E arth a n d P la n e ta ry Scien ce L etters, 83 (1 9 8 7 ) 3 6 3 -3 7 5 E lsevier S cien ce P u blish ers B .V ., A m sterd a m - P rinted in T h e N eth erla n d s 363 [41 Nankai Trough, Japan Trench and Kuril Trench: geochemistry of Fluids sampled by submersible “ Nautile” J a c q u e s B o u lè g u e \ J e a n T . Iiy a m a 2, J e a n -L u c C h a rlo u 3 a n d J a c q u e s J e d w a b 4 ' L a b o ra to ire d e G éoch im ie e l M éta llo g én ie ( C N R S VA 196), U n iversité P ierre et M a rie Curie, 4 p la c e Jussieu, 75252 P a ris C éd ex 05 (F rance) ' F aculty o f Science, U niversity o f Tokyo, B unkyo-ku, T o k y o 113 (Japan ) ' D 3 G M , I F R E M E R , C en tre d e B rest, B .P . 337, 29273 B rest C éd e x (F rance) 4 L a b o ra to ire d e G éochim ie, U n iversité L ib re d e B ruxelles, 5 0 A v. F.D. R oosevelt, B -I 0 5 0 B ru xelles (B elgique) R ev ised version accep ted February 5, 1987 D eep -w a ter sa m p les co lle cted du rin g the K a ik o project are o ften asso cia ted w ith b io lo g ica l c o m m u n itie s lo ca ted on g eo lo g ica l structures favorable to fluid ventin g. T h e evid e n c e o f fluid v en tin g are the tem p eratu re a n om alies, the decrease in su lfa te co n c en tr a tio n s, the c o n te n t in m eth an e and the lo w C j / f C j + C ,) ratio o f light hyd rocarbons. B ecause o f large d ilu tio n by a m b ia n t seaw ater du rin g sa m p lin g it is d iffic u lt to c o m p u te the c o m p o sitio n o f the a d v ectcd en d -m em b er pore fluid. Part o f this flu id sh ou ld origin ate in the “ p etroleu m w in d ow " , i.e. at tem perature a b ou t 60 ° C. M o d e lin g the upw ard flo w o f w ater, tak ing in to acco u n t the a n o m a lie s o f tem p eratu re m easured on the sea flo o r and th e g e o ch em ic a l a n o m a lies, leads to n o n -stea d y -sta te a d vection o f th e pore flu id. T h e o ccu rren ce o f a d eep co m p o n en t in th e flu id has im p lica tio n s for the g eo lo g ic a l and tecto n ic m o d els o f the su b d u ctio n z o n es o f f Japan. 1. Introduction 2. Sampling and analyses T he discovery of clam com m unities associated with fluid venting in the accretionary com plex of the O regon subduction zone has opened the dis­ cussion on the origin and the source depth of the advecting fluids [1,2], O ne of the purposes of the K aiko program was to test the possibility of a d ­ vection of fluids associated with the dew atering of the accretionary prism in the subduction zones off Japan. T here were some previous hints that such advection was occurring from the D SD P results off northern Jap an , It was found th at the light hydrocarbon ratio C , / C 2 was possibly lowered by inputs of petrogenetic hydrocarbons and that the 8n C of C H 4 was correlatively lowered . Hence, the bottom w ater sam pling during the K aiko legs was oriented tow ards u n derstanding these possible processes at locations where the presence of b io ­ logical com m unities indicate th at fluids m ay be venting. T he capability of handling tools from the subm ersible “ N au tile” was extensively used to fulfill this program . The w ater sam ples were collected either with titanium syringes handled directly above the sedi­ m ent interface or with an alum inium box corer and core sam plers lined w ith plastic by w ithdraw ­ ing the su pernatant water. T he use of syringes to attem p t to sam ple as m uch as possible of venting fluid associated with the biological com m unities posed constraints on the representativeness of the samples. The flux of fluid passing through the com m unities is sm all (see later) and so as not to dam age the instrum ent it was deployed in the free bottom w ater w ithout touching the solid sub­ strate; hence the fluid sam ples were a m ixture of m ostly bottom w ater with a small percentage of fluid advecting from the sedim ent. T he location of the sam ples is listed in Table 1 and shown in Figs. 1 and 2. The sam ples that are clearly associated with biological com m unities are HY4, 7, 8, 9, 10, 14, 15, 16. Fig. 2 shows the exact locations of the sam ples from the clam com m unity in the deep-sea fan of Tenryu C anyon together with the m easured tem peratures. Sam ple HY16 0012-82 1 X /8 7 / S 0 3 . 5 0 © 1987 E lsevier Scien ce P u blishers B.V. 364 ,5TI ;h v i o h y b . H Y ?. HYY} water= 1.19°C HYI 4' Fig. 2. D etailed sa m p lin g sch em e in the b iological com m u n ity o f T enryu d eep sea fan H Y = w ater sam ple: 5T L 5 N C and 5BC" are sed im ent sam ples. M easured tem peratures are also given ( ° C ) . A fter a d raw ing by A . Taira. T he black d ots -HYll-13 corresp on d to sam plin g lo c a tio n s and tem perature m easure­ m en ts (see |28]J. J 140°E' Fig. 1. G eneral lo ca tio n m ap o f d eep -w a ter sa m p les from s u b d u ctio n zo n e s o f Japan. was taken 2 m from sa m p le H Y I 5 so as to check for the effects of m ixing o f advecting fluids with seaw ater over sho rt distances. Som e of the o th e r sam ples m ay also be associated with fluid venting because of favorable geological structure. T his is the case for H Y 3 located in a region of small ledges, in the vicinity o f a thrust in the Z enisu Basin. In the vicinity of T en ry u C a n y o n , samples H Y I , 2 an d 5 will be considered as reference sam ples of local deep-sea water. TABLE 1 L ocatio n s and d ep th s o f w ater sa m p les o b ta in ed du rin g K aik o project S am ple L oca tio n D e p th (m ) Sam pler a C om m en ts H Y 11 H Y 12 3 8 ° 1 6 '4 2 N , 137 ° 2 4 '1 5 E 4187 T.S. N an k ai T rough sam e as H Y I 1 4187 1IY 13 3 3 ° 1 6 '5 3 N , 1 3 4 ° 2 4 '0 6 E 42 3 4 B.C. T.C. o live-grey sem i-c o n so lid a te d clayston e; N a n k a i T rough olive-grey sem i-c o n so lid a te d cla y sto n e HY2 3 3 ° 3 6 '6 0 N . 1 3 7 ° 3 2 '6 0 E 3844 T.S. T enryu C an y o n d eep -sea fan HYI 3 3 ° 3 6 '0 9 N , 1 3 7 ° 3 2 '6 0 E 3839 T.S. T enryu C an y o n d eep -sea fan; N -S ou tcro p HY5 HY4 3 3 ° 3 7 '6 2 N , 1 3 7 ° 3 2 '1 0 E 3676 T.S. top o f an ticlin e; sid e o f T enryu C an y o n 3 3 ° 3 6 '3 5 N . 1 3 7 ° 3 2 '2 5 E 3836 T.S. clam field , N W -SF . fault N 5 0 ° F . flexure; T en ryu C an yon HY7 sam e as H Y 4 3836 T.S. see Fig. 2 HY8 sam e as H Y 4 3836 T.S. sec Fig. 2 HY9 sa m e as H Y 4 3836 B .C see Fig. 2 H Y 10 sam e as H Y 4 3836 T .C . see Fig. 2 HY6 3 3 ° 3 6 '2 1 N , 1 3 7 ° 3 2 '0 3 E 3830 HY3 3 3 ° 11 '4 8 N , 1 3 7 ° 5 3 '0 1 E 4184 T.C . T.C . Z en isu Basin; coarse sa n d y s em ico n so lid a te d m ud: H Y 18 H Y 19 33 °4 5 'O O N , 1 4 2 ° 2 9 '5 6 E 5611 T.S. flank K ashim a S eam ou nt; calcareou s ou tcrop 5513 T .C . H Y 14 35 ° 4 8 '3 2 N , 1 4 2 ° 2 5 '7 2 H 35 ° 5 4 '6 2 N , 1 4 2 ° 3 0 '9 0 F . 5640 T.S. olive-green mud; K ashim a, innerw all o f Japan T rench c lam field; N 2 8 0 ° E flexure; innerw al o f Japan T rench, K ashim a H Y 15 4 1 ° 1 8 '0 5 N , 1 4 4 ° 4 4 '0 7 E 5129 T.S. clam field; sm all led ge on subvertical H Y 16 sam e as H Y 1 5 5129 T.S. 2 m o ff H Y 15 o live-grey m ud 300 m o ff clam field; T en ryu ca n y o n structure? fault indurated mud “ T .S . = titanium syringe; B.C. = box corer; T.O . = tube corer. 365 The syringes were subsam pled on board ship for 3H e / 4 H e and other rare gases m easurem ents, for light hydrocarbons and for 180 / 160 and D / H determ inations. Sam ples for n itrate were frozen upon recovery. T he determ inations of p H and alkalinity were done on board ship using Tris buffer as reference. Dissolved sulfide was de­ term ined by potentiom etry w ith an A g /A g 2S elec­ trode. Sam ples for trace elem ent chemistry' back at the shore-based laboratory were filtered (N ucleopore 0.1 jam) and acidified with u ltrapure nitric acid. The filters were stored for m icroscopic ex­ am ination. T he tem perature of the sam ples and the am bient seaw ater were determ ined in situ with the tem perature probe ( ± 0.005 °C ) attached to one of the arm s of “ N au tile” . D uring shipboard subsam pling and handling the tem perature was generally less than 7 ° C. G as overpressure was also found in several sam ples (H Y 3. 5, 7, 8, 11, 18). In the shore-based laboratory 180 / 160 and D / H were m easured by mass spectrom etry and light hydrocarbons by gas chrom atography. M eth­ ods for o th er elem ents are given in Table 2 to ­ gether with accuracies. T he filters were exam ined by SEM coupled with T R A C O R to identify m inerals. Some particles were also exam ined by X-ray m icrodiffraction. Results are given in Tables 3, 4 and 5. T otal dissolved sulfides were always less than 10 7 M. T he m ajor cations (L i*, N a 1, K / , R b / M g 2 t , C a 2*, S r2 + ) and anions (C l- , F - ) of seaw ater TABLE 2 A n a ly tica l m eth o d s E lem ent M eth o d Standard A ccu racy N a , K, Ca Mg FAA sea w a ter a 2-3% FAA seaw ater 3% Li. R b, Sr FAA seaw ater 5% ZCFAA seaw ater 5% Fe, M n. C u, C o 1 M o. Ba. Pb. C d ƒ Cl A g* T 0.2% F P seaw ater N O ,. SO„ Si IC MC seaw ater seaw ater 1-2% A ik H+T seaw ater 0.2% 1% 2% F A A = flam e ato m ic a b so rp tio n ; Z C F A A = Z eem a n corrected fla m eless a to m ic ab so rp tio n ; A g + T = titration w ith H g + u sing an A g -A g 2S electrod e; P = p o ten tio m e tr y w ith sp ecific e le c ­ trode; IC = ion ch rom atograp h y; M C = m o ly b d a te c o lo r im e ­ try; H + T = a cid titration. " Seaw ater w ith a d d itio n o f elem ent. showed no significant variation. The only excep­ tion is a 1-2% decrease of M g2 + in HY4, 7 and 8, accom panied by the decrease of the M g /C l ratio. C onsidering that elem ents such as Ca, Mg, Li, Sr show significant variations in the pore w aters of nearby sedim ents [4,5], this signifies that the frac­ tions of advected pore fluid collected in these sam ples are less than 10%. H ence the possibility of assessing the characteristics of such fluid will be lim ited. This point is discussed further below. 3. Geochemical evidence for advection of pore waters W e will first discuss the evidence for possible advection of fluids at the locations of sampling, then the characteristics of the fluids and possible controls on their com positions and last their origin and the rate and m ode of advection of pore fluid. T he sam ples show no significant variations in the com position of the m ajor dissolved cations and chloride which would have enabled us to place lim its on the contribution of pore w ater to the total samples. However, in the absence of such clear diagnostic features several other im portant param eters and species show sufficient variations from which we can deduce the participation of pore fluids. O f these the m ost straightforw ard evidence is the tem perature anom alies m easured in situ. We have found tem perature anom alies in the range 0 .1 -0 .4 5 ° C in the im m ediate vicinity of biological co m m u n itie s an d an o m alies in the range 0 .01-0.03° C in the overlying bo tto m waters. Even if some may be due to biological activity, it is doubtful that significant tem perature differences can be m aintained in open w aters if there is no input of fluids from the sedim ent. Similar tem per­ ature anom alies were also observed in the vicinity of fluid vents in the subduction zones off the coast of O regon , The small M g 2+ decrease in HY4, 7 and 8 can also be related to mixing of advected pore fluids and to the tem perature anom alies in the same site. T he occurrences of large concentrations of light hydrocarbons (about ten times the expected level for m ethane and m uch m ore for C 2 and C 3) are also related to advection of pore fluids (Table 5). T he m ethane contents are in the same range as found in sam ples from the vent areas in the sub- 366 TABLE 3 W ater sa m p les. M ain param eters: tem perature, p H and alkalinity (A ik ) and co m p o sitio n o f characteristic d isso lv ed elem en ts. The tem perature read ings are from the “ N a u tile ” p ro b e and the tem p eratu re d ifferen ces ( à T ) w ere th ose o b served du rin g sa m p lin g with reference to a m b ia n t d eep o cea n w aters Sam p le H Y ll H Y 12 Sam pler T.S. B.C. H Y 13 HY2 HYI HY5 HY4 T .C . T .S. T.S. T.S. 1.03 1.20 0 1.04 0 T .S. 1.20 0 7.83 2.37 8.21 8.06 7.90 2.47 2.46 2.32 2.23 .2 2.51 2.42 T(°C) AT( ° C) PH A ik b b SC O , Sio/' SO 2 “ 8.28 8.0 2.56 2.25 2.26 9.9 2.16 2.73 2.70 3.9 10. 2.73 2.34 12.7 13.0 2.70 2.73 2.75 7.39 NO, c 1.07 6.0 16.9 E ed 6.8 5.3 M nd 0.47 0.18 0.40 0.58 0.33 0.11 Cu d 0.75 0.66 0.85 1.35 1.45 Mo d 0.79 1.01 1.08 0 .7 2 Bad 1.42 0.98 1.8 Sam ple HY9 H Y 10 Sam pler T .C . T .C . T( ° ) B.C. 1.41 1.63 1.20 1.20 A T (°C ) 0.21 0.43 PH A ik b 7.74 7.92 2.34 0.74 22.4 1.19 0.77 0 .5 2 1.31 0.51 0.46 0.35 HY3 HYI 8 H Y 19 H Y 14 H Y 15 T .C T .S. T .C . T .S. T.S, T.S. 1.13 0.02 0 1.20 1.20 0 0 6.8 0.14 14.7 2.96 0.94 n.m . 5.39 4.2 4.77 16.5 9.21 Cu d 5.10 0.92 1.01 M od 1.23 0 .9 4 1.01 Ba d 1.12 1.79 0.79 6.50 1.73 7.5 17.6 2.59 1.01 0.80 5.03 Fe d M nd 1.85 19.3 2.33 12.3 0.23 2.72 2.46 2.30 9.8 2.40 7.97 0.86 3.45 2.68 0.0 9 7 2.32 12.1 2.54 8.02 2.49 2.87 2.52 2.19 N O ,“ c 2.55 0.85 2.26 7.1 sol a 2.23 2.52 2.80 2.33 14.7 11.2 8.05 1.89 8.30 7.7 0.40 2.5 2.54 S iO / 1.60 0.21 8.14 0.18 7.82 SCO / T .S. 1.41 0.11 12.5 2.39 2.43 2.37 HY8 4.1 9.6 HY6 HY7 T .S. 8.20 2.56 8.01 n.m . 2.30 10.2 4.56 9.8 2.74 2.18 0.76 0 .8 6 8.15 2.585 H Y 16 1.11 8.16 8.01 2.48 2.59 2.40 2.45 11.9 12.6 2.33 12.7 2.60 2.63 2.70 < 0.2 8.0 0.40 0.11 7.9 0.14 3.15 4.56 1.64 0.86 0.45 0.58 1.23 0.75 1.17 1.39 1.82 C on cen tra tio n scales: a 10 “ 2M; b 10 3 M ; c 10 5 M; d 10 7 M. Pb and Cd c o n cen tra tio n s w ere alway:s less than 10 * M , ex cep t H Y 9 w ith 3 x IO “ 8 M. n.m . — not m easured. For sam pler, see Table 1. TABLE 4 180 / u’0 and D / H o f w ater sa m p les (exp ressed in S r*c units versus SM O W standard) HYI 1 HY2 HYI HY5 HY4 HY7 HY8 S 's O 0.35 - 0 .2 4 -0 .3 9 - 0 .3 2 0.39 0.27 - 0 .3 0 S D 0.9 6 —1.19 - 1.31 - 0 .1 8 t 1.94 ■0.57 0.08 HY3 H Y 18 H Y 14 H Y 15 H Y 16 - 0 .2 5 -0 .9 7 -0 .3 0 3.91 - 0.37 0.47 6.39 0.45 6.29 - 6 .2 1 TABLE 5 M eth an e ( C , eth a n e (C 2 ) and propan e ( C , ) co n c en tr a tio n s in w ater sam ples expressed in n l/1 Sam p le HY4 HY7 HY3 c, 111 72 c\ 20 72 < 1 C, 3 < 1 C ,/ ( C , + C ,) 5 H Y 14 H Y 15 HYI 6 112 5 60 12 18 13 < 1 < 1 <1 < 1 13 5 6 6 82 duction area off O regon , The C , / ( C 2 + C ,) ratios are very low. which may be indicative of: (1) extrem e oxidation of a microbiologically p ro ­ duced hydrocarbon pool, m ostly of m ethane, in the upper 50 m of the sedimens [6,7]; or (2) input from therm ogenic hydrocarbons. In this case they should have been produced at tem peratures higher than 60° C. Therm ogenic hydrocarbons may also 367 have been partially oxidized in the u pper p a rt of the sedim ent. A m ixture of both sources of h y d ro ­ carbons is also possible. The decrease in concentrations of sulfate found in several sam ples collected above the seafloor (H Y 4, 6. 7, 8, 9, 3, 14, 15) is also characteristic of the advection of pore fluids. It is difficult to ascribe the decrease in sulfate contents to the sulfate-reducing bacteria, since the variations are not accom panied by an increase in dissolved nutrien ts an d total dissolved carbonates. In the area off Jap an such an expected correlation be­ tween S 0 4, nutrien ts and dissolved carbonates is found in sedim ents from the N ankai area collected during the D S D P program , The decrease in sulfate contents can be due to a contrib u tio n of advected pore w ater in the vicinity of biological com m unities since sim ilar sedim ents collected in the Tenryu C anyon (H Y I 2, 13) do not present this decrease of sulfate content. T he decrease in sulfate in the open w aters show th at the percentage of advected pore fluid th at was collected is at least 5% for HY8, 14 and 15, 8% for HY3 and 4 and m ore than 10% for HY7. T h u s te m p e ra tu re anom alies an d several anom alies of dissolved constituents point out that the w ater sam ples collected above biological com ­ m unities (H Y 4, 7, 8, 14, 15) or above geological structure (H Y 3) m ay represent a m ixture of n o r­ m al oceanic seaw ater and advected pore waters. In the N ankai T rough area the results obtained d uring the D SD P program [5,7] have shown that diagenesis has induced only lim ited changes in the pore w aters in the upp er hundred m eters of the sedim ent colum n. T he mixing of such pore w ater with deep-sea w ater should therefore not be ex­ pected to yield large deviations in concentrations of the m ajor elem ents. Also, the m etabolism of the Calyptogena may have induced changes in the advected pore w aters such as oxidation of light hydrocarbons and uptake of carbon dioxide, hy­ drogen sulfide , and of m etals such as Fe, Mn, C d, Mo . H ence it is difficult to assess the exact end-m em ber com position of the advecting fluid. different from th at of seaw ater collected at the sedim ent interface. The alkalinity (Aik) versus total dissolved carbonate ( S [ C 0 2]) diagram , (Fig. 3) shows that m ost of the free bottom w ater sam ples are in the range expected for deep Pacific Ocean w ater . T he expected variation ( A A lk /A 2 [ C 0 2]) for Pacific deep w aters was found for HY5 and H Y I in the Tenryu C anyon deep-sea fan. The super­ n a ta n t w aters (H Y 6, 9, 10, 12, 13) also show a sim ilar relation characteristic of carbonate dissolu­ tion and degradation of organic m atter , It is possible th a t m ostly carbonate dissolution is im ­ p o rta n t in sam ples HY2, 3, 11, 18 . The AlkS [ C 0 2] values for the sam ples where advection of pore w ater and changes due to clam m etabolism are expected do not generally follow the above trends. From the A lk -£ [C 0 2] relation and Alk-pH relation it can be deduced th at sam ples HY4, 7, 8, 14, 15. 16, associated with biological com m unities have higher alkalinity and lower S [ C 0 2] than expected. These may be due to advected pore w ater input and dissolved carbonate uptake by the ICO2 I 24 10-3m/V9 Fig. 3. A lk a lin ity (A ik ) vs. total d isso lv ed ca rb o n a te ( 2 [ C 0 2]) diagram . T h e nu m b ers corresp on d to the sam p le references. 4. Geochem ical characteristics of the fluids col­ lected during Kaiko A part from the characteristics discussed above the chem istry of the sam ples is generally not very T h e square su rrou n d in g the sam p le nu m ber corresp on d s app roxim atively to the a n alytical un certainty. T h e d ash ed lin es corresp on d to the relation ( A A l k / A S [ C 0 2 ]) = 0.93 exp ected for d eep P acific w aters (see ). T h e low er lin e corresp on d s to th e sup ern atan t w ater sam ples. T h e d o tte d lin e corresp on d s to th e relation du rin g carb on ate d isso lu tio n . 368 clams. Samples HY15 and HY16 were collected exactly at the sam e site, with a large dilution by seaw ater for HY16 (as shown by the values of AT, S 0 4 and C H 4). The Aik and 2 [ C 0 2] values imply that the advected pore w ater has in this case higher alkalinity and total dissolved carbonate. This is in agreem ent with pore waters characteris­ tics found during the D SD P [4,5]. Thus the al­ k a lin ity -to ta l dissolved carbonate relations show that the advected pore w ater has indeed higher concentrations than local seawater. However, the uptake of carbonate and ions (such as C a 2*) for m etabolism purpose by the clams [8.9] makes it difficult to deduce m ore accurately the properties of the advected pore water. The n itrate and silica concentrations are varia­ ble and some are not in the ranges expected for deep-sea w ater . Sam ples H Y 2 and 11 w'here the degradation of organic m atter was lim ited on the basis of the A lk -2 [C 0 2] relation have low n itrate and low silica concentrations; sam ples HYI and 5, where this relation is m ore extensive, have higher n itrate and silica concentration, as ex­ pected. The sam ples associated with the biological com m unities have very variable n itrate concentra­ tions and higher silica concentrations (except HY7). Since we have discovered that some silicate form ation was associated with the clam com m uni­ ties of the Tenryu C anyon it is, however, difficult to interpret this d ata (see further section). O ne can only argue th at the advection of pore w ater should bring high silica concentrations, which will favor silicate neogenesis. The redox potential properties of the waters associated with clams can be com puted from the two redox couples C 0 2/ C H 4 and S 0 4 “ / H S . T he first one gives Eh = —0.260 V and the second is in the range Eh = —0.250 V to - 0 .2 8 0 V. The N 2/ N H 4 couple cannot be assessed due to lack of N H 4 data. However, with N H 4 in the 10 5 M range and N 2 in equilibrium with the air the corresponding Eh should be about - 0.260 V. This is an indication that the N 2/ N H 4 may also be active in the vicinity of the clams, which would be in agreem ent with N 2 uptake by the clam s . The trace elem ent geochem istry is difficult to decipher. Several causes may affect their con­ centrations and they are very sensitive to the biological activity [8,9]. Also pollution due to the sam pling tools and to the subm ersible is affecting num erous metals. We have only given the results for the trace metals for which this last factor is negligible. As expected near the sedim ent-w ater interface, the concentrations of iron, copper and m anganese are m uch higher than in open ocean water. There is, however, no clear trend appearing for these metals. In the samples where sulfate depletion was observed and where hydrogen sulfide was produced, the iron concentrations are in the range expected for equilibrium with greigite (F e 3S4) or pyrite (FeS2) , In the same samples the concentrations of copper should be controlled by sulfide form ation as well. This is in agreem ent with the finding of m etal sulfides on the corre­ sponding filters (see further section below). The concentrations of m olybdenum are in the range expected for ocean w ater near the sea bottom -w ater interface , At the m easured con­ centrations of hydrogen sulfide ( ^ 1 0 7 M) and Eh conditions discussed above, the range of m olybdenum concentrations correspond to equi­ librium between M o 0 4 “ and M oS2. Barium concentrations are in the range found in deep-ocean w aters from near the sedim ent in­ terface , The ion activity product (I.A.P.) of barium sulfate o f the w ater samples, calculated with correction as given by C hurch , is quite variable: from 1.8 X 10“ 11 to 10.5 X 10“ " . Since the sulfate concentration is alm ost constant in the sam ples where no activity from vent organism s was noticed, the variations in I.A.P. correspond to local variations of the barium concentration. Thus we find that the barium concentrations in the free bottom waters are decreasing from the northern K urile w aters tow ards the south. These points are illustrated in Fig. 4, in a plot of I.A.P. versus Ba. The afore-m entioned “ non-biological” sam ples are linearly correlated in the I.A.P.-Ba plot (Fig. 4). T he sam ples where sulfate depletion was noticed are well off this correlation. This can be due to several causes discussed in the following. T heir positions suggest that deposition of B aS04 may have occurred in several cases. A simple mixing model between local seaw ater and an unknow n advected pore w ater can be com puted for both Ba and S 0 4. The com putations can be done by con­ sidering the constraints im posed on the mixing ratio of an advected pore w ater with local deep free w ater due to the decrease of sulfate, which lead to lim iting conditions for Ba, S 0 4, and I.A.P. 369 14 5- i- / « / \ 3,4 i loo 300 soo 700 900 B a [ n M ) Fig. 5. Ion activity prod u ct (l.A .P .) for B a S 0 4 versus barium co n c en tr a tio n for p o ssib le ad vected p ore w ater end-m em ber. T h e nu m b ered stars are the K aik o sam p les. SW T is the c o n d i­ 2D 60 80 100 120 140 Ba (nM) tion co rresp o n d in g to th e T en ryu C a n y o n area, S W K is that c o rresp o n d in g to the K ash im a S ea m o u n t area, an d S W K T is F ig. 4. Ion activ ity p rod u ct (I.A .P .) for B a S 0 4 versus barium co n c en tr a tio n in K a ik o sam ples. T h e I.A .P . sca le is m u ltip lied that co rresp o n d in g to the K urile T rench. T h e nu m bered areas c orresp on d to the c o m p u ted I.A .P . and Ba c o n d itio n s for the b y IO 11. T h e d a sh ed lin e corresp o n d s to th e relation b e tw een p ore w ater en d -m em b ers. T h e d ash ed lin e corresp on d s to the th e “ n o n -b io lo g ic a l” sa m p les (H Y I , 2. 5, 11, 12, 13, 18, 19). I.A .P .-B a relation for th e “ n o n -b io lo g ic a l” K aik o sam p les (see T h e rectan gle w ith the sa m p le nu m ber corresp o n d s to the Fig. 4). un certainty. T h e black rectan gles are th o se o f the o p e n ocean b o tto m w ater sam ples. for the advected pore w ater end-m em ber. The results of these calculations are given in Fig. 5. F o r sam ples HY3, 4, 6, 9 from the T enryu C anyon, H Y 14 from the K ashim a Seam ount and HY15 from the K urile T rench the pore w ater end-m em ­ ber have S 0 4 in the range 0 .3 -0 .6 X I O '2 M, Ba in the range 300-700 nM in the Tenryu and 700-1000 nM in the K ashim a and K urile areas. T he low sulfate values are in agreem ent w ith pore w ater com positions found in the D SD P sam ples from the sam e areas [4,5]. If we consider this end-m em ber it im plies that the passage of the advected w aters through the clam colonies in the Tenryu C anyon have led to B aS 0 4 deposition (seen from sam ples HY7, 8). This is shown by the lower Ba contents in HY7 and HY8 as com pared to local deep w aters (H Y 2 and H Y5) and the com puted com position of the deep com ponent (Fig. 5). In sam ple H Y10 redissolution of B aS 0 4 m ay have taken place next to the clam colonies. In the case of the K urile T rench there are two possi­ bilities: (1) redissolution of barium sulfate in the vicinity of the colonies (seen from H Y I6) or (2) deposition of B aS 0 4 linked to clam m etabolism (seen in this case from HY15). T hus one should expect a barium anom aly in connection with the biological com m unities. This is w hat we have found upon exam ination of the biological sam ples and the sedim ents , A last discussed feature of the deep-w ater sam ­ ples is their oxygen isotope and D / H characteris­ tics (Fig. 6). We will only discuss the N ankai area where several sam ples are available. T he ô 180 are in the range expected for the Pacific deep bottom waters. If mixing had occurred with w aters de­ rived from the upper part of the sedim entary colum n one should expect a slight shift tow ards m ore heavy oxygen isotope values as found during D S D P Leg 87 , In the N ankai T rough area, w here eight sam ples were obtained, the shift is rath er tow ards m ore light oxygen values which m ay reflect mixing with w aters originated from deeper in the sedim ent colum n as expected from the light hydrocarbons data. A puzzling effect is an enrichm ent in deuterium correlated with the decrease of 0 180 (called “ N ankai area mixing line” in Fig. 6). Since this shift is larger in sam ples 370 a n d of the biological activity o f the vent c o m m u n ­ ity. Some o f the conclusions are s u p p o r te d by authigenic m in erals observed on the filters from the syringes a n d the s u p e rn a ta n t waters. í; u« 0SM OW -2 5. M inerals found on the filters T h e overall results from exam ining the filters by optical m icroscopy, S E M coupled energy an a lyse r ( J E O L / T R A C O R ) an d X-ray m ic ro d if­ fraction. are sum m arize d in T ab le 6. M ost of the m inerals are those which can also be identified in the se dim e nts , a n d they are derived from the w e a thering of the J a p a n e se islands an d from b io ­ logical se d im e ntatio n. Sulfides are associated with the sam ples taken directly above the biological c o m m u n ities (H Y 4 . 8, 15). T h e sulfides identified are mostly pyrite, iron sulfides with som e small perc en ta g e of Z n a n d C u (Fig. 7a. b), p y rrhotite (Fig. 7c); a single crystal of M oS , has also been identified. T hese findings are in agre em e nt with the w ater chemistry. In places som e oxide ov er­ gro w th s are found (Fig. 7d)). C a lc ium c a rb o n ate s with very different m orphologies were also id e n ti­ fied. Several co rresp o n d to spicules from c o m ­ p o u n d ascidians [15J. A scidians were indeed id e n ­ tified on the shells of clanis  a n d u p o n e x a m in a ­ tion of p h o to g r a p h s taken from the “ N a u tile ” . C a lc ium c a r b o n a te s fra m b o id s were also fo u n d (Fig. 7e). they are alm ost p u re calcium c a r b o n a te a n d a thin organic m a tte r veil is left I Fig. 6. 5 D - 6 1!iO relationship in fluid sa m p les (tita n iu m syringe). D a ta in perm il versus SM O W . T h e m eteoric w ater lin e is given as w ell as the “ m ixing line" for the N a n k a i T rough sam ples. T h e area o f the rectangle for each sa m p le corresp o n d s to the m easurem en t un certainties. T h e sa m p le nu m ber is given near the rectangle. O n the 6 I80 scale “ P .D .B .W ." corresp o n d s to the range for P acific d eep b o tto m w aters and “ D S D P 8 7 " to the range fou nd in the upper sed im en ts from the N an k ai T rough investigated du rin g D S D P Leg 87 [14). w here m e th a n e was in larger c o n c e n tra tio n (H Y 4 ) this m ay be characteristic of the p o re fluid. It m ay also be a sa m pling artefact, however, n o t ex p la in a ­ ble. Thus, the geochem istry of the w ater sam ples also illustrates the in p u t of ad vected po re w'aters TABLE 6 M inerals observed by m icroscopic exam ination o f the filters Sam ple HY11 HY4 HY7 HY8 P rism atic rutile Black op aq u es (oxides) Sulfides W h ite m ica G reen o r black m ica G reen silicates Q u artz fragm ents D iato m (com plete o r fragm ents) G rap h ite and grap h ito id s Iro n oxyhydroxides W h ite clay m ineral M etal fragm ents (from subm arine o r sam pling devices) ? + - - + + + + + + + + - - - - + - - + + + + + + + + - — + + + + + + + + - + + + - - HY9 H Y 10 H Y 18 HY15 + + + - _ - - - - - - - - + - - + + - - + + - - - - + + + + + + + + + + + + + + + ? - = absent; + = frequent; + + = a b u n d an t; + + + = very ab u n d an t. - + + - + + + + - _ H Y 16 _ _ _ + + ? + + - - - - + + 371 F ig. 7. SHM p h o to g ra p h s o f particles fo u n d o n the filter c o rresp o n d in g to H Y 4. T h e scale bar is given in |im in the low er right angle o f each view , (a) C luster o f sm all pyrite crystals, (b ) C u b e o f pyrite covered by a Z n -F e su lfid e, (c ) P yrrhotite crystal, (d ) Sm all m agn etite crystal (w ith o u t T i) overgrow th s partially c o v erin g a C a -F e -M g alu m in o -silic a te crystal, (e) C alciu m ca rb o n a te (w ith ou t M g, M n. F e) fram b oid . (f) C a lciu m ca rb o n a te cry sta ls sta ck ed togeth er (traces o f F e, M g and M n). 372 upon acid dissolution. They are probably of bio­ logical origin, however, not identified. Stacks of calcium carbonate crystals, containing traces of Fe, Mg. M n were also found (Fig. 7f). They are probably of inorganic origin. X -ray m icrodiffrac­ tion revealed that vaterite, calcium carbonate m onohydrate as well as aragonite and calcite are present. Calcium carbonate hydrate suggests that it may be derived from m ethanehydrate yielding C a C 0 3, 6 H 20 (ikaite) which then decom poses at low er pressure , Such u nstable calcium carbonates have been identified in the deform ed sedim ents from the N ankai T rough during D SD P Leg 87  and in the vents o ff O regon coast  and are the precursors of pseudom orphes in an ­ cient subduction zones of Jap an and Oregon. Thus the presence of calcium carbonate hydrates seems to be related to the advection of m ethane. In the Tenryu C anyon biological vent com m un­ ity a white clay m ineral w ith a fluffy appearance was also abu n d an t. A lthough difficult to isolate, X-ray diffraction suggests that it m aybe a sm ec­ tite. This implies the presence of silica in much larger concentration than in seaw ater and hence its form ation may be related to advection of pore water. 6. Discussion A lthough the sam ples we have collected during the K aiko legs result from a com plex m ixing b e ­ tween am bient open ocean deep-sea w ater and pore w ater from the sedim ent, there are good geochemical d ata which show th at p art of these fluids have com ponents which may have a deep origin. The association of the biological com m uni­ ties with these geochem ical anom alies and with deep geological structures (from seismic data) also suggest a local inp u t of chem icals and n utrients by fluid venting. H ere we wish to discuss further the possibility of a deep com ponent input to surface sedim ents and to assess the characteristics of pore fluid m igration with special em phasis on the N ankai Trough area where d ata from D SD P are available. The m ethane concentrations, and the C , / ( C 2 + C \) ratios, of the samples raise several questions: (1) w hat is the origin of the hydrocarbons?; (2) w hat is the background concentration of m ethane in deep-sea w ater from subduction areas? This last question is difficult to answer straightforw ardly since what we thought was a background sam ple (H Y 3) may be associated with a structure favora­ ble to fluid venting according to the bottom p h o ­ tographs and its sulfate depletion. M ethane con­ centration in the same range as HY3 (50-70 nl/1) were found in w ater sam ples taken several miles from the subduction of the Oregon m argin , It is possible th at background concentration in sub­ duction area are several tim es larger than the expected 10 nl/1. In HY3 the C , / ( C 2 + C 3) ratio is very low and suggests that the hydrocarbon concentrations are influenced by therm ogenic m ethane input . T he concentration of light hydrocarbons introduced by advection of pore fluid may, however, have been reduced by oxida­ tion in the upper p art of the sedim ent colum n [8,20], W ith the help of the constraint due to sulfate depletions, the m ethane concentration that can be deduced for the advected com ponent in HY3 is about 600 nl/1, reflecting the low con­ centration due to oxidation. In the pore w aters of sedim ents where advection of therm ogenic hydro­ carbons was found, however, not from the Oregon m argin, concentrations in the range 10 100 x IO1 nl/1 were obtained , In the deep-w ater sam ples clearly associated with biological activity (HY4. 7. 14, 15, 16) the m ethane concentrations are in the range of HY3 or larger. One should expect this since dilution of advected fluid and increase of consum ption of light hydrocarbons have in­ fluenced this factor. T he C l/ ( C 2 + C 3) ratio is even lower than in HY3, showing clearly the input of a therm ogenic com ponent. If HY3 represents a background sam ple for the hydrocarbons we can solve a sim ple mixing model with an advected end-m em ber for C j, C 2 and C 3. In the case of HY4, 15, 16, the corresponding advected endm em ber should be C , = 4 0 0 nl/1 and C , / ( C 2 + C 3) = 2. These low concentrations are due to the oxidation o f the light hydrocarbons. If we take the pore w ater value of C , = 10-100 X IO3 nl/1, the am ount oxidized is larger than 95-99.5% of the hydrocarbons. Part of the m ethane encountered in the Kaiko sam ple can be derived from biogenic activity. However, in this case it is difficult to explain the C 2 and C 3 concentrations. The presence of ther­ mogenic hydrocarbons in the sam ples should rather be used as an argum ent related to the 373 plum bing of the advection of pore water. Since the clam colonies are located above thrusting struc­ ture, the therm ogenic hydrocarbons are evidence for the deep p enetration of these structures. The light hydrocarbons are m ost probably of therm ogenic origin. T he biochem ical studies have shown that the isotopic com position of the m ethane should be 0 13C = —50 to —30%o which is also characteristic of therm ogenic m ethane , T herm ogenic hydrocarbons are produced at tem ­ peratures higher than 5 0 -6 0 ° C. T he therm al gradient found in the Jap an T rench is in the range of 1 5 - 3 6 ° C /k m [21,22]. This yields depths in the range 1 .5 -3 km for the generation of the light hydrocarbons. This d ep th range com pares favor­ ably w ith th at deduced from the m e th a n e /e th a n e ratio versus d ep th diagram obtained during D SD P Hole 440. A ccording to that, the location of the source of therm ogenic hydrocarbons should be around 2.5 km , It is also in the sam e depth range as the vertical projection of the thrust faults found in the N ankai accretionary prism , and for dew atering in accretionary prism . The processes of m igration of w ater through the prism during dew atering are not well known. The location of vents in close association with structure related to thru st faults as at the base of the Tenryu C anyon deep-sea fan suggests it occurs as shear dew atering, i.e. due to fracture perm eabil­ ity. T he location of vents where erosional features allow the th ru st faults to reach the sedim ent-w ater interface also suggests th at the advection can be easily stopped o r diverted if buried under sedi­ m ent slump. A sim ple m odelization of the rate of advection of the fluid using a tw o-dim ension steady-state m odel has been attem p ted . It was found that it is not possible to sustain steady-state advection and th at the rate of advection should be at least 5000 c m /y r , In this case the tem perature anom alies observed can be o btained if cooling of the expelled fluid is effected by a lateral flux of seaw ater through the sedim ent near the vent loca­ tion [9,25]. T he geochem ical anom alies, such as sulfate depletions, are also difficult to explain if we have low advection rates. T he flux in the subduction areas off Jap an is probably non-stable. T his is in con trast w ith the stable steady-state fluxes com puted from the geochemical anom alies obtained off the coast of O regon  and the corresponding low rates of the pore fluid (1 -1 0 c m /y r) for the case of O regon . Sim ilar low rates have also been obtained to explain the th er­ mal behavior of the subduction in the northern Jap an T rench . T he case of fluid venting in the Tenryu C anyon area is well docum ented and geological as well geochemical characteristics can be com pared with sim ilar occurrences in the O regon margin. The finding of fluid venting and biological com m uni­ ties associated with a thrust in the K ashim a area (H Y 14) and in the Jap an T rench  can be sim ilarly understood. T he possible occurrence of fluid venting in the Zenisu Basin (HY 3) and the K urile T rench scarp (HY15, 16) is m ore difficult to understand. In the case of the Zenisu Basin advection of fluid is suggested by the sulfate de­ pletion and the low C , / ( C 2 + C 3) ratio. The Seabeam data, the seismic data and the possibility of a thrust structure in the Zenisu Basin , correspond to com pressive stresses which are favorable for fluid expulsion. A small clam colony at the apex of a fold above a thrust fault at the southernm ost deform ation front of the Zenisu area  may be related to this interpretation. In the K urile area, the tem perature and geochemical anom alies as well as the presence of clam colonies m ake a clear cut case for active venting. The geological setting, however, of the structure which would allow such venting is less clear. We can note that the deep biological com m unities were aligned along N 330° which is along the m ajor structural direction of the southw est inner wall of the Kurile Trench. The characteristics of the fluid venting suggest that this structure m ust have penetrated deeply. 7. Conclusions The deep w aters sam pled above biological com ­ m unities in the N ankai T rough and the Japan Trench areas can be related to fluid venting in connection with the geological setting which would favor advection of the fluids yielded by dew ater­ ing. In the upw ard path these fluids are enriched with molecules and elem ents of deep sedim entary origin: light hydrocarbons, trace metals. Hydrogen sulfide is also probably brought in the upper part of the sedim ent as shown by the dissolved sulfate depletion and the presence of m etal sulfides in the 374 samples. H ydrogen sulfide is utilized by bacteria associated with the gills of the clams from the biological com m unities . G eochem ical and bio­ geochemical d ata also show that part of the m ethane is also utilized for nutrient requirem ents. T he fluid sam ple com positions result from the m ixture of these different processes and sources. T he light hydrocarbon d ata suggest the depth scale of the fluid circulation: 1.5-3 km for the N ankai area. A model of fluid circulation suggests a non-steady-state advection rate of at least 5000 c m /y r. The relation of the fluid vents to overpres­ sure along the thrusting plane in the N ankai T rough and the deep origin of some of the fluid com ponents give a good base for the use of the electro kinetic properties of the venting fluids for prevision of earthquake in this area . The results obtained in this geochemical study have also im portant im plications concerning the budget of elem ents in the ocean. For instance, the budget of m ethane and light hydrocarbons will probably have to be reassessed. Thus the q u a n tita ­ tive im portance of the prelim inary results pre­ sented here will only be possible when a m ore com plete overview of the im portance of fluid vent­ ing in subduction areas will be obtained. vent sites a lon g the su b d u ction z o n e o ff O regon, B iol. Soc. W ash. Bull. 6. 4 7 5 -4 8 4 . 1986. 3 J.K . W helan and S. Sato. C , - C 5 hyd rocarbons from core gas p ock ets. D eep Sea D rillin g Project L egs 56 and 57. Japan T rench transect, in: Initial R eports o f the D e ep iÿea D rillin g Project 57, pp. 1 3 3 5 -1 3 4 7 . U .S . G overn m en t Print­ in g O ffice. W ash in gton D .C .. 1980. 4 G .W . M oore and J.M . G iesk es. Interaction betw een sed i­ m ent and interstitial w ater near Japan T rench, L eg 57, D e ep Sea D rillin g Project, in: Initial R eports o f the D eep Sea D rillin g Project 57, p. 1 2 6 9 -1 2 7 5 . U .S . G overn m en t Printing O ffice. W ash in gton D .C .. 1980. 5 H. K aw ahata. K. Fujioka and T. Ishikuza, S ed im en ts and interstitial w ater at Site 582 and 584. the N an k ai T rough and Japan T rench landw ard slope, in: Initial R eports o f the D e ep Sea D rillin g Project 87, pp. 8 6 5 -8 7 5 , U .S . G o v ern ­ m ent Printing O ffice. W ash in gton . D C .. 1986. 6 M . S ch oell, R ecen t ad van ces in p etroleum iso to p e g e o ch em ­ istry. Org. G e o ch em . 6, 645 -6 6 3 . 1984. 7 G .E . C layp ool. A .K . V uletich and K .A . K ven vold en . I so ­ top ic co m p o sitio n o f interstitial flu id s in sed im en t o f the N an k ai T rough, D e ep Sea D rillin g Project Leg 87. in: Initial R ep orts o f the D e ep Sea D rillin g Project 87. pp. 8 5 7 -8 6 0 . U .S . G overn m en t Printing O ffice. W ashington D C .. 1986. 8 J. B oulègue, b .L . B en ed en i, D . D ron , A . M a rio n i and R. L étolle. G eo ch em ica l and b io g e o c h e m ic a l o b serv a tio n s on the b io lo g ica l co m m u n ities associated w ith fluid ven tin g in the N an k ai T rou gh and Japan T rench su b d u ction zones, Farth Planet. Sei. Lett. 83. 3 4 3 -3 5 5 . 1983 (th is issue). 9 D D ron. J. B oulègue. A . T aira and C. R angin . G e o c h e m ­ istry o f the T enryu C an yon d eep -sea fan b io lo g ica l c o m ­ m u nity (K a ik o ), Earth Planet. Sei. L ett. 83. 3 5 6 -3 6 2 . 1983 Acknowledgements (th is issue). 10 W .S. Broecker and T .H . Peng, Tracers in the Sea. 69 0 pp. We acknowledge the invaluable assistance of the “ N autile” group and the help of R /V “ N ad ir” crew members. W e thank the K aiko Planning C om m ittee for giving access to shipboard sam ­ pling and H. Bougault, L. Floury, C. Levèque and M. Sibuet, all from IF R E M E R , for help with instrum entation. Part of the analytical study was done by Dr. A.M. de K ersabiec and F. Vidot. This study was partly supported by grants from P IR O C E A N /C N R S and IF R E M E R . We thank Dr. E. Suess for very helpful discussion on fluids in subduction areas. Hldigio Press. N e w Y ork. N Y .. 1980. 11 J. B oulègue. T race m etals (F e, C u. Z n. C d) in anoxjc en viron m en ts, in: Trace M etals in Sea W ater, N A T O C onf. Ser., M ar. Sei., pp. 5 6 3 -5 7 7 , Plen um Press. N e w Y ork, N Y .. 1983. 12 K .K . B ertine. T h e d e p o sitio n o f m olyb d en u m in an oxic w aters. M ar. C hem . 1. 4 3 - 5 3 , 1973. 13 T .M . C hurch. M arine Barite. 100 pp, P h .D . Thesis. U n iv e r ­ sity o f C alifornia at San D ie g o , 1970. 14 Y . M atsuhisa and R. M a tsu m o lo . O xygen iso to p e ratios o f interstitial w aters from the N a n k a i T rough and the Japan T rench. L eg 87, in: Initial R eports o f the D eep Sea D rillin g Project 57, pp. 8 5 3 -8 5 6 . U .S . G overn m en t Printing O ffiee. W ash in gton D C .. 1986. 15 B. K niprath and F. L afargue, Sp icu le form ation in the D id em n id ae, in: T h e M ech an ism o f B iom in eralization in A nim als and Plants, M . O m ori and N . W atab e. ed s.. T okai References U niversity Press. T o k y o . 1980. 16 L. I.aubier and S. O hta. D eep b iological c om m u n ities in the 1 L. D . K uim . F.. Suess, J.C. M oore. B.T. L ew is, S .D . R itger. su b d u ction z o n e o f Japan from b o tto m p h otograp h s taken D C . K adk o, T .M . T hornburg, R .W . b m b lev . W .D . R ugh. du rin g " N a u tile " d ives in the K aik o project. Earth Planet. G .J. M assoth. M .G . L angseth. G .R . C ochrane and R.L S cam m an. O regon su b d u ctio n zone: ventin g, fauna and carb on ates. Scien ce 231, 5 6 1 -5 6 6 . 1986. 2 F.. Suess. B. C arson. S.D . R itger. J.C. M oore. M .J. Jones. L .D K uim and G .R . C ochrane. B iological c o m m u n ities at Sei. Lett. 83. 3 2 9 -3 4 2 . 1987 (th is issue). 17 b. Suess. W Balzer, K .F. H esse. P.J. M uller. C .A . U ngerer an d G . W efer. C alciu m carb on ate hexahyd rate from organic rich sed im en ts of the A n tarctic shelf: glen d on ites. Scien ce 216. 1 1 2 8 -1 1 3 1 . 1984. precursors of 375 18 C .L . Stein and A .J. Sm ith, A u th ig e n ic ca rb o n a te n o d u les in 25 X . L e P ich ón , S. L allem an t and S. L allem an d , T ec to n ic the N a n k a i T rou gh , Site 583, in: Initial R ep o rts o f the c o n te x t o f flu id v e n tin g a lo n g Jap an ese trenches, A bstr., D e ep Sea D r illin g Project 87, pp. 6 5 9 -6 6 6 , U .S . G o v e r n ­ Int. K aik o C o n f. on S u b d u ction Z o n e s, N o v e m b e r 1 0 -1 5 , m en t P rin ting O ffice, W a sh in g to n D .C ., 1986. T o k y o 1986, 2 - 1 , p. 24. 19 M .J. W hiticar, E. S u ess an d H. W chner, T h erm o g en ic h y ­ d ro ca rb o n s in surface sed im en ts o f the B ran sfield strait, 26 M .W . H an and E. S u ess, C h em ical evid e n c e o f pore v en tin g in th e O regon su b d u ctio n zo n e, E O S 57, 1219, 1986. A nta rctica P eninsu la, N a tu r e 314, 8 7 - 9 0 , 1985. 27 B .H . R eck, E ffect o f w ater flo w from d ew aterin g subd u cted 2 0 R .Y . Stanier, M . D o u d o r o ff and E .A . A d elb erg , G en eral M icro b io lo g y , M a cM illa n , L o n d o n , 1971. sed im en ts o n therm al grad ien ts in the N o rth ea st Japan accretion ary prism , EO S 67, 379, 1986. 21 Sh ipboard S cie n tific Party, Sites 4 3 8 a n d 4 39: Japan D eep 28 X . Le P ich ón , T. Iiyam am a, J. B oulègue, J. C harvet, M. Sea T errace, L eg 57, in: Initial R ep o rts o f the D e ep Sea Faure, K. K ano, S. L allem ant, H. O kad a, C . R an gin . A. D rillin g Project 5 6 / 5 7 pp. 2 3 - 1 9 1 . U .S . G o v ern m en t P rin t­ T aira, T. U rab e an d S. U y ed a , N a n k a i T rou gh and Z en isu in g O ffice, W a sh in g to n D C .. 1980. R idge: a d ee p -se a su b m ersib le survey. Earthy Planet. Sei. 22 Sh ipboard S cie n tific Party. S ites 440: Japan T ren ch m id ­ slo p e T errace. L eg 57, in: Initial R ep o rts o f the D e ep Sea L ett. 83. 2 8 5 -2 9 9 , 1987 (th is issu e). 29 J.P. C ad et, K. K o b a y a sh i, J. A u b o u in , J. B oulègue, J. D rillin g Project 5 6 / 5 7 , pp. 2 2 5 -3 1 7 . U .S . G o v ern m en t D u b o is, H . H o tta , T . Ishii, L. Jolivet, Printing O ffic e, W a sh in g to n D .C ., 1980. L allem an d , N . N iitsu m a and H. S h im am u ra. D e ep scie n ­ 23 C .J. Bray and D E. K arig, P orosity o f sed im en ts in accre­ tionary prism s and so m e im p lic a tio n s for d ew aterin g p ro cesses. J. G eo p h y s. R es. 90, 7 6 8 -7 7 8 , 1985. 24 T .H . S h ip ley and G .F . M oore, S ed im en t accretion , su b d u c ­ tion and d ew a terin g at the base o f the trench slo p e o ff C o sta R ica a seism ic reflectio n view o f th e d é c o lle m e n t. J. G e o p h y s. R es. 91, 2 0 1 9 -2 0 2 8 . 1986. K. K on ish i. S. tific d iv es in Japan an d K uril T renches, Earth Planet. Sei. 83, 3 1 3 -3 2 8 , 1987 (th is issu e). 30 J. B oulègue, X . Le P ich ón and T. Iiyam a, P révision des trem b lem en ts de terre d a n s la région d e T ok ai (Jap on ), C .R . A cad . Sei. Paris, Ser. II 301 1 2 1 7 -1 2 1 9 , 1985.