Variations in stable isotopes and organic geochemistry in South Equatorial Atlantic during the last 30kyrs

Abstract:

In order to identify the glacial–interglacial changes in South Atlantic as cause of variation in oceanic denitrification and its response of nutrients transport towards the low latitudes, we used stable isotopes (δ¹³C and δ¹⁸O) of benthic and planktonic foraminifera and organic geochemistry (δ¹³C_org, δ¹⁵N, C_org and N_org) in a sediment core located on the continental slope off Northeast Brazil. Here we show that during the LGM occurred a reduction on denitrification rates (decrease on the δ¹⁵N signal) due the reduction in nutrients influx, which can be seen though the high benthic δ¹³C values. This might be related to a higher intake of nutrients in high latitude south that reduce the nutrient input into the thermocline toward lower latitudes. The interglacial period showed an opposite trend, with an increase in δ¹⁵N signal and a decrease in surface to deep gradient (Δδ¹³C), promoting an increased of oceanic ventilation and nutrient flux in the lower latitudes. However, the cold substages (H1 and YD) showed a drastic decrease in the denitrification rate as a result of Atlantic Overturning Circulation collapse and an increase of terrigenous input (high C/N and C_org values and low δ¹³C_org signal) as a consequence of the southward displacement of Intertropical Convergence Zone, which promoted an increase in the moisture at Northeast Brazil. Thus, we argue that the high latitude south have a large influence in nutrient influx within the thermocline toward the low latitudes and these variations could be associate with the ocean ventilation and terrigenous input during climate changes.