Towards determination of the source and magnitude of atmospheric pCO2 change across the early Paleogene hyperthermals

The early Paleogene greenhouse climate is punctuated by a series of extreme global warming events known as hyperthermals that are associated with massive additions of carbon to the ocean-atmosphere system. However, no existing proxies have suitable resolution to capture the change in atmospheric carbon dioxide (pCO2) across these events. Here, we reconstruct a nearly continuous record of pCO2 during the early Paleogene based on changes in terrestrial carbon isotope discrimination calculated from published high-resolution marine and terrestrial carbon isotope records. We calculate relatively stable baseline pCO2 = 569 + 250/−146 ppmv with significant increases in pCO2 at each of four hyperthermals. These background levels are significantly higher than most existing proxy estimates, but still lower than levels commonly assumed within carbon cycle models. Based on the pCO2 levels we calculate across each hyperthermal, we show that these events are associated with carbon additions most likely dominated by terrestrial organic matter oxidation or mantle-derived CO2. By matching the new high-resolution pCO2 data with global temperature data we calculate Earth-system sensitivity of ~0.8 to 1.6 KW−1 m2 across these hyperthermals. The slightly elevated ESS during the PETM and H2 suggests positive feedbacks through other greenhouse gases, changes in vegetation and/or oxidation of organic matter/methane may have amplified the temperature response to CO2 addition.