Habitat heterogeneity and vegetation response to the Middle Miocene Climatic Optimum in the Santa Cruz Formation (Patagonia)

CRIFO, C.; BARGO, M.S.; CUITIÑO, J. I.; KAY, R.F.; KOHN, M.; TRAYLER, R.; VIZCAÍNO, S.F.; ZUCOL A.F.

Seattle

Congreso; 37th Midcontinent Paleobotanical Colloquium 2020; 2020

The Middle Miocene Climatic Optimum (MMCO; ca. 17?14.5 Ma) is the most recent Cenozoic global warming event. It affected ecosystems that were broadly comparable to today?s, making it one of our best analogues for ongoing, anthropogenic climate change. Whereas vegetation responses to the MMCO in the northern hemisphere are well documented, southern hemisphere changes are poorly known. We use phytolith analysis to study the southernmost record of ecosystem change during the onset of the MMCO from fossiliferous horizons in the Atlantic coastal localities of the Santa Cruz Formation (SCF), Santa Cruz Province, Argentina. We infer vegetation change through time using samples collected through a vertical section, as well as multiple phytolith assemblages from a single stratigraphic level across several localities. This lateral sampling allows us to test the hypothesis that high faunal diversity at the onset of the MMCO was linked to habitat heterogeneity, presumably driven by niche differentiation. We then compare our result to faunal assemblages and climate proxy data from the SCF. Our record suggests that vegetation was distinctly heterogeneous during the MMCO global warming event, consistent with our hypothesis. Our study represents the first time that spatial variation in local vegetation during this event has been tested using direct paleobotanical evidence. Moreover, vegetation at the coast differed sharply from other inland early Miocene Patagonian sites, pointing to additional, regional heterogeneity in vegetation structure and response to climate change. Our data also suggest that plant communities responded in a non-analogue way to global warming and aridification, with an increase in open, dryadapted vegetation dominated by xerophytic woody taxa (shrubland) rather than grasses (grassland). We interpret dominant taxa in SCF grass communities as C3 (temperate) grasses that were not adapted to dry conditions, consistent with the hypothesis that open, grass-dominated habitats did not expand in Patagonia until after the early middle Miocene.