A LONG EARLY PALEOGENE TERRESTRIAL PALEOCLIMATE RECORD FROM THE BLACK PEAKS FORMATION, BIG BEND NATIONAL PARK, TX

Understanding the profound climatic transition between the Early Eocene Climatic Optimum and the preceding cooler Paleocene (65.5Ma to 56Ma) represents a critical challenge in deciphering the past behavior of the global climate system. In order to document changes in low-latitude North American climate through this period we have developed new litho and chemostratigraphic records from the fluvially deposited Paleocene Black Peaks Formation in Big Bend National Park (BBNP). Combining previously published magnetostratigraphy and biostratigraphy with new data from pedogenic carbonates, we have developed an age model for the deposits that indicates that the Black Peaks Fm. spans most of the Paleocene, from 63 to 55 Ma, without significant (>10⁶ yrs) unconformities and with relatively constant sediment deposition rates. As such, the Black Peaks Fm. is an ideal study site to record the continental expression of the climate record through the Paleocene.

Pedogenic carbonate C and O isotope records show substantial secular variation throughout the new record, which is correlated with global carbon isotope fluctuations documented in the deep sea. C and O isotope ratios are strongly positively correlated in the Big Bend record throughout most of the Paleocene, with the correlation weakening during the early Paleocene and early Eocene, suggesting strong coupling between the regional climate and global carbon cycling during most of the Paleocene. A transient 2‰ carbon isotope excursion low in the section occurs near the long-term carbon isotope minimum, and we tentatively assign it to the Late Danian Event. The presence of distinctive pedogenic nodules with radial fabric at and around this level suggests changes in soil conditions during this time. Chemical index of alteration data indicate relatively drier soil conditions at BBNP between this level and the long-term Paleocene Carbon Isotope Maximum. Climatic conditions in this region of subtropical North America were not conducive to storage and long-term burial of carbon during the late Paleocene, and we suggest that during this time enhanced moisture transport bypassing the southern continent may have contributed to wet climate and elevated organic carbon burial in mid-continental environments.