Precipitation seasonality in controlling the north‒south dipolar pattern of effective moisture variations on the eastern margin of the Tibetan Plateau during the Holocene

Abstract

Holocene climate change between different geographic units of the Asian continent exhibits spatiotemporal heterogeneity under the control of different atmospheric circulation systems. Precipitation/moisture variations between the northeastern and southeastern Tibetan Plateau show an out-of-phase pattern on decadal, centennial, and millennial time scales during the Holocene, influenced by interactions between the summer monsoon system and the westerlies. However, conflicting proxies exist in some regions on millennial time scale, and it is vital to detect and evaluate the detailed processes responsible for such a dipolar pattern. Here, we select and summarize well-dated Holocene records of pollen-based vegetation, oxygen isotope composition (δ¹⁸O) of authigenic carbonates and cellulose, hydrogen isotope composition (δD) of long-chain n-alkanes from leaf wax, and chronology of aeolian activity and soil development from various geological archives, including lake sediments, peat bogs, aeolian deposits, cave speleothems, and tree rings from the eastern margin of the Tibetan Plateau. We find a long-term decreasing trend in summer monsoon precipitation across the eastern Tibetan Plateau during the Holocene, indicated by declining arboreal pollen percentages, enhanced values of δ¹⁸O from lakes in the southeastern Tibetan Plateau (SETP), speleothems, and tree-ring cellulose, as well as enhanced δD values across the entire eastern margin of the plateau. A summary of the chronology of aeolian sand and paleosol on the northeastern Tibetan Plateau (NETP) reveals enhanced aeolian activity during the early Holocene and increased soil development from the middle Holocene onwards. These data, combined with those indicating increased lake levels and decreased lake-water δ¹⁸O values in the region, suggest that the effective moisture on the NETP increased throughout the Holocene. Our findings indicate that increased non-monsoon-season precipitation with the strengthening intensity of the westerlies, against a background of possible decreasing summer evaporation, provided increased moisture to the NETP, ultimately resulting in a Holocene dipolar pattern of humidity variations between the northern and southern parts of the eastern margin of the Tibetan Plateau. The inference that the seasonality of precipitation controlled moisture variation in the study region is supported by high-resolution climate proxies and meteorological data on centennial and decadal time scales during the last millennium. Additionally, we propose a conceptual framework of hydroclimatic processes in a land–lake system to reconcile contradictions between different indicators. The present study not only clearly summarizes the Holocene climate change on the eastern margin of the Tibetan Plateau, but it also highlights the critical importance of distinguishing terrestrial and lacustrine signals in paleolimnological studies, and of noting associated differences that may potentially result from proxy sensitivity to seasonal climate changes.