(Table 2) Composition of the deapest pore fluids from ODP Holes 168-1030B and 168-1031A

On the eastern flank of the Juan de Fuca Ridge, reaction between upwelling basement fluid and sediment alters hydrothermal fluxes of Ca, SiO2(aq), SO4, PO4, NH4, and alkalinity. We used the Global Implicit Multicomponent Reactive Transport (GIMRT) code to model the processes occurring in the sediment column (diagenesis, sediment burial, fluid advection, and multicomponent diffusion) and to estimate net seafloor fluxes of solutes. Within the sediment section, the reactions controlling the concentrations of the solutes listed above are organic matter degradation via SO4 reduction, dissolution of amorphous silica, reductive dissolution of amorphous Fe(III)-(hydr)oxide, and precipitation of calcite, carbonate fluorapatite, and amorphous Fe(II)-sulfide. Rates of specific discharge estimated from pore-water Mg profiles are 2 to 3 mm/yr. At this site the basement hydrothermal system is a source of NH4, SiO2(aq), and Ca, and a sink of SO4, PO4, and alkalinity. Reaction within the sediment column increases the hydrothermal sources of NH4 and SiO2(aq), increases the hydrothermal sinks of SO4 and PO4, and decreases the hydrothermal source of Ca. Reaction within the sediment column has a spatially variable effect on the hydrothermal flux of alkalinity. Because the model we used was capable of simulating the observed pore-water chemistry by using mechanistic descriptions of the biogeochemical processes occurring in the sediment column, it could be used to examine the physical controls on hydrothermal fluxes of solutes in this setting. Two series of simulations in which we varied fluid flow rate (1 to 100 mm/yr) and sediment thickness (10 to 100 m) predict that given the reactions modeled in this study, the sediment section will contribute most significantly to fluxes of SO4 and NH4 at slow flow rates and intermediate sediment thickness and to fluxes of SiO2(aq) at slow flow rates and large sediment thickness. Reaction within the sediment section could approximately double the hydrothermal sink of PO4 over a range of flow rates and sediment thickness, and could slightly decrease (by

Grain counts and XRD mineralogy of turbidite sand and hemipelagic mud from ODP Leg 168 sites

Sequences of late Pliocene to Holocene sediment lap onto juvenile igneous crust within 20 km of the Juan de Fuca Ridge in northwestern Cascadia Basin, Pacific Ocean. The detrital modes of turbidite sands do not vary significantly within or among sites drilled during Leg 168 of the Ocean Drilling Program. Average values of total quartz, total feldspar, and unstable lithic fragments are Q = 35, F = 35, and L = 30. Average values of monocrystalline quartz, plagioclase, and K-feldspar are Qm = 46, P = 49, and K = 5, and the average detrital modes of polycrystalline quartz, volcanic-rock fragments, and sedimentary-rock plus metamorphic-rock fragments are Qp = 16, Lv = 43, and Lsm = 41. Likely source areas include the Olympic Peninsula and Vancouver Island; sediment transport was focused primarily through the Strait of Juan de Fuca, Juan de Fuca Channel, Vancouver Valley, and Nitinat Valley. Relative abundance of clay minerals (<2-µm-size fraction) fluctuate erratically with depth, stratigraphic age, and sediment type (mud vs. turbidite matrix). Mineral abundance in mud samples are 0%-35% smectite (mean = 8%), 18%-59% illite (mean = 40%), and 29%-78% chlorite + kaolinite (mean = 52%). We attribute the relatively low content of smectite to rapid mechanical weathering of polymictic source terrains, with little or no input of volcanic detritus from the Columbia River. The scatter in clay mineralogy probably was caused by converging of surface currents, turbidity currents, and near-bottom nepheloid clouds from several directions, as well as subtle changes in glacial vs. interglacial weathering products.