LOW OXYGEN ISOTOPE VALUES OF FOSILL CELLULOSE INDICATE AN INTENSE MONSOON IN SOUTHEAST CHINA DURING THE LATE OLIGOCENE

Abstract:

Quantitative estimates of monsoon strength in south China are commonly reported from Neogene and Quaternary aged substrates. Similar data for the Paleogene, however, are generally lacking, leading to much uncertainty and debate over the early strength and evolution of the monsoon system. Here we report oxygen isotope measurements on tree-ring cellulose (δ¹⁸O_cell) extracted from 43 Oligocene age mummified wood fossils collected from Nanning, Guangxi, south China. Comparison with modern δ¹⁸O_cell values collected from trees currently living in Nanning Basin indicates significantly lower δ¹⁸O_cell values in the Oligocene than present. Today, such low δ¹⁸O_cell values are found only in trees growing at sites with low mean annual temperature (MAT) (e.g., eastern Canada: MAT = -3 to -8 ^oC), at high elevation (e.g., Nepal: elevation 3550 m), and in monsoon climates of east and southeast Asia that experience greater precipitation amounts than in Nanning Basin. Because independent evidence precludes very low MAT or high elevation in the region during the Oligocene, we propose these low Oligocene δ¹⁸O_cell values result from intense summer rainfall in south China during the Oligocene. Using existing relationships between δ¹⁸O_cell and precipitation amount, we estimate summer rainfall rates ~2 times greater during the late Oligocene than today. These data are consistent with an intense summer monsoon climate in south China during the Oligocene, and provide a rare quantitative estimate of monsoon precipitation in east Asia during the greenhouse to icehouse transition.