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

[摘要] A high resolution time-series analysis of stable carbon isotopic signaturesin particulate organic carbon (δ¹³C_POC) and associatedbiogeochemical parameters in sea ice and surface waters provides an insightinto the factors affecting δ¹³C_POC in the coastal westernAntarctic Peninsula sea ice environment. The study covers two austral summerseasons in Ryder Bay, northern Marguerite Bay between 2004 and 2006. A shiftin diatom species composition during the 2005/06 summer bloom tonear-complete biomass dominance of Proboscia inermis is strongly correlated with a large~10 ‰ negative isotopic shift in δ¹³C_POC that cannotbe explained by a concurrent change in concentration or isotopic signatureof CO₂. We hypothesise that the δ¹³C_POC shift may bedriven by the contrasting biochemical mechanisms and utilisation ofcarbon-concentrating mechanisms (CCMs) in different diatom species.Specifically, very low δ¹³C_POC in P. inermis may be caused by thelack of a CCM, whilst some diatom species abundant at times of higherδ¹³C_POC may employ CCMs. These short-lived yet pronouncednegative δ¹³C_POC excursions drive a 4 ‰ decrease in theseasonal average δ¹³C_POC signal, which is transferred tosediment traps and core-top sediments and consequently has the potential forpreservation in the sedimentary record. This 4 ‰ difference between seasonsof contrasting sea ice conditions and upper water column stratificationmatches the full amplitude of glacial-interglacial Southern Ocean δ¹³C_POC variability and, as such, we invoke phytoplankton specieschanges as a potentially important factor influencing sedimentary δ¹³C_POC. We also find significantly higher δ¹³C_POC in sea ice than surface waters, consistent withautotrophic carbon fixation in a semi-closed environment and possiblecontributions from post-production degradation, biological utilisation ofHCO₃⁻ and production of exopolymeric substances. This studydemonstrates the importance of surface water diatom speciation effects andisotopically heavy sea ice-derived material for δ¹³C_POC inAntarctic coastal environments and underlying sediments, with consequencesfor the utility of diatom-based δ¹³C_POC in the sedimentaryrecord.

[学科分类] 地球化学与岩石