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A Cryogenian chronology: Two long-lasting synchronous Neoproterozoic glaciations

Authors:

Abstract

The snowball Earth hypothesis predicts globally synchronous glaciations that persisted on a multimillion year time scale. Geochronological tests of this hypothesis have been limited by a dearth of reliable age constraints bracketing these events on multiple cratons. Here we present four new Re-Os geochronology age constraints on Sturtian (717-660 Ma) and Marinoan (635 Ma termination) glacial deposits from three different paleocontinents. A 752.7 ± 5.5 Ma age from the base of the Callison Lake Formation in Yukon, Canada, confirms nonglacial sedimentation on the western margin of Laurentia between ca. 753 and 717 Ma. Coupled with a new 727.3 ± 4.9 Ma age directly below the glacigenic deposits of the Grand Conglomerate on the Congo craton (Africa), these data refute the notion of a global ca. 740 Ma Kaigas glaciation. A 659.0 ± 4.5 Ma age directly above the Maikhan-Uul diamictite in Mongolia confirms previous constraints on a long duration for the 717-660 Ma Sturtian glacial epoch and a relatively short nonglacial interlude. In addition, we provide the first direct radiometric age constraint for the termination of the Marinoan glaciation in Laurentia with an age of 632.3 ± 5.9 Ma from the basal Sheepbed Formation of northwest Canada, which is identical, within uncertainty, to U-Pb zircon ages from China, Australia, and Namibia. Together, these data unite Re-Os and U-Pb geochronological constraints and provide a refined temporal framework for Cryogenian Earth history.
GEOLOGY
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A Cryogenian chronology: Two long-lasting synchronous
Neoproterozoic glaciations
Alan D. Rooney1, Justin V. Strauss1, Alan D. Brandon2, and Francis A. Macdonald1
1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
2Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204, USA
ABSTRACT
The snowball Earth hypothesis predicts globally synchronous glaciations that persisted on
a multimillion year time scale. Geochronological tests of this hypothesis have been limited by
a dearth of reliable age constraints bracketing these events on multiple cratons. Here we pres-
ent four new Re-Os geochronology age constraints on Sturtian (717–660 Ma) and Marinoan
(635 Ma termination) glacial deposits from three different paleocontinents. A 752.7 ± 5.5 Ma
age from the base of the Callison Lake Formation in Yukon, Canada, confirms nonglacial
sedimentation on the western margin of Laurentia between ca. 753 and 717 Ma. Coupled
with a new 727.3 ± 4.9 Ma age directly below the glacigenic deposits of the Grand Conglomer-
ate on the Congo craton (Africa), these data refute the notion of a global ca. 740 Ma Kaigas
glaciation. A 659.0 ± 4.5 Ma age directly above the Maikhan-Uul diamictite in Mongolia con-
firms previous constraints on a long duration for the 717–660 Ma Sturtian glacial epoch and
a relatively short nonglacial interlude. In addition, we provide the first direct radiometric age
constraint for the termination of the Marinoan glaciation in Laurentia with an age of 632.3
± 5.9 Ma from the basal Sheepbed Formation of northwest Canada, which is identical, within
uncertainty, to U-Pb zircon ages from China, Australia, and Namibia. Together, these data
unite Re-Os and U-Pb geochronological constraints and provide a refined temporal frame-
work for Cryogenian Earth history.
INTRODUCTION
After more than one billion years without
robust evidence of glaciation, Cryogenian (ca.
850–635 Ma) strata record arguably the most
extreme episodes of climate change in Earth’s
history. The widespread occurrence of low-
latitude glacial deposits on every paleoconti-
nent, coupled with the unique geochemistry
and sedimentology of cap carbonates (Hoff-
man et al., 1998; Bao et al., 2008), inspired the
snowball Earth hypothesis (Kirschvink, 1992).
However, the general paucity of radiometric
age constraints from multiple paleocontinents
for the onset and demise of Cryogenian gla-
ciations (Sturtian ca. 717–660 Ma, and Mari-
noan ending ca. 635 Ma), as well as reports of
putative pre-Sturtian glaciations (Frimmel et
al., 1996), has fostered doubts about the syn-
chronicity and global extent of these events
(e.g., Allen and Etienne, 2008; Kendall et al.,
2006). In particular, an apparent disagreement
between various geochronological constraints
has fueled the idea Cryogenian glaciations
were not particularly unique or extreme events;
however, it remains unclear if these age dif-
ferences represent true geological mismatches
or the combination of analytical error and/
or poor cross-calibration between different
geochronometers. Here we present four new
Re-Os ages from strata that bound Cryogenian
glacial deposits in northwest Canada, Zambia,
and Mongolia. We then integrate these data
with preexisting age constraints from multiple
paleocontinents to produce an updated global
Cryogenian chronology.
GEOLOGICAL SETTING
Black carbonaceous shales were sampled at
four separate localities on three different Neo-
proterozoic paleocontinents (Fig. 1; Table DR1
in the GSA Data Repository1). The Callison
Lake Formation of the Mount Harper Group
is exposed in the Ogilvie Mountains of Yukon,
Canada, and is composed of an ~400-m-thick
succession of mixed carbonate and siliciclastic
strata deposited in an episodically restricted
marine basin (Strauss et al., 2014; Fig. 1). Cur-
rent age constraints on the Mount Harper Group
include an Re-Os age of 739.9 ± 6.1 Ma from
the uppermost Callison Lake Formation and a
U-Pb chemical abrasion–isotope dilution–ther-
mal ionization mass spectrometry age on zircon
of 717.4 ± 0.1 Ma from the overlying Mount
Harper Volcanics (Macdonald et al., 2010a;
Strauss et al., 2014). To further refine the geo-
logical history of this succession, a black shale
horizon was sampled from the lower Callison
Lake Formation for Re-Os geochronology (Fig.
1; Fig. DR1 in the Data Repository).
The Katanga Supergroup of the Congo cra-
ton has been subdivided into the Roan, Nguba,
and Kundelungu Groups and comprises a mixed
carbonate and siliciclastic succession with two
diamictite horizons (Fig. 1; Wendorff, 2003;
Master et al., 2005). In the Chambishi area of
Zambia, the Mwashya subgroup of the Nguba
Group records subtidal deposition in a restricted
marginal marine setting and comprises an
~120-m-thick succession of black carbonaceous
shale with a gradational to locally disconformable
contact with the overlying glaciogenic deposits
of the Grand Conglomerate (Fig. 1; Selley et al.,
2005). Age constraints on the Katanga Super-
group are limited to a maximum age for the onset
of Roan Group sedimentation from a U-Pb sensi-
tive high-resolution ion microprobe (SHRIMP)
age of 883 ± 10 Ma (Armstrong et al., 2005) and
ages of ca. 760 Ma from various volcanics within
the Nguba Group (Key et al., 2001). Samples of
carbonaceous black shale were collected from
the Mwashya subgroup over a vertical interval of
6.49 m up to ~0.5 m below the Grand Conglom-
erate for Re-Os geochronology (MJCZ9 drill
core; Bodiselitsch et al., 2005).
The Tsagaan-Olom Group of the Zavkhan
terrane in southwest Mongolia consists of as
much as 2 km of carbonate-dominated strata that
host two glacial deposits, the Maikhan-Uul and
Khongor diamictites, which are considered to be
the Sturtian and Marinoan equivalents, respec-
tively (Fig. 1; Macdonald et al., 2009). Samples
for Re-Os geochronology were sampled at the
Taishir locality (Macdonald et al., 2009), 1.2 m
above the contact with the Maikhan-Uul diamic-
tite (Fig. 1) and within the Sturtian cap carbonate.
The Cryogenian–Ediacaran-age Hay Creek
and upper groups of the Windermere Super-
group are exposed in the Mackenzie Mountains,
northwest Canada, and host Marinoan-age gla-
cial deposits of the Stelfox Member of the Ice
Brook Formation (Fig. 1; Aitken, 1991; James et
al., 2001). A Marinoan age (ca. 635 Ma) for the
Stelfox Member is supported by carbon isotope
profiles and sedimentological characteristics of
the Ravensthroat and Hayhook cap carbonate.
Samples for Re-Os geochronology were col-
lected from the Sheepbed Formation near Shale
Lake (Aitken, 1991), 0.9 m above the contact
with the underlying Hayhook limestone. The
Sheepbed Formation consists of >700 m of
siliciclastics deposited in a proximal to distal
slope environment during a pronounced gla-
cioeustatic transgression (Fig. 1; Dalrymple and
Narbonne, 1996).
GEOCHRONOLOGY
Black shale from the lower part of the Cal-
lison Lake Formation of Yukon yields a Re-Os
1GSA Data Repository item 2015157, summary
of sampling techniques, detailed analytical meth-
ods, and data tables containing all isotopic and/or
geochronological data, is available online at www
.geosociety.org/pubs/ft2015.htm, or on request from
editing@geosociety.org or Documents Secretary,
GSA, P.O. Box 9140, Boulder, CO 80301, USA.
GEOLOGY, May 2015; v. 43; no. 5; p. 459–462; Data Repository item 2015157
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doi:10.1130/G36511.1
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Published online 27 March 2015
© 2015 Geological Society of America. For permission to copy, contact editing@geosociety.org.
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GEOLOGY
depositional age of 752.7 ± 5.5 Ma (all age
uncertainties also include the uncertainty in the
187Re decay constant, l, 2s, n = 5, mean square
of weighted deviates, MSWD, of 0.30) with an
initial 187Os/188Os (Osi) composition of 0.33 ±
0.03 (Fig. 2A). Regression of the Re-Os isotopic
composition data from the Mwashya subgroup
of Zambia yields a depositional age of 727.3
± 4.9 Ma (2s, n = 7, MSWD = 0.50) with an
unradiogenic Osi value of 0.35 ± 0.03 (Fig. 2B).
The basal Taishir Formation of Mongolia yields
a depositional Re-Os age of 659.0 ± 4.5 Ma
(2s, n = 6, MSWD = 0.67), with a moderately
radiogenic Osi value of 0.60 ± 0.01 (Fig. 2C).
Regression of the isotopic composition data
from samples of the Sheepbed Formation of
northwest Canada yields a Re-Os age of 632.3
± 5.9 Ma (2s, n = 5, MSWD = 0.58), with a
highly radiogenic Osi value of 1.21 ± 0.04 (Fig.
2D). These new Re-Os ages coupled with exist-
ing Re-Os and magmatic U-Pb age constraints
are combined to produce a refined geochrono-
logical framework for the Cryogenian as sum-
marized in Figure 3 (Table DR2).
DISCUSSION
The existence of a pre-Sturtian, global Kaigas
glaciation has been suggested from the apparent
relationship between inferred glacial deposits
and the following age constraints: a 741 ± 6 Ma
Pb-Pb zircon evaporation age in the Gariep belt
of the Kalahari craton (Frimmel et al., 1996); a
740 ± 7 Ma U-Pb SHRIMP age from near the
base of the Bayisi diamictite on the Tarim cra-
ton (Xu et al., 2009); and a 735 ± 5 Ma U-Pb
SHRIMP age from the Kundelungu Basin of
the Congo craton (Key et al., 2001). However,
previously published U-Pb and Re-Os ages from
time-equivalent strata in Laurentia (Karlstrom et
al., 2000; Macdonald et al., 2010a; Strauss et al.,
2014) and the Re-Os age presented here from the
Callison Lake Formation (Fig. 2A) document
nonglacial sedimentation from ca. 753 to 717
Ma on the western margin of Laurentia, argu-
ing against low-latitude glaciation during this
interval. Macdonald et al. (2010b) observed that
the 741 ± 6 Ma Pb-Pb evaporation age from the
Gariep belt (Frimmel et al., 1996) was sampled
from volcanic rocks that are not in direct contact
with glacial deposits and that conglomerate of
the Kaigas Formation was previously miscor-
related with glacigenic strata of the Numees
Laurentia
Congo
4
3
2
1
Rodinia at
ca. 715 Ma
2. Mackenzie Mtns., Canada 3. Ogilvie Mtns., Canada
811.5±0.1 Ma
732.2±4.7 Ma
662.4±4.3 Ma
777.7±2.5 Ma
716.5±0.2 Ma
717.4±0.1 Ma
Fifteenmile
Gp.
Reefal A.
Craggy
Dolostone
Callison
Lake
Fm.
Seela
Pass
Mt. Harper
Volcanics
Rapitan
Gp.
Hay Creek
Gp.
Undi.
Mount
Harper
Gp.
Hay Creek
Gp.
Rapitan
Gp.
Shezal
Sayeuni
Cu-cap
Redstone
River
Thund.
Ram
Head
T.S.
S.S.
Gayna
Coates Lk.
Gp.
Little Dal
Mt. Berg
Keele
Twitya
Gp.
SHPB
Cryogenian Ed.
Ton.
739.9±6.5 Ma
632.3±5.9 Ma
752.7±5.5 Ma
200 m
1. Kipushi Basin, Zambia
Nguba Gp.
U.R.
Gp.
Grand
Conglom.
Mwashya
Fm.
Bancroft
Fm.
Kakontwe
Fm.
Petit
Conglom.
Calcaire
Rose
Kunde-
lungu
Gp.
200 m
Maikhan-Uul Fm. Taishir Fm.
4. Zavkhan Basin, Mongolia
Ol
200 m
727.3 ± 4.9 Ma
200 m
Sturtian
Marinoan
Tsagaan Olom
Gp.
Siberia
Calcisiltite
Stratied diamictite
Evaporite
Limestone/Dolostone
Shale
Siltstone
Sandstone
Conglomerate
Unconformity
Vase-shaped microfossil
U/Pb zircon age
Re/Os ORR age
Re/Os ORR age this paper
Sample location
Massive diamictite
Basalt/Rhyolite/Diabase
Iron Formation
635 Ma cap carbonate
#
A
B
659.0±4.5 Ma
187Os/188Os
4.0
3.6
3.2
2.8
2.4
2.0
1.6
1.2
187Os/188Os
8.0
6.0
4.0
2.0
0
SpB-6B
SpB-6A
SpB-6C
SpB-6-8
187Os/188Os
187Re/188Os
Age = 632.3 ± 5.9 Ma
Osi = 1.21 ± 0.04
MSWD = 0.58
7.0
5.0
3.0
1.0
0 200 400 600
SpB-6-9
D
A1309-C
187Re/188Os
Age = 659.0 ± 4.5 Ma
Osi = 0.60 ± 0.01
MSWD = 0.67
60 100 140 180 220 260 300
A1309-A
A1309-B
A1309-E
A1309-D
A1309-I
C
187Os/188Os
187Re/188Os
100 200 300 400 500 600
7.5
6.5
5.5
4.5
3.5
2.5
1.5
Age = 752.7 ± 5.5 Ma
Osi = 0.33 ± 0.03
MSWD = 0.50
J1301-62.5-7
J1301-62.5-4
J1301-62.5-6
J1301-62.5-2
J1301-62.5-1
A
0 600400200
Age = 727.3 ± 4.9 Ma
Osi = 0.35 ± 0.03
MSWD = 0.50
MJ-31
MJ-34
MJ-33
MJ-36
MJ-32
MJ-35
MJ-37
B
187Re/188Os
Figure 1. A: Schematic stratigraphy of sample locations 1–4, adapted from Bodiselitsch et
al. (2005), Macdonald et al. (2009, 2010a), and Strauss et al. (2014). Geochronological con-
straints are from Jefferson and Parrish (1989), Macdonald et al. (2010a), Rooney et al. (2014),
and Strauss et al. (2014). Ton.—Tonian; Ed.—Ediacaran; Gp.—Group; Fm.—Formation;
Conglom.—Conglomerate; U.R.—Upper Roan; SHPB—Sheepbed Formation; Lk—lake; Cu-
cap.—Coppercap Formation; Thund.—Thundercloud Formation; T.S.—Ten Stone Formation;
S.S.—Snail Spring Formation; A.—assemblage; Mt.—Mount; Mtns.—mountains; #—includes
the Ravensthroat and Hayhook formations. B: Paleogeographic reconstruction of Rodinia at
ca. 715 Ma, modified from Li et al. (2013). ORR—organic-rich rock.
Figure 2. Re-Os isochron diagrams. A:
Lower Callison Lake Formation. B: Mwashya
subgroup (MJ-31–MJ-37, 172.61–179.10 m).
C: Taishir Formation. D: Sheepbed Forma-
tion. All data point error ellipses are 2s and
their diameters are larger than calculated
error ellipses. All isotopic composition and
elemental abundance data are presented in
Table DR1 (see footnote 1). MSWD—mean
square of weight deviates.
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Formation. Therefore, this age only provides
a maximum age constraint on the Sturtian-age
Numees Formation. Similarly, no definitive evi-
dence for glacial sedimentation has been dem-
onstrated in the Bayisi diamictite of the Tarim
block, and the 740 ± 7 Ma U-Pb age of Xu et
al. (2009) is potentially undermined by the fact
that the analysis includes many zircon grains
that are possibly inherited from underlying ca.
750 Ma volcanic rocks. Consequently, the 735 ±
5 Ma U-Pb SHRIMP age on the Luatama brec-
cia of the Congo craton (Key et al., 2001) has
remained a final holdout for the putative ca. 740
Ma Kaigas glaciation. However, this U-Pb age is
compromised not only by the inclusion of grains
that were possibly inherited from the underly-
ing ca. 765 Ma mafic volcanic rocks of the low-
ermost Mwashya subgroup (Key et al., 2001),
but also by the poorly constrained nature of the
contact between the Luatama breccia and the
glacial deposits (Selley et al., 2005). Our new
727.3 ± 4.9 Ma Re-Os age from directly below
the Grand Conglomerate is consistent with a
Sturtian age for the oldest Cryogenian glacial
deposits on the Congo craton (Figs. 2B and 3).
The Re-Os age of 752.7 ± 5.5 Ma for the basal
Callison Lake Formation provides a new deposi-
tional age constraint for the lower portion of this
unit and expands the range of vase-shaped micro-
fossil occurrences in northwest Canada (Strauss
et al., 2014). It is interesting that the Osi value
from this basin at the time of deposition was
markedly unradiogenic (Osi = 0.33), in contrast
to the Osi value presented for the upper Callison
Lake Formation at 739.9 ± 6.1 Ma (Osi = 0.60;
Strauss et al., 2014) and modern-day seawater
(Osi = 1.06; Peucker-Ehrenbrink and Ravizza,
2012). This Osi value suggests that the domi-
nant source of Os entering this basin was either
derived from the localized weathering of juve-
nile crustal material (e.g., basalt) or sourced from
hydrothermal inputs. Similarly, the unradiogenic
Osi value of 0.35 for the Mwashya subgroup is
suggestive of a significant influx of unradio genic
Os into this basin. These pre-Sturtian Osi values,
together with earlier work from northwest Can-
ada (Rooney et al., 2014), lend further support
to the so-called fire and ice mechanism of snow-
ball Earth initiation, whereby the weathering of
voluminous unradiogenic basalts at low latitudes
would have enhanced CO2 drawdown, resulting
in a climate sensitive enough to enter a global
glaciation (Goddéris et al., 2003).
The Re-Os age of 659.0 ± 4.5 Ma for the
Taishir Formation constrains the termination
of the Sturtian-age Maikhan-Uul glaciation in
Mongolia. This age is identical, within uncer-
tainty, to Re-Os and U-Pb zircon age constraints
for post-Sturtian horizons from northwest
Canada, China, and Australia, and confirms a
long duration (>55 m.y.) for the Sturtian glacial
epoch (Fig. 3; Zhou et al., 2004; Kendall et al.,
2006; Macdonald et al., 2010a; Lan et al., 2014;
Rooney et al., 2014). The Osi value of 0.60 is
moderately unradiogenic, in contrast to modern-
day seawater, and is similar to values reported
near the base of the post-glacial Twitya Forma-
tion in northwest Canada (0.54 versus 0.60).
An existing Re-Os date of 607.8 ± 4.7 Ma
from shale of the Windermere Supergroup,
Canada, was suggested to represent a termina-
tion age for the Marinoan glaciation (Kendall et
al., 2004). However, this Re-Os age is from an
isolated outcrop that is not in direct contact with
the underlying Marinoan-age glacial deposits or
cap carbonate (Kendall et al., 2004). Our new
Re-Os age of 632.3 ± 5.9 Ma from 0.9 m above
the distinctive Ravensthroat-Hayhook cap car-
bonate is within uncertainty of multiple ca. 635
Ma U-Pb zircon ages on the deglaciation of
the Marinoan snowball Earth event worldwide
(Figs. 2D and 3; Hoffmann et al., 2004; Condon
et al., 2005; Calver et al., 2013.
CONCLUSION
The four Re-Os ages presented herein help
refine our current Neoproterozoic chronology.
These data refute the evidence for an earlier
global Kaigas glaciation and suggest instead
that the initiation of the Sturtian glacial epoch
ca. 717 Ma marked the first unambiguous gla-
cial event in more than a billion years. Together
with existing geochronological data, the new
Re-Os ages constrain the onset and demise of
the long-lasting (717–660 Ma) Sturtian glacial
epoch and further bolster correlations for the
end-Cryogenian Marinoan glaciation ca. 635
Ma (Fig. 3). The long duration (>55 m.y.) of the
Sturtian glacial epoch implies a relatively short
Cryogenian nonglacial interlude (<25 m.y.),
consistent with a repeated trigger for glaciation
related to the tectonic background conditions
that drive weathering and the consumption of
CO2 on 1–10 m.y. time scales (Mills et al., 2011).
This updated Neoproterozoic chronology pro-
vides new constraints to test and refine climate
models of a long-duration glacial epoch and the
nature of a relatively short nonglacial interlude.
The Osi data presented here confirm enhanced
weathering of juvenile crustal material prior to
the onset of the Sturtian glacial epoch, consis-
tent with a basalt weathering trigger for initia-
tion of the Sturtian glacial epoch (e.g., Goddéris
et al., 2003; Rooney et al., 2014). These ages
confirm the central prediction of the snowball
Earth hypothesis of long-lived (~10 m.y.) gla-
ciation with globally synchronous deglaciation.
ACKNOWLEDGMENTS
We thank Sharad Master for access to the Zambian
drill core. Supported by the Massachusetts Institute
of Technology NASA Astrobiology Institute node,
the National Science Foundation (NSF) Sedimentary
Geology and Paleobiology Program (grant EAR-
1148058), an NSF Graduate Research Fellowship (to
Strauss), the Yukon Geological Survey, and Fireweed
Helicopters and Canadian Helicopters (support and
662.4±4.6
717.4±0.1
Age (Ma)
632.5±1.0*
632.3±5.9
635.3±1.1*
636.3±4.9
636.4±0.5
640.7±5.7
654.5±3.8
732.2±4.7
742.0±6.0
739.9±6.5
727.3±4.9
752.7±5.5
711.5±0.3
716.5±0.2
662.9±4.3
657.2±
6.9
659.0±4.5
659.6±
10.2
Sturtian Glacial Epoch
Marinoan Glaciation
U-Pb magmatic
zircon age
Re-Os ORR age
Reference
736.0±23
709.0±5.0
688.9±9.5
687.4±1.3
685.0±7.0
684.0±4.0
726.0±1.0
741.0±6.0
735.0±5.0
635.5±1.1*
1,2
3
4,2
5
1,2
6
7
5
8
3
9
11
10
12
12
13
685.5±0.4
14
15
16
17
17
18
19
3
10
21
22
715.8±2.5
19
23
24
25
3
719.5±0.3
20
Figure 3. Compilation of geochronological
constraints for Neoproterozoic strata for the
interval 753–630 Ma. No detrital U-Pb zircon
data are included, because they cannot con-
strain the onset or cessation of glaciation.
ORR—organic-rich rock. Asterisks—U-Pb
magmatic zircon ages including uncertain-
ties recalculated by Schmitz (2012); dag-
gers—Re-Os ages including uncertainty
in 187Re decay constant (l). Details of geo-
chronological data and accompanying refer-
ences (numbered) are provided in the Data
Repository (see footnote 1).
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transportation). We also thank Lyle Nelson, Uyanga
Bold, and Sarah Moon for assistance in the field;
Charlie Roots, Erik Sperling, Galen Halverson, Dave
Selby, and Paul Hoffman for fruitful conversations;
and four reviewers for constructive comments.
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Manuscript received 9 December 2014
Revised manuscript received 5 March 2015
Manuscript accepted 5 March 2015
Printed in USA
GSA DATA REPOSITORY ITEM 2015157
RHENIUM-OSMIUM GEOCHRONOLOGY
Black carbonaceous shale from outcrop and drill core was sampled from the
Callison Lake Formation, Mwashya subgroup (devoid of sulphide mineralization or
evidence of fluid flow and from drill core), Taishir and Sheepbed formations. For outcrop
samples a ca. 25 cm trench was dug to avoid sampling weathering material similar to
methods in Strauss et al. (2014) and to maximize the spread of 187Re/188Os values e.g.,
Kendall et al. (2009). Drill core samples (Zambia) were cut over an interval of 6.49 m
and each sample from outcrop and drill core was greater than 30 g yet thinner than 7 cm
vertically to minimize variations in initial 187Os/188Os values. Although we sampled > 6
m vertically, all initial 187Os/188Os values for the Mwashya subgroup displayed very little
variation (Table DR1).
Any weathered surfaces were removed with a diamond-encrusted rock saw and
samples were then hand-polished using a diamond-encrusted polishing pad to remove
cutting marks and eliminate any potential for contamination from the saw blade. The
samples were dried overnight at ~60 °C and then crushed to a fine (~30 μm) powder in a
SPEX 8500 Shatterbox using a zirconium grinding container and puck in order to
homogenize any Re and Os heterogeneity present in the samples (Kendall et al., 2009).
Re and Os isotopic abundances and compositions were determined at the Department of
Earth and Atmospheric Sciences, University of Houston following methodology by Selby
and Creaser (2003) and Cumming et al., (2013).
Between 0.25 and 1 g of sample was digested and equilibrated in 8 ml of CrVIO3-
H2SO4 together with a mixed tracer (spike) solution of 190Os and 185Re in carius tubes at
Rooney et al.
220 °C for 48 hours. Rhenium and osmium was isolated and purified using solvent
extraction (NaOH, (CH3)2CO, and CHCl3,), micro-distillation, anion column
chromatography methods, and negative mass spectrometry as outlined by Selby and
Creaser (2003) and Cumming et al. (2013). The CrVIO3-H2SO4 digestion method was
employed as it has been shown to preferentially liberate hydrogenous Re and Os yielding
more accurate and precise age determinations (Selby and Creaser, 2003; Kendall et al.,
2004; Rooney et al., 2011). Total procedural blanks during this study were 22.1 ± 4.2 pg
and 0.46 ± 0.05 pg for Re and Os respectively, with an average 187Os/188Os value of 0.183
± 0.06 (1σ, n = 4).
Isotopic measurements were performed using a ThermoElectron TRITON PLUS
mass spectrometer at the University of Houston via static Faraday collection for Re and
ion-counting using a secondary electron multiplier in peak-hopping mode for Os. In-
house Re and Os solutions were continuously analyzed during the course of this study to
ensure and monitor long-term mass spectrometer reproducibility. The University of
Houston Re standard solution measured on faraday cups yields an average 185Re/187Re
value of 0.59827 ± 0.00158 (1σ, n = 19), which is indistinguishable, within uncertainty to
that of Rooney et al. (2010). The measured difference in 185Re/187Re values for the Re
solution and the accepted 185Re/187Re value (0.5974) (Gramlich et al., 1973) is used to
correct the Re sample data for instrument mass fractionation and blank and spike
contributions. The Os isotope reference material used at NCIET is the Durham Romil
Osmium Solution (DROsS) yields an 187Os/188Os ratio of 0.10694 ± 0.00051 (1σ, n = 18)
that is indistinguishable, within uncertainty, to those reported in (Rooney et al., 2010).
Uncertainties for 187Re/188Os and 187Os/188Os are determined by error propagation
of uncertainties in Re and Os mass spectrometry measurements, blank abundances and
isotopic compositions, spike calibrations, and reproducibility of standard Re and Os
isotopic values. The Re-Os isotopic data, 2σ calculated uncertainties for 187Re/188Os and
187Os/188Os, and the associated error correlation function (rho) are regressed to yield a
Re-Os date using Isoplot V. 4.15 with the λ 187Re constant of 1.666 x 10-11a-1 (Ludwig,
1980; Smoliar et al., 1996; Ludwig, 2011).
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Cumming, V.M., Poulton, S.W., Rooney, A.D., and Selby, D., 2013, Anoxia in the
terrestrial environment during the late Mesoproterozoic: Geology, v. 41, p.583-
586.
Gramlich, J. W., Murphy, T. J., Garner, E. L., and Shields, W. R., 1973, Absolute
isotopic abundance ratio and atomic weight of a reference sample of rhenium:
Journal of research of the National Bureau of Standards, v. 77A, p. 691-698.
Kendall, B., Creaser, R. A., and Selby, D., 2009, 187Re-187Os geochronology of
Precambrian organic-rich sedimentary rocks: Geological Society, London, Special
Publications, v. 326, no. 1, p. 85-107.
Ludwig, K. R., 1980, Calculation of uncertainties of U-Pb isotope data: Earth and
Planetary Science Letters, v. 46, p. 212-220.
Ludwig, K.R., 2011, Isoplot/Ex, Version 4.15: A geochronological toolkit for Microsoft
Excel: Geochronology Center Berkeley, v. 4, p.1-70.
http://www.bgc.org/isoplot_etc/isoplot.html.
Rooney, A. D., Selby, D., Houzay, J.-P., and Renne, P. R., 2010, Re-Os geochronology
of a Mesoproterozoic sedimentary succession, Taoudeni basin, Mauritania:
Implications for basin-wide correlations and Re-Os organic-rich sediments
systematics: Earth and Planetary Science Letters, v. 289, no. 3-4, p. 486-496.
Rooney, A.D., Chew, D.M., Selby, D., 2011, Re-Os geochronology of the
Neoproterozoic-Cambrian Dalradian Supergroup of Scotland and Ireland:
Implications for Neoproterozoic stratigraphy, glaciation and Re-Os systematics:
Precambrian Research, v. 185, p. 202-214.
Selby, D., and Creaser, R. A., 2003, Re-Os geochronology of organic rich sediments: an
evaluation of organic matter analysis methods: Chemical Geology, v. 200, no. 3-4,
p. 225-240.
Smoliar, M.I., Walker, R.J., Morgan, J.W., 1996, Re-Os ages of Group IIA, IIIA, IVA
and IVB iron meteorites: Science, v. 271, p. 1099-1102.
Strauss, J.V., Rooney, A.D., Macdonald, F.A., Brandon, A.D., and Knoll, A.H., 2014,
740 Ma vase-shaped microfossils from Yukon, Canada: Implications for
Neoproterozoic chronology and biostratigraphy: Geology, v.42, p.659-662.
A
Samples
J1301-62.5
#1 #2 #3 #4 #5 #6 #7
10 cm
B
Samples
A1309
ABCDEFG
20 cm
H I
C
Samples
SpB-6
AB C 8
9
10 cm
10 cm
Figure DR1: Schematic diagram of sampling procedures for:
A) Callison Lake Formation, sample J1301-62.5 #3 was lost
when the carius tube neck broke in the oven and #5 was not
processed beyond sample powdering due to time
constraints; B) Taishir Formation, samples “G” and “H”
were more fissile and broke apart during transport out of the
field site and; C) Sheepbed Formation, samples SpB-6 #8
and #9 were part of a vertical transect (height of ca. 1 m) for
Os isotope stratigraphy that has not been fully processed yet.
Table DR1: Re and Os elemental abundance data and isotope composition data for all samples Fig. 2A-D
Sample Re (ng/g) ± Os (pg/g) ±
192Os ±187Re/188Os ±187Os/188Os ±rhoaOsi
b
J1301-62.5-1 11.911 0.110 411.01 1.60 127.34 0.53 186.10 2.17 2.677 0.028 0.702 0.331
J1301-62.5-2 17.933 0.120 419.32 1.85 111.35 0.50 320.44 3.18 4.381 0.045 0.789 0.341
J1301-62.5-4 15.263 0.110 311.05 1.66 76.36 0.48 397.68 3.00 5.355 0.039 0.872 0.341
J1301-62.5-6 19.278 0.110 430.67 2.24 111.56 0.61 343.83 3.91 4.681 0.040 0.775 0.346
J1301-62.5-7 22.191 0.130 400.75 2.01 90.41 0.48 488.34 3.00 6.491 0.031 0.877 0.335
MJ-31 3.296 0.037 54.61 0.43 11.77 0.14 557.18 4.66 7.148 0.069 0.910 0.368
MJ-32 8.273 0.080 249.33 1.27 74.63 0.48 220.54 2.46 3.036 0.046 0.957 0.348
MJ-33 11.599 0.053 240.02 1.21 60.57 0.38 381.02 5.00 5.004 0.063 0.925 0.361
MJ-34 14.027 0.067 255.91 1.33 59.60 0.37 468.24 6.17 6.052 0.076 0.925 0.346
MJ-35 11.926 0.171 387.54 1.91 118.56 0.65 200.15 2.14 2.808 0.043 0.749 0.369
MJ-36 7.947 0.126 174.65 1.07 45.57 0.33 347.00 5.38 4.593 0.081 0.739 0.365
MJ-37 0.826 0.072 37.76 0.47 12.62 0.32 130.27 6.13 1.931 0.036 0.800 0.343
A1309-A 0.928 0.005 55.26 0.20 18.97 0.08 97.34 1.09 1.680 0.013 0.863 0.600
A1309-B 1.885 0.008 95.70 0.36 32.12 0.13 116.77 1.22 1.891 0.014 0.969 0.596
A1309-C 1.042 0.004 60.30 0.22 20.63 0.09 100.45 1.12 1.712 0.015 0.964 0.598
A1309-D 9.644 0.048 328.34 1.36 101.62 0.41 188.82 1.99 2.688 0.025 0.885 0.594
A1309-E 3.720 0.015 164.98 0.65 54.06 0.22 136.92 1.44 2.121 0.022 0.942 0.602
A1309-I 5.718 0.024 153.99 0.69 44.44 0.18 256.00 2.69 3.428 0.037 0.968 0.589
SpB-6A 14.85 0.074 316.77 1.92 78.94 0.34 374.38 3.80 5.163 0.050 0.845 1.213
SpB-6B 5.45 0.028 371.99 0.99 121.46 0.23 89.29 0.91 2.155 0.050 0.923 1.213
SpB-6C 6.15 0.089 106.96 0.53 23.89 0.11 512.09 5.64 6.635 0.060 0.966 1.232
SpB-6-8 12.21 0.076 204.59 0.90 44.72 0.14 543.29 6.00 6.943 0.070 0.829 1.210
SpB-6-9 12.08 0.150 276.39 2.24 70.86 0.66 339.17 5.68 4.808 0.017 0.784 1.218
Ages are calculated using the λ187Re = 1.666 x 10-11y-1 (Smoliar et al., 1996).
All GPS coordinates are in datum WGS 84
Isochron
Uncertainties are given as 2σ for 187Re/188Os and 187Os/188Os and 192Os.
The uncertainty includes the 2 SE uncertainty for mass spectrometer analysis plus uncertainties for Os blank abundance and isotopic composition.
a Rho is the associated error correlation (Ludwig, 1980).
b Osi = initial 187Os/188Os isotope ratio calculated at 753 Ma, 727 Ma, 659 Ma and 632 Ma for the Callison Lake Formation, Mwashya subgroup and Taishir and Sheepbed formations, respectively.
Callison Lake Formation
GPS co-ordinates:
N64 39'48.0"
W139 43'48.6"
Mwashya subgroup
GPS co-ordinates:
Chambishi basin ~
N12 15'00.0"
E28 20'00.0"
Sheepbed Formation GPS
co-ordinates: N64
29'50.4"
W129 25'42.0"
Taishir Formation
GPS co-ordinates:
N46 40'20.1"
E96 33'39.5"
Paleocontinent Age (Ma) (+) (-) Technique Grains Relationship to Glacial Deposit Number Reference
South China 632.5 0.5 0.5 U-Pb ID-TIMS magmatic above Nantuo 1 Condon et al., 2005, Science, v.308, p. 95-98
South China 632.5 1.0 1.0 U-Pb ID-TIMS magmatic above Nantuo 2 Schmitz, M.D., Geological Time Scale 2012, v. 2, p. 1045-1082 - recalculation of above date
Laurentia 632.3 5.9 5.9 Re-Os isochron above Ice Brook 3 This paper
Kalahari 635.5 0.5 0.5 U-Pb ID-TIMS magmatic within Ghuab 4 Hoffmann et al., 2004, Geology, v.32, p. 817-820
Kalahari 635.5 1.1 1.1 U-Pb ID-TIMS magmatic within Ghuab 2 Schmitz, M.D., Geological Time Scale 2012, v. 2, p. 1045-1082 - recalculation of above date
South China 636.3 4.9 4.9 U-Pb SHRIMP magmatic within Nantuo 5 Zhang et al., 2005, Geology, v. 33, p. 473-476
Australia 636.4 0.5 0.5 U-Pb ID-TIMS magmatic above Cottons Breccia 6 Calver et al., 2013, Geology, v. 41, p. 1127-1130
Australia 640.7 5.7 5.7 Re-Os isochron possibly above Sturtian 7 Kendall et al., 2009, Precambrian Research, v. 172, p. 301-310
South China 654.5 3.8 3.8 U-Pb SHRIMP magmatic below Nantuo 5 Zhang et al., 2005, Geology, v. 33, p. 473-476
Australia 657.2 6.9 6.9 Re-Os isochron above Sturtian below Marinoan 8 Kendall et al., 2006, Geology, v. 34, p. 729-732
Mongolia 659.0 4.5 4.5 Re-Os isochron 1m above Maikhan Ul 3 This paper
Laurentia 659.6 10.2 10.2 Re-Os isochron below Port Askaig 9 Rooney et al., 2011,Precambrian Research, v. 185, p. 202-214
Laurentia 662.4 4.6 4.6 Re-Os isochron above Rapitan 10 Rooney et al., 2014, Proceedings of the National Academy of Sciences, v. 110, p. 51-56.
South China 662.9 4.3 4.3 U-Pb ID-TIMS magmatic below Nantuo above Tiesiao 11 Zhou et al., 2004, Geology, v. 32, p. 437-440.
Laurentia 684.0 4.0 4.0 U-Pb SHRIMP magmatic unknown 12 Lund et al., 2003, Geological Society of America Bulletin, v. 115, p. 349-372
Laurentia 685.0 7.0 7.0 U-Pb SHRIMP magmatic unknown 12 Lund et al., 2003, Geological Society of America Bulletin, v. 115, p. 349-372
Laurentia 685.5 0.4 0.4 U-Pb ID-TIMS magmatic within lower Scout Mountain 13 Keeley et al., 2013, Lithosphere, v. 5, p. 128-150
Laurentia 687.4 1.3 1.3 U-Pb ID-TIMS magmatic unknown 14 Condon and Bowring, 2011, Geological Society, London, Memoirs, Chapter v. 36, p. 135-149
Laurentia 688.6 9.5 6.2 U-Pb ID-TIMS magmatic within Rapitan 15 Ferri et al., 1999, British Columbia Ministry of Energy and Mines, Bulletin, v. 107, p.1-122
Laurentia 709.0 5.0 5.0 U-Pb SHRIMP magmatic unknown 16 Fanning and Link, 2004, Geology, v. 32, p. 881-884
Arabia 711.5 0.3 0.3 U-Pb ID-TIMS magmatic within Ghubrah 17 Bowring et al., 2007, American Journal of Science, v. 307, p. 1097-1145
South China 715.8 2.5 2.5 U-Pb ID-TIMS magmatic below Chang'an 18 Lan et al., 2014, Precambrian Research, v. 255, p. 401-411
Laurentia 716.5 0.2 0.2 U-Pb ID-TIMS magmatic within Rapitan 19 Macdonald et al., 2010, Science, v. 327, p. 1241-1243
Laurentia 717.4 0.1 0.1 U-Pb ID-TIMS magmatic below Rapitan 19 Macdonald et al., 2010, Science, v. 327, p. 1241-1243
Laurentia 719.5 0.3 0.3 U-Pb ID-TIMS magmatic base of Hula Hula 20 Cox et al., in review, Lithosphere
Arabia 726.0 1.0 1.0 U-Pb ID-TIMS magmatic Leger granite 17 Bowring et al., 2007, American Journal of Science, v. 307, p. 1097-1145
Congo 727.3 4.9 4.9 Re-Os isochron below Grand Conglomerate 3 This paper
Laurentia 732.2 4.7 4.7 Re-Os isochron below Rapitan 10 Rooney et al., 2014, Proceedings of the National Academy of Sciences, v. 110, p. 51-56.
Congo 735.0 5.0 5.0 U-Pb SHRIMP magmatic below Kundelungu 21 Key et al., 2001, Journal of African Earth Sciences, v. 33, p. 503-528
Laurentia 736.0 23.0 17.0 U-Pb ID-TIMS magmatic below Rapitan 22 McDonough and Parrish, 1991, Canadian Journal of Earth Sciences, v. 28, p. 1202-1216
Laurentia 739.9 6.5 6.5 Re-Os isochron below Rapitan 23 Strauss et al., 2014, Geology, v. 42, p. 659-662
Kalahari 741.0 6.0 6.0 Pb-Pb ID-TIMS magmatic below Numees 24 Frimmel et al. 1996, The Journal of Geology, v. 104, p. 459-469
Laurentia 742.0 6.0 6.0 U-Pb ID-TIMS magmatic top of Chuar 25 Karlstrom et al., 2000, Geology, v. 28, p. 619-622
Laurentia 752.7 5.5 5.5 Re-Os isochron below Rapitan 3 This paper
ID-TIMS, Isotope-Dilution Thermal Ionization Mass Spectrometry
SHRIMP, Sensitive High Resolution Ion Microprobe
SIMS, Secondary Ion Mass Spectrometry
Table DR2: Geological age constraints, techniques and data sources
All age uncertainties are 2σ.
... Countering this pattern and its application to Pannotia (Evans, 2021), the protracted Sturtian glaciation (ca. 717-663 Ma; e.g., Rooney et al., 2015;Cox et al., 2018;Lan et al., 2020) demonstrably coincides with the breakup of the supercontinent Rodinia rather than its amalgamation (e.g., Li et al., 2008Li et al., , 2013, whereas the shorter-lived Marinoan glaciation (ca. 650-635 Ma; e.g., Rooney et al., 2015;Hoffman et al., 2017;Bao et al., 2018) occurred prior to the proposed timing of Pannotia amalgamation. ...
... 717-663 Ma; e.g., Rooney et al., 2015;Cox et al., 2018;Lan et al., 2020) demonstrably coincides with the breakup of the supercontinent Rodinia rather than its amalgamation (e.g., Li et al., 2008Li et al., , 2013, whereas the shorter-lived Marinoan glaciation (ca. 650-635 Ma; e.g., Rooney et al., 2015;Hoffman et al., 2017;Bao et al., 2018) occurred prior to the proposed timing of Pannotia amalgamation. Likewise, refined chronostratigraphy (e.g., Boucot et al., 2013;Evans, 2021) suggests that the Late Paleozoic Gondwana glaciation likely preceded the peak of Pangea's amalgamation. ...
Article
Following a decade during which its presence was widely accepted, the existence of the putative Ediacaran supercontinent Pannotia has come into question since the turn of the millenium, largely due to the geochro-nology of Ediacaran-Cambrian orogens, which suggests that the supposed landmass had begun to break up well before it was fully assembled. Paleomagnetic data from this time interval have been used to both support and refute the existence of Pannotia, but are notoriously equivocal. Proxy signals for Ediacaran-Cambrian super-continent assembly and breakup, although collectively compelling, can be individually challenged, and efforts to detect the mantle legacy expected of supercontinent amalgamation, while promising, are inconclusive. Yet the existence of Pannotia is central to the nature, duration and evolution of the supercontinent cycle, and dictates the cycle's geodynamic pathway from the breakup of Rodinia to the assembly of Pangea. Hence, the question of Pannotia's existence, like that of Hamlet, is one of fundamental importance and demands far more attention than it has hitherto received.
... The Smalfjorden Formation has been correlated with glaciogenic deposits of the Marinoan glaciation (e.g., Rice et al., 2011) of Cryogenian age (ca. 645-635 Ma, Rooney et al., 2015;Shields-Zhou et al., 2016). The Mortensnes Formation on the other hand has been correlated with those of the Ediacaran Gaskiers glaciation (e.g., Rice et al., 2011), which was likely of short duration (ca. ...
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The Digermulen Peninsula in northeastern Finnmark, Arctic Norway, comprises one of the most complete Ediacaran–Cambrian transitions worldwide with a nearly continuous record of micro- and macrofossils from the interval of the diversification of complex life. Here, we report on the provenance and post-depositional alteration of argillaceous mudstones from the Digermulen Peninsula using rare earth elements and Sm–Nd and Rb–Sr isotopic systematics to provide an environmental context and better understand this important transition in Earth’s history. The studied sections comprise a mid-Ediacaran glacial–interglacial cycle, including the Nyborg Formation (ca. 590 Ma) and Mortensnes Formation (related to the ca. 580 Ma-old Gaskiers glaciation), and the Stáhpogieddi Formation (ca. 560–537 Ma), which yields Ediacara-type fossils in the Indreelva Member and contains the Ediacaran–Cambrian boundary interval in the Manndrapselva Member and basal part of the informal Lower Breidvika member (ca. 537–530 Ma). Three sample groups, (1) Nyborg and Mortensnes formations, (2) the lowermost five samples from the Indreelva Member and (3) the remaining samples from the Indreelva as well as from the Manndrapselva and Lower Breidvika members, can be distinguished, belonging to distinct depositional units. All samples have negative εNd(T) values (−6.00 to − 21.04) indicating a dominant input of terrigenous detritus with an old continental crust affinity. Significant shifts in Sm–Nd isotope values are related to changes in the sediment source, i.e. Svecofennian province vs Karelian province vs Svecofennian province plus in addition likely some juvenile (late Neoproterozoic volcanic) material, and probably reflect palaeotectonic reorganisation along the Iapetus-facing margin of Baltica. The combined Rb–Sr isotopic data of all samples yield an errorchron age of about 430 Ma reflecting the resetting of the Rb–Sr whole-rock isotope systems of the mudstones during the Scandian tectono-metamorphic event in the Gaissa Nappe Complex of Finnmark. Preservation of palaeopascichnids coincides with the sedimentation regimes of sample groups 2 and 3 while other Ediacara-type fossils, e.g. Aspidella-type and frondose forms, are limited to the sample group 3. Our results are similar to those of earlier studies from the East European Platform in suggesting oxic seafloor conditions during the late Ediacaran.
... Accompanying δ 18 O values can be found in supplement (Fig. S3 [see text footnote 1]). Radioisotopic dates of key stratigraphic columns are from Benus (1988), Krogh et al. (1988), Southworth et al. (2009), Rooney et al. (2015, Pu et al. (2016), Canfield et al. (2020), and Rooney et al. (2020). (C) Number of units that contain Ediacaran macrofossil occurrences from the mesostrat data set. ...
Ediacaran sediments record the termination of Cryogenian "snowball Earth" glaciations, preserve the first occurrences of macroscopic metazoans, and contain one of the largest known negative δ δ 13 C excursions (the Shuram-Wonoka). The rock record for the transition between the Proterozoic and Phanerozoic in North America is also physically distinct, with much of the continent characterized by a wide variety of mostly crystalline Proterozoic and Archean rocks overlain by Lower Paleozoic shallow-marine sediments. Here, we present quantitative macrostratigraphic summaries of rock quantity and type using a new comprehensive compilation of Ediacaran geological successions in North America. In keeping with previous results that have identified early Paleozoic burial of the "Great Unconformity" as a major transition in the rock record, we find that the Ediacaran System has greatly reduced areal extent and volume in comparison to the Cambrian and most younger Phanerozoic systems. The closest quantitative analogue to the Ediacaran System in North America is the Permian-Triassic interval, deposited during the culminating assembly and early rift-ing phases of the supercontinent Pangea. The Shuram-Wonoka carbon isotope excursion occurs against the backdrop of the largest increase in carbonate and total rock volume observed in the Ediacaran. The putatively global Gaskiers glaciation (ca. 580-579 Ma), by contrast, has little quantitative expression in these data. Although the importance of Ediacaran time is often framed in the context of glaciation, biological evolution, and geochemical perturbations, the quantitative expressions of rock area, volume, and lithology in the geologic record clearly demark the late Ediacaran to early Cambrian as the most dramatic transition in at least the past 635 m.y. The extent to which the timing and nature of this transition are reflected globally remains to be determined, but we hypothesize that the large expansion in the extent and volume of sedimentation within the Ediacaran, particularly among carbonates, and again from the Ediacaran to the Cambrian, documented here over ~17% of Earth's present-day continental area, provides important insights into the drivers of biogeochemical and biological evolution at the dawn of animal life.
... Former interpretations of a global Kaigas event might be the based on wrong correlations with Sturtian glacial deposits. Rooney et al. (2015) present evidence for non-glacial sedimentation during Kaigas times on the Tables 1 and 2. M. Zieger-Hofmann et al. Earth-Science Reviews xxx (xxxx) xxx western Laurentian margin, supporting the non-global scale of this glaciation. ...
Article
In order to present a conclusive new correlation model for Neoproterozoic glacial units of southern Namibia, ten different sections and a variety of samples thereof were studied and analysed with respect to field relationships, whole rock geochemistry, zircon U-Pb dating as well as Th-U ratios, Hf isotopic measurements and zircon grain size analyses, combined with the new method of LA-ICP-MS U-Pb dating of cap carbonates. This multi-method approach allowed for the construction of a correlation model for the Cryogenian and Ediacaran units of southern Namibia. Furthermore it revealed, that (1) sediments deposited during four glacial Neoproterozoic events in Southern Namibia did all show very similar detrital zircon features, allowing the interpretation of continuous recycling of the same material over the most part of the Neoproterozoic, which is also supported by the geochemical whole rock analyses, (2) cap carbonates are worth analysing concerning their U-Pb isotope ratios and can result in valuable age determinations, if reset and overprinted areas are recognised and avoided for laser ablation, (3) proving the Sturtian and the Marinoan age for the Numees Fm and the Namaskluft Mbr by U-Pb dating their overlying carbonate sequences was finally possible, (4) the Witvlei Grp sedimentation ends at 579 ± 52 Ma, which is the age for the stromatolites of the Okambara Mbr, representing the uppermost deposits of the Witvlei Grp, and this leading to (5) the start of Nama Grp sedimentation for southern Namibia was not earlier than 579 ± 52 Ma.
... The first of these episodes occurred at the beginning of the Paleoproterozoic era, approximately 2.5 Gya (Evans et al., 1997;Kirschvink et al., 2000). The next two occurred during the Cryogenian period of the Neoproterozoic era, about 700 Mya and with inception times roughly 50 Ma apart (Hoffman et al., 1998;Rooney et al., 2015;Prave et al., 2016). At other points in time, like the Cretaceous and the early Eocene "equable" climates, the Earth was warm and ice-free at high latitudes (Berner, 1990;Greenwood & Wing, 1995). ...
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Over its multibillion-year history, Earth has exhibited a wide range of climates. Its history ranges from snowball episodes where the surface was mostly or entirely covered by ice to periods much warmer than today, where the cryosphere was virtually absent. Our understanding of greenhouse gas evolution over this long history, specifically carbon dioxide, is mainly informed by deterministic and mechanistic models. However, the complexity of the carbon cycle and its uncertainty over time motivates study of non-deterministic models, where key elements of the cycle are represented by inherently stochastic processes. By doing so, we can learn what models of variability are compatible with Earth's climate record instead of how exactly this variability is produced, working backward. Here we discuss two simple stochastic models of variability in the carbon system and how they relate to Earth's snowball record in particular. The first, which is the most simple and represents CO2 concentration directly as a stochastic process, is instructive and perhaps intuitive, but is incompatible with this record. The second, which separates carbon source from sink and represents CO2 outgassing as a stochastic process instead of concentration, is more flexible. When outgassing fluctuates over longer periods, as opposed to brief and rapid excursions from a mean state, this model is more compatible with the snowball record. The contrast between these models illustrates what kind of variability may have characterized the long-term carbon cycle.
... The Neoproterozoic was a dynamic time in Earth's evolution, hosting the breakup of the supercontinent Rodinia and formation of the subsequent supercontinent Gondwana. This change in plate tectonic organization resulted in extreme global climate changes, including several global glaciation events (Hoffman and Li, 2009;Rooney et al., 2015). The Neoproterozoic was also a time when significant changes occurred to the global crustal volume and thickness (Dhuime et al., 2015;Cawood and Hawksworth, 2019) as well as the widespread appearance of blueschists (Stern, 2005;Palin and White, 2016), indicating fundamental changes in plate tectonic processes at this time (Brown, 2010;Palin and White, 2016). ...
Chapter
The Proterozoic Eon covers 40% of Earth's history, from 2500 to 541 Ma (million years ago), and was home to a series of major events in Earth's history. The tectonic configuration of the early stages of the Proterozoic is not known, but from 1500 Ma onwards there are reconstructions available. They are used here in a dedicated tidal model to simulate how the tides changed during the middle-late part of the Eon. We also revisit the Cryogenian period (715–630 Ma), when it has been proposed that Earth was home to vast near-global glaciations and extend the simulations into the later parts of the Eon. We show that the tides are far less energetic than today (about 40% of present-day tidal dissipation rates) for most of the period we could simulate, with the exception of a tidal maximum around 1250 Ma. The reason is that the period we simulated is home to the supercontinents Rodinia and Pannotia, and the results confirm the existence of a supertidal cycle linked to the supercontinent cycle.
Article
Sedimentary records suggest that the mid-Proterozoic (ca. 1.8–0.8 Ga) was persistently characterized by a greenhouse climate despite significantly lower solar luminosity compared to modern levels. To maintain greenhouse conditions, the partial pressure of carbon dioxide (pCO2) must have remained elevated, possibly indicative of key differences in the complexities of the carbon cycle compared to the modern. Modeling has suggested that high pCO2 was likely maintained by elevated rates of “reverse weathering:” marine authigenic clay formation, a process that consumes alkalinity and generates CO2. This process is kinetically slow in modern marine environments, yet is hypothesized to have been enhanced during the mid-Proterozoic due to the greater availability of important species for clay authigenesis such as silica and ferrous iron. This hypothesis is testable using the geological record, because enhanced reverse weathering would lead to the formation of abundant marine authigenic clays. However, the distribution of marine authigenic clays in the Proterozoic sedimentary record has not been paid sufficient attention. In this study, we report the presence of authigenic clays (glauconite and berthierine) from the Xiamaling Formation (ca. 1.4 Ga), North China. The glauconite-berthierine horizons occur as millimeter-to centimeter-thick laminae interbedded with muddy siltstone and feature detrital grains supported by the clay matrix. In places, these layers were partially reworked to form soft and cohesive intraclastic sands, suggesting a syndepositional origin. We hypothesize that marine iron cycling in the iron- and silica-rich mid-Proterozoic oceans may have facilitated the formation of authigenic iron-rich clay during the deposition of the Xiamaling Formation. The formation of iron-hydroxides on the seafloor—and the local increase in pH caused by subsequent dissimilatory iron reduction—could have resulted in the absorption of SiO2, Al(OH)3, and Fe(OH)2 to form soft, cohesive and noncrystalline Fe(OH)3-SiO2-Al(OH)3-Fe(OH)2 gels. These gels would have subsequently converted to glauconite/berthierine through aging. The transformation from glauconite-rich layers to berthierine-rich laminae was likely facilitated by a greater availability of Fe(II), and therefore higher Fe(II)/TFe and Fe/Si ratios. We suggest that the relatively rapid formation of syndepositional, seafloor berthierine and glauconite layers in the basal Xiamaling Formation is the result of enhanced reverse weathering during this time. This study provides an important geological support for carbon cycle models that invokes enhanced reverse weathering rates in the mid-Proterozoic ocean that may have helped to maintain a high-baseline pCO2 during this time.
Article
Most Neoproterozoic iron formations (NIF) are closely associated with global or near‐global “Snowball Earth” glaciations. Increasingly, however, studies indicate that some NIFs show no robust evidence of glacial association. Many aspects of non‐glacial NIF genesis, including the paleo‐environmental setting, Fe(II) source, and oxidation mechanisms, are poorly understood. Here, we present a detailed case study of the Jiapigou NIF, a major non‐glacial NIF within a Neoproterozoic volcano‐sedimentary sequence in North Qilian, northwestern China. New U–Pb geochronological data place the depositional age of the Jiapigou NIF at ~600 Ma. Petrographic and geochemical evidence supports its identification as a primary chemical sediment with significant detrital input. Major and trace element concentrations, REE + Y systematics, and εNd(t) values indicate that iron was sourced from mixed seawater and hydrothermal fluids. Iron isotopic values (δ56Fe = −0.04‰–1.43‰) are indicative of partial oxidation of an Fe(II) reservoir. We infer that the Jiapigou NIF was deposited in a redox stratified water column, where hydrothermally sourced Fe(II)‐rich fluids underwent oxidation under suboxic conditions. Lastly, the Jiapigou NIF has strong phosphorous enrichments, which in other iron formations are typically interpreted as signals for high marine phosphate concentrations. This suggests that oceanic phosphorus concentrations could have been enriched throughout the Neoproterozoic, as opposed to simply during glacial intervals.
Article
A rift basin developed in the Neoproterozoic Tarim Craton and it provides a key perspective for understanding the breakup of Rodinia. The rift-related successions with glacial deposits spanning ca. 200 m.y. in the North Tarim are studied through detrital zircon U-Pb dating and seismic interpretation. A total of 1503 ages from 11 samples are reported and two kinds of age distribution patterns are recognized. Samples of the Qiaoenbrak Group show unimodal distribution with most ages at 900–800 Ma; samples of the upper two formations show bimodal distribution: One age range at 900–600 Ma and another at 2000–1800 Ma. The emergence of Paleoproterozoic zircons indicates uplift and exhumation of the basement, which is compatible with plume upwelling. We find that there are prominent gaps between depositional ages and crystalline ages of detrital zircons, which corresponds with a plume environment instead of a back-arc environment. Besides, seismic and well data show that the Qiaoenbrak Group was deposited in (half-)grabens controlled by normal faults. The lower Sinian was less restricted but still affected by these faults, while the Upper Sinian thickens prominently towards the outer edge of the basin. Together, we propose a new two-stage plume-induced model for the Neoproterozoic rift in the Tarim Craton in which two different evolution paths are recognized: From rift to aulacogen for the Nanhua Qiaoenbrak Group, and from rift to passive margin for the Sinian System
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Biostratigraphy underpins the Phanerozoic time scale, but its application to pre-Ediacaran strata has remained limited because Proterozoic taxa commonly have long or unknown stratigraphic ranges, poorly understood taphonomic constraints, and/or inadequate geochronological context. Here we report the discovery of abundant vase-shaped microfossils from the Callison Lake dolostone of the Coal Creek inlier (Yukon, Canada) that highlight the potential for biostratigraphic correlation of Neoproterozoic successions using species-level assemblage zones of limited duration. The fossiliferous horizon, dated here by Re-Os geochronology at 739.9 +/- 6.1 Ma, shares multiple species-level taxa with a well-characterized assemblage from the Chuar Group of the Grand Canyon (Arizona, USA), dated by U-Pb on zircon from an interbedded tuff at 742 +/- 6 Ma. The overlapping age and species assemblages from these two deposits suggest biostratigraphic utility, at least within Neoproterozoic basins of Laurentia, and perhaps globally. The new Re-Os age also confirms the timing of the Islay delta C-13(carbonate) anomaly in northwestern Canada, which predates the onset of the Sturtian glaciation by >15 m.y. Together these data provide global calibration of sedimentary, paleontological, and geochemical records on the eve of profound environmental and evolutionary change.
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U-Pb zircon data from the uppermost Cottons Breccia, representing the Marinoan glacial-postglacial transition on King Island, Tasmania, provide the first direct age constraint on the Cryogenian-Ediacaran boundary in Australia. Zircons in four samples from the topmost meter of the Cottons Breccia, dated by sensitive high-resolution ion microprobe, exhibit two modes ca. 660 Ma and ca. 635 Ma. The younger component predominates in the uppermost sample, a possibly volcanolithic dolomitic sandstone, apparently lacking glacially transported debris, in the transition to cap carbonate. Chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) U-Pb dating of euhedral zircons from that sample yields a weighted-mean age of 636.41 +/- 0.45 Ma. Equivalence to published TIMS ash bed dates from Cryogenian-Ediacaran transitional strata in Namibia (635.51 +/- 0.82 Ma, within glacial deposit) and China (635.23 +/- 0.84 Ma, 2 m above glacial deposit) supports correlation of those strata to the Australian type sections and globally synchronous deglaciation at the end of the Cryogenian Period.
Article
The 3-27 m-thick cap carbonate overlying "Marinoan" Ice Brook Formation glacigene sediments and Keele Formation carbonate and terrigenous clastic rocks consists of two distinctive stratigraphic units. A lower, splintery, buff-weathering, microcrystalline dolostone of extensive lateral uniformity comprises mm-laminated peloidal sediment with local, low-angle, hummocky-like cross-stratification, micro-ridges, and synsedimentary tepees, all elongated perpendicular to depositional strike. This dolostone is unconformably overlain by an upper limestone that exhibits pronounced facies variation from inboard peloidal lime grainstone and mudstone to shelf-edge cementstone to outboard lime wackestone and mudstone. Calcite cementstones range from isolated crystal fans in laminated limestone to huge, decimetre-scale crystal arrays, to hemispherical and elongate crystal stromatolites wholly composed of acicular crystals that form decametre-scale reeflike structures. Crystal stromatolites are precipitates and replaced microbiolites that constructed biostromes and bioherms, like those on many flat-topped, reef-rimmed platforms. The calcite crystals have all the physical and chemical attributes of neomorphosed aragonite. This aragonite extensively replaced sediment and microbiolite just below the sea floor and grew upward into the overlying water column. Such interpreted massive synsedimentary replacement is rare in geological history and attests to the highly saturated state of the immediate postglacial ocean. All sediment is interpreted to have been CaCO3 originally. Low and constant δ¹⁸O values reflect diagenetic modification of these carbonates, although chemical attributes, such as Sr and C isotopes in some lithologies, are near pristine. Lower dolostones, virtually identical to most other coeval Marinoan caps worldwide, were part of a global precipitation event of remarkable similarity. Upper limestones are a more local phenomenon, deposited during sea-level rise in an aragonitic sea returning to equilibrium after global glaciation. Low ⁸⁷Sr/⁸⁶Sr ratios and varying δ¹³C values with carbonate sedimentary facies indicate that both units must have formed relatively rapidly, prior to significant fluvioglacial runoff, or that the influence of this runoff on the chemistry of seawater along continental shelves was minimal. The cap carbonate is thus interpreted to have formed in two steps: (1) during initial marine ice melting accompanied by oceanic overturn and upwelling, preceding continental margin rebound, and (2) during initial stages of sea-level rise accompanying continental deglaciation. While confirming brief, but extensive, carbonate precipitation from an ocean highly perturbed by global glaciation, the rocks also suggest that this event did not permanently affect either late Neoproterozoic ocean chemistry or the contained marine biosphere.
Article
Osmium, an ultra-trace element in seawater, is subject to strong authigenic enrichment in reducing marine sediments, and to a lesser degree of enrichment in oxic marine deposits. Temporal variations in sedimentary 187 Os/ 188 Os ratio preserve a rich archive from several types of deposi-tional environments that reflect changes on the Earth's surface. This chapter presents a compilation of marine Os isotope records from the Pleistocene to the Precambrian, and reviews the interpretations of these temporal variations, focusing on the most highly resolved events in the Cenozoic and Mesozoic. Chapter Outline 8.
Article
Uncertainties in the number and age of glacial deposits within the Port Nolloth Group have hindered both structural and stratigraphic studies in the Neoproterozoic Gariep Belt of Namibia and South Africa. These uncertainties are compounded by major lateral facies changes that complicate correlations locally. Herein, we report the results of integrated geological mapping, chemo- and litho-stratigraphic, and sedimentological studies that shed light on the age and stratigraphic architecture of the Port Nolloth Group. Particularly, we have distinguished an additional glacial deposit, herein referred to as the Namaskluft diamictite, which is succeeded by a ca. 635 Ma basal Ecliacaran cap carbonate. This interpretation indicates that the stratigraphically lower, iron-bearing Numees diamictite is not Marinoan or Gaskiers in age, as previously suggested, but is instead a ca. 716.5 Ma Sturtian glacial deposit. A Sturtian age for the Numees Formation is further supported by the discovery of microbial roll-up structures in the dark limestone of the Bloeddrif Member that caps the diamictite. A re-evaluation of the age constraints indicates that all Neoproterozoic iron formations may be of Sturtian age, and thus indicative of secular evolution of the redox state of the ocean.