We found significantly high atmospheric CH4 and CO2 concentrations at the surface of the RW in three out of four terminal areas of the glaciers studied (Fig. 3).
Significantly high CH4 (up to 6.7 ppmv) and CO2 (1060 ppmv) concentrations compared to background values (approximately 2.0 ppmv for CH4 and 430 ppmv for CO2) were recorded during continuous measurements of the portable gas analyzer near the first outlet of the Castner Glacier outflow on 12 June 2022 (Fig. 3d). We observed that the concentrations, especially for CH4, were higher when the inlet was close to the outflow water. Occasionally high CO2 concentrations were also recorded, although less frequently than the fluctuation in CH4. No clear relationship between the two gas concentrations was found in our short measurement period. A similar measurement was made at the same location in the previous year (26 June 2021), where little fluctuation was found in the measured gas concentrations (Fig. 3d). CH4 concentrations at the Gulkana Glacier remained stable, staying within a few ppmv of the background level, while we observed occasional fluctuations of CO2 concentrations in both years (Fig. 3a). At the Canwell and Matanuska Glaciers, slightly elevated CH4 concentrations of up to 2.2 and 2.9 ppmv, respectively, were recorded, especially when the gas inlet was located near the surface of RW (Fig. 3b/c). At both glaciers, several pulses of high CH4 and CO2 concentrations were recorded at unexpected times when chamber flux measurements were not conducted. Although the range of fluctuation in concentrations was smaller than in Castner Glacier, the fluctuations in CH4/CO2 concentrations showed an inverse correlation.
Dissolved CH4 concentrations in RW were all above background levels (0.046–0.052 µg C L− 1) at Castner (1.9 ± 0.2 µg C L− 1), Canwell (0.085 ± 0.002 µg C L− 1), and Matanuska (0.082 ± 0.002 µg C L− 1) (Table 3), with Castner Glacier being the highest of the four glaciers. In addition, at Canwell, the dissolved CH4 concentration at downstream of the ice tunnel (0.085 ± 0.002 µg C L− 1) was significantly higher than that at the upstream (0.049 ± 0.001 µg C L− 1). Markedly large CH4 fluxes (1.663 mmol− 1 m2 h− 1 on average) were observed from the RW surfaces at the Castner Glacier in 2022 (Table 1). Positive CH4 fluxes (i.e., emissions) from RW surfaces in the Canwell and Matanuska Glaciers (0.007 and 0.014 mmol m− 2 h− 1, respectively) were observed in 2022, while no significant CH4 fluxes were observed from sediment surfaces in 2022 and from any surface in 2021. CO2 was absorbed into the RW in both measuring years with the largest negative flux of 15.7 mmol m− 2 h− 1into the outflow water at the Castner Glacier in 2022 (Table 1).
The electric conductivity (EC) values of RW in the Castner Glacier were the highest (0.28 and 0.36 mS cm− 1 in 2021 and 2022, respectively) among the glaciers studied (Table 3). The pH of RW ranged from 7.4 to 8.0, with an exceptional value of 6.6 at the downstream sampling site of the Canwell Glacier in 2022.
The dissolved organic carbon (DOC) in the RW shows significantly higher values (0.28–0.40 mg L− 1) for the two sampling sites at the Castner Glacier than at the Canwell and Gulkana Glaciers (~ 0.20 mg L− 1) (Table 3). The higher values are comparable to those found in the snow cover of middle Alaska (~ 0.33 mg L− 1, near Fairbanks)[7], and are also consistent with the high concentration of CH4 is observed only at the Castner Glacier. The lower DOC values are similar to those from snow cover on sea ice in the Chukchi Sea and near Denali National Park and Preserve (0.17–0.24 mg L− 1)[7] and from glacier surface snow in southeast Alaska (0.20 mg L− 1)[8].
Table 2. CH4 and CO2 fluxes from RW and surrounding sediment surfaces measured by in-situ chamber method (N = number of measured locations).
Table 3. Water quality of RW. dis_CH4: Concentration of dissolved CH4 in outflow water, air_CH4: CH4 concentration of the ambient atmosphere at the site, EC: Electric conductivity, DOC: Dissolved Organic Carbon in the RW. Note that Castner, Gulkana DOC were sampled separately in late July. N: Number of repeated measurement times. EC and pH were measured once at each site.