Radiocarbon age anomalies, potential carbon sources analysis and correction of terrestrial gastropods in a loess sequence from Kunlun Mountains since 3.6 ka

Terrestrial mollusks serve as potential radiocarbon materials for dating loess deposits in the late Quaternary. However, the reliability of gastropod shell ¹⁴C ages is complicated by two issues: open-system behavior and the limestone problem. Due to lack of assessment on the preservation status of shell fossils and factors affecting old carbon, correction for age anomalies remains an unresolved question. This study investigated two gastropod shells, Cathaica rossimontana and Succinea sp., discovered from a loess section rich in charcoal in the North Kunlun Mountains of Central Asia. By analyzing local climate condition, leaching degree, and mineral composition, we revealed that the open systems have a negligible influence on shell fossils. The values of age anomalies of Cathaica rossimontana and Succinea sp. are respectively 1788 ± 240 years and 492 ± 100 years. Furthermore, we obtained the temporal variation patterns of different carbon sources in shell fossils through a mass balance model analysis. The results indicate that the proportion of carbonate sources ranges from 17.5% to 26.8% in Cathaica rossimontana, while it ranges from 6.0% to 7.8% in Succinea sp., which may result from their size and calcium demand. Succinea sp. is recommended for stratigraphic dating for its minor age anomalies. Our study further demonstrates that snails of the same species absorb a highly consistent proportion of limestone under stable environment. This provides a theoretical basis for correcting age anomalies in fossil shells. The corrected ages of Cathaica rossimontana, based on living specimens, are consistent with reliable charcoal ages. Our findings offer a potential approach to correct the old carbon in snail fossils and facilitate the reliable dating for loess deposits in Central Asia.