Offshore expansion of the Brazilian coastal upwelling zones during Marine Isotope Stage 5

摘要

Paleoceanographic reconstructions in upwelling regions can provide relevant information about changes in primary productivity, ocean atmosphere interactions and the carbon budget. Here, we assessed new data on planktonic foraminifera from a sediment core located near to coastal upwelling zones along the Brazilian coast. Our new data was combined with previous records to reveal the state of upwelling systems along the western South Atlantic margin throughout the last two deglacial and interglacial periods sensu lato. Despite the contemporary oligotrophic scenario of the Santos Basin, a remarkably high relative abundance of Globigerina bulbides and low temperatures at a depth of 100 m indicated upwelling conditions similar to current shelf up welling zones from 130 to 90 kyr BP. Comparing these results with previous studies, we argue that Brazilian shelf upwelling zones expanded offshore between 20 and 28 degrees S. We develop two conceptual scenarios to characterize the system: (1) during Marine Isotopic Stage (MIS 5), the system expanded along the continental margin between 20 and 28 degrees S following the eccentricity maximum; and (2) after 20 kyr BP, the system retracted to current continental shelf zones. We propose a new mechanism whereby variation of the Earth's eccentricity, which drives seasonality, is the main factor controlling expansion or retraction of the Brazilian upwelling system. Absence of such conditions in more recent periods supports our model and indicates that current upwelling zones are the remnants of a larger upwelling system. However, more studies are required to better define the latitudinal boundaries of the Brazilian upwelling system in the past and its possible influences over the regional carbon budget.