Fresh water seepage and pore water recycling on …geochem/urumu/Sagami Trough...Earth and Planetary...

Ž .Earth and Planetary Science Letters 138 1996 157–168

Fresh water seepage and pore water recycling on the seafloor:Sagami Trough subduction zone, Japan

U. Tsunogai a,) ,1, J. Ishibashi a, H. Wakita a, T. Gamo b, T. Masuzawa c,T. Nakatsuka c, Y. Nojiri d, T. Nakamura e

a Laboratory for Earthquake Chemistry, Faculty of Science, UniÕersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japanb Ocean Research Institute, UniÕersity of Tokyo, Minami-dai, Nakano-ku, Tokyo 164, Japan

c Institute for Hydrospheric-Atmospheric Science, Nagoya UniÕersity, Chikusa-ku, Nagoya 464-01, Japand National Institute for EnÕironmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305, Japan

e Tokyo UniÕersity of Fisheries, 4-5-7 Kounan, Minato-ku, Tokyo 104, Japan

Received 4 May 1995; accepted 22 November 1995

Abstract

Ž .We collected 19 samples of bottom sea water including 8 samples of shimmering fluid , using the Japanese submersibleŽ .Shinkai 2000, in the bathyal Calyptogena community area the Hatsushima area along Sagami Trough, at the northern end

of the Philippine Sea plate, in order to clarify the source of fluid seepage. Shimmering fluid samples showed a maximumtemperature of 11.68C, which is about 98C higher than the ambient sea water. The fluid chemistry of these samples is

Ž .summarized as follows: 1 alkalinity, H S, SiO , NH , ÝCO and CH show enrichment and Cl, SO , K, Na, Mg and Ca2 2 4 2 4 4Ž . Ž . 3 4show depletion compared with ambient sea water; 2 Cl depletion is about 20%; and 3 the content and Her He ratio of

dissolved helium show little anomaly compared with ambient sea water. The observed chemical composition andtemperature of the fluid is well explained by mixing of sea water, pore water and land-derived groundwater with atemperature of about 408C and a seeping flux of about 400 m3 per day. A preliminary estimation of the global chemical flux

Ž .implies that groundwater discharge to the ocean including pore water recycling is an important factor in controlling oceanchemistry.

Keywords: Philippine Sea Plate; hydrothermal conditions; pore water; discharge; He-3; sea water

1. Introduction

In recent years, ‘cold seepage’ on the seafloor inconnection with deep-sea biological communities hasbeen found in tectonically active subduction zones,

) Corresponding author. Fax: q81-3-3816-1784. E-mail:[email protected]

1 Also Research Fellow of the Japan Society for the PromotionŽ .of Science DC .

w x w xsuch as off Oregon 1,2 , off Peru 3 , the Nankaiw xTrough and Japan Trench 4,5 , and in tectonically

w xpassive areas, such as the Florida Escarpment 6 ,w x w xLouisiana slope 7 , Northern California 8 , and the

w xLaurentian Fan 9 . Activities of biological communi-ties associated with cold seep have been consideredas being supported by microbial chemosynthesis, thatoxidizes dissolved compounds in a manner similar tothat occurring in deep-sea hydrothermal communitiesw x2,10,11 .

0012-821Xr96r$12.00 q 1996 Elsevier Science B.V. All rights reservedŽ .SSDI 0012-821X 95 00228-6

( )U. Tsunogai et al.rEarth and Planetary Science Letters 138 1996 157–168158

The cold seep communities are located on thecontinental margin, in areas covered with thick, ter-rigenous, organic-rich sediments. This suggests thatcold seep flow from sedimentary layers could be animportant factor for discussing chemical transportfrom the sediment to the ocean. In order to estimateglobal chemical flux of the cold seepage, we mustdetermine the source of the fluids. For the source ofthe cold seep fluids, however, various sources havebeen proposed, such as pore water in accreted sedi-

Žments compacted by plate subduction off Oregon,.off Peru, the Nankai Trough and the Japan Trench

w x Ž2 , hydrocarbon in sediments Louisiana slope and. w xNorthern California 7 and land-derived groundwa-

Ž . w xter Florida Escarpment 6 . This is because geo-chemical data on the fluids is limited.

In this study, the source and flux of cold seepfluids in the Sagami Trough are discussed, based ongeochemical data of the fluid, as the first step inglobal chemical flux estimation. Preliminary estima-tions of the global chemical flux of cold seepage arepresented, using the Sagami Trough fluid as beingrepresentative of cold seepage on the seafloor.

2. Sagami Trough and the deep-sea biologicalcommunity

Sagami Trough is located in the northern conver-gence front of the Philippine Sea Plate, which issubducting beneath the plate of the Japanese Islands.The volcanic front of the Izu–Bonin arc runs in a

Fig. 1. Map showing the location of the Hatsushima cold seep area, together with the bottom topography off Hatsushima Island in theŽ .Sagami Trough Maritime Safety Agency, 1983 .

( )U. Tsunogai et al.rEarth and Planetary Science Letters 138 1996 157–168 159

north–south direction in the western part of SagamiTrough and includes such active volcanoes as Izu-

Ž .Oshima Island Fig. 1 . In 1984, the occurrence of awell developed biological community dominated bythe giant clam Calyptogena soyoae was found on the

Ž Xseafloor off Hatsushima Island 35800.1 N,X .139813.5 E; 1160 m deep; Hatsushima area by the

Ž .submersible Shinkai 2000 Fig. 1 . The communityextended over ca. 200 m from east to west and ca.600 m from south to north, including dense clampatches of a few to several meters in diameter.

Since 1986, various geochemical, geological, geo-physical and biological studies have been concen-trated in the Hatsushima area with the aim of clarify-ing the source of the cold seep fluid and the charac-

w xteristics of the biological communities 12,13 . Fromthese studies, the community is considered to beclosely associated with methane-rich fluid seep sys-

w xtems 12,14 . Active sulfate reduction and hydrogen

sulfide production using methane is proposed withinw xthe sediments just beneath the community 15 .

Chemo-autotrophic sulfur bacteria, living symbioti-cally in the clam’s gill tissues, are inferred to play animportant nutritional role in the seep communitiesw x12 . However, appropriate geochemical data was notavailable to establish the source of the seeping fluidin previous studies because of the limited advectionrate. Based on the high heat flow anomaly observed

w xin the Hatsushima area, Fujioka et al. 16 and Nakaw xet al. 17 proposed geothermal fluid circulation in

sediments as a source of the fluid seepage. On thew xother hand, Hashimoto et al. 13 and Yoshida and

w xTsukahara 18 proposed that tectonic compactionexpelled the pore water from the sediments.

This study analyzes 19 bottom sea water samplesŽ .taken in four dives Dives 607, 659, 720, and 721 of

Žthe Shinkai 2000 at two sites Colony site and.Deposit site in the Hatsushima area during the

Table 1Chemical and isotopic compositions of waters sampled

Units in: msmmolesrkg; msmmolesrkg and nsnmolesrkg, unless otherwise stated; —–snot measured; n.d.snot detected.) 1 Col.sColony site, Dep.sDeposit site. ) 2 Distance from seafloor to position of the sample inlet. ) 3 DT saverage temperatureavg

) 4 3 3 4 Ž y6 . ) 5 4anomaly during sampling. d He is the percentage deviation of the Her He ratio from the atmospheric value 1.40=10 . D Hew x ) 6is the saturation anomaly relative to atmospheric solubility 45 of the sample at ambient sea water temperature and salinity. Sampled

) 7 Ž w x.together with surface mud. D227-W bottom water collected 150 cm above the clam site: 12,15 .


Ž .period 1992–1993 Table 1 . The data on bottom seaw xwater taken by Sakai et al. 12 at the Colony site in

1986 are also discussed.The Colony site, on which previous studies were

centered, is one of the clam patches developed oversandy and muddy sediments; its pore water chem-

w x w xistry 15 and bottom sea water chemistry 12 havealready been reported. The Deposit site, located 200m north of the Colony site, is an area covered withyellowish-brown material whose origin has not been

w xdetermined 17 . During Dive 721 on November 27,1993, shimmerings generated by seeping fluid wereobserved at the Deposit site, and the shimmering

Ž .fluid was sampled D721 Nos. 1–8 . The tempera-ture of the shimmering fluid was 11.68C at maxi-mum, which is about 98C higher than that of theambient sea water. The average temperature anomalyof each sample, which is calculated by integration ofthe temperature during water sampling, and the dis-tance from the seafloor to the position of the sampleinlet are also listed in Table 1. Detailed descriptionsof the Deposit site and of Dive 721 may be found in

w xa preliminary dive report by Tsunogai et al. 19 .

3. Experimental

3.1. Sampling equipment

Water samples were collected by the submersibleShinkai 2000 of the Japan Marine Science and Tech-

Ž .nology Center JAMSTEC . An eight-port revolver-w xtype water sampler 20,21 was used. The sample

Ž .bottle, made of acrylic resin 700 ml , has a deadspace of about 20 ml in the check valve filled withdistilled water, and the recovered samples are dilutedto some extent by this water. The dilution is cor-rected by measuring the volume of the recoveredsample and that of the dead space. In dives otherthan Dive 721, a titanium inlet tube was attached tothe revolver-type sampler. A specially designed fun-nel-shaped inlet was successfully adopted for the

w xDive 721 sampling 19 . A temperature probe wasattached to the inlet tube to measure the temperatureof the incoming sampling fluids. Besides the re-volver-type sampler, cylinder samplers made of

Ž w x.acrylic resin M-type bottom sampler 21 were alsoused to collect background bottom sea water at theDeposit site.

3.2. Chemical analysis

After the Shinkai 2000 was back on board theŽ .tender ship Natsushima, titration alkalinity TA and

Ž .nutrients silicate and ammonia of water sampleswere measured. Aliquots of water samples werestored in lead glass bottles for dissolved heliummeasurements. For dissolved H S measurement, a2

portion of the sample was transferred to a polyethy-Ž .lene bottle and Zn AcO was added to precipitate2

ZnS. For dissolved ÝCO and CH measurements, a2 4

portion of the sample was transferred to 120 ml glassvial bottles, poisoned with 2 mmolrkg HgCl , and2

sealed with septum stoppers. The rest of the sampleswere then passed through 0.4 mm Sartorius filtersand stored in polyethylene bottles for the measure-

Ž .ment of major cations Na, K, Mg, Ca, Sr and Ba ,Ž .anions Cl and SO , SiO , ÝNH and sulfur isotope4 2 4Ž 34 .ratios d S of SO . The samples for major cations4

were acidified by ultra pure nitric acid. All sampleswere refrigerated. Both major and minor chemicalconstituents were analyzed by the methods listed in

w xTable 2 45–50 .

4. Results and discussion

4.1. Chemical features of fluids

Analytical results are shown in Table 1. Thewater samples, particularly the Dive 721 samples,

Table 2Methods and estimated errors of chemical analyses

ICP–AESs inductively coupled plasma–atomic emission spec-troscopy; GCrFIDsgas chromatography with flame ionizationdetector; MSsmass spectrometry.


show anomalies reflecting seeping fluid. SignificantŽ .enrichment of titration alkalinity TA , NH , Si,4

H S, ÝCO and CH and marked depletion of SO2 2 4 4

and Cl relative to sea water are observed. Slightdeviations from sea water are also found in K, Na,Mg, Ca and Ba concentrations.

Žw x . ŽFig. 2. Plots of SO concentration vs. average temperature anomaly DT and concentration of each component alkalinity, H S, Cl,4 avg 234 Ž . 3 . Ž w x.d S SO , SiO , NH , K, Na, Mg, Ca, Sr, Ba, Cl, SCO , CH , He and d He of water samples v s this study, `sdata from 124 2 4 2 4

Ž .and sea water I . The solid line is the least-squares fit to the samples.


Fig. 3. A plot of SO concentration vs. Cl concentration of4Ž . Ž .samples v and sea water I . The solid line is the least-squares

fit to the samples. Positions of seeping fluid end-member, porewater and fresh water are also shown. Bold arrows indicate theline of evolution of the composition of fresh water during ascentthrough the sedimentary layer.

As is shown in Figs. 2 and 3, the temperatureanomaly and most of the dissolved components arewell correlated with the SO concentration, suggest-4

ing mixing of two end-members: low-temperature,high SO content, bottom sea water and high-temper-4

ature, low SO content, seeping fluid. The least-4

squares fit lines in Fig. 2 are interpreted to be mixinglines of the two end-member components.

The d 34 S of sulfate dissolved in seafloor seepingfluids have provided specific information on its ori-

Ž w x.gin e.g. 22 . As shown in Fig. 2, the sample fluidsŽexcept sample D721-M, which may have been con-

.taminated by surface pore water during samplinghave almost the same d 34 S as ambient seawaterirrespective of SO concentration, confirming that4

the sulfate presented in samples is attributed to anŽaddition of ambient seawater during sampling or

.during ascent through the sedimentary layer . Theseeping fluid end-member is free of sulfate, whichcoincides well with the fact that sulfate is unstable inthe anaerobic condition of the sedimentary layer inthe continental margin, where land-derived organic

w xmaterials are abundant 23 .The temperature and chemical compositions of

Žthe seeping fluid end-member derived from normal-.izing the mixing lines of all samples to SO s0 are4

listed in Table 3. Chemical compositions of shallowŽ .-100 m pore waters in continental marginsŽ .rapidly deposited terrigenous types of sedimentsare also listed for comparison. The selected shallowpore waters have Cl concentrations similar to seawater, which implies that ordinary diagenetic reac-tions in the sedimentary layer, such as bacterialdegradation of organic matter and carbonate precipi-

w xtation 23 , are the main factors determining thecomposition. The chemical compositions of the seep-ing fluid and the shallow pore water show a commontrend in several species, enriched in TA, ÝCO ,2

CH , H S, NH and SiO and depleted in SO , Mg4 2 4 2 4

and Ca, compared with ambient sea water. Theresults imply that shallow pore water in SagamiTrough sediments may be a major component of theseeping fluid and that at least the enriched compo-nents, ÝCO , CH , H S, NH and SiO , are mostly2 4 2 4 2

Table 3Temperature and chemical composition of the seeping fluid end-member

Typical pore water compositions of terrigenous sediments aroundŽ .Japan Nankai Trough and Sagami Trough and ambient sea water

are also shown for comparison. ) 1 Data derived from the least-w xsquares fit lines of all samples normalized to SO s 04

Ž . ) 2 w x ) 3mmolrkg . ODP Leg 131 Site 808A 45 . D227-202ŽHatsushima area, outside the Calyptogena community patch:w x. ) 4 ) 515 . Helium data of D721 No. 1 was used. Data of

) 6 w xdissolved organic carbon. Air-saturated sea water 44 .


Fig. 4. A schematic diagram showing the process of fresh waterevolution in the sedimentary layer of the Sagami Trough.

of pore water origin. In contrast, in the case of thedepleted component of Cl, ordinary pore water–am-bient sea water mixing will not produce the observedcomposition, because significant Cl depletion willnot be produced by ordinary pore water–sediment

w xinteraction 23 . Thus, there must be another compo-nent which contributes to the high-temperature seep-ing fluid end-member from a deeper layer in thesediment. In Fig. 3, the relation between Cl and SO4

in the fluid samples is shown, together with seawater, pore water and proposed fluid end-membercompositions. To account for the low Cl and SO4

contents in the seeping fluid, a ‘fresh water’ compo-nent is suggested. The fluid end-member is inter-preted as being a mixture of the sulfate-free freshwater and sulfate-free pore water. This mixing pro-cess in the sedimentary layer is schematically shownin Fig. 4. As shown in the figure, the fresh watercomponent may be closely related with the source ofthe fluid seepage.

4.2. Mixing proportion of the fresh water component

If we use a and b to represent the mixing propor-tions of the two components, fresh water and pore

water, the concentration of Cl in the fluid end-mem-Žw x .ber Cl is expressed as follows, using concentra-em

Žw x .tions of that element in fresh water Cl and porefwŽw x .water Cl :pw

w x w x w xCl sa Cl qb Cl 1Ž .pwem fw

aqbs1 2Ž .Little or no chloride depletion is expected in an

ordinary interaction between pore water and sedi-w x w xment 23 . Thus, Cl can be approximated to Clpw

Ž .concentration of sea water 540 mmolrkg . In thecase of the low-Cl fresh water component, severalsources can be assigned, as will be discussed indetail in Section 4.4. Whatever the sources are,significant depletion of Cl compared with sea wateris expected for the fresh water component. Further-more, shallow pore water in Sagami Trough sedi-ments may be a major component of the seepingfluid, as discussed in Section 4.1, and the pore water

Ž .to fresh water ratio bra must be large. Thus, wew x Ž .use 0- Cl -100 mmolrkg .fw

Based on the estimated Cl range, the mixing ratioŽ .of fresh water a is estimated to be about 0.48–;

Ž .0.59, and the ratio of pore water to fresh water brain the seeping fluid is 0.7–;1.1. The straight linesdrawn in Fig. 2 are interpreted as mixing linesbetween sea water and the seeping fluid end-mem-ber.

ŽUsing this mixing ratio of fresh water as0.48–.;0.59 and the temperature of the end-member fluid

Ž .23"38C , the original temperature of the freshwater component is estimated to be about 34–;548C, assuming no heat source other than the freshwater in the fluid. Using the temperature of the freshwater component and the heat flow data reported forthe Hatsushima area, we can estimate the flux offresh water seepage in the Hatsushima area.

4.3. Seeping flux of fresh water in the Hatsushimaarea

w xKinoshita et al. 24 measured heat flow values atmore than 80 sites on the seafloor in the SagamiTrough from research vessels and found anoma-

Ž 2lously high heat flow average anomaly 400 mWrm ,. 2assuming conductive flow of heat in ca. 2 km of

the Hatsushima area, whereas a uniform heat flow of


50 mWrm2 was observed in another area of thewestern Sagami Trough. This anomaly correspondsto a total heat flow of 1.7=1010 calrday in theHatsushima area. Combining this total heat flow withthe estimated fresh water temperature, the seepingflux of the fresh water can be calculated by thesimple method described below.

Ž .In the calculation, we have assumed that: 1 theobserved heat flow anomaly is entirely attributed to

Ž .the ascent of high-temperature fresh water; 2 mostfresh water seepage is diffusive flow on the seafloorand heat flow of the diffusive seepage is equal to themeasured heat flow. The second assumption is based

Žon the observation that the shimmering site Deposit. Ž 2 w x.site is small about 10 m 19 compared with the

Ž 2 .whole area of high heat flow about 2 km . That isto say, a large portion of the fresh water may bedischarged as diffusive flow, which is hardly visibleon the seafloor, and a small portion of the freshwater may be recognizable as the shimmering fluid

Ž .at a very limited site Fig. 4 .Based on these assumptions, the total heat dis-

charge of the fresh water corresponds to a total heatflow of 1.7=1010 calrday in the Hatsushima area.Considering the heat transferred by the fresh water tobe 34–;54 calrg, the flux of the fresh waterseeping is estimated to be about 310–;500 m3rday.This flux is too large for steady-state seepage offluid from sediments due to tectonic compactionŽ 3 w x.less than 40 m rday in the Hatsushima area; 25 .Tectonic compaction may not be the driving force ofthe fluid migration. Using the estimated temperatureand flux of the fresh water, we discuss its source.

4.4. Source of the fresh water

Possible sources of low-Cl fresh water seeping onŽ . Ž .the seafloor are: 1 phase-segregated gas phaseŽ .hydrothermal fluid; 2 clay membrane ion-filtered

Ž . Ž . Ž .pore water; 3 methane gas hydrate clathrates ; 4Ždehydrated water from minerals primarily clay min-

. Ž .erals ; and 5 land-derived meteoric water.w xMassoth et al. 26 concluded that the low-Cl

submarine hydrothermal fluid found in the southernJuan de Fuca Ridge was a vapor-rich component ofhydrothermal fluid created by boiling. The tempera-

Ž .ture of the fresh water 34–;548C is likely to be aŽw x.result of magmatic heat contribution 16,17 ,

whereas the chemical data of seeping fluids do notŽw x.support this contribution 12,18 . In order to dis-

cuss the possibility of a magmatic heat contribution,we use helium data from the fluid sampled.

Ž4 3 .Helium isotopes He and He are useful signa-Žtures in discussing the origin of seeping fluid e.g.

w x.12 , especially from the viewpoint of a magmaticcontribution. Remarkable enrichment of heat and3He compared to sea water is a common feature ofseafloor-venting hydrothermal fluids. This is becausehydrothermal fluids extract heat and primordial he-lium, enriched in 3He, from mantle-derived magmaw x 327 . The Herheat ratios of the representative sub-marine hydrothermal fluids are centered in the range

y20 Ž . w xof 60;2400=10 molrJ 21,28–30 , irrespec-Ž .tive of tectonic setting MOR or arc volcano , output

Ž .temperature 20–4008C and sedimentation environ-Ž .ments covered with sediments or not . This is be-

cause 3He and heat in magmas are extracted at thew xsame rate through hydrothermal circulation 31 .

3 Ž .The d He values of the fluid samples Table 1are much smaller than those of the hydrothermal

Ž .fluids )600% . Furthermore, the difference in3Herheat ratio between the Sagami Trough fluidŽ y20 Ž . .about 5=10 molrJ in sample D721-1 andthe hydrothermal fluids is more than one order ofmagnitude. The significant deviation in 3Herheatratio of the Sagami Trough fluid suggests that theheat source is quite different from those of hy-drothermal fluids. In conclusion, the possibility of acontribution from magmatic heat sources and subse-quent boiling cannot be responsible for the source ofthe present fresh water.

The importance of clay membrane ion filtrationreaction in thick clay-rich sediments was mentioned

w xby Hanshaw and Coplen 32 . According to theirexplanation, shales and compacted clay minerals actas semi-permeable membranes to prevent the pas-sage of charged ionic species, such as Cl. As yet,however, clay membrane ion filtration reaction hasnot been shown to be important in any naturalenvironment. Also, its efficiency is too low to ac-count for the large fresh water flux of the Hat-sushima area. Ion-filtered pore water can be ne-glected as the source of the fresh water.

In view of recent findings of methane-rich seep-ing fluids on the seafloor in several convergent

Ž w x.marginal environments e.g. 2 , gas–hydrate disso-


Ž .ciation below the bottom-simulating reflector BSRand upward fluid expulsion are proposed as an im-portant source for the methane-rich cold seep fluidsw x33 . Enrichment of methane and depletion of Cl areproposed as common features of such pore water inthe sedimentary layer containing methane hydratew x34 . No evidence of methane hydrate itself, how-ever, has been reported in the Sagami Trough. Incomparison with other continental margin areas, the

Ž .relatively shallow water depth ;1000 m and highŽ w x.background geothermal gradient 50 Kr1000 m; 24

in Sagami Trough may not be suitable for methanehydrate formation.

Clay mineral transformation and dehydration reac-tions have been considered among the prime reac-

Ž w x.tions promoting hydrocarbon migration e.g. 35 .When low-Cl pore water was found in the NorthernBarbados accretionary complex, where no gas hy-drates are expected to occur, dewatering reactions ofclay minerals were invoked as an origin for the

w xlow-Cl pore water 36 . In the case of Sagami Trough,however, the sedimentary layer is too thin for theclay mineral dehydration reaction. The dehydrationreactions of clay minerals are both temperature- andpressure-dependent, and limited to high-temperatureand high-pressure conditions. At the commongeothermal gradient of approximately 50 Kr1000 m

w xin the Sagami Trough 24 , smectite starts to releasew xH O molecules at about 1000 mbsf 35 . This is2

deeper than the bottom of the sedimentary layer ofŽ .the Hatsushima area about 800 mbsf . Other clay

minerals are inferred to release H O at the same2

depth range. Clay mineral dehydration could not bean origin of low-Cl water.

w xManheim 37 has mentioned that the occurrenceof low-Cl pore water 120 km off the coast of Floridais a result of subsurface flow of groundwater fromthe land area. Although little quantitative discussionof seeping flux and the mechanism of lateral migra-tion was present in this study, the groundwater onland is a plausible source for fresh water on the

w xseafloor. The density difference 38 , temperaturegradient and hydrostatic head gradient between thefresh water and pore water could be responsible forthe migration.

Ž 3The seeping flux of fresh water about 400 m r.day in the present study could be compensated by a

Ž .total rainfall flux about 2000 mmryr of only 0.1

km2 in the land area of Izu Peninsula. Furthermore,the land groundwater 600–;1000 m deep beneathIto city, located in the northeast of the Izu Peninsu-lar, falls into the geothermal gradient temperature of40–;548C and has a Cl concentration of 0.2–;51

w xmmolrkg 39 , which is in good agreement with theestimated fresh water temperature of 34–;548C andassumed Cl concentration of 0–100 mmolrkg.Land-derived groundwater is highly probable as asource of the fresh water seepage.

The helium content and helium isotope ratio ofthe fresh water seepage, also suggest its groundwaterorigin, with values of almost air-saturated pure wa-ter, similar to those of groundwater. Using the in situradiogenic accumulation rates in groundwater of 2.1= 10y7 mmol4 Herkg yry1 and 3.4 = 10y15

3 y1 w xmmol Herkg yr 40 and an initial tritium concen-Ž y18tration of 10 TU tritium units10 TrH, half-life

3 .of 12 yr to He via beta-decay , groundwater withŽ 4age younger than about 3000 yr D He-35% and

3 . 4 3D He-25% is consistent with the He and HeŽ 4 3contents of the fresh water D He;35% and D He

.;40% . Except for groundwater with age from 20to 40 yr, which is enriched in anthropogenic tritiumŽ . w xfrom nuclear bomb tests 41 , the helium data forthe fresh water also supports its groundwater origin.In conclusion, the source of the fresh water can beplausibly explained as deep groundwater from a landarea through lateral migration.

4.5. Chemical flux of groundwater discharge andpore water recycling to ocean

The interaction with sediment will greatly changeŽthe chemical composition of water in sediments pore

. Ž w x.water from that of the original sea water e.g. 23 .As revealed in this study, pore water, being involvedin groundwater flow from land, is injected back to

Ž .the ocean Fig. 4 . The process of active pore waterrecycling could play an important role in controllingthe chemistry of the ocean. Here, we preliminaryestimate the chemical flux caused by groundwaterdischarge from the land area, including pore waterrecycling.

The rate of the global discharge of groundwater tothe ocean has been estimated as 30–;300 km3ryrw x42 . By using this estimation, we can calculate the

Ž .global chemical flux including pore water recycling


to the ocean. The flux of each element, e , is ex-i

pressed as follows:

Fx e sg e F q p e ys e F 3.1Ž . Ž . Ž . Ž . Ž .Ž .i i g i i p

sg e F q p e ys e bra F 3.2Ž . Ž . Ž . Ž . Ž .Ž .i g i i g

s F ra C e ybs e 3.3Ž . Ž . Ž .Ž .g i i

aqbs1 4Ž .Ž .where Fx e is the global chemical flux of compo-i

nent e to the ocean brought by groundwater dis-iŽ . Ž . Ž . Ž .charge; g e , p e , s e and C e are the averagei i i i

concentrations of element e in groundwater, porei

water, sea water and seeping fluid, respectively; Fg

and F are the global seeping flux of groundwaterp

and pore water; and a , and b are the averagemixing proportions of groundwater and pore water.This estimation has a merit that we can calculate thetotal flux without distinguishing the sourceŽ .groundwater or sediment of each element.

If we assume that the average chemical composi-Ž .tion, C e , and the average mixing ratio of eachi

Ž .component a , b of global seeping fluids are iden-tical to that of the end-member fluid of SagamiTrough, a can be approximated as 0.48–;0.59.

Ž . Ž .Using Eqs. 3.3 and 4 , and the above parameter,the global chemical flux of groundwater, includingpore water recycling, to the ocean can be calculated.The results are summarized in Table 4. Negativevalues indicate that removal to the sedimentary layerduring pore water circulation is larger than the totaloutput of pore water and groundwater. For most of

Table 4ŽThe calculated results of global groundwater seep flux including

.pore water recycling , assuming that the Sagami Trough seepageis the representative of world groundwater discharge to the ocean

The global fluxes of hydrothermal fluid and river to the oceanŽ . ) 1molryear are also shown. Calculated from hydrothermal

w x ) 2 w xfluid data of EPR 218N 43,51 . 52,53 .

the elements, groundwater and pore water recyclinghas almost the same order of flux as river and MOR.

One of the striking features of the calculated fluxis large flux in carbon compounds, especially

w xmethane. As stated by Gamo et al. 14 , the fluidseep activity generates a significant CH plume in4

the water column of Sagami Trough, which coin-cides well with our result. The estimated methaneflux of 1012 "1 molryr is at least one order ofmagnitude greater than the magmatic methane fluxfrom the mid-ocean ridge crest, which was consid-ered the primary source of methane in the deep

w xocean 43 . Groundwater seeping activity could be amajor factor controlling methane distribution in deepocean of the land margin area. The flux implies largedifferences in the regional distribution of methane inthe ocean.

High flux is the case not only for carbon com-pounds but also for many other elements. Groundwa-ter seepage and pore water recycling could be amajor factor controlling ocean chemistry. The pre-sent estimation however, has many problem. Due tothe large uncertainty in the estimation, the estimatedflux show enormous variations in some elements,such as K, Mg and Ca. Furthermore, the estimatedflux does not include uncertainties of spatial varia-tion in the mixing ratio of fresh water and pore waterŽ .bra and in the original composition of groundwa-ter, which are difficult to assume. The present esti-mation is preliminary and more detailed studies ofcold seep zones are needed to obtain a precise andaccurate estimate.

5. Summary and conclusions

The chemical composition and temperature of theHatsushima cold seep fluid are well explained by amixture of sea water, pore water and land-derivedgroundwater with a temperature of 34–;548C andseeping flux of 310;500 m3rday. Groundwater

Ž .seepage including pore water recycling to the oceancould be an important factor controlling ocean chem-istry.

Acknowledgements

We are very grateful to Dr. A. Ueda and Dr. A.Kawabata of Mitsubishi Materials Co. for measuring


sulfur isotope ratios. We are also grateful to K.Danno, Y. Imai and the Shinkai 2000 operationteam, as well as the Captain and crew members ofthe tender ship Natsushima for their skilful coopera-tion and expert seamanship. We are also grateful toDr. M. Kinoshita of Tokai University and Dr. K.Fujioka, Dr. M. Hattori and Dr J. Hashimoto of theDeep Sea Research Department of the JAMSTEC forfruitful discussions and suggestions. We thank themembers of the Laboratory for Earthquake Chem-istry, University of Tokyo, for numerous forms ofsupport. We also thank Dr. C.K. Paull and anony-mous reviewers for their constructive comments.[ ]MK

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