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

Suspended and sea ice particulate organic carbon (POC) collected from the R oss Sea, Antarctica, shows seasonal enrichments in C-13 unrelated to atmosp heric pCO(2) variations. The carbon isotopic composition of phytoplankton b loom-related POC (delta(13)C(POC)) collected from the southwestern Ross Sea during a 1992 bloom event is strongly correlated with POC concentration. T his correlation may be attributed to high growth rates, species-specific fr actionation, input from sea ice communities enriched in C-13, increased het erotrophic recycling, or high rates of bloom-related CO2[aq] drawdown in th e upper water column. While all C-13 enrichment, we favor the latter hypoth esis because these factors likely contribute to POC of the link between C-1 3 enrichment and partial isolation of the surface warer CO2 pool in areas o f sea ice melt-induced stratification of the upper water column. The CO2 dr awdown hypothesis is further supported by dissolved inorganic carbon delta( 13)C enrichments of up to 2-3 parts per thousand in the vicinity of phytopl ankton blooms. Patterns of isotopic composition and organic carbon concentr ation in Ross Sea surface sediments resemble that of the upper water column , particularly in areas of frequent, annually recurrent phytoplankton bloom s. Overall, surface sediment delta(13)C values are 1-3 parts per thousand e nriched relative to surface water POC, with the highest delta(13)C values f ound in areas that receive large amounts of phytoplankton bloom detritus. F uture downcore carbon isotopic analyses may prove useful as tracers of bloo m dynamics in melt-water-influenced areas, where large enrichments in C-13 coincide with high productivity.