SUBDUCTION-INDUCED PORE FLUID VENTING AND THE FORMATION OF AUTHIGENIC CARBONATES ALONG THE CASCADIA CONTINENTAL MARGIN: IMPLICATIONS FOR THE GLOBAL Ca-CYCLE

Palaeogeography, Palaeoclimatology, Palaeoecology, 71 (1989): 97 118 97 Elsevier Science P u b l i s h e r s B.V., Amsterdam - - P r i n t e d in The N e t h e r l a n d s

SUBDUCTION-INDUCED PORE FLUID VENTING AND THE FORMATION OF AUTHIGENIC CARBONATES ALONG THE CASCADIA CONTINENTAL MARGIN: IMPLICATIONS FOR

THE GLOBAL Ca-CYCLE

M . W . H A N 1 a n d E . S U E S S z

College of Oceanography, Oregon State University, Corvallis, OR 97331 (U.S.A.) (Received October 27, 1987; revised and accepted May 24, 1988)

A b s t r a c t

Han, M. W. and Suess, E., 1989. Subduction-induced pore fluid v e n t i n g and the formation of a u t h i g e n i c c a r b o n a t e s along the Cascadia c o n t i n e n t a l margin: Implications for the global Ca-cycle. Palaeogeogr., Palaeoclimatol., Palaeoecol., 71:97-118.

Pore fluid v e n t i n g associated with subduction-induced sediment deformation causes precipitation of calcium c a r b o n a t e as p r o m i n e n t c a r b o n a t e chimneys or cement in the accreted sediments across the active c o n t i n e n t a l m a r g i n off Oregon and Washington. A depletion of i n t e r s t i t i a l Ca 2 ÷ with a maximum decrease of 50% relative to seawater Ca 2÷ over only 1.5 m depth and reduction in porosity in the deformed sediments suggest t h a t i n t e r s t i t i a l Ca 2 ÷ is removed to form calcium c a r b o n a t e cement. In contrast, the pore waters of the undeformed abyssal plain sediments show no depletion in dissolved Ca 2 +. They are e i t h e r enriched to a maximum of 5% or show no change in dissolved Ca 2÷. Here the b a c k g r o u n d level of CaCO a c o n t e n t in the sediment is only 0.1 to 1%.

Calcium c a r b o n a t e p r e c i p i t a t i o n in the deformed sediments probably occurs as the result of upward m i g r a t i o n and oxidation of biogenic m e t h a n e and of the increase in c a r b o n a t e s a t u r a t i o n due to release of excess pore pressure during fluid venting. Upward advection of fluids at r a t e s of 1-28 cm y - ~ is predicted from diffusion-advection-reaction models applied to the downcore c o n c e n t r a t i o n profiles of dissolved Ca 2+ and NH4 + in the tectonically-deformed sediments. The r a n g e of predicted flow rates is related to the type of calcium c a r b o n a t e lithification; i.e. slow rates generate cement and fast rates generate chimneys,

C a r b o n a t e m i n e r a l precipitation associated w i t h pore fluid v e n t i n g requires direct t r a n s f e r of Ca 2 + from the oceanic b a s e m e n t to the a c c r e t i o n a r y complex. Such a m e c h a n i s m leads us to propose t h a t the a c c r e t i o n a r y complexes of the global plate s u b d u c t i o n zones are a major sink for crustal Ca 2 +. A global flux of crustal Ca 2 ÷ t h a t is removed by c a r b o n a t e m i n e r a l precipitation may be as m u c h as the h y d r o t h e r m a l Ca-input. This significant Ca-flux, not previously considered in the global geochemical budget, implies t h a t pore fluid v e n t i n g in subduction zones may also act as a global sink or source for o t h e r elements.

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

R e c e n t d i s c o v e r i e s o f c a r b o n a t e c h i m n e y s a n d b e n t h i c c o m m u n i t i e s o f t u b e w o r m s a n d

~Present address: Korea Ocean Research a n d Development Institute, Seoul H25-600 (Korea).

2Present address: GEOMAR Research Center, Wischhofstr.

1 3 D-23 Kiel 14 (F.R.G.).

g i a n t c l a m s a s s o c i a t e d w i t h p o r e f l u i d v e n t i n g i n t h e a c c r e t i o n a r y c o m p l e x e s o f s u b d u c t i o n z o n e s ( S u e s s e t a l . , 1 9 8 5 ; K u l m e t a l . , 1 9 8 6 ; S c h r o e d e r e t a l . , 1 9 8 7 ; B o u l e g u e e t a l . , 1 9 8 7 ; C a d e t e t a l . , 1 9 8 7 ; L e P i c h o n e t a l . , 1 9 8 7 ; O h t a a n d L a u b i e r , 1 9 8 7 ; P a u t o t e t a l . , 1 9 8 7 ) s u g g e s t a n e w m e c h a n i s m t h a t m a y a f f e c t t h e g l o b a l s e a w a t e r c o m p o s i t i o n . A d v e c t i n g p o r e f l u i d t h r o u g h t h e a c c r e t e d s e d i m e n t s w o u l d e x - c h a n g e a n d t r a n s f e r e l e m e n t s b e t w e e n t h e

subducting crust and the ocean water. One could foresee an important role of pore fluid venting at global subduction zones on the chemical mass balance of the oceans if the areal extent of fluid venting and the magnitude of mass transfer can be established. Although it is unclear at present if subduction-induced transport by fluid venting is similar in magni- tude to that of mid-ocean ridge hydrothermal fluxes, both are ultimately related to the major tectonic processes of plate generation and consumption.

Calcium carbonate precipitation and cemen- tation of near-surface sediments associated with pore fluid venting occurs not only in the accretionary complex of the northeast Pacific but also along several other convergent mar- gins: the Barbados accretionary complex (Moore et al., 1986), the N a n k a i Trough sediments (Stein and Smith, 1986), and the Peru convergent margin (Kastner et al., 1987).

This world-wide distribution suggests to us the need for a revision of the global Ca-balance (Thompson, 1983; Von Damm et al., 1985) by including - - as a first approximation - - subduction-induced calcium carbonate precipi- tation as a major sink for calcium.

Since prominent carbonate chimneys and crusts in the Cascadia accretionary complex off Oregon/Washington are associated with venting pore fluids, an understanding of the geochemical condition of the calcium carbon- ate system of the interstitial waters is essential in order to explain the formation of the pervasive carbonate lithification. Interstitial Ca 2 + in near-surface sediments is in general controlled by biogeochemical processes of organic matter decomposition which generates an excess of dissolved ECO2. Carbonate precip- itation, as oposed to dissolution in pelagic environments, was observed in hemipelagic environments of rapid burial of sediments rich in organic matter (Moor and Gieskes, 1980;

Suess et al., 1982; K a w a h a t a and Fujioka, 1986;

Stein and Smith, 1986).

The Cascadia accretionary complex receives hemipelagic sediments from the Columbia River drainage basin and the weathering of the

Coast Range at a fairly rapid rate of around 10 cm per 1000 y (Karlin, 1979; Krissek, 1982).

Characteristically these sediments are ex- tremely low in detrital and biogenic calcium carbonate. Therefore, the carbonates in the accretionary complex are not due to deposition but are the result of dynamic control on the dissolved calcium carbonate system associated with pore fluid venting. Moreover, the mor- phology and shape of the chimneys, crusts, concretions and interstitial cements strongly support an authigenic mechanism of formation (Scamman, 1981; Ritger et al., 1987). Venting apparently provides favorable geochemical conditions for calcium carbonate authigenesis in the accretionary complex.

In this paper we investigate, based on chemical compositions of pore water from the Cascadia subduction zone, the mechanism of carbonate mineral precipitation associated with pore fluid venting. We then evaluate the potential role of this process in removing crustal Ca 2+ and finally speculate on the implications for the global balance of Ca 2 + Geologic setting and methods

The J u a n de Fuca oceanic plate, generated at the J u a n de Fuca spreading Ridge, is presently being subducted off Oregon and Washington as it converges with the North American plate at a rate of 4 cm y-1 (Wells et al., 1984) (Fig.la). A portion of the sediments loaded on the subduct- ing plate is off-scraped and added onto the overriding North American plate forming the Cascadia accretionary complex along the lower continental slope (Silver, 1972; Carson et al., 1974; Kulm and Fowler, 1974).

The accretionary complex consists of a series of folded and thrusted ridges trending perpen- dicular to the convergence direction (Silver, 1972; Carson et al., 1974; Kulm et al., 1986). The ridges become progressively older from the west to the east across the complex. The youngest ridges of the deformation front are less than 0.3 m.y. old and rise from 400 to 1000 m above the adjacent abyssal plain (Carson et al., 1974;

Kulm et al., 1986).

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I -- 45°00 ' 44*30' I I l I ~" 134eW 130 126 142 38 124Q00 , Fig.la. Subduction zone off Oregon and Washington. The Juan de Fuca l~late is presently subducted off Oregon and Washington as it converges with the North American plate. An accretionary complex forms along the lower continental slope by off-scraping sediments from the subducting plate and adding them onto the overriding North American plate. The areas of investigation are marked by boxes along with DSDP sites of 174 and 176. b. Enlarged areas of investigation: Washington accretionary complex (Box A, above) and Oregon accretionary complex (Boxes B and C, below). Dots indicate core stations. Area A is the Washington overthrust region; there are two transects in this region with five core stations along the northern profile and four core stations along the southern profile. The circled core station along the southern profile denotes a sea mount, which is thought to be a mud volcano (L. D. Kulm, personal communication). Area B is the Oregon overthrust region; there are six core stations in this area. Area C is the Oregon underthrust region; there are seven core stations in this area. Depth contours are in meters.

The mode of occurrence and the morphology of the carbonates in the accretionary complex are diverse: slabs, crusts, and disseminated cement are found t h r o u g h o u t the near-surface sediments. Their mineralogies, isotopic compo- sitions, and fabric have been extensively stud- ied (Russell et al., 1967; Scamman, 1981; Ritger et al., 1987). Conical chimneys, 1-2 m in height protruding above the sea floor, were found associated with the benthic communities of tube worms and giant clams on the marginal ridge of the deformation front in the Oregon accretionary complex (Kulm et al., 1986). Other carbonate chimneys were discovered on the outermost continental shelf off northern Oregon (Scl/roeder et al., 1987).

Generally all carbonates are strongly de- pleted in the carbon isotope ~ 3C. The depletion is characteristic of methane-derived calcium carbonates (Kulm et al., 1986; Suess et al., 1987; Suess and Whiticar, this issue). Pure

carbonate precipitates which build up the chimney structures indicate an authigenic mode of formation. Hollow tubes and passages with numerous grooves and flutes within all the chimneys are thought to be the imprints of venting fluids (Schroeder et al., 1987).

Pore water samples and sediment cores were obtained from areas of carbonate lithification along four transects spanning the tectonic elements of the abyssal plain to the deforma- tion ridges (Fig.lb). Coring was done during two geological/geophysical cruises, the first in J u n e of 1983 on board the R~ V Wecoma and the second in August of 1984 on board the R / V Atlantis II.

The northern transects (Box A in Fig.lb) are in the central Washington overthrust area.

They consist of five stations along a northern profile and four stations along a southern profile (Fig.2a). Combined these stations repre- sent the abyssal plain environment (8306-9C,

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Fig.2. Locations of core s t a t i o n s from the W a s h i n g t o n o v e r t h r u s t region (a), the Oregon o v e r t h r u s t region (b), and the Oregon u n d e r t h r u s t region (c). Core locations are s h o w n in r e l a t i o n to the tectonic elements of the a c c r e t i o n a r y complex, deformation front, seaward ridge, sediment pond, and second ridge.

-10C a n d -3C), t h e c r e s t o f t h e s e a w a r d d e f o r m a - t i o n r i d g e (8306-4C), t h e p o n d e d s e d i m e n t s b e h i n d t h i s r i d g e (8306-12C a n d -5C), t w o m o r p h o l o g i c f e a t u r e s w i t h i n t h e s e d i m e n t p o n d (8306-14C = m o a t a n d 8306-11C = m o u n d ) , a n d a s e a m o u n t o n t h e a b y s s a l p l a i n (8306-2C).

T h e c e n t r a l t r a n s e c t is i n t h e O r e g o n o v e r - t h r u s t a r e a ( B o x B i n F i g . l b ) . T h r e e o f t h e t e c t o n i c e l e m e n t s w e r e s a m p l e d ( F i g . 2 b ) : t h e a b y s s a l p l a i n (8306-17C a n d -7A), t h e c r e s t o f t h e s e a w a r d r i d g e (8306-2A a n d -18C), a n d t h e f l a n k s o f t h e s e c o n d d e f o r m a t i o n a l r i d g e (8306- 4A, a n d -5A). N o p o n d e d s e d i m e n t b a s i n is d e v e l o p e d h e r e b e t w e e n t h e r i d g e s .

T h e s o u t h e r n t r a n s e c t is i n t h e O r e g o n u n d e r t h r u s t a r e a ( B o x C i n F i g . l b ) . H e r e t h e

TABLE I

101 t e c t o n i c e l e m e n t s w e r e c o r e d i n g r e a t e r d e t a i l t h a n i n t h e o v e r t h r u s t a r e a (Fig.2c). T w o c o r e s w e r e t a k e n j u s t s e a w a r d o f t h e d e f o r m a t i o n f r o n t w h i c h is e v i d e n t i n a s c a r p (8408-4, a n d -11), f o u r c o r e s w e r e t a k e n o n t h e s e a w a r d f l a n k o f t h e d e f o r m a t i o n a l r i d g e . T h e y r a n g e f r o m t h e f o o t o f t h e r i d g e (8306-26C), a c r o s s s e v e r a l s e d i m e n t e d l e d g e s (8306-27C, -24C a n d 8408-7) t o t h e l a n d - w a r d s i d e o f t h e r i d g e , w h e r e t h e p o n d e d s e d i m e n t s u n c o n f o r m a b l y o v e r l i e t h e r i d g e f l a n k (8408-10). C a r b o n i s o t o p e d a t a o f d i s s o l v e d Z C 0 2 , p o r e w a t e r n u t r i e n t s , a n d m e t h a n e c o n t e n t s f r o m t h i s t r a n s e c t a r e t h e s u b j e c t o f a s e p a r a t e p a p e r b y S u e s s a n d W h i t i c a r ( t h i s issue). A l l c o r e l o c a t i o n s a n d t h e i r t e c t o n i c e n v i r o n m e n t s a r e s u m m a r i z e d i n T a b l e I.

Core locations in the Oregon/Washington subduction zone

Station Latitude Longitude Water depth Setting

(°N) (°W) (m)

A. Central Washington overthrust area Northern profile

8306-9C 47°27.51 ' 126o34.55 ' 2370 8306-10C 47o27.42 ' 126°23.89 ' 2320 8306-11C 47027.49 ' 126°02.15 ' 1600 8306-12C 47o27.47 ' 126o03.28 ' 1807 8306-14C 47o27.50 ' 126°04.17 ' 1810

Southern profile

8306-2C 47°13.70 ' 126o08.90 ' 2320 8306-3C 47 ° 12.50' 126 ° 19.36' 2445 8306-4C 47o17.53 ' 126005.67 ' 1920 8306-5C 47017.74 ' 126o04.29 ' 2050 B. Oregon overthrust area

8306-2A 44o57.47 ' 125°21.87 ' 2376 8306-4A 44°55.91 ' 125o18.64 ' 2008 8306-5A 44°56.26 ' 125o18.45 ' 1990 8306-7A 44°52.82 ' 125o28.64 ' 2758 8306-17C 44057.00 ' 125o37.92 ' 2735 8306-18C 44°57.45 ' 125o21.96 ' 2376 8306-21C 44°56.10 ' 125°16.15 ' 1550 C. Oregon underthrust area

8306-24C 44°39.78 ' 125o19.07 ' 2420 8306-26C 44°39.00 ' 125°20.50 ' 2795 8306-27C 44o39.00 ' 125o19.66 ' 2623 8408-4 44o40.00 ' 125o21.60 ' 2860 8408-7 44o39.50 ' 125o19.70 ' 2550 8408-10 44o40.00 ' 125°17.10 ' 2180 8408-11 44o39.90 ' 125o21.50 ' 2846

abyssal plain abyssal plain

sediment pond behind seaward ridge mound in sediment pond

moat behind seaward ridge

sea mount on abyssal plain abyssal plain

top of seaward ridge

sediment pond behind seaward ridge

top of seaward ridge flank of second ridge flank of second ridge abyssal plain abyssal plain top of seaward ridge top of third ridge

foot of seaward ridge

second ledge on seaward ridge first ledge on seaward ridge abyssal plain

first ledge on seaward ridge sediment pond behind seaward ridge deformed abyssal plain

Pore waters were e x t r a c t e d from sediments by pressure filtration at the t e m p e r a t u r e of the bottom w a t e r (2°C) with a t e c h n i q u e of Hart- m a n n et al. (1973). Dissolved ammonium was d e t e r m i n e d by s t a n d a r d methods for n u t r i e n t analyses in s e a w a t e r (Grasshoff, 1976) and calcium by flame atomic absorption spectro- photometry. Dissolved ECO2 in pore w a t e r was acidified, helium-stripped and m e a s u r e d by on- line gas c h r o m a t o g r a p h y with a t h e r m a l con- d u c t i v i t y detector. Availability of pH data only at two core stations (8306-2C and 8306-3C) r e s t r i c t e d the estimation of c a r b o n a t e satura- tion state to these stations. C a r b o n a t e c a r b o n c o n t e n t was m e a s u r e d by H a P O J d i c h r o m a t e - LECO t e c h n i q u e described by Weliky et al.

(1983).

P o r e w a t e r chemistry data of the dissolved N H 4 ÷ and Ca 2 ÷ were used for the estimation of v e r t i c a l a d v e c t i o n rates. Dissolved ZCO 2, pH, and Ca 2+ were used for the c a l c u l a t i o n of calcium c a r b o n a t e s a t u r a t i o n state of pore waters following the r e l a t i o n s given by Skir- row (1975).

R e s u l t s

Pore w a t e r Ca, C a C O a and w a t e r c o n t e n t s Dissolved i n t e r s t i t i a l Ca 2 + shows contrast- ing c o n c e n t r a t i o n profiles with depth across all t r a n s e c t s of the a c c r e t i o n a r y complex (Fig.3):

Ca 2+ decreases significantly with depth, to a minimum of 50% of bottom w a t e r Ca 2 ÷ over only 1.5 m sediment thickness, in the coring stations located landward of the deformation f r o n t (crest and flanks of d e f o r m a t i o n a l ridges) while Ca 2 + remains c o n s t a n t or increases with depth, to a maximum of 5% of oceanic bottom w a t e r Ca 2 + over the same depth, in all stations from the abyssal plain and intra-basin sedi- ments. The Ca-content in the pore waters of the sea m o u n t located in the abyssal plain shows the strongest decrease.

T h o u g h depletion of i n t e r s t i t i a l Ca 2+ in c o n t i n e n t a l margin sediments has commonly been r e l a t e d to c a r b o n a t e mineral precipita- tion ( W a t e r m a n n et al., 1972; Suess et al., 1982;

Stein and Smith, 1986), such a drastic depletion over only 1.5 m sediment thickness has, to o u r knowledge, n o t been r e p o r t e d before. The decrease in i n t e r s t i t i a l Ca 2+ in the deformed sediments of the a c c r e t i o n a r y complex sug- gests significant c a r b o n a t e m i n e r a l precipita- tion and cementation.

No c h a n g e or e n r i c h m e n t of i n t e r s t i t i a l Ca 2 + in the undeformed abyssal plain and intra- basin sediments indicates no significant c a r b o n a t e mineral p r e c i p i t a t i o n there.

M e a s u r e m e n t s of CaCO3-content in sediments from selected stations show t h a t more CaCO a is c o n t a i n e d in the deformed sediments (2-5%) t h a n is in the undeformed sediments (0.1-1%) (Fig.4). A l t h o u g h we did n o t a t t e m p t to verify in detail the n a t u r e of these carbonates, we have obtained stable carbon isotope measure- ments of -1.98%o and -3.07%0 PDB from two bulk sediment samples of the deformation ridge (Station 8306-21C with subsurface sample intervals, 15-20 cm and 30-35 cm) with a total calcium c a r b o n a t e c o n t e n t of 2.5 and 2.6 wt-%, respectively. These data indicate the presence of some small p o r t i o n of methane-derived a u t h i g e n i c CaO 3 mixed with isotopically heav- ier biogenic CaCO 3. Samples obtained with D S R V A l v i n over the vent sites c o n t a i n more CaCO 3, r a n g i n g from 25 to 90%, and g r e a t e r a m o u n t s of methane-derived c a r b o n a t e s with 513C values of between -35%0 and -67%o PDB (Ritger et al., 1987).

The a p p a r e n t dependence of the i n t e r s t i t i a l Ca 2+ on sediment d e f o r m a t i o n is f u r t h e r confirmed by the r e l a t i o n s h i p between the depth gradients of i n t e r s t i t i a l Ca 2 + and w a t e r content. Figure 5 clearly shows t h a t the depletion of i n t e r s t i t i a l Ca 2+ coincides with the r e d u c t i o n in w a t e r content, whereas the e n r i c h m e n t of i n t e r s t i t i a l Ca 2 ÷ is observed in sediments whose w a t e r c o n t e n t changes little.

A l t h o u g h expulsion of pore fluid, due to sediment deformation, is the d o m i n a n t cause for the porosity r e d u c t i o n (Carson and Berglund, 1986), in situ c a r b o n a t e mineral p r e c i p i t a t i o n and the r e s u l t i n g c e m e n t a t i o n of sediments may in p a r t cause porosity r e d u c t i o n as well, especially in near-surface sediments.

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Oregon underthrust

Fig.3. Depth profiles of interstitial Ca concentration for the cores from the northern (a) and the southern (b) profiles of the Washington overthrust, from the Oregon overthrust (c), and the Oregon underthrust (d). In general, interstitial Ca decreases with depth at core stations from deformed sediments and increases or remains constant at core stations from undeformed sediments; refer to Table I for further station identification.

A l l of t h e s e c o n t r a s t i n g r e l a t i o n s h i p s o b s e r v e d b e t w e e n d e f o r m e d , a c c r e t e d s e d i m e n t s a n d u n d e f o r m e d a b y s s a l p l a i n s e d i m e n t s ; i.e. i n t e r - s t i t i a l C a - p r o f i l e s , C a C O 3 - c o n t e n t s , a n d w a t e r - c o n t e n t s , s u g g e s t t o u s c o n s i s t e n t l y t h a t t h e d e p l e t i o n o f i n t e r s t i t i a l C a 2 ÷ d i r e c t l y r e f l e c t s a u t h i g e n i c c a r b o n a t e m i n e r a l f o r m a t i o n .

M e c h a n i s m o f c a r b o n a t e p r e c i p i t a t i o n

T o s i m p l i f y o u r c o n s i d e r a t i o n o n t h e m e c h - a n i s m o f c a r b o n a t e m i n e r a l p r e c i p i t a t i o n , we w i l l o n l y c o n s i d e r C a 2 ÷ a n d CO3 2 a s t h e m a i n c o m p o n e n t s of t h e p r e c i p i t a t e s a l t h o u g h dolo- m i t e a n d m a g n e s i a n c a l c i t e s a r e c o m m o n

lOO

I - Q .

200

300

CaCO= Content (%)

1 2 3

• • ;2C •o ~ FI

• ,It =.2~e

• ll; [ ]

• • [] []

• [] • 26C

@ • • [ ] [ ]

27C

[] [ ]

17C []

[] 24C

[]

Undeformed 12C

Sediments

I Deformed Sediments 4

i

Fig.4. Downcore profiles of percent CaCO 3 in sediments from different tectonic settings. High CaCO3:content in deformed sediments and low CaCO3-content in undeformed sediments correspond, respectively, to an interstitial Ca- decrease and a Ca-increase.

0.005

0.000

~" -0.005

o

E - 0 . 0 1 0

v

0 - 0 . 0 1 5

-0.020

W8306-9C

0 -IOC -3C - i 2 C

-17C 0

%~] -IIC

-5C Undeformed Sediments

-18C

×

-14C +

-26C + X

-27C - 2 4 C X

[ -4c

I I

0.10

Deformed Sediments

-0.02~.~) 0 ^' 0.20 ^{, , 0.30}

WATER CONTENT (% wt / cm)

Fig.5. Water content- and interstitial Ca-gradients. Core stations where interstitial Ca decreases with depth show greater reduction in water c o n t e n t t h a n stations with interstitial Ca-enrichment.

(Russell et al., 1967; Scamman, 1981; Ritger et al., 1987). In order to identify the mechanism of carbonate mineral precipitation, we have to address briefly the origin of the two main

reactants. Calcium, ultimately of crustal origin, is transported into the accreted sedi- ments by upward advecting fluids. The CO32-- ion, which may be provided by oxidation of migrated methane, is equally brought to the near-surface depths by venting fluids. We pro- pose that oxidation of the migrated methane and the decrease in carbonate solubility (or conversely an increase in calcium carbonate saturation) due to the release of excess pore pressure play an important role in the forma- tion of carbonate mineral precipitates.

Migration and oxidation of methane

During biochemical degradation of organic matter in sediments, ZCO2 is generated by aerobic oxidation near the s e d i m e n t - w a t e r interface followed by anaerobic oxidation below, generally by sulfate reduction (Clay- pool, 1974; Berner, 1980). When sulfate is exhausted from the interstitial water, the process of biogenic methane formation takes place by CO2-reducing microbes, which utilize the pool of dissolved bicarbonate generated during sulfate reduction.

In the process of microbial methane forma- tion the residual bicarbonate is progressively enriched in 13C ( 5 1 3 C = - 4 0 to ÷10%o) and biogenic methane is depleted in 13C (&13C= - 9 0 to -70%o) (Claypool and Kvenvol- den, 1983). An increase in the pH due to removal of ZCO2 from pore water during biogenic methane production may cause pre- cipitation of carbonate in anoxic sediments.

The carbonate originated by this methano- genic process is thus enriched in ~3C as it is formed from ~3C-enriched bicarbonate (Clay- pool and Kaplan, 1974). However, carbonates with anomalously light 5 ~ 3 C values (513C = - 3 5 to -67%o) in the deformed sedi- ments (Schroeder et al., 1987; Ritger et al., 1987) indicate that they could not have origi- nated from this diagenetic process. Instead oxidation of biogenic methane appears to be the only process by which the strongly 13C- depleted carbonate ions could have been de- rived. The methane must have been trans-

ported t o w a r d the surface and be oxidized in order to precipitate c a r b o n a t e in near-surface sediments. The oxidation process and genera- tion of highly l SC-depleted b i c a r b o n a t e ions in the Oregon u n d e r t h r u s t pore fluids are docu- mented by Suess and W h i t i c a r (this issue).

A parcel of v e n t i n g pore fluid, driven by positive excess pore pressure, c o n t i n u a l l y ad- justs to the total h y d r o s t a t i c pressure on its path upward t h r o u g h the sediment column by releasing its excess pore pressure. In the case of m e t h a n e s a t u r a t i o n this release of pore pressure results in a c o r r e s p o n d i n g decrease in the a m o u n t of m e t h a n e dissolved in the pore fluid. M e t h a n e may thus escape t h r o u g h ebulli- tion and a c c e l e r a t e t r a n s p o r t from depth to the surface. As m e t h a n e r e a c h e s the near-surface sediments, it is oxidized by oxygen or sulfate- consuming microbes, providing an a b u n d a n t source of ~13C.depleted c a r b o n a t e ions ( 5 1 3 C = - 3 0 to -70%0) (Claypool, 1974) from which a n o m a l o u s l y 513C-depleted a u t h i g e n i c c a r b o n a t e minerals are formed. O c c u r r e n c e of pyrite in c a r b o n a t e s suggests t h a t a n a e r o b i c oxidation of m e t h a n e by sulfate reducing microbes is the more d o m i n a n t process in the Oregon s u b d u c t i o n zone t h a n aerobic oxida- tion (Ritger et al., 1987).

Anomalously light c a r b o n a t e minerals were also observed in Oregon c o n t i n e n t a l shelf (DSDP Leg 18 site 176) at 2 2 m subsurface (Claypool, 1974) and slope (Russell et al., 1967) sediments. M i g r a t i o n of m e t h a n e from depth to this horizon was proposed by Claypool (1974) to explain these 613C-depleted carbonates. Fluid advection from depth to present near-surface sediments was also proposed from the 11C- enriched dissolved ECO 2 in shallow pore fluids of the a c c r e t i o n a r y complex (Suess and Whiti- car, this issue). These a u t h o r s predict t h a t 30%

of the dissolved ZCO2 resulted from m e t h a n e oxidation. These estimates are consistent with fluid-induced c a r b o n a t e lithification in the deformed sediments.

The effect of pressure on carbonate solubility In the case of fluid filled porous media, the effective stress is the weight of the overburden,

105

which includes the weight of the w a t e r per unit area, minus the pore pressure (Rubey and Hubbert, 1959). Thus pore fluid v e n t i n g occurs when the pore pressure exceeds the weight of the h y d r o s t a t i c pressure and continues until the pore pressure equals the weight of the overlying water. This excess pore pressure is likely to be g e n e r a t e d in subduction zones where compressional stress is exerted by con- verging plates (Von Huene, 1985). During upward m i g r a t i o n of pore fluid this excess pore pressure must be released. The release may be one potential mechanism by which c a r b o n a t e minerals p r e c i p i t a t e in near-surface sediments of the deformed a c c r e t i o n a r y sediments, as the c a r b o n a t e mineral solubility is p r o p o r t i o n a l to pressure. The release of excess pore pressure in near-surface sediments is in t u r n p r o p o r t i o n a l to the rate of pore fluid venting. The faster the pore fluids escape, the more likely c a r b o n a t e minerals precipitate if p r e c i p i t a t i o n is only governed by the effect of pressure on c a r b o n a t e solubility.

The excess pore pressure gradient may be r e l a t e d to the r a t e of pore fluid v e n t i n g by Darcy's law (Bear, 1972):

u = K P / ~ z or = k P / n p g (1)

where u is the rate of pore fluid flow, K is the permeability of the porous medium, P is the excess pore pressure over any subsurface depth, z, k are the h y d r a u l i c conductivity, ~ is the viscosity of pore fluid, n is the porosity of sediments, p is the density of pore fluid and g is the g r a v i t a t i o n a l acceleration. From Eq.(1) the v e n t i n g rate and the excess pore pressure gradients are related by the expression:

P ( a t m ) / z ( m ) = 3.3u(m y 1) (2)

if k = 7 x l 0 - 1 ° m s 1, n=0.7, p = l . 0 5 g / c m 3, and g = 9.8 m/s 2 are fixed.

Suppose a p a r c e l of pore fluid flows from a subsurface depth of z to the sea floor at a w a t e r depth of 2.1km (which is the approximate w a t e r depth of the v e n t site where a u t h i g e n i c c a r b o n a t e chimneys and b e n t h i c organisms were found, Kulm et al., 1986), h y d r o s t a t i c pressure at this w a t e r depth is a p p r o x i m a t e l y

equivalent to 210atm (10m water column 1 atm). E q u a t i o n (2) predicts t h a t for a venting rate of 10my -1, the total pressure at 6 m subsurface would have to be 408 atm. This represents the sum of 198 atm from excess pore pressure plus 210 atm from hydrostatic pressure.

The 6 m thickness for the subsurface l a y e r where p r e c i p i t a t i o n might t a k e place, is arbi- trary. Based on porosity reduction, however, S c a m m a n (1981) assumed t h a t c e m e n t a t i o n would begin at shallow burial depth, probably within several meters of the s e d i m e n t - w a t e r interface. CaCO 3 p r e c i p i t a t i o n below this sub- surface depth is u n l i k e l y because m e t h a n e oxidation, by which c a r b o n a t e ions is provided for CaCO 3 precipitation, would n o t o c c u r below this depth due to the e x h a u s t i o n of i n t e r s t i t i a l sulfate available for m e t h a n e oxi- dation. If c a r b o n a t e mineral p r e c i p i t a t i o n was e n t i r e l y c o n t r o l l e d by t h e r m o d y n a m i c equili- b r a t i o n of pure calcite, the a m o u n t of CaCO3 which will be p r e c i p i t a t e d at a v e n t i n g r a t e of 10 m y - 1 would be e q u i v a l e n t to the difference in the equilibrium C a - c o n c e n t r a t i o n s between 408 atm and 210 atm, i.e., between the sediment surface and 6 m subsurface below sea floor.

Details of the t h e r m o d y n a m i c equilibrium model, q u a n t i f y i n g CaCO 3 p r e c i p i t a t i o n by the release of pore pressure, are described in the Appendix. The result of this t h e r m o d y n a m i c equilibrium model for CaCO3-precipitation shows the following l i n e a r r e l a t i o n s h i p be- tween ~/o CaCO3 and excess pore pressure over the 6 m subsurface i n t e r v a l (Fig.6);

Log(% CaCO3) = - 4.6564 + 1.0262 Log(P) (3)

o r

= - 4.6564 + 1.0262 Log(3.3 u z) (3)' This linear r e l a t i o n s h i p indicates t h a t the g r e a t e r the excess pore pressure (i.e., faster v e n t i n g rate) the g r e a t e r the CaCO3-precipita- tion, because the difference in the equilibrium C a - c o n c e n t r a t i o n s between the subsurface and the sediment surface will be e n h a n c e d by increasing pressure difference. The 6 m-thick- ness is only for i l l u s t r a t i o n and will be

-2 -3-

"~ -4 O L)

-5

..J -7

y = - 4.6564 ÷ 1.0262x R = 1.00

- 8 j ~ , ~ • ,

- 3 - 2 - 1 0 1 2

L o g P ( a t m )

Fig.6. T h e o r e t i c a l r e l a t i o n s h i p between the release of excess pore p r e s s u r e a n d the a m o u n t of potential CaCO 3 precipitation (in weight %) r e s p o n d i n g to t h e pore p r e s s u r e release. T h e r m o d y n a m i c equilibrium was as- s u m e d for pure calcite precipitation. T h e symbols []

r e p r e s e n t the r e l a t i o n s h i p between excess pore p r e s s u r e a n d p r e c i p i t a t i o n of CaCO 3 as predicted by e q u a t i o n s developed in t h e Appendix. W e t bulk density a n d porosity fraction of s e d i m e n t are a s s u m e d as 1.3 g/cm 3 and 0.7, respectively.

extended later in this section to a more appropriate subsurface depth in order to estab- lish a thermodynamic equilibrium model t h a t best takes into a c c o u n t the effect of the release of excess pore pressure on CaCO 3 precipitation.

The a m o u n t s of CaCO 3 formed from fluid v e n t i n g from only a 6-m thick sediment l a y e r are very small. Figure 6 shows t h a t the a m o u n t of CaCO 3 p r e c i p i t a t e d from the release of pore pressure can n o t a c c o u n t for the measured CaCO 3 c o n t e n t in some of the sediments even at the e x t r e m e v e n t i n g rate of 10 m y - 1. This is due to setting the subsurface base at 6 m which implies t h a t v e n t i n g pore fluids originate from this depth. The d i s a g r e e m e n t between predicted and measured a m o u n t s of CaCO3 precipitation, therefore, suggests a need for extension of the subsurface i n t e r v a l from which the fluids are generated. The r e l a t i o n of Eq.(3) will still hold at any depth below the 6 m subsurface as the model assumes an exclusive t h e r m o d y n a m i c c o n t r o l on c a r b o n a t e precipi- t a t i o n in response to the release of excess pore pressure.

The t h e r m o d y n a m i c equilibrium model for calcite p r e c i p i t a t i o n in case of the extended depth below 6 m is illustrated in Fig.7. This

Chimney formation ?

0 ~ u=lO m/yr

u=l m/yr 0 0.1

0 u=0.1 m/yr

~'-2

- 3

p/z=3.3 u

-4 i I i

0 1000 2000 3000 4000

Origin of fluid migration (m)

Fig.7. Theoretical relationship between CaCO3-content (in weight %) of lithified sediments and the proposed depths for the origin of pore fluids, at varying advection rates.

Given this relationship predicts carbonate precipitation in low limits because of the restrictive assumption of the thermodynamic control on the precipitation, CaCO 3- content of 1% may be assumed as significant and could be comparable to the formation of carbonate chimneys in real situation. The relationship then predicts that chimney formation is only likely when fluid venting rates are in the range of 10m yr ~ with the origin of fluid migration greater than 1 km subsurface.

d i a g r a m is o b t a i n e d by p l o t t i n g t h e t w o p a r a m e t e r s in Eq.(3), z a n d L o g (% CaCO3), on x- a n d y-axes. T h e r e l a t i o n s h i p b e t w e e n t h e s e t w o p a r a m e t e r s is i l l u s t r a t e d w i t h v a r y i n g v a l u e s of p o r e fluid a d v e c t i o n , u. It s h o w s t h a t s i g n i f i c a n t C a C O 3 p r e c i p i t a t i o n (>>1%) is likely at v e n t i n g r a t e s a n d s o u r c e d e p t h s of fluids at a r o u n d 1 0 m y 1 a n d 1 km, respec- tively. If o n e t a k e s i n t o a c c o u n t t h e r e s t r i c t i v e a s s u m p t i o n t h a t c a r b o n a t e p r e c i p i t a t i o n is e n t i r e l y c o n t r o l l e d by e q u i l i b r a t i o n of p u r e c a l c i t e in r e s p o n s e to t h e r e l e a s e of excess p o r e pressure, t h e n t h e c o n d i t i o n s specified in Fig.7 are l o w e r limits of c a r b o n a t e m i n e r a l p r e c i p i t a - tion. It is i n t e r e s t i n g to n o t e t h a t o n l y a fairly fast v e n t i n g r a t e of 10 m y 1 a n d a s o u r c e d e p t h for pore fluids of 1 k m m a y g e n e r a t e c h i m n e y s . S l o w e r r a t e s p r o v i d e C a C O 3 sufficient o n l y for c e m e n t a t i o n or even less as in t h e d i s s e m i n a t e d r h o m b s of CaCO3 of t h e O r e g o n m a r g i n sedi- m e n t s ( R i t g e r et al., 1987).

The i n t e r s t i t i a l c a l c i u m profile o b t a i n e d from D S D P site 174A in t h e C a s c a d i a a b y s s a l plain s h o w s a C a - m i n i m u m ( < 10 mM) at sub- b o t t o m d e p t h s of 50-350 m, b u t a d r a s t i c Ca-

107 e n r i c h m e n t , m u c h h i g h e r t h a n n o r m a l s e a w a t e r Ca 2 ÷ (>>10mM), at d e p t h g r e a t e r t h a n 350 m ( W a t e r m a n n et al., 1972; Claypool, 1974). Thus, v e n t i n g of p o r e fluids from t h e d e f o r m a t i o n of t h e s e s e d i m e n t s m u s t m i g r a t e v e r t i c a l l y from at l e a s t a d e p t h of 350 m a n d c a r r y c r u s t a l Ca 2 ÷ u p w a r d s in o r d e r to precipi- t a t e c a r b o n a t e in n e a r - s u r f a c e sediments.

T h e t h e r m o d y n a m i c e q u i l i b r i u m model for c a l c i t e s u g g e s t s t h a t p r e s s u r e c o n t r o l on car- b o n a t e s o l u b i l i t y w o u l d be a s i g n i f i c a n t mech- a n i s m for t h e f o r m a t i o n of the c a r b o n a t e c h i m n e y s h o u l d t h e v e n t i n g r a t e be in t h e r a n g e o f 10 m y 1 or g r e a t e r . A m i n i m u m flow r a t e of 277 ml m ~ d a y 1 was d i r e c t l y m e a s u r e d a b o v e a v e n t site in t h e d e f o r m a t i o n f r o n t of t h e O r e g o n a c c r e t i o n a r y complex (Suess et al., 1987). This r a t e is e q u i v a l e n t to o n l y 20 5 0 c m y I of v e r t i c a l m i g r a t i o n de- p e n d i n g on t h e p o r o s i t y . On the o t h e r h a n d , a v e n t i n g r a t e as h i g h as 2 0 0 c m y t was e s t i m a t e d from a s y s t e m a t i c c h a n g e in meth- a n e c o n c e n t r a t i o n s in timed w a t e r samples t h a t were o b t a i n e d by d e p l o y i n g a b e n t h i c c h a m b e r a b o v e t h e v e n t site (E. Suess, pers.

o b s e r v a t i o n ) .

E v i d e n t l y , t h e h i g h r e q u i r e d flow r a t e of 10 my ~ by t h e t h e r m o d y n a m i c model is j u s t b a r e l y a p p r o a c h e d in a n y of t h e m e a s u r e d flow rates. C o n s i d e r i n g t h e a s s u m p t i o n of t h e model t h a t c a r b o n a t e p r e c i p i t a t i o n be e n t i r e l y con- trolled by s o l u b i l i t y of p u r e c a l c i t e a n d t h e release of excess pore p r e s s u r e a s s o c i a t e d with pore fluid v e n t i n g , a n y closer a g r e e m e n t w o u l d be f o r t u i t o u s . W h a t is significant, h o w e v e r , is t h a t b o t h t h e m e a s u r e d a n d t h e model-derived flow r a t e s are c o n s i d e r a b l y faster t h a n a n y r e p o r t e d v e n t i n g r a t e s in o t h e r a c c r e t i o n a r y complexes, w h i c h are on t h e o r d e r of 1 cm y - at m o s t ( B o u l e g u e et al., 1987; Reck, 1987).

R i t g e r et al. (1987) p o i n t o u t t h a t n o r m a l l y u n d e r s a t u r a t e d b o t t o m waters, (with r e s p e c t to calcite), b a t h e t h e e n t i r e C a s c a d i a a c c r e t i o n - a r y complex. T h e r e f o r e , c a r b o n a t e m i n e r a l p r e c i p i t a t i o n r e q u i r e s s u p e r s a t u r a t i o n gener- a t e d from local c o n d i t i o n s w i t h i n t h e s e d i m e n t c o l u m n a n d n o t a t t h e s e d i m e n t w a t e r inter- face. C a l c u l a t i o n s of t h e i o n - c o n c e n t r a t i o n -

product (ICP), [Ca 2+] x [CO32-], using pH and dissolved ECO2 data in pore waters of the area indicate s u p e r s a t u r a t i o n of the pore waters with respect to calcite (Fig.8).

However, pore waters not only from the de- formed sediment but also from the undeformed abyssal plain sediment are s u p e r s a t u r a t e d with respect to calcite (Fig.8). Nonetheless, the degree of s u p e r s a t u r a t i o n in the deformed sediments is significantly higher t h a n in the undeformed abyssal plain sediments. Admitt- ing a large margin of error accumulated in these calculations and analyses, this system- atic difference in the degree of s a t u r a t i o n may suggest u n d e r s a t u r a t i o n of the pore waters from the undeformed abyssal plain with re- spect to calcite. A n o t h e r possibility is t h a t the s a t u r a t i o n in these pore fluids is controlled not by pure calcite but by a mixed carbonate mineral phase, whose solubility is higher t h a n t h a t of calcite.

S u p e r s a t u r a t i o n with respect to calcite in

.,c

D e g r e e o f Saturation

0 2 4 6 8 1 0 1 2

0 , i . i , i . 1 , i .

• []

1 0 0

Deformation front [ ] [ ]

[ ]

•D #

• []

Abyssal plain

2 0 0

Fig.8. Degree of s a t u r a t i o n of the pore waters from both the deformed and the undeformed abyssal plains with respect to calcite. Pore waters from b o t h the deformed (8306-2C) and the undeformed (8306-3C) abyssal plains are strongly supersaturated with respect to calcite. However, the pore waters from the deformed abyssal plain are systematically more s a t u r a t e d t h a n those from the undeformed abyssal plain. Skirrow's (1975) relations for concentrations of carbon species were used to calculate ICP, [Ca2+]×

[COa2-], of pore fluids. To finally o b t a i n the degree of s a t u r a t i o n the calculated ICPs were divided by K,p of CaCO 3 a t 0°C and 1 atm, which were the approximate conditions during pore water sampling from squeezing.

sediments devoid of calcium carbonate is quite common. An apparent increase in CaCO a- s a t u r a t i o n was also reported at depths of all sediments from the North American continen- tal margin off Nova Scotia and from the central Pacific (Sayles, 1987). This a u t h o r concluded t h a t the ICP increases cannot be adequately described by a single thermody- namic constant. Nevertheless, we t h i n k t h a t the relative difference in calcite s a t u r a t i o n between pore waters from deformed and unde- formed sediments is significant and is a n o t h e r indication for the enhanced calcium carbonate precipitation in sediments affected by fluid venting.

E v i d e n c e f o r v e n t i n g f r o m i n t e r s t i t i a l w a t e r

In addition to the first direct measurement of pore fluid venting rates at the vent site in the Cascadia accretionary complex (Suess et al., 1987), downcore c o n c e n t r a t i o n profiles of chemical species, such as Ca 2 + and NH 4 + can be used to ascertain vertical flow rates (Maris et al., 1984 and Bender et al., 1985). Rates of pore fluid venting were determined by applying one-dimensional diffusion-advection-reaction models to the downcore concentration profiles of interstitial Ca 2+ and NH4 + obtained from selected coring sites across the Cascadia accre- t i o n a r y complex. These stations are: 8408-4 from the abyssal plain of the Oregon u n d e r t h r u s t area; 8408-7, 8306-24C, and 8306- 26C from the flank of the seaward deformation ridge (also of the Oregon u n d e r t h r u s t area), and 8306-2C from the top of the sea m o u n t on the abyssal plain off Washington.

The model equation applied to interstitial Ca 2 + profiles may be written as

D d 2 C / d z 2 - u d C / d z - k C = O (4) where D is the diffusion coefficient of Ca 2 *, C is the c o n c e n t r a t i o n of interstitial Ca 2+, z is the vertical depth positive downward, u is the advection rate of pore fluid, and k is the coefficient of Ca-precipitation. The coordinate axis z is positive downward originating at the

s e d i m e n t - w a t e r i n t e r f a c e . T h e s o l u t i o n of Eq.(4) m a y be w r i t t e n as

C(z) = C1 exp(~lz) + C2 exp(,~2z) (5)

w h e r e ,

u +_ x / u 2 - 4 k D 2 =

2 D

C ( z = O) = C1 + C2, a n d

C ( z = h) = C , e x p ( ~ , h ) + C 2 exp (,tzh).

C ( z = 0 ) a n d C ( z = h ) a r e k n o w n f r o m t h e m e a s u r e d i n t e r s t i t i a l Ca c o n c e n t r a t i o n s a n d D is a s s u m e d as 3 × 10 -6 cm 2 s -1 ( L e r m a n , 1977).

T h e o n l y u n k n o w n t e r m s in Eq.(5) a r e u a n d k.

T h e l i n e a r c o n c e n t r a t i o n profile a t s t a t i o n 8408-4 (abyssal plain) was a s s u m e d to i n d i c a t e t h a t t h e C a - d i s t r i b u t i o n is e x c l u s i v e l y con- t r o l l e d by d i f f u s i o n a n d r e a c t i o n . A t this s t a t i o n , t h e r e f o r e , t h e u - t e r m was r e m o v e d a n d

109

s u b s e q u e n t c u r v e fitting of Eq.(5) to t h e m e a s u r e d Ca :+ profile g a v e a v a l u e for k (the coefficient o f C a - p r e c i p i t a t i o n ) . T h i s k v a l u e was u s e d in t h e o t h e r s t a t i o n s to d e t e r m i n e u b y fitting Eq.(5) to t h e m e a s u r e d Ca-profiles at t h o s e stations.

A d v e c t i o n r a t e s of p o r e fluids d e r i v e d in this m a n n e r f r o m t h e C a - m o d e l i n g r a n g e from 1 cm y - ~ to 6 cm y - 1 u p w a r d , b u t a r e m o s t l y of t h e m a g n i t u d e a r o u n d 1 cm y - 1 (Table II a n d Fig.9). S t a t i o n 8306-2C, h o w e v e r , t h e site of t h e sea m o u n t on t h e W a s h i n g t o n a b y s s a l p l a i n shows t h e f a s t e s t a d v e c t i o n r a t e w i t h 28 cm y - 1. In o r d e r to o b t a i n t h e best c u r v e fit a two- l a y e r m o d e l was a p p l i e d to t h e Ca-profile at this sea m o u n t station. T h i s sea m o u n t is c o n s i d e r e d to be a m u d v o l c a n o (L. D. Kulm, pers. comm.) w h i c h w o u l d r e a d i l y e x p l a i n t h e h i g h a d v e c t i o n r a t e .

Profiles of t h e m e a s u r e d i n t e r s t i t i a l NH4 +

T A B L E II

I n p u t p a r a m e t e r s u s e d for d i f f u s i o n - a d v e c t i o n - r e a c t i o n m o d e l l i n g of Ca- a n d N H 4 - d i s t r i b u t i o n s ; c o m p a r i s o n of c a l c u l a t e d a d v e c t i o n r a t e s

S t a t i o n 8408-4 8408-7 8306-24C 8306-26C 8306-2C

Ca N H 4 Ca NH4 Ca NH4 Ca N H 4 Ca NH4

D (10 -6 cm: s -1) 3 6 3 6 3 6 3 6 3

k (10 -4 y - ' ) 1 1 1 1 1

Jo (~mol/cm3yr) 0.013 0.013 0.013 0.013

a (1/cm) 0.006 0.05 0.01 0.01

C o 445 0 445 0 445 0 445 0 445 a

325

C h (NH4) 1.4 0.1 1.07 1.05

Ch (Ca) 325 420 338 290 325 a

250 Estimated advection rate

u (cm/yr) 0 0 - 6 . 0 - 6 . 7 - 1 . 0 - 0 . 9 - 1 . 5 - 1 . 1 - 0 . 1 a

- 28

6 0.013 0.01 b 0.01 0.02 0 b 0.5 1.2 0.5 b 1.2 1.39

- 0.04 b - 0.07

- 24

aTwo-layer model p a r a m e t e r s for Ca.

bThree-layer model p a r a m e t e r s for N H 4.

0 300

100.

£

200,

300

Ca (mg/I)

350 400 450

• , , ° ! , . . . I . . . .

1 0 0

200

3 0 0

200

Ca (mg/I)

300 400

'8306- I , I e ,

• l u=-I an/yr

/

500

0

100

200

300

Ca (mgJl)

320 340 360 380 400 420 440 460

, I i I i I • I • I • I . 410

0

1 0 0 '

1

200,

300

Ca (rag/I)

420 430 440

I , I 0 . I ,

F o *

u=-6 ~rCyr

450

2O

~.4o

60

80

Ca (rag/I)

200 300 400

0 m u 'd ~;I:i:.~ :; :rio nr~ /

U = ' ~ r:l

J u~-28 cm/yr

5O0

i

Fig.9. A p p l i c a t i o n o f a d i f f u s i o n - a d v e c t i o n - r e a c t i o n m o d e l to i n t e r s t i t i a l Ca-profiles. P o r e w a t e r a d v e c t i o n r a t e (u) w a s o b t a i n e d b y v i s u a l c u r v e f i t t i n g o f t h e m o d e l e q u a t i o n to t h e m e a s u r e d C a - c o n c e n t r a t i o n s . T w o - l a y e r m o d e l s w e r e a p p l i e d to t h e Ca-profile a t t h e c o r e s t a t i o n 8306-2C (sea m o u n t ) to o b t a i n t h e b e s t fit. V a l u e s o f t h e m o d e l p a r a m e t e r s u s e d i n t h e c u r v e f i t t i n g a r e l i s t e d in T a b l e II.