Stable isotopes of planktonic foraminifera from tropical Atlantic/Caribbean core-tops: Implications for reconstructing upper ocean stratification

Abstract

δ 18O values of nine tropical–subtropical planktonic foraminiferal species with different preferential habitat depths collected from 62 core–top samples along an east–west transect across the tropical Atlantic/Caribbean were used to test the applicability of interspecific δ 18O gradients for reconstructions of tropical upper ocean stratification. In general, the δ 18O difference (Δ δ 18O) between intermediate- and shallow-dwelling species decreases, and Δ δ 18O between deep and intermediate dwellers increases with increasing thermocline depth towards the west. The statistical significance of regional differences in Δ δ 18O highlights Δ δ 18O between the intermediate dwellers (in particular Globorotalia scitula and Globorotalia tumida) and the shallow dweller Globigerinoides ruber pink, as well as Δ δ 18O between the deep dwellers Globorotalia crassaformis or Globorotalia truncatulinoides dextral and intermediate dwellers as most sensitive to changes in tropical Atlantic thermocline depth. Based on the observed regional variations in interspecific Δ δ 18O, we propose a multispecies stratification index “STRA trop” = ( δ 18O intermediate − δ 18O shallow) / ( δ 18O deep − δ 18O shallow) for the tropical ocean. Statistically significant differences in STRA trop values between the E-Atlantic and the Caribbean suggest that this index may be a useful tool to monitor variations in tropical upper ocean stratification in the geological record.