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Salt marshes: An important coastal sink for dissolved uranium
Geochimicaet CosmochimicaActa, Vol. 60, No. 20, pp. 3879-3887, 1996 Copyright ,~ 1996 ElsevierScienceLtd Printed in the USA. All rights reserved 0016-7037/96 $15.00 + .00
Pergamon
PII S0016-7037 (96) 00211-6
Salt marshes: An important coastal sink for dissolved uranium T. M. CHURCh, M. M. SaraN,* M. Q. FLElSHER,~ and T. G. FERDELMAN:E College of Marine Studies, University of Delaware, Newark, DE 19716-3501, USA (Received January, 5, 1996; accepted in revised form June 20, 1996)
A b s t r a c t - - T h e global budget for marine uranium demands another geochemical sink other than deepsea systems, and the coastal environment may host some or all of this missing sink. In a previous paper (Sarin and Church, 1994), we have shown that some large subtidal estuaries are seasonal summer sinks at low salinities. In this paper, we show that intertidal salt marshes are even stronger sinks at all salinities, if for somewhat different reasons. Uranium was sampled in dissolved and particulate fractions over several tidal cycles and seasons for a lower Delaware Bay salt marsh (,Canary Creek, Lewes, Delaware, USA), and uniquely, during summer months, the dissolved uranium is nonconservative. Moreover, because uranium extraction is greater on higher tides and occurs over the entire salinity gradient, this processing appears associated with surface of vegetated high marsh. We hypothesize that either (1) uranium scavenging occurs during the process of tidal mixing and attendant flocculation of humic acids and iron oxides--favoring this process is the presence of sulfonate complexes in salt marsh humic substances, and iron coprecipitation during its extensive redox cycling in the salt m a r s h - - o r (2) uranium extraction occurs at the marsh surface during extensive flooding of the salt marsh surface sediments--favoring this process is the increase in sulfuric acidity at the summer salt marsh surface that could destabilize the tetracarbonate species of U (VI). The latter option is favored by both field observations of maximum removal at the surface during the spring and summer tide conditions, and selective extraction of sediment phases where uranium is found as adsorbed and complexed forms in the ascorbate-citrate and humic acid tractions, respectively. Mass balance calculations show that under steady-state conditions, nearly two-thirds of the uranium extracted from tidal waters is retained in the sediments, while one-third is exported as U-enriched particles during ebbing tides. Independent confirmation of this balance comes from the measured accumulation rate of uranium buried at depth. This represents the net inventory buried below the geochemically reactive surface responsible for the initial extraction and redistribution of uranium onto sediment or tidally exported phases. Extrapolated globally, uranium burial in salt marshes alone or total marine wetlands including mangroves could comprise at least 10% and perhaps as much as 50% the total marine sink for uranium, or on an area specific basis, up to 50 times their marine areal extent. 1. INTRODUCTION
has received considerable interest in recent years is the mechanism for its removal in anoxic and suboxic hemipelagic sediments (Anderson, 1987; Klinkhammer and Palmer, 1991 ; Sarkar et al., 1993; Barnes and Cochran, 1993). Distribution of uranium in Cariaco Trench sediments suggests that the removal of uranium from the water column involves diffusion of U ( V I ) into the sediments where it is slowly reduced to U (IV) and precipitated over a zone of several centimeters below the sediment-water interface (Anderson, 1987). This transformation occurs relatively late in the diagenetic sequence, after the microbially mediated dissolution of manganese and iron oxides, and may be induced by the onset of sulfate reduction with or without uranium reduction. For example, some investigators favor complexation of U ( V I ) by particulate organic matter with subsequent burial of the U-organic complex which is preserved under anoxic conditions (Baturin et al., 1971; Dorta and Rona, 1971; Kolodny and Kaplan, 1973; Degens et al,, 1977), Others prefer initial chemical reduction of U ( V I ) and precipitation of insoluble U ( I V ) (Bonati et al., 1971; Veeh et al., 1974; Carpenter et al., 1984; Yamada and Tsunogai, 1984). In fact, the first-order removal rate of uranium in incubated coastal sediments appears to be linear with sulfate reduction sug-
It has been well documented that uranium is enriched in deep-sea anoxic submarine sediments, the largest known abyssal sink in the marine budget of this element (Sackett et al., 1973; Degens et al., 1977; Cochran, 1982; Bernat and Church, 1989). Thus, besides its input to the ocean via rivers, the concentration of uranium in seawater might appear, to a large extent, to be controlled by its removal via anaerobic pathways in these sediments according to their areal distribution. In this context, the additional anoxic areas prevailing in continental shelf sediments and estuaries could contribute an additional sink accounting for uranium uptake from offshore/coastal waters (Cochran et al., 1986; Toole et al., 1987; Barnes and Cochran, 1993; Sarin and Church, 1994; Swarzenski et al., 1995). One aspect of the marine geochemistry of uranium which * Present address: Physical Research Laboratory, Ahmedebad,
380009, India. Present address: Lamont-Doherty Geological Observatory, Palisades, NY 10964, USA. * Present address: Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany
3879
3880
T. M. Church et al.
DELAWARE BAY
N SAK
-.-.°'--i of D
.... i C ._
Ebenezer Pond Morsh ~-~
L::::N 1
I 0
-
Kilometers
I I
~' FIG. 1. Sampling location of study site for uranium in tidal waters of Canary Creek salt marsh near Lewes, Delaware, USA.
gesting bacterial coreduction of uranium (Barnes and Cochran, 1993). The coastal salt marsh-estuarine system, exhibits a high degree of geochemical reactivity and is characterized by intense primary production, redox processes and tidal exchange (Lord and Church, 1983; Boulegue et al., 1982; Luther et al., 1986). An important biogeochemical question is what role salt marshes play in the recycling of some key elements such as uranium. Previous studies have shown that uranium is enriched in the near-surface sediments and microlayers of the salt marsh, including tidally exported particles (Church et al., 1981, 1986). The objective of the present study is to investigate the behavior of uranium during tidal
mixing in a Delaware salt marsh creek. Such a study could help to understand the fundamental aspects of uranium geochemistry in coastal redox systems, specifically its removal from intertidal waters and its subsequent enrichment in the marsh surface sediments. The paper also deals with the removal mechanism and burial rate of uranium in salt marshes and their significance in the marine geochemical budget of uranium. 2. SAMPLING AND ANALYSES Water samples were collected in the summers of 1983, 1984, 1985, and during the winter of 1984 from the tidal waters of the Canary Creek, a lower Delaware salt marsh (Fig. 1 ). The Canary
3881
Geochemistry of U in salt marshes Table 1.
Concentration of uranium isotopes and seston in Canary Creek, a lower Delaware salt marsh, during mid, spring, and neap tides in 1983.
Dissolved Sam pie Tidal Stage*
S(°/oo)
z3sU (dpm/l)
Particulate 23sU (dpm/g)
234U/z3sU A.R.
23~U/238U A.R.
Seston (rag/l)
Mid Spring Tide 1 2 3 4
20.9 29.2 29.3 25.8
l. 14_+0.03 1.73_+0.05 1.67+_0.03 1.38_+0.04
1.12_+0.02 1.15+0.01 1.15_+0.02 1.13_+0.02
0.93_+0.04 0.85_+0.03 0.65_+0.03 1.88_+0.06
1.07_+0.04 1.05_+0.04 1.08+_0.05 1.11_+0.02
18.29_+1.31 16.52_+1.70 30.05_+2.43 67.94+_3.25
Spring Tide 1 2 3 4
24.1 29.7 29.9 28.4
1.02_+0.03 1.70+_0.05 1.69_+0.03 1.36_+0.05
1.15_+0.02 1.16+0.02 1.12_+0.02 1.15_+0.02
0.99+_0.04 1.19_+0.04 1.49_+0.06 1.70+0.05
1.08_+0.03 1.06+_0.03 1.13_+0.02 1.09_+0.01
23.38_+1.01 33.31_+6.56 27.05_+1.38 56.78+_3.85
Neap Tide 1 2 3 4
24.0 29.2 27.6 15.8
1.21+0.04 1.63_+0.05 1,54_+0.04 0,61+_0,02
1.17+0.02 1.17_+0.02 1.17_+0.02 1,17_+0,02
1.08+0.04 1.01_+0.05 1.32_+0.06 1,82_+0.07
1.14+0.03 0.99_+0.04 1.10_+0.03 1.09_+0.03
33.07_+2.40 28.19_+4.01 38.54+_5.45 38.0l_+5,57
*Tidal Stage No. 1 = Low Slack- Mid Flood 2 = Mid Flood-High Slack 3 = High Slack- Mid Ebb 4 = Mid Ebb - Low Slack A.R. = Activity Ratio
Creek Marsh, located at the seaward edge of Delaware Bay is a high salinity tidal marsh dominated uniquely by the grass Spartina alterniflora. The surface of this marsh is inundated with lower Delaware Bay seawater ( - 3 0 % e ) during the highest (Spring) tides of each month. During the non-flooding stages, the surface of the marsh is directly exposed to the atmosphere. The headwaters of the Canary creek originate as fresh water spillways from Ebenezer Mill Pond. Thus, the system is a small-scale estuary ranging in salinity from 0%c at the pond to 30%e at the mouth. Three individual tidal cycles were sampled in the summer of 1983, covering the range from the neap to spring lunar cycle, over a salinity range of 16-30%~ (Table 1 ). During each tidal cycle, four integrated samples were collected by pumping the water at a constant rate for 3-h periods between successive low tides so as to compare the relative fluxes of uranium into and out of the marsh. During the summer of 1985, an entire salinity transect (0 to 30%0) along the Canary Creek was completed during one of the high tides (Table 2). Samples were collected from shallow wells and fresh water ponds to represent the zero salinity endmembers. In order to reveal the seasonal behavior of uranium and its exchange with the marsh sediments, samples were also collected during the winter of 1984 (Table 2). All samples were filtered in the field through 0.5 /zm absolute cartridge filters and were returned to the laboratory for subsequent analysis of uranium using standard radioehemical procedures (Krishnaswami and Sarin, 1976). Particulate samples of suspended sediment were collected during the summer of 1983 (Table 1) and analyzed for uranium isotopes. The silica and phosphate contents in the tidal waters were measured in the samples collected during the summer of 1985 (Fig. 5). In addition, a sediment core was obtained from the neighboring Great Marsh in August 1985, and the sediments were subjected to an extraction scheme designed specifically for salt marsh sediment (Ferdelman, 1988). Besides fractional determination of the major sulfide precipitates, fractions for humic acids (base extractable),
metal oxides (ascorbate-citrate), and a "total" leach fraction were included. "Total" uranium is also determined for comparison with earlier uranium distributions at the Great Marsh site (Church et al., 1981) and with a North Carolina salt marsh uranium profile (Benninger and Chanton, 1985). The method chosen for total uranium is digestion of ashed subsamples in boiling aqua-regia. This method is similar to the total metal determination for salt marsh sediments described by Sinex et al. (1980) and should digest all of the fractions from the sequential extraction. Although not strictly a complete digestion, this method does provide an accurate assessment of total uranium in the salt marsh sediment (Ferdelman et al., 1991 ). 3. RESULTS AND DISCUSSION T h e C a n a r y C r e e k tidal estuary has no m a j o r tributaries, but it is split at the E b e n e z e r and S a v a n n a h b r a n c h e s near its fresh water source. Both these b r a n c h e s are u n i f o r m l y s u r r o u n d e d b y the salt marsh. Since the input o f d i s s o l v e d u r a n i u m f r o m the fresh w a t e r e n d m e m b e r is m i n i m a l ( < 0 . 0 1 dprrdL, Tables 1 and 2 ) , u r a n i u m input to the Canary C r e e k is d o m i n a t e d by tidal m a r i n e waters. T h e s e m a r i n e waters, h o w e v e r , are already s o m e w h a t d e p l e t e d in u r a n i u m due to r e m o v a l p r o c e s s e s both on the m i d - A t l a n t i c s h e l f and in the D e l a w a r e estuary (Sarin and C h u r c h , 1994). T h e m o s t interesting aspect o f the data is r e v e a l e d by the uranium-salinity plots in the tidal creek waters (Figs. 2 and 3). T h e d i s s o l v e d u r a n i u m c o n c e n t r a t i o n s h o w s n o n c o n s e r vative behavior, during s u m m e r m o n t h s , o v e r almost the entire p o r t i o n o f the salinity gradient. Like the D e l a w a r e and C h e s a p e a k e bay estuaries ( S a t i n and Church, 1994), there
3882
T. M. Church et al. Table 2. Concentrations of uranium isotopes in the tidal Canary Creek of a salt marsh, Delaware Bay.
Sample
Salinity
238U
234U
234U/238U
(%)
(dpm/l)
(dl~n/l)
(A.R.)
1.87±0.05 1.82+0.05 1,665:0.05 1,185:0.03 1.665:0.05
1.145:0.02 1,145:0.01 1.155:0.02 1.125:0.02 1.16+0.02
1.73__.0.05 1.37 ±0.04 1,745:0.03
1.145:0.01 1.11 5:0.02 1,125:0.02
0.0021+0.0001 0.013 5:0.0006 0,058 5:0,002 0.026 5:0.0005 0.29 5:0,004 0.36 5:0,004 0.29 +0.005 0.35 5:0,005 0.45 5:0.007 0.44 5:0.006 0.70 5:0,008 1.33 -I-0.015 2.05 5:0.03
1.055:0.09 0,965:0.05 1.04+0.03 1,145:0.03 1.135:0.02 1,115:0.02 1.135:0.02 1.165:0.02 1.155:0.02 1,105:0.02 1.16+0.02 1.16+0.02 1.135:0.01
Winter (March) - 1984
MMS 1 MMS 2 MMS 3 MMS 4 RI 15
26.4 25.5 23.1 16.2 23.0
1.64+0.05 1.60+0.04 1.44+0.04 1,055:0.03 1.43 ±0.04 Summer (July) - 1984
JMS-1 JMS-2 RI 16"
24.5 26. i 27.1
1.52±0.04 1,245:0.04 1,555:0.03 Summer (June) - 1985
SMS SMS SMS SMS SMS SMS SMS SMS SMS SMS SMS SMS SMS
120"* 111 110"** 109 108 106 107 104 105 103 102 101 100
0.00 0.02 0.19 1.7 9.4 11.9 12.5 13.9 15.0 15,7 19.2 25.6 30.5
0.00205:0.0001 0,014 5:0.0006 0.056 5:0.002 0.023 5:0.0005 0.26 5:0,004 0.32 5:0,004 0.26 5:0.005 0.30 5:0.005 0.39 5:0.007 0.40 5:0.006 0.60 5:0.008 1.15 5:0.015 1.81 5:0.03
* Sample representing seawater end member. **Groundwater sample collected adjacent to the salt marsh. ***Sample from the fresh water pond. A.R. = Activity Ratio
is no evidence for uranium removal from tidal waters during the winter months (Fig. 4). The property-salinity plots of nutrients (Fig. 5) suggest that a large fraction of the silica is r e m o v e d from tidal waters, very similar to its nonconservative behavior in estuarine waters such as Delaware Bay (Sharp et al., 1982, 1984). The distribution of phosphate during tidal mixing in the Canary Creek is noteworthy in that it is removed at midsalinity regions during flooding tides and is desorbed at low salinities during ebbing tides (Fig. 5). Phosphate distribution is mainly dominated by its geochemical reactivity during tidal mixing ( E a s t m a n and Church, 1984), and at the marsh surface during summer months (Scudlark and Church, 1993 ). The nutrient measurements were made only on those samples collected during the s u m m e r of 1985. 3.1. Dissolved
Uranium
The data in Fig. 3 represent probably the first reported evidence for the widespread removal of uranium from flooding tidal waters over the full salinity range of any estuary, in this case a salt marsh. This contrasts subtidat estuarine
and shelf areas where uranium removal appears confined to either the lower ( S a t i n and Church, 1994) or higher (Toole et al., 1987; Barnes and Cochran, 1993) salinity areas. As in the upper reaches of Delaware and Chesapeake bays, large-scale removal of uranium occurs only during s u m m e r months w h e n the exposed surface sediments of the salt marsh exhibit a high degree of geochemical reactivity (Boulegue et al., 1982). In the winter months due to low temperature conditions, the diagenetic processes such as either sulfate reduction or sulfide oxidation with resultant acidity are at a m i n i m u m (Lord and Church, 1983). In response to these well-documented high oxidation rates in the near surface sediments (during summer m o n t h s ) , uranium may coprecipitate along with Fe(OH)3 at the sediment water interface. If the iron coprecipitation m e c h a n i s m for uranium removal from tidal waters is operative, its enrichment onto iron oxides formed in surface sediments may act as a geochemical reservoir of uranium. Ross-Carre ( 1 9 8 3 ) has described the cycling of metals in salt marshes based on the solid phase sulfide phases of the sediments in a model which hypothesizes metal complexat±on by humic thiol functional groups, which may result in
Geochemistry of U in salt marshes
3883
2.0
~,J
0 Mid T i d e o S p r i n g Tide
a.
~ Neop Tide • Fresh Woter
1.5
2,5
./'F 2.0
,. ,,/@/'/
1.5
N :3
,4~
Theoreticol Mixing Line ~
1.0
,,. I I "
/
~0
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t'~ O ffl
1.0
0.5
o.5
Q
I
0 0
0
6
12
18
SALINITY
t
I
24
30
(%)
F [ G . 2 . D i s s o l v e d 23~U concentration versus salinity from 1 6 - 3 0 % c in the tidal waters o f C a n a r y Creek. T h e n o n c o n s e r v a t i v e b e h a v i o r o f uranium is evident o v e r the restricted salinity gradient.
the transport of complexed uranium within the porewaters. Specifically, the recomplexation of uranium from carbonate ligands under the influence of low pH to humic sulphonate ligands and its subsequent precipitation, may also enhance uranium removal into the solid phases. In addition, there is the trapping of uranium by the marsh surface grasses via enrichment into the surface microlayer and particles in the flooding tidal water (Church et a]., 1986). The removal of uranium from the flooding tidal waters could also proceed either via enrichment onto iron oxides formed in surface microlayers or around Spartina stalks (Pellenbarg and Church, 1979). Thus, based on this evidence, the general scenario for uranium enrichment in salt marsh sediments either directly from solids or tidal waters could be ( 1 ) flocculation of humic substances or metal oxides with scavenging of the uranium in the creek waters and then subsequent removal by deposition onto the marsh surface, or after sur-
I
4
I
8
i
12
115
I
20
I
24
I
28
I
32
S A L I N I T Y (%0)
FIG. 4. Dissolved 238U concentration over an entire salinity gradient of the flooding tidal waters in Canary Creek Marsh during March 1984. Uranium exhibits conservative behavior during this late winter period when associated diagenetic processes are at a minimum.
face microlayer enrichment of the floccs onto Spartina stalks, and/or (2) removal of dissolved uranium from waters flooding the marsh surface sediments due to (bio)geochemical processes operational in the surface marsh sediments. This latter process might be envisioned to involve destabilization of the uranyl (VI) tetracarbonate complex under low pH due to the intense sulfide oxidation. The uranium is then easily scavenged by humic acids and iron oxides within the marsh surface. Furthermore, advective transport of creek waters through bioturbated creek banks may enhance uranium removal from the creek waters. Bioturbation rates, particularly
200
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I 8
I 12
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I I 16 20 S(%o)
2r4
r
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~1
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200
2 2.¢
~" I,~
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t
8
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12
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16
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20
t
24
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28
t
o_
5O
32
S A L I N I T Y (%o)
FIG. 3. Dissolved 23~U concentration over an entire salinity gradient of the flooding tidal waters in Canary Creek Marsh during June 1985. Large scale removal is evident during this period when the exposed surface sediments of the salt marsh exhibit a high degree of geochemical activity.
54
FIG. 5. Both silica and phosphate display nonconservative behavior due to biochemical and geochemical behavior during tidal mixing in Canary Creek salt marsh.
3884
T.M. Church et al. 0.16 _1 "
~
0,12
D
0.08
0 Mid Tide a Sprlng Tide ~, Neap Tide
T
I,I
I,--
C-¢:
0.04
0
I
I
I
1
I
2
5
4
LOW S l a c k Mid Flood
Mid Flood High S l a c k
High Slack M i d Ebb
Mid Ebb Low S l a c k
STAGE OF T I D E
FIG. 6. Particulate 2~U (expressed in units of dpm/l) in tidal waters collected during mid, spring and neap tides of Canary Creek salt marsh. For all tidal cycles, the low ebb tide samples show the greatest particulate uranium concentration.
by fiddler crabs, is highest in the summertime near the creek banks (Gardner et al., 1988) and sulfide oxidation is also enhanced in creek bank sediments (Luther et al., 1991 ).
3.2. Particulate Uranium Large increases in the particulate uranium concentration (expressed in units of dpm/L) are observed in the tidal waters for all the late ebb tidal samples, especially during the mid-Spring and maximum Spring tidal cycles (Fig, 6). The particulate uranium concentration is calculated as the product of the specific 23sU concentration in dpm/g and the suspended sediment concentration in mg/L (Table 1). Identical trends are observed for particulate uranium (dpm/g) and suspended sediment (mg/L), indicating that both the particulate loading and the particulate uranium concentration are greatest during late ebb tides. The elevated uranium concentrations observed in the ebbing particulates appear to originate from dissolved uranium scavenged from tidal seawaters, perhaps first via the surface microlayer, before export in particulate form (Church et al., 1986). However, due to relatively low concentration of particulate uranium and large errors associated with the data, the scavenging of uranium from tidal waters is not clearly decernible from the particulate 234U/23~U activity ratios (Table 1). Based on the three tidal cycles sampled, most of the uranium scavenging seems to take place during the Spring tides which cover the salt marsh surface. The primary mechanism appears to be dissolved uranium extraction by the surface layers of the marsh sediments, a minor flux of particulate uranium out of the marsh on ebbing tides, and net retention via burial of uranium in the salt marsh sediment.
3.3. Sedimentary Uranium The potential scavenging of dissolved uranium and its subsequent enrichment in the surface sediments of the salt
marsh is evident from the distribution of uranium in the near surface sediments (Church et al., 1981 ). These results show a broad subsurface maximum for both 23SU and the 234U/ =3sU activity ratio. The excess 234U/23sU ratio is clear evidence that this enrichment of uranium is, in fact, an "authigenic" fraction of marine uranium scavenged onto the marsh surface from tidal marine waters. Ferdelman (1988) used a wet chemical sequential extraction technique for the marsh sediments in order to determine the distribution of major reactant and product solid sulfur phases. In addition, the associated trace elements and uranium concentrations in these respective phases were determined. Uranium distribution in a neighboring salt marsh (Great Marsh) exhibits a clear association with the humic (base soluble) and metal oxide (ascorbate-citrate soluble) phases, which comprise the major fractions of the total uranium inventory (Table 3. Both these authigenic fractions yield ~34U/238Uvalues near seawater (1.15), as does the total uranium with a concentration 3 - 5 dpm/g-dry sediment in agreement with Church et al., 1981. Unlike iron, copper, or manganese, authigenic uranium distribution does not appear to be clearly associated with any of the major authigenic precipitates of sulfur, such as AVS, pyrite, or organic extractable (lipid) sulfur fractions which include elemental sulfur (Ferdelman et al., 1991 ). Details of the diagenetic redistribution of uranium among various sulfur or metal oxide phases is not apparent from this study. However, rather steady sedimentation of uranium associated with humic materials, authigenic minerals and metal oxides appears to be the likely process governing uranium removal from flooding tidal creek waters. Salt marsh uranium removal may occur as an indirect consequence of pyrite oxidation, due to reduced alkalinity resulting in the uncoupling of the stable uranyl carbonate complex. In the surface salt marsh sediments, 60 to 66% of the uranium exists in an adsorbed or complexed form (as defined by the ascorbate-citrate and base extractions). While the sequential extraction does not lead to direct insight into the uranium removal process, this observation strongly suggests that uranium may first be removed into the salt marsh sediments by simple adsorption-complexation reactions rather than any redox process. Thus, a more aquatic chemical mechanism for uranium removal by salt marsh sediments might be (1) recomplexation of tidal uranium from carbonate to phosphate/humic forms as suggested for upper estuaries (Sarin and Church, 1994), (2) mixing of these stronger uranium complexes with dissolved products of diagenesis including iron or humic acids diffusing up into the surface salt marsh, (3) scavenging of the resultant colloid flocculent (Eastman and Church, 1984), and (4) concentration and burial of uranium as an iron oxide or humate form. In any case, all the above processes are probably operative in that ( 1) the humic/flocculent scavenging mechanism and (2) the direct removal from flooding creek waters occurs as the water flows over and through low pH salt marsh surface sediments. The latter process, at least, is very important in that uranium removal occurs in the summertime when the salt marsh surface is most acidic. However, the former may also be likely as
3885
Geochemistry of U in salt marshes Table 3. Extractable uranium fraction in different phases of salt marsh sediments.*
Depth (cm)
Total
Base Ext.
Asc-Cit Ext.
AVS (6N ItCI)
Pyrite
Sum Seq Total
1.3
3.50±.05 (1.11±.02)
1.51±.03 (1.09-+.03)
0.80-+.02 (1.15±.02)
0.19+__.01 (1.23_+0.29)
0.08+.01 (1.18+__.08)
2.58
6.6
3.17±.03 (1.14_+.01)
1.48±.05 (1.16±.04)
0.4i_+.02 (1.17+.06)
0.12±.02 (2.25_+.23)
0.10±.01 (1.00±.10)
2.11
11.9
5.19±.15 (1.17±.03)
1.27±.04 (1.13±.05)
0.98+.03 (1.15_+.05)
0.30±.02 (1.01±.I1)
0.16±.02 (1.01±.12)
2.71
41.1
1.56±.03 (1.09±.02)
0.38±.03 (1.08±.08)
0.68±,92 (1.14±.03)
0.32± .03 (1.10±.10)
0.06±.01 (0.97±.21)
1.43
*All concentrations expressed in units of dpm/g-dry weight. Numbers in parentheses are ~U/238U activity ratios. The errors quoted are +1 a based on counting statistics (Ferdelman, 1988).
the salt marsh flocculation process displays strong seasonal patterns (Eastman, 1980). 3.4. Uranium Mass Balance in the Salt Marsh Mass balance calculations (Fig. 7) are used to estimate the source or sink term for uranium in the Canary Creek salt marsh (Fleisher, 1985). It is estimated that, on average, as much as 0.5 mg/m -~of dissolved uranium is removed from the tidal waters during summer months (Fig. 3). It is assumed that the uranium removal, if like other trace elements and nutrients, is rapid and less than the tidal mixing cycle of six hours (Eastman and Church, 1984). The amount of dissolved uranium scavenged by salt marsh particulate phases on an annual basis is calculated as the following product: the mean tidal prism for each tide cycle (m3/tidal cycle), the estimated number of tidal cycles (excluding those during October through March) for a given phase of the neap-spring lunar cycle flooding the marsh surface (tidal cycles for April through September), and the average removal of uranium (0.5 mg/tn 3) observed in the tidal prism
Dissolved Uroniurn
15k~/yr Particulate Uronlum
5 kcj/yr
-- '[-Okg/Y r Marsh
CQnory Creek
Sediments
~
_ . _ Delaware D¢Io::~ Boy ~, "~tlon
t ic Ocean
Average Annual Balance
oi.o,v,~ Removal 15*~_kg Particulate Export Sedimentary S i n k
\
-5 ± O.Skg 1 0 = 2 kg
FIG. 7. An annual mass balance of uranium in Canary Creek salt marsh. The dissolved annual uranium removed from tidal waters by this salt marsh is 15 kg based on the salt balance. The observed removal of uranium is balanced by the particulate export (5 kg/y) plus that accumulated in the surface sediments (10 ky/y).
(one tidal cycle). When forced for salt balance, the average amount of uranium processed, via particulate phases, is computed to be 15 kg U/y. (assuming 100 × 103 m 3 as the mean tidal prism and 300 tidal cycles for six months, April through September). Thus, it is assumed that, except during the months October through March, uranium removal occurs at this constant rate throughout the rest of the year. The particulate uranium flux (5 kg U/y) transported out of the marsh, during ebbing tides (Fig. 6), can be derived from the difference between the total amount of particulate uranium in the ebbing waters relative to the preceding flood tide. Within these reasonably approximate assumptions, the model calculations (Fig. 7) estimate that the net sedimentary accumulation rate of uranium on the Canary Creek marsh surface is 10 kg U/y when subtracted for the particulate transport out of the marsh. This translates into accumulation rate of 370 /~g • cm 2 • 103 y, considering the area of the marsh as 2.7 km 2. In fact, this calculation agrees with that computed from the sedimentary sink (420 p,g • cm 2 • 10 -3 y) based on a steady-state authigenic uranium concentration of 1.67/.tg/g in the deeper sections (Church et al., 1981). By using a density of 0.54 g/cm 3 and sedimentation rate as 0.47 cm/y this would yield a net burial rate of about 11 kg U/y, quite similar to that calculated above based on removal from tidal waters. It is important to note that the specific accumulation rate of uranium in this salt marsh is quite significant when compared to that reported for the known anoxic basin sediments in the marine environment (Sackett et al., 1973; Cochran, 1982; Anderson, 1987). Thus, the global geochemical budget for uranium in the marine environment may need to include an intertidal uranium sink. 4. CONCLUSIONS From this study, salt marshes represent one of the most effective marine systems for extracting uranium from seawater. On an annual basis, the amount of uranium removed from the tidal water column (75%) is nearly balanced by its sedimentary sink plus a smaller fraction (25%) of particulate
3886
T.M. Church et al.
uranium that is transported out of the marsh on ebbing tides. This efficiency is astounding considering the relatively short residence time for tidal waters flooding a high marsh. The fact that the process is seasonal in summer and is most efficient at high tides, suggests biogeochemical processes unique to the surface of the high marsh. The process appears related to conditions of strong sulfide oxidation and subsequent acidity which favor coprecipitation with iron oxides or newly formed organosulfur compounds at the marsh surface. Under such a mechanism the acidity appears to be a prerequisite for destabilizing the uranyl ( V ! ) tetracarbonate complex before extraction by these authigenic phases and thus, reduction o f uranium itself is not necessary. The sedimentary redox processes unique to salt marshes (and perhaps mangroves) may be significant aspects for the coastal retention and cycling of uranium. Considering just the areal extent of salt marshes (3.8 × 10~m 2, Woodwell et al., 1973), the estimated global uranium sink in salt marshes is 50 times more than its specific removal (per area) for the entire oceanic area and nearly 10% of the riverine input calculated by Satin et al. (1990). If mangroves are included, the global intertidal uranium could be as much as 50%. In total then, intertidal wetlands (salt marshes and mangroves), upper (polluted) estuaries, and coastal shelf waters may together represent important coastal sinks for the global marine budget o f uranium.
Acknowledgments--Support for this research was provided by NSF grant OCE-8411064. We thank Ms. S. L, Murray for her dedicated technical assistance. We also thank Drs. Benninger and Burnett for their insightful reviews which resulted in a significantly improved manuscript.
Editorial handling: B. P. Boudreau
REFERENCES
Anderson R. F. (1987) Redox behavior of uranium in an anoxic marine basin. Uranium 3, 145-164. Barnes C. E. and Cochran J. K. (1993) Uranium geochemistry in estuarine sediments; Controls on removal and release processes. Geochim. Cosmoehim. Acta 57, 555-569. Baturin G. N., Kochenov A. V., and Yu M. Senin (1971) Uranium concentration in recent ocean sediments in zones of rising currents. Geochim. Intl. 8, 281-286. Benninger L. K. and Chantor J.P. ( 1985 ) Fallout ~:~)-~4%Puand Natural 238U and 2~Pb in sediments of the North River Marsh, North Carolina. Eos 66, 276. Bernat M. and Church T. M. (1989) Uranium and thorium decay series in the modern marine environment. In Handbook of Environmental Isotope Geochem. Vol. 3, The Modern Marine Environment (ed. J. Fontes and P. Fritz), part A, Chap. 10, pp. 357-383. Elsevier. Bonati E., Fisher D. E., Joensuu D. O., and Rydell H. S. ( 1971 ) Postdepositioned mobility of some transition elements, phosphorus, uranium and thorium in deep-sea sediments. Geochim. Cosmochim. Acta 35, 189-201. Boulegue J., Lord C. J., and Church T. M. (1982) Sulfur speciation and associated trace metals (Fe, u/c) in the pore waters of Great Marsh, Delaware. Geochim. Cosmoehim. Aeta 46, 453-464. Carpenter R., Peterson M. L., Bennett J. T., and Somayajulu B. L. K. (1984) Mixing and cycling of uranium, thorium and 2~°Pbin Puget Sound sediments. Geochim. Cosmochim. Acta 48, 1949-1963.
Church T. M., Lord C. J., and Somayajulu B. L. K, ( 1981 ) Uranium, thorium and lead nuclides in salt marsh sediments, Estuarine Coastal Shelf Sci. 13, 267-275. Church T. M., Bernat M., and Sharma P. (1986) Distribution of natural uranium, thorium, lead, and polonium radionuclides in tidal phases of a Delaware Salt Marsh. Estuaries 9, 2-8. Cochran J. K. (1982) The oceanic chemistry of U- and Th-series nuclides. In Uranium Series Disequilibria: Applications to Environmental Problems (ed. M. lvanovich and R.S. Harmon), pp. 384-430. Clarendon. Cochran J. K., Carey A. E., Sholkovitz E. R., and Suprenant L. D. (1986) The geochemistry of uranium and thorium in coastal marine sediments and sediment pore waters. Geochim. Cosmochim. Acta 50, 663-680. Degens E. T., Khoo R., and Michaelis W. ( 1977 ) Uranium anomaly in Black Sea sediments. Nature 269, 566-569. Dorta C. C. and Rona E. (1971) Geochemistry of uranium in the Cariaco Trench. Bull. Mar. Sci. 21, 754-765. Eastman K. W. (1980) Mixing behaviour of iron, manganese, phosphorus, and humic acid in a salt marsh creek. M.S. thesis, Univ. Delaware. Eastman K. W. and Church T. M. (1984) Behaviour of iron, manganese, phosphate and humic acid during mixing in a Delaware salt marsh creek, Estuarine Coastal Shelf Sci. 18(4), 447-458. Ferdelman T. G. (1988) The distribution of sulfur, iron, manganese, copper and uranium in a salt marsh core as determined by a sequential extracting method. M.S. thesis, Univ. Delaware (pp. 27, 83, 122). Ferdelman T. G., Church T. M., and Luther G. W., III ( 1991 ) Sulfur enrichment of humic substances in a Delaware salt marsh sediment core. Geochim. Cosmochim. Acta 55, 979-988. Fleisher M. Q. (1985) The geochemistry of uranium in salt marshes: Tidal mixing and salt marsh-estuary exchange. M.S. thesis, Univ. Delaware (p. 141 ). Gardner W. D., Wolaver T. G., and Mitchell M. (1988) Spatial variations in the sulfur chemistry of salt marsh sediments at North Inlet, South Carolina. J. Mar. Res. 46, 815-836. Klinkhammer G. P. and Palmer M. R. ( 1991 ) Uranium in the oceans: Where it goes and why. Geochim. Cosmochim. Acta 55, 17991806. Kolodyny Y. and Kaplan I. R. (1973) Deposition of uranium in the sediment and interstitial water of an anoxic fjord. In Proceedings of the Symposium on Hydrageochemistry and Biochemisto', Vol. I (ed. E. Ingerson), pp. 418-442. The Clarke Co. Krishnaswami S. and Satin M. M. (1976) Procedure for the simultaneous determination of Th, Pb, Ra Isotopes, 2~Pb, 55Fe, 325i and L4C in marine suspended phases. Anal. Chim. Acta 83, 143-156. Lord C. J., II1, and Church T. M. (1983) The geochemistry of salt marshes: Sedimentary ion diffusion, sulfate reduction, and pyritization. Geochim. Cosmochim. Acta 47, 1381-1391. Luther G. W., II1, Church T. M., Scudlark J. R., and Cosman M. (1986) Inorganic and organic sulfur cycling in salt-marsh porewaters. Science 232, 746. Luther G.W., IlI, Ferdelman T.G., Kostka J.E., Tsamakis E.J, and Church T.M. (199l) Temporal and spatial variability of reduced sulfur species (FeS~, SsO~ ) and porewater parameters in salt marsh sediments. Biogeochem. 14, 57-88. Pellenbarg R. E. and Church T. M. (1979) The estuarine surfacemicrolayer and trace metal cycling in a salt marsh. Science 203, 1010-1012. Ross-Carre H. (1983) Les relations soufre-matiere orgenique-metaux darts les sediments reductuers actuels. Application a la formation due gite urasulfare (type role mobile) de Treville (Aude). Ph.D. dissertation, L'Universite Pierre et Marie Curie. Sackett W. M., Mo T., Spalding R. F., and Exner M. E. (1973) A reevaluation of the marine geochemistry of uranium. In Radioactive Contamination (~(the Marine Environment, pp. 10-14. IAEA. Sarin M. M. and Church T. M. (1994) Behavior of uranium during mixing in the Delaware and Chesapeake estuaries. Estuarine Coastal She!I" Sci. 39, 619-631, Sarin M. M., Krishnaswami S., Somayajulu B, L. K., and Moore W. S. (1990) Chemistry of uranium, thorium and radium isotopes
Geochemistry of U in salt marshes in the Ganga-Brahmaputra river system: Weathering processes and fluxes to the Bay of Bengal. Geochim. Cosmochim. Acta 54, 1387-1396. Sarkar A., Bhattacharya S. K., and Sarin M. M. ( 1993 ) Geochemical evidence for anoxic deep water in the Arabian Sea during the last glaciation. Geochim. Cosmochim. Acta 57, 1009-1016. Scudlark J. R. and Church T. M. (1993) Atmospheric input of inorganic nitrogen to Delaware Bay. Estuaries 16(4), 747-759. Sharp J. H., Culberson C. H., and Church T. M. (1982) The chemistry of the Delaware Estuary. General Considerations. Limnol. Oceanogr. 27(6), 1015-1028. Sharp J. H., Pennock J. R., Church T. M., Tramontano J. M., and Cifuentes L. A. (1984) The estuarine interaction of nutrients, organics, and metals: A case study in the Delaware Estuary. In The Estuary as a Filter (ed. V.S. Kennedy), pp. 241-257. Academic Press. Sinex S. A., Castillo A. Y., and Helz G. R. (1980) Accuracy of
3887
acid extraction methods for trace metals in sediments. Anal. Chem. 52, 2342-2346. Swarzenski P. W., McKee B. A. and Booth G. (1995) Uranium geochemistry on the Amazon shelf; Chemical phase partitioning and cycling across salinity gradient Geochim. Cosmochim. Acta 59, 7-18. Toole J., Baxter M. S., and Thompson J. (1987) The behaviour of uranium isotopes with salinity change in three U.K. estuaries. Estuarine Coastal She!I" Sci. 25, 283-297. Veeh H. H., Calvert S. E., and Price N. B. (1974) Accumulation of uranium in sediments and phosphorites on the South West African Shelf. Mar. Chem. 2, 189-202. Woodwell G. M., Rich P. H., and Hall C. A. S. (1973) In Estuaries. Brookhaven Syrup, Biol. Series 24 (ed. G. M. Woodwell and E. V. Pecan), pp. 221-224. Yamada M, and Tsunogai S. (1984) Post-deposition enrichment of uranium in sediment from the Bering Sea. Mar. Geol. 54, 263276.
Pergamon
PII S0016-7037 (96) 00211-6
Salt marshes: An important coastal sink for dissolved uranium T. M. CHURCh, M. M. SaraN,* M. Q. FLElSHER,~ and T. G. FERDELMAN:E College of Marine Studies, University of Delaware, Newark, DE 19716-3501, USA (Received January, 5, 1996; accepted in revised form June 20, 1996)
A b s t r a c t - - T h e global budget for marine uranium demands another geochemical sink other than deepsea systems, and the coastal environment may host some or all of this missing sink. In a previous paper (Sarin and Church, 1994), we have shown that some large subtidal estuaries are seasonal summer sinks at low salinities. In this paper, we show that intertidal salt marshes are even stronger sinks at all salinities, if for somewhat different reasons. Uranium was sampled in dissolved and particulate fractions over several tidal cycles and seasons for a lower Delaware Bay salt marsh (,Canary Creek, Lewes, Delaware, USA), and uniquely, during summer months, the dissolved uranium is nonconservative. Moreover, because uranium extraction is greater on higher tides and occurs over the entire salinity gradient, this processing appears associated with surface of vegetated high marsh. We hypothesize that either (1) uranium scavenging occurs during the process of tidal mixing and attendant flocculation of humic acids and iron oxides--favoring this process is the presence of sulfonate complexes in salt marsh humic substances, and iron coprecipitation during its extensive redox cycling in the salt m a r s h - - o r (2) uranium extraction occurs at the marsh surface during extensive flooding of the salt marsh surface sediments--favoring this process is the increase in sulfuric acidity at the summer salt marsh surface that could destabilize the tetracarbonate species of U (VI). The latter option is favored by both field observations of maximum removal at the surface during the spring and summer tide conditions, and selective extraction of sediment phases where uranium is found as adsorbed and complexed forms in the ascorbate-citrate and humic acid tractions, respectively. Mass balance calculations show that under steady-state conditions, nearly two-thirds of the uranium extracted from tidal waters is retained in the sediments, while one-third is exported as U-enriched particles during ebbing tides. Independent confirmation of this balance comes from the measured accumulation rate of uranium buried at depth. This represents the net inventory buried below the geochemically reactive surface responsible for the initial extraction and redistribution of uranium onto sediment or tidally exported phases. Extrapolated globally, uranium burial in salt marshes alone or total marine wetlands including mangroves could comprise at least 10% and perhaps as much as 50% the total marine sink for uranium, or on an area specific basis, up to 50 times their marine areal extent. 1. INTRODUCTION
has received considerable interest in recent years is the mechanism for its removal in anoxic and suboxic hemipelagic sediments (Anderson, 1987; Klinkhammer and Palmer, 1991 ; Sarkar et al., 1993; Barnes and Cochran, 1993). Distribution of uranium in Cariaco Trench sediments suggests that the removal of uranium from the water column involves diffusion of U ( V I ) into the sediments where it is slowly reduced to U (IV) and precipitated over a zone of several centimeters below the sediment-water interface (Anderson, 1987). This transformation occurs relatively late in the diagenetic sequence, after the microbially mediated dissolution of manganese and iron oxides, and may be induced by the onset of sulfate reduction with or without uranium reduction. For example, some investigators favor complexation of U ( V I ) by particulate organic matter with subsequent burial of the U-organic complex which is preserved under anoxic conditions (Baturin et al., 1971; Dorta and Rona, 1971; Kolodny and Kaplan, 1973; Degens et al,, 1977), Others prefer initial chemical reduction of U ( V I ) and precipitation of insoluble U ( I V ) (Bonati et al., 1971; Veeh et al., 1974; Carpenter et al., 1984; Yamada and Tsunogai, 1984). In fact, the first-order removal rate of uranium in incubated coastal sediments appears to be linear with sulfate reduction sug-
It has been well documented that uranium is enriched in deep-sea anoxic submarine sediments, the largest known abyssal sink in the marine budget of this element (Sackett et al., 1973; Degens et al., 1977; Cochran, 1982; Bernat and Church, 1989). Thus, besides its input to the ocean via rivers, the concentration of uranium in seawater might appear, to a large extent, to be controlled by its removal via anaerobic pathways in these sediments according to their areal distribution. In this context, the additional anoxic areas prevailing in continental shelf sediments and estuaries could contribute an additional sink accounting for uranium uptake from offshore/coastal waters (Cochran et al., 1986; Toole et al., 1987; Barnes and Cochran, 1993; Sarin and Church, 1994; Swarzenski et al., 1995). One aspect of the marine geochemistry of uranium which * Present address: Physical Research Laboratory, Ahmedebad,
380009, India. Present address: Lamont-Doherty Geological Observatory, Palisades, NY 10964, USA. * Present address: Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany
3879
3880
T. M. Church et al.
DELAWARE BAY
N SAK
-.-.°'--i of D
.... i C ._
Ebenezer Pond Morsh ~-~
L::::N 1
I 0
-
Kilometers
I I
~' FIG. 1. Sampling location of study site for uranium in tidal waters of Canary Creek salt marsh near Lewes, Delaware, USA.
gesting bacterial coreduction of uranium (Barnes and Cochran, 1993). The coastal salt marsh-estuarine system, exhibits a high degree of geochemical reactivity and is characterized by intense primary production, redox processes and tidal exchange (Lord and Church, 1983; Boulegue et al., 1982; Luther et al., 1986). An important biogeochemical question is what role salt marshes play in the recycling of some key elements such as uranium. Previous studies have shown that uranium is enriched in the near-surface sediments and microlayers of the salt marsh, including tidally exported particles (Church et al., 1981, 1986). The objective of the present study is to investigate the behavior of uranium during tidal
mixing in a Delaware salt marsh creek. Such a study could help to understand the fundamental aspects of uranium geochemistry in coastal redox systems, specifically its removal from intertidal waters and its subsequent enrichment in the marsh surface sediments. The paper also deals with the removal mechanism and burial rate of uranium in salt marshes and their significance in the marine geochemical budget of uranium. 2. SAMPLING AND ANALYSES Water samples were collected in the summers of 1983, 1984, 1985, and during the winter of 1984 from the tidal waters of the Canary Creek, a lower Delaware salt marsh (Fig. 1 ). The Canary
3881
Geochemistry of U in salt marshes Table 1.
Concentration of uranium isotopes and seston in Canary Creek, a lower Delaware salt marsh, during mid, spring, and neap tides in 1983.
Dissolved Sam pie Tidal Stage*
S(°/oo)
z3sU (dpm/l)
Particulate 23sU (dpm/g)
234U/z3sU A.R.
23~U/238U A.R.
Seston (rag/l)
Mid Spring Tide 1 2 3 4
20.9 29.2 29.3 25.8
l. 14_+0.03 1.73_+0.05 1.67+_0.03 1.38_+0.04
1.12_+0.02 1.15+0.01 1.15_+0.02 1.13_+0.02
0.93_+0.04 0.85_+0.03 0.65_+0.03 1.88_+0.06
1.07_+0.04 1.05_+0.04 1.08+_0.05 1.11_+0.02
18.29_+1.31 16.52_+1.70 30.05_+2.43 67.94+_3.25
Spring Tide 1 2 3 4
24.1 29.7 29.9 28.4
1.02_+0.03 1.70+_0.05 1.69_+0.03 1.36_+0.05
1.15_+0.02 1.16+0.02 1.12_+0.02 1.15_+0.02
0.99+_0.04 1.19_+0.04 1.49_+0.06 1.70+0.05
1.08_+0.03 1.06+_0.03 1.13_+0.02 1.09_+0.01
23.38_+1.01 33.31_+6.56 27.05_+1.38 56.78+_3.85
Neap Tide 1 2 3 4
24.0 29.2 27.6 15.8
1.21+0.04 1.63_+0.05 1,54_+0.04 0,61+_0,02
1.17+0.02 1.17_+0.02 1.17_+0.02 1,17_+0,02
1.08+0.04 1.01_+0.05 1.32_+0.06 1,82_+0.07
1.14+0.03 0.99_+0.04 1.10_+0.03 1.09_+0.03
33.07_+2.40 28.19_+4.01 38.54+_5.45 38.0l_+5,57
*Tidal Stage No. 1 = Low Slack- Mid Flood 2 = Mid Flood-High Slack 3 = High Slack- Mid Ebb 4 = Mid Ebb - Low Slack A.R. = Activity Ratio
Creek Marsh, located at the seaward edge of Delaware Bay is a high salinity tidal marsh dominated uniquely by the grass Spartina alterniflora. The surface of this marsh is inundated with lower Delaware Bay seawater ( - 3 0 % e ) during the highest (Spring) tides of each month. During the non-flooding stages, the surface of the marsh is directly exposed to the atmosphere. The headwaters of the Canary creek originate as fresh water spillways from Ebenezer Mill Pond. Thus, the system is a small-scale estuary ranging in salinity from 0%c at the pond to 30%e at the mouth. Three individual tidal cycles were sampled in the summer of 1983, covering the range from the neap to spring lunar cycle, over a salinity range of 16-30%~ (Table 1 ). During each tidal cycle, four integrated samples were collected by pumping the water at a constant rate for 3-h periods between successive low tides so as to compare the relative fluxes of uranium into and out of the marsh. During the summer of 1985, an entire salinity transect (0 to 30%0) along the Canary Creek was completed during one of the high tides (Table 2). Samples were collected from shallow wells and fresh water ponds to represent the zero salinity endmembers. In order to reveal the seasonal behavior of uranium and its exchange with the marsh sediments, samples were also collected during the winter of 1984 (Table 2). All samples were filtered in the field through 0.5 /zm absolute cartridge filters and were returned to the laboratory for subsequent analysis of uranium using standard radioehemical procedures (Krishnaswami and Sarin, 1976). Particulate samples of suspended sediment were collected during the summer of 1983 (Table 1) and analyzed for uranium isotopes. The silica and phosphate contents in the tidal waters were measured in the samples collected during the summer of 1985 (Fig. 5). In addition, a sediment core was obtained from the neighboring Great Marsh in August 1985, and the sediments were subjected to an extraction scheme designed specifically for salt marsh sediment (Ferdelman, 1988). Besides fractional determination of the major sulfide precipitates, fractions for humic acids (base extractable),
metal oxides (ascorbate-citrate), and a "total" leach fraction were included. "Total" uranium is also determined for comparison with earlier uranium distributions at the Great Marsh site (Church et al., 1981) and with a North Carolina salt marsh uranium profile (Benninger and Chanton, 1985). The method chosen for total uranium is digestion of ashed subsamples in boiling aqua-regia. This method is similar to the total metal determination for salt marsh sediments described by Sinex et al. (1980) and should digest all of the fractions from the sequential extraction. Although not strictly a complete digestion, this method does provide an accurate assessment of total uranium in the salt marsh sediment (Ferdelman et al., 1991 ). 3. RESULTS AND DISCUSSION T h e C a n a r y C r e e k tidal estuary has no m a j o r tributaries, but it is split at the E b e n e z e r and S a v a n n a h b r a n c h e s near its fresh water source. Both these b r a n c h e s are u n i f o r m l y s u r r o u n d e d b y the salt marsh. Since the input o f d i s s o l v e d u r a n i u m f r o m the fresh w a t e r e n d m e m b e r is m i n i m a l ( < 0 . 0 1 dprrdL, Tables 1 and 2 ) , u r a n i u m input to the Canary C r e e k is d o m i n a t e d by tidal m a r i n e waters. T h e s e m a r i n e waters, h o w e v e r , are already s o m e w h a t d e p l e t e d in u r a n i u m due to r e m o v a l p r o c e s s e s both on the m i d - A t l a n t i c s h e l f and in the D e l a w a r e estuary (Sarin and C h u r c h , 1994). T h e m o s t interesting aspect o f the data is r e v e a l e d by the uranium-salinity plots in the tidal creek waters (Figs. 2 and 3). T h e d i s s o l v e d u r a n i u m c o n c e n t r a t i o n s h o w s n o n c o n s e r vative behavior, during s u m m e r m o n t h s , o v e r almost the entire p o r t i o n o f the salinity gradient. Like the D e l a w a r e and C h e s a p e a k e bay estuaries ( S a t i n and Church, 1994), there
3882
T. M. Church et al. Table 2. Concentrations of uranium isotopes in the tidal Canary Creek of a salt marsh, Delaware Bay.
Sample
Salinity
238U
234U
234U/238U
(%)
(dpm/l)
(dl~n/l)
(A.R.)
1.87±0.05 1.82+0.05 1,665:0.05 1,185:0.03 1.665:0.05
1.145:0.02 1,145:0.01 1.155:0.02 1.125:0.02 1.16+0.02
1.73__.0.05 1.37 ±0.04 1,745:0.03
1.145:0.01 1.11 5:0.02 1,125:0.02
0.0021+0.0001 0.013 5:0.0006 0,058 5:0,002 0.026 5:0.0005 0.29 5:0,004 0.36 5:0,004 0.29 +0.005 0.35 5:0,005 0.45 5:0.007 0.44 5:0.006 0.70 5:0,008 1.33 -I-0.015 2.05 5:0.03
1.055:0.09 0,965:0.05 1.04+0.03 1,145:0.03 1.135:0.02 1,115:0.02 1.135:0.02 1.165:0.02 1.155:0.02 1,105:0.02 1.16+0.02 1.16+0.02 1.135:0.01
Winter (March) - 1984
MMS 1 MMS 2 MMS 3 MMS 4 RI 15
26.4 25.5 23.1 16.2 23.0
1.64+0.05 1.60+0.04 1.44+0.04 1,055:0.03 1.43 ±0.04 Summer (July) - 1984
JMS-1 JMS-2 RI 16"
24.5 26. i 27.1
1.52±0.04 1,245:0.04 1,555:0.03 Summer (June) - 1985
SMS SMS SMS SMS SMS SMS SMS SMS SMS SMS SMS SMS SMS
120"* 111 110"** 109 108 106 107 104 105 103 102 101 100
0.00 0.02 0.19 1.7 9.4 11.9 12.5 13.9 15.0 15,7 19.2 25.6 30.5
0.00205:0.0001 0,014 5:0.0006 0.056 5:0.002 0.023 5:0.0005 0.26 5:0,004 0.32 5:0,004 0.26 5:0.005 0.30 5:0.005 0.39 5:0.007 0.40 5:0.006 0.60 5:0.008 1.15 5:0.015 1.81 5:0.03
* Sample representing seawater end member. **Groundwater sample collected adjacent to the salt marsh. ***Sample from the fresh water pond. A.R. = Activity Ratio
is no evidence for uranium removal from tidal waters during the winter months (Fig. 4). The property-salinity plots of nutrients (Fig. 5) suggest that a large fraction of the silica is r e m o v e d from tidal waters, very similar to its nonconservative behavior in estuarine waters such as Delaware Bay (Sharp et al., 1982, 1984). The distribution of phosphate during tidal mixing in the Canary Creek is noteworthy in that it is removed at midsalinity regions during flooding tides and is desorbed at low salinities during ebbing tides (Fig. 5). Phosphate distribution is mainly dominated by its geochemical reactivity during tidal mixing ( E a s t m a n and Church, 1984), and at the marsh surface during summer months (Scudlark and Church, 1993 ). The nutrient measurements were made only on those samples collected during the s u m m e r of 1985. 3.1. Dissolved
Uranium
The data in Fig. 3 represent probably the first reported evidence for the widespread removal of uranium from flooding tidal waters over the full salinity range of any estuary, in this case a salt marsh. This contrasts subtidat estuarine
and shelf areas where uranium removal appears confined to either the lower ( S a t i n and Church, 1994) or higher (Toole et al., 1987; Barnes and Cochran, 1993) salinity areas. As in the upper reaches of Delaware and Chesapeake bays, large-scale removal of uranium occurs only during s u m m e r months w h e n the exposed surface sediments of the salt marsh exhibit a high degree of geochemical reactivity (Boulegue et al., 1982). In the winter months due to low temperature conditions, the diagenetic processes such as either sulfate reduction or sulfide oxidation with resultant acidity are at a m i n i m u m (Lord and Church, 1983). In response to these well-documented high oxidation rates in the near surface sediments (during summer m o n t h s ) , uranium may coprecipitate along with Fe(OH)3 at the sediment water interface. If the iron coprecipitation m e c h a n i s m for uranium removal from tidal waters is operative, its enrichment onto iron oxides formed in surface sediments may act as a geochemical reservoir of uranium. Ross-Carre ( 1 9 8 3 ) has described the cycling of metals in salt marshes based on the solid phase sulfide phases of the sediments in a model which hypothesizes metal complexat±on by humic thiol functional groups, which may result in
Geochemistry of U in salt marshes
3883
2.0
~,J
0 Mid T i d e o S p r i n g Tide
a.
~ Neop Tide • Fresh Woter
1.5
2,5
./'F 2.0
,. ,,/@/'/
1.5
N :3
,4~
Theoreticol Mixing Line ~
1.0
,,. I I "
/
~0
0
t'~ O ffl
1.0
0.5
o.5
Q
I
0 0
0
6
12
18
SALINITY
t
I
24
30
(%)
F [ G . 2 . D i s s o l v e d 23~U concentration versus salinity from 1 6 - 3 0 % c in the tidal waters o f C a n a r y Creek. T h e n o n c o n s e r v a t i v e b e h a v i o r o f uranium is evident o v e r the restricted salinity gradient.
the transport of complexed uranium within the porewaters. Specifically, the recomplexation of uranium from carbonate ligands under the influence of low pH to humic sulphonate ligands and its subsequent precipitation, may also enhance uranium removal into the solid phases. In addition, there is the trapping of uranium by the marsh surface grasses via enrichment into the surface microlayer and particles in the flooding tidal water (Church et a]., 1986). The removal of uranium from the flooding tidal waters could also proceed either via enrichment onto iron oxides formed in surface microlayers or around Spartina stalks (Pellenbarg and Church, 1979). Thus, based on this evidence, the general scenario for uranium enrichment in salt marsh sediments either directly from solids or tidal waters could be ( 1 ) flocculation of humic substances or metal oxides with scavenging of the uranium in the creek waters and then subsequent removal by deposition onto the marsh surface, or after sur-
I
4
I
8
i
12
115
I
20
I
24
I
28
I
32
S A L I N I T Y (%0)
FIG. 4. Dissolved 238U concentration over an entire salinity gradient of the flooding tidal waters in Canary Creek Marsh during March 1984. Uranium exhibits conservative behavior during this late winter period when associated diagenetic processes are at a minimum.
face microlayer enrichment of the floccs onto Spartina stalks, and/or (2) removal of dissolved uranium from waters flooding the marsh surface sediments due to (bio)geochemical processes operational in the surface marsh sediments. This latter process might be envisioned to involve destabilization of the uranyl (VI) tetracarbonate complex under low pH due to the intense sulfide oxidation. The uranium is then easily scavenged by humic acids and iron oxides within the marsh surface. Furthermore, advective transport of creek waters through bioturbated creek banks may enhance uranium removal from the creek waters. Bioturbation rates, particularly
200
\ 150
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I00
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::I.
..'-,.,
5C 2Z i
!
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I 8
I 12
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r
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t
8
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12
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20
t
24
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t
o_
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32
S A L I N I T Y (%o)
FIG. 3. Dissolved 23~U concentration over an entire salinity gradient of the flooding tidal waters in Canary Creek Marsh during June 1985. Large scale removal is evident during this period when the exposed surface sediments of the salt marsh exhibit a high degree of geochemical activity.
54
FIG. 5. Both silica and phosphate display nonconservative behavior due to biochemical and geochemical behavior during tidal mixing in Canary Creek salt marsh.
3884
T.M. Church et al. 0.16 _1 "
~
0,12
D
0.08
0 Mid Tide a Sprlng Tide ~, Neap Tide
T
I,I
I,--
C-¢:
0.04
0
I
I
I
1
I
2
5
4
LOW S l a c k Mid Flood
Mid Flood High S l a c k
High Slack M i d Ebb
Mid Ebb Low S l a c k
STAGE OF T I D E
FIG. 6. Particulate 2~U (expressed in units of dpm/l) in tidal waters collected during mid, spring and neap tides of Canary Creek salt marsh. For all tidal cycles, the low ebb tide samples show the greatest particulate uranium concentration.
by fiddler crabs, is highest in the summertime near the creek banks (Gardner et al., 1988) and sulfide oxidation is also enhanced in creek bank sediments (Luther et al., 1991 ).
3.2. Particulate Uranium Large increases in the particulate uranium concentration (expressed in units of dpm/L) are observed in the tidal waters for all the late ebb tidal samples, especially during the mid-Spring and maximum Spring tidal cycles (Fig, 6). The particulate uranium concentration is calculated as the product of the specific 23sU concentration in dpm/g and the suspended sediment concentration in mg/L (Table 1). Identical trends are observed for particulate uranium (dpm/g) and suspended sediment (mg/L), indicating that both the particulate loading and the particulate uranium concentration are greatest during late ebb tides. The elevated uranium concentrations observed in the ebbing particulates appear to originate from dissolved uranium scavenged from tidal seawaters, perhaps first via the surface microlayer, before export in particulate form (Church et al., 1986). However, due to relatively low concentration of particulate uranium and large errors associated with the data, the scavenging of uranium from tidal waters is not clearly decernible from the particulate 234U/23~U activity ratios (Table 1). Based on the three tidal cycles sampled, most of the uranium scavenging seems to take place during the Spring tides which cover the salt marsh surface. The primary mechanism appears to be dissolved uranium extraction by the surface layers of the marsh sediments, a minor flux of particulate uranium out of the marsh on ebbing tides, and net retention via burial of uranium in the salt marsh sediment.
3.3. Sedimentary Uranium The potential scavenging of dissolved uranium and its subsequent enrichment in the surface sediments of the salt
marsh is evident from the distribution of uranium in the near surface sediments (Church et al., 1981 ). These results show a broad subsurface maximum for both 23SU and the 234U/ =3sU activity ratio. The excess 234U/23sU ratio is clear evidence that this enrichment of uranium is, in fact, an "authigenic" fraction of marine uranium scavenged onto the marsh surface from tidal marine waters. Ferdelman (1988) used a wet chemical sequential extraction technique for the marsh sediments in order to determine the distribution of major reactant and product solid sulfur phases. In addition, the associated trace elements and uranium concentrations in these respective phases were determined. Uranium distribution in a neighboring salt marsh (Great Marsh) exhibits a clear association with the humic (base soluble) and metal oxide (ascorbate-citrate soluble) phases, which comprise the major fractions of the total uranium inventory (Table 3. Both these authigenic fractions yield ~34U/238Uvalues near seawater (1.15), as does the total uranium with a concentration 3 - 5 dpm/g-dry sediment in agreement with Church et al., 1981. Unlike iron, copper, or manganese, authigenic uranium distribution does not appear to be clearly associated with any of the major authigenic precipitates of sulfur, such as AVS, pyrite, or organic extractable (lipid) sulfur fractions which include elemental sulfur (Ferdelman et al., 1991 ). Details of the diagenetic redistribution of uranium among various sulfur or metal oxide phases is not apparent from this study. However, rather steady sedimentation of uranium associated with humic materials, authigenic minerals and metal oxides appears to be the likely process governing uranium removal from flooding tidal creek waters. Salt marsh uranium removal may occur as an indirect consequence of pyrite oxidation, due to reduced alkalinity resulting in the uncoupling of the stable uranyl carbonate complex. In the surface salt marsh sediments, 60 to 66% of the uranium exists in an adsorbed or complexed form (as defined by the ascorbate-citrate and base extractions). While the sequential extraction does not lead to direct insight into the uranium removal process, this observation strongly suggests that uranium may first be removed into the salt marsh sediments by simple adsorption-complexation reactions rather than any redox process. Thus, a more aquatic chemical mechanism for uranium removal by salt marsh sediments might be (1) recomplexation of tidal uranium from carbonate to phosphate/humic forms as suggested for upper estuaries (Sarin and Church, 1994), (2) mixing of these stronger uranium complexes with dissolved products of diagenesis including iron or humic acids diffusing up into the surface salt marsh, (3) scavenging of the resultant colloid flocculent (Eastman and Church, 1984), and (4) concentration and burial of uranium as an iron oxide or humate form. In any case, all the above processes are probably operative in that ( 1) the humic/flocculent scavenging mechanism and (2) the direct removal from flooding creek waters occurs as the water flows over and through low pH salt marsh surface sediments. The latter process, at least, is very important in that uranium removal occurs in the summertime when the salt marsh surface is most acidic. However, the former may also be likely as
3885
Geochemistry of U in salt marshes Table 3. Extractable uranium fraction in different phases of salt marsh sediments.*
Depth (cm)
Total
Base Ext.
Asc-Cit Ext.
AVS (6N ItCI)
Pyrite
Sum Seq Total
1.3
3.50±.05 (1.11±.02)
1.51±.03 (1.09-+.03)
0.80-+.02 (1.15±.02)
0.19+__.01 (1.23_+0.29)
0.08+.01 (1.18+__.08)
2.58
6.6
3.17±.03 (1.14_+.01)
1.48±.05 (1.16±.04)
0.4i_+.02 (1.17+.06)
0.12±.02 (2.25_+.23)
0.10±.01 (1.00±.10)
2.11
11.9
5.19±.15 (1.17±.03)
1.27±.04 (1.13±.05)
0.98+.03 (1.15_+.05)
0.30±.02 (1.01±.I1)
0.16±.02 (1.01±.12)
2.71
41.1
1.56±.03 (1.09±.02)
0.38±.03 (1.08±.08)
0.68±,92 (1.14±.03)
0.32± .03 (1.10±.10)
0.06±.01 (0.97±.21)
1.43
*All concentrations expressed in units of dpm/g-dry weight. Numbers in parentheses are ~U/238U activity ratios. The errors quoted are +1 a based on counting statistics (Ferdelman, 1988).
the salt marsh flocculation process displays strong seasonal patterns (Eastman, 1980). 3.4. Uranium Mass Balance in the Salt Marsh Mass balance calculations (Fig. 7) are used to estimate the source or sink term for uranium in the Canary Creek salt marsh (Fleisher, 1985). It is estimated that, on average, as much as 0.5 mg/m -~of dissolved uranium is removed from the tidal waters during summer months (Fig. 3). It is assumed that the uranium removal, if like other trace elements and nutrients, is rapid and less than the tidal mixing cycle of six hours (Eastman and Church, 1984). The amount of dissolved uranium scavenged by salt marsh particulate phases on an annual basis is calculated as the following product: the mean tidal prism for each tide cycle (m3/tidal cycle), the estimated number of tidal cycles (excluding those during October through March) for a given phase of the neap-spring lunar cycle flooding the marsh surface (tidal cycles for April through September), and the average removal of uranium (0.5 mg/tn 3) observed in the tidal prism
Dissolved Uroniurn
15k~/yr Particulate Uronlum
5 kcj/yr
-- '[-Okg/Y r Marsh
CQnory Creek
Sediments
~
_ . _ Delaware D¢Io::~ Boy ~, "~tlon
t ic Ocean
Average Annual Balance
oi.o,v,~ Removal 15*~_kg Particulate Export Sedimentary S i n k
\
-5 ± O.Skg 1 0 = 2 kg
FIG. 7. An annual mass balance of uranium in Canary Creek salt marsh. The dissolved annual uranium removed from tidal waters by this salt marsh is 15 kg based on the salt balance. The observed removal of uranium is balanced by the particulate export (5 kg/y) plus that accumulated in the surface sediments (10 ky/y).
(one tidal cycle). When forced for salt balance, the average amount of uranium processed, via particulate phases, is computed to be 15 kg U/y. (assuming 100 × 103 m 3 as the mean tidal prism and 300 tidal cycles for six months, April through September). Thus, it is assumed that, except during the months October through March, uranium removal occurs at this constant rate throughout the rest of the year. The particulate uranium flux (5 kg U/y) transported out of the marsh, during ebbing tides (Fig. 6), can be derived from the difference between the total amount of particulate uranium in the ebbing waters relative to the preceding flood tide. Within these reasonably approximate assumptions, the model calculations (Fig. 7) estimate that the net sedimentary accumulation rate of uranium on the Canary Creek marsh surface is 10 kg U/y when subtracted for the particulate transport out of the marsh. This translates into accumulation rate of 370 /~g • cm 2 • 103 y, considering the area of the marsh as 2.7 km 2. In fact, this calculation agrees with that computed from the sedimentary sink (420 p,g • cm 2 • 10 -3 y) based on a steady-state authigenic uranium concentration of 1.67/.tg/g in the deeper sections (Church et al., 1981). By using a density of 0.54 g/cm 3 and sedimentation rate as 0.47 cm/y this would yield a net burial rate of about 11 kg U/y, quite similar to that calculated above based on removal from tidal waters. It is important to note that the specific accumulation rate of uranium in this salt marsh is quite significant when compared to that reported for the known anoxic basin sediments in the marine environment (Sackett et al., 1973; Cochran, 1982; Anderson, 1987). Thus, the global geochemical budget for uranium in the marine environment may need to include an intertidal uranium sink. 4. CONCLUSIONS From this study, salt marshes represent one of the most effective marine systems for extracting uranium from seawater. On an annual basis, the amount of uranium removed from the tidal water column (75%) is nearly balanced by its sedimentary sink plus a smaller fraction (25%) of particulate
3886
T.M. Church et al.
uranium that is transported out of the marsh on ebbing tides. This efficiency is astounding considering the relatively short residence time for tidal waters flooding a high marsh. The fact that the process is seasonal in summer and is most efficient at high tides, suggests biogeochemical processes unique to the surface of the high marsh. The process appears related to conditions of strong sulfide oxidation and subsequent acidity which favor coprecipitation with iron oxides or newly formed organosulfur compounds at the marsh surface. Under such a mechanism the acidity appears to be a prerequisite for destabilizing the uranyl ( V ! ) tetracarbonate complex before extraction by these authigenic phases and thus, reduction o f uranium itself is not necessary. The sedimentary redox processes unique to salt marshes (and perhaps mangroves) may be significant aspects for the coastal retention and cycling of uranium. Considering just the areal extent of salt marshes (3.8 × 10~m 2, Woodwell et al., 1973), the estimated global uranium sink in salt marshes is 50 times more than its specific removal (per area) for the entire oceanic area and nearly 10% of the riverine input calculated by Satin et al. (1990). If mangroves are included, the global intertidal uranium could be as much as 50%. In total then, intertidal wetlands (salt marshes and mangroves), upper (polluted) estuaries, and coastal shelf waters may together represent important coastal sinks for the global marine budget o f uranium.
Acknowledgments--Support for this research was provided by NSF grant OCE-8411064. We thank Ms. S. L, Murray for her dedicated technical assistance. We also thank Drs. Benninger and Burnett for their insightful reviews which resulted in a significantly improved manuscript.
Editorial handling: B. P. Boudreau
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