Paleoceanographic reconstruction of the western equatorial Atlantic during the last 40 kyr

Abstract

The North Brazil Current in the western equatorial Atlantic is the main route for poleward heat transfer and therefore is a key component of the climate system. To understand the parameters that influence this region, we present a paleoceanographic reconstruction of the last 40kyr based on study of sediment core MC 10/3. This reconstruction is based on: i) the oxygen isotope composition of the planktonic foraminifer Globigerinoides ruber (white); ii) census counts of the planktonic foraminifera assemblage to estimate sea surface temperature (SST) via the Modern Analogue Technique (SSTMAT) and to deduce characteristics of the water column; and iii) values of δ18Oivc-sw (a proxy for sea surface salinity (SSS)). The oxygen isotope composition of G. ruber showed a strong shift of greater than 1‰ after 21calkyrBP. Such a change can be attributed mainly to a salinity reduction of two units, as well as a slight SST increase (~1°C), between 21 and 17.5kyrBP. This change may be related to a southward displacement of the Intertropical Convergence Zone (ITCZ) and its belt of convective activity, which would have increased precipitation over the region and diminished surface salinity. The structure of the planktonic foraminiferal assemblage also indicates such conditions. The productive/thermocline-dwelling Globigerinita glutinata showed higher abundance prior to 21calkyrBP. Other productive/deep-dwellers, i.e., Neogloboquadrina dutertrei, Globorotalia truncatulinoides (left- and right-coiling) and Globorotalia inflata, were slightly more abundant prior to 21calkyrBP and became less frequent with the development of low-salinity surface water that created more stratified and oligotrophic conditions in the water column. However, the warm/oligotrophic surface-dwellers (G. ruber, Globigerinoides sacculifer and Globigerinella siphonifera) became more abundant after 21calkyrBP. The species Globorotalia menardii was nearly absent during the Last Glacial Maximum (LGM), and its growth in abundance, mainly after 17.5calkyrBP, may indicate the resumption of the Agulhas leakage, which would have reached the study area via the South Equatorial Current (SEC). The return of high salinity values synchronous with the G. menardii increase also denotes the influence of Agulhas leakage into the western equatorial Atlantic.