Carbon sources during the Paleocene-Eocene Thermal Maximum

The Paleocene–Eocene Thermal Maximum (PETM; ~56 Ma) was a period of rapid 4-5ºC global warming and a global negative carbon isotope excursion (CIE) of 3-4.5‰, signaling the input of at least 1500 Gt of δ¹³C-depleted carbon into the ocean-atmosphere system. Carbon cycle modeling has indicated that the shape and magnitude of this CIE are generally explained by a large and rapid initial pulse, followed by ~50 kyr of ¹³C-depleted carbon injection. Crucially, some of suggested carbon sources, e.g. submarine methane hydrates and permafrost may respond to warming and act as positive carbon cycle feedbacks on millennial timescales. Previous analyses showed that warming started prior to the CIE at some high and mid-latitude sites, but is still unknown whether this is a global signal and timing and magnitude of such a warming remain poorly constrained.

We generated a new high-resolution TEX₈₆ and δ¹³C record from Ocean Drilling Program Site 959 in the eastern tropical Atlantic and find that initial warming preceded the PETM CIE by ~10 kyr. Moreover, cross-correlation functions on these new and published temperature-δ¹³C data imply that substantial (2-3 °C) warming lead ¹³C-depleted carbon injection by an average of 2-3 kyr globally.

Remarkably, a data compilation shows that global burial fluxes of biogenic Ba approximately tripled across all depths of the ocean studied, which on PETM time scales can only be explained by significant Ba addition to the oceans. Pore-waters below submarine methane hydrates are Ba-rich and require warming to dissociate. The simplest explanation for the temperature lead and Ba addition to the ocean is that methane hydrates dissociated as a response to initial warming and acted as a positive carbon cycle feedback during the PETM. This, however, naturally leads to the question what caused the initial global warming.

The absence of a CIE suggests a ¹³C-neutral CO₂ source, such as volcanism, is the most likely explanation. Recent findings directly connected hydrothermal vent activity in the Vøring and Møre Basins to the PETM CIE, explaining its’ exceptionally long duration (Frieling et al., 2016 PNAS) and Storey et al. (2007 Science) already showed based on absolute dating that the second and most active phase of North Atlantic Igneous Province (NAIP) roughly coincides with the PETM. We speculate the NAIP played a central role in Paleocene-Eocene climate change and, in addition to the thermogenic methane, supplied the CO₂ that drove the initial warming that lead to massive methane hydrate dissociation.