Factors influencing the stable carbon isotopic composition of suspended and sinking organic matter in the coastal Antarctic sea ice environment

A high resolution time-series analysis of stable carbon isotopic signatures in particulate organic carbon (delta C-13(POC)) and associated biogeochemical parameters in sea ice and surface waters provides an insight into the factors affecting delta C-13(POC) in the coastal western Antarctic Peninsula sea ice environment. The study covers two austral summer seasons in Ryder Bay, northern Marguerite Bay between 2004 and 2006. A shift in diatom species composition during the 2005/06 summer bloom to near-complete biomass dominance of Proboscia inermis is strongly correlated with a large ~10 parts per thousand negative isotopic shift in delta C-13(POC) that cannot be explained by a concurrent change in concentration or isotopic signature of CO2. We hypothesise that the delta C-13(POC) shift may be driven by the contrasting biochemical mechanisms and utilisation of carbon-concentrating mechanisms (CCMs) in different diatom species. Specifically, very low delta C-13(POC) in P. inermis may be caused by the lack of a CCM, whilst some diatom species abundant at times of higher delta C-13(POC) may employ CCMs. These short-lived yet pronounced negative delta C-13(POC) excursions drive a 4 parts per thousand decrease in the seasonal average delta C-13(POC) signal, which is transferred to sediment traps and core-top sediments and consequently has the potential for preservation in the sedimentary record. This 4 parts per thousand difference between seasons of contrasting sea ice conditions and upper water column stratification matches the full amplitude of glacial-interglacial Southern Ocean delta C-13(POC) variability and, as such, we invoke phytoplankton species changes as a potentially important factor influencing sedimentary delta C-13(POC). We also find significantly higher delta C-13(POC) in sea ice than surface waters, consistent with autotrophic carbon fixation in a semi-closed environment and possible contributions from post-production degradation, biological utilisation of HCO3- and production of exopolymeric substances. This study demonstrates the importance of surface water diatom speciation effects and isotopically heavy sea ice-derived material for delta C-13(POC) in Antarctic coastal environments and underlying sediments, with consequences for the utility of diatom-based delta C-13(POC) in the sedimentary record.

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