PCO2 Variations of Equatorial Surface Water Over the Last 330,000 Years: The δ13C Record of Organic Carbon

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We reconstructed past variations in CO2 partial pressure (local PCO2) in the surface waters of the East Atlantic equatorial upwelling zone over the last 330,000 years, based on the δ13C record of the (marine) organic matter in ‘Meteor’ core 16772. To deduce the initial δ13Corganic values of plankton and the CO2 solubility in surface water, the δ13C record was adjusted for i) past variations in (winter) sea surface temperature, ii) variations in the δ13C composition of inorganic carbon dissolved in the surface waters, using the δ13C values of G. ruber (white), and iii) isotopic fractionation during the degradation of settling organic matter in the water column and on the sediment surface.

The calculated paleo-PCO2 variations in the surface waters show a strong signal at the obliquity frequency and are approximately parallel to the VOSTOK ice-core record of atmospheric PCO2 over the last 140,000 years. Holocene PCO2 values varied within the range of modern local PCO2, which is 350–400 ppmv compared to a pre-industrial atmospheric pCO2 level of 280 ppmv. This positive anomaly demonstrates the persistent CO2 release from upwelled subsurface water. The glacial-to-interglacial amplitudes of local PCO2 (at the core site) exceeded those of atmospheric pCO2 by 20–60%, with values of less than 250 to 300 ppmv during cold isotopic stages, which indicate a decreased net carbon outgassing from the ocean to the atmosphere. The close correlation between high paleo-PCO2 and low paleo-nutrient contents and paleoproductivity (r=0.7–0.8) suggests that the local PCO2 variations resulted mainly from CO2 transfer by phytoplankton production, especially over the last 170,000 years.