Late Quaternary PCO2 Variations in the Angola Current: Evidence from Organic Carbon δ13C and Alkenone Temperatures

The δ13C record for organic carbon in Late Quaternary sediments of the Eastern Angola Basin shows a pronounced cyclicity with high values (-18.3 to -20‰) in glacial isotopic stages 2 to 4 and stage 6, and lower values (-20 to -21.2‰) in interglacial stages 1 and 5. Seasurface temperatures, derived from the alkenone Uk₃₇ index, vary in the opposite sense, ranging from 20–22°C in glacial stages up to 24–26°C in interglacial stages. The inverse relationship between δ13C_org and SST values suggests that the isotopic variations are not due to temperature-dependent isotopic fractionation during photosynthesis, in which case δ13C_org values would be expected to increase with increasing temperature. Relationships between sedimentary δ13C_org and C/N ratios indicate that differing proportions of marine and terrigenous organic matter can also be ruled out as a cause for the δ13C_org variability. Instead we conclude that changes in surface water CO₂ concentrations were responsible for the observed glacial to interglacial isotopic variations.Employing the relationships between planktonic δ13C_org and dissolved CO₂(aq) derived by POPP et al. (1989) and RAU et al. (1989) we estimate that the partial pressure of carbon dioxide (PCO₂) in the surface water of the Angola Current (AC) has been consistently higher than values given by the Vostok atmospheric CO₂ record for the last 160,000 years. This indicates that the AC has generally acted as a source region for atmospheric CO₂ throughout this period, which is consistent with the modern situation. However, the difference between surface-water and atmospheric pCO₂ was larger in interglacial (pCO₂ ≈ 100–120 µatm) than in glacial times (mean pCO₂ ≈ 70 µatm). This suggests that the AC was a weaker CO₂ source during glacial periods, presumably due to lower sea-surface temperatures and higher biological productivity.