seagrass meadow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2861, https://doi.org/10.5194/egusphere-egu2020-2861, 2020.

Marine sediments represent the most important sink for organic matter across geological time spans, in which carbon-containing molecules are sequestered away and can escape remineralization to CO₂ by microbial degradation. Strong associations between minerals such as iron oxides and organic matter reaching the seafloor play a fundamental role in this preservation and have been known for some decades. Despite the importance of this protective mechanism in the balances of the global carbon budget, very little is known of the fate of bound organic matter as it is shuttled across the redox gradient into the reducing layers of sediment, particularly with respect to the Fe-OM associations. This study focuses on measuring the selective affinity of ferric and ferrous iron species for various functional groups commonly associated with the degradation products of organic molecules in marine systems as the iron cycles from +3 (oxides: goethite, lepidocrocite and ferrihydrite) to +2 (sulfides: mackinawite) oxidation states. This approach involves following model compounds across an artificial iron redox shuttle while probing Fe-OM bonding via quantitative FTIR and calculating mass balances using elemental analysis. A predicted outcome of this study will be a greater understanding of the fate of mineral-bound organic matter as it traverses the sedimentary redox gradient and the importance of iron sulfides such as mackinawite in their preservation. 

How to cite: Tétrault, A. and Gélinas, Y.: Exploring the Affinity and Selectivity of Sedimentary Mackinawite (FeS) Towards Natural Organic Matter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3952, https://doi.org/10.5194/egusphere-egu2020-3952, 2020.

As sediments play an important role as either a sink or a source of phosphorus (P) for water column, it is important to elucidate the major P fractions and behaviors (i.e., mobilization and immobilization) in the sediments to better understand P cycles in local and global scale. We investigated major P speciation associated with the partitioning of organic carbon (C_org) oxidation in the sediments to elucidate the P dynamics at two contrasting sediments in the continental shelf (EB1) and rise (EC1) in the Ulleung Basin (UB), East Sea. Sulfate reduction (SR) pre-dominated C_org oxidation at shelf site (EB 1), comprising % of C_org oxidation, whereas Mn- and Fe-reduction combined accounted for >80% of C_org oxidation in Mn-oxide and Fe-oxide-rich basin site (EC 1). Under SR-dominated condition (EB 1), H₂S oxidation coupled to reductive dissolution of FeOOH to form precipitation of FeS induced the accumulation of dissolved iron and phosphate in the pore water. On the other hand, phosphate in the Mn- and Fe-oxide-rich basin sediments (EC 1) was depleted because the P released through organic matter decomposition or reductive dissolution of Fe oxide/Mn oxide was effectively adsorbed to the metal-oxides in the surface sediments. Sequential extraction of P phases revealed that Fe bound P (52-65% of total P) was the major phase in the surface sediments of both sites. Interestingly, the organic P (P_org) fraction was 2.4-times higher at the basin site (12 μmol g^-1) than at the shelf site (5 μmol g^-1). C_org : P_org ratios presented as redox proxies in sediments were 644 and 191 for EB1 and EC1, respectively,. The results indicate that P_org has an effective preservation relative to C_org under sub-oxic conditions (EC1), whereas P_org was preferentially regenerated under anoxic conditions (EB1). Overall, the dynamics of P in the UB sediments were largely regulated by the partitioning of C_org oxidation pathways (i.e., sulfate reduction vs. metal reduction) and resultant interaction between Fe/Mn-S-P.

How to cite: Mok, J.-S., Kim, B., Cho, H., An, S.-U., Lee, H.-J., and Hyun, J.-H.: Phosphorus dynamics associated with partitioning of organic carbon oxidation pathways in the surface sediments of the deep Ulleung Basin, East Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2812, https://doi.org/10.5194/egusphere-egu2020-2812, 2020.

Submarine groundwater discharge (SGD) as a pathway for water and chemical constituents between land and ocean is a rather young topic. For a long time it has been neglected by the scientific community and coastal managers. However, it has increasingly attracted attention since the turn of the millennium. Yet, SGD is mostly investigated either by terrestrial or marine disciplines although a broader, interdisciplinary approach would benefit SGD research. Moreover, so far reported SGD flux data at local to regional scale are a) hardly comparable as, to our best knowledge, only a few, mostly isolated studies directly compared available SGD methods in a quantitative fashion and b) flux data contain large uncertainties, either because they were up-scaled from local discrete (point) measurements to regional scales or because they were derived from modelling/ budgeting of regional or even global matter fluxes despite the known high spatial and temporal variability. 

In order to pave the way for a more standardized and interdisciplinary SGD research that would reduce inherited measurement/ extrapolation uncertainties, the Königshafen Submarine Groundwater Discharge Network (KiSNet)  seeks to contribute through three concrete aims:  

1. forming an interdisciplinary group of SGD experts to initiate and intensify collaborative ties across disciplines
2. improving individual methodologies by groundtruthing through interdisciplinary intercomparison, which includes a focus on spatial and temporal variability, and
3. providing a method catalogue which outlines optimal combinations for qualitative and quantitative SGD investigations that may serve as basis for future standardized SGD research.

The network will convene at the bay of Königshafen on Sylt, Germany, during two different points in time. Each time, all members of the network will apply qualitative (remote sensing, marine and terrestrial ground-based geophysics, biological indicators and socio-scientific methods) and quantitative (seepage meters, temperature rods, natural tracers, numerical simulation) methods from terrestrial and marine disciplines to investigate SGD synchronously and provide a robust basis to tackle above mentioned aims. 

Here, we will outline exact procedures, methods and anticipated results the network will produce and provide an overview on future actions the network anticipates.

How to cite: Mallast, U., Waska, H., and Moosdorf, N.: Königshafen Submarine Groundwater Discharge Network (KiSNet), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5424, https://doi.org/10.5194/egusphere-egu2020-5424, 2020.

Dimethylsulfonopropionate (DMSP) and dimethylsulfoxide (DMSO) are two compounds involved in the carbon and sulfur cycle and are the precursors of the climate cooling gas dimethylsulfide (DMS). Despite decades of research, their role as osmoregulator, cryoprotector or antioxidant within the phytoplankton cells remains uncertain in some part. Since the antioxidant cascade system from the DMSP reported by Sunda & al. (2002), more investigation need to be conducted to confirm or accurate these interactions. This study aims to improve the knowledge about DMSP and DMSO and their hypothetic role of antioxidant on three different classes of phytoplankton (Dinophyceae – Prymnesiophyceae – diatom) and one diatom no-DMSP producer Chaetoceros sp. as negative control. Laboratory cultures were submitted to three oxidative stress to produce Reactive Oxygen Species (ROS) with (1) increasing light intensity from 100 to 600 and up to 1200 µmole/m²s for a global and natural oxidative stress; (2) using the menadone bisulfite (MSB) to generate ·O₂ and (3) using 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) to inhibit the photosystem II (PSII). The PSII activity, the Chlorophyll a concentration (Chl a), the lipidic peroxidation (LOP), the ROS production and the pigment variation were analysed after 6h of incubation and related to the evolution of the DMSP and DMSO concentrations to better understand the cellular oxidative stress and its impact on the phytoplankton cell and DMSP and DMSO production.

How to cite: Gypens, N., Roberty, S., Borges, A. V., Cardol, P., and Royer, C.: An antioxidant function for dimethylsulfonopropionate (DMSP) and dimethylsulfoxide (DMSO) within three different phytoplankton groups, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6993, https://doi.org/10.5194/egusphere-egu2020-6993, 2020.

Extreme weather events such as tropical storms and hurricanes deliver large amounts of
freshwater (stormwater and river discharge) and associated dissolved organic carbon (DOC)
to estuaries and the coastal ocean, affecting water quality and carbon budgets. Hurricane
Harvey produced an unprecedented 1000-year flood event in 2017 that inundated the heavily
urbanized and industrialized Houston/Galveston region (Texas, USA). Within a week, storm-
associated floodwater delivered 105±10 Gg of terrigenous dissolved organic carbon (tDOC)
to Galveston Bay and the Gulf of Mexico continental shelves. In-situ decay constants of
8.75-28.33 yr ^-1 resulted in the biomineralization of ~70% of tDOC within one month of
discharge from the flood plain. The high removal efficiency of tDOC was linked to a diverse
microbial community capable of degrading a wide repertoire of dissolved organic matter
(DOM), and suggested hurricane-induced flood events affect net CO₂ exchange and nutrient
budgets in estuarine watersheds and coastal seas.

How to cite: Yan, G., Labonté, J., Quigg, A., and Kaiser, K.: Hurricanes accelerate dissolved organic carbon cycling in coastal ecosystems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6820, https://doi.org/10.5194/egusphere-egu2020-6820, 2020.

The Argentina Continental Margin represents a unique geologic setting where fundamental interactions between bottom currents and sediment deposition as well as their impact on biogeochemical processes and element cycling, in particular iron, can be studied. The aims of this study were to investigate 1) the consequences of different depositional conditions on biogeochemical processes and 2) diagenetic cycling of Fe mineral phases in surface sediments. Furthermore, it was 3) studied how sedimentary stable Fe isotope signatures (δ⁵⁶Fe) are affected during early diagenesis and finally 4) evaluated, under which conditions δ⁵⁶Fe might be used as proxy for microbial Fe reduction in methanic sediments. During RV SONNE expedition SO260, carried out in the framework of the DFG-funded Cluster of Excellence “The Ocean in the Earth System”, surface sediments from two depositional environments were sampled each using gravity corer and multi corer. One study site is located on the lower continental slope at 3605 m water depth (Biogeochemistry Site), while the other site is situated in a contourite system on the Northern Ewing Terrace at 1078 m water depth (Contourite Terrace Site). Sequential Fe extractions were performed on the collected sediments to determine four operationally defined reactive Fe phases targeting Fe carbonates, (easily) reducible Fe (oxyhydr)oxides and hardly reducible Fe oxides [1]. Purification of extracts for δ⁵⁶Fe analysis of the Fe carbonates and easily reducible Fe (oxyhydr)oxide fractions followed [2]. The dataset was combined with pore-water data obtained during the cruise and complemented by concentrations and stable carbon isotope signatures of dissolved methane determined post-cruise. The extent of the redox zonation and depth of the sulfate-methane-transition (SMT) differ between the two sites. It is suggested that sedimentation rates at the Biogeochemistry Site are low and that steady state conditions prevail, leading to a strong diagenetic overprint of sedimentary Fe phases. In contrast the Contourite Terrace Site is characterized by high sedimentation rates and a lack of pronounced diagenetic overprint [3]. Reactive Fe phases are subject to reductive dissolution at the SMT. Nevertheless, significant amounts of reactive Fe phases are preserved below the SMT as evidenced by the presence of dissolved Fe in the methanic sediments, and are available for deep Fe reduction possibly through Fe-mediated anaerobic oxidation of methane [4]. In this study, δ⁵⁶Fe signatures of reactive Fe phases in methanic sediments were determined for the first time. These data suggest significant microbial fractionation of Fe isotopes during deep Fe reduction at the Biogeochemistry Site, whereas at the Contourite Terrace Site the δ⁵⁶Fe signatures do not indicate remarkable microbial Fe isotope fractionation. It is concluded that the applicability of δ⁵⁶Fe signatures as tracer for microbial Fe reduction might be sensitive to the depositional regime, and thus may be limited in high sedimentation areas.

References:
Poulton, SW. and Canfield, DE., 2005. Chemical Geology 214: 209-221.
Henkel, S. et al., 2016. Chemical Geology 421: 93-102.
Riedinger, N. et al., 2005. Geochimica et Cosmochimica Acta 69: 4117-4126.
Riedinger, N. et al., 2014. Geobiology 12: 172-181.

How to cite: Melcher, A.-C., Henkel, S., Pape, T., Meixner, A., Kasemann, S. A., Köster, M., Volz, J., Frederichs, T., Miramontes, E., and Kasten, S.: Impact of depositional regimes on biogeochemical cycling of iron and stable Fe signatures in sediments from the Argentina Continental Margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8935, https://doi.org/10.5194/egusphere-egu2020-8935, 2020.

Benthic-pelagic coupling is responsible for the sudden appearance and disappearance of many coastal plankton blooms. Whether this signature is also reflected in pCO₂ and whether the processes involved are important for the carbon fluxes in the coastal ocean is unclear. To address these questions, we use an ecosystem model that accounts for benthic-pelagic coupling of three different functional phytoplankton groups. Coupled with the water column model GOTM, we investigate the air-sea CO₂ fluxes in the Baltic Sea and compared them with observations. We show that the variability is very well captured by the model. The relative importance of the life cycle processes in regulating carbon fluxes is demonstrated.

How to cite: Chavan, H. and Hense, I.: Biologically driven variability in coastal carbon fluxes - A model study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9026, https://doi.org/10.5194/egusphere-egu2020-9026, 2020.

Our study examines the features of photosynthetic processes that occurred in the coastal area of the south-eastern Baltic Sea during the advanced phase of intensive summer bloom of 2018. We aim for a better understanding of short-time variability in primary production coupled with planktonic composition and phytoplankton functional activity in relation to location of nutrients sources on the coast and sub-mesoscale eddies, which appear over the coastal slope of Cape Taran and move alongside the Sambia Peninsula coast. These two-day studies, conducted on board of research vessels, represent a snapshot of a highly variable ecosystem alongside the Sambia Peninsula and Curonian Spit at the end of summer. Satellite images of sea surface temperatures and chlorophyll «a» concentration were also used for identification of spatial variations and eddies; the circulation conditions were derived from the operational system SatBaltyk. Across the coastal area, the effects of physico-chemical conditions influenced the phytoplankton composition and photosynthetic activity. In the south, the hot weather as well as the impacts of the Vistula Lagoon and the Amber combine affected the increase of nutrients and caused the strongest cyanobacterial bloom. In the Cape Taran area, the plankton community was transformed as a result of sub-mesoscale eddies development. Substantial gradients of nutrients, composition, biomasses and functional activity of phytoplankton along transects through eddies field was shown.

Collecting of samples was done with a support of the state assignment of Shirshov Institute of Oceanology, Russian Academy of Sciences (topic no. 0149-2019-0013). Data processing was supported by the state assignment of Shirshov Institute of Oceanology, Russian Academy of Sciences (topic no. 0149-2019-0006) and RFBR grant ( no.19-05-50090).

How to cite: Kudryavtseva, E., Bukanova, T., Aleksandrov, S., Mosharov, S., Dmitrieva, O., Melnik, A., Krek, A., and Ezhova, E.: Variability in planktonic community caused by sub-mesoscale eddies and spatial features of the Baltic Sea coast, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21523, https://doi.org/10.5194/egusphere-egu2020-21523, 2020.

The Bohai Sea and Yellow Sea are semi-enclosed basins strongly affected by human activities due to climate change and growing industries in China. Changes of hydrology, nutrient concentrations and sources and resulting ecosystem responses are therefore progressively intensifying during the last decades. In order to characterize nutrient sources and dynamics and to estimate the anthropogenic impact, we investigated nutrient concentrations and dual isotopes of nitrate in spring and summer 2018 in Bohai Sea and Yellow Sea. Furthermore, we sampled suspended matter and surface sediments and determined organic carbon, nitrogen and stable nitrogen isotopic ratios.

In spring, the water column was well mixed and the study area was mainly affected by the Yellow River diluted water and the Yellow Sea Warm Current water, which were the main nitrate sources. In summer, the water was stratified, and the Yellow River and Changjiang River diluted water supplied nutrients to an even larger region than in spring. During this season, the Yellow Sea Cold Water mass formed the bottom water of the Yellow Sea where nutrients became enriched. In contrast to other polluted marginal seas, the stable isotopic ratios of dissolved and particulate nitrogen are relatively low in the study area, which could be due to nutrient supply from the atmosphere or the open ocean. Using nitrogen isotopes, we developed a box model of reactive nitrogen for the Bohai Sea and quantified the input of atmospheric and riverine reactive nitrogen, submarine groundwater and water exchange with the Yellow Sea, constraining the budgets of reactive nitrogen combining mass fluxes with an isotopic balance. Including the isotopic balance improved the mass balance based only on nutrient concentrations.

How to cite: Tian, S., Gaye, B., Tang, J., Luo, Y., Sanders, T., Dähnke, K., and Emeis, K.-C.: Nutrient sources in the Bohai Sea and Yellow Sea: results from seasonal sampling in 2018, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21448, https://doi.org/10.5194/egusphere-egu2020-21448, 2020.

The Benguela upwelling system (BUS) offshore Namibia is among the most productive ocean regions worldwide and is a globally important reservoir of biodiversity and biomass. The forcing of cold, nutrient-rich deep waters up the coastal shelf leads to high rates of primary productivity in surface waters, intense carbon remineralization and consequently to (bottom water) oxygen depletion on the shelf that varies temporally and spatially with the intensity of the upwelling.
Recurring events of deoxygenation have a severe impact on marine ecosystems, for instance increased mortality and altered biogeochemical cycles of key elements such as carbon (C), iron (Fe), phosphorus (P) and sulfur (S). Therefore, it is crucial that we establish a clear mechanistic framework of the impact of oxygen depletion on (global) biogeochemical cycles, not only to allow for the reconstruction of climate-ocean feedbacks in upwelling regions in the past, but to enable predictions of future behavior.
During an expedition with RV Pelagia in February of 2019, we collected water column and sediment samples from the shelf and slope off Namibia (100 to 1517 m water depth, bottom water O₂ between 0.5 and 175 µmol L^-1) and measured nutrient fluxes in on-board sediment incubations to understand the early diagenetic behavior of those key elements and trace metals underlying the (periodically) oxygen-depleted waters of the BUS.
We analyzed dissolved concentrations as well as solid-phase speciation of key elements such as iron (Fe), manganese (Mn), phosphorus (P) and sulfur (S) to understand the chemical and physical processes controlling their distribution along the depth/redox-transect.
Our results show intense P cycling on the shelf, as evidenced by very high pore-water P concentrations, an enhanced efflux of PO₄ to suboxic bottom waters and indications of phosphorite formation at depth in the sediment. N/P ratios well below Redfield indicate N depletion and (relative) P accumulation in the water column, a shift in nutrient stoichiometry that can impact the composition of microbial communities in such waters. Meanwhile, the slope sediments are overlain by oxic bottom waters, retain P more efficiently and exhibit N/P ratios close to Redfield stoichiometry.
The capacity of the sediment to buffer toxic sulfide and prevent its release to the water column was dependent on the abundance of sulfide oxidizers at the sediment surface. Furthermore, the variable accumulation of sulfide affected Fe speciation and sedimentary P retention.
Overall, we show an intimate coupling between sedimentary cycles of essential elements in the Benguela upwelling system, a stark contrast between shelf and slope environments that is further enhanced by local variation of oxygen depletion and a very strong role of microbes in driving the cycles.

How to cite: Ungerhofer, K. A., Reichart, G.-J., and Kraal, P.: Coupled benthic cycling of iron, phosphorus and sulfur in the Benguela upwelling system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19291, https://doi.org/10.5194/egusphere-egu2020-19291, 2020.

Coastal and shelf sediments trap and bury significant quantities carbon (Berner, 1982) and provide an conditions allowing for the long-term storage of carbon. Through burying this carbon these sediments potentially provide a climate mitigation services. Currently our understanding of the spatial distribution of C within the surficial sediments of  coastal and shelf seas is limited. Using Scotland’s EEZ as a natural laboratory in conjunction with the tiered seabed mapping methodology developed by Smeaton and Austin (2019), we show that coastal and shelf sediments are highly heterogenous in both sediment type and C content. The tiered approach utilised in this study is ideally suited to global applications where data availability may differ significantly. Improved spatial mapping of seabed C will provide policy makers with a new tool for the targeted management and protection of these globally important C stores.

Berner, R. A., 1982, Burial of organic carbon and pyrite sulfur in the modern ocean: Its geochemical and environmental significance.Am. J. Sci.282,451–473 (1982)

Smeaton, C. and Austin, W.E.N., 2019. Where’s the Carbon: Exploring the Spatial Heterogeneity of Sedimentary Carbon in Mid-Latitude Fjords. Frontiers in Earth Science, 7, p.269.

How to cite: Austin, W. and Smeaton, C.: Towards a Sedimentary Carbon Stock Estimate for Scotland's EEZ: A Tiered Mapping Approach. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9573, https://doi.org/10.5194/egusphere-egu2020-9573, 2020.

Consisted of shallower Sunda Shelf, deeper Zengmu Basin, and Nansha Trough Basin, the southern South China Sea (SCS) provides an ideal scene for oceanography studies. Spreading all over nearly from 50 to 3000m at depth, a total of 93 surface sediment samples were collected to analyze the environmental factors constraining the foraminiferal distribution pattern in the southern South China Sea (SCS). Species distributions and stable isotopic compositions were combined to reveal the controlling factors, such as depth, salinity, substrate, runoff, currents, and cold seep activities. Water depth is the dominant factor controlling both assemblage composition and δ¹⁸O of benthic foraminiferal tests. The 1000 m isobath separates the sites into two clusters (Cluster A and B), which are dominated by deep-water species and shallow-water species, respectively. The sites in the deep-water zone (Cluster A) are characterized by lower absolute abundances, species richness and Shannon Index values (a measure of diversity), and higher proportions of planktonic foraminifers compared with the sites in the shallow-water zone (Cluster B). Increasing proportions of agglutinated tests with depth and rapidly decreasing proportions of planktonic foraminifera in the Nansha Trough Basin provide evidence of calcium dissolution occurring at a depth corresponding with previous reports. Oxygen stable isotopes (δ¹⁸O_B) of benthic foraminifera become more positive with depth only up to 1000 m and remain constant beyond. Differences in the proportion of agglutinated and porcelaneous tests in the shallow-water zone suggest that terrestrial runoff from nearby river systems (Mekong River and northern Borneo rivers) and seasonal surface currents (SCS Southern Cyclonic Gyre and SCS Southern Anticyclonic Gyre) jointly influence the distribution patterns of foraminifera in the study area. Enrichment of planktonic δ¹⁸O is a response to cold waters brought by the SCS southern cyclonic gyre during winter. An upwelling current (Winter Natuna Off-Shelf Current) containing higher amounts of organic matter/nutrients contributes to the depleted δ¹³C of planktonic foraminifera and to the abnormal abundance of foraminifera at the sites within its area of influence. The dominance of the foraminifer Melonis barleeanus at sites belonging to Subcluster A1 and the stable isotope compositions of benthic foraminifera (δ¹⁸O > 0, δ¹³C < 0) across the sites suggest the influence of active cold seeps in the southern SCS. This research highlights the complexity of environmental variables that interact to influence the foraminiferal assemblages and geochemistry in the southern South China Sea, which could serve as a model for paleoenvironmental and palaeoceanographic reconstructions.

How to cite: Yin, J., Liu, C., and Yang, X.: Distribution and constraining factors of planktonic and benthic foraminifers in bottom sediments of the southern South China Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12503, https://doi.org/10.5194/egusphere-egu2020-12503, 2020.

In order to estimate sediment organic carbon budget in coastal oceans and continental shelves, a first step is to estimate how much of the deposited organic matter is retained within a sediment matrix, for further remineralization and preservation on a geological timescale, rather being physically flushed away by benthic flow¹. This question becomes more challenging for the regions where ‘mobile’ layers (e.g. fluff layer, fluid mud and nepheloid layer) are formed due to the massive organic matter inputs, and often frequent resuspension and deposition². Organic matter remineralization and preservation in sediments has been mostly investigated but often overlooks the role of flow-induced shear stresses on suspending the organic matter. While such flow influences in sediment organic matter budget may have little influence on sediment organic matter budget in deep oceans, it cannot be neglected in shallow-water coastal seas and continental shelves where cyclic resuspension, deposition and frequent storm events occur^3,4. To our knowledge, the resistance strengths of organic matter in sediments against flow resuspension has received little attention.

To investigate this knowledge gap, various organo-clay aggregates and organo-clay-sand aggregates formed under different flow conditions were investigated by a series of laboratory flume⁵ and high resolution X-ray Microcomputed Tomography (micro-CT) experiments⁶. Herein, a novel methodology is proposed, which successfully establishes quantitative relationships between the resuspension resistance strengths of these organic aggregates and a wide range of flow intensities, from moderate to storm conditions. The results provide a basis for computing resuspension under a range of flow conditions and, hence improving estimates of the organic matter budget in the coastal zone.  

References

1. Burdige, D. J. Preservation of organic matter in marine sediments: Controls, mechanisms, and an imbalance in sediment organic carbon budgets? Chem. Rev. 107, 467–485 (2007).
2. McKee, B. A., Aller, R. C., Allison, M. A., Bianchi, T. S. & Kineke, G. C. Transport and transformation of dissolved and particulate materials on continental margins influenced by major rivers: Benthic boundary layer and seabed processes. Cont. Shelf Res. (2004). doi:10.1016/j.csr.2004.02.009
3. Burdige, D. J. Burial of terrestrial organic matter in marine sediments: A re-assessment. Global Biogeochem. Cycles 19, 1–7 (2005).
4. Nicholls, R. J. & Cazenave, A. Sea-level rise and its impact on coastal zones. Science (2010). doi:10.1126/science.1185782
5. Thompson, C. E. L., Couceiro, F., Fones, G. R. & Amos, C. L. Shipboard measurements of sediment stability using a small annular flume-core mini flume (cmf). Limnol. Oceanogr. Methods (2013). doi:10.4319/lom.2013.11.604
6. Zhang, N. et al. Nondestructive 3D Imaging and Quantification of Hydrated Biofilm-Sediment Aggregates Using X-ray Microcomputed Tomography. Environ. Sci. Technol. 52, 13306–13313 (2018).

How to cite: Zhang, N., Thompson, C., and Townend, I.: A novel approach to quantifying resuspension resistance of sediment organic matter against coastal flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15864, https://doi.org/10.5194/egusphere-egu2020-15864, 2020.

Upwelling systems are very productive regions of the ocean that strongly contribute to the local economies holding very different fisheries. These dynamic systems are characterized by a high degree of spatial and temporal variability of biogeochemical properties, including carbon, which is generally poorly represented in coarse-resolution global models. The importance of the marine carbon system characterizing these systems has been demonstrated in different regions from multiple perspectives. For the first time, we evaluate the drivers of the spatiotemporal variability of the seawater partial pressure of CO₂ (pCO₂) in the Canary-Iberian Upwelling System (25.5-45ºN, 5.5-20.5ºW) to better understand the inorganic carbon cycle in this highly-productive upwelling region. To do so, we first coupled a regional high-resolution ocean circulation model CROCO with the ocean biogeochemical model PISCES and run a climatological simulation. A first-order Taylor expansion was applied over this simulation to compute the contribution of four variables to the pCO₂ spatiotemporal variability: salinity-normalized dissolved inorganic carbon (sC_T), salinity-normalized total alkalinity (sA_T), temperature (T) and freshwater fluxes (FW). Modeled pCO₂ is in agreement with that of recent data-based monthly climatologies (open ocean RMSE: 5.2-10.8 µatm; coastal ocean RMSE: 7.9-18.7 µatm), measured data from the Surface Ocean CO₂ Atlas (SOCAT) (RMSE: 6.6-13.9 µatm) and computed pCO₂ from measured A_T and pH at the European Station for Time series in the Ocean Canary islands (ESTOC) (RMSE: 5.1 µatm). The spatial distribution of the pCO₂ anomalies relative to the domain mean shows two different areas with opposite anomalies: positive anomalies around the coast in the entire domain and in open ocean south of 33ºN and negative anomalies in open ocean north of 33ºN. This pattern is mainly driven by the contribution of the T component and a minor influence of sA_T and FW, with the sC_T component largely counteracting the effects of the other drivers but contributing to the positive anomaly along the Iberian coast. The seasonal variability is controlled by T and sC_T, with a minor influence of sA_T and a negligible importance of FW. The seasonal cycle shows a direct covariation between the T contribution and the δpCO2 (monthly mean minus annual mean of pCO₂) and an inverse covariation between the sC_T contribution and the δpCO₂ that counteracts the effect of T in the δpCO₂ amplitude. A decrease in the δpCO₂ amplitude was found from open ocean (depths > 200m) to coastal ocean (depths < 200m) determined mainly by a decrease in the influence of the T driver and, less significant, also by a reduction of the sC_T contribution. The general agreement between modeled and observed contributions to pCO₂ variability at the ESTOC time-series station, in terms of both phase and amplitude, lends credibility to our deconvolution and model, which has been applied across the Canary-Iberian Upwelling System, to assess the processes behind the spatiotemporal variability of pCO₂.

How to cite: Broullón, D., Nolasco, R., Reboreda, R., Dubert, J., Gehlen, M., Orr, J., and Pérez, F. F.: Mechanisms driving seawater pCO2 spatiotemporal variability in the Canary-Iberian Upwelling System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18139, https://doi.org/10.5194/egusphere-egu2020-18139, 2020.

Besides hosting large mining complexes, the Rio Doce basin is widely exploited for agricultural activities and, industrial supplies. The Rio Doce is one of the main water bodies in the southeast region of Brazil, with an estimated sedimentary load of 11.22x10⁶ tons/year and just only it’s sediment transport capacity, associated with the activities along its watershed, justify a deep study at the continent-ocean interface. However, in addition, in November 2015 the collapse of the Fundão tailing dam, a property of the Samarco mining company, described as one of the largest environmental disasters in Brazilian history, mobilized around 55 million m³ of mining waste through the Rio Doce basin. A reddish, fine granulometry mud, composed of silica, hematite, magnetite, manganese oxides and organic matter was transported in the river system through more than 600 km and released in the ocean. In this sense, the present study evaluated the distribution of major and trace elements in six marine sediments located in the discharge zone of the Rio Doce, three in the continental shelf and three in the slope, after the arrival of the mine tailings. The sediments cores M125-39-2, M125-43-2, M125-44-2, M125-49-2, M125-50-2 and M125-55-8 were collected with a multi-corer during the RV Meteor cruise M125. The major and trace metals were determined through the total digestion method (USEPA 3052) and analyzed by an ICP-OES, also were determined the granulometry, total organic carbon (TOC), total nitrogen (TN), δ¹³C and δ¹⁵N. The core M125-39-2 closer to the discharge zone of the Rio Doce registered the Mariana event. Two distinct events can be suggested in this core, one associated with the deposition of the mining tailings from the dam rupture and the second by the possible subsequent remobilization of these materials under high rainfall conditions, where an increase in Fe, Al, Si, Ti, As, Pb among other elements was recognized. Interpolation of the δ¹³C and δ¹⁵N with TOC and TN led to identified two distinct groups in this core, one with a mixed organic matter source (bottom of M125-39-2) and the other with a marine isotopic signature (top of M125-39-2).Also, the granulometric data and the elemental ratios when interpreted together show that the influence of the Rio Doce discharge was predominant to the M125-39-2 core, consistent with an abrupt, localized increase of the terrestrial contribution.  The most superficial centimeter of the core, M125-50-2 presented an increase in the concentrations of Fe, Al, Si, K and, Ti, as in the other trace elements concentrations. The proximity to the source area, the patterns of marine currents and winds in the region were fundamental for the accumulation of major and trace elements from the tailing dam rupture in the core M125-39-2. Finally, organic matter content and the granulometry, despite their secondary role in this study, are factors with some potential that could enhance the adsorption of metals from the ore plume.

How to cite: Díaz, R., Santarém, N., Moreira, M., Dias, B., Vieira da Silva Filho, E., Bahr, A., and Albuquerque, A. L.: Distribution of major and trace elements in marine sediments deposited next to Doce river discharge after the break of the Mariana tailing dam., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16497, https://doi.org/10.5194/egusphere-egu2020-16497, 2020.

Continental shelves cover less than 5% of the global ocean surface, but play a crucial role in the marine global biogeochemical cycling. Coastal ecosystem dynamics are governed and constrained to a wide extent by the biogeochemical processes occurring in the benthic domain. Such processes define the so called benthic-pelagic coupling (hereafter BPC), i.e. two-way exchange of organic matter (particulate and dissolved) and inorganic compounds. The physically mediated exchanges structuring the BPC are constituted by the sinking and resuspension fluxes of particulate organic matter and by the diffusion of inorganic nutrients. Despite its importance and the continuous enhancement of model resolution, the BPC in global marine ecosystem models is generally roughly approximated. Moreover, observational data focusing on the BPC dynamics are fairly scanty in time and space, thereby hampering model parameterization and validation. The main objectives of this study are to develop and test a numerical model addressing BPC processes and to evaluate ecosystem dynamics in marine areas with different climatic and ecological characteristics. In particular, we here focused on two key interaction processes: the sinking velocity of particulate matter and the diffusive fluxes of inorganic dissolved nutrients at the benthic-pelagic interface. The benthic sub-model has been calibrated accounting for the complex pelagic food web and for the main ecological and physical characteristics of continental shelf areas in different sites: Gulf of Trieste (Italy), St. Helena Bay (South Africa), Svinoy Fyr (Norway). At each study area, the one-dimensional coupled BFM-NEMO modelling system was setup by prescribing temperature and salinity vertical profiles in NEMO, while the shortwave radiation acts as a primary forcing of BFM. Model results have been validated with available in situ data.

Sensitivity tests has been performed to investigate the role of the BPC exchanges in determining the pelagic biogeochemical cycles and to carry out a comparative analysis accounting for each site characteristics.

How to cite: Amadio, C., Zavatarelli, M., Lovato, T., and Butenschoen, M.: Numerical modelling of the benthic-pelagic coupling in coastal marine ecosystems at contrasting sites , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13130, https://doi.org/10.5194/egusphere-egu2020-13130, 2020.

The coastal carbonate system regulates the pH of the coastal waters and controls the circulation of CO₂ between land-sea interfaces and open sea system. In the context of the ELME (Evaluation of the Lebanese Marine Environment: A multidisciplinary study) project, a seasonal survey of the carbonate system has been started in 2019 through the sampling of 3 different transects starting from the coast towards the open sea, offshore two Lebanese cities (Beirut and Tyre) to evaluate the spatio-temporal variations of this system in coastal areas. The carbonate chemistry is being studied by measuring both total alkalinity (A_T) and total dissolved inorganic carbon (C_T), together with other critical parameters in coastal ecosystems such as temperature, salinity, pH, dissolved oxygen, nutrients (phosphates, nitrates, nitrites, silicates), and chlorophyll a. The preliminary results show that the highest carbonate system inventories (2546.4 and 2266 µmol kg^-1 for A_T and C_T respectively) were measured in transects influenced by discharges of dumpsite and port areas (offshore Beirut) where positive and significant correlations (p << 0.005) have been recorded with nutrients, particularly with nitrites (> 10 µmol kg^-1). Furthermore, TrOCA approach was used to estimate the anthropogenic CO₂ concentrations (C_ANT) below the mixed layer depth. The results demonstrate that all waters in both studied areas are contaminated by C_ANT, even the deep ones (> 400 m) located in the furthest monitored station, with values greater than 70 µmol kg^-1. This fact raises concerns about the effects of such relatively high C_ANT concentrations on coastal organisms therein. This work presents the preliminary results of an ongoing study. The continuity of this project will help to assess the relationship between land-based anthropogenic pressures and the coastal biogeochemistry in a changing Eastern Mediterranean Sea.

How to cite: Hassoun, A. E. R., Fakhri, M., Habib, M., Ouba, A., Jemaa, S., Mahfouz, C., Jaber, H., Ghanem, A., Tannous, M., and El Kheir, M.: Spatio-temporal survey of the coastal carbonate system offshore Lebanon-Levantine Mediterranean Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19618, https://doi.org/10.5194/egusphere-egu2020-19618, 2020.

Human-induced catchment changes have affected the sedimentary processes in marginal seas, which will impact the transport and burial processes of materials and inevitably impact marine biogeochemical cycles. Polycyclic aromatic hydrocarbons (PAHs) in surface sediments from the East China Sea (ECS) at two time nodes (2006 and 2018) were compared to understand the response of PAHs to human-induced catchment changes. PAH concentrations in the ECS ranged from 8–414 ng g^-1 (dry weight), with a mean value of 112±77 ng g^-1, relatively lower than that in 2006 (38–308 ng g^-1, with a mean of 122±60 ng g^-1). Sharp decreases in sediment loads have triggered erosion in subaqueous delta and changed the distribution of sediment components, which may eventually influence the distribution pattern of PAHs. The obvious spatial differentiation of PAHs between 2006 and 2018 suggested that the depositional center of PAHs shifted from the estuary to the inner shelf area. PAH deposition patterns in the ECS were primarily influenced by riverine input before 2006, but are now dominated by winnowing processes related to long-distance transport due to sharply decreased sediment loads. Dam construction in the river catchment intercepted large amounts of sediments and PAHs, shifting the Changjiang-derived PAH depositional center from the ocean to reservoirs. Overall, depositional patterns of PAHs in the ECS were largely altered by human-induced catchment changes, which may cause significant impacts on the region’s biogeochemical cycles and ecosystem health.

How to cite: Wang, C., Hao, Z., Feng, Z., Zhang, C., and Zou, X.: Rapid changes in distribution and fate of Polycyclic aromatic hydrocarbons (PAHs) in sediments from the East China Sea and their response to human-induced catchment changes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2156, https://doi.org/10.5194/egusphere-egu2020-2156, 2020.

Despite the potential importance in the oceanic carbon cycle and benthic ecosystem, global feature of lateral supply of aged organic matter hosted on lithogenic particles derived from sediment resuspension has not been systematically examined. We compiled concentrations and fluxes of lithogenic material in the ocean in a global-scale by using literature data of sediment trap studies to understand the contribution of resuspended sediment to sinking particulate matter. We find that these contributions are significant in various oceanic settings, particularly over continental margins. Lithogenic material flux decreased with increasing distance from the margins and above the seafloor. Examination of Δ¹⁴C values of sinking POC revealed strong relationships with parameters that represent contribution of resuspended sediment. We then derive estimates for the contribution of aged POC from sediment resuspension to sinking POC based on these relationships and global lithogenic material flux data.

How to cite: Kim, M., Hwang, J., Eglinton, T. I., and Druffel, E. R. M.: Lateral particle supply as a key vector in the oceanic carbon cycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4740, https://doi.org/10.5194/egusphere-egu2020-4740, 2020.

Gulf of Ob - the closing estuary of the Ob River, where fresh and saltwater are mixing. This is a very large and long stretch of water: about 800km in length and 30 to 90 km in width. The impressive size of the Gulf of Ob and the impact on the Kara Sea (runoff 530 km3 / year) give to Ob Estuary regional significance. River Ob bringing the largest amount (75%) of freshwater to the Gulf of Ob - an important industry flux and transport artery of Western Siberia, which in turn creates anthropogenic load on the estuary (surfactants, oil products, excess amounts of organic substances).

Changes in salinity, acidity, alkalinity in frontal zones cause a chain reaction of subsequent physicochemical processes leading, in turn, to the deposition of more than 90% of sedimentary material and dissolved organic matter inputted by Ob. Inorganic forms flows of phosphorus from the sediment at the frontal zone is low, which is explained by the high content of Ferrum(III+) oxide.

The fluxes of silicon and nitrogen did not significantly change, however, high absolute values of the silicon content in the mixing zone of fresh and sea waters are observed, which may be associated with the phenomenon of "avalanche" sedimentation observed in this zone.

This work was supported by the grant of the Russian Science Foundation 19-17-00196 Dissolved transformation runoff in estuarine regions of Russian rivers of various climatic zones

How to cite: Borisenko, G.: Geographic features of the distribution of bottom fluxes of nutrients (N, P, Si) in the frontal zone of the Ob River estuary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-123, https://doi.org/10.5194/egusphere-egu2020-123, 2020.

A key challenge in understanding how climate change will impact continental shelf ecosystems is to understand the physical and chemical drivers of primary productivity in these systems and to assess the natural variability on spatial and temporal scales. Presently the impacts of climate change on ecosystem processes/services along the western Irish shelf are poorly known due to a lack of in situ data, this is most notable for the contribution from picoplankton. In this project, we were able to take advantage of the annual WESPAS Fisheries surveys along the Western Shelf waters from 47°N northwards to 58°30’N onboard the Celtic Explorer to obtain biogeochemical data for this region. A number of key Essential Ocean Variables (EOVs) have been measured annually since 2016; including nutrients, baseline optical measurements of CDOM and FDOM, phytoplankton abundance via flow cytometry (Accuri C6) and Chlorophyll concentration in surface waters. Utilizing data from the Sentinel series of satellite allows us then to examine in more detail the potential drivers of picoplankton abundance and their impact on C and other elemental biogeochemical cycles in these waters.

The overarching aim of this work is to provide baseline data for developing biogeochemical climatologies for this region and for determining Good Environmental Status (GES) as per the EU Marine Strategy Framework Directive.

This publication/presentation* has emanated from research supported in part by a research grant from Science Foundation Ireland (SFI) under Grant Number 13/RC/2092 and is co-funded under the European Regional Development Fund and by PIPCO RSG and its member companies.

How to cite: Mullins, M. and Croot, P. P.: Biogeochemistry of the Western Irish Shelf: The role of picoplankton as assessed by flow cytometry and remote sensing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8379, https://doi.org/10.5194/egusphere-egu2020-8379, 2020.

Mesoscale eddies formed at Eastern boundary upwelling systems (EBUS) are important vehicles for nutrients and carbon to the open oligotrophic ocean that influence the biogeochemistry on relatively small spatial scales (on the order of 100 km). They impact upper-ocean chemistry and biology through a number of processes. For example, in cyclonic eddies upward nutrient supply to the euphotic zone typically results in intensified primary productivity and changes in community structure, both of which affect export fluxes of carbon to the deep ocean. Therefore, the factors that control the (sub)mesoscale dynamics of the upper ocean are essential to understanding the efficiency of the biological carbon pump. However, the governing dynamical processes are largely unknown, and so is the overall biogeochemical and ecosystem response. To investigate the horizontal and vertical variability of phytoplankton and heterotrophic bacteria within and around mesoscale eddies, we collected samples along a zonal corridor of the westward propagation of eddies between the Cape Verde Islands and Mauretania as well as from a cyclonic eddy along this transect at high spatial resolution. In the eddy, we generally observed enhanced primary production, based on ¹⁴C incorporation, and heterotrophic microbial activity, based on ³H leucine incorporation, compared to the surrounding waters. Similarly, microbial heterotrophic respiration rates obtained from optode‐based oxygen consumption measurements during dark incubations were highest inside the eddy. However, the detailed eddy survey revealed a patchy distribution of all microbial process rates. The rates were highest in the Northern and Western periphery of the eddy where depth-integrated primary and heterotrophic production were more than three times higher than in the eddy core. The patchy distribution was also apparent from flow cytometry data, which showed higher relative abundances of larger eukaryotic phytoplankton (nanoplankton) compared to picoplankton in the most productive regions of the eddy. The higher activities were additionally accompanied by a higher relative abundance of high nucleic acid containing bacteria, which are considered the more active members of the given community compared to low nucleic acid-containing bacteria. The enhanced primary production, particularly in the Northern and Western eddy peripheries, will fuel export production particularly in these regions. To gain further insight into the organic carbon dynamics, data on the spatial distribution and the lateral and vertical fluxes of dissolved and particulate organic matter are currently underway. While our data confirm previous studies of enhanced biological activity within eddies formed in EBUS regions, it also indicates that the effect of variable phytoplankton and heterotrophic bacterial distributions and activity within an eddy leads to consequences for the spatial and temporal representativeness of measurements from only a few samples. This study thus contributes to a more comprehensive view on the functioning of eddy dynamics and it will facilitate modelling efforts on the role that eddies play in the ocean carbon budget.

How to cite: Becker, K. W., Devresse, Q., and Engel, A.: Variations of microbial activity and diversity in mesoscale eddies formed in the Eastern boundary upwelling system off West Africa, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19914, https://doi.org/10.5194/egusphere-egu2020-19914, 2020.

Studying the carbon dynamics of estuarine sediment is crucial to understanding of carbon cycle in the coastal ocean. This study is designed to investigate the spatial variability of organic (TOC) and inorganic carbon (TIC), and to explore the mechanisms regulating their dynamics in the Yellow River Estuary (YRE) and Liao River Estuary (LRE). Based on data of the surface sediment cores, we found that TIC (6.3-20.1 g kg^-1) was much higher than TOC (0.2-4.4 g kg^-1) in the YRE, but TIC (0.4 - 4.2 g kg^-1) much lower than TOC (0.1 - 8.7 g kg^-1) in the LRE. Both TOC and TIC were generally higher to the north than to the south in the YRE, and higher offshore than nearshore in the LRE, primarily due to the differences in kinetic energy level (i.e., higher to the south and nearshore). The ranges of C:N and δ¹³C_org were smaller in the YRE (2.1 - 10.1 and -24.26‰ ~ -22.66‰) than in the LRE (0.8 - 13.4 and -27.80‰ ~ -22.12‰). Our analysis suggested that TOC was mainly from marine sources in the YER, except in the southern shallow bay where approximately 75% of TOC was terrigenous. The contribution of terrestrial sources TOC was much higher in the nearshore area than in the offshore area in the LRE. The overall low levels of TOC were due to profound resuspension that could cause enhanced decomposition. On the other hand, high levels of TIC resulted partly from higher rates of biological production, and partly from decomposition of TOC associated with sediment resuspension. The isotopic signiture in TIC seems to imply that the latter is dominant in forming more TIC in both the YRE and LRE, and there may be transfer of OC to IC in the water column. Further studies with integrative and quantitative approaches are needed not only to assess the spatial and temporal variations of major carbon forms in the water column and sediments, but also to quantify the contributions of various sources and transformations among the different carbon pools, which aims to better understand the carbon cycle in northern China in a changing climate.

How to cite: Yu, Z., Wang, X., Hu, L., and Yao, W.: Spatial variability and driving factors of carbon in typical estuarine sediments in northern China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4972, https://doi.org/10.5194/egusphere-egu2020-4972, 2020.

A high resolution three-dimensional (physical-biogeochemical) numerical model of the Northern Adriatic Sea has been implemented by coupling the European general circulation model - NEMO (Nucleus for European Modeling of the Ocean, https://www.nemo-ocean.eu/), with the marine biogeochemical model BFM (Biogeochemical Flux Model, bfm-community.eu/).

The modeling system is implemented with a horizontal resolution of about 800 m and a vertical resolution of 2 m, in z coordinates. The NEMO model is off-line nested at its open boundary with the Mediterranean Sea physical model of the Copernicus Marine Environment Monitoring Service (CMEMS, http://marine.copernicus.eu/).

The BFM component of the modeling system now includes a detailed and explicit representation of the benthic biogeochemical cycling (benthic fauna, organic matter, nutrients), as well as the dynamics of the benthic-pelagic processes.

The inclusion of the benthic dynamics in the 3D biogeochemical modeling of a shallow coastal basin, such as the Northern Adriatic Sea, represents an innovative application in the field of coastal and shelf biogeochemistry, since benthic biogeochemical processes can significantly constrain the coastal environmental dynamics.

Simulations have been performed in hindcasting mode with interannually varying physical (surface heat and water fluxes, including river runoff) and biogeochemical (river nutrient load) forcing. Results are validated against available observations from in situ and satellite platforms for sea surface temperatures, chlorophyll-a and dissolved inorganic nutrients, in order to explore the sensitivity of the pelagic environment to the inclusion of an explicit benthic dynamics and to evaluate issues related to model coupling and error/prediction limits.

The study is carried out in the framework of the European Project H2020 "ODYSSEA" (Operating a network of integrated observatory systems in the Mediterranean SEA, http://odysseaplatform.eu/), with the final goal to build an on-line forecasting modeling system of the Northern Adriatic Sea.

How to cite: Scroccaro, I., Zavatarelli, M., and Lovato, T.: Modeling the biogeochemical dynamics of the Northern Adriatic Sea with an explicit benthic-pelagic coupling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13458, https://doi.org/10.5194/egusphere-egu2020-13458, 2020.

Abstract: EGU 2020

Session: BG4.1: Biogeochemistry of coastal seas and continental shelves (Helmuth Thomas)

Mona Norbisrath¹, Kirstin Dähnke¹, Andreas Neumann¹, Justus van Beusekom¹, Nele Treblin¹, Bryce van Dam¹, Helmuth Thomas¹

¹Institute for Coastal Research, Helmholtz-Zentrum Geesthacht

Contact: mona.norbisrath@hzg.de

In-situ investigation of alkalinity - denitrification coupling in the sediment - water column interface

As a shallow shelf sea, the North Sea is very vulnerable to anthropogenic impacts like rising CO₂ concentrations, increasing nutrient inflows and coincident oxygen loss.

Two important processes that determine the role of the coastal ocean as a net sink for anthropogenic CO₂ are alkalinity and denitrification. Alkalinity, the acid binding capacity of the ocean, buffers natural and anthropogenic changes in the oceans’ CO₂ and pH system. Denitrification, an anaerobic microbial process in which organic matter is respired, uses NO₃^- instead of O₂ as a terminal electron acceptor. Denitrification reduces NO₃^- to N₂ and in turn produces alkalinity.

Eutrophication, caused by leaching of excess fertilizer nutrients into coastal seas, leads to enhanced denitrification and therefore to enhanced alkalinity as well as an increased uptake of CO₂. However, the quantitative relationship between denitrification and alkalinity production and its control under changing environmental conditions is yet to be determined.

In the German Bight, denitrification is usually restricted to anoxic sediments. In this study, we therefore focus on in-situ experiments in the sediment - water column interface. Batch core incubations in combination with the isotope pairing technique (IPT) and labelled nitrate additions were used to detect denitrification and gauge its effect on alkalinity production during a cruise on RV Heincke (HE541) in September 2019 in the German Bight. To quantify denitrification, the production of all three N₂ isotope species (²⁸N₂, ²⁹N₂ and ³⁰N₂) is measured using a membrane inlet mass spectrometer (MIMS). We expect an increase of denitrification rates with nitrate concentrations and incubation times, and we will quantify benthic denitrification. We will further evaluate the assumption of concurrent increases in alkalinity production and will investigate the benthic-pelagic coupling of these processes. Investigating the in-situ interaction of metabolic alkalinity and denitrification will give an estimation of the alkalinity impact on the reduction of anthropogenic CO₂ in the atmosphere.

How to cite: Norbisrath, M.: In-situ investigation of alkalinity - denitrification coupling in the sediment - water column interface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1555, https://doi.org/10.5194/egusphere-egu2020-1555, 2020.

The Earth system has entered a new geological epoch, the Anthropocene. The oceans’ capacity to regulate atmospheric carbon dioxide (CO 2 ) at various
timescales is amongst the most crucial players to maintain climate on Earth in a habitable range. The biogeochemical property exerting this regulatory mechanism is alkalinity, the oceans’ CO 2 and pH buffer capacity. The proposed project will investigate how the oceans’ alkalinity is impacted firstly by human measures, required by the Paris agreement (COP 21) to mitigate climate change via bioenergy production and its downstream effects on shallow oceans, and secondly by climate change, in particular by increased weathering in the Arctic because of ice retreat.

How to cite: Thomas, H., Norbisrath, M., Treblin, N., van Dam, B., Pätsch, J., and Emeis, K.-C.: The Ocean's Alkalinity: Connecting geological and metabolic processes and time-scales, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2855, https://doi.org/10.5194/egusphere-egu2020-2855, 2020.

Investigation of different anaerobic respiratory pathways and their impacts on the release ratio of DIC/alkalinity at selected North Sea regions

Tentative authors: Nele Treblin^1,2, Michael E. Böttcher³, Tristan Zimmermann¹, Daniel Pröfrock¹, Mona Norbisrath¹, Bryce van Dam¹, Helmuth Thomas¹

¹Institute for Coastal Research, Helmholtz Center Geesthacht

²Alfred-Wegener-Institute Helmholtz Center for Polar and Marine Research

³Leibniz Institute for Baltic Sea Research Warnemünde

Coastal sediments play a crucial role in carbon metabolism, which decreases with increasing distance from the shoreline. The North Sea, a NW European shelf sea, represents a relatively shallow, well-ventilated (on annual timescales) system, connected to the Baltic Sea and the North Atlantic. Especially the southern part of the North Sea receives a large amount of organic matter (OM), both from riverine input and internal North Sea sources. After the depletion of oxygen due to aerobic OM respiration, anaerobic metabolic activities become dominant in the sediment. In the absence of oxygen, electron acceptors, such as NO₃^-, Fe³⁺, Mn⁴⁺ and SO₄^2-, facilitate not only the release of respired CO₂, but also of alkalinity, furthermore enhanced by potential dissolution of sedimentary carbonates. Therefore, under these conditions, benthic-pelagic coupling may impact on the potential to absorb CO₂ from the atmosphere.

To investigate the described processes, porewater and sediment samples, collected from six different stations in the German Bight (North Sea) during the RV Heincke cruise HE541 in September 2019, have been analyzed for their vertical concentration profiles of nutrients, various trace metals, sulfur, DIC and alkalinity.

Benthic oxic and anoxic zones have been identified based on the vertical concentration gradients. Furthermore, alkalinity and DIC are set in relation to anaerobic metabolic activities. Finally, active reworking and ventilation becomes pivotal in areas such as the North Sea. Thus, the influence of bioturbation on anaerobic respiration is also considered.

How to cite: Treblin, N., Böttcher, M. E., Zimmermann, T., Pröfrock, D., Norbisrath, M., van Dam, B., and Thomas, H.: Investigation of different anaerobic respiratory pathways and their impacts on the release ratio of DIC/alkalinity at selected North Sea regions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1556, https://doi.org/10.5194/egusphere-egu2020-1556, 2020.

The modern increase in atmospheric CO₂ driven by fossil fuel combustion and land-use change is warming our atmosphere and surface oceans. The absorption of this excess CO₂ by the oceans decreases seawater pH in a process known as ocean acidification (OA), which represents a threat to marine ecosystems with adverse impacts on coral health. It is important to understand how modern climate change impacts interannual and interdecadal climatic cycles and atmospheric phenomena which are originating in the Pacific and modulating global climate. There is a scarcity of data necessary to study the impacts of these changes on natural variability on longer timescales. In this study, we present multi-proxy (e.g. Sr/Ca, δ¹⁸O, δ¹³C, B/Ca) reconstructions of sea surface temperature (SST), surface seawater carbonate chemistry, with implications for pH variability of the South Pacific back to preindustrial times. This region of the Pacific is interesting for tracking the development of OA because of the well-constrained interannual to interdecadal SST and SSS variability from existing coral-based reconstructions. Massive corals (Porites sp.) from Rotuma and Tonga will be analyzed to extend the currently available SST reconstructions and expand the spatio-temporal coverage beyond the instrumental records. New monthly-resolved SST records will provide larger analyses exploring the influence of interannual and decadal-interdecadal climatic fluctuations on CO₂ absorption and pH variation. We aim to quantify the anthropogenic impact on SST, pH and the ocean carbonate system to achieve a better understanding of the status in the South Pacific under open ocean conditions.

How to cite: Todorovic, S., C. Wu, H., K. Linsley, B., Kuhnert, H., and Dissard, D.: Preindustrial to modern variability of sea surface temperatures and CO2 uptake in the South Pacific, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21539, https://doi.org/10.5194/egusphere-egu2020-21539, 2020.

Aiming to support the prediction of future climate developments, this project investigates the role on geological timescale of the ocean plankton in reducing atmospheric carbon concentration by exporting carbon to the deep-sea. While it is well-known that the transition from the Eocene to the Oligocene brought significant climate changes and, in connection, also a change of the oceans’ carbon export production, the important role of phytoplankton and the links to changing ocean circulation are still poorly understood, as is, similarly, the impact on those changes on the diversity of the plankton contributing to the carbon pump. Investigating the nature of this interaction will provide significant insight into the functions of the oceans as climate regulators.

To address those question, we are generating diversity and absolute abundance data for diatoms and radiolarians, biogeographic data for radiolarians, as well as oxygen and carbon isotope data on planktic and benthic foraminifera, and on the fine fraction (<45µm, i. e. coccoliths), as well as other proxies to estimate surface and deep ocean temperatures and export productivity. These will be generated as paired data from individual samples in various deep-sea drilling sites in and around the Southern Ocean (as it is the focal point of the climatic/oceanographic changes at that period). These data will then be compiled and confronted to an ocean circulation model.

Here we will present our results so far (oxygen and carbon isotope on the bulk fine fraction, as well as radiolarian and diatom diversity estimates), based on two main localities from the antarctic (ODP Site 689B from the Weddell Sea) and the subantarctic (ODP Site 1090B on the southern flank of the Agulhas ridge) South Atlantic. A comparison with a newly generated, database-driven diversity analysis of the same groups in the same region, using the Neptune (NSB) database, will also be shown. While the exhaustive taxonomical compilation made on these two sites for the diatoms records three times more species than what was recorded in the literature for the Southern Ocean biome, it still shows an evolutionary turnover at the Eocene-Oligocene, just as the classic, NSB-driven analysis does. The fine fraction oxygen isotope at both sites 689B and 1090B show a pattern similar to that recorded in planktonic foraminifera in neighbour sites, indicating a significant drop in SST close to the Eocene-Oligocene boundary, while the fine fraction carbon isotope signal in the antarctic site shows a subsequent decrease indicating changes in exported productivity 2Myr after the global cooling.

How to cite: Rodrigues de Faria, G., Lazarus, D., Struck, U., Asatryan, G., Renaudie, J., and Ozen, V.: Paleogene Polar Plankton and export productivity changes between the Eocene and Oligocene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5924, https://doi.org/10.5194/egusphere-egu2020-5924, 2020.

Clouds and their interaction with short- and longwave radiation represent one of the major uncertainties in our understanding of global climate change. The presence of clouds, particularly of bright low-level water clouds, doubles the Earth’s albedo and they are responsible for half of the solar radiation reflected into space.
Contrary to spaceborne, polar-orbiting observations which are of great detail at fixed time we focus on spaceborne time-resolved measurements of the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) aboard Meteosat Second Generation. We present an innovative method to track warm low-level clouds. The method widely used in experimental fluid mechanics and known as particle image velocimetry (PIV) [1, 2] relies on basic pattern matching. The principle of pattern matching is usually referred to as cross-correlation. It tells us something about displacements and enables the reconstruction of cloud trajectories. Thereby, we quantify cloud development and in combination with the CLAAS-2 dataset [3] we characterize temporal changes of cloud properties.

References
[1] Keane, R. D., Adrian, R. J.: Theory of cross-correlation analysis of PIV images. Applied Scientific Research 49, 191–215 (1992). DOI: 10.1007/BF00384623

[2] Tropea, C., Alexander, L., Yarin, L., (Eds.), F.: Handbook of experimental fluid mechanics. Springer (2007)

[3] Benas, N., Finkensieper, S., Stengel, M., van Zadelhoff, G.-J., Hanschmann, T., Hollmann, R., Meirink, J. F.: The MSG-SEVIRI-based cloud property data
record CLAAS-2. Earth System Science Data 9(2), 415–434 (2017). DOI: 10.5194/essd-9-415-2017

How to cite: Seelig, T., Hu, Y., Deneke, H., and Tesche, M.: Quantifying cloud development from geostationary observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3544, https://doi.org/10.5194/egusphere-egu2020-3544, 2020.