The carbon isotopic record of the C 37:2 alkenone in the South Atlantic: Last Glacial Maximum (LGM) vs. Holocene

Abstract

The carbon isotopic fractionation ( ε p) of the C 37:2 alkenone was analysed for 19 South Atlantic sediment samples from the Last Glacial Maximum (LGM). Our study covers the equatorial and subtropical ocean including the coastal upwelling regions off South Africa, the equatorial upwelling, and the oligotrophic western basins. The results were compared to the Holocene ε p records from the respective core locations ( Andersen, N., Müller, P.J., Kirst, G., Schneider, R.R., 1999. Alkenone δ 13C as a proxy for past PCO 2 in surface waters: results from the Late Quaternary Angola Current. In: Fischer, G., Wefer, G. (Eds.), Use of Proxies in Paleoceanography: Examples from the South Atlantic. Springer, Berlin, pp. 469–488; Benthien, A., Andersen, N., Schulte, S., Müller, P.J., Schneider, R.R., Wefer, G., 2002. Carbon isotopic composition of the C 37:2 alkenone in core–top sediments of the South Atlantic Ocean: Effects of CO 2 and nutrient concentrations. Glob. Biogeochem. Cycles 16, 10.1029/2001GB001433 ). Generally, alkenone ε p was lower during the LGM compared to the Holocene. Higher glacial ε p values were only found in sediments from the Angola Basin and in one sample from the eastern crest of the Walvis Ridge. Considering present understanding of LGM–Holocene changes in surface-water conditions (i.e. nutrient level, primary productivity, phytoplankton assemblages), the observed glacial/interglacial difference in ε p indicates that multiple factors controlled the isotopic fractionation in alkenone producing algae depending on the regional setting. In the oligotrophic areas of the South Atlantic the lower than Holocene glacial ε p values can be partly explained with a decrease in surface-water PCO 2 during the LGM. In contrast, the Holocene to LGM decrease in ε p values in the coastal upwelling areas as well as in the eastern tropical Atlantic most probably reflects much higher glacial haptophyte growth rates induced by an increase in surface-water nutrient concentrations. The exceptional opposite trend of the ε p differences in the Angola Basin can be explained by a shift in the phytoplankton community towards a greater dominance of diatoms under glacial conditions, thus leaving less nutrients available for haptophytes. In this way, the isotopic record of alkenones indicate lower haptophyte growth rates during the LGM although other palaeoceanographic proxies point to enhanced productivity and higher nutrient levels.