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

[1] We compared ocean atlas values of surface water [PO43-] and [CO2(aq)] against the carbon isotopic fractionation (epsilon(p)) 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 epsilon(p) 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 C-37: 2 alkenones. Instead, we found inverse relationships between epsilon(p) and [PO43-] for the two ocean basins ( for the Atlantic, epsilon(p) = - 4.6*[PO43-] + 15.1, R = 0.76; for the Pacific, epsilon(p) = - 4.1*[PO43-] + 13.7, R = 0.64), suggesting that epsilon(p) 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 epsilon(p) values, indicating high nutrient concentrations, coincide with high contents of total organic carbon and C-37 alkenones, low surface water temperatures, and decreased bulk delta(15)N values, suggesting an increased upwelling of nutrient-rich cool subsurface waters as the main cause for the observed epsilon(p) decrease.