Recognition of two potential warming events in the Paleocene terrestrial record of North America and associated mammalian responses

Numerous early Cenozoic transient warming events (“hyperthermals”) have been recognized in the marine record but few have been located in fossiliferous terrestrial strata. Locating warming events is critical for understanding how terrestrial animals and plants responded to past climate change. We recognize two carbon isotope excursions (CIEs) in lower Paleocene (Danian) strata of the San Juan Basin, New Mexico, U.S.A., based on sustained deceases in carbon isotope values in bulk organic carbon. The upper excursion correlates closely with the Latest Danian Event (LDE) hyperthermal, previously known with confidence only from the marine record. We correlate the lower excursion to a smaller CIE in the marine record tied to a 405,000 year eccentricity cycle, and modest warming. Both excursions in the San Juan Basin occur in red bed intervals bounded by drab-colored rocks, suggesting transient seasonal drying. Both are also associated with marked decreases in percent total organic carbon, possibly representing decreased primary productivity. The lower excursion is associated with a strong pulse of mammalian faunal turnover. Strata above the LDE are unfossiliferous, precluding local turnover analysis, but a strong, regional pulse of turnover occurs at or near the LDE. Our results suggest warming and/or drying during these events resulted in ecological perturbations, accelerating mammalian origination and extinction rates. Additionally, the first representatives of two North American mammalian families (Arctocyonidae, Plesiadapidae) reached Europe around the time of the LDE. We hypothesize this was in response to short-term warming, opening a previously impassable high-latitude dispersal route connecting the continents.