Chapter 22
Stable Sulfur Isotopic Compositions of Chromium-Reducible Sulfur in Lake Sediments 1
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Brian Fry , Anne Giblin, Mark Dornblaser, and Bruce Peterson Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543
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We measured δ S values of water column sulfates and reduced sedimentary sulfides (chromium reducible sulfur) in twelve low sulfate lakes from the northern United States and Nova Scotia. The results show that the net isotopic fractionation occurring during sulfate reduction is smaller than that observed in marine environments, with maximum differences (Δδ S) between sulfate and CRS ranging from 0.4 to 18.8%ovs. the 40-70%ovalues typical of marine sediments. In lakes with 12-83 μΜ sulfate, the observed fractionation was not well correlated with sulfate level, but was inversely correlated with two measures of carbon supply available to sulfate reducing bacteria. The data are in agreement with theoretical models that predict smaller isotopic fractionation when increasing carbon availability leads to increased demand for very limited sulfate supplies. We suggest that the observed Δδ S estimate of isotopic fractionation in low sulfate lakes reflects the ratio of sulfate to carbon supply available to sulfate reducing bacteria in sediments. 34
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Sulfate reducing bacteria produce sulfides that are stored in sediments as iron sulfides or organic sulfur. These sulfides are typically depleted in S vs. the initial sulfate because of isotopic fractionation during the process of dissimilatory sulfate reduction. Thisfractionationis measured most simply as the isotopic difference between sulfate and sulfides, i.e., Aô S, and the magnitude of this isotopic fractionation ranges from about 2-70 %o in marine andfreshwatersystems. Laboratory and field studies of sulfate-rich systems have repeatedly shown that this large range in fractionation values is controlled by the rate of sulfate reduction (7, 2), with fractionation decreasing as carbon supply or temperature increases the overall reduction rate. This inverse relationship applies when the reduction rate is 34
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Current address: Department of Biology, Florida International University, University Park, FL 33199
0097-6156/95/0612-0397$12.00/0 © 1995 American Chemical Society Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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398
GEOCHEMICAL TRANSFORMATIONS OF SEDIMENTARY SULFUR
expressed on a per cell basis (3) or on an areal basis that sums over the community of sulfate-reducing bacteria (4). Particularly in marine systems where sulfate levels are high (28mM in full strength seawater) and carbon supply usually limits sulfate reduction rates, maximalfractionationsof 40-70 %o are observed, e.g., Chanton (4), Hartman and Neil sen (5), Goldhaber and Kaplan(6). In sharp contrast to marine systems, lakes have generally much lower sulfate levels (usually less than ΙΟΟμΜ), and smaller fractionations in the 2-20 %o range are observed (7, 8, 9,10). We began the present study to determine whether the difference in sulfate levels between freshwater and marine systems was sufficient to explain the large variation in observed isotopic fractionations, or whether other factors such as carbon supply were involved. We have surveyed sediments of 12 North American lakes for amounts and isotopic compositions of sulfidic, chromium reducible sulfur (CRS). Previous studies have shown that CRS is primarily composed of sulfide-derived sulfur in most lake sediments, with little detectable contribution from organic sulfur compounds (77, 72,13). Using the CRS assay that is specific for the sulfide products of sulfate reduction, we investigated sulfate concentrations, temperature, bottom water anoxia, and carbon availability as possible controls of isotopic fractionation. We advance the hypothesis that the magnitude of sulfur isotopic fractionation observed in lake sediments is not solely governed by sulfate levels, but rather records the balance of sulfate and carbon supplies available to benthic sulfate reducing bacteria.
Methods The twelve lakes studied were diverse in terms of their geography, trophic status, sulfate concentrations, and hypolimnetic oxygen deficits (Table I). The lakes were sampled for water and sediment chemistry at various times during the period between 1987-1991. Oxygen concentrations and temperature were measured with an Orbisphere 2714 meter and stirring probe. Dissolved S 0 analysis was analyzed by ion chromatography (Dionex 2010): precision was 0.3 %. Sediment cores were taken by S C U B A divers with small (6.5 cm) diameter piston corers (14). The cores were sectioned under N and frozen until analysis (75). Lake sediments did not contain appreciable amounts of carbonates , and organic carbon content was analyzed on a Perkin-Elmer 240C C H N elemental analyzer. CRS (chromium-reducible sulfur), which includes all monosulfides, pyrite (FeS ), and elemental S, was determined withfreeze-driedmud by heating in an acid Cr(II)Cl solution (16). Sulfate in water samples was precipitated with barium chloride, filtered, ashed, and decomposed to S 0 with V 0 for mass spectrometric analysis (77). Zinc sulfide precipitates captured from CRS analyses were similarly decomposed to S 0 (for a complete description of isotopic analyses, see (14)). All S isotopic determinations were measured on a Finnigan M A T 251 isotope ratio mass spectrometer. Results are reported as ô S values versus the Canyon Diablo Troilite standard using conventional notation, e.g. Fry (8). Duplicate samples varied by < 0.2 %o 5 S. 2
4
2
2
2
2
2
5
2
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Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
N44.20W74.08 N43.47W74.52
N41.45W70.02 N41.35W70.36 N41.58W69.71
N44.22W65.12
ADIRONDACK^ Heart Dart
CAPE COD Cliff Mares Gull
NOVA SCOTIA Mountain
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N42.44W77.34 N42.47W77.43
FINGER LAKES Canadice Conesus
4
3
2
-
N46.00W89.37
22
67 73 83
12 62
160 230
20 32 32 33 35
48
2
4
19
51 45 49
1 52
146 144
17 25 14 28 32
40
12.9
8.8 12.5 13.9
27.9 20.3
3.1 5.4
25.2 14.2 17.2 18.3 20.2
" 14.6
2
8.5
48.8 15.7 226.3
269.0 90.7
112.9 297.6
167.9 78.8 126.0 208.6 107.7
86.5
3 2
9.4
7.6 8.9 13.7
8.3 5.4
3.1 3.6
6.3 6 1 7.2 7.0 6.7
4.2
4
Epilimnetic S0 '
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+ = Recurrent anoxia, +/- = anoxic bottom water in some years, - = anoxic bottom waters never observed Total Carbon, 0-1 cm CRS concentration at depth of Ô S minimum Lowest ô S value of CRS in upper 10 cm of core
-
+/+
-
+ +/-
+
-
+ +/-
-
+
+/-
N45.48W89.40 N46.02W89.40
N49.65W93.73
Latitude/Longitude
WISCONSIN Bird Trout Basin 2 Trout Basin 3 Trout North Crystal
ELA Lake 240
Lakes
1
Table I. Characteristics of Study Lakes Sediments S0 " (uM) Anoxia Epilimnetic Hypolimnetic % C μπιοί CRS/g dry wt
Downloaded by FUDAN UNIV on February 26, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0612.ch022
-2.5
-4.0 -4.1 8.6
7.9 2.4
-15.7 -14.1
1.4 -4.7 2.3 0.7 -2.4
0.7
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4
11.9
11.6 13.0 5.1
0.4 3.0
18.8 17.7
4.9 10.8 4.8 6.3 9.1
3.5
Ô S CDT CRS Δ5 SsULFATE-CRS
so
s*
I t
1'
î
>
400
GEOCHEMICAL TRANSFORMATIONS OF SEDIMENTARY SULFUR
Results We generally found different isotopic patterns for CRS-ô^S in lakes with anoxic vs. well-oxygenated bottom waters. Sediments from lakes with anoxic bottom waters usually accumulated more CRS with less overall isotopic fractionation (smaller difference between epilimnetic sulfate 5 S and CRS-ô^S) than did sedimentsfromlakes with well-oxygenated bottom waters. This contrast was evident even in a single lake where we collected coresfromoxic and anoxic subbasins (Figure 1). In Trout Lake, Wisconsin, CRS concentrations were higher in sediments underlying anoxic bottom waters, while CRS accumulation in sediments of the oxic basin was lower, even though organic C contents were similar in both cores analyzed. The isotopic values of CRS in the anoxic basin were uniform with depth and close to that of starting sulfate (Figure 1). The CRS isotopic profile in sediments of the oxic basin was more complex, and showed lowest values at 3 cm, a point just above the CRS concentration maximum, followed by increasing values with depth (Figure 1). The contrasts in the isotopic profiles might indicate reduction of most available sulfate in the anoxic basin, with little regard to isotopic composition, but less rapid sulfate consumption that allows largerfractionationin near-surface sediments of the oxic basin. To investigate these possibilities, we measured sulfate concentrations in hypolimnetic waters of Trout Lake. The results show steep concentration decreases in the hypolimnion of the regularly anoxic basin, with a further decline in porewater sulfate to near-zero levels in the top 5 cm of sediments (Figure 2). This very limited sulfate availability for benthic sulfate reducing bacteria led to a lowfractionation,calculated eitherfromthe increase of 5 S values in hypolimnetic sulfate (3.6 %ofractionationfactor calculated using closed system assumptions, following Mariotti et al. (18)) or as the 4.8 %o A5 S difference between epilimnetic sulfate and minimum ô S-CRS value in the upper 10 cm of sediments. We observed a two-fold larger Ad S value (10.8 vs. 4.8 %o) in the oxic basin of Trout Lake, where sulfate reduction in bottom sediments was not sufficiently strong to deplete sulfatefromthe hypolimnion (Basin 2, Figure 2). We also examined sedimentsfromseveral other lakes with varying degrees of bottom water hypoxia. In lakes with recurrent or occasional anoxia, the CRSô S values were usually close to those of epilimnetic sulfate, and CRS isotopic values were fairly constant with depth or increased (Figure 3, top 3 left panels). In contrast, sedimentsfromlakes with well-oxygenated bottom waters usually showed an isotopic minimum in the upper five cm and a larger overall isotopic difference between epilimnetic sulfate and CRS values (Figure 3, top 3rightpanels). There were exceptions to these generalized patterns of CRS isotopic composition. For example, two lakes that had occasionally anoxic bottom waters (Dart Lake and Lake 240, Figure 3) showed contrasting CRS patterns with Dart Lake lacking a sub-surface isotopic minimum as do other lakes with strongly anoxic bottom waters, while Lake 240 showed a sub-surface isotopic minimum that is usually seen in lakes with well-oxygenated bottom waters (Figure 3). In other lakes such as Mountain Lake, we lacked detailed CRS profiles, but the trend of the existing data are consistent with a subsurface isotopic minimum (Figure 3).
Downloaded by FUDAN UNIV on February 26, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0612.ch022
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Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
22.
FRY ET AL.
C R S (/xmol/g) 0
50
100
401
Chromium-Reducible Sulfur in Lake Sediments
150
200
250
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g
β 20 ο
20
Ο
Λ 30 +-> fX ϋ 40 Ω
Trout Lake Basin 3
Trout Lake Basin 3
50
50
60 CRS 50
100
(/xmol/g) 150
200
250
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