Sciency Thoughts: Understanding current flows in hydrothermal vents.

Hydrothermal vents are areas on the sea floor where water warmed by the heat of the Earth’s interior escapes into the wider ocean. These waters contain high levels of dissolved minerals and nutrients rare in the wider oceans, and therefore support unique ecosystems able to survive without any input of solar energy. The most extreme examples of hydrothermal vents are the black smokers found around volcanically active ocean ridges. These vents produce water with temperatures reaching hundreds of degrees centigrade, kept liquid only by the high pressures present on the ocean floor, and opaque due to the high levels of minerals present. However hydrothermal vents are also present on areas of the ocean floor far from the active ridges, or any other form of volcanic influence, apparently emitting water that has been transported hundreds of kilometres beneath the ocean floor, which is far harder to explain.

In a paper published in the journal Nature Communications on 26 June 2015, Dustin Winslow of the Earth and Planetary Sciences Department at the University of California Santa Cruz and Andrew Fisher of the Earth and Planetary Sciences Department and Institute for Geophysics and Planetary Physics at the University of California Santa Cruz describe the results of a computer model of water flows supplying water to hydrothermal vents on the ocean floor, based upon observations of vents around the Juan de Fuca Ridge off the northwest coast of North America, which appears to solve the mystery of water supply to distant hydrothermal vents.

Marine sediments tend to be very rich in clay minerals, and are thus effectively impermeable to the flow of water. However the volcanic rocks that lay beneath these sediments tend to be highly porous, allowing the free transport of water. Therefore areas of exposed rock can act as either entrances or exits for water moving through subsurface aquifers, which are isolated from the general ocean in areas where the volcanic bedrock is covered by sediments. Observational evidence from around the Juan de Fuca Ridge suggest that whether an area of exposed volcanic rock serves as an entrance or exit to the aquifer is driven by the size of the exposure rather than other factors, such as proximity to volcanic heating, or the temperature or current speeds of the water passing over the entrances, with hydrothermal vents (i.e. areas where water exits from the aquifers) always occurring from exposed rocks with limited areas.

Geometry and configuration of 3D domains. Domains represent a section of upper ocean crust, oriented with the long-axis parallel to the spreading ridge, consistent with conditions at a field site on the eastern flank of the Juan de Fuca Ridge. A conductive volcanic rock section (red, lower permeability) is overlain by a crustal aquifer (orange, higher permeability) and marine sediments (blue, lower permeability) and two volcanic rock outcrops penetrate through the sediment (light blue). Heat is applied to the base, following a lithospheric cooling trend. The sides and base are no-fluid flow boundaries, and the top is free flow (fluid and heat) with pressure varying as a function of seafloor depth. Winslow & Fisher (2015).

Winslow and Fisher simulated flows between outcrops of different sizes, through subsurface aquifers being heated gently but evenly and constantly from below. They initially tried modelling systems where the current was present at the start of the experiment, on the basis that establishing a system and maintaining it are not the same, but found that even where no current was present at the outset of the experiment, a current flow from the larger opening to the smaller was quickly established.

Larger areas of permeable rock exposure allow both the entrance and exit of water across their surface, whereas smaller openings allow movement in only one direction. As warmer water rises from the vent into the water column warm water exiting from the vent quickly comes to dominate the flow of water at smaller vents, particularly if these are raised above the surrounding area (i.e. sticking up through the mud). This in turn leads to a draw on the waters of the aquifer, effectively pulling water through from the larger areas of exposure.

Simulation results at dynamic steady state. This simulation, showing one quarter of the domain illustrated in the top figure, has one large outcrop and one small outcrop. Domain colours show domain temperatures, including influence of rolling/mixed convection in basement aquifer and thermal influence of recharging/discharging outcrops. Inset diagrams show fluid flow vectors within and around outcrops (length indicates flow rate), with vectors plotted on a natural-log scale, the longest vector (exiting the top of the discharging outcrop) corresponding to a flow rate of 14m per year. Fluid flow through the sediment is so slow that it would generate no detectable thermal or geochemical anomalies. Vertical exaggeration (VE) of main image is times three; VE of inset images is times two. Winslow & Fisher (2015).

Hydrothermal vents are thought to account for about 25% of heat loss from the Earth’s interior, which if correct suggests that a very large amount of water is exiting from such vents every day. Despite this such vents are notoriously hard to find, with efforts to locate them with satellites having been largely inconclusive, and most known vents having been discovered by exploratory missions involving deep-sea submersibles. Winslow and Fisher’s findings suggest that a preference for such vents occurring on smaller outcrops is a genuine natural phenomenon, rather than a product of the observation methods being used. This would explain the difficulty in finding such vents using satellites, as very small discrete vents, no matter how numerous, would fall below the area/size at which it was possible to detect them using remote sensing technology.

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Sunday, 28 June 2015

Understanding current flows in hydrothermal vents.