Land snail shell morphology as a new Paleomonsoon proxy

Multi-proxy climate records from loess deposits in northern China and speleothem δ¹⁸O records from caves in southern China are widely used to reconstruct changes in the Asian summer monsoon (ASM) on orbital timescales. However, ASM variations inferred from these records display different dominant periods, complicating our understandings of the forcing mechanism of the ASM. This is partly because of the uncertainty and complexity inherent in interpreting the climatic significance of the commonly-used physicochemical indicators for loess, such as those influenced by pedogenic processes and sedimentation rates. New climate proxies are required to address this problem, especially biological proxies, which potentially offer the advantages of greater sensitivity and a more comprehensive response to climate change. The shell morphology of land snails can respond to the natural selection pressure on populations caused by drought environments, leading to its ability to record environmental humidity. Here, we evaluate the potential of the morphology of land snail shells as a monsoonal climate proxy. Specifically, we investigate the variations in the morphology of the fossil shells of Cathaica orithyia from the Chinese Loess Plateau over the past ~ 470 kyr, and compare them with previous ASM records. Our findings indicate that the stratigraphic record of shell aperture morphology seemingly shows a precessional cyclicity (23 kyr), similar to that of Chinese speleothem δ¹⁸O records, as well as obliquity (41 kyr) and eccentricity (100 kyr and 400 kyr) cycles. Our results validate the use of shell morphology as a dependable proxy of aridity or humidity, which may be influenced by multiple climatic factors. Importantly, our shell morphology record suggests that the ASM variations may be forced by multiple orbital periodicities, among which precession predominates.