| Summary: | Newly analyzed core material from Svalbard presents the most expanded sedimentary section spanning the Paleocene Eocene Thermal Maximum (PETM) studied to date. Carbon isotopic analysis of the bulk organic matter extracted from core BH9-05 details the onset of the negative carbon isotope excursion (CIE) of approximately 4.2 over 19,000 years (8 m of section, sampled every 30 cm) and its recovery over 50 m of section, representing 150,000 years. The CIE of terrestrial higher plant n-alkanes (~6) is larger than that of the bulk organic carbon (4.2), suggesting the CIE of the atmospheric CO2 is in the range of 4.2 to 6. We use a novel approach to modeling the excursion, forcing an Earth system model of intermediate complexity to conform to the total organic carbon isotope record, yielding rates of carbon release at the PETM for a specified isotopic composition representing end-member potential sources (methane or fossil organic matter). We find that the peak rate of carbon addition is only a small fraction of the current rate of fossil fuel burning (9 Pg C/yr) whether the source is methane (0.3 Pg C/yr; ä13C = -60) or organic matter (1.7 Pg C/yr; ä13C = -22). Model/data comparison, especially the observed and modeled seafloor carbonate dissolution record, favors the higher peak rate and larger (~13,000 Pg C) cumulative addition associated with an organic-matter source, such as rapid oxidation of peat/coal/marine organic matter, thermal alteration of marine organic matter during emplacement of the N. Atlantic Volcanic Province, or a mix of relatively 13C enriched (volcanic) and relatively 13C depleted (methane) sources. However, model sensitivity analysis shows that while the rate and amount of carbon added (for a specified source type) is relatively insensitive to key model uncertainties, the predicted seafloor carbonate dissolution response is quite sensitive to the presumed initial ocean alkalinity and seafloor carbonate distribution (i.e., the oceans buffer capacity against CO2 addition). Given the limited coverage of the existing deep-sea record, we cannot definitively rule out the alternative scenario involving a smaller cumulative addition (~2500 Pg C) from an isotopically lighter source (methane).
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