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Spatio-temporal variations in organic carbon composition driven by two different major phytoplankton communities in the Ross Sea, Antarctica

2023-01-01T00:00:00Z

Title: Spatio-temporal variations in organic carbon composition driven by two different major phytoplankton communities in the Ross Sea, Antarctica Authors: Kim Jaehong; La, Hyoung Sul; Kim, Jeong-Hoon; Jo Naeun; Lee Junbeom; Kim, Bo Kyung; Son, Wuju; Kim Kwanwoo; Jang Hyo-Keun; Park Sanghoon; Yoo, Hyeju; Kim Jaesoon; Park, Jisoo; Ahn So Hyun; Lee Sang Heon Abstract: The compositions of organic carbon could be important in determining biological carbon pump efficiency. However, little information on them in relation to each algal assemblage is currently available in the Ross Sea. Here, we inves-tigated the seasonal variations in organic carbon composition and the relative abundance of each organic carbon, in-cluding particulate organic carbon (POC), dissolved organic carbon (DOC), and transparent exopolymer particles (TEPs), characterized by different algal groups in the Ross Sea. The average POC and DOC contributions to the total organic carbon (TOC = POC + DOC) were 13.8 & PLUSMN; 3.7 % and 86.2 & PLUSMN; 3.7 % in mid-January 2019 and 20.9 & PLUSMN; 4.1 % and 79.1 & PLUSMN; 4.1 % in February-March 2018, respectively. The carbon content of TEP (TEP-C) contributed 19.6 & PLUSMN; 11.7 % and 4.6 & PLUSMN; 7.0 % of POC and TOC in mid-January and 36.2 & PLUSMN; 14.8 % and 9.0 & PLUSMN; 6.7 % in February-March, respectively. We found that the organic carbon compositions were affected by seasonal variations in the phytoplankton bloom phase, physical characteristics, and phytoplankton community structure. DOC concentra-tions and contributions to the TOC increased as phytoplankton cells became senescent in mid-January and decreased in February-March when phytoplankton were relatively active. From February-March, the deepened mixed layer depth encouraged TEP formation, subsequently increasing the TEP contributions. Regardless of the sampling season, all organic carbon concentrations per unit Chl-a were significantly higher in P. antarctica-abundant groups. The DOC contributions to the TOC were correspondingly higher at the P. antarctica-abundant stations in mid-January, which indicates that P. antarctica could be also important in the DOC contributions in the Ross Sea. The rapid alteration in environmental characteristics and phytoplankton community structures in the Ross Sea due to climate change could affect the organic carbon pool at the euphotic layer which consequently could determine the efficiency of the biological pump.

Biogeochemistry of iron in coastal Antarctica: isotopic insights for external sources and biological uptake in the Amundsen Sea polynyas

2023-01-01T00:00:00Z

Title: Biogeochemistry of iron in coastal Antarctica: isotopic insights for external sources and biological uptake in the Amundsen Sea polynyas Authors: Hung-An Tian; Mathijs van Manen; Zach B. Bunnell; Jung, Jinyoung; Lee, SangHoon; Kim, Tae-Wan; Gert-Jan Reichart; Tim M. Conway; Rob Middag Abstract: Seasonal phytoplankton blooms in the Antarctic Amundsen Sea Polynyas are thought to be supported by an external supply of iron (Fe) from circumpolar deep waters, benthic sediments, and/or ice shelf meltwaters. However, largely due to the limited amount of Fe data reported for the Amundsen Sea Polynyas, understanding of the sources and processes that affect the biogeochemistry of Fe in this region (notably within the ice shelf system) remains limited. Here, we present the first investigation of dissolved Fe isotope distributions (δ56Fe) along the conveyer belt of waters into and through the Amundsen Sea, via the Dotson Ice Shelf, from samples collected during austral summer (2017-2018). Our dataset allows us to characterize and compare the dissolved δ56Fe signatures of incoming modified Circumpolar Deep Water (mCDW) and of sedimentary sources on the continental shelf. The range in dissolved δ56Fe (-1 to +0.1 ‰) observed in the Amundsen Sea close to the seafloor, coupled with elevated dissolved Fe concentrations (up to 1.6 nmol/L), suggests that Fe is released from shelf sediments via a combination of reductive and non-reductive processes, with non-reductive dissolution input being relatively more important (20？56 %) than reductive dissolution (4-12 %). Near the Dotson Ice Shelf, the δ56Fe in the mCDW inflow (-0.70 ‰) was lower than the mCDW outflow (-0.23 ‰), whereas any change in dissolved Fe concentrations was negligible. We speculate that this shift in dissolved δ56Fe underneath the ice shelf is driven by a combination of enhanced preservation (and addition) of lithogenic colloidal Fe(III) and/or complexation with Fe-binding ligands, together with a differential loss of Fe2+. We also found distinct δ56Fe signatures in surface waters of the polynya, with apparent preferential uptake of isotopically light Fe in a bloom dominated by diatoms leading to a relatively heavy remnant dissolved δ56Fe signature of +1.06 ‰, compared to a bloom dominated by haptophytes where more modest and variable isotope fractionation was observed. The different isotopic composition between the two regions could be related to the dominance of different species, but this remains speculative. Despite prominent biological uptake, we suggest that other factors such as rapid recycling (e.g., adsorption and regeneration), bacterial regeneration, and complexation with organic ligands, together with the supply of lithogenic particles also play important roles in setting surface dissolved δ56Fe in the Amundsen Sea Polynyas. Overall, this study provides a further understanding of the external Fe sources and the biogeochemical processes in the Amundsen Sea and thus a baseline on how changing conditions in Antarctica can affect Fe cycling in the Southern Ocean and beyond.

The biogeochemistry of zinc and cadmium in the Amundsen Sea, coastal Antarctica

2023-01-01T00:00:00Z

Title: The biogeochemistry of zinc and cadmium in the Amundsen Sea, coastal Antarctica Authors: Tian Hung-An; van Manen Mathijs; Wille Flora; Jung, Jinyoung; Lee, SangHoon; Kim, Tae-Wan; Aoki Shigeru; Eich Charlotte; Brussaard Corina P. D.; Reichart Gert-Jan; Conway Tim M.; Middag Rob Abstract: The trace metals zinc (Zn) and cadmium (Cd) are both involved in the metabolic processes of marine phyto-plankton, and as such, both metals play important roles in ocean biogeochemical cycles. In Antarctica, the Amundsen Sea (AS) experiences rapid ice shelf melting, and the Amundsen Sea polynya (ASP) hosts seasonal phytoplankton blooms in austral summer, with important implications for atmospheric carbon dioxide draw-down. However, the effects of ice melting and phytoplankton blooms on the biogeochemistry and distributions of Zn and Cd in the ASP remain poorly studied. Here, we present the first combined dataset of dissolved and particulate Zn and Cd in the AS (including the inflow and outflow to and from the Dotson and Getz ice shelves) collected as part of the GEOTRACES process study GPpr12. We use this dataset to assess the sources of both elements to the AS region and characterize the particle composition in the ASP. We find that the main source of both dissolved Zn and Cd in the AS is Circumpolar Deep Water (CDW), with an additional small flux of both metals from shelf sediments. By contrast, aerosol deposition, ice shelf melt, and sea ice melt are all deemed insignificant sources for either Zn or Cd in the AS. Labile particulate Zn and Cd dominate the total particulate pool in the surface layer, indicating that biological uptake is a predominant process for the cycling of both metals in the ASP, whereas sediment resuspension and ice shelf melt do not supply a significant amount of either particulate Zn or Cd. Additionally, we use two commonly used approaches to estimate biogenic and lithogenic particulate concentrations. We find high biogenic particulate concentrations at the surface, decreasing with depth, indicating remineralization plays an important role in the cycling of particulate metals. In contrast, lithogenic particulate metal concentrations remain low throughout the water column. We also show that the estimated uptake ratios of Zn and Cd relative to phosphate in the surface layer are lower than reported for the open Southern Ocean, likely related to the spatial and temporal variability of Fe in the AS. Overall, these new observations provide insight into the biogeochemistry of both Zn and Cd in the AS, a region that is subject to the influence of rapid climate change, which may have implications for the larger-scale cycling of trace metals in the Southern Ocean. Specifically, the amount of Zn and Cd supplied to the surface ASP will increase, given that the volume of CDW that flows towards the Dotson Ice Shelf is predicted to increase.

The role of the Dotson Ice Shelf and Circumpolar Deep Water as driver and source of dissolved and particulate iron and manganese in the Amundsen Sea polynya, Southern Ocean

2022-10-20T00:00:00Z

Title: The role of the Dotson Ice Shelf and Circumpolar Deep Water as driver and source of dissolved and particulate iron and manganese in the Amundsen Sea polynya, Southern Ocean Authors: Van Manen, Mathijs; Aoki, Shigeru; Brussaard, Corina P.D.; Conway, Tim M.; Eich, Charlotte; Gerringa, Loes J.A.; Jung, Jinyoung; Kim, Tae-Wan; Lee, SangHoon; Lee, Youngju; Reichart, Gert-Jan; Tian, Hung-An; Wille, Flora; Middag, Rob Abstract: Coastal areas around Antarctica such as the Amundsen Sea are important sources of trace metals and biological hotspots, but are also experiencing the effects of climate change, including the rapid thinning of ice sheets. In the central Amundsen Sea Polynya (ASP), both bio-essential dissolved Fe (DFe) and dissolved Mn (DMn) were found to be depleted at the surface, indicating substantial biological uptake and/or precipitation. Close to the Dotson Ice Shelf (DIS) there were elevated surface concentrations of DMn (>3 nM) but surprisingly not for DFe (<0.3 nM). While Fe-binding ligand data suggests that ligands were abundant near the DIS, these were most likely not strong enough to outcompete scavenging and thus increase DFe substantially in the outflow. In contrast to the dissolved phase, particulate Fe (PFe) and Mn (PMn) concentrations (both labile and refractory fractions) were elevated over the entire water column close to the DIS and partly in the central ASP. We hypothesize that DFe was released from the DIS and immediately established an equilibrium with the labile particulate Fe (L-PFe)pool, via (reversible) scavenging, as indicated by a positive correlation between L-PFe and DFe in the outflow. This scavenging results in relatively low DFe concentrations, but the pool of labile PFe likely buffers the DFe pool when DFe is decreasing, e.g. due to uptake by phytoplankton. The DFe distribution also shows that inflowing modified circumpolar deep water (mCDW) and benthic sediments are clear and important sources for both DFe and DMn in the ASP. Refractory Fe and Mn likely have a lithogenic source, whereas the labile fractions are mostly biogenic in surface waters, and authigenic in deep waters (>100 m depth). We compared different uptake ratios, underlining that uptake ratio estimates do not necessarily capture natural variability and it is likely better to use a range of values. In the future, climate change may increase the heat flux of mCDW and thereby the melting of the DIS. This will most likely cause an increased input of Fe and Mn into the ASP, which may fuel increased levels of primary productivity in the ASP.