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

a r t i c l e i n f o 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 Δδ 18 O, a proxy for water column stratification indicates increased vertical mixing of the glacial ocean. The spatial distribution of values of δ 18 w (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. Large-scale temperature oscillations observed in oxygen isotope records from Greenland ice cores have been found to reflect extensive reorganisations of the ocean–atmosphere system over glacial– interglacial timescales (Bond et al., 1993). In the search for a mechanism to explain the processes governing such global changes, attention has increasingly focused on the tropics which, as a major source of sensible heat, latent heat and water vapour to the atmosphere, have the potential to alter oceanic balance by the export of heat and salinity to the high latitudes through thermohaline circulation (Broecker, 1997). As an important component of thermo-haline circulation, the tropical Atlantic Ocean is an important driving force for climatic change and the accumulation of excess salt in this region ultimately impacts upon the density of high …