Reconstruction of Late Pleistocene Surface Water pCO2 in the Eastern Equatorial Atlantic Based on Alkenone and Boron Isotopes

Reconstructing the long-term history of the partial pressure of atmospheric carbon dioxide (pCO2) is a big challenge in paleoclimatic and paleoceanographic research. Hence, a number of proxies have been proposed to estimate paleo-CO2 concentrations at time periods prior to those covered by ice cores. Two of the most prominent proxy approaches are (a) the carbon isotopic fractionation of C37-alkenones produced by certain haptophyte algae and (b) the boron isotopic composition in planktonic foraminifer shells. Both techniques have limitations and uncertainties due to their specific methodology. In past paleoceanographic studies, either one or the other approach has been applied individually. Here, we present a study where we directly compare alkenone carbon isotopic fractionation over several late Pleistocene glacial cycles with published boron isotope data at the same location (ODP site 668B, Sierra Leone Rise). The site is presently characterised by an air-sea equilibrium in CO2 as well as low productivity. We show that over the last 800,000 years alkenone isotope data are not primarily controlled by atmospheric pCO2 levels. We further demonstrate that the species assemblage of alkenone producing algae has no influence on their isotopic signal. In contrast, the boron isotope proxy has been shown to have a great potential for estimating atmospheric pCO2 at this location.