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CR7.1

Decreasing sea-ice coverage, increasing permafrost-derived inputs and increasing ice sheet and glacier discharge will continue to affect high latitude environments in the coming decades under all future climate scenarios. Such changes at the interface between the ocean and the cryosphere raise questions about the downstream effects in marine ecosystems, as increased meltwater discharge is likely to impact not only coastal hydrology but also biogeochemistry, sediment transport and ecosystem services such as fisheries and carbon sequestration. However, the impact of increasing melt on fjord and coastal environments is poorly constrained, impacting our ability to make predictions regarding the consequences of future climate change. In order to understand the effect of changing cryosphere-derived inputs on high latitude fjords and marine coastal environments, knowledge concerning the physical and biochemical perturbations occurring in the sea ice and water column and the structure, function and resilience of affected ecosystems must be integrated. In this session we explicitly welcome cross-disciplinary attempts to understand how far reaching the effects of sea-ice, permafrost derived material and glacial changes are on marine biogeochemistry, productivity, biodiversity, and ecosystem services. Topics may include, yet are not limited to, the effect of sea-ice, permafrost, and glacier discharge on sea-ice and water column structure, primary and secondary production, community structure, macronutrient and micronutrient availability, microbial processes, the carbonate system, and the biological carbon pump. Modelling experiments, and studies based on long-term observational records including sediment traps and proxy reconstructions from marine sediment cores are also welcome.

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Co-organized by BG4/CL4/OS3
Convener: Sofia Ribeiro | Co-conveners: Jade HattonECSECS, Mark HopwoodECSECS, Letizia Tedesco, Anna Pienkowski, Jonathan HawkingsECSECS, Susann Henkel, Hong Chin NgECSECS
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| Thu, 07 May, 16:15–18:00 (CEST)

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Chat time: Thursday, 7 May 2020, 16:15–18:00

D2641 |
EGU2020-12659
| solicited
Maya Bhatia, Stephanie Waterman, David Burgess, Patrick Williams, Megan Roberts, Charvanaa Dhoonmoon, and Erin Bertrand

When glaciers melt, they contribute significant quantities of water and ice, sediments and dissolved chemicals to the ocean. Recent efforts in Greenland and Antarctica show that both the delivery of materials to the marine environment, as well as local changes to ocean circulation induced by the input of freshwater, have the potential to profoundly impact key processes such as primary and secondary production, and by extension the biological carbon pump. Yet, extensive knowledge gaps remain about the chemical composition of glacial meltwater runoff at the ice-ocean interface, the spatial extent of its influence within coastal environs, and the mechanisms by which glaciers affect surface marine microbial communities. Nowhere are these knowledge gaps more prominent than in the Canadian Arctic Archipelago (CAA) – a region where the role of glacial meltwater in marine biogeochemical cycles is almost fully unexplored – despite the fact that it is a hotspot for glacial retreat and meltwater runoff to the ocean. Here, we conduct a regional comparative study of the nearshore coastal zone of glaciated and non-glaciated fjords and of multiple glaciers of varying type (land-terminating, tidewater) and size draining large ice caps. Our study site in Jones Sound, NU is home to the Inuit hamlet of Grise Fiord. Traditional knowledge from this community indicates that the termini of tidewater glaciers in this region are rich in wildlife, providing habitual hunting grounds for its citizens. Guided by this information, we combined shipboard measurements of temperature, salinity, turbidity, and chlorophyll a with bottle samples characterizing oxygen, sediment, carbon, nutrient, metal, and biological community composition to elucidate how these properties evolve with distance from the shore. Results from this study substantially further our understanding of glacier-ocean impacts in the CAA and beyond, while also providing data critical to accurate future projections of high-latitude marine ecosystem productivity and function in this era of climate change.

D2642 |
EGU2020-3638
| Highlight
Judith Braun, Sian Henley, Marie Porter, Tim Brand, and Keith Davidson

Rapid warming has been observed across the Arctic region in recent decades, resulting in a significant reduction in sea ice extent and duration which is most pronounced in the marginal ice zone. These sea ice changes and the resultant changes in ocean mixing are affecting nutrient supply, uptake and cycling and are predicted to have large-scale consequences for the distribution and magnitude of primary production throughout the Arctic Ocean. The objective of this study is to quantify the uptake of inorganic and organic nitrogen in the Barents Sea marginal ice zone during winter, spring and summer 2018, in the context of the seasonal transition in sea ice coverage and upper ocean dynamics.

We conducted three cruises in January, April and June 2018 along a 30 °E transect covering the full range of sea ice conditions and water masses observed in the Barents Sea. We measured the concentration of inorganic and organic nitrogen compounds throughout the water column and conducted nitrogen uptake experiments on water samples taken from the euphotic zone using 15N-labelled nitrate, ammonium, urea and amino acids.

These uptake rates are used to calculate nitrate-based new production and regenerated production based on ammonium, urea and amino acids, and these calculations are used to estimate the f-ratio. Here we will present initial results on the supply of inorganic and organic nitrogen forms and their uptake rates by different phytoplankton communities at different times of year. These rates and the derived f-ratio provide a measure for productivity of the ecosystem over the winter to summer transition. We will discuss the effects of sea ice cover and water mass structure with respect to the polar front on new production and phytoplankton community composition, which play an important role in regulating the magnitude of primary production in the Barents Sea. These results will contribute to our understanding of how biological and biogeochemical processes in the Arctic may respond to ongoing changes in the physical environment as climate change proceeds.

D2643 |
EGU2020-9393
Craig Smeaton and William Austin

Fjords are recognized as globally significant hotspots for the burial (Smith et al., 2015) and long-term storage (Smeaton et al., 2017) of marine and terrestrially derived organic carbon (OC). By trapping and locking away OC over geological timescales, fjord sediments provide a potentially important yet largely overlooked climate regulation service. The proximity of fjords to the terrestrial environment in combination with their geomorphology and hydrography results in the fjordic sediments being subsidized with organic carbon (OC) from the terrestrial environment. This terrestrial OC (OCterr) transferred to the marine environment has traditionally be considered lost to the atmosphere in the form of CO2 in most carbon (C) accounting schemes yet globally it is estimated that 55% of OC trapped in fjord sediments is derived from terrestrial sources (Cui et al., 2016). So is this terrestrial OC truly lost? Here, we estimate the quantity of OCterr held within North Atlantic fjords with the aim of better understanding the recent and long-term role of the terrestrial environment in the evolution of these globally significant sedimentary OC stores. By understanding this subsidy of OC from the terrestrial to the marine environment we can take the first steps in quantifying the terrestrial OC stored in fjords and the wider coastal marine environment.

Cui, X., Bianchi, T.S., Savage, C. and Smith, R.W., 2016. Organic carbon burial in fjords: Terrestrial versus marine inputs. Earth and Planetary Science Letters451, pp.41-50.

Smeaton, C., Austin, W.E., Davies, A., Baltzer, A., Howe, J.A. and Baxter, J.M., 2017. Scotland's forgotten carbon: a national assessment of mid-latitude fjord sedimentary stocks. Biogeosciences.

Smith, R.W., Bianchi, T.S., Allison, M., Savage, C. and Galy, V., 2015. High rates of organic carbon burial in fjord sediments globally. Nature Geoscience8(6), p.450.

 

D2644 |
EGU2020-7114
Mimmi Oksman, Anna Bang Kvorning, and Sofia Ribeiro

In the past few decades, warming of the Arctic region has resulted in an abrupt increase of freshwater discharge from the Greenland Ice Sheet into its surrounding ocean. Greenland fjords are modulated by ice-ocean interactions and are very productive ecosystems that sustain important fisheries and other societal ecosystem services. While many studies are ongoing to understand seasonal and inter-annual changes, very little is known about the long-term impacts of freshwater discharge on primary producers and overall Arctic marine ecosystem functioning and structure. This long-term perspective is particularly important because freshwater runoff is expected to increase in the future along with rising atmospheric temperatures. Here, we present records from three marine sediment cores from the Godthåbsfjord that were used to reconstruct past marine productivity and freshwater discharge. The results based on diatom assemblages, BSi and TOC indicate marked fluctuations in past fjord productivity since the end of the Medieval Climate Anomaly, and periodical bursts of freshwater into the fjord resulting in a lowered productivity.

D2645 |
EGU2020-11144
Heike H. Zimmermann, Stefan Kruse, Kathleen R. Stoof-Leichsenring, Luise Schulte, Dirk Nürnberg, Ralf Tiedemann, and Ulrike Herzschuh

Marine protists are a phylogenetically diverse group of single-celled eukaryotes that respond sensitively to changes in environmental conditions. Yet, our understanding how long-term climate variability has shaped the taxonomic composition is mostly unknown, especially of non-biomineralizing groups, such as green algae, since traditional micropaleontological studies are limited to the analysis of microfossil remains with often hardly discernable morphological differences between species (e.g. diatoms). Here we present a sedimentary ancient DNA (sedaDNA) record of the marine sediment core SO201-2-12KL, which was retrieved from the eastern continental slope of Kamchatka at 2173 m water depth (N 53.992660°, E 162.375830°) and covers the past 19.9 thousand years. We applied sedaDNA metabarcoding to 63 samples using a diatom-specific, short plastid marker that is part of the rbcL gene. Additionally, we used metagenomic shotgun sequencing on a subset of 26 samples to investigate the overall taxonomic composition of protists. Metagenomic shotgun sequencing revealed a variety of unicellular plankton groups mostly from green algae (especially Bathycoccus) and diatoms. At 11.1 cal kyr BP only single sequences assigned to green algae, diatoms and coccolithophorids could be detected. Metabarcoding showed strong variability in the richness of diatom sequence variants, which was highest during Heinrich Stadial 1 and the Younger Dryas. From about 11.4 cal kyr BP diatom taxonomic diversity strongly decreased until about 10.7 cal kyr BP. This was associated with highest taxonomic and phylogenetic turnover recorded over the past 19.9 cal kyr. Concomitant with this we recorded sequences assigned to Skeletonema subsalsum, a coastal diatom associated with low salinities or freshwater. Tentatively, as we wait for the confirmation by further sequencing, we suggest that the reduced protist diversity during the Early Holocene resulted from sea surface freshening, which led to a strengthened vertical stratification which could have reduced past productivity due to limited nutrient supply from deeper waters to the photic zone.

D2646 |
EGU2020-2744
Kate Winter, John Woodward, Stuart Dunning, and Robert Raiswell

Glacial systems transfer sediment, rich in essential nutrients and bioavailable iron (BioFe) from continental sources to the ocean through iceberg transport, resulting in Heinrich layers during deglacial phases. The Southern Ocean is currently nutrient-rich but iron limited, so any increase in continental sediment supply could enhance primary productivity in the Southern Ocean and ultimately drawdown atmospheric CO2; potentially limiting predicted global temperature rise. The contribution of continental sediments to this negative climate feedback loop, termed the ‘Fe hypothesis’ has yet to be considered in IPCC reports. As other sources of BioFe are decreasing, global temperatures are warming, and CO2 levels are rising at glacial terminations it is now critical to assess BioFe flux through the glacial system. In this project, we use established laboratory procedures to extract nanoparticulate Fe from Antarctic sediments, recovered from nunataks, glacial moraines and debris bands in wind scoured blue ice areas in and around the Sør Rondans Mountains in East Antarctica (72°S, 24°E). These coastal margin mountains channelise ice (and entrained sediments) from the polar plateau towards the local Roi Baudouin Ice Shelf, at flow speeds of 30 – 60 m a-1. Concentrations of FeA - comprising fresh ferrihydrite (potentially bioavailable) and FeD - comprising all remaining (oxyhydr)oxide Fe are reported at all sites, with elevated concentrations along nunataks that define the edges of deep subglacial valleys which support ice flows over 2 km thick. Terrestrial surveys of these nunataks, combined with ice penetrating radar data and numerical modelling studies show how these nutrient-rich sediments are entrained and transported through the ice, from continental Antarctica, to the Southern Ocean, where ice-influenced phytoplankton blooms have been reported.

D2647 |
EGU2020-22400
Maria Van Leeuwe, Mairi Fenton, Ee Davey, Amber Annett, Mike Meredith, and Jacqueline Stefels

The Western Antarctic Peninsula (WAP) is one of the fastest warming oceanic regions on earth, with a recorded increase in winter temperature of 6˚C since 1950. Coinciding with the warming of shelf water the amount of sea ice that is formed over winter shows a general declining trend. The consequences of this decline for biogeochemical processes are poorly understood. Microalgal composition and production was studied in sea ice in four consecutive winters from 2013-2016, at Ryder Bay, located at the southern part of the WAP.

Sea ice was sampled over the period of ice formation in autumn until ice melt in spring. Microalgal composition was studied by means of their pigment signature and microscopy; production capacity was studied by fluorescence analyses and C13-incorporation studies. At the onset of ice formation, the sympagic algal communities consisted of a mixture of species. Over the course of winter, heterotrophic flagellates became dominant. In spring, biomass increased strongly in the bottom layers and reached a maximum concentration of more than 700 µg Chl.a l-1 in December 2014. These communities were mainly diatom-dominated. In spring, algal samples were also taken from under ice and pelagic communities. For the first time, we are able to present data that show essential differences in seeding potential of sea ice for diatom and flagellate species. The downstream effects of predicted changes in sea-ice cover and associated ice-algal communities on biogeochemical exchange processes with the marine ecosystem will be discussed.

D2648 |
EGU2020-21368
| solicited
| Highlight
Letizia Tedesco, Eva Leu, Marc Macias-Fauria, Christopher J. Mundy, Dirk Notz, Janne Søreide, Malin Daase, Jakob Doerr, and Eric Stephen Post

Arctic food webs are short and relatively species poor, rendering them vulnerable to changes or perturbations at any individual trophic level. High-latitude warming represents one major source of potential perturbation to Arctic marine and terrestrial food webs, which may experience cascading effects derived from changes in primary production through so-called “bottom-up” effects. We synthesize current knowledge on i) the changing Arctic marine icescape, ii) the drivers of biological changes for Arctic marine primary production, iii) the different pulses of Arctic marine primary production, iv) patterns of marine trophic and phenological changes, and iv) some mechanisms through which sea-ice dynamics ostensibly influence terrestrial primary productivity. We deliver a set of predictions for key productivity indicators, propose a semi-quantitative model of the expected future changes in primary production in the ice-covered Arctic Ocean, and close with an overview of the challenges ahead for reaching a holistic and comprehensive understanding of the ecosystem dynamical consequences and associated impacts on human life of warming-related sea-ice decline.

D2649 |
EGU2020-1606
Angelika Renner, Allison Bailey, Marit Reigstad, Arild Sundfjord, and Sigrid Øygarden

The shelf break north of Svalbard represents a major gateway for the inflow of nutrient-rich Atlantic Water (AW) to the Arctic Ocean. In this region, AW leaves the surface and subducts below Polar Surface Water (PSW). The supply of nutrients to the euphotic layer therefore varies strongly by season but also interannually, depending on e.g. rates of advection of sea ice and PSW over the AW boundary current. Additionally, the presence of sea ice can limit light availability in spring and early summer. Here, we present results from repeat sampling of hydrography, macronutrients (nitrate/nitrite, phosphate and silicic acid), and chlorophyll a along a transect at 31 E, 81.5 N in the period 2012-2017. Such time series are scarce but invaluable for investigating the range of variability in hydrography and nutrient concentrations. Measurements were done in late summer/early autumn, giving an indication of the nutrient consumption by primary producers over summer. The different years were characterised by very distinct sea ice conditions, both during the productive season and during the field campaigns. This impacted hydrography and primary production and thus nutrient concentrations in the surface and AW layers at the end of summer.

D2650 |
EGU2020-10718
Alba Filella Lopez de Lamadrid and Anja Engel

Freshwater discharge around Greenland has more than doubled during the last decade. Understanding the associated physical and biogeochemical impacts in the ocean is of great importance for future predictions of ocean circulation, productivity and feedbacks within the Earth system. In summer 2019 we performed several cross-shore sections passing through the highly variable environments and physical regimes along the east Greenland coastline. Microbial communities showed distinct latitudinal and meridional distributions. Water mass characteristics played a major role in controlling the abundances of organisms with few groups appearing in significant numbers in coastal (colder and fresher) waters. Surface polar waters rich in dissolved organic carbon (DOC) flow south in the East Greenland Current maintaining a high DOC signal in inshore waters. Further optical analyses on the DOC fraction will determine what fractions of this material originate from long scale transport out of the Arctic. Of particular interest was an enhanced production of gel particles rich in carbon in an area extending across Denmark Strait, from close to Scoresby Sund to north of Iceland. Significant concentrations (e.g. 80 µg X.G. eq. L-1) of these transparent exopolymer particles (TEP) were even found deeper than 100m, which is highly unusual. Given the role of TEP as a binding agent for sinking particles, enhancing the sinking of carbon in the water column, it is of interest to know why such a TEP hotspot arises. We hypothesize that it could be either related to circulation through the Strait or the timing of bloom dynamics in this region prior to our cruise.  Our main conclusion from preliminary data analysis is that the east Greenland coastal system is highly dynamic with mixed properties reflecting various degrees of mixing between southward flowing Polar Water and warmer Atlantic water masses.

D2651 |
EGU2020-3949
Hong Chin Ng, Lucie Cassarino, Rebecca Pickering, Malcolm Woodward, Samantha Hammond, and Katharine Hendry

The biogeochemical cycling of nutrient silicon (Si) in the northern high latitudes has received increasing attention over recent years. This is in large part due to the discovery of silicon limitation of diatoms over seasonal timescales, the potential role of melting glaciers in supplying a significant amount of this nutrient to the coastal ocean, and the rapid environmental changes the polar ocean is experiencing as a result of global climate warming. However, our understanding of the nutrient Si in the polar ocean is severely restricted by the lack of knowledge of the benthic Si cycling and its controlling processes, which is due to the limited number of seafloor observations in the region. In this study, we address this knowledge gap through the acquisition of sediment pore water profiles and the execution of incubation experiments on sediment cores collected from the Greenland continental margin and the Labrador Sea.

Our results indicate a net (benthic) flux of dissolved silica (DSi) out of the sediment into the overlying seawater at the study sites. A new global compilation also reveals that benthic Si flux observed at our marginal sites are substantially higher than in the open ocean. This is likely because benthic flux in the open ocean is solely maintained by molecular diffusion along a concentration gradient, while there are additional processes: pore water advection and rapid dissolution of certain siliceous sponge groups, and other reactive silica phases, that contribute to the elevated benthic Si flux on the Greenland margin. This finding has important implications for existing evaluations of oceanic Si budgets, which have not accounted for any processes other than diffusion in the global estimation of benthic Si flux. Our results also suggest that strong benthic Si flux observed on the Greenland margin, combined with wind-driven coastal upwelling, could be a significant source of this nutrient to both the diverse (benthic) sponge communities and (planktonic) diatom productivity in the region. The magnitude of this benthic cycling could potentially rival the other continental inputs of Si in the northern high latitudes, as the first estimation of total benthic Si flux from the western Greenland shelf alone (0.04–0.27 Tmol year-1) is in the same order of magnitude as the total Si export from Greenland Ice Sheet (0.2 Tmol year-1) and the pan-Arctic rivers (0.35 Tmol year-1) respectively.

D2652 |
EGU2020-9134
Rachael Ward, Kathrine Hendry, Jemma L. Wadham, Jon R. Hawkings, Robert M. Sherrell, and Amber Annett

The accelerated melting of the Greenland Ice Sheet could potentially enhance fluxes of key nutrients, to the surrounding oceans, impacting marine biogeochemical processes and ecosystems. Iron (Fe) is one key micronutrient for marine phytoplankton that may be affected by this increase in meltwater flux, with high export of dissolved and particulate Fe from glacial meltwaters into fjords and a potentially significant increase in the supply of labile and potentially bioavailable Fe to the Greenlandic shelf. However, biogeochemical processing within estuarine-like fjord systems may result in depletion of nutrients, acting as a sink of micronutrients before they can reach the coastal ocean. The extent to which glacially derived micronutrients, specifically Fe, reach coastal waters remains an unanswered question.

Here, we address this question by assessing the concentration of dissolved (<0.45 µm) and labile particulate (determined using the Berger leach) bio-essential trace metals (Fe, Cd, Mn, Ni, Cu, Zn) in two contrasting glaciated fjords in southwest Greenland; one fed predominantly by marine terminating glaciers and the other by a land terminating glacier. We investigate the difference in size fractionated concentrations between fjords and the transport of these metals from stations close to glacial termini down to the fjord mouths. Our findings reveal that each micronutrient exhibits a distinctive behaviour, with some metals enhanced in meltwaters (e.g. dissolved Fe and Mn) and some depleted (e.g. dissolved Cd), relative to marine waters. The spatial variability in our dataset highlights that concentration of Fe and other trace metals (Cd, Mn, Ni, Cu, Zn) enriched in meltwaters become depleted towards the mouth of the fjords, with non-conservative loss from surface waters. Despite this depletion, the concentrations of these metals in waters that reach the coastal zone are significantly higher than typical surface ocean values, both in dissolved and labile particulate form. These data can ultimately be used in combination with a physical understanding of the fjord systems to constrain the capacity of fjords to enhance productivity downstream and deliver micronutrients into coastal and open ocean systems. Furthermore, the direct comparison of land- and marine-terminating glacial fjords could provide valuable information on the potential future impact of retreating glacial systems with enhanced melting.

D2653 |
EGU2020-8962
Jade Hatton, Hong Chin Ng, Alexander Beaton, Lorenz Meire, and Katharine Hendry

Meltwaters from glaciers and ice sheets in the Arctic region potentially provide significant fluxes of key nutrients to downstream ecosystems, and hence recent acceleration of melting has implications for the high-latitude biogeochemical cycles. Previous work has shown that silicon (Si) exported from glacial environments also has a distinct isotopic signature compared to those in non-glacial rivers. However, the extent to which glacially-derived Si and other bioavailable nutrients reach the open ocean is much debated, due to biological uptake and complex physical processes within heterogeneous fjord environments.

We assess the impact of glacial meltwater from marine and land-terminating glaciers on fjord biogeochemistry, with a focus upon Si cycling, by sampling two fjords (Godthåbsfjord and Ameralik Fjord) in southwest Greenland over two melt seasons. We combine silicon isotope measurements with a range of complementary physical and chemical parameters to evaluate the role of glacially derived nutrients and benthic recycling on biological productivity within the two contrasting fjord environments. Data from two consecutive melt seasons also enables us to begin to assess inter-annual variability. In addition, continuous measurements of silicic acid and nitrate concentrations along Godthåbsfjord during the 2019 melt season, obtained using novel sensor technology, allow us to assess intra-annual variations in nutrient export. Our targeted field campaigns have provided a suite of nutrient, trace element and isotopic data that will improve the current understanding of the complex biogeochemical cycling within fjord environments, and allow a better assessment of the importance of glacial meltwater on nutrient export and primary productivity in downstream ecosystems.

D2654 |
EGU2020-853
Anna Bang Kvorning, Tania Beate Thomsen, Mimmi Oksman, Marit-Solveig Seidenkrantz, Christof Pearce, Thorbjørn Joest Andersen, and Sofia Ribeiro

The Greenland Ice Sheet has been losing mass at an increasing rate over the past decades due to atmospheric and oceanic warming. As a result, freshwater discharge from the Greenland Ice sheet has doubled in the last two decades and is expected to strongly increase in the future, with a large impact on the functioning of coastal marine ecosystems. While glacier runoff delivers nutrients and labile carbon into the fjords, an increase in sediment inputs is expected to have a negative impact in primary productivity, due to increased turbidity and subsequent reduction in available light for photosynthesis. Bridging modern satellite, historical and paleo-records is a key approach, as our capacity to project future scenarios requires an understanding of long-term dynamics, and insight into past warm(er) climate periods that may serve as analogues for the future. We will present results from a master’s project developed within the framework of project GreenShift: Greenland fjord productivity under climate change. Two high-resolution sediment core records from two contrasting fjord systems in NE and SW Greenland were analysed to assess the impact of Greenland Ice Sheet melt on sediment fluxes and primary productivity, focusing on the time period from the Little Ice Age until present. The overall goal of this work is to gain a better understanding of the possible linkages between GIS melt and productivity in Greenland fjord systems, with a view to improve future projections. We followed a multiproxy approach including grain-size distribution, organic carbon and biogenic silica fluxes; and dinoflagellate cyst analyses. Our preliminary results show an overall trend towards sea-surface freshening in recent decades for both fjords influenced by land-terminating (NE) and marine-terminating (SW) glaciers, alongside with important differences both in terms of sedimentary organic composition and dinoflagellate cyst assemblages.  

D2655 |
EGU2020-20051
Tiia Luostarinen, Sofia Ribeiro, Kaarina Weckström, Mikael Sejr, Lorenz Meire, Petra Tallberg, and Maija Heikkilä

Understanding environmental factors affecting diatom species composition at seasonal resolution can contribute to the improvement of paleo sea-ice reconstruction. We recorded diatom species succession over one full year (May 2017‒May 2018) using automated sediment traps installed in two contrasting Greenlandic fjords: seasonally ice-covered Young Sound in high arctic NE Greenland and nearly sea-ice free Godthåbsfjord in subarctic SW Greenland. The two study sites had distinct seasonal regimes in terms of both sediment and diatom fluxes. In Young Sound, diatom fluxes peaked during the ice-melt in June–July (max. 880×106 valves m-2 d-1), but were very low (0.11­–12.7×106 valves m-2 d-1) for the rest of the year. The pattern was very different in Godthåbsfjord, where diatom fluxes were more stable throughout the year and at maximum 320×106 valves m-2 d-1 in summer. A total of 60 diatom taxa were present in Young Sound and 50 in Godthåbsfjord, with 19 and 22 sympagic or pelagic species, respectively. The diatom assemblage in Young Sound is strongly dominated by the pennate sea-ice species Fragilariopsis oceanica, Fragilariopsis reginae-jahniae and Fossula arctica, which exhibited pulse-like deposition in the trap during and after the ice melt. In Godthåbsfjord, the fluxes were dominated by resting spores of centric Chaetoceros, while the rest of the assemblage was characterized by the cold-water indicator species Detonula confervacea spore, Fragilariopsis cylindrus and Thalassiosira antarctica var. borealis spore accompanied by some warmer-water species. Some sea-ice indicator species were also observed in Godthåbsfjord, but at very low counts and throughout the year, likely transported from the inner fjord, which experiences seasonal sea-ice coverage. We show that F. oceanica, F. reginae-jahniae and F. arctica exhibit similar seasonal behaviour and are clearly linked to sea ice. On the other hand, Fragilariopsis cylindrus seems to have a more flexible niche, and is not an unequivocal ice indicator. Similarly, Pauliella taeniata has a differing niche, and does not favour our study locations probably due to its preference for lower salinities. We underscore the importance of taking into account ecological and seasonal preferences of the individual diatom species when reconstructing past sea-ice conditions qualitatively or quantitatively.

D2656 |
EGU2020-7536
Kristýna Jachnická, Tyler J. Kohler, Lukáš Falteisek, Petra Vinšová, Marie Bulínová, Jemma L. Wadham, Guillaume Lamarche-Gagnon, Andrew J. Tedstone, Jonathan R. Hawkings, Anne M. Kellerman, Karen A. Cameron, and Marek Stibal

Glaciers and ice sheets host diverse microbial life within the hydrologically connected supraglacial, englacial, and subglacial habitats. Microbial cells are collected from the entire glacial ecosystem by seasonally-generated meltwater and exported by proglacial streams. Over the course of the melt season, a subglacial drainage system develops beneath outlet glaciers from the Greenland Ice Sheet (GrIS). This system evolves from an inefficient distributed network to a more efficient channelized pathway. The extent and interconnectivity of the subglacial drainage system with the surface and sediment bed is hypothesized to differ with catchment size.

In this study, we ask whether microbial export from GrIS outlet glacier systems depend on catchment size and whether they evolve with subglacial hydrology over time. We hypothesize that larger catchments will have proportionally greater subglacial drainage, which may be reflected in a greater proportion of subglacial microbes compared to smaller catchments, where the supraglacial inputs might have a higher influence on the exported meltwater. We also expect that changes in assemblage structure are likely to coincide with the evolution of the subglacial drainage system of larger catchments as the season progresses, with supraglacial inputs increasing in importance as the channelized efficient system fully develops. To test these hypotheses, we sampled three outlet glaciers of the GrIS with different catchment sizes (from biggest to smallest: Isunnguata Sermia, Leverett and Russell glaciers) over the 2018 summer. Meltwater samples were taken at the same time each day over a period of three weeks to catch temporal patterns of microbial assemblages. DNA was extracted from samples, and 16S rRNA gene amplicons sequenced to characterize assemblage structure.

This study will help us better understand the meltwater hydrology of the GrIS by describing patterns in its microbial export and the degree of influence from supra- and subglacial systems. In this current age of glacier recession, it is furthermore important to make these characterizations as we might not have opportunity in near future to investigate them in the same unchanged environment.

D2657 |
EGU2020-10685
James Ward, Felipe Sales de Freitas, Hong Chin Ng, Katharine Hendry, Sandra Arndt, Rebecca Pickering, Sian Henley, Rachael Ward, Christian März, and Johan Faust

Biogeochemical cycling of silicon (Si) in the high latitudes has an important influence on the marine Si budget. The Barents Sea is divided aproximately equally into Arctic and Atlantic water (ArW and AW respectively) domains.  However, increases in the temperature and inflow of AW across the Barents Sea opening is driving an expansion of the AW realm. While the sensitivity of pelagic processes pertaining to primary production is receiving increasingly more attention, less is known of the effect on the benthic Si cycle. This knowledge gap could prove integral, as the flux of Si across the sediment-water interface (SWI) from Arctic shelf sediments could be up to 20% higher than that of riverine sources. This benthic flux is largely controlled by early diagenetic processes in sediment pore waters, including biogenic silica (bSi) dissolution and authigenic precipitation.

To improve our understanding of benthic Si dynamics in the Barents Sea and examine its sensitivity to future change, we analysed pore water and sediment samples from both the AW and ArW realms between 2017-2019 for dissolved silica (dSi) concentrations and stable silicon isotopic compositions. Moreover, to determine the composition and content of bSi, as well as Si sorbed onto metal oxides, we conducted a sequential digestion of surface sediment. Following this we coupled our analyses with reaction transport modelling to further improve our mechanistic understanding of the system and to quantitatively disentangle the relative importance of these diagenetic processes to pore water Si chemistry and benthic fluxes.

Our work suggests that both interannual and spatial variability of dSi are increased in the southern, AW region of the Barents Sea. Benthic flux estimates for the southern sites have been found to more than double (~30 to 100 mmol m-2 yr-2) between cruise years, compared to a more consistent flux in the north (~80 mmol m-2 yr-2). Therefore, future Atlantification of the northern region may enance the variability of dSi supply from the benthos to bottom waters, with potential consequences for diatom productivity in the region.

D2658 |
EGU2020-8107
Katja Laufer, Alexander Michaud, Hans Røy, and Bo Barker Jørgensen

Glacial runoff is a significant source of Fe to high-latitude marine environments. The amount and characteristics of glacially derived Fe depends on bedrock lithology, glacial comminution as well as glacier type. Because much of the Fe that comes from glaciers is in the particulate phase or will become particulate once in contact with saline and oxic fjord water, much of the glacial Fe ends up in fjord sediments in close proximity to the glaciers. Within these sediments, the glacially derived Fe undergoes redox-cycling driven by indirect (abiotic reaction with metabolic products, such as hydrogen sulfide) and direct interactions with microorganisms. This redox-cycling has the potential to alter the characteristics of the glacially derived Fe and thereby also its fate, for example if it is buried in the sediment or exported to the water column.

We investigated the amount and reactivity of Fe(III) minerals from the meltwater plume, meltwater streams, icebergs, and sediments at stations with increasing distance from the glacier in three different fjords on the west coast of Spitsbergen, Svalbard. Two of the fjords have large tidewater- glaciers at their head and possess differing bedrock lithology (Kongsfjorden and Lilliehöökfjorden). The third fjord, Dicksonfjorden, has land-terminating glaciers with a bedrock lithology similar to the glaciers at the head of Kongsfjorden, thus providing insight into the impact of glacial retreat on benthic biogeochemical processes. Results from sequential and time-course extractions showed that Fe(III)-mineral reactivity increased with distance from the glacier fronts and decreased with sediment depth at each station in all three fjords. Fe(III)-oxide reactivity from different glacial sources (meltwater plume and iceberg material from tidewater glaciers and meltwater stream material from land-terminating glaciers) differed based on source type and Fe(III) from all glacial sources was generally less reactive compared to surficial sediments distal to the glacier front. While the general trends were the same for all three fjords, based on pore water profiles of dissolved Fe, we found a lower potential for Fe-export to the water column when only land-terminating glaciers were present. This difference highlights that glacial retreat potentially impacts the function of fjord sediments as a source of Fe to the water column. We conclude that glacial runoff supplies large quantities of Fe minerals to fjord sediments, but benthic recycling of Fe by microorganisms transforms the relatively unreactive glacially-derived Fe(III)-oxides to a more reactive form. Microbially driven recycling of reactive Fe(III)-oxides in fjord sediments may play a role in liberating Fe to the water column, predominantly at the mouth of the fjord, and might represent an unquantified source of Fe to Fe-limited marine phytoplankton.

D2659 |
EGU2020-1528
Elizabeth Jones, Angelika Renner, Melissa Chierici, Ingrid Wiedmann, and Martin Biuw

Coastal oceans and shelf regions play a key role in marine productivity and represent an important part of the global carbon cycle, however due to strong seasonal and temporal dynamics, the processes controlling the biogeochemical cycling remain largely unresolved. This study presents the first time series measurements of hydrography, carbonate chemistry and macronutrients in a sub-Arctic fjord, Kaldfjorden (69.75 ºN, 18.68 ºE), of northern Norway during a full annual cycle. The influence of freshwater inputs and biological production were the dominant controls on fjord carbonate chemistry. Meteoric water freshened the upper layers during spring and summer, accounting for variability in surface water total alkalinity. Remineralisation of organic matter and deep mixing into carbon-rich subsurface water resupplied the water column with total inorganic carbon and macronutrients throughout the winter. Surface water saturation states (Ω) of aragonite were lowest 1.64 ± 0.04 during winter and early spring. Rapid CT drawdown in the spring phytoplankton blooms exhausted the winter stock of nitrate to drive high Ω of 2.26-2.33 and CO2 undersaturation (ΔfCO2(sea-air) was -58 ± 33 μatm) in the surface layer from April. Decreases in surface water alkalinity from July to October are consistent with coccolithophore blooms in the fjord. Kaldfjorden was estimated to be an annual net sink of atmospheric CO2. Climate change may intensify the natural variability in these important marine environments and gathering seasonal baseline data helps to unravel the contribution of fjords to oceanic CO2 cycling.

D2660 |
EGU2020-17797
Anna J. Pienkowski, Katrine Husum, Simon Belt, and Lukas Smik

An understanding of modern sea-ice proxy distributions relative to measured environmental parameters underpins accurate palaeo reconstructions necessary for correct future projections. We here present new data on highly-branched isoprenoid (HBI) lipid biomarkers produced by sea-ice diatoms (IP25, IPSO25) and phytoplankton (HBI III, HBI IV) in marine surface sediments taken in a south-north transect east of Svalbard as part of the Nansen Legacy project. Collectively, these biomarkers can be used to reconstruct seasonal spring sea-ice (SpSIC) and the seasonal sea-ice edge. Eight sites at ~78-83°N were sampled by multicorer. All cores contain abundant biomarkers, except the northernmost station. Biomarker-based SpSIC shows a general south-north increase, mimicking observational sea-ice concentration satellite-based means (1988-2017). The HBI T25 index suggests ice edge phytoplankton blooms at southern stations, agreeing with the general pattern of increased phytoplankton HBIs previously reported from the eastern Barents Sea. As a next step, these new biomarker findings will be used to reconstruct longer-term (Holocene) variability in sea-ice in this region. 

D2661 |
EGU2020-13904
Maija Heikkilä, Anna Pieńkowski, Sofia Ribeiro, and Kaarina Weckström

The Arctic cryosphere is changing rapidly due to increased runoff from land, changing sea-ice regime and the degradation of the circumpolar permafrost zone. Fjords and other nearshore areas form a productive zone that is vital for both Arctic biodiversity and local communities, rendering the understanding of Arctic coastal ecosystem change from a long-term perspective crucial.

The ACME working group provides a community platform to critically assess and refine available coastal marine proxies that can be used to reconstruct cryosphere changes and their multifaceted ecosystem impacts. ACME seeks to promote a leap forward in the accuracy of paleo reconstructions that are central for deciphering cryosphere-biosphere interactions in the Arctic region at relevant timescales.

The goals for the three-year (2019‒2022) Phase I of the working group are:

- To build a community-refined database that contains a network of proxies commonly used for sea ice, primary production, and meltwater runoff reconstructions in Arctic coastal and fjord environments.

- To facilitate knowledge transfer and collaborations between proxy specialists and the integration of the field and satellite monitoring community.

- To further critical methodological understanding and data handling skills of the next-generation of Arctic paleoceanographers and paleoenvironmental researchers.

D2662 |
EGU2020-11519
Vera Pospelova, Catherine Lalande, Maija Heikkilä, and Louis Fortier

Studies of dinoflagellate cysts or ciliates in sediment traps provide essential information on weekly, monthly, seasonal, annual, and/or multi-annual changes in their fluxes in relation to measured or implied environmental parameters. Such information is essential for understanding ecological preferences of individual taxa which is the foundation for performing reliable (paleo)environmental high-resolution regional reconstructions. Up to date, sediment trap studies are rare, and only three of those deal with dinoflagellate cysts production in ice-covered conditions: in Antarctic waters (Harland and Pudsey, 1999); Arctic fjords in the Svalbard archipelago (Howe et al., 2010); and Hudson Bay (Heikkilä et al., 2016). All these studies consistently show a very limited or no cyst recovery from the samples that were collected during the ice-covered intervals. However, the timing of individual species production (e.g. cysts of Pentapharsodinium dalei, Islandinium minutum, and Spiniferites elongatus) within the ice-free condition is inconsistent as it varies from region to region. In this session, we will present our preliminary results on dinoflagellate cyst continuous bi-weekly record at the Beaufort Sea shelf break from September 2014 to August 2015.

D2663 |
EGU2020-1193
Laura Halbach, Liane G. Benning, Eva L. Doting, Martin Hansen, Hans Jakobsen, Lars C. Lund-Hansen, Lumi Haraguchi, Brian K. Sorrell, Thanassis Zervas, and Alexandre M. Anesio

Large blooms of purple-brownish pigmented glacier algae cover the ablation zones of the Greenland Ice Sheet (GrIS) and amplify its melt by lowering the ice surface albedo and increasing its solar radiation absorption. The darkening effect of these Zygnematophycean algae can be mainly attributed to their phenolic pigments, which absorb in the visible (VIS) and UV light ranges. Currently, a mechanistic understanding of the factors regulating the production of these pigments and their implications for the large-scale biologically-driven albedo reduction on the GrIS is missing. Here, we reveal how light (VIS vs. UV range) controls the phenolic pigment production, endo- and exometabolome, gene expression and photosynthetic performance of glacier algae. Two different algal communities (a mixed natural microbial community collected from snow-free ice and a laboratory-grown community of the ice algae Mesotaenium berggrenii without its original dark pigmentation) were used for a set of in situ incubations on Mittivakkat glacier in SE-Greenland. Pulse-amplitude-modulated (PAM) fluorometry revealed an overall higher photosynthetic performance (electron transport rate) at higher irradiances for the field population containing purpurogallin-like pigments compared to the lab community without dark pigmentation. The lab population showed a low maximum quantum efficiency of photosystem II under in situ light conditions, indicating a photo-damaging effect from high intensities of UV light in the absence of purpurogallin-derived phenolic pigments. Our study highlights the intracellular shading effect by purpurogallin-derived pigments, which are key for the survival of glacier algae on the ice and forms a cornerstone of understanding the large-scale variability in the biological darkening of the GrIS.

D2664 |
EGU2020-10968
Maurice Levasseur, Martine Lizotte, Virginie Galindo, Margaux Gourdal, and Michel Gosselin

Biogenic sources of sulfur are important precursors of aerosols in the Arctic during the summer months. Recent studies show that peaks in ultrafine particle formation events often coincide with hotspots of dimethyl sulfide (DMS) emissions from the marginal ice zone. During the last 10 years, we explored the diversity of DMS sources associated with the ice and at the marginal ice zone in the Canadian Arctic, and assessed how the projected changes in sea ice extent, thickness, and other properties could strengthen or weaken these emissions. Results from four Arctic expeditions presenting DMS concentrations and dynamics in snow, sea ice, melt ponds, under-ice water, and at the ice edge will be shown and discussed in the context of ongoing and future changes in the cryosphere. The analysis of the pooled dataset points toward an increase in DMS emissions in a warmer Arctic with a potential cooling feedback on climate.

D2665 |
EGU2020-8940
Jan Hartmann, Susann Henkel, Sabine Kasten, Adrián Silva Busso, and Michael Staubwasser

Polar regions are critical for future climate evolution, and they experience major environmental changes. A particular focus of biogeochemical investigations in these regions lies on iron (Fe). This element drives primary productivity and, thus, the uptake of atmospheric CO2 in vast areas of the ocean. Due to the Fe-limitation of phytoplankton growth in the Southern Ocean, Antarctica is a key region for studying the change of iron fluxes as glaciers progressively melt away. The respective climate feedbacks can currently hardly be quantified because data availability is low, and iron transport and reaction pathways in Polar coastal and shelf areas are insufficiently understood. We show how novel stable Fe isotope techniques, in combination with other geochemical analyses, can be used to identify iron discharges from subglacial environments and how this will help us assessing short and long term impacts of glacier retreat on coastal ecosystems.

Stable Fe isotopes (δ56Fe) may be used to trace Fe sources and reactions, but respective data availability is low. In addition, there is a need to constrain δ56Fe endmembers for different types of sediments, environments, and biogeochemical processes.

δ56Fe data from pore waters and sequentially extracted solid Fe phases at two sites in Potter Cove (King George Island, Antarctica), a bay affected by fast glacier retreat, are presented. Close to the glacier front, sediments contain high amounts of easily reducible Fe oxides and show a dominance of ferruginous conditions compared to sediments close to the ice-free coast, where surficial oxic meltwater discharges and sulfate reduction dominates. We suggest that high amounts of reducible Fe oxides close to the glacier mainly derive from subglacial sources, where Fe liberation from comminuted material beneath the glacier is coupled to biogeochemical weathering. A strong argument for a subglacial source is the predominantly negative δ56Fe signature of reducible Fe oxides that remains constant throughout the ferruginous zone. In situ dissimilatory iron reduction (DIR) does not significantly alter the isotopic composition of the oxides. The composition of the easily reducible Fe fraction therefore suggests pre-depositional microbial cycling as it occurs in subglacial environments. Sediments influenced by oxic meltwater discharge show downcore trends towards positive δ56Fe signals in pore water and reactive Fe oxides, typical for in situ DIR as 54Fe becomes less available with increasing depth.

We found that a quantification of benthic Fe fluxes and subglacial Fe discharges based on stable Fe isotope geochemistry will be complicated because (1) diagenetic processes vary strongly at short lateral distances and (2) the variability of δ56Fe in subglacial meltwater has not been sufficiently well investigated yet. However, isotope mass balance models that consider the current uncertainties could, in combination with an application of ancillary proxies, lead to a much better quantification of Fe inputs into polar marine waters than currently available. This would consequently allow a better assessment of the flux and fate of Fe originating from the Antarctic Ice Sheet.

Henkel et al. (2018) Diagenetic iron cycling and stable Fe isotope fractionation in Antarctic shelf sediments, King George Island. GCA 237, 320-338.

D2666 |
EGU2020-3917
Aga Nowak, Andy Hodson, Stephen Hudson, Elisabeth Isaksson, Arwyn Edwards, David Pearce, and Sara Rassner

This poster describes new studies into the microbial ecosystems found in Blue Ice Areas (BIA) around the periphery of the Antarctic Ice Sheet. These habitats are located on the Antarctic fringe, often at high elevation, and therefore represent the first opportunity for cells entombed in old glacier ice advected from Antarctic interior, to be revived by the increased availability of solar radiation (blue ice) and water (subsurface melt).

Our study is the first to consider how two different types of BIA host two different habitats; those associated with nunataks or mountain ranges, and those associated with ice surfaces only. The difference between them is the availability of debris and water for microbial processes. In the former scenario, debris is blown onto the blue ice from local sources to provide a source of both microorganisms and energy. In the latter case, the lack of an external debris source is compensated by the possibility subsurface melting due to the optical properties of blue ice. In these systems a far greater proportion of the cells are liberated from ancient glacier ice.

To explore these overlooked ecosystems, we visited high elevation (c.1200m) BIAs in Dronning Maud Land, near Troll Research Station. Our expedition during Antarctic summer season 2019-2020 yielded microbial and biogeochemical data from both debris-free and debris-rich BIAs, as well as shallow glacier ice cores of different ages. In addition, we explored a variety of cryoconite holes entombed within the above BIAs.

Our results show that BIA ecosystems are characterized by tremendous heterogeneity between their cryoconite holes. While some show signs of photosynthesis, others are dominated by bacterial production. Furthermore, optical properties of the BIA and physical properties of the ice itself (fracturing) control subsurface meltwater production and water movement, influencing the sub-ice ecosystems and their biogeochemistry.  The poster will present how we are exploring the revival of these ice bound organisms on their way from the Antarctic Ice Sheet interior towards the coast.