CRITICAL ZONE MEDIATION OF CARBON CYCLE RESPONSE THROUGH THE PALEOCENE-EOCENE THERMAL MAXIMUM

Anthropogenic perturbation of the carbon cycle is now a primary driver of the climate system. Carbon cycle science is in consensus that the ultimate fate of anthropogenic carbon will be burial as carbonate and organic carbon in sediments, but the sequence of feedbacks and processes that will promote this burial and govern atmospheric CO₂ concentrations in the intervening >10⁵ years is poorly constrained. Although the marine branch of the carbon cycle stores the vast majority of exogenic C and supports the largest sediment burial fluxes of carbon, most of the key uncertainties in short- and long-term carbon cycle projections involve processes within the critical zone (terrestrial vegetation, soils, and groundwater). Evidence for substantial changes in critical zone processes during the ~100 kyr period of intensive global change at the Paleocene-Eocene boundary (the Paleocene-Eocene thermal maximum, PETM) is abundant, and may offer a case study elucidating patterns of change in critical zone biogeochemistry associated global warming. We synthesize data from continental and coastal sites with well-resolved PETM records and highlight patterns of change consistent with reduced carbon storage and enhanced cycling rates in these settings, suggesting potential for positive critical zone feedbacks on the PETM carbon cycling perturbation. Model simulations suggest that such changes would lead to enhanced convergence between simulated and observed records of ocean/atmosphere carbon chemistry, but that significant discrepancies remain to be addressed. Understanding the mechanistic drivers and geographic extent of changes in critical zone carbon storage during the PETM will be critical to further solidify evidence for the nature of these responses and their potential role in mediating past and future global change.