Earth and Planetary Science Letters 389 (2014) 209–220
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
Trace element content of sedimentary pyrite as a new proxy for
deep-time ocean–atmosphere evolution
Ross R. Large a,∗ , Jacqueline A. Halpin a , Leonid V. Danyushevsky a ,
Valeriy V. Maslennikov b , Stuart W. Bull a , John A. Long c,d , Daniel D. Gregory a ,
Elena Lounejeva a , Timothy W. Lyons e , Patrick J. Sack f , Peter J. McGoldrick a ,
Clive R. Calver g
a
Centre for Ore Deposit and Exploration Science (CODES), University of Tasmania, Private Bag 126, Hobart, Tasmania 7001, Australia
Institute of Mineralogy, Russian Academy of Science, Urals Branch, Miass, Russia
School of Biological Sciences, Flinders University, PO Box 2100, Adelaide, South Australia 5001, Australia
d
Museum Victoria, PO Box 666, Melbourne, Victoria 3001, Australia
e
Department of Earth Sciences, University of California, Riverside, CA 92521, USA
f
Yukon Geological Survey, PO Box 2703 (K-102), Whitehorse, Yukon Y1A 2C6, Canada
g
Mineral Resources Tasmania, PO Box 56, Rosny Park, Tasmania 7018, Australia
b
c
a r t i c l e
i n f o
Article history:
Received 29 July 2013
Received in revised form 8 December 2013
Accepted 15 December 2013
Available online xxxx
Editor: G. Henderson
Keywords:
palaeo-oceanography
sedimentary pyrite
trace elements
ocean chemistry
oxygenation proxy
selenium
a b s t r a c t
Sedimentary pyrite formed in the water column, or during diagenesis in organic muds, provides an
accessible proxy for seawater chemistry in the marine rock record. Except for Mo, U, Ni and Cr,
surprisingly little is known about trace element trends in the deep time oceans, even though they
are critical to developing better models for the evolution of the Earth’s atmosphere and evolutionary
pathways of life. Here we introduce a novel approach to simultaneously quantify a suite of trace elements
in sedimentary pyrite from marine black shales. These trace element concentrations, at least in a firstorder sense, track the primary elemental abundances in coeval seawater. In general, the trace element
patterns show significant variation of several orders of magnitude in the Archaean and Phanerozoic,
but less variation on longer wavelengths in the Proterozoic. Certain trace elements (e.g., Ni, Co, As,
Cr) have generally decreased in the oceans through the Precambrian, other elements (e.g., Mo, Zn, Mn)
have generally increased, and a further group initially increased and then decreased (e.g., Se and U).
These changes appear to be controlled by many factors, in particular: 1) oxygenation cycles of the Earth’s
ocean–atmosphere system, 2) the composition of exposed crustal rocks, 3) long term rates of continental
erosion, and 4) cycles of ocean anoxia. We show that Ni and Co content of seawater is affected by global
Large Igneous Province events, whereas redox sensitive trace elements such as Se and Mo are affected
by atmosphere oxygenation. Positive jumps in Mo and Se concentrations prior to the Great Oxidation
Event (GOE1, c. 2500 Ma) suggest pulses of oxygenation may have occurred as early as 2950 Ma. A flat to
declining pattern of many biologically important nutrient elements through the mid to late Proterozoic
may relate to declining atmosphere O2 , and supports previous models of nutrient deficiency inhibiting
marine evolution during this period. These trace elements (Mo, Se, U, Cu and Ni) reach a minimum in
the mid Cryogenian and rise abruptly toward the end of the Cryogenian marking the position of a second
Great Oxidation Event (GOE2).
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Whole rock variation of trace elements (TE) — in particular Mo,
U, V, Cr and Ni — in black shales deposited in marine environments
have been successfully used to interpret palaeo-redox conditions
on the ocean floor (Lyons et al., 2003; Tribovillard et al., 2006;
Reinhard et al., 2013), including temporal changes in oxygenation
*
Corresponding author.
E-mail address: [email protected] (R.R. Large).
0012-821X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.epsl.2013.12.020
of the global ocean (Holland, 1984; Anbar et al., 2007; Scott et
al., 2008; Partin et al., 2013). Most of the redox sensitive TE, as
well as multitude of other TE in black shales, are concentrated in
early-formed pyrite (Huerta-Diaz and Morse, 1992; Large et al.,
2007, 2009; Gregory et al., in press). Laser Ablation-Inductively
Coupled Plasma Mass Spectrometer (LA-ICPMS) techniques developed to study zonation in the composition of pyrite (FeS2 ) in various ore systems (Danyushevsky et al., 2011) have demonstrated
how this information can be used to study potential sources of
metals (Meffre et al., 2008), track changes in the chemistry of hydrothermal fluids (Large et al., 2009), determine the evolution of
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R.R. Large et al. / Earth and Planetary Science Letters 389 (2014) 209–220
ore deposits (Thomas et al., 2011) and establish the effectiveness
of pyrite as a trap for TE in contaminated sediments (Gregory
et al., 2013, in press). We suggest here, that in the same way that
hydrothermal pyrite tracks changes in chemistry of ore fluids, sedimentary pyrite, which is a common trace mineral in carbonaceous
marine black shales (Fig. 1a–d), can be used to track first-order
changes in the chemistry of seawater and ultimately oxygenation
of the ocean–atmosphere system. Whereas bulk shale studies require samples of particular palaeo-redox facies (Lyons et al., 2003;
Scott et al., 2008; Algeo and Rowe, 2012), the focus on earlyformed pyrites developed here allows sampling of a wider range
of redox conditions, enabling a more complete record of seawater
variation. For this study we have analysed over 1880 sedimentary
pyrites from 131 black shale samples covering 77 time intervals
from 3515 Ma to present day (Supplementary Tables 1–4).
2. TE analysis of sedimentary pyrite as a proxy for seawater
chemistry
2.1. Incorporation of TE in sedimentary pyrite
Sedimentary pyrite forms in seafloor muds when H2 S, produced by the microbial reduction of marine sulfate, reacts with
iron to form pyrite (FeS2 ), arguably by way of an iron monosulfide precursor, such as mackinawite or greigite (Rickard and
Luther, 1997; Rickard, 2012). Experimental studies (Huerta-Diaz
and Morse, 1992; Morse and Arakaki, 1993) have shown that
many TE are incorporated into the precursor iron monosulfide
at an early stage, including (in approximate order of incorporation) As, Hg, Mo, Co, Cu, Mn, Ni, Cr, Pb, Zn and Cd, which are
absorbed from seawater and local pore waters in seafloor muds
(Larget et al., 2007, 2009; Gregory et al., in press). This TE enrichment process is controlled by the amount of pyrite produced
and the amount of trace metals available from seawater and pore
waters (Huerta-Diaz and Morse, 1992). However it is recognised
that microbially controlled reduction and sequestration of TE and
sulfate on the seafloor are complex biochemical processes. Variables like Eh/pH conditions in the sediment/water column, degree of bio-productivity and availability of preferred electron acceptors will influence which TE the microbes reduce and coprecipitate with pyrite at any given time (Kulp and Pratt, 2004;
Mitchell et al., 2012).
Various textural forms of pyrite are typical of syngenetic/diagenetic sedimentary origins, including framboids, microcrystalline
clusters, small nodules and layers of fine crystals (Fig. 1a–d).
Late diagenetic pyrites typically overgrow earlier pyrite generations (Thomas et al., 2011; Rickard, 2012) and commonly have
a rounded anhedral surface which may be expressed as elongate nodules or various spheroidal and nodular forms (Large et
al. 2007, 2009). Recent studies have shown that many of the
TE originally thought to be present in the structure of pyrite,
are actually present as micro-inclusions (Ciobanu et al., 2009;
Large et al., 2009; Deditius et al., 2011) abundant in early forms
of pyrite (Fig. 1a–c). Whereas Ni, Co, and Se are commonly present
in the pyrite structure, As, Cu, Zn, Pb, Bi, Sb, Tl, Mo, Ag, Cd, Mn,
Hg, and Te may be either in the structure or as sulfide microinclusions, and Ti, V, U, Ba, Sn, W, and Cr, are invariably present
within micro-inclusions of matrix material. This earliest formed
inclusion-bearing sedimentary pyrite is the most TE rich, and subsequent, more crystalline generations with fewer inclusions (late
diagenetic to early metamorphic) may contain lower TE concentrations and trend toward a pure FeS2 composition (Huerta-Diaz and
Morse, 1992; Large et al., 2007; Large et al., 2009; Rickard, 2012).
Trace elements that are loosely held in the pyrite structure or occur as nano-inclusions (e.g., Zn, Cu, Pb, Ag, Mo) are readily expelled
from the pyrite during metamorphic recrystallisation, whereas TE
Fig. 1. a–d, Various textures of sedimentary pyrite; a, cluster of framboids, b, cloud
of fine framboids and microcrystals, c, nodule, d, fine microcrystals concentrated
in a sedimentary bed. e–j, LA-ICPMS images of TE distribution in pyrite nodule
(c above) and surrounding matrix. Note the high concentration of Se in pyrite relative to matrix in (e), compared with low concentration of V in the pyrite relative to
matrix in j. k, time resolved LA-ICPMS spectra of selected TE in a sedimentary pyrite
analysis. The laser was switched on at 31 s after the start of the analysis. Ablation
continued for 60 s. The intensity of the signal for each isotope is taken as the difference between the mean signal with laser off (background, the first 30 s of each
acquisition) and the mean signal with the laser on. As the vertical axis is counts
recorded by the detector on ICPMS, differences in intensity of the signals from different isotopes do not directly reflect differences in concentrations. The plot shows
the differing response from TE in the pyrite structure (Pb, Co, Sb and Se) compared
to TE in micro-inclusions (Ti, Zn and Cr). Copper shows a dual response.
R.R. Large et al. / Earth and Planetary Science Letters 389 (2014) 209–220
211
Table 1
Summary of TE variations in the Archean and Proterozoic oceans based on sedimentary pyrite composition.
Eon/Era
Trace elements enriched* in ocean
Trace elements depleted* in ocean
Little change in mean trace element
concentration*
Across GOE2 (c. 660 Ma)
Bi, Ba, W, Sn
Ag, Cr, V, Tl
Neoproterozoic to 660 Ma#
Mo, Se, Zn, Cd, Ni, Co, As, Cu, Mn,
Hg, Pb, Sb, Te, U
Cu, Bi, Ba
Co, Mn, Pb, U, V, W, Sn
Mesoproterozoic
Palaeoproterozoic
As, Tl
Cu, Bi, U, Cr, V, W
Across GOE1 (c. 2500 Ma)
Mo, Se, Tl, Mn, U
Archean
Ni, As, Co, Cu, Hg, Bi, Cr, W, Sn, Sb, Te
Mo, Zn, Ni, As, Se, Cd, Tl, Hg, Cr, Ag,
Sb, Te
Mo, Co, Mn, Hg, U, Cr, Ba, V, W, Te
Mo, Zn, As, Cd, Co, Tl, Hg, Ba, Ag, Sb,
Te
Zn, Cd, Ni, Co, As, Cu, Ag, Sb, Hg, Te,
Pb, Bi, Cr, Ba, Sn
Mo, Zn, Se, Cd, Mn, U, Ba
*
#
Zn, Ni, Se, Cd, Cu, Pb, Bi, Sn, Ag, Sb
Ni, Se, Mn, Pb, Sn
V, W
Tl, Pb, V, Ag
Compared with the range of average values through time (0 to 3.5 Ga).
Neoproterozoic prior to GOE2 (Tonian-Cryogenian).
that are stoichiometric in the pyrite structure (e.g., Co, Ni, As, Se)
tend to remain and even become enriched during metamorphism
(Large et al., 2007, 2011b).
2.2. Sample selection, rationale and analysis
In this study, samples of sedimentary pyrite were selected on
the basis of: (1) metamorphic grade of the host shale, (2) texture
of pyrite and (3) Co/Ni ratio of pyrite, as outlined below.
1. Black shales of mid greenschist facies or below were chosen.
Post lithification metamorphism and/or hydrothermal activity
leads to recrystallisation of earlier pyrite forms and commonly
results in pyrite overgrowths with euhedral outer surfaces and
well developed crystal growth zones with alternation of TE
chemistry (Large et al., 2009, 2013; Thomas et al., 2011). These
latter euhedral and zoned pyrite types have been studiously
avoided in this current study, as their chemistry reflects the
composition and conditions of the metamorphic or hydrothermal fluids rather than the precursor seawater chemistry during
sedimentation. Samples with pyrrhotite were also rejected, as
previous studies have shown a marked loss of TE during conversion of pyrite to pyrrhotite (Thomas et al., 2011).
2. Pyrite was selected that retained textures indicative of syngenesis or formation during early diagenesis. Textures indicative of diagenetic pyrite include; framboids, microcrystalline
pyrite in clouds in the shale, microcrystalline pyrite aligned
parallel to bedding, small cubic euhedral and bladed crystals,
small porous patches and rounded or elongate zoned nodules (Fig. 1a–d) (Huerta-Diaz and Morse, 1992; Large et al.,
2007, 2009; Guy et al., 2010). Pyrite textures indicative of
metamorphic and/or hydrothermal growth, including medium
and large sized euhedral crystals (>1 mm), clear crystals without inclusions, and pyrite which cuts across the bedding as
blebs or veins, were not selected for analysis. Textural discrimination between diagenetic and metamorphic pyrite textures was assisted by etching pyrite with nitric acid (Large
et al., 2007), coupled with the LA-ICPMS mapping technique
when required (Fig. 1e–j, Supplementary Fig. 7) (Large et al.,
2009, 2013; Thomas et al., 2011).
3. Chemical screening of the pyrite was undertaken following LAICPMS analysis. On a Co versus Ni plot, samples which showed
a linear array parallel to Co/Ni = 1, and with Co/Ni < 2 were
selected as suitable. This is based on previous work (Bajwah
et al., 1987; Large et al., 2009) that have shown diagenetic
pyrite commonly has Co/Ni 2, whereas hydrothermal pyrite
has higher Co values that trend away from the Co/Ni = 1 line.
More than 80% of our data was deemed acceptable on this basis.
All LA-ICPMS analyses of pyrite were performed at the Centre for Ore Deposit and Exploration Science (CODES), University of
Tasmania (see Supplementary Information for LA-ICPMS methodology). LA-ICPMS element maps clearly show those TE that are
evenly distributed through the pyrite (e.g., Se in Fig. 1e), compared with those that are zoned (Cu, Fig. 1h), those with a
patchy distribution (As, Fig. 1i), and those dominated by inclusions
(Mo and Zn, Fig. 1f–g). The time-resolved signal of an individual LA-ICPMS analysis (Fig. 1k) provides information on the likely
form of elements incorporation in the pyrite (Large et al., 2009;
Thomas et al., 2011). Removal of the effects of matrix microinclusions leads to the pyrite compositions measured on several
pyrite grains from the same black shale sample commonly exhibiting a variation of less than one order of magnitude (ppm scale),
compared with the overall dataset variation of up to four orders of
magnitude for any given TE (see example in Supplementary Fig. 1).
2.3. Proof of concept
Five separate lines of evidence support our contention that variations in the TE composition of early-formed sedimentary pyrite in
black shales are a good first order proxy for TE variations in the
ocean through time (Table 1). The physical/chemical connection
between early-formed sedimentary pyrite and seawater underpins
our first line of evidence. Syngenetic pyrite which forms in euxinic water columns adsorbs TE from the seawater (Huerta-Diaz
and Morse, 1992) is an obvious proxy for seawater TE chemistry (Berner et al., 2013). However, diagenetic pyrite in anoxic to
dysoxic bottom water conditions forms by biogenic reduction of
sulfate in the pore waters of seafloor muds within the first few
centimetres of the seafloor (Wilkin et al., 1996). In this environment the muds contain from 60 to 85% seawater (Baldwin and Butler, 1985; Harrold et al., 1999) and thus ensure a strong connection
between pyrite chemistry and seawater chemistry, even though
sedimentary clays and organic matter are likely to play a role in
partitioning TE into pyrite. The seawater content of seafloor muds
remains over 40% in the top 100 to 200 m (Baldwin and Butler,
1985) and thus diagenetic pyrites forming at considerable depth
will maintain a seawater connection influenced by matrix interactions. The significance of matrix/pore water interaction in effecting
a change in sedimentary pyrite chemistry can be evaluated from
data on pyrite TE depth profiles in recent sedimentary environments from Huerta-Diaz and Morse (1990, 1992). Their data, collected from a range of anoxic to euxinic sedimentary environments
in the Gulf of Mexico, shows that most TE in sedimentary pyrite
increase with depth from 0 up to 100 cm depth. However, the increase is variable, depending on sedimentary environment and the
TE measured, and is commonly in the range of 1.4 to 6 times (less
than one order of magnitude), compared to the 2 to 4 orders of
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R.R. Large et al. / Earth and Planetary Science Letters 389 (2014) 209–220
Fig. 2. a, Changes in mean LA-ICPMS analyses of Mo, As, Ni, Co and Se in the paragenetic stages of sedimentary pyrite from the Neoproterozoic Khomolkho Formation, Siberia
from Large et al. (2007). Py1 = syngenetic to early diagenetic pyrite; py2 = early diagenetic pyrite; py3 = late diagenetic pyrite; py4 = metamorphic pyrite; py5 = late
metamorphic pyrite; pyrrhotite = late metamorphic pyrrhotite replacing pyrite. b, Range in TE content (Se, As, Ni and Mo) plotted against pyrite framboid size for a Late
Permian black shale from the Perth Basin, Western Australia. The range in individual TE for each framboid size is similar.
magnitude variation exhibited by sedimentary pyrite in our samples from different epochs and eras through time.
The second line of evidence comes from a study of the LAICPMS composition of a range of different textural and paragenetic
pyrite types, from syngenetic, through diagenetic to metamorphic,
in the Neoproterozoic Khomolkho Formation in the Lena Goldfield, Siberia (Large et al., 2007). This study demonstrated a general
trend of steady to decreasing mean TE content in pyrite with time,
from syngenetic to diagenetic to metamorphic through the paragenesis (Fig. 2a). The changes during diagenesis for all five TE are
substantially less than one order of magnitude. This suggests that
the first order (2 to 4 orders of magnitude) temporal changes are
unlikely to relate to diagenesis in the seafloor muds, but more
likely reflect the changes in TE content of seawater within and
overlying the muds.
The third line of evidence, based on our framboid data from
the Perth Basin in Late Permian marine black shales (Thomas
et al., 2004) shows there is no substantial difference in the TE
(range ppm) of small (5 μm) syngenetic framboids, compared
with larger (6 to 35 μm) pyrite framboids (Fig. 2b). All sizes of
framboids in this Late Permian black shale sample show similar TE
contents (40 to 800 ppm Ni and As; 3 to 80 Mo and Se; Fig. 2b)
suggesting that early-formed sedimentary pyrite is a first order
recorder of TE variations irrespective of its framboid size and mode
of origin (syngenetic or diagenetic).
The fourth line of evidence is exhibited in Fig. 3, which demonstrates a first order positive relationship between the LA-ICPMS TE
composition of sedimentary pyrite from an actively forming sedimentary basin the Cariaco Basin on the Venezuela shelf (Lyons
et al., 2003; Piper and Perkins, 2004), and the TE composition of
R.R. Large et al. / Earth and Planetary Science Letters 389 (2014) 209–220
Fig. 3. Positive correlation (large dashed field) between mean TE concentrations in
current ocean and mean TE concentrations in modern sedimentary pyrite from Cariaco Basin. Vanadium, U and Ba do not fall in the same field as the other traces, as
they are concentrated in non-sulfide inclusions within the pyrite.
213
The final proof of concept for the approach proposed here involves comparing our data on the TE composition of pyrite with
the TE content of whole rock black shale samples, particularly
for Mo, which is commonly used to interpret palaeo-ocean conditions and changes in oxygenation of the atmosphere/ocean system through time (Lyons et al., 2003; Tribovillard et al., 2006;
Scott et al., 2008). A small group of black shale samples from the
Devonian Popovich Formation, Nevada, reveal similar down-hole
trends (Fig. 4); those TE concentrated in pyrite such as Mo and
Se are commonly between 10 to 100 times the whole rock value,
depending on the percentage of sedimentary pyrite and organic
matter in the sample.
In order to evaluate the deep time variability of TE in sedimentary pyrite in black shales, we compare Mo data on whole rock
euxinic and/or organic-rich black shales from Scott et al. (2008)
with our data on the Mo content of sedimentary pyrite in black
shales over the same time interval (Fig. 5a and b). The two data
patterns are remarkably similar, with several times more Mo in
pyrite compared with the whole rock values, even though Mo is
present both in the sedimentary matrix and in the sedimentary
pyrite (Fig. 1) (Chappaz et al., 2014). A second comparison is made
with the Ni/Fe ratio measured on whole rocks and minerals in marine Banded Iron Formations (BIFs) (Konhauser et al., 2009) (Fig. 5c
and d). Again the two datasets show similar patterns, although the
Konhauser et al. (2009) plot contains no Phanerozoic data due to
the lack of BIFs in this era. In summary, our pyrite TE plots have
reproduced the temporal trends in the previous whole rock data
and confirm the value of sedimentary pyrite as a proxy for deep
time variations of TE in the oceans.
3. Temporal trends in TE from sedimentary pyrite
Individual TE time curves derived from LA-ICPMS analysis of
1885 pyrite spots on 131 black shale samples are given in the
Supplementary Information (Supplementary Tables 2 and 3; Supplementary Figs. 3–5). We have selected a subset of the TE to
focus our discussion; Mo, Ni, Co, As and Se measured in sedimentary pyrite, and U and Cr measured in the sedimentary matrix
(Figs. 6–8). In the summary figures to follow, we use a rolling TE
mean to highlight the long-term TE trends (Fig. 6b–f, see also Supplementary Fig. 2) at the expense of the variation in individual
sample sets (Fig. 6a). In general terms (see Mo, Ni, Co, As and Se
in Fig. 6b–f), most TE exhibit significant cyclic variation of several orders of magnitude in the Archaean and Phanerozoic, but less
variation on longer wavelengths in the Proterozoic. In the Precambrian, there are four general TE trends:
Fig. 4. Down hole comparison between LA-ICPMS Mo and Se analyses from whole
rock and sedimentary pyrite in black shales from drill hole CD 12c, Carlin District
Nevada (Large et al., 2011a).
modern ocean water (http://www.mbari.org/chemsensor/summary.
html). Relative to seawater, TE in Cariaco Basin pyrite are enriched
between 5 and 8 orders of magnitude (Supplementary Table 1).
Trace elements Mo, As, Ni, Zn, Cu and Se which are enriched in
the current ocean (>100 ng/kg) are also enriched in the currently
forming sedimentary pyrite (>100 ppm), whereas Co, Pb, Sb, Tl,
Cd and Ag which are in lower abundance in the ocean (>1 and
<100 ng/kg) are also in lower abundance in sedimentary pyrite
(>10 and <100 ppm). The least abundant TE in the ocean (Te,
Bi, Au) are also the least abundant in Cariaco Basin sedimentary
pyrite. Barium, V and U, which are present within silicate inclusions in the sedimentary pyrite, plot outside the pyrite correlation
field.
Trend 1: a flat-to-broadly increasing pattern through the Archaean
and Proterozoic: only Mo, Zn, Mn, and Cd show this pattern.
Although this pattern is not unidirectional, it suggests that
supply of these TE to the ocean has exceeded drawdown into
seafloor muds through most of the Precambrian.
Trend 2: A variable, but generally decreasing pattern through the
Archaean and Proterozoic: Ni, Co, As, Te, Tl, Hg, Sb, Pb, Bi, Cr,
Sn, and W. This pattern suggests drawdown has exceeded supply for the Precambrian.
Trend 3: a general increase through the Mesoarchaean to early
Palaeoproterozoic followed by a decrease through the Mesoand Neoproterozoic: Se, U, and V. In the first half of the period supply exceeded drawdown, with reversal in the second
half.
Trend 4: a relatively flat pattern with no obvious trend through
the Archaean and Proterozoic: Cu, Ag, and Ba. In this case TE
supply and drawdown were roughly matched through the period but with perturbations.
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R.R. Large et al. / Earth and Planetary Science Letters 389 (2014) 209–220
Fig. 5. Comparison of published data on temporal variation in whole rock Mo and Ni analyses on black shales and BIFs with LA-ICPMS analyses of sedimentary pyrite in black
shales. a, Mo whole rock black shale data from Scott et al. (2008) and b, Mo in sedimentary pyrite derived from this study. c, Ni/Fe whole rock and mineral data from BIFs
(Konhauser et al., 2009) and d, Ni in sedimentary pyrite derived from this study.
All TE show high frequency and high amplitude variations through
the Phanerozoic suggesting geologically rapid changes in ocean
chemistry over this latter period of Earth history. It should be emphasised that the four trends discussed above are a first attempt to
group the data, but in fact each TE shows its own individual pattern (Supplementary Figs. 3 and 5), that is probably controlled by
a number of factors, which are beyond the scope of analysis here.
A brief discussion of the temporal trends of the five TE shown in
Fig. 6, and comparison between these pyrite TE trends and those
previously published for whole rock datasets follows below.
Molybdenum (Trend 1) is a conservative TE in the current
ocean, present as the soluble MoO4 = species (Helz et al., 1996).
The general rise in the Mo time curve (Fig. 6b) is attributed to
oxygenation of the atmosphere, particularly in the Neoarchean and
Phanerozoic, leading to increased continental oxidative weathering
such that supply of Mo to the oceans exceeded the drawdown
into seafloor sediments (Anbar et al., 2007; Scott et al., 2008;
Dahl et al., 2011). Our time curve shows that Mo in pyrite commenced at a relatively low 0.5 to 1 ppm at 3500 Ma and increased
through a series of steps to reach 10 to 70 ppm at 1000 Ma. The
step up at around 2500 Ma likely corresponds to the Great Oxidation Event (GOE1) (Farquhar et al., 2001; Holland, 2006) when
appreciable O2 first accumulated in the atmosphere. In the Neoproterozoic there is a trough in the Mo time curve (referred to as
the Cryogenian TE trough hereafter) that concludes with the Sturtian glaciation and proposed Snowball Earth (Hoffman et al., 1998).
A rapid rise corresponding to a second Great Oxidation Event (referred to here as GOE2) occurred during the Neoproterozoic when
oxygen rose to significant levels that ultimately supported the rise
of animal diversity. Our TE data suggests a threshold was reached
by 660 Ma, earlier than recent estimates that place this event either shortly after the Marinoan Glaciation (630 Ma) (Sahoo et al.,
2012) or prior to the late Ediacaran (580–550 Ma) (Fike et al.,
2006; Canfield et al., 2007; Scott et al., 2008).
Nickel (Trend 2) is a nutrient TE present as Ni2+ in the modern ocean. The Ni time curve (Fig. 6c) shows an overall decrease
through the Precambrian to 660 Ma, but most of the drop occurs in the Archaean, decreasing from around 2500–10,000 ppm to
just under 1000 ppm through the Palaeoproterozoic. This pattern,
also seen in Ni/Fe ratio in BIFs (Fig. 5c), is attributed to cooling
upper mantle and decreased eruption of Ni-rich komatiite ultramafic rocks from about 2700 Ma (Konhauser et al., 2009). Cobalt
(Trend 2) shows a similar pattern to Ni, but with more variation (Fig. 6d). Most of the segments on the time curve when Ni
and Co in pyrite exceeds 1000 ppm and 500 ppm respectively,
overlap with episodes of Large Igneous Province (LIP) eruptions of
mafic lavas and associated intrusives, both subareal and submarine
(Prokoph et al., 2004) (Fig. 7). The komatiitic LIPs of the Archaean
are particularly enriched in Ni and Co, and thus contributed to the
sustained high levels of Ni and Co to the global oceans and sedimentary pyrite (Fig. 7). The highest Phanerozoic Ni and Co were
measured in sedimentary pyrite of 250 Ma from the Perth Basin
Western Australia; values up to 48,000 ppm Ni and 16,000 Co
were recorded, indicating the likelihood of widespread enrichment
of the global ocean in Ni and Co due to the Siberian Trap eruptions. It is noteworthy that these Siberian eruptions are associated
with the largest nickel ore bodies on Earth, Norilsk and Talnack.
Arsenic (Trend 2) (Fig. 6e) also exhibits a declining trend
through the Precambrian. However, the As pattern has two segments; 1) a continuous decrease from a mean of 5000 to 300 ppm
in pyrite over the period 3000 to 1800 Ma; 2) followed by a rise to
10,000 ppm at 1400 Ma and fall to the Cryogenian TE trough, rising again at GOE2 around 660 Ma. The cause for the sharp decline
in As through Late Archean and Early Proterozoic is not sourcerock controlled, as unlike Ni and Co, As is not enriched in ultramafic or mafic rocks (Reimann and Caritat, 1998), and alternative
As-bearing Archean source rocks are not known. A possible explanation for the As trend relates to the fact that As forms a soluble
arsenate complex (HAsO24− ) in oxic to anoxic ocean conditions and
soluble polysulfide complex (H2 As3 S−
6 ) in euxinic ocean conditions
(see Supplementary Table 5). Thus higher levels of volatile elements, S and As, associated with a degassing Earth in the Archean,
may have contributed to stable arsenic polysulfide complexes in
the mainly H2 S-bearing Archean ocean, which declined in concentration through the Early Proterozoic as H2 S levels in the ocean
dropped and As was drawn-down into seafloor pyrite. An increase
in As through the Mesoproterozoic (Fig. 6e) supports a switch to
anoxic conditions as the SO24− /H2 S ratio of the ocean continued to
increase, followed by an As decline as euxinic conditions took hold
again in the Neoproterozoic (Poulton and Canfield, 2011).
Selenium (Trend 3) (Fig. 6f), like Mo, is sourced from the oxidative weathering of continental pyrite, with release of selenite
and selenate species (SeO23− and SeO24− ; see Supplementary Fig. 6),
which accumulate in the ocean. Selenium is a nutrient TE, with
R.R. Large et al. / Earth and Planetary Science Letters 389 (2014) 209–220
215
Fig. 6. a–f, Temporal trends of Mo, Ni, Co, As and Se measured by LA-ICPMS analysis of sedimentary pyrite in black shales. Each symbol represents a single pyrite analysis in
(a). The curves presented in (b–f) are derived from a 19 point rolling mean (or moving average).
significant biological uptake and eventual accumulation on the
seafloor in both organic matter and by substitution for S in sedimentary pyrite (Diener and Neumann, 2011; Mitchell et al., 2012).
Se shows strong narrow peaks around 3000 and 2500 Ma, rising to
a flat trend between 2500 and 1700 Ma, with a major perturbation
from 1800 to 1300 Ma, then declines to a trough at 900 to 660
Ma, before rising again at GOE2. The stoichiometric substitution
of Se for S in pyrite, its homogeneous distribution in sedimentary
pyrite (Fig. 1e, Supplementary Fig. 7), high enrichment in pyrite
versus the black shale matrix (Supplementary Table 4), low variability in pyrite from the same sample (Supplementary Fig. 1), and
tendency to remain in pyrite even after significant recrystallisation
(Large et al., 2007; Thomas et al., 2011), indicate the Se content
of sedimentary pyrite may be a superior oxygenation proxy compared to other commonly used proxies, as further discussed below.
3.1. Selenium as a proxy for deep time atmosphere–ocean oxygenation
In Fig. 8 we compare the temporal trend of Se in pyrite with
Mo, U and Cr, three of the commonly employed whole rock
proxies for atmosphere–ocean oxygenation (Anbar et al., 2007;
Scott et al., 2008; Partin et al., 2013; Reinhard et al., 2013). All
four redox-sensitive TE show related trends through the Archean
and Proterozoic.
The temporal trend of U (Fig. 8c) is broadly similar to Se
(Fig. 8b), stepping up through the late Archean and Early Proterozoic to a broad peak around 1700–1400 Ma, and then decreasing
into the Cryogenian TE trough from 1000 to 660 Ma. The broad U
peak around the mid Proterozoic may relate to the abundance of
hot U-bearing granites in the Paleo- and Mesoproterozoic (Wilson
and Akerblom, 1982; Wyborn et al., 1992) that would have presented an enriched source rock of U for the contemporary oceans.
Chromium (Fig. 8d), like Ni and Co (Fig. 7), is elevated in the
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R.R. Large et al. / Earth and Planetary Science Letters 389 (2014) 209–220
Fig. 7. Intervals of major LIPs (Prokoph et al., 2004), juxtaposed with the time curves
for Co and Ni. Most highs for Ni and Co in sedimentary pyrite coincide with the
periods of high numbers of LIPs (green bars). The extreme minimum in Ni and Co
around 400 Ma is a good match for a period of minimum LIP activity from 300 to
500 Ma, whereas the maximum at 250 Ma matches the Siberian Trap LIP eruptions.
Archean due to the abundant ultramafic/mafic crustal components
and shows a decline from 2200 to 1200 Ma before rising to a
plateau around 800 Ma. The Cr data shows a very similar trend
to that recently published for organic-rich shales using whole-rock
analytical techniques (Reinhard et al., 2013).
Through the Archean, Se (Fig. 8b) averages around 10 ppm in
pyrite but with a spike at 2970 Ma and steps upwards between
2650 and 2500 Ma. The latter steps, to over 100 ppm Se, are many
tens of millions years earlier than the accepted position of GOE1
between 2450 to 2300 Ma (Farquhar et al., 2001; Holland, 2006;
Pufahl and Hiatt, 2012) and suggest earlier oxygenation pulses
(Anbar et al., 2007; Wille et al., 2007; Scott et al., 2008). The
spike at 2970 Ma, which is from pyrite in black shales from the
Witwatersrand Basin, may define the first pulse of global oxygenation, and supports other recent evidence for an oxygenation pulse
determined by Cr isotopes and redox sensitive TE in the Pongola
Supergroup (Crowe et al., 2013). From 1700 to 700 Ma, Se shows
a gradual decline (Fig. 8b). This trend suggests the possibility of a
gradual drop in atmosphere oxygen through the Meso- and Neoproterozoic but with minor perturbations (particularly at 1700 to
1300 Ma). Our U and Cr curves measured on the black shale matrix
(Fig. 8c and d) show a similar decreasing trend during this period,
consistent with published whole-rock data (Partin et al., 2013).
Reinhard et al. (2013) have calibrated the palaeoredox proxies Cr
and Mo in the Proterozoic to delineate anoxic settings, where Cr is
reduced and buried, compared to euxinic settings, where both Cr
and Mo are reduced/sulfidised and buried. On this basis, our data
(Fig. 8a and d) may be interpreted to suggest that anoxic settings
were dominant in the Proterozoic from 2150 to 700 Ma, whereas
euxinic settings (anoxic + H2 S) were dominant from 1200 to 800
Ma. This interpretation fits with the concept of decreasing atmosphere oxygenation through the Meso- and Neoproterozoic, with
minimum O2 levels around 800 Ma, corresponding to the Cryogenian TE trough.
Fig. 8. Temporal trends of a, Mo in sedimentary pyrite, b, Se in sedimentary pyrite,
c, U in shale matrix, and d, Cr in shale matrix; measured by LA-ICPMS displayed as
a 19 point rolling mean.
3.2. TE deficiency in the Meso- to Neoproterozoic Oceans
Anbar and Knoll (2002) present the hypothesis that under the
conditions of a deep sulfidic ocean for most of the Meso- and Neo-
R.R. Large et al. / Earth and Planetary Science Letters 389 (2014) 209–220
217
Fig. 9. Schematic of the major findings of this paper related to redox sensitive TE variations and oxygenation of the ocean–atmosphere. The red line is the estimated
oxygenation curve based on the Se content of sedimentary pyrite. The baseline % PAL (Present Atmospheric Level) is from Sahoo et al. (2012).
proterozoic (Canfield, 1998) nutrient TE would be scarce, thus limiting the ecological distribution of eukaryotic algae and adversely
effecting marine evolution. Our data on the biologically important
nutrient TE (Mo, Zn, Co, Cd, Se, Ni, Ag, Mn, U, V and Cr; Supplementary Figs. 3–5) supports this hypothesis and shows that
although these TE exhibit various trends in detail, they all show
a relative depletion from 1200 to 700 Ma compared to their early
Phanerozoic values, and in most cases their Neoarchean to Palaeoproterozoic values. For the redox sensitive TE that are employed
as atmosphere–ocean oxygenation proxies, Mo, U, Cr and Se there
are two possible causes for these trends; (1) low values of these
TE represent gradual drawdown of these elements in a predominantly sulfidic deep ocean (Anbar and Knoll, 2002; Reinhard et
al., 2013), with limited renewal by continental erosion due to low
levels of tectonic activity over the period 1800 to 700 Ma. Atmospheric oxygen remains constant in this scenario (Canfield, 2005;
Holland, 2006) Alternatively, (2) the decline in these redox sensitive TE signal a gradual decrease in atmosphere oxygenation from
1800 to 800 Ma, with the lowest O2 levels forming the Cryogenian TE trough between 800 and 660 Ma, prior to the rise at
GOE2. Other workers (Shen et al., 2003; Planavsky et al., 2011;
Poulton and Canfield, 2011; Partin et al., 2013) provide evidence
for sustained low levels of O2 during the mid Proterozoic, although
none have suggested a significant O2 decline during this period.
In considering these two scenarios, there is little evidence to
support a static tectonic regime, as this period of the middle to late
Proterozoic saw the amalgamation (1300–1000 Ma) and breakup
(900–700 Ma) of the super continent Rodinia (Li et al., 2008;
Condie et al., 2011). Active erosion during these events would
involve a continuous supply of nutrient TE to the oceans. However the normalised 87 Sr/86 Sr curve of riverine runoff compared
with mantle influence (Shields, 2007), indicates a decline in erosive supply of Sr which parallels the Se and U time curves. On
the other hand, low O2 may have caused the nutrient deficiency,
which is corroborated by carbon isotope evidence for low productivity (Anbar and Knoll, 2002). Canfield (2005) suggests that
atmospheric oxygen rose to a level of 5 to 10% Present Atmospheric Level (PAL) around 1800 Ma and remained constant until
about 700 Ma. Planavsky et al. (2011) suggest a lower level of 1 to
10% PAL throughout the Meso- and Neoproterozoic. Based on our
data, and these previous estimates, we speculate that atmospheric
O2 may have decreased from a level around 10–20% PAL at 1800
Ma to the Cryogenian TE trough at 1–5% PAL between 800 and 660
Ma (Fig. 9).
4. Strengths, weaknesses and caveats of sedimentary pyrite
proxies for ocean chemistry
The TE variation in sedimentary pyrite, determined by LAICPMS, has the potential to enable the development of several ocean–atmosphere proxies through time including: ocean TE
chemistry, ocean pH, ocean sulfate content, ocean–atmosphere
oxygenation, ocean anoxic events, ocean productivity and ocean
hydrothermal activity. Although there are strengths associated with
this approach, there are also several weaknesses and caveats.
4.1. Strengths
Sedimentary pyrite concentrates many of the redox sensitive
and nutrient TE, in particular, Se, Mo, Cu, Ni, Ag, As, Cd, Co, Mn, Sb
well above their content in the sedimentary matrix of black shales
(Fig. 4, Supplementary Information). This means, that by using the
LA-ICPMS technique, the minimum (down to ppb levels) and maximum (over wt% levels) of TE in pyrite can be measured accurately,
enabling proxies for both maximum and minimum levels of TE in
the oceans to be developed. This has not been previously possible.
The LA-ICPMS technique used here enables 22 TE to be analysed
simultaneously. This enables the trends of a suite of redox sensitive elements (e.g., Mo, Se, Cu, U and Cr) to be compared on the
same sample set. Furthermore, TE in both the pyrite and sedimentary matrix are quantified, and thus TE concentrated in the matrix,
(e.g., U, V, W, Sn and Ba) can also be tracked through time.
Pyrite analyses involve only one mineral, and thus there are less
complications than whole rock analyses of black shales, where TE
are partitioned amongst a number of minerals and organic matter,
of varying proportions, plus seawater and pore fluids. Sedimentary
pyrite forms in black shales from sub-oxic to euxinic environments,
and thus sampling is not restricted to a particular redox facies in
order to apply the technique. This enables a much larger population of black shales to be included in deep time research. A detailed petrographic study of sedimentary pyrite allows the effects
218
R.R. Large et al. / Earth and Planetary Science Letters 389 (2014) 209–220
of late diagenesis, metamorphism and hydrothermal overprints to
be fully assessed and thus avoided in sample selection.
4.2. Weaknesses
Sedimentary pyrite develops both in the water column (syngenetic) and in the sea floor muds (diagenetic), and yet the technique proposed here does not take into account the modification
of seawater TE chemistry by interaction with minerals, other than
pyrite, in the muds. Our analyses are a mixture of syngenetic and
early diagenetic framboidal pyrites plus nodular pyrites that developed during late diagenesis, probably several meters below the
seafloor. Diagenetic nodular pyrite that forms during late diagenesis, commonly contains lower TE concentrations (Fig. 2; Large et
al., 2007), but no more than an order of magnitude less than syngenetic and early diagenetic pyrite in the same sedimentary unit.
We argue that seawater is the principal component of pore fluids, and as framboidal pyrite develops in the top few centimetres
of the muds, when seawater is >60% of the muds, then a strong
chemical connection between pyrite and seawater is maintained.
These same assumptions apply in the use of strontium, carbon and
oxygen isotopes of sedimentary carbonate as a proxy for their respective isotope variations in the palaeo-ocean (Veizer et al., 1999;
Shields and Veizer, 2002).
The TE in early formed (syngenetic and early diagenetic) pyrite
can be strongly modified by later diagenetic, metamorphic or hydrothermal processes and thus have little relationship to the original seawater chemistry. This is a major issue, especially for those
elements loosely held in the pyrite structure, such as Mo (Chappaz
et al., 2014). However, the approach proposed here is to undertake petrographic studies of pyrite textures in order to screen out
samples with evidence of metamorphic and hydrothermal influences (e.g., Thomas et al., 2011). Selenium in pyrite is very robust
to metamorphic influences and is the best of the redox sensitive TE
(Fig. 9). Black shales and associated pyrite are of little use as seawater proxies for samples above middle greenschist facies due to
major TE exchange with metamorphic fluids (Pitcairn et al., 2006;
Large et al., 2011b).
Our data indicate a wide variation in the TE content of sedimentary pyrite from the same sample. However, many TE show a
variation of less than one to two orders of magnitude. This degree
of variation could be considered to relate to ocean TE variation,
or to the inherent variability of syngenetic and early diagenetic
pyrite, and the effects of diagenesis and low-grade metamorphism.
The variation of one order of magnitude in a single sample is
significantly less that the variation of up to 4 orders of magnitude in the full dataset through time. Studies on single drill core
(Supplementary Fig. 1) show the variation in individual samples
compared to the mean variation through a stratigraphic section,
and clearly demonstrate that robust trends in the TE are related to
stratigraphic position.
In addition to the concentrations of TE in seawater, a number
of other factors may have an effect on the TE content of diagenetic pyrite, including; the matrix composition of the sediment,
the amount of pyrite produced, pH of pore fluids, degree of bioproductivity and preferred electron acceptors. Although these factors
are assumed here to have a second order effect, they each need
further evaluation.
4.3. Caveats
The most coherent TE data on early-formed sedimentary pyrite
is obtained by the analysis of pyrite framboids, and these must be
the first priority in any analytical strategy. However many black
shales of greenschist facies do not contain framboidal pyrite, but
may contain diagenetic nodules or partially recrystallised framboids. Great care is required to record the textural and paragenetic
changes in pyrite in order to undertake analyses on the earlyformed pyrite or their partially recrystallised equivalents.
In order to place pyrite TE analyses in a temporal context, high
quality geochronology is required. For Phanerozoic samples, fossil
assemblages can yield detailed chronological resolution. In Precambrian settings, the U–Pb analysis of zircon can provide high
quality constraints on the timing of eruption of tuffs and/or maximum depositional ages for silt- and sand-dominated strata that
may be interbedded with black shale sequences. U-bearing authigenic mineral phases also provide at least a minimum age of
deposition of the host rocks, and in some cases may date diagenesis. Where available, we have preferred chronological constraints
based on these techniques in order to minimise uncertainties related to whole rock dating methods, however the age resolution
and uncertainties vary significantly (Supplementary Table 2). A major caveat of any temporal analysis such as this is the accuracy and
resolution of chronological constraints.
5. Conclusions
The use of sedimentary pyrite as a proxy for TE trends in
the palaeo-ocean has opened a new window into deep time, and
presents exciting possibilities. Several lines of evidence indicate
that for any particular black shale depositional environment, syngenetic pyrite that forms in the water column and diagenetic pyrite
that forms in the top few meters of the seawater-saturated muds,
will have a similar first order TE composition that relates to the
TE composition of the seawater at that time. Processes during diagenesis do not substantially change the mean first order TE content of sedimentary pyrite. However, metamorphic processes that
cause recrystallisation of pyrite and conversion to pyrrhotite at
moderate to high metamorphic grades, may be accompanied by
major changes in many TE concentrations. Therefore, this study
has concentrated on early-formed (syngenetic to diagenetic) pyrite
and avoided samples with obvious metamorphic or hydrothermal
pyrite and/or pyrrhotite.
Our data suggest that for most TE, temporal variations in concentration have been over 2 to 4 orders of magnitude. The concentrations of some TE, like Ni, Co and Cr, generally decrease through
time, but with significant variation, commonly related to changes
in the composition of the exposed crust. Other redox sensitive TE
like Mo and Se, generally increase through time, and are principally controlled by oxygenation trends in the ocean–atmosphere
system.
A schematic diagram of our major findings is given in Fig. 9.
This figure uses the Se content of sedimentary pyrite as a proxy for
variations in atmosphere oxygenation. This proxy is based on the
premise that oxidative weathering of pyrite in continental rocks
leads to the release of selenium as both the selenate and selenite species. Under neutral to alkaline oxygenated conditions the
selenate species remains highly soluble, where it can be readily transported via river systems to the ocean. However under
low oxygen conditions, continental weathering releases little soluble Se, because the reduced forms of Se (selenide, elemental Se0
and organo-selenium complexes) are relatively insoluble. Thus significant increases in atmospheric oxygen, accompanied by active
erosion, can lead to a major increase in the supply of soluble Se
to the ocean, to be subsequently trapped in sedimentary pyrite in
seafloor muds.
The Se curve derived from sedimentary pyrite, along with
curves for other redox sensitive TE (e.g., Mo, U, Tl, Mn, Cd, Ag),
show significant increases in TE abundance of one to two orders
of magnitude at GOE1 and GOE2 (Fig. 9). However our data for
some TE shows sharp rises prior to GOE1, suggesting pulses of
R.R. Large et al. / Earth and Planetary Science Letters 389 (2014) 209–220
oxygenation commenced as early as 2970 Ma. Through the Proterozoic, TE variations in sedimentary pyrite are subdued, compared
with the Archaean, suggesting steady state or declining oxygen levels reaching a minimum in the Cryogenian prior to GOE2. From
1800 to 700 Ma a number of biologically important TE (e.g., Se, Cr,
Ag, Mo, Cu, Ni, Mn) show a flat to decreasing trend suggesting that
nutrient TE deficiency may be the reason for limited eukaryotic
development and radiation, thus adversely effecting marine evolution during the middle to late Proterozoic. This period ended with
the Sturtian glaciation, and from 700 to 540 Ma many of the redox sensitive TE rose by 1 to 2 orders of magnitude in response
to GOE2, accompanied by diversification of phytoplankton and the
appearance of eukaryotic clades (Shields-Zhou and Och, 2011); the
prelude to the Cambrian Explosion.
Acknowledgements
The following colleagues have provided black shale samples toward this project: Robert Scott, Ray Coveney Jr., Jeffrey Abbott,
Bradley Guy, David Huston, Lex Lambeck, Jeffrey Steadman, Charles
Makoundi, Sean Johnson, Richard Batchelor, Stuart Smith, Mikhail
Krupenin, Jerry Sharrock, Peter Sorjonen-Ward, Luba Leonova, Valeriy Murzin and Sergey Karpov. Funding was provided by an ARC
Centre of Excellence grant CE0561595 and AMIRA International
project grant P923 to RRL. Ron Fuge (University of Wales), Thomas
Kulp (Binghamton University), Bernd Lottermoser, Patrick Quilty,
Jocelyn McPhie (University of Tasmania), Brian Skinner (Yale University) and Noel White (Consultant) are thanked for their comments on earlier versions of the manuscript. We thank B. Kamber
and three other reviewers, and Editor G. Henderson, for excellent
comments for revision and improvement of the final manuscript.
This is YGS publication number 019.
Appendix A. Supplementary material
Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.epsl.2013.12.020.
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