Are basal Ediacaran (635 Ma) post-glacial “cap dolostones” diachronous?

doi:10.1016/j.epsl.2007.03.032

Earth and Planetary Science Letters

Volume 258, Issues 1–2, 15 June 2007, Pages 114-131

https://doi.org/10.1016/j.epsl.2007.03.032 Get rights and content

Abstract

A layer of shallow-water dolostone (“cap dolostone”) with idiosyncratic sedimentary structures was deposited across continental margins world-wide in the aftermath of the terminal Cryogenian snowball Earth. The dolostone has a global average thickness of 18.5 m and is interpreted stratigraphically in different ways in the current literature: as diachronous (top and bottom) and tracking glacioeustatic flooding, as semi-diachronous (bottom diachronous, top isochronous) and outlasting the flood, or as isochronous (top and bottom) and recording ocean-wide changes over time subsequent to deglaciation. Each interpretation carries a different implication for the timescale of cap dolostones and their isotopic signatures, and therefore for their origin.

In northern Namibia, we studied the Keilberg cap dolostone (635 Ma) across the Otavi carbonate bank and down a contiguous submarine paleoslope to estimated depths of ∼ 0.5 km. We find giant wave ripples and other wave-generated structures in all areas, including the lower slope, pointing to a base-level change of large amplitude. No other formation in the carbonate succession contains wave-generated bedforms on the lower slope.

Carbon isotope records from the bank are similar in shape and absolute value, irrespective of thickness. Slope records are also similar to one another, but different in shape and value from those on the bank. If the cap is isochronous, lower-slope waters were enriched in ¹³C by 2–3‰ compared with the bank, which seems improbable. If diachronous, the lower slope, upper slope and bank records collectively describe a sigmoidal δ¹³C curve over time with a net decline of 4.4‰. In addition, a lateral gradient of 1.0‰/100 km existed from the inner to outer bank.

If the flooding was rapid (< 10 kyr), as suggested by ice-melting models, the δ¹³C change may reflect strong surface warming, methane release, and kinetic isotope effects associated with rapid carbonate production. If the transgression was prolonged (> 100 kyr), as implied by actualistic interpretation of paleomagnetic reversals in this and other cap dolostones, the δ¹³C change could record Rayleigh distillation associated with the drawdown of a large atmospheric CO₂ reservoir, built up during the preceding snowball glaciation. Either way, the sedimentology and isotopes support the diachronous interpretation, and are inconsistent with the semi-diachronous and isochronous models. The base-level rise of ∼ 0.5 km implies a glacioeustatic origin, meaning that cap dolostone sedimentation was synchronous with land ice melting. This leaves the actualistic interpretation of reversal frequency and speed in cap dolostones in conflict with ice-melting models.

Introduction

The last snowball Earth episode ended in 635 Ma [1] with the deposition of meters to decameters of dolomite ([CaMg]CO₃) on continental margins and inland seas world-wide (Table 1). “Cap dolostones” [2], [3] represent a unique perturbation in the saturation state of the ocean. They sharply overlie terminal glacigenic strata (or an equivalent hiatus) and typically underlie limestones or fine-grained clastics of deeper water origin. The genesis of cap dolostones and their significance with respect to the glaciation continue to be debated [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], but many agree that they accompanied a major marine transgression (rise in relative sea level), presumed to reflect the melting of continental ice sheets. In terms of sequence stratigraphy, cap dolostones form the transgressive systems tract of the post-glacial depositional sequence [9]. If the average ice thickness over all continents, continental shelves and banks (40% of global surface area) was 1.0 km [18], [19], [20], [21], [22], its melting would have caused a glacioeustatic rise of 0.6 km, equivalent to ∼ 0.45 km after hydroisostatic adjustment (assuming densities of 3.3 and 2.9 g cm^− 3 for mantle and crust, respectively).

Cap dolostones and related limestones (“cap carbonates” sensu lato) display a panoply of unusual sedimentary features, which occur in a broadly consistent vertical sequence on different paleomargins (Fig. 1). The sequence has been interpreted in three basically different ways. If the cap dolostone is isochronous (Fig. 2a), meaning that its base and top are approximately the same age everywhere, the vertical sequence must represent basin-wide environmental changes over time. For example, deposition of cap dolostone has been attributed to time-dependent gas-hydrate destabilization and methane oxidation [12]. If isochronous, the cap dolostone must have been deposited in different water depths. If its deposition followed glacioeustatic flooding [12], the timescale would not be limited by the estimated ice-sheet meltdown time of ∼ 2–10 kyr [21]. A second possibility is that the cap dolostone is semi-diachronous (Fig. 2b), meaning that its base is diachronous and tracked the glacioeustatic transgression, but its top is isochronous. For example, the switch in sedimentation from cap dolostone to overlying limestone has been attributed to ocean-wide mixing of saline glacial deep water and a low-density meltwater plume [15]. If the stable density stratification outlasted the glacioeustatic flood [15], the timescale for cap dolostone sedimentation would once again not be limited by the meltdown time. A third option is that cap dolostones are wholly diachronous (Fig. 2c), with bottoms and tops that are older or younger according to paleoelevation. If cap dolostone sedimentation tracked the glacioeustatic flood, it could have been deposited in the same range of water depths everywhere. Environmental changes over time would be reflected by differences between sections from lower and higher areas. A basin-wide event might register at the top of the dolostone in low areas and at the bottom in high areas. The timescale for deposition of any given section would be only a fraction of the overall cap dolostone timescale, which itself must be less than the meltdown time, assuming that dolostone sedimentation began when some grounded ice had already melted. The timescale over which the deposition occurred is critical to any interpretation of cap dolostones and their isotopic characteristics as this will determine the relative importance of ocean mixing, warming, gas exchange, and carbonate or silicate weathering.

In this study, we employ field sedimentology and δ¹³C_carb isotopic measurements from paleogeographically well-characterized sections of the 635-Ma Keilberg (pronounced kile´-bairg) cap dolostone in northern Namibia [23], [24], to determine if it is diachronous, semi-diachronous or isochronous. The diachronous transition from cap dolostone to limestone [17] will be treated in detail in a forthcoming paper incorporating data from Namibia and Canada.

Section snippets

Paleogeography and development of the Otavi carbonate bank

Between ∼ 770 Ma [24] and ∼ 580 Ma [25], [26], the (present) southwestern promontory of the Congo craton was a slowly-subsiding marine bank (platform), maintained by the deposition of ∼ 3 km of dominantly shallow-water carbonate strata of the Otavi Group (Fig. 3). The strata are exposed in an arcuate fold belt that rims the craton in northern Namibia. The southern edge of the bank is intersected in a structurally simple homocline on the south flank of Kamanjab Inlier (Fig. 4). To the south lies a

Stratigraphic expression of the Ghaub glaciation

Two discrete glacial episodes left their imprint on the Otavi bank [31], [24]). The first (Chuos Formation) occurred during the stretching stage, the second (Ghaub Formation) during thermal subsidence. South of the bank, a unit containing ice-rafted debris (IRD), stratigraphically equivalent to the Ghaub (gowp) Formation, hosts a volcanic ash containing zircons dated by U–Pb ID-TIMS at 635.5 ±0.5 Ma [23], [1]. The cap dolostone (Keilberg Member) above the Ghaub Formation (or its equivalent

Sedimentology of the Keilberg cap dolostone on the bank and slope

The Keilberg cap dolostone [31] is a laterally continuous layer of very pale grey, tannish or pinkish tinted, erosion-resistant dolomite. It is 65–75 m thick on the outer bank and tapers to 15–20 m on the inner bank. The taper is not due to differential accommodation. On the contrary, it is thicker on the elevated rim of the bank and thinner in the more-eroded interior (Fig. 5). On the slope, the cap dolostone tapers from 65–105 m on the upper slope to 5–10 m on the lower slope. Its overall

Carbon isotopes

Stable carbon isotope ratios were measured by methods previously described [24] in micro-drilled powders of closely-spaced samples from four sections on the bank, one on the upper slope, and four on the lower slope (Fig. 7). Bank section B1 is at the bank-edge, B4 is ∼ 100 km from the bank-edge, and B2 and B3 are at an intermediate distance (Fig. 4a). B2 overlies a basement structural high, Makalani Ridge (Fig. 3), and is nearly 3× thicker than B3, ∼ 300 km to the east, which is not known to

Sedimentary structures

Sedimentary structures (giant wave ripples, low-angle cross-stratification, reverse-graded peloids) indicate that the bulk of the cap dolostone on the lower slope originated above wave base, the same as on the bank. Given that the estimated paleo-relief of at least 0.5 km exceeds the depth of wave base at the most extreme [42], the top of the bank must have been above sea level when the dolostone was accumulating on the lower slope. A diachronous cap dolostone and a large base-level rise are

Conclusions

The existence of giant wave ripples and low-angle cross-stratification at an estimated paleodepth of 0.5 km below the carbonate bank-edge provides sedimentological evidence that the post-glacial Keilberg cap dolostone is diachronous, and was associated with a major marine flooding of glacioeustatic origin.

Carbon isotope records from the lower slope, upper slope and bank strongly support a diachronous interpretation and are incompatible with the semi-diachronous and isochronous models for this

Acknowledgements

This study was funded by the U.S. National Science Foundation grant EAR-0417422 (Geobiology and Environmental Geochemistry Program), Harvard University and Hamilton College. We are grateful to the Geological Survey of Namibia, Edila Köhler of farm Danubé 59, and other farm owners and residents for the privilege of doing field work in Namibia. Enthusiastic field assistance was provided by Mary Beth Day, Woody Fischer, Tim Fox, John Higgins, Matt Hurtgen, Gabe Jostrom, Francis Macdonald, Corey

References (80)

S.M. Porter et al.
Chemostratigraphy of Neoproterozoic cap carbonates from the Volta Basin, West Africa
Precambrian Res.
(2004)
E. Font et al.
Chemostratigraphy of the Neoproterozoic Mirassol d'Oeste cap dolostones (Mato Grosso, Brazil): an alternative model for Marinoan cap dolostone formation
Earth Planet. Sci. Lett.
(2006)
M.T. Hurtgen et al.
Sulfur cycling in the aftermath of a 635-Ma snowball glaciation: evidence for a syn-glacial sulfidic deep ocean
Earth Planet. Sci. Lett.
(2006)
Y. Donnadieu et al.
Is there a conflict between the Neoproterozoic glacial deposits and the snowball Earth interpretation: an improved understanding with numerical modeling
Earth Planet. Sci. Lett.
(2003)
R.I.F. Trindade et al.
Paleolatitude of glacial deposits and paleogeography of Neoproterozoic ice ages
C. R. Geoscience
(2007)
N. Eyles et al.
‘Zipper-rift’: a tectonic model for Neoproterozoic glaciations during the breakup of Rodinia after 750 Ma
Earth-Sci. Rev.
(2004)
S. Xiao et al.
The Neoproterozoic Quruqtagh Group in eastern Chinese Tianshan: evidence for a post-Marinoan glaciation
Precambrian Res.
(2004)
S.A. Kasemann et al.
Boron and calcium isotope composition in Neoproterozoic carbonate rocks from Namibia: evidence for extreme environmental change
Earth Planet. Sci. Lett.
(2005)
C.R. Calver et al.
The late Neoproterozoic Grassy Group of King Island, Tasmania: correlation and paleogeographic significance
Precambrian Res.
(2000)
A.J. Kaufman et al.
Stable isotope record of the terminal Neoproterozoic Krol platform in the Lesser Himalayas of northern India
Precambrian Res.
(2006)

F.A. Corsetti et al.

The relationship between the Neoproterozoic Noonday Dolomite and the Ibex Formation: new observations and their bearing on ‘snowball Earth’

Earth-Sci. Rev.

(2005)

D. Condon et al.

U–Pb ages from the Neoproterozoic Doushantuo Formation, China

Science

(2005)

G.E. Williams

Sedimentology, stable-isotope geochemistry and palaeoenvironment of dolostones capping late Precambrian glacial sequences in Australia

J. Geol. Soc. Aust.

(1979)

M.J. Kennedy

Stratigraphy, sedimentology, and isotopic geochemistry of Australian Neoproterozoic postglacial cap dolostones: deglaciation, δ¹³C excursions, and carbonate precipitation

J. Sediment. Res.

(1996)

J.P. Grotzinger et al.

Anomalous carbonate precipitates: is the Precambrian the key to the Permian?

Palaios

(1995)

J. Bertrand-Sarfati et al.

Lower Cambrian apatitic stromatolites and phospharenites related to the glacio-eustatic cratonic rebound (Sahara, Algeria)

J. Sediment. Res.

(1997)

P.F. Hoffman et al.

A Neoproterozoic snowball Earth

Science

(1998)

N.P. James et al.

Late Neoproterozoic cap carbonates: Mackenzie Mountains, northwestern Canada: precipitation and global glacial meltdown

Can. J. Earth Sci.

(2001)

M.J. Kennedy et al.

Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth's coldest intervals?

Geology

(2001)

P.F. Hoffman et al.

The snowball Earth hypothesis: testing the limits of global change

Terra Nova

(2002)

J.A. Higgins et al.

Aftermath of a snowball Earth

Geophys. Geochem. Geosyst.

(2003)

G. Jiang et al.

Stable isotopic evidence for methane seeps in Neoproterozoic postglacial cap carbonates

Nature

(2003)

G. Jiang et al.

Stratigraphy, sedimentary structures, and textures of the late Neoproterozoic Doushantuo cap carbonate in South China

J. Sediment. Res.

(2006)

A.J. Ridgwell et al.

Carbonate deposition, climate stability, and Neoproterozoic ice ages

Science

(2003)

G.A. Shields

Neoproterozoic cap carbonates: a critical appraisal of existing models and the plumeworld hypothesis

Terra Nova

(2005)

W.B. Harland

Evidence of late Precambrian glaciation and its significance

D.A.D. Evans

Stratigraphic, geochronological, and paleomagnetic constraints upon the Neoproterozoic climatic paradox

Am. J. Sci.

(2000)

W.R. Peltier et al.

Climate dynamics in deep time: modeling the “snowball bifurcation” and assessing the plausibility of its occurrence

D. Pollard et al.

Climate-ice sheet simulations of Neoproterozoic glaciation before and after collapse to Snowball Earth

K.H. Hoffmann et al.

U–Pb zircon date from the Neoproterozoic Ghaub Formation, Namibia: constraints on Marinoan glaciation

Geology

(2004)

G.P. Halverson et al.

Toward a Neoproterozoic composite carbon-isotope record

Geol. Soc. Amer. Bull.

(2005)

B. Goscombe et al.

Event geochronology of the Pan-African Kaoko Belt, Namibia

Precambrian Res.

(2005)

D.R. Grey et al.

⁴⁰Ar/³⁹Ar thermochronology of the Pan-African Damara Orogen, Namibia, with implications for tectonothermal and geodynamic evolution

Precambrian Res.

(2006)

P.F. Hoffman

28th DeBeers Alex. Du Toit Memorial Lecture: On Cryogenian (Neoproterozoic) ice-sheet dynamics and the limitations of the glacial sedimentary record

S. Afr. J. Geol.

(2005)

G.P. Halverson et al.

A major perturbation of the carbon cycle before the Ghaub glaciation (Neoproterozoic) in Namibia: prelude to snowball Earth?

Geophys. Geochem. Geosyst.

(2002)

Z.X. Li, S.V. Bogdanova, A.S. Collins, A. Davidson, De Waele, B., R.E. Ernst, I.C.W. Fitzsimons, R.A. Fuck, D.P....

K.-H. Hoffmann et al.

A preliminary note on a revised subdivision and regional correlation of the Otavi Group based on glaciogenic diamictites and associated cap dolostones

Commun. Geol. Surv. Namib.

(1996)

M.J. Kennedy et al.

Two or four Neoproterozoic glaciations?

Geology

(1998)

A.H. Knoll et al.

A new period for the geologic time scale

Science

(2004)

P.F. Hoffman, G.P. Halverson, in press. Otavi Group of the Northern Platform and the Northern Margin Zone. In: Miller,...

Cited by (212)

Questioning the role of methane in the wake of a snowball Earth: Insights from isotopically anomalous cap dolostone cements with a complex diagenetic history
2024, Geochimica et Cosmochimica Acta
Methane has been widely regarded as an important source of greenhouse gas that modulated Earth’s paleoclimate. Notably, it has been proposed that a massive release of methane via clathrate destabilization may have played a critical role during and/or in the immediate aftermath of the Marinoan deglaciation. One key piece of supporting evidence for this hypothesis comes from the isotopically anomalous (δ¹³C_carb < –30 ‰) methane-derived authigenic calcite (MDAC) cements within post-Marinoan cap dolostones in South China. However, the origin of these MDAC cements remains hotly contested, including a clumped isotope study that reinterpreted them as resulting from late hydrothermal fluids. To further evaluate these critical yet controversial cements, a detailed investigation by secondary ion mass spectrometry (SIMS) was conducted. Here we report the largest range of δ¹³C_carb values (–53.1 ‰ to +6.3 ‰) yet measured in Precambrian carbonates. Our study shows that the MDAC cements are post-depositional, void-filling, and rich in Mn. Importantly, integrated petrographic and SIMS results consistently show that MDAC cements post-date disrupted dolomite laminae that bear surprisingly positive δ¹³C_carb values up to +6.3 ‰. This is the first report that authigenic carbonates with positive δ¹³C_carb signals are found within post-Marinoan cap dolostone. Although the precise age of these authigenic carbonates remains ambiguous, integrated chemostratigraphic and SEM-SIMS results suggest that the dolomite laminae formed after the deposition of the uppermost cap dolostone interval, when the seawater δ¹³C had already evolved to positive values; and the MDAC formed even later. The dolomite laminae and MDAC cements, therefore, represent distinct post-depositional, exogenous, diagenetic carbon signals that are irrelevant to the deglaciation. Our new results challenge the hypothesis that methane played a central role at the end of or in the immediate aftermath of the Marinoan glaciation. Rather, the infiltration of methane-bearing fluids within cap dolostones could have occurred much later.
A late Ediacaran ice age: The key node in the Earth system evolution
2023, Earth-Science Reviews
Glacial depositions are widely discovered in the late Ediacaran strata all over the world, but the temporal and spatial distributions of the Ediacaran glaciations remain poorly constrained, preventing our further understanding of the nature and consequence of this ice age. In this study, we reviewed the global Ediacaran glacial depositions, the environmental changes, and biological evolution during the ice age. Available geochronological and biostratigraphic data suggest that the Ediacaran ice age might have extended from >580 Ma to <560 Ma, which is broadly coincident with a true polar wander (TPW) event with rapid rotation of continents. The TPW event may provide a possible solution for the spatiotemporal distributions of Ediacaran glaciations. Moreover, the late Ediacaran ice age bracketed the Shuram Excursion (SE), the largest negative carbonate carbon isotope excursion in Earth's history, which has been attributed to the massive oxidation of dissolved organic carbon or methane in the deep ocean, as well as witnessed the diversification of Ediacaran biota, the earliest, complex, macroscopic life forms in Earth's history. Termination of this ice age brought metazoans into the fossil records, starting the prelude of the ‘Cambrian Explosion’, thus implying the late Ediacaran ice age as a critical node in the history of Earth System.
Snowball earth: The African legacy
2023, Journal of African Earth Sciences
The stratigraphic association of late Precambrian glacial deposits with shallow-marine carbonate successions, which are biased toward the warmer parts of the surface ocean, was known in Africa and elsewhere since the 1930's, but geologists under-appreciated it as a paradox in physical climatology. A physical explanation for global freeze-up emerged from one-dimensional energy-balance climate models in 1969, after which it was widely discussed and refined. In 1981, it was suggested that a global freeze-up could be self-terminating after millions of years through slow accumulation of atmospheric CO₂, a greenhouse gas. The idea of a self-reversing global freeze-up, or ‘snowball Earth,’ was first applied geologically by Kirschvink (1992) based on paleomagnetic evidence in South Australia. He cited no paper in physical climatology and geologists remained unaware of developments in that science until after 2000. A self-reversing global freeze-up involves strong predictions in duration, synchroneity and termination state that have now been tested in the geological record. Inflections in growth rate and disciplinary diversity of peer-reviewed papers in 1997 implies that climate physics and geology were galvanized to test both the Siderian (ca 2430 Ma) and Cryogenian (717−635 Ma) snowball Earth hypotheses by new field research conducted in South Africa and northern Namibia, respectively.
Facies and stratigraphy of the basal Ediacaran cap carbonate, Naukluft Mountains, Namibia
2023, Precambrian Research
The termination of the Marinoan Snowball Earth glaciation constitutes a dramatic interval of climate change. This field study seeks to investigate this climate transition by characterizing the basal-Ediacaran cap carbonate succession of the Tsabisis Formation overlying the Blässkranz Formation glacial deposits exposed in the Naukluft Mountains of Namibia by using sedimentologic and stratigraphic field observations supplemented by chemostratigraphic analyses. Measured sections and lateral bed/unit tracing delineate stratigraphic stacking patterns and regional facies changes. In downdip areas Marinoan diamictite transitions upward into dolostone intermixed with sandstone and extrabasinal clasts that is gradually overlain by fine grained laminated dolostone. Updip localities show the diamictite is overlain by intercalated sandstones, gravels, and shales before an abrupt change to laminated dolostone of the cap carbonate. A succession of stromatolites, which become strongly elongate upward, prograde into the laminated dolostone in the updip localities. The stromatolites are overlain by laminated dolostone that grades upward into rhythmite with intercalations of shale. Near the top of the cap, rhythmites may be reworked into tabular intraclast conglomerate, locally intercalated with hummocky cross stratified sandstone, which passes upward into the shale and limestone members of the Tsabisis Formation. The Tsabisis Formation is overlain by the Noab Formation, dominated by sedimentary breccia, laminated dolostone, and sandstone. The lateral and vertical distribution of facies indicate a retreat of the shoreline and glacially sourced siliciclastics near the base of the cap carbonate, a shallowing succession to fair-weather wave base at the top of the stromatolite facies, and a second shallowing succession to storm wave base near the top of the cap carbonate. Maximum flooding occurred soon after the initiation of carbonate deposition and two sequence boundaries mark higher stratigraphic levels within the cap carbonate. The depositional patterns of the Tsabisis Formation cap carbonate resemble that of other Precambrian mixed carbonate-siliciclastic platforms and shows multiple base-level fluctuations that could be attributed to a range of mechanisms. Some of the base-level changes may have been directly coupled to events driven by deglaciation but distinguishing these from conventional controls on accommodation space in the rock record is challenging and multiple depositional models are feasible.
Application of the δ<sup>44/40</sup>Ca-δ<sup>88/86</sup>Sr multi-proxy to Namibian Marinoan cap carbonates
2023, Geochimica et Cosmochimica Acta
Several studies have exploited the stable calcium (Ca) and strontium (Sr) isotope compositions of marine carbonate rocks to investigate ancient carbon cycle dynamics, seawater geochemistry, and controls on carbonate production and preservation. However, the two proxies have rarely been applied together. Based on knowledge achieved to date, the δ^44/40Ca-δ^88/86Sr multi-proxy can distinguish signatures imparted by mass-dependent fractionation and reservoir mixing, including early diagenesis and shifts in the isotopic composition of seawater. In detail, the δ^44/40Ca-δ^88/86Sr multi-proxy represents a five-isotope system because the determination of δ^88/86Sr values requires analysis of fractionation-corrected ⁸⁷Sr/⁸⁶Sr ratios, which provide additional constraints on mixing. Here, we apply the novel δ^44/40Ca-δ^88/86Sr multi-proxy to two Marinoan (ca. 635 Ma) “cap carbonate” sequences from Namibia. Cap carbonates were widely deposited after the Neoproterozoic Marinoan Snowball Earth Event, but controversy surrounds their origin. We find that the rocks archive primary environmental signals deriving from a combination of seawater-glacial meltwater mixing and kinetic isotope effects. In an outer platform section, dolostone δ^44/40Ca and δ^88/86Sr values define a line predicted for kinetic mass-dependent isotope fractionation. This dolostone mostly precipitated from meltwater. Moreover, stratigraphically higher samples exhibiting the fastest precipitation rates display elevated ⁸⁷Sr/⁸⁶Sr ratios, consistent with long-held expectations that a rapid deglacial weathering pulse forced cap carbonate formation. An inner-platform dolostone shows greater effects from water-mass mixing but still reveals that precipitation rates increased up-section. Overlying limestones show greater Ca and Sr contributions from seawater. Amplification of local coastal processes during global ice sheet collapse offers a simple but sufficient proposition to explain the Ca isotope heterogeneity of cap carbonates. Detection of kinetic isotope effects provides evidence for predominantly rock-buffered diagenesis and further offers a basis for developing the δ^44/40Ca-δ^88/86Sr multi-proxy as an indicator of saturation state and pCO₂.
The origin of cap carbonate after the Ediacaran glaciations
2023, Global and Planetary Change
Cap carbonate is the most unique component of a Snowball Earth glaciation, i.e., two Cryogenian global glaciations. In the canonical model, cap carbonate precipitation requires a deglacial extremely high atmospheric pCO₂ level, which is unique for a Snowball Earth glaciation but did not occur in other non-Snowball Earth glaciations. However, some Ediacaran glacial successions also have a carbonate unit directly overlying the glacial deposits, although the Ediacaran glaciation is widely accepted to be a non-Snowball Earth ice age. Therefore, occurrence of Ediacaran glacial cap carbonate is counterintuitive, challenging the traditional model of cap carbonate precipitation. Here, we investigate two Ediacaran glacial cap carbonates, i.e. the Zhoujieshan cap carbonate above the Hongtiegou glacial deposits in the Chaidam Block, and the Hankalchough cap carbonate overlying the Hankalchough glacial deposits in the Tarim Block. The Zhoujieshan cap carbonate shows a transitional contact with the underlying glacial deposits, and has a nearly invariant carbonate carbon isotope value (δ¹³C_carb) of ∼ + 1‰. The Hankalchough cap carbonate from the Luobupo section has a persistent δ¹³C_carb value of ∼ − 5‰, but display a ∼ 15‰ spatial isotopic gradient in the Quruqtagh area. Combining with other occurrences, the Ediacaran glacial cap carbonate differs from the Snowball Earth cap carbonate in paleogeographic distributions, sedimentary structures, and mineralogical and geochemical compositions. We noticed that the Ediacaran glacial cap carbonates were restricted at ∼30–50° N/S, which may reflect a rapid transition from glaciation to carbonate deposition in subtropics. Thus, we speculate that the Ediacaran glaciation might have extended to latitude low enough, while the subsequent deglaciation and global warming may associate with an expansion of carbonate precipitation toward higher latitudes. Thus, the Ediacaran glacial cap carbonate precipitation occurred when carbonate precipitation and glaciation spatially overlapped in subtropics. Alternatively, considering paleomagnetic evidence for a ∼ 90° reorientation of all continents due to the Ediacaran true polar wander (TPW) event, some Ediacaran glacial cap carbonates could also be driven by glaciated continents moving into lower latitudes, where glacier melt and carbonate precipitated. Either way, a typical Snowball Earth glaciation may lead to global cap carbonate depositions, while most of Phanerozoic glaciations are characterized by glacial deposits in mid- to high-latitude and carbonate precipitation in tropics, i.e., showing no spatial overlapping of glaciation and carbonate precipitation. Thus, the enigmatic cap carbonate precipitation after the Ediacaran glaciations imply a unique climatic condition that differs from either a Snowball Earth glaciation or a Phanerozoic ice age.

View all citing articles on Scopus

View full text

Are basal Ediacaran (635 Ma) post-glacial “cap dolostones” diachronous?

Abstract

Introduction

Section snippets

Paleogeography and development of the Otavi carbonate bank

Stratigraphic expression of the Ghaub glaciation

Sedimentology of the Keilberg cap dolostone on the bank and slope

Carbon isotopes

Sedimentary structures

Conclusions

Acknowledgements

Precambrian Res.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

C. R. Geoscience

Earth-Sci. Rev.

Precambrian Res.

Earth Planet. Sci. Lett.

Precambrian Res.

Precambrian Res.

Earth-Sci. Rev.

U–Pb ages from the Neoproterozoic Doushantuo Formation, China

Science

Sedimentology, stable-isotope geochemistry and palaeoenvironment of dolostones capping late Precambrian glacial sequences in Australia

J. Geol. Soc. Aust.

Stratigraphy, sedimentology, and isotopic geochemistry of Australian Neoproterozoic postglacial cap dolostones: deglaciation, δ13C excursions, and carbonate precipitation

J. Sediment. Res.

Anomalous carbonate precipitates: is the Precambrian the key to the Permian?

Palaios

Lower Cambrian apatitic stromatolites and phospharenites related to the glacio-eustatic cratonic rebound (Sahara, Algeria)

J. Sediment. Res.

A Neoproterozoic snowball Earth

Science

Late Neoproterozoic cap carbonates: Mackenzie Mountains, northwestern Canada: precipitation and global glacial meltdown

Can. J. Earth Sci.

Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth's coldest intervals?

Geology

The snowball Earth hypothesis: testing the limits of global change

Terra Nova

Aftermath of a snowball Earth

Geophys. Geochem. Geosyst.

Stable isotopic evidence for methane seeps in Neoproterozoic postglacial cap carbonates

Nature

Stratigraphy, sedimentary structures, and textures of the late Neoproterozoic Doushantuo cap carbonate in South China

J. Sediment. Res.

Carbonate deposition, climate stability, and Neoproterozoic ice ages

Science

Neoproterozoic cap carbonates: a critical appraisal of existing models and the plumeworld hypothesis

Terra Nova

Evidence of late Precambrian glaciation and its significance

Stratigraphic, geochronological, and paleomagnetic constraints upon the Neoproterozoic climatic paradox

Am. J. Sci.

Climate dynamics in deep time: modeling the “snowball bifurcation” and assessing the plausibility of its occurrence

Climate-ice sheet simulations of Neoproterozoic glaciation before and after collapse to Snowball Earth

U–Pb zircon date from the Neoproterozoic Ghaub Formation, Namibia: constraints on Marinoan glaciation

Geology

Toward a Neoproterozoic composite carbon-isotope record

Geol. Soc. Amer. Bull.

Event geochronology of the Pan-African Kaoko Belt, Namibia

Precambrian Res.

40Ar/39Ar thermochronology of the Pan-African Damara Orogen, Namibia, with implications for tectonothermal and geodynamic evolution

Precambrian Res.

28th DeBeers Alex. Du Toit Memorial Lecture: On Cryogenian (Neoproterozoic) ice-sheet dynamics and the limitations of the glacial sedimentary record

S. Afr. J. Geol.

A major perturbation of the carbon cycle before the Ghaub glaciation (Neoproterozoic) in Namibia: prelude to snowball Earth?

Geophys. Geochem. Geosyst.

A preliminary note on a revised subdivision and regional correlation of the Otavi Group based on glaciogenic diamictites and associated cap dolostones

Commun. Geol. Surv. Namib.

Two or four Neoproterozoic glaciations?

Geology

A new period for the geologic time scale

Science

Stratigraphy, sedimentology, and isotopic geochemistry of Australian Neoproterozoic postglacial cap dolostones: deglaciation, δ¹³C excursions, and carbonate precipitation

⁴⁰Ar/³⁹Ar thermochronology of the Pan-African Damara Orogen, Namibia, with implications for tectonothermal and geodynamic evolution