Origin of Deep Methane from Active Faults along the Itoigawa–Shizuoka Tectonic Line between the Eurasian and North American Plates: ¹³C/¹²C and ¹⁴C/¹²C Methane Profiles from a Pull-Apart Basin at Lake Suwa

Stable and radiocarbon isotope analyses provide valuable insights into carbon dynamics in hydrosphere ecosystems. A lake containing seeps of deep methane (CH4) may provide an ideal environment for elucidating such dynamics, including the involvement of deep carbon in the surface ecosystem and its food web. In this study, we performed carbon isotope analysis of seep gas, lake water, and a cyanobacterial bloom sample collected at Lake Suwa, a pull-apart basin on the Itoigawa–Shizuoka tectonic line, Japan. CH4 collected from two seep sites 100 m from the lakeshore was 14C-depleted (Δ14C = −972.1 to −989.8‰), indicating that the origins of seep CH4 and benthic bubble CH4 (Δ14C = +21.7‰) collected near the lakeshore differed. The results suggested that benthic bubble CH4 was produced by methanogenic archaea in benthic sediments and that seep CH4 was produced by methanogenic archaea using relic carbon. The Δ14C values of dissolved inorganic carbon (DIC) in surface lake water collected at seep sites (Δ14C = −103.1 to −630.6‰) and near the lakeshore (Δ14C = −100.9‰) suggested that the influence of deep carbon occurred not only around the seep sites but also at the lakeshore. The Δ14C values of the cyanobacterial bloom sample also showed values similar to those of DIC (Δ14C = −110.3‰), suggesting that 10–63% of DIC in surface water and 10% in primary producers originated from relic carbon, including CH4 and CO2, and was incorporated into the surface ecosystem of Lake Suwa.