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Climate Reconstruction of the Mint Canyon Flora (Miocene) Based on Leaf Physiognamy: Implications for the Evolution of Mediterranean Climate and Vegetation in California

ARENS, Nan Crystal, Museum of Paleontology and Dept. of Integrative Biology, University of California, Berkeley, CA 94720-4780; SUETAKE, N. M.; CHENG, Y.; CHEW, T.; GOMEZ-FARIAS, I.; HAUSELT, M. M.; KUMBHANI, S. R.; LO, S.; LUCERO, C. A.; MURPHY, S. K.; and TU, L. K.

Mediterranean climate is characterized by warm, dry summers and cool, wet winters, usually in the absence of a freezing season. This climate regime is believed responsible for the dramatic endemic species richness in the California flora, particularly woody dicots of the Rhamnaceae, Ericaceae and Fagaceae. Paleontological work suggests that species richness in the California Mediterranean flora arises from an assemblage of plants, the Madro-Tertiary Geoflora, composed of species adapted to low rainfall and non-freezing temperature.

The Mint Canyon Formation consists of terrestrial deposits, Langhian (mid-Miocene, about 15 Ma) in age. Extrusive rocks in the upper portion of the formation match volcanics from the Mojave Desert, where the Mint Canyon formation was likely deposited. The right-lateral strike-slip faulting displaced fossil-bearing beds to their present location during the Pliocene. The Miocene age of the Mint Canyon flora, its geographic location near the heart of the ancestral Madro-Tertiary range, and the presence of many genera important in the modern California Mediterranean flora make it a key to the question: Is development of the Mediterranean climate coincident with the diversification and spread of Madro-Tertiary elements?

Previous climatic reconstructions of the Mint Canyon flora focused on the affinities of modern taxonomic analogs and assume no change in environmental tolerance during the dramatic radiation of California Mediterranean genera. Climate-Leaf Analysis Multivariate Program (CLAMP) uses leaf physiognomy -- independent of taxonomic identity -- to draw paleoclimatic inferences. We use CLAMP (a canonical correspondence model) and leaf margin analysis (a linear model) to reconstruct climate of Mint Canyon flora, which includes more than 60 form species.

CLAMP analysis reconstructs mean annual temperature (MAT) of 14C for the Mint Canyon flora (in contrast with leaf margin analysis, which reconstructs MAT of 16-17C), warm month mean temperature is 25C. The growing season is approximately 8 months long with mean monthly growing season precipitation of 36mm, with much of that precipitation (20 mm) coming during the three wettest months. Although this method does not distinguish between summer and winter seasonal rain, CLAMP analysis does allow us to reject the hypothesis that annual precipitation was about evenly divided between winter cyclonic storms and summer connective showers as is observed in the deserts of Arizona, New Mexico and northern Mexico today. This suggests that elements of the California Mediterranean flora may have been tracking the seasonal climate as it developed in coastal California during the Miocene.

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