Isotopic Constraints on Sources and Transformations of Nitrate in the Mount Everest Proglacial Water

Abstract

Glacier melting exports a large amount of nitrate to downstream aquatic ecosystems. Glacial lakes and glacier-fed rivers in proglacial environments serve as primary recipients and distributors of glacier-derived nitrate (NO₃^-), yet little is known regarding the sources and cycling of nitrate in these water bodies. To address this knowledge gap, we conducted a comprehensive analysis of nitrate isotopes (δ¹⁵N_NO₃, δ¹⁸O_NO₃, and Δ¹⁷O_NO₃) in waters from the glacial lake and river of the Rongbuk Glacier-fed Basin (RGB) in the mountain Everest region. The concentrations of NO₃^- were low (0.43 ± 0.10 mg/L), similar to or even lower than those observed in glacial lakes and glacier-fed rivers in other high mountain regions, suggesting minimal anthropogenic influence. The NO₃^- concentration decreases upon entering the glacial lake due to sedimentation, and it increases gradually from upstream to downstream in the river as a soil source is introduced. The analysis of Δ¹⁷O_NO₃ revealed a substantial contribution of unprocessed atmospheric nitrate, ranging from 34.29 to 56.43%. Denitrification and nitrification processes were found to be insignificant in the proglacial water of RGB. Our study highlights the critical role of glacial lakes in capturing and redistributing glacier-derived NO₃^- and emphasizes the need for further investigations on NO₃^- transformation in the fast-changing proglacial environment over the Tibetan Plateau and other high mountain regions.