Seawater recharge into oceanic crust: IODP Exp 327 Site U1363 …afisher/CVpubs/pubs/Wheat2013_G… · Received 13 September 2012; Revised 15 March 2013; Accepted 23 March 2013; Published

Article

Volume 14, Number 6

17 June 2013

doi:10.1002/ggge.20131

ISSN: 1525-2027

Seawater recharge into oceanic crust: IODP Exp 327 SiteU1363 Grizzly Bare outcrop

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

Samuel M. HulmeMoss Landing Marine Laboratory, Moss Landing, California, USA

Andrew T. FisherEarth and Planetary Sciences andC-DEBI,University of California SantaCruz, SantaCruz, California, USA

Beth N. OrcuttCenter for Geomicrobiology, Aarhus University, Aarhus, Denmark

Now at: Bigelow Laboratory for Ocean Sciences, East Boothbay, Maine, USA

Keir BeckerUniversity of Miami, Miami, Florida, USA

[1] Systematic differences in sediment thermal and pore water chemical profiles from Integrated OceanDrilling Program Site U1363 document mixing and reaction within the basaltic crust adjacent to GrizzlyBare outcrop, a site of hydrothermal recharge into 3.6 My-old basaltic crust. A transect of seven holeswas drilled ~50 m to ~750 m away from the base of the outcrop. Temperatures at the sediment-basementinterface increase from ~6�C to >30�C with increasing distance from the outcrop, and heat flow issuppressed within several hundred meters from the outcrop. Calculated fluid compositions at thesediment-basement interface are generally explained by mixing between bottom seawater and alteredcrustal basement fluids, with a composition similar but not identical to fluids from seeps at Baby Bareoutcrop, located ~45 km to the northeast. Reactions within upper basement and overlying sediment affect avariety of ions (Mn, Fe, Mo, Si, PO4

3-, V, and U) and d13DIC, indicating a diagenetic influence and diffusiveexchange with overlying sediment pore waters. The apparent 14C age of basal pore fluids is much older thanbottom seawater. Collectively, these results are consistent with seawater recharge at Grizzly Bare outcrop;however, there are strong gradients in fluid composition within 50 m of the outcrop, providing evidence forcomplex flow paths and vigorous mixing of young, recently recharged seawater with much older, more reactedbasement fluid. The proximity of these altered fluids to the edge of the outcrop raises the possibility for fluidseepage from the outcrop in addition to seawater recharge.

Components: 9,900 words, 7 figures, 3 tables.

Keywords: ridge flank; hydrothermal; outcrop; chemical exchange; low temperature; pore water.

Index Terms: 0450 Biogeosciences: Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424); 1034Geochemistry: Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424); 1829 Hydrology: Groundwaterhydrology; 3017 Marine Geology and Geophysics: Hydrothermal systems(0450, 1034, 3616, 4832, 8135, 8424);3015 Marine Geology and Geophysics: Heat flow (benthic).

©2013. American Geophysical Union. All Rights Reserved. 1957

Received 13 September 2012; Revised 15 March 2013; Accepted 23 March 2013; Published 17 June 2013.

Wheat, C. G., S. M. Hulme, A. T. Fisher, B. N. Orcutt, and K. Becker (2013), Seawater recharge into oceanic crust:IODP Exp 327 Site U1363 Grizzly Bare outcrop,Geochem. Geophys. Geosyst., 14, 1957–1972, doi:10.1002/ggge.20131.

1. Introduction

[2] The eastern flank of the Juan de Fuca Ridge hasbeen the subject of numerous mapping, heat flow,sediment coring, seismic, and submersible expedi-tions, and targeted for three scientific drillingexpeditions (Ocean Drilling Program (ODP) Leg168 and Integrated Ocean Drilling Program (IODP)Exp 301 and 327) [e.g., Coggon et al., 2010;Cowen et al., 2003; Davis et al., 1992, and 1997;Fisher et al., 2003a, 2005, and 2011; Hutnaket al., 2006; Lever et al., 2010, Mottl et al., 1998,Orcutt et al., 2010, and Wheat et al., 2000 and2010]. A central goal of these studies has beento elucidate coupled hydrogeologic, geochemical,and microbiological processes and propertieswithin a ridge-flank setting, where fluid, heat, andsolute fluxes are driven by lithospheric coolingand focused by basaltic basement roughness,crustal faulting and sediment accumulation [e.g.,Fisher and Wheat, 2010].

[3] Many of the studies in this area have focused onfluid flow and related processes that occur on 3.5 to3.6 My-old seafloor, as part of a regional, warm(up to ~65�C) ridge-flank hydrothermal system(RFHS). Seawater enters the volcanic oceanic crustthrough a seamount, flows laterally within the crustalrock below thick sediment, and emerges through an-other seamount after undergoing considerableheating, reaction, and diffusive exchange with over-lying sediment pore waters [Wheat et al., 2000;Fisher et al., 2003a; Hutnak et al., 2006]. This isthe only known RFHS for which the geometry ofthe primary fluid flow path, from recharge to dis-charge, is known with confidence.

[4] Considerable sampling has been accomplishedat the discharge end of this system, with shallowgravity, piston and push coring and direct samplingof springs and boreholes, allowing the geochemicalcomposition of crustal basement fluids to be resolved[e.g., Mottl et al., 1998; Wheat et al., 2002; Wheatet al., 2010]. Heat flow data collocated with seismicdata and sediment core samples have allowed an as-sessment of thermal conditions, demonstrating nearisothermality of upper basement temperatures, anda monotonic increase in seafloor heat flow, close toBaby Bare outcrop [e.g., Davis et al., 1992; Wheat

et al., 2004a; Hutnak et al., 2006]. In contrast, muchless is known about what happens during the firstphases of water-rock-sediment interaction, soonafter seawater enters the crust at a RFHS.

[5] On the basis of co-located thermal and seismictransects radiating from Grizzly Bare outcrop,Grizzly Bare outcrop is a site of seawater rechargeto the crust [Fisher et al., 2003a; Hutnak et al.,2006]. One of these transects was positioned in thedirection of a buried ridge that extends to the north,upon which Baby and Mama Bare outcrops resideand parallel to the present spreading center to thewest. This transect was selected for measurementsand coring on IODP Expedition 327 because thesediment-basement interface is relatively well im-aged, thermal data from this transect are consistentwith seawater recharge into the crust and it trends to-wards Baby Bare outcrop. Seven holes (in four loca-tions) were drilled at Site U1363 along this transect.Thermal measurements were made and sedimentwas collected for pore water extraction and analy-sis to assess thermal and chemical changes in theupper basaltic basement. These data representsthe first detailed assessment of upper basementconditions near a recharge site for a RFHS.

[6] Forty to fifty kilometers north of Site U1363,along the trend of a buried basement ridge, thereare monotonic changes in fluid composition alongan inferred fluid flow pathway even in heteroge-neous permeable upper basaltic crust [Wheat et al.,2000; Hulme and Wheat, 2013]. We hoped to findsimilarly systematic changes in chemical andthermal conditions along the Site U1363 transect,allowing us to quantify rates of basement fluidalteration, elucidate processes related to crustalevolution, and assess microbial impact as cool oxicseawater evolves into a reducing, warm and alteredcrustal basement fluid. Results presented in thispaper suggest that fluid flow, mixing, and reactionsat Site U1363 are more complex than anticipated.

2. Regional Setting and Processes

[7] Seafloor spreading along the Juan de Fuca Ridge(Figure 1A), to the west of the field area, has createdextensive bathymetric relief associated with faulting

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GeochemistryGeophysicsGeosystemsG3G3 WHEAT ET AL.: SEAWATER RECHARGE INTO BASALTIC CRUST 10.1002/ggge.20131

and the formation of abyssal hills. The volcanicoceanic crust is exposed close to the activespreading center, but sediment is generally thickerand more continuous towards the continental marginto the east where rapid sedimentation in this near-continental region has resulted in the burial of volca-nic crust at an unusually young age [Underwoodet al., 2005]. Sediment is typically 250–600 mthick where the seafloor is 3.5 to 3.6 My old, eastof the active spreading center, but there are volcanicoutcrops that penetrate through this thick sediment,

remnants of off-axis volcanic events [Karsten et al.,1998; Becker et al., 2000] that now serve as entryand exit points for RFHS fluids. Sediment in thisarea is generally orders of magnitude lesspermeable than the upper basaltic crust [Spinelliet al., 2004; Becker et al., 2008; Fisher et al.,2008], limiting rapid flow of seawater into andhydrothermal fluids out of the crust to locationswhere highly permeable conduits such as basementoutcrops or faults are exposed at the seafloor.

[8] A cluster of three basaltic outcrops penetratesthrough sediment on 3.5 to 3.6 My-old seafloor100 km east of the Juan de Fuca Ridge near 48�N:Papa Bare, Mama Bare, and Baby Bare outcrops[Davis et al., 1992; Mottl et al., 1998; Beckeret al., 2000]. Baby Bare outcrop (Figure 1A) is themost extensively surveyed and smallest of the threefeatures. Here, highly altered fluid discharges at arate of 5–20 l/s and releases 2–3 MW of heat [Mottlet al., 1998; Wheat et al., 2004a]. Mama and PapaBare outcrops are also known sites of hydrothermaldischarge, although surveying of these seamountsis limited.

[9] No hydrothermal recharge sites have beenidentified on or near these three seamounts.However, basement fluids are thought to rechargethrough Grizzly Bare outcrop, 52 km to the south,based on thermal and geochemical data (Figure 1).Grizzly Bare outcrop is conical in shape, 3.5 km indiameter, and rises 450 m above the surroundingseafloor. Grizzly Bare outcrop was identifiedinitially as a site of regional hydrothermal rechargebased on systematic differences in basement fluidcomposition and patterns of seafloor heat flowimmediately adjacent to the edifice [Wheat et al.,2000; Fisher et al., 2003a; Hutnak et al., 2006].Seafloor heat flow is depressed within a few kilome-ters of the edge of basalt exposure along severaltransects [Fisher et al., 2003a; Hutnak et al.,2006]. Seismic reflection data allow determinationof sediment thicknesses at locations where heat flowwas measured. Downward continuation of seafloorheat flow data along these lines suggested basementtemperatures of 60–65�C within several kilometersof the outcrop edge. Temperatures are much lower,and seafloor heat flow values are suppressed, closerto the outcrop edge, where sediment is 100–200 mthick. In contrast, warm altered fluids dischargingfrom Baby Bare Outcrop cause a monotonic upwardsweep of isotherms with proximity to the outcrop,resulting in extremely high seafloor heat flowadjacent to the volcanic outcrop [Davis et al.,1992; Fisher et al., 2003a; Wheat et al., 2004a].

47o 18

'47

o 16

' N

128o 02'128o 06'

2800260024002200Depth (m-msl)

2600

2500

24002300

2200

GeoB00-170

GrizzlyBare

outcrop

U1363A, B

U1363C, D

0.0 0.5 1.0 km

U1363E, F

U1363G

B

Juan

de

Fuc

a R

idge

NorthAmerica

130o W 126o

48o

50o

52o N

46o

Site U1363

BabyBareoutcrop

GrizzlyBareoutcrop

A

Figure 1. The eastern flank of the Juan de Fuca Ridgehas been the site of several drilling expeditions that includesediment and basalt recovery and borehole observatoryinstallations (CORKs). Seven holes were drilled nearGrizzly Bare outcrop on IODP Exp 327 (Site U1363).

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[10] Regional scale fluid flow in upper volcaniccrustal rocks from Grizzly Bare to Baby Bareoutcrops is driven by a “hydrothermal siphon,” thedifference in pressure at the base of more denserecharging (cool) and less dense discharging (warm)fluids. This is the same forcing that is likely to drivemost sedimented ridge-flank hydrothermal discharge,helping to explain the global ridge-flank heat flowanomaly [Fisher and Harris, 2010; Fisher andWheat, 2010]. However, hydrothermal circulationbetween Grizzly Bare and Baby Bare outcrops hasvirtually no regional influence on lithospheric heatloss [Davis et al., 1999; Fisher et al., 2003a; Hutnaket al., 2006] because the amount of fluid (and associ-ated advective heat) that discharges from Baby Bareoutcrop is relatively small.

3. Methods

[11] Procedures for sediment and basement coringduring the expedition, and information about corerecovery, are described in more detail elsewhere[Fisher et al., 2011]. Of the seven boreholesdrilled at Site U1363, three were washed throughthe sediment section without coring to determinethe thickness of the sediment prior to using theadvanced piston coring (APC) system (Table 1).The APC system often provides higher quality corethan does the extended core barrel (XCB) system,which is compatible with the same bottom holeassembly, but the XCB corer also allows collectionof a small amount of upper basement rock, andpotentially the sediment-basement interface itself.Neither core system is especially effective at recov-ering undisturbed core material from unlithifiedsand-rich lithologies, which tend to either becirculated away by the drilling and coring process,or become remobilized and settle out in the core

liner. In general, the sediment cover in this areacomprises hemipelagic mud (clayey silt to silty clay),thin-bedded turbidites (sand-silt-clay), and thick-bedded medium sand turbidites [Fisher et al. 2011].

[12] Sediment pore waters were extracted bysqueezing cored sediment that was processedwithin a nitrogen-filled glove-bag at room tempera-ture [Fisher et al., 2011]. Pore water compositionwas determined using standard colorimetric,titration, inductive coupled plasma emission andmass spectrometery, and ion chromotraphy tech-niques and are tabulated [Fisher et al., 2011],except for the carbon isotopic measurements whichwere made on four 30- to 60-ml samples sealed inglass bottles and poisoned with mercuric chloride(Table 2). These carbon isotopic measurements weremade at the National Ocean Sciences AcceleratorMass Spectrometry Facility from the dissolved inor-ganic carbon in the samples. Samples for methanegas concentrations were also collected immediatelyafter core recovery from each location where a porewater whole-round core was collected; however, nodetectable methane concentrations were recorded[Fisher et al. 2011]. Due to filter contaminationissues, samples for nitrate concentrations measuredshipboard were considered to be unreliable, and aseparate pore water aliquot was collected on shore.These samples were collected from sediment whole-round cores that had been frozen at �80�C then sub-sequently thawed in a 4�C cold room. Pore waterwas extracted from these samples using acid-washedRhizon samplers (Rhizosphere Research Products,Netherlands), and the samples were analyzed usingthe same analytical methods as used shipboard.

[13] Ten determinations of in situ temperature wereattempted using two tools, the APCT-3 tool and theSET probe [Fisher et al., 2011]. The APCT-3 tool isthe third generation of an instrumented APC coring

Table 1. Summary of Borehole Configurations and Thermal Results

Hole Sediment Thickness (m) a Ntempb CTR c (m2-K/W) Heat Flow (W/m2) Tsbi

d (�C)

U1363A 58.0 – 36.6 – –U1363B 57.0 2 36.0 0.288 12.2U1363C NA 2 – 0.194 –U1363D 231.2 – 159.3 – 32.7U1363E 36.0 – 22.4 – –U1363F 35.0 2 21.7 0.242 7.1U1363G 17.5 1 10.4 0.415 6.1

aSediment thickness determined from tagging basement with drill bit.bNumber of successful sediment temperature determinations used to determine heat flow and sediment-basement interface temperature (along with

bottom water temperature).cCTR= cumulative thermal resistance, calculated from empirical relation developed for Sites 1026 and 1027 in Davis et al. [1999].dTsbi= temperature at the sediment-basement interface.

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shoe [Heesemann et al., 2006]. An autonomous datalogger, temperature sensor, and power supply arehoused inside an annular cavity in a custom-designed APC coring shoe, and a modified core-catcher subassembly is attached above the shoe. Theinstrument is programmed to collect data at regularintervals (typically every 1 to 5 s). The instrumentedcoring shoe is held just above the mudline to collectbottom water temperature data, then lowered intoplace so that the core barrel seals the base of thedrillstring, which is used as a hydraulic accumulatorfor piston coring. After the tool is fired into the forma-tion, it is left in place for 6–7 min, to allow partialthermal equilibration following frictional heatingresulting from insertion of the coring shoe. Data arerecovered when the APCT-3 tool is returned to thesurface, along with the associated piston core.

[14] The SET probe is a modified version of aconically tipped, push-in probe first deployedduring ODP Leg 168 [Davis et al., 1997]. Like theAPCT-3 tool, the SET probe is programmed priorto deployment to record data at a fixed time interval.The SET probe is deployed down the drill string inlieu of a conventional core barrel. It is latched intoposition with the tip of the tool extending ~1 mbeyond the end of the bit, then pushed into theformation using the drill string, decoupled from thedrill string, and allowed to record data for 6–7 min.The tool is pulled from the formation using the drillstring and recovered by wireline. Because the diame-ter of the SET probe is only about 1 cm at the tip, itsthermal response time is much faster than that of theAPC coring shoe, meaning that a larger fraction ofthe thermal decay associated with probe insertionoccurs during the limited measurement period.

[15] Data from both tools were processed usingsoftware that compares measured tool response to

synthetic cooling curves based on tool geometry, toolproperties, and formation properties. Processing isgraphically interactive, which allows the user toidentify irregularities in temperature time-records.Measured temperature values were compared tomodeled tool response curves, then extrapolated toinfinite time to estimate equilibrium temperatures.Each measurement was processed using a rangeof sediment thermal conductivity values, becauseExpedition 327 thermal conductivity data fromsandy turbidites were highly scattered and often oflow quality. Heat flow values were determined bycombining calculated equilibrium sediment tempera-tures with a cumulative thermal resistance versusdepth function, as determined for ODP Sites 1026and 1027 based on seafloor heat flow, sedimentthickness, and upper basement temperature data[Davis et al., 1999]. Comparison of lithologic andthermal conductivity data collected during ODPLeg 168 and IODP Expeditions 301 and 327suggests that lithologies encountered during the threedrilling expeditions were similar.

4. Results

[16] Sediment pore water chemical profiles areinfluenced by diagenetic reactions and exchangewith basement fluids as observed at ODP Sites1026 and 1027 and IODP Site U1301 [Daviset al., 1997; Fisher et al., 2005]. However, incontrast to these sites, pore waters from the baseof the sediment column at IODP Site U1363 arecooler and typically less altered, with Mg concen-trations of 36 to 28 mmol/kg (Figure 2). Sulfatereduction in the sediment column results in anincrease in alkalinity, both showing an apex nearthe middle of the sediment column. At the base of

Table 2. Measured Carbon Isotopic Values and Calculated Values for the Uppermost Fluid in Basaltic Basement

Measured Measured Measured Basement Endmemberc,d Endmemberc,e

DIC DIC alkalinity alkalinity calculated calculatedd13C (%) Δ14C mmol/kg mmol/kg 14C age d13C (%)

2Aa U1363G 3H6 �13.1 �792.69 2.91 2.67 12,000 �122Ba U1363G 3H6 �13.11 �799.78 2.91 2.67 12,000 �123b U1363F 4H3 �7.94 �820.72 1.81 1.20 10,000 �1.84b U1363F 4H3 �10.92 �801.3 1.81 1.20 9,400 �6.31 U1363D 4X3 �11.46 �916.87 1.27 0.98 18,000 �8.9

aDuplicate samples.bTwo separately processed samples from the same core.cAssumes that the alkalinity and DIC are equivalent, given the pH of the pore waters.dCalculated assuming the measured 14C is a result of adding depleted C from the pore water to the value in the basement, thus 14Cmeasured

Concmeasured =14Cpore water Concpore water +

14Cbasement Concbasement.eCalculated assuming the measured d13C is a result of adding �20%. d13C from the pore water to the value in the basement, thus d13C measured

Concmeasured = d13Cpore water Concpore water + d

13Cbasement Concbasement.

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the sediment column, sulfate and alkalinity valuestrend towards bottom seawater values because ofexchange with a less altered basement fluid.Concentrations of Ca generally increase downhole,with the exception of a profile affected by carbonateprecipitation, resulting from the high alkalinityproduced by microbial sulfate reduction. Otherelements, such as K and Rb, show changes in porewater concentrations with depth, ending near thesediment-basement contact at concentrations thatare altered relative to seawater. Because of the Ksampling artifact from squeezing [de Lange et al.,1992], we reduced the measured pore waterconcentration by 1.4 mmol/kg for all values inFigure 2.

[17] The composition of the basement fluid wascalculated for most holes by linear extrapolationof the deepest four to ten samples to the depth ofuppermost basement (Holes U1363B, D, and F)when gradients exist or by averaging the deepestfour samples (Table 3). This analysis is adequatefor most chemical species, but not for several traceelements (Pb, Cr, Co, Cu, and Zn) [e.g., Wheatet al., 2004b]. The number of samples used forthese extrapolations differed from hole to hole andwith ionic species, based on the reactivity of eachion and the shape of the associated profile. In thecase of Hole U1363G, the deepest sample wascollected right at the sediment-basement interface,so the composition of this fluid is considered to bethat of the upper basement fluid. At this hole,

samples were collected at a “curation” depth greaterthan 18 m. These “deeper” samples are interpretedto be a sediment slurry that was sucked into the coreliner during the APC coring process (as was appar-ent from visual inspection of cores after recovery).Interestingly, the compositions of these slurrysamples match those from the sample collected at17.5 m depth and basal pore water chemical trendsat the sediment-basement interface.

[18] In situ sediment temperatures were determinedin Holes U1363B, U1363C, U1363F, and U1363G(Table 1, Figure 3). Two sediment temperatures plusa bottom-water determination were used to assess thethermal gradient and seafloor heat flow for theseholes, except for Hole U1363G, which was only17.5 m deep, allowing for a single sediment temper-ature determination just above basement. Sedimenttemperatures were particularly well determined atthe base of Holes U1363F and U1363G, wheremeasurements were made adjacent to the basementcontact. Data from Holes U1363B and U1363Cwere not collected as close to the basement contact,but the depth to basement is known with confi-dence, and the consistency of measured thermalgradients allows confident extrapolation (Figure 4;Table 1).

[19] Heat flow determinations from Holes U1363Cand U1363F are similar to values determined fromnearby surface probe measurements, and the valuedetermined for Hole U1363G is similar to surface

0 10 20 30 40 50 600

50

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Mg and Ca (mmol/kg)

Dep

th (

m)

GF

B

C/D

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wat

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g -

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Alkalinity andSulfate (mmol/kg)

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C/D

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ulfa

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0 4 8 12

K (mmol/kg)Rb ( mol/kg)

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wat

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- F

illed

Figure 2. Selected pore water chemical depth profiles from IODP Site U1363 and seawater values. Pore waters areaffected by sediment diagenetic processes and diffusive fluxes from the overlying seawater and underlying basalticbasement fluids. Measured data are in Fisher et al. [2011] and extrapolated basement fluids compositions are presentedin Table 3. Because of the K sampling artifact from squeezing [de Lange et al., 1992], we reduced the measured porewater concentration by 1.4 mmol/kg.

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probe data from a position 65 m away (Figure 4B).The heat flow determined for Hole U1363B is about50% greater than that determined during the surfaceprobe survey [Hutnak et al., 2006]. Basement tem-peratures estimated from extrapolation of boreholethermal data are generally similar to values calcu-lated from surface probe thermal data and estimatedsediment thickness values from seismic reflectiondata (Figure 4C). The consistency of these results isimpressive considering the difficulty in imaging thesediment-basement interface from seismic reflectiondata along a sloped surface adjacent to Grizzly Bareoutcrop. The key result from these thermal studiesis confirmation that upper basement temperaturesare suppressed within several hundred meters ofthe edge of Grizzly Bare outcrop [Figure 4, Fisheret al., 2003a, Hutnak et al., 2006].

5. Discussion

5.1. Thermal Data: GeneralHydrologic Pattern

[20] The prevailing paradigm of RFHS is thatseawater warms as it enters the basement, reactswith volcanic crustal rocks, is affected by microbialprocesses, and exchanges with overlying sedimentpore waters. Thus, the temperature of the basementfluid is affected by transport processes, whichinclude advection and conduction, whereas thefluid composition is affected by advection, diffu-sion, and reaction.

[21] Fisher et al. [2003a] and Hutnak et al. [2006]show heat flow profiles adjacent to both recharging

Table 3. Composition of Fluids in Basaltic Basement at IODP Site U1363 and Baby Bare Outcrop Compared to theComposition of Bottom Seawater

Site Bottom a U1363G b,c U1363F b U1363B b U1363C/D b Baby a

Seawater Bare

Latitude (N) --- 47º17.312’ 47º17.326’ 47º17.352’ 47º17.574’ 47º42.37’Longitude (W) --- 128º2.170’ 128º2.137’ 128º2.106’ 128º2.762’ 127º47.15’Distance From Outcrop (m) --- 50 115 177 770 52,000Sediment Thickness (m) --- 17.5 35.0 57.0 231.2 0Deepest Pore Water (m) --- 17.5 32.2 53.3 222.7 ---Basement Temp (�C) 1.8 6 7 12 33 64Mg (mmol/kg) 52.6 36 35 35 28.6 0.98Chlorinity (mmol/kg) 542 544 536 546 551 554Sulfate (mmol/kg) 28.1 25.2 25.8 27.3 26.7 17.8Alkalinity (mmol/kg) 2.5 2.67 1.20 1.71 0.98 0.43Br (mmol/kg) --- 0.82 0.80 0.79 0.81 ---pH 7.9 7.2 7.3 7.3 7.1 8.3Na (mmol/kg) 467 473 464 474 471 473Na/Cl (mol/mol) 0.861 0.869 0.865 0.869 0.854 0.853Ca (mmol/kg) 10.3 21.0 22.6 23.3 35.0 55.2Sr (mmol/kg) 86 85 83 90 101 110Ba (mmol/kg) 0.15 0.35 0.07 0.59 1.04 0.43K (mmol/kg) 10.1 9.2 8.9 8.5 4.6 6.88Li (mmol/kg) 26.6 62 17.9 18.2 16.7 9.0Rb (mmol/kg) 1.37 1.41 1.7 1.33 0.53 1.12Cs (nmol/kg) 2.2 2 3 3.1 2.2 5.3Si (mmol/kg) 190 420 300 310 350 360B (mmol/kg) 410 460 580 450 270 570Nitrate (mmol/kg) 39.2 2.5 2 1 2 0.8Ammonium (mmol/kg) 0.3 107 110 170 220 76Phosphate (mmol/kg) 2.8 2.8 0.2 2.0 0.5 0.3Mn (mmol/kg) 0.001 55 6 79 100 2.9Fe (mmol/kg) 0.001 2.7 0 0 3 <0.05V (nmol/kg) 38.4 6 14 6 5 10U (nmol/kg) 12 4.8 7.2 1.4 2 0.6Mo (nmol/kg) 100 870 560 550 280 297d 13C �0.6 �12 �4 -- �8.9 �0.614C Age (Years) 2300 12,000 10,000 -- 18,000 12,000 d

aData are from Wheat and Mottl [2000], Wheat et al. [2002], and Sansone et al. [1998].bData are from Fisher et al., 2011.cSamples from deeper than 17.5 m result from the piston sucking in and homogenizing basal sediment without further penetration even though

depths are recorded as deeper than 17.5 m.d[Walker et al., 2007].

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and discharging outcrops, and compare valuesobserved to those inferred from conductive andcoupled (fluid-heat) numerical models. Hutnaket al. [2006] also show calculated temperatures alongthe sediment-basement interface, as inferred fromdownward continuation of seafloor heat flow data.In areas of observed hydrothermal discharge, heatflow values increase monotonically with increasingproximity to the exposed basement outcrop, and tem-peratures at the sediment-basement interface remainelevated, well above bottom water values [e.g., BabyBare outcrop, Figure 10 of Hutnak et al., 2006].

[22] Transects of thermal profiles from areas ofinferred recharge have a contrasting pattern. Inlocations with the most vigorous recharge, heatflow is suppressed within ~500 m of the outcrop

edge, and temperatures at the sediment-basementinterface decrease towards bottom seawater temper-ature as the outcrop is approached [e.g., GrizzlyBare outcrop, Figure 12 of Hutnak et al., 2006].However, the monotonic reduction of heat flowmay not extend all the way to the outcrop edge,for two reasons. First, conductive refraction tendsto generate a local, positive heat flow anomaly atthe break in slope between the flat seafloor andthe sloped basalt edifice. In addition, secondaryhydrothermal convection within the outcrop gener-ates local anomalies [elevated and suppressedvalues, e.g., Figure 22 of Hutnak et al., 2006;Kawada et al., 2011]. Some thermal transectsaround Grizzly Bare outcrop show a mixed thermalpattern, with small local anomalies superimposedon a broader pattern consistent with recharge

5 10 15 20 250

50

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Temperature (°C)

Dep

th (

mb

sf)

Hole U1363A, B

Hole U1363E, F

Hole U1363G

Hole U1363C

A

5 10 15 20 250

50

100

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Hole U1363B

Hole U1363C

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Temperature (°C)

Cu

mu

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al resistance (m

2 K/W

)

B

Figure 3. Thermal data from IODP Site U1363. Interpreted temperature and heat flow values are listed in Table 1.

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[Figures 12–14 from Hutnak et al., 2006]; one ofthe transects radiating from Grizzly Bare outcrop,to the northwest of the Site U1363 transect, showsevidence of discharge [Figure 15 fromHutnak et al.,2006].

[23] The thermal transect selected for placement ofthe Site U1363 transect, and verified with boreholesmeasurement, shows a clear recharge signature: azone of low heat flow located several hundred metersfrom the outcrop edge, and sediment-basement tem-peratures that decrease towards bottom seawatervalues close to the outcrop. The monotonically ele-vated heat flow values seen adjacent to Baby Bareoutcrop, where vigorous discharge has been con-firmed, are clearly absent [Fisher et al., 2003a;Hutnak et al., 2006; Wheat et al., 2004a]. However,there are three elevated heat flow values close to theedge of Grizzly Bare outcrop along this transect.There is no evidence of upward flow through sedi-ment (shallow thermal gradients in sediment areclearly conductive), and heat flow is suppressed

farther from the outcrop (Figure 4). As describedearlier, conductive refraction at the break in slopeand secondary convection in basement could bothcontribute to the near-outcrop anomaly, which issuperimposed on the broader thermal indicationsof recharge. The Site U1363 transect is locatedbetween transects showing an even strongerrecharge signature (to the southeast), and one show-ing a modest discharge signature (to the northwest)[Fisher et al., 2003a, Hutnak et al., 2006]. SiteU1363 appears to be located within the transitionbetween these regimes.

5.2. Chemical Data: Transport and MixingClose to Grizzly Bare Outcrop

[24] Although thermal and chemical data are broadlyindicative of RFHS recharge through Grizzly Bareoutcrop [Wheat et al., 2000; Fisher et al., 2003a;Hutnak et al., 2006], data presented in the presentstudy suggest complexity in flow paths, includingrecharge, fluid flow, and mixing in and out of theplane of the Site U1363 transect. If there wereconservative mixing between recharging and reactedhydrothermal fluids along this transect, one mightexpect to observe a linear trend in a cross plot oftemperature and Mg concentration at the sediment-basement interface. End-members would be definedby bottom seawater (cold, high Mg) and alteredhydrothermal basement fluid (warm, low Mg),similar in composition to fluids sampled from BabyBare springs and nearby instrumented boreholes(Table 3). A linear mixing trend such as this wasanticipated along the Site U1363 transect becauselaboratory studies of seawater-basalt experiments atlow temperatures indicate very slow reaction rates[Seyfried, 1977], consistent with observations froma variety of settings around the globe [Elderfieldet al., 1999; Fisher and Wheat, 2010]. Data fromHoles U1363C/D (located ~750 m from the outcropedge) lie on a mixing trend of this kind (Figure 5),consistent with advective transport and conservativemixing. However, data from the other holes(U1363B, F, and G) closer to the outcrop fall belowthis linear mixing trend (Figure 5), indicating thateither there is a preferential loss of either heat orMg.We see similar trends when plotting temperatureversus other ions (e.g., Ca, alkalinity, and K).

[25] The non-conservative loss of Mg (and other sol-utes) is unlikely in this setting, based on experimentaland environmental data at the observed temperatures[Seyfried, 1977; Fisher and Wheat, 2010]. For exam-ple, old altered basement fluids at ODP Site 1023(~15�C) are less altered (47. 2 mmol Mg/kg) than

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Figure 4. Reflection seismic and thermal data adjacentto Grizzly Bare outcrop and Site U1363. Location oftransect is shown in Figure 1B. A. Seismic data fromZühlsdorf et al. [2005] and Hutnak et al. [2006], withisotherms estimated from downward continuation ofheat flow data. B. Seafloor heat flow data (small squares)from Hutnak et al. [2006]. Large squares are boreholedata reported in this study. C. Extrapolated temperaturesat the sediment-basement interface from downward con-tinuation of the seafloor and borehole heat flow data.

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the cooler boreholes at Site U1363, which shouldexperience less alteration because at lower tempera-tures reactions are more sluggish. In contrast, heatloss is more likely because of the very high thermaldiffusivity of crustal rocks in comparison to solutediffusivity. As a result, a crustal region adjacent to arecharge channel can cool without significant geo-chemical modification. A similar decoupling of heatcontent and fluid composition in basaltic crust onridge flanks has been documented in boreholes asthey return to their natural state after being perturbedby drilling operations [Wheat et al., 2003; Wheatet al., 2010], and as altered fluids rise from deeperin the crust towards the seafloor [e.g., ODP Sites1030 and 1031; Elderfield et al., 1999].

[26] Our interpretation of Site U1363 thermal andgeochemical data is that recharging seawater entersthe crust in the vicinity of Site U1363, reducingupper crustal temperatures, but mixing, reactionsand fluid flow in and out of the plane of the drillingtransect and from below lead to complex geochemi-cal and thermal patterns at the sediment-basement in-terface. Further from the outcrop (Holes U1363C/D)vigorous local convection and mixing result in a fluidcomposition consistent with simple mixing trends. Itshould not be surprising that fluid pathways are com-plex in a system such as this one, particularly consid-ering the heterogeneity in upper crustal stratigraphyand structure in this region. This interpretation is con-sistent with variations seen in basement fluid samplesfrom ODP Sites 1026 and 1027, ~45 km to the north

[Davis et al., 1997]. Although these sites are just2.2 km apart, basement fluids from these sites arechemically distinct, suggesting a lack of mixingbetween them. Contrasts in fluid temperatures andcompositions within <1 km of Grizzly Bare outcropare similarly profound with oxic seawater in closeproximity to warm reduced basement fluids. Thepotential for strong vertical and lateral gradients inredox and other geochemical characteristics inbasement provide conditions conducive to microbialactivity, similar to strong redox boundaries that arecommonly found at the sediment-basement interface[Engelen et al., 2008].

5.3. Extent of Basementand Sediment Reactions

[27] The extent of reaction within basaltic basementis assessed by comparing the fluid composition inupper basement to a mixing trend defined byregionally warm and altered basement fluids (e.g.,having a chemical composition similar to springsfrom Baby Bare outcrop) and bottom seawater. Asdiscussed above, if conservative mixing processesare dominant, then data for a chemical speciesplotted versus Mg would lie on a mixing line, giventhe slow rate of reaction for Mg at temperatures<25�C. Such trends are evident for some ions(Figure 6), but others fall off a simple mixingtrend, indicating that reactive processes and/ordiffusive exchange with overlying pore watersare important in controlling fluid concentrations.Probable reactions that take place are assessedby comparing the results with several data sets:(1) water-rock experiments [Seyfried, 1977;Seyfried and Bischoff, 1979], (2) estimated fluidcompositions for upper basement at ODP Sites1023–1025 with temperatures (15–39�C) thatoverlap those from IODP Site U1363 and ODPSites 1028 and 1029 [Elderfield et al., 1999], (3)fluids collected following restoration of ambientcrustal conditions in IODP Hole U1301A [Wheatet al., 2010], and (4) basal sediment pore watergradients that provide a measure of exchange withupper basement fluids [e.g., Elderfield et al., 1999;Wheat et al., 2000; Hulme and Wheat, 2013].

[28] The Ca-Mg data from Site U1363 generally lieon the mixing line between reacted altered base-ment fluids and seawater (Figure 6). No suchmixing scenario is suggested for experimental dataor data from elsewhere along the ODP Leg 168 dril-ling transect, for which the primary influence oncomposition seems to be reaction temperature.Along the Leg 168 drilling transect, the removal

0

20

40

60

80

0 10 20 30 40 50 60Mg (mmol/kg)

25 C

70 C

1363

C,D

ODP

1030/31

Figure 5. Temperature and magnesium concentrationfor selected fluids. The green squares represent the calcu-lated values for the upper basaltic basement fluids fromSite U1363 (Table 3). Black circles are similar data fromODP Leg 168 [Elderfield et al., 1999]. The red and bluedata are experimental data after about 1 year of reaction[Seyfried, 1977; Seyfried and Bischoff, 1979]. The yellowline represents conservative mixing of bottom seawaterwith spring fluids from Baby Bare outcrop (Table 3).The arrow highlights the loss of heat in fluids nearerGrizzly Bare outcrop and at ODP Sites 1030/31.

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of Mg from seawater is balanced by a crustal Casource, maintaining charge balance, with the extentof reaction determined mainly by the temperatureof reaction [e.g., Mottl and Wheat, 1994; Elderfieldet al., 1999; Fisher and Wheat, 2010]. The fluidresidence time must also be important, given thatneither the 25�C nor the 70�C experiments[Seyfried, 1977], which were conducted for 1.5and 0.5 years, respectively, reached concentrationsequal to those from borehole samples at similartemperatures (Figure 5). The 14C ages of samplesfrom the ODP Leg 168 transect are thousands ofyears old [Elderfield et al., 1999]. Even if the actualfluid “age” is considerably younger [e.g., Stein andFisher, 2003], sufficient time has passed for thesefluids to react with basalt, implying these fluidsare “equilibrated” with basalt and with diffusivefluxes of Mg and Ca to/from overlying sedimentpore fluids. In other words, the diffusive exchangewith overlying sediment pore waters for these ionsappears insignificant relative to rates of reaction[Wheat et al., 2000; Hulme and Wheat, 2013].

[29] A cross plot of alkalinity and Mg data from theSite U1363 transect also places data close to amixing line, suggesting that mixing near the out-crop influences alkalinity (Figure 6). However,there appears to be some removal of alkalinity (datafalling below the mixing line), consistent withexperimental data that show depletions in CO2. In

contrast, Mg concentrations are higher (relativelyless reacted), indicating that reactions removingalkalinity are more rapid than those removing Mg.

[30] Similarly, K-Mg data from Site U1363 holesclose to the outcrop (T<15�C) fall along the mixingline, suggesting minimal reaction relative to mixing.Experimental data are consistent with this pattern.However, at Hole U1363C/D, where the upper base-ment temperature is ~33�C, K is removed relative toMg, consistent with observations from the high-temperature end of the ODP Leg 168 transect, andperhaps indicating a greater sediment influence. SiteU1363 Li-Mg data (not shown) also fall on a mixingline, with the exception of data from Hole U1363G,which has a higher Li concentration relative to thetrend. Although, Li tends to be removed from fluidsby reaction with basaltic crust at these cool tempera-tures, the diffusive flux of Li from the overlying sed-iment tends to increase the concentration in basementfluids with time [Wheat et al., 2000]. Thus, the longerthe fluids reside in basement, the larger the influencefrom sediment and the higher the Li concentration.

[31] Sulfate concentrations in RFHS can beexplained by the integrated diffusive loss to the over-lying pore waters with minimal removal in basalticbasement by microbial processes [Elderfield et al.,1999; Wheat et al., 2000; Lever et al., 2010; HulmeandWheat, 2013]. For example, there is a monotonicdecrease in sulfate concentrations along the transect

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Figure 6. Dissolve ion versus magnesium concentrations for selected fluids. The green squares represent thecalculated data for the upper basaltic basement fluids from Site U1363 (Table 3). Black circles are similar data fromODP Leg 168 [Elderfield et al., 1999]. The red and blue data are experimental data after about 1 year of reaction[Seyfried, 1977; Seyfried and Bischoff, 1979]. The yellow line represents conservative mixing of bottom seawater withspring fluids from Baby Bare outcrop (Table 3). These data highlight mixing and reaction within the basaltic crust anddiffusional inputs from overlying sediment pore waters.

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from Baby Bare outcrop to the north side of MamaBare outcrop as a result of continuous diffusive lossto overlying sediment pore waters along a flow path[Wheat et al., 2000; Hulme and Wheat, 2013].Because of this diffusive control, a plot of SO4 versusMg data from the ODP Leg 168 transect does notform a linear mixing trend. Most of the Site U1363data lie above the Mg-SO4 mixing line (Figure 6).This trend suggests that the altered seawater fromHole U1363 C/D is less influenced by diffusive lossof sulfate to the overlying sediment than that expectedbetween mixing of reacted basement fluid and seawa-ter. Thus, these warm altered fluids must be sourcedfrom a location where the basement fluid is relativelyevolved with respect to thermally controlled elements(e.g., loss of Mg and gain of Ca), but less affected bysedimentary diffusive exchange relative to fluids thatseep from Baby Bare outcrop. This suggests that thesefluids may have originated at least several kilometersfrom Hole U1363 C/D where basement temperatureare ~65�C, or may have undergone reaction deeperin the crust [e.g., ODP Sites 1030 and 1031;Elderfield et al., 1999]. Reactions at greater depthcannot be the sole explanation for the chemical data,because sulfate values are depressed relative to bot-tom seawater, requiring at least some sulfate removalby exchange with overlying sediment.

[32] Mn-Mg data from Site U1363 fall well above themixing line, as do the other environmental data andthe 70�C experimental data (Figure 6). This indicatesthat Mn is one of the more reactive elements withinbasaltic crust; however, it seems likely that elevatedMn values from Site U1363 are also influenced byexchange with sediment pore waters. Likewise, esti-mated concentrations of Mo and ammonium (and Siand Fe; not shown) in upper basement fluids appearto be influenced by sediment pore waters.

[33] One of the goals for sampling close to thesediment-basement interface at Site U1363 was todocument variations in the redox state in thebasement fluid with distance from the outcrop.Nitrate is a particularly sensitive redox indicator,and steep nitrate gradients were detected near theseafloor (in the upper 20 cm) in pore fluids recov-ered while gravity coring in this area (C.G. Wheatand M. Mottl, unpublished data). Unfortunately,we were unable to document steep nitrate gradientsin basal sediment recovered from Site U1363,perhaps because we did not recover high-qualitysamples from close enough to the sediment-basement interface, or because there is so littlenitrate in upper basement fluids. If the latter inter-pretation were correct, then nitrate is consumed

very rapidly in upper basement fluids in this area.Likewise phosphate, V, and U (not shown) inbasement fluids appear to be consumed rapidlyalong the Site U1363 transect (Figure 6).

[34] By examining all of these relationships, exper-imental data, and data from other field sites, we geta better understanding of which elements are mostreactive in this setting. The sulfate data indicate thataltered fluids from Site U1363 show less evidencefor diffusive loss than do fluids that seep from BabyBare springs. Furthermore, there is evidence thatthose ions that are greatly affected by diffusiveexchange show more alteration closer to GrizzlyBare outcrop, perhaps indicating a greater influenceof sediment sources (e.g., Mo, Si, Rb, Li, andsulfate: Table 3). This interpretation requires somemovement of reacted basement fluids across thedrill sites from warmer and/or deeper parts of thecrust towards the outcrop (Figure 7). Thermal datasuggest that there may be discharge of hydrother-mal fluids along one of five transects measuredadjacent to Grizzly Bare outcrop [Figure 15 inHutnak et al., 2006], but the amount and extent ofdischarge close to the Site U1363 transect must bemodest or else there would be less suppression ofupper basement temperatures. More generally,these result emphasize the complexity of crustalflow paths around Grizzly Bare outcrop, withnewly recharged and older, more reacted fluidsmaintaining distinct geochemical characteristicsand gradients, even as flow occurs from rechargeto discharge sites.

5.4. Residence Time and Apparent 14C Ages

[35] Four 30 to 60 ml pore water samples from nearthe sediment-basement interface were analyzed fortheir 14C age (Table 2). As with the other chemicaldata, we used these data to extrapolate to the 14Cage of the fluid in upper basaltic basement, assum-ing that the alkalinity is a measure of DIC dilutionof the end-member with the added C from depleted14C from basal sediment. Although the 14C of indi-vidual samples shows a monotonic change (older)away from the outcrop, when the values arecorrected for dilution with diagenetic DIC themonotonic trend vanishes. Surprising, the 14C ageof these fluids is tens of thousands of years, similarto values from the ODP sites and boreholes[Elderfield et al., 1999; Walker et al., 2007].

[36] The d 13C for the DIC in upper basalticbasement was similarly calculated (Table 2). Weassumed a diagenetic d 13C of �20%, consistentwith a plankton/sediment source [McCorkle et al.,

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1985; Rau et al., 2001; Zeebe, 2007]. Thecalculated values range from �1.8% to �12%without a systematic trend with distance from theoutcrop. These values span those measured bySansone et al. [1998], who analyzed spring fluidsfrom Baby Bare outcrop reporting near seawater d13C values (~�0.6%) that are slightly different thanseawater values measured reported byWalker et al.,2007; �1.3 and �1.5%. Walker et al., 2007analyzed borehole fluids from ODP Hole 1026Band harpoon fluids fromBaby Bare outcrop reportingd 13C values of�5.8 to�9.7%, that were interpretedto reflect systematic carbonate precipitation with in-put of basaltic CO2. Spring fluids from Baby Bareoutcrop were devoid of sediment artifacts (e.g., noexcess ammonium and dissolved silica), unlikeborehole fluids from Hole 1026B [Wheat et al.,2004b]. Data to determine if sediment influencescould affect the Baby Bare harpoon d 13C datawere not provided [Walker et al., 2007]. An alter-native explanation for these depleted d 13C valuesis a sediment source, stemming from diffusive

exchange with pore waters. Samples from SiteU1363 are affected by sedimentary carbon andwhen corrected for values in basement fluids theyare less depleted than the measured values. Notethat one datum from Site U1363 is not consistentwith Rayleigh fractionation with reasonable amountsof basaltic C inputs.

[37] Nevertheless, the carbon isotopic story iscomplex and does not follow those elements thatare controlled by diffusive exchange with overlyingpore water (e.g., sulfate) or reaction with basalt(e.g., Mg). The complete data set and in particularthe two types of isotopic C data, suggest a com-plex mixing scenario near a recharge area for aRFHS, yet the isotopic DIC data as a whole areinconsistent, possibly resulting from sediment arti-facts. If there are not artifacts, the DIC data aresimilar to other ions that are thought to be highlyreactive in this setting. This conclusion is notanticipated because pore water alkalinity gradientsnear the sediment-basement interface are much

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Volcaniccrust

to Bab

y Bar

e

Vigorous mixing(in and out of profile)

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Mg = 53Ca = 10

T = 2

Figure 7. Conceptual interpretation of regional thermal and geochemical data from Site U1363, and results ofanalytical and numerical studies [e.g., Wheat et al., 2000; Fisher et al., 2003a; Hutnak et al., 2006]. The seismicreflection profile and isotherms are the same as shown in Figure 4A. Mg and Ca concentrations (mmol/kg) and approx-imate upper basement temperatures (�C) are shown in boxes above drill sites. Similar parameters are also shown forrecharging bottom seawater (large blue arrow). Cold recharging fluid and ambient (reacted, warm) basement fluidsmix and react, and there is diffusive exchange for many solutes with the overlying sediment. The most intenserecharge appears to occur off the Site U1363 transect, but it is close enough to impart a recharge signature. Inset showsa cartoon of a hypothetical, three-dimensional flow system, intended to place the Site U1363 drilling transect incontext. There are likely to be a small number of recharge and possible discharge sites distributed irregularly aroundthe perimeter of Grizzly Bare outcrop. The Site U1363 transect must be relatively close to a recharge zone because ofthe observed suppression of basement temperatures near the outcrop, but the fluid composition indicates that acomponent of flow in and out of the transect profile, and/or exchange with fluids from greater depth must occur. Someof the fluid recharging at Grizzly Bare outcrop is thought to discharge at Baby Bare outcrop, 52 km to the north [Wheatet al., 2000; Fisher et al., 2003a].

1969


smaller than those of other ions. Also, carbonateprecipitation is ubiquitous and relatively quick inthis environment where Ca, alkalinity, and tem-perature variations are systematic. A highly reac-tive DIC pool is not consistent with systematicalkalinity values nor microbial rates [Lever et al.,2010]: additional studies are required to sort out theDIC story.

6. Summary and Conclusions

[38] We present new pore water chemical andthermal data from IODP Site U1363 at the baseof Grizzly Bare outcrop, a site where there isevidence for seawater recharge into the crust aspart of a regional RFHS. A transect of boreholesextending radially from the outcrop reveals amixture of seawater and thermally altered fluids,similar in composition to warm (64�C) hydro-thermal fluids sampled ~50 km to the northeast.Surprisingly, only 50 m from exposed basalt onGrizzly Bare outcrop, this mixture indicates66% (recharged) seawater and 34% thermallyaltered fluids (based on Mg data). This mixturedecreases to about 50:50 at a distance of ~750 mfrom the basaltic outcrop. However, the tempera-tures of these fluids are much lower that onewould find based on conservative mixing. Thisrequires the conductive loss of heat from thesefluid mixtures during transport. Not all of themeasured ions are consistent with the ratioscited above, because of non-conservative exchangeor reactions.

[39] Results from the Site U1363 transect do not in-dicate a simple pattern of monotonic change withdistance from the outcrop. Instead, we see evidencefor complex flow paths, mixing of distinct watertypes, and reaction. The data indicate a strongsedimentary signature close to the outcrop, consis-tent with a potential flow path towards the outcropeven though the thermal budget clearly requiresnearby seawater recharge. There was no evidencefor fluid seepage within thin sediment aroundGrizzly Bare outcrop during a site survey expedi-tion [Zühlsdorff et al., 2005; Hutnak et al., 2006;Wheat and Mottl, unpublished data], but this is alarge feature, and it is possible that diluted alteredbasement fluids exit the seafloor somewhere on ornear the edifice, consistent with thermal data thatshow evidence for complex fluid flow patternsadjacent to the edifice.

[40] Results from this study illustrate how challeng-ing it can be to “map” hydrologic flow paths in the

ocean crust. Even around seamounts and otherbasement outcrops where inflow or outflow areknown to occur, the primary channels through whichfluids, heat, and solutes move through the crustremain poorly constrained by sampling at a smallnumber of locations. Resolving these flow pathwaysis important, because they can result in large gradi-ents in redox-sensitive solutes (anoxic, altered, andwarm basement fluids relative to cold, oxic bottomseawater) and thus have significant implications forthe evolution of the oceanic lithosphere andsubseafloor microbial activity and ecology. Thepotential for strong vertical and lateral gradients inredox and other geochemical characteristics in base-ment provides conditions conducive to microbialactivity, similar to strong redox boundaries that arecommonly found at the sediment-basement interface[Engelen et al., 2008]. The Site U1363 transectis the first to systematically sample close to aknown site of hydrothermal recharge on a RFHS.Resulting samples and data suggest that careful siteselection and extensive spatial coverage will berequired to resolve the complexity and implicationsof recharge guided by seamounts for linkedthermal-chemical-microbial-geological processes.

Acknowledgments

[41] This work was made possible through the IntegratedOcean Drilling Program and the dedicated personnel that makeit possible to collect quality deep-sea sediment cores. Shore-based funding was provided from the U.S. Science SupportProgram and the Center for Dark Energy Biosphere Investiga-tions (C-DEBI). This work was supported by grants from theU.S. National Science Foundation, OCE 0939564 (withsubawards to CGW and ATF), OCE-1030061 (CGW), andOCE-1031808 (ATF) and grants from the U.S. ScienceSupport Program (CGW, SMH, ATF) and the Danish NationalResearch Foundation and the Max Planck Society (BNO). Thisis C-DEBI contribution i57.

References

Becker, K., and A. T. Fisher (2008), Borehole packer tests atmultiple depths resolve distinct hydrologic intervals in 3.5Ma upper oceanic crust, eastern flank of Juan de FucaRidge. J. Geophys. Res., doi:10.1029/2007JB005446.

Becker, N. C., C. G. Wheat, M. J. Mottl, J. L. Karsten, andE. E. Davis (2000), A geological and geophysical investi-gation of Baby Bare, locus of a ridge-flank hydrothermalsystem in the Cascadia Basin. J. Geophys. Res., 105:23557–23568.

Coggon, R., D. A. H. Teagle, C. E. Smith-Duque, J. C. Alt, andM. J. Cooper (2010), Reconstructing past seawater Mg/Caand Sr/Ca from mid-ocean ridge flank calcium carbonateveins, Science, 327, 1114–1117.

1970


Cowen, J. P., S. J. Giovannoni, F. Kenig, H. P. Johnson, D.Butterfield, M. S. Rappe, M. Hutnak, and P. Larn (2003),Fluids from aging ocean crust that support microbial life,Science, 299, 120–123.

Davis, E. E., et al. (1992), FlankFlux: an experiment to studythe nature of hydrothermal circulation in young oceaniccrust, Can. J. Earth Sci., 29(5), 925–952.

Davis, E. E., et al. (1997), Proc. ODP, Init. Repts. [CD-ROM], 168: Ocean Drilling Program, College Station,Texas.

Davis, E. E., D. S. Chapman, K. Wang, H. Villinger, A. T.Fisher, S. W. Robinson, J. Grigel, D. Pribnow, J. Stein,and K. Becker (1999), Regional heat-flow variationsacross the sedimented Juan de Fuca Ridge easternflank: constraints on lithospheric cooling and lateralhydrothermal heat transport, J. Geophys. Res., 104(B8),17,675–617,688.

de Lange, G. J., R. E. Cranston, D. H. Hydes, and D. Boust(1992), Extraction of pore water from marine sediments: Areview of possible artifacts with pertinent examples fromthe North Atlantic, Mar. Geol., 109, 53–76.

Elderfield, H., C. G. Wheat, M. J. Mottl, C. Monnin, andB. Spiro (1999), Fluid and geochemical transport throughoceanic crust: A transect across the eastern flank ofthe Juan de Fuca Ridge, Earth Planet. Sci. Lett., 172,151–169.

Engelen, B., et al. (2008) Fluids from the OceanicCrust Support Microbial Activities within the DeepBiosphere, Geomicrobiol. J., 25: 1, 56–66, doi:10.1080/01490450701829006.

Fisher, A. T., et al. (2003a), Hydrothermal recharge anddischarge across 50 km guided by seamounts on a youngridge flank, Nature, 421, 618–621.

Fisher, A. T., E. E. Davis, and K. Becker (2008), Borehole-to-borehole hydrologic response across 2.4 km in the upperoceanic crust: implications for crustal scale properties. J.Geophys. Res., doi:10.1029/2007JB005447.

Fisher, A. T., T. Urabe, A. Klaus, and the Expedition 301Scientists (2005), Proc. IODP, 301: College Station, Texas,(Integrated Ocean Drilling Program Management Interna-tional, Inc.), doi:10.2204/iodp.proc.301.103.2005.

Fisher, A. T., and R. N. Harris (2010), Using heat as a tracerof subseafloor fluid flow, Geofluids, doi:10.1111/4j.1468-8123.2009.00274.X.

Fisher, A. T., and C. G. Wheat (2010), Seamounts as conduitsfor massive fluid, heat, and solute fluxes on ridge flanks,Oceanography, 23(1), 74–87.

Fisher, A.T., T. Tsuji, K. Petronotis, and the Expedition 327Scientists (2011), Proc. IODP, 327: Tokyo (IntegratedOcean Drilling Program Management International, Inc.),doi:10.2204/iodp.proc.327.2011.

Heesemann, M., H. Villinger, A. T. Fisher, A. Trehu, andS. Witte (2006), Data report: Testing and deployment ofthe new APC3 tool to determine Insitu temperatures whilePiston coring, In T. S. Collett, M. Riedel, M. Maloneet al., Proc. IODP, Expedition 311, Integrated OceanDrilling Program, College Station, Texas, doi:10.2204/iodp.proc.311.108.2006.

Hulme, S., and C. G. Wheat (2013), Quantifying geochemicaland fluid fluxes across the rough basement transect of theeastern Juan de Fuca Ridge flank, Submitted to Geochem.Geophys. Geosyst..

Hutnak, M., A. T. Fisher, L. Zuhlsdorff, V. Spiess, P. Stauffer,and C. W. Gable. 2006. Hydrothermal recharge anddischarge guided by basement outcrops on 0.2-3.6 Maseafloor east of the Juan de Fuca Ridge: observations and

numerical models. Geochem. Geophys. Geosyst., 7,Q07O02, doi:10.1029/2006GC001242.

Karsten, J. L., N. Becker, M. J. Mottl, and C. G. Wheat (1998),Petrology of Baby Bare and Mama Bare lavas, Geophys. Res.Lett., 25, 117–120.

Kawada, Y., N. Seama, and T. Urabe (2011), The role ofseamounts in the transport of heat and fluids: Relationsamong seamount size, circulation patterns and crustal heatflow, Earth Planet. Sci. Lett., 306, 55–65.

Lever, M. A., V. B. Jeuer, Y. Morono, N. Masui, F.Schmidt, M. J. Alperin, F. Inagaki, K.-U. Hinrich,and A. Teske (2010), Acetogenesis in deep subsefloorsediments of the Juan de Fuca Ridge flank: Asynthesis of geochemical, thermodynamic, and gene-basedevidence, Geomicrobio. J., 27, 183–211, doi:10.1080/01490450903456681.

McCorkle, D. C., S. R. Emerson, and P. D. Quay (1985),Stable carbon isotopes in marine porewaters, Earth Planet.Sci. Lett., 74, 13–26.

Mottl, M. J., and C. G. Wheat (1994), Hydrothermal circula-tion through mid-ocean ridge flanks: Fluxes of heat andmagnesium. Geochim. Cosmochim. Acta, 58: 2225–2237.

Mottl, M. J., et al. (1998), Warm springs discovered on 3.5 Ma-old oceanic crust, eastern flank of the Juan de Fuca Ridge,Geology, 26, 51–54.

Orcutt, B. N., W. Bach, K. Becker, A. T. Fisher, M. Hentscher,B. M. Toner, C. G. Wheat, and K. J. Edwards (2010),Colonization of subsurface microbial observatoriesdeployed in young ocean crust, ISME J., 1–12, doi:10.1038/ismej.2010.157.

Rau, G. H., F. P. Chavez, and G. E. Friederich (2001),Plankton 13C/12C variations in Monterey Bay, California:Evidence of non-diffusive inorganic carbon uptake ofphytoplankton in an upwelling environment, Deep-Sea Res.I, 48, 79–94.

Sansone, F. J., M. J. Mottl, E. J. Olson, C. G. Wheat, and M. D.Lilley (1998), CO2-depleted fluids from mid-ocean ridge-flank hydrothermal springs, Geochim. Cosmochim. Acta,62, 2247–2252.

Seyfried, W. E. (1977). Seawater–basalt interaction from 25–300◦C and 1–500 bars: implications for the origin of submarinemetal-bearing hydrothermal solutions and regulation of oceanchemistry. Ph.D. thesis, University of Southern California,242 pp.

Seyfried, W. E., and J. L. Bischoff (1979), Low temperaturebasalt alteration by seawater: An experimental studyat 70_C and 150_C, Geochim. Cosmochim. Acta, 43,1937–1947.

Spinelli, G. A., E. G. Giambalvo, and A. T. Fisher (2004), Hy-drologic properties and distribution of sediments, in Hydro-geology of the Oceanic Lithosphere, edited by E. E. Davis,and H. Elderfield, pp. 151–188, Cambridge University Press,Cambridge, UK.

Stein, J. S., and A. T. Fisher (2003), Observations and modelsof lateral hydrothermal circulation on a young ridgeflank: numerical evaluation of thermal and chemicalconstraints, Geochem. Geophys. Geosys., 4(3), 1026doi:10.1029/2002GC000415.

Underwood, M., K. D. Hoke, A. T. Fisher, E. R. Giambalvo, E.E. Davis, and L. Zühlsdorff (2005), Provenance, stratigraphicarchitecture, and hydrogeologic effects of turbidites innorthwestern Cascadia Basin, Pacific Ocean, J. Sediment.Res., 75(1), 149–174.

Walker, B. D., M. D. McCarthy, A. T. Fisher, and T. P.Guilderson (2007), Dissolved inorganic carbon isotopiccomposition of low-temperature axial and ridge-flank

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hydrothermal fluids of the Juan de Fuca Ridge, Mar. Chem.,108, 123–136.

Wheat, C. G., and M. J. Mottl (2000), Composition of pore andspring waters from Baby Bare: Global implications ofgeochemical fluxes from a ridge flank hydrothermal system,Geochim. Cosmochim. Acta, 64, 629–642.

Wheat, C. G., H. Elderfield, M. J. Mottl, and C. Monnin(2000), Chemical composition of basement fluids withinan oceanic ridge flank: Implications for along-strike andacross-strike hydrothermal circulation, J. Geophys. Res.,105, 13437–13447.

Wheat, C. G., M. J. Mottl, and M. Rudniki (2002), TraceElement and REE Composition of a Low-Temperature RidgeFlank Hydrothermal Spring, Geochim. Cosmochim. Acta, 66,3693–3705.

Wheat, C. G., H. W. Jannasch, M. Kastner, J. N. Plant, andE. H. DeCarlo (2003), Seawater Transport and reaction inupper oceanic basaltic basement: Chemical data fromcontinuous monitoring of sealed boreholes in a mid-oceanridge flank environment, Earth Planet. Sci. Lett., 216,549–564.

Wheat, C. G., M. J. Mottl, A. J. Fisher, D. Kadko, E. E.Davis, E. Baker (2004a), Heat flow through abasaltic outcrop on a sedimented young ridge flank.

Geochem. Geophys. Geosyst., 5, Q12006, doi:10.1029/2004GC000700.

Wheat, C. G., H. W. Jannasch, M. Kastner, J. N. Plant, E. H.DeCarlo, and G. Lebon (2004b), Venting formation fluidsfrom deep sea boreholes in a ridge flank setting: ODP Sites1025 and 1026. Geochem. Geophys. Geosyst., 5, Q08007,doi:10.1029/2004GC000710.

Wheat, C. G., H.W. Jannasch, A. T. Fisher, K. Becker, J. Sharkey,and S. Hulme (2010), Subseafloor seawater-basalt-microbereactions: Continuous sampling of borehole fluids in aridge flank environment, Geochem. Geophys. Geosyst., 11,Q07011, doi:10.1029/2010GC003057.

Zeebe, R. E. (2007), Modeling CO2 chemistry, d13C,and oxidation of organic carbon and methane insediment porewater: Implications for paleo-proxies inbenthic foraminifera, Geochim. Cosmochim. Acta, 71(13), 3238–3256.

Zühlsdorff, L., M. Hutnak, A. T. Fisher, V. Spiess, E. E. Davis,M. Nedimovic, S. Carbotte, H. Villinger, and K. Becker(2005), Site Surveys related to IODP Expedition 301:ImageFlux (SO149) and RetroFlux (TN116) expeditionsand earlier studies, in Proc. IODP, edited by A. T. Fisher,et al., doi:10.2204/iodp.proc.2301.2102.2005, IntegratedOcean Drilling Program, College Station, Texas.

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Seawater recharge into oceanic crust: IODP Exp 327 Site U1363 …afisher/CVpubs/pubs/Wheat2013_G… · Received 13 September 2012; Revised 15 March 2013; Accepted 23 March 2013; Published