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A unifying model for Neoproterozoic–Palaeozoic exceptional fossil preservation through pyritization and carbonaceous compression

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ARTICLE
Received 28 May 2014 | Accepted 4 Nov 2014 | Published 17 Dec 2014

DOI: 10.1038/ncomms6754

A unifying model for Neoproterozoic–Palaeozoic
exceptional fossil preservation through pyritization
and carbonaceous compression
James D. Schiffbauer1, Shuhai Xiao2, Yaoping Cai3, Adam F. Wallace4, Hong Hua3, Jerry Hunter5, Huifang Xu6,
Yongbo Peng7 & Alan J. Kaufman8

Soft-tissue fossils capture exquisite biological detail and provide our clearest views onto the
rise of animals across the Ediacaran–Cambrian transition. The processes contributing to
fossilization of soft tissues, however, have long been a subject of debate. The Ediacaran
Gaojiashan biota displays soft-tissue preservational styles ranging from pervasive pyritization
to carbonaceous compression, and thus provides an excellent opportunity to dissect the
relationships between these taphonomic pathways. Here geochemical analyses of the Gaojiashan fossil Conotubus hemiannulatus show that pyrite precipitation was fuelled by the
degradation of labile tissues through bacterial sulfate reduction (BSR). Pyritization initiated
with nucleation on recalcitrant tube walls, proceeded centripetally, decelerated with
exhaustion of labile tissues and possibly continued beneath the BSR zone. We propose that
pyritization and kerogenization are regulated principally by placement and duration of the
decaying organism in different microbial zones of the sediment column, which hinge on
post-burial sedimentation rate and/or microbial zone thickness.

1 Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA. 2 Department of Geosciences, Virginia Tech, Blacksburg, Virginia
24061, USA. 3 Early Life Institute, State Key Laboratory of Continental Dynamics, and Department of Geology, Northwest University, Xi’an 710069, China.
4 Department of Geological Sciences, University of Delaware, Newark, Delaware 19716, USA. 5 Nanoscale Characterization and Fabrication Laboratory,
Institute of Critical Technology and Applied Science, Virginia Tec; h, Blacksburg, Virginia 24061, USA. 6 NASA Astrobiology Institute, Department of
Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA. 7 Department of Geological Sciences, Indiana University, Bloomington, Indiana
47405, USA. 8 Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, Maryland 20742, USA.
Correspondence and requests for materials should be addressed to J.D.S. (email: schiffbauerj@missouri.edu).

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S

oft-tissue fossils in the geological record are rare relative to
the profusion of shelly hard parts. While shelly organisms
produce mineralized structures in vivo, soft tissues require
authigenic mineralization to enter the fossil record. The factors
contributing to soft-tissue mineralization can be partitioned into
two distinct but complementary categories: those that facilitate
and those that drive preservation. Facilitating factors are typically
palaeoenvironmental in nature, serving to chemically or
mechanically delay or inhibit aggressive aerobic degradation
(noted in ref. 1 as ‘distal environmental and diagenetic
conditions’). While independently invoked as responsible for
soft-tissue preservation, facilitating factors are neither mutually
exclusive nor sufficient to guarantee fossilization. These
conditions can only enable soft-tissue preservation, because
delaying degradation is only part of the preservational puzzle.
Driving factors refer to constructive mineralization processes that
replicate or stabilize soft tissues (‘proximal causes’ of ref. 1),
ensuring their survivability through geological time and
diagenetic alteration. Counterintuitively, many mineralization
processes are dependent on microbially mediated degradation to

a

d

b

Py

Ca cement

c

Figure 1 | Taphonomic representations of Conotubus hemiannulatus.
(a) Pervasively pyritized (rusty weathered colour) Conotubus on bedding
plane. Scale bar, 5 mm. (b) Longitudinally fractured Conotubus specimen
showing carbonate cement infill (labelled Ca cement), with pyritized
(labelled Py) nested funnel walls visible (arrows). Scale bar, 1 mm.
(c) Specimen of Conotubus preserved as two-dimensional carbonaceous
compressions with aluminosilicate coating. Scale bar, 5 mm. (d) Interpretive
schematic of Conotubus showing flexible, funnel-in-funnel tube structure
and morphology. Figures in b,c are reproduced with permission from
Elsevier (modified from ref. 28 and ref. 1, respectively).
2

provide and locally concentrate necessary chemical constituents.
Each case of soft-tissue preservation is thus a race between
destructive decay and constructive mineralization processes2.
Superlative fossils occur in a narrow window where neither
degradation obliterates nor mineralization overprints important
biological details, a scenario contingent on both appropriate
settings and rapid stabilization processes.
Spanning across the Ediacaran–Cambrian transition, when
Konservat–Lagerstätten (deposits with exceptional soft-tissue
preservation3) are most abundant4,5, numerous mineralization
pathways fulfil the role of soft-tissue stabilizers6,7. Two of these
pathways—Beecher’s
Trilobite-type
pyritization
(threedimensional pervasive pyritization) and Burgess Shale-type
kerogenization (two-dimensional carbonaceous compression)—
are particularly important. Pervasive pyritization is commonly
facilitated by rapid burial, minimal ambient organic material,
periodic or persistent anoxia/dysoxia, reactive iron and sulfate
availability, low bioturbation and bacterial sulfate reduction
(BSR)-mediated decay1,8–14 (BSR: CH3COO  þ SO2–
4 -2
HCO–3 þ HS–). By and large, kerogenization has been attributed
to many of the same facilitating conditions, mostly related to
rapid burial into anoxic/dysoxic palaeoenvironments15. While
numerous
other
palaeoenvironmental
and
diagenetic
considerations have been invoked for kerogenization, such as
interactions with clays6,16 or ferrous iron17, high alkalinity15,18
and oxidant restriction (that is, lack of sulfate for BSR) through
early diagenetic sealing18 (though see also ref. 19), the common
association of kerogenized fossils with pyrite1,6,12,16,20–24 bolsters
the interrelationship of these taphonomic processes.
Fossils in the late Ediacaran (B551–541 Ma) Gaojiashan
Lagerstätte illustrate a preservational gradient from pervasive
pyritization to compressed kerogenization1, including threedimensional pervasive pyritization, incomplete pyritization
and carbonaceous compression with associated pyritization.
As such, this Lagerstätte offers an opportunity to establish a
comprehensive taphonomic model marrying these pathways. To
this end, we geochemically investigate the abundant Gaojiashan
fossil Conotubus hemiannulatus, which shows preservation in
each of these taphonomic styles. Our data form the basis for the
proposed unifying model, which invokes sedimentation rate, and
thus time the decaying carcass spends in specific microbial zones,
to regulate taphonomic styles along the pyritization–kerogenization gradient.
Results
General taphonomic observations. Among hundreds of Conotubus specimens examined in this study, B80% are preserved
three-dimensionally through complete or incomplete pyritization
(Fig. 1a,b), with the remaining preserved through two-dimensional kerogenization1 (Fig. 1c). Pervasively pyritized Conotubus
specimens possess secondary cracks filled with calcite cements,
and thin rinds (o20 mm) of iron oxide along these cracks
(Figs 2g,3c and 4e). Viewed in longitudinal and transverse crosssections (Figs 2a,3a and 4b), these specimens show a bimodal size
distribution of pyrite crystals. Generally, a micrometric size class
of crystals ranges from B10 to B250 mm, and a millimetric size
class of crystals ranges from B800 mm to a few mm. Micrometric
pyrite crystals are mainly found at the outer edge of the fossil
(Figs 2a and 3a) and sometimes along non-continuous central
voids (Fig. 3a) or fractures (Figs 2a and 4b), whereas millimetric
pyrite crystals comprise the bulk of the tube interior (Fig. 2a). In
some cases, millimetric crystals appear to be amalgamations of
micrometric crystals (Fig. 3a). In others, millimetric crystals
appear to have subtle textural variations along their outer edges,
possibly indicative of later overgrowth (Fig. 2b,c).

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a d

>–19 ‰
–19 to <–21 ‰
–21 to <–23 ‰
–23 to <–25 ‰
–25 to <–27 ‰
–27 to <–29 ‰
–29 to <–31 ‰
–31 to <–33 ‰
≤–33 ‰

Left transverse transect
Mean = –28.3‰

e

Right transverse transect
35
Mean = –24.0‰

4
Point position (mm)

25
3

15

2

Frequency (%)

b

5

1

c

5
0

0
–40

–30

–20

–30
–10 –40
δ S (‰-VCDT)

–20

–10

34

f

0

Frequency (%)
15

5

Longitudinal transect
35
Mean = –25.5‰

25

g

δ34S (‰-VCDT)

–10

–20

–30

–40
0

1

2

3

4

5

6

7

8

9
FeS2-Fe-Ca-Si

Point position (mm)

d34S

Figure 2 | SEM/EDS and SIMS
PY data for Conotubus specimen GJS-Cono001. (a) Backscatter Z-contrast electron micrograph montage of
longitudinal section, with all SIMS spots (circles) plotted at analysis location. Microdrilling location for IRMS d34SPY assessment indicated by star. Point
and star colour corresponds to d34SPY bins shown in upper left key. Scale bar, 1 mm. Insets (b,c) at lower left show higher magnification split-frame of
black rectangle region highlighting subtle textural difference associated with pyrite overgrowth (arrows in each panel); backscatter electron (b) and
secondary electron (c) images. Scale bar, 250 mm. (d–f) SIMS transects, with dashed lines indicating mean values and histograms showing frequency of
points by 1% bins. Note generalized U-shaped profile in d (left transect), with higher values towards fossil edges and lower values centrally located.
To show precision of each 10-cycle point sample mean, error bars mark±1 standard error. These errors do not include the analytical uncertainties of the
Balmat pyrite standard, which are relatively small. (g) Energy-dispersive X-ray elemental overlay of iron-sulfur (gold), iron but no sulfur (red), calcium
(blue) and silicon (green). Map region corresponds to red rectangle in a. Scale bar, 500 mm.

a
4

Point position (mm)

–23 to <–25 ‰
–25 to <–27 ‰
–27 to <–29 ‰
–29 to <–31 ‰
–31 to <–33 ‰
≤–33 ‰

b

Transverse transect
Mean = –28.8‰

35

3

25

2
15
1

Frequency (%)

>–19 ‰
–19 to <–21 ‰
–21 to <–23 ‰

5
0
–40

0
–30
–20
–10
δ34S (‰-VCDT)

0

c

FeS2-Fe-Ca-Si

d34S

Figure 3 | SEM/EDS and SIMS
PY data for Conotubus specimen GJS-Cono002. (a) Backscatter Z-contrast electron micrograph montage of
longitudinal section, with all SIMS spots (circles) plotted at analysis location. Microdrilling location for IRMS d34SPY assessment indicated by star.
Point and star colour corresponds to d34SPY bins shown in upper left key. Scale bar, 1 mm. (b) SIMS transect, with dashed line indicating mean value,
and histogram showing frequency of points by 1% bins. To show precision of each 10-cycle point sample mean, error bars mark±1 s.e. These errors
do not include the analytical uncertainties of the Balmat pyrite standard, which are relatively small. (c) EDS elemental overlay of iron-sulfur (gold),
iron but no sulfur (red), calcium (blue) and silicon (green). Map region corresponds to red rectangle in a. Scale bar, 500 mm.
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a

–19 to <–21 ‰
–21 to <–23 ‰
–23 to <–25 ‰
–25 to <–27 ‰
–27 to <–29 ‰
–29 to <–31 ‰

b

c

d

Frequency (%)
0

5

15

25

35 0

δ34S (‰-VCDT)

–10

5

15

25

35

Diameter
Mean = –21.8‰

Circumference
Mean = –25.2‰
–20

–30

–40
0

e

FeS2-Fe

1

2

3
Point position (mm)

f

Ca

4

5

0

1

g

Si

Figure 4 | SEM/EDS and d34SPY data for Conotubus specimen GJS-Cono003. (a,b) Backscatter Z-contrast electron micrograph montage of transverse
section, with all SIMS spots (circles) plotted at analysis location. Microdrilling for IRMS d34SPY was made on the reverse side of the specimen at a location
corresponding to the star. Point and star colour corresponds to d34SPY bins shown in upper right key. Scale bar in a, 2 mm, in b, 500 mm. (c,d) SIMS
circumference and diameter transects, respectively, with dashed lines indicating mean values, and histograms showing frequency of points by 1% bins.
Circumference d34SPY plot follows clockwise from straight arrow in b. Using dashed red line in b as a reference, note slight separation of d34SPY values:
higher d34SPY values to the right and lower d34SPY values to the left of the dashed line in b. Diameter d34SPY plot follows top-to-bottom. To show the
precision of each 10-cycle point sample mean, error bars mark±1 s.e. These errors do not include the analytical uncertainties of the Balmat pyrite standard,
which are relatively small. (e–g) EDS elemental overlay of iron-sulfur (gold), iron but no sulfur (red), calcium (blue) and silicon (green). Map region
corresponds to red rectangle in a. Scale bar in g is 500 mm, applicable for e and f.

Sulfur isotopic data. Pyrite sulfur isotopic values (d34SPY),
measured using secondary ion mass spectroscopy (SIMS), range
between –7.6 and –37.9%-VCDT (Supplementary Table 1 and
Fig. 2a,d–f; Fig. 3a,b; Fig. 4b–d). Although not a hard-and-fast
rule, pyrite with d34SPY values Z–19% tend to be associated with
fractures and/or textural variations in the pyritized fossil, which
may correspond to later overgrowth (for instance, see adjacent
red points in Fig. 2a longitudinal transect positioned just below
arrows indicating textural variation in Fig. 2b,c). Disregarding
obvious fracture-associated 34S-enriched points, the transverse
d34SPY transects exhibit a generalized U-shaped profile, with
localization of greater values along the tube-wall edges and lower
values towards the center of the fossil (Fig. 2d,e). This pattern is
not discernable in all specimens (Figs 3b and 4c,d). Microdrilled
pyrite assessed via isotope ratio mass spectroscopy (IRMS)
yield d34SPY values broadly similar to mean SIMS values
(Supplementary Table 1). Carbonate-associated sulfate sulfur
isotopic composition (d34SCAS) of host rock was assessed at
þ 33.6%-VCDT (Supplementary Table 1), comparable to contemporaneous units in Oman25. Thus, D34SCAS-PY is appreciably
high (ranging from þ 41.2 to 71.5%).
4

Carbon and oxygen isotopic data. Three-dimensionally but
incompletely pyritized Conotubus specimens show an outer rim
of pyrite, with multiple generations of compositionally distinct
carbonate cements surrounding a central void in the tube interior
(Fig. 5). In the only chemically analysed cross-section with this
preservational style but representative of other similarly preserved
specimens, the outer first generation cements consist of large
centripetally terminating calcite crystals, indicating inward carbonate growth from the pyrite rim. The inner second generation
cements are zoned rhombohedral crystals, with alternating
ultraviolet-luminescent zones of ferroan dolomite and dull zones
of dolomite þ calcite. Carbon and oxygen isotopic compositions
of microlaminae in the host rock and for the two generations of
fossil carbonate cements were determined (Supplementary
Table 2 and Fig. 6). The mean d13C value of the darker coloured
microlaminae (6.1±0.9% VPDB) is greater than that of the
lighter coloured microlaminae (4.4±1.0%) with slight overlap in
ranges, whereas their mean d18O values are broadly similar
(darker microlaminae ¼ –5.4±1.2%; lighter microlaminae ¼
–5.9±0.3%) with mostly overlapping ranges. While similar in
carbon isotope composition, the cements in the Conotubus

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a

b
Host rock
Laminae

CV

c
IC
Py rim
OC

e

f

Ca-Mg

Fe-Si

d

C104

Calcite
Dolomite

Light laminae
Dark laminae
D104

Relative intensity

g

Outer cement
Inner cement
10

15

20

25

30

Diffraction angle, 2 (degrees)

Figure 5 | Ultraviolet photoluminescence and in situ XRD data for Conotubus specimen 1GH2-70A. (a) Overview photomicrograph of transverse
slab of Conotubus and surrounding matrix of laminated rock. Note pyrite or iron oxide rim (gold colour), outer carbonate cement (dark colour), inner
carbonate cement (light colour) and central void. Scale bar, 10 mm. (b) Backscattered Z-contrast electron micrograph montage of region defined by
white rectangle in a. Fine pyrite or iron oxide rim (Py rim) seen as bright discontinuous specks, surrounding carbonate cements (IC: inner cement;
OC: outer cement) and central void (CV). Scale bar, 1 mm. (c) Ultraviolet photoluminescence microscopy of black rectangle region in a, showing
zoned carbonate crystals in inner cement. Scale bar, 1 mm. Inset (d) shows higher magnification view of white rectangle in c for better detail of crystal
zoning. Scale bar, 500 mm. (e,f) EDS elemental maps of white rectangle region in b, with calcium (orange) and magnesium (green) overlay shown in
e and iron (red) and silicon (blue) overlay shown in f. Scale bar in f is 1 mm, applicable for e. (g) In situ XRD patterns of host rock light- and dark-coloured
laminae, outer carbonate cement within fossil and inner carbonate cement within fossil. Characteristic peak positions of calcite and dolomite are
shown in orange and green vertical dashed lines, respectively. The strongest diffraction peaks from calcite and dolomite are labelled with C104 and
D104, respectively.

tube and the host rock have distinct d18O values. Between
fossil-interior cement generations, the outer calcite cements have
slightly greater mean d13C and d18O values (d13C ¼ 6.1±0.1%,
d18O ¼ –8.2±0.2%) than the inner dolomite þ calcite cements
(d13C ¼ 5.2±0.4%, d18O ¼ –9.0±0.2%).
Compaction. Three-dimensional preservation of Conotubus
allows an estimate of the compaction ratio of the host sediments.
Viewed perpendicular to bedding plane, the calcisiltite/calcilutite
microlaminae warp around nearly circular Conotubus tubes
(Figs 4a,b,5 and 6). Measurements of microlaminae surrounding
Conotubus tubes in comparison with their thickness extending
beyond the fossil yields an estimated sediment compaction ratio
of B1.85:1. Conotubus tubes show negligible compaction, with a
major:minor axis ratio of B1.15:1, although an oblate crosssection may be biological26.

Discussion
Previous analyses of pyritized Palaeozoic Lagerstätten suggest
rapid burial into anoxic sediments, which in turn reduces
bioturbation, impedes organic deterioration and emplaces the
decaying carcasses within the sulfidic BSR zone below the
oxic-anoxic boundary in the sediment profile. Further,
pyritization-conducive sediments typically have low organic
carbon content and abundant reactive iron, serving to focus
BSR and pyrite precipitation on decaying carcasses8,9,13,14,27.
While the Gaojiashan biota is similar to these younger deposits
in that pyritized fossils are found in sediments deposited by
rapid burial events11, there are some key differences. First, the
full pyritization–kerogenization preservational gradient from
three-dimensional pervasive pyritization to two-dimensional
kerogenized compressions, with intermediate or admixed
preservational modes, appears to be unique to the Gaojiashan
Lagerstätte. Second, the tubular fossils of the Gaojiashan biota

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a

b

8

Light-coloured lamina below fossil (a)
Light-coloured lamina above fossil (a)
Dark-coloured lamina (a)
Light-dark laminae (b)
Outer cement within conotubus (a)
Inner cement within conotubus (a)

c

7

5

δ13C (‰-VPDB)

6

4

3

2
–9.5

–8.5

–7.5

–6.5

–5.5

–4.5

–3.5

–2.5

δ18O (‰-VPDB)

d13C

d18O

Figure 6 | Host rock and fossil carbonate IRMS
and
data. (a) Microdrill map of slab 1GH2-70A with Conotubus specimen. Scale bar,
10 mm. (b) Microdrill map of slab (two continuous pieces) without Conotubus fossils. Scale bar, 50 mm. (c) Cross-plot of all d13C and d18O analyses.
Convex hulls and nested convex hulls (shaded) show groupings of data by location of microsampling. Yellow points show light-coloured laminae
(single circle below fossil and double circle above fossil in a; dashed circle from slab in b). Blue points show dark-coloured laminae (single circle
contiguous to fossil in a; dashed circle from slab in b). Red and green diamonds show outer and inner cements in a, respectively. Keys also applicable
for microdrill location maps.

notably do not retain their most labile soft parts10,28, showing no
evidence of the soft-bodied organisms that lived within these
tubes. While some sites of pervasive pyritization show glimpses of
highly labile soft-tissue preservation, such as pygnogonid and
crustacean musculature from the Hünsruck Slate29,30, pervasive
pyritization of Conotubus only captures the three-dimensional
exterior morphologies of the tubes. The original histology of
Conotubus tubes is unknown. However, they have been
interpreted as supportive, refractory tissues, either nonbiomineralized or weakly biomineralized28.
Building upon the Raiswell et al.31 diffusion–precipitation
model for pyritization, we can begin to elucidate the progression
of pervasive fossil pyritization and provide insights into early
diagenetic conditions responsible for the taphonomic styles
observed in the Gaojiashan Lagerstätte. According to this
model31, the locus of mineralization is controlled by the
intersection of two diffusion fields. The first arises as a
consequence of BSR, driving sulfide outward into the sediment
porewaters. The second originates from sediment sources,
carrying reactive iron toward the decaying organic nucleus. The
intersection of these two diffusion fronts results in supersaturated
6

porewaters with respect to iron sulfide at some distance from the
organic nucleus. The spatial trends in both pyrite crystal
morphology and d34SPY values of Conotubus fossils are
consistent with centripetal pyrite growth controlled by the
diffusion–precipitation dynamics of mineralization31. Similar to
the formation of pyrite rims around chert nodules32 and
carbonate concretions8,31, sulfate reducing bacteria (SRB)
metabolized a centrally located organic nucleus, in this case the
labile tissues of the Conotubus organism, generating an outward
diffusion of sulfide. The sulfide meets with an inward-diffusing
reactive iron sourced from ambient porewater, forming a reaction
front where pyrite-precursor iron monosulfide33 precipitation
occurs. As this process proceeds, the organic nucleus is
progressively exhausted, resulting in an inward shift in the
reaction front (Supplementary Fig. 1).
One complicating issue, however, is that the recalcitrant tube
walls of Conotubus would form a barrier to impede but not
entirely halt diffusion, resulting in accumulation of sulfide and
reactive iron on either side of the tube walls. As such, we must
consider another important factor controlling the locus of
pyritization: heterogeneous nucleation facilitated by an organic

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Pervasive pyritization

Microbial zonation with sediment depth

a

b

Incomplete pyritization

c

Partial kerogenization/pyritization

d

Diffuse pyrite kerogenization

Sediment–water interface
Aerobic

O2
NO–3
Mn2+

Fe/Mn/DN
Se

2–
Fe2+ SO4

dim

en

tat

BSR

Methanogenesis
Time

ion

rat
e

BSR pyritization
Pyrite overgrowth

Termination of
BSR pyritization
Carbonate infill

Limited pyritization
Kerogenization

HS–
Kerog.

Fe-S
complexes
CH4

Rapid event burial
Background sedimentation

100 101 102 103 104
Conc. (μM)

Figure 7 | Pyritization–kerogenization taphonomic gradient. Fossil examples (a–d, upper panels) and proposed taphonomic model (lower panels) with
sedimentary microbial zonation. Diagonal lines track the position of the decaying organisms in the sediment column, and the slope of each line represents
sedimentation rate. Grey upper diagonal line corresponds to rapid event burial, and black lower diagonal line corresponds to post-burial sedimentation.
Brackets indicate the length of time the decaying organism resides in the BSR zone (gold colour), and experiences BSR-mediated pyrite growth, although
pyrite overgrowth may continue into underlying methanogenesis zone (blue colour). From left to right, each diagram indicates a shortened residence time
in the BSR zone as compared with the previous panel, which could result from either, or both, changes in post-burial sedimentation rate and BSR zone
thickness. (a) Pervasively pyritized Conotubus in transverse cross section. Scale bar, 2.5 mm. Diagram shows initial rapid event deposition, followed by slow
post-burial sedimentation. (b) Polished transverse cross-section of pyritized Conotubus showing carbonate in the center and thin outer lamina (arrow)
representing nested tube wall. Scale bar, 1 mm. Corresponding diagram shows increase in post-burial sedimentation rate and reduction in BSR zone
thickness, leading to comparatively earlier termination of pyritization. (c) Specimen of Conotubus with admixed taphonomic mode of pyritization (black
arrows) and carbonaceous compression (white arrows). Scale bar, 5 mm. Diagram shows further increase in post-burial sedimentation rate and reduction
in BSR zone thickness, yielding partial pyritization and onset of kerogenization once carcass exits BSR zone. (d) Specimen of Conotubus preserved via
complete kerogenization with diffuse pyrite. Scale bar, 5 mm. Diagram shows highest post-burial sedimentation rate and thinnest BSR zone, with limited
pyritization, and earliest onset of kerogenization. Relative abundances of chemical species at right follows that of refs 47,48 after ref 67. Fe–S complexes
curve shows possible continued pyrite overgrowth from downward diffusion of HS–/Fe2 þ . Figures in a–d are reproduced with permission from Elsevier
(modified from ref. 1 and ref. 28).

substrate. The individually conserved nested Conotubus tube
walls (Figs 1b and 7b) may have promoted pyritization by
providing a naturally favourable substrate for the initiation of
iron sulfide nucleation—an unaccounted-for factor in the
traditional model31.
The ability of an organic substrate, such as the tube walls, to
promote mineralization can be readily justified within the
constructs of classical nucleation theory. For a cube-shaped
nucleus, the relationship between the free energy barrier opposing
the formation of a stable nucleus in solution (homogeneous case)
Þ, where ahom
and on a foreign substrate is: DGhet ¼ DGhom ð2aahet
hom
and ahet are the interfacial energies of the homogeneous and
heterogeneous nuclei. Although ahet depends on the balance
between the liquid-nucleus, liquid-substrate and substratenucleus interfacial tensions, a simple analysis shows that if
ahomEahet, the free energy barrier opposing nucleation at a
surface is reduced to half of the homogeneous barrier. As
nucleation rate shows an exponential dependence on DG*, this
translates into a substantial increase in the rate of surface
nucleation. Indeed, nucleation rates depend strongly on the
chemical nature and physical structure of the organic interface.
For instance, NH3þ terminated surfaces can be completely passive
to the formation of amorphous SiO2; however, under identical
conditions, NH3þ /COO– surfaces promote SiO2 deposition to the
extent that the organic may be coated by a nearly-coherent
nanoscale layer of amorphous material within a few hours34.
Moreover, as evidenced by calcite nucleation on polysaccharide

matrices35, significant variations in the surface nucleation rate
may occur between biopolymers with similar functionalities, with
the specific order of substrate preference depending chiefly on
supersaturation.
During the earliest stages of microbially induced degradation,
when nucleation sites on the tube surface are most abundant,
pyritization is dominated by precipitation of abundant micrometric crystals as are observed at the outer edges of the
Conotubus specimens. The size and location of the crystallites
indicates that the organic tube-wall surface likely plays a role in
directing the onset of nucleation, and that the pyrite supersaturation state, s, was relatively high as compared with the later
stages of mineralization when pyritization proceeds through
coarsening of existing crystals rather than formation of new
crystals. This interpretation
is supported
by the nucleation rate

2
equation, Jn ¼ Ae  DG =kB T ¼ Ae  B=s , where A is a preexponential constant (whose units represent the number of
molecules attaching to a critically-sized nucleus per unit time and
surface area), B is a constant that describes the shape of the
nucleus, T is temperature, kB is the Boltzmann constant, DG* is
free energy matching the thermodynamic barrier opposing
activity product
nucleation and s is supersaturation ðln ion
solubility product Þ. This
equation shows that an increase in supersaturation state generally
results in a higher number of nuclei per unit area of substrate or
volume of solution in the case of homogeneous nucleation. As
compared with any soft body tissues of the Conotubus organism,
the tube walls must have provided the most chemically favourable

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nucleation sites for initiation of pyrite growth. Pyrite crystals in
these regions are isotopically heavier (d34SPY typically 4–25%;
Fig. 2a,d,e) than those in the tube interior, indicating a
progressive decline in BSR rate throughout the pyritization
process and corresponding relaxation in diffusion-limited sulfate
availability. The inferred sulfate limitation in initial pyritization
may seem contradictory to the high D34SCAS-PY values
(Supplementary Note 1). However, oxidative recycling of sulfide
and oxidation of detrital pyrite entrained in microturbidites14
likely supplied 34S-depleted sulfate for pyritization, thereby
contributing to the high D34SCAS-PY values. As BSR continues
over time, labile tissues are diminished and BSR necessarily slows
because of decreasing availability of metabolizable organic carbon
rather than sulfate exhaustion or deep burial, which would
instead yield a shift towards higher d34SPY values. As a result of
slowed BSR, sulfide production rate would drop, supersaturation
levels with respect to iron sulfide would decrease and nucleation
would be disfavoured. Thus, iron sulfide precipitation is focused
on overgrowing existing crystals, leading to the formation of
millimetric crystals. During this shift, d34SPY values would
decrease as the BSR system switches from sulfate limitation to
organic limitation. The expected result is a U-shaped d34SPY
profile as observed in some specimens (Fig. 2d,e), although this
profile can be obscured by pyrite overgrowth (see Discussion
below).
From the understanding of how the pyritization process
proceeds and expectations for resultant sulfur isotopic trends
(Supplementary Note 1), we can then shift our focus to resolving
the geochemical constraints for pervasive pyritization. With the
first dissociation constant of H2S at 1.05  10  7 (K1 ¼ [H þ ]
[HS  ]/[H2S]) and seawater pH typically constrained to B7.5–8.5,
HS– is the dominant dissolved sulfide species. Thus, we can write
the precipitation of pyrite as Fe2 þ þ S0 þ HS–-FeS2 þ H þ ,
such that the equilibrium solubility product for pyrite is given as
Ksp ¼ [Fe2 þ ][HS  ]/10  pH ¼ 10  16.35 (ref. 36). With activity
coefficients for HS– and Fe2 þ in seawater at 0.67 and 0.26
(ref. 37), we can then calculate the apparent solubility product for
K ð10  pH Þ
0
¼ g sp2 þ gHS  ¼ 8:110  24 ðM2 Þ at the low end of
pyrite as Ksp
Fe
normal seawater pH. As such, given the Raiswell et al.31 model
assumption of the estimated ratio of reservoir concentrations (C0)
of sulfur to iron, C0S :C0Feo0.1 (that is, [Fe2 þ ]Z10  [HS–]), for
localized pyrite formation at the site of decay31, reactive iron
must be greater than approximately 9  10  12 M to drive pyrite
precipitation, well below modern observations of anoxic
sediments ato10  4 M (refs 8,31). Because the majority of the
fossils in the Gaojiashan Lagerstätte exhibit three-dimensional
pervasive pyritization1, we can surmise an excess of available
sediment/porewater reactive iron. Assuming a decay constant
of 0.1–1.0 per year (reported for organic decay in marine
sediments38) and an organic nucleus radius of 0.1–1.0 cm
(appropriate for Conotubus with a maximum diameter of
1.2 cm (ref. 28)), porewater dissolved sulfide concentrations of
o10  4 M are required for soft tissue pyritization (Fig. 3 in ref.
31). As localized pyritization of highly labile tissues occurs when
C0S :C0Feo0.1 (ref. 31), Fe2 þ content in porewater may have been
up to 10  3 M. We can thus reasonably constrain porewater Fe2 þ
content in Gaojiashan sediments between 9  10  12 M and
10  3 M, justified by the lack of labile soft-tissue pyritization in
this Lagerstätte.
In incompletely pyritized but three-dimensional Conotubus
tubes, alkalinity generated during BSR (Supplementary Table 3)
and bacterial Fe(III) reduction (BFeR: CH3COO  þ
8 Fe(OH)3-2 HCO3 þ 8 Fe2 þ þ 15 OH– þ 5 H2O) could
contribute to carbonate infill assuming appropriately basic
microenvironmental
pH
levels
to
facilitate
calcite
8

precipitation32,39. While similar relatively heavy d13C values of
host rock and fossil cement render BSR- or BFeR-sourced
bicarbonate as insignificant contributors, dissolved sulfide is
observed to enhance crystallization of calcite and dolomite40,
possibly contributing to early calcite cementation. In addition,
SRB exopolysaccharides commonly incorporate metal cations
(such as Mg and Fe), which could facilitate precipitation of highMg calcite and ankerite41. The B2% difference between
carbonate cement d18O values within the Conotubus specimen
and host carbonates, but broadly similar d13C values (Fig. 6c),
suggest that the cements may have been slightly influenced by
diagenetic fluids without significant alteration of carbon isotopic
composition. Slightly lower d18O and d13C values in the inner
versus outer cements indicates a stronger influence of diagenetic
fluids on the inner cementation, consistent with petrographic
evidence showing a later origin than the outer cements. The
relatively small isotopic difference between the cements and
sedimentary matrix suggests that carbonate cementation may
have occurred shortly after the cessation of pyritization, playing a
constructive preservational role by enhancing the rigidity of
partially pyritized tubes before sediment compaction.
While we have established that both pervasive and incomplete
pyritization must have predated sediment compaction based on
warping host rock microlaminae encasing the Conotubus fossils,
we can further constrain the duration of the pyritization process.
To do so, we must consider requisite levels of sulfate, reactive iron
and metabolizable organic material. On the basis of the size of
Conotubus specimens, with a maximum diameter of 12 mm and
length of 3–80 mm (ref. 28), the amount of sulfate required for
complete pyritization will follow Msulfate ¼ 2 MVr
, where V is the
0
pyrite

volume of the Conotubus tube, r is the density of pyrite (E5 g per
cm3) and Mpyrite is the molar mass of pyrite (E 120 g mol  1).
Using V ¼ p*r2*l for the volume of a cylinder, with Conotubus
radius (r) and length (l) simplified as 5 and 40 mm, the total
sulfate required is 0.26 moles. The amount of reactive iron needed
is 0.13 moles. The amount of organic carbon needed varies by the
electron donor compound used (largely fermentative end
products; Supplementary Table 3). For simplicity, we use BSR
of acetate, which, in accordance with the above constraints,
requires 0.52 moles of organic C. If we then assume that the
interior volume of Conotubus tube is completely full of soft tissue
(an overestimate), with a nested tube-wall thickness of 1 mm
(Fig. 1b), the volume of soft tissue that contains metabolizable
organic material using the same dimensions as above is B2 cm3.
Assuming that the soft tissue was pure amorphous carbon
(density ¼ B1.8 g cm  3), 0.52 moles would occupy a volume of
B3.5 cm3, a substantially larger volume than is available within
the Conotubus tube. As such, given realistic carbon contents in
animal tissues, we can deduce that there is a deficit of endogenous
carbon to account for the volume of pyrite precipitated—which is
clearly problematic, but disregarded for now and revisited in our
model description below. On the basis of these calculations and
high-end rates of sulfate reduction in Ediacaran sediments
(6.935 mmol cm  3 per year according to ref. 42, with pervasive
anoxic conditions and higher metabolizability of organics before
the evolution of vascular plants32,42–44), we can calculate an
approximate timeframe for generating sufficient HS– to
pervasively pyritize a Conotubus tube with a 3.14 cm3 total
volume. If the 0.26 moles of SO24  is derived entirely
endogenously within a Conotubus tube, a total of 82.8 mmol
HS– per cm3 is required for pyritizing the full volume of the tube.
At a rate of 6.935 mmol cm  3 per year, a total of B12 years
would be required to generate enough HS– for pyritization. Even
at modern rates of sulfate reduction, such as measured rates of
0.1 mmol cm  3 per year (ref. 45), this yields 828 years to generate

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enough HS– for pyritization of moderately sized Conotubus tubes.
The decaying organism is a BSR hotspot, which should pull in
2 þ from the surrounding microenvironment
both SO2–
4 and Fe
(as opposed to assumed steady-state equilibrium31), thus these
cited bulk-sediment rates may overestimate the pyritization time.
It is important to note, however, that these estimates assume that
HS– is: (1) a rate-limiting factor for pyrite precipitation and (2)
entirely derived from BSR in and around the decaying Conotubus
organism in the BSR zone. As discussed below, these assumptions
may not be valid.
From these and previously reported observations1, the data
presented herein allow for a pragmatic approach to consider the
timing, processes, and interrelationship of pyritization and
kerogenization. First, the pyritization process is likely initiated
in the BSR zone in the sediment column. The sediment column
typically consists of an aerobic respiration zone just beneath the
water–sediment interface, followed by an anaerobic zone with
nitrate, manganese and iron reduction, then the BSR zone, and
finally the methanogenesis zone at depth46 (Fig. 7). For fossil
pyritization to occur in the BSR zone, factors directly responsible
for BSR-mediated pyrite mineralization—the availability of
sulfate, reactive iron, metabolizable organic material and
positioning of the carcass within the BSR zone—must overlap
with commonly invoked facilitating conditions such as rapid
sedimentation and burial, low bioturbation and sediment anoxia.
These factors are intimately related. For example, sedimentation
rate influences the availability of reactive iron, sulfate and organic
carbon in the sediments, thus controlling the thickness of the BSR
zone and ultimately how much time the decaying carcass spends
here. Because sedimentation rate plays such an important role, we
propose a unifying model in which sedimentation rate was the
primary factor controlling the pyritization–kerogenization
gradient as observed in the Gaojiashan biota (Fig. 7).
To help explain the model, we focus on four representative
preservational styles along the pyritization–kerogenization
gradient: (1) pervasive pyritization, (2) incomplete threedimensional pyritization, (3) partial kerogenization with abundant pyrite association and (4) kerogenization with limited or
highly diffuse pyrite association. For complete pyritization
(Fig. 7a), initial pyrite mineralization must occur rapidly to
circumvent compression, and must continue to fill the entire
volume of the organism. If full pyritization occurs entirely in the
BSR zone, then the decaying carcass must stay in the BSR zone
for a sufficient amount of time, requiring a thick BSR zone and/or
a slow sedimentation rate following burial. The initial smothering
event that entrapped the Conotubus organism rapidly placed it
below the oxic zone of aggressive aerobic decay, and subsequent
sedimentation proceeded sufficiently slowly to ensure a long
enough duration in the BSR zone for complete pyritization. This
scenario is consistent with the d34SPY data, with greater outeredge values suggesting an initial period of relatively rapid BSR,
ensuring three-dimensional structural integrity, followed by a
progressive decrease in BSR rate (Supplementary Fig. 1) accompanying greater isotopic fractionation and lower d34SPY values.
Volumetric considerations, however, suggest a shortage of
organic fuel from the Conotubus soft-tissue itself, which would
require either an exogenous source of carbon or production of
de novo organic carbon within the organic nucleus (for instance,
by in situ chemoautotrophs) to continue fuelling further sulfide
production once the labile soft tissues of Conotubus were
exhausted. Alternatively, pyritization could continue in the
methanogenesis zone where BSR is disfavoured but residual or
downward-diffusing porewater HS– (or Fe–S complexes47)
support pyrite overgrowth and continued pyritization. This
extended period of overgrowth may not show any clear textural
change, but would obscure the anticipated U-shaped d34SPY

profile because residual porewater HS– tends to have greater d34S
values. In combination, the observed subtle textural variations
(Fig. 2b,c) and higher d34SPY values along millimetric crystal
edges may support this alternative scenario. Regardless, the keys
for complete pyritization are rapid burial and initial pyritization
to maintain structural integrity, followed by prolonged pyrite
overgrowth within and perhaps below a relatively thick BSR zone
to completely fill Conotubus tubes with pyrite.
Incomplete but three-dimensional pyritization (Fig. 7b)
requires the same facilitating taphonomic conditions as complete
pyritization, including rapid placement in the BSR zone and
initiation of pyritization to maintain three-dimensionality, but
instead experiences closure of the pyritization process before
complete infilling of the Conotubus tube with pyrite. As such,
temporal limitation of one or more of the required chemical
components, such as diffusive extinction of sulfate, may limit the
extent of the BSR zone. With or without BSR zone restriction,
the decaying Conotubus organism is likely pushed down into the
methanogenesis zone due to high post-burial sedimentation rate,
such that pyritization in the BSR zone is prematurely terminated.
Alternatively, these incompletely pyritized tubes may persist
within the BSR zone, but with cessation of pyritization due to
exhaustion of metabolizable organic material and a shortage of
exogenous carbon input. The interior space originally occupied by
the Conotubus animal then becomes a central void, which is
subsequently infilled with carbonate cements. The timing of
carbonate cementation may depend on the electron donor used in
BSR (Supplementary Table 3). Two scenarios are plausible:
(1) the BSR respiration pathway does not produce acidity and
carbonate cementation is synchronous with pyritization; or
(2) carbonate cementation follows dissipation of a lower pH
microenvironment generated with BSR production of acidity. In
either case, no organic material remains or at least none is
preserved through kerogenization. Regardless of which alternative
is more realistic, the true preservational distinction between
complete and incomplete three-dimensional pyritization hinges
on the duration, not the initiation, of pyrite formation. With
regard to complete pyritization, we suggest that continuing pyrite
overgrowth is necessary for complete Conotubus tube infill,
whereas incompletely pyritized tubes instead are infilled with
carbonate cements after premature termination of pyritization.
At the other end of the Gaojiashan preservational spectrum are
kerogenized carbonaceous compressions. While models for
kerogenization invoke the same facilitating taphonomic conditions for pyritization, three-dimensional structural rigidity is not
preserved, and instead organic tissues are stabilized to form
geologically robust but two-dimensionally compressed carbonaceous remains. In the kerogenization taphonomic modes,
pyritization is necessarily limited for tissues to be preserved as
carbonaceous compressions. The potential key is a rapid postburial sedimentation rate and/or a narrower BSR zone, such that
the fossilizing Conotubus tube is much more quickly moved
through the BSR zone with little pyritization, thus positioning the
decaying organism within the methanogenesis zone shortly
following the onset of decay. In such a case, Conotubus tubes
that exhibit an appreciable amount of pyritization (Fig. 7c) may
have stayed in the BSR zone long enough to facilitate localized
pyritization but not extensively sustained for pyritization to
maintain three-dimensional structural integrity. Kerogenized
Conotubus with only diffuse pyrite (Fig. 7d) likely experience
the majority of preservation within the methanogenesis zone
during the earliest stages of decay, thus spending only very
limited time in the BSR zone due to a high post-burial
sedimentation rate and/or a narrowed BSR zone. Following
these conditions, the fossil is predominantly comprised of
carbonaceous remains from kerogenization within the

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methanogenesis zone. Methanogens are incapable of decomposing higher molecular weight organic compounds, commonly
using acetate as the terminal electron acceptor48 (generalized:
CH3COO– þ H2O-CH4 þ HCO–3 ). If the breakdown of highmolecular weight compounds via bacterial fermentation
(generalized: C6H12O6 þ 4 H2O-2 CH3COO– þ 2 HCO–3 þ 4
H þ þ 4 H2) is mostly limited to metabolically labile tissues, and
aggressive aerobic degradation (generalized: CH3COO– þ O2-2
CO2 þ H2O þ OH–)48 was limited as a result of rapid burial, only
the most recalcitrant structures such as the Conotubus tube walls
will enter the fossil record. A few alternative scenarios should also
be considered. First, the Conotubus remains may indeed undergo
degradation within the BSR zone, but instead occupy
microenvironments that are substantially limited in either or
both reactive iron or sulfate thus hindering pyrite precipitation.
Second, the invocation of minimal ambient organic material13 for
pyritization may not hold in cases of kerogenization. That is, SRB
may prefer disseminated organic sources if Conotubus is
emplaced within comparatively more organic-rich sediments,
with Conotubus thus avoiding BSR-degradation and localized
pyritization. Regardless, in either case, organic degradation is
clearly limited to labile tissues, such that only the recalcitrant
tissues are eventually preserved as kerogens.
Previous biostratinomical analysis of the late Ediacaran
Gaojiashan Lagerstätte has established that event deposits played
an important role in the preservation of Gaojiashan fossils11, but
the post-burial mineralization processes leading to the observed
pyritization–kerogenization gradient of preservation remain
elusive. On the basis of geochemical analyses of threedimensionally pyritized Conotubus specimens, we have
established that the pyritization process proceeded centripetally
from a rim of micrometric pyrite nucleated on the tube wall and
fuelled by SRB degradation of labile tissues within the tube. This
process can be described using a diffusion–precipitation
model31,32, modified to account for the role of an organic tube
in promoting the onset of pyritization by providing a preferable
substrate for nucleation. We suggest that preservation along the
pyritization–kerogenization gradient can be understood as
variations in the placement and duration of the carcass within
the BSR and methanogenesis zones. The amount of time spent in
each zone is principally influenced by post-burial sedimentation
rate and/or BSR zone thickness, the latter of which is also related
to the availability of metabolizable organic material and sulfate.
The fossils that show three-dimensional pyritization, whether
pervasive or incomplete, spend more time in the BSR zone. In
contrast, those that are two-dimensional carbonaceous
compressions, either with extensive or diffuse pyrite association,
are more rapidly buried in the methanogenesis zone. In each case,
degradation is required for preservation, and each mode may be
viewed as a complex balance between decay and mineralization.
These preservation processes, whether pyritization or
kerogenization, are governed by the same suite of facilitating
taphonomic conditions, are primarily influenced by microbial
decay pathways, and differ principally in the duration of
interactions with the BSR and methanogenesis zones. Thus, the
scientific conversation regarding contributing factors of Beecher’s
Trilobite-type pyritization and Burgess Shale-type carbonaceous
compression should be shifted from facilitating factors such as a
lack of bioturbation49,50 and bed-capping carbonate cements18, to
those that drive mineralization and stabilization processes and are
directly responsible for fossil preservation.
Methods
Materials. The Conotubus hemiannulatus specimens examined in this study were
collected from the Gaojiashan Member of the Dengying Formation at Gaojiashan
in Ningqiang county, southern Shaanxi Province, South China28. Located in the
10

northwest margin of the Yangtze Platform (Supplementary Fig. 2A), the
Gaojiashan region has numerous exposures of Ediacaran–Cambrian strata with
broadly similar stratigraphy (from lower to upper): siliciclastic-dominated
Doushantuo Formation; dolostone-dominated Dengying Formation; and
phosphorite and carbonate of the basal Cambrian Kuanchuanpu Formation.
The Dengying Formation is tripartite (Supplementary Fig. 2B), consisting of
(from lower to upper): thick-bedded, vuggy, peritidal dolostones of the
B300-m-thick Algal Dolomite Member; thin-bedded calcareous siltstones,
mudstones and limestones of the B55-m-thick Gaojiashan Member that hosts
the Gaojiashan Lagerstätte; and thick-bedded, vuggy, microbially laminated-tostromatolitic, peritidal dolostone of the B200-m-thick Beiwan Member.
Correlation with the radiometrically dated Dengying Formation in the Yangtze
Gorges suggests that the Gaojiashan Lagerstätte is constrained between 551 and
541 Myr ago51,52.
Gaojiashan fossils mostly occur within millimetric beds of fine-grained
calcisiltite-siltstone of the Gaojiashan Member, interpreted to have been deposited
below storm wave base. Biostratinomic analysis of the Gaojiashan biota indicates
that rapid event deposition played a key role in the exceptional preservation of
Gaojiashan fossils11. The Gaojiashan biota is dominated by weakly biomineralized
to non-biomineralized tubular and ribbon-like fossils (for example, Cloudina,
Conotubus, Gaojiashania, Shaanxilithes and Sinotubulites), and also includes
protolagenid microfossils, algal debris, ichnofossils and microbial mat
textures11,28,53–59. The focus of this study, Conotubus hemiannulatus, is currently
only known from the Gaojiashan Lagerstätte. Showing similarities in construction
(Fig. 1d) and epibenthic life-mode, Conotubus has been interpreted as a possible
evolutionary precursor of Cloudina28,55. Conotubus differs from Cloudina in its
tube composition, as Conotubus was likely only weakly biomineralized at most28.
The fossiliferous unit occurs B18–49 m above the base of the Gaojiashan Member,
with the Conotubus–dominated biofacies within the 26–39 m range corresponding
to positive d13C and negative d18O excursions60. Rare earth elemental
geochemistry60 suggests that the Gaojiashan Member was deposited in a restricted
shallow sea environment, with riverine influx, common storm deposition
influence11, and high bioproductivity.
The samples studied herein were collected from calcareous siltstones 28–45 m
above the base of the Gaojiashan Member (Supplementary Fig. 2B). Three pyritized
Conotubus hemiannulatus specimens, collected 28–29 m above the base of the
Gaojiashan Member (%1 in Supplementary Fig. 2B), were trimmed to sub-25-mm
pieces, vacuum embedded in low-viscosity epoxy, and then ground and polished to
make two longitudinal cross-sections and one transverse cross-section. These three
specimens (reposited at the University of Missouri, Columbia; specimen numbers:
GJS-Cono001 to GJS-Cono003) were imaged and compositionally analysed using
scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDS).
Sulfur isotopic compositions of pyrite (d34SPY, reported as %-VCDT) were
measured in situ using SIMS and on microdrilled powders using IRMS. A fourth
specimen, collected at 37–38 m above the base of the Gaojiashan Member (%3 in
Supplementary Fig. 2B), on a larger slab was polished to make a transverse
cross-section. This specimen (reposited at the Virginia Tech; specimen number:
1GH2-70A) was analysed using SEM/EDS, ultraviolet photoluminescence
microscopy and in situ X-ray diffraction (XRD). Both specimen number 1GH-70A
and a non-fossiliferous slab, collected at 44–45 m above the base of the Gaojiashan
Member (%4 in Supplementary Fig. 2B), were microdrilled for IRMS d13C and
d18O analyses (see Fig. 6a,b for drilling maps). An additional slab, collected 32 m
above the base of the Gaojiashan Member (%2 in Supplementary Fig. 2B), was
analysed via IRMS for d34SCAS (data reported in Supplementary Table 1).
Scanning electron microscopy. All SEM and EDS analyses (Fig. 2a–c,g; and
Fig, 3a,c; Fig. 4a,b,e–g; Fig. 5b,e,f) were conducted using an FEI company Quanta
600 field-emission variable-pressure SEM with an integrated Bruker AXS Quantax
400 high-speed silicon-drift EDS detector (Virginia Tech Institute for Critical
Technology and Applied Science Nanoscale Characterization and Fabrication
Laboratory). The specimens were analysed in high-vacuum mode (B6  10  6
Torr), B10 mm working distance, 20 keV beam accelerating voltage, 5.0 spot size
(approximation of beam diameter and specimen current) and a system take-off
angle of 35° for EDS analyses. EDS point scans were conducted for 100 s live-time,
and elemental maps were collected for 600 s live-time.
Secondary ion mass spectroscopy. All SIMS analyses followed d34SPY methods
described in ref. 32, using a Cameca 7f GEO magnetic sector system (Virginia Tech
Institute for Critical Technology and Applied Science Nanoscale Characterization
and Fabrication Laboratory). A Cs þ primary beam with 1 nA current at an energy
of 20 kV was used to sputter the specimen, and the 32S and 34S isotopes were
detected using dual Faraday cup detectors from a B10 mm analytical spot. Mass
resolution of B2,000 m per Dm was used to resolve the 32S from 16O2 mass
interference. A total of 214 spots across three specimens were analysed
(Supplementary Table 1; Figs 2a,d–f; Fig. 3a,b; Fig. 4b–d), and measured d34SPY
values are reported as % deviation from VCDT (Vienna Cañon Diablo Troilite;
34S/32S ¼ 0.0450045 ref. 61) calibrated via a Balmat pyrite standard
(d34SBalmat ¼ 15.1%-VCDT; 34S/32S ¼ 0.04568407 refs 62,63). Each SIMS spot
measurement consisted of 10 cycles of 34S/32S ratios acquired from the same
physical spot after B2-min presputter to remove potential surface contamination.

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6754

The duration of individual spot analyses was B5 min, not including presputter.
One-sigma errors of sample d34SPY measurements were calculated from the
instrumental standard error report of the 10 cycles, converted to %-VCDT.

Ultraviolet photoluminescence and X-ray diffraction. Photoluminescence
microscopy and in situ XRD analyses were conducted on a UV microscope
(Olympus BX51) and a Rigaku Rapid II X-ray diffraction system with a 2D image
plate, in the S.W. Bailey X-ray Diffraction Laboratory, Department of Geoscience,
University of Wisconsin. The polished cross-sectional slab of Conotubus, specimen
number 1GH2-70A, was used in the analyses. All diffraction patterns were collected using reflection mode. Single-crystal diffraction patterns were also obtained
to understand the crystallographic relationship between the dolomite and calcite in
the zoned crystals (Fig. 5c,d,g).

Isotope ratio mass spectroscopy. Carbon and oxygen IRMS analyses were
carried out at the University of Maryland and the Northwest University in Xi’an.
Approximately 100 mg microdrilled powder was allowed to react for 10 min at
90 °C with anhydrous H3PO4 in a Multiprep inlet system connected to an
Elementar Isoprime dual inlet mass spectrometer for d13C and d18O analysis.
Isotopic results are expressed in the standard d notation as % deviation from
VPDB. Uncertainties determined by multiple measurements of NBS-19 were better
than 0.05% (1s) for both C and O isotopes.
Carbonate-associated sulfate sulfur IRMS analysis was carried out at Indiana
University. After trimming to remove visible veins and weathering rinds and
ultrasonically cleaning in distilled deionized water, 56.3 g of whole rock was
crushed and pulverized into powder of o200 mesh size (74 mm) using a splitdiscus mill. Sulfate extraction followed the recommended protocol of ref. 64. Ultrapure Milli-Q water (18 MO), purified NaCl, and distilled HCl were used to create
solutions for leaching and acid digestion of the sample. The powder was immersed
in 10% NaCl solution under constant magnetic stirring at room temperature until
no sulfate was present in filtrate solutions. After water-leaching steps, the carbonate
sample was dissolved in 10% HCl solution. The slurry was decanted and vacuum
filtered through a 0.45-mm cellulose membrane filter. The acid-leached sulfate was
collected as BaSO4 by adding saturated BaCl2 solution. All collected BaSO4 samples
were further purified by the DDARP method of ref. 65. BaSO4 was converted to
SO2 using an Elemental Analyzer at 990 °C and the d34SCAS measurement was
conducted on a Thermo-Electron Delta V Advantage mass spectrometer in
continuous-flow mode.
In addition, pyrite from samples GJS-Cono001 to GJS-Cono003 was
microdrilled and analysed for d34SPY (s.d. ¼ ±0.3%) to compare with SIMS
results. Pyrite extraction followed the chromium reduction method of ref. 66.
Sulfur isotopic results are expressed in the standard d notation as % deviation
from VCDT.

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Acknowledgements
This research was supported by funding through NASA Exobiology and Evolutionary
Biology Program, NASA Astrobiology Institute (N07-5489), National Science Foundation (EAR-0824890, EAR095800, EAR1124062), Chinese Academy of Sciences, National
Natural Science Foundation of China (41202006; 41030209; 41272011), Chinese Ministry
of Science and Technology, Virginia Tech Institute for Critical Technology and Applied
Sciences and China Postdoctoral Science Foundation (2013M531410). We would like to
thank K.L. Shelton and J.W. Huntley for insightful discussion.

Author contributions
J.D.S. designed the research with input from S.X. and Y.C. S.X. supervised the research.
J.D.S, S.X., Y.C. and H.H. performed the fieldwork. Sample preparation was performed
by J.D.S. and Y.C., SEM and EDS analysis was performed by J.D.S., SIMS analysis was
performed by J.D.S. and J.H., ultraviolet and XRD analysis was performed by H.X., IRMS
analysis performed by Y.C., Y.P. and A.J.K. and geochemical data analysis was performed
by J.D.S., S.X. and A.F.W. J.D.S., with significant input from all of the authors, wrote the
paper.

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How to cite this article: Schiffbauer, J. D. et al. A unifying model for Neoproterozoic–
Palaeozoic exceptional fossil preservation through pyritization and carbonaceous
compression. Nat. Commun. 5:5754 doi: 10.1038/ncomms6754 (2014).

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