| Description |
Paleoclimate records indicate that the early Cenozoic Era was punctuated by several abrupt, transient hyperthermal events, most notably the Paleocene-Eocene Thermal Maximum (PETM, ca. 55.5 Ma) and the Eocene Thermal Maximum 2 (ETM2, ca. 53.6 Ma). These abrupt warming events occur coeval to a significant perturbation to the global carbon cycle. Although the timing and magnitude of these ancient hyperthermal events were similar to projected future changes associated with anthropogenic carbon emissions, the application of these events as analogues to anthropogenic global change is complicated by widely divergent interpretations of the rate, patterns, and sources of carbon release. Recently drilled scientific cores that span these hyperthermals were collected at three terrestrial sites from the Bighorn Basin, Wyoming, USA. Pedogenic carbonate isotope data from the Gilmore Hill (GMH) site exhibit relatively stable background δ13C and δ18O values over a host of multicolored floodplain lithologies but lack the expected >2‰ shift of the ETM2 carbon isotope excursion (CIE). Bulk sedimentary organic matter at Basin Substation (BSN) reveals a dramatic decrease in total organic carbon (TOC) preservation through the estimated PETM interval while δ13C values are variable and lack the expected >2‰ PETM CIE. High-resolution carbon isotope data from Polecat Bench (PCB) pedogenic carbonate show that the PETM is characterized by a sustained >5‰ negative CIE, and the onset of this event is characterized by two massive releases of between 600 and 10,000 gigatons of carbon (Gt C), each occurring in one thousand years or less. We hypothesize that the ETM2 interval is located within poorly cemented, highly weathered sands that characterize the upper 25 m of core at GMH and may represent a climatic response to the warming event. The variability and lack of discernible CIE in our BSN δ13C record suggests that additional external mechanisms, such as water stress or organic matter source, likely impacted the preserved TOC. The PCB record, however, shows volatile behavior in Earth's carbon cycle leading up to initiation of the PETM and indicates that the PETM is a strong analogue to current and future carbon cycle perturbation in both magnitude and rate of change. |
