Carbon isotopic composition of the C37:2 alkenone in core top sediments of the South Atlantic Ocean: Effects of CO2and nutrient concentrations

Abstract

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 (εp) values were correlated with the overlying surface water concentrations of aqueous CO2 ([CO2(aq)]), phosphate, and nitrate. We found εp positively correlated with 1/[CO2(aq)] and negatively correlated with [PO43−] and [NO3−]. However, the relationship between εp 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]that the isotopic fractionation in haptophytes is related to nutrient‐limited growth rates. The relatively high variability of the εp−[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.