METHANE-DERIVED CO 2 IN PORE FLUIDS EXPELLED FROM THE OREGON SUBDUCTION ZONE

(1)

Elsevier Science P u b l i s h e r s B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s

M E T H A N E - D E R I V E D CO 2 IN PORE FLUIDS EXPELLED FROM THE O R E G O N S U B D U C T I O N Z O N E

E. S U E S S 1'3 and M. J. WHITICAR 2

l College of Oceanography, Oregon State University, Corvallis, OR 97331 (U.S.A.) 2Bundesanstalt ffir Geowissenschaften und Rohstoffe, 3000 Hannover 51 (F.R.G.)

(Received J a n u a r y 9, 1988; revised and accepted M a r c h 22, 1988)

A b s t r a c t

Suess, E. a n d Whiticar, M. J., 1989. M e t h a n e - d e r i v e d CO 2 in pore fluids expelled from t h e Oregon s u b d u c t i o n zone.

Palaeogeogr., Palaeoclimatol., Palaeoecol., 71:119 136.

Pore fluids e x t r a c t e d from n e a r - s u r f a c e s e d i m e n t s of t h e d e f o r m a t i o n front along the Oregon s u b d u c t i o n zone have, in general, the dissolved n u t r i e n t p a t t e r n c h a r a c t e r i s t i c of bacterial sulfate reduction. However, in c e r t a i n locations there are peculiar a m m o n i u m d i s t r i b u t i o n s a n d a n o m a l o u s l y 13C_depleted dissolved ECO 2. T h e s e carbon isotope and n u t r i e n t p a t t e r n s are a t t r i b u t e d to the c o n c u r r e n t microbially-mediated oxidation of s e d i m e n t a r y organic m a t t e r (POC) and m e t h a n e (CH4) o r i g i n a t i n g from depth. In c o n t r a s t to the oxidation of s e d i m e n t a r y organic m a t t e r in the sulfate zone, utilization of m e t h a n e as t h e carbon source by s u l f a t e - r e d u c i n g bacteria would g e n e r a t e only h a l f as m u c h total carbon dioxide for e a c h mole of sulfate c o n s u m e d a n d would n o t g e n e r a t e a n y dissolved a m m o n i u m . The isotopically light ECO 2 released from m e t h a n e oxidation depletes the total metabolic carbon dioxide pool. Therefore, NH~, ECO 2 and 6 j 3C of i n t e r s t i t i a l carbon dioxide in t h e s e pore fluids distinctly reflect t h e combined c o n t r i b u t i o n s of each of t h e two c a r b o n s u b s t r a t e s u n d e r g o i n g mineralization; i.e. m e t h a n e and s e d i m e n t a r y organic matter. By appropriately p a r t i t i o n i n g the n u t r i e n t a n d s u b s t r a t e relationships, we c a l c u l a t e t h a t in the area of the m a r g i n a l ridge of the Oregon s u b d u c t i o n zone as m u c h as 30% of t h e ECO 2 in pore fluids m a y r e s u l t from m e t h a n e oxidation.

The c a l c u l a t i o n also predicts t h a t the c a r b o n isotope s i g n a t u r e of the carbon dioxide derived from m e t h a n e is between

- 35%0 a n d - 63%0 PDB. S u c h a n isotopically light gas g e n e r a t e d from w i t h i n t h e a c c r e t i o n a r y complex could be the residue of a biogenic m e t h a n e pool. Fluid a d v e c t i o n is required to carry s u c h m e t h a n e from depth to the present near- surface sediments. T h i s m e c h a n i s m is c o n s i s t e n t with large-scale, tectonically-induced fluid t r a n s p o r t envisioned for accreted s e d i m e n t s of the world's c o n v e r g e n t plate boundaries.

I n t r o d u c t i o n

The deep-sea submersible A l v i n has provided s t r u c t u r a l and stratigraphic data on the frame- work of plate subduction along the n o r t h e a s t Pacific convergence zone and has visited deep sites of fluid venting in the u n d e r t h r u s t tectonic setting of the Oregon margin. Communi- ties of clams and tube worms, authigenic

3present address: GEOMAR Research Center, Wischhofstr.

1 3. 2300 Kiel 14 (F.R.G.).

carbonate minerals, methane-enriched bottom waters, and biological tissues with extreme 12C isotope enrichment were collected at this accretionary complex where the J u a n de Fuca oceanic plate plunges beneath the North American continental plate (Suess et al., 1985;

Kulm et al., 1986; Schroeder et al., 1987).

Equally exciting new information on the chemistry of fluids and vent organisms was gathered by the submersible Nautile from the subducting plate boundaries of the northwest Pacific (Le Pichon et al., 1987; Boulegue et al., 1987). The drilling vessel Joides Resolution

0031-0182/89/$03.50 ' ~ 1989 Elsevier Science P u b l i s h e r s B.V.

(2)

r e c e n t l y p e n e t r a t e d t h e d ~ c o l l e m e n t b e t w e e n t h e c o n v e r g i n g C a r i b b e a n a n d A t l a n t i c p l a t e s n e a r t h e B a r b a d o s d e f o r m a t i o n f r o n t a n d also f o u n d a n o m a l o u s p o r e w a t e r ion a n d m e t h a n e c o n c e n t r a t i o n s , as e v i d e n c e for a c t i v e f l u i d m o v e m e n t a l o n g f a u l t planes ( M o o r e et al., 1986). It a p p e a r s t h a t in m o d e r n a n d a n c i e n t a c c r e t e d deposits m e t h a n e and, perhaps, h i g h e r h y d r o c a r b o n s dissolved in v e n t i n g pore fluids play a u n i q u e role in p r o v i d i n g e n e r g y a n d c a r b o n for t h e b e n t h i c c o m m u n i t i e s of l a r g e v e n t o r g a n i s m s a n d for t h e l i t h i f i c a t i o n of a c c r e t i o n a r y deposits by c a r b o n a t e c e m e n t (Hart and Suess, this issue). We h a v e e a r l i e r hypothesized, i n f e r r e d from p o r e fluid c h e m i c a l anomalies, t h a t s u c h a m e t h a n e - b a s e d biogeo- c h e m i c a l s y s t e m o p e r a t e s at t h e O r e g o n subduc- t i o n zone (Suess et al., 1985; K u l m et al., 1986).

We will n o w s u b s t a n t i a t e a n d e x t e n d this r e a c t i o n m e c h a n i s m by p r e s e n t i n g stable carbon isotope d a t a of ECO 2 from p o r e fluids o f t h e d e f o r m a t i o n front. T h e s e fluids c o n t a i n dissolved m e t a b o l i c c a r b o n dioxide w h i c h is depleted w i t h 13C b e y o n d w h a t is e x p e c t e d from m i n e r a l i z a t i o n of n o r m a l s e d i m e n t a r y o r g a n i c m a t t e r . T h e p o r e w a t e r d a t a are e x p l a i n e d by a m i x i n g model w h e r e b y c o n c u r r e n t m i c r o b i a l o x i d a t i o n o f s e d i m e n t a r y o r g a n i c m a t t e r (POC) a n d t h e c h e m i c a l l y m o r e r e d u c e d m e t h a n e c a r b o n s u b s t r a t e (CH4) a c c o u n t for t h e observed n u t r i e n t a n d isotope p a t t e r n s . M o r e o v e r , t h e isotope s i g n a t u r e of t h e p o s t u l a t e d m e t h a n e - d e r i v e d ECO 2 is c o n s i s t e n t w i t h t h a t of m e t h a n e - d e r i v e d dolomite a n d m a g n e s i a n c a l c i t e ce- m e n t s of t h e O r e g o n a c c r e t i o n a r y complex. T h e 513C c h a r a c t e r i s t i c s of t h e d i a g e n e t i c carbonates a n d t h e m e t a b o l i c c a r b o n dioxide r a n g e from - 35 to - 66%0 P D B a n d i n d i c a t e a r e s i d u a l b i o g e n i c s o u r c e for t h e m e t h a n e .

C o r i n g s i t e s a t t h e O r e g o n s u b d u c t i o n z o n e T h e s t y l e o f s e d i m e n t a c c r e t i o n a l o n g por- t i o n s of t h e O r e g o n s u b d u c t i o n z o n e i n c l u d e s s t a c k e d s e q u e n c e s of l a n d w a r d d i p p i n g a n d s e a w a r d d i p p i n g p a c k a g e s of h e m i p e l a g i c sedi- m e n t s of t h e A s t o r i a fan s e p a r a t e d by t h r u s t p l a n e s ( K u l m a n d F o w l e r , 1974). T h e m o r p h o -

PACI PLZ

5 2 ° N

5 0 °

4 8 °

4 6 °

4 4 °

4 2 °

4 0 °

- - 3 8 °

1 3 4 ° W 1 3 0 ° 1 2 6 ° 1 2 2 °

w E

0 P l a i n M a r g i n a l R i d g e

Vent Site I~ i B a s i n

4

~ T p ~ ~

6 - ~ - - -

8

I I I I I

k m 0 5 I 0 k m

Qu = Q u a t e r n a r y , Tp = Pilocene Tmv = Late Miocene,

u n d i f f e r e n t i a t e d oceanic c r u s t

Fig.1. (Top). Northeastern Pacific plates; the Juan de Fuca plate is subducted as it converges with the North American plate. An accretionary complex forms by off-scraping sediments from the oceanic plate. In the area of investiga- tion (box) the tectonic style of subduction is by under- thrusting. (Bottom),The convergence-induced lateral stress causes large-scale dewatering of the accretionary complex along the main d~collement surface and numerous landward dipping fault planes. An area of extensive venting of fluids is located just off the ridge crest (Vent Site). The tectonic elements of convergence are: undeformed abyssal plain, marginal ridge, and ponded basin.

(3)

logical expression of the accreted stacks is a series of n o r t h - s o u t h trending t h r u s t ridges with intervening basins of ponded sediments.

In the study area (Fig.lA and B) the initial deformation front is a seaward-facing scarp at the toe of the c o n t i n e n t a l slope t h a t rises 4-6 m above the abyssal plain. A second scarp, exposed a few hundred meters landward, ap- parently is the outcrop of the d4collement surface which separates the deformed off- scraped deposits of the t h r u s t ridge from the

more gently dipping s t r a t a which are being subducted with the converging oceanic plate.

A transect of gravity cores and pore water profiles was obtained across these tectonic elements of the Oregon subduction zone (Fig.2;

Table I). Two cores [8408-04 and 8408-11] were raised from the gently dipping s t r a t a between the escarpments, which showed not evidence of deformation. The main u n d e r t h r u s t ridge rises steeply, more t h a n 800 m, above the abyssal plain. The seaward face of this ridge is cut by

E q u a t o r i a l b l e r c a t o r P r o j e c t i o n • S c a l e = i O 0 . O 0 i n c h e s / d e g r e e

4 4 °43

NECOR/SeaBeam 2 9 - 0 c t - 8 7 11=50

4 4 ° 4 0

4 4 ° 3 7

1 2 5 ° 2 0 1 2 5 " 1 5

AI T8L20 GRIDDED DIVE AREA

D I V E . L I M D I V E . O R D 1 5 0 - N B L 7

Fig.2. S e a B e a m bat:hymetry (20 m depth c o n t o u r s ) a n d core l o c a t i o n s in t h e u n d e r t h r u s t tectonic s e t t i n g off Oregon. Cores 8408-04 and 8408-11, at t h e toe of the c o n t i n e n t a l slope, are s e a w a r d of t h e d e f o r m a t i o n front. Cores 8306-26, 8306-27 and 8306- 24 are on the s e a w a r d face of the u n d e r t h r u s t ridge. Cores 8708-02 and 8708-03 are located at t h e v e n t site, and Core 8408-10 at the w e s t e r n edge of' t h e ponded s e d i m e n t basin.

(4)

T A B L E I

S t a t i o n l o c a t i o n s a n d w a t e r d e p t h s o f c o r i n g t r a n s e c t a c r o s s d e f o r m a t i o n f r o n t o f t h e O r e g o n s u b d u c t i o n z o n e S t a t i o n L a t i t u d e L o n g i t u d e W a t e r d e p t h

( ° N ) ( ° W ) ( m )

8306-24 44°39.78 ' 125o19.07 ' 2420

8306-26 44o39.00 ' 125o20.50 ' 2795

8306-27 44°39.00 ' 125019.66 ' 2623

8408-04 44°40.00 ' 125o21.60 , 2860

8408-10 44o40.00 ' 125°17.10 ' 2180

8408-11 44039.90 ' 125°21.50 ' 2846

8708-02 44040.55 ' 125o17.43 ' 2040

8708-03 44o40.55 ' 125o17.43 ' 2040

canyons and exposes mainly rocks but in part is covered by patches of sediment which accumulate on morphological ledges. Three cores [8306-24, 8306-26, and 8306-27] were collected from these patches on the seaward face of the u n d e r t h r u s t ridge. Along the landward flank of the ridge, just off its crest, the benthic communities of tube worms and giant clams were observed during

Alvin

dives (Suess et al., 1985; 1987a). Two cores (8708-02 and 8708-03) were t a k e n from the sediment patch surround- ing the vent site; core 8708-03 was collected within 10 m of the giant white clam and tube worm communities. Finally, one core (8408-10) was t a k e n from the westernmost part of the sediment pond near the unconformity with the u n d e r t h r u s t ridge, about 200 m distant from the vent site.

Sampling and analyses

Interstitial waters were extracted from sediments of these eight stations by pressure filtration at the temperature of the bottom water close to 2°C with a technique initially described by H a r t m a n n et al. (1973). For the highly compacted sediments at the u n d e r t h r u s t ridge a hydraulic press and sample cell was used similar to the system adopted by the Ocean Drilling Program (Manheim and Sayles, 1974). Total dissolved carbon dioxide (ECO2) was determined by on-line gas chromatography and quantified by thermal conductivity after

acidification and He-stripping of the interstitial waters. Dissolved chloride was determined by the standard Mohr-titration, sulfate gravi- metrically as BaSO 4, ammonium by standard methods for n u t r i e n t analyses in seawater (Grasshoff, 1976) and calcium and magnesium by flame atomic absorption spectrometry. The results are reported in units of g/kg for chlorinity and mmol/L (mM), mg/L or ~mol/L (~M) for the other dissolved species along with precision estimates (Table II). Samples for stable carbon isotope measurements of ZCO2 and deuterium/hydrogen ratios of water were drawn into V a c u t a i n e r s ® and treated with mercuric chloride. The dissolved ZCO 2 was prepared for 13C/12C measurement by the standard method of acidification and collection at 25°C. The interstitial HEO was reduced to hydrogen for D/H measurement by freezing the HEO over onto zinc-filled 6 m m glass tubes which were evacuated, sealed and heated to 460°C (Coleman et al., 1982). All isotope mass ratios were measured using a F i n n e g a n MAT 251 mass spectrometer. The results are reported in the usual delta n o t a t i o n (Table III):

s a m p l e -

Ra/Rb

standard I~0"

(1)

where

Ra/Rb

are the 13C/12C and D/H ratios relative to the PDB standard for carbon and the SMOW-(H20 ) standard for hydrogen.

The organic-carbon-to-organic-nitrogen ratio (C/Nby atoms) of sedimentary organic m a t t e r of the Oregon margin sediments was needed to evaluate the C/N rgeneration ratio of POC during microbial sulfate reduction. For this purpose organic carbon and carbonate carbon contents were determined on five of the cores using the H3POJdichromate-LECO technique described by Weliky et al. (1988). Basically, c a r b o n a t e - c a r b o n is measured as the CO2 liberated during t r e a t m e n t of the sediment with phosphoric acid, and organic carbon is measured as the CO2 evolved during subse- quent oxidation of the remaining sediment and phosphoric acid mixture with dichromate. The

(5)

T A B L E I1

P o r e w a t e r d a t a f r o m e i g h t g r a v i t y cores a c r o s s t h e O r e g o n m a r g i n d e f o r m a t i o n front; t h e a c c u r a c y of c h l o r i n i t i e s m a r k e d by (*) is affected by d i l u t i o n a n d / o r loss by e v a p o r a t i o n , t h e r e f o r e t h e N a - c o n t e n t s of t h e s e s a m p l e s w e r e u s e d to c a l c u l a t e the Ca 2 ~ and Mg 2" d e c r e a s e

Core D e p t h i n t e r v a l N H 4 ECO 2 SO 4 Ca Mg C1

s t a t i o n (em) (~M) (mM) (mM) (mg/L) (rag/L) (g/kg)

8408-04 20 23 192 4.47 27.16 432 1310 19.34

55 58 501 7.91 24.94 417 1290 19.15

90-93 689 9.88 23.14 400 1290 18.84"

125 128 1022 13.84 20.84 376 1280 18.74"

160 163 1154 15.83 19.45 356 1250 18.93"

195 198 1340 18.35 17.67 323 1250 19.06

8404-10 0 5 26 2.80 27.88 428 1310 19.10

20 25 150 3.93 27.08 425 1290 19.42

50 55 296 5.59 26.02 401 1250 19.39

215-220 1508 20.53 11.46 172 1201 20.69*

8408-11 0-5 33 3.18 28.17 418 1260 18.95

20 25 172 4.36 27.19 410 1290 19.24

65 70 699 10.42 22.32 379 1270 19.10

120-125 955 12.78 20.09 342 1250 19.35

8708-02 30-40 33 3.12 27.71 419 1301 19.22

60 70 38 3.17 27.62 417 1293 19.29

92 98 85 3.27 27.28 409 1284 19.29

115 120 124 3.29 26.70 405 1274 19.37

].40 147 256 5.54 24.75 380 1259 19.26

].66 172 441 9.05 19.19 333 1247 19.47

195 200 722 11.66 14.20 270 1177 19.20

215 220 711 14.92 11.36 249 1196 19.41

8708-03 7 12 42 3.33 27.24 419 1313 19.46

20 26 87 3.56 26.59 416 1281 19.31

32 39 126 4.16 25.87 402 1286 18.94

56-61 197 4.13 24.44 391 1272 19.05

82-87 300 5.25 23.36 361 1242 19.14

112 117 463 6.88 18.93 303 1191 19.22

8306-24 5 10 71.0 3.10 26.86 410 1284 19.10

25 30 230.0 5.20 25.39 406 1309 19.28

60 65 539.0 8.60 22.04 384 1249 19.09

85 90 740.0 10.60 19.97 338 1184 18.60"

125 130 758.0 12.80 15.42 313 1177 19.45

160 165 1114.0 15.40 12.72 286 1149 19.45

175 180 1039.0 15.30 11.54 275 1134 19.45

8306-26 5 10 126.0 4.(}0 26.94 415 1294 19.74

25 30 178.0 4.90 25.84 412 1293 19.41

45 50 389.0 7.80 24.52 397 1287 18.92"

70 75 824.0 13.00 22.71 377 1270 19.54

92 98 923.0 15.30 21.01 358 1285 19.17

100 105 1037.0 16.80 20.08 335 1282 19.52

105 110 1013.0 17.00 19.57 334 1292 19.67

110 115 1002.0 18.10 19.25 338 1276 19.45

8306-27 20-25 162.0 3.'75 26.09 403 1288 19.48

50 55 261.0 4.83 24.68 395 1289 19.41

70 75 339.0 5.78 23.91 382 1331 19.63

90-95 402.0 6.63 22.84 361 1278 19.49

105 110 437.0 7.19 22.62 353 1254 19.34

130 135 516.0 8.45 21.53 337 1269 18.36

145 150 633.0 9.45 20.59 320 1231 19.16

(6)

T A B L E II (continued)

S t a n d a r d s e a w a t e r C1- T i t r a t i o n w i t h AgNO3 0.06% or 0.01%0

SO 4 G r a v i m e t r y < 1% or 0.15 m M

Mg F l a m e AAS 1% o r 0.5 m M

Ca F l a m e AAS 1% o r 0.1 m M

ZCO2 GC-Therm. Cond 2% o r 0.05 m M

NH4 C o l o r i m e t r y 3% or 0.5 #M

PO4 C o l o r i m e t r y 0.5% or 0.1 ~M

P r o p a g a t i o n of a n a l y t i c a l u n c e r t a i n t i e s c a u s e t h e ZCO 2 e s t i m a t e s in F i g s . 4 A - D a n d 5 A - D to be precise by ___ 12%; i.e.

+ 0 . 3 m M for e s t i m a t e s in t h e r a n g e of ZCO z of s e a w a t e r a n d + 1.5 m M for e s t i m a t e s > 10.0 mM.

TABLE III

S t a b l e c a r b o n i s o t o p e s of dissolved ZCOz a n d deuteri- r a n / h y d r o g e n r a t i o s o f H 2 0 f r o m selected p o r e w a t e r s a m p l e s of t h e O r e g o n s u b d u c t i o n zone. Note: e x t r e m e 1 sC.

d e p l e t i o n o f ZCO z in deep p o r e w a t e r s of S t a t i o n s 8306-24, 8306-27 a n d 8708-02

Core Depth j 1 3 C 0 2 ( . . . . ) ~'~ C O 2 5D(water)

no. (cm) (%0 PDB) (raM) (%0 ^SMOW)

8306-24 25-30 - 15.2 5.2 1.2

85-90 - 23.1 10.6 2.6

160-165 - 26.3 15.4 0.7

8306-26 25-30 - 13.1 4.9 1.5

70-75 - 18.5 13.8 - -

100-105 - 18.4 16.8 - 1.4

8306-27 20-25 - 13.3 3.75 0.5

50 55 - 19.2 4.83 1.3

70-75 - 20.7 5.78 - 1.2

90-95 - 21.3 6.63 0.2

105-110 - 2 3 . 4 7.19 0.3

130-135 - 24.3 8.45 - -

145-150 - 24.7 9.45 - 0.2

8408-04 125-130 - 18.3 13.84 0.1

160-163 - 16.6 15.83 0.1

8708-02 60-70 - 13.5 3.17 - -

115-120 - 23.9 3.29 - -

215-220 - 34.8 14.92 - -

CO2 is detected by a thermal conductivity detector. Total nitrogen was determined using the micro-Kjeldahl digestion method described by Bremner (1960). Inorganic fixed ammonium- nitrogen was determined by the method of Silva and Bremner (1966). In this procedure, the organically-bound nitrogen is first removed with potassium hypobromite. The residue is

then treated with hydrofluoric acid to destroy the silicate lattice structure of clay minerals.

The solution is made basic with KOH and the nitrogen is distilled as NH4 and detected as in the micro-Kjeldahl technique. Organic nitro- gen is calculated as the difference between the

1 . 5 -

I . O - E

i

E E

0 . 5 -

0

0 ---

0

~¢~i:;i :::.:. ~ • - o 4 / - z 6 / - H

%:::::.:i:!.i:L ~ o - 24

o) ~.'..:: ~:v~ ~ D - 2 7 '

- - ~'!!i:::~ ~ x - fo

~ ~ ~ ^-02/03

0

~ o "::!::.:

~ a o ":::':';~.

® ":(:.z

,~ 2~ 2'5 3b

S u l f a t e ( r a M )

Fig.3. D i s s o l v e d a m m o n i u m a n d sulfate c o n c e n t r a t i o n s in t h e p o r e w a t e r s s h o w two g r o u p i n g s : the solid s y m b o l s w h i c h d e n o t e s t a t i o n s w i t h a m m o n i u m g e n e r a t e d as expected f r o m m i c r o b i a l s u l f a t e r e d u c t i o n of n o r m a l s e d i m e n t a r y o r g a n i c m a t t e r (POC) a n d t h e o p e n s y m b o l s w h i c h d e n o t e s t a t i o n s w i t h s t r o n g n e g a t i v e a m m o n i u m a n o m a l i e s c o m p a r e d to t h e o b s e r v e d s u l f a t e r e d u c t i o n . T h i s deficit reflects t h e lack of a m m o n i u m r e l e a s e d f r o m t h e a n a e r o b i c o x i d a t i o n of m e t h a n e (CH~). The r e g r e s s i o n line i n d i c a t e s t h e r e g e n e r a t i o n r a t i o N H ~ : - S O ~ - = 7 : - 5 3 u s e d in Eq.(2). The stippled a r e a c o v e r s t h e r e g e n e r a t i o n r a t i o s N H 4: SO4 + 2- b e t w e e n 7 . 0 7 : - 5 3 ( C / N = 1 5 ) a n d 9 . 6 4 : - 5 3 ( C / N = l l ) as e s t i m a t e d f r o m t h e o r g a n i c c a r b o n a n d o r g a n i c n i t r o g e n a n a l y s e s of t h e O r e g o n m a r g i n cores. The a r r o w s i n d i c a t e s a m p l e s f r o m w h i c h 513CO 2 d a t a w e r e o b t a i n e d (for r e s u l t s see Table III).

(7)

total nitrogen and fixed nitrogen. The results of the C/N ratios are incorporated into Fig.3, the data, however, are reported elsewhere (Han, 1987).

Pore w a t e r chemistry: substrates for metabolic C02 production

Pore waters from the sediments of the marginal ridge and the undeformed toe of the continental slope show patterns of dissolved constituents characteristic of mineralization of organic m a t t e r by microbial sulfate reduction, i.e., an increase of ZCO2, and NH~ and a corresponding decrease of dissolved SOl with depth in core (Fig.3; Table II). This is expected for hemipelagic environments of the type represented by the Astoria fan deposits (Waterman et al., 1972; H a r t m a n n et al., 1973;

Claypool, 1974; Suess, 1976). Also, as in similar continental nmrgin settings, the interstitial dissolved Ca z+ and Mg 2+ contents decrease due to formation of diagenetic carbonate minerals at greater depth in the sediment.

Dolomite, high- and low-Mg calcites, and aragonite are ubiquitous in the Oregon margin sediments (Russell et al., 1967; Scamman, 1981; Ritger et al., 1987; Han and Suess, this issue).

We assume for this discussion t h a t along the Oregon subduction zone all of the Ca 2+ and Mg 1+ decrease observed in pore waters is consumed by diagenetic carbonate mineral formation. This assumption is justified and discussed in detail in this volume by Han and Suess, and elsewhere (Han, 1987). It most likely overestimates the ECO2 removal from pore fluids, because it does not consider a l t e r a t i o n of volcano-clastics and terrigenous alumino- silicates. In many cases, however, there is a 1:1 molar decrease of both Ca/+ and Mg 2÷ as would be expected from dolomite formation.

Since this diagenetic mineral is common in the carbonate crusts, concretions, chimneys and cements of the lithified accretionary deposits (Ritger et al., 1987), we t h i n k t h a t the above assumption is well-justified. The simple stoi- chiometric nutrient-regeneration-mineral-pre-

cipitation may then be approximated by the reaction:

2[(CHzO)(NH3)x] + SO2- + Ca 2 + + Mg 2 +

[CaMg(CO3)z] + xNH•+ H2S (2)

Hereby x denotes the inverse of the carbon-to- nitrogen regeneration ratio. Hydrogen sulfide is not considered because it takes part in a series of reactions involving more complex solid phases whose stoichiometry cannot be as readily approximated as t h a t of dolomite or calcites (Berner, 1977, 1980). Ammonium, except for ion-exchange, accumulates in the anoxic pore fluids and is a sensitive indicator for the oxidation of sedimentary organic m a t t e r (POC) by microbial sulfate reducers.

At certain localities across the main under- t h r u s t ridge of the Oregon subduction zone, the regenerated n u t r i e n t pattern is anomalously depleted in NH~ and ZCO2 relative to the observed interstitial SO ] - removal. Only the NH~-distribution is shown in Fig.3; the open symbols are for samples from the deformed sediments, the closed symbols denote the tectonically undeformed sediment samples from the toe of the continental slope. The NH~- deficit seen in the pore fluids of the deformed sediments is larger t h a n t h a t of the most nitrogen-depleted organic matter of the Oregon margin sediments [lower limit C/N (by atoms) = 15; Fig.3]. The upper limit in this figure shows a regenerative ratio of - S O l :NH~=

-53:9.64 which corresponds to the mean ratio of C-organic/N-organic(by ,tom~ = 11 in five of the sediment cores analyzed. It is commonly assumed t h a t uptake of ammonium by ion- exchange significantly reduces the ratio between SO]--consumption and NH~-release (Rosenfeld, 1981; Boatman and Murray, 1982).

This process is likely the reason for the overall deviation between the C/N-ratio of POC in the sediments and t h a t actually observed in the pore water; i.e. the discrepancy in Fig.3 between the trend followed by the solid symbols and t h a t covered by the stippled area.

Eighteen samples out of a total of 50 were selected for 613ZCO2 analyses. These samples

(8)

are indicated by h o r i z o n t a l arrows in Fig.3 and the results listed in Table III. The ZCO 2- depletion in the samples r e l a t i v e to sulfate r e d u c t i o n not i l l u s t r a t e d here, remains signifi- c a n t even a f t e r a d j u s t m e n t for r e m o v a l by c a r b o n a t e f o r m a t i o n based on Ca 2 +- and Mg 2 +- losses. We a t t r i b u t e the anomalies in NH~

and ZCO2 to the oxidation of m e t h a n e because in utilizing such a c a r b o n substrate, microbial sulfate r e d u c e r s g e n e r a t e only h a l f as m u c h metabolic ZCO2 as they g e n e r a t e from utilizing normal s e d i m e n t a r y o r g a n i c m a t t e r (POC).

M e t h a n e oxidation g e n e r a t e s no NH~ at all.

Anaerobic m e t h a n e oxidation is by now well- established as an e a r l y diagenetic process in anoxic hemipelagic sediments (Devol, 1983;

I v e r s e n and J o r g e n s e n , 1985; W h i t i c a r and Faber, 1986). Aerobic m e t h a n e consumption has been shown by mussels at seep communities (Childress et al., 1986). The different ZCO 2- r e g e n e r a t i o n and S O 2 - - r e d u c t i o n ratios are evident from the following comparison:

M e t h a n e oxidation:

CH4 + SO~- = ZCO2 + 2H20 + S 2 - (3) ZCO 2 : - SO 2- = 1:1

S e d i m e n t a r y o r g a n i c m a t t e r oxidation:

2[(CH20)(NH,)x] + SO 2- = 2ZCO 2 + 2H20 + 2xNH~+ S 2 - ZCO2: - SO 2- = 2:1

Therefore, t h e N H + and ZCO2-anomalies in pore fluids from the localities at the under- t h r u s t ridge may reflect the combined contributions of each of the two s u b s t r a t e s - - POC and CH 4 ^-^- u n d e r g o i n g c o n c u r r e n t mineralization.

The c o n t r i b u t i o n s can be c a l c u l a t e d from the NH~--anomaly in the pore fluids, the dissolved SO2--change, the Ca 2 ÷ and Mg 2÷ losses, and the organic-carbon-to-organic-nitrogen ratios of the POC s u b s t r a t e as follows:

ZCO 2 = k + ANH~x C/N + (4)

(ASO 2- - ANH+x 1 / 2 C / N ) - (ACa 2 + + AMg 2 +)

H e r e b y is ZCO 2 the predicted total dissolved carbon dioxide c o n t e n t in any pore w a t e r sample. This prediction assumes equal diffusiv- ities for all dissolved components. The con- s t a n t k denotes the q u a n t i t y of ZCO 2 buried with the oceanic bottom water; usually an a m o u n t of ~2.6 mM. The term, ANH 4 x C/N, quantifies the metabolic ZCO 2 c o n t r i b u t e d by the oxidation of POC; h e r e b y C/N is the r e g e n e r a t i o n r a t i o of the POC-substrate; i.e.

C / N b y atoms = 15. The next term, ASO 2- - A N H ~ x l / 2 C / N , is the a m o u n t of metabolic ZCO2 derived from CH4. The expression, ANH~x 1/2C/N, is e q u i v a l e n t to the a m o u n t of SO 2- reduced during POC oxidation and the f a c t o r 1/2 scales C/N to S O 2 - / N H ~ according to the Redfield stoichiometry. The difference b e t w e e n the total change in sulfate and t h a t consumed by POC is the SO 2- consumed in CH4-0xidation, because Eq.(3) specifies t h a t ASO 2- :AZCO2 mctha,e = 1 : 1. The last term, ACa 2 + ÷ AMg 2 +, is a simple m e a s u r e for the a m o u n t of ZCO 2 removed by c a r b o n a t e m i n e r a l formation, as discussed previously.

The results of p a r t i t i o n i n g of metabolic ZCO 2 according to Eq.(4) are shown in Figs.4 and 5. Core 8708-02 (Fig.4A) is discussed in g r e a t e r detail because it shows all the im- p o r t a n t features. The discussion applies, correspondingly, to the o t h e r cores. In Core 8708-02, at about 130 cm below the seafloor, a d i s c o n t i n u i t y separates the onlapping ponded sediments above from the deformed ridge sediments below. In the deeper sediments the NH~-anomaly is significant and used to predict the ZCO 2 c o n t r i b u t e d from CH4-0xida- tion (heavy solid line). T h e predicted ZCO 2 and m e a s u r e d values (solid symbols connected by dashed line) agree very well. The gross ZCO 2 produced prior to c a r b o n a t e mineral precipitation is shown by the dotted line. The precipitation of diagenetic carbonates, from the sum of the Ca 2+ and Mg 2+ losses, reduces the gross ZCO 2 prediction to the level shown by the h e a v y solid line w i t h o u t symbols. The open symbols show the predicted ZCO 2 c o n t r i b u t i o n from CH4-oxidation according to the term [ASO 2- - A N H + 1/2C/N] from Eq.(4). This pre-

(9)

Z C O z ( m M )

o I o 2 o

8 7 0 8 - 0 2

A

- ~ - ×~ ...

...

~ C O 2 ( m M )

IO 2 0

. ~ i i L L L I I

s4os-,,c

'i \

~ C O z ( r a M )

IO 2 0

i i i i i i i ~ i J

8 4 0 8 - 0 4

" , B

Z C O z [ r n M )

rO 2 o

I I r i i k I I I

8 5 0 6 - 2 6

Fig.4. Predicted partitioning for ZCO 2 according to Eq.(4) in pore waters of cores 8708-02 (A), 8408-04 (B), 8408-11 (C), and 8306-26 (D): The dotted line (Fig.4A only) shows the gross ZCO 2 resulting from seawater burial, POC- and CH 4- oxidation prior to C a - M g - c a r b o n a t e precipitation. The heavy solid lines without symbols show the predicted ZCO2-content after correction for carbonate mineral precipitation. The dashed lines with closed symbols indicate the measured ZCO2-contents. The contributions of CH4-derived ECO 2 are shown by the open symbols, prior to carbonate precipitation, and by the t h i n solid lines after correction for carbonate precipitation. The horizontal p a t t e r n denotes a discrepancy between predicted and measured ZCO 2 which is greater t h a n expected from the u n c e r t a i n t y of the analytical data (see Table II).

diction does not take into account the carbonate mineral precipitation and therefore should be evaluated vis-a-vis the gross ECO 2 production. Finally, the CH4-derived ZCO 2, corrected for carbonate mineral precipitation, is shown by the trend of the small solid symbols connected by the thin solid line. In the ponded sediment section of Core 8708-02, the measured ZCO 2 contents change little with depth as do the predicted values, a l t h o u g h the two dis- agree by more t h a n the estimated u n c e r t a i n t i e s (horizontally shaded area). Tentatively, we t h i n k t h a t this may reflect a partial oxidation of methane by dissolved oxygen r a t h e r t h a n by

~C02 (raM)

=

8 4 0 8 - 1 0

', C 0

\

i

z

J z

E, C O z ( m M ) % C 0 2 ( r a M )

o ~o 2 0 o Lo z o

i i i i i i ~ L • o k i i i L - - I I ~ I I

i

4

2

T C O z ( r a M )

io z o

_ i i i i i

.~ ~ 8 7 0 8 - 0 3

i D

Fig.5. Predicted partitioning for ZCO 2 according to Eq.(4) in pore waters of cores 8306-27 (A), 8306-24 (B), 8408-10 (C), and 8708-03 (D). The measured, predicted, and CH4-derived ECO 2 contents are shown by the same symbols as in Fig.4;

the gross ZCOz-content prior to carbonate mineral precipitation is omitted.

sulfate and thus the ammonium anomaly would not accurately predict the a m o u n t of CH4-derived ECO 2. The upper sediment is coarse-grained and the site of active fluid venting and aerobic m e t h a n e oxidation a likely process.

In the remaining seven cores (Figs.4B-D and 6A-D) the measured and predicted ZCO2 are shown and, when applicable, the total methane- derived ZCO 2 (open symbols) and the precipitation-corrected methane-derived ZCO2 contents (small solid symbols connected by thin solid lines). For simplicity, the gross ECO2, generated prior to carbonate precipitation,, is omitted.

Along the transect, from the deformation front to the sediment pond, the agreement between predicted and measured ECO2 is quite good. Three distinct groupings are evident:

First, Cores 8408-04, 8306-26, and 8408-11 contain no to insignificantly small contributions of methane-derived ZCO2 in their pore fluids• A

(10)

~ C O z m m o l / k g

5 I0 15 20 25

+ 2 ~ ~ = ... ...,...,

~i!~::~[~i~::~::;i~i~i~::;i~!~i~!~::;::~!;i;:: ... . ... :::::::::::::::::::::::::::::::::::::

- 2 % ° - 4

_ -8 ! ~ i ! : : . ~ ° o x

E3 -12

a. i m+

o~ - 16

0 e

--- ::iiiii::::::: + • :::;:!:!:~;;;!!!!!~!i!~i::iii:: F;5~::!!~::~::;!;i~!i::

~o- -20

- 2 2 7oo

iiiiiiiil; ,,.

-24 i~ii:ii!ii?i~i~i~i~i!:. •

- 2 8 ;::;~::~i~::~::~::~i;::~::;i;ii::i::i::iii::~ ... x

•••:•••:••.•.

•••••••.:.

•••••.:••.

••:•••••:•••••:.•.

•••••••.••••••••••:••••.:•:•••:

•••• ".'.'.'.'.":':':.:-:... - 3 5 o °

- 3 2 :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: =======================================================

Fig.6. Total CO 2 and &13CO2 of pore fluids and simple two- component mixing of three possible XCO2-sources: Dissolu- tion of biogenic calcium carbonate which does not significantly affect the $13CO2; decomposition of sedimentary organic carbon (POC) which adds "light" ZCO2 of between - 2 0 % and -22%0 PDB; anaerobic oxidation of methane which generates extremely light ZCO2 depending whether the methane is thermogenic (-35%0 PDB) or biogenic ( - 65%0 PDB). Note: Stations 8408-04 ( • ) and 8306- 26( + ) show 13C-depletions as expected from POC decomposition; more strongly 13CO=-depleted samples are from stations affected by fluid venting [8708-02 ( • ) , 8306-24 ( x ), and 8306-27 ( • ) ] ; Station 8306-17 (©) [not discussed in this paper] from the abyssal plain bordering the Oregon overthrust to the north also shows very "light" ZCO 2, probably derived from methane oxidation; S W = seawater.

sand layer at the bottom of Core 8306-26, with elevated contents of diagenetic carbonates (Han and Suess, this issue), shows some evidence for methane-derived CO 2. In this interval the predicted and measured ZCO 2 deviate significantly from each other. We are u n c e r t a i n of the reason for this discrepancy, but have observed the same phenomenon repeatedly within coarse-grained sediments in other parts of the Oregon-Washington subduction zone. Second, Cores 8306-27 and 8306-24 contain up to 30% methane-derived metabolic

ECO2, with significant proportions predicted all the way to the sediment-water interface.

Third, cores 8708-03, 8708-02 and 8408-10 contain significant amounts of methane-derived ECO2 only in pore fluids of the deeper core sections. These are overlain by sediments which contain no methane-derived ZCO2 but perhaps show evidence for aerobic methane oxidation. The overlying sediment cover varies in thickness.

In summary, then, we hypothesize - - based on pore fluid chemical anomalies - - t h a t the sediments at the toe of the continental slope up to and including the scarps undergo early diagenesis of normal sedimentary organic matter by sulfate reduction and the sediments covering the seaward face of the main u n d e r t h r u s t ridge and the ponded basin at the ridge crest show evidence for methane-derived metabolic E C O 2.

This interpretation is supported by the anomalous 13C.depletio n in the dissolved ECO 2 of selected samples of pore fluids from Cores 8306-24, 8306-26, 8306-27, and 8708-02 of the u n d e r t h r u s t ridge and 8408-04 from the toe of the c o n t i n e n t a l slope (Fig.6; Table III). We consider it particularly significant t h a t certain carbon isotope values of the dissolved ECO2, between -23%0 and -34%0 PDB, are more negative t h a n the minimum values of -22%0 measured for POC from this area (Hedges and Mann, 1979; Hedges and Van Green, 1982). The latter would be the lowest value t h a t could reasonably be explained by addition of metabolic ZCO 2 solely from sedimentary POC sources (Claypool and Kaplan, 1974; Reeburgh, 1982). This trend is exemplified by the stable carbon isotope composition of interstitial ZCO 2 of cores 8408-04 and 8306-26 unaffected by m e t h a n e oxidation. In fact, the two samples 8408-10/125-130 and/160-163 we used to calcu- late the isotopic composition of ECO2 generated from POC. The values ranged from - 19.1 to - 21.4%o PDB, based on these, an average of -20%0 was used in the isotope balance discussed later. It has been shown for m a n y recent and ancient anoxic environments t h a t the 513 C distribution in such pore fluids is explainable by

(11)

a mixing process between buried oceanic ZCO2 (~13C = + 0.50/00) and metabolic Z C O 2 generated from mineralization of POC (~13C=-20%o PDB) (McCorkle, 1987). In oxic and suboxic environments, where metabolic ZCO2 dissolves skeletal carbonates - - an important benthic process causing the sedimentary lysocline - - skeletal-derived ZCO2 also effects the total dissolved ZCO2 pool (Emerson and Bender, 1981; Emerson et al., 1982; McCorkle, 1987). As described above, sulfate reduction produces alkalinity, and thus does not drive CaCO 3 dissolution. In the unlikely case t h a t skeletal CaCO 3 dissolution would contribute ZCO2 it is similar isotopically to the oceanic ZCO2 and therefore would not cause a negative carbon isotope shift. The shallow pore fluids of the Oregon subduction zone which show a strong negative 513C-shift are a rare occurrence and, so far, seem to be unique to the marginal ridge sediments which are affected by fluid expulsion from depth. We suggest t h a t these isotope signatures clearly reflect the mineralized CH 4- substrate just; as their NH2-defieieney reflects the lack of organic nitrogen during CH 4- oxidation.

Carbon isotope b a l a n c e for ~C02

By including CH4-substrate mineralization, the ZCO: in pore fluids of the marginal ridge and their peculiar carbon isotope composition can be modelled as a three-component mixing process. The sources are: oceanic ECO: from buried seawater bicarbonate, metabolic ZCO:

from POC-mineralization and from CH4-mineralization. With most of the parameters k n o w n in this system, or at least reasonable assump- tions possible, we consider in the following discussion a carbon isotope balance and solve for the ~13C of the hypothetical CH4-derived portion of ZCO 2.

In the isotope balance the q u a n t i t y of ZCO:

is t h a t actually measured in the pore fluid and which is generated by mineralization of one or both of the organic substrates, plus the buried oceanic ZCO2, minus the q u a n t i t y removed by carbonate mineral formation. The a m o u n t of

methane-derived ECO2 is a critical q u a n t i t y for the isotope balance. It is calculated from the ammonium and dissolved sulfate relationship (as previously shown) and adjusted for the ECOE-removal by mineral precipitation. This adjustment is necessary for all the ZCOz- sources because otherwise the different terms are weighted unevenly.

Isotope fractionation during lithification by carbonate minerals is not yet t a k e n into account because, at most, only 1-2%o of abso- lute change in the dissolved ZCO 2 is expected at the ambient temperatures of the Oregon margin sites. This fractionation, though, is less t h a n the overall significance of the isotope balance.

The conditions for isotope balance of the pore water ECO2 are expressed as follows:

(~13C02( . . . . )[~~C02 ( .. . . )] = ~13C02 (sw)[ c]

+ t~l 3C02

(POC)[ b]

+ t~13C02 {meth) X [a] (5) Hereby [a], [b] and [c] are the estimated concentrations of the three ZCO 2 sources as follows:

[ECO2 (meth)] = methane-derived CO 2 [mM]

•13C02 (meth)= (~13C of methane-derived CO 2 (0/ooPDB)

[ZC02 (Poc)]= POC-derived C O 2 [raM]

513CO2 (Poc)= 513C of POC-derived CO 2, calculated from two samples of Core 8408-04

= - 2 0 (%o PDB)

[ZC02 (sw)] = inorganic ZCO2 of buried sea water, assumed to be 2.6 [mM]

513CO2 (sw)= 513C of buried seawater ZCO 2, assumed to be + 0.5 (%o PDB)

On the left-hand side of Eq.(5) the measured parameters are:

[ZCO2 ( .. . . )] = dissolved ZC02 [mM]

~13C02( . . . . )=(~13C of ZCO 2 in pore water (%0 PDB)

Solving for the u n k n o w n parameter 313CO2 (meth), yields a most interesting sequence of 513C values for the ECO2 from mineralization of the postulated CHa-substrate in cores 8306-24, 8306-27, and 8708-02 (Table IV). First,

(12)

TABLE IV

Predicted 51aCO2 of methane-derived ZCO2 based on pore water isotope balance and calculated amount of methane- derived ZCO 2. The three stations, 8306-24, 8306-27, and 8708-02 are affected by venting of methane-charged fluids.

The uncertainty for the predicted 313C of the ZCO 2 is based on a propagation of analytical errors of the pore water data (Table II)

Depth ECO2( .... ) 513CO2t .... ) Methane-derived (cm) (mM) (%0 PDB)

613CO2(pred)

(%0 PDB)

Core 8306-24

25-30 0.61 - 15.2 -37___4 85-90 2.09 -23.1 - 5 9 + 6 160-165 4.42 -26.3 - 5 1 ± 5

Core 8306-27

20-25 0.51 -13.3 - 3 4 ± 4 50-55 0.91 - 19.2 - 50 + 5 70-75 1.10 - 20.7 - 54 ± 5 90-95 1.52 -21.3 - 4 9 _ 5 105-110 1.57 - 23.4 - 60 ± 6 130-135 2.01 - 24.3 - 59 + 6 145-150 2.10 -24.7 - 6 3 ± 6

Core 8708-02

60-70 0.20 (1.11)* -13.5 - 3 4 ± 4 115-120 0.49 (1.09) -23.9 - 5 0 + 5 215-220 6.43 - 34.8 - 57 ± 6

( )*: from discrepancy between predicted and measured ZCO 2 assumed to be methane-derived by aerobic oxidation;