Sedimentary sterols as biogeochemical indicators in the Southern Ocean

Abstract

Abundances and isotopic compositions of sterols and of total organic carbon in surface sediments were measured at 18 stations in the Ross Sea, Antarctica. Ten sterols, 5α-cholestan-3β-ol (cholestanol), cholest-5-en-3β-ol (cholesterol), cholest-5,22E-dien-3β-ol, 24-methyl-5α-cholest-22E-en-3β-ol (brassicastanol), 24-methyl-cholest-5,22E-dien-3β-ol (brassicasterol), 24-ethyl-5α-cholestan-3β-ol (sitostanol), 24-ethyl-cholest-5-en-3β-ol (sitosterol), 24-ethyl-cholest-5,22E-dien-3β-ol (stigmasterol), 4α,23,24-trimethyl-5α-cholestan-3β-ol (dinostanol), and 4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol (dinosterol), are most widely distributed. Polytopic vector analysis of the variations in abundance resolved four sources for these compounds: an assemblage of phytoplankton characteristic of the Ross Sea Polynya, diatoms and associated consumers, zooplankton, and processes associated with heterotrophic dinoflagellates. Concentrations of stanols were strongly correlated with those of dinosterol and dinostanol. Concentrations of total organic carbon (TOC) ranged from 0.1% to 1.2% and were lowest on crests and banks and higher in basins. The mole fraction of organic carbon occurring as sterols ranged from 3 to 1100 ppm. Values were lowest at stations with anomalously old TOC (estimated from regional variations in the radiocarbon age of acid-insoluble organic carbon), thus pointing to weathering and redistribution of surface sediments as important factors in the differential degradation of sterols and TOC. The difference in first order rate constants for the degradation of these materials is ca. 0.002 yr −1. Stigmasterol and the C 27 sterols were significantly enriched in 13C relative to other sterols. The abundance of 13C in TOC at four western stations was 4‰ higher than elsewhere. Abundances of 13C in all sterols at these stations is also 4‰ higher than elsewhere, indicating enrichment of 13C in the entire biological community. Independent observations of P CO 2 in surface waters, together with known relationships between isotopic fractionation and the concentration of dissolved CO 2, show that the isotopic zonation in organic carbon is due entirely to dynamic drawdown of CO 2 in western surface waters. At those locations, late melting of ice produces salinity gradients that inhibit mixing of CO 2 from deeper waters.