Carbon 13/Carbon 12 ratios of sedimentary organic matter from the Ross Sea, Antarctica: A record of phytoplankton bloom dynamics

Abstract

Suspended and sea ice particulate organic carbon (POC) collected from the Ross Sea, Antarctica, shows seasonal enrichments in 13 C unrelated to atmospheric p CO 2 variations. The carbon isotopic composition of phytoplankton bloom‐related POC (δ 13 C POC ) collected from the southwestern Ross Sea during a 1992 bloom event is strongly correlated with POC concentration. This correlation may be attributed to high growth rates, species‐specific fractionation, input from sea ice communities enriched in 13 C, increased heterotrophic recycling, or high rates of bloom‐related CO 2 [aq] drawdown in the upper water column. While all these factors likely contribute to POC 13 C enrichment, we favor the latter hypothesis because of the link between 13 C enrichment and partial isolation of the surface water CO 2 pool in areas of sea ice melt‐induced stratification of the upper water column. The CO 2 drawdown hypothesis is further supported by dissolved inorganic carbon δ 13 C enrichments of up to 2–3‰ in the vicinity of phytoplankton blooms. Patterns of isotopic composition and organic carbon concentration in Ross Sea surface sediments resemble that of the upper water column, particularly in areas of frequent, annually recurrent phytoplankton blooms. Overall, surface sediment δ 13 C values are 1–3‰ enriched relative to surface water POC, with the highest δ 13 C values found in areas that receive large amounts of phytoplankton bloom detritus. Future downcore carbon isotopic analyses may prove useful as tracers of bloom dynamics in melt‐water‐influenced areas, where large enrichments in 13 C coincide with high productivity.