FOSSIL PHYTOLITH ASSEMBLAGES FROM SOUTHERN PATAGONIA INDICATE CHANGING HABITATS DURING THE MIDDLE MIOCENE CLIMATIC OPTIMUM

The Middle Miocene Climatic Optimum (MMCO; ca. 17–14.5 Ma) was characterized by high temperature and pCO₂, thought to promote warm and wet climates at high latitudes. The Santa Cruz Formation (SCF; 47-52°S) of coastal Patagonia (ca. 17.8–16.6 Ma) represents the southernmost sedimentary record in the world spanning the onset of the MMCO. SCF fossiliferous horizons yield one of the most species rich and well-preserved Cenozoic vertebrate assemblages known. Plant macrofossils, and well preserved phytolith assemblages also occur in the SCF within the same strata as the faunas. These linked fossil records allow us to, for the first time, compare in detail animal and plant paleoecology, vegetation structure, and local climate through the MMCO global warming event in southern South America. Importantly, isotopic data from fossil enamel and bones have been used to estimate Mean Annual Temperature (MAT) and Mean Annual Precipitation (MAP) and can be directly compared to the vegetation data from phytoliths.

Here, we present preliminary results from analysis of phytolith assemblage composition (% plant functional types) from the main stratigraphic section of coastal SCF, documenting vegetation change during the early part of the MMCO, and interpreting it in light of current climatic reconstructions based on isotopic and faunal data. With the exception of the oldest assemblage, most phytolith assemblages analyzed so far are more or less dominated by forest indicators, including palms. Although there is no clear pattern of variation in overall assemblage composition (e.g., forest indicator: open habitat indicator ratio) through time, different forest indicators dominate different assemblages, which could be explained by vegetation shifts through time, spatial heterogeneity, or both. Grass phytolith assemblages consist mainly of morphotypes from C₃ pooid grasses throughout the section, but are characterized by a substantial decrease in morphotype size from older to younger levels, which may reflect decreased water availability. These results are consistent with isotopic data indicating dominantly C₃ vegetation and a shift toward a more arid climate characterized by decreased MAP as well as increased MAT during the onset of the MMCO, but suggest that this drying did not result in an expansion of open-habitat grasses.