Stable phytoplankton community structure in the Arabian Sea over the past 200,000 years

Glacial to interglacial climate changes have been related to organic carbon cycling in oceanic surface waters1, and this possible link has led to the development of sedimentary tracers of past marine biological production. For example, sediment records of organic carbon2, opal3 and biogenic barium4 have been used to reconstruct past variations in production in different oceanic regimes, but these tracers cannot be used to discriminate between the relative contributions of different phytoplankton groups. Such a discrimination would provide greater insight into the operation of the biological ‘pump’ transporting material down out of surface waters, and into the possible influence of the structure of oceanic food chains on carbon fluxes. Several organic biomarker compounds have now been established for tracing the contribution of different planktonic groups to organic carbon in sediments5,6,7. Here we show that four such biomarkers—dinosterol, alkenones, brassicasterol and chlorins, which represent dinoflagellates, prymnesiophytes, diatoms and chlorophyll-producers, respectively—have concordant concentration maxima that coincide with organic carbon maxima over the past 200,000 years in a sediment core from the northeastern Arabian Sea. Not only do these organic tracers track changes in ocean production in this region, but the similar distributions of dinosterol and brassicasterol indicate that the relative contributions of the dominant members of the phytoplankton community (diatoms and dinoflagellates) to production were roughly uniform on timescales greater than 3,000–4,000 years over the past 200,000 years.