Carbon Isotope Record of Trace n‐alkanes in a Continental PETM Section Recovered by the Bighorn Basin Coring Project (BBCP)

Continental sedimentary records of early Paleogene hyperthermals are typically limited to weathered, often discontinuous, outcrop exposures. In 2011, the Bighorn Basin Coring Project (BBCP) collected the first continuous terrestrial records of the Paleocene‐Eocene Thermal Maximum (PETM) in the Bighorn Basin, Wyoming. Organic matter preservation was poor during the PETM, even in core material. Concentrations of leaf waxes during the PETM are too low for compound‐specific carbon isotope analysis by conventional means. However, the recent development of picomolar‐scale compound‐specific isotope analyses (pico‐CSIA) has reduced sample requirements and enabled measurements of carbon isotope ratios of n‐alkanes across the PETM in the Basin Substation core. While the prominent, negative carbon isotope excursion in total organic carbon that typically identifies the PETM in the sedimentary record is absent from the core, lithostratigraphic, biostratigraphic, and chemostratigraphic data suggest that the most likely position of the PETM is from ~87.82‐ to ~50‐m composite depth. This ~40‐m interval coincides with the lowest weight percent organic carbon, n‐alkane abundances, and n‐alkane δ13C values and the highest n‐alkane average chain lengths. Comparison of the n‐alkane isotope record from the core with that from organic‐rich rocks exposed in the SE Bighorn Basin suggests that n‐alkanes in the core fail to express the full magnitude of the carbon isotope excursion. We hypothesize that floodplain sediments at Basin Substation contain a mixture of PETM and reworked fossil n‐alkanes. Low total organic carbon suggests that PETM climate accelerated organic matter decay rates and floodplains may have acted as a carbon source during the PETM. Key Points Organic matter preservation was poor during the PETM in the Bighorn Basin, WY, even in less weathered core material Floodplain sediments in the core likely contain a mixture of contemporaneous PETM and reworked fossil n‐alkanes Low organic carbon suggests that organic matter decay rates outpaced plant productivity and soils may have acted as a carbon source during the PETM