Black smokers are the dramatic result of seawater being heated to high temperatures (generally 250° to 350°C) by magmatic systems, then discharging at the seafloor. However, not all seawater that circulates through the oceanic crust is heated to high temperatures. “Diffuse flow” is a catchall term to describe low-temperature (< 0.2° to ~ 100°C) fluids that slowly discharge through sulfide mounds, fractured lava flows, and assemblages of bacterial mats and macrofauna. Diffuse-flow fluids are generally mixtures of cold seawater and either magmatically heated fluids, conductively heated seawater, or both. Although the limited data indicate that 50–90% of the hydrothermal heat loss occurs as diffuse flow at the seafloor, modeling results coupled with geochemical data suggest that nearly 90% of the heat loss ultimately stems from magmatically heated fluids. There is a critical need to obtain more diffuse-flow measurements to improve models of heat and geochemical fluxes, better understand subsurface fluid flow dynamics, and determine the extent of the subsurface biosphere as well as the spatial and temporal variability of diffuse flow. New measurement techniques and diffuse-flow models provide insight into the characteristics of these subsurface fluids and their manifestation at the seafloor.

Bemis, K., R.P. Lowell, and A. Farough. 2012. Diffuse flow on and around hydrothermal vents at mid-ocean ridges. Oceanography 25(1):182–191, https://doi.org/10.5670/oceanog.2012.16.

Baker, E.T., G.J. Massoth, S.L. Walker, and R.W. Embley. 1993. A method for quantitatively estimating diffuse and discrete hydrothermal discharge. Earth and Planetary Science Letters 118:235–249, https://doi.org/10.1016/0012-821X(93)90170-E.
1. Butterfield, D.A., G.J. Massoth, R.E. McDuff, J.E. Lupton, and M.D. Lilley. 1990. Geochemistry of hydrothermal fluids from Axial Seamount Hydrothermal Emissions Study Vent Field, Juan de Fuca Ridge: Subseafloor boiling and subsequent fluid-rock interaction. Journal of Geophysical Research 95:12,895–12,921, https://doi.org/10.1029/JB095iB08p12895.
2. Butterfield, D.A., K.K. Roe, M.D. Lilley, J.A. Huber, J.A. Baross, R.W. Embley, and G.J. Massoth. 2004. Mixing, reaction and microbial activity in the sub-seafloor revealed by temporal and spatial variation in diffuse flow vents at Axial Volcano. Pp. 269–289 in The Subseafloor Biosphere at Mid-Ocean Ridges. W.S.D. Wilcock, E.F. DeLong, D.S. Kelley, J.A. Baross, and S.C. Cary, eds, Geophysical Monograph Series, vol. 144, American Geophysical Union, Washington, DC.
3. Cooper, M.J., H. Elderfield, and A. Schultz. 2000. Diffuse hydrothermal fluids from Lucky Strike hydrothermal vent field: Evidence for a shallow conductively heated system. Journal of Geophysical Research 105:19,369–19,375, https://doi.org/10.1029/2000JB900138.
4. Converse, D.R., H.D. Holland, and J.M. Edmond. 1984. Flow rates in the axial hot springs of the East Pacific Rise (21°N): Implications for the heat budget and the formation of massive sulfide deposits. Earth and Planetary Science Letters 69:159–175, https://doi.org/10.1016/0012-821X(84)90080-3.
5. Corliss, J.B., J. Dymond, L.I. Gordon, J.M. Edmond, R.P. von Herzen, R.D. Ballard, K. Green, D. Williams, A. Bainbridge, K. Crane, and T.H. van Andel. 1979. Submarine thermal springs on the Galápagos Rift. Science 203:1,073–1,082, https://doi.org/10.1126/science.203.4385.1073.
6. Coumou, D., T. Driesner, and C. Heinrich. 2008. The structure and dynamics of mid-ocean ridge hydrothermal systems. Science 321:1,825–1,828, https://doi.org/10.1126/science.1159582.
7. Craft, K., and R.P. Lowell. 2009. A boundary layer model for submarine hydrothermal flows at on-axis and near-axis locations. Geochemistry Geophysics Geosystems 10, Q12012, https://doi.org/10.1029/2009GC002707.
8. Craft, K., R.P. Lowell, and L.N. Germanovich. 2008. Models of diffuse flow near mid-ocean ridge axes. Eos, Transactions, American Geophysical Union 89(53):Fall Meeting Supplement Abstract V41B-2073.
9. Crowell, B.W., R.P. Lowell, and K.L. Von Damm. 2008. A model for the production of sulfur floc and “snowblower” events at mid-ocean ridges. Geochemistry Geophysics Geosystems 9, Q10T02, https://doi.org/10.1029/2008GC002103.
10. Curewitz, D., and J.A. Karson. 1997. Structural settings of hydrothermal outflow: Fracture permeability maintained by fault propagation and interaction. Journal of Volcanology and Geothermal Research 79:149–168.
11. Delaney, J.R., V. Robigou, R.E. McDuff, and M.K. Tivey. 1992. Geology of a vigorous hydrothermal system on the Endeavour Segment, Juan de Fuca Ridge. Journal of Geophysical Research 97:19,663–19,682, https://doi.org/10.1029/92JB00174.
12. Di Iorio, D., J.W. Lavelle, P.A. Rona, K. Bemis, G. Xu, L.N. Germanovich, R.P. Lowell, and G. Genc. 2012. Measurements and models of heat flux and plumes from hydrothermal discharges near the deep seafloor. Oceanography 25(1):168–179, https://doi.org/10.5670/oceanog.2012.14.
13. Edmond, J.M., C. Measures, R.E. McDuff, L.H. Chan, R. Collier, B. Grant, L.I. Gordon, and J.B. Corliss. 1979. Ridge crest hydrothermal activity and the balances of the major and minor elements in the ocean: The Galapagos data. Earth and Planetary Science Letters 46:1–18, https://doi.org/10.1016/0012-821X(79)90061-X.
14. Farough, A. 2011. A parameterized approach to partitioning between focused and diffuse heat output and modeling hydrothermal recharge at the East Pacific Rise 9°50’N. M.S. Thesis, Virginia Tech, Blacksburg, VA, 79 pp.
15. Fontaine, F.J., and W.S.D. Wilcock. 2006. Dynamics and storage of brine in mid-ocean ridge hydrothermal systems. Journal of Geophysical Research 111, B06102, https://doi.org/10.1029/2005JB003866.
16. Fornari, D.J., M. Tivey, H. Schouten, M. Perfit, D. Yoerger, A. Bradley, M. Edwards, R. Haymon, D. Scheirer, K. Von Damm, and others. 2004. Submarine lava flow emplacement at the East Pacific Rise 9°N: Implications for uppermost ocean crust stratigraphy and hydrothermal fluid circulation. Pp. 187–217 in Mid-Ocean Ridges: Hydrothermal Interactions Between the Lithosphere and Oceans. C.R. German, J. Lin, and L.M. Parson, eds, Geophysical Monograph Series, vol. 148, American Geophysical Union, Washington, DC.
17. Foustoukos, D.I., N.J. Pester, K. Ding, and W.E. Seyfried Jr. 2009. Dissolved carbon species in associated diffuse and focused flow hydrothermal vents at the Main Endeavour Field, Juan de Fuca Ridge: Phase equilibria and kinetic constraints. Geochemistry Geophysics Geosystems 10, Q10003, https://doi.org/10.1029/2009GC002472.
18. Germanovich, L.N., D. Di Iorio, G. Genc, R.S. Hurt, R.P. Lowell, J.F. Holden, D.A. Butterfield, and E.J. Olson. 2009. Direct measurements of hydrothermal heat output at Juan de Fuca Ridge. Eos, Transactions, American Geophysical Union 90(52):Fall Meeting Supplement Abstract OS13A-1179.
19. Germanovich, L.N., R.P. Lowell, and P. Ramondenc. 2011. Magmatic origin of hydrothermal response to earthquake swarms: Constraints from heat flow and geochemical data. Journal of Geophysical Research 116, B05103, https://doi.org/10.1029/2009JB006588.
20. Goto, S., T. Gamo, H. Chiba, K. Fujioka, and K. Mitsuzawa. 2007. Contribution of heat outputs from high- and low-temperature hydrothermal sources to the neutrally buoyant plume at the TAG hydrothermal mound, Mid-Atlantic Ridge. Earth, Planets and Space 59:1,141–1,146.
21. Goto, S., M. Kinoshita, and K. Mitsuzawa. 2003. Heat flux estimate of warm water flow in a low-temperature diffuse flow site, southern East Pacific Rise 17°25’S. Marine Geophysical Research 24:345–357, https://doi.org/10.1007/s11001-004-3957-x.
22. Holland, M.E., J.A. Baross, and J.F. Holden. 2004. Illuminating subseafloor ecosystems using microbial tracers. Pp. 291–303 in The Subseafloor Biosphere at Mid-Ocean Ridges. W.S.D. Wilcock, E.F. DeLong, D.S. Kelley, J.A. Baross, and S.C. Cary, eds, Geophysical Monograph Series, vol. 144, American Geophysical Union, Washington, DC.
23. Humphris, S.E., D.J. Fornari, D.S. Scheirer, C.R. German, and L.M. Parson. 2002. Geotectonic setting of hydrothermal activity on the summit of Lucky Strike Seamount (37°17’N, Mid-Atlantic Ridge). Geochemistry Geophysics Geosystems 3(8), 1049, https://doi.org/10.1029/2001GC000284.
24. Kelley, D.S., J.A. Baross, and J.R. Delaney. 2002. Volcanoes, fluids, and life at mid-oceanic ridge spreading centers. Annual Review of Earth and Planetary Sciences 30:385–491, https://doi.org/10.1146/annurev.earth.30.091201.141331.
25. Lang, S.Q., D.A. Butterfield, M.D. Lilley, H.P. Johnson, and J.I. Hedges. 2006. Dissolved organic carbon in ridge-axis and ridge-flank hydrothermal systems. Geochimica Cosmochimica Acta 70:3,830–3,842, https://doi.org/10.1016/j.gca.2006.04.031.
26. Lowell, R.P., B.W. Crowell, K.C. Lewis, and L. Liu. 2008. Modeling multiphase, multicomponent processes at oceanic spreading centers. Pp. 15–44 in Magma to Microbe: Modeling Hydrothermal Processes at Ocean Spreading Centers. R.P. Lowell, J.S. Seewald, A. Metaxas, and M.R. Perfit, eds, Geophysical Monograph Series, vol. 178, American Geophysical Union, Washington, DC.
27. Lowell, R.P., A. Farough, L.N. Germanovich, L.B. Hebert, and R. Horne. 2012. A vent-field-scale model of the East Pacific Rise 9°50’N magma-hydrothermal system. Oceanography 25(1):158–167, https://doi.org/10.5670/oceanog.2012.13.
28. Lowell, R.P., S.R. Gosnell, and Y. Yang. 2007. Numerical simulations of single-pass hydrothermal convection at mid-ocean ridges: Effects of the extrusive layer and temperature dependent permeability. Geochemistry Geophysics Geosystems 8, Q10011, https://doi.org/10.1029/2007GC001653.
29. Mittelstaedt, E., A. Davaille, P.E. van Kaken, N. Gracias, and J. Escartín. 2010. A noninvasive method for measuring the velocity of diffuse hydrothermal flow by tracking moving refractive index anomalies. Geochemistry Geophysics Geosystems 11, Q10005, https://doi.org/10.1029/2010GC003227.
30. Ondeas, H., M. Cannat, Y. Fouquet, A. Normand, P.M. Sarradin, and J. Sarrazin. 2009. Recent volcanic events and the distribution of hydrothermal venting at the Lucky Strike hydrothermal field, Mid-Atlantic Ridge. Geochemistry Geophysics Geosystems 10, Q02006, https://doi.org/10.1029/2008GC002171.
31. Pruis, M. 2004. Energy and volume flux into the deep ocean: Examining diffuse hydrothermal systems. PhD Thesis, University of Washington.
32. Pruis, M.J., and H.P. Johnson. 2004. Tapping into the sub-seafloor: examining diffuse flow and temperature from an active seamount on the Juan de Fuca Ridge. Earth and Planetary Science Letters 217:379–388, https://doi.org/10.1016/S0012-821X(03)00607-1.
33. Ramondenc, P., L.N. Germanovich, and R.P. Lowell. 2008. Modeling the hydrothermal response to earthquakes at oceanic spreading centers. Pp. 97–121 in Magma to Microbe: Modeling Hydrothermal Processes at Oceanic Spreading Centers. R.P. Lowell, J.S. Seewald, A. Metaxas, and M.R. Perfit, eds, Geophysical Monograph Series, vol. 178, American Geophysical Union, Washington, DC.
34. Ramondenc, P., L.N. Germanovich, K.L. Von Damm, and R.P. Lowell. 2006. The first measurements of hydrothermal heat output at 9°50’N, East Pacific Rise. Earth and Planetary Science Letters 245:487–497, https://doi.org/10.1016/j.epsl.2006.03.023.
35. Rona, P.A., and R. Light. 2011. Sonar images hydrothermal vents in seafloor observatory. Eos, Transactions, American Geophysical Union 92(20), https://doi.org/10.1029/2011EO200002.
36. Rona, P.A., and D.A. Trivett. 1992. Discrete and diffuse flow heat transfer at ASHES vent field, Axial Volcano, Juan de Fuca Ridge. Earth and Planetary Science Letters 109:57–71, https://doi.org/10.1016/0012-821X(92)90074-6.
37. Rona, P.A., R.P. Denlinger, M.R. Fisk, K.J. Howard, G.L. Taghon, K.D. Klitgord, J.S. McClain, G.R. McMurray, and J.C. Wiltshire. 1990. Major off-axis hydrothermal activity on the northern Gorda Ridge. Geology 18:493–496, https://doi.org/10.1130/0091-7613(1990)018<0493:MOAHAO>2.3.CO;2.
38. Rudnicki, M.D., and H. Elderfield. 1992. Theory applied to the Mid-Atlantic Ridge hydrothermal plumes: The finite difference approach. Journal of Volcanology and Geothermal Research 50:161–172, https://doi.org/10.1016/0377-0273(92)90043-D.
39. Sarrazin, J., P. Rodier, M.K. Tivey, H. Singh, A. Schultz, and P.M. Sarradin. 2009. A dual sensor device to estimate fluid flow velocity at diffuse hydrothermal vents. Deep Sea Research Part I 56:2,065–2,074, https://doi.org/10.1016/j.dsr.2009.06.008.
40. Scheirer, D.S., T.M. Shank, and D.J. Fornari. 2006. Temperature variations at diffuse and focused flow hydrothermal vent sites along the northern East Pacific Rise. Geochemistry Geophysics Geosystems 7, Q03002, https://doi.org/10.1029/2005GC001094.
41. Schultz, A., J.M. Delaney, and R.E. McDuff. 1992. On the partitioning of heat flux between diffuse and point source venting. Journal of Geophysical Research 97:12,229–12,314, https://doi.org/10.1029/92JB00889.
42. Schultz, A., P. Dickson, and H. Elderfield. 1996. Temporal variations in diffuse hydrothermal flow at TAG. Geophysical Research Letters 23:3,471–3,474, https://doi.org/10.1029/96GL02081.
43. Shank, T.M., D.J. Fornari, K.L. Von Damm, M.D. Lilley, R.M. Haymon, and R.A. Lutz. 1998. Temporal and spatial patterns of biological community development at nascent deep-sea hydrothermal vents (9°50’N, East Pacific Rise). Deep-Sea Research Part II 45:465–515, https://doi.org/10.1016/S0967-0645(97)00089-1.
44. Sohn, R.A. 2007. Stochastic analysis of exit fluid temperature records from the active TAG hydrothermal mound (Mid-Atlantic Ridge, 26°N): 1. Modes of variability and implications for subsurface flow. Journal of Geophysical Research 112, B07101, https://doi.org/10.1029/2006JB004435.
45. Tivey, M.K. 1995. Modeling chimney growth and associated fluid flow at seafloor hydrothermal vents sites. Pp. 158–177 in Seafloor Hydrothermal Systems: Physical, Chemical, Biological, and Geological Interactions. S.E. Humphris, R.A. Zierenberg, L.S. Mullineaux, and R.D. Thomson, eds, Geophysical Monograph Series, vol. 91, American Geophysical Union, Washington, DC.
46. Tivey, M.K. 2007. Generation of seafloor hydrothermal vent fluids and associated mineral deposits. Oceanography 20(1):50–65, https://doi.org/10.5670/oceanog.2007.80.
47. Tivey, M.K., A.M. Bradley, T.M. Joyce, and D. Kadko. 2002. Insights into tide-related variability at seafloor hydrothermal vents from time-series temperature measurements. Earth and Planetary Science Letters 202:693–707, https://doi.org/10.1016/S0012-821X(02)00801-4.
48. Trivett, D.A., and A.J. Williams III. 1994. Effluent from diffuse hydrothermal venting, Part 2: Measurement of plumes from diffuse hydrothermal vents at the southern Juan de Fuca Ridge. Journal of Geophysical Research 99:18,417–18,432, https://doi.org/10.1029/94JC00096.
49. Tolstoy, M., F. Waldhauser, D.R. Bohnenstiehl, R.T. Weekly, and W.-Y. Kim. 2008. Seismic identification of along-axis hydrothermal flow on the East Pacific Rise. Nature 451:181–184, https://doi.org/10.1038/nature06424.
50. Veirs, S.R., R.E. McDuff, and F.R. Stahr. 2006. Magnitude and variance of near-bottom horizontal heat flux at the Main Endeavour hydrothermal vent field. Geochemistry Geophysics Geosystems 7, Q02004, https://doi.org/10.1029/2005GC000952.
51. Von Damm, K.L., and M.D. Lilley. 2004. Diffuse flow hydrothermal fluids from 9°50’N East Pacific Rise: Origin, evolution and biogeochemical controls. Pp. 245–268 in The Subseafloor Biosphere at Mid-Ocean Ridges. W.S.D. Wilcock, E.F. DeLong, D.S. Kelley, J.A. Baross, and S.C. Cary, eds, Geophysical Monograph Series, vol. 144, American Geophysical Union, Washington, DC.
52. Wankel, S.D., L.N. Germanovich, M.D. Lilley, G. Genc, C.J. DiPerna, A.S. Bradley, E.J. Olson, and P.R. Girguis. 2011. Influence of subsurface biosphere on geochemical fluxes from diffuse hydrothermal fluids. Nature Geoscience 4:461–468, https://doi.org/10.1038/ngeo1183.
53. Wilcock, W.S.D., S.D. Archer, and G.M. Purdy. 2002. Microearthquakes on the Endeavour Segment of the Juan de Fuca Ridge. Journal of Geophysical Research 107(B12), 2336, https://doi.org/10.1029/2001JB000505.

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