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Orbitally forced ice sheet fluctuations during the Marinoan Snowball Earth glaciation

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Abstract

Two global glaciations occurred during the Neoproterozoic. Snowball Earth theory posits that these were terminated after millions of years of frigidity when initial warming from rising atmospheric CO2concentrations was amplified by the reduction of ice cover and hence a reduction in planetary albedo. This scenario implies that most of the geological record of ice cover was deposited in a brief period of melt-back. However, deposits in low palaeo-latitudes show evidence of glacial-interglacial cycles. Here we analyse the sedimentology and oxygen and sulphur isotopic signatures of Marinoan Snowball glaciation deposits from Svalbard, in the Norwegian High Arctic. The deposits preserve a record of oscillations in glacier extent and hydrologic conditions under uniformly high atmospheric CO2concentrations. We use simulations from a coupled three-dimensional ice sheet and atmospheric general circulation model to show that such oscillations can be explained by orbital forcing in the late stages of a Snowball glaciation. The simulations suggest that while atmospheric CO2concentrations were rising, but not yet at the threshold required for complete melt-back, the ice sheets would have been sensitive to orbital forcing. We conclude that a similar dynamic can potentially explain the complex successions observed at other localities.
LETTERS
PUBLISHED ONLINE: 24 AUGUST 2015 | DOI: 10.1038/NGEO2502
Orbitally forced ice sheet fluctuations during the
Marinoan Snowball Earth glaciation
Douglas I. Benn1,2*, Guillaume Le Hir3, Huiming Bao4, Yannick Donnadieu5, Christophe Dumas5,
Edward J. Fleming1,6, Michael J. Hambrey7, Emily A. McMillan6, Michael S. Petronis8,
Gilles Ramstein5, Carl T. E. Stevenson6, Peter M. Wynn9and Ian J. Fairchild6
Two global glaciations occurred during the Neoproterozoic.
Snowball Earth theory posits that these were terminated
after millions of years of frigidity when initial warming from
rising atmospheric CO2concentrations was amplified by the
reduction of ice cover and hence a reduction in planetary
albedo1,2. This scenario implies that most of the geological
record of ice cover was deposited in a brief period of
melt-back3. However, deposits in low palaeo-latitudes show
evidence of glacial–interglacial cycles4–6. Here we analyse the
sedimentology and oxygen and sulphur isotopic signatures
of Marinoan Snowball glaciation deposits from Svalbard, in
the Norwegian High Arctic. The deposits preserve a record
of oscillations in glacier extent and hydrologic conditions
under uniformly high atmospheric CO2concentrations. We
use simulations from a coupled three-dimensional ice sheet
and atmospheric general circulation model to show that such
oscillations can be explained by orbital forcing in the late stages
of a Snowball glaciation. The simulations suggest that while
atmospheric CO2concentrations were rising, but not yet at
the threshold required for complete melt-back, the ice sheets
would have been sensitive to orbital forcing. We conclude
that a similar dynamic can potentially explain the complex
successions observed at other localities.
The Wilsonbreen Formation in northeast Svalbard contains a
detailed record of environmental change during the Marinoan, the
second of the major Cryogenian glaciations (650–635 Ma; refs 7,8).
At this time, Svalbard was located in the Tropics on the eastern
side of Rodinia9,10. The <180 m thick Wilsonbreen Formation was
deposited within a long-lived intracratonic sedimentary basin11. It
is subdivided into three members (W1, W2 and W3) based on
the relative abundance of diamictite and carbonate beds7,8 (Fig. 1
and Supplementary Figs 1 and 2). The occurrence throughout the
succession of lacustrine sediments containing both precipitated car-
bonate and ice-rafted detritus, and intermittent evaporative carbon-
ates and fluvial deposits, indicates that the basin remained isolated
from the sea, consistent with eustatic sea-level fall of several hundred
metres and limited local isostatic depression (Supplementary Infor-
mation; ref. 12). This makes it ideal for investigating environmental
change within a Neoproterozoic panglaciation, as it provides direct
evidence of subaerial environments and climatic conditions.
We made detailed sedimentary logs at ten known and new
localities extending over 60 km of strike (Fig. 1 and Supplemen-
tary Fig. 1; see Methods). Seven sediment facies associations were
identified, recording distinct depositional environments that varied
in spatial extent through time (Supplementary Fig. 3 and Supple-
mentary Information). These are: FA1: subglacial, recording di-
rect presence of glacier ice; FA2: fluvial channels; FA3: dolomitic
floodplain, recording episodic flooding, evaporation and microbial
communities; FA4: carbonate lake margin, including evidence of
wave action; FA5: carbonate lacustrine, including annual rhythmites
and intermittent ice-rafted debris; FA6: glacilacustrine, consisting
of ice-proximal grounding-line fans (FA6-G) and ice-distal rainout
deposits (FA6-D); and FA7: periglacial, recording cold, non-glacial
conditions. Further descriptions are provided in the Supplementary
Information. The vertical and horizontal distribution of these facies
associations (Fig. 1) allows the sequence of environmental changes
to be reconstructed in detail:
(1) The base of the Wilsonbreen Formation is a well-marked
periglacially weathered horizon with thin wind-blown sands
(Supplementary Fig. 4a,b). This surface records very limited
sediment cycling in cold, arid conditions.
(2) At all localities, the weathering horizon is overlain by fluvial
channel facies (FA2) and mudstones, marking the appearance
of flowing water in the basin and implying positive air temper-
atures for at least part of the time (Supplementary Fig. 5a).
(3) Glacilacustrine deposits (FA6-D) record flooding of the basin
and delivery of sediment by ice rafting (Supplementary
Fig. 4c,d). Far-travelled clasts are common, indicating transport
by a large, continental ice sheet.
(4) Warm-based, active ice advanced into the basin, indicated by
traction tills and glacitectonic shearing (FA1; Supplementary
Fig. 4e–g). (1–4 make up Member W1).
(5) Ice retreat is recorded by a second periglacial weathering
surface (FA7). This is overlain by fluvial channel, flood-
plain, lake-margin and carbonate lacustrine sediments of
W2 (FA2-5; Supplementary Fig. 5), recording a shifting mo-
saic of playa lakes and ephemeral streams. Lakes and river
channels supported microbial communities. Millimetre-scale
carbonate-siliciclastic rhythmites indicate seasonal cycles of
1Department of Geology, The University Centre in Svalbard (UNIS), N-9171 Longyearbyen, Norway. 2School of Geography and Geosciences, University of
St Andrews, St Andrews KY16 8YA, UK. 3Institut de Physique du Globe de Paris, 75238 Paris, France. 4Department of Geology and Geophysics, E235
Howe-Russell Complex, Louisiana State University, Baton Rouge, Louisiana 70803, USA. 5Laboratoire des Sciences du Climat et de l’Environnement,
CNRS-CEA, 91190 Gif-sur-Yvette, France. 6School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK.
7Institute of Geography and Earth Sciences, Aberystwyth University, Aberystwyth SY23 3DB, UK. 8Natural Resource Management, Environmental
Geology, New Mexico Highlands University, Las Vegas, New Mexico 87701, USA. 9Lancaster Environment Centre, University of Lancaster, Lancaster
LA1 4YQ, UK. Present address: CASP, West Building, 181A Huntingdon Road, Cambridge CB3 0DH, UK. *e-mail: Doug.Benn@unis.no
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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO2502
010
180
150
100
50
m
0
km
20 30 40 50 60
D1
W3
W2
W1
E4
DRA
DIT
REIN
KLO
McD
ORM
PIN
AND
BAC
SLA
Marine carbonate FA1: subglacial (till) FA2: fluvial
FA6: (proximal)
FA7: periglacial
FA6: glacilacustrine (distal)FA1: (glacitectonite)
FA2-5: (carbonate) fluvial and lacustrine
Figure 1 | Sedimentary architecture and palaeoenvironments of the
Wilsonbreen Formation. Regional correlation of facies associations and
members W1, W2 and W3 across northeast Svalbard. From north to south,
study locations are: DRA, Dracoisen; DIT, Ditlovtoppen; AND, East
Andromedafjellet; REIN, Reinsryggen (informal name); KLO, Klofjellet;
McD, MacDonaldryggen; BAC, Backlundtoppen–Kvitfjellet ridge; PIN,
Pinnsvinryggen (informal name); SLA, Slangen and ORM, Ormen.
photosynthesis. The environment seems to have been closely
similar to that of the present-day McMurdo Dry Valleys in
Antarctica, although with less extreme seasonality owing to its
low latitude13.
(6) Water levels and glacier extent underwent a series of oscil-
lations, recorded by switches between glacilacustrine diamic-
tite (FA6-D) and fluvial, lacustrine and lake-margin sediments
(FA2-5) in W2. Sedimentation rates inferred from annual
rhythmites in W2 suggest that each retreat phase may have
lasted 104years.
(7) A second major ice advance marks the base of W3, with
widespread deposition of subglacial tills and glacitectonism of
underlying sediments. Basal tills are absent from the northern-
most locality, but close proximity of glacier ice is recorded by
grounding-line fans (FA6-G; Supplementary Fig. 4h,i).
(8) Ice retreated while the basin remained flooded and glaci-
genic sediment continued to be delivered to the lake by
ice rafting. Thin laminated carbonates (FA5) in W3 indi-
cate periods of reduced glacigenic sedimentation, indicative
of minor climatic fluctuations over timescales of 103years
(Supplementary Fig. 5g).
(9) A sharp contact with overlying laminated ‘cap’ carbonate
(Supplementary Fig. 2) records the transition to post-glacial
conditions. At some localities, basal conglomerates provide ev-
idence of subaerial exposure, followed by marine transgression.
The cap carbonate closely resembles basal Ediacaran carbonates
elsewhere, and marks global deglaciation, eustatic sea-level rise
and connection of the basin to the sea1,12,14.
Environmental and atmospheric conditions during deposition
of W2 and W3 can be further elucidated by isotopic data from
carbonate-associated sulphate in lacustrine limestones (Fig. 2
and Supplementary Fig. 6). These exhibit negative to extremely
negative 117O values, with consistent linear co-variation with δ34S,
indicating mixing of pre-glacial sulphate and isotopically light
sulphate formed in a CO2-enriched atmosphere15,16. The observed
0
−1.6
−1.4
−1.2
−1.0
−0.8
−0.6
−0.4
−0.2
0.0
y = 0.106x − 3.37
Heavy
endmember
Towards light
endmember
r2 = 0.59
W2 limestones, existing data
W2 limestones, new data
W3 limestones, all new data
51015202530
CAS δ34S (% )
%
CAS Δ17O (% )
%
Figure 2 | Co-variation of 117O and δ34S from carbonate-associated
sulphate in W2 and W3. ‘Existing data’ (ref. 16) and new data define a
mixing line between pre-glacial sulphate (top) and an isotopically light
sulphate formed by oxidation of pyrite including incorporation of a
light-117O signature from a CO2-enriched atmosphere. Data from W2 and
W3 lie on closely similar trend lines, indicating no detectable change in
pCO2between deposition of the two members.
values could reflect non-unique combinations of pCO2,pO2, O2
residence time and other factors, but a box model17 indicates pCO2
was most likely 10 to 100 mbar (1 mbar =1,000 ppmv).
These values are far too high to permit formation of low-
latitude ice sheets in the Neoproterozoic, but they are consistent
with a late-stage Snowball Earth. For an ice-free Neoproterozoic
Earth, model studies indicate mean terrestrial temperatures in the
range 30–50 C for pCO2=10 to 100 mbar (ref. 18). Formation of
low-latitude ice sheets requires much lower pCO2, on the order of
0.1–1 mbar (refs 2,19,20). Once formed, however, ice sheets can
persist despite rising CO2from volcanic outgassing, as a result of
a high planetary albedo. This hysteresis in the relationship between
pCO2and planetary temperature is a key element of Snowball Earth
theory. It implies that W2 and W3 were deposited relatively late in
the Marinoan, after volcanic outgassing had raised pCO2from 0.1 or
1 mbar to 10 or 100 mbar. Modelled silicate weathering and volcanic
outgassing rates indicate that this would require 106to 107years21.
The consistent co-variation of 117O and δ34S in lacustrine
limestones in both W2 and W3 suggests no detectable rise in
atmospheric pCO2, as this would alter the slope of the mixing
line (Fig. 2). This implies that the glacier oscillations recorded
in W2 and W3 occurred during a relatively short time interval
(<105years21) towards the end of the Marinoan. In turn, this implies
that the remainder of the Wilsonbreen Formation (including the
basal weathering horizon) represents many millions of years, during
which pCO2built up from the low values necessary for inception
of low-latitude glaciation to those indicated by the geochemical
evidence. The weathering horizon provides direct evidence of cold,
arid conditions during this interval, before the appearance of fluvial
and glacilacustrine sediments in the basin.
The evidence for ice sheet advance/retreat cycles at low latitudes
in a CO2-enriched atmosphere motivated a series of numerical
simulations to test the hypothesis that these cycles were linked
to Milankovitch orbital variations. We employed asynchronous
coupling of a three-dimensional ice sheet model and an atmospheric
general circulation model using the continental configuration of
ref. 22. We first ran simulations with a modern orbital configuration
to examine ice sheet behaviour through a large range of pCO2
values from 0.1 to 100 mbar (ref. 23; Supplementary Figs 7–10).
Consistently with previous results2,20, at low pCO2(0.1 mbar), global
ice volume reaches 170 ×106km3, but substantial tropical land
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NATURE GEOSCIENCE DOI: 10.1038/NGEO2502 LETTERS
Surface NH
Surface SH
Land ice surface
(×1013 m2)
0
−1,000
−600
−200
1,000
600
200
1,000
2,000
3,000
4,000
Ice thickness (CSO, 30 kyr of simulation)Ice thickness (WSO, 20 kyr of simulation)
Ice thickness (WSO minus CSO)
0°
20° N
20° S
40° N
a
40° S
60° S
0°
20° N
20° S
40° N
40° S
60° S
0°
20° N
20° S
40° N
40° S
60° S
2.5
3.0
5.5
6.0
0 10,000 20,000
Time (yr)
30,000 40,000
[CSO][WSO] [WSO][CSO]
m
m
Latitude
Latitude
Latitude
b
cd
0.50
0.50
0.50
Figure 3 | Modelled ice sheet oscillations in response to orbital forcing. a,b, Shaded contours show land ice thickness obtained with 20 mbar of carbon
dioxide in response to changes of orbital forcing (WSO (a)) and CSO (b), warm/cold summer orbit for the northern hemisphere) over the course of two
precession cycles (40kyr of simulation). In the light brown continental areas without ice, the white line is used to represent the old ice sheet extension
(WSO case). The Svalbard area is indicated by a red circle. c, Ice thickness variation in 10 kyr (WSO case after 20 kyr minus CSO case after 30kyr of
simulation). In acthe continental outline is shown by the 0.5m elevation contour. d, Surface of hemisphere covered by ice (m2) through time ([WSO] and
[CSO] indicate which orbital configuration is used).
areas remain ice free as a result of sublimation exceeding snowfall
(Supplementary Fig. 10a). Ice volume remains relatively constant
for pCO2=0.1 to 20 mbar (Supplementary Fig. 10b), owing to an
increase in accumulation that compensates for higher ablation rates
(Supplementary Fig. 13). In contrast, above 20 mbar, ice extent in the
eastern Tropics significantly decreases (Supplementary Fig. 10c). At
pCO2=100 mbar, most of the continental ice cover disappears, except
for remnants over mountain ranges (Supplementary Fig. 10d).
To test the sensitivity of the tropical ice sheets to Milankovitch
forcing, experiments with changing orbital parameters were initial-
ized using the steady-state ice sheets for pCO2=20 mbar. Although
obliquity has been invoked as a possible cause of Neoproterozoic
glaciations24, this mechanism remains problematical and cannot
account for significant climatic oscillations at low latitudes25,26 . We
therefore focused on precession as a possible driver, and used two
opposite orbital configurations, favouring cold and warm summers,
respectively, over the northern tropics (CSO: cold summer orbit
and WSO: warm summer orbit; Supplementary Fig. 14). Switch-
ing between these configurations causes tropical ice sheets to ad-
vance/retreat over several hundred kilometres in 10 kyr (Supple-
mentary Movie 1), with strong asymmetry between hemispheres
(Fig. 3). Shifting from WSO to CSO causes ice retreat in the South-
ern Hemisphere and ice sheet expansion in the Northern Hemi-
sphere (Supplementary Fig. 14c,d). Significant ice volume changes
occur between 30N and S, but are less apparent at higher lati-
tudes. This reflects higher ablation rates in the warmer low lati-
tudes (Supplementary Fig. 14e,h), and higher ice sheet sensitivity
to shifting patterns of melt. Larger greenhouse forcing at the end of
the Snowball event implies increasing ice sheet sensitivity to subtle
insolation changes. Given a strong diurnal cycle23, our simulations
also predict a significant number of days above 0C in the tropics
(Supplementary Fig. 15), consistent with geological evidence for
ice rafting, liquid water in lakes and rivers, and photosynthetic
microbial communities.
Our results show that geological evidence for glacial–interglacial
cycles5–7 is consistent with an enriched Snowball Earth theory. Ter-
mination of the Marinoan panglaciation was not a simple switch
from icehouse to greenhouse states, but was characterized by a
climate transition during which glacial cycles could be forced
by Milankovitch orbital variations. The geochemical evidence
presented here implies that at least the upper 60–70% of the
Wilsonbreen Formation was deposited in 105years, on the as-
sumption that a trend in pCO2would be evident over longer
timescales21. Rates of CO2build-up, however, may have slowed in
the later stages of Snowball Earth owing to silicate weathering of
exposed land surfaces, so it is possible that the oscillatory phase was
more prolonged.
Initiation of low-latitude glaciation in the Neoproterozoic re-
quires low pCO2(0.1–1 mbar; refs 2,19,20), implying that the os-
cillatory phase was preceded by a prolonged colder period (106
to 107years) during which pCO2increased gradually as a result of
volcanic outgassing21. This timescale is in agreement with recent
dates indicating the Marinoan lasted 15 million years27. The basal
weathering horizon is consistent with a period of low temperatures
and limited hydrologic cycle before the oscillatory phase2,19 .
Further work is needed to refine the upper and lower limits
of pCO2conducive to climate and ice sheet oscillations in Snow-
ball Earth. Factors not included in the present model, such as
supraglacial dust or areas of ice-free tropical ocean28–30 , can be
expected to make the Earth system more sensitive to orbital forcing.
While many details remain to be investigated, our overall conclu-
sions remain robust.
The Neoproterozoic Snowball Earth was nuanced, varied and
rich. We anticipate that detailed studies of the rock record in other
parts of the world, in conjunction with numerical modelling studies,
will continue to yield insight into the temporal and regional diversity
of this pivotal period in Earth history.
Methods
Methods and any associated references are available in the online
version of the paper.
Received 17 February 2015; accepted 8 July 2015;
published online 24 August 2015
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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO2502
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Acknowledgements
This work was supported by the NERC-funded project GR3/ NE/H004963/1 Glacial
Activity in Neoproterozoic Svalbard (GAINS). Logistical support was provided by the
University Centre in Svalbard. This work was granted access to the HPC resources of
CCRT under allocation 2014-017013 made by GENCI (Grand Equipement National de
Calcul Intensif). We also thank D. Paillard and P. Hoffman for stimulating discussions
and valuable insights.
Author contributions
Field data were collected and analysed by I.J.F., D.I.B., E.J.F., M.J.H., E.A.McM., M.S.P.,
P.M.W. and C.T.E.S. Geochemical analyses were conducted by H.B. and P.M.W. Model
experiments were designed and conducted by G.L.H., Y.D., C.D. and G.R. The
manuscript and figures were drafted by D.I.B., I.J.F. and G.L.H., with contributions from
the other authors.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints.
Correspondence and requests for materials should be addressed to D.I.B.
Competing financial interests
The authors declare no competing financial interests.
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NATURE GEOSCIENCE DOI: 10.1038/NGEO2502 LETTERS
Methods
Sedimentology. Lithofacies were classified based on grain size, internal
sedimentary structures and deformation structures, and bounding surfaces.
Detailed stratigraphic logs were made in the field, supplemented by drawings and
photographs of key features. Samples were taken for polishing and thin sectioning,
to allow detailed examination of microstructures in the laboratory. In addition,
data were collected on clast lithology, shape, surface features and fabric.
Diamictites of the Wilsonbreen Formation are commonly very friable, allowing
included clasts to be removed intact from the surrounding matrix, allowing
measurement of both clast morphology and orientation, using methods developed
for unlithified sediments. Clast morphology (shape, roundness and surface texture)
was measured for samples of 50 clasts to determine transport pathways. Clast fabric
analysis was performed by measuring a-axis orientations of samples of 50 clasts
with a compass-clinometer, and data were summarized using the eigenvalue or
orientation tensor method. Oriented samples for measurement of anisotropy of
magnetic susceptibility (AMS) were collected using a combination of field-drilling
and block sampling. AMS was measured using an AGICO KLY-3 Kappabridge
operating at 875 Hz with a 300 A m1applied field at the University of Birmingham
and an AGICO MFK-1A Kappabridge operating at 976Hz with a 200 A m1
applied field at New Mexico Highlands University.
Geochemistry. Laboratory procedures for extracting, purifying and measuring the
triple oxygen (δ18O and 117O) and sulphur (δ34 S) isotope composition of
carbonate associated sulphate (CAS) in bulk carbonates are detailed in ref. 16.
Briefly, fresh carbonate-bearing rock chips were crushed into fine grains and
powders using mortar and pestle. Rinsing the fines with 18 Mwater revealed little
water-leachable sulphate in any of the Wilsonbreen carbonates. Subsequently,
about 10 to 30 g carbonates were slowly digested in 1–3 M HCl solutions. The
solution was then centrifuged, filtered through a 0.2µm filter, and acidified before
saturated BaCl2droplets were added. BaSO4precipitates were collected after >12 h
and purified using the DDARP method (see Supplementary Information). The
purified BaSO4was then analysed for three different isotope parameters: 117O, by
converting to O2using a CO2-laser fluorination method; δ18O, by converting to CO
through a Thermal Conversion Elemental Analyzer (TCEA) at 1,450C; and δ34 S,
by converting to SO2by combustion in tin capsules in the presence of V2O5
through an Elementar Pyrocube elemental analyser at 1,050 C. The 117O was run
in dual-inlet mode, whereas the δ18O and δ34 S were run in continuous-flow mode.
Both the 117O and δ18 O were run on a MAT 253 at Louisiana State University,
whereas the δ34S was determined on an Isoprime 100 continuous-flow mass
spectrometer at the University of Lancaster, UK. The 117O was calculated as
117Oδ017 O0.52×δ018 O in which δ01,000ln (Rsample/Rstandard ) and Ris the
molar ratio of 18O/16 O or 17O/16O. All δvalues are in Vienna Standard Mean Ocean
Water VSMOW and Vienna Canyon Diablo Troilite (VCDT) for sulphate oxygen
and sulphur, respectively. The analytical standard deviation (1σ) for replicate
analysis associated with the 117O, δ18O and δ34 S are ±0.05h,±0.5hand ±0.2h,
respectively. As the CAS is heterogeneous in hand specimens, the standard
deviation is for laboratory procedures. δ34S values were corrected against VCDT
using within-run analyses of international standard NBS-127 (assuming δ34S values
of +21.1h). Within-run standard replication (1 s.d.) was <0.3h. All geochemical
data are included in Supplementary Table 1.
Numerical modelling. Model runs were conducted with a coupled
atmospheric general circulation model (LMDz) and ice sheet model (GRISLI:
GRenoble Ice Shelf and Land Ice model). LMDz (spatial resolution 4in latitude
×5in longitude with 38 vertical levels) was run with prescribed continental ice
to climatic equilibrium. GRISLI has a 40km grid size and is driven with
downscaled climatic fields of surface air temperature, precipitation and
evaporation. To capture ice sheet–climate feedbacks, LMDz is rerun using the new
ice sheet distribution and topography. This procedure was repeated each 10 kyr to
investigate orbital forcing.
Surface mass balance (accumulation minus sublimation and melting) was
computed from monthly mean temperature, precipitation and evaporation rate.
Melt rate is calculated using the positive-degree-day method.
No sea ice dynamics treatment is specified, the sea ice cover is prescribed and a
thickness of 10 m is imposed. Ice albedo is fixed at 0.6, whereas snow albedo varies
from 0.9 from 0.55 as a function of the zenith and ageing process. Land ice/snow
free surface has the characteristic of a bare soil (rocky regolith) with an
albedo of 0.3.
Code availability. Code for the GCM LMDz can be accessed at:
http://lmdz.lmd.jussieu.fr. Code for the ISM GRISLI (GRenoble Ice Shelf and Land
Ice model) is not available.
Further details of the methods and modelling procedures are provided in the
Supplementary Information in the online version of the paper.
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... This work focuses on marine deglaciation. The impact of continental deglaciation is not considered in the model simulation as it may not be critically important since the tropical land ice had probably been ablated before the marine deglaciation started according to Benn et al. (Benn et al., 2015). ...
... where T i is the monthly mean surface temperature for the i th month. Although it varies from location to location, it has been found that on average 8 mm of ice can be melted per PDD (Benn et al., 2015). It is often assumed that 50-60% of the meltwater is refrozen to the ice (Tarasov and Peltier, 1997), so 4 mm of net melting of ice per PDD is assumed herein. ...
... The exact location and geographical extent of the ridge are not important as long as it can obstruct the zonal ocean currents in the tropical region. The continental ice sheets in the tropical region might have been ablated at the start of the deglaciation (Benn et al., 2015) and is therefore not considered. ...
Article
During the Neoproterozoic snowball Earth events, the climate was cold and the oceans were covered by marine ice of ~1000 m thick (sea glacier). Extremely high CO2 level was required in order to trigger the deglaciation these events. It is unclear how long it would take for the sea-ice cover to be completely ablated after the deglaciation started, and what the physical state of the ocean looked like. Here we use a fully coupled general circulation model, CCSM3, to evaluate the rate of poleward retreating of the marine ice and estimate an upper-bound timescale for the marine deglaciation. It is found that the deglaciation will take at most ~300 to 1500 years, depending on the surface albedo of ice; as the albedo decreases, the deglaciation will be prolonged due to the lower CO2 level required to start the deglaciation. Such fast calving/melting of sea ice allows a fresh water lid of ~800 m to develop at the end of the deglaciation.
... However, in the terrestrial realm, glacial advances and retreats have been documented late in the Marinoan ice age when PCO2 was high and these were linked to model solutions of Milankovitch forcing (Benn et al., 2015). In the marine realm, the persistence of ice cover throughout the glaciation is directly contradicted by sedimentological evidence of open water within glacigenic deposits of both Sturtian (Spence et al., 2016) and Marinoan (Lang et al., 2018) times. ...
... Bao et al., 2012). Furthermore, Svalbard is the only location where the anomaly is present in the underlying Marinoan deposits where it has been used to infer high syn-glacial carbon dioxide levels when the sulphate was created (Bao et al., 2009;Benn et al., 2015). Additionally, insights into sediment accumulation rates allows a reconciliation between the traditionally contrasting models of rapid versus slow cap carbonate formation. ...
... It displays interstratified limestone and glacigenic sediments near its top and is succeeded by a thin limestone interpreted as an offshore deposit, typical for a Sturtian cap (Fairchild et al., 2016a). The upper glacigenic unit, the Wilsonbreen Formation (Figure 2), is non-marine (Benn et al., 2015;Fleming et al., 2016;Fairchild et al., 2016b), and is overlain by a cap dolostone which is the subject of this paper (Figure 3). This is correlated with the Marinoan cap carbonate, the lower boundary of which was used to define the base of the Ediacaran System in South Australia (Knoll et al, 2006). ...
Article
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Two cap carbonates overlying Cryogenian panglacial deposits are found in North‐East Svalbard of which the younger (635 Ma) forms the base of the Ediacaran Period. It is represented by a transgressive succession in which laminated dolostone, typically around 20 m thick (Member D1), is succeeded transitionally by a similar thickness of impure carbonates (Member D2). In Spitsbergen, there is evidence of microbially influenced sediment stabilisation and carbonate precipitation in the lower part of D1, whilst the upper part of D1 and D2 show centimetre‐decimetre‐scale graded units with undulatory lamination interpreted as evidence of storm activity. Carbonate originated as possible freshwater whitings, as well as microbial precipitates. Exhumed and eroded hardgrounds display replacive 10‐30 μm dolomite crystals with cathodoluminescence characteristics consistent with early diagenetic manganese and iron reduction. Regionally, carbon isotope values consistently decrease by around 2‰ from around ‐3‰ over 30 m of section which is both a temporal and a bathymetric signal, but not a global one. An exponential decline in carbonate production predicted by box models is fitted by a semi‐quantitative sedimentation model. A mass‐anomalous 17O depletion in carbonate‐associated sulphate in dolomite, inherited from precursor calcite, decreases from ‐0.6 to ‐0.3‰ in the basal 15 m of section and then approaches background values. The post‐glacial anomalous 17O depletion in carbonate‐associated sulphate and barite elsewhere has been interpreted in terms of ultra‐high pCO2 at the onset of deglaciation. Such anomalies, with larger amplitude, have been reported in Svalbard from underlying lacustrine and tufaceous limestones representing a hyperarid glacial environment. The anomalous sulphate could be produced contemporarily, or the internally drained landscape may have continued to release 17O‐anomalous sulphate as it was transgressed during cap carbonate deposition. The late Cryogenian to earliest Ediacaran record in Svalbard provides the most complete record of the basal 17O‐depletion event in the world.
... The consequences of the South Qinling rift system have been explored using climate-ice-sheet-carbon modeling. We simulated the pCO 2 evolution by including two major constraints: (a) the duration of snowball Earth, which is 57 Myr for the Sturtian (Condon et al., 2005;Hoffman et al., 1998;Zhou et al., 2019) in comparison to <16 Myr for the Marinoan (Nelson et al., 2020) and (b) the continental ice sheet response to orbital forcing as a function of CO 2 levels (Benn et al., 2015) to compute the weathering efficiency and the coverage of land ice (Supporting Information). Our simulations reveal that, despite the presence of a massive ice sheet (170 millions of km 3 ; Figure S10 in Supporting Information S1), the meltwater flux remains too limited to counteract the sluggish hydrological cycle (Abbot et al., 2012). ...
... bar) based on simulations. The interplay between the hydrological cycle and high pCO 2 caused slow warming, which would reduce snow albedo (de Vrese et al., 2021) and the accumulation of dusty ice in the (Benn et al., 2015) (a) Temporal pCO 2 evolution with different background degassing rates in the context of a hard snowball Earth. Dashed lines represent the amount of carbon between short and long snowball events. ...
... Dashed lines represent the amount of carbon between short and long snowball events. Melting thresholds for short and long snowball events are based on their duration (16 and 57 Myr, respectively), all being encompassed within thresholds estimated by climate models from 0.29 (Pierrehumbert, 2004) to 0.05 bars (Benn et al., 2015) assuming a white surface (snow and ice). Melting thresholds for a mudball earth (Abbot & Pierrehumbert, 2010) can be estimated to be at least 1/3 of a clean snowball earth (i.e., 0.1 instead of 0.29 bar) while de Vrese et al. (2021) suggests lower pCO 2 for initiating melting. ...
Article
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The Cryogenian Period (717–635 Ma) experienced two low‐latitude “snowball Earth” glaciations, the Sturtian and the Marinoan of contrasting 57 and <16 Myr durations, respectively. A lack of reliable age controls on extensional tectonics and associated magmatic rocks during the Marinoan has hampered an understanding of the deglaciation. Furthermore, although deglaciation is generally assumed to have occurred once ongoing magmatism accumulated enough atmospheric CO2, as suggested by cap carbonates, specific geologic evidence linking volcanic events with deglaciation are lacking. Here, we present high‐precision zircon geochronology with chemical abrasion‐isotope‐dilution isotope ratio mass spectrometry that indicates an extensive and thick sequence of rift‐related magmatic rocks in South Qinling, Central China, erupted 2–6 Myr before the termination of the Marinoan. Climate modeling proposes a scenario explaining why the Marinoan was the shorter snowball and how volcanism may have driven the deglaciation.
... Orbital forcing has presumably operated throughout Earth history with evidence found as old as ca. 2480 Myr ago [1][2][3][4] , and may have influenced the course of severe "snowball Earth" glaciations 5,6 , notably during the Cryogenian Period, about 720 to 635 Myr ago. Under snowball conditions, astronomical-induced variations in insolation due to Earth's precession, obliquity, and eccentricity should be ongoing, but the range of associated climate variability and mechanisms in an icecovered ocean are poorly understood. ...
... Under snowball conditions, astronomical-induced variations in insolation due to Earth's precession, obliquity, and eccentricity should be ongoing, but the range of associated climate variability and mechanisms in an icecovered ocean are poorly understood. Recent ice sheet and atmospheric modeling results indicate that orbital forcing should be a viable climate driver under a wide range of atmospheric concentrations of CO 2 between 0.1 and 200 mbar, and therefore was likely to have modulated ice sheet volume throughout much of the Cryogenian Period 5 . ...
... While evidence for orbital forcing has been suggested in Cryogenian glacial successions 5 , evidence for multiple, internally consistent timescales of orbital forcing has not been demonstrated. This is likely because most glacial deposition occurs irregularly 7 , forming glacial diamictites (that is, lithified sediments comprising chaotic mixtures of a wide range of clast-and grain-size distribution) only rarely preserving stratification. ...
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The snowball Earth hypothesis—that a runaway ice-albedo feedback can cause global glaciation—seeks to explain low-latitude glacial deposits, as well as geological anomalies including the re-emergence of banded iron formation and “cap” carbonates. One of the most significant challenges to snowball Earth has been sedimentological cyclicity that has been taken to imply more climate dynamics than expected when the ocean is completely covered in ice. However, recent climate models suggest that as atmospheric CO2 accumulates, the snowball climate system becomes sensitive to orbital forcing. Here we show the presence of nearly all Milankovitch (orbital) cycles preserved in stratified banded iron formation deposited during the Sturtian snowball Earth. These results provide evidence for orbitally forced cyclicity of global ice sheets that resulted in periodic oxidation of ferrous iron. Orbital glacial advance and retreat cycles provide a simple mechanism to reconcile both the sedimentary dynamics and the enigmatic survival of multicellular life during snowball Earth.
... Well-dated geological data indicate that the Earth experienced orbitally paced climate changes since at least the late Precambrian -1.4 billion years ago during the Proterozoic Eon (Benn et al., 2015;Zhang et al., 2015; 40 Hoffman et al., 2017;Meyers and Malinverno, 2018), and all along the Phanerozoic (Lisiecki and Raymo, 2005;Liebrand et al., 2011;Miller et al., 2011;Kent et al., 2017Kent et al., , 2018Olsen et al., 2019;Drury et al., 2020;Westerhold et al., 2020). These changes reflect the variations in the Earth's axis of rotation -precession and tilt, and in the geometry of the Earth's orbit around the sun -eccentricity, driven by gravitational interactions within the Solar system (Berger, 1977;Laskar et al., 2011). ...
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Abrupt climate changes are defined as sudden climate changes that took place over tens to hundreds of years or recurred at millennial timescales; they are thought to involve processes that are internal to the climate system. By contrast, astronomically forced climate changes involve processes that are external to the climate system and whose multi-millennial quasi-periodic variations are well known from astronomical theory. In this paper, we re-examine the main climate variations determined from the U1308 North Atlantic marine record, which yields a detailed calving history of the Northern Hemisphere ice sheets over the past 3.2 Myr. The magnitude and periodicity of the ice-rafted debris (IRD) events observed in the U1308 record allow one to determine the timing of several abrupt climate changes, the larger ones corresponding to the massive iceberg discharges labeled Heinrich events (HEs). In parallel, abrupt warmings, called Dansgaard–Oeschger (DO) events, have been identified in the Greenland records of the last glaciation cycle. Combining the HE and DO observations, we study a complex mechanism giving rise to the observed millennial-scale variability that subsumes the abrupt climate changes of last 0.9 Myr. This process is characterized by the presence of Bond cycles, which group DO events and the associated Greenland stadials into a trend of increased cooling, with IRD events embedded into every stadial, the latest of these being an HE. These Bond cycles may have occurred during the last 0.9 Ma when Northern Hemisphere ice sheets reached their maximum extent and volume, thus becoming a major player in this time interval's climate dynamics. Since the waxing and waning of ice sheets during the Quaternary period are orbitally paced, we conclude that the abrupt climate changes observed during the Middle Pleistocene and Upper Pleistocene are therewith indirectly linked to the astronomical theory of climate.
... Well-dated geological data indicate that the Earth experienced orbitally paced climate changes since at least the late Precambrian -1.4 billion years ago during the Proterozoic Eon (Benn et al., 2015;Zhang et al., 2015; 40 Hoffman et al., 2017;Meyers and Malinverno, 2018), and all along the Phanerozoic (Lisiecki and Raymo, 2005;Liebrand et al., 2011;Miller et al., 2011;Kent et al., 2017Kent et al., , 2018Olsen et al., 2019;Drury et al., 2020;Westerhold et al., 2020). These changes reflect the variations in the Earth's axis of rotation -precession and tilt, and in the geometry of the Earth's orbit around the sun -eccentricity, driven by gravitational interactions within the Solar system (Berger, 1977;Laskar et al., 2011). ...
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Abrupt climate changes constitute a relatively new field of research, which addresses variations occurring in a relatively short time interval of tens to a hundred years. Such time scales do not correspond to the tens or hundreds of thousands of years that the astronomical theory of climate addresses. The latter theory involves parameters that are external to the climate system and whose multi-periodic variations are reliably known and almost constant for a large extent of Earth history. Abrupt changes, conversely, appear to involve fast processes that are internal to the climate system; these processes varied considerably during the past 2.6 Myr, and yielded more irregular fluctuations. In this paper, we re-examine the main climate variations determined from the U1308 North Atlantic marine record, which yields a detailed calving history of the Northern Hemisphere ice sheets over the past 3.2 Myr. The magnitude and periodicity of the ice-rafted debris (IRD) events observed in the U1308 record allow one to determine the timing of several abrupt climate changes, the larger ones corresponding to the massive iceberg discharges labeled Heinrich events (HEs). In parallel, abrupt warmings, called Dansgaard-Oeschger (DO) events, have been identified in the Greenland records of the last glaciation cycle. Combining the HE and DO observations, we study a complex mechanism that may lead to the observed millennial-scale variability corresponding to the abrupt climate changes of last 0.9 Myr. This mechanism relies on amended Bond cycles, which group DO events and the associated Greenland stadials into a trend of increased cooling, with IRD events embedded into every stadial, the latest of these being an HE. These Bond cycles may have occurred during the last 0.9 Ma when Northern Hemisphere ice sheets reached their maximum extent and volume, thus becoming a major player in this time interval’s climate dynamics. Since the waxing and waning of ice sheets during the Quaternary period are orbitally paced, we conclude that the abrupt climate changes observed during the Mid and Upper Pleistocene are therewith indirectly linked to the astronomical theory of climate.
... The lithology of the Fiq Formation indicates a "soft" Snowball Earth scenario (Allen and Etienne, 2008). The hydrological cycle during the Snowball Earth events should not be completely shut down (Allen, 2007;Rieu et al., 2007), and should be sensitive to astronomical forcing (Benn et al., 2015;Mitchell et al., 2021b), at least in low-latitude areas where Oman was located (Kempf et al., 2000;Kilner et al., 2005). During the warm and humid periods, chemical weathering would enhance the production of clay minerals, which are efficiently transported by large runoff fluxes to the ocean, leading to higher GR values. ...
Article
Cyclostratigraphy is a powerful tool in chronostratigraphic studies, and has shown its potential in Precambrian time. The Cryogenian Period is one of the most dynamic and intriguing intervals in Earth history and it is a witness to the most extreme climate changes on the planet, known as two Snowball Earth events. However, a robust chronostratigraphic framework is still lacking especially for the younger Marinoan Snowball Earth event, hampering the stratigraphic correlation and comparison among geographically separated records, as well as the understanding of the initiation and termination of this global-scale glaciation. The Cryogenian Fiq Formation in Oman is thought to be Marinoan-equivalent. The duration of the nonglacial units in the Fiq Formation could provide a conservative age constraint on the previously not well-determined onset age of the Marinoan glaciation. In this work, a cyclostratigraphic study has been conducted on three drill cores of the Fiq Formation. The multi-taper method (MTM) spectral analysis, the correlation coefficient (COCO), and the evolutionary correlation coefficient (eCOCO) analyses were performed on the high-resolution gamma-ray (GR) data from the three cores. A hierarchy of Milankovitch cycles (eccentricities, obliquities, and precessions) has been identified, which was used to estimate the optimal sedimentary accumulate rates and the duration of the Fiq Formation in each core. Based on the cyclostratigraphic results, the time scale of the Fiq Formation is at least ~6 Myr. Integrating the radiometric ages and the cyclostratigraphic data from other continents, the onset age of the Marinoan glaciation is suggested to be 650–641 Ma. This new result helps improve the chronostratigraphic framework for the Cryogenian Period and also provides critical temporal constraints for the geological and numerical models that explore the possible mechanisms for the Snowball Earth events.
Article
When a snowball Earth deglaciates through a very high atmospheric CO2 concentration, the resulting inflow of freshwater leads to a stably stratified ocean, and the strong greenhouse conditions drive the climate into a very warm state. Here, we use a coupled atmosphere–ocean general circulation model, applying different scenarios for the evolution of atmospheric CO2, to conduct the first simulation of the climate and the three-dimensional ocean circulation in the aftermath of the Marinoan snowball Earth. The simulations show that the strong freshwater stratification breaks up on a timescale of the order of 103 years, mostly independent of the applied CO2 scenario. This is driven by the upwelling of salty waters in high latitudes, mainly the Northern Hemisphere, where a strong circumpolar current dominates the circulation. In the warmest CO2 scenario, the simulated Marinoan supergreenhouse climate reaches a global mean surface temperature of about 30 ∘C under an atmospheric CO2 concentration of 15×103 parts per million by volume, which is a moderate temperature compared to previous estimates. Consequently, the thermal expansion of seawater causes a sea-level rise of only 8 m, with most of it occurring during the first 3000 years. Our results imply that the surface temperatures of that time were potentially not as threatening for early metazoa as previously assumed. Furthermore, the short destratification timescale found in this study implies that Marinoan cap dolostones accumulated during the deglacial period, given that they were deposited under the influence of a freshwater environment.
Article
Global Neoproterozoic glaciations are related to extreme environmental changes and the reprise of iron formation in the rock record. However, the lack of narrow age constraints on Cryogenian successions bearing iron-formation deposits prevents correlation and understanding of these deposits on a global scale. Our new multiproxy data reveal a long Cryogenian record for the Jacadigo Group (Urucum District, Brazil) spanning the Sturtian and Marinoan ice ages. Deposition of the basal sequence of the Urucum Formation was influenced by Sturtian continental glaciation and was followed by a transgressive interglacial record of >600 m of carbonates that terminates in a glacioeustatic unconformity. Overlying this, there are up to 500 m of shale and sandstone interpreted as coeval to global Marinoan glacial advance. Glacial outwash delta deposits at the top of the formation correlate with diamictite-filled paleovalleys and are covered by massive Fe and Mn deposits of the Santa Cruz Formation and local carbonate. This second transgression is related to Marinoan deglaciation. Detrital zircon provenance supports glaciostatic control on Cryogenian sedimentary yield at the margins of the Amazon craton. These findings reveal the sedimentary response to two marked events of glacioeustatic incision and transgression, culminating in massive banded iron deposition during the Marinoan cryochron.
Article
Otavi Group is a 1.5−3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran−Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group. Subsidence accommodating Otavi Group resulted from S−N crustal stretching (770−655 Ma), followed by post-rift thermal subsidence (655−600 Ma). Rifting under southern Swakop Group continued until 650−635 Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S−N plate divergence. Rift related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746 Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks. Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770−717 Ma; (2) glacial early Cryogenian/Sturtian, 717−661 Ma; (3) non-glacial middle Cryogenian, 661−646±5 Ma; (4) glacial late Cryogenian/Marinoan, 646±5−635 Ma; and (5) non-glacial early Ediacaran, 635−600±5 Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585 measured sections (totaling >190 km of strata) and >8,764 pairs of δ13C/δ18Ocarb analyses (tabulated in Supplementary On-line Information). Each glaciation began and ended abruptly, and each was followed by anomalously thick ‘catch-up’ depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5−9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3−8.8 m Myr−1, despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters ≤1.6 km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice-grounding line is required in the 110 Sturtian measured sections in our survey. In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9 m regional average thickness, n=140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement ≤90 m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over ≥0.6 km of paleobathymetric relief. Late Tonian shallow-neritic δ13Ccarb records were obtained from the 0.4-km-thick Devede Fm (~770−760 Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (~737−717 Ma) in Swakop Group. Devede Fm is isotopically heavy, +4−8‰ VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746 Ma volcanics and shows two negative excursions bridged by heavy δ13C values. The negative excursions could be correlative with Russøya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia. Middle Cryogenian neritic δ13C records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0−3.5‰. Early Ediacaran neritic δ13C records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (≤+11‰) with extreme point-to-point variability (≤10‰) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform δ13C records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9±1.5‰. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater. Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60−0.58 Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding ≥1.85 km of megakarstic relief, and kmthick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.
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This work presents the detailed geology of Svalbard. It arises from about 50 years of research in many aspects of Svalbard geology by the author, with many colleagues and collaborators. The work is divided into four parts, the first being introductory, setting the scene and outlining the main geological features and the principal geological conventions used throughout. Part two divides Svalbard into eight regions, describing each in a separate chapter. Part three looks at historical events and environments, and part four comprises a summary of the economic aspects of Svalbard geology, plus indexes and reference lists.
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