FIRST RECOGNITION OF CLIMATE HYPERTHERMALS IN THE LOWER PALEOCENE RECORD OF THE SAN JUAN BASIN, NEW MEXICO, USA

The greenhouse climate of the early Paleogene is punctuated by numerous short-term, transient warming events, known as “hyperthermals.” The largest and best known of these is the Paleocene-Eocene Thermal Maximum (PETM). The PETM is characterized by dramatic faunal and floral changes linked to rapid warming including mammalian immigration, dwarfing of mammals, and extreme but transient floral change. In order to understand whether these patterns are repeating or are unique to the PETM, biotic records from other hyperthermals must be studied. Hyperthermals can be identified by rapid negative excursions in δ¹³C values. Here we report two large (3-4‰) negative shifts in δ¹³C values in the lower Paleocene of the San Juan Basin based on the carbon isotope content of bulk organics in fine-grained rocks of the Nacimiento Formation. Bulk carbon δ¹³C values reflect values in ancient vegetation, which are controlled by atmospheric values and by local environmental conditions. The lowest of the excursions occurs at the top of the Pantolambda cavirictum-Mixodectes pungens zone (Tj5) and the highest at the top of the M. pungens zone (Tj6). These excursions are present in both Torreon Wash and Escavada Wash. The upper excursion occurs at the top of polarity chron C27n and is correlated to the Latest Danian Event (LDE), previously known with confidence only from the marine record. The lower Tj5 excursion is previously unrecognized. Both of these excursions occur at the top of lighter-colored variegated intervals just below an abrupt transition to dark gray intervals. The LDE carbon excursion is restricted to the uppermost part of C27n in our sections, based on previous paleomagnetic work, which contrasts with the marine record where the LDE reportedly continues into C26r. This suggests that the upper part of the excursion is not recorded in bulk organics in our sections. Preliminary paleontological results based on pre-existing collections suggest that dwarfing occurred in the phenacodontid “condylarth” Tetraclaenodon puercensis. However, larger samples with better stratigraphic control are needed. Recognition of these hyperthermals in the continental record holds promise for a better understanding of how the continental biota responded to the repeated, transient warming events of the Paleogene.