A global deglacial negative carbon isotope excursion in speleothem calcite

Abstract:

δ¹³C values of speleothem calcite decreased globally during the last deglaciation defining a carbon isotope excursion (CIE) despite relatively constant δ¹³C values of carbon in the ocean-atmosphere system. The magnitude of the CIE varied with latitude, increasing poleward from ~2‰ in the tropics to as much as 7‰ at high latitudes. This recent CIE provides an interesting comparison with CIEs observed in deep time.

A substantial portion of this CIE can be explained by the increase in atmospheric pCO₂ that accompanied deglaciation. The dependence of C₃ plant δ¹³C values on atmospheric pCO₂ predicts a 2‰ δ¹³C decrease driven by the deglacial pCO₂ increase. I propose that this signal was transferred to caves and thus explains nearly 100% of the CIE magnitude observed in the tropics and no less than 30% at the highest latitudes in the compilation. An atmospheric pCO₂ control on speleothem δ¹³C values, if real, will need to be corrected for using ice core data before δ¹³C records can be interpreted in a paleoclimate context. The decrease in the magnitude of the equilibrium calcite-CO₂ carbon isotope fractionation factor explains a maximum of 1‰ of the CIE at the highest northern latitude in the compilation, which experienced the largest deglacial warming. Much of the residual extratropical CIE was likely driven by increasing belowground respiration rates, which were presumably pronounced at high latitudes as glacial retreat exposed fresh surfaces and/or vegetation density increased. The largest increases in belowground respiration would have therefore occurred at the highest latitudes, explaining the meridional trend. This work supports the notion that increases in atmospheric pCO₂ and belowground respiration rates can result in large CIEs recorded in terrestrial carbonates, which, as previously suggested, may explain the magnitude of the PETM CIE as recorded by paleosol carbonates.