A critical review of marine sedimentaryδ13Corg‐pCO2 estimates: New palaeorecords from the South China Sea and a revisit of other low‐latitude δ13Corg‐pCO2 records

Abstract

In an attempt to understand better the local biogeochemistry of the South China Sea (SCS) and to unravel the contribution of this marginal low‐latitude basin to changes in atmospheric CO2 concentrations, we analyzed the carbon isotopic composition of organic matter (δ13Corg) in four sediment cores from throughout the SCS covering the last 220 kyr. Higher values (around −19.5 to −20.5‰) mark glacial stages, while lower values (around −21 to −22.5‰) are characteristic of interglacials. Following well established procedures, the δ13Corg records are converted to local pCO2 estimates. On the basis of these and other low‐latitude δ13Corg−pCO2 estimates from the literature, we present a critical evaluation of the use of δ13C of bulk sedimentary organic matter to hindcast past changes in local CO2(aq). Three crucial pitfalls are identified. (1) Given the present inability to quantify precisely the time‐varying amount of terrigenous Corg input to marine sediments, absolute values of pCO2 estimates based on bulk sedimentary Corg are questionable. (2) None of the low‐latitude sedimentary δ13Corg−pCO2 records shows the expected correlation between temporal changes in upwelling intensity and CO2 estimates, most likely due to the antagonistic influences of CO2(aq) and phytoplankton growth rate on δ13Corg. (3) A detailed comparison of marine δ13Corg−pCO2 records with the Vostok CO2 record reveals significant differences in phasing, specifically at the end of the last deglaciation and during the oxygen isotope stage 5/4 transition. However, in areas where equilibrium between oceanic and atmospheric CO2 occurs, for example the SCS and the Mediterranean, the timing of changes in δ13Corg should agree with the CO2 record from ice cores if δ13Corg is a reliable proxy for changes in CO2(aq). Taken together, the compilation of records presented here cautions the use of δ13Corg as an unambiguous tracer of dissolved molecular CO2 in the surface ocean and calls for a re‐evaluation of the role of the low‐latitude ocean on temporal changes in atmospheric CO2.