Age and pedogenesis of alpine grassland soils on the northeastern Qinghai-Tibetan Plateau: Insights from optical dating

Abstract

Grassland soils in alpine regions of the Qinghai-Tibetan Plateau (QTP) constitute a crucial component of the QTP ecosystem. Understanding their formation requires accurate chronologies and insights into key pedogenic processes. This study applied multi-grain (MG) and single-grain (SG) post-infrared infrared stimulated luminescence (pIRIR) dating to alpine grassland soils around the Gonghe Basin in the northeastern (NE) QTP to gain new insights into their ages and pedogenic processes. In addition, ¹⁴C dating was performed on soil organic matter, with the resulting ¹⁴C ages compared with optical ages to evaluate their reliability for soil age determination. pIRIR dating showed that bioturbation-induced soil reworking is common in alpine grassland soils. SG pIRIR dating allows more accurate soil age estimation by effectively identifying grains associated with original deposition and pedoturbation, while ¹⁴C dating yields underestimated ages due to younger carbon contamination. We proposed an SG pIRIR-based approach that can be applied to alpine grassland soils to constrain their ages and quantify bioturbation. Combined with a synthesis of regional alpine loess and palaeosol/soil chronologies and a comparison with regional climatic records, the influence of climate on alpine soil pedogenesis and bioturbation was explored. Our results showed that pedogenesis in the studied profiles started at ∼11–5 ka, following an aeolian dust aggradation pedogenic mode. The intensity of soil mixing decreases with depth, with the most intensive mixing occurring in a near-surface zone of tens of centimetres depth. Integrating SG dating results with a new conceptual model, we for the first time estimated the recent and past downward soil mixing rates and the timing of intensified bioturbation for alpine soils on the QTP. Chronological synthesis revealed that alpine soil development on the NE-QTP was most pronounced since ∼6 ka. Effective moisture is a key factor that affects both soil development and bioturbation intensity in alpine grassland soils.