Carbon isotopic fractionation (εp) of C37 alkenones in deep-sea sediments: Its potential as a paleonutrient proxy

Abstract

We compared ocean atlas values of surface water [PO3-4] and [CO2(aq)] against the carbon isotopic fractionation (εp) of alkenones obtained from surface sediments of the South Atlantic and the central Pacific (Pacific data are from Pagani et al. [2002]). We observed a positive correlation between εp and 1/[CO2(aq)], which is opposite of what would be expected if the concentration of CO2(aq) were the major factor controlling the carbon isotopic fractionation of C37:2 alkenones. Instead, we found inverse relationships between εp and [PO3-4] for the two ocean basins (for the Atlantic, εp = -4.6*[PO3-4] + 15.1, R = 0.76; for the Pacific, εp = -4.1*[PO3-4] + 13.7, R = 0.64), suggesting that εp is predominantly controlled by growth rate, which in turn is related to nutrient concentration. The similarity of the slopes implies that a general relationship between both parameters may exist. Using the relationship obtained from the South Atlantic, we estimated surface water nutrient concentrations for the past 200,000 years from a deep-sea sediment core recovered off Angola. Low εp values, indicating high nutrient concentrations, coincide with high contents of total organic carbon and C37 alkenones, low surface water temperatures, and decreased bulk δ15N values, suggesting an increased upwelling of nutrient-rich cool subsurface waters as the main cause for the observed εp decrease. Copyright 2004 by the American Geophysical Union.