Are Internal Dynamics Responsible for the Late Paleocene and the Early Eocene Hyperthermals?

The hyperthermal events during the Late Paleocene and the Early Eocene, including, the Paleocene-Eocene Thermal Maximum (PETM; 56 Ma) and the Eocene Thermal Maximum 2 and 3 (ETM2; 54.06 Ma and ETM3; 52.87 Ma), are abrupt transitions that might be the result of the Earth system crossing a tipping point. When a system passes a tipping point, a small initial change gets amplified by positive feedbacks, eventually resulting in a large abrupt response. Such tipping points can be preceded by critical slowing down, which could be detected as an increase in the autocorrelation and standard deviation in the paleorecords prior to the transition, suggesting a gradual loss of resilience in the system. For our analysis, we employ high-resolution records of stable oxygen and carbon isotopes from the South Atlantic Ocean as proxy reconstructions of the carbon cycle and the climate systems during the Early Paleogene. To study the degree of causal interaction between the two systems, we use Convergent Cross Mapping (CCM), a state-space reconstruction-based method using a new moving window approach to reveal how causation might change in time. Our combined analysis using resilience indicators and windowed-CCM reveal a loss of resilience and an increase in the causal interaction between the carbon cycle and the climate system before the PETM. After the PETM, the windowed-CCM analysis indicates a tight coupling between the carbon cycle and the climate during the early Eocene. Moreover, we also detect a loss of resilience in the paleorecords starting around 300 kyr and 260 kyr prior to ETM2 and ETM3 respectively. Positive feedbacks from organic carbon sources such as marine methane hydrates have been suggested to be involved in the hyperthermals. In the modern climate system, these might likewise represent carbon cycle tipping elements. Our resilience indicators suggest that the internal rather than external mechanisms were responsible for the abrupt transitions. Furthermore, the CCM analysis in conjunction with the absence of major positive feedbacks such as the presence of polar ice caps during this time could be employed to stipulate that these hyperthermal events may be caused by the increase in coupling between the carbon cycle and climate systems not unlike a positive feedback eventually pushing both systems towards a tipping point.