Atmospheric CO₂ sink caused by enhanced chemical weathering in the Rongbuk glacier runoff at the initial ablation, Mt. Qomolangma (Everest)

River networks convey carbon (C) from terrestrial ecosystems to the ocean and function as processors that transfer and emit C into the water (Cole et al., 2007, Hotchkiss et al., 2015, Ran et al., 2021). C fluxes in inland waterways include contributions from terrestrial inputs, aquatic primary production, burial, emissions, and export to oceans (Pilla et al., 2022). Carbon dioxide (CO₂) emissions occurring at the interface between water and air play a crucial role in the aquatic C cycle. The CO₂ emission from rivers and streams into the atmosphere is estimated between 0.56 and 3.56 petagrams (Pg) of C per year (Cole et al., 2007, Raymond et al., 2013, Sawakuchi et al., 2017).

Although this range is notable, it exceeds the amount of C transported from land to the ocean by rivers (0.70–0.95 Pg C yr⁻¹) and is in equal proportion to the amount of organic carbon (OC) burial (0.6–3.6 Pg C yr⁻¹) (Cole et al., 2007, Aufdenkampe et al., 2011, Raymond et al., 2013, Drake et al., 2018, Pilla et al., 2022). Land-based C inputs and strong biological processes are the main causes of high CO₂ concentrations in rivers. However, in alpine rivers fed by glaciers, which are ubiquitous globally, biological processes are far less active than those in the tropics and subtropics due to low temperatures and high mineral sediment loads (St. Pierre et al., 2019). The freshwaters supplied by glaciers primarily flow through mineral-rich terrain where the lack of terrestrial vegetation and fertile soils restrict the amount of OC emitted into the rivers. A high erosion rate and fine-textured sediment cause glacier erosion to become a veritable mineral surface area production factory, which accelerates the process of chemical weathering (Anderson, 2007).

Glacier meltwater harbors multiple potential C sources, such as the dissolution of atmospheric gases, C trapped in ice bubbles, mechanical grinding and organic matter (OM) remineralization (Pain et al., 2021). Controlled by basin lithology, carbonate and silicate weathering turn glacier-fed freshwaters into an atmospheric CO₂ sink (Yan et al., 2023, Zhu et al., 2024), while sulfide oxidation coupled with carbonate weathering (SO − CW) or organic matter oxidation makes it an atmospheric CO₂ source (Sharma et al., 2019, Shukla et al., 2023). However, modeling the magnitude of C exchange in meltwater remains challenging due to the coupling of biotic and abiotic processes (Christiansen et al., 2021). Investigating C cycles comprehensively in glacierized regions demands focusing on chemical weathering and biological processes in diverse geological settings. The glaciers ecosystem is dominated by cryophilic and cold-tolerant microorganisms inhabiting unique habitats with distinctive organisms and biogeochemical activity (Kohler et al., 2024). Microbial communities from different glacier habitats, interacting via meltwater, participate in ecological processes such as C sequestration and release, and are important in regional and global elemental biogeochemical cycles (Margesin and Collins, 2019, Zhang et al., 2024). Thus, the unique microbial community in the glacial environment, along with the low temperatures and chemical weathering processes, indicates that glaciers could play a role in the global C cycle and should not be overlooked.

Inland waters are of crucial importance in C cycling and greenhouse gas emissions. However, the role of glacier-fed rivers in terms of CO₂ emissions or absorption remains unclear (Song et al., 2024). Current research on greenhouse gas (GHG) emissions at the water–air interface of rivers primarily focuses on large, high-order rivers, with limited studies conducted on small, low-order headwater rivers (Butman and Raymond, 2011, Teodoru et al., 2014, Liu and Raymond, 2018). Due to substantial exposed bedrock and a lack of OC, the CO₂ emissions (−15.1 to 132 mmol CO₂ m²/d) from high-elevation streams in the Rocky Mountains were significantly reduced (Crawford et al., 2015). Similar findings were also reported in Alpine catchments in Switzerland (Robison et al., 2023).

Research on the hydrochemistry and C emission processes of glacial meltwater mainly focuses on periods of intense ablation (Zhang et al., 2021, Konya et al., 2024), while little is known about initial glacial-fed runoff since the initial ablation stage marks the crucial transition from frozen to the liquid phase. The C sink properties of glacier-fed rivers change dramatically at different stages of ablation. According to Meire et al. (2015), meltwater in the Greenland Ice Sheet is often CO₂ undersaturated, ranging from 74 μatm to 350 μatm, particularly during the summer season when there is high discharge. The measured CO₂ sink strength of Canada’s Lake Hazen watershed is –33.33 mmol/m²/d during the ablation season from June to August, while it is only −2.5 mmol/m²/d based on annual median fluxes in the Valsorey Basin, southwestern Switzerland (St. Pierre et al., 2019, Robison et al., 2023). The difference of one order of magnitude might partly result from the different measuring seasons. Therefore, extrapolating the exchange volume during periods of intense ablation to the whole year might induce considerable uncertainty. Observations of exchange fluxes at the beginning and end of glacial melting can provide reliable data to support the exploration of annual CO₂ emissions from meltwater.

The Tibetan Plateau (TP) contains extensive glaciers (97,605 km²) in the middle and low latitudes, averaging over 4000 m a.s.l. (Yao et al., 2012, RGI Consortium Randolph Glacier Inventory - A Dataset of Global Glacier Outlines, 2023). These glaciers feed over ten major Asian rivers, supplying drinking water to billions (Qu et al., 2017). Research on the C biogeochemical cycle of glacial-fed freshwaters has largely focused on high-latitude environments, which exhibit different diurnal or seasonal cycles as glacierized watersheds in more temperate climates (Christiansen and Jørgensen, 2018, Lamarche-Gagnon et al., 2019, St. Pierre et al., 2019, Konya et al., 2024; Table 1). Global warming is causing various changes in the TP ecosystems (Yao et al., 2022). Previous research has extensively examined the chemical composition of stream water in glacier basins, including the dynamics of organic and inorganic carbon (Li et al., 2019, Zhou et al., 2019). However, there is a lack of studies on the dynamics of CO₂ in glacier-fed rivers on the TP (Wang et al., 2014, Yan et al., 2023, Shukla et al., 2023), with few continuous direct CO₂ flux measurements and unclear underlying drivers. Research on TP glacier-fed rivers indicates that C (mostly in the form of CO₂) emissions can exhibit significant spatial and temporal variations, ranging from −21.65 mmol/m²/d in the southeast to 71.0 mmol/m²/d in the north (Zhang et al., 2021, Du et al., 2022; Table 1). These spatial variations complicate accurate CO₂ release/uptake estimation and its integration into TP carbon models

Glaciers on the TP are facing substantial melting. Glacier-fed freshwaters significantly influence downstream aquatic ecosystems and are vital components of regional C cycles (Wadham et al., 2019, Du et al., 2024). Conducting observational research on the C cycle of glacial meltwater across the TP is essential. The Himalayas are the center of glacier distribution in the southern TP, and the Rongbuk Glacier on the north slope of Qomolangma (Everest) is representative of glaciers in this region. In this study, we selected the Rongbuk Glacier meltwater runoff (RBM), a typical glacial-fed river, to investigate the emission characteristics of CO₂ in the initial stage of ablation. The main aims of this study were (1) to explore the temporal and spatial variations of the partial pressure of CO₂ (pCO₂) and the CO₂ efflux rate (FCO₂) at the water–air interface, and (2) to examine the impact of biogeochemical processes on glacial-fed runoff pCO₂ and FCO₂. This study will improve our understanding of C cycling in glacier meltwater runoff and provide support for more accurate estimates of regional and global C budgets from the Rongbuk Glacier and other high-altitude glaciers on the TP.