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Geochemistry of the Tenryu Canyon deep-sea fan biological community (Kaiko)
356
Earth and Planetary Scwnee Letters. 83 (1987) 356-362 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
[41
Geochemistry of the Tenryu Canyon deep-sea fan biological community (Kaiko) D o m i n i q u e D r o n ~, J a c q u e s Boul6gue ~, A s a h i k o T a i r a 2 a n d C l a u d e R a n g i n 3 J Lahoratotre de G~ochtmte et M&allog~;nie (CNRS UA 196). University; Pierre et Marie Curie. 4 phlce .lussieu, 75252 Paris C~;dex 05 (1.'rance) ' Ocean Research Institute. Unit,ersttv of Takyo. Nakano-Ku Minaret Dai 1-15-1. l?~k.vo (Japan) ~ D~;partenlent de Gg'ototectonique (('NRS U.4 215), Um1,ersit~; Pwrre et Marie Curie. 4 place Juss'teu. 75252 Paris (.'¢;dex 05 (France) Revised version accepted February 5, 1987 The fenryu Canyon deep-sea fan biological community is characterized by both reduced and oxidized sedimcnts in the immediate vicinity of the pore water vents. The upper sediments in contact with the clams are reduced, the organic mfitter is enriched in sulfur, and inorganic sulfides (Fe, Cu. Zn) arc forming. Towards the outer fringes of the communities the sediment is oxidized and metals generally associated v, ith fcrro-manganese oxides are concentrated. Several metals. Cd, Pb, Mo show distributions which are strongly influenced by the metabolism of the clam colony. Comparison of water and sediment geochemistry leads to the conclusion that there should be a downward flux of oxygenated seaw~,ter on the boundaries of the colony and an upward flux of chemically more reduced deep pore water at the location of the colon.,,'. Trace metals anomalies as well as 81"N anomalies of organic matter may bc useful to prospect for extinct venting aret, s in ancient subduction zones.
I. Introduction
Several benthic biological communities, with clams as their principal organism have been discovered atop pore water vents on the landward slope of the subduction areas off Japan [1-3]. Similar occurrences have been previously described from the subduction zone off the Oregon margin [4]. A survey of the Tenryu Canyon deepsea fan community was done including water, sediment and biological sampling by the submersible "Nautile" (Figs. 1 and 2). At other locations (Kashima Seamount, Japan Trench, Kurile Trench) sediment recovery was very low because of dewatered and quite indurated substrate, preventing penetration of the core tubes. In this report we present data on bottom water, supernatant water, sediment and clam composition. which enable us to deduce possible water flow patterns and geochemical behavior of elements in the vicinity of the biological community. 2. Sampling and analyses All samples were taken with the mechanical arms of the submersible "Nautile" handling tube (X)I2-821X/87/$03.50
'~' 1987 Elsevier ~ience Publishers B.V.
corer and box corer. They were precisely located as shown in Fig. 2. We collected four sediment samples. 5BC and 5NC were taken directly within the clam colonies, 5TI was taken on the outskirt of the colonies, 3TI is located 300 m off the vent site (see Fig. 1). Supernatant waters were sep-
,,~/3T I [
:I ._
i
& ,
i:ig. 1. Sampling area in the Tenryu Canyon deep-sea fan. Samples associated with the biological community are noted by a black circle. At this kuaation the direction of a thrust structure (N50 ° ) and of a possible fault (NW-SE) system are also given. The samples taken away from the biological community correspond to the star: HY6 = water sample: 3"I"1= sediment.
357 Dv
./
1,24 °
I., ~ ' ° 7 "+-
.........
-.+?cl
j
'+'°
water=l.19°C
+ ~_f
Fig. 2. Detailed sampling scheme in the biological community of Tenryu Canyon deep-sea fan. HY = water sample; 5TI, 5NC and 5BC = sediment samples. Measured temperatures are also given in ° C . After a drawing by A. Taira (see [1,2] for further details).
arated from over the sediments. The samples are HY9, HY10 and HY6 corresponding to 5BC, 5TI and 3TI respectively. Free bottom water samples (HY4, HY7) were collected directly above the clam colonies using titanium syringes [1]. The chemistry of these water samples is discussed elsewhere [1]. We showed that the compositions of the water samples collected above the clam colonies result from the mixture of advected pore water with free deep-sea water. Several modification of the advected pore water were due to clam metabolism: uptake a n d / o r fixation of hydrogen sulfide, methane and nitrogen. The sediments were frozen on board, and freeze-dried on shore in the laboratory (within one month of sampling). Wet chemical analyses of the sediment were done after washing and desalting with distilled water. Si was measured by colorimetry, AI, Cr, Mn, Fe, Co, Ni, Cu, Zn, Rb, Mo, Cd, Ba, Pb were measured by flame atomic absorption spectrometry a n d / o r flameless Zeeman corrected atomic absorption spectrometry. Accuracies are + 0.5% for Si, A1, Fe, + 2% for other elements, AI, Na, K, Ca, Mg were measured after H F attack of the sediment. The metal cations were measured after a combined H C I - H N O 3 attack of the sediment so as to avoid contribution from the silicates. Trace elements present in carbonates and other minerals were also measured with an electron microprobe on polished sections of the sediment. Total carbon, inorganic and organic carbon and the same fractions for sulfur were analyzed by step combustion and
chromatography of evolved CO 2 and SO 2 (accuracy +0.3%). However, with this method we cannot distinguish between sulfide sulfur in inorganic compounds and sulfur in non-heme iron-sulfur protein compounds. Rock-Eval analyses of different sediment fraction were also performed at IFP [5]. X-ray diffraction and IR spectrometry were performed for identification of the mineral phases. In addition ESR and ESCA were also performed on the bulk sediment samples to investigate Mn, Fe, Cu and Mo. The clam samples were investigated (as freeze-dried subsamples of selected tissues) using the same methods [6]. The organic fraction of the sediments, as well as the tissues of clams were analyzed for t3C/12C and tSN/14N isotopic ratios [6]. The main results of the chemical analyses of the sediment are given in Table 1, 2 and 3. The analyses of the water samples are given in [1] and the complete analyses of the clam are given in [6] and some selected results are given in Fig. 6.
3. Sediment composition Quartz and detrital alumino-silicates from weathering of the Japanese islands are the main components of all the sediments. Sample 5NC is mostly coarse grains due to the fact that it was obtained by scooping the sediment with a biological net. The other sediments have different characteristics. The grain size is quite different between samples (Fig. 3). 5BC is a black, soupy mud in which the clams were standing upward. In 5NC and 5BC samples, the coarse quartz and plagioclase grains are often blackened by a sulfur-rich organic matter (seen on 5NC). The amount of the < 80 ~ m size fraction is limited and is composed by silicates, quartz and phyllic minerals. The particles on filters from water samples HY4 and HY7 (see [1, Table 6]) have the same composition as the < 80 ~m size fraction. Sulfides are present in the > 80~m fraction as well as miilimetric lignic remains which are not common in other sediment samples. The sulfides are mostly rich in Fe, with traces of Z, Cu > Ni, Mo, Cd. The high sulfide content is restricted to sample 5BC (Table 1). 5NC, which was also taken from a location with clam colonies has numerous organic remains among which shell debris and serpulid tubing debris can be recognized (Fig. 3b).
358
..
°
m
l
q
•
,
I ~.
,
N
"t
!
'/
I
N
m Fig. 3. Photographs of the sediment under the microscope (natural light). Scale = 300 ,am: except in (b) where scale = 2500 # m . (al 5BC ( > 80 ~m); (b) 5 N C note the remains of a serpulid tube; (c) 5T1 ( > 80 a m ) : (d) 3T1 > 80 ,am).
Sample 5T1 is a brown, soupy mud which is found in the immediate vicinity of the clam colonies. The detrital fraction is similar to that of 5BC, but with fewer large grains (Fig. 3c). The organic matter content is partly found as remains from copepods (300-350 /~m), radiolarians (100300 ~m), forams (80-180 p.m), algae debris, fish scales, ascidians (300 ~m), and mixed organic
debris. The organic matter seems more fresh than in other samples (from visual observation) and its accumulation may be due to bioturbation. An examination of the filters (HY10, [1]) and the examination of polished section of the sediment showed that oxides containing Fe and Mn are quite frequent, and sulfides are scarce. The oxides also contain traces of Zn, Cu > Cr, Ni.
TABLE 1 Element content of the sediments Sample
Si
AI
5BC
25.4
7.3
Sto t
5T1
29.3
6.7
< 0.1
3T1
26.8
7.0
< 0.1
0.29
S.,,,rg
S,,rs
('t,,,
C.,,,,s
(?ors
613C,,r~
S 1~ N,r~
0.025
1.0
0.16
0.85
- 25.5
-- 0.2
< 0.1
0.027
1.5
0.61
0.89
< 0.1
0.018
0.93
0.32
0.61
25
4- 1.0
0.26
Si, AI, S and C are in wt.%; tot = total: inorg = inorganic; org := organic, al~(- in %, vs. PDB; 6nSN in %<, vs. atmosphere.
359
Sample 3T1 is an olive-green mud with few grains > 300 ~m. As seen in Fig. 3d, the biogenic matter content is less than in 5T1. The inorganic phases are almost the same as those in sample 5TI. Sulfides are absent. Radiolarian debris are found and forams are very scarce. Except for the presence of sulfides and sulfurrich organic matter in association with the clam colony (5BC), oxides in the vicinity of the colony (5T1), and differences in color and texture, the sediments from the Tenryu Canyon deep-sea fan are characteristic of detrital continental material with high sedimentation rate.
4. Geochemistry of organic matter The sediments in the vicinity of the clam colonies (5BC, 5T1) have a higher organic content than those away from colonies (Table 2) (3T1). The maximum temperature from Rock Eval (Tm,x in Table 2) of kerogen pyrolysis, which is higher when the organic matter is more matured [5], is also different in the two areas. The organic matter is less matured at the contact with the clam colony. The slight difference between 5BC and 5T1 may be due to the larger content in trace metals of 5T1, especially Mn which is known to delay pyrolytic decomposition [7] or to slight differences in the inorganic matrix [8] or may reflect actual differences of immature benthic biomass away
from the center of the vent. The results for C content and Tm~x can be compared with those obtained in the upper meters of sediments from the Nankai Trough during the DSDP Leg 87 [9]. The samples we obtained during Kaiko may have a lower content of land-derived organic matter. In all samples the inorganic carbon content is less than the organic carbon content. The calcium carbonate content+ which should covary with inorganic carbon, was already presumed to be noticed as very small from visual observation. The higher inorganic carbon content in 5T1 is in agreement with the observation of carbonate debris in the sample. The 613C and 615N of organic matter in sediments in contact with clam colony is slightly less than in Nankai Trough sediments. At least in the case of 8tSN this may reflect excretions from the clams [6]. The 813C is in the range of mixed marine-continental organic matter found in other sediments from the same area [10]. This mixed origin is also indicated by the hydrogen index versus oxygen index diagram (Fig. 4). This diagram indicates that the organic m a t t e r of 5T1 and 5BC is in between type II (marine) and type 1II (continental) organic matter. It is distinctly different from the type found in the Nankai Trough (3T1) which has a more continental character. The sulfur content of organic matter is higher in the vicinity of the clams. This is also in agreement with the black soupy aspect of the mud in sediments of this environment.
TABLE 2 Organic matter content of sediments expressed in C (v,t.%) for different size fractions. Also given is Tmax. ( ° C) the pyrolysis temperature at the maximum generation of the hydrocarbons from kerogen during Rock-Eval measurements
c(z)
T,,,.,, (°c)
5BC Total > 50 ,am < 50 ,am
0.85 0.42 0.94
403 405 398
5TI Total > 80 ,am < 80,am
0.89 1.48 -
419 442 -
3TI Total > 50 ,am > 50 ,am
0.80 0.69
416 435
-
H.I.Ii~m: rl " •oo ii
II Ioo
a00 O.L~
Fig. 4. Hydrogen Index (H.I. in mg H C / g Co~8) versus Oxygen Index (O.I. in mg C O 2 / g Corg) for sediments from the Tenryu Canyon and Nankai area. Black triangle = 5BC; black square 5TI; black circle = 3T1; open circle = DSDP Site 582, upper 20 m; open diamonds = DSDP Site 583 upper 20 m. Roman numerals correspond to kerogen types.
360 1104,
5. Geochemistry of trace metals
The trace element content of the sediments (Table 3) is highly variable. Iron contents are slightly lower than those found in the Nankai Trough sediments [11,12]. Co, Ni and Zn contents are in the lower range of the concentrations observed in the Nankai Trough [11]. In order to have a basis for geochemical comparison we have normalized the trace element content on a quartz and alumino-silicate free basis (Fig. 5). It appears that the oxide-rich sediment, 5T1, which is next to the pore water vent, is much richer in elements which are generally enriched in ferromanganese oxides: Fe, Mn, Ba, Cu, Zn, Cr, Ni, Co and possibly Mo. Most of these elements have lower concentration in 5BC than in 3T1. Pb, Cd and Mo are comparatively enriched in 5BC, Ba and Co are slightly depleted. This pattern of concentrations can be compared with the geochemistry of pore waters, supernatant waters and local seawater (see Table 3 and [1]). It appears that HY10, which corresponds to 5T1 is depleted in Fe, Cu, Mn and Mo as compared with HY9 and HY6. This is in agreement with the uptake of these metals in the oxide phases found at 5T1. Ba shows a higher concentration in HY10 than in other samples, it is also high in 5T1. This observation is in agreement with the conclusion that BaSO4 should form in the environment of the vents (probably biologically) [1] and that it may redissolve in HY10. thus enabling uptake by the oxides. This would explain the net enrichment of 5T1. The enrichment of Cd. Mo and Pb in 5BC is large as compared with oceanic siltstones. This can be explained by uptake and release by the clams. As shown in Fig. 6
I
I
i ~]~.
:_
•---~...
I
\-...
58t"
571
......
37-f
• .
J,
|-'ig. 5. Trace element content of the sediments (on a quartz and alumino-silicate free basis). All concentrations in ppm.
F
~:
Bo
o..
"'Zn
.:
~Cd
1
Mn I
,0L
L
Mo
gdls
viscera
rnonHe
Fig. 6. Trace element content of selected tissues. All concentrations in ppm (dry weight).
TABLE 3 Trace metal contents of the sediments in the HCI-HNO 3 soluble fraction (corrected for carbonates)..All results are in ppm except t:e which is in weight percent (dry weight of total sediment). Concentrations of trace metals in sediment pore water (I-'IY6, 9. 10) are in 10 7 m o l / k g Sample
Mn
Fe
Cu
Rb
Mo
Ba
('r
('o
Ni
Zn
('d
Pb
5BC 5T1 3T1 11Y9 HY10 HY6
340 1000 395 5.39 4.77 9.21
2.73 2.93 2.81 17.6 4.2 16.5
39 196 46.5 5.10 0.92 1.(11
160 102 200 17 17 17
7.0 5.0 0.9 1.23 (I.94 1.01
14.5 700 35 1.12 1.79 0.79
82 74 115
9 15 12
34 48 46
86 1115 92
0.82 0.18 (I.39
38 22 37
361
(see also [6]) Cd and Mo are present in high concentrations in the gills and other tissues. Thus there is a biological concentration factor in the 5BC sediments. The effect of trace metal uptake from pore waters by the clam is important [1] and it is then reflected in the sediment content probably owing to excretion or organic input at the death of the animal (as shown by the 8aSN of organic matter). 6. D i s c u s s i o n
and conclusion
The information obtained from the study of the geochemistry of the sediments and from waters [1] and clams [6] enable us to propose a scheme of the near-surface plumbing of the vents from the Tenryu Canyon deep-sea fan. This is presented in Fig. 7. The geological structure enabling upward flow of pore water containing nutrients is the key parameter for the establishment of the biological community. In the Tenryu Canyon it is provided by an emergent thrust fault, identified on seismic lines, and crosscut by numerous right-lateral strike-slip faults. Location at the mouth of the Tenryu Canyon has enabled important erosion of the deformation front [2]. There the clam colony, in symbiosis with bacteria which fix and metabolize C H 4, C O 2, H 2 S , N 2 [6], can be thrived in areas limited in widths by the thrust plane intersection with the seafloor and partly along the transverse faults. Such area is characterized by coarser sediments which may be due to disturbance by the upward flow a n d / o r bioturbation by the clam colony. The rapid lateral variations of
I I
.
. . . . . . .
'
-
"
T
,"
"°.v
~.,.~
¢~.~
~li
;
'"
"
x
*
Fig. 7. Schematic representation of sediment facies and water flows in the Tenryu Canyon deep-sea fan biological community. The large arrows downward correspond to possible seawater and pore water flow. The emplacement of the sediment samples are given (dashed line with sample reference).
the geochemical properties (sediment lithology, trace elements, organic matter) are in agreement with this limited scatter of the upward flow where the colonies are found. The intense biological activity and the presence of hydrogen sulfide [1], allow the deposition of a black, putrid mud layer in the interstices between the clams. This layer is only slightly richer in organic matter content than are Nankai Trough sediments. However, it has a lower 81SN due to the metabolic input of clams. This sediment also c o n t a i n s metallic sulfide (Fe, Cu, Zn > Pb, Cd, Mo) which may be produced via detoxification of the clam colony [6]. The water flowing from the clam colony into seawater still contains high concentrations of Fe and Mn [1]. This enables oxide formation and sedimentation surrounding of the clam colony. It occurs as a brown layer. This layer is also enriched in fresh organic matter due to the intense biological activity and scavenging observed in this area [2]. Interestingly, the sediment is not turning anoxic as one might expect. The sulfate content of HY10 is in the range of seawater [1]. These characteristics can be explained if there is locally a downward flux of oxygenated seawater in the vicinity of the clam colony. The oxygen available to the clam is needed for methane oxydation [6,13]. The postulated oxygenated seawater lateral flux may thus help fulfil a nutritional requirement. The lower concentration of Cd in 5T1 as compared to 3T1 also suggests that the oxygenated water flow will possibly extract Cd from the sediments surrounding the colonies and make it available to the clams [14]. The Cd contents enable to compute that the 10 g / m 2 of Cd needed to sustain the clam colony can be provided by leaching of a minimal 10 cm x 10 m 2 slab of 3T1 type sediment. Cd has been shown to be very important to the metabolism of the clams; hence the seawater flow may provide one of the limiting trace nutrients of the colony [6]. Thus, the biological activity linked to pore fluid venting in the deep-sea fan of Tenryu Canyon leaves a clear geochemical imprint in the upper part of the sediment. The imprint is evident in a zonation, from reduced sediment along the very' path of the upward flux of pore fluids and to an oxidized rim at the outer edge of the reduced zone. The width of this oxidized zone is probably less than 10 m based on field observations done
362
from the submersible. Once formed, the oxides can be expected to be stable enough to leave an anomalous geochemical imprint, at least in overall trace metal content (see Fig. 5). This will be more effective in subduction areas with high sedimentation rates. In that case the remobilization of metals by diagenesis will be limited. Hence it should be possible to identify such venting zones in sediments from ancient subduction areas by a combination of sedimentological criteria (coarse grain, linear structure along the vent area) and geochemical anomalies (in Fe, Mn, Ba, Cu, Zn, Ni, Co at the boundaries). These geochemical anomalies reflect the special metabolism of biological communities utilizing nutrients from expelled pore waters. Organo-geochemical anomalies, mostly the lowering of 6aSN of organic matter in the coarse grain zone. linked to biological activity should also be employed to diagnose venting areas. If the vents are not located below the carbonate compensation depth, the formation of anomalously low 8~~C carbonate from methane oxidation will also be a very efficient reconnaissance tool [15].
Acknowledgements This study was done with the analytical help of A.M. de Kersabiec, D. Dubarry and F. Vidot (UPMC). We have also benefited from help from the Laboratoire de Grologie (ENS, Uhn) and Laboratoire de Prtrologie (UPMC). Isotopic compositions of organic matter were provided by the courtesy of Dr. A. Mariotti (INRA-UPMC). Rock-Eval measurements were provided by the courtesy of Dr. B. Durand and Dr. J.Y. Huc (I.F.P.). The box corer sampler was obtained owing to M. Sibuet. We thank E. Suess for several discussions on pore fluids during his stay in Paris while on sabbatical leave.
References 1 J. Boulrgue, J.T. liyama, J.L. Charlou and J. Jedwab, Nankai Trough, Japan Trench and Kurile Trench: geochemistry of fluids sampled by submersible "Nautile", Earth Planet. Sci. Lett. 83, 363-375, 1987 (this issue). 2 X. Le Pichon, T. liyamama, J. Boulrgue, J. Char'vet, M. Faure, K. Kano, S. Lallemant, H. Okada, C. Rangin, A. Taira, T. Urabe and S. Uyeda, Nankai Trough and Zenisu
Ridge: a deep-sea submersible survey. Earth Planet. Sci. Lett. 83, 285-299, 1987 (this issue). 3 J.P. Cadet, K. Kobayashi, J. Aubouin, J. Boulrgue, J. Dubois, H. Hotta, T. Ishii, L. Jolivet, K. Konishi, S. Lallemand, N. Niitsuma and H. Shimamura, Deep scientific dives in Japan and Kuril Trenches, Earth Planet. Sci. Lett. 83, 313-328, 1987 (this issue). 4 E. Suess, B. Carson, S.D. Ritger, J.C. Moore, M.J. Jones, L.D. Kulm and G.R. Cochrane, Biological communities at vent sites along the subduction zone off Oregon, Biol. Soc. Wash. Bull. 6, 475-484, 1986. 5 J. Espitalie, J.L. Laporte, M. Madec, F. Marquis, P. Leplat, J. Pauleu and A. Boutefeu, Une mrthode rapide de caract~risation des roches mrres, de leur potentiel p&rolier et de leur degr6 d'rvolution, Rev. Inst. Fr. Petr. 32, 23-42. 1977.
6 J. Boul~gue, E.L. Benedetti. D. Dron, A. Mariotti and R. L&olle, Geochemical and biogeochemical observations on the biological communities associated with fluid venting in the Nankai Trough and Japan Trench subduction zones, Earth Planet. Sci. Lett. 83, 343-355, 1987 (this issue). 7 J.R. Disnar and J. Trichet. The influence of various divalent cations on the thermally induced evolution of organic matter from an algal mat, Org. Geochem. 6, 865-871. 1984. 8 B.J. Katz, Limitation of Rock-Eval pyrolysis for typing organic matter, Org. Geochem. 4, 195-203. 1983. 9 P.K. Mukhopadhyay, J. Rullkotter, R.G. Schaefer and I).H. Welte, Facies and diagenesis of organic matter in Nankai Trough sediments, Deep Sea Drilling Project Leg 87 A, in: Initial Reports of the Deep Sea Drilling Project 87, pp. 877-889. U.S. Government Printing Office. Washington. D.C.. 1986. 10 T. Machihara, Characterization of insoluble organic matter in sediments from the Nankai Trough. Deep Sea Drilling Project Leg 87A, in: Initial Reports of the Deep Sea Drilling Project 87, pp. 891-896. U.S. Government Printing Office. Washington, I).C, 1986. 11 It. Kax~ahata, K. Fujioka and T. lshikuza, Sediments and interstitial water at Site 582 and 584, the Nankai Trough and Japan "l-tenth landward slope, in: Initial Reports of the Deep Sea Drilling Project 87. pp. 865-875, U.S. Government Printing Office, Washington. I).C.. 1986. 12 Y. Sano and H. Wakita. 3 H e / a H e ratios of pore gases in pore waters sites 583 and 584, in: Initial Reports of the Deep Sea Drilling Project 87, pp. 861-864, U.S. Government Printing Office, Washington, D.C.. 1986. 13 R.Y. Stanier, M. Doudoroff and E.A Adelberg. General Microbiology. MacMillan. London, 1971. 14 J. Boulrgue. Trace metals (Fe, Cu. Zn, Cd) in anoxic environments, in: Trace Metals in Sea Water. NATO Conf. Ser.. Mar. Sci., pp. 563-577, Plenum Press, New York. N.Y.. 1983. 15 L.D. Kulm, E. Suess, J.C. Moore, B.T. l,ewis. S.D. Ritger. I).C. Kadko, T.M. Thornburg, R.W. Embley, W.D. Rugh, G.J. Massoth, M.G. Langseth, G.R. Cochrane and R . L Scamman. Oregon subduction zone: venting, fauna and carbonates. Science 231. 561-566, 1986.
Earth and Planetary Scwnee Letters. 83 (1987) 356-362 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
[41
Geochemistry of the Tenryu Canyon deep-sea fan biological community (Kaiko) D o m i n i q u e D r o n ~, J a c q u e s Boul6gue ~, A s a h i k o T a i r a 2 a n d C l a u d e R a n g i n 3 J Lahoratotre de G~ochtmte et M&allog~;nie (CNRS UA 196). University; Pierre et Marie Curie. 4 phlce .lussieu, 75252 Paris C~;dex 05 (1.'rance) ' Ocean Research Institute. Unit,ersttv of Takyo. Nakano-Ku Minaret Dai 1-15-1. l?~k.vo (Japan) ~ D~;partenlent de Gg'ototectonique (('NRS U.4 215), Um1,ersit~; Pwrre et Marie Curie. 4 place Juss'teu. 75252 Paris (.'¢;dex 05 (France) Revised version accepted February 5, 1987 The fenryu Canyon deep-sea fan biological community is characterized by both reduced and oxidized sedimcnts in the immediate vicinity of the pore water vents. The upper sediments in contact with the clams are reduced, the organic mfitter is enriched in sulfur, and inorganic sulfides (Fe, Cu. Zn) arc forming. Towards the outer fringes of the communities the sediment is oxidized and metals generally associated v, ith fcrro-manganese oxides are concentrated. Several metals. Cd, Pb, Mo show distributions which are strongly influenced by the metabolism of the clam colony. Comparison of water and sediment geochemistry leads to the conclusion that there should be a downward flux of oxygenated seaw~,ter on the boundaries of the colony and an upward flux of chemically more reduced deep pore water at the location of the colon.,,'. Trace metals anomalies as well as 81"N anomalies of organic matter may bc useful to prospect for extinct venting aret, s in ancient subduction zones.
I. Introduction
Several benthic biological communities, with clams as their principal organism have been discovered atop pore water vents on the landward slope of the subduction areas off Japan [1-3]. Similar occurrences have been previously described from the subduction zone off the Oregon margin [4]. A survey of the Tenryu Canyon deepsea fan community was done including water, sediment and biological sampling by the submersible "Nautile" (Figs. 1 and 2). At other locations (Kashima Seamount, Japan Trench, Kurile Trench) sediment recovery was very low because of dewatered and quite indurated substrate, preventing penetration of the core tubes. In this report we present data on bottom water, supernatant water, sediment and clam composition. which enable us to deduce possible water flow patterns and geochemical behavior of elements in the vicinity of the biological community. 2. Sampling and analyses All samples were taken with the mechanical arms of the submersible "Nautile" handling tube (X)I2-821X/87/$03.50
'~' 1987 Elsevier ~ience Publishers B.V.
corer and box corer. They were precisely located as shown in Fig. 2. We collected four sediment samples. 5BC and 5NC were taken directly within the clam colonies, 5TI was taken on the outskirt of the colonies, 3TI is located 300 m off the vent site (see Fig. 1). Supernatant waters were sep-
,,~/3T I [
:I ._
i
& ,
i:ig. 1. Sampling area in the Tenryu Canyon deep-sea fan. Samples associated with the biological community are noted by a black circle. At this kuaation the direction of a thrust structure (N50 ° ) and of a possible fault (NW-SE) system are also given. The samples taken away from the biological community correspond to the star: HY6 = water sample: 3"I"1= sediment.
357 Dv
./
1,24 °
I., ~ ' ° 7 "+-
.........
-.+?cl
j
'+'°
water=l.19°C
+ ~_f
Fig. 2. Detailed sampling scheme in the biological community of Tenryu Canyon deep-sea fan. HY = water sample; 5TI, 5NC and 5BC = sediment samples. Measured temperatures are also given in ° C . After a drawing by A. Taira (see [1,2] for further details).
arated from over the sediments. The samples are HY9, HY10 and HY6 corresponding to 5BC, 5TI and 3TI respectively. Free bottom water samples (HY4, HY7) were collected directly above the clam colonies using titanium syringes [1]. The chemistry of these water samples is discussed elsewhere [1]. We showed that the compositions of the water samples collected above the clam colonies result from the mixture of advected pore water with free deep-sea water. Several modification of the advected pore water were due to clam metabolism: uptake a n d / o r fixation of hydrogen sulfide, methane and nitrogen. The sediments were frozen on board, and freeze-dried on shore in the laboratory (within one month of sampling). Wet chemical analyses of the sediment were done after washing and desalting with distilled water. Si was measured by colorimetry, AI, Cr, Mn, Fe, Co, Ni, Cu, Zn, Rb, Mo, Cd, Ba, Pb were measured by flame atomic absorption spectrometry a n d / o r flameless Zeeman corrected atomic absorption spectrometry. Accuracies are + 0.5% for Si, A1, Fe, + 2% for other elements, AI, Na, K, Ca, Mg were measured after H F attack of the sediment. The metal cations were measured after a combined H C I - H N O 3 attack of the sediment so as to avoid contribution from the silicates. Trace elements present in carbonates and other minerals were also measured with an electron microprobe on polished sections of the sediment. Total carbon, inorganic and organic carbon and the same fractions for sulfur were analyzed by step combustion and
chromatography of evolved CO 2 and SO 2 (accuracy +0.3%). However, with this method we cannot distinguish between sulfide sulfur in inorganic compounds and sulfur in non-heme iron-sulfur protein compounds. Rock-Eval analyses of different sediment fraction were also performed at IFP [5]. X-ray diffraction and IR spectrometry were performed for identification of the mineral phases. In addition ESR and ESCA were also performed on the bulk sediment samples to investigate Mn, Fe, Cu and Mo. The clam samples were investigated (as freeze-dried subsamples of selected tissues) using the same methods [6]. The organic fraction of the sediments, as well as the tissues of clams were analyzed for t3C/12C and tSN/14N isotopic ratios [6]. The main results of the chemical analyses of the sediment are given in Table 1, 2 and 3. The analyses of the water samples are given in [1] and the complete analyses of the clam are given in [6] and some selected results are given in Fig. 6.
3. Sediment composition Quartz and detrital alumino-silicates from weathering of the Japanese islands are the main components of all the sediments. Sample 5NC is mostly coarse grains due to the fact that it was obtained by scooping the sediment with a biological net. The other sediments have different characteristics. The grain size is quite different between samples (Fig. 3). 5BC is a black, soupy mud in which the clams were standing upward. In 5NC and 5BC samples, the coarse quartz and plagioclase grains are often blackened by a sulfur-rich organic matter (seen on 5NC). The amount of the < 80 ~ m size fraction is limited and is composed by silicates, quartz and phyllic minerals. The particles on filters from water samples HY4 and HY7 (see [1, Table 6]) have the same composition as the < 80 ~m size fraction. Sulfides are present in the > 80~m fraction as well as miilimetric lignic remains which are not common in other sediment samples. The sulfides are mostly rich in Fe, with traces of Z, Cu > Ni, Mo, Cd. The high sulfide content is restricted to sample 5BC (Table 1). 5NC, which was also taken from a location with clam colonies has numerous organic remains among which shell debris and serpulid tubing debris can be recognized (Fig. 3b).
358
..
°
m
l
q
•
,
I ~.
,
N
"t
!
'/
I
N
m Fig. 3. Photographs of the sediment under the microscope (natural light). Scale = 300 ,am: except in (b) where scale = 2500 # m . (al 5BC ( > 80 ~m); (b) 5 N C note the remains of a serpulid tube; (c) 5T1 ( > 80 a m ) : (d) 3T1 > 80 ,am).
Sample 5T1 is a brown, soupy mud which is found in the immediate vicinity of the clam colonies. The detrital fraction is similar to that of 5BC, but with fewer large grains (Fig. 3c). The organic matter content is partly found as remains from copepods (300-350 /~m), radiolarians (100300 ~m), forams (80-180 p.m), algae debris, fish scales, ascidians (300 ~m), and mixed organic
debris. The organic matter seems more fresh than in other samples (from visual observation) and its accumulation may be due to bioturbation. An examination of the filters (HY10, [1]) and the examination of polished section of the sediment showed that oxides containing Fe and Mn are quite frequent, and sulfides are scarce. The oxides also contain traces of Zn, Cu > Cr, Ni.
TABLE 1 Element content of the sediments Sample
Si
AI
5BC
25.4
7.3
Sto t
5T1
29.3
6.7
< 0.1
3T1
26.8
7.0
< 0.1
0.29
S.,,,rg
S,,rs
('t,,,
C.,,,,s
(?ors
613C,,r~
S 1~ N,r~
0.025
1.0
0.16
0.85
- 25.5
-- 0.2
< 0.1
0.027
1.5
0.61
0.89
< 0.1
0.018
0.93
0.32
0.61
25
4- 1.0
0.26
Si, AI, S and C are in wt.%; tot = total: inorg = inorganic; org := organic, al~(- in %, vs. PDB; 6nSN in %<, vs. atmosphere.
359
Sample 3T1 is an olive-green mud with few grains > 300 ~m. As seen in Fig. 3d, the biogenic matter content is less than in 5T1. The inorganic phases are almost the same as those in sample 5TI. Sulfides are absent. Radiolarian debris are found and forams are very scarce. Except for the presence of sulfides and sulfurrich organic matter in association with the clam colony (5BC), oxides in the vicinity of the colony (5T1), and differences in color and texture, the sediments from the Tenryu Canyon deep-sea fan are characteristic of detrital continental material with high sedimentation rate.
4. Geochemistry of organic matter The sediments in the vicinity of the clam colonies (5BC, 5T1) have a higher organic content than those away from colonies (Table 2) (3T1). The maximum temperature from Rock Eval (Tm,x in Table 2) of kerogen pyrolysis, which is higher when the organic matter is more matured [5], is also different in the two areas. The organic matter is less matured at the contact with the clam colony. The slight difference between 5BC and 5T1 may be due to the larger content in trace metals of 5T1, especially Mn which is known to delay pyrolytic decomposition [7] or to slight differences in the inorganic matrix [8] or may reflect actual differences of immature benthic biomass away
from the center of the vent. The results for C content and Tm~x can be compared with those obtained in the upper meters of sediments from the Nankai Trough during the DSDP Leg 87 [9]. The samples we obtained during Kaiko may have a lower content of land-derived organic matter. In all samples the inorganic carbon content is less than the organic carbon content. The calcium carbonate content+ which should covary with inorganic carbon, was already presumed to be noticed as very small from visual observation. The higher inorganic carbon content in 5T1 is in agreement with the observation of carbonate debris in the sample. The 613C and 615N of organic matter in sediments in contact with clam colony is slightly less than in Nankai Trough sediments. At least in the case of 8tSN this may reflect excretions from the clams [6]. The 813C is in the range of mixed marine-continental organic matter found in other sediments from the same area [10]. This mixed origin is also indicated by the hydrogen index versus oxygen index diagram (Fig. 4). This diagram indicates that the organic m a t t e r of 5T1 and 5BC is in between type II (marine) and type 1II (continental) organic matter. It is distinctly different from the type found in the Nankai Trough (3T1) which has a more continental character. The sulfur content of organic matter is higher in the vicinity of the clams. This is also in agreement with the black soupy aspect of the mud in sediments of this environment.
TABLE 2 Organic matter content of sediments expressed in C (v,t.%) for different size fractions. Also given is Tmax. ( ° C) the pyrolysis temperature at the maximum generation of the hydrocarbons from kerogen during Rock-Eval measurements
c(z)
T,,,.,, (°c)
5BC Total > 50 ,am < 50 ,am
0.85 0.42 0.94
403 405 398
5TI Total > 80 ,am < 80,am
0.89 1.48 -
419 442 -
3TI Total > 50 ,am > 50 ,am
0.80 0.69
416 435
-
H.I.Ii~m: rl " •oo ii
II Ioo
a00 O.L~
Fig. 4. Hydrogen Index (H.I. in mg H C / g Co~8) versus Oxygen Index (O.I. in mg C O 2 / g Corg) for sediments from the Tenryu Canyon and Nankai area. Black triangle = 5BC; black square 5TI; black circle = 3T1; open circle = DSDP Site 582, upper 20 m; open diamonds = DSDP Site 583 upper 20 m. Roman numerals correspond to kerogen types.
360 1104,
5. Geochemistry of trace metals
The trace element content of the sediments (Table 3) is highly variable. Iron contents are slightly lower than those found in the Nankai Trough sediments [11,12]. Co, Ni and Zn contents are in the lower range of the concentrations observed in the Nankai Trough [11]. In order to have a basis for geochemical comparison we have normalized the trace element content on a quartz and alumino-silicate free basis (Fig. 5). It appears that the oxide-rich sediment, 5T1, which is next to the pore water vent, is much richer in elements which are generally enriched in ferromanganese oxides: Fe, Mn, Ba, Cu, Zn, Cr, Ni, Co and possibly Mo. Most of these elements have lower concentration in 5BC than in 3T1. Pb, Cd and Mo are comparatively enriched in 5BC, Ba and Co are slightly depleted. This pattern of concentrations can be compared with the geochemistry of pore waters, supernatant waters and local seawater (see Table 3 and [1]). It appears that HY10, which corresponds to 5T1 is depleted in Fe, Cu, Mn and Mo as compared with HY9 and HY6. This is in agreement with the uptake of these metals in the oxide phases found at 5T1. Ba shows a higher concentration in HY10 than in other samples, it is also high in 5T1. This observation is in agreement with the conclusion that BaSO4 should form in the environment of the vents (probably biologically) [1] and that it may redissolve in HY10. thus enabling uptake by the oxides. This would explain the net enrichment of 5T1. The enrichment of Cd. Mo and Pb in 5BC is large as compared with oceanic siltstones. This can be explained by uptake and release by the clams. As shown in Fig. 6
I
I
i ~]~.
:_
•---~...
I
\-...
58t"
571
......
37-f
• .
J,
|-'ig. 5. Trace element content of the sediments (on a quartz and alumino-silicate free basis). All concentrations in ppm.
F
~:
Bo
o..
"'Zn
.:
~Cd
1
Mn I
,0L
L
Mo
gdls
viscera
rnonHe
Fig. 6. Trace element content of selected tissues. All concentrations in ppm (dry weight).
TABLE 3 Trace metal contents of the sediments in the HCI-HNO 3 soluble fraction (corrected for carbonates)..All results are in ppm except t:e which is in weight percent (dry weight of total sediment). Concentrations of trace metals in sediment pore water (I-'IY6, 9. 10) are in 10 7 m o l / k g Sample
Mn
Fe
Cu
Rb
Mo
Ba
('r
('o
Ni
Zn
('d
Pb
5BC 5T1 3T1 11Y9 HY10 HY6
340 1000 395 5.39 4.77 9.21
2.73 2.93 2.81 17.6 4.2 16.5
39 196 46.5 5.10 0.92 1.(11
160 102 200 17 17 17
7.0 5.0 0.9 1.23 (I.94 1.01
14.5 700 35 1.12 1.79 0.79
82 74 115
9 15 12
34 48 46
86 1115 92
0.82 0.18 (I.39
38 22 37
361
(see also [6]) Cd and Mo are present in high concentrations in the gills and other tissues. Thus there is a biological concentration factor in the 5BC sediments. The effect of trace metal uptake from pore waters by the clam is important [1] and it is then reflected in the sediment content probably owing to excretion or organic input at the death of the animal (as shown by the 8aSN of organic matter). 6. D i s c u s s i o n
and conclusion
The information obtained from the study of the geochemistry of the sediments and from waters [1] and clams [6] enable us to propose a scheme of the near-surface plumbing of the vents from the Tenryu Canyon deep-sea fan. This is presented in Fig. 7. The geological structure enabling upward flow of pore water containing nutrients is the key parameter for the establishment of the biological community. In the Tenryu Canyon it is provided by an emergent thrust fault, identified on seismic lines, and crosscut by numerous right-lateral strike-slip faults. Location at the mouth of the Tenryu Canyon has enabled important erosion of the deformation front [2]. There the clam colony, in symbiosis with bacteria which fix and metabolize C H 4, C O 2, H 2 S , N 2 [6], can be thrived in areas limited in widths by the thrust plane intersection with the seafloor and partly along the transverse faults. Such area is characterized by coarser sediments which may be due to disturbance by the upward flow a n d / o r bioturbation by the clam colony. The rapid lateral variations of
I I
.
. . . . . . .
'
-
"
T
,"
"°.v
~.,.~
¢~.~
~li
;
'"
"
x
*
Fig. 7. Schematic representation of sediment facies and water flows in the Tenryu Canyon deep-sea fan biological community. The large arrows downward correspond to possible seawater and pore water flow. The emplacement of the sediment samples are given (dashed line with sample reference).
the geochemical properties (sediment lithology, trace elements, organic matter) are in agreement with this limited scatter of the upward flow where the colonies are found. The intense biological activity and the presence of hydrogen sulfide [1], allow the deposition of a black, putrid mud layer in the interstices between the clams. This layer is only slightly richer in organic matter content than are Nankai Trough sediments. However, it has a lower 81SN due to the metabolic input of clams. This sediment also c o n t a i n s metallic sulfide (Fe, Cu, Zn > Pb, Cd, Mo) which may be produced via detoxification of the clam colony [6]. The water flowing from the clam colony into seawater still contains high concentrations of Fe and Mn [1]. This enables oxide formation and sedimentation surrounding of the clam colony. It occurs as a brown layer. This layer is also enriched in fresh organic matter due to the intense biological activity and scavenging observed in this area [2]. Interestingly, the sediment is not turning anoxic as one might expect. The sulfate content of HY10 is in the range of seawater [1]. These characteristics can be explained if there is locally a downward flux of oxygenated seawater in the vicinity of the clam colony. The oxygen available to the clam is needed for methane oxydation [6,13]. The postulated oxygenated seawater lateral flux may thus help fulfil a nutritional requirement. The lower concentration of Cd in 5T1 as compared to 3T1 also suggests that the oxygenated water flow will possibly extract Cd from the sediments surrounding the colonies and make it available to the clams [14]. The Cd contents enable to compute that the 10 g / m 2 of Cd needed to sustain the clam colony can be provided by leaching of a minimal 10 cm x 10 m 2 slab of 3T1 type sediment. Cd has been shown to be very important to the metabolism of the clams; hence the seawater flow may provide one of the limiting trace nutrients of the colony [6]. Thus, the biological activity linked to pore fluid venting in the deep-sea fan of Tenryu Canyon leaves a clear geochemical imprint in the upper part of the sediment. The imprint is evident in a zonation, from reduced sediment along the very' path of the upward flux of pore fluids and to an oxidized rim at the outer edge of the reduced zone. The width of this oxidized zone is probably less than 10 m based on field observations done
362
from the submersible. Once formed, the oxides can be expected to be stable enough to leave an anomalous geochemical imprint, at least in overall trace metal content (see Fig. 5). This will be more effective in subduction areas with high sedimentation rates. In that case the remobilization of metals by diagenesis will be limited. Hence it should be possible to identify such venting zones in sediments from ancient subduction areas by a combination of sedimentological criteria (coarse grain, linear structure along the vent area) and geochemical anomalies (in Fe, Mn, Ba, Cu, Zn, Ni, Co at the boundaries). These geochemical anomalies reflect the special metabolism of biological communities utilizing nutrients from expelled pore waters. Organo-geochemical anomalies, mostly the lowering of 6aSN of organic matter in the coarse grain zone. linked to biological activity should also be employed to diagnose venting areas. If the vents are not located below the carbonate compensation depth, the formation of anomalously low 8~~C carbonate from methane oxidation will also be a very efficient reconnaissance tool [15].
Acknowledgements This study was done with the analytical help of A.M. de Kersabiec, D. Dubarry and F. Vidot (UPMC). We have also benefited from help from the Laboratoire de Grologie (ENS, Uhn) and Laboratoire de Prtrologie (UPMC). Isotopic compositions of organic matter were provided by the courtesy of Dr. A. Mariotti (INRA-UPMC). Rock-Eval measurements were provided by the courtesy of Dr. B. Durand and Dr. J.Y. Huc (I.F.P.). The box corer sampler was obtained owing to M. Sibuet. We thank E. Suess for several discussions on pore fluids during his stay in Paris while on sabbatical leave.
References 1 J. Boulrgue, J.T. liyama, J.L. Charlou and J. Jedwab, Nankai Trough, Japan Trench and Kurile Trench: geochemistry of fluids sampled by submersible "Nautile", Earth Planet. Sci. Lett. 83, 363-375, 1987 (this issue). 2 X. Le Pichon, T. liyamama, J. Boulrgue, J. Char'vet, M. Faure, K. Kano, S. Lallemant, H. Okada, C. Rangin, A. Taira, T. Urabe and S. Uyeda, Nankai Trough and Zenisu
Ridge: a deep-sea submersible survey. Earth Planet. Sci. Lett. 83, 285-299, 1987 (this issue). 3 J.P. Cadet, K. Kobayashi, J. Aubouin, J. Boulrgue, J. Dubois, H. Hotta, T. Ishii, L. Jolivet, K. Konishi, S. Lallemand, N. Niitsuma and H. Shimamura, Deep scientific dives in Japan and Kuril Trenches, Earth Planet. Sci. Lett. 83, 313-328, 1987 (this issue). 4 E. Suess, B. Carson, S.D. Ritger, J.C. Moore, M.J. Jones, L.D. Kulm and G.R. Cochrane, Biological communities at vent sites along the subduction zone off Oregon, Biol. Soc. Wash. Bull. 6, 475-484, 1986. 5 J. Espitalie, J.L. Laporte, M. Madec, F. Marquis, P. Leplat, J. Pauleu and A. Boutefeu, Une mrthode rapide de caract~risation des roches mrres, de leur potentiel p&rolier et de leur degr6 d'rvolution, Rev. Inst. Fr. Petr. 32, 23-42. 1977.
6 J. Boul~gue, E.L. Benedetti. D. Dron, A. Mariotti and R. L&olle, Geochemical and biogeochemical observations on the biological communities associated with fluid venting in the Nankai Trough and Japan Trench subduction zones, Earth Planet. Sci. Lett. 83, 343-355, 1987 (this issue). 7 J.R. Disnar and J. Trichet. The influence of various divalent cations on the thermally induced evolution of organic matter from an algal mat, Org. Geochem. 6, 865-871. 1984. 8 B.J. Katz, Limitation of Rock-Eval pyrolysis for typing organic matter, Org. Geochem. 4, 195-203. 1983. 9 P.K. Mukhopadhyay, J. Rullkotter, R.G. Schaefer and I).H. Welte, Facies and diagenesis of organic matter in Nankai Trough sediments, Deep Sea Drilling Project Leg 87 A, in: Initial Reports of the Deep Sea Drilling Project 87, pp. 877-889. U.S. Government Printing Office. Washington. D.C.. 1986. 10 T. Machihara, Characterization of insoluble organic matter in sediments from the Nankai Trough. Deep Sea Drilling Project Leg 87A, in: Initial Reports of the Deep Sea Drilling Project 87, pp. 891-896. U.S. Government Printing Office. Washington, I).C, 1986. 11 It. Kax~ahata, K. Fujioka and T. lshikuza, Sediments and interstitial water at Site 582 and 584, the Nankai Trough and Japan "l-tenth landward slope, in: Initial Reports of the Deep Sea Drilling Project 87. pp. 865-875, U.S. Government Printing Office, Washington. I).C.. 1986. 12 Y. Sano and H. Wakita. 3 H e / a H e ratios of pore gases in pore waters sites 583 and 584, in: Initial Reports of the Deep Sea Drilling Project 87, pp. 861-864, U.S. Government Printing Office, Washington, D.C.. 1986. 13 R.Y. Stanier, M. Doudoroff and E.A Adelberg. General Microbiology. MacMillan. London, 1971. 14 J. Boulrgue. Trace metals (Fe, Cu. Zn, Cd) in anoxic environments, in: Trace Metals in Sea Water. NATO Conf. Ser.. Mar. Sci., pp. 563-577, Plenum Press, New York. N.Y.. 1983. 15 L.D. Kulm, E. Suess, J.C. Moore, B.T. l,ewis. S.D. Ritger. I).C. Kadko, T.M. Thornburg, R.W. Embley, W.D. Rugh, G.J. Massoth, M.G. Langseth, G.R. Cochrane and R . L Scamman. Oregon subduction zone: venting, fauna and carbonates. Science 231. 561-566, 1986.