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We estimate the rate of upward pore fluid flow and chlorinity (Cl−) concentrations at depth through a joint analysis of Cl− concentration and temperature versus depth profiles obtained from gas hydrate related test sites drilled and cored in the Krishna-Godavari Basin off the eastern coast of India. Cl−, measured on conventional core samples, decreases with depth at all sites but some of the Cl−profiles show a prominent convex shape, whereas in situ temperature profiles, obtained by the Advanced Piston Coring Temperature probe, are mostly linear at all sites established during the National Gas Hydrate Program 02 Expedition (NGHP-02). Assuming a one-dimensional, time-dependent model for the advection of pore fluid including the sedimentation effect and depth-dependent diffusivity, we estimate darcy velocity and the Cl− concentrations at depth. The best-fit darcy velocity of 1.2–1.6 × 10−11 m/s was estimated for the sites along the crest of the regional anticlinal structure in the NGHP-02 Area B, which was significantly faster than those on the flanks of the anticline. Because the thermal diffusion coefficient is much larger than the chloride ion diffusion coefficient, estimated darcy velocities are not great enough to generate nonlinear temperature profiles with depth, which is consistent with observed linear thermal profiles. • We estimate pore fluid flow rate through analysis of Cl− & T profiles in hydrate-bearing sediments of Krishna-Godavari Basin. • Some Cl− profiles show convex shape, whereas temperatures are mostly linear, constraining pore fluid flow rate. • Maximum flow rate is 1.2~1.6 × 10−11 m/s on anticlines in Area B, significantly faster than in flank or in other areas. [ABSTRACT FROM AUTHOR]

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. • The decrease of Cl– concentration with depth is attributed to upward advection of low-Cl– fluid derived from clay mineral dehydration. • 10–20% of migrating fluid is trapped in the thick gas hydrate zone. • The excursions in Cl– concentrations in the gas hydrate zone are attribute to the dissociation of gas hydrate during core recovery. [ABSTRACT FROM AUTHOR]

Detailed reconstruction of Indian summer monsoons is necessary to better understand the late Quaternary climate history of the Bay of Bengal and Indian peninsula. We established a chronostratigraphy for a sediment core from Hole 19B in the western Bay of Bengal, extending to approximately 80 kyr BP and examined major and trace element compositions and clay mineral components of the sediments. Higher δ18O values, lower TiO2 contents, and weaker weathering in the sediment source area during marine isotope stages (MIS) 2 and 4 compared to MIS 1, 3, and 5 are explained by increased Indian summer monsoonal precipitation and river discharge around the western Bay of Bengal. Clay mineral and chemical components indicate a felsic sediment source, suggesting the Precambrian gneissic complex of the eastern Indian peninsula as the dominant sediment source at this site since 80 kyr. Trace element ratios (Cr/Th, Th/Sc, Th/Co, La/Cr, and Eu/Eu*) indicate increased sediment contributions from mafic rocks during MIS 2 and 4. We interpret these results as reflecting the changing influences of the eastern and western branches of the Indian summer monsoon and a greater decrease in rainfall in the eastern and northeastern parts of the Indian peninsula than in the western part during MIS 2 and 4. Key Points: The sedimentation rate was estimated for a sediment core from Hole 19B in the western Bay of Bengal, extending to approximately 80 kyr BPIndian monsoonal fluctuations on glacial‐interglacial time scale implied from chemical and mineral propertiesTrace element ratios reflect changing in influences of the eastern and western branches of the Indian summer monsoon [ABSTRACT FROM AUTHOR]