Doubling of the surface carbon inventory across the PETM?

Abstract:

The Paleocene-Eocene Thermal Maximum event (PETM) is the best-studied geologic example for the environmental and climate effects of large-scale release of carbon from the solid Earth to the climate system, yielding invaluable observational constraints on how climate and environment may respond under continued anthropogenic carbon emissions. While the evidence for PETM carbon release are incontrovertible there is continuing debate about the source and amount of carbon released as well as the change in CO₂ greenhouse gas climate forcing and implied climate sensitivity. Recent evidence suggest that a pulse of volcanic CO₂ outgassing from the North Atlantic Igneous Province may have been the principle source of more than 10,000 petagram (PgC) of total geologic carbon release (Gutjahr et al., 2017), contrasting with the long-standing paradigm of clathrate destabilization releasing 2,000-4,000 PgC methane (Dickens et al., 1995). Here we use first principle theory, biogeochemical box modeling and Earth System modeling to demonstrate strong reductions in the ocean’s vertical carbon isotope gradient in response carbon addition, lending sensitivity to the carbon isotope difference, ∆δ¹³C, between surface dwelling and benthic foraminifera as a proxy to reconstruct the total mass of added carbon. Assessing the two ocean drilling sites with adequate planktic/benthic δ¹³C data coverage, we indeed find a persistent large-scale suppression of ∆δ¹³C at ODP Site 1209 but no such change is evident at DSPD Site 401. Notably, the normal ∆δ¹³C at Site 1209 recovers gradually on a timescale of 100 thousand years, suggesting that >10,000 PgC of added carbon from geologic sources (>-20‰) were rapidly removed from the climate system, presumably via a tight weathering and/or organic carbon burial feedback with atmospheric CO₂. If Site 401 is taken as representative, however, carbon addition was likely <5,000 PgC from a methane source (<-40‰). Our models and theory consistently suggest that the near doubling of the surface carbon inventory implied by the geologic carbon scenario would cause an observable global suppression of ∆δ¹³C. Resolving this observational discrepancy between Site 1209 and 401 may be the clearest path to identify the PETM carbon sources and to constrain the long-term sensitivity of climate and environment to carbon input.