Identifying the natural and agricultural impacts on the glaciochemistry of the Aru ice core on the northwestern Tibetan Plateau

Glaciochemical data sourced from ice cores in polar regions and the Alps have been extensively examined. However, quantitative studies on glaciochemical records of the Tibetan Plateau (TP) are scarce. To address this, we investigated annual variations in the major soluble ions (Ca²⁺, Mg²⁺, Na, K, NH⁺⁺₄⁺, Cl⁻, NO₃⁻, and SO₄²⁻) in the Aru ice core on the northwestern TP from 1850 to 2016. Applying a positive matrix factorization model, the sources of the major soluble ions and three factors to evaluate natural and agricultural impacts were identified. Factor 1, crustal dust with high loadings of Mg²⁺ (81.9%) and Ca²⁺ (68.7%), significantly positively correlated with wind speed and significantly negatively correlated with δ¹⁸O and net accumulation recorded by the ice core, suggesting that strong winds contributed to crustal dust transport from arid and semi-arid regions of Central Asia and deposition in the Aru glacier. However, relatively warm and wet climate prevented the transport of crustal dust. Factor 2 comprised salt lakes with high dominant loadings of Na (75.3%), SO⁺₄²⁻ (64.1%), Cl⁻ (60.8%), NO₃⁻ (52.2%), and K (49.4%). Declining lake water levels exposed salt lake minerals, which were carried to glaciers under the dynamic conditions of strong winds, whereas warming resulted in an expansion of glacial meltwater and lake water volume, which decreased the contribution of salt lake sediments. Therefore, the contribution of salt lake deposition decreased. Factor 3 was agricultural sources with a high loading of NH⁺₄⁺ (82%), whose trend aligned closely with the population number and N productions from agricultural sources in South and Central Asia, suggesting that NH₃ emissions from agricultural practices are a critical contributor to Factor 3. This study quantified the proportional contribution of natural and agricultural sources to glaciochemical composition, advancing our understanding of glaciochemical records in ice cores from source recognition to quantification.