Changes in sediment redox conditions following the BP DWH blowout event

Abstract

Following the blowout of the Macondo well, a pulse in sedimentation resulted in changes in sedimentary redox conditions. This is demonstrated by downcore and temporal changes in the concentration of redox sensitive metals: Mn, Re, and Cd. Sediment cores collected in the NE Gulf of Mexico (GoM) reveal increased sedimentation after the Deepwater Horizon blowout. The formation of mucous-rich marine snow in surface waters and subsequent rapid deposition to underlying sediments is the likely cause. Respiration of this material resulted in decreased pore-water oxygen concentration and a shoaled redoxcline, resulting in two distinct Mn peaks in sediments following the event, one typically in the top 10 mm, with the other at 20–30 mm. Most cores near the wellhead reveal this non-steady state behavior for up to two years after the event. Associated with the Mn minimum between the two Mn peaks, a modest (15–30%) enrichment of Re consistent with reducing sediments typically exists. A three-year time series of three stations following the event reveal that sediment Re increased 3–4 times compared to the pre-impact baseline value for two years, indicating sediments are increasingly more reducing for two years. In the third year, Re concentration decreased, suggesting a return towards pre-impact conditions. In select sites where the density of benthic foraminifera was determined, an assemblage-wide decrease occurred coincident with reducing conditions as determined by redox sensitive metals, demonstrating the important consequences of changing redox conditions on benthic ecosystems. Determination of redox sensitive metals will continue to constrain the temporal evolution of reducing conditions, which will serve to document the long-term effects of the spill, and the possible return to pre-event conditions.

Full text

Author's Accepted Manuscript
Changes in sediment redox conditions follow-
ing the BP DWH blowout event
D.W. Hastings, P.T. Schwing, G.R. Brooks, R.A.
Larson, J.L. Morford, T. Roeder, K.A. Quinn, T.
Bartlett, I.C. Romero, D.J. Hollander
PII: S0967-0645(14)00371-3
DOI: http://dx.doi.org/10.1016/j.dsr2.2014.12.009
Reference: DSRII3779
To appear in: Deep-Sea Research II
Cite this article as: D.W. Hastings, P.T. Schwing, G.R. Brooks, R.A. Larson, J.L.
Morford, T. Roeder, K.A. Quinn, T. Bartlett, I.C. Romero, D.J. Hollander, Changes
in sediment redox conditions following the BP DWH blowout event, Deep-Sea
Research II, http://dx.doi.org/10.1016/j.dsr2.2014.12.009
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Title: Changes in sediment redox conditions following the BP DWH Blowout event
1
2
Authors: D.W. Hastings
a
, P.T. Schwing
b
, G.R. Brooks
a
, R.A. Larson
a, b
, J. L. Morford
c
, T.
3
Roeder
a
, K. A. Quinn
b
, T. Bartlett, I.C. Romero
b
, D.J. Hollander
b
4
5
a) Eckerd College. 4200 54
th
Ave South, Saint Petersburg, FL, 33711. U.S.A. Phone +1
6
727-864-7884. Email hastindw@eckerd.edu
7
b) University of South Florida, College of Marine Science. 140 7
th
Ave. South, Saint
8
Petersburg, FL, 33701. U.S.A
9
c) Franklin & Marshall College, Chemistry Department, P.O. Box 3003, Lancaster, PA
10
17604-3003. U.S.A.
11
12
Keywords: Oil spill, Gulf of Mexico, Deepwater Horizon, paleoredox, trace metal, rhenium,
13
manganese
14
15
Bounding coordinates: 27.0
0
- 30.0
0
N, 85.0
0
– 88.0
0
W
16
17
Corresponding author: David Hastings
18
19
In Review: Deep Sea Research Part II special volume on the Deepwater Horizon oil spill
20
21
Abstract
22
Following the blowout of the Macondo well, a pulse in sedimentation resulted in changes in
23
sedimentary redox conditions. This is demonstrated by downcore and temporal changes in the
24
concentration of redox sensitive metals: Mn, Re, and Cd. Sediment cores collected in the NE
25
Gulf of Mexico (GoM) reveal increased sedimentation after the Deepwater Horizon blowout.
26
The formation of mucous-rich marine snow in surface waters and subsequent rapid deposition to
27
underlying sediments is the likely cause. Respiration of this material resulted in decreased pore-
28
water oxygen concentration and a shoaled redoxcline, resulting in two distinct Mn peaks in
29
sediments following the event, one typically in the top 10 mm, with the other at 20-30 mm.
30
Most cores near the wellhead reveal this non-steady state behavior for up to two years after the
31
event. Associated with the Mn minimum between the two Mn peaks, a modest (15-30%)
32
enrichment of Re consistent with reducing sediments typically exists. A three-year time series of
33
three stations following the event reveal that sediment Re increased 3-4 times compared to the
34
pre-impact baseline value for two years, indicating sediments are increasingly more reducing for
35
two years. In the third year, Re concentration decreased, suggesting a return towards pre-impact
36
conditions. In select sites where the density of benthic foraminifera was determined, an
37
assemblage-wide decrease occurred coincident with reducing conditions as determined by redox
38
sensitive metals, demonstrating the important consequences of changing redox conditions on
39
benthic ecosystems. Determination of redox sensitive metals will continue to constrain the
40
temporal evolution of reducing conditions, which will serve to document the long-term effects of
41
the spill, and the possible return to pre-event conditions.
42
43
44
45

2
46
47
1. Introduction
48
The uncontrolled release of oil following the blowout event on April 20, 2010 on the Deepwater
49
Horizon drilling platform was unique in many respects. It was the largest offshore accident in the
50
history of the U.S. petroleum industry with over 600 million L of oil (Atlas and Hazen, 2011)
51
and 1.0-1.5 x 10
10
moles of natural gas (Valentine et al., 2010) released into the marine
52
environment at a depth of 1544 m, accompanied by over 6.8 million L of dispersant (Kujawinski
53
et al., 2011). This was the largest accidental release of petroleum and gas into the marine
54
environment, at the deepest depth, with the greatest amount of dispersant added during the spill.
55
The dispersant was added both at depth and at the surface.
56
The exact fate of the released oil is difficult to determine; best estimates are that 35-60% reached
57
the surface, where it evaporated, was deposited on the coast, or was incorporated into flocculent
58
material (Ryerson et al., 2012; Thibodeaux et al., 2011). Subsurface intrusions of natural gas and
59
oil formed, at 1000-1300 m and at ~ 400 m (Joye et al., 2011). These estimates leave about one
60
third of the total oil released unaccounted for.
61
During and following the event, a large amount of mucous-rich marine snow contaminated with
62
oil formed in the surface waters (Passow et al., 2012). Several mechanisms have been proposed
63
to explain the formation of the marine snow following the event, including coagulation of
64
phytoplankton with oil droplets; coagulation of suspended matter with the oil droplets; and
65
production of mucosoid material from the degraders of the oil, which grew and multiplied
66
rapidly following the event (Passow et al., 2012).
67

3
Increased microbial activity following the event has been well documented at the surface and in
68
the water column (Edwards et al., 2011; Joye et al., 2014; Redmond and Valentine, 2011;
69
Valentine et al., 2010; Ziervogel et al., 2012). The consequent microbial production of sticky
70
transparent exopolymeric particles (TEP) enhances the aggregation process, as would the release
71
of exopolymeric substances (EPS) by phytoplankton (Passow, 2000; Verdugo and Santschi,
72
2010). Marine snow may have formed within the subsurface oil intrusions as well as at the
73
surface (Passow, 2014; Passow et al., 2012; Ziervogel et al., 2012).
74
The large flocs of marine snow observed in the upper water column were the precursors of what
75
is observed as a substantial sedimentation pulse (Schrope, 2013). While exact documentation of
76
the sedimentation event remains challenging due to the rapid time scale, there is evidence in the
77
benthic environment of increased sedimentation over an extensive area in the NE Gulf of Mexico
78
(GoM) close to the wellhead following the spill (Montagna et al., 2013; White et al., 2012;
79
Ziervogel et al., 2012). The swift transport of particles from the surface to the sediments is a
80
well-established process with relatively large sinking particles (e.g. Alldredge and Silver, 1988;
81
Asper et al., 1992). Consistent with this pulse, a visually distinctive, brown, fine-grained surface
82
layer in the top 1-2 cm with dark brown or black bands is seen in the sediment cores affected by
83
the blowout event.
84
A single mechanism for this increase in sedimentation has not been identified. The formation and
85
aggregation of marine snow following the event, sinking rapidly to the bottom, is one likely
86
scenario. Mucous-rich marine snow was observed following the spill (Passow et al., 2012), and
87
in simulations of the spill (Passow, 2014), which is consistent with the rapid sedimentation to
88
depth. Changes in the outflow of the Mississippi River is another feasible mechanism. The
89
Mississippi River was diverted in an effort to protect sensitive coastlines, and with a modest
90

4
increase in Mississippi River outflow (Bianchi et al., 2011), siliciclastic input to the GoM may
91
have increased. About 5% of the total oil released was burned (Ryerson et al., 2012); the
92
pyrogenic remnant waste could be another contributor to the sedimentation pulse, albeit a minor
93
one.
94
Our hypothesis was that the pulse of organic rich material to the sea floor associated with the
95
marine snow event resulted in increased respiration of organic carbon in the sediments, which in
96
turn resulted in decreased oxygen in sediment pore waters. We use changes in the relative
97
concentration of redox sensitive elements, Mn, Re, and Cd, to constrain changes in the redox
98
state of marine sediments following the blowout event.
99
1.2 Background: Redox sensitive metals
100
Authigenic enrichment of redox sensitive metals occurs under reducing conditions since these
101
metals undergo a change in redox state and either become less soluble, are adsorbed to surfaces,
102
or form insoluble metal sulfides. These metals have been exploited in numerous studies to
103
constrain paleoredox conditions (e.g. Crusius et al., 1996; Dean, 1989; Morford et al., 2012;
104
Morford et al., 2001; Nameroff et al., 2002; Tribovillard et al., 2006). Bulk Mn is depleted, and
105
Re, Cd, V, Mo, and U are all authigenically enriched under reducing conditions providing an
106
excellent proxy for low oxygen and anoxic environments. In this study, we focus on Mn, Re and
107
Cd. Several excellent review articles (Algeo and Rowe, 2012; Morford and Emerson, 1999;
108
Tribovillard et al., 2006) describe these processes in detail; a summary of their geochemical
109
behavior is provided below.
110
1.2.1 Manganese
111

5
The redox chemistry of manganese (Mn) in marine sediments has been studied extensively, in
112
part due to the importance of Mn cycling across the redoxcline in reducing environments. Solid
113
Mn(IV) oxides delivered from overlying seawater are readily reduced to dissolved Mn(II) when
114
pore water oxygen is consumed. The soluble Mn(II) diffuses upward, and where pore water
115
oxygen is present, is then oxidized to Mn(IV) oxide completing the redox cycle of Mn in the
116
sediment (e.g. Burdige and Gieskes, 1983; Froelich et al., 1979; Gobeil et al., 1997). A distinct
117
peak in bulk Mn typically marks the top of the redoxcline. Recent findings of abundant
118
porewater Mn(III) in hemiplegic sediments requires a revision of this classic redox model to
119
include one electron transfer reactions for the Mn cycle (Madison et al., 2013). Dissolved
120
Mn(III) intermediates are produced by oxidation of Mn(II) by dissolved oxygen as well as
121
dissimilatory MnO
2
reduction by organic matter (Madison et al., 2013).
122
1.2.2 Rhenium
123
Rhenium (Re) behaves conservatively in seawater, is mobile under oxic conditions, and
124
precipitates under mildly reducing conditions (Crusius et al., 1996). It is ideally suited as a redox
125
tracer since its detrital concentration is very low relative to the authigenic deposition; oxic
126
marine sediments have low concentrations of 0.6 ppb or less (Boyko et al., 1986; Koide et al.,
127
1986). Re is not enriched in ferromanganese nodules (Koide et al., 1986), does not show an
128
association with either Fe or Mn oxides (Morford et al., 2005; Schaller et al., 2000), and
129
hydrothermal processes play a negligible role in Re geochemistry (Ravizza et al., 1996). Re is
130
enriched in reducing sediments because dissolved Re(VII)O
4-
is reduced and precipitates in the
131
solid phase, most likely as Re(IV)O
2
(Crusius et al., 1996). Re enrichment occurs under both
132
anoxic (Colodner et al., 1993) and suboxic conditions, below the zones of U and Fe reduction
133
and prior to sulfate reduction (Crusius et al., 1996; Morford et al., 2005).
134

6
1.2.3 Cadmium
135
Cadmium (Cd) is a nutrient-like element in seawater, with water column profiles analogous to
136
dissolved phosphate implying an association of Cd with biogenic soft parts. Degradation of Cd-
137
rich organic material results in enrichment of Cd at or just below the sediment –water interface.
138
Authigenic enrichment occurs where Mn is depleted, just below the redox front (Gobeil et al.,
139
1997). Enrichment of Cd is a consequence of precipitation of an insoluble sulfide phase at very
140
low sulfide levels below common analytical detection limits of several µ mol/kg (Rosenthal et al.,
141
1995a).
142
1.2.4 Barium
143
Barium (Ba) in marine sediments is controlled by several factors. Ba preservation declines under
144
suboxic conditions, with high organic carbon respiration and/or low bottom water oxygen
145
(McManus et al., 1998). Thus, the accumulation rate of marine barite in marine sediments has
146
been used to reconstruct past changes in ocean productivity (e.g. McManus et al., 1999; Paytan
147
and Griffith, 2007). Authigenic barite forms in the water column, as well as within marine
148
sediments and around hydrothermal vents and cold seeps (Griffith and Paytan, 2012), many of
149
which are found in the Northern GoM and is thus of particular importance (Feng and Roberts,
150
2011; Joye et al., 2010). Mississippi River water is enriched in barium relative to GoM water
151
(Hanor and Chan, 1977).
152
Of special relevance to this work, barium is highly enriched in drilling mud, which relies on
153
barite to increase the density (Trocine and Trefry, 1983). In May 2010, an attempt was made to
154
plug the leaking well with more than 10
5
bbl/day (>200 x 10
5
L/day) of high density drilling mud,
155
the so-called “top kill” (National Commission on the BP Deepwater Horizon Oil Spill, 2011). An
156
enrichment of Ba was seen in deep-water plume samples in May 2010 associated with this effort
157

7
(Joung and Shiller, 2013). Enrichment of Ba in surficial sediments close to the wellhead is
158
evidence of this effort.
159
2. Methods
160
2.1 Site Description
161
We collected sediment cores at five stations in the NE GoM aboard the R/V Weatherbird II on a
162
series of cruises from August 2010 to August 2013 (Figure 1; Table 1). Cores were retrieved
163
with an Ocean Instruments MC-800 multi-corer, which collects up to eight 10 cm diameter cores
164
without disturbing the sediment-water interface. Cores were carefully extruded using a calibrated
165
threaded rod at 2 mm resolution in the top 20 mm, and at 5 mm resolution below 20 mm.
166
Sediment samples from stations close to the wellhead were retrieved with a box core aboard the
167
RV Pelican leg PE-1031 on May 5-9, 2010, two weeks after the explosion, which we use as a
168
pre-event control since it precedes the large sedimentation pulse. Cores were extruded at 2-3 cm
169
resolution.
170
Box cores from the Fisk Basin (PE07-5I; 817 m depth; 27°33.0
0
N, 92°10.1
0
W) and Garrison
171
Basin (PE07-2; 1570 m depth; 26°40.5
0
N, 93°55.5
0
W) were collected on the R/V Pelican in
172
2007; a box core beneath a long-term sediment trap mooring in the NGoM was collected in
173
January 2009 (PE09-004 MC1). Linear sediment accumulation rates are relatively high (20–40
174
cm/kyr) due to large inputs of terrigenous material from the Mississippi River. AMS
14
C dates
175
with bomb radiocarbon confirm that the core-top samples include the most recently deposited
176
sediments (Richey et al., 2007).
177
2.2 Visual Core Descriptions
178

8
Cores collected in the NE GoM on August 2010 and later reveal a surficial dark brown layer, 1-
179
10 cm thick, overlying a lighter, tan colored layer, which extends to the base of the sediment core.
180
Within the dark brown surface layer, at least one and sometimes two distinct darker brown-black
181
bands are present. The sediment-water interface for core DSH-08 (February 2011) was at an
182
incline, with a 5-7 mm offset from one side to another. Thus, the interval that we report at 7 mm
183
depth was at the sediment water interface and exposed to bottom water on one side of the core
184
barrel.
185
2.3 Solid Phase Analyses
186
Subsamples (~0.2 g) were freeze dried, weighed, then digested in a Milestone Ethos EZ
187
microwave oven in closed Teflon
®
digestion vessels with 10mL concentrated trace metal grade
188
HNO
3
at 165°C and high pressure (~25 bar) for 15 minutes according to standard methods (US
189
EPA method 3051a). The digest was diluted 1:10 with MQ ultrapure H
2
O, and filtered with 0.45
190
µm PVDF syringe filters. Since HF was not used in the digestion method, the digest does not
191
include refractory components such as aluminosilicates, but does include authigenic phases,
192
crude oil, organic phases, FeMn oxides, and carbonates.
193
The samples were analyzed using an Agilent 7500cx ICP-MS with an octopole reaction cell in
194
helium mode for Mn, Fe, and Cd to reduce isobaric interferences and in no-gas mode for Mo, Ba,
195
and Re. Prior to analysis, samples were spiked with an internal standard containing Ge, In, and
196
Bi in order to correct for instrumental drift during analysis. Elemental concentrations were
197
determined using a 6-point external calibration line. Triplicate samples were typically measured
198
for one or two depth intervals in each core with an average relative precision of ±2%, ±4%, ±4%,
199
and ±4% (1 σ) for Mn, Re, Cd and Ba, respectively. Long-term analytical precision based on
200
analyzing the same sample 3 times each run over 1.5 years of ICP MS analyses is ±3%, ±5%,
201

9
±5%, and ±2% (1 σ) for Mn, Re, Cd and Ba, respectively. A complete data set collected under
202
this project, including all elements determined by ICP-MS, is stored with the Gulf of Mexico
203
Research Initiative Information and Data Cooperative (GRIIDC) and available at
204
https://data.gulfresearchinitiative.org. The data set used in this paper is available as a
205
supplementary data set at http://www.pangaea.de
206
2.4 Benthic foraminifera
207
Subsamples of sediment were freeze-dried, weighed and washed with a sodium
208
hexametaphosphate solution through a 63-µm sieve to disaggregate the clay particles from
209
foraminifera tests. The coarse fraction remaining on the sieve was dried and weighed again. All
210
benthic foraminifera were picked from the samples, identified, and counted. Total assemblage
211
density is reported, as opposed to living community density for direct comparison of up-core
212
(post-DWH) to down-core (pre-DWH control) records (Osterman, 2003; Scott and Medioli,
213
1980). The total assemblage density approach was also appropriate since these records were to
214
be used as reference records when determining any persistent sedimentary (physical, chemical,
215
biological) features related to the DWH event in future sedimentary records on the decadal time-
216
scale (Osterman, 2003; Scott and Medioli, 1980). Foraminifera assemblage density values were
217
reported in individual per unit volume (indiv/cm
3
) (Sen Gupta et al., 2009). The values were
218
normalized to the known wet volume of each sample based on the core diameter (10 cm) and the
219
thickness of each sample (2 or 5 mm).
220
3. Results:
221
3.1 Pre-impact results
222

10
Our first task is to establish natural, pre-event levels of redox sensitive metals and the normal
223
depth of the redoxcline before the spill. Determining this is a challenge, since sediment cores
224
near the wellhead were not sampled at high resolution before the event. We identify pre-event
225
conditions using sediment cores at three sites located in the Northern GoM: Garrison and Fisk
226
Basin, and site PE 09-004, where a sediment trap is moored. We also obtained a suite of four
227
cores near the wellhead taken less than two weeks after the explosion and before substantial
228
sedimentation occurred; the site closest to the wellhead is presented here. Authigenic metal data
229
are presented graphically in figures 2a-d, and in supplementary data table 1.
230
Garrison Basin (Figure 2a; 1570 m): Mn values are at a low baseline value in the top 120 mm,
231
increase to a distinct peak at 162 mm, then decrease back to baseline levels. Both Re and Cd
232
remain low with no enrichment at depth. These profiles indicate a redoxcline at 170-180 mm,
233
where bulk Mn begins to decrease, with no enrichment of either Re or Cd associated with
234
reducing conditions. Ba values between 200 -300 ppm indicate levels that would be expected in
235
sediments not affected by drilling activities or cold seeps.
236
Fisk Basin (Figure 2b; 817 m): Shallower and situated closer to the mouth of the Mississippi
237
River than Garrison Basin, Fisk Basin shows a shallower redoxcline, at 90-120 mm, as indicated
238
by the Mn peak centered at 80 mm. Re is constant at 0.4 ppb from the surface to 110 mm, where
239
it increases to the base of the core. Cd decreases from the surface to 170 mm, and then increases
240
to 0.4 ppm at the bottom of the core, likely associated with CdS(s) formation. In both Garrison
241
and Fisk Basin, Ba decreases from surface values of 300 and 600 ppm, respectively, to ~200
242
ppm in the top 10-40 mm.
243

11
Sediment trap site PE09-004 (Figure 2c; 1132m): Closer to the wellhead than either Garrison
244
or Fisk Basin, this site adds insight into pre-impact profiles of redox sensitive metals at a depth
245
similar to impacted sites. A distinct Mn peak at 77 mm with Re enrichment directly below this
246
depth indicates a redoxcline somewhat shallower than the two other pre-impact sites. The
247
absence of Cd enrichment is consistent with no sulfide, and therefore no CdS(s).
248
Proximal to DWH wellhead PE 1031-6 (Figure 2d; 1380 m): Sediments taken two weeks after
249
the explosion 7 km from the DWH wellhead at site PE 1031-6, show Mn decreasing from 12.6
250
mg/g at the surface (0-20 mm), to 2.7 mg/g at 65 mm. Re increases from 0.6 ppb in the top
251
20mm to 1.4 ppb at 35 mm, followed by more substantial increases to 2.6 ppb at 100 mm and 5.7
252
ppb at 125 mm. The sample resolution for this core is coarse, 20-30 mm, and is not adequate to
253
resolve the characteristic Mn peak, or allow a detailed comparison to our sampling at 2 mm
254
resolution. Sediments from three additional sites sampled at the same time close to the wellhead
255
were also analyzed as part of our effort and provide insight into pre-event [Re].
256
3.2 Post-Impact Results
257
Time series: Three sites were sampled five to six times between August 2010 – August 2013,
258
allowing us to determine how the event impacted surficial sediments and to constrain the
259
temporal evolution of reducing conditions. While Mo, V, and U are enriched in suboxic and
260
anoxic sediments and are frequently part of the suite of redox sensitive trace metals, downcore
261
changes in these elements were associated primarily with changes in Mn oxide (e.g. Hem, 1978)
262
and did not provide geochemical insight into other processes. We do not discuss changes on Mo,
263
V, and U since they primarily reflect changes in bulk Mn in the surficial sediments affected by
264
the depositional event. Rather than present figures of all 17 downcore metal profiles from each
265
time different stations were sampled, we describe general trends and present downcore results
266

12
that are representative of element profiles at the three different stations. In each figure, the
267
surface 30 mm is shown in greater detail to reveal more subtle changes in down core
268
concentrations. A complete data set with metal data from each station are provided in
269
supplementary data table 1. Temporal evolution of reducing conditions is best described by
270
changing [Re] over time.
271
3.2.1 DSH-10 Bulk Mn is typically characterized by two peaks in the top 5 -30 mm, separated by
272
about 15 mm (figures 3a, b, c). In December 2010 and August 2013 profiles, there is a broad Mn
273
maximum spread between 5 – 19mm, rather than two distinct peaks; the surficial Mn peak at
274
11mm in the February 2011 profile is characterized by just one point, with a relatively small
275
increase of 3 mg/g (25%). In most profiles, there is a modest Re enrichment of 0.1-0.3 ppb
276
coincident with the Mn minimum defined by the two peaks. This is either not significant or
277
nonexistent in September 2011, August 2012, and August 2013. Except for September 2011, Re
278
increases to a distinct relative maximum just below the second Mn peak, then decreases before
279
increasing again at depth, typically at ~100 mm. For August 2010 and February 2011, a
280
secondary Cd peak of 0.08 ppb and 0.05 ppb exists at 5 mm and 19 mm, respectively, in addition
281
to the broad Cd enrichment below the deeper peak common in all profiles. This secondary Cd
282
peak is reduced in the September 2011 and August 2012 profiles.
283
3.2.2 DSH-08 As with DSH-10 there are two Mn maxima in the surface 30 - 40 mm for samples
284
taken in December 2010 and February 2011. Re increases from the surface by ~0.2-0.4 ppb to a
285
maximum coincident with the Mn minimum (figure 4a, b). In February 2011, there is also a Re
286
peak at 29 mm, at the base of the second Mn peak. The last three sample dates (September 2011,
287
August 2012, August 2013) show a single Mn peak, or a very subtle second peak, rather than a
288
distinct double Mn peak, with a more gradual decline in Mn to the baseline values at depth. In
289

13
August 2013, Re has a distinct peak at 15 mm where Mn is decreasing. Below the relative
290
minimum at 19 mm, Re then gradually increases to the base of the core.
291
3.2.3 PCB-06 In December 2010 (figure 5a), double Mn peaks are evident, close to the surface
292
at 5 mm and at 21 mm, a feature which is less pronounced in February 2011 (figure 5b), and
293
subdued, or no longer present, in the last three sample efforts (September 2011, August 2012,
294
August 2013. In December 2010 a modest Re enrichment of 0.1 ppb is coincident with the Mn
295
minimum at 7 mm (figure 5a). Re enrichment above background values is at the surface in
296
February 2011; surface Re continues to be above background values in September 2011.
297
Subsurface Re peaks at 50-70 mm are evident in all but the February 2011 core. For December
298
2010, February 2011, and September 2011 a secondary Cd peak exists at 15 to 17 mm, in
299
addition to the broad Cd enrichment below the deeper peak common in all profiles. Minor
300
variations in Re patterns in August 2012, and 2013 cores seem to show the opposite patterns:
301
modest Re increases are not coincident with Mn minima.
302
3.2.4 SW-01, April 2012: Mn is characterized by two peaks, at 15 mm and 81 mm with a
303
relative minimum at 35 mm (figure 6). Re mirrors the Mn profile with a clear maximum at
304
35 mm, a minimum at 60-80 mm, then increasing to a maximum of 2.4 ppb at 142 mm. At or
305
below the redoxclines as defined by the Mn minima, Cd increases to a maximum of 0.22 ppm at
306
35 mm and again at 102 mm. Ba shows an unusual downcore profile, with two distinct maxima;
307
Ba values are consistent with normal, pre-impact concentrations (265 ppm) at the surface, with
308
distinct maxima at 5 mm (1100 ppm) and at 51 mm (1000 ppm).
309
6. Discussion
310
6.1 Pre-impact geochemistry
311

14
It is well established that the downward flux of organic carbon to the sediments, and subsequent
312
oxidation, is the driving force for early diagenesis in marine sediments, resulting in the reduction
313
of a series of electron acceptors: oxygen, nitrate, Mn oxides, Fe oxides, then sulfate, in order of
314
decreasing energy per mole organic carbon (e.g. Emerson et al., 1980; Froelich et al., 1979).
315
After oxygen is nearly depleted, nitrate is reduced, followed by reduction of Mn(IV) oxides to
316
Mn(II). The dissolved Mn(II) diffuses upward, is reoxidized to Mn(IV) by pore water oxygen,
317
and then trapped within the sediment. Under steady-state conditions, this leads to the formation
318
of a single Mn peak, which typically defines the depth of the redoxcline (e.g. Burdige and
319
Gieskes, 1983). The redox state of surface sediments and the depth of the redoxcline is
320
controlled by the flux of organic carbon to sediments, bottom water oxygen concentration, and
321
the degradation rate constant for the carbon (e.g. Emerson et al., 1985). This single Mn peak
322
typical of continental slope sediments is clearly observed in cores we sampled before the event
323
from the Sediment Trap Site, Fisk Basin, and Garrison Basin, at 77 mm, 80 mm, and 165 mm,
324
respectively. Re increases below the Mn peak, consistent with Re enrichment in mildly reducing
325
sediments. This characteristic Mn peak is considerably shallower in cores near the wellhead (e.g.
326
PE-1031-6) since they are closer to the Mississippi River, with its high nutrient load and
327
resulting high productivity and high sedimentation rate.
328
6.2 Post impact – organic geochemistry
329
Following the blowout event, our working hypothesis is that the accumulation of marine snow
330
resulted in a substantial sedimentation pulse. In addition to an increase in sedimentation rate, the
331
quality of the organic carbon associated with the sediment pulse and the oil-associated marine
332
snow was likely more labile and would be remineralized more quickly than the relatively low
333
quality partially degraded fecal pellets that make up most of the organic carbon rain. “High
334

15
quality,” fresh diatom-derived organic carbon is remineralized > 300% faster than “low quality”
335
carbon from fecal pellets, showing that different organic matter makes a significant difference in
336
respiration rates and residence time of carbon on the ocean floor (Mayor et al., 2012). At least
337
50% of the recently deposited aliphatic hydrocarbons in each of the three times series cores
338
degraded between the December 2010 and February 2011 sampling (Romero, personal
339
communication).
340
The changing influence of the Mississippi River is potentially a confounding factor. Nutrient-and
341
sediment-laden Mississippi River water would enhance productivity in the NE GoM and could
342
potentially be responsible for the increased sedimentation rate. In response to the spill, fresh
343
water from the Mississippi River was diverted in 2010 in an effort to minimize damage to the
344
coasts (Bianchi et al., 2011). A year later, in May 2011, the Mississippi River flooded to record
345
levels, a 100-year flood event (Falcini et al., 2012) while 2012 experienced a serious drought as
346
revealed by USGS riverine discharge data
(http://water.usgs.gov/).
The most substantial
347
changes in sedimentation are recorded shortly after the blowout (Schrope, 2013) not during the
348
later changes in river discharge in 2011 or 2012.
349
6.3 Post impact: Mn geochemistry
350
The sedimentation pulse mixed with fresh organic carbon would result in increased microbial
351
respiration with consumption of oxygen, and a shoaling of the redoxcline. As the redox boundary
352
migrates upward, the new Mn oxide peak is shallower, leaving behind a relic Mn peak at depth.
353
The observation of two or more Mn peaks, and the concept of non-steady state diagenesis was
354
explored to describe paleoredox variations and changes in paleoproductivity (Finney et al., 1988),
355
changes in the modern carbon cycle (Kuzyk et al., 2011) and in turbidite sequences in the North
356
East Atlantic (Colley et al., 1984; Thomson et al., 1993). Pulsed sedimentation to the seafloor is
357

16
not uncommon (e.g. Gobeil et al., 1997; Gobeil et al., 2001; Honjo et al., 1982). It is worth
358
noting that the redox boundary can migrate upward quickly but downward more slowly due to
359
relatively fast oxidation of organic matter in contrast to the slower diffusion of oxygen (Gobeil et
360
al., 2001). We rely on the well-known behavior of Mn cycling in reducing sediments, along with
361
redox sensitive elements, Re and Cd, to constrain changes in reducing conditions of sediments.
362
Exceptions where no double peak for Mn is observed include DSH-08 and PCB-06 in September
363
2011 and PCB-06 profiles in August 2012; the relic Mn peak is subtle in the August 2012 DSH-
364
08 Mn profile. This is likely due to the reduction of the Mn oxide associated with the relic peak
365
under the more reducing conditions. There is a wide range in reported kinetics for the reduction
366
of Mn oxide with first order rate constants ranging from 0.002 d
-1
for pelagic sediments (Burdige
367
and Gieskes, 1983) and 8-60 day
-1
for coastal environments (Aller, 1980). A pulse of organic
368
carbon to the sea floor can trigger reduction of Mn and Fe oxides within just a few days with a
369
first order rate constant of 0.159 d
-1
(Magen et al., 2011), demonstrating that Mn oxides in
370
surficial sediments can be rapidly reduced.
371
6.4 Post impact: Re enrichment
372
Re is the clearest indicator of reducing conditions, since detrital Re is very low, has no known
373
role in biogeochemical processes, and is not associated with cycling of Mn oxides (Morford et
374
al., 2005; Schaller et al., 2000). For cores not impacted by the blowout event, down core Re is
375
effectively constant in the surficial 70 to 100 mm at 0.3-0.4 ppb (Garrison Basin; figure 2a), 0.4
376
ppb (Fisk Basin; figure 2b) and 0.5 ppb (Sediment Trap site; figure 2c). Below the Mn peak, Re
377
increases as sediments becoming more reducing with depth. Surface sediments from the suite of
378
cores (PE 1031) sampled just after the blowout close to the wellhead are progressively more
379
reducing at shallower water depths and closer to the Mississippi River. This is shown by
380

17
decreasing Mn content and greater Re enrichment in surface (0-20 mm) sediments in cores at
381
shallower depths and closer to the mouth of the Mississippi River.
382
Several features of the Re profiles provide insight into the evolution of reducing conditions
383
following the impact. A modest Re enrichment of 0.1- 0.3 ppb typically occurs below the
384
shallowest Mn peak, and coincident with the Mn minimum, characteristically at 15-20 mm at
385
sites between August 2010 and February 2011. This infers reducing conditions relatively shallow
386
in sediments. The loss of this feature in the September 2011 and 2012 cores could be a result of
387
oxidation of the organic carbon and diffusion of O
2
into the sediments, or the re-emergence of
388
bioturbation with recovery of normal conditions.
389
A distinct Re peak at 30-40 mm at site DSH-10 (figures 3a, b, c), and at 50-70 mm for all but one
390
profile from PCB-06 (figure 5a), suggests non steady state behavior, and is consistent with
391
substantially more reducing conditions following the sediment pulse. The absence of high-
392
resolution Re data from cores preceding the impact makes a comparison with normal conditions
393
difficult. Nonetheless, the subsurface Re peak implies non steady state conditions.
394
6.4.1 Evolution of Re enrichment
395
We examined the change in Re for the three stations for which a 36-month long time series exists.
396
Since Re changes substantially with depth, as well as over time, choosing a consistent depth
397
interval is important. For DSH-10 and PCB-06, we chose the distinct Re peak centered at 40mm
398
and 60mm, respectively. For DSH-08 where no distinct Re peak exists, we chose [Re] at 50 mm,
399
approximately where the Re peaks occurred at the two other sites.
400
At each site, subsurface authigenic Re concentrations at ~ 50 mm increase substantially over two
401
years, increasing 3-4 times compared to the pre-impact value for Re (table 2; figure 7). We
402

18
determined the pre-impact value for Re based on the suite of cores (PE-1031) collected at
403
different water depths just after the explosion occurred, and before substantial sedimentation or
404
deposition of oil (see supplementary data table 1). These pre-impact Re values are indicated at
405
the y-axis on figure 7. The increase in Re demonstrates a clear change in sedimentary redox
406
conditions after the impact, and continuing over the subsequent two years. The Re enrichment
407
suggests persistent reducing conditions due to continued remineralization of organic carbon. The
408
pulse of organic carbon must be incorporated into the sediment, and not simply at the sediment
409
water interface, for it to have an effect on the oxygen content within the sediment (Gobeil et al.,
410
1997). Since the accumulation of sediment from the pulse was at least 20 mm, this criterion is
411
satisfied. The final Re “peak” value for both DSH 10 and PCB 06 on August 2013, three years
412
after the initial sample, is substantially lower, indicating a return towards pre-impact conditions
413
at DSH 10 and PCB 06 or integration of the subsurface Re peak into adjacent depths.
414
6.5 Post Impact: Cd enrichment
415
Changing redox conditions is also a control of the dynamic down core concentration profiles of
416
Cd. Distinct Cd peaks of ~ 0.1 ppb at 15-17 mm in cores from PCB-06 in December 2010,
417
February 2011 and September 2011 are consistent with the presence of low concentrations of
418
sulfide and formation of authigenic CdS (Rosenthal et al., 1995b). This near-surface Cd
419
enrichment is also observed in the August 2010 DSH-10 profile and at DSH-08 in December
420
2010. These peaks are in addition to the deeper, broader, and greater Cd enrichments at ~60 mm
421
and deeper. The shallow, secondary Cd peaks are absent or substantially diminished in later
422
cores from August 2012 and 2013, which suggests oxidation of the free sulfide, burndown of the
423
CdS, and return towards pre-impact conditions.
424
6.6 Post Impact: Ecological impact
425

19
Down core changes in Mn and Re reveal clear evidence of changes in redox conditions following
426
the event. What is the ecological impact of such changes on the benthic community? Given that
427
redox changes occurred at the mm scale, benthic foraminifera are potentially sensitive indicators
428
of the changes in reducing conditions. We have documented changes in the density of benthic
429
foraminifera at two sites, DSH-08 and PCB-06, in December 2010 and February 2011
430
(supplementary data table 2). Where Mn and Re indicate significant reducing conditions by Mn
431
depletion and Re enrichment, there is a clear decrease in density of the dominant genera of
432
benthic foraminifera.
433
At site DSH-08 in December 2010, a 40-60% reduction in both Bulimina spp. and Uvigerina spp.,
434
the most abundant genera of benthic foraminifera, occurs between 13 – 17 mm (see
435
supplementary data table 2). This is coincident with the Mn minimum, and a modest (0.20 ppb;
436
35%) Re enrichment (figure 4a). At the surface, a significant decrease in density of benthic
437
foraminifera occurs, while a modest (0.16 ppb; 30%) Re increase and Mn minimum occurs,
438
consistent with reducing conditions (figure 4a). On the subsequent sampling date, February 2011,
439
a dramatic reduction in the most abundant genera, Bulimina spp., Uvigerina spp., occurs at 13
440
mm, coincident with the Mn minimum and a 0.14 ppb increase in Re (figure 4b). A similar large
441
reduction in “other species”, primarily Brizalina spp. and Bolivina spp., also occurs at this depth.
442
Site PCB-06 reveals a similar relationship between redox sensitive metals and benthic
443
foraminiferal density. In December 2010 a 60% reduction in Brizalina spp. and Bolivina spp.
444
(“other species”) occurs at 8 mm, coincident with the Mn minimum and a 0.07 ppb (15%)
445
increase in Re. In February 2011, a dramatic decrease in density of the dominant three genera
446
occurs in the surface 0-8 mm, entirely consistent with decreases in Mn in that depth range, and
447
an unusual 0.18 ppb (30%) increase in Re in the top 5 mm (figure 5b).
448

20
Decreases in Uvigerina spp. in core PCB-06 December 2010 at shallow depths (~4 mm), are not
449
exactly coincident with changes in Re or Mn. This could be an offset between the two multicores,
450
since the redox sensitive metals and the foraminifera were sampled in different cores.
451
Alternatively, another mechanism may be responsible for changes in foraminiferal density. PAH
452
compounds and other toxic organic compounds found in the crude oil and/or dispersant may be
453
another reason for the changes in foraminiferal density.
454
7. Conclusions:
455
Downcore concentration profiles of redox sensitive metals from sites in the NE GoM sampled
456
before the blowout event, just after the explosion near the wellhead, or distal from the wellhead
457
after the event, are typical of continental slope sediments. Bulk Mn is distinguished by a single
458
peak caused by reduced Mn(II) at depth diffusing up to oxic sediments where it precipitates, thus
459
defining the depth where sediments become reducing. Immediately below the Mn peak, Re
460
increases above detrital values, followed by increases in authigenic Cd reflecting a CdS phase.
461
Water depth and distance from the Mississippi River have a substantial influence on the depth of
462
the redoxcline and the behavior of these redox sensitive metals.
463
Following the blowout event, a substantial marine snow event which led to a large sedimentation
464
pulse was recorded in NE GoM sediments. Subsequent respiration of the organic carbon
465
associated with the marine snow resulted in reducing conditions, as evidenced by downcore
466
changes in redox sensitive metals Mn, Re and Cd.
467
After the event, at three time series sites (DSH-08, DSH-10, and PCB-06) and at SW-01, double
468
Mn peaks are typically present, at ~5 mm and at ~20-30 mm, consistent with the non-steady state
469
behavior associated with a shoaling redoxcline and a relic Mn peak. Some profiles do not show
470

21
two distinct peaks but reveal shoulders with relatively high Mn values over a broad depth range
471
of 10-15 mm. These double Mn peaks result from the observed changes in the input of organic
472
carbon and possibly a more labile carbon source. Double peaks of Re and Cd are present in
473
several profiles, indicating non-steady state behavior for these redox sensitive metals as well.
474
Re concentrations at the subsurface maximum (40-60 mm) increase over two years at all three
475
sites, demonstrating that reducing conditions persist over time. In the last sampling effort in
476
August 2013, [Re] decreased at two sites, suggesting a possible return towards pre-impact
477
conditions. Cd enrichment indicates depths where CdS is precipitating due to presence of sulfide
478
and reducing conditions. In August 2012 no double peak for Mn is seen at PCB-06, and the
479
secondary peak in DSH-08 is minor, another indication that the surface sediments (< 30 mm)
480
may have begun toward return to pre-impact conditions.
481
An assemblage-wide decrease in benthic foraminiferal density occurred at the same depth
482
intervals as reducing conditions suggesting an important consequence of changing redox
483
conditions in sediments to biotic communities.
484
Acknowledgments:
485
We dedicate this work to Benjamin Flower, who passed away in July 2012. Ben was careful and
486
caring, hard working and honest, and dedicated to his family, friends, and colleagues. He was an
487
enthusiastic ultimate (Frisbee®) player, who embraced the “Spirit of the Game” in both his
488
professional and personal life.
489
Many thanks to the numerous Eckerd College undergraduate students who helped in the
490
laboratory and at sea including Shannon Hammaker, Chloe Holzinger, Farley Miller, Claire
491
Miller, and Corday Selden. Grateful acknowledgements to Alan Shiller, who provided important
492

22
insight at a critical time. A special thanks to Luke McKay and Andreas Teske (University of
493
North Carolina) for collecting and providing valuable sediment samples collected just after the
494
explosion. Thanks to the exceptional crew of the R/V Weatherbird II for their capable assistance
495
at sea collecting samples, and staying safe during the field operations.
496
We would like to thank the NSF Rapid Grant program for providing the funding for the Deep
497
Sea Instruments MC-800 Multi-corer. We acknowledge the British Petroleum/Florida Institute of
498
Oceanography (BP/FIO) Gulf Oil Spill Prevention, Response, and Recovery Grants Program for
499
providing funding for the initial research cruises during 2010 and 2011. This research was made
500
possible by funding from The Gulf of Mexico Research Initiative to both the Deep-C Consortium
501
and to the C IMAGE Consortium. The complete data set can be accessed at the GRIIDC website:
502
https://data.gulfresearchinitiative.org/.
503
504
Figure captions 505
Figure 1 Map of sites sampled for analysis. 506
Figure 2. Pre-impact redox sensitive metal and Ba profiles at (a) Garrison Basin; (b) Fisk Basin, 507
(c) USGS sediment trap site (PE09-04), 1132m (d) PE 1031-6, near well head. 508
Figure 3. Redox sensitive metal and Ba profiles at time series site DSH 10, 1520m for 509
(a) August 2010 (b) December 2010 and (c) August 2012. 510
Figure 4. Redox sensitive metal and Ba profiles at time series site DSH 08, 1143m for 511
(a) December 2010 and (b) February 2011. 512
Figure 5. Redox sensitive metal and Ba profiles at time series site PCB 06, 1011m for 513
(a) December 2010; (b) February 2011 514
Figure 6. Redox sensitive metal and Ba profiles at site SW-01, 1187m; April 2012. 515
Figure 7. Temporal evolution of Re enrichment following the blowout. [Re] associated with the distinct 516
subsurface Re peak is plotted for three stations (see text). Estimates of pre-event [Re] are indicated on the y-axis. 517
No clear Re peak exists at site DSH 08; we chose [Re] at ~50 mm depth, close to the depths where Re peaks 518
were found at the two other sites. For Feb 2011 PCB-06 and Sept 2011 DSH-10, there was no distinct Re 519
maxima; [Re] at the same depth as the other dates is shown. These are depicted by an open symbol and are 520
not included in the trend line. 521
522

23
523
Table 1: Sample sites
524
Site ID Latitude Longitude
Water depth
(m)
Collection
date
DSH-08 29° 7.367N 87°
52.064W 1143 Dec 2010
Feb 2011
Sept 2011
Aug 2012
Aug 2013
DSH-10 28° 58.743N 87° 53.497W 1520 Aug 2010
Dec 2010
Feb 2011
Sept 2011
Aug 2012
Aug 2013
PCB-06 29° 7.362N 87° 15.973W 1011 Dec 2010
Feb 2011
Sept 2011
Aug 2012
Aug 2013
SW-01 28° 13.252N 89° 4.17W 1187 Aug 2012
DWH-01
Wellhead
28° 43.462N 88°23.237W 1577 Aug 2012
Aug 2013
Pre –event cores
Fisk Basin
PE07 6-II
27°33.0'N 92°10.1'W 817 July 2007
Garrison Basin
PE07-2
26°40.5'N 93°55.5'W 1570 July 2007
W. Florida Slope
NT 1200
27°57.982 86° 1.388 1200 June 2011
PE 1031-6 28° 46.557N 88° 24.293W 1380 May 2010
PE 1031-11 28° 49.454N 88° 27.378W 1138 May 2010
PE 1031-12 28° 51.179N 88° 29.194W 879 May 2010
PE 1031-23 28° 50.574N 88° 40.713W 732 May 2010
Sed Trap Site
PE09-004 MC1
27º 31.508N 90º 10.126W 1132 Jan 2009

24
525
526
Table 2: Time series of Re values at three stations following the blowout event.
527
Date Re (ppb)
DSH-10
Re (ppb)
PCB-06
Re (ppb)
DSH-08*
Before event 1.39 1.1 1.0
Aug-2010 2.15 ND ND
Dec-2010 2.36 1.85 0.94*
Feb-2011 2.64 1.33* 0.97*
Sep-2011 1.89* 2.54 1.79*
Aug-2012 3.79 3.95 2.15*
Aug 2013 2.64 1.87 2.45*
528
* no distinct Re peak. 529
ND: no data since station was not sampled on that date 530
531

25
532
533

26
534
535
536
Figure 1 537
0
50
100
150
200
0 10 20
Depth (mm)
Mn (mg/g)
0.0 0.5 1.0
Re (ppb)
0.0 0.2 0.4
Cd (ppm)
0 100 200 300
Ba (ppm)
(a) Pre-impact: Garrison Basin, 1570m
(b) Pre-impact: Fisk Basin, 817 m
0
50
100
150
200
0 5 10 15
Depth (mm)
Mn (mg/g)
0.0 0.5 1.0
Re (ppb)
0.0 0.2 0.4
Cd (ppm)
reducing
0 500
Ba (ppm)

27
538
539
540
541
542
Figure 1 (cont’) 543
544
545
(c) Pre-impact: USGS sediment trap site PE09-04, 1132m
0
50
100
150
200
0 20 40
Depth (mm)
Mn (mg/g)
0.0 0.5
Re (ppb)
0.0 0.2 0.4
Cd (ppm)
MnOx
reducing
0 2000 4000
Ba
(ppm)
0
50
100
150
0
10
Depth (mm
)
Mn (mg/g)
0
2
4
Re (ppb)
0.0
0.2
0.4
Cd (ppm)
0
100
200
300
Ba
(ppm)
(d) Pre-impact: near well head, PE 1031-6, 1380m

28
546
547
0
10
20
30
0 10 20
Depth (
mm)
Mn (mg/g)
0.0 0.5 1.0
Re (
ppb
)
0.0 0.2 0.4
Cd (ppm)
0
50
100
150
200
0 10 20
Depth
(mm)
0.0 1.0 2.0 0.0 0.2 0.4
(a) DSH-10, August 2010
reducing
reducing
0
200
400
Ba (ppm)
0
200
400

29
Figure 2 548
0 1 2 0.0 0.2 0.4
0
50
100
150
0 10 20
Depth (mm)
0.0 0.5 1.0
Re (ppb)
0.0 0.2 0.4
Cd (ppm)
0
10
20
30
0 10 20
Depth (
mm
)
Mn (mg/g)
(b) DSH-10, December 2010
reducing
reducing
0 200 400 600
Ba (ppm)
0 200 400 600

30
549
550
Figure 2 (cont) 551
552
553
0
10
20
30
0 10 20 30
Depth (mm)
Mn (mg/g)
0.0 0.2 0.4
Cd (ppm)
0.0 0.5 1.0
Re (ppb)
0
50
100
150
200
0 10 20 30
Depth (mm)
0.0 0.2 0.4
0 1 2 3 4
(c) DSH-10, August 2012
reducing
0 200 400
Ba (ppm)
0 200 400

31
554
555
0
10
20
30
40
0 10
Depth (
mm
)
Mn (mg/g)
0.0 0.2 0.4
Cd (ppm)
0.0 0.5 1.0
Re (ppb)
0 500 1000
Ba (ppm)
0
50
100
0 10
Depth
(mm)
0.0 0.2 0.4
0 1 2 0 500 1000
(a) DSH-08, December 2010
Reducing;
fewer
forams
0 1 2
0.0 0.5 1.0
Re (ppb)
0
10
20
30
40
0 5 10
Depth (mm)
Mn (mg/g)
0.0 0.2 0.4
Cd (ppm)
0 500 1000
Ba (ppm)
0
50
100
150
0 5 10
Depth (mm)
0.0 0.2 0.4 0.6 0 500 1000
(b) DSH-08, February 2011
reducing
reducing;
fewer
forams
reducing
reducing

32
Figure 4 556
557
558
0
10
20
30
0 10
Depth (mm)
Mn (mg/g)
0.0 0.2 0.4
Cd (ppm)
0 200 400
Ba (ppm)
0.0 0.5 1.0
Re (ppb)
0
50
100
150
0 10
Depth (mm)
0.0 0.2 0.4 0 200 400
0 1 2 3
(a) PCB-06, December 2010
reducing
reducing
0
10
20
30
0 10
Depth (mm)
Mn (mg/g)
0.0 0.2 0.4
Cd (ppm)
0 200 400
Ba (ppm)
0.0 0.5 1.0
Re (ppb)
0
50
100
150
200
0 10
Depth (mm)
0.0 0.2 0.4 0 200 400
0 1 2 3
reducing
(b) PCB-06, February 2011
Foram
mortality

33
Figure 5 559
560
Figure 6 561
562
0
50
100
150
200
0 10 20 30
Depth (mm)
Mn (mg/g)
0.0 0.2 0.4
Cd (ppm)
0 500 1000 1500
Ba (ppm)
0 1 2
Re (ppb)
reducing
reducing
SW01, April 2012

563
564
565
Figure 7 566
567
0
1
2
3
4
Mar-2010 Aug-2010
Feb
[Re] ppb
DWH event
34
Feb
-2011 Aug-2011 Feb-2012 Aug-2012
Feb
DSH 10
PCB 06
DSH 08
Feb
-2013 Aug-2013

35
568
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571
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