Heat £ow, sediment and pore £uid chemistry, and...

Heat £ow, sediment and pore £uid chemistry, andhydrothermal circulation on the east £ank of Alarcon Ridge,

Gulf of California

A.T. Fisher a;b;*, E. Giambalvo a, J. Sclater c, M. Kastner c, B. Ransom c,Y. Weinstein c, P. Lonsdale c

a Earth Sciences Department, University of California, Santa Cruz, CA 95064, USAb Institute of Tectonics, University of California, Santa Cruz, CA 95064, USA

c Geological Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093, USA

Received 10 July 2000; accepted 20 March 2001

Abstract

New seismic, heat flow, sediment and pore fluid geochemistry data from the east flank of Alarcon Ridge, at themouth of the Gulf of California, provide evidence for vigorous hydrothermal circulation within young oceanic crustformed at a moderate-rate spreading center. Data and samples were collected 9^20 km from the ridge axis to assess thehydrologic state of 0.30^0.65 Ma seafloor. Conductive heat flow values are 15^55% of that input at the base of thelithosphere. Heat flow is highest near the center of a sediment-covered trough, and lowest along the trough margins,suggesting that trough-bounding faults and basement exposures may help to focus hydrothermal recharge. Sedimentand pore fluid geochemistry data, in combination with reactive transport modeling, indicate that conditions within theshallow sediments are dominantly diffusive and reactive in two locations, but that bottom seawater recharges throughthe thin sediment layer with velocities on the order of 2^10 mm/yr at other sites. Seafloor heat flow appears to beentirely conductive, which is consistent with the slow rate of seepage inferred from pore fluid observations andmodeling. Fluid recharge through sediments requires that basement is underpressured relative to hydrostaticconditions. We interpret these observations and inferences to indicate vigorous fluid flow in basement, and secondaryseepage through overlying sediments. The heat flow deficit along the 11-km Alarcon Basin transect averages 440 mW/m2, equivalent to heat output of 5 MW per kilometer of spreading axis. This heat output is similar to the combinedfocused and diffusive heat output of a single basement outcrop on the east flank of Juan de Fuca Ridge, and suggeststhat sites of concentrated heat and fluid outflow may exist on the east flank of Alarcon Ridge. ß 2001 Published byElsevier Science B.V.

Keywords: hydrothermal conditions; Gulf of California; crust; heat £ow; pore water; geochemistry

1. Introduction

Sea£oor hydrothermal circulation results inenormous transfers of heat and solutes betweenoceanic crust and the overlying ocean [1]. Spec-

0012-821X / 01 / $ ^ see front matter ß 2001 Published by Elsevier Science B.V.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 3 1 0 - 7

* Corresponding author.

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www.elsevier.com/locate/epsl

tacular manifestations of this process includeblack smoker vents and biological communitiesfound at ridge crests, where magma associatedwith crustal formation provides large drivingforces. Hydrothermal circulation is also commonon ridge £anks away from the spreading axis, butindications of this activity tend to be more subtle.Evidence of ridge £ank circulation includes low-temperature £uid seepage and conductive heat£ow values that are suppressed below, or elevatedabove, that expected for crust of a particular age.

While the temperatures of £uid circulationalong ridge £anks are generally lower than thoseat the ridge crest, £ank circulation results in al-most 70% of the global sea£oor heat £ow anom-aly (the di¡erence between expected and measuredheat £ow) and a volume £ux much larger thanthat associated with high-temperature ridge sys-tems [2,3]. Because much of the sea£oor has notbeen studied, we do not have a quantitativeunderstanding of the distribution of hydrothermalcirculation on ridge £anks, nor what processesand properties control its extent and intensity.Heat £ow and geochemical data are particularlyscarce from oceanic crust younger than 1 Ma,because the sediment cover necessary for penetra-tion of heat £ow and coring tools tends to be thinand incomplete.

We present results of a 1998 expedition to theAlarcon Basin at the southern end of the Gulf ofCalifornia, during which swath mapping, seismicpro¢ling, dredging, CTD casts, coring, and heat£ow measurements were used to characterize thephysical state of young oceanic crust. This area isparticularly interesting because the young crust oneither side of Alarcon Ridge records the tectonichistory of the region as Baja California was splitfrom the rest of North America and plates werereorganized over the last 5 Ma [4]. Relatively highsedimentation rates have made it possible to con-duct coring and heat £ow surveys over oceaniccrust that is extremely young. In contrast to otherspreading centers formed close to continents,Alarcon Ridge is a `normal' mid-ocean ridgerather than a sedimented spreading center (e.g.,Guaymas Basin, Gulf of California; EscanabaTrough, Gorda Ridge; Middle Valley, Juan deFuca Ridge). Thus insights gained from Alarcon

Ridge may be applicable to a large part of themid-ocean ridge system.

2. Geological setting and previous work

Alarcon Ridge is located at the southern end ofthe Gulf of California and the northern end of theEast Paci¢c Rise, just north of the Tamayo trans-form (Fig. 1A). Magnetic anomalies show thatoceanic crust has been accreted by Alarcon Ridgesince the beginning of Chron C2An (3.6 Ma),with a mean spreading rate of 50 mm/yr overthe last 1 Ma [5]. Although marine geological in-vestigations in this area date back over 30 years[6,7], the complex tectonic and geological historyof the southern gulf were poorly constrained untilmodern swath mapping techniques, combinedwith magnetics pro¢les, seismic surveys, and im-proved navigation, made it possible to resolve¢ne-scale sea£oor features.

There have been numerous heat £ow studies inthe central Gulf of California (e.g., [8^11]), butheat £ow in the southernmost gulf is poorly re-solved. Several workers [12^14] report individualheat £ow values near or within Alarcon Basin(92^293 mW/m2), including borehole measure-ments made during Deep Sea Drilling Project(DSDP) Leg 65. These measurements were madefar from the ridge crest or across adjacent fracturezones, but the data are generally consistent with asimple 1/kt cooling model for oceanic lithosphere.These results contrast with those found in similarcrustal settings of intermediate spreading rate andincomplete sediment cover (e.g., [15,16]), whereheat £ow is often much lower than conductivepredictions. It was suggested that, south of theTamayo Fracture zone, the high sedimentationrate may have helped to hydrologically seal thecrust at an unusually young age [13]. Hydrother-mal circulation in basement was inferred to benecessary to reduce seismic velocities near thebase of the crust at the mouth of the Gulf ofCalifornia [13], and basement is known to be hy-drologically active because DSDP Hole 482C,drilled 12 km from the ridge crest through 143m of sediments and 47 m of basalt, turned intoa `man-made hot spring' soon after drilling [17].

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3. Survey design and methods

The Alarcon Basin ¢eld area was surveyed us-ing swath mapping, magnetics, gravity, and sin-gle-channel seismic instruments during the ¢rsthalf of the 1998 Alarcon Basin program. We ranheat £ow and coring operations based on newbathymetric maps and seismic lines that indicatedthe extent of sediment coverage. We focused heat£ow and coring work on 0.30^0.65 Ma crust onthe east £ank of Alarcon Ridge, where basementstructure is simply de¢ned, and su¤cient sedimentcover is available for safe penetration of heat £owand coring instruments. Navigation was facili-tated by a P-code di¡erential-geographic positionsystem in combination with dynamic positioning.This allowed coring and heat £ow stations to beco-located, and subsequent seismic lines to be runimmediately over heat £ow and coring sites. Seis-mic sources were a single, 210-inch, gas injection(GI) airgun (single-channel) and a hull-mountedtransducer (3.5 kHz), and the seismic survey speedwas 7 kts.

Sea£oor heat £ow was measured using a violin-bow probe [18,19] with a 3.5-m lance that housed11 thermistors spaced at 30-cm intervals. The in-strument included a separate bottom-water ther-mistor and in situ thermal conductivity capabil-ities. Data were collected during two instrumentlowerings positioned along two lines, with multi-ple penetrations during each lowering (Fig. 1B).Heat £ow data were processed using the methoddescribed by Villinger and Davis [20]. Subsea£oordepths were corrected for di¡erences in sedimentthermal conductivity through calculation of cu-mulative thermal resistance values, and plots oftemperature versus thermal resistance (Bullardplots) were used to calculate heat £ow. If condi-tions within the shallow sediments are conductiveand at steady state, a Bullard plot will be linear,and the slope of the least-squares, best-¢tting linewill indicate sea£oor heat £ow.

Records from the shallowest thermistors com-monly indicated equilibrium temperatures that de-viated signi¢cantly from linear trends suggestedby the deeper thermistors, probably because shal-low sediments are disturbed by the weight standabove the thermistor lance [20]. We used a param-

eter called `scatter' (h) [21] to evaluate theconsistency of individual Bullard plots madewith di¡erent numbers of thermistors, whereh=

��M 2=�N31�p

, M2 is based on the mis¢t be-

Fig. 1. (A) Southern Gulf of California and location of Alar-con Ridge. Small square is area of panel B. Deep Sea Drill-ing Site locations shown with open circles and numbers.(B) Swath-mapped bathymetry of east £ank of Alarcon Ba-sin, showing general structural trend running northeast^southwest. Structure curves to the northwest approachingtransform to the north of the ridge. Active ridge crest isshallow region in the northwest corner of the ¢gure. Crossesindicate locations of heat £ow measurements; circles are lo-cations where cores were collected. Labeled heat £ow andcore locations are discussed in the text and shown in Fig. 4.

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tween equilibrium temperatures and a best-¢ttingstraight line on a Bullard plot, and N is the num-ber of thermistors used in the analysis(69N9 11). We favored the solution that usedthe largest number of thermistors by startingwith N = 11. We calculated new values of h as Nwas reduced (by eliminating the shallowest ther-mistors from the analysis), and selected ¢nal heat£ow values when a subsequent reduction in N didnot reduce h. We used the range in heat £owvalues calculated in this way to estimate uncer-tainties for individual measurements (Table 1).The mean of these uncertainties is 3.0% for allstations, and the largest uncertainty is 8.6%. Allvalues were corrected for instrument tilt (9 11³tilt, 9 2% correction).

The heat £ow values have not been correctedfor sedimentation because the sedimentation his-tory of the basin is not known. Calculations[22,23] based on apparent sediment thickness(9 40 m) and crustal ages indicate corrections of9 3% if sedimentation was continuous since thevolcanic crust was accreted (sedimentationrate = 0.06^0.1 mm/yr), or 9 5% if all sedimentaccumulated in the last 100 ka (0.2^0.4 mm/yr).

Cores were collected at several sites where heat£ow had been measured (Fig. 1B) using two sys-tems: a 9-cm-diameter, gravity coring system witha 4.0-m barrel, and an eight-tube, multi-coringsystem capable of recovering the upper 1.1 m ofsediment virtually undisturbed. Multi-core sam-pling allows detailed analyses of local variability

Table 1Heat £ow and coring summary from 1998 Alarcon Basin survey

Station Pen Latitude Longitude Na Tilt Qcorrb Quncert

c Core typed Pro¢le typee

(³N) (min N) (³W) (min W) (³) (mW/m2) (mW/m2)

HF-1 1 23 29.899 108 20.980 11 3 133 1 MC, GC 1HF-1 2 23 29.714 108 20.691 9 5 262 3HF-1 3 23 29.529 108 20.403 11 7 280 7HF-1 4 23 29.343 108 20.115 7 8 340 10HF-1 5 23 29.155 108 19.823 9 10 339 8HF-1 6 23 28.969 108 19.533 11 7 373 12 MC 1HF-1 7 23 28.722 108 19.147 6 11 333 16HF-1 8 23 28.473 108 18.761 10 9 401 21 MC, GC 1HF-1 9 23 28.226 108 18.376 9 7 272 13HF-1 10 23 28.039 108 18.086 9 7 301 5HF-1 11 23 27.884 108 17.839 9 5 322 4 MC, GC 2HF-1 12 23 27.788 108 17.690 11 4 201 1 MC, GC 2HF-1 13 23 27.436 108 17.140 11 8 255 11 MC 1HF-1 14 23 27.188 108 16.754 11 6 194 12HF-1 15 23 26.940 108 16.368 10 6 147 1HF-1 16 23 26.692 108 15.984 10 8 164 3HF-2 1 23 27.956 108 19.328 9 3 253 8HF-2 2 23 28.231 108 19.200 10 7 304 26HF-2 3 23 28.507 108 19.012 9 11 322 9HF-2 4 23 28.534 108 18.857 9 5 357 12HF-2 5 23 28.598 108 18.953 9 6 427 6 MC 1HF-2 6 23 28.660 108 19.050 7 5 402 6HF-2 7 23 28.688 108 18.892 10 5 378 8HF-2 8 23 28.965 108 18.706 8 10 380 15aN = number of thermistors used in ¢nal heat £ow value.bQcorr = ¢nal heat £ow value corrected for instrument tilt.cQuncert = di¡erence between highest and lowest heat £ow value calculated using 6^11 thermistors and the ¢nal value shown inprevious column; a conservative estimate of uncertainty.dMC = multicore, GC = gravity core.ePro¢le types described in the text, characteristic curves shown in Fig. 4. Type 1 pro¢les interpreted to indicate domination by£uid recharge; type 2 pro¢les interpreted to indicate domination by reaction and di¡usion.

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of chemical and physical properties. Both systemsrecovered sediments in butyrate tubes.

After being brought on deck, cores for porewater analyses were immediately capped, tapedand taken to a cold room (4^9³C) for processing.Cores for physical properties were sampled imme-diately on deck and those for organic carbonanalyses were frozen until analyzed on shore.

Cores collected for pore waters were sectionedin a nitrogen glove bag at discrete depth intervalsand samples were immediately placed in centri-fuge tube and spun in a refrigerated ultracentri-fuge to separate the sediment and pore water.Pore waters were then extracted from the sampletubes in a nitrogen-¢lled glove bag and aliquotswere immediately analyzed for alkalinity (by titra-tion) and ammonium (by spectrophotometry),and tested for sul¢de (precipitated as CdS). Theremainder of each sample was sealed in glass am-poules and archived for analysis of other chemicalcomponents in the onshore laboratory.

Sediment water content and porosity were de-termined by sampling known volumes of sedimentand comparing wet and dry masses; dry masseswere corrected for salt content prior to porositydeterminations. Because of the high water contentof the surface sediments, the uncertainty of thesemeasurements was probably þ 5% in the upper5 cm, but decreased to þ 2% with depth.

The total organic carbon content of the sedi-ment was determined, by di¡erence, using totalcarbon data from a Perkin-Elmer CHN Analyzerand data for carbonate obtained from a coulom-eter. Di¡erent aliquots of the same ¢nely ground,freeze-dried sample were used for both methods.Precision of analyses, as determined by repeatedmeasurements of the same sample, was þ 2%.

4. Heat £ow and seismic results

The ¢rst line of heat £ow measurements andsediment cores was oriented parallel to thespreading direction and perpendicular to primarysea£oor structure, forming a transect 9^20 kmfrom the active ridge (Fig. 1B). The transectcrossed a sediment-covered trough 7 km wide,then continued over the adjacent abyssal hill to-

wards the southeast. A second line of heat £owmeasurements and cores was run down the centerof the trough perpendicular to the ¢rst line, par-allel to the overall structural trend (Fig. 1). Thetrough is bounded to the northwest and southeastby abyssal hills rising 40^80 m above the adjacentsea£oor. This is the third trough southeast of theridge, and the ¢rst within which smooth bathyme-try and seismic data suggested that sediment cov-er was continuous.

Shallow sediment thermal conductivities variedlinearly with depth from 0.70 W/m³C near thesea£oor to 0.85 W/m³C at 4.5 m below the sea-£oor (mbsf) (Fig. 2A). Seven conductivity mea-surements near 3.0^3.5 mbsf indicated higher con-ductivities, 0.9^1.2 W/m³C, probably indicative ofa sandy (turbidite?) layer. Because this higher-conductivity layer was not measured during allpenetrations, it must be discontinuous or thinnerthan the 30-cm thermistor spacing of the heat£ow lance.

All heat £ow measurements are consistent withconductive, steady-state conditions in shallowsediments, and values vary from 133 to 427mW/m2 (Table 1; Figs. 2^4; Bullard plots forall measurements are available at: emerald.ucsc.edu/Va¢sher/alarcon). The highest values arefound near the center of the trough, and thereare locally elevated values over the abyssal hillnear the southeast end of the ¢rst heat £ow line(Figs. 3 and 4). Conductive heat £ow in the sur-vey area is 15^55% of that input at the base of thelithosphere for crust of this age (Fig. 4), suggest-ing that much of the heat loss in this area is ad-vective. Heat £ow is lowest at the trough marginsimmediately adjacent to the edges of abyssal hills.

Seismic lines run along the ¢rst heat £ow andcoring transect (pro¢le L^LP) illustrate gross sedi-ment and crustal structure, including several no-table features (Fig. 4). Sediment thickness esti-mated from the single-channel seismic line isrelatively constant within the main trough (0.05s-twt, about 37 m), but thins over the hills toeither side of the trough. Sediment may be thickerlocally near the edges of the trough where base-ment appears to deepen (Fig. 4). Picking sedimentthickness from these records is di¤cult near theedges of the abyssal hills because of side echoes.

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The 3.5-kHz records reveal coherent sedimentarylayering within the trough and over the top of theabyssal hill to the southeast (16^18 km from theridge axis). Temperatures within uppermost base-ment calculated from the measured heat £ow andsediment thicknesses estimated from the seismicrecords are 5^20³C. The lowest apparent base-ment temperatures are associated with low heat£ow and thick sediments at the edges of thetrough (HF1-1 and HF1-12; Fig. 4).

Narrow (300^500-m-wide) zones of acousticallytransparent sediment are apparent on both 3.5-kHz and single-channel seismic records (Fig. 4).There is continuity of the shallowest layers withinthese zones, but layering becomes less distinctwith depth and disappears entirely near basement.A crossing line run parallel to basement structurealong the second heat £ow transect indicates thatthese features are spatially restricted and approx-imately circular in plan view.

Similar sediment structures were noted along ashallow, buried basement ridge on 1.0 Ma crusteast of the Juan de Fuca Ridge using GI gun,water gun, and Parasound (4-kHz) records. Thesestructures were interpreted to indicate elevatedsediment porosity and an associated reduction inseismic impedance [24]. Pore £uids squeezed fromcores in that area indicated that £uid was seepingfrom the sea£oor at about 2 mm/yr (Ocean Drill-ing Program (ODP) Sites 1030 and 1031 [25]).Zu«hlsdor¡ et al. [24] hypothesized that £uid £owhad altered sediment properties, and that thecharacteristic seismic signature could be used toinfer locations of sea£oor seepage. Sedimentolog-ical and geotechnical tests of sediments from theJuan de Fuca £ank seepage area demonstratedthat porosities and permeabilities were elevatedrelative to those at similar depths at nearby sites[26], but also indicated that di¡erences in proper-ties could be explained entirely by di¡erences in

Fig. 2. Thermal conductivity and example Bullard plots (temperature versus cumulative thermal resistance) from the 1998 Alar-con Basin survey. Temperatures are plotted as deviations from bottom water values, and depth-shifted so that the best-¢tting line(slope = heat £ow) passes through 0³C at the sea£oor. (A) In situ thermal conductivity data from all measurements. Filled circlesindicate measurements used to generate the linear thermal conductivity versus depth function shown with a solid line. Opencircles indicative of locally elevated thermal conductivity at 3.0^3.5 mbsf were omitted from the function ¢t. (B) Bullard plot ofstation HF1-1. All 11 thermistors used in heat £ow calculation. (C) Bullard plot of station HF1-8. The shallowest thermistor(open circle) was omitted from heat £ow calculation (as described in the text). (D) Bullard plot of station HF1-12. All 11 ther-mistors used in heat £ow calculation. (E) Bullard plot of station HF2-5. The shallowest two thermistors (open circles) were omit-ted from heat £ow calculation (as described in the text).

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primary lithology. Hemipelagic clay, which wouldbe deposited preferentially over topographichighs, tends to retain its initial porosity to greaterdepths than do the ¢ne-grained turbidites, whichtend to be deposited in lower-standing areas. Var-iations in sea£oor elevation of only a few tens ofmeters could result in a spatial di¡erences in de-position of these distinct sediment types.

Although the single-channel seismic recordsfrom Alarcon Basin do not allow an unequivocalassessment of local basement topography, thereappear to be small basement highs below severalof the transparent zones (Fig. 4C). In addition,swath maps of the unsedimented crust close tothe ridge crest reveal numerous circular, volcanicfeatures dotting the sea£oor, with topographic re-lief of 20^100 m and diameters up to several hun-dred meters. These features are more commonalong the west £ank of Alarcon Ridge, but arepresent along the east £ank as well. For this rea-son, and because the pore £uid data (describedbelow) are most consistent with di¡usive, reactive,and slow hydrologic recharge conditions within

the shallow sediments, we suggest that small base-ment highs may be responsible for the narrowzones of acoustically transparent sediment. Oneexplanation is that basement highs accumulate ¢-ner-grained hemipelagic sediments than does thesurrounding sea£oor, as observed on the Juan deFuca Ridge £ank. A variation on this interpreta-tion is that small basement highs result in sedi-ments that are su¤ciently tilted so as to disruptseismic layering. In either case, when the relativemagnitude of topographic relief is reduced, fol-lowing initial sedimentation, more continuouslayering is established across these basement highs(Fig. 4). Detailed seismic, coring, and geotechni-cal work will be required to test applicability ofthese hypotheses to the east £ank of AlarconRidge.

5. Sediment and pore water results

Sediments recovered by coring in Alarcon Basinare mainly hemipelagic mud. Mineral X-ray dif-

Fig. 3. Heat £ow values (mW/m2) and hand-drawn heat £ow contours (50 mW/m2 interval). L^LP indicates track for seismic linesand heat £ow pro¢le shown in Fig. 4.

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fractograms and organic carbon (OC) analysesindicate that surface and near-surface sedimentsare compositionally similar throughout the studyarea. Sediment carbonate contents are generally

low (V0.3 wt%), with slightly elevated values(0.4^0.6 wt%) found throughout the cores, andvalues up to 2.0 wt% occurring near 2 mbsf atHF1-1 (Figs. 1B and 4). OC contents are gener-

Fig. 4. (A) Measured heat £ow values (open squares) along pro¢le L^LP. Solid curve is expected heat £ow based on crustal age[2]. Dashed line is conductive sea£oor heat £ow from a numerical model. Conductive model was based on layering of sedimentsand basement interpreted from seismic pro¢les, as shown in the line drawing. Line drawing is vertically exaggerated by 4r. Theconductive solution was calculated using a constant bottom water temperature, heat £ow in at the base of the system varyingwith crustal age, and appropriate sediment and basement thermal conductivity values. Circles indicate locations where cores werecollected. (B) 3.5-kHz pro¢le along line L^LP (Fig. 3), perpendicular to strike of primary tectonic features. Note narrow zones ofacoustically transparent sediment. (C) Single-channel seismic record along line L^LP, used to pick sea£oor and basement depthsand create the ¢nite-element grid used for the conductive model.

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ally uniform and relatively high, 4.0^4.5 wt% inthe ¢rst 2 m of sediment, except at HF1-12 (Fig.1), where values are 0.5^2.0 wt% lower. Thesedi¡erences could have resulted from local varia-tions in the deposited proportions of hemipelagicsediment, perhaps resulting from winnowing of¢ne-grained sediments along the edge of the sedi-mented trough (Fig. 4).

Given the small size of the area investigated(Fig. 1B) and the similarity of sediments recov-ered along the transect, pore water pro¢les acrossthe trough were expected to reveal similar geo-chemical characteristics. Because the recoveredsediments exhibited high OC contents, we antici-pated measuring large gradients in pore water al-kalinity, ammonium, and sulfate related to thebacterial degradation of organic matter.

Concentration^depth pro¢les of Alarcon Basinpore waters can be classi¢ed into two groups thatwe have designated `type 1' and `type 2' pro¢les(Fig. 5). In type 1 pro¢les, the concentrations ofpore water species analyzed do not deviate, ordeviate only slightly, from those of bottom sea-water. In contrast, type 2 pro¢les show evidenceof signi¢cant di¡usion and water^sediment reac-tion. Di¡erences between the two groups are par-ticularly striking for ammonium, alkalinity, sul-fate, and calcium concentrations, all of whichare in£uenced by the bacterially mediated decom-position of sedimentary organic matter. The onlytwo sites with type 2 pro¢les are HF1-11 andHF1-12 at the southeast edge of the trough (Table1, Fig. 4A).

The most probable causes for the observed de-viations in geochemical behavior are di¡erences inmicrobiological reactions, temperature, and hy-drologic regime. The ¢rst possibility can be dis-counted on the basis of: (1) the observed compo-sitional and sedimentological homogeneity, and(2) the short distance (1.2 km) between stationsthat exhibit strong biologically mediated organicmatter degradation (type 2) and those showinglittle biological activity (type 1). Interestingly,the type 1 site that shows the weakest geochemicalindication of bacterial activity (HF1-8) has ahigher OC content (4.0^4.5 wt% in the upper 2mbsf) than does the type 2 site that shows thestrongest evidence of bacterial activity (HF1-12;

the OC content is 3.7 wt% at the top of the coreand gradually decreases to 2.4 wt% at the corebottom).

There are only small di¡erences in sedimenttemperatures between the various sites. Withmaximum sediment thicknesses 9 40 m and heat£ow suppressed well below lithospheric valuesthroughout the study area (Figs. 3 and 4), temper-atures at the top of basement are 9 20³C, too lowto cause signi¢cant thermal degradation of buriedorganic matter.

The explanation we prefer for the observedcontrast in pore water gradients along the transectis that di¡erences in hydrologic regime eithermask (type 1) or fail to mask (type 2) reactionswithin the shallow sediments. Type 1 pro¢les areattributed to seawater downwelling that erases theconsequences of reaction, while type 2 pro¢lesindicate a lack of seawater downwelling. In Sec-tion 6 we describe how hydrothermal circulationin basement may lead to secondary rechargethrough shallow sediments, and estimate the mini-mum recharge velocities required by the porewater pro¢les.

6. Alarcon Basin hydrogeology

6.1. Geothermal analyses

If heat loss along the Alarcon Basin transectwas entirely conductive, sea£oor refraction wouldlead to elevated heat £ow at the edges of abyssalhills, and locally lower values over sea£oor highs(e.g., [27]). To quantify the magnitude of conduc-tive refraction, we prepared a two-dimensionalnumerical model that includes basement relief,sediment thickness, and sediment and basementthermal conductivity values (Fig. 4A). Heat inputat the base of the model varied with crustal age.Because of refraction and the pinching out ofsediments at the edges of basement outcrops,modeled conductive heat £ow tends to be sharplyelevated along the edges of the sediment-¢lledtrough and relatively low over the basementhigh 16^18 km from the ridge crest. In contrast,the greatest measured heat £ow values are foundnear the center of the trough, and there is locally

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elevated heat £ow over the basement high to thesoutheast. The lowest heat £ow values were mea-sured at the base of abyssal hills on either side ofthe trough, suggesting that faults associated withthese feature may help to focus recharge of coldbottom water. Slightly elevated heat £ow over thebasement high 16^18 km from the ridge (Fig. 4)may indicate local advective redistribution of heatwithin basement beneath this feature.

We assume that there is a component of across-strike £uid £ow within basement, with £uid enter-ing the outcrop northwest of the heat £ow trans-ect and £owing to the southeast, and use an ana-lytical model of coupled heat and £uid £ow[28,29] to estimate the lateral speci¢c discharge(volume £ux per unit area) required to explainthe observed suppression in heat £ow. Assuminga 40-m-thick sediment layer, a 500-m-thick perme-

Fig. 5. Pore water concentrations from cores collected at two sites (HF1-8 and HF1-12) illustrating pro¢le types, and reactivetransport model results used to interpret the observations. Type 1 pro¢les are interpreted to indicate seawater recharge, and type2 pro¢les are interpreted to indicate mainly di¡usion and reaction. The distribution of type 1 and type 2 pro¢les is listed in Table1. Example model results are based on the following conditions: basement temperature = 5³C; basement £uid = seawater; reactiveTOC in the sediments = 1.5%; sedimentation rate = 0.06 mm/yr. Curves show modeled pore water pro¢les for various £ow rates.Other boundary conditions and properties give somewhat di¡erent results, but the overall trends are the same, as described inthe text. (A) Observed Ca. (B) Observed alkalinity. (C) Observed SO4. (D) Observed NH4. (E) Modeled Ca. (F) Modeled alkalin-ity. (G) Modeled SO4. (H) Modeled NH4.

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able zone in upper basement, and appropriate £u-id and sediment thermal properties, the data from9^14 km from the ridge crest (Fig. 4A) suggestspeci¢c discharge of 2^3 m/yr. Assuming a thinnerpermeable zone requires a proportionately greaterrate of discharge, as would thinner sediment cov-er. Some crustal heat is probably extracted by£uids £owing along strike, since depression ofheat £ow suggests that £uids enter basement onboth sides of the trough, but limited observationsdo not allow us to map £uid pathways in detail atthis time.

The mean heat £ow de¢cit (the di¡erence be-tween lithospheric conductive and measured val-ues, Fig. 4A) over the 11-km heat £ow transect is440 mW/m2 ( þ 100 mW/m2, 1 S.D.). Integrationalong the transect results in a total heat de¢cit of5 MW per kilometer of ridge crest. This heat out-put is equivalent to that of a single black smokervent, and is similar to that estimated for com-bined focused and di¡usive heat advection fromthe Baby Bare outcrop on the east £ank of Juande Fuca Ridge [30].

6.2. Geochemical analyses

We interpret sediment pore waters exhibitinglittle or no chemical di¡erence from seawater(type 1) to indicate localized sites of downwelling,whereas sites with large geochemical gradients(type 2) are thought to indicate places where £u-id^sediment reactions and di¡usional processesdominate. We have run a set of numerical, reac-tive transport simulations to help place con-straints on these interpretations. We used a modi-¢ed version of GIMRT (Global ImplicitMulticomponent Reactive Transport) [31] to con-duct a series of one-dimensional simulations thatincorporate sediment burial, £uid advection, spe-cies-speci¢c molecular di¡usion, organic matterdegradation (via SO4 reduction), and inorganicmineral diagenesis including calcite precipitationand dissolution. We focused on the in£uence ofthese reactions on pore water concentrations ofCa, alkalinity, SO4, and NH4 because these spe-cies showed the strongest contrast between thevarious sites and are particularly sensitive to or-ganic matter degradation (Fig. 5). A conservative

tracer such as chloride is commonly used for mak-ing sea£oor seepage calculations based on pore£uids (e.g., [32,33]), but chloride concentrationsdid not vary signi¢cantly in the cores recoveredfrom the £ank of Alarcon Ridge. At the otherextreme, biogenic silica was so abundant andhighly reactive in Alarcon Basin sediments thatit provides a relatively poor constraint on seepagerates.

All simulations were started with a 40-m-thicksediment layer having a constant porosity of 70%.Several sets of upper and lower boundary temper-atures and £uid compositions were tested. At theupper boundary we used seawater chemistry anda bottom water temperature of 2³C. At the lowerboundary, we tested three sets of conditions: sea-water at 5³C, reacted basement £uid at 15³C, andreacted basement £uid at 20³C. The compositionswe assumed for the reacted basement £uid werethose of ODP Sites 1024 and 1025 (Juan de FucaRidge £ank), with temperatures of 15³C and20³C, respectively [34]. The sediment thermal gra-dient was held constant and conductive. Sedimentwas advected through the system at a constantsedimentation rate (values of 0.06^0.1 mm/yrwere tested) and the models were run for 1.0^1.5 Myr, long enough to establish dynamicsteady-state conditions. We also varied the quan-tity of reactive organic matter in the sedimentcolumn from 0.4 to 4.0%. We ran simulationsthat included no £uid £ow through the sediments,and simulations with £uid seepage rates of 2 mm/yr up and down and 10 mm/yr up and down. Allof these parameters in£uenced the modeled pore£uid pro¢les, several of which are compared toobservational data (Fig. 5). Because of the widerange of free parameters, we did not attempt to`best ¢t' model results to observations. Instead,we used the model results to suggest a range ofconditions that are consistent with observations.

Type 1 pro¢les were most consistent with sim-ulations that include downward seepage at rateson the order of 2^10 mm/yr. Upward seepage ismuch less e¤cient at suppressing pore water gra-dients unless velocities are considerably greater(Fig. 5), but higher upward velocities are incon-sistent with heat £ow pro¢les. Relatively slowseepage into the sediment was more consistent

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with observations when we modeled the systemwith lower fractions of reactive OC (which resultin a slower rates of SO4 reduction) or with lessreacted (and lower-temperature) basement £uid.Higher values for TOC or the extent of basement£uid reaction required higher downward seepagerates to match shallow pore water observations.Several combinations of TOC content, sedimenta-tion rate, and basement £uid composition werealso found to simulate type 2 pro¢les, but thesepro¢les were best represented by simulations thatincluded no £uid £ow or very slow down£ow(Fig. 5).

Vertical £uid £ow can also in£uence conductiveheat £ow values (e.g., [11,35]), but curvature inthermal gradients due to £uid recharge is di¤cultto detect with a 3.5-m probe. Recharge at rates6 30 cm/yr would be rapid enough to produce ageochemical signal, but too slow to perturb shal-low thermal gradients. Thus the geochemistry andheat £ow data are entirely consistent with £uidrecharge into the sea£oor at rates on the orderof 2^10 mm/yr.

We also note variations in chemical gradientswith depth in Alarcon Basin sediments (Fig. 5).These variations could result from subtle di¡er-ences in physical or chemical conditions withdepth, transient or lateral £uid £ow, or somecombination of these conditions. More data andmore comprehensive modeling will be required totest these possibilities and to evaluate di¡erencesin the location or intensity of pore £uid £ow with-in shallow sediments.

6.3. Comparison to other sites

The hydrologic regime apparent in Alarcon Ba-sin is similar to that documented on a sedimentpond on 7.3 Ma crust on the west £ank of theMid-Atlantic Ridge (MAR) [29,36]. In that set-ting, elevated basement outcrops on either sideof the sediment pond are thought to allow crus-tal-scale £ow of seawater through basement. Heat£ow in the MAR sediment pond is suppressed by75^80% below that expected based on crustal age,and pop-up pore pressure instruments deployedwithin the sediment pond detected negative pres-sure gradients, indicative of slow seawater re-

charge [36,37]. Limited chemical analyses of sedi-ment pore waters were also consistent withdownward seepage of bottom seawater [38]. Theinterpretation that basement is underpressured be-low the sediment pond is consistent with the re-charge of ocean bottom water for 17 years intoDSDP Hole 395A, drilled through the edge of thepond and into the underlying basalt [39^41]. Wesuggest that hydrologic conditions responsible for£uid £ow within young crust in Alarcon Basin aresimilar to those documented near DSDP Site 395.

Thermal, chemical, and hydrologic conditionseast of Alarcon Ridge are also similar to thoseat other sites where young crust (6 1 Ma) createdat an intermediate-rate spreading center is incom-pletely blanketed by sediments: the GalapagosMounds ¢eld south of the Galapagos SpreadingCenter [15,42,43], and the west £ank of the Juande Fuca Ridge [16]. In these cases, mean heat £owis depressed signi¢cantly below that expectedbased on crustal age. However, heat £ow mea-surements at each of these sites include scatteredvalues considerably greater than predicted basedon crustal age. These high values tend to be asso-ciated with the edges of abyssal hills, and havebeen interpreted to indicate localized £uid up£owalong faults [16,42]. The Alarcon Basin data set islimited thus far, but we note only low heat £owvalues on the edges of the abyssal hills, and noevidence for hydrothermal discharge along hill-bounding faults or outcrops or through sedi-ments.

7. Summary and conclusions

New seismic, heat £ow, sediment and pore £uidgeochemistry data from the east £ank of AlarconRidge provide evidence for vigorous hydrother-mal circulation within young oceanic crust formedat a moderate-rate spreading center. Heat £owvalues are 15^55% of that input at the base ofthe lithosphere for crust of this age. Heat £ow ishighest near the center of a sediment-¢lled trough,and lowest along the trough margins, suggestingthat trough-bounding faults and basement expo-sures may help to focus hydrothermal recharge.Sediment and pore £uid geochemistry data, in

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combination with reactive transport modeling, in-dicate that conditions within the shallow sedi-ments are dominantly di¡usive and reactive intwo locations, but at other sites where coreswere collected, the data suggest that bottom sea-water recharges through the sediments at veloc-ities of about 2^10 mm/yr. Sea£oor heat £owappears to be entirely conductive, which is con-sistent with the low seepage velocities inferredfrom geochemical observations. The total £uid£uxes through sediments are probably small rela-tive to direct basement recharge through basaltexposures, but recharge through sediments re-quires that basement is underpressured relativeto local hydrostatic conditions.

Similar conditions have been detected on otheryoung ridge £anks. A combination of heatingfrom below, lateral and vertical di¡erences in per-meability, and basement relief and exposure re-sults in vigorous £uid £ow through basement,and slow recharge of seawater through overlyingsediments. That conductive heat £ow is consis-tently lower than that predicted based on crustalage requires that heat be removed advectivelyfrom basement. There are likely to be componentsof £uid £ow moving across strike beneath thesediment-covered trough, and along strike of ma-jor tectonic structures. The heat £ow de¢cit alongthe 11-km heat £ow and coring transect in Alar-con Basin is 440 mW/m2, equal to 5 MW perkilometer of ridge. This is equivalent to the com-bined advective and di¡usive heat output througha single basement outcrop on the east £ank of theJuan de Fuca Ridge. It seems likely that there aresites of di¡use and focused ridge £ank dischargein Alarcon Basin. Careful surveys will help tolocate £uid discharge areas in this area, and toassess the mechanisms by which heat and solutesare exchanged between the crust and ocean.

Acknowledgements

We are grateful for the advice and cooperationof colleagues from the Department of Geology atCICESE, Ensenada, Mexico, without whom theAlarcon Basin expedition would not have beenpossible. Gretchen Robertson and Holger Mi-

chaelis provided analytical assistance. We alsothank SIO ship operations, and the o¤cers andcrew of the R/V Roger Revelle, particularly Capt.Chris Curl and Marine Technicians Seth Mogtand Bob Wilson. Funding in support of this proj-ect was provided by the UCSC Committee onResearch, the CULAR/UCSC program (STB-UC 98-181), and NSF/OCE-9819242.[EB]

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