Outflow From the Amazon: Late Quaternary Changes in the Dynamics of the Surface Water Currents of the Western Equatorial Atlantic

Stable isotope analyses (δ18O) and Mg/Ca palaeothermometry were performed on 4 different species of planktonic foraminifera in order to assess changes in the upper water hydrography of the equatorial Atlantic Ocean throughout the late Quaternary. The Amazon Fan and Ceara Rise, situated at the mouth of the Amazon River, directly underlie the modern flowpath of the North Brazil Current (NBC), a warm water boundary current which flows northwestwards along the Brazilian continental shelf. As a conduit for the cross-equatorial transport of heat and salinity, the NBC is a key component of the heat budget of the North Atlantic and thus of thermohaline circulation. Increases in the latitudinal thermal gradient, associated with the migration of the Intertropical Convergence Zone (ITCZ), result in a periodic weakening of the northward heat flux by way of a seasonal retroflection of the NBC which turns eastwards to form the North Equatorial Counter Current (NECC). Fifteen ODP cores drilled on the Amazon Fan and Ceara Rise were sampled at depths representative of 5 timeslices, selected for their significance in establishing the extent of palaeoclimatic change during glacial-interglacial cycles (modern; early Holocene; Younger Dryas; Last Glacial Maximum (LGM) and Marine Isotope Stage 3). The resulting spatial and temporal distributions of calculated values of δ18Ow (isotopic composition of ambient seawater) are used to infer variations in surface water currents and demonstrate directional shifts in the dispersal of freshwater outflow from the Amazon River. Sea surface temperature reconstructions reveal progressive climatic amelioration over the last 30,000 years, indicating a temperature increase of ~ 3.2 ± 1.1°C since the LGM. In conjunction with this warming, values of Δδ18O, a proxy for water column stratification, are suggestive of increased vertical mixing in the glacial ocean. Spatial variations in δ18Ow imply an oceanward shift in the river outflow plume during cold climates. This directional change, driven by surface water currents, implies a continuation in the formation of the NECC which would cause the curtailment of cross-equatorial heat and salinity transport through a weaker or non-existent NBC. We suggest that a prolongation in the retroflection of the NBC could have resulted from a mean southward migration of the ITCZ during glacial periods.