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.

Automated summary

We investigated the carbon isotopic composition (δ13C) of snail shells in Hainan Island, China, and found that it mainly reflects the δ13C of C3 plants and is related to precipitation. We established a new equation between δ13C and precipitation, which can potentially be used for quantitative reconstruction of paleo-precipitation in the East Asian monsoon region.