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A high resolution time-series analysis of stable carbon isotopic signatures in particulate organic carbon (δ13CPOC) and associated biogeochemical parameters in sea ice and surface waters provides an insight into the factors affecting δ13CPOCin 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 δ13CPOC that cannot be explained by a concurrent change in concentration or isotopic signature of CO2. We hypothesise that the δ13CPOC shift may be driven by the contrasting biochemical mechanisms and utilisation of carbon-concentrating mechanisms (CCMs) in different diatom species. Specifically, very low δ13CPOC in P. inermis may be caused by the lack of a CCM, whilst some diatom species abundant at times of higher δ13CPOC may employ CCMs. These short-lived yet pronounced negative δ13CPOC excursions drive a 4‰ decrease in the seasonal average δ13CPOC 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 δ13CPOC variability and, as such, we invoke phytoplankton species changes as a potentially important factor influencing sedimentary δ13CPOC. We also find significantly higher δ13CPOCin 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 Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. and production of exopolymeric substances. This study demonstrates the importance of surface water diatom speciation effects and isotopically heavy sea ice-derived material for δ13CPOC in Antarctic coastal environments and underlying sediments, with consequences for the utility of diatom-based δ13CPOC in the sedimentary record. [ABSTRACT FROM AUTHOR]