Department of Earth System Science

Seasonal trade wind-induced upwelling along the southern margin of the Caribbean Sea occurs in response to the annual migration of the Intertropical Convergence Zone. Laminated, high deposition rate sediments of the Cariaco Basin, a small anoxic basin on the Venezuelan continental shelf, clearly record large changes in the past intensity of this upwelling. Because sediments of the Cariaco Basin are largely unbioturbated, they offer a natural opportunity to study the stable isotopic records of multiple planktonic foraminiferal taxa and to evaluate their sensitivity to both the modern hydrography and temporal changes in upwelling intensity and climate. Oxygen isotope data (δ¹⁸O) from four dominant foraminiferal taxa are presented for the time period covering the last 28 kyr. The δ¹⁸O data from Globigerina bulloides, after correction for nonequilibrium precipitation, are used as a monitor of sea surface conditions during the winter-spring upwelling season. The δ¹⁸O data from white Globigerinoides ruber are used as a measure of annual-average conditions in the near surface, while pink G. ruber data are consistent with use as an index of endmember conditions during the summer-fall nonupwelling season. Data from the deeper dwelling Neogloboquadrina dutertrei yield information on conditions near the base of the local thermocline. During the last glacial, δ¹⁸O data from G. ruber and generally reduced interspecific differences indicate cooling of surface waters over the Cariaco Basin by up to 4°C. This longer-term cooling does not appear to be related to changes in upwelling intensity along the coast but may instead reflect more regional cooling of the larger Caribbean. Superimposed on this pattern, between 12.6 and ∼10 ka, is a convergence of δ¹⁸O data between G. bulloides and N. dutertrei, implying much stronger upwelling during the last deglaciation. This scenario is consistent with other evidence for high productivity at this time. At ∼14 ka, a sharp δ¹⁸O depletion event observed in all taxa seems to have been produced by increased freshwater discharge to the southern Caribbean, suggesting either higher regional rainfall or the influence of glacial melting in the Andes. Minimum δ¹⁸O values of pink G. ruber around 6–7 ka record warmer summer sea surface temperatures and/or decreased salinity in the mid-Holocene.