Evidence for carbon addition

Clear evidence for massive addition of 13C-depleted carbon at the onset of the PETM comes from two observations. First, a prominent negative excursion in the carbon isotope composition (δ13C) of carbon-bearing phases characterizes the PETM in numerous widespread locations from a range of environments. Second, carbonate dissolution marks the PETM in sections from the deep-sea.
The total mass of carbon injected to the ocean and atmosphere during the PETM remains the source of debate. In theory, it can be estimated from the magnitude of the δ13C excursion, the amount of carbonate dissolution on the seafloor, or ideally both. However, the shift in the δ13C across the PETM depends on the location and the carbon-bearing phase analyzed. In some records of bulk carbonate, it is about 2‰; in some records of terrestrial carbonate or organic matter it exceeds 6‰.[11] Carbonate dissolution also varies throughout different ocean basins. It is extreme in parts of the north and central Atlantic Ocean but far less pronounced in the Pacific Ocean. With available information, estimates of the carbon addition range from about 2500 to over 6800 gigatons [12]
The timing of the PETM δ13C excursion has been calculated in two complementary ways. The iconic core covering this time period is the ODP's Core 690, and the timing is based exclusively on this core's record. The original timing was calculated assuming a constant sedimentation rate.[13] This model was improved using the assumption that 3He flux is constant; this cosmogenic nuclide is produced at a (roughly) constant rate by the sun, and there is little reason to assume large fluctuations in the solar wind across this short time period.[14] Both models have their failings, but agree on a few points. Importantly, they both detect two steps in the drop of δ13C, each lasting about 1,000 years, and separated by about 20,000 years. The models diverge most in their estimate of the recovery time, which ranges from 150,000[13] to 30,000[14] years. There is other evidence to suggest that warming predated the δ13C excursion by some 3,000 years.