Origin of low-chloride fluid in sediments from the eastern continental margin of India, results from the National Gas Hydrate Program Expedition 02

Abstract

Abstract National Gas Hydrate Program Expedition 02 was conducted in 2015 by D/V Chikyu in the western Bay of Bengal, India. Analyses of interstitial water in sediment from four drilling sites along the eastern and western transect in the Krishna–Godavari Basin show that dissolved chloride (Cl−) concentrations decreased with depth from seawater values (>550 mM) near the sediment surface to 300–400 mM at 350–400 m below seafloor (mbsf), suggesting the upward advection of low-Cl– fluid. Excursions of Cl− concentrations toward low values in gas hydrate zones were attributed to the presence of fresh water released by dissociation of gas hydrate during core recovery. At Sites in the western transect, Cl− concentrations below the gas hydrate reservoir (around 280 mbsf) were ca. 60 mM and ca. 45 mM lower than those from above the reservoir, and δ18O and δD values were 1–2‰ and 0.7–1.3‰ higher than those from above the reservoir, respectively. These results suggest that a fraction of migrating low-Cl– fluid is trapped in the gas hydrate zone, causing decreases of δ18O and δD in the residual water due to isotopic fractionation during gas hydrate formation. From mass-balance calculations, we estimated that 10–20% of the advected fluid is trapped at the gas hydrate zone. At all sites, the δ18O and δD values below the gas hydrate zone, representing relatively unaltered low-Cl– fluid, were 0.5–1‰ higher and ca. 13–15‰ lower than those of seawater, respectively. This trend is consistent with water derived from the dehydration of clay minerals, which generally occurs in deeply buried sediments. The contribution of clay mineral dehydration is relatively large at the western transect compared to that in the eastern transect. Our results imply that fluid advection contributes to the accumulation of gas hydrate in the study area.