Emerging proglacial lake constraints on mercury transport and transform patterns in glacial meltwater runoff on China's Tibetan plateau.

The rapid expansion of proglacial lakes on the Tibetan Plateau introduces uncertainties in mercury (Hg) cycling following glacial retreat. This study investigated Hg dynamics in a glacierized watershed by combining comprehensive sampling of glaciers and proglacial lakes to assess their role in Hg transport and methylation. The total Hg (THg) and methylmercury (MeHg) concentrations in aquatic systems ranged from 0.71 to 3.35 ng/L and 0.01-0.11 ng/L, respectively. Compared with glacial meltwater, glacial lake water contained lower THg concentrations (1.20 ng/L) but higher MeHg concentrations (0.09 ng/L), indicating active Hg methylation. Sediment THg (4.34-69.15 ng/g) exhibited spatial heterogeneity, reflecting divergent Hg inputs from supraglacial and subglacial sources. Elevated THg and MeHg in central lake sediments suggest substantial Hg deposition and transformation, likely driven by meltwater-derived organic carbon. Hg isotopic signatures analyses further revealed that Hg speciation is governed by meltwater inputs, atmospheric deposition, bedrock weathering. Additionally, metagenomic analysis has revealed that sediments in periglacial regions, particularly those in proglacial lakes, exhibit a high potential for Hg methylation, indicating that microbial activity may also be a significant factor influencing regional Hg cycling. As proglacial lakes expand, their capacity to modulate Hg fluxes may intensify, with potential glacial lake outburst floods (GLOFs) further altering Hg transport pathways. These findings highlight growing risks to hydrochemical stability in alpine watersheds and the broader Third Pole region under climate-driven glacier loss.