Middle Miocene monsoon seasonality inferred from Thai rhino-teeth stable isotopes

The present climate of Thailand is controlled by the Southeast Asian Monsoon system. Although modeling paleoclimatic fluctuations in monsoon variability has improved, records of past climate changes are crucial to observe the first occurrence or intensification of the monsoon wind system. Different time spans have been suggested from 30 Ma (Ramstein et al. 1997; Fluteau et al. 1999) in the Indochina area to the end of the Miocene (8-6 Ma)(Quade et al. 1989; Kroon et al. 1991; Cerling et al. 1993)in the Himalayan area due to different hypotheses such as (i) Paratethys ocean shrinkage (Ramstein et al. 1997; Fluteau et al. 1999, (ii) Himalayan and Tibetan plateau uplift(Kutzbach et al. 1989; Ruddiman &\ Kutzbach 1989; Prell &\ Kutzbach 1989) and (iii) chemical weathering and atmospheric [CO₂](Prell &\ Kutzbach 1989; Raymo &\ Ruddiman 1992). Here we reconstruct middle Miocene seasonality in precipitation using high-resolution \delta¹⁸O and \delta¹³C records from CaCO₃ of rhinocerotid tooth enamel in Thailand. \delta¹⁸O of mammal tooth enamel, precipitated under constant body temperature, is a powerful precipitation/evaporation proxy, since temperature corrections are unnecessary. The results show that middle Miocene seasonality in rainfall was similar to today, although the amount of precipitation was higher, which is in agreement with atmospheric circulation models (Fluteau et al. 1999). References Cerling, T. E., Wang, Y. & Quade, J. Expansion of C4 ecosystems as an indicator of global ecological change in the late miocene. Nature 361, 344-345 (1993). Fluteau, F., Ramstein, G. & Besse, J. Simulating the evolution of the Asian and African monsoons during the 30 Myr using an atmospheric general circulation model. J. Geophys. Res. 104, 11995-12018 (1999). Kroon, D., Steens, T. & Troelstra, S. R. Onset of monsoonal related upwelling in the western Arabian Sea as revealed by planktonic foraminifers. Proc. Ocean. Drill. Program Sci. Results 117, 257-263 (1991). Kutzbach, J. E., Guetter, P. J., Ruddiman, W. F. & Prell, W. L. Sensitivity of climatic uplift in southern Asia and in American West: Numerical experiments. J. Geophys. Res. 94, 18393-18407 (1989). Prell, W. L. & Kutzbach, J. E. Sensitivity of the Indian monsoon to forcing parameters and implications for its evolution. Nature 360, 647-652 (1992). Quade, J., Cerling, T. E. & Bowman, J. R. Development of asian monsoon revealed by marked ecological shift during the latest miocene in northern pakistan. Nature 342, 163-166 (1989). Ramstein, G., Fluteau, F., Besse, J. & Joussaume, S. Effect of orogeny, plate motion and land sea distribution on Eurasian climate over the past 30 million years. Nature 386, 788-795 (1997). Raymo, M. E. & Ruddiman, W. F. Tectonic forcing of late Cenozoic climate. Nature 359, 117-122 (1992). Ruddiman, W. F. & Kutzbach, J. E. Forcing on the late Cenozoic uplift northern hemisphere climate by plateau uplift in southern Asia and the American West. J. Geophys. Res. 94, 18409-18427 (1989).