Supplementary information (D. A. Fike, J. P. Grotzinger, L. M. Pratt, & R. E. Summons)
Oxidation of the Ediacaran Ocean
Huqf lithostratigraphy
The Nafun Group varies in composition and facies depending on paleogeographic position1.
The basal Masirah Bay consists of deeper-water black and grey shales that interfinger with postglacial cap carbonates. These grade upward into siliciclastic shales that shallow upward and
interfinger with tidal- and storm-emplaced sandstones and siltstones. The Masirah Bay passes
conformably into the peritidal limestones and dolostones of the Khufai Formation which form a
prograding shallow-marine ramp succession1. Khufai carbonates are unconformably overlain by
the Shuram Formation. The basal Shuram is comprised of outer shoreface calcareous sandstones
and limestones which grade upward into red shales, while the upper Shuram contains
interstratified carbonates that become more abundant into the basal Buah Formation. The Buah
is a simple shallowing-upward carbonate succession that culminates in shallow subtidal
stromatolitic and oolitic grainstone facies.
The Khufai-Shuram contact is at least locally unconformable2 based on field observations
and regional seismic data that show incision. This interpretation is supported by the locally
variable profile of δ13Ccarb across the Khufai-Shuram boundary2, indicating spatially variable
erosion/deposition and by the presence of inflection points (Figure 1a,c-d) or jumps (Figure 1b)
in carbon and sulphur isotopes across the contact, suggesting discontinuous deposition. The age
of the topmost Khufai is constrained to be younger than ~600 – 620 Myr, based on U/Pb ages
from detrital zircons3, 4. Based on sequence stratigraphic correlation, we interpret the
unconformity to encompass the period of Gaskiers glaciation (~580 Myr [ref 5]). However, see
ref [4] for an alternative view that suggests the basal Shuram is ~ 600 Myr. Our inference
suggests the basal Shuram is younger than 580 Myr, consistent with an estimate3 of ~560 - 570
Myr from models of sedimentation rates coupled to U/Pb ages from ash beds in overlying strata
and from global chemostratigraphic correlation.
Overlying the Nafun Group is the Ara Group, a series of six carbonate-evaporite cycles6, 7.
The contact between the Buah and overlying Ara Group is marked by a disconformable, karstic
surface. Additional ages3, 6 from overlying Ara carbonate units indicate ongoing deposition from
~ 547 through ~541 Myr. At the base of the fourth Ara carbonate unit the Ediacaran-Cambrian
boundary has been identified6 based on a 7‰ negative excursion in carbonate δ13C and the
disappearance of Ediacaran Namacalathus and Cloudina fossil assemblages.
Detailed description of observed trends (Figure 1a-e)
Observation of δ13Ccarb provides a reference framework for interpreting other isotope proxies
and for global correlations. Above Abu Mahara glacial strata, δ13Ccarb rises to ~6‰ and falls to
0‰ just prior to the Masirah Bay-Khufai boundary. δ13Ccarb then rises to +3‰ where it plateaus
for most of the Khufai, except for a negative excursion to 0‰ near the limestone-dolostone
transition from deeper to shallower water facies in the mid-Khufai. The uppermost Khufai is
marked by δ13Ccarb = 2.6‰; whereas the base of the Shuram is characterized by δ13Ccarb = 0.0‰,
decreasing to -12.0‰ at the nadir of the Shuram excursion. Above this level, δ13Ccarb rises
gradually to -4.5‰ at the Shuram-Buah transition. δ13Ccarb rises throughout most of the Buah,
hovering briefly around 0‰ before peaking at 6.5‰ in the upper Buah. δ13Ccarb decreases to 0‰
in the uppermost Buah.
In contrast, δ13Corg in the form of total organic carbon (Figure 1b) reveals a very different
pattern. Above Abu Mahara glacial strata, δ13Corg = -31.1‰ and gradually increases to ~ -29‰
in the upper Masirah Bay. From the uppermost Masirah Bay there is a decrease in δ13Corg to ~ 33‰ in the mid-Khufai, followed by an increase to -31‰ at the top Khufai. Beginning in the
base of the Shuram, δ13Corg increases slightly to -26‰ transitioning upward from the interbedded
sandstones and limestone to the basal Shuram red shales, an interval over which δ13Ccarb plunges
from 0 to -12‰ and then rises to ~ -8‰. Throughout the rest of the Shuram, δ13Corg gradually
decreases to ~ -31‰ while δ13Ccarb rises to -4‰. The increased scatter in the data through this
part of the section may be due to very low TOC levels (Figure S3), characteristic of most
sections through the Shuram and Buah formations. Beginning in the lower Buah, δ13Corg begins
to show sympathetic covariation with δ13Ccarb lasting through the uppermost Buah (r2 = 0.72).
Examination of δ34SSO4 (Figure 1c, in the form of carbonate-associated-sulphate) reveals
significant variability in the basal section, with an increase in δ34SCAS from 9‰ above Abu
Mahara glacial strata to 22‰ in the lower Masirah Bay. From this point through the upper
Khufai, δ34SCAS generally follows the same trends as δ13Ccarb. There is a 5‰ decrease in δ34SCAS
in the upper Masirah Bay coincident with the 6‰ negative trend in δ13Ccarb. Following this,
δ34SCAS increases to ~21‰ at the Masirah Bay-Khufai boundary and remains approximately
constant until the limestone-dolostone transition in the Khufai, coincident with the rebound and
plateau in δ13Ccarb. From the base of the Khufai dolostone, δ34SCAS increases linearly to ~29‰ at
the top Khufai, while δ13C rises from 0 to +3. The lowermost Shuram calcareous sandstones are
characterized by δ34SCAS = 28.0‰. This decreases to ~ 24‰ in the basal red shales as δ13Ccarb
plunges to ~ -12‰. Here, the covariance with δ13Ccarb ends as δ34SCAS decreases gradually to
~21‰ in the mid-Buah while δ13Ccarb recovers from the Shuram excursion. The upper Buah is
marked by an increase in δ34SCAS to ~26‰.
Above Abu Mahara glacial strata, δ34Spyr (Figure 1d) increases from 4‰ to 17‰ in the mid
Masirah Bay. There is a gradual decrease in δ34Spyr to -10‰ in the upper Khufai. δ34Spyr
increases slightly in the basal Shuram calcareous sandstones and then decreases through the red
shales to -22.5‰ in the upper Shuram interbedded carbonates. There is an increase in δ34Spyr to ~
-10‰ in the uppermost Shuram and lower Buah, followed by a drop to -25‰ in the uppermost
Buah.
Examining Δδ34S, the fractionation between coeval sulphate and pyrite, provides a more
meaningful way to interpret δ34Spyr by eliminating covariation with sulphate δ34S. Here we find
Δδ34S increases from less than 1‰ at the top of the post-glacial cap carbonate to ~ 35‰ at the
top Khufai. The basal Shuram is characterized by Δδ34S = 39‰, which decreases slightly in the
lower Shuram and remains relatively constant (35.2 ± 4.1‰, n = 49) throughout the duration of
the Shuram excursion. As the Shuram excursion ends in the upper Buah, Δδ34S increases to an
average of 47‰ (n = 14), with 10 samples having fractionations indicative of BSD.
Evaluating diagenesis:
Evidence for primary δ13Ccarb during the Shuram excursion
We examined two geochemical proxies (δ18O and Mn/Sr) to assess the likelihood that the
13
δ Ccarb signal of the Shuram excursion was primary (Figure S2a-b). Although there is a
correlation between δ13Ccarb and δ18Ocarb for samples during the Shuram excursion (r2 = 0.6), the
range of δ18O values for the samples during the excursion falls within the variability of those
before and after the excursion (Figure S2a). This is consistent with either a partial resetting of
the δ18O signal8,9 (e.g., by meteoric diagenesis) or secular trends10 in marine δ18O over the
Neoproterozoic. Examination of whole rock Mn/Sr values (Figure S2b) shows that samples that
define the Shuram excursion are within the variability of sample values that bound the excursion.
In addition, most excursion samples have Mn/Sr < 1, well below the threshold8, 9 for alteration of
primary δ13Ccarb. This indicates that meteoric diagenesis was insufficient to significantly reset
the Mn/Sr ratio and, therefore, highly unlikely to have altered the primary composition of
δ13Ccarb. Based on these data, the remarkable uniformity of the Shuram excursion across Oman
(inclusive of both surface and subsurface datasets), and the presence of a single negative
excursion of similar magnitude in other globally correlative strata, we conclude that the δ13Ccarb
signal is primary.
Evidence for primary δ34SSO4 preserved in Oman
We have examined δ34SSO4 versus several indicators of diagenesis (δ18Ocarb, Mn/Sr, δ13Ccarb,
and the concentration of carbonate-associated sulphate [SO4] (Figure S2c-f). With the exception
of one point (circled in red), there appears to be no indication of diagenetic alteration in our
δ34SSO4 data. This point has been excluded from Figure 1 to avoid re-scaling the x-axis;
however, it is included in supplementary table S1. The anomalous point comes from the
Marinoan-equivalent cap carbonate and may reflect the unusual depositional environment rather
than post-depositional alteration. Several samples have anomalously high [SO4] (> 2000ppm)
although well within the range reported from modern carbonates. Of these, only the one sample
indicated as possibly altered has a Δδ34S that is small enough such that post-depositional pyrite
oxidation might have contributed to the measured [SO4].
Supporting material for oxidation during stage II (Shuram excursion)
Additional evidence for stage II oxidation is found in the ~8‰ decrease in δ34SSO4 across the
Shuram excursion, while ∆δ34S remains nearly constant. This is consistent with the oxidation of
marine sulfides accompanying DOC oxidation during the Shuram excursion. Further support for
a more oxidizing environment during deposition of stage II strata comes from pyrite and total
organic carbon (TOC) abundances (Figure S3): the Masirah Bay and Khufai formations have
relatively abundant pyrite (0.78 ± 0.58 wt%) and TOC (0.70 ± 0.49 wt%), whereas the Shuram
and Buah formations have significantly less pyrite (0.064 ± 0.055 wt%) and TOC (0.100 ± 0.030
wt%). It should be noted that pre- (Masirah Bay/Khufai) and post-excursion (Shuram-Buah)
data cluster into distinct groupings independent of lithology (siliciclastic vs. carbonate). Both
the Masirah Bay-Khufai and Shuram-Buah formations represent upward shallowing sequences of
siliciclastic-carbonate packages that are sedimentologically similar; because they plot so
distinctly (Figure S3), it suggests that they were formed under different redox conditions, with
Shuram-Buah deposition occurring in a significantly more oxidizing environment.
Construction of δ13Ccarb correlation and associated paleobiological indices (Figure 2)
This figure correlates various globally-distributed Ediacaran-age strata that contain the
Shuram excursion. Because of variability in sediment accumulation rates, the duration of
periods of non-deposition and/or the magnitudes of erosional episodes, the geometry of the
δ13Ccarb curve varies between sections11. Nevertheless, the major components of the curve,
particularly the large magnitude of the negative δ13Ccarb, are present in each section, and help
build a composite model. Such composite models provide a useful means to visualize the degree
to which globally disparate sections record the same chemostratigraphic signal. The figure was
constructed using δ13Ccarb from Oman (this study; ref 6) and U/Pb ages from Oman6 and those
from China12 and Namibia13 that have been previously correlated to the Oman δ13Ccarb
chemostratigraphy. The Oman δ13Ccarb record was then fit linearly between successive
geochronological tie-points to construct a time line for the section, incorporating a reasonable
estimate3 for the age of the Khufai-Shuram unconformity. Next, the δ13Ccarb data from Namibia
and China, possessing multiple geochronological constraints were added and aligned to the
Oman record by fixing δ13Ccarb maxima and minima (e.g., nadir of the Shuram excursion, Buah
positive, etc.) with a linear stretch between tie-points and geochronologic data. Data from
geochronologically unconstrained sections (Australia, USA) were overlain on this record using a
linear stretch between tie-points (δ13Ccarb maxima and minima). The absence of high-resolution
age constraints for the interval of 630 – 555 Myr is problematic. Therefore, while we believe the
shape of the curve will remain accurate, the scaling of this portion of the plot is approximate and
the details of timing are likely to change as more dates become available. δ13Ccarb data from:
Oman (this study); China12, 14; Australia15,16; Namibia13, 17; and Death Valley18. U/Pb zircon ages
from Oman6, Namibia13, 19, and China12. The range for acanthomorph acritarchs20, 21 is derived
from several Australian sections and the upper Doushantuo Formation, China. Additional
work22,23,24 (indicated by the dashed line) shows other acritarch species evolved during stage 2
oxidation, but how these organisms are related to the complex acritarch fauna20 of Stage 2 and 3
remains unclear. The range5, 25 for sessile and segmented fronds is for strata in Newfoundland
(Mistaken Point) above the Gaskiers glaciation5. The range for small metazoa and bilaterian
embryos from the Doushantuo Formation, China summarized in ref[12]. The range26 for
macroscopic mobile bilaterian is obtained from strata from the White Sea, while the range12 for
macroscopic multicellular algae is taken from the upper Doushantuo Formation. Finally, the
range of weakly calcified metazoa is a composite observation based on both Oman6 and
Namibia13.
Methods
Sample preparation
All samples examined in this study are drill cuttings sampled every 2m from the Miqrat-1
drill hole in the South Oman Salt Basin (Figure S1). Samples were soaked in distilled deionized
(DI) water and then rinsed 3x in DI to remove any water soluble drilling contamination. Next,
samples were rinsed 3x in methanol, dichloromethane, and hexane to remove any soluble organic
contamination. Samples were powdered using a SPEX 8510 Shatterbox with an alumina ceramic
container.
Carbon extraction and isotopic analysis
Carbonate carbon and oxygen isotopes were measured according to the methods described
by Ostermann & Curry27. Organic carbon isotopes were analyzed as samples of total organic
carbon (TOC). For TOC analyses, powdered samples were acidified in 6N HCl for 24 hours to
remove carbonate minerals, filtered, rinsed with DI, dried, and the residuum was loaded into tin
cups for isotopic analysis. Samples were flash combusted at 1060C in a Carlo Erba NA1500
Elemental Analyser fitted with an AS200 autosampler. The resulting CO2 gas was analyzed by
continuous flow using a Delta plus XP Isotope Ratio Mass Spectrometer. Carbon isotopes are
reported as δ13C = (Rstandard/Rsample – 1) * 1000, where R = the ratio of 13C/12C, in units of per mil
(‰) relative to the V-PDB standard. Calibration of δ13Corg was done by comparison with
international standards (IAEA-CH-6, NBS-22) and in-house references (“Acetanilide” and “Penn
State Kerogen”) interspersed with the sample analyses.
Sulphur extraction and isotopic analysis
Powdered samples were rinsed with DI to remove soluble sulphates (e.g., from sulphide
oxidation) and then soaked for 24 hours in DI to remove water-soluble sulphate minerals (e.g.,
anhydrite). Sulphate δ34S was examined in the form of carbonate-associated
sulphate28,29,30,31,32,33 (CAS), also known as structurally-substituted sulphate, which is sulphate
trapped in the carbonate mineral matrix, either through substitution for the carbonate ion or in
crystal defects. CAS was obtained by dissolving the powdered sample in 6N HCl for 2 hours at
~60C under nitrogen gas or in 6N HCl for 24 hours at room temperature. No δ34S offset was
observed between these methods. Following dissolution, samples were filtered to remove
insoluble residues and an excess of 1 M BaCl2 solution was added to the solute to precipitate
BaSO4. The insoluble residue was kept for pyrite analysis.
Pyrite was extracted as chromium-reducible sulphur34. Pyrite extraction was performed
under nitrogen gas by the addition of 6N HCl and 0.4M reduced chromium chloride solution.
The reaction was allowed to proceed for 2 hours with the sulphide collected as silver sulphide
after bubbling through a sodium citrate buffer (pH 4) and into a silver nitrate (1 M) trap. Rinsed,
filtered, and dried BaSO4 and Ag2S precipitates were then combined with an excess of V2O5 and
analyzed for S-isotope composition at Indiana University on a Finnigan MAT 252 gas source
mass spectrometer fitted with a peripheral elemental analyzer (EA) for on-line sample
combustion35. Sulphur isotope compositions are expressed in standard δ-notation as permil (‰)
deviations from V-CDT, with analytical error of <0.07‰, calculated from replicate analyses of
samples and laboratory standards. Samples were calibrated using the international standards
NBS-127 (20.3‰) and S3 (-31.5‰), as well as four internal standards: silver sulphide (EREAg2S: -4.3‰), chalcopyrite (EMR-CP: 0.9‰), and two barium sulfate standards (BB4-18:
39.5‰; PQB: 38.0‰).
Supplementary figures
Figure S1: Map of the Sultanate of Oman. Neoproterozoic rocks crop out in the Oman
Mountains in the north, in the Huqf area along the east-central coast, as well as near Mirbat in
the south. Neoproterozoic deposits are also found throughout much of the subsurface in Oman,
where they have not been exposed to oxidative weathering and alteration. Samples from this
study are from the drill hole Miqrat-1, spanning a depth range from 3200 – 4244 m.
Figure S2: Evaluation of δ13Ccarb and δ34SSO4 diagenesis. Samples from the Shuram excursion
are in black, while samples bounding it are in blue. a-b) Evaluation of diagenesis of the δ13Ccarb
signal. a) Cross plot of δ13Ccarb vs. δ18Ocarb. b) Cross plot of δ13Ccarb vs. whole-rock Mn/Sr. c-f)
Evaluation of diagenetic alteration of δ34SSO4 data. Sample encircled in dashed red line is likely
to have been diagenetically altered. c) Cross plot of δ34SSO4 vs. δ18Ocarb. d) Cross plot of δ34SSO4
vs. whole-rock Mn/Sr. e) Cross plot of δ34SSO4 vs. δ13Ccarb. f) Cross plot of δ34SSO4 vs.
concentration of carbonate-associated sulphate [SO4] (ppm). See above for discussion and Table
S1 for data.
Figure S3: Abundance of pyrite and TOC by formation. Masirah Bay (blue) and Khufai shales
and carbonates (black), deposited before the Shuram excursion, define a high pyrite-high TOC
realm, whereas all the Shuram (red) and Buah (magenta) shales and carbonates, deposited during
and after the Shuram excursion, are confined to a low pyrite-low TOC area (dashed circle). See
text for discussion and Table S1 for data.
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