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Late Paleocene through middle Eocene climates in lowland North America

The most complete sequence of late Paleocene (60 Ma) through middle Eocene (48–50 Ma) leaf assemblages in North America occurs in the coastal plain from Texas northeastward into southeastern United States (Fig. 1). This sequence, placed in the Wilcox Group, interdigitates with marine sediments and represents sedimentation on a Paleogene coastal plain at a paleolatitude within 5° of the present-day latitude of c. 30°. The leaf assemblages are in the process of being analyzed physiognomically with CLAMP (Climate Leaf Analysis Multivariate Program). This analysis is based on c. 170 modern samples of leaves, most collected proximal to meteorological stations to provide calibration to environmental parameters (cf. Wolfe & Spicer 1999). These parameters include (and standard errors for estimates) mean annual temperature (MAT, ±1.8°C), warm-month mean temperature (WMMT, ±2.0°C), growing season length (GSL, ±1 month), mean growing season precipitation (MGSP, ±40 cm), total precipitation of the three consecutive wettest months during the growing season (3 Wet, ±17 cm), and total precipitation of the three consecutive driest months during the growing season (3 Dry, ±11 cm). Obviously if MAT and WMMT can be estimated, then mean annual range of temperature (MART) and cold-month mean temperature (CMMT) can also be estimated. At c. 59–60 Ma, the MAT in central Texas was about the same as today, i.e., 19°C. However, the climate was much milder, with a mean annual range of c. 11°C (vs. c. 20° today), and much wetter, with MGSP of c. 300 cm (vs. 80–120 cm today). This precipitation was about as seasonal as today, with the 3 Wet/3 Dry totals being 120/60 (vs. c. 30/15 today). By 57–58 Ma, MAT had increased to c. 23–24°C, and the climate had become even milder (MART 6–7°C) and wetter (MGSP >400 cm) but still maintaining a seasonal character. A maximum MAT of c. 27°C was reached between 56–57 Ma and maintained to at least c. 55 Ma, accompanied by a decrease in MART to c. 2°C. Precipitation perhaps increased to >500 cm and was still somewhat seasonal. The climatic regime at 30°N was essentially equatorial. Along the northern border of the coastal plain, extensive bauxitic laterites developed in Arkansas. If any temperature “spike” occurred, it is not evident, but considering the already high level of MAT at this latitude, any shortterm temperature increase as postulated for high latitudes might not occur at such warm latitudes. By c. 50 Ma, MAT had declined to 22–23°C in central Texas, but, just as significantly, MGSP had undergone a major decrease to c. 130 cm with a marked dry season (3 Dry = 15 cm). Indeed, this dryness appears greater than that evidenced in the overlying floras of the Claiborne Group in the middle Eocene. The only other certain lowland North American floras in the 48–60 Ma interval are those distributed along the west coast. During the late Paleocene, floras are known from south-central Alaska (Chickaloon Fm. and flora) south to southern British Columbia (Huntingdon Fm., Kanaka Creek flora) and adjacent Washington (Chuckanut Fm.: Bellingham Bay and Slide Member floras); early Eocene floras are known on the Yakutat block, then positioned off southeastern Alaska (Kulthieth, including the flora of the Kulthieth and Kushtaka Fms.), and southern British Columbia (Huntingdon Fm.: Terminal Dock and similar floras). Middle Eocene lowland floras are known from the Yakutat block (Charlotte Ridge flora), southern British Columbia (Huntingdon Fm.: Kitsilano Beach flora), south of Seattle (Franklinian floras in the Green River Canyon section of the Puget Group), and near San Diego (Del Mar flora). The main plant-bearing beds of the Chickaloon are bracketed