200 000 yr diatom records from Atlantic upwelling sites reveal maximum productivity during LGM and a shift in phytoplankton community structure at 185 000 yr

Abstract

Time series of diatom accumulation rates (DAR) and assemblage composition for the last 200 000 yr from the Atlantic major coastal upwelling loci of the eastern boundary currents and eastern Equator are compared together with independent proxies of export production, to upper ocean productivity and to nutrient concentration and utilization. Fluctuations in DAR are concomitant at all sites and in good correspondence to the other independent productivity proxies indicating glacial periods as the most productive times of the past 200 000 yr in both hemispheres of the eastern Atlantic. Maximum productivity, about one order of magnitude higher than at present, occurred at the last glacial maximum (LGM). Despite the ‘Atlantic’ similarities in both coastal and equatorial upwelling types, important regional and site specific variations emerge. In the southern Hemisphere site, a contradiction of proxies, diatoms and Corg, is noted at the base of stage 6 (185 000 yr). The good correlation found between the Corg and the benthic foraminifera fluxes is interpreted as a reflection of a shift in the phytoplankton community structure related to either a change in the N:P:Si ratio, or similar nutrient conditions associated with increased water column stratification.