Carbon isotopic fractionation (ε p ) of C 37 alkenones in deep‐sea sediments: Its potential as a paleonutrient proxy

We compared ocean atlas values of surface water [PO 4 3− ] and [CO 2 (aq)] against the carbon isotopic fractionation (ε 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 ε p and 1/[CO 2 (aq)], which is opposite of what would be expected if the concentration of CO 2 (aq) were the major factor controlling the carbon isotopic fractionation of C 37:2 alkenones. Instead, we found inverse relationships between ε p and [PO 4 3− ] for the two ocean basins (for the Atlantic, ε p = −4.6*[PO 4 3− ] + 15.1, R = 0.76; for the Pacific, ε p = −4.1*[PO 4 3− ] + 13.7, R = 0.64), suggesting that ε 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 ε 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 δ 15 N values, suggesting an increased upwelling of nutrient‐rich cool subsurface waters as the main cause for the observed ε p decrease.