Concurrent hydrological closure and hominin presence in the Early Pleistocene Nihewan Basin (northern China): insights from stable isotopes

Abstract. The Nihewan Basin in northern China contains rich Early Pleistocene Palaeolithic sites, representing one of the earliest locations of hominins outside Africa. Here, we present the first long-term stable oxygen (δ¹⁸O_eq.cal) and carbon (δ¹³C_eq.cal) isotope record derived from ostracod shells, preserved in the composite 86.2-m NH-T sediment section at the northeastern part of the basin, with a timeframe between ca. 1.67 and 0.78 Ma. The study aimed at reconstructing the long-term climatic changes and hydrological dynamics of the Early Pleistocene Nihewan Basin and to assess their impact on hominin activities. Unexpectedly, we found a strong covariance of δ¹⁸O_eq.cal and δ¹³C_eq.cal values, clearly suggesting that the basin was mostly hydrologically closed. The dominance of evaporation implies that δ¹⁸O_eq.cal shifts track the hydrological state at the section location between closed settings with higher water levels (more standing waters) and open settings with low water levels (more flowing waters) instead of regional changes in precipitation/evaporation ratios alone. Moreover, we observed the concurrence of high δ¹⁸O_eq.cal and δ¹³C_eq.cal values and the increase in the marine-land temperature gradient (ΔT), indicating enhanced East Asian Summer Monsoon (EASM)-driven precipitation which led to wetter climate and increased biogenic productivity. Conversely, low δ¹⁸O_eq.cal and δ¹³C_eq.cal values reflect decreased EASM-driven precipitation and drier climate and reduced biogenic productivity. The new stable isotope data, combined with the synthetic archaeological record, suggest that hominin activities in the Nihewan Basin mostly coincided with periods of higher δ¹⁸O_eq.cal and δ¹³C_eq.cal values when more standing waters bodies and wetter climate prevailed in the region.