Elemental geochemical constraints on paleoprecipitation in westerlies-dominated arid Asia: A comparative surface soil study from NE Iran and SW Tajikistan

Abstract

Westerlies-dominated arid Asia spans the dry mid-latitude Asian interior from NE Iran across the Central Asian interior to NW China and the southern Mongolian Plateau. Although elemental geochemical proxies are widely used in monsoonal Asia, their applicability to loess deposits in this broader arid domain remains insufficiently tested, especially in its western and central sectors. Here, we investigate the geochemical characteristics of modern surface soils (0–2 cm) from the Iranian Loess Plateau and SW Tajikistan to evaluate their relationships with present-day climatic conditions and their implications for paleoclimatic reconstruction. Specifically, we aim to (i) characterize elemental geochemical parameters and their paleoclimatic significance and (ii) identify robust proxies for paleoprecipitation reconstruction. Our results indicate (i) low chemical weathering intensity in both regions; (ii) a coupled influence of provenance and climate on weathering processes; and (iii) in NE Iran, strong relationships between the mean annual precipitation and the indices C [(Fe + Al + Mn + Cr + Co + Ni)/(Ca + Mg + Na + K + Sr + Ba)] (r = 0.73, p < 0.001), Rb/Sr (r = 0.72, p < 0.001), and Re [(CaO + MgO + K₂O + Na₂O)/(Fe₂O₃ + MnO₂)] (r = −0.71, p < 0.001), supporting their use for quantitative precipitation reconstruction. By contrast, in SW Tajikistan, soil moisture reflects the combined effects of atmospheric precipitation, seasonal temperature variability, and temperature-controlled snow/ice meltwater, obscuring a unique precipitation–proxy relationship and restricting these indices to qualitative indicators of past moisture conditions. Overall, this modern-process calibration provides an elemental geochemical framework for paleoclimate interpretation in western and central arid Asia, and underscores the need for region-specific proxy validation under contrasting hydrological regimes.