Sedimentology and stable isotope ratios of planktonic foraminifera from two cores off Northeastern Brazil

The stable isotope composition of planktonic foraminifera correlates with evidence for pulses of terrigenous sediment in a sediment core from the upper continental slope off northeastern Brazil. Stable oxygen isotope records of the planktonic foraminiferal species Globigerinoides sacculifer and Globigerinoides ruber (pink) reveal sub-Milankovitch changes in sea-surface hydrography during the last 85,000 yr. Warming of the surface water coincided with terrigenous sedimentation pulses that are inferred from high XRF intensities of Ti and Fe, and which suggest humid conditions in northeast Brazil. These tropical signals correlate with climatic oscillations recorded in Greenland ice cores (Dansgaard-Oeschger cycles) and in sediment cores from the North Atlantic (Heinrich events). Trade winds may have caused changes in the North Brazil Current that altered heat and salt flux into the North Atlantic, thus affecting the growth and decay of the large glacial ice sheets.

Stable carbon and oxygen isotope ratios of benthic and planktic foraminifera from the Atlantic Ocean

Benthonic foraminifera in late Pleistocene deep-sea cores show significant variation in delta 13C with depth in sediment. This, and the report by Sommer et al., (in prep) of delta 13C variations in planktonic foraminifera, indicate that the delta13C in dissolved oceanic CO2 undergoes a significant change in a few thousand years. This is in apparent contradiction to the estimated 300 ka residence time for carbon in the ocean. It is suggested that this is a consequence of changes in the terrestrial plant biomass, which has a delta13C of about -25?. Postulated changes in world vegetation, particularly in tropical rainforests during the Late Pleistocene, were sufficient to produce change of the magnitude observed. Rapid expansions of forests between 13 ka and 8 ka ago may have resulted in the striking accumulation of aragonite pteropods in Atlantic Ocean sediments of the age. Rapid deforestation during an interglacial-glacial transition probably caused the intense carbonate dissolution which is observed in Equatorial Pacific Ocean sediments deposited over this interbal. The current rate of injection of fossil fuel CO2 into the atmosphere is substantially greater than the rate at which it was added during post-interglacial aridification in the tropics.