Geochemistry G3 Volume 9 Geophysics 2 December 2008 …afisher/CVpubs/pubs/WheatFisher... ·...

Massive, low-temperature hydrothermal flow from a basalticoutcrop on 23 Ma seafloor of the Cocos Plate: Chemicalconstraints and implications

C. Geoffrey WheatGlobal Undersea Research Unit, University of Alaska Fairbanks, P. O. Box 475, Moss Landing, California 95039, USA([email protected])

Andrew T. FisherEarth and Planetary Sciences Department and Institute for Geophysics and Planetary Physics, University of California,1156 High Street, Santa Cruz, California 95064, USA ([email protected])

[1] Systematic variations in pore water chemical and thermal profiles from sediment gravity cores indicatethe presence of a ‘‘cool’’ (10–20�C) ridge-flank hydrothermal system within basement surroundingDorado outcrop, a small basaltic edifice on 23 Ma seafloor of the Cocos Plate. Dorado outcrop is locatedwithin a 14,500-km2 region of cool seafloor, where 60–90% of the lithospheric heat is removedadvectively. Pore water chemical profiles from sediments on and near Dorado outcrop indicate a range ofdiffusive, advective, and diagenetic influences, including evidence for upward fluid seepage at up toseveral meters per year. The chemical composition of fluid that discharges from Dorado outcrop is onlyslightly different from that of bottom seawater. Pore water nitrate and geological constraints suggest aminimum volumetric flux per unit width of basement of 1800 m3 a�1 cm�1 and a total seawater flowthrough Dorado outcrop of �3000 kg a�1. This flow rate is orders of magnitude greater than that estimatedfrom Baby Bare outcrop, a similarly sized basement edifice on younger seafloor on the eastern flank of theJuan de Fuca Ridge. The nearest likely basement recharge site is Tengosed Seamount located �20 kmaway. Calculated rates for the specific discharge at Dorado outcrop are consistent with young 14C ages,suggesting a residence time in basement no greater than a few hundred years. If the fluid exiting fromDorado outcrop is characteristic of ridge-flank hydrothermal circulation in general (cool, relativelyunaltered), these systems can have an important influence on global geochemical budgets for many solutes(e.g., chloride, magnesium, sulfate, potassium, lithium, boron, silica, phosphate, manganese, and iron)because the rate of fluid discharge is so large. Ridge flank fluids having the same composition of fluidexiting Dorado outcrop also may contribute to subseafloor microbial processes within basaltic basementand the overlying sediment, and suggest that oxidation reactions within basaltic crust can continue wellbeyond 10 Ma in some settings.

Components: 10,687 words, 5 figures, 2 tables.

Keywords: hydrothermal; ridge flank; Cocos Plate; seawater fluxes; pore water; nitrate.

Index Terms: 3017 Marine Geology and Geophysics: Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424); 3021

Marine Geology and Geophysics: Marine hydrogeology; 1050 Geochemistry: Marine geochemistry (4835, 4845, 4850).

Received 25 June 2008; Revised 30 September 2008; Accepted 21 October 2008; Published 2 December 2008.

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Q12O14, doi:10.1029/2008GC002136

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Wheat, C. G., and A. T. Fisher (2008), Massive, low-temperature hydrothermal flow from a basaltic outcrop on 23 Ma

seafloor of the Cocos Plate: Chemical constraints and implications, Geochem. Geophys. Geosyst., 9, Q12O14,

doi:10.1029/2008GC002136.

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Theme: Formation and Evolution of Oceanic Crust Formed af Fast Spreading RatesGuest Editors: D. A. H Teagle and D. Wilson

1. Introduction

[2] The hydrothermal heat flux from seafloor ridgeflanks, areas far from the magmatic and thermalinfluence of seafloor spreading, is �8 TW, com-prising �30% of the total heat flux from theoceanic lithosphere [e.g., Parsons and Sclater,1977; Stein and Stein, 1994]. Global estimates ofadvective seafloor cooling are based on the differ-ence between heat flow observations and conduc-tive predictions from lithospheric cooling models,the latter being well constrained by global bathy-metric and heat flow data sets. The comparison ofheat flow observations and conductive coolingmodels suggests that the largest fraction of ridge-flank heat loss occurs within young plates, throughseafloor <10 Ma in age, but measurable advectiveheat loss continues on average until seafloor rea-ches 65 Ma. Some older seafloor sites continue tolose heat advectively beyond this age, and hydro-thermal circulation redistributes heat regionally andlocally at numerous sites beyond 100 Ma [e.g., VonHerzen, 2004].

[3] Although ridge-flank hydrothermal circulationhas been studied for several decades, it has provendifficult to resolve several important characteristicsof fluid, heat, and solute transport within thesesystems. It has been particularly challenging toidentify and sample sites of low-temperature,ridge-flank hydrothermal egress and to quantifythe global geochemical impacts of these circulationsystems. There are subtle differences in the com-position of most ridge-flank hydrothermal fluids,relative to bottom seawater, because typical fluidreaction temperatures are low (�10–20�C), lead-ing to slow inorganic reaction kinetics, and resi-dence times are short. But because fluid flows areso large (approaching or exceeding riverine flowson a global basis), even small differences in fluidcomposition can have a major influence on thebudgets and cycling of biochemically importantsolutes in seawater [e.g., Wheat et al., 2003; Wheatand Mottl, 2004].

[4] The small number of ridge-flank hydrothermalsites that has been studied fall into two generalcategories. Most of these sites are located on youngseafloor where sediment cover is patchy and base-ment rocks are exposed across wide areas, leadingto a large fraction of advective heat loss, and lowbasement fluid temperatures [e.g., Williams et al.,1974; Langseth et al., 1984; Johnson et al., 1993;Villinger et al., 2002]. In cool and open circulationsystems such as these, it is difficult to trace fluidpathways, locate or link hydrothermal recharge anddischarge sites, determine rates of transport orresidence times, or collect samples of pristinehydrothermal fluids.

[5] At the other extreme are warmer and morerestricted ridge-flank systems where basement tem-peratures are higher and fluids are more altered[e.g., Mottl and Wheat, 1994; Wheat and Mottl,1994]. Fluid pathways and flow rates are morereadily quantified in such settings, but the associ-ated advective heat fluxes are relatively small andhave little influence on regional heat loss. Al-though chemical fluxes from warm ridge flankhydrothermal systems such as these may influencesome geochemical budgets, these systems are notcharacteristic of processes that remove large quan-tities of lithospheric heat on a global basis [e.g.,Wheat and Mottl, 2000].

[6] Both kinds of ridge-flank hydrothermal sys-tems are found in the FlankFlux region on theeastern flank of the Juan de Fuca Ridge, cool/openand warm/restricted [e.g., Davis et al., 1992;Wheatand Mottl, 1994; Elderfield et al., 1999; Wheat etal., 2000, 2004; Hutnak et al., 2006]. At thewestern end of this region close to the ridge(�1.5Ma seafloor), seafloor heat flow is suppressedby 60–90% relative to lithospheric predictions, andtemperatures in the uppermost basaltic basementaquifer are low [Davis et al., 1992; 1999; Hutnaket al., 2006]. Here, the composition of the basalticformation fluid is slightly different from that ofoverlying seawater, as inferred from pore watersamples squeezed from sediment cores recovered

GeochemistryGeophysicsGeosystems G3G3

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from just above basaltic basement [e.g., Elderfieldet al., 1999]. Because there is extensive basalticexposure in this area, it is difficult to resolvepatterns and pathways of fluid circulation.

[7] In contrast, at the eastern end of the FlankFluxregion (3.5–3.6 Ma seafloor), fluid flow in basalticbasement below thick sediment redistributes heatlocally [Davis et al., 1992; Spinelli and Fisher,2004], and a few basaltic outcrops (e.g., Baby Bareoutcrop) focus discharge of warm, highly alteredbasaltic formation fluids [Wheat and Mottl, 2000;Wheat et al., 2004]. Sampling warm formationfluids in this area is easier because discharge isso restricted; however, the small flows have virtu-ally no influence on regional heat loss from theplate [Davis et al., 1999; Fisher et al., 2003a;Hutnak et al., 2006]. Hydrothermal circulation inthis area is currently a weak vestige of what wasonce amuchmore open and efficient system, like thewestern FlankFlux region, before Pleistocene sedi-mentation buried large areas of basaltic basement[Underwood et al., 2005;Hutnak and Fisher, 2007].

[8] In the current study we present pore watergeochemical data from a ridge-flank hydrothermalsystem on 23 Ma seafloor of the Cocos Plate,eastern Pacific Ocean (Figure 1). This hydrother-mal system has characteristics that make it ideal forquantifying the global impacts of low-temperaturefluid circulation. Similar to the eastern FlankFluxarea, fluid recharge and discharge are focusedthrough a small number of basement outcrops thatpenetrate thick, low-permeability sediments [Fisheret al., 2003b; Hutnak et al., 2007]. But like thewestern FlankFlux area, fluid circulation extracts60–90% of lithospheric heat across a large region,and fluid temperatures are generally low [Hutnak etal., 2008]. We present data from sites and sampleson and adjacent toDorado outcrop (Figure 1), a smallbasement edifice that discharges cool hydrothermalfluid from the underlying crust. Fourteen gravitycores were collected on and near Dorado outcrop;twelve of these coreswere instrumentedwith thermalprobes that allowed determination of the local ther-mal gradient [Hutnak et al., 2008]. We present porewater chemical data from these cores to constrain therate of seawater flow through the outcrop andsurrounding crust, and provide insights as to theinfluence of low-temperature ridge-flank hydrother-mal fluids on oceanic geochemical budgets.

2. Geologic and Geophysical Setting

[9] The seafloor of the Cocos Plate offshore theNicoya Peninsula, Costa Rica, is underlain by 18–

24 Ma lithosphere generated at the fast spreadingEast Pacific Rise (EPR) to the west, and themedium spreading Cocos Nazca Spreading Center(CNS) to the south [e.g., Meschede et al., 1998;Ranero and von Huene, 2000; Barckhausen et al.,2001] (Figure 1). The boundary between EPR- andCNS-generated seafloor, known as the ‘‘plate su-ture,’’ separates areas having isochrons that aresubparallel or perpendicular to the nearby MiddleAmerica Trench. Here the TicoFlux surveys exam-ined regional patterns and processes and assessedthe influence of seamounts, faults, and other localfeatures in guiding fluid flow [Fisher et al., 2003b;Hutnak et al., 2007, 2008] (Figure 1a). Swath-mapdata overlain on satellite gravimetric data [Smithand Sandwell, 1997] helped to locate basalticbasement outcrops (Figure 1b). The largest out-crops (seamounts) were apparent in satellite-basedbathymetric maps, and swath data revealed addi-tional smaller outcrops. Basaltic outcrops are mostcommon on the EPR-generated seafloor northwestof the plate suture and on CNS-generated seafloorlocated to the south of a ridge-jump trace. Multi-channel seismic reflection data were used to locateregional tectonic features and to map sedimentthickness and basement relief. Multipenetrationheat flow stations, and additional measurementsmade with outrigger probes attached to sedimentcore barrels, were generally located along seismicreflection profiles to assess the temperatures inupper basaltic basement and constrain patterns ofhydrothermal circulation. Coring operations tar-geted buried basement highs and areas of thinsediment adjacent to basaltic outcrops, with theintent of characterizing the composition of forma-tion fluids in areas with upward fluid seepage.

[10] TicoFlux surveys define an abrupt thermaltransition between warmer and cooler parts of theCocos Plate. This transition is more closely asso-ciated with the distribution of basement outcropsthan with major tectonic boundaries [Fisher et al.,2003b; Hutnak et al., 2007] (Figure 1b). The Tico-Flux and earlier surveys delineated a 14,500 km2

region within which seafloor heat flow was typi-cally 10–40 mW/m2, just 10–40% of lithosphericvalues [Hutnak et al., 2008]. Ten seamounts andother basaltic outcrops mapped within this coolregion of the survey area. Collectively, these sea-mounts and outcrops comprise �260 km2 ofexposed basaltic basement, <2% of the seafloorarea. Heat flow and seismic surveys oriented radi-ally away from individual outcrops indicate thatsome basaltic outcrops/seamounts allow hydrother-


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Figure 1



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mal recharge whereas others allow hydrothermaldischarge [Hutnak et al., 2007, 2008], a patternobserved on younger seafloor east of the Juan deFuca Ridge [Davis et al., 1992; Wheat et al., 2000;Fisher et al., 2003a; Wheat et al., 2004; Hutnak etal., 2006]. Cold fluid recharge is indicated by adecrease in seafloor heat flow and a downwardsweeping of isotherms where sediment thins inproximity to a basaltic outcrop. In contrast, warmfluid discharge results in extremely high seafloorheat flow and an upward sweeping of isothermsadjacent to an area of exposed basaltic basement.In the latter case, the temperature of the sediment-basement contact often remains nearly isothermalas the contact shallows toward the seafloor.

[11] The focus of the present study is Doradooutcrop, a 0.25-km2 area of basement exposureon 23-Ma seafloor of the cool side of the CocosPlate (Figure 1c). Dorado outcrop is the seafloorexpression of a much larger basaltic edifice that ismostly buried by sediments. This feature is similarin some ways to Baby Bare outcrop in the easternFlankFlux area. Dorado and Baby Bare outcropshave similar shapes and areas of basement expo-sure. Both extend 50 to 80 m above the seafloor,have a seafloor expression that is elongated in thenorth-south direction, parallel to an active spread-ing center to the west, are surrounded by 400–500 m of widely continuous sediments, are sepa-rated from potential recharge sources by tens ofkilometers (Baby Bare outcrop and Grizley Bareoutcrop are separated by 52 km; Dorado outcropand Tengosed Seamount are separated by 20 km)(Figure 1b), and were likely formed during off-axis volcanism [Karsten et al., 1998; Becker etal., 2000; Barckhausen et al., 2001; Silver et al.,2004].

[12] However, the two outcrops are different inseveral critical respects. Baby Bare outcrop lieson 3.5 Ma crust that was generated at an interme-diate spreading rate. The temperature in uppermostbasaltic basement around Baby Bare outcrop is

60–65�C and measured heat flow values within afew kilometers from the outcrop are near thosepredicted by lithospheric cooling models. Herewarm hydrothermal formation fluids that seep fromthe crust are highly altered and discharge at 4–13 kg/s [Mottl et al., 1998]. In contrast, Doradooutcrop overlies 23 Ma crust that was formed at afast spreading rate. Hydrothermal fluids in basalticbasement within and surrounding this feature aregenerally 10–20�C. The mass flux of hydrothermalfluid from Dorado outcrop appears to be aboutthree orders of magnitude greater than that fromBaby Bare outcrop, as discussed below.

3. Sampling and Analytical Methods

[13] Fourteen gravity cores, many equipped withself-contained temperature recorders, were attemp-ted on and near Dorado outcrop (Figure 1c). Mostof these cores recovered 0.4–3.8 m of sediment,one core recovered basalt chips, and one core failedto penetrate the seafloor and returned empty andwith a damaged core cutter, indicating that Doradooutcrop has a thin and patchy sediment cover withlikely areas of exposed basalt. Two piston cores(PC-44 and �48) were collected far from Doradooutcrop (locations shown in Figure 1b) and provideuseful regional data for comparison to thosederived from Dorado gravity cores.

[14] Sediment pore water was extracted by splittingthe gravity cores and placing sediment not affectedby smearing along the edge of the core liner intocentrifuge tubes. These tubes were cooled to 1–4�Cand spun for 5 min at �11,000 rpm using a rotorhead that was precooled to �20�C so that samplesdid not warm during centrifugation. Pore watersrecovered after centrifugation were filtered through0.45 mm filters and stored in a variety of acid-washed plastic and glass containers for ship andshore-based analyses. Some aliquots were acidifiedwith subboiled 6N HCl. Sediment samplesintended for pore water C isotopic analyses were

Figure 1. Site maps. (a) The TicoFlux area is located off the coast of Coast Rica on 18–24 Ma seafloor of theCocos Plate, comprising lithosphere produced at both the Cocos-Nazca Spreading Center and the East Pacific Rise.The red box indicates the map area in Figure 1b. (b) Multibeam data are overlain on gravitational bathymetry toillustrate seafloor relief (modified from Hutnak et al. [2008]). Dorado outcrop and Tengosed Seamount are locatedwithin a 14,500 km2 area of cool seafloor, where the heat flux is generally 10–40% of lithospheric predictions (northand west of the white line that delineates areas with cool and warm seafloor). Most of the pore water data discussed inthis study were collected by gravity coring on and near Dorado outcrop, but data are also presented from two pistoncores (PC-44 and PC-48) collected on warmer crust at the locations shown. (c) Fourteen gravity cores were taken onand near Dorado outcrop. Pore water data from all of these cores are shown in Figure 2, and results from Cores GC-36, �40, �43, and �50 are highlighted and discussed in the text.

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processed within a nitrogen-filled glove bag. Aftercentrifugation, aliquots for C isotopic analysis weretransferred within a nitrogen-filled glove bag intoevacuated containers containing a few milligramsof HgCl2.

[15] Ship-based analyses included ion selectiveelectrodes (pH), potentiometric titration (alkalinityand chlorinity), and colorimetric (phosphate) tech-niques. Shore-based analysis included inductivelycoupled plasma (ICP) emission spectrometry, ionchromatography, colorimetric, and mass spectro-metric techniques (Tables 1 and 2; auxiliary mate-rial includes all of the data).1 Stable isotopicanalyses were conducted by D. McCorkle atWoods Hole Oceanographic Institute and radiocar-bon measurements were made at the National

Ocean Sciences Accelerator Mass SpectrometryFacility in Woods Hole.

4. Geochemical Results

4.1. Evidence for Pore Water FlowThrough Sediments

[16] Pore water chemical profiles for alkalinity,phosphate, nitrate, and manganese are presentedin Figure 2. Data are shown for all cores collectednear Dorado, and results are highlighted for fourcores that illustrate the influence of particularconditions (Cores GC-36, �40, �43, and �50).Systematic variations in pore water chemical pro-files indicate a range of diagenetic effects caused orlimited by the degree to which basaltic formationfluids seep upward through thin sediment (Figure 2).In general, if the flow of basaltic formation fluidsthough sediment is rapid, diagenetic reactions andthe associated diagentic flux from the sediment to/

Table 1. Concentrations of Bottom Seawater and the Estimated Concentration of Basaltic Formation Fluid FromDorado Based on Systematic Differences in Pore Water Chemical Profiles as the Basis for Calculating a Global RidgeFlank Flux Relative to the Riverine Fluxa

Units SeawaterDorado BasalticFormation Fluid River Fluxb (%)

Baby Bare BasalticFormation Fluid

Temperature �C 2 10 64Alkalinityc mmol/kg 2.52 2.38 2 0.43Chlorinityc mmol/kg 539.5 541 �110 554.4Cad mmol/kg 10.24 10.2 4 55.2Mgd mmol/kg 52.6 52.4 17 0.98Sulfatee mmol/kg 27.9 27.3 87 17.8Nad,f mmol/kg 462 462 380 473Kd mmol/kg 10.17 9.8 130 6.88Lid mmol/kg 26.0 25.5 17 9.0Srd mmol/kg 88 88 19 110Bad mmol/kg 0.13 0.13 0.1 0.43Bd mmol/kg 418 420 �17 570Silicad mmol/kg 170 <320 �11 360Phosphateg mmol/kg 2.90 2.0 14 0.3Nitrateg mmol/kg 42.3 40.9 0.2 0Mnd mmol/kg 0.0 0.3 �28 2.9Fed mmol/kg 0.0 0.5 �10 <0.05d18Oh % �2.5 �2.9 0.3dDh % �0.69 �0.76 1

aThe formation fluid composition for Baby Bare is provided for comparison.

bNumbers in bold are calculated from the anomaly based on data from Dorado outcrop. Nonbold numbers are calculated assuming a 1%

chemical anomaly in formation fluids (Ca, Na, Sr, and Ba). Both calculations use a conservative ridge flank hydrothermal fluid flux of 8 � 1015 kga�1 (comparable to the estimated flux of 2.1 � 1016 kg a�1 by Wheat et al. [2003]), and riverine fluxes from Mackenzie [1992], Li [2000], Edmondet al. [1979], Spivack and Staudigel [1994], Berner and Rao [1994], Von Damm et al. [1985], Elderfield and Schultz [1996], and Treguer et al.[1995]. Positive fluxes are into the oceanic crust.

cPotentiometric titrations.

dInductively coupled plasma emission spectrometry.

eIon chromatography.

fCharge balance.gColorimetric.

hMass spectrometric techniques.

1Auxiliary materials are available in the HTML. doi:10.1029/2008GC002136.



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from the pore water are small relative to theadvective flux. In this case, diagenetic reactionshave a minimal effect on solute concentrationsduring ascent, and the pore water compositionremains close to that in basaltic formation fluids.For example, Cores GC-40 and GC-50 were takenwhere sediment thins along the southwestern sideof Dorado and have systematic pore water chem-ical profiles consistent with rapid upward seepage.These profiles are similar to those observed at theMariana Mounds in the western Pacific Ocean,where there are seafloor springs [Wheat andMcDuff, 1994]. However, if the speed of porewater seepage is slower, such that the advectiveflux is of the same magnitude as the diageneticflux, then diagenetic reactions affect pore waterprofiles (e.g., GC-43); it may still be possible toestimate the composition of the basaltic formationfluid from such profiles, depending on the sedi-ment thickness and the seepage rate. Finally, if the

advective flux is much smaller than the diageneticflux, then diagenetic reactions dominate pore waterprofiles, making it difficult to estimate the compo-sition of the basaltic formation fluid even if acomplete sediment section is recovered. CoreGC-36 was taken 2 km northeast of Dorado, wheresediment cover is �100 m thick and chemicalprofiles are consistent with diffusion and reactionbeing the dominant diagenetic processes. Thesesediments are sufficiently thick to prevent upwardfluid seepage, even if basement is slightly over-pressured [e.g., Wheat and Mottl, 1994; Spinelli etal., 2004].

[17] Measured heat flow values are consistent withfluid flow in basement around Dorado outcrop.Away from Dorado outcrop heat flow values arelow, generally 20–30 mW/m2, whereas on or nearDorado outcrop (e.g., GC-40 and GC-50) valuesexceed 600 mW/m2 [Hutnak et al., 2008]. Al-

Figure 2. Pore water profiles for selected chemical species (alkalinity, phosphate, nitrate, and manganese) withresults highlighted from four of the fourteen cores collected on and near Dorado outcrop. Cores GC-40 and �50 weretaken on/near Dorado outcrop and are affected by the upward seepage of a basaltic formation fluid that is slightlyaltered relative to bottom seawater. Core GC-36 was collected �2 km northeast of Dorado outcrop. Pore waterchemical profiles from this core are characteristic of diagenetic profiles in the absence of fluid seepage. Data fromcore GC-43 illustrate an intermediate case of fluid seepage.

Table 2. Concentration, Isotopic Compositions, and Calculated Ages for Seawater and Pore Water Samples FromNear the Base of the Sampled Section in Cores From Areas With Upwelling Formation Fluidsa

TCO2 (mmol/kg) d13C (% PDB) 14C (y) 14C Error (y) Heat Flowb (W/m2) Temperaturec (�C)

Seawater 2.336 �0.19 2165 25 N.A. 2GC-50 2.234 �0.37 2495 25 1.0 10PC-44d 0.862 �5.13 35600 280 0.63 75PC-48e 2.181 �4.73 14650 63 0.13 40–45

aLocations are shown in Figure 1. Analyses were conducted by D. McCorkle at WHOI and at the National Ocean Sciences Accelerator Mass

Spectrometry Facility in Woods Hole.bHeat flow data tabulated and reported by Hutnak et al. [2008].

cUpper basement temperature below sediment cores estimated by downward continuation of measured thermal gradient [Hutnak et al., 2007].

Temperature values are higher in areas of higher heat flow and/or thinner sediment.dCore was collected 88 km NE of GC-50 [e.g., Friedmann, 2003; Wheat and McManus, 2005].

eCore was collected 142 km SSE of GC-50 [e.g., Wheat and McManus, 2005].



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though some of the elevated heat flow near theoutcrop may result from rapid upward fluid seepage,high heat flow results mainly from relatively iso-thermal conditions in uppermost basement (typicalbasement temperatures of 10–20�C), as the sedi-ment cover thins adjacent to a local area of basementexposure. Isothermal basement conditions arecaused by vigorous local convection, which canalso be associated with overpressured fluids andlead to seepage at chemically significant rates wheresediments are thin. None of the seepage rates esti-mated from chemical data in the present study aresufficiently rapid so as to result in curved thermalgradients within the upper few meters of sediment.

[18] Chemical profiles from GC-40, �43, and �50are consistent with the upward seepage of a basalticformation fluid having a nitrate concentration closeto that of bottom seawater [e.g., Gieskes andBoulegue, 1983; Bender et al., 1985; Wheat andMcDuff, 1994]. In the present case, as this fluidseeps upward through the sediment near the edge ofthe outcrop, microbial activity within the sediment

reduces the concentration of nitrate. Slower flowresults in lower pore water nitrate concentrations.Nitrate is fully depleted in GC-36, in which there isno evidence for advective transport. Pore water dataare also consistent with a basaltic formation fluidthat has phosphate and alkalinity concentrationslower than those of seawater, whereas manganeseconcentrations are elevated above seawater con-centrations (Table 1, Figure 2). Similar argumentscan be used to describe pore water phosphate,alkalinity, and manganese profiles in response tomicrobial degradation of organic matter in thecontext of upward pore water seepage.

[19] The rate that formation fluids seep upwardsthough the sediment is quantified using an advec-tion-diffusion-reaction model. An example, basedon nitrate profiles, constrains order-of-magnitudefluid seepage rates and provides a visualization ofexpected nitrate profiles at a variety of flow con-ditions and sediment thicknesses. A more preciseestimate of flow rates is not warranted at presentbecause of uncertainties in the sediment thicknessand rate of denitrification near the sediment-basaltcontact.

[20] We use first order reaction terms for nitrification:

Ds;NO3@2NO3=@z

2 � v@NO3=@zþ gn knO2 ¼ 0 ð1Þ

and denitrification

Ds;NO3@2NO3=@z

2 � v@NO3=@z� kdNO3 ¼ 0; ð2Þ

where Ds,NO3 is the sediment diffusion coefficientfor nitrate, NO3 is the nitrate concentration, z isdepth, v is the pore water velocity, gn is theRedfield ratio of nitrate produced to oxygenconsumed during nitrification, kn is the rate ofnitrification, and kd is the rate of denitrification[Wheat and McDuff, 1994]. The equations aresolved subject to the boundary conditions:

NO3 z¼0ð Þ ¼ NO3 0ð Þ ð3Þ

and

NO3 z¼zbotð Þ ¼ NO3 botð Þ; ð4Þ

where NO3(0) is the nitrate concentration in bottomseawater and NO3(bot) is the nitrate concentration inthe basaltic formation fluid [e.g., Wheat andMcDuff, 1994, Appendix A].

[21] Simulated and measured nitrate profiles areshown in Figure 3. A 100-m-thick sediment section

Figure 3. Comparison of observed and calculated porewater nitrate concentrations, based on a one-dimen-sional advection-diffusion-reaction model. The bottomseawater concentration is indicated by the red filledcircle. This plot illustrates the effects of pore wateradvection on nitrate profiles. Closer fits to the data arepossible by varying the depth to basaltic basement, flowrate, and rates of reaction. The background simulation(0 cm a�1, no flow) was calculated with a depth tobasaltic basement of 100 m, whereas simulations withflow are based on a sediment thickness of 5 m. The flowof seawater within the sediment is sufficiently fast nearand on the outcrop that there is little change in porewater composition relative to the composition in basalticformation fluid.



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is used for the case of no flow, as in the location ofcore GC-36. Here the rate of denitrification mustbe sufficiently rapid to remove the nitrate from thepore water in the upper 25 cm of the sediment(assuming that the upper tens of centimeters ofsediment was not lost during gravity coring oper-ations, consistent with observations). A slower rateof denitrification is suggested at core locationswhere there is evidence for seepage, perhaps be-cause of differences in sedimentation or compac-tion histories associated with basement relief.These calculations suggest that seepage rates maybe as high as several meters per year through thethin sediment immediately adjacent to and onDorado outcrop (Figure 3). These calculations alsosuggest that there is a net flux of nitrate into thecrust during seawater circulation in basement, priorto upward seepage through sediments, which couldresult from processes within basaltic basement orthe overlying sediments. Nitrate could be removedfrom basaltic formation fluids by diffusion to theoverlying sediment pore waters [e.g., Wheat andMcDuff, 1995; Bender et al., 1985] or by microbialreactions within basaltic basement [Huber et al.,2006; Santelli et al., 2008]. At much higher tem-peratures than those observed at Dorado outcrop,nitrate concentrations in seawater can be reducedwithin basaltic basement in the presence of reducediron [e.g., Gieskes et al., 1983].

4.2. Chemical Constraints for SeawaterCirculation Through the Basaltic Crust

[22] Pore water data are consistent with a basalticformation fluid that has a nitrate concentration of41 mmo/kg (40.9 ± 0.6 mmo/kg). This value isbased on pore water samples from GC-40 and �50(excluding the deepest sample) that were extractedfrom sediment at least 50 cm below the seafloor,thus minimizing artifacts caused by nitrification.This average is slightly less than the seawaterconcentration of 42.3 mmo/kg (Table 1). Thisinterpretation constrains a conceptual hydrologicmodel (Figure 4). High concentrations of nitrate inbasaltic formation fluids require that seawaterenters basaltic basement though outcrops or otherareas of basement exposure with minimal sedimentcontact or interaction because diagenetic reactionsinvolving sediment and pore water would rapidlydeplete the fluid of nitrate (e.g., GC-36). Theclosest area of basement exposure to Dorado out-crop, based on nearly complete swathmap coveragearound Dorado outcrop, is Tengosed Seamount, amuch larger outcrop located 20 km to the east(Figure 1b). In contrast to patterns of seafloor heatflow observed near Dorado outcrop, heat flowprofiles oriented radially adjacent to TengosedSeamount show abrupt decreases in heat flowimmediately adjacent to areas of exposed base-ment, resulting from downward sweeping of iso-

Figure 4. Conceptual model of the crustal hydrogeologic system that results in fluid discharge at Dorado outcrop.Cold oxygenated seawater enters basaltic basement at Tengosed Seamount, consistent with a depressed heat flowsurrounding the seamount [Hutnak et al., 2007, 2008]. This basaltic formation fluid flows toward Dorodo outcrop,located �20 km away. Here the heat flow is elevated relative to local values, even through the circulating fluid isrelatively cool, because mixed convection and the rapid flow rate in basement helps to keep the sediment-basementinterface relatively isothermal. As seawater transits within basaltic basement between Tengosed Seamount andDorado outcrop, the fluid warms by �8–18�C, reacts with basaltic crust, undergoes diffusive exchange withoverlying sediment pore water, and may be altered by microbial processes. Gravity cores targeted the thinnestsediment on the Dorado outcrop with the intent of sampling fluids from a region with upward fluid seepage.



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therms, consistent with the seamount being a site ofseawater recharge [Hutnak et al., 2008]. There maybe other (more distant) recharge sites that supplyformation fluids that discharge at Dorado outcrop,but we focus discussion on Tengosed Seamountbecause it is the nearest edifice through which thereis thermal evidence of recharge and associatedbasement cooling. There may be additionalsmaller outcrops near Dorado outcrop that remainunmapped, but observational and modeling stud-ies suggest that smaller outcrops are favored sitesfor hydrothermal discharge, whereas larger out-crops are favored sites of hydrothermal recharge[e.g., Fisher et al., 2003a; Hutnak et al., 2006,2008].

[23] Seawater that recharges through TengosedSeamount flows within upper basaltic basement

where it warms and reacts via biotic or abioticprocesses (Figure 4). There is a diffusive flux ofnitrate from the basaltic formation fluid to overly-ing sediment pore water, with nitrate being con-sumed by microbial processes in the sediment. The(slightly) warmed and altered formation fluid thenseeps through thin sediment or discharges fromareas of basement exposure on Dorado outcrop. Ifthe discharge rate is sufficiently rapid, the forma-tion fluid composition is maintained until the fluidexits the seafloor and is diluted by bottom seawa-ter. The observation that formation fluids seepingfrom Dorado outcrop have nitrate concentrationsonly slightly different from that of seawater, de-spite steep nitrate gradients within the sediment,requires that the advective flux of nitrate throughthe upper basaltic crust be much greater than thesum of removal fluxes from diffusion exchangewith the overlying sediment pore water and anyreactions occurring within basaltic basement.

[24] We expand upon this conceptual model toconstruct a coupled fluid-solute transport and con-sumption model to estimate the advective fluxthough basaltic basement (Figure 5). This modelis consistent with the general processes and hydro-logic pattern outlined above [e.g., Wheat andFisher, 2007] and at a variety of ridge flank sites[e.g., Baker et al., 1991, Wheat and McDuff, 1995;You et al., 2003]. The volumetric flux per unitwidth perpendicular to flow (m3 a�1 cm�1) withinupper basaltic basement is determined from thediffusive flux at the sediment-basalt interface (Fd)and the concentrations of bottom seawater (Csw)and formation fluid. For simplicity, reactions with-in upper basaltic basement are ignored, althoughadditional losses may occur through biotic andabiotic processes (requiring volumetric fluxes larg-er than those calculated herein). The diffusive fluxperpendicular to fluid flow is modeled as [Berner,1980],

Fd ¼ �Ds@C=@z ð5Þ

where Ds is the porosity- and temperature-depen-dant sediment diffusion coefficient [Li and Gre-gory, 1974] and dC/dz is the basal pore waterchemical gradient. The nitrate gradient immedi-ately above the sediment-basalt interface isassumed to be the same as that at the seafloor. Aconservative estimate for the seafloor gradient is 4mmol nitrate/kg/cm. This value is based on datafrom the shallowest pore water samples andseawater concentrations from gravity cores thatare not affected by advection.

Figure 5. Model used to estimate fluid fluxes withinthe basaltic basement between Tengosed Seamount andDorado outcrop, based on a similar model applied toMiddle Valley [Wheat and Fisher, 2007]. Plot showshow the nitrate concentration in basement fluids(vertical axis) is expected to change as a function offlow rate (horizontal axis), given different nitrategradients at the sediment-basement interface (differentcurves). Assuming a conservative estimate for thenitrate gradient (4 mmol/kg/cm, same as that at theseafloor), the calculated seawater flux per unit width is1800 m3 a�1 cm�1(the intersection of the 4 mmol/kg/cmcurve and a nitrate concentration of 41 mmol/kg (redhorizontal line)). One would use one of the other twocurves if the nitrate gradient was four times greater orweaker, as labeled. If the nitrate concentration in theformation fluid is lower than inferred, then thecalculated fluid flux through basement would be lower.The arrows indicate the displacement of the redhorizontal line for this case.



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[25] The model consists of a diffusive flux througha length (l), defined by the distance to the nearestpossible recharge site, a unit width (w) of onecentimeter, and an advective flux within upperbasaltic basement. The advective flux along theflow path is calculated from the nitrate concentra-tion in seawater, seawater density (r), and thevolumetric flux (Q). Provided the volumetric fluidflux is conserved, the chemical flux out (Fo) of thebox is the sum of diffusive and advective fluxes:

Fo ¼ Fdlw� CswQr ð6Þ

The calculated composition of the formation fluidthat exits the box is compared with the estimatedcomposition of 41 mmol nitrate/kg in the formationfluids. A volumetric flux per unit width of 1800 m3

a�1 cm�1 results in a calculated fluid concentrationof 41 mmol nitrate/kg, matching the observed data(Figure 5). The calculated volumetric flux per unitwidth scales linearly with the assumed nitrategradient above basement. For example, even if weuse a gradient that is one fourth the size, the calculatedvolumetric flux per unit width is �450 m3 a�1 cm�1.In contrast, if a component of the formation fluid isderived from a seamount or outcrop more distant than20 km (the distance from Dorado outcrop toTengosed Seamount), then the volumetric flux mustbe greater to maintain the near-seawater nitrateconcentration. This analysis does not account forseawater-basalt and microbial reactions within thebasaltic basement. Similar to the case of a more distalrecharge site, higher volumetric fluxes would berequired if these reactions were to have a significantinfluence on the nitrate concentration in the formationfluid. Last, we highlight the case for a nitrateconcentration of 35 mmol/kg, which is the concentra-tion of the sample at the base of GC-50. Even if thisconcentration represents the concentration in forma-tion fluids, thus requiring an oxygenated formationfluid that allows nitrification to generate higher nitrateconcentrations than are observed in the shallowerportion of the sediment core, the calculated volu-metric flux per unit width remains high (300 m3 a�1

cm�1).

[26] Given a characteristic volumetric flux per unitwidth (1800 m3 a�1 cm�1), we calculate thespecific discharge (volume flux per area) throughupper basaltic basement assuming an effectiveheight for the interval through which most seawaterflows (heffective). The effective height is the thick-ness of the basaltic layer multiplied by the effectiveporosity (8effective), the fraction of rock comprisinga well-connected pore network (probably much

less than the bulk porosity of upper basement). Ifthe effective porosity is 1 to 5% and flow occurswithin the upper 500 m of basaltic crust, then thecalculated specific discharge is 35 to 7 km a�1,respectively, and the nominal travel time fromrecharge to discharge (Tengosed Seamount to Dor-ado outcrop) is 0.6 to 3 years, respectively.

[27] Such a short travel time is consistent with the14C age of the sediment pore water from near thebase of GC-50 (Table 2). The 14C age of this sample,thought to be representative of the 14C age of thebasaltic formation fluid, is only 340 years older thana bottom seawater sample. This age must beconsidered the maximum travel time from rechargeto discharge sites because diffusive exchange withold carbon reservoirs during transport increases theapparent age of the fluid [e.g., Sanford, 1997].Diffusive exchange processes during flow throughheterogeneous oceanic crust are likely to increasethe apparent age of the fluid by at least one to twoorders of magnitude [e.g., Fisher, 2004].

[28] The apparent 14C age of the basaltic formationfluid from Dorado outcrop is considerably youngerthan the ages of basaltic formation fluids from theFlankFlux area, (4000 years) [e.g., Elderfield etal., 1999; Walker et al., 2008], and the ages of twoadditional TicoFlux pore water samples (PC-44and PC-48, Table 2, Figure 1b). These TicoFluxpore water samples were collected by piston coringin areas of upward fluid seepage on warmer partsof the Cocos Plate. Fluids collected from thesecores are more chemically altered than the basalticformation fluids from near Dorado outcrop, muchlike pore waters recovered in the eastern FlankFluxarea. The sample from PC-44 (which is about aswarm and altered as formation fluid from BabyBare outcrop) has a 14C age �33,000 years olderthan bottom seawater, whereas the sample fromPC-48 (which is not as warm or altered as that fromPC-44, but is both warmer and more altered thanthe GC-50 sample from near Dorado outcrop) has a14C age �12,000 years older than bottom seawater(Table 2).

[29] These warmer, older, more altered TicoFluxpore waters were collected in areas where there isno seafloor heat flow deficit [Fisher et al., 2003b;Hutnak et al., 2007, 2008] (Figure 1b) and in areaswhere hydrothermal circulation is less open andnot as efficient in mining lithospheric heat relativeto that observed near Dorado outcrop. Interestingly,some of the western FlankFlux fluids that werecollected in areas where the measured heat flow isa small fraction of the theoretical (lithospheric) value



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also have 14C ages on the order of 10,000 years(ODP Sites 1023, 1024, and 1025) [Elderfield et al.,1999] even though potential recharge sites are tensof kilometers away, similar to the distance betweenDorado outcrop and Tengosed Seamount. Thissuggests that formation fluids that circulate throughbasaltic basement and discharge from the seafloorat Dorado outcrop move very rapidly compared tothose in the FlankFlux area, spending relativelylittle time within the basaltic aquifer betweenrecharge and discharge sites. This observationillustrates that the heat flow deficit alone is anincomplete measure of fluid flow rates withinbasaltic crust. The apparent age of the formationfluid is likely a function of the heat flow deficit andcrustal permeability.

[30] If fluids flow through the entire 500-m-long-axis width of Dorado outcrop, the volumetricfluxes calculated from chemical data suggest atotal flow on the order of 3000 kg/s, two to threeorders of magnitude more fluid than is flowingfrom Baby Bare outcrop (4–13 kg/s) [Mottl et al.,1998]. This chemical-based estimate of the volu-metric fluid flux is at the lower end of the rangecalculated for discharging outcrops in this areabased on the regional thermal deficit, 1000–20,000 kg/s [Hutnak et al., 2008]. The advectiveheat flux associated with a flow of 3000 kg/s is100–210 MW, assuming that formation fluids arewarmed by 8–18�C relative to bottom water. Thisflux is equivalent to the heat output from a medi-um-sized black smoker vent field [Baker, 2007].The fluid flux from a single hydrothermal blacksmoker vent is on the order of 5 kg/s (assumingflow rate of 1 m/s through an 8-cm diameter orifice).Thus the fluid flux from the 0.25 km2 area of Doradooutcrop may be as great as that from �600 blacksmokers.

4.3. Implications for Global ChemicalFluxes From Ridge Flanks

[31] The impact of low-temperature, ridge flankhydrothermal processes on global budgets has beenassessed and reviewed in earlier studies [e.g.,Elderfield and Schultz, 1996; Wheat and Mottl,2004, and references therein]. Because of the vastvolume of seawater that flows through ridge flanks,these flows are important to global oceanic budgetsfor some chemical species. Estimates of basementfluid composition from Dorado outcrop are usefulin this regard, although they are based on extrap-olation of sediment pore water compositions. Weuse the extrapolated basement fluid composition

from Dorado outcrop to estimate the global impactof such flows, assuming that the chemical compo-sition of these fluids is consistent with ridge flankfluids having similar temperatures that are thoughtto be responsible for the majority of the lithospher-ic heat flow deficit. This last assumption is justifiedby the observations that basement temperatureseems to have the greatest control on ridge flankfluid composition [e.g., Wheat and Mottl, 1994],and that basement temperatures near Dorado out-crop are similar to those seen on young ridge flankswhere sediment cover is patchy and hydrogeologicconditions are thought to be more typical [e.g.,Langseth et al., 1984; Davis et al., 1992].

[32] If the entire convective heat loss from ridgeflanks (�8 TW) [Parsons and Sclater, 1977; Steinand Stein, 1994] were to result from warmingseawater by 8–18�C (resulting in fluid that reactswith basement at 10–20�C), then the flux ofseawater through the oceanic crust would be 3 to8 � 1015 kg a�1, somewhat less than the flux of21 � 1015 kg a�1 calculated from the global heatflow, sediment thickness, and crustal age data sets[Wheat et al., 2003]. Given a seawater flux of 5 �1015 kg a�1 (based on warming the hydrothermalfluid by 13�C), we estimate the chemical flux fromlow-temperature, ridge-flank hydrothermal pro-cesses from the measured difference in seawaterand the basaltic formation fluid (average pore watercompositions from GC-40 and �50) at Doradooutcrop (Table 1). For many of the chemicalspecies, calculated fluxes associated with ridge-flank hydrothermal fluids comprise a significantfraction of riverine fluxes.

[33] For some dissolved elements (e.g., calcium,sodium, strontium, and barium) the composition ofbasaltic formation fluid appears to be little differentfrom that of bottom seawater, indicating that thereis little influence of rapid, cool hydrothermalcirculation on the concentration of these elements.In Table 1 we assume a 1% change in the concen-tration of these elements (differences that would bemeasurable in pristine samples collected from afocused discharge site on basement), to quantifythe potential global impact of low-temperaturereactions for these four elements. The estimatedflux for two of the elements, sodium and strontium,is a substantial portion of the riverine flux, yet itremains to be determined if these assumed chem-ical anomalies are typical of natural systems.

[34] Clearly, there is a need to collect pristine fluidsfrom many such hydrothermal systems, and fo-cused discharge sites like Dorado outcrop offer



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critical opportunities to sample and understandthese systems. Results from one system alone willnot resolve global fluxes due to the complexity ofthese systems. For example, pore water data fromnear Dorado outcrop are consistent with a netremoval of nitrate during hydrothermal circulation,as observed at other ridge-flank sites [e.g., Wheatand Mottl, 2004], and as indicated by the highnitrogen content of alteration products in oceanicbasalts [Busigny et al., 2005]. In contrast, theremay be ridge flank hydrothermal systems in whichthe crust is a net source of nitrate to the oceans[Gieskes and Boulegue, 1983; Erzinger and Bach,1996]. It remains to be determined which conditionis more widespread on a global scale.

[35] Around Dorado outcrop, the concentration ofnitrate decreases from a bottom seawater value of42.3 mmol/kg to 41 mmol/kg prior to discharge. Aportion of this flux stimulates microbial processesin the overlying sediment and other nitrate-remov-ing processes also could occur within basalticbasement. Because we assumed the same rate ofnitrate removal for both the seawater-sediment andsediment-basalt interfaces, using the same nitrategradient at the top and base of the sedimentsection, the calculated flux of nitrate into thesediment is about twice the early diagenetic fluxcalculated from the uppermost sediment. The sameargument should apply to other microbially medi-ated ions such as phosphate [e.g., Wheat et al.,2000] and in some cases ammonium, sulfate,manganese, and iron. Likewise, highly sediment-reactive elements such as silica and germanium areexpected to behave similarly at both interfaces[e.g., Wheat and McManus, 2005, 2008].

[36] Diffusive exchange with overlying sedimentcan account for all of the documented nitrate lossin basement around Dorado outcrop, indicating thatthere might be little reaction within basaltic base-ment in this area. Bach and Edwards [2003]document the extent of basaltic Fe and S oxidationbased on drilling studies at numerous sites, andprovide constraints for the oxidation of theseelements using dissolved oxygen and nitrate. How-ever, they restrict their calculations to a convectiveheat loss of 3 TW, which is the amount of heatadvected from crust aged 1 to 10 Ma. With thisrestriction, their calculations lead to a predictionfor the complete removal of dissolved oxygen andnitrate from circulating seawater during the first10 Ma of crustal aging. In contrast, formation fluidfrom Dorado loses only �3% of its initial seawaternitrate concentration during ridge-flank circulation,

suggesting that crustal oxidation can extend wellbeyond 10 Ma.

[37] There is a need to collect pristine hydrother-mal fluid from features like Dorado outcrop, ide-ally by sampling focused seepage sites or springson bare basalt, so that uncertainties in estimates offormation fluid compositions and processes can bereduced. Areas of focused, low-temperature, ridge-flank discharge offer critical opportunities to avoidthe confounding influence of diagenetic reactionsin sediments during pore water seepage. There isalso a need to locate and collect fluid samples fromsimilar discharge sites on seafloor across a range ofcrustal ages, to assess whether or not the fluid andchemical fluxes documented at Dorado outcrop aretypical of global processes, and to quantify prop-erties and processes that influence the chemicalcomposition of basaltic formation fluids in thesesystems. Only through systematic surveys, sam-pling, and analyses of this kind can we quantifywith certainty the integrated impacts of ridge-flankhydrothermal circulation on global geochemicalbudgets.

5. Conclusions

[38] Pore water chemical data were collected bygravity coring on and near Dorado outcrop, a smallbasement edifice that discharges cool hydrothermalfluid from the underlying crust on 23 Ma seafloorof the Cocos Plate. Pore water samples collectedjust above basaltic basement were used to infer thecomposition of the basaltic formation fluid, whichis only slightly different from that of bottomseawater in this area, and flow rates through thethin sediment cover adjacent to areas of outcropexposure. Pore water profiles indicate a range ofdiffusive, advective, and diagenetic influences thatare the basis for one-dimensional reactive transportmodeling. A comparison of model calculations andpore water data suggest that seepage rates throughsediments on and adjacent to Dorado outcrop are asgreat as several meters per year.

[39] Chemical and geologic constraints are used toestimate flow rates through basaltic basement thatallow Dorado outcrop fluids to remain relativelyunaltered compared to seawater. Given the nearestlikely basement recharge site is Tengosed Sea-mount, �20 km away, we estimate a minimumvolumetric flux per unit width of basement of1800 m3 a�1 cm�1, and a total fluid flow throughDorado outcrop of at least 3000 kg a�1. This flowis orders of magnitude greater than that estimated



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to discharge from Baby Bare outcrop, on youngerseafloor on the eastern flank of the Juan de FucaRidge. The 14C age of pore water thought to becharacteristic of basement formation fluids nearDorado outcrop suggests an extremely short resi-dence time in the crust, no more than a fewhundred years. This short residence time contrastswith much longer residence times estimated using14C measurements on the eastern flank of the Juande Fuca Ridge, and on other parts of the CocosPlate where basement fluids are warmer and morealtered. These differences in fluid residence timesand chemical compositions suggest significant dif-ferences in flow rates, and perhaps differences inthe geometry and nature of flow paths within thecrust.

[40] If the hydrothermal fluid exiting from Doradooutcrop is characteristic of ridge-flank hydrother-mal circulation in general (10–20�C, relativelyunaltered), these systems can still have an importantinfluence on the geochemical fluxes and budgets formany solutes on a global basis, particularly forchloride, magnesium, sulfate, potassium, lithium,boron, silica, phosphate, manganese and iron. Theimplications of these analyses for subseafloormicrobial processes and communities remain tobe determined. Thermal and chemical conditionswithin basaltic basement near Dorado outcropappear to be similar to those in many young ridgeflanks. This suggests that microbial conditions maybe similar as well.

[41] Unfortunately, there are few samples of fluidsfrom these kinds of ridge-flank hydrothermal sys-tems, and most samples (including those analyzedin the present study) are from sediment pore water.We need to locate and sample focused dischargesites for cool, ridge-flank hydrothermal systems inorder to develop and test hypotheses like thosepresented. A challenge in the past has been toidentify focused discharge sites in areas wherethe fluid temperatures and fluid compositions aretypical of global systems, but this study shows thatsuch sites do exist. The key is to find places wherethere are thick sediments and relatively few areasof basement exposure, then use thermal data todistinguish between recharge and discharge loca-tions to map fluid pathways. Not all such sites willhave fluid circulation that is vigorous enough toremove a significant fraction of lithospheric heat,but these sites must be sampled in order to under-stand how heat advection from ridge flanks influ-ences global geochemical budgets and subseafloormicrobiology.

Acknowledgments

[42] We would like to that the crew of the R/VMelville, Chris

Moser, and our scientific collaborators for helping to obtain

sediment cores, pore water, heat flow, bathymetric, seismic

and other data during the TicoFlux II expedition. This work

was supported by NSF OCE–0002031 and OCE–0400462

(CGW) and OCE–0550713 and OCE–0727952 (ATF). This

paper benefited from thoughtful reviews by J. Gieskes,

D. Teagle, and an anonymous reviewer.

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