UNDERSTANDING ASTRONOMICALLY FORCED CARBON CYCLE FEEDBACKS THROUGH THE LENS OF AN EARTH SYSTEM MODEL (Invited Presentation)

Astronomical cycles in geological records demonstrates the influence of astronomical forcing on Earth’s climate-carbon dynamics. Proxies suggest that during greenhouse climates such as the early Cenozoic, isotopically light carbon is released during periods of warmth (at eccentricity maxima) and re-sequestered during the following cooling (at eccentricity minima). However, the dominant carbon sources and sinks at play on orbital timescales remain unclear--particularly in absence of large dynamic ice sheets. Methods: In an Earth system model, we apply 4-Myr-long transient astronomical forcing to examine how various climate sensitive (in)organic carbon feedbacks respond and influence the astronomical expression in key oceanographic variables (temperature, pCO₂, δ¹³C of DIC, and wt% CaCO₃). Among others, we assess the impact of marine productivity, CaCO₃ compensation, terrestrial weathering, organic matter burial, and phosphorus cycling. Results: While most processes are driven by changes in the local conditions -controlled by obliquity and precession, these high frequency signals convert to low frequency eccentricity cycles in pCO₂, benthic δ¹³C, and wt% CaCO₃ as a result of the lowpass filtering effect of the ocean carbon and nutrient reservoir. The magnitude of early Cenozoic δ¹³C variability can be explained by an astronomically forced input-burial fluxes of marine organic carbon, however, it does not explain the presence of the short (100 kyr) eccentricity cycles in benthic δ¹³C, nor the reduced preservation of CaCO₃ during periods of warmth as recorded in the proxy data. In addition, the modelled phasing of the forcing and proxies depends on the paleogeography. Implications: This provides support for the hypothesis that additional feedbacks that are not yet(!) included here (e.g., terrestrial carbon or methane) were likely important controls during greenhouse astronomical climate variability.