ChemicalGeology, 99 (1992) 1-27
Elsevier Science Publishers B.V., Amsterdam
1
Carbon-sulfur-iron systematics of the uppermost deep-water
sediments of the Black Sea
Timothy W. Lyons~ and Robert A. Berner
Department of Geology and Geophysics, Yale University, P.O. Box 6666, New Haven, CT 06511. USA
(Received June 5, 1991; revised and accepted November 20, 1991 )
ABSTRACT
Lyons, T.W. and Berner, R.A., 1992. Carbon-sulfur-iron systematics of the uppermost deep-water sediments of the Black
Sea. In: P.A. Meyers, L.M. Pratt and B. Nagy (Guest-Editors), Geochemistry of Metalliferous Black Shales. Chem.
Geol., 99: 1-27.
Box cores recovered during Leg 4 of the 1988 R / V "Knorr" Black Sea Oceanographic Expedition from deep-water
regions of the basin were dominated by coccolith-rich, microlaminated (Unit 1) sediment and muddy, gray turbidite
layers. Both organic carbon (OC) and pyrite sulfur values for Unit i display narrow ranges, with mean concentrations of
5.3 _+ 1. l ( 1a) wt% and 1.3 + 0.3 wt%, respectively. Unit I is not enriched in pyrite-S relative to sediments deposited under
oxygenated bottom waters (normal marine sediments) with comparable OC concentrations. Carbon-sulfur relationships
(evaluated on a calcium carbonate-free basis to avoid spurious correlations resulting from dilution effects) demonstrate
that OC and pyrite-S are essentially decoupled. These observations, combined with the persistence of elevated pore-water
sulfide to depth and a strong correlation between pyrite-S and the detrital Fe component argue strongly for limitation of
pyrite formation in Unit 1 by the availability of reactive Fe. U n i t - / F e limitation is further indicated by degree-of-pyritization (DOP) studies (a measure of the extent to which the original potentially reactive Fe has been transformed to
pyrite). These studies show sulfidation of reactive Fe ranging from 57% to 78%, with DOP values independent of OC
concentration. U n i t - / D O P profiles suggest that the majority of the pyrite is formed in the sulfidic water column and/or
very close to the sediment-water interface. Pyrite-S concentrations of Unit 1, when compared with the particulate reduced
sulfur fluxes measured in time-series sediment traps, are compatible with predominantly water-column pyrite formation.
Because of the limitations in the supply of reactive Fe associated with the comparatively high supply of OC, the microlaminated sediment is characterized by C/S ratios greater than those typical of Holocene oxically deposited sediments.
The turbidite muds of the deep basin display high reduced S values (relative to Holocene normal marine sediments) in
samples with low OC concentrations (low C/S ratios). This reflects pyrite formation under anoxic-sulfidic bottom-water
conditions in a probable upper-slope source region, as well as during transport and final deposition, Intermediate DOP
values for the turbidites, in part a product of their rapid rate of deposition, reveal that Fe limitation is ultimately not a
factor and that further pyrite should form during burial. However, the very rapid rate at which Unit-/pyrite forms suggests
fundamental differences between the overall reactivities of the Fe phases associated with the microlaminated and turbiditic sediments. The signature of water-column anoxia with regard to sedimentary pyrite formation is clearly indicated by
the high DOP values of Unit I and the comparatively high levels of S associated with the low concentrations of OC of the
turbidite muds. This agrees with similar conclusions based on studies of ancient sedimentary rocks.
1. Introduction
Correspondence to: T.W. Lyons, Department of Geological Sciences, University of Michigan, Ann Arbor, MI
48109, USA.
~Present address: Department of Geological Sciences,
University of Michigan, Ann Arbor, MI 48109, USA.
0009-2541/92/$05.00
The use of C - S - F e systematics as they relate
to sedimentary pyrite formation has become an
increasingly popular practice in the interpre-
© 1992 Elsevier Science Publishers B.V. All rights reserved.
2
T.W. LYONS AND R.A. BERNER
tation of paleoenvironments of ancient finegrained sedimentary sequences (Dill and Nielsen, 1986; Gautier, 1986; Anderson et al., 1987;
Fisher and Hudson, 1987; Leventhal, 1987;
Davis et al., 1988; Beier and Hayes, 1989; Dean
and Arthur, 1989 ). The formation of sedimentary pyrite begins with the bacterial reduction
of sulfate under anoxic pore-water or watercolumn conditions. This reaction can be expressed in simplified terms as:
±53 (CH20)106(NH3)16 ( H 3 P O 4 ) Jr" S O 2 16
+~H3POa+H
2HCO~ + H S - + 33NH3
+
where ( C H 2 0 ) I 0 6 ( N H 3 ) I 6 ( H 3 P O 4 )
is an
idealized representation of sedimentary organic matter. Pyrite ultimately forms via the
reaction of the resultant H S - with Fe delivered as detrital mineral phases (see Goldhaber
and Kaplan, 1974; Berner, 1984).
The factors limiting pyrite production vary
as a function of the diagenetic/depositional
conditions of formation, thereby creating a potential for paleoenvironmental indicators. Sulfate limitation generally prevails in freshwater
to brackish depositional settings (Berner and
Raiswell, 1984), while normal marine sediments (those deposited under oxygenated bottom waters ) are typically characterized by limitations of the availability of bacterially
metabolizable organic matter (Berner, 1970,
1982, 1984; Goldhaber and Kaplan, 1974; Lin
and Morse, 1991 ). In contrast, the extent of
reactive-Fe availability is the principal control
on pyrite formation in the sediments of euxinic basins (anoxic and H2S-containing)
(Raiswell and Berner, 1985; Fisher and Hudson, 1987; Raiswell et al., 1988; Dean and Arthur, 1989 ). Fe can also be limiting in biogenic
deposits (Berner, 1984) and extremely organic-rich sediments such as tidal marshes
(King et al., 1982; Giblin and Howarth, 1984;
Giblin, 1988).
Sediment C-S relationships are summarized schematically in Fig. 1, a plot of wt% organic C vs. wt% pyrite-& Trends for three fun-
damental depositional systems are included in
this figure. As a consequence of sulfate limitation, fresh to slightly brackish samples typically plot in a low-S region over a broad range
of organic C concentrations. Conversely, organic C and pyrite-S are generally coupled in
sediments deposited under oxygenated marine
conditions. Despite scatter in the data, a strong
positive correlation between organic C and pyrite-S has been recognized for a wide variety of
modern normal marine deposits (Goldhaber
and Kaplan, 1974; Berner, 1982; Lin and
Morse, 1991 ). The Holocene normal marine
data define a line passing through the origin
and yield a mean C/S weight ratio of 2.8. Sediments deposited under anoxic-sulfidic (euxinic) bottom-water conditions are represented
in very general terms by two lines in Fig. 1, defining two separate scenarios for euxinic-basin
pyrite formation. In both cases, the lines are
characterized by non-zero S intercepts (i.e.
high pyrite-S associated with low organic C
f
0
0
wt. % organic carbon
Fig. 1. Schematic plot of organic C vs. pyrite-S displaying
the generalized distributions of data for three fundamental depositional systems: normal marine (oxygenated bottom waters), euxinic (anoxic, sulfidic bottom waters) and
fresh water.
C-S-Fe SYSTEMATICS OF THE UPPERMOST DEEP-WATER SEDIMENTS OF THE BLACK SEA
contents). This positive intercept reflects pyrite formation within the water column, as well
as at the sediment-water interface, in intimate
contact with the ubiquitous dissolved sulfide
of the water column [the syngenetic pyrite
component of Raiswell and Berner (1985)].
Unlike burial (diagenetic) pyrite formation,
this fraction is independent of the quantity of
locally deposited organic matter (i.e. that with
which it accumulates and is buried) (Berner
and Raiswell, 1983; Leventhal, 1983; Berner,
1984; Raiswell and Berner, 1985 ). The euxinic
line of zero slope represents strongly Fe-limited syngenetic pyrite formation in association
with decoupled Fe and organic C deposition.
Therefore, sulfur concentrations show little
variation over the range of carbon values.
Greater variability in the detrital reactive Fe
flux would produce scatter in the sulfur values.
The line of positive slope reflects either: ( 1 ) a
syngenetic pyrite pool augmented by a contribution from carbon-limited diagenetic (burial) pyrite formation with sufficient reactive
Fe, or (2) predominantly syngenetic pyrite
formation under Fe-limited conditions where
the deposition of reactive Fe is closely coupled
with organic C deposition (see Raiswell and
Berner, 1985 ).
With a few recent exceptions (Boesen and
Postma, 1988; Calvert and Karlin, 1991; Middelburg, 1991 ), systematic studies of pyrite
formation and applications of modern diagenetic principles in present-day euxinic settings
are few in number. Our current level of understanding stems largely from investigations of
ancient sedimentary sequences dating from
periods of more widespread oceanic anoxia
(Schlanger and Jenkyns, 1976; Fischer and
Arthur, 1977; Berry and Wilde, 1978; Jenkyns,
1988), as well as analysis of the existing data
set for the Holocene Black Sea, a frequently
cited modern analog.
The Black Sea, as the world's largest modern
permanently anoxic basin (Spencer et al.,
1972), has been the single most important
source of C-S-Fe geochemical data from a
3
modern euxinic environment (e.g., Vinogradov et al., 1962; Hirst, 1974; Rozanov et al.,
1974; Vaynshteyn et al., 1986 ) - - primarily as
a result of the 1969 "Atlantis II" cruise (Degens and Ross, 1974) and the efforts of Soviet
scientists over many decades. These data have
been employed by a number of workers (most
notably Berner and Raiswell, 1983; Leventhal,
1983; Raiswell and Berner, 1985) to address
C-S-Fe relationships in Black Sea sediments
and to provide a modern context for the investigation of anoxically-deposited, organic-rich
sediments (black shale environments) of the
geologic record.
The 1988 R / V "Knorr" Black Sea Oceanographic Expedition has provided a unique opportunity to study uppermost Holocene sediments collected over a broad region of the
southern Black Sea basin from a variety of depositional settings. The purpose of this paper
is to re-evaluate C-S-Fe relationships in the
sediments of the deep-water regions of the
basin in light of newly acquired data. The intent, therefore, is to redefine the the Black Sea
"paradigm" as it specifically relates to pyrite
formation in anoxic-sulfidic depositional settings and to demonstrate its value for paleoenvironmental interpretation. The use of C-S-Fe
sediment geochemistry has become an increasingly popular practice in both oil and mineral
exploration. Prior to this and other recent
studies in modern oxygen-deficient settings, we
have largely relied on the "ancient as the key
to the present". While one could argue at length
over details concerning the validity of the Black
Sea model as an analog for the deposition of all
black, laminated, organic- and metal-rich
shales of the record, it remains perhaps our best
hope for understanding specifics such as nutrient cycling, trace-metal redox behavior, biological mechanisms, and rates and styles of
sediment deposition in anoxic depositional
settings. This study represents important confirmation from a modern euxinic basin of the
great potential of a number of geochemical
methods for paleoenvironmental interpreta-
4
T.W.
tion. The sediments of the deep basin will be
contrasted with those of the basin margin in
future publications.
AND
R.A. BERNER
1991; Lyons and Berner, 1990a, b). The sediment cores specifically emphasized in this
study were collected by box core ( ~ 50 × 50
X 50 cm) at five deep-water stations during
Leg 4 (June 21-July 8) of the 1988 expedition
(Fig. 2). The water depths at stations 7, 9, 1 I,
14and 18A (BC 2) are 1949, 2094, 2175, 2218
and 2150 m, respectively. The box cores were
subsampled immediately upon arrival on deck
using thin-walled plastic core liner for later
chemical and sedimentological analysis. In addition, Plexiglas®-enclosed "sediment slabs"
of ~ 3.8-cm thickness 'were collected from a
majority of the box cores for on-board X-radiography and photography. These Plexiglas ®
subsamplers were 30 cm long, thus enabling
only partial sampling of' some box cores.
2. Sample locations and descriptions
The strongly stratified water column of the
deep portions of the modern Black Sea is characterized by a thick permanent layer of anoxic-sulfidic water (in excess of 2000 m in the
central basin) underlying a ~ 100-m oxic to
suboxic cap (Murray et al., 1989). These euxinic conditions preclude the possibility of
benthic biological activity in the deep basin
(with the exception of microbially mediated
processes) and result in well-preserved primary features. Sediments encountered in the
lower-slope/abyssal-plain regions of the basin
fall into two main categories: ( 1 ) microlaminated, coccolith-rich sediments reflecting
"normal" deep-water deposition; and (2)
sandy and muddy resedimented (mass flow)
deposits derived from the basin margin. These
sediment types, as well as those from marginal
positions in the basin, are discussed at length
in Lyons ( 1991 ) along with preliminary C - S Fe results (see also Calvert and Karlin, 1990,
......
LYONS
2.1. Microlaminated (Unit 1) deposits
Box-core subsamples from Station 9 (30 cm
in length) and Station 14 (32.5 cm) were
comprised entirely of alternating white and
dark-gray to black mm.-scale laminae in association with surface "fluff " layers (particulate benthic boundary layers) ~ 2.5 cm thick
(Fig. 3). The microlaminated sediment, de-
:: " :
:.¢ ~:~US S R .~,::"~~"
: :e,. ,:...~@:~" ' :'+.......
: k:?~, ..., .: ~..:~:,+".'-:~
:...,7,!:?"
1
46 ° N
t o m a n i ~ }'
~
~ }: "> " :
•~:
:+-#
• - .:~:~.
44 °
--
_
40 °
,,
.,s,....,.q";:, . .
15":(r
B
42 °
,a4.
u
l
" "
i
?'i " :
g
a
r
i
a
:
~
?
~
'
i
i:i
~ 7 ~ '
26 ° E
m
t
28 °
30 °
t
32 °
t
1
34 °
36 °
,
38 °
,
40 °
L+]
42 °
Fig. 2. Location map showing the distribution of Leg-4 ( 1988 Black Sea Oceanographic Expedition ) box-coring stations
described in the text.
C-S-Fe SYSTEMATICS OF THE UPPERMOST DEEP-WATER SEDIMENTS OF THE BLACK SEA
Fig. 3. Photograph of the upper portion of the Unit-/box
core collected at Station 9. A well-developed surface fluff
layer is associated with the microlaminated sediment of
Unit 1. Note the cm-scale light and dark banding, reflecting variations in the relative proportions of carbonate and
siliciclastic sediment. Smearing along the sides of the Xray slab has obscured some of the fine detail.
fined as Unit 1 in the nomenclatural scheme of
Ross et al. (1970) and Ross and Degens
(1974), and the associated surface "flocculent" layer comprise the uppermost Holocene
non-turbiditic deposition in the deep Black Sea.
This hemipelagic horizon is typically on the
order of 30 cm thick and overlies the organic
C-rich Unit-2 sapropel. The light laminae are
rich in calcium carbonate and dominated by
the coccolithophore species Emiliania huxleyi,
while the dark layers are predominantly siliciclastic (Hay, 1988).
Despite disparate absolute ages revealed for
the base of Unit 1 by different dating tech-
5
niques (radiocarbon vs. varve counts) (Ross
et al., 1970; Ross and Degens, 1974; Degens et
al., 1978, 1980; Calvert et al., 1987, 1991; Hay,
1988; Hay et al., 1990; Jones, 1990), sediment-trap studies suggest that the light-dark
pairs are strongly related to seasonal variability in sedimentation and might, therefore, be
annual varve couplets (for a detailed discussion, see Honjo et al., 1987; Hay et al., 1990).
Varve chronologies indicate an age of ~ 10001100 yr B.P. for the base of Unit 1 and a corresponding sedimentation rate of ~ 25-30 cm/
1000 yr (Degens et al., 1978; Hay, 1988)
(these estimates may be revised as detailed
varve counts of 1988 cores become available).
Conversely, Jones (1990) determined an age
of ~ 3200 yr for the approximate U n i t / - U n i t
2 boundary using accelerator mass spectrometry (AMS) radiocarbon dating (see also Calvert et al., 1991 ). Recent 14C analyses have revealed surface-sediment "ages" of insufficient
magnitude to account for the existing disparity
(Jones, 1990; Calvert et al., 1991 ). Additionally, the radiocarbon ages of carbonate C and
coexisting organic C collected from Unit-/
sediment are in good agreement, suggesting a
lack of reworked carbon in these two reservoirs of the microlaminated horizon (Calvert
et al., 1987, 1991 ; Jones, 1990). At present, the
discrepancy between varve and radiocarbon
chronologies has not been completely resolved; however, Crusius and Anderson ( 1991,
1992 ) found reasonable agreement between the
data of Jones (1990) and Calvert et al. ( 1991 )
and their 21°Pb-derived U n i t - / m a s s accumulation rates. Calvert et al. ( 1991 ) determined
a sedimentation rate of 16 cm/1000 yr from
the slopes of AMS ~4C profiles for stations 9
and 14.
The sediment analyzed from Station 18A
(BC 2) consists of a surface flufflayer ( 1.5 cm)
and ~ 17 cm of homogeneous gray mud conformably overlying at least 12 cm of Unit-/
sediment (Fig. 4). Unit-/ deposition is characterized by remarkable uniformity over broad
regions of the deep Black Sea. This microlam-
6
T.W. LYONS AND R.A. BERNER
X-radiographs from stations 9 and 14 depicted
in Fig. 5 [see Lyons (:1991) for a more detailed discussion ].
2.2. Turbiditic muds
Fig. 4. Photograph ofa Station-18A subcore (actually BC
2). A homogeneous gray mud layer (turbidite) with a
surface fluff interval rests conformably on Unit-/
sediment.
inated horizon contains recognizable internal
sequences (ranging from cm-scale laminae
packets down to what may be individual couplets) that are readily correlated between the
three Unit-1 stations of Leg 4 ( 9, 14 and 18A),
including correlation of the microlaminated
sediment below the gray m u d layer at Station
18A with material near the top of the cores at
stations 9 and 14. This represents correlation
over a distance of ~ 500 km. The ability to correlate Unit-1 sediment on a fine scale is obvious from the subcore photograph and paired
The homogeneous gray m u d layer at Station
18A (BC 2) (Fig. 4) is an excellent example
of the other dominant sediment type encountered in the abyssal regions of the Black Sea
during Leg 4. Although the exact mechanism (s) of emplacement remains uncertain,
these gray deep-basin muds, as well as the
sandy deposits of the abyssal region, are believed to be mass-flow accumulations representing resedimentation of basin-margin facies (Shimkus and Trimonis, 1974; Lyons,
1991 ). Subcores of 43.:5- and 38.5-cm length
from stations 7 and 11, respectively, were
dominated by two distinct muddy gray "turbidite" layers (Fig. 6) (the basal several centimeters of the subcore at Station 11 appeared
non-turbiditic and somewhat Unit /-like, but
were significantly richer in siliciclastic sediment than the typical microlaminated sediment of the central abyssal plain). It is unclear
whether the muddy turbidites collected at the
two stations are in any sense correlative. The
sediment-water interfaces at both stations were
marked by well-developed fluff layers. Furthermore, the two m u d layers in both cores
were separated by thin horizons of what appeared to be compacted fluffmaterial (Fig. 6 ).
These "compacted fluff layers" presumably
represent episodes of background sedimentation between two turbidite "events". However, Moore and O'Neill ( 1991 ), using the activities of natural and Chernobyl radionuclides
measured in Leg-4 box cores, interpreted the
Black Sea fluff layers not as steady-state
(background) deposition, but rather as the
Fig. 5. a. Subcores from stations 14 (a~) and 9 (a2) placed side by side revealing the strong correlation of Unit-/microlaminated material between these two sites. These stations were separated by ~ 210 kin.
b. X-radiographs of the Unit-/subcores depicted in (a). Note the pronounced correlation of what appear to be individual
ram-scale "varve" couplets.
C-S-Fe SYSTEMATICS OF THE UPPERMOST DEEP-WATER SEDIMENTS OF THE BLACK SEA
(bl)
(b2)
8
T.W. LYONS AND R.A. BERNER
displayed remarkably conformable (erosionfree) basal contacts (see Fig. 4). In the subcores collected at stations 7 and 18A, the turbidites appeared strongly homogeneous under
both visual and X-radiographic examination.
In contrast, Station-I/ muds displayed some
faint darker-gray color bands, as well as a strikingly obvious lamination when viewed by Xradiography. However, grain-size analyses of
the turbidites from both stations 11 and 18A
(BC 2) revealed a predominance of fine grain
sizes (fine silt to clay) and strong textural homogeneity (graded textures were virtually absent) (Lyons, 1991). Complete descriptions
and interpretations of Leg-4 turbidites are included in Lyons ( 1991 ).
3. Analytical methods
Fig. 6. Photograph of subcore collected at Station 7. Two
homogeneous gray mud intervals (turbidites) underlie a
very well-developed surface fluff layer. Note the laminated nature of both the lower portion of the surface fluff
and the thin horizon of what appears to be compacted fluff
material separating the two gray muds.
products of high-productivity-high-flux events.
They further argued, based on nuclide inventories, that fluff-generating events do not occur more frequently than every 20 years. The
Leg-4 radionuclide data of Crusius and Anderson ( 1991 ) and Moore and O'Neill ( 1991 ), as
well as the positions of Black Sea muddy turbidites within the U n i t - / " v a r v e " chronology
(see Arthur et al., 1988), indicate that turbidite emplacement is a short-lived phenomenon. Crusius and Anderson ( 1991 ) employed
21°Pb to estimate that the Station-18A (BC 2)
turbidite was deposited in approximately 1945.
The muddy turbidites at stations 7, 11 and
18A (BC 2) were medium gray in color and
The subcores collected for later chemical
analysis (solid-phase and pore-water) were refrigerated and processed on board under a nitrogen atmosphere (extruded, 2-cm intervals)
at ~ 8 ° C. Efforts were made to avoid the sediment immediately adjacent to the walls of the
core liner where smearing might have occurred
during subsampling. The Station-11 sediment
was extruded subsequent to the cruise using a
frozen archive core that had undergone notable compaction (dewatering) during the
freeze-thaw process (sample intervals ranged
from ~5 to ~8 cm when corrected for
compaction ).
Following partial removal of the pore-water
phase (via centrifugation), the sediments were
immediately frozen for storage. In the laboratory, total inorganic and organic carbon were
measured using both air- and freeze-dried
samples and the method of Krom and Berner
(1983). This procedure involves the determination of total carbon using a LECO® carbon
analyzer before and after ashing (at 450°C).
Carbon in the ashed subsample represents inorganic (carbonate) carbon; organic C is calculated by difference. Results from the application of this technique to modern marine
C-S-Fe SYSTEMATICS OF THE UPPERMOST DEEP-WATER SEDIMENTS OF THE BLACK SEA
sediments agree strongly with replicate values
derived from acid digestion methods, even for
the carbonate-rich Black Sea sediments of this
study [see also Westrich ( 1983 ) for a comparison of carbon analytical methods]. Furthermore, the carbon data reported here are very
similar to other available Black Sea data (e.g.,
G.A. Cutter, pers. commun., 1990; Calvert et
al., 1991 ), including values determined using
different methodologies. Carbonate mineral
concentrations were calculated from measured
values of inorganic C assuming a CaCO3 stoichiometry. After careful consideration, it was
found to be unnecessary to correct dry sediment weights for salt content because significant quantities of pore water were removed
(via centrifugation ) prior to drying.
Total reduced inorganic sulfur [pyriteS + acid-volatile sulfide-S (AVS-S) + elemental S] was measured using the chromium reduction method described by Zhabina and
Volkov (1978) and Canfield et al. (1986).
This method has been shown to be specific to
the reduced inorganic phases of sulfur - - liberation of organic S and sulfate-S has not been
observed (Canfield et al., 1986). The evolved
H2S was trapped as ZnS and analyzed by iodimetric titration. Sulfur yields on the order of
96-98% have been achieved for freshly-ground
pyrite standards using this technique. Replicate chromium reduction analyses using dried
Black Sea sediments typically resulted in a high
degree of reproducibility ( _ 2% or better). For
a number of samples, however, the chromium
reduction extraction was preceded by an AVS
distillation using wet, homogenized, freshlythawed sediment and 6 N HC1 containing 15%
SnC12 (room temperature, stirred, 1.5-hr extraction; the details of this procedure are discussed in Berner et al., 1979; Chanton and
Martens, 1985; Morse and Cornwell, 1987).
This HC1-SnC12 extraction yielded sulfur recoveries averaging 98% for CdS standards.
Aliquots of filtered pore water were fixed on
board for pore-water sulfide analysis using a
Zn-acetate-NH4OH solution in a nitrogen-
9
filled glove bag immediately following sediment centrifugation. Total dissolved sulfide
was later determined using the methylene blue
technique of Cline (1969) with a detection
limit of 3/z~r. Additional pore-water aliquots
were acidified on board, using redistilled HCI,
for later analysis of dissolved sulfate and Fe.
Sulfate was determined gravimetrically as
BaSO4 using ~ 10% BaC12 and 2-10 ml of acidified pore water (as outlined in Westrich,
1983). Interstitial dissolved Fe was measured
spectrophotometrically by means of the ferrozine method of Stookey (1970) with a detection limit of < 1 ~tM. The agreement between
duplicate analyses of both dissolved sulfate and
dissolved Fe was generally very good. Bottom
waters were sampled using a Niskin ® bottle
attached to the frame of the box corer. The
sample bottle was rigged to trip at the time of
coring; these samples were acidified for later
analysis.
In order to assess the availability of reactive
(towards dissolved sulfide) Fe in deep-basin
Black Sea sediments and, consequently, the role
of Fe limitation in pyrite formation, degree-ofpyritization (DOP) values (Berner, 1970;
Raiswell et al., 1988 ) were determined for the
majority of the samples herein discussed. DOP
is a measure of the extent to which the original
total reactive Fe has been transformed to pyrite and can be expressed as:
DOP-
(pyrite-Fe)
(pyrite-Fe) + (extractable Fe )
where pyrite-Fe concentrations are calculated
from measured values of pyrite-S. "Extractable Fe" represents the remaining unsulfidized
portion of the Fe pool that has the potential to
react with H2S. This "reactive" Fe component
is defined here as the fraction of total solidphase Fe that is solubilized during a boiling 12
N HC1 distillation (brought to a boil and then
simmered for exactly 1 min) (Berner, 1970;
Raiswell et al., 1988 ). Calibration of this technique to pure Fe-mineral phases has revealed
nearly complete Fe extraction from Fe-oxides
10
(including magnetite) and comparatively minor removal of silicate Fe (with the possible
exceptions of chlorite, nontronite and, perhaps, biotite) (Berner, 1970; R. Raiswell, pers.
comm., 1989 ). Although somewhat arbitrary,
this approach is reproducible and has great
value in comparative studies.
Recently, Canfield (1989a), using a variety
of carefully calibrated wet-chemical extractions, studied the speciation and reactivity of
Fe phases in modern marine sediments in detail. Although the standard boiling-concentrated HC1 extraction was employed in the
present study to render our results more amenable to broader comparison, Canfield's extraction scheme might allow for a more sophisticated investigation of iron sulfidation and
availability. Finally, Leventhal and Taylor
(1990) performed a systematic comparative
study of DOP methods using a number of different "extractable Fe" distillations for both
modern and ancient sediments. Leventhal and
Taylor found comparable DOP values for
Black Sea DSDP Leg-42 samples from a wide
range of core depths using boiling 12 N HC1 for
I min, room temperature 1 N HC1 for 24 hr
and sodium dithionite in a citrate buffer for 24
hr [see Canfield (1988, 1989a) for a discussion of the Na-dithionite method]. However,
the levels of Fe extracted via Na-dithionite
were decidedly the lowest of the three sets of
reported values.
4. Results and interpretations
4.1. Unit 1
4.1.1. Unit 1 solid-phase carbon-sulfur data.
The down-core distributions of solid-phase organic and inorganic carbon and total reduced
inorganic sulfur in subcores from stations 9 and
14 are provided in Fig. 7 and Table 1. Previously published data indicate the presence of
only minor amounts of elemental sulfur in
deep-basin Black Sea sediments (Vinogradov
et al., 1962). Kluckhohn et al. (1990) found
T.W. LYONS AND R.A. BERNER
good agreement between total S and pyrite-S
below the upper 6 cm in Unit-/sediment collected during Leg 3 of the 1988 expedition. The
small difference observed between the two
fractions in the upper 6 cm was attributed to
the possible presence of organic and elemental
S and very minor measured values of AVS-S.
Furthermore, our study has revealed that sulfur in the form of acid-volatile Fe-monosultides comprises only a few percent of the total
reduced inorganic S in the upper several centimeters of Unit-/sediment and becomes negligible with depth. Given these observations,
the sulfur data in Fig. 7 and Table 1, with the
possible exception of values from the upper few
centimeters, are interpreted to represent exclusively pyrite-S.
The profiles depicted in Fig. 7 reveal a
somewhat random down-core distribution of
carbon and sulfur at both stations, reflecting
the nonsteady-state nature of the particle flux
(see Hay and Honjo, 1989). However, the organic C profile at Station 9 is roughly a mirror
image of the inorganic (7 trend, thus suggesting
a relatively strong inverse relationship between the two that will be addressed later in
this discussion (see p.16). The centimeterscale, light and dark "bands" (multicouplet
packets) correlated between the subcores from
stations 9 and 14 (Fig. 5) derive from longerterm (multiyear) fluctuations in E. huxleyi
production and/or siliciclastic input superimposed on the seasonal variability (e.g., peak
bloom periods; Hay and Honjo, 1989). This
correlation is reflected in the general similarities between the inorganic C profiles of the two
stations. The mean concentrations of pyrite-S,
calcium carbonate (as calculated from the inorganic C data ) and organic C for Station 9 are
1.4, 54.5 and 5.7 wt%, respectively, and 1.2,
50.2 and 4,9 wt% for Station 14. The organic
C value of 8.6 wt%, an extreme, for the 2-4-cm
interval of Station 9 likely relates, at least in
part, to the inclusion of material from the surface fluff layer.
C-S-Fe SYSTEMATICSOF THE UPPERMOSTDEEP-WATERSEDIMENTSOF THE BLACKSEA
(a)
Wt. %
0
0
-
2
,
4
.
.
.
.
11
Wt. %
(b)
6
. .
8
10
.
2
4
6
8
0
10
10
Depth
(cm)
Depth
(cm)
20
20
10
[
~----~T--org
Station 14
Station 9
30
M Sulfur
• C-inorg
30
Fig. 7. Depth distributions of pyrite-S and solid-phase organic and inorganic carbon in U n i t - / sediment collected at
stations 9 (a) and 14 (b). The cores were extruded and processed using 2-cm intervals.
4.1.2. Unit I pore-water data. The depth distribution of dissolved sulfate and sulfide in pore
waters from subcores collected at stations 9 and
14 are presented in Fig. 8. The profiles from
both stations display only minor down-core
decreases in dissolved sulfate despite organic
C concentrations generally on the order of 4-6
wt%. The comparatively low extent of diagenetic (burial) sulfate reduction indicated by
the observed trends suggests low degrees of organic-matter reactivity (with respect to sulfate-reducing bacteria) (see also Canfield,
1989b). Interstitial sulfate concentrations were
actually greater than measured bottom-water
values over the length of the Station-9 subcore
and the upper portion of the subcore from Station 14. This observation, while difficult to explain, probably reflects errors in sampling and/
or analysis associated with the bottom-water
procedures (e.g., pretripped sample bottles
yielding water samples from shallower, less saline depths). In line with this idea, Sweeney
and Kaplan (1980) reported ~18.0 m M
SO42- at 2000 and 2050 m from two deepwater Black Sea water-column profiles. Despite low extents of overall post-depositional
sulfate reduction, variance from the Sweeney
and Kaplan bottom-water sulfate values in the
uppermost measured sediment intervals at stations 9 and 14 likely derives, to a large degree,
from active sulfate reduction within the surficial layers of sediment.
Interstitial sulfide trends in Fig. 8 reveal
negligible to relatively small increases above
values typical of central-basin Black Sea deep
waters (256 and 349 # M a t 2000 and 2050 m,
respectively, at two central basin stations;
Sweeney and Kaplan, 1980). [The Sweeney
and Kaplan data are typical of the low end of
the concentration range reported by Brewer
and Spencer (1974) for samples from 2000 m
and deeper from a large number of deep-water
stations sampled during the R / V "Atlantis II"
cruise. Nicholson (1988) and Nicholson et al.
T.W. LYONSAND R.A. BERNER
12
TABLE 1
Solid-phase C, S and Fe data for Unit-/sediment at station 9 and 14
Depth
(cm)
(wt%)
DOP* ~
C/S* ~
Corg
C,nors
CaCO3 .3
sulfur .4
HCl-soluble Fe .5
total reactive Fe .6
8.62
5.34
6.37
5.64
7.04
5.19
3.35
5.70
5.70
5.71
4.54
5.55
5.73
3.88
5.91
4.89
6.04
4.66
7.63
8.89
7.19
6.75
6.64
8.02
7.91
6.53
32.36
49.25
40.79
50.34
38.89
63.66
74.10
59.92
56.30
55.39
66.88
65.94
54.49
1.70
1.42
1.84
1.63
1.78
1.02
0.70
1.50
1.24
1.62
0.92
1.13
1.38
0.62
0.47
0.55
0.45
0.52
0.25
0.20
0.38
0.34
0.45
0.26
0.31
0.40
2.10
1.71
2.15
1.87
2.07
1.14
0.81
1.69
1.42
1.86
1.06
1.29
1.60
0.70
(/.72
0.74
0.76
0.76
0.78
0.75
0.78
0.76
0.76
0.75
0.76
0.75
5.07
3.76
3.46
3.46
3.95
5.09
4.76
3.80
4.60
3.52
4.95
4.92
4.28
5.37
4.72
4.56
5.59
4.86
3.79
5.82
5.13
4.72
4.75
4.65
5.98
4.15
4.93
4.08
4.54
5.16
4.76
6.96
8.05
5.54
6.61
6.04
6.79
7.51
6.61
5.62
6.02
33.99
37.87
43.01
39.71
58.01
67.12
46.21
55.11
50.36
56.61
62.59
55.09
46.87
50.20
1.21
1.53
1.44
1.69
0.99
0.78
1.34
1.26
1.43
0.97
0.76
0.97
1.24
1.20
0.78
0.83
0.68
0.73
0.40
0.31
0.50
0.44
0.57
0.44
0.39
0.42
0.64
0.55
1.83
2.16
1.93
2.20
1.27
0.99
1.67
1.54
1.82
1.29
1.05
1.26
1.72
1.59
0.57
0.62
0.65
0.67
0.69
0.69
0.70
0.72
0.69
0.65
0.63
0.68
0.63
0.66
4.44
3.09
3.16
3.31
4.90
4.84
4.34
4.07
3.30
4.88
6.14
6.18
3.35
4.31
Station 9:
2-4
4-6
6-8
8-10
10-12
12-14
14-16
16-18
18-20
20-22
22-24
24-26
Mean
Station 14:
0-2
2-4
4-6
6-8
8-10
10-12
12-14
14-16
16-18
18-20
20-22
22-24
24-26
Mean
* ~Degree-of-pyritization = (pyrite-Fe) / [ (pyrite-Fe) + (HCl-soluble Fe ) ]
,2 ( wt% organic C ) / (wt% pyrite-S ).
* 3Calculated from wt% inorganic C.
*4"Pyrite" sulfur (see text).
* 5Extraction: boiling, 12 N HCI for 1 min.
,6Total reactive Fe = (HCl-soluble Fe ) + (pyrite-Fe).
( 1988 ) reported dissolved sulfide values in the
range of 300-400/zM at depths o f 2000-2200
m from a number of basinal sites. ] The observed dissolved-sulfide trends are compatible
with the relatively low rates o f sulfate reduction (compared with the rates expected in
nearshore marine sediments with similar concentrations of organic C) implied by the sul-
fate data. Albert et al. (1988 ) reported sulfate
reduction rates on an areal basis of ~ 60/tmol
m -2 hr -~ for Unit-/ sediments collected at
stations in the central deep basin during Leg 2
of the 1988 expedition, but found maximal
rates at the sediment-water interface with a 4fold decrease by 20 cm. Sulfate reduction rates
reported for deep-basin sediments of the Black
13
C-S-Fe SYSTEMATICS OF THE UPPERMOST DEEP-WATER SEDIMENTS OF THE BLACK SEA
(a) Pore-Water Sulfate (mM)
14 . 1,5 .
1,6~
17
18
(b) Pore-Water Sulfide (p.M)
19
o
100 200 300~'400 500 600 700 800
10
lO
Depth
(cm)
Depth
(cm)
20
20
,!
I
" Station9 1
[] Station 14
30
Fig. 8. Down-core profiles of dissolved sulfate (a) and sulfide (b) in pore waters collected from Unit-/ sediment at
stations 9 and 14. Arrows indicate the measured bottom-water dissolved-sulfate value for both stations (this study; essentially the same concentration was found for both stations) and a "mean" deep-water ( > 2000-m depth) dissolved-sulfide
concentration based on the data of Brewer and Spencer (1974), Nicholson (1988), Sweeney and Kaplan (1980), and
Nicholson et al. (1988).
Sea are typically highest very close to the sediment-water interface (Vaynshteyn et al., 1986;
see also Sorokin, 1962, 1964). These reduction-rate maxima in the surficial sediment layers coincide with the dramatic drop in organic
C concentration observed in the deep basin in
association with the transition from the surface fluff layer to the underlying microlaminated sediment (Station 9 of this study; see also
Calvert and Karlin, 1991 ).
Dissolved Fe concentrations were low in
subcores from stations 9 and 14. A partial data
set from Station 9 reveals a fairly systematic
increase from ~ 0 . 2 / ~ / ( 2 - 4 cm ) to ~ 3.0 # M
dissolved Fe by 20-22 cm, while interstitial Fe
in a partial profile from Station 14 ranges from
~0.5/~M (0-2 cm) to ~ 1.8/IM (6-8 cm) and
appears to remain at essentially 1.8 pdhr to the
deepest measured value ( 14-16 cm). These
low levels of dissolved Fe were expected given
the down-core persistence of moderately high
concentrations of dissolved sulfide and the insolubility of Fe-sulfides. Bottom-water Fe was
below detection at stations 9 and 14.
In summary, organic-matter reactivity and
not concentration appears to limit sulfate reduction in Unit-/ sediments. The concentrations of interstitial sulfate measured at stations 9 and 14 indicate that sulfate is not a
limiting factor in Fe-sulfide formation (see
Boudreau and Westrich, 1984) and that methanogenesis is not likely in Unit-/ sediment.
Appreciable methane production is believed to
occur only when dissolved sulfate is essentially
exhausted (Martens and Berner, 1974). This
is consistent with the findings of W.S. Reeburgh (pers. commun., 1989) indicating that
there is a net flux of methane into the sediments of the abyssal regions of the Black Sea.
The low concentrations of AVS-S in Unit 1 are
not unexpected given the comparatively slow
sediment accumulation rates, the measured
14
levels of pore-water sulfide and the thermodynamically unstable nature of "FeS" (AVS)
relative to pyrite in the presence of excess hydrogen sulfide. The moderately high and relatively constant levels of dissolved sulfide observed at depth in subcores from stations 9 and
14 suggest: ( 1 ) that all reactive Fe has been
previously consumed so that Fe is limiting pyrite formation in the microlaminated (Unit I )
deposits; and (2) that further bacterial sulfate
reduction at depth is inhibited by a paucity of
reactive organic matter [see Morse et al.
( 1992 ) for a relevant discussion of Fe-limited
sediments from Baffin Bay, Texas, U.S.A. ].
4.1.3. Unit-1 reactive Fe. The solid-phase data
of Table 1 include HCl-soluble Fe and total reactive Fe [HCl-soluble Fe + pyrite-Fe (and
AVS-Fe ) ] for U n i t - / s e d i m e n t collected at stations 9 and 14. DOP values calculated from
these data are given in Fig. 9 and Table 1. The
relatively high DOP values support the contention of Fe-limited pyrite formation. Calvert
and Karlin (1991) also reported high DOP
values and discussed limitations in the availability of reactive Fe for the deep-water microlaminated (Unit 1) sediments of the modern
Black Sea. Despite some scatter at depth, DOP
at both sites appears to increase systematically
from surface m i n i m a and level off by ~ 8 cm
below the sediment-water interface. Consequently, DOP is insensitive to variations in the
concentrations of organic C, as well as total reactive Fe. The observed trends suggest some
degree of post-depositional pyrite formation in
the upper layers of sediment at both stations
(down-core DOP increases of ~ 0.70-0.77 at
Station 9 and ~ 0.57-0.70 at Station 14 translate to increases in pyrite content of 10 and 23
wt%, respectively). The pattern below ~ 8 cm
implies that a relatively fixed fraction of the
"reactive" Fe pool is not readily sulfidized despite comparatively slow sedimentation rates
and dissolved-sulfide concentrations approaching 700/tM (Station 14). This assump-
T.W. LYONS AND R.A. BERNER
Degree of Pyritization (DOP)
0.5 0.6 0.7 0.8 0.9
0.4
1
{{{{{
1.0
o {ii
Depth
~{
•
o
Station 9
Station 14
3O
Fig. 9. Depth distributions of degree-of-pyritization
(DOP, see text for explanation) values from Unit-/sediment collected at stations 9 and 14. Note that the DOP
scale is somewhatexpanded (0.40-1.00).
tion indicates that the boiling, concentratedHC1 extraction might result in an overestimation of the residual "readily" reactive component of the total solid-phase Fe pool in Unit I.
4.1.4. Unit 1 carbon-sulfur relationships. C-S
relationships for Unit-/ sediments are summarized in a scatter plot of wt% organic C vs.
wt% pyrite-S (Fig. 10). The C-S plot of Fig.
10 includes the data from stations 9 and 14
provided in Fig. 7 and 'Table 1, as well as data
from three 2-cm subturbidite microlaminated
intervals from a subcore collected at Station
18A (BC 2). The "normal marine regression
line" depicted in this figure was plotted using
a mean C / S ratio of 2.8 (Goldhaber and Kaplan, 1974; Berner, 1982; Raiswell and Berner,
1986 ). The data from all three stations fall below the normal marine line, revealing that Unit
1 (at least in the observed organic C range) is
not enriched in pyrite-S relative to sediments
C - S - F e SYSTEMATICS O F THE U P P E R M O S T DEEP-WATER SEDIMENTS OF THE BLACK SEA
Normal Marine Regression Line N
~
rite-S and organic C displayed by Station-9
sediment when assessed alone (r2=0.62,
r 2 =0.77 when the outlier point is excluded)
(Fig. 1 la).
The linear relationship in the C-S plot for
Station 9 (Fig. 1 la) suggests some degree of
carbon limitation associated with pyrite formation. However, the correlation is likely a
/
/
2
"
•
[]
[]
/
/
2
4
6
•
•
o
Station 9
_Station14
o
Station 18A
wt. % organiccarbon
8
15
10
(a) 2.0
1.8
Fig. 10. Scatter plot ofwt% organic C vs. wt% pyrite-S for
Unit-/sediment from stations 9, 14 and 18A. The Holocene normal marine regression line (Goldhaber and Kaplan, 1974; Berner, 1982; Lin and Morse, 1991 ) has been
included for comparison (C/S = 2.8 ).
Station9
==7//
1.6
~
1.4
~
1.2
~
t.0
0.8
with comparable organic C concentrations deposited under oxygenated bottom-water conditions. Because the C-S plot includes data
from intervals spanning the length of the subcore, it might be argued that pyrite formation
is not complete in some instances. However,
the DOP relationships of Fig. 9 indicate that
pyrite-S concentrations are not likely to increase dramatically. While correcting the data
from the surface layers to the maximum DOP
values would move several points closer to the
normal marine line, the overall distribution of
data on the C-S plot would not change appreciably. C/S ratios are given for each sample interval from stations 9 and 14 (Table 1 ). The
mean C/S weight ratio calculated for these two
stations (the ratio of the mean wt% organic C
divided by the mean wt% pyrite-S) is 4.15.
A simple linear regression fit of the data displayed in Fig. 10, when evaluated collectively,
yields a correlation coefficient (r 2 ) between
pyrite-S and organic C of 0.45 and a y-intercept of ,-, 0.2 wt% S. Exclusion of the one outlier point (organic C > 8 wt%), which may include surface fluff layer material characterized
by incomplete diagenesis (Pilskaln, 1990),
does not modify the extent of correlation in any
appreciable way. The suggestion of correlation
observable in Fig. 10 appears to largely hinge
on the stronger linear relationship between py-
0.6
y o 7.37171~2 +
0.4
4
5
0.22718x R^2 - 0.624
6
7
8
wt. % organiccarbon
(b) 2.0
Station 9
='7"~
•
1.8
1.6
03
1.4
*~ 1.2
1.0
0.8
•
0.6
20
y . 0.19495+ 2 5926e-?.x R^2 - 0.788
30
40
5O
60
wt. % non-carbonate
(c) 9
70
Station9
g
.£
8
._=o
7
6
~
I
y - 12.595 - 1.0584x R^2 - 0,803
,
,
4
,
L
5
,
=
6
,
•
=
7
•i
~
,
i
8
,
9
wt. % organiccarbon
Fig. 11. Three scatter plots of data from Unit-/sediment
collected at Station 9. Simple linear regression fits have
been included with the data of each plot.
a. Wt% organic C vs. wt% pyrite-S.
b. Wt% non-carbonate (predominantly siliciclastic sediment ) vs. wt% pyrite-S.
c. Wt% organic C vs. wt% inorganic C (Ca-carbonate
carbon).
16
T.W. LYONS AND R.A. BERNER
spurious one arising from the dilution effects
of high calcium carbonate concentrations. Fig.
I lb demonstrates a strong positive correlation
between pyrite-S and the non-carbonate (i.e.
predominantly siliciclastic) component of the
microlaminated sediment at Station 9. This relationship reflects the Fe-limited conditions of
pyrite formation and a detrital source of reactive Fe that appears to be closely coupled with
the siliciclastic sediment fraction. For example, there is a good positive correlation
( r 2 = 0 . 8 5 ) between the carbonate-free component and total reactive Fe. As observed by
Raiswell and Berner (1985) for basinal Black
Sea sediments, there is no significant correlation between reactive Fe and organic C concentrations in U n i t - / d e p o s i t s when plotted on
a CaCOa-free basis (r 2 = 0.02 and 0.26 for stations 9 and 14, respectively). Because of limitations in the availability of reactive Fe, a correlation of r 2 -- 0.99 for wt% pyrite-S vs. wt%
total reactive Fe was observed for Station 9,
with a nearly constant ratio between the two
components ( F e / S ~ 1.2 compared to 0.87 for
stoichiometric pyrite) (plot not shown). Furthermore, correlations of r E -----0.87 for pyrite-S
vs. HCl-soluble Fe and r 2 = 0.82 for pyrite-S vs.
HCl-soluble Fe on a carbonate-free basis (for
the nine intervals where a "constant" asympNormal MarineRegressionLine
4
[
J.-'"
• 2 i-
"~
/
[
1t
[]
•
o-
r-~ st~on9
/
/
J
CaCO3-free basis
oV . . . .
? ., - - T W - . . . . .
0
2
4
6
8
10 12
]
J a Station14 /
h°_ station 18A /
~.--r~.,
14
16
.-
18
20
wt. % organic carbon
Fig. 12. Plot of wt% organic C vs. wt% pyrite-S graphed
on a CaCO3-free basis for U n i t - / s e d i m e n t from stations
9, 14 and 18A. The regression line included with this plot
represents the best fit for a large number of Holocene normal marine sediments (e.g., Goldhaber and Kaplan, 1974;
Berner, 1982; Lin and Morse, 1991 ) ( C / S = 2 . 8 ) .
totic DOP value is attained) were found for
Station 9.
Organic C shows a strong negative correlation with respect to inorganic C (calcium carbonate) in sediments from Station 9
(r2=0.80)
(Fig. l lc). This relationship is
likely a product, at least in part, of the dilution
effect of calcium carbonate, suggesting that the
total organic C flux is ostensibly decoupled
from the coccolith CaCO3 contribution. This
observation is in contrast to the positive relationship between C a C O 3 and organic C, perhaps reflecting E. huxleyi production, reported
by Raiswell and Berner (1985) for surficial
deep-water Black Sea sediments (see also
Shimkus and Trimonis, 1974). At this juncture, a coupling between terrigenous sedimentation and the total organic C flux (i.e. an appreciable flux of recycled and terrestrial organic
C to the deep basin; e.g., see Pelet and Debyser, 1977; Simoneit, 1977; Calvert and Fontugne, 1987; Hay, 1988; Beier and Wakeham,
1990; cf. Lee et al., ]L980) cannot be discounted. In sum, the relationships observed in
Fig. 1 lb and c would act in concert to produce
a spurious positive linear relationship between
organic C and pyrite-S, as evidenced by Fig.
lla.
A plot of wt% organic C vs. wt% pyrite-S
presented on a CaCO3-free basis (Fig. 12 ) corroborates the supposition of a false C-S coupling in Unit-/ sediment. Here, the distribution of pyrite-S concentrations appears
independent of organic C (however, the data
from stations 9 and 14 are separated into two
fairly distinct groups, suggesting a very subtle
trend between sites). When viewed in light of
the other available evidence, this strongly scattered arrangement of data below the normal
marine line (r2=0.18 for a simple linear
regression fit using the data from all three stations) suggests Fe-limited pyrite formation in
association with a nonsteady-state Fe flux for
the microlaminated (Unit 1) deposits of the
deep Black Sea. At Station 14, pyrite-S correlates positively quite well with wt% non-car-
17
C-S-Fe SYSTEMATICS OF THE UPPERMOST DEEP-WATER SEDIMENTS OF THE BLACK SEA
bonate (r 2 = 0 . 6 7 ) , but there is no systematic
relationship between organic C and CaCO3
( r 2 = 0 . 1 4 ) (plots not shown). As a consequence, an obvious coupling, real or otherwise, between pyrite-S and organic C is also
lacking in sediment collected at Station 14
(r 2 ----0.08) (as revealed in Fig. 10).
to those of the Station-18A turbidite. As described earlier (see p. 8 ), grain-size analysis of
the turbidite from Station 18A (BC 2), as well
as for the two m u d layers of Station 11, revealed dramatic textural homogeneity and a
predominance of fine grain sizes (fine silt to
clay) (Lyons, 1991 ). Note the elevated concentrations of organic C (stations 7 and 18A)
and inorganic C (CaCO3, Station 7) associated with the fluff layers (surface and subsurface). Also note the contrast between the turbidite and Unit-/chemistries at Station 18A.
The turbidite sulfur values in the subcores represented in Fig. 13 are comparable to those of
Unit 1; however, both organic and inorganic
carbon concentrations are much lower than
Unit-/values (compare Fig. 7 ).
4.2. Turbidites
4.2.1. Turbidite solid-phase carbon-sulfur data.
The down-core distributions of solid-phase organic and inorganic carbon and total reduced
inorganic sulfur are given in Fig. 13 and Table
2 for two subcores from the deep-water region
of the Black Sea basin (stations 7 and 18A [ BC
2 ]; see Fig. 2 ). Both cores contain muddy gray
turbidite horizons that display remarkable CS compositional homogeneity. The two m u d
layers of Station 7, although believed to represent separate turbidite "events", exhibit nearly
identical C-S chemistries that are very similar
(a)
00
4.2.2. Turbidite pore-water data. The pore
waters at Station 7 are characterized by a downcore monotonic decrease in dissolved sulfate
(Fig. 14 ) associated with a concomitant rise in
Wt. %
(b)
1 2 3 4 5 6 7
' " ' i~"~/~ff'Laie;-i-t]
Turbidite A
lO~-[
(Cm~o
IS
Depth
Turbidite B
00
Wt. %
4
6
2
•
,
l
Depth(cm)
.
•
40
7
8
.
,,
Turbidite
.
.
[] C-org
/
Sulfur
o C- norg
|
/
J
.
.
{t{ {
2O
•
,
J
.
30 1
.
Siatlon 18A
a C-org
• Sulfur
o C-inorg
30
Fig. 13. Depth distribution of solid-phase total reduced inorganic sulfur, as well as organic and Inorganic carbon, in
sediment subcores collected at stations 7 (a) and 18A (b). The upper 2-cm interval from the Station-18A subcore includes material from the surficial fluff layer.
T.W. LYONSAND R.A. BERNER
18
TABLE 2
Solid-phase C, S and Fe data for turbiditic sediment at stations 7 and 18A (BC 2)
Depth
( wt% )
DOP* i
C/S* 2
(cm)
Cor~
C~,o~g
CaCO3 .3
sulfur .4
HCl-soluble Fe .5
total reactive Fe .6
7.07
3.16
1.29
1.34
1.52
2.54
2.13
1.44
1.45
1.44
1.41
1.14
1.08
1.11
1.36
1.35
1.34
1.30
1.86
3.01
2.62
1.37
1.29
1.14
2.15
1.64
1.21
1.19
1.22
1.19
1.49
1.48
1.53
1.23
1.34
1.28
1.32
1.54
25.07
21.88
11.38
10.75
9.53
17.92
13.65
10.10
9.90
10.15
9.91
12.39
12.37
12.77
10.23
11.20
10.68
11.03
12.83
1.40
1.31
1.19
1.20
1.12
1.18
1.28
1.18
1.17
1.17
1.13
1.15
1.12
1.20
1.14
1,18
1,15
1,18
1,19
1.01
1.39
1.83
1.89
1.83
1.53
1.67
1.84
1.86
1.81
1.83
1.94
1.79
1.86
2.01
2.02
1.98
1.74
1.77
2.23
2.53
2.87
2.94
2.81
2.56
2.79
2.87
2.88
2.83
2.81
2.94
2.77
2.91
3.00
3.05
2.98
2.77
2.81
0.55
0.45
0.37
0.36
0.35
0.40
0.40
0.36
0.35
0.36
0.35
0.34
0.35
0.36
0.33
0.34
0.34
0.37
0.37
5.05
2.41
1.08
1.12
1.35
2.15
1.66
1.22
1.24
1.23
1.24
0.99
0.97
0.93
1.19
1.14
1.16
1.10
1.51
1.99
1.53
1.66
1.75
2.07
2.13
1.90
1.9l
1.87
16.58
12.76
13.86
14.59
17.26
17.80
15.83
15.90
15.57
1.42
1.51
1.51
1.51
1.50
1.52
1.53
1.53
1.50
1.56
1.75
1.82
1.78
1.83
1.74
1.70
1.68
1.73
2.80
3.07
3.14
3.10
3.14
3.06
3.03
3.01
3.04
0.44
0.43
0.42
0.43
0.42
0.43
0.44
0.44
0.43
1.94
1.23
1.27
1.32
1.17
1.13
1.30
1.30
1.33
Station 7:
0-2
2-4
4-6
6-8
8-10
10-12
12-14
14-16
16-18
18-20
20-22
22-24
24-26
26-28
28-30
30-32
32-34
34-36
Mean
Statton 18A:
0-2
2-4
4-6
6-8
8-10
10-12
12-14
14-16
Mean
2.75
1.86
1.91
1.99
1.75
1.72
1.98
1.99
1.99
* ~Degree-of-pyritization= (pyrite-Fe) / [ (pyrite-Fe) + ( HCl-soluble Fe ) ].
* -' ( wt% organic C ) / (wt% total reduced inorganic S ).
* 3Calculated from wt% inorganic C.
*4Total Cr-reducible S (total reduced inorganic S).
*SExtraction: boiling, 12 NHCI for 1 min.
*6Total reactive F e = (HCl-soluble Fe) + (pyrite-Fe).
dissolved sulfide from ~ 200 # M for the 0-2cm interval to ~ 2 5 0 0 / z M b y 30-32 cm (Fig.
14). The observed turbidite pore-water trends
indicate high rates of bacterial sulfate reduction and, particularly when contrasted with
pore-water profiles of Unit 1 (Fig. 8 ), confirm
the effectiveness of remobilization and export
of reactive organic matter to deep-water settings via turbidite processes. As discussed for
the microlaminated sediments at stations 9 and
14, the sulfate concentrations in the uppermost intervals were somewhat greater than the
measured bottom-water value (Fig. 14). Interstitial Fe concentrations at Station 7 were low
C-S-Fe SYSTEMATICSOF THE UPPERMOSTDEEP-WATERSEDIMENTSOF THE BLACKSEA
Pore-Water Sulfate (mM)
4
6
8
10
12
14
Degree of Pyritization (DOP)
16
18
lo
0.3
U
0.4
0.5
0.6
0.7
10
Depth
(cm)
"
xx
20
Depth
(cm)2o
,f
30
/
f
Station 7
400
19
---D-- Sulfate
I
---~--
•
Sulfide
1000
2000
Pore-Water Sulfide (I.tM)
Station 7. The cores were processed and the pore waters
analyzed using 2-cm intervals. Arrows are included to indicate the measured bottom-water sulfate value at Station
7 (this study) and a " m e a n " deep-water ( > 2 0 0 0 - m
depth) dissolved sulfide concentration based on the published results of other Black Sea studies (see Fig. 8).
and displayed a scattered down-core distribution within a narrow range of values (0.4-5.3
~m).
4.2.3. Turbidite reactive Fe. DOP values along
with data for HCl-soluble Fe and total reactive
Fe for the turbidites at stations 7 and 18A are
presented in Fig. 15 and Table 2. Pyrite-Fe is
calculated from the values of total chromiumreducible sulfur since black AVS enrichments
were not observed in the turbidite muds at stations 7 and 18A. The mean turbidite DOP at
Station 7 is 0.35+0.01 (_+la) [data from
samples incorporating the surface fluff layer
(0-4 cm ) and the subsurface "compacted fluff
layer" ( 10-14 cm ) were omitted from the calculation of the mean ]. The mean DOP for the
turbidite at Station 18A is 0.43_+0.01 ( l a ) ,
I
[] Station 18A I
3000
Fig, 14. Down-core profiles o f dissolved sulfate and sulfide in pore waters collected from turbiditic sediment at
Station 7
40
I
Fig. 15. Depth distribution of DOP values for U n i t - / a n d
turbiditic sediments collected at stations 7 and 18A. Flufflayer material is included in the subcores (see Fig. 13 ).
while the values at the base of the core are typical of U n i t - / s e d i m e n t . At both stations, the
Fe ( D O P ) data reveal a striking degree of
compositional homogeneity with depth compatible with that observed for the C and S data
(Fig. 13).
4.2.4. Comparison of turbidite and Unit 1 carbon-sulfur relationships. A compositional
comparison between deep-basin turbidites and
the microlaminated sediment is provided in the
C-S plot of Fig. 16a, where C and S are plotted
as weight percent of total sediment. This figure
includes U n i t - / d a t a from stations 9, 14 and
18A and turbidite values from stations 7, 18A
and five sample intervals spanning the two turbidites at Station 11. While similarities in the
sulfur compositions are apparent, dramatic
differences in the organic C concentrations result in a strong separation of the two data sets.
The turbidite data are clustered in two high-S,
low-C groups. The five "turbidite" points (circled in the Fig. 16 plots) falling outside these
20
T.W, LYONS
(a) 3
R.A. BERNER
Fig. 16b includes the simple linear regression
fit for the microlaminated samples. The turbidite data fall just below the very weak trend
suggested by the U n i t - / v a l u e s ( r 2 = 0.18 ).
Normal Marine Regression L m e k ~ /
_•
AND
2
t
5. Discussion
/
-
I" Oe,tt
J
~
0
2
'i'I
4
6
Turbidites I
8
10
wt. % organic carbon
Best Fit Line:
Unit 1 dataonly
2
I
~
• TM
m j ~ m
•
m
k
o
~
0
.
2
4
6
8
i
10
.
.
.
12
.
.
14
L
16
.
t8
wt % organic carbon
Fig. 16. Scatter plots o f wt% organic C vs. wt% reduced
inorganic S for turbiditic and Unit-1 sediments at stations
7, 9, I1, 14 and 18A. Circled"turbidite'" data points represent those samples in which fluff-layer material is incorporated in the 2-cm interval. Arrows have been included
to indicate the direction the data would shift with further
post-depositional pyrite formation.
a. Wt% on a total sediment basis. The Holocene normal
marine regression line has been included for comparison
(C/S=2.8).
b. Wt% on a CaCO3-free basis. A simple linear regression
fit for the Unit-/samples is included.
two main groups of data include fluffmaterial;
the point lying far to the right on the graph represents the high organic C value for the 0-2-cm
fluff-layer interval at Station 7. The tight clustering of the remaining turbidite data is a
product of the strong homogeneity of these
sediments. The net result of the combined Unit
I/turbidite data set displayed in Fig. 16a is a
broad spread in the carbon values and a distribution of turbidite sulfur data that lies, with
the exception of the one extreme fluff-layer
sample, entirely above the "normal marine
regression line". In contrast to the CaCO3-rich
sediments of Unit 1, the position of the turbidite field does not shift dramatically when
plotted on a carbonate-free basis (Fig. 16b).
5. I. Previous Black Sea studies
Leventhal (1983) investigated C-S relationships in Black Sea sediments using the data
of Hirst (1974) from anoxic sites of deposition over a range of water depths. The resultant C-S plot reveals a distribution of data with
sulfur values lying well above the normal marine line, with a positive linear relationship between carbon and sulfur (organic C values
range from < 1 to ~ 6 wt%). Furthermore, a
non-zero S intercept was observed which Leventhal (1983) attributed to Fe-sulfide precipitation within the sulfidic water column, as well
as at the sediment-water interface. To date,
pyrite-S enrichments of this variety relative to
sediments deposited under oxygenated bottom waters have not been found for U n i t - / d e posits collected during the 1988 R / V "Knorr"
cruise (Calvert and Karlin, 1990, 1991;
Kluckhohn et al., 1990: Lyons and Berner,
1990a; this study).
Raiswell and Berner ( 1985 ) calculated deepbasin sulfur values for the Black Sea using data
from the solid-phase Fe speciation study of
Rozanov et al. (1974). These calculated values vary, on a weight percent basis, between
0.79 and 1.69 [mean S = 1 . 0 8 + 0 . 2 3 (1~)].
This range is compatible with the present study,
but significantly lower than the data used by
Leventhal (1983). When plotted on a carbonate-free basis, Raiswell and Berner (1985)
found a distribution of data that spanned a relatively wide range of organic C values and that
intersected the normal marine line in the vicinity of ~ 3-4 wt% organic C. Raiswell and
Berner (1985) also reported fairly low b O P
values ( ~ 0.10-0.50) that increased with increasing concentrations of organic C. Regres-
C-S-Fe SYSTEMAT1CS OF THE UPPERMOST DEEP-WATER SEDIMENTS OF THE BLACK SEA
sion of the C-S data resulted in a best-fit line
with a positive slope and a non-zero S intercept. These relationships were interpreted to
reflect the addition of C-limited burial pyrite
to the original syngenetic fraction. Careful
comparison of the organic-C and CaCO3 concentrations [provided by Rozanov et al.
(1974) with the Fe values] to data from the
present study indicate that the Black Sea plots
of Raiswell and Berner ( 1985, fig. 3) contain
data from both Unit-/ sediment and a large
number of turbidite samples.
5.2. Black Sea carbon-sulfur-iron
relationships
The relationships observed in Fig. 16 of this
study have the appearance of being strongly
analogous to those described for the Black Sea
by Berner (1984) and Raiswell and Berner
( 1985 ). The inclusion of turbidite data results
in a broad spread of organic C data and the
requisite low organic C values for a more complete understanding of deep-basin pyrite formation. As a consequence of the strong Fe limitation described earlier (see pp. 14 and 16),
the U n i t - / d a t a plot at S concentrations below
the line for normal marine sediments. By contrast, the low organic-C turbidite data display
sulfur enrichments relative to sediments accumulating under oxygenated bottom waters and
plot above the normal marine line. Low C/S
ratios are a commonly observed phenomenon
associated with anoxic deposition (e.g., Berner and Raiswell, 1983; Gautier, 1986), particularly for sediments with relatively low organic C concentrations. With higher organic C,
the availability of reactive Fe becomes the
dominant factor in producing euxinic C/S ratios greater than those typical of normal marine sediments. This situation is observed for
Unit 1 in the Black Sea.
However, the question remains whether the
positions of the turbidite data in Fig. 16a and
b reflect pyrite formed in the sediments of the
shallow basin margin and transported to abys-
2[
sal depths or, instead, indicate real trends in
water-column and deep-sea diagenetic sedimentary pyrite formation. Fig. 16a, when evaluated alone, has the appearance of a scatter plot
for Fe-limited syngenetic pyrite formation (a
non-zero S intercept with relatively invariant
sulfur concentrations over a range of organic C
concentrations). However, regardless of the
relationship suggested in Fig. 16a, the DOP
values for turbidite samples at stations 7 and
18A fall roughly in the range of 0.30 to 0.45
(Fig. 15 ), indicating that Fe limitation should
not be a factor in these sediments and that any
similarities between the S concentrations of the
turbidite and Unit-/samples shown in Fig. 16a
occur largely by chance.
When plotted on a CaCO3-free basis (Fig.
16b ), the position of the turbidite data relative
to the Unit-/values calls to mind the relationship reported by Raiswell and Berner ( 1985 ):
namely, that C-limited pyrite that formed during burial, as implied by the apparent linear relationship between C and S, is added to the
original concentration of water-column pyrite,
indicated by the positive S intercept. However, it is unlikely that the relationship in Fig.
16b simply derives from the addition of diagenetic pyrite. The U n i t - / D O P trends of Fig.
9 suggest that only a relatively minor portion
of the total measured pyrite-S was added during burial - - even for these sediments with relatively high concentrations of organic C. The
diagenetic pyrite indicated for Unit 1 is not
sufficient to account for the slope of the line
weakly defined by the U n i t - / d a t a in Fig. 16b.
Furthermore, the relatively high surficial DOP
values are indicative of largely water-column
sulfidation and/or pyrite formation at or very
close to the sediment-water interface.
While the position of the turbidite data with
respect to the normal marine line in Fig. 16 appears to be an expression of their overall euxinicity, the comparatively low DOP values of
the turbidites may be more a function of sediment source and perhaps the rapid rates at
which they accumulate than their associated
22
low organic C concentrations. Recall that sulfate reduction rates and the concomitant rise
in pore-water sulfide are greater in turbidite
sediments than in Unit i (compare Figs. 8 and
14). The sediments collected at two basinmargin stations during Leg 4 have previously
been proposed as representatives of a possible
general sediment-source region for Black Sea
deep-basin m u d d y turbidites (Lyons, 1991;
Lyons and Berner, 1993). These anoxic, upper-slope stations are located at ~ 200- and
~ 230-m water depth, at sites proximal to the
impingement of the water-column oxic-anoxic interface with the basin-margin substrate.
The sediments of these marginal stations reveal chemical characteristics, including degrees of reactive Fe sulfidation, very similar to
those of the turbidite muds of the deep basin,
and the radionuclide results of Crusius and
Anderson (1991) and Moore and O'Neill
( 1991 ) are also consistent with a basin-margin
turbidite source. However, the sulfur scenario
is complicated by the possibility of additional
pyrite formation associated with transport to
the deep basin, as well as post-depositionally
within the turbidite. As organic C is consumed
by sulfate reduction, and pyrite is formed, the
points in Fig. 16 representing the turbiditic
samples would move to the "northwest" in the
diagram. This is demonstrated by the arrows
in Fig. 16. It is worth noting that turbidite DOP
values comparable to those of Unit I would
yield solid-phase reduced sulfur concentrations similar to the U n i t - / s u l f u r values (if the
comparison is made on a CaCO3-free basis).
Nevertheless, to attribute the intermediate
DOP values of the turbidites to their rapid deposition alone would require a gross oversimplification of the mechanistic details. The rapid
rate of U n i t - / F e sulfidation is clear from earlier discussions, with much of the pyrite formation occurring in the water column or very
close to the sediment-water interface. The turbidite muds, whether or not from an anoxic
source region, would have been in contact with
water-column sulfide during transport to the
T.W. LYONS AND R.A. BERNER
deep basin, as well as pore-water sulfide [with
concentration levels likely well in excess of
those in the microlaminated sediments (e.g.,
Station 7)] for periods perhaps on the order
of decades (e.g., Station 18A). Despite prolonged exposure to dissolved sulfide, the turbidites reveal DOP values substantially below
those of U n i t - / s e d i m e n t . These observations
suggest fundamental differences between the
overall reactivities of the Fe phases associated
with the microlaminated and turbiditic sediments. While additional details are speculative
and certainly beyond the scope of this discussion, the possibility that U n i t - / p y r i t e formation might be closely linked to Fe redox cycling
and associated precipitation and sulfidation of
rapidly reactive hydrous ferric oxides in the
oxic-anoxic interface region of the water-column must be pursued further. Therefore, to say
that turbidite muds are not Fe-limited relative
to the microlaminated deposits, we must speak
in terms of reactivity on different time scales
- - a difference not readily discernible with the
boiling, concentrated-HC1 DOP extraction
technique.
On a final note, Muramoto et al. (1991)
measured fluxes of reduced sulfur in the deep
Black Sea using time-series sediment traps.
When corrected for additional organic C loss
associated with diagenetic and further watercolumn microbial oxidation (see also Hay and
Honjo, 1989), Muramoto et al. ( 1991 ) found
reduced sulfur to comprise 1.1 wt% of the total
particulate flux. This value agrees well with the
measured mean of 1.3 wt% for Unit-/pyrite-S
at stations 9 and 14 (this study). Muramoto et
al. ( 1991 ) reported a strong positive linear relationship with an S intercept close to zero for
a scatter plot of organic C vs. reduced S in trap
samples. This correlation does not necessarily
represent carbon-limited pyrite formation, but
rather was attributed to scavenging of sulfides
a n d / o r reactive Fe by organic aggregates. S
isotopic constraints argue against significant
resuspension of sediment sulfides and suggest
that sediment-trap particulate reduced sulfur
C-S-Fe SYSTEMATICS OF THE UPPERMOST DEEP-WATER SEDIMENTS OF THE BLACK SEA
fluxes are forming at and just below the oxicanoxic interface in the water column (Muramoto et al., 1991 ). Recently determined S isotopic values and down-core trends for Unit 1
pyrite (Lyons, 1992 ) are compatible with predominantly water-column pyrite formation.
Finally, Tambiev ( 1987 ) and Muramoto et al.
( 1991 ) reported the occurrence of framboidal
pyrite within the anoxic water column of the
Black Sea.
6. Conclusions
Details concerning sedimentary pyrite formation in the deep-water regions of the modern Black Sea have been addressed using systematic
evaluations
of
the
C-S-Fe
relationships of the uppermost deposits of the
basin floor. The microlaminated, "slowly" depositing, calcium carbonate-rich (Unit 1 ) sediments of the abyssal floor are characterized by
low rates of sulfate reduction below the surficial layers despite a mean organic C concentration of 5.3 wt%. This observation reflects a
surprisingly low degree of organic matter reactivity given the extent ofbasinal anoxia and is,
at least in part, a function of the Unit-/accumulation rate. C-S scatter plots for Unit-/sediment have been constructed on a CaCO3-free
basis to avoid spurious correlations deriving
from the effects of CaCO3 dilution. The distribution of organic-C and pyrite-S data for Unit1 samples collected during Leg 4 of the 1988
R / V "Knorr" Expedition (when plotted on a
CaCO3-free basis) reveals pyrite-S concentrations essentially independent of associated organic C. These data all lie below the normal
marine regression line, indicating C/S ratios
for Unit 1 that are greater than those typical of
oxically deposited sediments. By contrast,
many ancient euxinic sediments display comparatively low C/S ratios (i.e. S enrichments
relative to normal marine deposits), particularly at low levels of organic C. The discrepancy can be explained in terms of the amount
of reactive Fe relative to organic C. If there is
23
too much carbon and not enough iron, the C/
S ratio will be high. In the Black Sea, pyrite formation in Unit-/ sediment is limited by the
availability ofdetrital reactive Fe phases. Consequently, pyrite-S concentrations distinctly
higher than those typical of normal marine deposits are not found associated with the relatively high organic C values of the microlaminated sediment. Comparatively low levels of
total reactive Fe would be expected given the
low amount of silicate detritus (or high
CaCO3) in Unit 1; this relationship, along with
"high" organic C values, becomes a critical
factor in determining the position of Unit 1type data on a C-S plot.
DOP values are relatively high (0.57-0.78)
and independent of the organic C content, thus
supporting the contention of Fe limitation in
Unit 1. The DOP profiles suggest that much of
the pyrite formation occurs in the sulfidic water
column and/or very close to the sedimentwater interface. However, small down-core increases in U n i t - / D O P values are suggestive of
a minor amount of additional diagenetic (burial) pyrite formation. Regardless of the possible contribution of diagenesis, the general pyrite-S concentrations of Unit 1, when compared
with the particulate reduced sulfur fluxes measured in Black Sea time-series sediment traps
(Muramoto et al., 1991 ), are compatible with
predominantly water-column pyrite formation. The DOP values for Unit 1 fall within the
low to intermediate range of values characteristic of ancient sediments interpreted (by independent paleoecological means) to reflect
oxygen-deficient deposition (see Raiswell et
al., 1988).
The relative worth of Black Sea sediment
studies towards an increased understanding of
modern euxinic settings, as well as paleoenvironmental interpretation, is hampered by the
narrow range of measured organic C in Unit 1.
The inclusion of deep-basin turbidite data with
values from Unit 1 on a C-S scatter plot results in a broader range in organic C values. In
the low range of organic (7 exhibited by the tur-
24
bidite muds, the impact of water-column anoxia on turbidite sulfur concentrations stands
in strong contrast to the low sulfur values of
normal marine sediments. The turbidite data
plot at sulfur values above the normal marine
line, but are still characterized by relatively low
values of DOP. With further pyritization, the
turbidite data would plot even further above
the line. The observed extents of pyritization
are believed to reflect: ( 1 ) an anoxic, rapidlydepositing, upper-slope source region; (2) the
rapid rate at which the turbidites accumulate
and, perhaps most importantly; and (3) Fe
phases reactive on different time scales relative to those of the microlaminated deposits.
In a sense, therefore, these are perhaps not
"typical" euxinic sediments.
An on-going S-isotope investigation of the
sediments herein described has supported the
contention of predominantly water-column
formation for the pyrite of the microlaminated
sediment and may provide further insight with
regard to many of the questions and conclusions presented in this paper. The complexities of the basinal system (e.g., turbidite vs.
Unit 1 deposition, dilution effects) are revealed in the C - S - F e systematics described
here. Given the degree of complexity demonstrated for these modern Black Sea sediments,
C-S plots must be applied to studies of the
geologic record with great care. It is clear that
any collection of data from a euxinic setting,
modern or ancient, likely integrates results
from sediments from different sources with
different geochemical behaviors and histories.
Finally, this study also provides important
confirmation from a modern euxinic setting of
the great potential for DOP as a paleoenvironmental indicator (see Raiswell et al., 1988 ), as
well as the value of the Black Sea for understanding processes related to an anoxic water
column.
Acknowledgements
Principal financial support of this project has
been supplied by National Science Foundation
T.W. LYONS AND R.A. BERNER
grants OCE 85-08472 and OCE 88-22977. Additional funding has been provided by the Shell
Development Company. Travel support from
IGCP Project 254 (Metalliferous Black Shales
and Related Ore Deposits) is also gratefully
acknowledged, as are the captain and crew of
the R / V "Knorr" for a highly successful cruise.
We thank G. Ravizza, J. Muramoto and R.F.
Anderson for valuable discussions. This contribution has benefited from the insightful reviews of D.E. Canfield and R. Raiswell. Musical inspiration (for T.W.L.) courtesy of The
Cure and (for R.A.B.) from S. Rachmaninoff
(Symphony #2 ),
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