MONSOON SEASONALITY IN THE LATE EOCENE OF SOUTH CHINA: EVIDENCE FROM SUB-ANNUAL δ18O RECORDS

Late Eocene mollusk shells from the Nadu Formation, Bose Basin, in southern China show a strong seasonal cycle in δ¹⁸O. Ranges in the δ¹⁸O of fossil bivalves and a gastropod are, 5.3, 5.9 6.2, and 7.0‰ respectively. These δ¹⁸O amplitudes are greater than that generated by a seasonal temperature cycle, given the limited temperature range for unionid bivalve growth. The pattern seen suggests that the δ¹⁸O of the ambient water was at its most negative in the middle-to-late summer. Minimum δ¹⁸O values of shell suggest that water δ¹⁸O values were very similar to the modern monsoon-driven summer rain in south China.

The δ¹⁸O ranges of recent bivalves and a gastropod are 3.7, 4.6, 3.2 and 3.2‰, respectively, less than the range observed in the fossil shells. The reduced range may be due to a reduced δ¹⁸O cycle in modern river water due to the catchment effect or groundwater input to the river. However, the minimum shell δ¹⁸O values are identical to those of the fossil shells, reflecting similar δ¹⁸O values of summer river waters if the annual temperature cycle was similar to today. This is likely because of the low sensitivity of calculated water δ¹⁸O values to temperature differences – on the order of 1‰ for each 4.3°C.

The average δ¹⁸O ratio for all fossil samples is -6.2‰, with minimum and maximum δ¹⁸O ratios of -10.9‰ and -2.4‰ respectively. From the average shell value a river water δ¹⁸O of -5.9‰ can be calculated based on an empirical relationship between shell and river water δ¹⁸O. The weighted average δ¹⁸O ratio for modern rainwater in Liuzhou, China is -6.3‰. The isotopic cycles present in Late Eocene bivalves from the Nadu formation have δ¹⁸O cycles that reflect significant changes in the seasonal δ¹⁸O of river water, with δ¹⁸O minima occurring in the mid-late summer. The average δ¹⁸O value calculated for Late Eocene river water is similar to that in the region today. Minimum shell δ¹⁸O values are identical for modern and fossil shells, reflecting similar wet-season river δ¹⁸O values, which dominate river recharge. The fossil data strongly suggest the presence of a monsoon climate during the latest Eocene. This implies the presence of a plateau that was high and vast enough to drive a monsoon system and is in agreement with studies that report an early high elevation for the Tibetan Plateau.