Major cation/chlorine ratio and stable chlorine isotopic compositions of sediment interstitial water in the Brazos-Trinity Basin IV from the Gulf of Mexico (IODP 308)

Abstract

Interstitial water from the Integrated Ocean Drilling Program Expedition 308 (IODP 308), drilling in the Brazos-Trinity Basin IV on the slope of Gulf of Mexico, were analyzed for major anion and cation concentrations and δ37Cl isotopic compositions. In the Brazos-Trinity Basin IV, interstitial water samples from the Hole U1319 show chlorine content mostly centering around 560mM, and δ37Cl values from −0.81‰ to +0.71‰ with an average of −0.18‰, which are close to the seawater values (0‰). Integrated with the Na/Cl and Br/Cl ratios, the low δ37Cl of interstitial water show no relationship with the dissolution of halite or addition of relict brine. The concentrations of K+, Na+, Ca2+ and Mg2+ in the interstitial water show a similar decreasing pattern at shallow depths, reaching minima at ∼13.5m, corresponding to the alkalinity maximum. Relationships between alkalinity and Na+, K+, Ca2+, Mg2+ concentrations suggest that only Ca2+ concentration being controlled by the precipitation of diagenetic carbonates, while Na+, K+ and Mg2+ concentrations can be used as a geochemical indicator to reveal the fluid processes in the sedimentary basin. Abnormity low Na/Cl, K/Cl, and Mg/Cl ratios of interstitial water show a strong link to well-known cycles of sea level change to well imaged deposits in the fill of Brazos-Trinity Basin IV, which suggests that a lateral fluid with low cation/Cl ratios was probably intruded in the sediment strata during low stand sea level period in the basin.