Loess deposits record stable Mid-Pleistocene hydroclimate during phases of human occupation of Central Asia

Palaeolithic tools preserved in the loess-palaeosol sections of southern Tajikistan as early as ~800 ka evidence the episodic presence of ancient hominins across major Quaternary climate shifts, such as the Mid-Pleistocene and Mid-Brunhes Transitions (MBT). The richest assemblage of lithic tools found in the region, the Karatau Culture, is found mainly in palaeosols associated with Marine Isotope Stages (MIS) 15, 13, and 11, with intervening glacial periods as well as previous and subsequent interglacial periods characterised by a near absence of tools, except for MIS 14 which contains a smaller number of artefacts. Curiously, the disappearance of the Karatau culture coincides with an abrupt increase in magnetic susceptibility in the palaeosol units. Currently, the cause of the alternating phases of occupation and their possible connection to wider-scale climate remain unclear.

The Khovaling Loess Plateau loess-palaeosol sequences provide an opportunity to understand the climatic and environmental context of the appearance and disappearance of early hominins. Since the Khovaling Loess Plateau is located in a transitional zone between climate systems (Mid-Latitude Westerlies, Siberian High and Indian Monsoon) regional climate may be sensitive to global climate reorganisations within the Quaternary. Based on the observed abrupt increase in magnetic susceptibility following MIS 11, it has been hypothesized that monsoon incursions may have occurred during some interglacials, and that these incursions may have ceased after MIS 11, coinciding with the disappearance of the Karatau culture. However, evidence for potential monsoon incursions is highly debated, and the cause for the change in the magnetic susceptibility record remains unclear. In this study, we apply a novel multi-frequency magnetic susceptibility approach, complemented by elemental composition data from XRF and XRD, and by paleoclimate simulations, to investigate possible variations of the hydroclimate in Central Asia. The simulations, performed with the fully-coupled HadCM3 global climate model, allow us to assess the relative and combined effects of orbital, greenhouse gas and ice sheet forcings on the hydroclimate variability including possible moisture transport pathway changes in Central Asia around MIS 13 and 11.

Based on the combined evidence, we argue that the abrupt increase in bulk magnetic susceptibility after MIS 11, observed across different sites in southern Tajikistan, is best explained by a sediment provenance change. It appears to be unrelated to any change in rainfall seasonality, and to a lesser degree, intensity. We demonstrate that relative frequency dependence of magnetic susceptibility (χ_FD %) is the most suitable proxy for calculating quantitative palaeoprecipitation estimates in this region. Our magnetic susceptibility results, calibrated against a modern-analogue based transfer function, indicate that the demise of the Karatau culture coincides with an approximate +25% increase in regional annual mean precipitation. Combined with the other proxy data, this result indicates a relatively stable regional climate across periods of hominin occupation and the MBT.