Seamounts as Conduits for Massive Fluid, Heat, and Solute Fluxes on Ridge Flanks Oceanography

74 Nw eifuku Volcano, Champagne vent. The white-smoker chimneys are ~ 20-cm across and ~ 50-cm high, and venting fluids measure 103°C. white flocculent mats and elemental sulfur coatings surround the chimneys, and liquid Co 2 droplets rise from the seafloor. Measurements of conductive heat flow typically fall below predictions of lithospheric cooling models until an average lithospheric age of ~ 65 Ma. This deviation from model predictions, and the variability of seafloor heat flow commonly observed on ridge flanks, is generally attributed to hydrothermal circulation. This circulation also contributes to geochemical reactions that facilitate the exchange of solutes between the crust and ocean, redistribute elements within the crust, and alter the physical state of plates as they

This deviation from model predictions, and the variability of seafloor heat flow commonly observed on ridge flanks, is generally attributed to hydrothermal circulation.This circulation also contributes to geochemical reactions that facilitate the exchange of solutes between the crust and ocean, redistribute elements within the crust, and alter the physical state of plates as they age (e.g., Staudigel et al., 1981;Wilkens et al., 1991;Alt, 2004).
Earth's geothermal heat output is about 44 TW, with most heat loss occurring through ocean basins (e.g., Sclater et al., 1980;Pollack et al., 1993).Seafloor hydrothermal heat output is on the order of 10 TW, ~ 25% of Earth's total geothermal heat output, and ~ 30% of the oceanic lithospheric heat output (Figure 1A).Only a small fraction of this advective heat output occurs at high temperatures at mid-ocean ridges; the vast majority occurs at lower temperatures (generally 5-20°C) on ridge flanks, suggesting an associated fluid discharge of ~ 10 16 kg yr -1 (Figure 1B) (C.Stein et al., 1995;Mottl, 2003;Wheat et al., 2003).This low-temperature flow rivals the discharge of all rivers to the ocean (4 x 10 16 kg yr -1 ), and is about three orders of magnitude greater than the sum of high-temperature hydrothermal discharges at mid-ocean ridges (~ 10 13 kg yr -1 ).Resulting ridge-flank chemical fluxes impact biogeochemical cycles for numerous solutes (e.g., Wheat et al., 2003), and may help to sustain vast subseafloor microbial ecosystems (Edwards et al., 2005;Huber et al., 2006).
Unfortunately, there are few direct measurements of fluid fluxes and chemical compositions from typical ridgeflank hydrothermal systems.Instead, local and regional fluid and heat flows on ridge flanks have been calculated from seafloor heat flow deficits, and fluid composition has been inferred from pore fluid samples squeezed from sediments collected just above the basalt contact (e.g., Elderfield et al., 1999;Wheat and Mottl, 2004).
One reason for the lack of direct measurements and samples from typical ridge-flank hydrothermal systems is the difficulty in locating sites of lowtemperature discharge.Vent fields on mid-ocean ridges are often located by detecting small thermal, chemical, and particle anomalies tens of meters above the seafloor (e.g., E. Baker and Massoth, iNtRoduCtioN Oceanic heat flow is generally highest close to mid-ocean ridge spreading centers and decreases as the lithosphere ages.This broad pattern is a natural consequence of lithospheric cooling, as predicted by conductive models based on plate thickness, basal temperature, thermal conductivity, and heat capacity (e.g., Davis and Lister, 1974;Parsons and Sclater, 1977).Seafloor heat flow is also highly variable near spreading centers, where sediment cover is thin or patchy and volcanic rocks are exposed at the seafloor across large areas.Conductive seafloor heat flow through ridge flanks, areas far from the thermal influence of magmatic emplacement at seafloor spreading centers, tends to be less variable, particularly where sediment cover is more continuous.aBstR aCt.Seamounts play a fundamental role in facilitating the exchange of fluids, heat, and solutes between the oceanic lithosphere and the overlying ocean.
Global heat flow compilations indicate that much of the seafloor loses a significant fraction of lithospheric heat because of fluid flow from the crust, and most of this advective heat loss occurs on ridge flanks, areas far from the thermal influence of magmatic emplacement at seafloor spreading centers.The driving forces available to move fluid between the crust and ocean are modest, and most of the seafloor is blanketed by low-permeability sediment that prevents vertical fluid flow at thermally significant rates.Thus, most of the thermally important fluid exchange between the crust and ocean must occur where volcanic rocks are exposed at the seafloor; little fluid exchange on ridge flanks occurs through seafloor sediments overlying volcanic crustal rocks.Seamounts and other basement outcrops focus ridge-flank hydrothermal exchange between the crust and the ocean.We describe the driving forces responsible for hydrothermal flows on ridge flanks, and the impacts that these systems have on crustal heat loss, fluid composition, and subseafloor microbiology.
We show data collected from two ridge-flank areas that illustrate how the extent of fluid exchange, lithospheric heat loss, and chemical reaction and transport depend on the rate of fluid flow, fluid residence time, and temperature in crustal hydrologic systems.Seamounts are ideal places to sample crustal fluids as they exit the crust and enter the ocean, to determine their chemical and microbial characteristics, and to assess the importance of this global hydrogeologic system on the evolution of Earth's lithosphere, ocean, and biosphere.1987).Low-temperature discharge on ridge flanks is unlikely to create easily identifiable plumes because differences in physical and chemical properties between crustal fluids and ocean bottom water can be so small (Wheat et al., 1997).Seamounts and other basement outcrops provide readily identifiable windows into ridge-flank conditions and processes that are important, but otherwise would be difficult to quantify.
In this paper, we describe the critical role that seamounts play in global-scale, ridge-flank hydrothermal processes.
Seamounts were initially defined as submarine volcanic constructions rising at least 1000 m above the surrounding seafloor (Menard and Ladd, 1963).Some of the basaltic edifices discussed in this paper do not meet this strict definition, either because they never were 1000 m high or because they are now mostly buried by marine sediments.To be consistent with other studies in this special issue of Oceanography, we adopt a more recent definition (Schmidt and Schmincke, 2000;Staudigel and Clague, 2010) and refer to all volcanic edifices on the seafloor that were originally ≥ 100 m tall as "seamounts." In the next section, we describe the driving force of ridge-flank hydrothermal circulation flowing through seamounts, and discuss the potential for Andrew T. Fisher (afisher@ucsc.edu)(1980).Green short dashed line = Mottl (2003).Most of this advective heat loss occurs on ridge flanks, areas far from the thermal influence of magmatic emplacement at seafloor spreading centers.(B) Global cumulative fluid discharge necessary to advect the amount of heat estimated by Mottl (2003), plotted as function of age.Curves are shown for assumed temperature differences between bottom water and hydrothermal fluid of 5°C, 20°C, and 65°C.Lower thermal values are most typical of ridge-flank hydrothermal circulation that mines lithospheric heat.The higher thermal value is characteristic of weak ridge-flank circulation that results in significant local fluid and rock alteration, but has little regional thermal influence.(C) Calculated fluid discharge per seamount, assuming that all of the fluid flow estimated earlier passes through seamounts.The number of hydrothermally active seamounts is estimated to be somewhere between 10 4 and 10 6 , based on mapping and seamount population estimates by wessel (2001) and hillier and watts (2007), and the observation that, of the seamounts and outcrops that have been surveyed, a significant fraction appear to be hydrothermally active (Fisher et al., 2003a(Fisher et al., , 2003b;;hutnak et al., 2008;Villinger et al., 2002).also shown are estimates of the fluid flux through Baby Bare and dorado seamounts, as discussed in the text.seaMouNts as CoNduits FoR RidGe-FLaNk hydRotheRMaL CiRCuLatioN the importance of seamounts in extracting Lithospheric heat Most of the seafloor is blanketed by finegrained marine sediment (Figure 2A).
Sediment cover is generally absent on and near seafloor spreading centers where the crust is young, and below subtropical gyres, but becomes increasingly continuous as seafloor spreads and ages.Typical marine sediment has a permeability much lower than that of the upper volcanic crust (Spinelli et al., 2004;Fisher et al., 2008; Figure 2B).
Hydrothermal flow through the sediment and volcanic crust is regulated by pressure gradients on ridge flanks.(e.g., Wheat and Mottl, 2004).

Physics of Ridge-Flank hydrothermal Circulation: the hydrothermal siphon
The  , 2004).(a) distribution of seafloor sediment types.sediment tends to be thinner near mid-ocean ridges (defined by 1 m.y.isochrons, black lines) and thicker on older seafloor, close to large continental areas affected by terrigenous inputs, where upwelling supports high biogenic productivity, and at high latitudes affected by ice-rafted debris. in contrast, thin sediment exists below subtropical gyres, particularly in the North atlantic and south Pacific oceans.(B) typical permeability of seafloor sediments as a function of sediment thickness.The dashed vertical line shows the typical lower limit on permeability of upper oceanic basement rocks (see recent compilation of basement observations in Fisher et al., 2008).(C) typical fluid seepage speeds through marine sediments of various kinds (solid curves, as labeled), based on typical excess fluid pressures of 20 kPa, and minimum seepage speeds necessary to detect fluid seepage via pore water solute (green curve) or thermal data (red curve).also shown is the typical flow rate needed to remove a significant fraction of lithospheric heat on a regional basis (blue curve).For the vast majority of seafloor areas covered with sediments, fluid flow is not capable of extracting a significant fraction of lithospheric heat, except by bypassing the sediment layer entirely through seamounts or other basement outcrops.
can shut down a hydrothermal siphon.Fisher, 2008).seafloor heat flow is generally higher adjacent to discharging seamounts, and lower adjacent to recharging seamounts.Between seamounts, heat flow may be equal to or less than conductive plate cooling values, depending on the regional efficiency of lithospheric heat extraction.
Perhaps the presence of a local basement high that penetrated surrounding sediments helped to focus the discharge of altered crustal fluids.In contrast, samples from DSDP Hole 418A, located just ~ 4 km away where the top of basement is lower, were much less altered and contained less common secondary mineralization (Donnelly et al., 1980;Staudigel et al., 1981).et al., 1998;Wheat et al., 2004b).Mama and Papa Bare are also known sites of hydrothermal discharge.
No hydrothermal recharge sites have been identified on these three features.
Instead, fluids recharge through Grizzly Bare, 52 km to the south (Figure 5A).
Grizzly Bare is conical in shape, 3.5 km in diameter, and rises 450 m above the surrounding seafloor.Fluid flow in basement between Grizzly Bare and Baby Bare may be facilitated by enhanced basement permeability in a direction parallel to the primary crustal fabric, as expressed by abyssal hill topography (Wheat et al., 2000;Fisher et al., 2003aFisher et al., , 2008)).
Grizzly Bare was identified initially as a site of regional hydrothermal recharge based on patterns of seafloor heat flow immediately adjacent to the edifice.Seafloor heat flow is depressed within a few kilometers of the edge of basalt exposure along several transects of measurements (Fisher et al., 2003a).
Seismic reflection data allow determina- to the area of exposed volcanic rocks (Davis et al., 1992;Fisher et al., 2003a).
Driving forces available to sustain This calculation assumes that crustal permeability along the recharging and discharging flow paths is no lower than 10 -12 m 2 , consistent with regional measurements (e.g., Becker and Davis, 2004;Fisher et al., 2003aFisher et al., , 2008) ) and results of numerical modeling experiments (Hutnak et al., 2006).

Hydrothermal circulation between
Grizzly Bare and Baby Bare has virtually no regional influence on lithospheric heat loss (Davis et al., 1999;Fisher et al., 2003a;Hutnak et al., 2006).Suppressed and elevated seafloor heat flow adjacent to Grizzly Bare and Baby Bare, respectively, extend only a few kilometers from the edge of exposed basalt, and regional (background) heat flow away from outcrops is constant along a swath of 3.5-3.6million-year-old seafloor extending 100 km to the north and south (Hutnak et al., 2006).It is likely that the hydrothermal circulation system between the two seamounts was considerably more active, and had a larger regional influence on lithospheric heat loss, when sediment cover was thinner and less complete in this area, prior to the recent phase of rapid sedimentation in the Pleistocene.It is the relatively slow and restricted nature of fluid flow today between Grizzly Bare and Baby Bare that allows the circulating fluids to be warmed and react extensively with basalt and overlying sediment pore waters while in transit (e.g., Mottl et al., 1998;Elderfield et al., 1999;Wheat et al., 2000).
The hydrothermal fluid seeping from Baby Bare is almost completely depleted in Mg (having exchanged Mg for Ca during water-rock reaction) and has lost most of its initial seawater alkalinity.
This composition is consistent with an extensive reaction time in basement, as suggested by a measured radiocarbon age of thousands of years (Elderfield et al., 1999;Walker et al., 2007).The actual travel time may be decades to centuries because dispersive exchange with regions containing old carbon adjacent to primary flow paths causes the fluid to appear older (Sanford, 1997;J. Stein and Fisher, 2003).Reaction for decades to centuries at 64°C would alter the fluid as observed, based on seawaterbasalt experimental data (Seyfried and Bishoff, 1979).
In most ridge-flank settings, the composition of hydrothermal crustal fluids is inferred mainly from the composition and compositional gradients observed in pore fluids from basal sediment.But for Baby Bare, data also are available from springs on the seamount (Mottl et al., 1998;Wheat and Mottl, 2000) and from nearby boreholes (Elderfield et al., 1999;Wheat et al., 2004a)

Million-year-old seafloor on the eastern Flank of the east Pacific Rise
The Cocos Plate seafloor offshore the Nicoya Peninsula, Costa Rica (Figure 5B), comprises a northern region formed at the East Pacific Rise (EPR), and a southern region formed at the Cocos-Nazca Spreading Center (CNS) (e.g., Meschede et al., 1998;Barckhausen et al., 2001).Recent surveys of the area included swath mapping to locate seamounts, and multichannel seismic reflection data to delineate regional tectonic features, sediment thickness, and crustal relief (Fisher et al., 2003b;Hutnak et al., 2007).Multipenetration  5; data from wheat and Fisher, 2008;wheat et al., 2004b).Nitrate (a) and Mg (B) data from pore fluids collected near and on Baby Bare.Basement formation fluid within Baby Bare is highly altered relative to bottom seawater, having exchanged Mg for Ca almost entirely, and having all nitrate lost during lateral transport, probably as a result of diffusive exchange with overlying sediment.where there is rapid upward seepage, pore fluid compositions in shallow sediments are similar to basement fluids sampled from nearby springs and boreholes.Nitrate (C) and Mg (d) data from pore fluids collected near and on dorado.The composition of basement formation fluids in this area is similar to bottom seawater, so rapid upward seepage results in nearly constant Mg and nitrate concentrations in shallow sediment.in contrast, where seepage is slow or there is no seepage, microbial processes remove much of the nitrate in sedimentary pore waters, but have no effect on Mg concentrations.evidence for upward fluid seepage (Wheat and Fisher, 2008).
These surveys documented an abrupt thermal transition between warm and cool areas of the plate, consistent with shallow fluid circulation in the volcanic crust (Figure 5B; Fisher et al., 2003b;Hutnak et al., 2007).Regional maps include ten seamounts in 14,500 km 2 of cool seafloor, comprising just 260 km 2 of basalt exposure, less than 2% of the cool region.Surveys show that some seamounts recharge, whereas others discharge (Hutnak et al., 2007(Hutnak et al., , 2008)).(Dorado), one that both recharges and discharges, and one that shows evidence neither for recharge nor discharge (Hutnak et al., 2008).

Dorado is physically similar to
Baby Bare, the small surface expression of a larger volcanic edifice that is mostly buried by thick sediment.
Heat flow around Dorado is generally less than 20 mW m -2 (about one-fifth of lithospheric model predictions), but rises to 1000 mW m -2 (ten times lithospheric model predictions) along the outcrop edge as hydrothermal fluid rises rapidly.In contrast, heat flow is locally suppressed adjacent to the much larger Tengosed Seamount (the nearest mapped basement edifice; Figure 5B), located 20 km to the east, because of the rapid penetration of cold bottom water into exposed crustal rocks (Hutnak et al., 2008).Large-scale fluid flow in this area extracts lithospheric heat and results in a regional conductive deficit of 800-1,400 MW.The estimated power output per discharging outcrop is 200-350 MW, similar to that determined from plume and point studies of high-temperature vent fields at seafloor spreading centers (E. Baker, 2007).But, in contrast to high-temperature systems, the heat advected from Dorado and other outcrops in this area is conveyed by fluids that are moderately warmer than bottom seawater (Wheat and Fisher, 2008).Data from this area suggest a regional fluid flow of 4 to 80 x 10 3 L s -1 through discharging seamounts, equivalent to 1 to 20 x 10 3 L s -1 through Dorado and similar features (Hutnak et al., 2008).This fluid-flow rate is ~ 1000 times greater than that inferred to be seeping from Baby Bare (Mottl et al., 1998;Wheat et al., 2004b).
Driving forces available to sustain flow between Tengosed and Dorado are estimated based on the hydrothermal siphon concept described earlier (Figure 3).Given typical hydrothermal fluid temperatures in the upper volcanic crust around Dorado of 10-30°C, and a regional sediment thickness of 500 m, the maximum pressure difference available to drive fluid from Tengosed to Dorado is ≤ 30 kPa.This pressure difference is smaller than was calculated between Grizzly Bare and Baby Bare, despite the much greater fluid flux through seafloor around Tengosed and Dorado, requiring that basement permeability be considerably greater in the latter area, perhaps by two to three orders of magnitude (Fisher et al., 2003a;Hutnak et al., 2008).The reason for higher crustal permeability in this area is not known, but may be a consequence   (Hutnak et al., 2008;Wheat and Fisher, 2008).1026B and nearby Baby Bare (Cowen et al., 2003;Huber et al., 2006;Nakagawa et al., 2006).These studies indicate that organisms within crustal fluids comprise a mixture of microbes commonly found in seawater, marine sediments, and elsewhere in the crust, including thermophilic bacteria and archaea.

syNthesis aNd iMPLiCatioNs
Little is known about the potential for crustal microbial populations at Dorado.Analyses of satellite gravimetric and ship track data suggest that there could be as many as 10 5 seamounts having a radius of ≥ 3.5 km and height ≥ 2 km (Wessel, 2001), and perhaps 10 6 to 10 7 features > 100 m in height (Hillier and Watts, 2007).Given the ubiquity of these features on ridge flanks, it is surprising how little we know about which seamounts are hydrologically active-how many recharge and how many discharge.Observational and modeling studies suggest that smaller features are favored sites of discharge, perhaps because it is easier to maintain warm conditions during upflow when that flow occurs through a smaller edifice, whereas larger features are more likely to remain cold (Fisher et al., 2003a;Hutnak et al., 2007Hutnak et al., , 2008)) seamounts as Conduits for Massive Fluid, heat, and solute Fluxes on Ridge Flanks B y a N d R e w t.F i s h e R a N d C .G e o F F R e y w h e at Measurements of conductive heat flow typically fall below predictions of lithospheric cooling models until an average lithospheric age of ~ 65 Ma.

Figure 1 .
Figure 1.Calculated ridge-flank hydrothermal advective heat output, fluid discharge, and discharge per seamount.(a) Global cumulative advective heat output as a function of seafloor age (data from Mottl, 2003, and references therein).Red dotted line = C. stein et al. (1995).Blue dashed line = sclater et al.(1980).Green short dashed line =Mottl (2003).Most of this advective heat loss occurs on ridge flanks, areas far from the thermal influence of magmatic emplacement at seafloor spreading centers.(B) Global cumulative fluid discharge necessary to advect the amount of heat estimated byMottl (2003), plotted as function of age.Curves are shown for assumed temperature differences between bottom water and hydrothermal fluid of 5°C, 20°C, and 65°C.Lower thermal values are most typical of ridge-flank hydrothermal circulation that mines lithospheric heat.The higher thermal value is characteristic of weak ridge-flank circulation that results in significant local fluid and rock alteration, but has little regional thermal influence.(C) Calculated fluid discharge per seamount, assuming that all of the fluid flow estimated earlier passes through seamounts.The number of hydrothermally active seamounts is estimated to be somewhere between 10 4 and 10 6 , based on mapping and seamount population estimates by wessel (2001) and hillier and watts(2007), and the observation that, of the seamounts and outcrops that have been surveyed, a significant fraction appear to be hydrothermally active(Fisher et al., 2003a(Fisher et al., , 2003b;;hutnak et al., 2008;Villinger et al., 2002).also shown are estimates of the fluid flux through Baby Bare and dorado seamounts, as discussed in the text.
the next section) indicate that the driving forces for ridge-flank hydrothermal circulation tend to be no more than a few hundreds of kilopascals, and are often much less.Pressure differences this small cannot sustain fluid flow through marine sediment at rates sufficient to extract a significant fraction of lithospheric heat once sediment thickness exceeds 10 -20 m.Thus, the vast majority of fluid flow that is responsible for advective heat loss from oceanic lithosphere must bypass sediment rather than flow directly through sediment.Seamounts help to connect the oceanic crustal hydrogeologic system to the overlying ocean, along with fracture zones, large igneous provinces, and other basement outcrops.The qualitative importance of seamounts to ridge-flank hydrogeology has been appreciated for decades, but researchers generally avoided collecting heat flow data in proximity to these features (or discounted such data when they were collected), because many seafloor thermal studies were intended to test lithospheric cooling models.In this context, the confounding influence of advective heat extraction around seamounts was to be avoided, not quantified.In addition, the lack of continuous multibeam bathymetric maps and precise navigation upon which seismic and heat flow surveys could be collocated often limited quantitative interpretation of heat flow data collected close to seamounts.Improvements in the availability and quality of bathymetric, seismic, and closely spaced transects of heat flow data, and renewed interest in understanding the dynamics and impacts of ridge-flank hydrothermal circulation, have focused attention in recent years on the physical and chemical composition of fluid flow through seamounts on ridge flanks

Figure 2 .
Figure 2. sediment type, permeability, and limits on fluid flow through seafloor sediments on ridge flanks (figures modified from spinelli et al., 2004).(a) distribution of seafloor sediment types.sediment tends to be thinner near mid-ocean ridges (defined by 1 m.y.isochrons, black lines) and thicker on older seafloor, close to large continental areas affected by terrigenous inputs, where upwelling supports high biogenic productivity, and at high latitudes affected by ice-rafted debris. in contrast, thin sediment exists below subtropical gyres, particularly in the North atlantic and south Pacific oceans.(B) typical permeability of seafloor sediments as a function of sediment thickness.The dashed vertical line shows the typical lower limit on permeability of upper oceanic basement rocks (see recent compilation of basement observations inFisher et al., 2008).(C) typical fluid seepage speeds through marine sediments of various kinds (solid curves, as labeled), based on typical excess fluid pressures of 20 kPa, and minimum seepage speeds necessary to detect fluid seepage via pore water solute (green curve) or thermal data (red curve).also shown is the typical flow rate needed to remove a significant fraction of lithospheric heat on a regional basis (blue curve).For the vast majority of seafloor areas covered with sediments, fluid flow is not capable of extracting a significant fraction of lithospheric heat, except by bypassing the sediment layer entirely through seamounts or other basement outcrops.
The hydrothermal siphon concept applies to ridge-flank hydrothermal circulation in general, not only through seamounts, but also in the crust, where fluid temperature and pressure differences tend to be small when large areas of basement are exposed because the extent of circulation keeps the crust cool on a regional basis.There are additional hydrothermal circulation systems associated with individual seamounts on ridge flanks(Harris et al., 2004).Lateral thermal gradients associated with bathymetric relief can drive low-temperature fluid circulation through these features, without seamount-to-seamount flow.These flows may impact local thermal conditions, and the global fluid flux associated with these systems could be large.But because of low fluid temperatures and short residence times, these flows are likely to have a smaller influence on crustal fluid composition than systems discussed in this study.

Figure 3
Figure 3. schematic illustrations of the causes and impacts of ridge-flank hydrothermal circulation guided by seamounts.within recharging and discharging seamounts (a and B, respectively), fluid temperatures (T fluid ) and densities (ρ fluid ) remain relatively constant so long as vertical transport (descent and ascent) is sufficiently rapid.The difference between fluid pressures (P fluid ) at the base of recharging and discharging columns of fluid (C) is the primary driving force for ridge-flank hydrothermal circulation.This pressure difference is available to move fluid laterally within the reactive basement aquifer (d), mining heat, reacting with the crust, and providing nutrients and/or energy to microbial communities living in pore spaces (wheat andFisher, 2008).seafloor heat flow is generally higher adjacent to discharging seamounts, and lower adjacent to recharging seamounts.Between seamounts, heat flow may be equal to or less than conductive plate cooling values, depending on the regional efficiency of lithospheric heat extraction.
closely spaced boreholes have yielded rocks indicating very different alteration histories, often as a function of basement relief.For example, crustal samples recovered from Deep Sea Drilling Project (DSDP) Hole 417A, which penetrated basement that is topographically elevated relative to the surrounding area, are highly altered, consistent with an extensive history of warm ridgeflank hydrothermal flow (Donnelly

Figure 4 .Figure 5 .
Figure4.Concentrations of (a) phosphate and (B) magnesium versus temperature in ridge-flank fluids from the upper basaltic crust.These plots comprise a global compilation of pore water and borehole samples collected during scientific ocean drilling, by gravity/piston coring near basaltic outcrops, and by sampling seafloor seeps and springs.Phosphate data were published previously(wheat et al., 2003), whereas the magnesium data were compiled for the present study.data are classified qualitatively to indicate quality, as assessed by the authors: red circle = highest quality, blue square = moderate quality, and black diamond = fair quality.Factors involved in this analysis include the number of samples, the proximity of samples to the sediment-basalt interface, magnitude of pore water gradients, errors in estimated gradients, and availability of thermal data at the same location as concentration data to estimate the temperature of upper basaltic rocks.Purple lines indicate trends consistent with the highest-quality data.horizontal green lines indicate typical concentrations in bottom seawater, which is generally at a temperature of ~ 2°C.
tion of sediment thicknesses at locations where heat flow was measured, and downward continuation of thermal conditions shows that isotherms are swept downward by cold, recharging fluid in shallow crust adjacent to the outcrop edge.In contrast, warm fluid discharge from Baby Bare causes extremely high seafloor heat flow, and an upward sweeping of isotherms, adjacent this flow are quantified through consideration of fluid properties and pressures at recharge and discharge sites (Figure3).Given a regional basement fluid temperature of ~ 64°C, bottom water (fluid recharge) temperature of ~ 2°C, and a typical sediment thickness of ~ 600 m, the available driving pressure for hydrothermal flow between Grizzly Bare and Baby Bare is ~100 kPa.
. Mg concentrations from shallow sediment pore waters on and around Baby Bare show upward curvature indicative of fluid seepage at a range of speeds (Figure 6).The speed of fluid seepage is great relative to the time required for molecular diffusion of solutes to affect chemical profiles, but generally too slow to have an influence on conductive heat transport.Similar to Mg, redox-sensitive ions such as nitrate and dissolved oxygen are absent in Baby Bare basement fluids, likely lost as a result of diffusion into the overlying sediment pore waters and consumption by microbial processes.Because reactions involving nitrate in the sediment are relatively rapid, nitrate in bottom seawater is entirely consumed within the upper few centimeters of sediment.Likewise, nitrate that diffuses from the basaltic formation fluid upward and into overlying sediment is consumed rapidly.This explains why sediment pore water nitrate profiles from Baby Bare are not influenced significantly by the different speeds of upward seepage (Figure 6).

Figure 6 .
Figure6.Pore water profiles from gravity and piston cores collected on and near Baby Bare and dorado (locations shown in Figure5; data from wheatand Fisher, 2008;wheat et al., 2004b).Nitrate (a) and Mg (B) data from pore fluids collected near and on Baby Bare.Basement formation fluid within Baby Bare is highly altered relative to bottom seawater, having exchanged Mg for Ca almost entirely, and having all nitrate lost during lateral transport, probably as a result of diffusive exchange with overlying sediment.where there is rapid upward seepage, pore fluid compositions in shallow sediments are similar to basement fluids sampled from nearby springs and boreholes.Nitrate (C) and Mg (d) data from pore fluids collected near and on dorado.The composition of basement formation fluids in this area is similar to bottom seawater, so rapid upward seepage results in nearly constant Mg and nitrate concentrations in shallow sediment.in contrast, where seepage is slow or there is no seepage, microbial processes remove much of the nitrate in sedimentary pore waters, but have no effect on Mg concentrations.
Bare/Grizzly Bare area, cold fluid recharge is indicated by lower seafloor heat flow and downwardsweeping isotherms where sediment thins in proximity to basement exposure.In contrast, warm fluid discharge causes extremely high seafloor heat flow, and upward-sweeping isotherms, adjacent to areas of exposed basement.In the latter case, the temperature of the sediment-basalt contact often remains nearly isothermal as the contact shallows toward the seafloor.Surveys of five of the ten mapped seamounts on the cool part of the Cocos Plate indicate recharge through two, discharge through one of the crust being formed at a fast rate, with longer periods of magmatic activity at the spreading center and greater horizontal continuity in the crust.The rapid rate of fluid flow around Dorado is evident in the radiocarbon age of the basement fluid, only a few hundred years older than bottom " seaMouNts aRe ideaL PLaCes to saMPLe CRustaL FLuids as they exit the CRust aNd eNteR the oCeaN, to deteRMiNe theiR CheMiCaL aNd MiCRoBiaL ChaRaCteRistiCs, aNd to assess the iMPoRtaNCe oF this GLoBaL hydRoGeoLoGiC systeM oN the eVoLutioN oF eaRth's LithosPheRe, oCeaN, aNd BiosPheRe.
and Fisher, 2008).The short residence time of crustal hydrothermal fluids near Dorado, coupled with low crustal temperatures, explains why crustal fluid alteration is modest.Pore water profiles from sediments at the edge of Dorado generally show little difference from seawater values, whether or not there is significant fluid seepage (Wheat and Fisher, 2008; Figure 6D).One exception to this rule is nitrate, which reacts rapidly in the sediment because of microbially mediated processes.As a result, nitrate concentrations in sediment pore fluids around Dorado are lower than those of bottom seawater, with profile shapes affected by the rate of fluid seepage (Figure 6C).There has been no direct sampling of low-temperature fluids discharging from Dorado.Collecting pristine fluids as they exit exposed basalt would allow determination of formation-fluid composition without sedimentary (and associated sampling and processing) artifacts.For many elements, this fluid composition may be only 1% or 2% different from seawater, differences that approach analytical limits.These subtle differences in fluid composition could have a significant influence on global geochemical budgets because the associated fluid flows are so large

Figure
Figure 3D summarizes the key characteristics of ridge-flank hydrothermal circulation guided by seamounts.Networks of seamounts permit rapid fluid circulation to bypass thick and relatively continuous sediment across much . Only two field studies have been completed so far that allow identification of specific hydrothermal recharge and discharge sites, separated laterally by tens of kilometers, and only one field site has been sampled to characterize microbial processes and community structure.At each of these two sites, work has focused on a single pair of features, leading to a highly idealized view of fluid flow patterns.We don't know how seamounts make the hydrogeologic transition between an early stage dominated by volcanic construction and magmatic heating, and a later stage in which lithospheric cooling provides the primary heat source to drive hydrothermal circulation.Researchers have not resolved the influence of hydrothermal circulation through seamounts on subduction processes and recycling of elements from the ocean and crust into the mantle.These and many other aspects of seamount evolution, hydrogeology, geochemistry, and microbiology remain intriguing frontiers in oceanographic research.aCkNowLedGMeNts We thank Oceanography guest editors Hubert Staudigel and Anthony Koppers for encouragement, critical questions, and thoughtful suggestions as this manuscript was being prepared and revised.J. Alt, K. Becker, and C. Stein provided helpful reviews.This research was supported by National Science Foundation grants OCE-0550713 and OCE-0727952 (ATF) and OCE-0727119 and OCE-0727120 (CGW).