Observations and models of lateral hydrothermal circulation on a young ridge flank: Numerical evaluation of thermal and chemical constraints

Abstract

[1] We used a two-dimensional coupled heat and fluid flow model to investigate large-scale, lateral heat and fluid transport on the eastern flank of the Juan de Fuca Ridge. Cool seawater in the natural system is inferred to enter basement where it is exposed close to the spreading center and flow laterally beneath thick sediments, causing seafloor heat flow to be depressed relative to that input at the base of the plate. The flow rate, and thus the properties of permeable basement (the flow layer), controls the efficiency of lateral heat transport, as quantified through numerical modeling. We simulated forced flow in this layer by pumping water through at a fixed rate and quantified relations between flow rate, thickness of the permeable basement, and the extent of suppression of seafloor heat flow. Free flow simulations, in which fluid flow was not forced, match heat flow constraints if nonhydrostatic initial conditions are used and flow layer permeabilities are set to the high end of observed values (10 À11 to 10 À9 m 2 ). To match seafloor heat flow observations, the models required lateral specific discharge of 1.2 to 40 m/yr for flow layer thicknesses of 600 to 100 m, respectively. The models also replicate differences in fluid pressures in basement, and the local distribution of pressures above and below hydrostatic. Estimated lateral flow rates are 10Â to 1000Â greater than estimates based on radiocarbon ages of basement pore waters. Estimated lateral flow rates based on thermal and chemical constraints can be reconciled if diffusion from discrete flow zones into less permeable stagnant zones in the crust is considered.

Carbon dioxide sequestration in deep-sea basalt

“…Surface heat flow surveys identify a recharge site with depressed isotherms at a distant seamount on the Juan de Fuca plate, which implies a lateral bulk permeability of 10 Ϫ10 to 10 Ϫ12 m 2 in the basement along that flow path (37). Based on coupled thermal modeling, high-permeability (Ϸ10 Ϫ9 to 10 Ϫ10 m 2 ) channels with lateral flow rates of up to 40 m/year match the observed thermal and flow regimes and require just one-sixth of the ocean crust to be permeable to 600 m thickness (35,38). Other drilling results have observed active 1-to 10-m-thick fluid aquifers within basalt basement, persisting to 600 m thickness (39,40).…”

“…Bulk permeability estimates in the shallow basement range from derived from pore water chemistry (35). Recent cross-hole flow experiments in basalt basement estimate lateral bulk permeability to be 10 Ϫ12 m 2 , an order of magnitude lower than boreholescale packer tests, which has been attributed to permeability anisotropy within the upper basement (36).…”

Expedition 301 synthesis: hydrogeologic studies

“…The second explanation is inconsistent with the lack of a spatial trend in heat flow values ≥5 km from outcrops and buried basement highs. Where advection has been inferred to explain heat flow suppression on the order of 15%-20% in other locations, there is generally lower heat flow near areas of recharge (e.g., Langseth and Herman, 1981;Stein and Fisher, 2003). The low values adjacent to Grizzly Bare outcrop extend only a few kilometers from the outcrop, and background heat flow away from this feature is identical to that away from Baby Bare and other outcrops to the north (Zühlsdorff et al).…”

“…It is not possible for large volumes of fluid to recharge the crust, flow laterally across tens of kilometers, and then be stored indefinitely. In fact, discharge is required as a complement to recharge in order for a flow system such as this to be self-sustaining; it is the difference between pressures at the base of recharging and discharging columns of crustal fluid that creates a "hydrothermal siphon" capable of operating at a crustal scale (e.g., Stein and Fisher, 2003;.…”

Tracing the life history of a marginal sea—On “The South China Sea Deep” Research Program

“…As discovered by ocean drillings, the microbial metabolism is extremely slow in the "deep biosphere" [141,142], and the water circulation is extremely sluggish in the "subseafloor ocean" [143]. In result, the deep-sea carbon cycling, controlled by microbes and DOC, is by several orders of magnitude lower than in the upper ocean, and therefore remains a common loophole in all carbon cycling models of current studies on global change.…”