Removal of Refractory Dissolved Organic Carbon in the Amundsen Sea, Antarctica

Abstract

the removal mechanism of refractory deep-ocean dissolved organic carbon (deep-Doc) is poorly understood. the Amundsen Sea polynya (ASp) serves as a natural test basin for assessing the fate of deep-Doc when it is supplied with a large amount of fresh-Doc and exposed to strong solar radiation during the polynya opening in austral summer. We measured the radiocarbon content of Doc in the water column on the western Amundsen shelf. the radiocarbon content of Doc in the surface water of the ASP reflected higher primary production than in the region covered by sea ice. The radiocarbon measurements of DOC, taken two years apart in the ASP, were different, suggesting rapid cycling of Doc. the increase in Doc concentration was less than expected from the observed increase in radiocarbon content from those at the greatest depths. Based on a radiocarbon mass balance, we show that deep-Doc is consumed along with fresh-Doc in the ASp. our observations imply that water circulation through the surface layer, where fresh-Doc is produced, may play an important role in global Doc cycling.The concentration of dissolved organic carbon (DOC) in the deep ocean (>1000 m, hereafter deep-DOC) is vertically uniform, but varies horizontally from ~34 to 48 μmol kg −1 (ref. 1 ). The 14 C ages of deep-DOC range from ~4000 yr in the North Atlantic to ~6500 yr in the North Pacific 2-6 . The old 14 C ages, together with its resistance to bacterial consumption 7 , imply that deep-DOC is refractory. A decrease in deep-DOC concentration of ~29% along the deep water circulation path is consistent with a very slow consumption rate 8 . Bacterial degradation, photochemical degradation, and adsorption to particles have been demonstrated as potential removal mechanisms 8-13 . However, the removal processes of deep-DOC are still not clearly understood.The Amundsen Sea in the west Antarctic is characterized by a relatively broad and deep continental shelf and extensive perennial sea ice cover 14 (Fig. 1). Circumpolar Deep Water (CDW) flows onto the Amundsen Shelf along the seafloor [15][16][17] . A part of the intruding CDW mixes with the overlying water to form modified CDW (MCDW). In the surface layer above ~400 m, water flows north, northwest, and off the shelf in general 18,19 . The western Amundsen Sea harbors a highly productive polynya, the Amundsen Sea Polynya (ASP) [20][21][22] . Peak annual production can be as high as ~3 gC m −2 d −1 with an average annual production of 105 gC m −2 yr −1 (ref. 22 ). High primary production in summer supplies considerable DOC (fresh-DOC) to the euphotic layer 20 . Deep-DOC supplied to the surface layer of the Amundsen Sea via CDW intrusion and mixing with the overlying water is subjected to biogeochemical processes such as the addition of extensive fresh-DOC 20 and microbial 23 and/or photochemical degradation 10 . The turnover time of the waters in the western Amundsen Sea embayment was suggested to be a few decades based on the 14 C content of dissolved inorganic carbon (DIC) 24 . Because of this unique enviro...

Molecular-Level Chemical Characterization of Dissolved Organic Matter in the Ice Shelf Systems of King George Island, Antarctica

“…It was similar to the DOC levels in previous studies conducted during austral fall: 43.3 ± 2 µM in April (Carlson et al, 2000) and 45.1 ± 1.6 µM from February to April in Ross Sea (Bercovici et al, 2017). Moreover, from spring to summer, large ranges of DOC values have been recorded in the Antarctic seawater; for example, 41-64 µM DOC was detected in samples collected from November to February (Fang et al, 2020), whereas DOC varied between 40 and 102 µM in the upper mixed layer (Doval et al, 2002). The DOC values measured in this study were 1.1 to 1.9 times lower than those in the austral summer.…”

Glacial Ice Melting Stimulates Heterotrophic Prokaryotes Production on the Getz Ice Shelf in the Amundsen Sea, Antarctica

“…On the other hand, relatively high Chl‐a concentrations in the WG were associated with the pycnocline formed by the salinity gradient in the shallow water column (30–50 m depth) due to the inflow of sea‐ice meltwater (Figures 2i–2k), which increased the residence time of phytoplankton in the surface water column with sufficient light and nutrients to support high phytoplankton biomass. The Chl‐a concentration in the ASP (average, 2.26 ± 0.70 μg L −1 ) (Table 1 and Figure 2r) was lower than the values reported in December 2013 to January 2014 (average, 8.89 ± 1.11 μg L −1 ) (Y. Lee et al., 2016) because phytoplankton blooms started further northeast of the ASP and the sampling sites at ASP along our sampling transect were not characterized by the blooms at the time of sampling (Fang et al., 2020).…”

“…The Chl-a concentration in the ASP (average, 2.26 ± 0.70 μg L −1 ) (Table 1 and Figure 2r) was lower than the values reported in December 2013 to January 2014 (average, 8.89 ± 1.11 μg L −1 ) (Y. Lee et al, 2016) because phytoplankton blooms started further northeast of the ASP and the sampling sites at ASP along our sampling transect were not characterized by the blooms at the time of sampling (Fang et al, 2020).…”

Marine Organic Carbon and Radiocarbon—present and Future Challenges

“…The processes controlling DIC in the ocean are CO 2 exchange with the atmosphere, photosynthesis, respiration and production and dissolution of carbonate. The processes controlling DOC in seawater are more complex, including river input, microbial production of recalcitrant DOC (Jiao et al 2010), chemoautotrophy (Ingalls et al 2006Hansman et al 2009), hydrothermal processes (McCarthy et al 2011;Lang et al 2006;Shah Walter et al 2018;Estes et al 2019), enhanced refractory DOC utilization by priming (Bianchi et al 2011;Fang et al 2020;Shen and Benner 2018), gel formation (Verdugo et al 2004) and dissolution/aggregation of POC (Smith et al 1992;Druffel et al 2016).…”