Effect of fluid-sediment reaction on hydrothermal fluxes of major elements, eastern flank of the Juan de Fuca Ridge

• Published in: Geochimica et cosmochimica acta 2002-05, Vol.66 (10), p.1739-1757
• Main Authors: Giambalvo, Emily R., Steefel, Carl I., Fisher, Andrew T., Rosenberg, Nina D., Wheat, C.Geoffrey
• Format: Article
• Language: English
• Subjects: Marine
• ISSN: 0016-7037
• DOI: 10.1016/S0016-7037(01)00878-X

On the eastern flank of the Juan de Fuca Ridge, reaction between upwelling basement fluid and sediment alters hydrothermal fluxes of Ca, SiO sub(2(aq)), SO sub(4), PO sub(4), NH sub(4), 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 SO sub(4) 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 NH sub(4), SiO sub(2(aq)), and Ca, and a sink of SO sub(4), PO sub(4), and alkalinity. Reaction within the sediment column increases the hydrothermal sources of NH sub(4) and SiO sub(2(aq)), increases the hydrothermal sinks of SO sub(4) and PO sub(4), 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 SO sub(4) and NH sub(4) at slow flow rates and intermediate sediment thickness and to fluxes of SiO sub(2(aq)) at slow flow rates and large sediment thickness. Reaction within the sediment section could approximately double the hydrothermal sink of PO sub(4) over a range of flow rates and sediment thickness, and could slightly decrease (by less than or equal to 10%) the size of the hydrothermal source of Ca.