Many foraminifera form shells made of calcium carbonate. The elemental and isotopic composition of these shells varies greatly from inorganically precipitated calcites, suggesting a strong biological control on the process of CaCO₃ precipitation. Moreover, this composition differs, sometimes greatly, between species, which may indicate that the controls on calcite chemistry is not fixed among all species. For paleoceanographic application, a better grip on this inter-species variability in calcite chemistry is necessary. Here we present the latest insights in environmental controls on element incorporation, biomineralization mechanisms and evolutionary patterns in biomineralization. An integrated understanding of foraminiferal calcification will also allow predicting their response to changes in marine inorganic carbon chemistry (e.g. ocean acidification), which in turn, is necessary to assess the contribution of changes in foraminiferal calcification rates to (surface) marine inorganic carbon cycling.

How to cite: de Nooijer, L., Pacho Sampedro, L., Francois, D., Karancz, S., and Reichart, G.-J.: Foraminiferal biomineralization: mechanisms, calcite chemistry and evolution, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19534, https://doi.org/10.5194/egusphere-egu24-19534, 2024.

Sulfur over calcium ratio (S/Ca) in foraminiferal calcite has been suggested as a potential tool to reconstruct seawater carbonate ion concentration ([CO₃^2-]). The approach of using sulfur incorporation as a proxy for the carbon system was based on benthic foraminiferal controlled growth experiments, which suggested that foraminifera incorporate more sulfur when there is less [CO₃^2-] available in the seawater. With sulfate ([SO₄^2-]) being proposed to be the dominant form in which sulfur is incorporated in the calcium carbonate of the foraminiferal shells, S/Ca would provide an independent parameter for the reconstruction of seawater inorganic carbon chemistry. To further explore the potential of this proxy, we used five planktonic foraminiferal species collected from the field. S/Ca values in planktonic foraminifera collected from core-top sediments that span a large range of growth conditions (temperature, salinity, [CO₃^2-] and [HCO₃^-]) reveal an opposite trend with [CO₃^2-] compared to the results from the benthic foraminifera culture experiments. Moreover, we found an additional effect of incorporation of Mg on S/Ca ratios, or a combined effect on both. Using the ratio of S to Mg overcomes this issue and S/Mg ratios correlate with [CO₃^2-]. Still, these correlations are likely affected by multiple parameters and/or incorporation pathways other than only SO₄^2- as suggested by our inorganic calcite growth experiments. Results of this study suggest a critical evaluation of the use of foraminiferal S/Ca, considering the aqueous species involved during uptake and potentially combining other elements that may share controls.

How to cite: Karancz, S., Uchikawa, J., de Nooijer, L. J., Wolthers, M., Conner, K., Hite, C., Brummer, G.-J. A., Lattaud, J., Haghipour, N., Rosenthal, Y., Zeebe, R. E., Sharma, S., and Reichart, G.-J.: Sulfur incorporation in (foraminiferal) calcite, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19571, https://doi.org/10.5194/egusphere-egu24-19571, 2024.

Ocean water yields an integrated global signal of geological and biological processes operating on our planet, which in turn control the marine C cycle, oceanic alkalinity budget and atmospheric CO₂ levels. Therefore, reconstructing the chemical and isotope composition of seawater and/or coastal waters through time represents one of the main research objectives of earth system evolution studies.

Here we present stable and radiogenic Sr isotope variations (δ^88/86Sr and ⁸⁷Sr/⁸⁶Sr) measured in waters and carbonates from modern and Holocene coastal marine system in South Australia (Coorong Lagoon/Murray River Estuary) that is connected to the Southern Ocean, thus exhibiting large gradients in water chemistry, salinity and carbonate saturation (Mosley et al. 2023; Shao et al. 2021). The studied hydrological system shows a large salinity range from brackish (<20 psu) to normal marine (~35 psu) and hypersaline (~110 psu), with the salinity changes being tightly linked to DIC and Alkalinity, and thus CaCO₃ saturation state (SI values) of local waters, calculated via PHREEQC. The primary research aim was to assess how spatial and temporal changes in salinity, carbonate chemistry and CaCO₃ saturation (dissolution vs precipitation of carbonates), monitored throughout the year (in spring, summer, fall and winter), impact the Sr isotope composition in the present-day coastal marine system. Such knowledge is, in turn, important for a better calibration and assessment of the δ^88/86Sr proxy for paleo-oceanographic and environmental applications including past marine alkalinisation/acidification events and/or paleo-salinity reconstructions (Farkas et al. 2024; Shao, 2022).

Importantly, our results from seasonal sampling and monitoring showed that the δ^88/86Sr in waters is positively correlated with their SI values (carbonate saturation) and salinity, with the heaviest or most positively fractionated stable Sr isotope signatures of +0.48 ± 0.03‰ (thus above ‘normal seawater’ of +0.39 ‰) measured in summer season (hot and dry period) in hypersaline (>70 psu) and oversaturated (SI ~1) waters. In contrast, the isotopically light and systematically lower stable Sr isotope signatures (< 0.35‰) are documented in brackish waters that are also undersaturated with respect to CaCO₃ minerals (SI < 0). Overall, these results point to the primary control of carbonate dissolution versus precipitation phenomena, and thus CaCO₃ saturation, on the δ^88/86Sr proxy in the modern coastal marine system.

Finally, we will also illustrate how a coupled δ^88/86Sr and ⁸⁷Sr/⁸⁶Sr approach can be applied for paleo-salinity reconstructions of the coastal marine systems, such as the Coorong Lagoon, based on the Sr isotope analysis of recent and fossil carbonate archives (bivalve shells) recovered from local sediment cores (Shao, 2022). Briefly, available results and geochemical modeling of the Sr isotope data from Holocene fossil shells suggest that over the last ~2400 years the Coorong Lagoon become progressively more evaporitic, exhibiting a temporal shift from a purported brackish paleo-lagoon to the present-day hypersaline carbonate producing system.   

References

Farkas et al. (2024) Treatise on Geochemistry, Third Edition. Elsevier (Book Chapter 00086)

Mosley et al. (2023) Marine Pollution Bulletin, 188, 1-16.  

Shao (2022) PhD Thesis, University of Adelaide 

Shao et al. (2021) GCA, 293, 461-476.  

How to cite: Farkas, J., Shao, Y., Mosley, L., Tyler, J., Tibby, J., and Eisenhauer, A.: Calibrating stable Sr isotope proxy for paleo-oceanographic studies: Insights from δ88/86Sr variability in modern and Holocene coastal marine system in South Australia , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7103, https://doi.org/10.5194/egusphere-egu24-7103, 2024.

The chemical history of seawater provides key information on Earth’s geologic processes and is fundamental for robust CO₂ reconstructions. The knowledge of the secular evolution of the oceanic boron isotope budget is particularly important for CO₂ reconstruction from boron isotopes. The boron isotope composition of seawater (δ¹¹B_sw) is homogeneous, but varies on multi-million year time scales, given its residence time of approximately 10 million years. To date, the secular evolution of the oceanic boron isotope budget has been difficult to constrain, posing a major uncertainty for boron-based pH and CO₂ reconstructions from Earth’s geologic past and critically limiting our understanding of the global biogeochemical cycling of this important element through time. Evaporitic minerals bearing fluid inclusions – and halites in particular – have provided important insights on past variations in major and minor ion composition, and present a highly appealing archive for reconstructing δ¹¹B_sw (as well as other isotopic systems) given their direct origin from seawater. However, the interpretation of their signatures is not straightforward due to the possibility of fractionation during evaporation, crystallisation, and local biogeochemical interactions. Here we present data illuminating the evolution of boron isotopes and various other elements during evaporite formation from laboratory experiments and natural modern evaporitic settings across the globe, accompanied by new analytical developments for high-precision single fluid inclusion measurement using laser ablation. These data enable us to critically evaluate the evaporite archive, paving an avenue to robust seawater and CO₂ reconstructions from Earth’s geological past.

How to cite: Jurikova, H., Bodnar, R., Branson, O., Dumont, M., Evans, D., Gázquez, F., Kirichenko, Y., Lazar, B., Liang, M.-C., Lowenstein, T., Sendula, E., Voigt, C., Xu, C., Zheng, X., and Rae, J.: Ocean chemistry archived in modern evaporites: implications for robust seawater and CO2 reconstructions from Earth’s past, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13608, https://doi.org/10.5194/egusphere-egu24-13608, 2024.

Despite the growing recognition of in-situ silicate alteration (dissolution and formation) in marine sediments, its global significance and controlling factors are still poorly understood. By compiling information from scientific ocean drilling programs and applying numerical modelling, we aim to 1) provide constraints on the environmental parameters of silicate dissolution in marine sediments, 2) identify silicate phases responsible for the hyper porewater alkalinity (>56 meq/L) commonly observed from productive continental margin sediments, and 3) investigate the interplay between silicate dissolution, clay formation, and carbonate authigenesis as well as their effect on marine carbon cycling. 

Through numerical modelling, we show that alkaline conditions resulting from combined iron and sulfate reduction favour formation of smectite group clay minerals, while the acidic conditions arising from organic matter fermentation promote dissolution of saponite and several mica-group silicates. This result resonates with previous observations of reverse weathering (i.e. clay formation) in shallow iron- and/or sulfate reducing sediments, while silicate weathering (i.e. silicate dissolution) has been reported deeper in methanogenic sediment columns. 

Using pore fluid composition data, we show that marine silicate weathering is primarily driven by dissolution of K- and Mg-containing silicate minerals. Especially, higher-than-seawater Mg concentrations were observed in almost all sites that have hyper alkalinity and the weathering process contribute more than one-third of the measured alkalinity. No apparent difference was observed for porewater Ca concentrations when comparing sites with and without hyper alkalinity, which hints for complicated feedbacks through authigenic carbonate formation. 

The global dataset analysed revealed that sites with high alkalinity correspond to locations with a medium distance from shore. While such a pattern cannot be easily explained by supply of organic matter nor by silicate phases alone, we interpret this observation to be the result of sediment maturity. Our inference is further strengthened by observations of higher alkalinity at sites with greater thermal history within the methanogenesis zone, a factor that measures how much time and temperature a sediment parcel has experienced under subsurface conditions. Collectively, we conclude that substantial dissolution of marine silicate phases occurs when the sediments have been transported some distance offshore and buried below sulfate reduction zone for a prolonged period and/or experience sufficiently high geothermal heating.

We simulated alteration of silicate and carbonate phases within a complete early diagenetic sequence to understand how dissolved carbon is converted to alkalinity under variable organic matter degradation rates. We show that authigenic carbonate formation is effective in control downcore DIC/alkalinity level with a moderate organic matter degradation rate. Only a very limited amount of carbonic acid produced by reverse weathering can diffuse away from sediments. Under a scenario with fast organic matter fermentation, dissolution of silicates (such as phlogopite) becomes the only buffer for porewater pH that converts most of the dissolved inorganic carbon produced from organic matter fermentation to carbonate alkalinity. Consequently, marine weathering sustained by silicate mineral dissolution increases the alkalinity production by as much as 16%, with most of the alkalinity leaking to surface oxic sediments instead of being sequestrated as carbonate minerals. 

How to cite: Hong, W.-L., Sun, X., Huang, T.-H., and Torres, M.: The role of marine silicate alteration in regulating carbon cycling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4178, https://doi.org/10.5194/egusphere-egu24-4178, 2024.

Global warming has the potential to release large quantities of methane (CH₄) from marine sediments, representing a positive carbon cycle-climate feedback [1]. Unambiguous evidence of this feedback in the geological record will improve understanding of the potential risk it poses for exacerbating anthropogenic global warming. For example, climate driven sedimentary CH₄ release is one of the hypothesized mechanisms for the onset of the Paleocene-Eocene Thermal Maximum (PETM) [2]. Increased sedimentary barium (Ba) burial rates have been interpreted as evidence of this mechanism [2,3], but these records are also sensitive to other processes [4]. Stable Ba isotope variations are a new geochemical tool that may improve interpretations of such records, potentially leading to clearer geological insights into the significance of this carbon cycle-climate feedback.

This study aims to determine (1) the flux and isotope composition of Ba across the sediment-water interface associated with seafloor CH₄ venting and (2) the significance of these fluxes for the marine Ba inventory. To achieve this, Ba concentration and isotope data is presented for seawater samples at different altitudes above the seafloor (1m to 60m) collected across the Regab pockmark, a methane cold seep offshore Congo. Samples were collected with a remotely operated vehicle, providing a high resolution of sample collection within the benthic boundary layer, spanning areas of varying CH₄ venting fluxes.

The measured Ba isotope values from all sites possess δ^138/134Ba values +0.20 to +0.40 ‰ and [Ba] values 80.0 – 90.6 nmol kg^-1, which are typical of ambient seawater from this depth range. Furthermore, no difference in dissolved Ba isotopes or Ba concentrations with altitude at each location is observed and there is no significant difference in seawater [Ba] and Ba isotope composition between locations featuring different dissolved seawater CH₄ concentrations.

These results are interpreted to show that there is no resolvable difference in the [Ba] vs. δ^138/134Ba relationship over the pockmark, and any Ba fluxes are too small to resolve in a circulating water column. This likely reflects the quantitative removal of pore water Ba by barite precipitation within the upper sediments, preventing significant Ba release to the water column.

The findings indicate CH₄ seeps do not seem to significantly impact either the dissolved Ba concentration or isotope composition of the ocean, and consequently makes the use of sedimentary Ba concentration and isotope records as a tracer of past CH₄ release events questionable. These insights suggest caution should be held when developing Ba isotopes as a novel tracer of past large-scale seafloor methane release i.e. PETM sediments. The study also provides insights on the influence of methane seep environments on Ba isotopes and the factors governing the stable isotope distribution of Ba in both modern and ancient sediments and oceans.

Reference:

[1] James et al., (2017), Limnology and Oceanography, 61, S283-S299

[2] Dickens et al., (2003), GSA Special Paper, 369, 11-23

[3] Frieling et al., (2019), Palaeoceanography and paleoclimatology, 34, 546-566

[4] Bridgestock et al., (2019), Earth and Planetary Science Letters, 510, 53-63

How to cite: Petrou, E., Bridgestock, L., Henderson, G. M., Hsieh, Y.-T., Bayon, G., and Lemaitre, N.: Examining the utility of barium isotopes as a tracer of large-scale seafloor methane venting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18309, https://doi.org/10.5194/egusphere-egu24-18309, 2024.

The North Atlantic Ocean plays a critical role in the global circulation system, regulating the penetration of surface waters into the deep ocean, but also in key ocean biochemical cycles (e.g. carbon, oxygen, nutrients). Meridional heat and salt transport (i.e. buoyancy) drive the formation of different water masses and their circulation pathways. A relatively unknown but important element controlling the net meridional export of heat, salt and other chemical species into the North Atlantic is the Mediterranean Outflow Water (MOW): the salt injector. In this work, we present the first high resolution systematic study of traditional (T, S, Nutrients) and novel (Nd isotopes, alkalinity, pH) geochemical parameters of MOW waters from its source area at the Strait of Gibraltar up to the northern Iberian margin (Cantabric Sea). During the TRANSMOW cruise in spring 2021, over 500 seawater samples were collected along the main MOW pathway following its northward flow. A comprehensive suite of geochemical parameters including ε_Nd, alkalinity, pH and preformed nutrients were analyzed for these samples. We show that MOW can be ‘traced’ unequivocally using ε_Nd as a conservative tracer, a feature that opens a new set of possibilities to better estimate the contribution of MOW export to higher latitudes in the North Atlantic Ocean. Other parameters directly linked to the carbon cycle (alkalinity and pH) are also controlling the distinctive chemical properties of the Mediterranean waters.. One of the key advantages of these geochemical tracers is that they allow to better quantify export and mixing rates of MOW with North Atlantic waters. Using statistical tools such as the Optimum Multi-Parameter Analysis (OMPA) on an array of conservative tracers we have quantified mixing rates and exports between different water masses. These results will be fundamental to better constrain paleoreconstructions in the sedimentary record using different proxies such as Nd and B isotopes (for water mass distribution and pH), B/Ca ratios (for seawater carbonate ion saturation) and even new experimental proxies such as Na/Ca (for salinity).

How to cite: Pena, L. D., Campderrós, S., García-Solsona, E., Paredes-Paredes, E., Frigola, J., Rodríguez-Díaz, C. N., Lucas, A., Calvo, E., Pelejero, C., and Cacho, I.: Geochemical characterization of Mediterranean Outflow Waters in the modern ocean: Nd isotopes, carbon cycle and new export constraints, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16921, https://doi.org/10.5194/egusphere-egu24-16921, 2024.

We present a novel mechanistic representation of both organic and inorganic marine particles within the Bern3D Earth system model of intermediate complexity, which is based on the columnar particle flux model MSPACMAM. This new approach moves away from the assumption of globally and temporally invariant sinking profiles. Instead, the new scheme calculates sinking speeds and remineralisation and dissolution rates based on local temperature, density, and seawater chemistry. When combined with an improved representation of dynamic iron release and scavenging, this scheme introduces new dynamic feedbacks in the response of the biological pump to climate and circulation changes in the Bern3D model. Particle remineralisation and dissolution rates are now functions of temperature, oxygenation, saturation state, and sinking speed. The sinking rate is in turn modulated by changes in export production (amount and composition) as well as the viscosity of seawater. In addition to light and macronutrients, export production is affected by iron availability, which is depending on the rate of iron removal through scavenging and iron release from decaying particles and sediments, both processes that depend on organic particle fluxes and local oxygen concentrations. We demonstrate the non-linear interactions between these new dependencies in transient and steady-state simulations of various climatic boundary conditions. The newly introduced particle concentrations sensitive to changes in temperature and density result in shallower carbonate dissolution and deeper organic particle remineralisation in the Southern Ocean under full glacial conditions. In idealized scenarios of anthropogenic climate change, there is a smaller decline of export production but faster oxygen depletion than with the old static particle decay scheme. In addition, organic particle fluxes affect sedimentary iron release, which can lead to a positive feedback on export production if the released iron reaches the surface ocean.

How to cite: Adloff, M., Dinauer, A., Laufkötter, C., Pöppelmeier, F., Jeltsch-Thömmes, A., and Fortunat, J.: Impacts of dynamically simulated biogenic particles and iron on the marine carbon cycle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16877, https://doi.org/10.5194/egusphere-egu24-16877, 2024.

Part of the carbon taken up by the ocean is transformed into biogenic particulate matter, that eventually leaves the surface ocean, settling toward the seafloor. Planktonic organisms secreting a calcium carbonate (CaCO₃) shell occupy a key, but ambivalent role in this scheme. First, the precipitation of their shell generates CO₂, thereby reducing the ocean CO₂ sink, while the sinking of their shell constitutes a direct export of carbon to the deep ocean. Meanwhile, the dissolution of their shells generates alkalinity, which in turn boosts the capacity of seawater to take up more CO₂ from the atmosphere.

CaCO₃ is present in the ocean under two main mineral forms: calcite (relatively stable) and aragonite (relatively soluble). Aragonite, produced in today’s oceans mostly by pteropods, a group of pelagic snails, has a very poorly understood role in the marine carbon cycle, and key questions remain unanswered: what controls their soft parts degradation and shell dissolution in the upper kilometer of the water column? How do both processes interact?

During the BEYΩND expedition that took place in March 2023 across the Southern Atlantic, we sampled pteropods using a multinet at 5 different depth ranges in the upper kilometer. Retrieved pteropods were representative of different life stages (adults, juveniles), some still well preserved, some dead with various stages of decomposition. Individuals were then preserved into ethanol, and later scanned with a micrometric resolution using microtomography. From the scans, the post-mortem degradation of the different body parts can be appreciated, shell micro-ornamentations can be seen, and possibly gut contents, which may influence dissolution and degradation processes. From the collected scans, a reactive transport model is then applied to predict in 3D the rates at which both organic matter degradation and aragonite dissolution occur, as well as how they interact.

How to cite: Sulpis, O., Chaurand, P., Kruijt, A., Cala, B., Peijnenburg, K. T., van Dijk, R., van der Burg, D., and Humphreys, M.: Post-mortem pteropod degradation in the Southern Atlantic twilight zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9768, https://doi.org/10.5194/egusphere-egu24-9768, 2024.

Phytoplankton acclimate to increased nutrient stress by decreasing their cellular quotas (nutrient:carbon ratios). Reducing cellular quotas reduces the export efficiency of the limiting nutrient, helping sustain biological productivity. Here we present a version of the Community Earth System Model with phytoplankton group specific, fully variable C:N:P:Fe:Si ratios constrained by field observations of particulate organic matter stoichiometry and individual cell spectroscopy. We compare the results of a steady-state fully fixed stoichiometry model to the fully variable model and find that using a fixed Redfield stoichiometry leads to a decrease of 1PgC/yr carbon export, increase of 18 ppm atmospheric CO2, decrease of 55 TgN/yr nitrogen fixation, and decrease of 27/yr TgN nitrogen fixation. We also investigate the impacts of variable nutrient acquisition on global patterns of nutrient limitation and find that the weaker ability of phytoplankton to acclimate to N stress by lowering their cellular quotas relative to other nutrients pushes marine ecosystems towards nitrogen limitation. Only when the nutrient supply ratios are highly skewed, exceeding the ability of the phytoplankton to acclimate, do other nutrients become growth-limiting, as with iron in the High Nitrate, Low Chlorophyll (HNLC) regions. We show that in the oligotrophic gyres, variable plankton stoichiometry, given sufficient time, pushes the marine ecosystems towards co-limitation, as non-limiting nutrients are more efficiently drawn down and exported (higher cellular quotas), relative to the growth-limiting nutrient (lower cellular quotas).

How to cite: Wiseman, N., Moore, J. K., and Martiny, A. C.: Phytoplankton variable elemental composition modifies the marine biological pump and largely determines the global patterns of nutrient limitation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12166, https://doi.org/10.5194/egusphere-egu24-12166, 2024.

Marine organisms, from plankton to fish, provide a wealth of ecosystem services, including carbon sequestration in a process known as the ocean’s biological carbon pump (BCP). The BCP brings carbon from the atmosphere to the ocean depths where it is stored for decades to centuries. Although parts of the ocean’s BCP are under threat from human activities,  BCP carbon sequestration rarely features as an objective in efforts to protect ocean spaces. Moreover, although BCP carbon sequestration services could support discussions of conservation and climate finance,  its economic value has yet to be estimated in space and time.

Biogeochemical modeling and mapping efforts have grown in recent years, and emerging results could potentially help to fill in important spatially explicit and economic knowledge gaps that could inform the protection of the BCP. We developed a new metric to map and quantify the global ocean’s BCP long-term carbon sequestration and computed its value on a potential carbon market. We show the  global spatial patterns and valuation in relation to geopolitical and management boundaries, and highlight options for governance and management. Our results highlight potential opportunities for preserving the climate services of the BCP both nationally and in areas beyond national jurisdiction , and can be used to inform discussions about marine protected areas, environmental impact assessment, and conservation finance.

How to cite: Berzaghi, F., Pinti, J., Aumont, O., Maury, O., and Wisz, M.: Distribution and valuation of the biological carbon pump and its carbon sequestration:  Implications for international area-based management and climate finance, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-723, https://doi.org/10.5194/egusphere-egu24-723, 2024.

The North Pacific (>20°N) stands out as a significant carbon sink, contributing to approximately 25% of the global oceanic CO₂ uptake and absorbing around 0.5 Pg C yr^-1 from the atmosphere. Despite the well-established importance of the biological carbon pump in maintaining this regional carbon sink, our current understanding of the strength and efficiency of the biological pump in this vast region remains incomplete. Historical studies have primarily relied on extrapolations from a limited number of observations.

In this study, we utilize data from 85 BGC-floats, covering over 160 annual cycles, to constrain essential fluxes relevant to the biological pump in the North Pacific, including net primary production, the export of distinct biogenic carbon, and air-sea CO₂ flux. Furthermore, we combine the output from a well-constrained regional ecosystem model (ROMS-CoSiNE-Iron Model) to gain mechanistic insights into how the food-web dynamics drive the strength and efficiency of the biological carbon pump across different ecosystems.

Overall, our study offers an integrated perspective on the North Pacific biological pump by leveraging high-resolution observations from the BGC-float array and simulation from an improved ecosystem model.

How to cite: Huang, Y. and Chai, F.: Integrated Perspective of the Biological Pump in the North Pacific: Synergy from BGC-Float Observations and Ecosystem Model Simulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4985, https://doi.org/10.5194/egusphere-egu24-4985, 2024.

Diatoms, a ubiquitous group of phytoplankton, account for approximately 40% of particulate organic carbon (POC) exported via the ocean biological carbon pump, which modulates atmospheric CO₂. Diatoms are represented in global biogeochemical models as effective vectors for sinking POC, with their large size and dense skeletons made of biogenic Silica (BSi) thought to allow rapid transfer of organic carbon to the ocean interior. However, we observe this not to be the case across large parts of the Southern Ocean mesopelagic zone. Here we present direct flux measurements from different sectors of the Southern Ocean demonstrating that silica and carbon cycles in the Southern Ocean mesopelagic are strongly decoupled, with a weak mechanistic link between BSi and POC fluxes. By combining Marine Snow Catcher flux measurements, in-situ pump, and CTD particulate data, we show that for a large part of the productive season, diatoms do not represent efficient vectors of sinking POC through the mesopelagic, yet POC is still efficiently transferred to depth. We suggest that processes influencing flux attenuation differ between the upper mesopelagic and deep ocean, with rapid BSi flux attenuation in the upper mesopelagic caused by elevated rates of BSi remineralization or negation of biomineral ballast effects by particle processes such as buoyancy regulation or fragmentation. Biomineral ballast may yet play an important role in shaping the efficiency of sinking POC transfer in the deep ocean. More broadly, these results highlight the need to understand the nuanced role this key taxon plays in transferring carbon through the mesopelagic, a region that is highly vulnerable to climate change effects and key in shaping the efficiency of downward carbon transport. As diatoms appear to be inefficient at delivering carbon to the deep ocean, projected losses in the strength of the Southern Ocean BCP due to shifts in phytoplankton community composition to smaller size classes may be less than previously predicted.

How to cite: Williams, J., Giering, S., Baker, C., East, H., Espinola, B., Le Moigne, F., Villa, M., Pabortsava, K., Blackbird, S., Pebody, C., Saw, K., Moore, M., Henson, S., Sanders, R., and Martin, A.: Reassessing the role of diatoms in carbon transfer through the Southern Ocean Twilight Zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12043, https://doi.org/10.5194/egusphere-egu24-12043, 2024.

The marine biological carbon pump (BCP) stores carbon in the ocean interior, isolating it from exchange with the atmosphere and thereby coregulating atmospheric carbon dioxide (CO₂). As the BCP commonly is equated with the flux of organic material to the ocean interior, termed “export flux,” a change in export flux is perceived to directly impact atmospheric CO₂, and thus climate. Here, we recap how this perception contrasts with current understanding of the BCP, emphasizing the lack of a direct relationship between global export flux and atmospheric CO₂. We argue for the use of the storage of carbon of biological origin in the ocean interior as a diagnostic that directly relates to atmospheric CO₂, as a way forward to quantify the changes in the BCP in a changing climate. The diagnostic is conveniently applicable to both climate model data and increasingly available observational data. It can explain a seemingly paradoxical response under anthropogenic climate change: Despite a decrease in export flux, the BCP intensifies due to a longer reemergence time of biogenically stored carbon back to the ocean surface and thereby provides a negative feedback to increasing atmospheric CO₂. This feedback is notably small compared with anthropogenic CO₂ emissions and other carbon-climate feedbacks. A comprehensive view of the BCP's impact on atmospheric CO₂, is a prerequisite for assessing the effectiveness of marine CO₂ removal approaches mediated by biology.

How to cite: Landolfi, A., Frenger, I., Kvale, K., Somes, C. J., Oschlies, A., Yao, W., and Koeve, W.: Misconceptions of the marine biological carbon pump in a changing climate: Thinking outside the “export” box, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9848, https://doi.org/10.5194/egusphere-egu24-9848, 2024.

Calcifying organisms produce calcium carbonate (CaCO₃) shells and skeletons. When they die, biogenic CaCO₃ is vertically exported from the euphotic zone and dissolves throughout the water column and in sediments. The alkalinity generated from this process can influence the ocean’s buffer capacity for absorbing atmospheric CO₂. However, the magnitude and driver of surface CaCO₃ export and subsequent dissolution in the ocean’s interior – a process called the carbonate pump – are highly uncertain. We present key drivers of pelagic CaCO₃ dissolution constrained by an inverse ocean biogeochemistry model combined with multiple observation databases. Within the upper twilight zone (shallower than 300 m), we found a tight association between particulate organic carbon remineralization rates and the CaCO₃ dissolution efficiency (the fraction by which the surface exported CaCO₃ dissolves), which is further supported by the observed particle flux and concentration data. In the deep ocean (deeper than 300 m), dissolution of CaCO₃ is primarily driven by conventional thermodynamics of CaCO₃ solubility with reduced fluxes of CaCO₃ burial to marine sediments beneath more corrosive North Pacific deep waters. Shallow CaCO₃ dissolution, shown to be sensitive to ocean export production, can increase the neutralizing capacity for respired CO₂ by up to 6% in low-latitude thermocline waters. Without shallow dissolution, the ocean might lose 20% more CO₂ to the atmosphere through the low-latitude upwelling regions – the world’s largest area of CO₂ outgassing in the contemporary climate. Our work identifies a previously overlooked sensitivity of oceanic CO₂ uptake to the biological pump. We suggest that Earth system models need to include the respiration driven CaCO₃ dissolution processes for a better projection of future oceanic carbon sink.

How to cite: Kwon, E. Y., Dunne, J., and Lee, K.: Present-day global patterns of the ocean carbonate pump and the key drivers of CaCO3 dissolution, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13592, https://doi.org/10.5194/egusphere-egu24-13592, 2024.

Zooplankton fecal pellets constitute a major component of passively sinking particles in the ocean. The sinking of zooplankton fecal pellets provides an efficient vehicle for the transfer and sequestration of particulate organic carbon in the deep sea, which has been widely reported in different ocean regions. However, most existing studies focus on the sinking flux of fecal pellets within the upper ocean, while the lower mesopelagic and bathypelagic zones are rarely investigated. Here, we report the spatiotemporal flux variation of zooplankton fecal pellets collected by two sediment traps deployed in mesopelagic and bathypelagic zones (500 m and 2190 m, respectively) of the southern South China Sea from June 2020 to May 2023. The average fecal pellet numerical flux is 3.21*10⁴and 4.64*10⁴ pellets m^-2 d^-1 at 500 m and 2190 m, respectively, corresponding to an average fecal pellet carbon flux from 0.43 to 0.84 mg C m^-2 d^-1 at these two depths. Fecal pellet fluxes display distinct seasonal patterns due to the control of the East Asian monsoon system, with higher fluxes in winter and spring, and lower fluxes in summer and autumn. Higher fecal pellet fluxes combining with the presence of extra-large pellets are found in bathypelagic zone, which is attributed primarily to in-situ reworking and repackaging of sinking particulate matter by deep-dwelling zooplankton communities, as well as lateral inputs from adjacent high productive continental coasts and shelves. We compare our results with global deep-sea (>500 m) fecal pellet flux data reported from different sediment-trap stations with distinct marine primary productivity and zooplankton biomass. Furthermore, we will report on the state of our progress on carbon isotope analysis (¹³C, ¹⁴C) for disentangling the source-to-sink dynamics of fecal pellets and its role in the deep-sea carbon export and sequestration.

How to cite: Li, J., Liu, Z., Lin, B., Zhao, Y., Zhang, X., Cao, J., Zhang, J., Song, H., Blattmann, T., Haghipour, N., and Eglinton, T.: Zooplankton Fecal Pellet Flux and Carbon Export in the Deep South China Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7805, https://doi.org/10.5194/egusphere-egu24-7805, 2024.