Unravelling the Pleistocene climatic evolution in the Siwaliks: implications for hominin settlement in the upper Soan Valley, Pakistan

Since the early Pleistocene, climatic variability has profoundly influenced population dynamics, including the migration of hominins into and out of Africa. In South Asia, Quaternary sediments, particularly those found in the Siwaliks, provide a valuable record of climatic changes relevant to hominin habitation. While the upper Soan Valley in Pakistan has been extensively studied for evidence of hominin to modern human activities, the associated environmental and climatic changes remain underexplored. This study aims to elucidate the climatic evolution of the Quaternary sediments in the Riwat area, with the primary objective of reconstructing the environmental conditions that facilitated long-term hominin settlement and survival. To address this knowledge gap, we analyzed the geochemistry and clay mineralogy of 46 sediment samples utilizing spectroscopy and X-ray diffractometry (XRD), focusing on the Plio-Quaternary period. Our findings reveal three distinct climatic stages. The first stage (Stage I), characterized by cold conditions during the Pliocene, and was further divided into semi-arid to semi-humid substages based on C-values. This was followed by a cold to humid climatic stage (Stage II) during the early to middle Pleistocene, marked by moderate to high chemical weathering, which was also categorized into semi-arid to semi-humid substages. Within Stage II, a warm and humid phase was inferred during the early to middle Pleistocene, culminating in the coldest and most arid conditions of Stage III during the late Pleistocene to Holocene. The climatic changes observed are indicative of the uplift of the Himalayan-Tibetan Plateau and the influence of the southwestern monsoon on the Indian subcontinent. Moderate to intense monsoonal activity characterized the early to middle Pleistocene, creating conditions conducive to early hominin settlement and adaptation due to the availability of resources. Conversely, environmental conditions during periods of weakened monsoons exhibited an arid climate from the late Pleistocene to Holocene (corresponding to MIS 1-4 in oceanic records). This study enhances our understanding of hominin-climate interactions and has implications for the role of climate in shaping diverse hominin habitation patterns, both globally (out of Africa) and regionally (South Asian) contexts. • Moderate to high chemical weathering prevailed in the upper Soan Valley, upper Indus Basin • Three main climatic stages are evolved in the upper Soan Valley, since the late Pliocene to Holocene. • Changes from arid, semiarid to semi humid, humid and arid stages in dominant fluvial settings. • Semiarid, humid and arid climatic phases favored the early hominin occupation in South Asia.