Hydrogeology of sea-floor cooling

Much of the Earth’s internal heat production escapes from the ocean floor, by a combination of direct cooling of new lavas at ridges, hydrothermal pumping of seawater through oceanic crust and conduction.  These processes are responsible for the increase in density of oceanic lithosphere that causes the ocean floor to gradually deepen away from spreading axes, thereby adding a gravitational force (ridge-slide force) to help drive plate tectonics.  The cooling also ensures that oceanic lithosphere is sufficiently cool at destructive margins for metamorphic processes in subduction zones to increase its density above that of the mantle, thereby largely driving plate tectonics through slab-pull force.  More than 70% of internal heat loss through the oceans is dissipated through crust that is younger than 1 Ma.  Much of that emanates from huge hydrothermal geysers, about which a great deal has been revealed in recent years.  What of the other 30% that escapes through older crust?  The older it is, the more it is literally blanketed by sediments that should act to block circulation of seawater, because they are so fine grained and impermeable.  It might seem as if heat lost would have to be by conduction alone.  That is not sufficient to explain the shape of the ocean basins.  However, some recent work near the Juan de Fuca Ridge in the NE Pacific by a team from the USA, Canada and Germany (Fisher, A.T. and 12 others 2003.  Hydrothermal recharge and discharge across 50 km guided by seamounts on a young ride flank.  Nature, v. 421, p. 618-621) shows that basic principles of hydrogeology guide seawater to increase heat loss.  Outflow is not through the sedimentary cover, but through seamounts, which are outcrops of the underlying igneous part of the crust.  Like many springs on land, the water that flows from them can come from far afield.  The sedimentary cover acts as an aquiclude, making the crystalline crust a confined aquifer, but for any flow to operate water must infiltrate the ocean floor.  Fisher and colleagues have found that some seamounts have higher heat flow than others, and are sites of outflowing warm water.  Some have anomalously low heat flow and may well be sites where seawater is infiltrating.  Dating outflowing water using 14C reveals that it is very young, and must have flowed rapidly, yet in their study area there are no signs of significant recharge through the sediments.  One seamount, 50 km from another which discharges water is the only likely source.  So, it seems as if the distribution and number of sea mounts on the oceanic part of a plate might bear greatly on the processes that eventually take place when the plate is subducted.  “Pimply” plates could have cooled more than smooth plates with an unbroken blanket of inefficiently conductive sediments.

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