Planktonic foraminifera as recorders of past surface-water stratification

Abstract

The δ 18 O difference between shallow- and deep-living planktonic foraminifera is a proxy for the stratification of surface waters. In Holocene sediments from the Atlantic this difference increases from about 0‰ in subpolar regions to ∼3‰ in the tropics. The δ 18 O values of the shallow dwellers Globigerinoides sacculifer and Neogloboquadrina pachyderma mainly reflect surface-water conditions. The calcification depth of the deep-living species Globorotalia truncatulinoides (right-coiling) is affected by the stratification of the water column. In the subtropics, shell formation of G. truncatulinoides (right-coiling) begins in weakly stratified surface waters. In the tropics, the initial shell is secreted below the mixed layer. This hydrographically induced difference in calcification depth allows the use of the isotopic difference between G. truncatulinoides and the shallow species as a proxy for the stratification of surface waters. The Δδ 18 O between G. truncatulinoides and G. sacculifer from tropical gravity Core GeoB 1523-1 was significantly lower during isotope stage 2. This suggests that the glacial temperature stratification of western equatorial Atlantic surface waters was significantly reduced relative to the Holocene.