PALEOCLIMATE RECONSTRUCTION AND THE PATTERN OF CLIMATE CHANGE AT EARLY PALEOCENE FROM LEAF PHYSIOGNOMY AT SAN JUAN BASIN, NEW MEXICO FOLLOWING THE CRETACEOUS-PALEOGENE BOUNDARY

Early Paleocene terrestrial paleoclimate reconstructions using leaf physiognomy show a decrease in mean annual temperature (MAT) across the Cretaceous-Paleogene (K-Pg) boundary continuing until the middle Paleocene. Additionally, decreasing early Paleocene MAT may be linked to relatively low diversity floras during the early Paleocene. However, these paleoclimate reconstructions are centered on the Northern Great Plains of North America with comparatively little research from southern North America limiting our understanding of regional paleoclimatic changes following the K-Pg boundary. The San Juan Basin (SJB), located in northwestern New Mexico, preserves a nearly continuous sequence of fossiliferous early Paleocene terrestrial deposits making the SJB an ideal location to apply paleobotanical paleoclimate proxies to and early Paleocene flora from southern North America. Here we present leaf physiognomy paleoclimate estimates for the first ~1.5 myr of the early Paleocene from the SJB.

Fossil leaves were collected from the Ojo Alamo Sandstone and lower Nacimiento Formation (~66.0 to 64.5 Ma). Digital leaf physiognomy (DiLP) and leaf margin analysis (LMA) were used to estimate MAT; DiLP and leaf area analysis (LAA) were used to estimate mean annual precipitation (MAP). Leaf life span was also estimated using the relationship between petiole width and leaf mass per area (M_a). Paleoclimate estimates were warm (DiLP: 20.0 – 27.4 °C, LMA: 24.3 – 17.7 °C) and relatively wet (DiLP: 1544 – 1906 mm/yr, LAA: 1514 – 2330 mm/yr) corresponding with a modern tropical seasonal forest. Majority of morphotypes were deciduous with an average M_a of 68.8 g/m². While MAT overall decreases from the K-Pg boundary similarly to results from the Northern Great Plains, several changes in MAT were observed; DiLP estimates indicate decreasing MAT from 66.0 – 65.2 Ma (27.4 – 20.2 °C), increasing MAT from 65.2 – 63.8 Ma (20.2 – 23.5), and decreasing MAT to 64.6 Ma (23.5 – 20.0). When compared to sites from northern North America, the SJB had higher MAT and MAP. Our results suggest a latitudinal temperature gradient across North America and a regional mechanism for climatic cooling during the early Paleocene.