Boron concentration and isotopic constraints on processes affecting the chemistry of interstitial water in normal- and over-pressured basins, Gulf of Mexico

The Integrated Ocean Drilling Program Expedition 308 (IODP308) drilled normal-pressured sediments from the Brazos–Trinity Basin IV and over-pressured sediments from the Ursa Basin on the northern slope of the Gulf of Mexico. The interstitial water samples from the normal-pressured basin show B concentrations and B isotopic compositions ranging from 255 to 631 μM (0.6 to 1.5 times of seawater value) and from + 29.1 to + 42.7‰ (relative to NIST SRM 951), respectively. A wider range is observed both for B concentrations (292 to 865 μM, 0.7 to 2.1 times of seawater value) and δ11B values (+ 25.5 to + 43.2‰) of the interstitial water in the over-pressured basin. The down-core distribution of B concentrations and δ11B values in the interstitial waters are sensitive tracers for assessing various processes occurring in the sediment column, including boron adsorption/desorption reactions involving clay minerals and organic matter in sediments as well as fluid migration and mixing in certain horizons and in the sediment column. In the normal-pressured basin adsorption/desorption reactions in shallow sediments play the major role in controlling the B content and B isotopic composition of the interstitial water. In contrast, multiple processes affect the B content and δ11B of the interstitial water in the over-pressured Ursa Basin. There, the stratigraphic level of the maxima of B and δ11B correspond to seismic reflectors. The intruded fluids along the seismic reflector boundary from high to low-topography mix with local interstitial water. Fluid flow is inferred in the Blue Unit (a coarse sandstone layer, connecting the high- to low-pressured region) from the freshening of interstitial water in Ursa Basin Site U1322, and upward flow by the overpressure expels fluid from the overburden above the Blue Unit.