Estuaries
Vol. 23, No. 1, p. 1-9
February 2000
Iron, Sulfur, and Carbon Diagenesis in Sediments of Tomales Bay, California R. M. CHAMBERS1
Department of Biology Fairfield University Fairfield, Connecticut 06430 j. T. HOLLIBAUGH
Department of Marine Science University of Georgia Athens, Georgia 30602 C. S. SNIVELY
Center far Environmental Studies San Francisco ,State University Tiburon, California 94920 J. N. PLANT
Department of Oceanography University of Hawaii Honolulu, Hawaii 96822 ABSTRACT: Analysis o f 3-m s e d i m e n t cores revealed that profiles of carbon (C)~ sulfur (S)~ and iron (Fe) v a r i e d w i t h relative distance f r o m marine and terrestrial s e d i m e n t sources in T o m a l e s Bay, California. Despite relatively high sedimentation rates t h r o u g h o u t the bay (historically 3 - 3 0 m m yr t)~ sulfate reduction of d e p o s i t e d organic matter l e d to free-sulfide accumulation in s e d i m e n t s only at the location farthest f r o m terrestrial r u n o f f , the source of reactive iron. Acid-volatile sulfide concentrations in all s e d i m e n t s ( < 1 0 ixnrol g 1) w e r e low relative to concentrations o f chronfiumreducible sulfide (up to 400 ixnrol g 1 farthest f r o m the reactive iron source). A calculated index of iron availability, u s e d to describe s e d i m e n t resistance to build-up of f r e e sulfide, was l o w e s t at this location. Recent, upward shifts in reactive Fe concentration and in the relative contribution of terrestrial organic carbon ( m e a s u r e d as a shift in 8~aC of bulk s e d i m e n t organic matter) in all cores indicated that erosion a n d transport of s e d i m e n t s f r o m the w a t e r s h e d surr o u n d i n g T o m a l e s Bay increased after E u r o p e a n s e t t l e m e n t in the 1850s.
e n b o r n et al. 1987; Canfield 1989a), the diagenetic reactions involving reactive iron and sulfide must be influenced by s e d i m e n t a t i o n rate and c o m p o sition of inorganic material (Fe a n d S sources) (Morse a n d Cornwell 1987; Canfield 1989b; Morse et al. 1992). Reactive iron species play an i m p o r t a n t role in m e d i a t i n g the fixation or eventual oxidation of sulfides in carbon-rich s e d i m e n t s (Canfield et al. 1992; H a e s e et al. 1997), but source materials for G, S, and Fe in estuaries are quite different. Sulfate is deposited to s e d i m e n t s primarily f r o m m a r i n e waters, iron is deposited f r o m terrestrial sources, a n d c a r b o n is deposited f i o m b o t h m a r i n e a n d terrestrial sources. Because of these different sources, analyses of C, S, a n d Fe in estuarine sediments theoretically can be used to identify contributions of terrestrial r u n o f f as a source of iron a n d c a r b o n to
Introduction
T h e diagenetic reactions of c a r b o n (C), sulfur (S), and iron (Fe) are linked closely in estuarine sediments, primarily t h r o u g h pathways of anaerobic respiration. O r g a n i c c a r b o n is oxidized to carb o n dioxide by sulfate-reducing bacteria, generating h y d r o g e n sulfide. T h e p r o d u c e d sulfide can be re-oxidized to sulfate or react with different species of iron to yield iron-sulfide minerals. Since the relative i m p o r t a n c e of sulfate r e d u c t i o n to estuarine s e d i m e n t m e t a b o l i s m is influenced by the rate of s e d i m e n t burial a n d the c o n c e n t r a t i o n and reactivity of organic material deposited (C source) (Goldh a b e r a n d Kaplan 1975; C h a n t o n et al. 1987; Ed1 C o r r e s p o n d i n g a u t h o r ; tcle: 203/254-4000, ext. 2543; fax: 208/254-4253; e-mail:
[email protected]. 9 2000 Estuarine Research Federation
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R.M. Chambers et al.
sediment metabolism. If the relative deposition of terrestrial and m a r i n e materials has c h a n g e d with time, these analyses may d o c u m e n t the history of land-use c h a n g e in the s u r r o u n d i n g watershed (Ittekkot 1988; Bird et al. 1995). While organic c a r b o n often is used as a tracer of t e r r e s t r i a l i n p u t to e s t u a r i n e e n v i r o n m e n t s (Hedges and Van G r e e n 1982), few studies have considered land-use c h a n g e as a significant factor influencing the distribution of inorganic S and Fe in estuaries (e.g., H o w a r t h et al. 1991). Cornwell and S a m p o u (1995) did n o t observe any consistent trends in C, S, and Fe distributions in r e c e n t estuarine sediments that could be traced to changing sediment deposition rates, a l t h o u g h they were able to detect spatial differences in Fe-S mineral formarion based on the C source. Studies d o c u m e n t ing geological differences a m o n g depositional env i r o n m e n t s as r e c o r d e d in s e d i m e n t a r y rock (e.g., Bein et al. 1990) address the changes in C, S, and Fe distributions at a time scale too long to identify patterns associated with land-use change. Studies of sediment C, S, and Fe have b e e n completed for many estuaries on the East Coast of N o r t h A m e r i c a ( G o l d h a b e r et al. 1977; C h a n t o n et al. 1987; Fallon 1987; Hines et al. 1991; Cornwell and S a m p o u 1995; G a g n o n et al. 1995) and in E u r o p e (Leventhai 198S; S ~ r e n s e n a n d J ~ r g e n s e n 1987; J ~ r g e n s e n et al. 1990; Canfield et al. 1998; T h a m drup et al. 1994). O n the West Coast of N o r t h America, research is extensive on continental shelf sediments but generally is lacking for estuarine s e d i m e n t s (Skyring 1987; g r a n d e s a n d Devol
1997).
T h e objective of this study was to d e t e r m i n e the relative i m p o r t a n c e of m a r i n e and terrestrial sedim e n t sources to the diagenesis of carbon, sulfur, and iron in a West Coast estuary. Specifically, we examined the distribution of C, S, and Fe in sedim e n t cores from Tomales Bay, California, where sedimentation rates are high and sediment composition was hypothesized to vary along a linear gradient of source materials (marine and terrestrial). We hypothesized that temporal gradients of sediment composition would r e c o r d the history of land-use change since E u r o p e a n settlement of California in the middle of the 19th century. Methods
T h e study site was Tomales Bay, California, a small, n a r r o w estuary f o r m e d at the j u n c t i o n of the N o r t h A m e r i c a n and Pacific tectonic plates (Fig. 1). Hydrographically, the bay is separated into an outer portion, which exchanges water with the coastal ocean every tidal cycle, and an i n n e r portion, which does n o t (Smith et al. 1987). T h e principal tributary, Lagunitas Creek, flows year.round,
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drains 80% of the watershed s u r r o u n d i n g the inner p o r t i o n of the bay, and is the major terrestrial source of sediments (Smith et al. 1991). Based on the tributary's location at the head of this linearshaped bay, we expected a g r a d i e n t in sediment deposition would occur along the longitudinal axis of the bay. Most r u n o f f occurs during the rainy season, N o v e m b e r - A p r i l . gaywide sedimentation rates have been variable over the last 140 yr, averaging S m m yr -1 with some extreme rates up to SO m m yr -1 (Rooney 1995). We selected three stations for s e d i m e n t coring that were equally spaced along the longitudinal axis of the i n n e r bay, at different distances from terrestrial and marine s e d i m e n t sources (Fig. 1). Cores were collected in March and S e p t e m b e r 1998 using a diver.operated piston corer (Sansone et al. 1994). A PVC derrick and a cable maintained the piston at the sediment-water interface, and an 8.9-cm diam polycarbonate core liner was p u s h e d into the sediment to an a p p r o x i m a t e d e p t h of S m. A water jet was used to dislodge s e d i m e n t surr o u n d i n g the core during its retrieval. Expansion plugs were placed in the top and b o t t o m of each core for their transport to the processing laboratory. S e d i m e n t cores were extruded manually, and subcores were collected at 20-cm intervals for sep-
Sediment Diagenesis of Tomales Bay
arate analyses. Porosity and dry bulk density were d e t e r m i n e d on known volumes of s e d i m e n t by wet weighing and again after drying at 100~ Grab samples of surface sediments for stable isotopic carbon analysis were collected 8 kin, 10 kin, 12 kin, 14 kin, 16 kin, and 18 km from the m o u t h of the bay in N o v e m b e r 1999. CARBON ANALYSES T h e p e r c e n t organic matter of dried, weighed sediment samples was d e t e r m i n e d as weight loss on ignition after ashing for 6 h at 450~ Organic carb o n concentrations and bulk isotopic compositions of surface and deep sediments were d e t e r m i n e d (in triplicate) by oxidizing ground, decalcified sedi m e n t s with C u O in e v a c u a t e d q u a r t z tubes. Evolved carbon dioxide was distilled cryogenically; isotopic composition was d e t e r m i n e d using either a Finnigan MAT 252 or a Finnigan Delta S isotopic ratio m o n i t o r i n g mass spectrometer. All isotopic compositions were r e p o r t e d in standard 8-notation relative to the PDB standard. Analytical precision of samples, expressed as a coefficient of variation a r o u n d sample means, always varied less than 9%. SULFUR ANALYSES
P o r e w a t e r f r o m e a c h d e p t h was e x t r a c t e d t h r o u g h a sediment squeezer ( R e e b u r g h 1967) using pressurized nitrogen (60 psi), and dissolved sulfate concentrations were d e t e r m i n e d on 0.2 loomfiltered samples by weighing sulfates precipitated with barium. Dissolved sulfide concentrations were d e t e r m i n e d immediately u p o n extraction using the m e t h o d of Cline (1969). Sulfate r e d u c t i o n rates in the sediments were measured using the radioisotope injection technique (Jergensen 1978). Single syringe cores from each s e d i m e n t d e p t h were injected with labeled sSS-sulfate and incubated for 94 h at the a m b i e n t s e d i m e n t temperature (~12~ After terminating the incubation by freezing, r e d u c e d inorganic sulfur c o m p o u n d s were extracted using a single-step p r o c e d u r e (Fossing and J o r g e n s e n 1989), and r e d u c t i o n of labeled sulfate was detected by scintillation counting. T h e ratio of label i n c o r p o r a t i o n to label injection was used to calculate the rate of sulfate r e d u c t i o n in the sediments (Chambers et al. 1994). Reduced inorganic sulfur in sediments [acid-volatile sulfur (AVS), iron monosulfide (FeS), and c h r o m i u m - r e d u c i b l e sulfur (CRS, assumed to be pyrite, FeS2) ] was d e t e r m i n e d on split replicates of subcores at each depth using a two-step reaction p r o c e d u r e (Fossing and J o r g e n s e n 1989). Sediments were placed in nitrogen-purged reaction vessels, to which 1 N HG1 was added. H y d r o g e n sulfide liberated at r o o m t e m p e r a t u r e (AVS) was collected by r u n n i n g the gas train t h r o u g h vials
3
containing NaOH. After 1 h, the vials were replaced, and a r e d u c e d c h r o m i u m solution and c o n c e n t r a t e d HC1 were added. Reactants were h e a t e d to boiling for 1 h, and liberated h y d r o g e n sulfide was trapped in collection vials. AVS and CRS concentrations were d e t e r m i n e d spectrophotometrically against p r e p a r e d sulfide standards. T h e coefficient of variation a r o u n d sample means averaged 85% for AVS and 9% for GRS. IRON ANALYSES
A two-step sequential extraction p r o c e d u r e was completed for determinations of different forms of reactive iron in replicate sediment samples. First, sediments were incubated for 9 h in a 0.2 M solution of a m m o n i u m oxalate, adjusted to p H 2. Extracted iron concentrations were d e t e r m i n e d on GF/F-filtered suspensions using the ferrozine technique (Stookey 1970). T h e filters plus sediments were resuspended in a 0.1 M solution of sodium citrate (pH 4), then 0.8 g sodium bisulfite was added (dithionite reagent). Following a second 2-h extraction, iron concentrations were d e t e r m i n e d on filtered suspensions. T h e oxalate p r o c e d u r e extracts iron monosulfide (FeS), magnetite (FesO4) , and poorly crystalline iron oxides----lepidocrocite ( y F e O O H ) and ferrihydrite (Fe(OH)s); some additional, u n r e a c t e d crystalline iron oxides also may be dissolved ( T h a m d r u p and Canfield 1996). After oxalate extraction, the dithionite p r o c e d u r e extracts remaining crystalline iron oxides---goethite ( o t F e O O H ) a n d h e m a t i t e (F%O~) ( C a n f i e l d 1989b). T h e coefficient of variation a r o u n d sample m e a n s averaged 5% for oxalate extractions and 4% for dithionite extractions. Results CARBON PROFILES
O r g a n i c m a t t e r was a p p r o x i m a t e l y 7% d r y weight at the sediment surface at all three coring locations, decreasing to n e a r 6% within the u p p e r 90 cm (Fig. 9a). Between 90 cm and 300 cm, organic matter decreased m o r e gradually, from 6% to about 4%. Depth profiles of the stable isotopic signature of organic carbon were different a m o n g stations, with m o r e depleted carbon deposited nearest the terrestrial source (station 16) (Fig. 2b). Further, a distinct shift to m o r e depleted carbon was observed at all stations, occurring deepest near the terrestrial source. T h e stable isotopic signature of organic carbon in surface sediments was closely correlated with distance from the terrestrial source (r 2 - 0.98) (Fig. 2c). T h e 81sC value 18 km from the m o u t h of the bay (nearest Lagunitas Greek) was -95.0%o; 8 km from the m o u t h of the bay the 81SG value was -22.0%o.
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SULFUR PROFILES
B e c a u s e o f e x t e n s i v e b i o i r r i g a t i o n , sulfate c o n c e n t r a t i o n s at all s t a t i o n s w e r e r e l a t i v e l y c o n s t a n t t h r o u g h o u t t h e u p p e r 40 c m o f s e d i m e n t (Fig. 8a). B e l o w this z o n e , s u l f a t e d e p l e t i o n at all s t a t i o n s was g r a d u a l , with s u l f a t e c o n c e n t r a t i o n s d e c r e a s i n g to n e a r 0 m m by 200 cm. S u l f a t e d e p l e t i o n with d e p t h is c a u s e d by s u l f a t e r e d u c t i o n as a p a t h w a y f o r carb o n o x i d a t i o n , b u t t h e h i g h e s t r a t e s o f sulfate red u c t i o n , u p to 2 n m o l c m s h 2 w e r e m e a s u r e d in t h e u p p e r 20 c m o f s e d i m e n t (Fig. 3 b ) . T h e s e r a t e s w e r e s i m i l a r at all s t a t i o n s , a n d d e c r e a s e d d r a m a t ically b e l o w 50 c m to r a t e s < 0.1 n m o l c m ~ h 2. I n t h e c o r e s f r o m s t a t i o n 12 a n d s t a t i o n 8, h o w e v e r , we m e a s u r e d l o c a l i z e d h i g h r a t e s o f s u l f a t e r e d u c t i o n m u c h d e e p e r t h a n 100 cm, w h e r e sulfate c o n c e n t r a t i o n s h a d d e c r e a s e d to n e a r 0 p~M (Fig. 3). T h e s e " h o t s p o t s " f o r sulfate r e d u c t i o n o c c u r at
0.5
1
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2
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S R R , n m o l c m "3 h -1 Fig. 3. Depth profiles of a) porewater sulfate a n d b) sulfate reduction rates m e a s u r e d in s e d i m e n t cores.
the top of the sediment zone where high concent r a t i o n s o f m e t h a n e b e g i n to a c c u m u l a t e ( K J . Sansone personal communication). We interpret these h o t s p o t s as z o n e s o f s u l f a t e r e d u c t i o n a s s o c i a t e d with anaerobic methane oxidation (Reeburgh 1980), a p r o c e s s p o t e n t i a l l y e n h a n c e d by a d d i t i o n o f t h e sulfate tracer. F r e e s u l f i d e s c o u l d n o t b e d e t e c t e d e i t h e r by s m e l l o r by c h e m i c a l analysis in p o r e w a t e r f r o m s t a t i o n s 16 a n d 12, b u t s u l f i d e was d e t e c t e d b e l o w 60 c m at s t a t i o n 8, typically < 100 l.tlVi to 800 cm. B e l o w 60 c m t h e a v a i l a b i l i t y o f r e a c t i v e i r o n app e a r s to l i m i t t h e f o r m a t i o n o f i r o n - s u l f l d e m i n e > als at this l o c a t i o n (see b e l o w ) . A c i d - v o l a t i l e s u l f u r c o n c e n t r a t i o n s w e r e always less t h a n 10 > t o o l g-1 s e d i m e n t at all s t a t i o n s , a n d d e c r e a s e d to < 0.5 p.mol g 1 b e l o w 150 c m (Fig. 4a). D e s p i t e t h e g e n eral s i m i l a r i t y in s u l f a t e r e d u c t i o n r a t e s a m o n g stations, p r o f i l e s o f r e d u c e d s u l f u r in t h e s e d i m e n t s w e r e q u i t e d i f f e r e n t . I n all s e d i m e n t cores, CRS, o r p y r i t e sulfur, a c c u m u l a t e d with d e p t h , b u t CRS also increased with distance from the terrestrial source
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of s e d i m e n t s (Fig. 4b). T h e h i g h e s t c o n c e n t r a t i o n of CRS f r o m station 8 (over 400 IJ,m o l S g 2) was m o r e t h a n d o u b l e the h i g h e s t c o n c e n t r a t i o n f r o m station 16. N o effects o f b i o t u r b a t i o n o n the pyrite profiles were obvious, in that CRS i n c r e a s e d b o t h within a n d b e l o w the u p p e r s e d i m e n t z o n e w h e r e sulfate concentrations were relatively constant (Figs. 3a a n d 4b). IRON PROFILES
T h e a m o u n t s o f i r o n extracted with a m m o n i u m oxalate were lowest at station 8, i n t e r m e d i a t e at station 12, a n d h i g h e s t at station 16 (Fig. 5a). Because AVS c o n c e n t r a t i o n s were low (Fig. 4a) a n d m a g n e t i t e is typically a m i n o r c o m p o n e n t o f m a r i n e s e d i m e n t s (Canfield a n d B e r n e r 1987), we ass u m e the oxalate e x t r a c t i o n recovers m o s t of the p o o l of p o o r l y crystalline iron. Profiles o f this easily extracted i r o n e x h i b i t e d sharp d r o p s in c o n c e n t r a tion b e t w e e n 0 cm a n d 40 cm at station 8, b e t w e e n 80 cm a n d 190 cm at station 19, a n d b e t w e e n 180 a n d 990 cm at station 16. A m o u n t s o f i r o n extracted by the d i t h i o n i t e r e a g e n t typically were five times g r e a t e r t h a n by a m m o n i u m oxalate (Fig. 5b).
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C o n c e n t r a t i o n s t e n d e d to d e c r e a s e slightly with d e p t h ; the c o n c e n t r a t i o n s o f crystalline i r o n oxides at station 8 were a b o u t h a l f the c o n c e n t r a t i o n s at stations 19 a n d 16. T h e m e a s u r e d c o n c e n t r a t i o n s o f i r o n f r o m seq u e n t i a l e x t r a c t i o n s were used to calculate the total a m o u n t of i r o n available for r e a c t i o n with free sulfide, to c o m p a r e with the a m o u n t of i r o n alr e a d y p r e c i p i t a t e d as iron-sulfide minerals. Because i r o n a p p e a r s to limit iron-sulfide m i n e r a l form a t i o n b e l o w 60 cm at station 8, we m a d e the ass u m p t i o n t h a t i r o n chemically extracted at these d e p t h s r e p r e s e n t e d i r o n that was n o t reactive with free sulfide. This would p e r h a p s i n c l u d e u n r e a c t i v e i r o n oxides a n d a m o r p h o u s i r o n silicates (Canfield 1989b). T h e average a m o u n t s of i r o n e x t r a c t e d at d e p t h at station 8 were s u b t r a c t e d f r o m the total extracted i r o n at each station, as were the a m o u n t s of AVS-iron. Available i r o n is the p o o l o f u n r e a c t e d i r o n oxides t h a t can r e a c t with flee sulfides; by definition the p o o l size d r o p s to 0 lJomol g 1 with d e p t h at station 8. T h e d e c r e a s e in available i r o n with d e p t h is m a t c h e d by increases in sulfidized
6
R.M. Chambers et al.
0
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350
, 0,2 Index
, 0,4
0,6
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of Iron Availability
Fig. 6. Depth profiles of the Index of Iron Availability determined from sediment cores. iron (iron-sulfide minerals) (Fig. 4). T h e I n d e x of I r o n Availability, a m e a s u r e of sulfidization in sediments (sensu Boesen and Postma 1988), expresses the ratio of available iron to available + sulfidized iron. Profiles of the index decrease with d e p t h a n d with distance f r o m the terrestrial s e d i m e n t source (station 16 to station 8) (Fig. 6). W h e n the i n d e x nears 0, reactive iron is no l o n g e r available a n d free sulfides are detected in the porewater. Discussion
O r g a n i c c a r b o n fuels sulfate r e d u c t i o n in estuarine sediments, which leads to general decreases in p e r c e n t organic m a t t e r over time and s e d i m e n t d e p t h (Fig. 9a). U p to 90% of the sulfide p r o d u c e d in the b i o t u r b a t e d z o n e of Tomales Bay is r e m o v e d f r o m the p o r e w a t e r e n v i r o n m e n t a n d / o r re-oxidized to sulfate ( C h a m b e r s et al. 1994). This is d e m o n s t r a t e d by the relatively constant sulfate concentrations to a p p r o x i m a t e l y 40 cm d e p t h (Fig. 3a). This extent of b i o t u r b a t i o n is substantial given the fairly high s e d i m e n t a t i o n rates of 3 m m yr ~ or m o r e in Tomales Bay (Rooney 1995). Rapid sedim e n t a t i o n moves c a r b o n m o r e quickly into anaerobic zones in the s e d i m e n t s (Canfield 1989a) a n d can p r e c l u d e the d e v e l o p m e n t of infaunal p o p u lations of b i o t u r b a t o r s t h r o u g h suffocation a n d / o r the toxic effects of liberated sulfides ( C h a n t o n et al. 1987; Giblin et al. 1995). Free-sulfide p r o d u c t i o n a n d persistence is typical of m a n y estuarine e n v i r o n m e n t s with high organic loading, but Tomales Bay s e d i m e n t s receive high organic loading and still m a i n t a i n an active infauhal community. H i g h s e d i m e n t a t i o n and low sulfide are conditions m o r e typical of n e a r s h o r e river deltas, where physical mixing of s e d i m e n t by strong currents in the overlying waters k e e p s sul-
fide c o n c e n t r a t i o n s low despite rapid oxidation of organic m a t t e r by sulfate r e d u c t i o n (Aller et al. 1986; Canfield 1989b). In oceanic systems, sedim e n t a t i o n provides organic c a r b o n as a food source to the benthos, thereby increasing the n u m ber of organisms and, concomitantly, b i o t u r b a t i o n ( B o u d r e a u 1994). As n o t e d by Giblin et al. (1995), b i o g e o c h e m i c a l processes a p p e a r to control benthic c o m m u n i t y structure in m a r i n e e n v i r o n m e n t s , n o t the o t h e r way a r o u n d . Below the b i o t u r b a t e d zone, the p r e d o m i n a n c e of sulfate r e d u c t i o n over sulfide oxidation is shown by decreases in p o r e w a t e r sulfate c o n c e n t r a t i o n with d e p t h (Fig. 3a) and increases in r e d u c e d sulfur minerals (Fig. 4). T h e availability of iron for reaction with free sulfide is critical to sulfur aliagenesis (Kostka a n d L u t h e r 1995; Anschutz et al. 1998). Canfield (1989b) d e m o n s t r a t e d the influence of e x t r e m e s in reactive iron availability, f r o m the iron-rich Mississippi delta and f r o m an ironp o o r location in L o n g Island Sound. Reactive metal oxides can act as buffers against p o r e w a t e r sulfide a c c u m u l a t i o n a n d at s o m e d e p t h s may be coupled directly with organic m a t t e r oxidation (Aller et al. 1986; H i n e s et al. 1991; T h a m d r u p et al. 1994; T h a m d r u p a n d Canfield 1996). D e p e n d i n g on location, T o m a l e s Bay s e d i m e n t s exhibit conditions of reactive iron sufficiency, where free sulfide is absent (stations 16 a n d 12) a n d reactive iron deficiency, where free sulfide is p r e s e n t (station 8) (Fig. 6). We calculated the degree of pyritization as the ratio of pyrite iron to the total reactive iron in sediments, defined h e r e as the sum of pyrite iron plus iron extracted by 1 N HC1 (Raiswell et al. 1994). A l t h o u g h these ratios increased with d e p t h a n d with distance f r o m terrestrial s e d i m e n t sources in Tomales Bay, they n e v e r e x c e e d e d 0.4 ( u n p u b lished data), a value used to infer iron limitation of the f o r m a t i o n of iron-sulfide minerals (Morse a n d Emeis 1990; but see Canfield et al. 1992). At all d e p t h s below 90 cm at station 8, however, the degree of pyritization was greater that 0.8, indicating that m o r e of the available iron was tied up in pyrite, relative to s e d i m e n t s f r o m o t h e r locations in the bay. A g r a d i e n t in the a m o u n t of terrestrial s e d i m e n t d e p o s i t i o n - - t h e p r i m a r y reactive iron source f r o m the h e a d to the m o u t h of Tomales Bay explains in p a r t the o b s e r v e d difference in sulfur diagenesis. T h e g r a d i e n t in terrestrial s e d i m e n t deposition also is d e m o n s t r a t e d in the differences in c o m p o sition of deposited organic m a t t e r (Fig. 2c). M o r e terrestrial organic m a t t e r ( m o r e negative 81sC values) was f o u n d in surface s e d i m e n t s nearest the h e a d of T o m a l e s Bay (nearest the source of r u n o f f f r o m Lagunitas Creek); 81sC values were less neg-
Sediment Diagenesis of Tomales Bay
ative toward the m o u t h of the bay (Fig. 1). Sedim e n t deposition n e a r the m o u t h may include a greater fraction of organic matter from phytop l a n k t o n derived from coastal upwelling (Smith and H o l l i b a u g h 1997) a n d / o r eelgrass (Zostera ma ri,za), which grows there in extensive beds. Terrestrial s e d i m e n t deposition tends to buffer Tomales Bay sediments from free-sulfide accumulation by increasing the a m o u n t s of iron that can react to oxidize sulfide a n d / o r p r o d u c e Fe-S minerals. Spatial differences in a m o u n t s of reactive iron and types of organic matter deposited along the axis of the bay highlight the relative importance of m a r i n e and terrestrial sediment sources to sulfur diagenesis (Figs. 2 and 6). A l t h o u g h the types of organic matter deposited at each location are different (Fig. 2c), the a m o u n t s are a b o u t the same (Fig. 2a) as are the rates of anaerobic oxidation via sulfate r e d u c t i o n (Fig. 3). T h e major difference a m o n g sites appears to be the a m o u n t s of reactive iron available for pyrite formation, which decrease toward the m o u t h of the bay. Using the observed shift in extractable iron as a temporal m a r k e r for sedimentation (Fig. 5a), the integrated areal a m o u n t s of s e d i m e n t a r y pyrite that have accumulated s u b s e q u e n t to the shift are similar a m o n g stations (17 tool m e, 10 tool m ~, and 14 tool m -~ from the head to the m o u t h of the bay). Even t h o u g h the concentrations of iron in these recent sediments vary a m o n g stations by a factor of 4, the variation is an a p p a r e n t c o n s e q u e n c e of the iron sedimentation rate. Given similar a m o u n t s of organic matter and rates of sulfate reduction, less reactive iron deposited at the m o u t h of the bay translates into m o r e pyrite f o r m a t i o n per unit of sediment (Fig. 4b). In all cores, we also observed a distinct shift in the isotopic signature of organic c a r b o n (Fig. 2b), suggesting a historical c h a n g e in sediment composition. Similar to the pattern observed for iron, the shift o c c u r r e d deepest in the sediments near the h e a d of the bay, decreasing toward the m o u t h of the bay. Because the shift was not associated with sulfate profiles (Fig. ga), we conclude it was n o t a diagenetic change but instead records a historical change in the types of sediments deposited. We know that land use c h a n g e d dramatically in California with E u r o p e a n settlement in the 1850s, when large portions of the Tomales Bay watershed were converted from a stable Mediterranean-type b i o m e to agriculture for potato and dairy farming (Plant 1995). T h e conversion seems to have created a m o r e o p e n terrestrial ecosystem where erosion and transport of sediments has increased the m a g n i t u d e and c h a n g e d the composition of materials delivered from the watershed to Tomales Bay.
7
T h e role reactive iron plays in mediating the fixation and eventual oxidation of sulfides in marine sediments may affect n u t r i e n t cycling in estuaries. For example, when reactive iron is depleted, the presence of free sulfides in sediments will inhibit coupled nitrification-denitrification. This leads to a m m o n i u m retention in the system and possible release of estuaries from n i t r o g e n limitation (Joye and H o l l i b a u g h 1995); excess n i t r o g e n potentially could fuel p h y t o p l a n k t o n blooms and p r o m o t e estuarine eutrophication. Typically, changes in land use increase deposition of sediments from surr o u n d i n g watersheds to estuarine systems and provide a reactive iron source as a buffer to sulfide p r o d u c t i o n . With sufficient iron buffering, as seen in Tomales Bay, removal of n i t r o g e n via denitrification in the sediment would keep the system nitrogen limited. Unfortunately, increased watershed r u n o f f of inorganic nutrients associated with sedim e n t transport in some estuaries may also stimulate p h y t o p l a n k t o n blooms, leading to increased sedimentation of labile organic carbon ( J o r d a n et al. 1991; Conley et al. 1998). Sulfate r e d u c t i o n then m i g h t be stimulated b e y o n d the buffering capacities of reactive iron, leading to a positive feedback a m o n g a m m o n i u m retention, labile c a r b o n p r o d u c t i o n in the water column, and sulfide generation in the sediments. This scenario has been played out countless times in urbanized watershedestuarine systems that n o w experience water colu m n hypoxia. In Tomales Bay, a l l o c h t h o n o u s deposition of reactive iron from the s u r r o u n d i n g rural watershed appears to have offset any effects of p h y t o p l a n k t o n stimulation associated with nutrient-rich r u n o f f and the bay remains n i t r o g e n limited post-European settlement. T h e relative magnitude and composition of materials transported to estuaries will influence the net impact of landuse c h a n g e on estuarine eutrophication. ACKNOWLEDGMENTS Funding was provided by National Science Foundation grant OCE 89-14833 to S. V. Smith; C. S. Snivelywas funded as an REU student, supplementary to National Science Foundation grant OCE-8914921 to J. T. Hollibangh. Thanks to members of the Tomales Bay LMERresem-ch crew for field work, and to Pat Wong for lab assistance. Helpflfl reviews of previous versions of the manuscript were provided by S. V. Smith, S. g. Joye, and two anonymous reviewers. LITERATURE CITED ATJv.e, R. C.,J. E. MACI~IN,ANDR. rE Cox,JI~. 1986. Diagenesis of Fe and S in Amazon inner shelf muds; apparent dominance of Fe reduction and implications for the genesis of ironstones. Cc~tinental She~'Researda 6:263-289. ANSCHI_rrz, I~, S. ZHONO, B. SUNDBY,A. MuccI, AND C. GOBEIL. 1998. Burial efficiency of p h o s p h o r u s a n d the g e o c h e m i s t r y o f iron in c o n t i n e n t a l ram-gin secthnents. Liwr~ology ard Ocear~ograp/zsv 43:53-64. BEIN, A., A. AUMOoI-LABIN, AND E. SASS. 1990. Sulfm- sinks a n d
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Received fc,r cor~sideration, J~ne 3, 1998 Accepted for pu blication, Jul7 4, 1999
