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 (δ 13C POC) and associated biogeochemical parameters in sea ice and surface waters provides an insight into the factors affecting δ 13C 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 ‰ negative isotopic shift in δ 13C POC that cannot be explained by a concurrent change in concentration or isotopic signature of CO 2. We hypothesise that the δ 13C POC shift may be driven by the contrasting biochemical mechanisms and utilisation of carbon-concentrating mechanisms (CCMs) in different diatom species. Specifically, very low δ 13C POC in P. inermis may be caused by the lack of a CCM, whilst some diatom species abundant at times of higher δ 13C POC may employ CCMs. These short-lived yet pronounced negative δ 13C POC excursions drive a 4 ‰ decrease in the seasonal average δ 13C 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 ‰ difference between seasons of contrasting sea ice conditions and upper water column stratification matches the full amplitude of glacial-interglacial Southern Ocean δ 13C POC variability and, as such, we invoke phytoplankton species changes as a potentially important factor influencing sedimentary δ 13C POC. We also find significantly higher δ 13C 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 HCO 3− and production of exopolymeric substances. This study demonstrates the importance of surface water diatom speciation effects and isotopically heavy sea ice-derived material for δ 13C POC in Antarctic coastal environments and underlying sediments, with consequences for the utility of diatom-based δ 13C POC in the sedimentary record.