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

Abstract. A high resolution time-series analysis of stable carbon isotopic signatures in particulate organic carbon (δ¹³C_POC) and associated biogeochemical parameters in sea ice and surface waters provides an insight into the factors affecting δ¹³C_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 δ¹³C_POC that cannot be explained by a concurrent change in concentration or isotopic signature of CO₂. We hypothesise that the δ¹³C_POC shift may be driven by the contrasting biochemical mechanisms and utilisation of carbon-concentrating mechanisms (CCMs) in different diatom species. Specifically, very low δ¹³C_POC in P. inermis may be caused by the lack of a CCM, whilst some diatom species abundant at times of higher δ¹³C_POC may employ CCMs. These short-lived yet pronounced negative δ¹³C_POC excursions drive a 4 ‰ decrease in the seasonal average δ¹³C_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 δ¹³C_POC variability and, as such, we invoke phytoplankton species changes as a potentially important factor influencing sedimentary δ¹³C_POC. We also find significantly higher δ¹³C_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 δ¹³C_POC in Antarctic coastal environments and underlying sediments, with consequences for the utility of diatom-based δ¹³C_POC in the sedimentary record.