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

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2019-11, Vol.20 (11), p.4922-4938
• Main Authors: Hulme, Samuel M., Wheat, C. Geoffrey
• Format: Article
• Language: English
• Subjects: Advection; Ammonia; Basalt; Biogeochemical cycle; Biogeochemical cycles; Biogeochemistry; Chemical analysis; Chemical composition; Core sampling; Deep sea drilling; diagenesis; Drilling; Earth; Fluctuations; Fluids; Gravity; Hydrologic processes; hydrothermal; Iron; Juan de Fuca; Manganese; Minerals; ocean drilling; Ocean floor; Oceanic crust; Oceans; Outcrops; Pore water; ridge flank; Seawater; Sediment; Sediment samples; Sediments; Solutes; Submersibles; Sulfates; Temperature; Tracers; Water analysis; water‐rock reactions; Zinc
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2019GC008408

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. Plain Language Summary Seawater circulates below the seafloor and through the basaltic crust and discharges at the seafloor through rock outcrops that are not covered with sediment. As this seawater circulates within the crust, it warms and is chemically altered. On a global scale this discharge redistributes 25% of the heat lost from the Earth and affects biogeochemical cycles in the ocean. Potential mechanisms for changing the composition of the circulating fluid include reactions with basaltic minerals, microbial metabolic processes, and diffusive exchange with pore waters in the overlying sediment. While these mechanisms are commonly invoked, there has not been a comprehensive study to quantify the contributions of these various sources and sinks in changing crustal fluid chemistry, and by extension, its impact on ocean chemistry. This study takes advantage of a diverse and unique dataset along a known flow path in oceanic crust. Our analysis hi