[M-IS20] Interactions of Geosphere-Hydrosphere-Biosphere and Deep-sea Methane Environments

[MIS20-P02] Halogen geochemistry of interstitial water in a gas hydrate field, Amazon Fan offshore Brazil

*Satoko Owari¹, Marcelo Ketzer², Daniel Praeg³, Sebastian Migeon³, Alexis Gilbert⁴, Frederico Rodrigues⁹, Jean Arias⁵, Swanne Gontharet⁶, Cleverson Silva⁷, Tadeu dos Reis⁸ (1.Tokyo University of Marine Science and Technology, 2.Linnaeus University, 3. Géoazur, 4.Utrecht University, 5.Federal University of Rio Grande, 6.Sorbonne University, 7.Universidade Federal Fluminense, 8.Universidade do Estado do Rio do Janeiro , 9.Universidade do Vale dos Sinos, Unisinos)

Keywords:Interstitial water, Amazon Fan, Gas hydrate, Halogens

The Amazon fan offshore Brazil receives a large amount of terrestrial sediment from the Amazon River, which is known as the largest supplier of organic-rich sediment to the ocean. A high methane flux within the fan has resulted in an upper slope gas hydrate system indicated by a discontinuous bottom-simulating reflection (BSR) and associated with seafloor gas venting both within and above the gas hydrate stability zone. The AMAGAS-AMARYLLIS I campaign of the R/V Marion Dufresne in May- June 2023 recovered cores of sediment above a BSR patch, of which we here study cores MD23-3616, -3625, -3635, and -3641. Three of these cores targeted sites of gas venting: water column gas flares had been observed at the site of MD23-3616 during a campaign in 2016 but had ceased in 2023, while flares were observed at sites MD23-3625 and -3641 during the 2023 campaign; core MD23-3641 also contained gas hydrates. Site MD23-3635 is background site not associated with gas venting. Interstitial water was extracted from cored sediments aboard ship using rhizon samplers, and the amounts of dissolved SO₄, and halogens (Cl, I, and Br) were measured in Japan.
Chloride is a conservative ion; therefore, the concentration is altered by input of water molecules or consumption from interstitial water, such as gas hydrate formation/dissociation. Two venting sites (3616, 3625) and one venting-free site showed approximately the same Cl concentration with standard seawater (Cl of IAPSO: 560 mM). Cl concentration at Site 3641 (venting site) showed approximately 80 mM lower than the IAPSO Cl value near the depth of gas hydrate bearing sediments. This is interpreted to indicate freshening of interstitial water by gas hydrate decomposition. I and Br are used as tracers for methane and organic matter in gas hydrate bearing sediments because they have an affinity for organic matter. Two venting sites (3616, 3625) showed a high iodine gradient (4.9 - 5.0 µmol/m³m); in contrast, the venting-free site (3635) showed gradients up to 5 times lower (1 µmol/m³m). I and Br at Site 3641 showed extremely low concentrations, suggesting that they were strongly diluted by decomposition water and in situ concentration was altered after recovery. The iodine profiles thus indicate significant differences between gas venting sites and the gas venting-free site, although Br did not show clear differences among the sites. Our results provide new information on the halogen geochemistry of interstitial water in areas of methane venting in gas hydrate settings and the determinants of methane cycling in sediments in the Amazon fan.