Latitudinal δ13Corg variations in sinking matter and sediments from the South Atlantic: effects of anthropogenic CO2 and implications for paleo-PCO2 reconstructions

Latitudinal variations of δ13Corg of plankton, sinking particles and surface sediments of the southern Atlantic Ocean have been compiled and compared to literature-derived [CO2 (aq)] values in surface waters. We observed less variability of the sediment-δ13C data at a given latitude compared to the plankton values which more sensitively record seasonal changes. In the tropical–subtropical Atlantic with low seasonality, we measured a relatively small latitudinal variation (−18.5 to −22.5‰) of the sediment-δ13C. In contrast, a steep gradient of both isotope values and [CO2 (aq)] was found south of 40°S. South of 60°S, the sediment-δ13C values were between −23 and −26.5‰ corresponding to generally higher [CO2 (aq)] values. The surface sediments were generally heavier by 0.5–3.2‰ in the tropical–subtropical Atlantic and 3.0–4.6‰ in Southern Ocean, respectively, compared to sinking matter; they were also heavier by up to 3.2‰ than freshly sedimented phytodetritus sampled in the Southern Ocean. These differences may at least be partly attributed to the anthropogenic PCO2 increase leading to an enhanced effect in the Southern Ocean due to increasing CO2 solubility. We estimated the preindustrial [CO2 (aq)] from sediment-δ13Corg to be lower by 1–2 μmol l−1 in warm waters and up to 8 μmol l−1 in cold waters compared to present day measurements. We found that the isotope values of the surface sediments were generally linked to surface water [CO2 (aq)] (preindustrial), resulting in a relationship rather similar to that derived from plankton data [Rau, G.H., 1994. Variations in sedimentary organic δ13C as a proxy for past changes in ocean and atmospheric CO2 concentrations. In: Zahn, R., Pedersen, T.F., Kaminski, M.A., Labeyrie, L. (Eds.), Carbon Cycling in the Glacial Ocean, NATO Asi Series, Global Environmental Change, Vol. 17]. However, estimations of past [CO2 (aq)] from sediment-δ13Corg may have a large uncertainty especially in the Southern Ocean. Here, our data show the weakest correlation, probably due to effects of changing growth rates and productivity, in particular at the frontal zones.