[A-CG50] Biogeochemical linkages between the surface ocean and atmosphere

[ACG50-04] Tracing the sources of excess methane in subsurface seawater using both stable carbon and hydrogen isotope ratios as tracers

*Fumiko Nakagawa¹, Urumu Tsunogai¹, Kotaro Takada¹, Masanori Ito¹, Shintaro Sato², Sohiko Kameyama³ (1.Graduate School of Environmental Studies, Nagoya University, 2.Graduate School of Science, Hokkaido University, 3.Graduate School of Environmental Science, Hokkaido University)

Keywords:methane, ocean, hydrogen isotopic composition, carbon isotopic composition

Methane (CH₄) is one of the most significant greenhouse gases, but its sources and its behavior remain partially unresolved. Aquatic environments such as oceans and lakes are one of the major sources of CH₄. Therefore, understanding the origins and dynamics of CH₄ in aquatic environments is critical. CH₄ is generally produced in anaerobic conditions and is oxidized in aerobic environments. Therefore, CH₄ in aquatic environments is thought to be produced in anaerobic sediments and rapidly oxidized in the water column. However, surface seawater, despite its aerobic nature, often contains dissolved CH₄ concentrations that exceed atmospheric equilibrium. This contradiction, referred to as the "methane paradox," has been recognized for decades. Sasakawa et al. (2008) used the δ¹³C of CH₄ as an indicator and concluded that sinking particles are the primary source of this excess CH₄. On the other hand, Klintzsch et al. (2023) demonstrated CH₄ emissions from cultured phytoplankton in laboratory experiments and found that the δ¹³C signature of CH₄ released from phytoplankton was broadly consistent with the δ¹³C of excess CH₄ observed in seawater, as reported by Sasakawa et al. (2008). Based on these findings, Klintzsch et al. (2023) proposed phytoplankton as a major source of excess CH₄ in seawater.
In this study, we analyzed δ²H, along with δ¹³C, to elucidate the origins of excess CH₄ in open ocean waters. Seawater samples were collected during the KH23-3 cruise aboard the R/V Hakuho-maru in the subarctic and subtropical regions of the western North Pacific, during the KS24-12 cruise aboard the R/V Shinsei-maru in the waters near Japan around 35°N, and during the MR24-07 cruise in the subtropical western North Pacific. CH₄ concentrations and their δ¹³C and δ²H values were measured at various depths. Notably, this is the first study to reveal δ²H values of excess CH₄ in seawater.
Our results confirmed that many surface seawater samples exhibited CH₄ supersaturation compared to atmospheric equilibrium. The mean δ¹³C value of excess CH₄ was determined to be −40.4 ± 2.4 ‰, while the mean δ²H value of excess CH₄ was −42.3 ± 4.7 ‰, which is significantly higher than the δ²H values of typical organic matter in marine environments (δ²H = −118.23 ± 35 ‰; Lecuyer et al., 1998). Sasakawa et al. (2008) attributed the high δ¹³C values (−33 ‰) observed in excess CH₄ to oxidative degradation during its release into the oxic seawater from reducing environments within sinking particles, where CH₄ is initially produced with δ¹³C values of approximately −60 ‰ to −80 ‰. The high δ²H values observed in this study are consistent with this interpretation. Conversely, it is difficult to reconcile such high δ²H values with CH₄ produced by phytoplankton. Our findings, supported by δ²H data, suggest that excess CH₄ in surface seawater is more likely derived from micro-reducing environments within sinking particles.