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Intrashell oxygen isotope (δ18Os) analyses of terrestrial snails have been carried out over two decades. However, the intraspecies/interspecies differences are not yet well understood. Here, we conducted a high‐resolution intrashell δ18Osstudy on 43 shells from Cathaica fasciolaand Bradybaena ravida, and 1449 δ18Osdata were obtained. These large amounts of data demonstrate the reproducibility of intrashell δ18Os. Our results suggest that C. fasciolaand B. ravidahave similar growing seasons mainly from March to October in the studied region. The ecological habits of the two species could explain the distinctive variations in their δ18Ossequences. Therefore, our study may interpret the discrepancy between the studies of land snail species and the stable/clumped isotopes of their shells. Moreover, the calculated average growth rate is ∼290 μm/day (from 250 to 330 μm/day) for nonadult C. fasciola. Thus, the use of snail shells for reconstructing high‐resolution terrestrial climate changes is promising. Land snails are sensitive to climatic and environmental changes. Therefore, the oxygen isotopic composition of snail shells (δ18Os) has often been used to study environmental changes. Researchers started to collect multiple samples from a single shell for δ18Osanalyses about two decades ago. However, it is still unclear whether high‐resolution intrashell δ18Osresults are repeatable and whether the influences of different individuals/species may exist. To address these issues, we collected 43 snail shells from Cathaica fasciolaand Bradybaena ravida. In total, 1449 δ18Osdata were obtained. The large amount of data showing similar variations in the intrashell δ18Osfrom different shells demonstrates the repeatability. Accordingly, our data suggest that the two species have similar growing seasons, mainly from March to October, although their ecological habits are different. In addition, the ecological habits of these two species could explain the subtle differences in their δ18Os. Taken together, our study could resolve the discrepancy between previous work on land snail species and stable/clumped isotopes. Furthermore, the estimated average growth rate is ∼290 μm/day (from 250 to 330 μm/day) for nonadult C. fasciola. Overall, snail shells are promising records for exploring high‐resolution terrestrial climate changes in the future. The intrashell δ18Osof land snails is reproducible, which mainly records the δ18O of precipitationCathaica fasciolaand Bradybaena ravidahave different ecological habits but similar growing seasonEcological habits could explain the discrepancy between studies of land snail species and stable/clumped isotopes The intrashell δ18Osof land snails is reproducible, which mainly records the δ18O of precipitation Cathaica fasciolaand Bradybaena ravidahave different ecological habits but similar growing season Ecological habits could explain the discrepancy between studies of land snail species and stable/clumped isotopes

The seasonal rainfall regime is a key factor control on local ecological and social processes and is commonly thought to be stable under long‐term climate changes. Here we present a unique high‐resolution rainfall record from the Thai‐Malay Peninsula, combined with a state‐of‐the‐art transient climate simulation, demonstrating a fundamental rainfall regime shift from summer to winter during the Holocene. Transient model simulation and new sensitivity experiments further reveal that westward migration of the boundary between summer and winter rainfall regimes results in a summer to winter rainfall regime shift forced by distinct changes in summer and winter monsoons. Our findings suggest that the seasonal rainfall regime could be unstable under climate change around the boundaries of rainfall regimes in the tropics and possibly worldwide, which might be more critical for shaping both past and future ecological environments. The observed seasonal rainfall regime, which varies over time, challenges the assumption of stable seasonal patterns in reconstructions of paleorainfall, suggesting that a reappraisal of the understanding of past ecological‐social changes with respect to rainfall may be needed. In this study, we have used a high‐resolution rainfall record from the Thai–Malay Peninsula and a state‐of‐the‐art transient paleoclimate simulation to describe a novel summer to winter rainfall regime shift during the Holocene era in the tropics, which may apply to regime‐boundary regions worldwide and thus provide a new perspective on dramatic ecological and social changes recorded around regime boundary regions for both past and future studies. A unique rainfall record in the Thai‐Malay and a transient simulation suggest a summer to winter rainfall regime shift during the HoloceneTransient paleoclimate simulation simulation reveals that this shift was caused by westward migration of the boundary between summer and winter rainfall regimesThis regime shift could occur in regions around the boundaries of rainfall regimes in the tropics and possibly worldwide A unique rainfall record in the Thai‐Malay and a transient simulation suggest a summer to winter rainfall regime shift during the Holocene Transient paleoclimate simulation simulation reveals that this shift was caused by westward migration of the boundary between summer and winter rainfall regimes This regime shift could occur in regions around the boundaries of rainfall regimes in the tropics and possibly worldwide

Abstract: The expansion of inland Asia deserts has considerably influenced the environmental, social and economic activities in Asia. Aridification of inland Asia, especially timing of the initiation of Asian desertification, is a contentious topic in paleoclimatology. Late Cenozoic eolian loess-red clay sequences on the Chinese Loess Plateau, which possess abundant paleoclimatic and paleo-environmental information, can be regarded as an indicator of inland Asia desertification. Here we present a detailed magnetostratigraphic investigation of a new red clay sequence about 654 m in Zhuanglang located at the western Chinese Loess Plateau. Sedimentological, geochemical, mineralogical, and quartz morphological lines of evidence show that the red clay is of eolian origin. Magnetostratigraphic correlations indicate that this core sequence spans from 25.6 to 4.8 Ma, and typical eolian red clay appears as early as 25 Ma. This extends the lower limit of the red clay on the Chinese Loess Plateau from the previously thought early Miocene back into the late Oligocene. This new red clay record further implies that the inland Asia desertification was initiated at least by the late Oligocene. This sequence provides a unique high-resolution geological record for understanding the inland Asia desertification process since the late Oligocene.