Experimental and field studies have revealed that the stable carbon isotope fractionation associated with photosynthesis by plankton largely depends on the concentration of dissolved molecular carbon dioxide, [CO2(aq)]. This has led to suggestions that the isotopic composition of marine sedimentary organic carbon (d13Corg) can be used to determine past CO2 levels in the surface layer of the ocean (review by Rau, 1994). Here we compare paleoceanic PCO2 records from two sediment cores in the Angola and Cape Basins to the Vostok atmospheric CO2 record (Barnola et al., 1991) in order to explore the Late Quaternary sink/source conditions.
Core GeoB 1016 (11°46'S, 11°41'E, 3411 m) is from the Angola Current (AC), which presently is a source region for atmospheric CO2 throughout the year (Takahashi et al., 1993). Core GeoB 1711 (23°19'S, 12°23'E, 1967m) is from the Benguela Current system and characterizes the outer
filamentous mixing area of the coastal upwelling regime off Namibia. Offshore subtropical waters at this latitude tend to be a CO2 source in summer and a sink in winter (Takahashi et al., 1993). We adopted the McCabe/Popp model (Popp et al., 1989) to convert d13C measurements of sedimentary organic carbon (bulk and C37:2 alkenones) and of planktonic foraminiferal calcite (Globigerinoides ruber) to [CO2(aq)]. The CO2 partial pressure (PCO2) that would be in equilibrium with the estimated [CO2(aq)] was calculated using Henry's law and SST values derived from alkenone (UK37) analyses (Müller et al., 1994).
On this basis, the bulk organic carbon d13C record of core GeoB 1711 from the Cape Basin documents a permanent near-equilibrium between the CO2 partial pressure in the surface waters and the atmosphere during the last 140,000 years. In contrast, a permanent oceanic source for carbon dioxide during the last 200,000 years is recorded at site GeoB 1016 in the Angola Basin (Müller et al., 1994). The alkenone d13C measurements in this sediment core generally confirm the PCO2 variations deduced from the bulk organic carbon signal. Larger PCO2 differences up to 100 ppmv are only obvious during isotope stage 6. During glacial times, particularly during stages 2 and 6, the d13C values of coccolithophorid organic carbon (estimated as alkenone d13C plus 3.8 ”, Jasper & Hayes, 1990) are generally lower than the bulk d13C values. This might be the result of an increased contribution of terrestrial C4 plants during glacial periods of extended savannah vegetation. Another possible explanation is that the growth rates of other primary producers than coccolithophorides (e.g. diatoms and dinoflagellates) were generally higher in the more productive glacial surface waters at this site (Schneider et al., 1994) resulting in higher bulk organic carbon d13C values (Laws et al., 1995) and, consequently, too low paleo-PCO2 estimates.
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