Ramona Schneider: Palaeoenvironments of ancient humans in Central Asia across Mid-Pleistocene global climate transitions

Abstract

Formative events in early human evolutionary history are often explained by climatic influences that significantly changed the habitats and migration corridors of early hominins. In Central Asia, Mid-Pleistocene hominin occupation is extensively demonstrated by stone tools embedded in loess-palaeosol sedimentary deposits that are up to 1 million years old. The presence of these ancient humans is contemporaneous with major reorganisations of global climate, most importantly the Mid-Pleistocene Transition (MPT; 1,2–0,7 Ma) and Mid-Brunhes Transition (MBT; 0.43 Ma). However, the response of regional climates and ecosystems to global changes, and the potential impact of this on early human migration, occupation patterns and evolution in Central Asia, are largely unknown.

This thesis uses a multi-climate-proxy approach to quantitatively reconstruct climatic and environmental change in southern Central Asia over the Middle Pleistocene from loess-palaeosol sections in Tajikistan. Combined U/Th-dating and clumped isotope measurements are used to assess the age and formation temperatures of soil carbonate concretions in the loess sequences. Anisotropy of magnetic susceptibility is used to measure magnetic fabrics in loess and to infer directions of dust-transporting winds. A new multi-frequency magnetic susceptibility proxy is developed and combined with a modern-analogue transfer function to reconstruct palaeoprecipitation. Estimates of past rainfall are evaluated against fully-coupled palaeoclimate model simulations, and sediment elemental composition indicating weathering intensity.

Regional climate in southern Central Asia was remarkably resilient over the major Mid-Pleistocene global climate transitions, with consistent dominance of wintertime westerly precipitation and southerly dust-transporting wind directions driven by synoptic-scale westerly circulation. This stable regional climate adjusted to global transitions: increasing glacial erosion is recorded in loess chemical and mineral magnetic properties and reflects the imprint of the MPT with an overall cooling trend and intensified glacial periods, and moderately enhanced precipitation after the MBT. Extensive hominin occupation is limited to certain Mid-Pleistocene interglacial periods in Tajikistan and does not correlate with any major regional climate change, except for an occupation hiatus coinciding with the post-MBT precipitation increase. Our results indicate that migration-occupation patterns of ancient humans in Central Asia are not simply a function of local climate changes. Extra-regional factors such as blocked established migration routes or the appearance of other favourable habitats may also contribute to the timing and spatial distribution of hominin occupation patterns in southern Central Asia.