Evidence for sulfidic deep water during the Late Permian in the
East Greenland Basin
Jesper K. Nielsen* Department of Geology, University of Tromsø, Dramsveien 201, 9037 Tromsø, Norway
Yanan Shen Centre de Recherche en Géochimie et en Géodynamique, Université du Quebec à Montréal, C.P. 8888,
Succursale Centre-Ville, Montréal, Quebec H3C 3P8, Canada
ABSTRACT
A detailed study of the size distribution of framboidal pyrites in the black shales of the
Upper Permian Ravnefjeld Formation was performed to evaluate the redox state of the
Late Permian ocean. In contrast to framboidal pyrites in bioturbated sediments, the smaller and less variable size distribution of pyrite framboids in the laminated shales of the
Ravnefjeld Formation provides persuasive evidence for sulfidic (H2S-rich) bottom-water
conditions in the East Greenland Basin. However, the S isotope compositions of both pyrite
populations show a similar distribution. The widespread d34S values of pyrites (241.2‰
to 228.2‰) in the black shales of the Ravnefjeld Formation indicate a large fractionation
(up to 52.7‰) relative to seawater sulfate, and may record different pathways of sulfur
cycling in sulfidic water columns as well as within sediments. The new data from the East
Greenland Basin indicate that environmental stress such as widespread sulfidic conditions
could have caused the biotic crisis in the Late Permian.
Keywords: pyrite, framboids, sulfidic, stable sulfur isotopes, end-Permian mass extinction.
INTRODUCTION
It is well known that the greatest mass extinction over the history of life occurred at the
Permian-Triassic (P-Tr) boundary, when
.50% of all invertebrate families and perhaps
90% or more of existing species perished in
the oceans (e.g., Erwin, 1994). Numerous
models have been proposed to explain the
trigger or cause of the P-Tr mass extinction
(Jin et al., 2000; Erwin et al., 2002, and references therein). Among them, many have advocated that environmental changes, in particular global oceanic anoxia, may have played
a key role in driving the mass extinction (e.g.,
Wignall and Hallam, 1992; Knoll et al., 1996;
Wignall and Twitchett, 1996; Isozaki, 1997;
Kump et al., 2003). A few studies have documented a widespread anoxic condition coeval with mass extinction. For example, evidence for oceanic anoxia in the Late Permian
to Middle Triassic comes from detailed sedimentological and geochemical studies on
deep-sea pelagic successions in Japan (Kajiwara et al., 1994; Isozaki, 1994, 1997; Kato
et al., 2002). Preliminary S and Fe speciation
analyses on the sedimentary rocks of the Lower Triassic Vardebukta Formation in western
Spitsbergen also suggest the existence of a
euxinic depositional environment (Wignall et
al., 1998). In contrast, sedimentological constraints on the carbonate-dominated successions spanning the P-Tr boundary interval in
Iran led Heydari et al. (2003) to argue that
shallow and moderately deep waters of the
Late Permian open ocean were well oxygenated. Therefore, a more complete understand*E-mail: [email protected].
ing of paleo–redox conditions during the Late
Permian requires detailed examination across
multiple basins.
In combination with geologic data, the redox state of ancient oceans can be evaluated
by geochemical study of siliciclastic rocks;
however, the investigation of Late Permian
ocean chemistry has been difficult because of
the rarity of preservation of pelagic sediments
worldwide. In the East Greenland Basin, however, shale-dominated successions of Late
Permian age are well preserved and thus provide an excellent opportunity for paleoenvironmental reconstruction. In this study we examine size distributions of framboidal pyrites
from siliciclastic sedimentary rocks of the Upper Permian Ravnefjeld Formation in the East
Greenland Basin. We integrate sedimentology
and size distribution of framboids with S isotope data. These complementary approaches
provide new insights into the history of ocean
chemistry and mass extinction in the Late
Permian.
GEOLOGIC SETTING
The Ravnefjeld Formation, Foldvik Creek
Group, was deposited in the late Paleozoic
East Greenland Basin (;400 km long and 80
km wide), which formed by rifting and thermal contraction of the crust (Surlyk et al.,
1986) (Fig. 1). The Foldvik Creek Group unconformably overlies folded and faulted Devonian and Lower Permian rocks. During major transgression-regression events, deposition
of fluviomarine conglomerates of the Huledal
Formation was slowly replaced by deposition
of the hypersaline carbonates and evaporites
of the Karstryggen Formation (Surlyk et al.,
1986; Stemmerik, 2001) (Fig. 1). The area
was subsequently subaerially exposed and
eroded, followed by the deposition of the basinal shales of the Ravnefjeld Formation along
with coeval platform carbonates of the Wegener Halvø Formation (Stemmerik, 2001).
Topographic highs to the southeast and north
of the Triaselv area, along with a major fault
system at the western boundary of the basin
(Fig. 1), are thought to have controlled sedimentation (Surlyk et al., 1986; Surlyk, 1990).
An increased siliciclastic supply and a relative
fall of sea level led to deposition of the greengray bioturbated shales and sandstones of the
Schuchert Dal Formation above the Ravnefjeld Formation (Surlyk et al., 1986; Stemmerik, 2001) (Fig. 1).
Our samples were collected from the shallow drill core of GGU 303102 at the Triaselv
area, east of Schuchert Dal (Fig. 1). Lithostra-
Figure 1. A: Simplified map of studied area (location shown in inset) in East Greenland
Basin showing location of studied core. B: Cross section of Upper Permian Foldvik Creek
Group (modified from Surlyk et al., 1986); vertical extent of core through Ravnefjeld Formation is shown by box.
q 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; December 2004; v. 32; no. 12; p. 1037–1040; doi: 10.1130/G20987.1; 4 figures; Data Repository item 2004164.
1037
Figure 2. Lithostratigraphy and framboid size distributions of Upper Permian Ravnefjeld
Formation.
tigraphically, the Ravnefjeld Formation is divided into five units, including two organicrich, laminated shale units (L1 and L2) and
three bioturbated units (B1, B2, and B3) (Piasecki and Stemmerik, 1991) (Fig. 2). The differences between these lithofacies reflect the
extent to which the sediment was bioturbated
and are distinguishable in the core (location in
Fig. 1A; vertical extent in Fig. 1B). Laminated
units are finely layered and consist of organicrich black shales, whereas bioturbated units
are dominated by gray shales and siltstones
showing clear burrow structures and crossbedding. The five units of the Ravnefjeld Formation identified from the core are recognized
in all outcrops with approximately the similar
thickness, and they show the same general depositional pattern throughout the basin (Piasecki and Stemmerik, 1991). Therefore, our
results from the drill-core material are representative of the deep-water ocean chemistry in
the East Greenland Basin.
The interpretations of depositional environment for the Ravnefjeld Formation are controversial and range from anoxic (Piasecki and
Stemmerik, 1991) to oxic bottom-water conditions (Perry et al., 1995), though both models agree that the bioturbated units were deposited under oxic conditions. The absence of
stratigraphically diagnostic faunas makes the
interbasinal correlation difficult for the Ravnefjeld Formation. However, a low-diversity
conodont fauna comprising Mesogondolella
(Neogondolella) rosenkrantzi, Merrillina divergens, and Xaniognathus abstractus in the
coeval carbonate-dominated Wegener Halvø
1038
Formation indicates that the Ravnefjeld Formation shales were deposited in the late Wujiapingian (Stemmerik et al., 2001).
ANALYTICAL METHODS
We collected 50 samples for S isotope analyses of pyrites in sedimentary rocks of the
Upper Permian Ravnefjeld Formation (Figs. 1
and 2). Pyrite was quantitatively extracted by
the chromium reduction method of Canfield et
al. (1986). S isotope analyses were performed
at the Coastal Science Laboratories, United
States, with reproducibility better than
60.5‰. The S isotope results are reported relative to the Cañon Diablo troilite meteorite
(CDT) standard.
We chose 11 samples from both laminated
and bioturbated units for diameter measurements of framboidal pyrites. Framboid diameters were measured on polished thin sections
by using a scanning electron microscope
(Philips SEM 515) in backscattered-electron
operational mode. The easily recognizable
shape and structure of framboidal pyrites allowed us to measure the diameter directly
from the SEM screen.
RESULTS AND DISCUSSION
Size of Framboidal Pyrites and
Paleo–Redox Conditions
Wilkin et al. (1996) demonstrated that size
distributions of framboidal pyrites may be indicative of bottom-water oxygen levels. The
principle behind this approach arises from differences in pyrite formation between euxinic
and oxic depositional environment. In modern
euxinic environments such as the Black Sea,
framboidal pyrites are predominantly formed
immediately beneath the redoxcline and show
a high growth rate where both dissolved Fe
and sulfide concentrations are high. Because
of the hydrodynamic instability, the pyrites
formed in the upper zone of the sulfidic water
column sink to the seafloor so quickly that
they cannot attain appreciable sizes (Wilkin et
al., 1996, 1997). Therefore, framboidal pyrites
from euxinic environments are characterized
by small size and a narrow size range (Wilkin
et al., 1996). In contrast, in normal marine
sediments with oxic bottom water, the lower
growth rate and longer growth time allow
larger framboids to develop (Wilkin et al.,
1996, 1997).
In a similar approach, Wignall and Newton
(1998) proposed that the maximum framboid diameter (MFD) may be a useful indicator of paleo–redox conditions: greater MFD is expected
in normal marine sediments compared to those
in euxinic sediments. Indeed, the diameter measurement of framboidal pyrites has been successfully used to investigate both modern and
ancient depositional environments (Passier et al.,
1997; Wilkin et al., 1997; Wignall and Newton,
1998; Wilkin and Arthur, 2001).
A comparison of framboid size distributions
in both laminated and bioturbated units of the
Ravnefjeld Formation is shown in Figure 2
and Table DR11. ‘‘Box-and-whisker’’ plots are
useful in describing the size distribution of
framboidal pyrites in sedimentary successions
(Wilkin et al., 1997; Wignall et al., 1998). The
boxes in Figure 2 range from quartile Q 5
0.25 to quartile Q 5 0.75, comprising 50% of
the data. A bold line and a dashed line in the
box represent the median and mean value, respectively. The lines extending to the left and
right of the boxes indicate minimum and maximum values of framboid diameter. Framboid
sizes in the bioturbated units (B1, B2, and B3)
show quite similar distributions in contrast to
those in laminated units (L1 and L2) (Fig. 2).
More than 90% of framboidal pyrites from
both laminated units are ,5 mm in diameter,
and the maximum diameter is ;12 mm (Fig.
2; Table DR1, see footnote 1). In contrast, the
framboids in bioturbated sedimentary rocks
from the Upper Permian Ravnefjeld Formation exhibit more variation and larger sizes;
the maximum diameter is ;33 mm (Fig. 2;
Table DR1, see footnote 1).
Our framboid data show a sharp decline in
size and MFD from the bioturbated B1 unit to
the laminated L1 unit as well as from B2 to L2
1GSA Data Repository item 2004164, Table
DR1, framboid size distribution, and Table DR2, S
isotope compositions of pyrites, is available online
at www.geosociety.org/pubs/ft2004.htm, or on request from [email protected] or Documents
Secretary, GSA, P.O. Box 9140, Boulder, CO
80301-9140, USA.
GEOLOGY, December 2004
Figure 3. Relationship
between mean size and
standard deviation of
framboidal pyrites from
laminated and bioturbated sedimentary rocks of
Ravnefjeld Formation.
(Fig. 2). Likewise, they show sharp increases
in size and MFD from L1 to B2 and from L2
to B3 (Fig. 2). The relationship between mean
size and standard deviation shows more clearly
the difference of pyrite framboid distribution
between laminated and bioturbated sedimentary rocks of the Upper Permian Ravnefjeld Formation (Fig. 3). It is clear that the framboidal
pyrites from the laminated shales plot distinctly
from the bioturbated sediments, which show
much larger mean size (Fig. 3).
Collectively, our data show a remarkable
difference of pyrite framboid distribution between bioturbated and laminated units of the
Ravnefjeld Formation (Figs. 2 and 3). Consistent with sedimentary structures and bioturbation, the distribution of framboidal pyrites
in B1, B2, and B3 shows the oxic bottomwater conditions when these sediments were
deposited. By contrast, the smaller size and
less variable range of framboidal pyrites as
well as the smaller MFD observed in laminated shales provide independent evidence for
the development of sulfidic deep water during
the Late Permian in the East Greenland Basin.
Stable Sulfur Isotopes
Sulfate reduction is a metabolic process
during which sulfate is reduced to sulfide and
the sulfide is isotopically enriched in 32S owing to large kinetic isotopic effects. It has been
demonstrated that S isotope fractionation during sulfate reduction by either pure cultures or
natural populations is 4‰–46‰ (e.g., Habicht
and Canfield, 1997; Detmers et al., 2001).
Therefore, the S isotope compositions of H2S
produced and ultimately fixed in metal sulfides (mostly pyrite) in nature (Berner, 1984)
provide indications of biological processes as
well as the environment of sulfide formation
(e.g., Canfield and Thamdrup, 1994; Passier et
al., 1997; Shen et al., 2003).
The S isotope compositions of the pyrites
from both laminated and bioturbated units are
independent of sedimentary facies and are sig-
GEOLOGY, December 2004
nificantly enriched in 32S: the d34S values
range from 241.2‰ to 228.2‰ (Fig. 4; Table DR2 [see footnote 1]). Our isotopic analysis of the laminated evaporites from the underlying Karstryggen Formation yielded a
d34S value of 111.5‰ (sample GGU
446338), consistent with the isotopic composition of coeval seawater sulfate worldwide
(Strauss, 1999). Therefore, the fractionations
between sulfates and pyrites are 39.7‰–
51.7‰ for laminated shales and 44.5‰–
52.7‰ for bioturbated sediments of the Ravnefjeld Formation (Fig. 4), and they may
record different metabolic pathways of sulfur
cycle in natural environment.
Under the sulfidic bottom-water conditions
at the time that laminated sedimentary units
L1 and L2 were deposited, sulfides produced
by sulfate reduction in anoxic water columns
became depleted in 34S, as shown by their
strongly negative d34S values (from 240.2‰
to 228.2‰) that resulted from bacterial sulfate reduction (Fig. 4; Table DR2, see footnote
1). However, relative to seawater sulfate, the
fractionations recorded in biogenic pyrites in
laminated shales are greater than the maximum fractionation of 46‰ demonstrated for
sulfate reducers. These differences may be attributable to the isotopic modification during
the oxidative part of the sulfur cycle. It has
been shown that sulfides produced by sulfate
reduction can escape into the redox transition
zone and be oxidized into intermediate sulfur
compounds (Jørgensen, 1990). The disproportionation of the intermediate sulfur compounds such as thiosulfate (S2O232 ), sulfite
(SO232 ), or elemental sulfur (S0) to sulfate and
hydrogen sulfide can generate products that are
more depleted in 34S than the original sulfide
from which the intermediate compounds were
formed (Jørgensen, 1990; Canfield and Thamdrup, 1994; Cypionka et al., 1998). Through a
repeated oxidation-disproportionation cycle,
the fractionation between seawater sulfate and
sulfide can be significantly augmented. There-
Figure 4. S isotope compositions of sedimentary pyrites from Upper Permian Ravnefjeld Formation.
fore, the overall isotopic effects during sulfate
reduction and the attendant oxidative part of
the sulfur cycle such as reoxidation and disproportionation adequately explain the variations
in d34S values of pyrites from the laminated
shales of the Ravnefjeld Formation (Fig. 4).
Likewise, the mixture of reduced-sulfur
species produced by different pathways of sulfur metabolism within the sediments can account for the similar isotopic records in the
bioturbated units (B1, B2, and B3) (Fig. 4).
Under oxic bottom-water conditions, bioturbation may have facilitated isotopic modification during the oxidative part of the sulfur
cycle. Through irrigation of the buried sediments, bioturbation may promote the delivery
of oxidants such as iron oxides or manganese
oxides required by continuous reoxidation of
H2S. Subsequent disproportionation of sulfur
intermediate compounds can add a component
extremely depleted in 34S to the sulfur reservoir and result in unusually low d34S values
for pyrites (Fig. 4).
Similar extreme 34S depletions between pyrites in oxic and sulfidic sediments have been
observed, for example, in the modern Black
Sea. There, pyrites from estuarine, oxic shelf,
and sulfidic deep-sea sediments show comparable maximum fractionations relative to
seawater sulfate (Wijsman et al., 2001). However, unlike the sulfidic Black Sea sediments,
the laminated shales of L1 and L2 show relatively widespread S isotope compositions
from 240.2‰ to 228.2‰ (Fig. 4). In the sulfidic Black Sea sediments, notably Unit I, the
strongly negative and uniform d34S values
(237‰ to 239‰) indicate that pyrite formed
dominantly near the redox boundary and that
the isotopic composition of the water-column
sulfide was consistent over space and time
(Lyons, 1997; Wilkin and Arthur, 2001). In
contrast, in the laminated shales of L1 and L2,
some of the relatively 34S-enriched pyrites
1039
could reflect the addition of diagenetic pyrites
formed within the sediment, which usually
characterize a greater range of S isotope values (Wilkin and Arthur, 2001; Wijsman et al.,
2001). Therefore, the remarkable similarities
between the Upper Permian sedimentary rocks
and the sulfidic Black Sea sediments in terms
of both framboid size distribution and S isotope composition of pyrites provide compelling evidence for sulfidic deep waters in the
East Greenland Basin.
IMPLICATIONS
The documentation of sulfidic conditions
in the East Greenland Basin may pave the
way to improve our understanding of the
end-Permian mass extinction and environment. The pulsed Late Permian mass extinction began near the end of the Capitanian or
in the earliest Wujiapingian and culminated
at the end of the Changxingian (e.g., Knoll
et al., 1996; Erwin et al., 2002). In good accord with the timing of the biotic crisis, sulfidic deep-water conditions were established
in the high-paleolatitude Boreal Ocean in the
late Wujiapingian, as evidenced by our geochemical data from the Upper Permian Ravnefjeld Formation. Elsewhere, the pelagic
chert sequences in Japan record superanoxic
and sulfidic conditions of the Panthalassa
Ocean from the early Wujiapingian to the
Middle Triassic (Isozaki, 1994, 1997; Kato et
al., 2002). Moreover, evidence for anoxia
during the Late Permian–Early Triassic
comes from both high-paleolatitude and lowpaleolatitude sections (e.g., Wignall and
Twitchett, 1996; Wignall et al., 1998; Woods
et al., 1999). Therefore, it is likely that marine anoxia may have been extremely extensive and intense in the Late Permian. Consequently, environmental stress such as
sulfidic deep water permitted few habitable
areas in the marine realm and could have
caused the end-Permian mass extinction.
ACKNOWLEDGMENTS
The research was supported by the Faculty of Science, University of Copenhagen, and Statoil North Norway (Nielsen), the Danmarks Grundforskningsfond, and
a National Research Council Fellowship (to Shen). We
thank three reviewers for constructive comments. This
paper forms a contribution to the TUPOLAR project on
the ‘‘Resources of the sedimentary basins of North and
East Greenland’’ funded by the Danish Natural Science
Research Council.
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Manuscript received 22 July 2004
Revised manuscript received 6 August 2004
Manuscript accepted 9 August 2004
Printed in USA
GEOLOGY, December 2004