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The carbon isotopic record of the C37:2 alkenone in the South Atlantic: Last Glacial Maximum (LGM) vs. Holocene

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Benthien, A. , Andersen, N. , Schulte, S. , Müller, P. , Schneider, R. and Wefer, G. (2005): The carbon isotopic record of the C37:2 alkenone in the South Atlantic: Last Glacial Maximum (LGM) vs. Holocene , Palaeogeography palaeoclimatology palaeoecology, 221 , pp. 123-140 . doi: 10.1016/j.palaeo.2005.02.008
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Abstract:

The carbon isotopic fractionation (εp) of the C37:2 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 εp records from the respective core locations (Andersen et al., 1999 and Benthien et al., 2002). Generally, alkenone εp was lower during the LGM compared to the Holocene. Higher glacial εp 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 LGMHolocene changes in surface-water conditions (i.e. nutrient level, primary productivity, phytoplankton assemblages), the observed glacial/interglacial difference in εp 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 εp values can be partly explained with a decrease in surface-water PCO2 during the LGM. In contrast, the Holocene to LGM decrease in εp 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 εp 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.

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