Constraints on the rate of carbon injection across the PETM – towards a theoretical framework for hyperthermals

Abstract:

As an episode of rapid global warming associated with the release of massive quantities of carbon to the atmosphere and oceans, the Paleocene-Eocene Thermal Maximum (PETM, 56 Ma) is considered a potential analog for modern anthropogenic carbon emissions. Although there is still some debate regarding the exact mass and isotopic composition of the carbon source responsible for this event, numerical models have been used to interpret existing records as being consistent with a total release of between 3000 and 6000 Pg C. Despite this estimate being comparable to potential future fossil fuel CO₂ release, the rate of carbon release during the PETM is still a major area of uncertainty and precludes any straightforward comparisons between the paleo-record and the modern. Here we use the Earth system model cGENIE to quantify the consequences of differing carbon emissions rates on the isotopic record of different carbon reservoirs. We test a range of emissions scenarios – from years to millenia and constant versus pulsed emissions rates – and trace the resulting carbon isotope records within the atmosphere, with depth in the ocean, and in the sedimentary record. We identify the characteristic relationship between the difference in carbon isotope excursion sizes between atmospheric CO₂ and dissolved inorganic carbon (DIC) and the duration of emissions. From available isotope records spanning the PETM we hence constrain the duration of carbon emissions to less than 2500 years. Our experiments also provide a general interpretive framework for assessing rates of carbon emissions for other events.