Carbon isotope evidence for increased levels of multiple greenhouse gasses during the initial Eocene thermal maximum

Nearly a decade ago, it was shown that the abrupt, negative shift in marine and terrestrial carbon isotope ratios at the Paleocene/Eocene boundary provides indirect evidence for an abnormally large release of methane, probably from gas hydrates, to the ocean/atmosphere system. This release coincides with a dramatic, transient, warming event now termed the initial Eocene thermal maximum (IETM), inspiring obvious questions about the causal links between methane hydrate dissociation and climate warming during this event. However, uncertainties regarding the amount of methane which might have escaped to the atmosphere, and the short residence time of methane in the modern atmosphere, make direct and indirect radiative warming due to heightened atmospheric methane levels an imperfect explanation for IETM warming, which averaged ∼4-6° globally and persisted for ∼100 ky. Here, we provide evidence, based on a mechanistic examination of the divergence of marine and terrestrial δ ¹³C records during the IETM, for transient increases in atmospheric levels of CO₂ and/or water vapor during this event. Exact quantification of these increases is not possible, but doubling of CO₂ levels and/or relative humidity increases on the order of 20% are indicated. These changes in atmospheric chemistry appear to have been sustained for at least ∼80 ky, and likely contributed to climatic warming during the IETM. Further, elevated productivity of the terrestrial biosphere should have accompanied increased atmospheric CO₂ and/or H₂O levels, as indicted by the observed changes in terrestrial carbon isotope systems. Terrestrial biosphere fertilization might thus have accounted for a substantial carbon sink and a negative feedback on IETM climate warming by atmospheric carbon compounds. However, paleosol sections documenting this event show unchanged or decreased accumulation of organic matter, suggesting that organic matter turnover rates in soils increased and that the terrestrial carbon sink during the IETM was limited to the net increase in standing biomass, minus any reductions in soil carbon storage. Because standing biomass constitutes a transient carbon sink, long term carbon cycle recovery from the IETM must have occurred through other mechanisms, such as elevated silicate weathering or marine organic carbon burial.