Seamounts Enhance the Global Influence of Ridge-flank Hydrothermal Circulation First Workshop Seamount Biogeosciences Network Scripps Institution of Oceanography 24 -25 March 2006 A. T. Fisher University of California, Santa Cruz Earth Sciences Department and Institute for Geophysics and Planetary Physics
Retro. Flux (2000): Acknowledgements: E. Davis, C. G. Wheat, M. Mottl, K. Becker, M. Hutnak, R. Macdonald, A. Cherkaoui, L. Christiansen, M. Edwards Image. Flux (2000): V. Spiess, L. Zühlsdorff, H. Villinger Tico. Flux I and II (2001 -02): R. Harris, C. Stein, E. Silver, K. Wang, C. G. Wheat, M. Hutnak, A. Cherkaoui, M. Pfender, G. Spinelli, M. Underwood IODP 301 (2004), Thompson/ROPOS (2004), Atlantis/Alvin (2005): T. Urabe, A. Klaus, A. Bartetzko, K. Becker, R. Coggon, M. Dumont, B. Engelen, S. Goto, V. Heuer, S. Hulme, M. Hutnak, F. Inagaki, G. Iturrino, S. Kiyokawa, M. Lever, S. Nakagawa, M. Nielsen, T. Noguchi, W. Sager, M. Sakaguchi, B. Steinsbu, T. Tsuji, C. G. Wheat, J. Alt, W. Bach, J. Baross, J. Cowen, S. D’Hondt, E. E. Davis, D. Kadko, M. Mc. Carthy, J. S. Mc. Clain, M. J. Mottl, M. Sinha, G. Spinelli, V. Spiess, R. Stephen, D. Teagle, H. Villinger, L. Zühlsdorff, R. Meldrum Funding agencies/sponsors:
Selected references • Hutnak, M. , A. T. Fisher, Zühlsdorff, et al. , Hydrothermal recharge and discharge guided by basement outcrops on 0. 2 -3. 6 Ma seafloor east of the Juan de Fuca Ridge: observations and numerical models, Geochem. , Geophys. , Geosystems, in press, 2006. • Hutnak, M. , A. T. Fisher, C. A. Stein, et al. , The thermal state of 18 -24 Ma upper lithosphere subducting below the Nicoya Peninsula, northern Costa Rica margin, in Interplate Subduction Zone Seismogenesis, edited by T. Dixon, C. Moore, Columbia University Press, New York, in press, 2006. • Fisher, A. T. , Marine hydrogeology: future prospects for major advances, Hydrogeol. J. , 13: 69 -97, DOI: 10. 1007/s 10040 -004 -0400 -y, 2005. • Harris, R. N. , Fisher, A. T. , Chapman, D. , Seamounts induce large fluid fluxes, Geology, 32 (8), 725 -728, doi: 10. 1130/G 20387. 1, 2004. • Fisher, A. T. , E. E. Davis, Hutnak, et al. , Hydrothermal circulation across 50 km on a young ridge flank: the role of seamounts in guiding recharge and discharge at a crustal scale, Nature, 421: 618 -621, 2003. • Fisher, A. T. , Stein, C. A. , Harris, et al. , Abrupt thermal transition reveals hydrothermal boundary and role of seamounts within the Cocos Plate, Geophys. Res. Lett. , 30 (11), 1550, doi: 10. 1029/2002 GL 016766, 2003. Find copies of these and related papers at http: //es. ucsc. edu/~afisher
Most of the seafloor is hydrogeologically active… modified from Fisher (2005)
Seafloor hydrogeology influences. . . …the physical state and evolution of the crust and mantle, including volatile cycling at subduction zones; …the chemical evolution of the oceans; …heat loss and thermal evolution of Earth; and …development and evolution of remarkable biological communities, both on and within the crust. Focus of this presentation is: seafloor hydrothermal circulation
Seafloor hydrothermal circulation is… …the passage of warm (or hot) water through rock of the oceanic crust; …generally a result of heating from below, although it can also result immediately adjacent to newly-erupted magma; …largely responsible for the presence of about 1/2 of the elements that make the ocean "salty"; …thought likely to have occurred very early in Earth history - and may occur on other planetary bodies in our solar system. This presentation will focus on largescale, ridge-flank systems (high-temperature flows, single-seamount circulation systems are also important)
How BIG is the ocean crustal hydrothermal fluid reservoir? (based on geometrical considerations) Reservoir Volume in storage (km 3) Percent of total Oceans 1. 4 billion 97. 2 Glaciers, ice sheets 30 million 2. 1 Ocean crust 20 -30 million 1 -2 Groundwater (continental) 9 million 0. 6 Rivers, lakes 100 thousand 0. 009 Soil water 70 thousand 0. 005 Atmosphere 10 thousand 0. 001
How BIG is the global hydrothermal fluid flux? (based on heat flux considerations) Location/kind Volume flux (km 3/yr) Rain+snow on land+sea 400, 000 Evaporation+transpiration 400, 000 River discharge 40, 000 Ridge-flank (>1 Ma) 2, 000 -20, 000 Groundwater discharge 6000 Glacial melting/freezing 6000 Ridge-axis (>1 Ma) 40 Large enough to "recycle" the ocean every 100 k-500 k yrs
What does the oceanic crust look like? A very permeable aquifer…
Ridge flank hydrothermal systems are subtle… …fluid temperatures are very low (mean ~5 -20°C), so systems are hard to detect; …driving force is heat rising slowly from deep inside the Earth - generally not directly related to volcanic activity; …result in much larger fluid flows than on the ridge axis, chemical impact less well understood; …may help to support vast, subseafloor ecosystems.
What is the extent of the subseafloor biosphere? • Studies of cell densities in marine sediments suggest a roughly log-depth distribution • Extrapolation globally suggests an enormous biomass - highly speculative • What about basement microbiology? modified from Parkes et al. (1994), D’Hondt et al. (2003); Fisher et al. (2005)
Two inferences and two questions… • Global heat flow anomaly requires enormous fluid fluxes (alters crust, results in large solute fluxes, may influence subseafloor biosphere, etc. ); • Vast majority of flow occurs on ridge flanks, far from spreading centers, at relatively low temperatures… …how does water enter and exit oceanic basement once thick sediments accumulate across vast distances? …what provides the necessary driving force(s) to move huge volumes of fluid through oceanic crust on a global basis? Seamounts!
Example 1: Eastern flank of Juan de Fuca Ridge (3. 5 Ma seafloor) IODP Exp. 301
Bathymetry in Second Ridge and Southern Outcrop areas Second Ridge area • Regional sediment thickness is 400 -600 m of turbidites, hemipelagic mud, relatively impermeable • Three Bares - well studied, known sites of discharge. • BB vents at 5 -20 L/s, 2 -3 MW power output • What about larger outcrops to the south? modified from Fisher et al. (2003) Southern Outcrop area Young water, 14 C age ~ 4. 3 ka
Bathymetry in Second Ridge area • Seafloor is generally flat, slopes gently to the southeast Three Bares - basement ourcrops extend 60 -200 m above surrounding sediments • Mama Bare and Baby Bare located above a buried basement ridge; Papa Bare is on another ridge modified from Fisher (2005), data from Davis et al. , (1997)
Basement relief of Second Ridge area, based on dozens of closely-spaced seismic lines Basement surface map Local discharge site modified from Zühlsdorff et al. (2005)
Baby Bare Outcrop • Very high heat flow, fluid seepage from basement (5 -20 L/s), thin sediment cover… modified from Johnson et al. (2003), Wheat et al. (2004)
Bathymetry, seismic coverage, heat flow modified from Fisher et al. (2003)
Variations in heat flow and upper basement temperatures around outcrops modified from Fisher et al. (2003)
Does fluid recharging the crust through Grizzly Bare vent at Baby Bare? • Driving forces are too small to bring BB vent fluid through thick sediments; chemistry is also wrong for this - BB fluids interact mainly with basalt at ~65 -70 °C • No closer outcrops to the west modified from Fisher et al. (2003) Second Ridge area Southern Outcrop area
Consistent trend in basement fluid chemistry from south to north 87/86 Sr SO 4 SW 0. 7092 28. 1 BB 0. 7075 17. 8 1026 0. 7073 17. 0 MB 0. 7071 16. 3 MORB ~0. 705 ~0 Second Ridge area data from Wheat et al. (2000) Southern Outcrop area
Primary driving force for ridge-flank hydrothermal circulation… …the "hydrothermal siphon" Can generate ∆P = 10's-100's k. Pa (thus basement permeability must be large, because fluxes are enormous)
What driving forces, flow rates, and basement permeability are implied? • Driving force is a “hydrothermal siphon, ” the difference in pressure at depth below recharging and discharging columns of water. Magnitude depends on depth of circulation and temperature difference • Calculate magnitude of driving force based on mass flux, 5 -20 L/s exiting from BB, at least as much entering GB (5 -50 L/s) • 14 C provides estimate of maximum travel time (4. 3 ka), but requires enormous correction to account for flow channeling
Driving force calculations Available driving forces modified from Fisher et al. (2003)
Required flow rate and lateral permeability in basement Apparent and actual fluid ages may differ by 100 x or more because of diffusive/dispers ive losses modified from Fisher et al. (2003) Apparent (14 C) age Actual age? Range of available driving forces:
Example 2: Cocos Plate west of Costa Rica (18 -24 Ma seafloor) Middle America Trench (MAT)
Results from Tico. Flux I and II (2001 -02) 70 -90% of lithospheric heat is "missing" ~100% of lithospheric heat is measured modified from Hutnak et al. (2006)
Why does hydrothermal circulation extract so much heat in this area, and why is this process so much less effective on CNS-generated seafloor than on EPRgenerated seafloor of similar age on the same plate? Three hypotheses: (1) Normal faulting associated with flexure at the outer slope of the MAT increases permeability and associated fluid circulation and advective heat loss within EPR-generated seafloor. (2) Heat flow is higher on CNS-generated crust because passage over the Galapagos hot spot added heat to the plate, compensating for earlier hydrothermal cooling. (3) Basement outcrops common on EPR-generated seafloor in Tico. Flux area, but absent on CNS-generated seafloor immediately south of the plate suture, provide pathways for recharge and discharge of hydrothermal fluids.
Satellite data show some seamounts, but emphasize mainly Cocos Ridge • Prior to Tico. Flux, only satellite bathymetric data available for most of the region • Area near the MAT looks relatively "smooth" far from Cocos hot spot track
New swath-map data show numerous seamounts on EPR-generated seafloor modified from Hutnak et al. (2006)
A global process • Satellite gravity data reveal ~15, 000 seamounts • Only features larger than D~3. 5 km are detected; may be 80 -100 k seamounts total! • Seamounts are not the only outcrops: fracture zones, LIPS, etc… modified from Wessel (2001), Harris et al. (2004)
Open questions • • What fraction of seamounts are hydrologically are active? What are the fluid, heat, solute fluxes associated with seamounts (specific, total)? What initiates seamount-seamount circulation? How variable is circulation (rate, direction)? What controls the initial direction of flow (does it change, large versus small diameter, height)? What is the distribution of focused versus diffuse flow? What is the geometry of circulation within seamounts? Between seamounts? What are the maximum length scales? How does fluid flow relate to seamount and crustal structure? How does fluid flow relate to subseafloor microbiology? Seafloor micro-, macrobiology? Many of these questions and numerous others apply to hightemperature hydrothermal circulation through seamounts
Summary and conclusions • • • Seafloor hydrothermal circulation on ridge flanks comprises enormous fluid and heat fluxes. Circulation occurs at rates sufficient to extract a significant fraction of lithospheric heat out to 65 Ma on average (circulation continues in basement to older ages in many places). Fluids can travel 10's of kilometers between basement outcrops, in some cases mining >70% of lithospheric heat. The driving force for this circulation is a hydrothermal siphon, generates small-moderate driving forces (10's to a few 100 k. Pa), requires high basement permeability. Seamounts are a critical part of the global convection cycle, penetrate low-permeability sediments, allow the siphon to form, are natural "windows" into subsurface processes. Little is known about the physics, chemistry, biology of ridgeflank convection through seamounts, need to explore more of these important features.
Questions? modified from Fisher (2005)
Global thermal fluxes… modified from Stein and Stein (1995), Fisher (2005)
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