Carbon isotope composition of land snail shells as a proxy for precipitation amount in the East Asian Monsoon region: A case study from Hainan Island

The carbon isotopic composition of land snail shells (delta C-13(shell)) potentially reflects changes in vegetation composition and thus precipitation. To better assess the response of delta C-13(shell) to East Asian monsoon (EAM) precipitation in different regions, we investigated the delta C-13 of modern snail shells and the corresponding surface soil and vegetation in Hainan Island, in the tropical monsoon region of China. This region spans a large mean annual rainfall gradient (1250-2300 mm), but it has a very small range of mean annual temperature (similar to 2 degrees C). We found that the most depleted delta C-13(shell) value (-18.5 parts per thousand) reported in this study is more negative than previously reported in the literature, and that delta C-13(shell) in Hainan Island mainly reflects the delta C-13 of C3 plants, indicating the C3 dietary preference of snails. After confirming that delta C-13(shell) is a reliable precipitation proxy in Hainan Island, we produced a new general equation between delta C-13(shell) and precipitation, combining our data from Hainan Island with that from previous studies in the EAM region: delta C-13(shell) =-0.0025 (+/- 0.0002) MAP -9.0841 (+/- 0.23) (n = 131, r =-0.73, p < 0.01). A comparison of the slope of the linear equation for Hainan Island (delta C-13(shell) =-0.003 (+/- 0.0007) MAP -7.5619 (+/- 1.27), n = 18, r =-0.74, p < 0.01) with that of Bao et al. (2019), with a larger temperature range (>10 degrees C), enabled us to validate that temperature in the EAM region has little effect on delta C-13(shell) compared to precipitation. We also conducted a theoretical evaluation of the effect of temperature on delta C-13(shell) using a flux balance model, which confirmed that the delta C-13(shell)-precipitation proxy is insensitive to temperature variations, considering temperature effects on the fractionation coefficient and the delta C-13 of C3 plants, where C3 plants are the dominant plant type. Our results potentially enable the quantitative reconstruction of paleo-precipitation via the delta C-13(shell) of snail fossils in the sedimentary deposits of the EAM region.