Dual stable isotope characterization of excess methane in oxic waters of a mesotrophic lake

Abstract

To determine the source of excess methane in oxic, surface‐water columns often found in freshwater environments, we measured the in situ concentration and stable isotopic compositions (δ13C and δ2H) of methane in Lake Biwa, a mesotrophic lake in Japan. The values from the littoral zone and lake‐floor sediments were determined, besides those in the water column of the pelagic zone. Furthermore, we conducted incubation experiments to measure microbial oxidation rates and alterations in the isotopic signatures of methane. We found significant vertical and seasonal variations in both in situ concentrations and stable isotopic compositions of methane measured in the pelagic zone. We concluded that active microbial oxidation was primarily responsible for the variation in δ13C and δ2H values of methane in the pelagic water column. As a result, we defined a new indicator Δ(2,13) to characterize the sources of dissolved methane, in which variations in both δ13C and δ2H during methane oxidation had been corrected. The excess methane in oxic, surface‐water columns exhibited Δ(2,13) values similar to those in the littoral zone. We concluded that excess methane at the surface of the pelagic zone originated from the littoral zone via lateral transport. Anoxic near sediments and inflowing rivers were responsible for methane enrichment in water of the littoral zone and in the surface water columns of the pelagic zone.

Volatile‐Rich Hydrothermal Plumes Over the Southern Central Indian Ridge, 24°49’S: Evidence for a New Hydrothermal Field Hosted by Ultramafic Rocks

“…(2010) and Tsunogai et al. (2020). Analytical precisions were around ±3% for methane concentrations and 0.2‰ for δ 13 C‐CH 4 .…”

“…The methane concentration and stable isotopic ratios were measured by using an isotope-ratio mass spectrometer (IRMS) coupled with a gas chromatograph (GC) at Nagoya University, Japan. The detailed analytical method for measuring methane concentration and stable carbon isotopes was described in Hirota et al ( 2010) and Tsunogai et al (2020). Analytical precisions were around ±3% for methane concentrations and 0.2‰ for δ 13 C-CH 4 .…”

Hydrogen and carbon isotope fractionation factors of aerobic methane oxidation in deep-sea water

“…Seawater samples were collected during R/V Mirai cruise MR17-03C in 2017 (Brisbin et al, 2020) above two active deep-sea hydrothermal vent sites (Fig. 1), including the Hatoma Knoll site (depth: 1531 m) (Toki et al, 2016) and the Daisan-Kume Knoll ANA site (depth: 1079 m) (Makabe et al, 2016;Uyeno et al, 2020). Hatoma Knoll is characterized by vigorous venting of fluids at 323 • C with boilingderived bubbles.…”

“…Assuming variations in the isotope ratios of CH 4 in the environment are attributable solely to CH 4 oxidation, can also be calculated using another equation without the parameter f from discriminations of isotope ratios observed (Tsunogai et al, 2020), as follows:…”

Hydrogen and carbon isotope fractionation factors of aerobic methane oxidation in deep-sea water

“…Assuming variations in the isotope ratios of CH4 in the environment are attributable solely to CH4 oxidation, Λ can be calculated also using another equation without the parameter f from discriminations of isotope ratios observed [Tsunogai et al, 2020], as follows:…”