A critical review of marine sedimentary d13C-pCO2 estimates: New palaeorecords from the South China Sea and a revisit of other low-latitude d13Corg-pCO2 records.

Kienast, M.; Calvert, S. E.; Pelejero, C.; Grimalt, J. O.
Global Biogeochemical Cycles
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 (d13Corg ) in four sediment cores from throughout the SCS covering the last 220 kyr. Higher values (ca. -19.5 to -20.5‰) mark glacial stages, while lower values (ca. -21 to -22.5‰) are characteristic of interglacials. Following well established procedures, the d13Corg records are converted to local pCO2 estimates. Based on these and other low-latitude d13Corg-pCO2 estimates from the literature, we present a critical evaluation of the use of d13C 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 CO2 estimates based on bulk sedimentary Corg are questionable. 2) None of the low-latitude sedimentary d13Corg-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 d13Corg. 3) A detailed comparison of marine d13Corg-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, e.g. the SCS and the Mediterranean, the timing of changes in d13Corg should agree with the CO2 record from ice cores if d13Corg is a reliable proxy for changes in CO2(aq). Taken together, the compilation of records presented here cautions the use of d13Corg 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.
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