Landscape dynamics and human-environment interactions in the northern foothills of Cho Oyu and Mount Everest (southern Tibet) during the Late Pleistocene and Holocene

Abstract

Here we present an integrated earth surface process and paleoenvironmental study from the Tingri graben and the archaeological site of Su-re, located on the southern rim of the Tibetan plateau, spanning the past ca. 30 ka. The study area is characterized by cold climate earth surface processes and aridity due to its altitude and location in the rain shadow of the Mount Everest–Cho Oyu massif and is thus sensitive to climatic and anthropogenic perturbations. In this highly dynamic geomorphic environment, paired-cosmogenic nuclide results from boulders on a massive hummocky moraine in the southern Tingri graben reveal complex exposure histories that limit our capability of directly dating the corresponding glacial advance, and shed a note of caution on previously published single-nuclide-based exposure ages along the northern Himalaya. Based on geomorphic considerations, however, the moraine clearly represents the local last glacial maximum, and likely coincided with a ∼344 ± 109 m drepression of discontinuous permafrost zone relative to today during the global last glacial maximum (gLGM). This greatly intensified permafrost and periglacial hillslope processes and led to fluvial aggradation of the valley floors of ≥12 m. We observe formation of a thick (≥50 cm) pedo-complex starting at ca. 6.7 ka before present (BP) and erosional truncation at ca. 3.9 ka BP. Widespread landscape instability and erosion characterize the region subsequent to 3.9 ka and intensifies in the 15th century AD. Several lines of (geo)archaeological evidence, including the presence of pottery sherds, sling-shot projectiles and hammer stones within the sedimentary record, indicate human presence at Su-re since ca. 3.9 ka BP. Our data suggest that in the Su-re-Tingri area climatic conditions were warm and moist enough to allow vegetation expansion and soil formation only from ca. 6.7–3.9 ka, followed by weakening of the Indian summer monsoon (ISM) strength between ca. 4.2 and 3.9 ka, which is a prominent climatic event in the wider Asian monsoon region, and reflected in the investigation area by the 3.9 ka erosional boundary. Merging our Holocene landscape reconstruction with the geoarchaeological evidence, we speculate that the combined effect of Little Ice Age (LIA) cooling and an anthropogenic overuse of the landscape led to climatically induced landscape degradation and ultimately to an anthropogenically triggered ecological collapse in the 15th century. Such a scenario is in-line with regional historical data on declining monastery construction and migration of the ethnic group of the Sherpas.From an earth surface dynamics perspective, we find that transient landscape processes on the southern rim of the Tibetan plateau are strongly linked to millennial scale changes in the ISM intensity and duration. We identify three types of unidirectional non-linear ISM-landscape interactions. Given that the Tibetan plateau is the largest high-altitude landmass on our planet and our limited understanding of several of the key earth surface processes on the plateau, we pinpoint the need for more long-term (Quaternary scale) empirical data particularly on permafrost and periglacial processes and human-environment interactions.