Geochemistry of selenium in a coastal salt marsh

0016-7037/91/$3.00

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Geochemistry of selenium in a coastal salt marsh DAVID J. VELIIVSKY*and GREGORY

A. CUTTER

Department of Oceanography, Old Dominion University, Norfolk, VA 23529. USA (Received November 20, 1989; accepted in revised$orm October 19, 1990)

Abstract-The cycling of sedimental selenium was examined over a one-year period in the Great Marsh, Delaware (USA). While total selenium and elemental selenium decrease with depth in the sediments at similar rates, Se(IV + VI) exhibits pronounced seasonality related to the redox conditions of the marsh. Porewater selenium reflects the diagenetic cycling of Se(IV + VI) in the sediments and suggests that a partial remobilization of sedimentary selenium occurs when the upper sediments become oxidizing. Diagenetic and mass-balance models indicate that the major sources of selenium to the marsh are creek waters and atmospheric deposition, while total selenium may be removed from the sediments via the flux of volatile selenium compounds. of selenium in marine sediments. Thus, to fully elucidate the geochemistry of selenium in marine sediments, speciation in porewaters and solid phases must also be determined. The types of reactions that affect selenium in sediments can be both biotically and abiot~cally controlled. These reactions involve conversions between particulate and dissolved phases and include redox reactions that can change the oxidation state of selenium. Examples of such processes include (1) reduction of selenite (Se IV) and selenate (Se VI) to elemental selenium (Se’), (2) scavenging of selenite (Se IV) by Fe and Mn oxides, (31 release or oxidation of organic selenide (Se -11) during diagenesis of organic matter, (4) precipitation of achavalite (FeSe) or ferroselite (FeSe& and (5) incorporation of selenium into solid phases such as pyrite. In temperate latitudes, salt marsh sediments are exposed to varying conditions related to tidal movements and periodic exposure to the atmosphere (FREY and BAMN, 1985). The salinity, temperature, pE and pH of marsh sediments show temporal variations on time scales of hours to months (LORD and CHURCH, 1983; FREY and BASAN, 1985; CASEY and LASAGA, 1987; SCUULARKand CHURCH, 1988). These variations allow redox conditions in the marsh to cycle over similar time periods (i.e., seasonal changes in the oxidizing and reducing conditions within the sediment). In this respect, salt marsh sediments provide ideal natural laboratories for examining the behavior of selenium during the early diagenesis of marine sediments. In particular, processes such as dissolution and precipitation of certain mineral phases (e.g., iron oxides and pyrite: LORD and CHURCH, 1983; KING et al.. 1985: CUTTER and VELINSKY, 1988) can be studied over precisely resolved depth ranges (O-40 cm) and relatively short time scales (months). The information obtained from studying marsh sediments may supply a basis for the investigation of processes in coastal and oceanic sediments where temporal changes are longer and sampling logistics are far more complicated and expensive. The purpose of this paper is to examine the processes which affect the distribution and speciation of selenium in salt marsh sediments. For this investigation, the speciation of solid phase selenium (i.e., Se(IV) -t Se(VI), Se’, pyrite-Se, and organic

INTRODUCITON SELENIUMIS INVOLVEDIN numerous biogeochemical processes in the marine environment. While there have been many studies of selenium in the water column (e.g., MEASURES et al., 1983, 1984; CUTTER and BRULAND, 1984; TAKAYANAGIand WONG, 1985), there has been little research on the geochemistry of selenium in marine sediments. SOKOLOVAand PILIPCHUK(I 973) and TAMARI(1978) reported a positive relationship between total sedimentary selenium (ZSe), pyrite-iron, and organic carbon (OC) from sediments of the North Pacific. They concluded that the correlation between ZSe and OC indicates that selenium is biologically active (i.e., taken up and incorporated by phytoplankton; WRENCH, 1978), whereas the relationship between lZlSeand pyrite-iron may result from the similar chemical properties of selenium and sulfur (GOLDSCHMIDT,1954). Recently, BEIZILE and LEBEL( 1988) reported total selenium profiles in sediments from seven stations within the St. Lawrence Estuary. Their results showed a slight relationship between XSe, iron, and the carbon/nitrogen (C/N) ratio of the sediments. TAKAYANAGIand BELZILE(1988) demonstrated that the loss of oxalic acid-leachable selenium in the upper 2 cm of the sediments is balanced by the upward flux of porewater selenium. The oxalic-leachable selenium is that selenium which is presumably associated with iron and manganese oxides. This indicates that the cycling of Fe/Mn oxides can be important in controlling the distribution of selenium in marine sediments (BALISTRIERIand CHAO, 1990). Selenium, however, can exist in a variety ofoxidation states (-II, 0, IV, VI) in both organic and inorganic compounds, complicating the study of selenium geochemistry. Since the different oxidation states of selenium show markedly different solubilities and affinities for solid phases, changes from one oxidation state to another may affect the potential mobility

* Present uddre”w Carnegie Institution of Washington, Geophysical Laboratory, 525 I Broad Branch Road, NW, Washington. DC, 20015, USA. 179

180

D. J. Velinsky and G. A. Cutter

Se) and porewater selenium were determined on samples taken from the Great Marsh (Lewes, DE) over a one-year period. STUDY AREA The Great Marsh is located near Lewes, Delaware (USA) on the southern shore of Delaware Bay (Site 1, Fig. I). The area is dominated by the short form of Spartina alternijlora and has been relatively undisturbed by human activity since the 193Os,when mosquito control ditches were built. Tidal inundations of the marsh occur only during the highest tide ofeach month, and otherwise the marsh surface remains in contact with the atmosphere. Aspects of the biogeochemistry of the Great Marsh system have been examined by SWAIN (197 I), CHURCH et al. (I 98 l), BOULEGUEet al. (1982), LORDand CHURCH (1983), LUTHERand CHURCH(1988),and CUTTERand VELINSKY (1988), and are summarized below. The exposure of the marsh sediment surface to the atmosphere and infusion of oxygen during S. alterniflora photosynthesis cause iron and sulfur to be cycled seasonally between oxidized and reduced compounds. LQRD and CHURCH(1983) divided the chemical cycle in the marsh sediments into three seasonal phases. In the spring/ early summer, infusion of photosynthetic oxygen by .S. alterniforu (to a depth of ca. 15 cm) produces a subsurface oxidation of iron

monosulfides and pyrite (CUTTERand VELINSKY,1988). This process releases sulfate and protons into the porewaters (LUTHER and CHURCH, 1988), and facilitates the precipitation of iron oxides and elemental sulfur. By late summer/early fall, the temperature of the marsh increases and sulfate reduction becomes the dominant reaction controlling the geochemistry of the marsh. Iron oxides, elemental sulfur, and protons are consumed, with reduced iron (Fe+‘) and sulfide building up in the porewaters. In the winter, both reduction and oxidation rates decrease, allowing the formation of sulfide phases (e.g., iron monosulfides and pyrite) due to the downward diffusion of Fe++ and upward movement of sulfide in the porewaters. The high concentration of dissolved sulfide (up to 7 mM) causes dissolved iron to be totally consumed by the precipitation of iron monosulfides and pyrite at this time. Below the oxic surface layers, pyrite accounts for 70-80% of the total sulfur and reactive iron (LORD and CHURCH, 1983; CUTTERand VELINSKY,1988). This marsh system is not sulfur limited but can be iron limited at depth due to the seasonally changing redox conditions (LORD and CHURCH, 1983). Lateral heterogeneity within this study area (Site I, Lordsville) is minimal. Pyrite and reactive iron analyses from duplicate cores show agreement to within 12% (CUTTERand VELINSKY,1988; TSAMAKIS, 1990). In comparison to seasonal changes in marsh sediments (LUTHERand CHURCH, 1988; KOSTKAet al., 1989; data presented below), lateral heterogeneity is relatively minor.

*)-

DELAWARE

Walminpton

BAY

OLD MILL CREEK CANARY

CREEK

rCK IiOG GUT

0.0

ll

,

RED MILL POND

38”38’45”N

.?*ln*‘U,

\\

1 Kilometer

4 N I 1 w-o__...

FIG. I. Location map of the Great Marsh, Lewes, Delaware. Samples were taken within a meter of each other at Site 1 (Lordsville).

Ge~hernis~~

of selenium in a salt marsh

METHODS

Table

1.

Great

Marsh porewater

selenium

data.

Sample Collection Sediment, porewater, and creek water samples were collected on 4 April 1985, 19 June 1985,5 December 1985,26 March 1986, and 26 June 1986. These dates were chosen to represent typical seasonal conditions in the marsh. Cores (30 to 50 cm deep) were obtained by carefully driving a butyrate core liner (6 cm in diameter) into the sediment. Ail cores were spaced within a meter of each other. By measuring outside and inside core lengths, core compaction was determined to be less than 10%. Core compaction was assumed to be linear and depth intervals were corrected accordingly. The cores were imm~iately sealed from the atmosphere and returned to the laboratory for processing. Within one hour of collection, the cores were extruded and sectioned in a nitrogen-purged glovebox (Lord, 1980), and the porewater obtained using REEBURGH(I 967) sediment squeezers with 0.4 pm Nuciepore filters. The temperature of the cores was maintained as close to ambient marsh temperature as possible. Sampling intervals for the top 30 cm were 2.5 cm. and below 30 cm intervals were increased to 5 cm. Due to the low concentration of dissolved selenium, two cores were squeezed at 5 cm intervals over their entire length to obtain sufficient porewater volume. The first I to 2 mL of porewater was discarded, and the remainder collected in pre-cleaned Teflon bottles. The Dorewater was acidified to IJH 1.5 with 6 M HCI, and stored eithe; under a nitrogen atmosphere or by quick freezing with liquid nitrogen. The total volume of porewater collected by this method ranged from 30 to 80 mL per 5 cm section (two cores combined). Porewaters were analyzed for total dissolved selenium and selenium speciation within 24 hours ofcollection. Sediment samples were frozen immediately after squeezing.

Deptha

CSeb

Depth

CSe

Sampling o-11.5 11.5-14.4 14.4-17.3 L7.3-20,2 20.2-23.0 23.0-25-g 25.9-35.7

date:

4/4/85 ND 0.19 0.43 0.48 ND 0.16 ND

Sampling O-6 6-12 12-18 la-24 27-30 33-36

date: 3/26/W ND ND ND ND ND ND

Sampling

date:

6/19/85 0.12 0.29 1.94 0.10

Sampling o-5.9 5.9-11.7 11.7-17.6 17.6-23.4 23.4-29.3 29.3-34.0

date: 6/26/86 1.30 3.94 0.54 ND 0.16 ND

date:

12/5/85 ND 0.44 ND 0.09 ND

o-5.5 5.5-10.9 10.9-16.4 16.4-21.8

Sampling o-a 8-16 16-24 24-32 32-48

a - Depths in centimeters: b - Concentrations nM Se (porewater); ND - not detectable

in

Analytical Methods The determination of dissolved selenium speciation (total, Se IV. Se VI, Se -II + 0) is described by CUTTER (1978,1983) and VELINSKY (1987). The technique involves the selective generation of hydrogen selenide using sodium borohydride and acidification, liquid nitrogencooled trapping, and atomic absorption detection. The precision is better than 5% (relative standard deviation), with a detection limit of 0.06 nM Se using a 50 g water sample. If sample volumes permitted, all determinations were made in triplicate. Only a few porewater samples had sufficiently high selenium ~oncentmtions for speciation measurements to be performed. The determinations oftotal selenium @Se) and selenium speciation in sediments were performed using the methods developed by CUTTER (1985) and VELINSKYand CUTTER(1990). In brief, portions of the previously squeezed sediments were dried at 40°C, ground with an agate mortar and pestle, and sieved through a 150 pm nylon mesh screen. Prior to grinding, most root material was removed from the sediment. This might unde~stimate the concentration and phase speciation of selenium in the surface sections (O-l 5 cm) of the marsh. However, the amount of selenium in the plants is a small fraction of the total selenium in the sediments (<0.3%) and would most likely not affect the dist~bution of selenium with depth (VELINSKY,1987). For CSe, an aliquot (ca. 0.1 g) was digested using a three-step nitric-perchloric acid procedure (CUTTER, 1985). After evaporation of most of the acid, the remaining residue was taken up in 4 M HCI. The resultant solution was analyzed by hyd~de-~eneration/atomic absorption spectrometry. The concentration of Se(IV + VI) was determined with a sodium hydroxide leaching technique (CUTTER, 1985). Approximately 0.3 g of dried sediment was leached with 2 G NaOH for four hours. The solution was then adjusted to oH 2 and immediately separated from the sediment using centrifugaiion. The supernatant was passed through an XAD-8 resin column to remove

organic selenium, and the eluant was analyzed as above. Precision of the ZSe and Se(IV + VI) methods was generally less than 7 and 10% (relative standard deviation), respectively. The detection limit for both the ZSe and Se(IV + VI) methods is 0.0 I nmol Se g-l when analyzing 0.3 g (dry weight) of sediment. Elemental selenium (Se’) was extracted from a sediment aliquot (ca. 0.3 g) with a I M sodium sulfite solution adjusted to pH 7 (VELINSKY and CUTTER, 1990). After extraction, the sulfite solution was

separated from the residue by centrifugation and was oxidized using nitric acid. The solution was then evaporated to near dryness and taken up in a 4 M HCI solution. The solution was analyzed for total dissolved selenium as described above. Pyrite-selenium was determined by reduction to hydrogen selenide using acidic chromium (II). The generated hydrogen selenide was separated from hydrogen sulfide with a Porapak+PS chromatographic column interfaced to the atomic absorption spectrometer. Selenium in pyrite can exist as ferroselite (FeSe?) or as a mixed Se-S phase (e.g., FeSeS). Analytically, the sep aration of ferroselite and py~te-confining selenium is dif&cult due to their similar chemical properties. Therefore, selenium determined by this method is termed “pyrite-Se.” Organic selenium was calculated as the difference between ZSe and Se(IV + VI) f Se0 + pyrite-Se. Precision for the Se” and pyrite-Se methods was better than IO and 15%(RSD), respectively. The detection limit for Se” is 0.01 nmol Se g-’ with a sample size of 0.3 g (dry weight) and 0.32 nmol Se g-’ for pyrite-Se when analyzing a 80 mg (dry weight) sample. Ancittary data used in this research, such as porewater sulfate, chloride, pH, and sulfide were provided by J. Scudlark and G. Luther III of the University of Delaware. RESULTS AND DISCUSSION

Dissolved Selenium in Pore and Creek Waters Profiles of porewater dissolved selenium exhibit distinct seasonality (Table 1, Fig. 2). Changes in the concentration of porewater selenium appear reiated to the redox characteristics ofthe marsh environment. Maximum concentrations were observed in June 1985 and 1986 when the marsh was most oxidizing, with low or unde~~ble concent~tions when the marsh was reducing (April and December 1985 and March 1986). For example, in April 1985 the concentration of porewater selenium reached a maximum of only 0.48 nM Se (porewater), while in June 1985 a sharp maximum of 1.94 nM Se was observed. This trend was again observed in 1986 with concentrations reaching 3.94 nM Se by June 1986.

D. J. Velinsky and G. A. Cutter

182

noted that the contribution of porewater selenium to the total sedimentary selenium (and its speciation) is negligible due to the separation of the porewaters from the solid phases and the low concentration of selenium in the porewaters. XSe will be examined first to set the stage for the discussion of the different chemical forms of selenium in sediments which follows.

The speciation of dissolved selenium was determined in several of the June 1985 and 1986 porewater samples where sufficient volumes were obtained. In June 1985 two depth intervals (6- 11 and 1 l- 16 cm) were analyzed for Se(IV). Only the 1 l- 16 cm interval had detectable Se(N) (0.47 nM Se), which accounted for 25% of the total dissolved selenium (Table 1). In June 1986 dissolved Se(IV + VI) was determined in all sections of the core. In the top 6 cm, Se(IV + VI) was 0.66 nM Se or 51% of the total dissolved selenium, while below this depth the concentration of Se(IV + VI) was undetectable. The concentration of dissolved organic selenium (calculated as the difference between total dissolved selenium and dissolved Se(IV + VI)) increases with depth and accounts for 100% of the total dissolved selenium below the surface section. These limited speciation data indicate that dissolved Se(IV + VI) comprises ca. 50% ofthe total dissolved selenium at the surface of the marsh and that this fraction decreases in concentration with depth. While the porewater selenium concentrations change dramatically over the period of this study, adjacent creek waters have fairly constant dissolved selenium concentrations. Concentrations range from 0.98 to 1.09 nM Se, with no detectable seasonal differences. The average total dissolved selenium of 1.03 f 0.05 nM Se (n = 4) is similar to those determined in other estuarine and freshwater environments (MEASURES and BURTON, 1978; CUTTER, 1989). The speciation of total dissolved selenium from the creek waters was determined for the December 1985 and March 1986 sampling periods. While the concentration of dissolved selenite is low for both time periods (co.06 nM Se), the concentration of dissolved selenate increases from 0.16 to 0.82 nM Se from December to March. respectively. Dissolved organic selenium decreases from 80% in December to 35% of the total dissolved selenium by March. Selenium Data iments form a during

Total sedimentary selenium Concentrations of total selenium in the marsh sediments @Se) in all cores, regardless of season, exhibit a general decrease with depth (Table 2, Fig. 3). The concentration of ZSe ranges from 2.66 to 12.4 nmol Se g-’ for all depths. Surface sections exhibit an average concentration of 7.45 f 1.72 nmol Se g-’ (n = 5), while concentrations at 34 cm average 4.69 + 0.99 nmol Se g-’ (n = 5). Below 34 cm, concentrations decrease to as low as 2.66 nmol Se g-’ at 40 cm (Table 2). Because all profiles have data at 34 cm, this depth will be used as a reference level for further discussions. A broad subsurface selenium maximum is seen generally between 15 and 25 cm. This broad maximum is also present in the organic carbon and nitrogen data (CUTTER and VELINSKY, 1988). The existence of the selenium maximum may reflect a depositional artifact rather than a diagenetic effect. Given a sedimentation rate of 0.47 cm a-’ (CHURCH et al., 198 I), the deep selenium maximum corresponds to the 1930s when mosquito ditches were dug in the marsh. The resultant change in the drainage pattern of the marsh may have led to an increased accumulation of total selenium (and carbon and nitrogen) during this period.

Sedimentary selenite + selenate In April 1985 the concentrations of Se(IV + VI) in the sediments are less than 8% of the ZSe, with concentrations ranging from 0.19 to 0.48 nmol Se g-’ (Table 2). Distinct trends are not obvious in the April profile (Fig. 4). However, from spring to summer, the profile changes dramatically. The June 1985 profile (Fig. 4) shows a gradual increase in the

in Sediments

on the different chemical forms of selenium in sedin conjunction with total selenium @Se) results help coherent picture of the reactions selenium undergoes early diagenesis of salt marsh sediments. It should be

Total Porewater (nM 4 0

1

2

3

4

Selenium

Se)

012340123

4

0

1

2

3

4

i

6 June 1985

i FIG. 2. Depth distribution of total porewater selenium in the Great Marsh. Sediments at specific intervals, and data are plotted versus the mean depth of each section.

were sectioned

and squeezed

Geochemists Table 2.

Deptha

183

of selenium in a salt marsh

Sreat Marsh solid phase selenium data.

ES&

Pyr-Ssd

SE0

Ss(IV+VI)C

Sampling date: b/4/85 o-2.9 8.864.25 2.9-5.8 7.35kO.25 5.0-8.6 7.4721.27 8.6-11.5 5.834.25 11.5-14.4 5.7020.25 14.4-17.3 5.19+0.25 17.3-20.2 6.33iO.25 20.2-23.0 5.5720.25 23.0-25.9 3.8020.13 25.9-28.8 3.80t0.25 28.8-32.3 4.18+0.51 32.3-35.7 4.05+0.38

0.48l~O.OE9 ND 0.355+0.089 ND 0.355+0.089 0.380+0.063 0.190+0.089 ND ND ND 0.317 ND

4.69kO.38 3.2920.13

SamplinS date: 6/19,'85 O-2.7 5.57 2.7-5.5 5.32kO.25 5.5-8.2 6.59kO.38 8.2-10.9 5.45~0.25 10.9-13.7 5.9520.63 13.7-16.4 4.1820.13 16.4-19.1 5.32 19.1-21.8 6.71iO.63 21.8-24.6 6.21fl.13 24.6-27.3 4.439.13 27.3-30.0 3.9QO.38 30.0-34.4 3.67 34.4-38.8 3.29iQ.25 38.8-42.0 2.66tO.38

0.139 0.317+0.063 0.671 1.2720.01 1.65+0.03 1.39kO.03 0.532+0.063 0.659 0.355 0.456 0.431+0.013 0.266+0.051 0.228 0.114+0.013

3.80 3.8OtO.13 4.0520.38 3.2920.38 3.049.25 2.2820.25 2.9120.25 3.54~0.38 3.41fl.25 2.79TO.25 1.7720.13 1.65kO.13 [email protected] l.Ol+O.l3

ND ND

Sampling date: L2/5/85 O-3.2 5.83 5.074.38 3.2-6.4 6.08~0.25 6.4-9.6 9.6-12.8 5.83kO.13 12.8-16.1 7.22+0.25 7.2220.13 16.1-19.3 19.3-22.5 7.22kO.13 22.5-25.7 9.37kO.38 6.33+0.38 25.7-28.9 28.9-32.1 5.329.25 5.1920.25 32.1-35.3 35.3-38.6 5.199.25 38.6-41.7 5.4QO.25 41.7-48.1 4.6920.38

0.405+0.051 0.?98+o.S1 0.633 0.836+0.051 0.836+0.051 0.?09+0.063 0.494+0.023 0.342+0.025 0.215+0.013 0.367+0.025 0.317+0.025 0.152+0.038 0.114+0.013 0.165+0.013

WA NA NA NA

NA NA NA

NA

NA NA NA

NA

NA

Sampling data: 3/26/86 o-3 9.37iO.89 3-6 10.0 6-9 12.0t1.3 9-12 12.4fl.b 12-15 8.11 15-U 9.8820.25 18-21 9.12*1.01 21-24 8.3620.76 24-27 11.1~1.4 27-30 a.11+0.51 30-33 6.3QO.25 33-36 5.70+0.13 36-39 4.439.13 39-4s 4.6920.38

0.583~0.051 0.747 0.861+0.114 1.90 fl.13 0.469fJ.038 0.735tO.025 0.241~0.025 0.393+0.038 0.519+0.013 0.570+0.051 0.494+0.025 0.329~0.038 0.342+0.038 0.215~0.038

4.56fl.38 4.8lfl.25 4.OQO.25 r.aQo.31 3.93fl.13 3.93fl.38 2.79fl.13 3.554.30 3.29*0.25 2.03+0.25 1.77H.13 1.77fl.38 1.39+a.38 0.76j1.13

ND

Sampling date: 6/26/86 o-2.9 7.60+0.51 2.9-5.9 8.87 5.9-8.8 8.11~0.51 8.8-11.7 10.0*0.4 11.7-14.7 7.60*0.51 14.7-17.6 8.3620.38 17.6-20.5 3.95*0.13 20.5-23.4 8.2320.38 23.4-26.4 6.70+0.51 26.4-29.3 5.70 29.3-31.6 6.08s.13 31.6-34.0 5.19~0.13

0.595+0.038 1.39 20.13 2.15 2.150+0.13 2.150+0.13 0.811+0.038 0.570 0.59QO.025 0.545+0.038 0.291kO.038 0.380+0.051 0.380+0.051

4.05+0.25 4.694.25 2.79fl.01 4.43fl.51 2.41fl.25 3.29ti.13 2.79fl.38 3.29fl.25 2.28f3.25 1.52&0.13 l.le~o.2s 1.279.25

4.31kO.13 2.66~0.38 4.05~0.13 2.2820.13 3.04tO.25 3.5biQ.25 1.65~0.13 1.52to.25 1.39+0.13 0.89+0.25

NA NA NA NA NA NA NA NA

ND ND ND ND ND ND ND ND ND

ND 0.481+0.076 0.393

ND ND ND

0.735+0.114 0.81Q.0.076 ND ND ND ND ND ml ND

NA NA NA NA NA NA NA

ND

0.089 PID AD 0.469 2.41kO.38 0.646 ND

0.342 ND 0.557 0.621 0.950

ND ND ND ND ND ND ND ND ND ND

0.266 Em

Org-sea

3.80+0.51 3.67 3.9320.25 2.91 1.279.25 2.5320.25 3.04+0.38 1.90 1.90 2.03 2.03+0.63 2.79

1.6QO.63 1.14*0.13 1.7750.63 0.89+0.38 1.2720.76 ND 1.01+0.25 2.53tO.76 2.41kO.76 1.27+0.13 1.65tO.38 1.77kO.25 1.65+a.25 1.6QO.38

NA NA NA NA NA NA Nh NA NA NA NA NA NA NA

4.3QO.89 4.439.89 6.9QO.89 3.55~0.63 3.67d.25 4.819.38 3.80+1.01 3.93kO.76 7.47t1.39 5.19+0.63 4.179.38 3.04+0.51 2.15+o.38 2.66fl.38

2.9120.63 2.79+0.63 3.04+0.51 3.429.63 3.179.63 4.31+0.38 2.53fl.25 4.31+0.51 4.059.63 3.8O+O.S1 4.31Ho.25 3.fSfl.25

a - Depth in centimeters; b - Total rclaniwd:all concantrr~iono .=a in nanomolas per gram dry vafght; c - Sclantta+Sslenats;d - Pyrite-salanium: a - Otgar,tcralsnium calculaxted as the dtffarancs between Zse and the suniof S.(IV+VI)+Ss"+Pyr-Se; ND - Not detectable; NA - Not analyzed.

D. J. Velinsky

184

and G. A. Cutter

Total Selenium (nanomole Se g-l ) 0

5

10

15 0

5

10

1s

0

5

10

15 0

5

10

15 0

5

10

15

j % 4 April 198.5

FIG. 3. Depth distribution of total selenium in sediments of the Great Marsh. 3.0 cm intervals, and data are plotted versus the mean depth of each section.

concentration of Se(IV + VI) from the surface (0.14 nmol Se g-‘, 2.4% of the ZSe) to a maximum at a depth of 12 cm (1.65 nmol Se g-‘, 28% of the ZSe). Below 18 cm, Se(IV + VI) accounts for less than 10% of the ZSe (Table 2, Fig. 4). The winter profile of Se(IV + VI) exhibits lower concentrations compared to the June 1985 or 1986 profiles (Table 2). Concentrations increase from 0.41 nmol Se g-’ at the surface to a maximum of 0.84 nmol Se g-’ between 5 and 14 cm. Below this maximum, concentrations and percentages of Se(IV + VI) in the sediments decrease gradually to 7% of the ZSe at 34 cm (Table 2, Fig. 4). From December 1985 to March 1986, concentrations of Se(IV + VI) do not change appreciably. By June 1986 the Se(IV + VI) concentrations increase throughout the upper 15 cm of the core (Table 2, Fig. 4). The concentration of Se(IV + VI) in the surface section is 0.60 nmol Se g-’ (8% of the ZSe) and increases gradually to a broad maximum of 2.15 nmol Se g-’ (23% of the ZSe) centered at 12 cm. The concentrations of Se(IV + VI)

Sediments

were sectioned

at 2.6 to

in the sediments then rapidly decrease to less than 10% of the ZSe at 30 cm. The spatial and temporal distributions of Se(IV + VI) in the marsh sediments appear to be related to changes in the redox environment of the marsh. When the marsh sediments are reducing (e.g., April 1985), FeS forms (CUTTER and VELINSKY, 1988) and Se(IV + VI) is generally less than 10% of the ZSe. During oxidizing periods (e.g., June 1985), as indicated by low or undetectable FeS in the upper 15 cm of the marsh, Se(IV + VI) exhibits concentrations of up to 33% of the ZSe (Table 2). Moreover, Se(IV + VI) is predominantly found in the seasonally oxic portion of the sediment (0- 15 cm). Below I5 cm, in the permanently anoxic sediment, concentrations of Se(IV + VI) in the sediments are less than 10% of the ZSe.

Elemental selenium The distributions of Se” in all cores are similar. Concentrations uniformly decrease with depth and do not display

Selenite+Selenate (% of total selenium) 0

0

25

50

75 100 0

25

50

75 100 0

25

50

75 100 0

25

FIG. 4. Depth distribution of selenite + selenate in sediments normalized to Be. to 3.0 cm intervals, and data are plotted versus the mean depth of each section.

50

75 100 0

Sediments

25

50

were sectioned

75 10

at 2.6

185

Geochemistry of selenium in a salt marsh

Elemental Selenium (% of total selenium) 2.5

50

75 loo

0

25

50 75 loo 0

25

50

75 loo

0

25

50

75 loo

FIG. 5. Depth distribution of elemental selenium in sediments normalized to Be. Sediments were sectioned at 2.6 to 3.0 cm intervals, and data are plotted versus the mean depth of each section.

systematic seasonal differences (Fig. 5, Table 2). Surface concentrations of Se’ (Table 2) average 4.28 f 0.42 nmol Se g-’ (n = 4) or 58% of the ZSe (range = 49 to 68%), below which concentrations decrease to an average of 1.27 f 0.25 nmol Se g-i (n = 4) or 50% of the ZSe at 34 cm. In the June 1985 profile (Fig. 5) Se” accounts for up to 7 1% of the ZSe near the surface and decreases to 46% at a depth of 36 cm. Because Se’ occupies a large region in pE/pH stability field diagrams, GEERING et al. (1968) LAKIN (1973), HOWARD ( 1977), and ELRASHIDI et al. ( 1987) suggested that Se” should be the dominant form of Se in sediments. The Se” data confirmed this prediction. Further, the Se” data along with elemental sulfur (Sea) data for the Great Marsh (CUTTER and VELINSKY, 1988) indicate that seasonal redox changes do not affect Se” to the same extent as S”. This is postulated to result from differences in oxidation-reduction potentials of Se and S (LAKIN, 1973).

Pyrite-selenium The data for pyrite-Se (Table 2) are limited because the concentrations of pyrite-Se in most samples were below the detection limit of the method (VELINSKY and CUTTER, 1990). Overall, concentrations range from 0.27 to 2.41 nmol Se g-’ and account for 4 to 26% of the ZSe. The pyrite-Se data are in contrast to the pyrite data for the same cores (CUTTER and VELINSKY, 1988). While pyrite is the dominant form of sulfur, pyrite-Se is only a minor fraction of the total Se.

Acid volatile selenium Attempts were made to determine acid volatile selenium (AVSe) from marsh sediments. HOWARD (1977) suggests that FeSe (achavalite) could form during alkaline oxidation of FeS-Se” or FeS-HSe-. In contrast, ELRASHIDI et al. (1987) predict that PbSe and SnSe would be stable mineral phases in neutral and alkaline soils. At all depths, AVSe was below the detection limit (~0.32 nmol Se g-l). The absence of AVSe indicates that this phase of Se is either not stable in these sediments or is possibly degraded during storage. The former

conclusion (1977).

is in agreement

with the calculations

by HOWARD

Organic selenium Organic selenium is calculated as the difference between total selenium (De) and the sum of the inorganic forms of selenium (i.e., Se”, pyrite-Se, and Se(IV + VI)). The concentrations of organic selenium from April 1985, June 1985, March 1986, and June 1986 sampling periods are listed in Table 2. Because organic selenium is calculated as a difference, the errors associated with these values are potentially large (Table 2) and interpretations are made with caution. Overall, the concentrations of organic selenium exhibit no consistent trends with depth, and range from 0.89 to 7.47 nmol Se g-’ (Table 2). The concentrations of organic selenium were normalized to total selenium @Se) and plotted versus depth in Fig. 6. In April 1985, the calculated amount of organic selenium is 47 t 11% (n = 12) of the ZSe throughout the entire core. A similar trend is seen in the March 1986 profile (Fig. 6), with an average of 5 1 + 11% (n = 12) of the ZSe as organic selenium. The June 1985 and 1986 profiles show a depletion of organic selenium relative to the XSe in the upper 20 cm of the marsh. Below this depth, organic selenium in the two profiles exceeds 50% of the ZSe. It should be noted that the apparent seasonal variability in the percent organic selenium may be an artifact of the calculation which defines this fraction (i.e., the seasonal changes in sedimentary Se(IV + VI) and lack of seasonal changes in Se”). Mode1 Calculations The transport of Se through the marsh includes the import of Se from creek waters and deposition from the atmosphere, and the export of Se via pore fluids, gaseous emissions, and surface runoff. Within the sediments, oxidation/reduction processes could transform the different chemical forms of Se and affect their transport through the marsh system. A genera1 schematic model of Se pathways in a salt marsh is shown in

D. J. Velinsky and G. A. Cutter

186

Organic Seknium (% of total selenium) 0

25 50 75 loo 0

2s

so 7s I

10

s Y

20.

S E 30. P

IMarch

1986

FIG. 6. Depth distribution of organic selenium in sediments normalized to Be. Sediments were sectioned at 2.6 to 3.0 cm intervals, and data are plotted versus the mean depth of each section. Fig. 7. This schematic is used to guide mass balance calculations and approaches to diagenetic modeling of Se in salt marsh sediments. Input of selenium to the marsh As seen in Fig. 3, the concentration of total selenium in the sediments varies between 2.66 and 12.4 nmol Se g-‘, and shows no distinct seasonal variation. These concentrations are up to an order of magnitude higher than the estimated crustal abundance of Se ( 1.14 nmol Se g-‘: TUREKIAN and WEDEPOHL, 196 1). The major process for the incorporation of Se into marsh sediments is presumably the uptake of Se by plants from creek waters (PETERSONand BUTLER, 1962; RO~ENF’ELDand BEATH, 1964). Furthermore, a recent investigation by OREMLANDet al. (1989) indicates that certain anaerobic bacteria can reduce dissolved selenate and selenite to insoluble elemental selenium. Other important mechanisms for the input of Se to the salt marsh are the adsorption of dissolved selenite and selenate (from creek and rain waters) onto Fe/Mn oxides (BALISTRIERIand CHAO, 1990) and or-

ganic matter, and the de~sition of particulate selenium from creek water and the atmosphere. Overall, the ultimate sources of Se to this marsh are postulated to be the atmosphere (wet and dry deposition) and adjacent creek waters (dissolved and particulate). Although quantifying these inputs is difficult, estimates will be made to indicate their relative importance. Using a present-day wet depositional flux of 0.19 nmol Se cm-’ a-’ (CUTTERand CHURCH, 1986) and a dry depositional flux of 0.04 nmol Se cmb2 a-’ (ROSS, 1985), a rota1 atmospheric flux to the marsh surface of 0.23 nmol Se crne2 a-’ is obtained. It should be pointed out that the wet de~sitional flux is taken from samples collected at a location about 10 km from the marsh site. The dry deposition fiux is taken from data compiled by ROSS (1985) for a remote continental location and may vary by a factor of 4 between intermediate and remote/continental areas. Taking the average Se concentration (C) in the top 2.5 cm section to be 7.45 nmol Se g-’ (-t23%, IZ= 5), a sedimentation rate (0) of 0.47 cm a-’ (CHURCH et al., 1981), a porosity ($) of 0.85 (LORD and CHURCH, 1983), and a sediment dry density (P) of 1.8 g cme3 (CHURCH, pers. comm.), the mass accumulation rate (BERNER, 1980), R, can be calculated using R (nmol Se cme2 a-‘) = C o p (1 - 4).

FIG. 7. Schematic representation of the geochemical cycle of selenium in a salt ma=h environment The major speciation of selenium is represented, and may be in both the dissolved and particuIate state. Major processes include (1) input from creek waters; (2) input from atmospheric deposition; (3) removal and recycling at the sediment surface; (4) diagenetic reactions within the sediment; (5) export to

creek waters; and (6) gaseous emissions ‘tothe atmosphere.

(1)

Assuming steady-state input (i.e., that the changes in the concentmtion of BSe are diagenetj~lly controll~), this equation predicts that 0.95 f 0.25 nmol Se crnm2 a-’ is deposited at the marsh surface in order to support the surface selenium concentration. Thus, atmospheric deposition (wet and dry) can account for only 24% of this input, and, by difference, 76% (0.72 nmol Se crne2 a-‘) of the input must be from creek waters. The key assumption in Eqn. (1) is that the changes in the concentration of total selenium with depth are diagenetically controlled (i.e., the inputs of selenium have been constant over the past 72 years that it took to deposit 34 cm of sediment) and not related to source variations. This assumption is most likely valid for two reasons. First, the concentration of total dissolved selenium in the creek waters, which accounts for approximately 76% of the Se input, is well within the

Geochemistry of selenium in a salt marsh range of concentrations in other unpolluted freshwater (CUTTER, 1989), estuarine (MEASURES and BURTON, 1978; TAKAYANAGI and COSSA, 1985), and oceanic environments (CUTTER and BRULAND, 1984; MEASURES et al., 1984). Thus, increasing the Se input from creek waters appears unlikely. Secondly, the concentration of Se with depth in ice core samples from Greenland (WEISS et al., 1971) shows relatively constant values over the last century. These results suggest that the atmospheric input of Se has also remained constant over the period represented in the upper 34 cm of the marsh. Therefore, the decrease of total selenium in the sediments with depth is more likely due to diagenetic processes than to source-related changes. As stated previously, Se can be taken up into the marsh via biological fixation by S. alternijlora. Selenium can be incorporated into plant tissue as seleno-amino acids (e.g., seleno-methionine) which are used for biochemical oxidationreduction reactions (Stadtman, 1974). To estimate the biological fixation rate of Se, an average plant production rate for short S. alternijlora in the Great Marsh of 0.57 g plant cm-* a-’ (on a dry weight basis) was used (ROMAN and DAIBER, 1984). The concentration of Se in S. alterniflora was determined to be 0.76 + 0.25 nmol Se g-’ (n = 5) from samples (whole plants) taken in April and June 1985 and March 1986. This yields a Se incorporation rate of0.43 nmol Se cm-’ a-‘. These calculations indicate that the biological fixation of Se can be an important mechanism for the incorporation of Se into marsh. Many researchers have found a correlation between organic carbon and selenium (TOURTELOT, 1964; VINE and TOURTELOT, 1970; SOKOLOVA and PILIPCHUK, 1973; TAMARI, 1978). A correlation between organic matter and Se may also be indicative of biotic uptake of Se or possible scavenging of Se onto organic matter. However, regression analyses show only a weak positive correlation between the concentration of organic carbon (CUTTER and VELINSKY, 1988) and either ZSe (r* = 0.29, P < 0.001, n = 67) or organic selenium (r* = 0.08, P = 0.04, n = 5 1) in sediments from the Great Marsh. Therefore, either additional mechanisms besides plant uptake are helping to incorporate Se into these sediments, or processes are fractionating organic carbon and selenium during diagenesis.

Internal cycling of selenium in marsh sediments The internal cycle of Se in the salt marsh sediments may involve the transformation of Se between elemental selenium, selenite f selenate, acid volatile selenium, and pyrite-Se. The oxidation/reduction reactions which change the chemical species of Se can be both abiotic or biologically mediated. In addition, these transformations can include dissolved intcrmediates (i.e., porewater selenium). The only chemical form of Se in the sediments that shows distinct seasonal changes due to redox cycling is Se(IV + VI) (Fig. 4). Internal sources and sinks of Se(IV + VI) include solid phase Se” and organic selenium, as well as porewater selenium. The chemical oxidation of Se” is dependent upon its allotropic form and particle size (SCHULEK and KOROS, 1960; GEERING et al., 1968). The red and black amorphous allotropes of Se are the easily oxidized, while the grey hex-

agonal

187

allotrope is the most inert. GEERING et al. (1968) state that the red form of Se” would be the likely allotrope in sediments because this form initially precipitates from solution. While the abiotic oxidation of Se” is slow (GEERING et al., 1968; HOWARD, 1977), biological oxidation can also be important. SARATHCHANDRAand WATKINSON (198 1) show that a strain of Bacillus megagaterium,isolated from soils, can OXidize Se” to Se(IV). Following from these results and those by GEERING et al. (1968), we speculate that the oxidation of Se” to Se(IV + VI) is likely to be biologically mediated, although abiotic pathways do exist. The seasonal decrease in Se(IV + VI) in the marsh sediments can be related to biological uptake, conversion to either Se” or other selenium phases, and remobilization to the porewaters. Biological uptake of Se(IV) has been documented by PETERSON and BUTLER ( 1962) for higher plants, and by FODA et al. (1983) for bacteria. Bacteria can reduce Se(IV) and Se(VI) to organic selenium (SHAMBERGER, 1983) as well as to Se” (LEVINE, 1925; KOVAL’SKII and ERMAKOV, 1970). Recently, OREMLAND et al. (1989) demonstrated, using slurry experiments, that bacterial reduction of selenate can produce Se”. Their results indicate that the dissimilatory reduction of selenate to Se” can be an important mechanism for the incorporation and retention of Se in sediments. A decrease in Se(IV + VI) in the sediments could also be due to its conversion to gaseous forms. REAMER and ZOLLER ( 1980) show that Se(IV), and to a lesser extent Se(V1) and Se”, is converted to volatile methylated species by micro-organisms in sewage sludge and soils. Overall, it appears that a host of abiotic and biotic mechanisms could account for the loss or gain of Se(IV + VI) in the marsh sediments. The internal transformation of Se(IV + VI) in the marsh sediments can be quantified with a mass balance approach. These calculations assume that changes in Se(IV + VI) in the upper sections of the marsh are related to seasonal variations in the redox environment and that area1 variations in Se(IV + VI) are small. For each sampling period the standing stock (nmol Se cm-*) of Se(IV + VI) was determined by calculating the depth integrated concentrations from the surface to 20 cm using a digitizing planimeter. The transformation rate was determined by subtracting the standing stock for one profile from the previous profile and dividing by the time between sampling. However, these values are actually net rates since they include both production and consumption reactions. From April to June 1985, sedimentary Se(IV + VI) forms at a rate of 0.04 nmol Se cm-* d-‘. while from June to December 1985 it is lost at a rate of 0.01 nmol Se cm-’ d-‘. Between December 1985 and March 1986, Se(IV + VI) is produced at a rate of 0.01 nmol Se cm-’ d-‘. From March to June 1986, the amount of Se(IV + VI) in the sediments further increases at a rate similar to that calculated for the previous year (0.04 nmol Se cm-* d-l). Concurrent with the changes in Se(IV + VI), changes also occur in porewater selenium (Figs. 2 and 4). It is postulated that when the upper I5 cm of the marsh sediment is aerated by the S. alterniflora in the spring, Se” and/or organic selenium are oxidized to Se(IV + VI) with a concurrent partial remobilization of Se to the porewaters. Selenium speciation data for the porewaters indicate both inorganic and organic

D. J. Velinsky and G. A. Cutter

188

forms of Se are present. For example, in June 1986, 5 1% of the total dissolved selenium (Table 1) was present as organic selenium in the upper 6 cm of the marsh, and this fraction increased to 100% below 6 cm. The above data indicate a dynamic redox cycle for sedimentary selenium with a close coupling between the solid and dissolved phases of selenium in the sediment.

Export of selenium from the marsh sediments The profiles of total sedimentary selenium @Se) for all periods show a general decrease with depth (Fig. 3). To calculate a depth-integrated loss of BSe, a simple first-order decay model was used (BERNER, 1980): -W

ase/az -

k[Se] = 0.

(2)

This method is similar to those which have been used to model organic carbon (OC) decomposition in marine sediments (BERNER, 1980; MARTENS and KLUMP, 1984) and assumes that the rate of selenium loss is first order with respect to sedimentary selenium:

asqat =

-k[Se]

(3)

where k is a pseudo-first-order rate constant. The distribution of Se in sediments has not been extensively studied in the past so there are few data or models with which to compare. The use of a first-order decay model does not imply a specific mechanism to explain the decrease in ZSe with depth but is used simply as an approach to determine the loss rate of ZSe in the marsh. This model (Eqn. 2) assumes that porosity is constant with depth and bioturbation in the sediments is neglected. With the boundary conditions Se = Se, at z = 0, the general solution is Se, = Se0 exp[(-k/w)z].

(4)

However, because BSe does not decrease to zero as the depth goes to infinity, as Eqn. (4) would predict, a more complex model was employed to fit the data (MARTENS and KLUMP, 1984). In this approach, ZSe was broken into two classes, a “labile” fraction (Se,) and a “refractory” fraction (Se,). Equations (3) and (4) now apply only to Se, with Se0 replaced by Se,,,_,(= Se,, - Se,). Equation (4) is then rewritten as Se, = (Se0 - Se,) exp[(-k/w)z]

+ Se,.

(5)

To obtain values for Se1,oand k/w, the ZSe data (Table 2) was fitted by an error minimization computer program which produces best-fit Sea, k/w, and Se, values. The depth-integrated loss rate of labile ZSe (Se& can be obtained by substitution of Eqn. (5) into Eqn. (2) and integrating over 34 cm:

dSe,&t

= -kFSel,o exp[(-klw)z]az

which upon integration nium:

yields the loss rate of “labile”

= wFSe,( 1 - exp[(-k/w)z]),

(6) sele-

(7)

where z is the depth interval (0 to 34 cm) and F is a concentration to mass conversion factor (BERNER, 1980) equal to P(1 - @).

(8)

Substitution of the fitted values into Eqn. (7) gives a depth integrated loss of ZSe of 0.33 nmol Se cm-* a-‘. The question arises as to what chemical form or forms of Se are controlling the loss of total selenium in the sediments. The only form of Se that decreases in a similar fashion as ZSe is solid phase Se”. To check if Se” could account for the loss of ZSe, a model similar to that described above was applied to the Se“ data (Table 2). Diagenetic modelling (see above) yields a rate of 0.37 nmol Se cm-* a-’ for the loss of Se”. This rate accounts for essentially 100% of the ZSe loss and indicates that the loss of ZSe in these sediments appears to be controlled by the loss of Se”. Although the loss of ZSe in these sediments can be accounted for by Se’, other more mobile Se intermediates must be involved in this process. Se” is incorporated in the sediments as an insoluble solid and must therefore be removed through either gaseous or aqueous intermediates that can be transported out of the sediment. One mechanism that could remove Se” from the marsh involves the conversion of Se” to soluble forms and subsequent diffusion and/or advection of dissolved selenium across the sediment-water interlace. The porewater and sedimentary Se(IV + VI) profiles suggest that the interconversion of selenium is occurring. During oxidizing events in the marsh, a small but important fraction of the Se” may be oxidized to sedimentary Se(IV + VI), and concurrently a fraction of this is released into the porewaters. For porewater export of Se to be significant, the Se concentration in the porewater must be greater than that in the surrounding creek water. Comparing the average total dissolved selenium from the creek waters of 1.03 nM Se to the porewater profiles in Fig. 2, it is apparent that this is not the case for all but one sampling time (26 June 1986). Otherwise, porewater selenium gradients indicate a flux into the sediment from the surrounding waters. Also, it is possible that porewater advection could remove dissolved Se from the marsh sediments. Although advective flow rates have not been determined for this marsh, calculations based on other marsh systems indicate that the loss of Se from this mechanism would be small (co.0 1 nmol Se cm-* a-‘; VELINSKY, 1987). Since the loss of Se via porewaters appears to be minor, the biogenic emission of volatile Se compounds could account for the loss of Se from the marsh. Such a pathway would include the conversion of sedimentary selenium to methylated selenium compounds (e.g., dimethylselenide. DMSe, and dimethyldiselenide, DMDSe) via microbial processes. These volatile selenium compounds could then flux out of the sediment. In a recent study of the Kesterson Reservoir, Cooke and Bruland (1987) observed significant amounts of DMSe in the water column. They estimated that approximately 30% of the Se introduced to this system may be lost by the production of DMSe and subsequent evasion to the atmosphere. Though it has been demonstrated that microbial processes produce volatile Se compounds in sediments and waters (LEWIS et al., 1974; CHAU et al., 1976; REAMER and ZOLLER, 1980; COOKE and BRULAND, 1987; KARLSON and FRANKENBERGER, 1988), no data on the flux of these compounds from salt marsh sediments exist. The arguments presented above suggest that a gaseous selenium flux of approximately 0.3 nmol Se cm-* a-’ may be

189

Geochemistry ofselenium in a salt marsh

required to account for the loss of Se from the sediments. As an attempt to independently calculate the flux of volatile Se compounds from the marsh, the correlation between total sedimentary selenium and sulfur (CUTTER and VELINSKY, 1988) and an estimate of the gaseous sulfur flux from the marsh surface were used. Selenium fluxes are estimated using sulfur data because the metabolic pathways which volatilize Se and S are thought to be similar (SHAMBERGER, 1983; ROSS, 1985). The total selenium to total sulfur ratios (atomic) for all cores range from 8.3 to 55 X 10m6, with an average value of 20 + 11 X 10m6 (n = 68). Generally, Se/S values decrease with depth in the marsh. The gaseous sulfur flux from s. alternifora-dominated marshes has been measured by a number of researchers for various locations on the east coast of North America (ANEJA et al., 1979; ADAMS et al., 1981; CARROLL, 1983; STEUDLER and PETERSON, 1985; COOPER et al., 1987; de MELLO et al., 1987). Only those studies in which most or all of the dominant S gases (H$, DMS, COS. DMDS, CS2, and CH,SH) were measured are considered for this calculation. Using the average Se/S ratio stated above and a range of S emissions of 1.6 to 18.1 Fmol S cm-’ a-‘, a Se gas flux of 0.04 to 0.38 nmol Se cm-’ a-’ is calculated. This method of calculating the volatile flux of Se from the Great Marsh is complicated by several factors. CHAU et al. (1976) found no direct correlation between concentrations of sedimentary selenium and volatile selenium emissions. Moreover, volatilization rates are dependent upon the types of microorganisms present within the sediment and the oxidation state of the available Se. REAMER and ZOLLER ( 1980) and DORAN and ALEXANDER (1977) show that Se(W) is incorporated and volatilized faster than Se”. Also, it has been assumed that organisms take up and volatilize Se and S in the same ratio as that found in the sediments (i.e., no fractionation between Se and S); the literature contains no data which can be used to check this assumption. In spite of these assumptions, this flux calculation indicates that volatile selenium emissions from the marsh sediment might be of the same order of magnitude as the calculated loss of ZSe from the marsh. In fact, many investigators (see ROSS, 1985; COOKE and BRULAND, 1987; MOSHER and DUCE, 1987) have suggested the loss of Se from aquatic ecosystems via volatilization. However, the importance of this process in salt marshes can only be fully validated when direct flux measurements are obtained. SUMMARY

AND

CONCLUSIONS

The geochemical cycle of Se in the Great Marsh is presented in Fig. 7 and the results of the various model calculations are given in Table 3. This cycle depicts the external inputs/ outputs of Se and the internal changes between the different oxidation states of Se. The depth profiles of the different chemical forms of Se in the sediments along with profiles of porewater selenium over seasonal cycles have revealed the following information: 1) The dominant forms of Se in marsh sediments are Se” and organic Se, and these distributions show no strong seasonal variability. Concentrations of Se(IV + VI) account for a maximum of 30% of the ZSe in the sediments and show strong seasonal variations that coincide with

TABLE 3. Summary of model resultsapb INPUTS (1) Creek waters - 0.73 + 0.19 (76%) - 0.23 f 0.06 (24%)

(2) Atmospheric

INTERNALREACTIONS WITHIN THE SEDIMENTS (3) Loss of total selenium Loss of elemental selenium (4) Net change of Se(VI+IV)

= 0.33 + 0.09 - 0.37 + 0.10 - + 0.08

EXPORTS (5) Porewater

flux

(6) Gaseous flux

-

< 0.01

- 0.04 to 0.38

a - See Figure 7; b - Units are nmole Se cmsL yr-' except (4) which is nmole Se cms2 day-'. Values in ( ) are percentages of the total input.

the redox environment of the marsh sediment. Pyriteselenium is a minor fraction of the ZSe with depth and shows no coherent trend with depth or season. 2) Porewater selenium reflects the diagenetic cycling of sedimentary Se(IV + VI) and suggests that a partial remobilization of sedimentary selenium occurs when the upper sediments become oxidizing. 3) The sources of Se to the marsh are dominated by creek water inputs and to a lesser extent atmospheric deposition. Once the Se is “fixed” into the marsh sediment by both biological and abiological processes, a gradual decrease in ZSe with depth occurs (0.33 nmol Se cmm2 a-‘). 4) Mass-balance calculations in conjunction with diagenetic modelling indicate that the loss of ZSe is related to the decrease in Se“. A possible removal mechanism of Se from this marsh is estimated to be the emission of volatile selenium (0.04 to 0.4 nmol Se cm-* a-‘). Acknowledgments-We thank our colleagues, Tom Church and George Luther III (University of Delaware), for the use of their field site and laboratory in Lewes, and J. Scudlark for his invaluable assistance in the field and laboratory. Chris Krahforst and Jeff Busa provided valuable field assistance. We also thank David Burdige with help in data interpretation, diagenetic modeling, and critical manuscript review. Paul Koch and L. J. Graham provided manuscript review. Reviews and comments by Drs. J. D. Burton, J. W. Morse. and an unknown reviewer were appreciated. Support for this research was from the Electric Power Research Institute (Grant 2020-l) to GAC. This research is part of DJV’s Ph.D dissertation at Old Dominion University. Editorial handling: F. J. Miller0 REFERENCES

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