2. First Workshop Seamount Biogeosciences Network Scripps Institution of Oceanography 24-25 March 2006 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 A. T. Fisher University of California, Santa Cruz Earth Sciences Department and Institute for Geophysics and Planetary Physics Seamounts Enhance the Global Influence of Ridge-flank Hydrothermal Circulation

3. Acknowledgements: RetroFlux (2000): E. Davis, C. G. Wheat, M. Mottl, K. Becker, M. Hutnak, R. Macdonald, A. Cherkaoui, L. Christiansen, M. Edwards ImageFlux (2000): V. Spiess, L. Zühlsdorff, H. Villinger TicoFlux 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 Funding agencies/sponsors: 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. McCarthy, J. S. McClain, M. J. Mottl, M. Sinha, G. Spinelli, V. Spiess, R. Stephen, D. Teagle, H. Villinger, L. Zühlsdorff, R. Meldrum

4. Selected references Hutnak, M., A.T. Fisher, Z ü hlsdorff, 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/s10040-004-0400-y, 2005. Harris, R. N., Fisher, A.T., Chapman, D., Seamounts induce large fluid fluxes, Geology, 32 (8), 725-728, doi:10.1130/G20387.1, 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:10.1029/2002GL016766, 2003. Hutnak, M., A.T. Fisher, Z ü hlsdorff, 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/s10040-004-0400-y, 2005. Harris, R. N., Fisher, A.T., Chapman, D., Seamounts induce large fluid fluxes, Geology, 32 (8), 725-728, doi:10.1130/G20387.1, 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:10.1029/2002GL016766, 2003. Find copies of these and related papers at http://es.ucsc.edu/~afisher

5. Most of the seafloor is hydrogeologically active… modified from Fisher (2005)

6. 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 the 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

7. 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 large- scale, ridge-flank systems (high-temperature flows, single-seamount circulation systems are also important)

8. How BIG is the ocean crustal hydrothermal fluid reservoir? Reservoir Volume in storage (km 3 ) Percent of total Oceans1.4 billion97.2 Glaciers, ice sheets30 million2.1 Ocean crust20-30 million1-2 Groundwater (continental) 9 million0.6 Rivers, lakes100 thousand0.009 Soil water70 thousand0.005 Atmosphere10 thousand0.001 (based on geometrical considerations)

9. How BIG is the global hydrothermal fluid flux? Location/kind Volume flux (km 3 /yr) Rain+snow on land+sea400,000 Evaporation+transpiration400,000 River discharge40,000 Ridge-flank (>1 Ma) 2,000-20,000 Groundwater discharge6000 Glacial melting/freezing6000 Ridge-axis (>1 Ma) 40 Large enough to "recycle" the ocean every 100k-500k yrs (based on heat flux considerations)

10. What does the oceanic crust look like? A very permeable aquifer…

11. 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.

12. Studies of cell densities in marine sediments suggest a roughly log-depth distribution Studies of cell densities in marine sediments suggest a roughly log-depth distribution Extrapolation globally suggests an enormous biomass - highly speculative Extrapolation globally suggests an enormous biomass - highly speculative What about basement microbiology? What about basement microbiology? modified from Parkes et al. (1994), D’Hondt et al. (2003); Fisher et al. (2005) What is the extent of the subseafloor biosphere?

13. 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.); 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… 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!

14. Example 1: Eastern flank of Juan de Fuca Ridge IODP Exp. 301 (3.5 Ma seafloor)

15. Bathymetry in Second Ridge and Southern Outcrop areas Regional sediment thickness is 400-600 m of turbidites, hemi- pelagic mud, relatively impermeable Regional sediment thickness is 400-600 m of turbidites, hemi- pelagic mud, relatively impermeable Three Bares - well studied, known sites of discharge. Three Bares - well studied, known sites of discharge. BB vents at 5-20 L/s, 2-3 MW power output BB vents at 5-20 L/s, 2-3 MW power output What about larger outcrops to the south? What about larger outcrops to the south? modified from Fisher et al. (2003) SecondRidgearea SouthernOutcroparea Youngwater, 14 C age ~ 4.3 ka

16. Bathymetry in Second Ridge area Seafloor is generally flat, slopes gently to the south- southeast Seafloor is generally flat, slopes gently to the south- southeast Three Bares - basement ourcrops extend 60-200 m above surrounding sediments Mama Bare and Baby Bare are located above a buried basement ridge; Papa Bare is on another ridge Mama Bare and Baby Bare are located above a buried basement ridge; Papa Bare is on another ridge modified from Fisher (2005), data from Davis et al., (1997)

17. Basement surface map Basement relief of Second Ridge area, based on dozens of closely-spaced seismic lines modified from Zühlsdorff et al. (2005) Local discharge site

18. Baby Bare Outcrop Very high heat flow, fluid seepage from basement (5-20 L/s), thin sediment cover… 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)

19. Regional view of outcrop setting

20. Bathymetry, seismic coverage, heat flow modified from Fisher et al. (2003)

21. Variations in heat flow and upper basement temperatures around outcrops modified from Fisher et al. (2003)

22. 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 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 No closer outcrops to the west modified from Fisher et al. (2003) SecondRidgearea SouthernOutcroparea

23. ~0.705 ~0 MORB 87/86 Sr SO 4 SW0.709228.1 BB0.707517.8 10260.707317.0 MB0.707116.3 Consistent trend in basement fluid chemistry from south to north data from Wheat et al. (2000) SecondRidgearea SouthernOutcroparea

24. …the "hydrothermal siphon" Can generate ∆P = 10's-100's kPa Primary driving force for ridge-flank hydrothermal circulation… (thus basement permeability must be large, because fluxes are enormous)

25. 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 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) 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 14 C provides estimate of maximum travel time (4.3 ka), but requires enormous correction to account for flow channeling

26. Driving force calculations Available driving forces modified from Fisher et al. (2003)

27. Required flow rate and lateral permeability in basement Apparent ( 14 C) age Actualage? Apparent and actual fluid ages may differ by 100 x or more because of diffusive/dispers ive losses Range of available driving forces: modified from Fisher et al. (2003)

28. Example 2: Cocos Plate west of Costa Rica Middle America Trench (MAT) (18-24 Ma seafloor)

29. modified from Hutnak et al. (2006) Results from TicoFlux I and II (2001-02) 70-90% of lithospheric heat is "missing" ~100% of lithospheric heat is measured

30. 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 EPR- generated seafloor of similar age on the same plate? Three hypotheses: (1) (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) (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) (3)Basement outcrops common on EPR-generated seafloor in TicoFlux area, but absent on CNS-generated seafloor immediately south of the plate suture, provide pathways for recharge and discharge of hydrothermal fluids. Three hypotheses:

31. Satellite data show some seamounts, but emphasize mainly Cocos Ridge Prior to TicoFlux, only satellite bathymetric data available for most of the region Prior to TicoFlux, only satellite bathymetric data available for most of the region Area near the MAT looks relatively "smooth" far from Cocos hot spot track Area near the MAT looks relatively "smooth" far from Cocos hot spot track

32. New swath-map data show numerous seamounts on EPR-generated seafloor modified from Hutnak et al. (2006)

33. modified from Wessel (2001), Harris et al. (2004) A global process Satellite gravity data reveal ~15,000 seamounts Satellite gravity data reveal ~15,000 seamounts Only features larger than D~3.5 km are detected; may be 80-100k seamounts total! Only features larger than D~3.5 km are detected; may be 80-100k seamounts total! Seamounts are not the only outcrops: fracture zones, LIPS, etc… Seamounts are not the only outcrops: fracture zones, LIPS, etc…

34. Open questions What fraction of seamounts are hydrologically are active?What fraction of seamounts are hydrologically are active? What are the fluid, heat, solute fluxes associated with seamounts (specific, total)?What are the fluid, heat, solute fluxes associated with seamounts (specific, total)? What initiates seamount-seamount circulation? How variable is circulation (rate, direction)?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 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 distribution of focused versus diffuse flow? What is the geometry of circulation within seamounts? Between seamounts? What are the maximum length scales?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 seamount and crustal structure? How does fluid flow relate to subseafloor microbiology? Seafloor micro-, macrobiology?How does fluid flow relate to subseafloor microbiology? Seafloor micro-, macrobiology? What fraction of seamounts are hydrologically are active?What fraction of seamounts are hydrologically are active? What are the fluid, heat, solute fluxes associated with seamounts (specific, total)?What are the fluid, heat, solute fluxes associated with seamounts (specific, total)? What initiates seamount-seamount circulation? How variable is circulation (rate, direction)?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 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 distribution of focused versus diffuse flow? What is the geometry of circulation within seamounts? Between seamounts? What are the maximum length scales?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 seamount and crustal structure? How does fluid flow relate to subseafloor microbiology? Seafloor micro-, macrobiology?How does fluid flow relate to subseafloor microbiology? Seafloor micro-, macrobiology? Many of these questions and numerous others apply to high- temperature hydrothermal circulation through seamounts

35. Seafloor hydrothermal circulation on ridge flanks comprises enormous fluid and heat fluxes.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).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.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 kPa), requires high basement permeability.The driving force for this circulation is a hydrothermal siphon, generates small-moderate driving forces (10's to a few 100 kPa), 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.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 ridge- flank convection through seamounts, need to explore more of these important features.Little is known about the physics, chemistry, biology of ridge- flank convection through seamounts, need to explore more of these important features. Seafloor hydrothermal circulation on ridge flanks comprises enormous fluid and heat fluxes.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).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.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 kPa), requires high basement permeability.The driving force for this circulation is a hydrothermal siphon, generates small-moderate driving forces (10's to a few 100 kPa), 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.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 ridge- flank convection through seamounts, need to explore more of these important features.Little is known about the physics, chemistry, biology of ridge- flank convection through seamounts, need to explore more of these important features. Summary and conclusions

36. Questions? modified from Fisher (2005)

37. modified from Stein and Stein (1995), Fisher (2005) Global thermal fluxes…