PALEOSOL RECORDS OF MIDDLE MIOCENE CLIMATE CHANGE IN THE SANTA CRUZ FORMATION, PATAGONIA, ARGENTINA: A PALEO-ANALOG FOR FUTURE CLIMATE CHANGE?

The middle Miocene thermal maximum (~16 Ma) was a period of global climate unusually warm and wet for the Neogene, and is of interest as a paleo-analog for future climate change from anthropogenic global warming. In Argentina, the Santa Cruz Formation, in addition to its rich fossil vertebrate assemblages, contains a paleosol record of middle Miocene climate change. The early Miocene in the Santa Cruz Formation is characterized by a thick sequence of weakly to strongly developed grassland paleosols (Orthents and Ustolls) as well as paleosols with root traces and profile forms of open shrubland vegetation (Cambid). The middle Miocene thermal maximum is characterized by paleosols (Udepts, Udalfs) developed under open woodland-shrubland vegetation, during a period of climatic warmth and humidity. The late middle Miocene is characterized by a greater diversity of pedotypes, all suggestive of an arid and cooler environment (Argids, Cambids, and Ustepts).

Modern soils in the Santa Cruz province are mostly Aridisols, Mollisols, Entisols, and Andisols; at the study site, the modern soils are Argids, like those found in the early and late Miocene. Alfisols like those that characterize the middle Miocene have limited extent in modern Patagonia, except in the Andean foothills and isolated locations in northern Patagonia. The modern landscape falls within the semi-desert and shrub-steppe Patagonian biozone, also similar to the paleoenvironments reconstructed for the late Miocene in this study. The woodland vegetation and Alfisols that characterize the middle Miocene thermal maximum are more like the montesteppe or deciduous forest now found to the north and west, respectively, in Patagonia.

A future climate change of the magnitude experienced in the middle Miocene could have similar effects on the regional soil and biozone distribution in Patagonia, increasing woodlands to the east and south in addition to increasing temperature and precipitation levels. Understanding how arid environments, like the open shrubland maintained in coastal Patagonia at present, respond to relatively small climate changes, like that of the middle Miocene, has important ramifications for understanding the possible effects of coming global climate change.