A 1700-year peatland-based hydroclimate record from the Hengduan Mountains in the southeastern Tibetan Plateau reveals changing dynamics of the summer monsoon interface

Abstract

The Indian summer monsoon (ISM) and the East Asian summer monsoon (EASM) affect precipitation patterns across the broad Asian continent. The southern Hengduan Mountains region in the southeastern Tibetan Plateau is located at the ISM–EASM interface, but how these two monsoon systems affect long-term regional precipitation variability remains unclear. Here, we develop a 1700-year record of centennial-scale hydroclimate variability from a rarely reported Sphagnum-dominated subalpine peatland in the southern Hengduan Mountains, using Sphagnum-specific cellulose δ¹³C and δ¹⁸O proxies that are supported by characterization of modern proxy–environment relationships.

Based on the modern variability of cellulose δ¹³C and δ¹⁸O in Sphagnum growth increments, we show that δ¹³C is a reliable proxy for moisture availability, following the “water film” mechanism. In contrast, δ¹⁸O is controlled by multiple mechanisms and can increase due to either higher precipitation δ¹⁸O and stronger evaporative enrichment under drier conditions or increased plant use of evaporated pool water caused by pool expansion under wetter conditions. Using the time-varying δ¹³C–δ¹⁸O correlation as a constraint, we infer from our coupled cellulose δ¹³C–δ¹⁸O records several centennial-scale moisture shifts, including wet shifts during 700–1200 CE, 1500–1800 CE, and after 1950 CE, and dry shifts during 300–700 CE, 1200–1500 CE, and 1800–1950 CE, with a generally wetter Medieval Warm Period than the Little Ice Age. The temporal pattern is consistent with previously published regional pollen-climate records but shows disparities from those lake-based terrestrial input records. Based on our novel single-stem isotope analysis, we hypothesize that the latter might be additionally affected by changes in extreme event characteristics.

We find that our new hydroclimate record shows an in-phase relationship with ISM records in the Indian subcontinent, except during 1500–1800 CE and in recent decades, during which our analysis presents evidence for an increased influence from EASM system. We attribute the transitions in large-scale hydroclimate patterns during these two periods to La Niña mean-state and anthropogenic radiative forcing, respectively, which weaken zonal wind flows over the Bay of Bengal—the key passage of the ISM. This study highlights the non-stationary relationship between hydroclimate and Asian summer monsoon dynamics at the ISM–EASM interface.