Evidence for a prolonged retroflection of the North Brazil Current during glacial stages

The importance of the role played by the tropics in driving and propagating climate change between hemispheres has long been the focus of attention in a bid to evaluate ocean–atmosphere interactions on glacial–interglacial timescales. The Amazon Fan and Ceara Rise in the western equatorial Atlantic Ocean lie directly in the flowpath of the North Brazil Current (NBC) which, as a conduit for the cross-equatorial transport of heat and salinity, is a key component for the heat budget of the North Atlantic. Mg/Ca palaeothermometry and stable oxygen isotope analysis of planktonic foraminifera sampled from 15 sites across the Amazon Fan and Ceara Rise reveal variations in oceanic surface currents and of the dispersal of freshwater from the River Amazon over five timeslices (modern; early Holocene; Younger Dryas; Late Glacial and Marine Isotope Stage 3). Sea surface temperature reconstructions reveal progressive climatic amelioration over the last ~ 30 ka, indicating a temperature increase of ~ 3.2 ± 1.1 °C since the Late Glacial. In conjunction with this warming, values of Δδ18O, a proxy for water column stratification indicates increased vertical mixing of the glacial ocean. The spatial distribution of values of δ18w (the isotopic composition of ambient seawater) is used to infer surface current variations and demonstrates an oceanward shift in the river outflow plume in cold climates representing a prolongation or possibly permanent continuation in the duration of the seasonal retroflection of the NBC causing the curtailment of cross-equatorial transport. A prolongation of this retroflection could have resulted from a mean southward migration of the southern boundary of the ITCZ.