6/26/15, "A 'hydrothermal siphon' drives water circulation through the seafloor. New study explains previous observations of ocean water flowing through the seafloor from one seamount to another," UC Santa Cruz, Tim Stephens
About 25 percent of the heat that flows out of the Earth's interior is transferred to the oceans through this process, according to Andrew Fisher, professor of Earth and planetary sciences at UC Santa Cruz and coauthor of the study. Much of the fluid flow and heat transfer occurs through thousands of extinct underwater volcanoes (called seamounts) and other locations where porous volcanic rock is exposed at the seafloor.
Fisher led an international team of scientists that in the early 2000s discovered the first field site where this process could be tracked from fluid inflow to outflow, in the northeastern Pacific Ocean. In a 2003 paper published in Nature, Fisher and others reported that bottom seawater entered into one seamount, traveled horizontally through the crust, gaining heat and reacting with crustal rocks, then discharged into the ocean through another seamount more than 50 kilometers away.
"Ever since we discovered a place where these processes occur, we have been trying to understand what drives the fluid flow, what it looks like, and what determines the flow direction," Fisher said.
For the new study, first author Dustin Winslow, a UCSC Ph.D. candidate who graduated this month, developed the first three-dimensional computer models showing how the process works. The models reveal a 'hydrothermal siphon' driven by heat loss from deep in the Earth and the flow of cold seawater down into the crust and of warmed water up out of the crust.
"Dustin's models provide the best, most realistic view of these systems to date, opening a window into a hidden realm of water, rock, and life," Fisher said.
The models show that water tends to enter the crust ('recharge') through seamounts where fluid flow is easiest due to favorable rock properties and larger seamount size. Water tends to discharge where fluid flow is more difficult due to less favorable rock properties or smaller seamount size. This finding is consistent with field observations suggesting that smaller seamounts are favored as sites of hydrothermal discharge.
"This modeling result was surprising initially, and we had to run many simulations to convince ourselves that it made sense," Winslow said. "We also found that models set up to flow in the opposite direction would spontaneously flip so that discharge occurred through less transmissive seamounts. This seems to be fundamental to explaining how these systems are sustained."
Winslow's project was funded by the U.S. National Science Foundation through a graduate fellowship and as part of the Center for Dark Energy Biosphere Investigations (C-DEBI). UCSC is a partner in C-DEBI, which is headquartered at the University of Southern California." via Hockey Schtick
"Outcrops smaller than ~2 km in diameter, which are thought to be abundant globally but are generally undetectable with satellite gravimetric data26, may have a disproportionate influence on lithospheric heat extraction....The vast majority of sites where this process occurs remain unmapped and unexplored."...
6/26/15, "Sustainability and dynamics of outcrop-to-outcrop hydrothermal circulation," Nature Communications, Dustin M. Winslow and Andrew T. Fisher
(end of parag. 3): "These results suggest that outcrops smaller than ~2 km in diameter, which are thought to be abundant globally but are generally undetectable with satellite gravimetric data26, may have a disproportionate influence on lithospheric heat extraction. This may explain why so few sites of ridge–flank hydrothermal discharge, a global process responsible for 25% of Earth’s geothermal heat loss, have been identified to date: the vast majority of sites where this process occurs remain unmapped and unexplored."...
Conditions observed in northeastern Pacific Ocean:
"Ridge–flank hydrothermal circulation through the volcanic ocean crust is responsible for the majority of the seafloor heat-flux deficit1, drives solute fluxes between the crust and the ocean2, 3, and supports a vast and diverse crustal biosphere4, 5. Basement outcrops allow massive hydrothermal flows to bypass marine sediments that generally have much lower permeability than the underlying volcanic rocks6, 7, 8, 9. Although bare volcanic rock is common close to seafloor spreading centres, where the crust is young, widely spaced rock outcrops provide the primary pathways for hydrothermal exchange of fluid, heat and solutes between crust and the ocean on older and more heavily sedimented ridge flanks8, 10, 11, 12.
Flow between rock outcrops, which can be separated laterally by tens of kilometres, is driven by a hydrothermal siphon, where the primary impelling force is generated by the difference in density between recharging (cool) and discharging (warm) columns of crustal fluid 8, 13, 14.
However, factors controlling flow sustainability, rate and direction in these hydrothermal siphon systems have not previously been identified or explained.
Computer simulations of ridge–flank hydrothermal siphons can be used to determine the physical parameters that allow these systems to function, and how system properties influence fluid and heat transport. In comparison with earlier one and two-dimensional (2D) models of similar systems8, 12, 14, 3D simulations provide a more accurate representation of crustal and outcrop geometries, and lateral heat extraction adjacent to 3D fluid flow paths. In comparison with steady-state models, transient simulations include more realistic flow behaviours such as mixed convection and simultaneous axisymmetric and asymmetric flow around outcrops.
Here we present the first transient, 3D simulations of outcrop-to-outcrop hydrothermal siphons on the seafloor, and explore the parameter space under which a siphon is sustained.
The outcrop geometry and the range of sediment and basement properties simulated are guided by conditions observed 100 km east of the Juan de Fuca Ridge15, northeastern Pacific Ocean, where thermal, geochemical and hydrogeological field observations show that a hydrothermal siphon is presently active8, 16. Through these models, we identify key controls on system behaviour (outcrop size and permeability), and provide a mechanistic explanation as to why some outcrop-to-outcrop systems sustain hydrothermal siphons, whereas others do not. Simulations indicate that, for the geometry and range of properties tested, a significant contrast in outcrop properties is required for a hydrothermal siphon to be sustained, and that discharge is favoured through the outcrop that is more restrictive to flow. This helps to explain field observations indicating that small outcrops tend to be sites of hydrothermal discharge8, 11, 17, and suggests that small outcrops may play an especially important role in extracting lithospheric heat from the oceanic crust."...
"Hydrothermal circulation within the sea floor, through lithosphere older than one million years (Myr), is responsible for 30% of the energy released from plate cooling, and for 70% of the global heat flow anomaly."...
Letters to Nature:
2/6/2003, "Hydrothermal recharge and discharge across 50 km guided by seamounts on a young ridge flank,"Nature
"Hydrothermal circulation within the sea floor, through lithosphere older than one million years (Myr), is responsible for 30% of the energy released from plate cooling, and for 70% of the global heat flow anomaly (the difference between observed thermal output and that predicted by conductive cooling models)1, 2. Hydrothermal fluids remove significant amounts of heat from the oceanic lithosphere for plates typically up to about 65 Myr old3, 4. But in view of the relatively impermeable sediments that cover most ridge flanks5, it has been difficult to explain how these fluids transport heat from the crust to the ocean. Here we present results of swath mapping, heat flow, geochemistry and seismic surveys from the young eastern flank of the Juan de Fuca ridge, which show that isolated basement outcrops penetrating through thick sediments guide hydrothermal discharge and recharge between sites separated by more than 50 km. Our analyses reveal distinct thermal patterns at the sea floor adjacent to recharging and discharging outcrops. We find that such a circulation through basement outcrops can be sustained in a setting of pressure differences and crustal properties as reported in independent observations and modelling studies."