STABLE PHYTOPLANKTON COMMUNITY STRUCTURE IN THE ARABIAN SEA OVER THE PAST 200,000 YEARS

Glacial to interglacial climate changes have been related to organic c arbon cycling in oceanic surface waters', and this possible link has l ed to the development of sedimentary tracers of past marine biological production. For example, sediment records of organic carbon(2), opal( 3) and biogenic barium(4) have been used to reconstruct past variation s in production in different oceanic regimes, but these tracers cannot be used to discriminate between the relative contributions of differe nt phytoplankton groups. Such a discrimination would provide greater i nsight into the operation of the biological 'pump' transporting materi al down out of surface waters, and into the possible influence of the structure of oceanic food chains on carbon fluxes. Several organic bio marker compounds have now been established for tracing the contributio n of different planktonic groups to organic carbon in sediments(5-7). Here we show that four such biomarkers-dinosterol, alkenones, brassica sterol and chlorins, which represent dinoflagellates, prymnesiophytes, diatoms and chlorophyll-producers, respectively-have concordant conce ntration maxima that coincide with organic carbon maxima over the past 200,000 years in a sediment core from the northeastern Arabian Sea. N ot only do these organic tracers track changes in ocean production in this region, but the similar distributions of dinosterol and brassicas terol indicate that the relative contributions of the dominant members of the phytoplankton community (diatoms and dinoflagellates) to produ ction were roughly uniform on timescales greater than 3,000-4,000 year s over the past 200,000 years.