Ocean Drilling Program, United States

We report results from shipboard analyses and postcruise studies on gas and interstitial pore waters from Ocean Drilling Program (ODP) Sites 1150 and 1151, both penetrating approximately 1200 m of subseafloor sediment from the upper Japan Trench forearc. Apart from concentrations of hydrocarbons in the gas phase, pore waters were analyzed for various element concentrations and stable isotopic composition of Cl and B. Both drill sites are characterized by two opposing trends in the downhole element profiles in the pore waters. A strong depletion of Na, K, Cl, and salinity is contrasted by an equally profound increase in specific mobile elements such B, Li, or Sr with depth. For example, although chlorinity drops to about one-half the seawater (SW) value, the same fluid shows a approximately 10-fold enrichment of boron and approximately 40-fold increase in Li (relative to SW). The strong variations in concentration occur in the deep, tectonically deformed part of the two drill sites, which show abundant fractures and two shear zones (the latter at Site 1150). The heavily fractured portions are further characterized by low 37Cl values (down to -1.1) and high 11B values ( approximately 40-46). In the gas phase, the fractured intervals show significant increases in hydrocarbon contents and C1/C2 ratios. The profound excursions in gas and water profiles can be best explained by deep-seated processes. These may include clay mineral diagenesis, alteration of tephra from the Japan and Izu arcs, transformation of biogenic silica from abundant diatoms, and maturation of organic matter. The strong enrichment of some mobile elements (e.g., Sr, B, and Li) attests that enhanced fluid flow through permeable shear zones in the Japan Trench forearc (most prominently at Site 1150) may be an efficient mechanism to return those elements from the deep forearc back into the ocean.