[M-IS22] Interactions of Geosphere-Hydrosphere-Biosphere and Deep-sea Methane Environments

[MIS22-04] Deep fluid input into gas hydrate areas of the Japan Sea: Insights from Li isotope composition of sedimentary pore fluids

*Yusuke Miyajima¹, Yasumasa Sakai², Akira Ijiri², Kyoko Yamaoka¹, Daisuke Araoka¹, REO IKAWA¹, Hideyoshi Yoshioka¹ (1.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 2.Kobe University)

Keywords:Japan Sea, Lithium isotopes, Methane hydrate, Oxygen isotopes, Pore fluid

Fluid flow originating from clay mineral dehydration and oceanic crust/mantle wedge alteration is common in gas hydrate areas in fore-arc and strike-slip settings (e.g., Scholz et al., 2009; Nishio et al., 2015). However, the role of fluid migration in gas hydrate formation in back-arc basins is poorly understood. To test deep fluid input and reveal its source in gas hydrate areas of Joetsu and Sakata, northeastern Japan Sea, we examined the concentration and stable isotopic composition of lithium (Li) in pore fluids of deep subseafloor sediments. Six sediment cores were collected from hydrate-bearing and reference sites at water depths between ~530 and 980 m using the drilling vessels Poseidon-1 and Chikyu. Pore fluids were extracted from the core sediments at 1 to 140 m below seafloor (bsf) and analyzed for element concentrations and Li isotope ratios after chromatographic separation. At the reference sites, the pore fluid Li concentration decreased from a seawater-like value (22 μM) to 14 μM in the uppermost sediments at 1–9 mbsf and then increased downcore to 48 μM in maximum, suggesting the presence of Li-rich fluids in the deep sediments. The Li concentration at the hydrate sites was higher than those at the reference sites and showed a clear increase downcore to 78 μM in maximum. At both reference and hydrate sites, the Li isotope ratio, δ⁷Li value, of the pore fluids ranges between 28.7‰ and 19.0‰. The δ⁷Li value showed a clear decrease in the upper 30 and 60 m sediments at Joetsu and Sakata, respectively, and remained almost constant downward. The depth profiles of the Li concentration and δ⁷Li value suggest the advection of ⁶Li-rich fluids regardless of the hydrate occurrence. Positive correlations between the Cl/Li ratio and δ⁷Li value indicate the two-component mixing of seawater-like pore fluids with Cl-depleted deep fluids. Extrapolating the linear regressions between the Cl/Li ratio and δ⁷Li value of the seawater and pore fluids at the hydrate sites, we calculated endmember δ⁷Li values for the deep fluids as 7.2–16.1‰. Assuming the Li isotope fractionation between bulk sediments (0‰) and the deep fluids, the endmembers could be produced at ~40–180°C (Hindshaw et al., 2019), corresponding to ~1000–3000 mbsf. A similar deep Li endmember was also estimated in a previous study using pore fluids from shorter cores and hydrate-dissociating water at Joetsu (Nishio et al., 2009). Except for the Joetsu hydrate site, the δ⁷Li values show positive correlations with the δ¹⁸O values, which cannot be explained by clay mineral dehydration. The Li endmember could originate from the alteration of the Miocene to Pliocene sediments or the leaching from deeper igneous rocks.
This study was conducted as a part of the methane hydrate research project funded by the Ministry of Economy, Trade and Industry (METI), Japan.