PLANKTONIC-FORAMINIFERA AS RECORDERS OF PAST SURFACE-WATER STRATIFICATION

The delta(18)O difference between shallow- and deep-living planktonic foraminifera is a proxy for the stratification of surface waters. In H olocene sediments from the Atlantic this difference increases from abo ut 0 parts per thousand in subpolar regions to similar to 3 parts per thousand in the tropics, The delta(18)O values of the shallow dwellers Globigerinoides sacculifer and Neogloboquadrina pachyderma mainly ref lect surface-water conditions, The calcification depth of the deep-liv ing species Globorotalia truncatulinoides (right-coiling) is affected by the stratification of the mater column. In the subtropics, shell fo rmation of G. truncatulinoides (right-coiling) begins in weakly strati fied surface waters. In the tropics, the initial shell is; secreted be low the mixed layer. This hydrographically induced difference in calci fication depth allows the use of the isotopic difference between G. tr uncatulinoides and the shallow species as a proxy for the stratificati on of surface waters. The Delta delta(18)O between G. truncatulinoides and G. sacculifer from tropical gravity Core GeoB 1523-1 was signific antly lower during isotope stage 2. This suggests that the glacial tem perature stratification of western equatorial Atlantic surface waters was significantly reduced relative to the Holocene.