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Sufficient oxygen for animal respiration 1,400 million
years ago
Shuichang Zhang
a,1
, Xiaomei Wang
a
, Huajian Wang
a
, Christian J. Bjerrum
b,c
, Emma U. Hammarlund
d,e
,
M. Mafalda Costa
f,g
, James N. Connelly
f,g
, Baomin Zhang
a
, Jin Su
a
, and Donald E. Canfield
d,e,1
a
Key Laboratory of Petroleum Geochemistry, Research Institute of Petroleum Exploration and Development, China National Petroleum Corporation, Beijing
100083, China;
b
Department of Geosciences and Natural Resource Management, Section of Geology, University of Copenhagen, DK-1350 Copenhagen,
Denmark;
c
The Nordic Center for Earth Evolution at the Department of Geosciences and Natural Resource Management, Section of Geology, University of
Copenhagen, DK-1350 Copenhagen, Denmark;
d
Institute of Biology, University of Southern Denmark, DK-5230 Odense, Denmark;
e
The Nordic Center for
Earth Evolution at the Institute of Biology, University of Southern Denmark, DK-5230 Odense, Denmark;
f
Centre for Star and Planet Formation, University of
Copenhagen, DK-1350 Copenhagen, Denmark; and
g
Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen, Denmark
Contributed by Donald E. Canfield, November 27, 2015 (sent for review November 2, 2015; reviewed by Lee Kump and Jennifer Morford)
The Mesoproterozoic Eon [1,600–1,000 million years ago (Ma)] is
emerging as a key interval in Earth history, with a unique geo-
chemical history that might have influenced the course of biological
evolution on Earth. Indeed, although this time interval is rather
poorly understood, recent chromium isotope results suggest that
atmospheric oxygen levels were <0.1% of present levels, suffi-
ciently low to have inhibited the evolution ofanimal life. In contrast,
using a different approach, we explore the distribution and enrich-
ments of redox-sensitive trace metals in the 1,400 Ma sediments of
Unit 3 of the Xiamaling Formation, North China Block. Patterns of
trace metal enrichments reveal oxygenated bottom waters during
deposition of the sediments, and biomarker results demonstrate the
presence of green sulfur bacteria in the water column. Thus, we
document an ancient oxygen minimum zone. We develop a simple,
yet comprehensive, model of marine carbon−oxygen cycle dynam-
ics to show that our geochemical results are consistent with atmo-
spheric oxygen levels >4% of present-day levels. Therefore, in
contrast to previous suggestions, we show that there was sufficient
oxygen to fuel animal respiration long before the evolution of
animals themselves.
atmosphere
|
Mesoproterozoic
|
oxygen minimum zone
|
trace metals
|
biomarkers
Some aspects of the history of atmospheric oxygen on Earth
are well understood. For example, before about 2,300 million
years ago (Ma), atmospheric oxygen was likely less than 0.001%
of present atmospheric levels (PAL) (1, 2), whereas, after about
550 Ma, levels have been greater than about 20% PAL (3–5),
sufficient to sustain large-animal respiration. The intervening
history, however, has been both poorly studied and poorly con-
strained. This history is of critical importance as it allows one to
establish possible links between changing oxygen levels and an-
imal evolution, where molecular clock estimates showing an
evolution of crown-group metazoans (including the ancestors of
modern Porifera and Placozoa) sometime during the Cryogenian
Period (720–635 Ma) (6, 7). Indeed, there is a long-standing
suggestion that rising atmospheric oxygen concentrations in the
late Neoproterozoic Eon (1,000–542 Ma) (8–11) enabled animal
respiration, thus explaining the timing of animal evolution.
The oxygen levels required for early animal respiration were
lower than those needed to sustain large motile animals and
were probably ≤1% PAL (10, 12, 13). Recent chromium isotope
results suggest oxygen levels of <0.1% PAL through the Meso-
proterozoic Eon (1,600–1,000 Ma) and until about 700 Ma, when
rising levels then spurred animal evolution (11). In contrast, we
present evidence that oxygen was ≥3.8% PAL at 1,390 Ma,
sufficient to fuel early animal respiration.
Study Site and Sample Collection
The Xiamaling Formation (see Fig. S1) was deposited below
storm wave base in a tropical setting between 10°N and 30°N
(14, 15). Pre-Xiamaling sediments were deposited on a passive
margin, but occasional ash layers in the Xiamaling Formation have
led to suggestions of a back-arc setting (15). Sediments of the
Xiamaling Formation, however, are highly laminated with no
evidence for mass flows or turbidites, and volcanoclastics are
rare. Therefore, deposition in a tectonically quiet environment
is indicated, consistent with continued deposition on a passive
margin. There is also no evidence for storm wave interaction
with the sediment, so a water depth of >100 m is likely. The
sediment package was never heated to above 90 °C, thus pre-
serving organic biomarkers (16). High-precision zircon data yield
an age of 1,384.4 ±1.4 Ma for a tuff layer 210 m below the top of
the formation and 1,392.2 ±1.0 Ma for a bentonite layer 52 m
below the tuff layer (16).
As one of our objectives was to study biomarker distribu-
tions within the Xiamaling sediments, most of our data come
from cores obtained with fresh water as drilling fluid. In some
cases, we compared our inorganic geochemical results to
results obtained from outcrop samples, where the outer
weathered layer was first removed. Details of our sediment
sampling procedures and analytical methods are described in
Supporting Information.
Significance
How have environmental constraints influenced the timing of
animal evolution? It is often argued that oxygen first increased to
sufficient levels for animal respiration during the Neoproterozoic
Eon, 1,000 million to 542 million years ago, thus explaining the
timing of animal evolution. We report geochemical evidence for
deep-water oxygenation below an ancient oxygen minimum
zone 1,400 million years ago. Oceanographic modeling constrains
atmospheric oxygen to a minimum of ∼4% of today’s val-
ues, sufficient oxygen to have fueled early-evolved animal
clades. Therefore, we suggest that there was sufficient atmo-
spheric oxygen for animals long before the evolution of ani-
mals themselves, and that rising levels of Neoproterozoic
oxygen did not contribute to the relatively late appearance of
animal life on Earth.
Author contributions: S.Z., X.W., H.W., E.U.H., J.N.C., and D.E.C. designed research; S.Z.,
X.W., H.W., C.J.B., E.U.H., M.M.C., B.Z., J.S., and D.E.C. performed research; S.Z., X.W.,
H.W., C.J.B., M.M.C., J.N.C., and D.E.C. analyzed data; C.J.B. and D.E.C. developed and per-
formed O
2
model calculation; and S.Z.,X.W., C.J.B., E.U.H., J.N.C., and D.E.C. wrote thepaper.
Reviewers: L.K., Pennsylvania State University; and J.M., Franklin and Marshall.
The authors declare no conflict of interest.
Freely available online through the PNAS open access option.
1
To whom correspondence may be addressed. Email: sczhang@petrochina.com.cn or dec@
biology.sdu.dk.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1523449113/-/DCSupplemental.
www.pnas.org/cgi/doi/10.1073/pnas.1523449113 PNAS Early Edition
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EARTH, ATMOSPHERIC,
AND PLANETARY SCIENCES
Evidence for Bottom Water Oxygenation
In a zone from 260 to 300 m depth, shales with high total organic
carbon (TOC) alternate with low-TOC cherts (Fig. 1 and Fig. S2).
These sediments are distinct from the low-TOC sediments de-
posited below this interval and the intermediate-TOC sediments
deposited above. The high-TOC zone is best expressed in out-
crop samples, due to the low sample recovery of shales during
coring (Fig. 1). The alteration between the high-TOC shales and
low-TOC cherts likely represents orbitally forced changes in
trade wind intensity, as this influenced the strength of coastal
upwelling (16).
The high-TOC shales within the high-TOC zone, and partic-
ularly in the depth range of 270–295 m, are enriched in the redox-
sensitive elements molybdenum (Mo) and uranium (U) (Fig. 1
and Fig. S2) but are either depleted or unenriched in vanadium
(V). The gray shales in this interval show small to negligible
enrichments in Mo and U, but, like the black shales, many also
show depletion in V. Our focus, however, will be on the geo-
chemical environment surrounding black shale deposition. In
modern environments, V is commonly released from sediments
depositing under low-oxygen (but still oxygenated) conditions
(17, 18) and under normal bottom water oxygen levels where
oxygen only penetrates a few millimeters into the sediment.
Vanadate is transported to sediments as the vanadate ion
[H
2
V(VI)O
4
−
] adsorbed onto Mn oxides. The vanadate ion is re-
leased as the Mn oxides are reduced to Mn
2+
(18), and, where
oxygen is limiting, Mn oxides do not readily reform at the
sediment surface (18–20), allowing vanadate to escape to the
overlying water.
Vanadate is also released into anoxic waters following Mn
oxide reduction, but in the absence of oxygen, the vanadate is
reduced to the vanadyl form [V(IV)O
2+
] (18). This form is highly
surface-reactive and removed to the sediment together with
vanadyl ions formed from vanadate ions transported across the
chemocline into the anoxic waters. Thus, in oxygen minimum
zones (OMZs) where anoxic waters overlay sediments, and in
euxinic basins, V is enriched together with Mo and U (17, 21–
23). In summary, coenrichments of V, Mo, and U occur under
anoxic water column conditions, whereas V release from sedi-
ments only occurs under bottom water oxygenation (see Sup-
porting Information for further discussion).
Thus, trace metal patterns demonstrate bottom water oxy-
genation during deposition of unit 3 of the Xiamaling Forma-
tion. Our results complement a previous study from the 1,410 Ma
Kaltasy Formation, Volgo-Ural region, Russia, where Fe speci-
ation and trace metal abundances (these sediments also indicate
V release relative to crustal average values) indicate bottom
water oxygenation in sediments deposited deeper than storm
wave base (likely >150 m depth) (24).
Evidence for an Oxygen Minimum Zone Setting
The chemical environment of the Xiamaling Formation is fur-
ther constrained by exploring the abundance of 2,3,6-trimethyl
aryl isoprenoids (2,3,6-TMAI). These biomarkers are breakdown
products of isorenieratane, whose precursors are isorenieratene
and β-isorenieratene, which are themselves carotenoid pigments
associated with “brown”strains of green sulfur bacteria (GSB)
(Chlorobiaceae) (25). These organisms are obligate anaerobic
phototrophs that frequent modern and ancient sulfidic (and likely
also ferruginous) water columns, using the oxidation of sulfide and
ferrous iron to gain energy for building cell biomass (26–30). El-
evated abundances of 2,3,6-TMAI’s(Fig.1andFigs. S3 and S4)
suggest that phototrophic low-light-adapted GSB populations oc-
cupied the Xiamaling water column during much of unit 3 de-
position. Thus, biomarker evidence, combined with trace metal
dynamics, suggest that unit 3 of the Xiamaling Formation de-
posited in a true OMZ setting with deep oxygenated water
overlain by anoxic water with either H
2
SorFe
2+
in the photic zone.
There is some evidence, however, that β-isorenieratane can
form from the late diagenetic aromatization of partially hydro-
genated β-carotene (31), with the β-carotene sourced from algae
Unit
4
2
3
TOC (wt %)
0 5 10 15 20
Outcrop
Core
320
300
280
260
240
220
(m)
Lithology
Si
i
S
i
Si
Si
i
Si
Si
Si
S
i
Si
S
i
Si
S
i
Si
S
i
Si
S
i
Si
Si
Si
Si
Si
Si
Si
Si
Si Si
Si
S
S
i
Si
S
i
S
S
S
S
S
S
060
40
20
V/Al x10
-4
Mo/Al x10
-4
0123
U/Al x10
-4
0.0 0.5 1.0 1.5
C
18
-TMAI
C
19
-TMAI
TMAI (μg/g TOC)
02468
Black shale
Red sandy mudstoneGreen sandy mudstone Brown marlstone
Green chert
Si
Dark grey chert
Si
Grey silty mudstone
etinotneB-KenotsdumydnaskraD
Black silty mudstone
S
S
S
1392.2 ± 1.0 Ma
-35 -33 -31 -29 -27 -25
δ
13
C
kerogen (‰, VPDB)
Fig. 1. Geochemistry, biomarker, and δ
13
C of kerogen for the Xiamaling Formation from the top of unit 4 to the bottom of unit 2. TOC, from both the core
and outcrop samples are presented. Data include both black shales and other sediment types like cherts. Vertical dotted lines represent concentration ratios
relative to crustal average (see Supporting Information and Tables S1 and S2).
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www.pnas.org/cgi/doi/10.1073/pnas.1523449113 Zhang et al.
or cyanobacteria. The source of β-isorenieratane, and ulti-
mately the 2,3,6-TMAIs, can, in principle, be tested through the
δ
13
C of the 2,3,6-TMAI compounds as GSB produce relatively
13
C-enriched biomass through the reductive citric acid cycle in
carbon fixation (see review in ref. 32). Unfortunately, we were
unable to measure the isotopic compositions of the 2,3,6-TMAIs,
but the δ
13
C of insoluble organic matter (kerogen) tends to heavier
values in sync with peak abundances of C
18
-TMAIorC
19
-TMAI
(Fig. 1). This observation is consistent with the addition of GSB
biomass to the carbon pool. Therefore, we are confident that our
C
18
-TMAI and C
19
-TMAI biomarkers represent the presence of
GSB in the ancient Xiamaling Formation water column. We note,
however, that although illuminating the geochemical environment,
the recognition of an OMZ setting is not critical to constraining
atmospheric oxygen concentrations as described immediately
below. The most critical point is the recognition that Xiamaling
Formation sediments deposited in oxygenated deep waters as
constrained from trace metal distributions as described above.
Constraining Atmospheric Oxygen Levels
The presence of oxygenated bottom waters during Xiamaling
Formation deposition allows constraints on levels of atmospheric
oxygen. The water supplied to OMZs originates as oxygen-
saturated surface waters that are mixed during winter months into
the thermocline in extratropical latitudes (33). This water loses
oxygen to respiration as it flows along isopycnal surfaces to the
OMZ (Fig. 2). There is sufficient respiration to consume all of
the oxygen flowing to the anoxic portion of an OMZ, but in-
sufficient respiration to consume the oxygen from water flowing
to the deeper depths. Our goal is to determine the minimum
amounts of atmospheric oxygen required to allow oxygenated
waters to persist in these deeper waters. Knowing this, we derive
a lower limit for atmospheric oxygen levels, assuming that the
upper mixed layer of the ocean was in oxygen equilibrium with
the atmosphere.
The oxygen loss to respiration is obtained by combining the
transit time of water from its place of ventilation and the rate of
oxygen respiration in the water. The transit time of water to the
OMZ is approximated by calculating the so-called “water age,”
which is assessed as the volume of water confined within adjacent
layers of constant density (isopycnal surfaces) ratioed by the flux
of water into this volume (34). The shortest water ages give the
lowest estimates for atmospheric oxygen (see below), and these
are found in the South Atlantic, with ages of 5–6 y in the upper
∼100 m of the water column, increasing to about 25 y by 400 m
water depth (34). In contrast, water ages in the North Pacific
range from 7−20 y in the upper 100 m to well over 100 y by 400 m
depth (34). Water ages for restricted and semirestricted marine
water bodies generally fall within these ranges (see Supporting
Information for summary of water ages).
The rate of respiration of marine organic matter decreases as a
function of water depth (34), and the rate at a given depth de-
pends on the export flux of organic matter from the upper water
column (also called new production), the settling rate of the
organic matter, and its degradability. In our modeling, we ex-
plored export production values ranging from 10% to 200% of
present-day Equatorial Atlantic average values (Supporting In-
formation). Of this range, we view 20–150% as a good estimate
for the Xiamaling Formation, recognizing that during the Mes-
oproterozoic Eon, marine carbon isotope values suggest a carbon
cycle operating at rates broadly similar to today (35).
We also explored in our modeling a wide range of particle
settling rates. We highlight that the settling rates of small, pre-
dominantly prokaryote-derived, organic particles 1.39 Ga were
likely much lower than today (36) where dense, eukaryote-derived,
algal tests and animal-derived fecal material dominate the settling
flux (37). Settling velocities of between 1 m·d
−1
and 6 m·d
−1
en-
compass the range predicted for micrometer-sized planktonic
cells, and their aggregates, through a planktonic bloom (38). We
focus on this range of settling velocities in interpreting our results.
Organic matter degradability in the modern ocean is assessed
from the attenuation of particle-settling fluxes with water depth
as revealed through sediment trap experiments. Recent studies,
accounting for particle capture efficiency, reveal that upper water
column particles degrade more rapidly in low latitudes with higher
temperatures than in high latitudes with lower temperatures (39).
We least-squares fit the low latitude, the high latitude, and the
combined trap data with a continuous reaction model, generating
parameters describing the aging of particles as they settle. From
these parameters, we calculate the decomposition rate of settling
organics with depth as a function of new production and particle
settling rate (see Supporting Information for details).
In our calculations, new production is equal to the export flux
of organic matter at the base of a nominal mixed-layer depth of
50 m, and organic matter is consumed as the particles sink. At a
unique water depth for each calculation, the carbon-settling flux
matches the burial flux of organic matter into Xiamaling For-
mation sediments as constrained from precise sediment chro-
nology and sediment TOC content, which varies from 10 wt% to
20 wt% in the black shales (Fig. 1). The rate of oxygen respira-
tion at this depth is then combined with the appropriate water
age for the depth to give the total amount of oxygen respired.
Atmospheric oxygen values are calculated from this amount of
respired oxygen assuming water saturation with the atmosphere at
the temperature corresponding to the diagnosed water depth, where
we use the average temperature−depth distribution as found in the
tropical to subtropical South Atlantic (see Supporting Information).
The Xiamaling Formation formed during a prolonged ice-free pe-
riod of Earth history, and ocean temperatures during this time may
have been higher than those in the modern ocean. Higher tem-
peratures ultimately yield higher estimates of atmospheric O
2
,as
explored below, so focusing on present-day temperatures provides a
conservative minimum estimate of atmospheric O
2
concentration.
Our estimates of atmospheric O
2
are also minimum values, as our
calculation of oxygen levels does not account for excess oxygen in
the water as required by the geochemical data.
An example of model results is shown in Fig. 3 A−C. These
results were generated with a Xiamaling Formation TOC con-
tent of 15 wt%, organic reactivity from the combined sediment
trap data, and water age and temperature depth distributions for
the South Atlantic (Supporting Information). Within the most
likely Mesoproterozoic ranges of new production and particle
settling rates (as shown in the outlined box in Fig. 3 A−C), and
for water depths of >100 m (dotted lines in Fig. 3 Aand B), as
is likely for the Xiamaling Formation depositional environment,
anoxic
oxic
Thermocline
σT1
σT2
oxic
anoxi
c
Fig. 2. Cartoon representation of our oxygen respiration model. The car-
toon shows the origin of OMZ water at the thermocline, and its transit to the
OMZ along layers of constant density σ
t
. The cartoon also shows how both
the flux of organic carbon and its respiration rate attenuate with depth in
the water column. Finally, the cartoon shows our convergence criteria where
depth is determined by matching the TOC flux through the water column
with the TOC burial flux in Xiamaling Formation sediments.
Zhang et al. PNAS Early Edition
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3of6
EARTH, ATMOSPHERIC,
AND PLANETARY SCIENCES
our calculations show oxygen consumption of between about
21 μM and 40 μM, translating into minimum atmospheric oxygen
levels of 8–15% PAL.
Variations in water age, organic matter reactivity, and tem-
perature will change these results, as shown in a series of sen-
sitivity analyses (Fig. 3 D−F). Here we document high and low
estimates of atmospheric oxygen with variations in water age−depth
distributions (Supporting Information), temperature (present day
and +10 °C), and organic matter reactivity as revealed in the high,
low, and combined sediment trap data. Higher water tempera-
tures, as might be expected during this apparently ice-free time
in Earth history, yield higher estimates for minimum atmo-
spheric oxygen levels, whereas lower water ages yield lower es-
timates. We view 10 y as a conservative minimum water age; this
is less than or equal to the water age found in the modern open
ocean at water depths of >100 m, and similar to or less than
those found in modern semienclosed basins (see Supporting
Information). Higher organic matter reactivity (Fig. 3F) produces
higher oxygen estimates, whereas lower reactivity (Fig. 3D)
produces lower estimates.
With a water age of 10 y, minimum oxygen estimates range
from 1.3% PAL (low reactivity, high latitude, Fig. 3D) to 6.2%
PAL (high reactivity, low latitude, Fig. 3F) with 3.8% PAL for
the combined data (Fig. 3E). As temperature is considered the
prime variable controlling the latitudinal distribution of organic
matter reactivities, and because the Xiamaling Formation de-
posited in a low-latitude setting, we favor the model results from
the combined or low-latitude reactivities. Therefore, our sensi-
tivity analysis reveals 3.8–6.2% PAL as a likely minimum atmo-
spheric oxygen level, whereas 1.3% PAL is an unlikely but possible
minimum estimate.
Our most likely minimum estimate of 3.8–6.2% PAL is very dif-
ferent from the <0.1% PAL oxygen suggested for this time from Cr
isotope results (11). This result arises, ultimately, from a general lack
B
A
C
010 20 30 40 50 60 70 80
0
10
20
30
40
50
60
70
80
90
100
()
2
010 20 30 40 50 60 70 80
0
10
20
30
40
50
60
70
80
90
100
τo (yr)
atmos. O2 (% PAL)
0 5 10 15
0
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200
100
80
60
00.1 0.2 0.3 0.4 0.5
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00.1 0.2 0.3 0.4 0.5
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1
1.2
1.4
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1.8
2
0
10
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60
70
80
D
010 20 30 40 50 60 70 80
0
10
20
30
40
50
60
70
80
90
100
τo (yr)
atmos. O2 (% PAL)
0 5 10 15
0
5
10
High latitude
F
E
0 5 10 15
0
5
10
15
20
Low latitude
Combined
New production (xPL)New production (xPL) New production (xPL)
Atmospheric O2 (%PAL)Atmospheric O2 (%PAL)Atmospheric O2 (%PAL)
Atmospheric O2 (%PAL)
Depositional water depth (m) AOU (µM)
Settling velocity (x PL) t0 (yr)
Fig. 3. (A) Calculated minimum oxygen utilization (AOU) for water masses circulating to oxygenated waters beneath the anoxic portion of an OMZ. This
calculation is calibrated to a Xiamaling Formation organic carbon burial flux calculated from 15 wt% TOC, with organic matter reactivity as calculated from
the mean of all sediment trap data, and a temperature−depth distribution as for the present tropical Atlantic Ocean. The black box outlines new production
rates ranging from 0.2 to 1.5 times present values (xPL), with the present value as observed in the equatorial Atlantic (Supporting Information), and particle
settling rates between 1 m·d
−1
and 6 m·d
−1
(they are presented as a fraction, xPL-present level, of the diagnosed particle settling rates from the sediment trap
data as described in Supporting Information). The dotted line indicates the solutions for the 100 m depth contour. (B) Values of AOU converted to percent
present atmospheric oxygen levels (%PAL) assuming water saturation with air at the temperature for the water depth diagnosed in the calculation. (C) Water
depths diagnosed by matching the particle settling flux with the burial flux of organic matter into Xiamaling Formation sediments. The stippled line rep-
resents water depths of less than 100 m, which we consider unlikely given the deep-water depositional setting of the Xiamaling Formation. (D) Sensitivity
analyses depicting the high and low oxygen estimates for a series of calculations with variable water residence times (τ
o
) assuming low organic matter re-
activity as found in modern low latitudes. Calculations with the present-day temperature−depth distributions are shown in blue shades, and calculations with
this temperature profile +10 °C are shown in brown. A large overlap is observed. The darkened area outlines the most likely range of water ages. (E)AsinD,
but assuming organic matter reactivity from the combined sediment trap data. (F)AsinD, but assuming high organic matter reactivity as found in modern
high latitudes. See Constraining Atmospheric Oxygen Levels and Figs. S5–S8 and Tables S3–S7 for details.
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of fractionated Cr (with δ
53
Cr values in the range of −0.25‰to
0‰, compared with crustal values estimated at −0.1‰to −0.2‰)
in pre-Neoproterozoic iron-enriched marine sediments. Fraction-
ations are imparted during the oxidative weathering of Cr(III)
minerals on land, and fractionated Cr is transported to the sea and
is believed to be captured in the iron-enriched sediments (40).
Therefore, no fractionation would indicate no oxidative weathering
on land and low levels of atmospheric oxygen. We note, however,
that other data sets report evidence for fractionated Cr in pre-
Neoproterozoic sediments, and, in particular, many banded
iron formations in the time window from 1,800 Ma to 3,000 Ma
have δ
53
Crvaluesrangingupto0.2–0.3‰(40, 41). The dis-
crepancy between these data sets has yet to be evaluated, but we
note that no standard procedures are used to correct for a detrital
Cr component. Therefore, we believe that the terrestrial and ma-
rine geochemical behavior of Cr is still poorly understood, and, in
particular, it is unclear how chromium isotope signals are trans-
ferred from the land to marine sediments and which sediments best
preserve such isotope signals.
Consequences of Elevated Mesoproterozoic Oxygen Levels
Atmospheric oxygen levels of 3.8–6.2% PAL (or even 1.3% PAL)
are sufficient to support the respiration of sponges, considered
good candidates for early evolved animals, whose oxygen re-
quirements are in the range of ≤1–4% PAL (12). Such levels are
also sufficient to support small motile animals such as annelid
worms, which may require even less oxygen (13). Thus, it appears
that sufficient oxygen existed to support animal metabolism long
before the evolution of stem-group animals themselves, which, as
noted above, from molecular phylogenetic evidence, occurred
around 780 Ma (13). Thus, our results support the idea that ox-
ygen itself did not limit the late emergence of animal life (1, 42).
ACKNOWLEDGMENTS. We thank Yu Wang, Caiyun Wei, Huitong Wang,
Dina Holmgaard Skov, Heidi Grøn Jensen, Susanne Møller, Jørgen Kystol,
and Anne Thoisen for technical support. We thank the Danish National Re-
search Foundation (Grant DNRF53), the ERC (Oxygen Grant 267233), Danish
Agency for Science, Technology and Innovation (Grant 12-125692), the Scien-
tific Research and Technological Development Project of China National Pe-
troleum Corporation (CNPC 2014A-0200 and CNPC 2014E-3209), and the State
Key Program of National Natural Science Foundation of China (41530317).
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