Albert Benthien

Stable Carbon Isotopic Composition of the C37:2 Alkenone: A Proxy for CO2(aq) Concentration in Oceanic Surface Waters?

We tested the applicability of the carbon isotopic composition of C37:2 alkenones (δ13C37:2) as a proxy for dissolved carbon dioxide CO2(aq) in oceanic surface waters. For this purpose we determined (δ13C37:2 in suspended particulate organic matter (POM) and surface sediments from the South Atlantic. In opposite of what would be expected from a diffusive CO2 uptake model for marine algae we observed a positive correlation between 1/[CO2(aq)] and the isotopic fractionation (ep) calculated from (δ13C37:2. This clearly demonstrates that CO2(aq) is not the primary factor controlling ep at the sites studied. On the other hand we found a negative correlation between ep and the phosphate concentration in the surface waters (0–10 m) supporting the assumption of (1997) that ep is primarily related to nutrient-limited algal growth rather than to [CO2(aq)]. Reconstructing past CO2(aq) levels from (δ13C37:2 thus requires additional proxy information in order to correct for the influence of haptophyte growth on the isotopic fractionation. In the eastern Angola Basin, we previously used δ15N of bulk organic matter as proxy for nutrient-limited growth rates. As an alternative the Sr/Ca ratio of coccoliths has been recently suggested as growth-rate proxy which should be tested in future studies.

Delta 13C measured on alkenones of surface sediment samples GeoB1008-6 to GeoB3603-1 from the South Atlantic

We have analyzed the stable carbon isotopic composition of the diunsaturated C37 alkenone in 29 surface sediments from the equatorial and South Atlantic Ocean. Our study area covers different oceanographic settings, including sediments from the major upwelling regions off South Africa, the equatorial upwelling, and the oligotrophic western South Atlantic. In order to examine the environmental influences on the sedimentary record the alkenone-based carbon isotopic fractionation (Ep) values were correlated with the overlying surface water concentrations of aqueous CO2 ([CO2(aq)]), phosphate, and nitrate. We found Ep positively correlated with 1/[CO2(aq)] and negatively correlated with [PO43-] and [NO3-]. However, the relationship between Ep and 1/[CO2(aq)] is opposite of what is expected from a [CO2(aq)] controlled, diffusive uptake model. Instead, our findings support the theory of Bidigare et al. (1997, doi:10.1029/96GB03939) that the isotopic fractionation in haptophytes is related to nutrient-limited growth rates. The relatively high variability of the Ep-[PO4] relationship in regions with low surface water nutrient concentrations indicates that here other environmental factors also affect the isotopic signal. These factors might be variations in other growth-limiting resources such as light intensity or micronutrient concentrations.