THE MIDDLE MIOCENE CLIMATE TRANSITION IN LOUISIANA: PALEOENVIRONMENTAL EVIDENCE FROM UNGULATE ENAMEL STABLE ISOTOPES

The Middle Miocene climate transition (MMCT; ~14.5-13.0 Ma) was a pronounced episode of cooling associated with the reestablishment and advance of the Eastern Antarctic ice cap following the warmest interval since middle Eocene times. In the interest of characterizing North American Gulf Coast environmental conditions during this interval, we analyzed the tooth enamel carbonate carbon and oxygen stable isotope composition of three ungulate taxa (Prosynthetoceras francisi, Merychippus sp., and Teleoceras sp.) from two sites in the late Barstovian Fleming Formation at Fort Polk (Vernon Parish, Louisiana).

Bulk sampling δ¹³C (-10.99±0.94‰ and -10.92±1.4‰) of 29 enamel samples indicates C₃ diets for the three taxa, and are consistent with habitation of dry, open biomes (e.g. woodland savanna). This interpretation is consistent with published pollen and phytolith data. δ¹³C values from Prosynthetoceras francisi were of particular interest, as they are on average lower than those of Merychippus sp. and Teleoceras sp., suggesting preference for wetter or more heavily vegetated portions of the biome. The δ¹³C of thirty-two serial samples from two taxa (Merychippus sp. and Teleoceras sp.) have low variability (1.37‰ and 1.11‰, respectively), suggesting little seasonal variation in precipitation in the region during the MMCT. Mean Annual Precipitation is also resolvable when referencing the values from these taxa, indicating MAP around 300-500 mm/year, which is considerably lower than today but still consistent with habitat interpretations.

The δ¹⁸O of thirty-two serial samples from two taxa (Merychippus sp. and Teleoceras sp.) have marked variability (4.16‰ and 2.09‰, respectively), suggesting either temperature seasonality or seasonal changes in water vapor source in the region during the MMCT. If this variability is attributed only to temperature seasonality we find that Mean Annual Range of Temperature is 12.55°C, suggestive of considerably more tropical-like seasonality than today.