Orbital forcing of the Eocene Hyperthermals and the Paleocene-Eocene Thermal Maximum

Multimillion-year proxy records across the Eocene show prominent variations on orbital time scales. The cycles, which have been identified at numerous sites across the globe, preferentially concentrate spectral power at eccentricity and precessional frequencies. It is evident that these cycles are an expression of changes in global climate and carbon cycling paced by astronomical forcing. Importantly, there is robust evidence that orbital forcing is also the pacemaker for a long sequence of transient Eocene climate events, called hyperthermals. Little is known though about the link between orbital forcing and the carbon cycle-climate system. Here I will analyze climate and carbon cycling changes across the Eocene hyperthermals in relation to astronomical forcing using a variety of proxies. Furthermore, I will apply the analysis to the largest hyperthermal event throughout the Cenozoic, the Paleocene-Eocene Thermal Maximum (PETM, 56 Ma). The PETM was associated with about 5 K global surface warming and an estimated total carbon release of several thousand Pg, rendering the PETM an event that is widely considered the best analog for present/future carbon release. Next, I will compare the Eocene hyperthermals and the PETM, pointing out commonalities in their response to orbital forcing. Moreover, carbon vs. oxygen isotope excursions show very similar slopes during the hyperthermals, as well as the PETM, pointing to a common origin. The results underline that the PETM is not an isolated event, but rather part of a sequence of early Cenozoic hyperthermals. I will also discuss the conundrum that the observed duration of the PETM appears to be much longer than predicted by models that use first order assumptions. Understanding the long duration of the PETM in relation to orbital forcing is also critical for predicting the long-term consequences of anthropogenic carbon release. In that context, I will identify a remarkable pattern in the forcing and response to the short eccentricity cycle and the duration/nature of the hyperthermals vs. the PETM.