Subseafloor Fluid and Chemical Fluxes Along a Buried-Basement Ridge on the Eastern Flank of the Juan de Fuca Ridge

Abstract

Hydrothermal circulation of low-temperature fluids within oceanic crust affects global biogeochemical cycles. We present data from a warm temperature (64 °C) hydrothermal system on the eastern flank of the Juan de Fuca Ridge to assess whether chemical fluxes to the ocean from these systems arise from the basaltic crust or the overlying sediment pore waters. Extensive sampling of this system included fluid chemical data from deep sea drilling, gravity coring, and submersible operations from five sites on a buried ridge that is parallel to the spreading center to the west. These data were subjected to a transport (advection-diffusion) model to constrain chemical and fluid fluxes along this five-site transect. Solutes (K, Cl, sulfate, Ba, Sr, Cs, Mo, and Y) that are nonreactive within the basaltic crust constrain the volumetric fluid flux per unit width within the basaltic crust from 0.05 to 0.2 m3 ·year−1 · cm−1, consistent with a recent tracer study. Using this average fluid flux, reactive fluxes were determined for twenty-four solutes and partitioned among seawater, sediment and basaltic sources and sinks. Only Ca, Ce, and Gd were released from basaltic basement to the ocean, whereas other solutes (sulfate, Mg, K, Li, Rb, Cd, U, Y, Yb, Gd, and La) were consumed in the basaltic crust, and still others (Cl, Ba, Sr, Cs, Mo, Mn, Fe, Co, NH3, and Zn) had a sediment origin with a net flux to the ocean. Diffusive exchange with the overlying sediment had a greater impact than seawater-basalt reactions for some solutes.