Greenhouse gas emissions from supraglacial waters and possible impact of microbial activities in Parlung No. 4 Glacier, southeast Tibetan Plateau

Supraglacial waters, such as cryoconite holes (CH) and streams, are widely distributed on glaciers, yet their role in the cryospheric carbon cycle remains poorly understood. In this study, we investigate supraglacial waters and greenhouse gas (GHG) dynamics on a southeast Tibetan Plateau glacier (from 29 June to 13 September 2023) combining in situ CO 2 and CH 4 flux measurements with metagenomics. The results show that, at the campaign scale, supraglacial water acts as a weak CO 2 sink (average: −1.65 ± 2.69 mmol/(m 2 d)), and CH 4 exchange was overall neutral, with a slight tendency towards net release (average: 0.82 ± 6.86 μmol/ (m 2 d)), despite substantial spatial and temporal variability. These fluxes were close to 0 and 1–4 orders of magnitude lower than those reported for larger proglacial ecosystems (e.g. lakes, runoff and forelands). Because of this, supraglacial waters remain at the lower end of reported global glacial GHG fluxes, forming an important low-flux baseline for the glacial carbon cycle. Mechanistically, we found a contradiction between microbial functional potential and observed fluxes. The functional potential for CO 2 production was likely constrained by the limited availability of labile organic carbon in this ultra-oligotrophic environment. In contrast, continuous meltwater dilution, with efficient gas exchange, helps sustain the net CO 2 sink. For CH 4 , although the methanotroph-dominated community suggested a potential sink, the observed weak release tendency was likely supported by external inputs from nearby methanogenic hotspots, such as cryoconite piles, and hydrological transport. These baseline data are key for assessing the cryospheric carbon budget and coupling physical and hydrological drivers with microbial processes to better understand GHG dynamics in supraglacial waters.