Neoproterozoic glacial epochs – Snowball Earth, or limited glaciation?

Tek Jung Mahat 7 November 2017Department of Geography

MA = a million years (Megayear) ago

GA = a billion years (Gigayear) ago)

The geological clock: a projection of Earth's 4,5 Ga history on a clock

Glacio-epochsSchematic representation of glacio-epochs in Earth history and their

relationship to phases of supercontinent assembly and break up (Tectonic influences on long-term climate change: geotectonic setting of Archean, Proterozoic and Phanerozoic glaciations)

Archean glacio-epochs (c. 4–2.5 Ga)There are fundamental uncertainties regarding Archean climates because of the dearth of sedimentary deposits and climate modelling yields very different, opposed perspectives. Glaciation is recorded at about 2.9 and 2.8 Ga but is restricted to southern Africa. The geodynamic setting indicates a passive margin setting. A systematic search is needed for new deposits in other basins.

Paleoproterozoic glacio-epoch (c. 2.4 Ga)The well-developed relationship between Paleoproterozic rifting and glacial deposits suggests either a causal relationship between rift-related uplift and climatic cooling or selective preservation of glaciated rift deposits. Deposits are dominantly submarine debrites associated with thick turbidites. They are part of very thick marine tectono-stratigraphic successions, often associated with volcanics, recording the changing interplay of subsidence rates and sediment supply as rifting progresses. An association between glacials and banded iron formations may reflect deposition in semi-enclosed basins with incipient spreading centres.

Paleoproterozoic to Mesoproterozoic non-glacial interval (c. 2.3–0.75 Ga)The absence of any extensive glacial record during the long Paleoproterozoic–Mesoproterozoic interval between about 2.3 Ga and 750 Ma represents a large gap in Earth's glacial history. Glaciation should have been a common phenomenon given the known formation of several large landmasses and the orographic effects of associated high standing orogens, but this does not appear to be the case other than briefly and locally in Australia at about 1.8 Ga. Possibly the rock record has not been sufficiently well examined, deposits were extensively reworked and not preserved along active plate margins, or as yet unknown processes acted to suppress glaciation.

Glacio-epochs of the last billion years and their relationship to

supercontinent cycleA. three distinct global pulses of glaciation in the Neoproterozoic glacio-epoch

B. Variation in 13C over last 1.5 Ga (largest excursions occur in the Neoproterozoic are coincident with breakup of Rodinia)

C. Variation in Cosmic Ray Flux (curve) a timing of Earth's periodic crossings of spiral arms of the Milky Way

D. Estimated global temperature trends

E. Variation in atmospheric carbon dioxide

Schematic glaciated rift basin during Rodinia breakup after 750 Ma (fault activity and sedimentation in a marine rift basin)

Neoproterozoic glacio-epoch (0.75 Ga to 545 Ma)• Snowball Earth hypothesis:

severe Neoproterozoic glaciation occurred at low latitudes (most controversial and polarized area of debate). Researchers claim, during the Snowball Earth, some 650 ma ago, earth was either completely frozen or was almost completely frozen. Then, the earth was covered by a single sheet of ice extending from pole to pole. Scientists however think that Snowball Earth was not a single incident and that it happened multiple times with the duration of each event varying.

Estimated changes in global mean surface temperature, based on energy-balance calculations, and ice extent through one complete snowball event.

Neoproterozoic glacio-epoch (0.75 Ga to 545 Ma)Neoproterozoic glaciations occurred against an overall tectonic backdrop of active crustal extension as Rodinia broke apart

The Neoproterozoic glacio-epoch and the break up of Rodinia

RodiniaBreakupstages

Mid-life Rodinia stretching to the high latitude at above a mantle superplume

continued continental rifting on lower-latitude Rodinia

onset of Rodinia breakup, and pan-Rodinian “Sturtian” glaciation

continuing Rodinia breakup and sea-level rises

Rodinia breakup near completion, and the global “Marinoan” glaciation

Rodinia breakup completion, early Gondwanaland assembly, and the “Gaskiers” glaciation.

formation of Gondwanaland, high continental topography, and the lowering of sea level.

Slide Title

Breaking and integrating super-continents

Rodinia was a supercontinent formed about 1.1 ga ago. 750 ma ago, Rodiniabroke into three pieces that drifted apart as a new ocean formed between the pieces. Then, about 600 ma ago, those pieces came back together with a big crunch known as the Pan-African orogeny (mountain building event). This formed a new supercontinent, with the name of Pannotia. By about 550 ma ago, Pannotia was breaking up into several small fragments, Laurentia (the core of what is now North America), Baltica (northern Europe), and Siberia, among others, and one very large piece. This large piece, containing what would become China, India, Africa, South America, and Antarctica, was called Gondwana. It is considered a supercontinent in its own right because it is so big, but it is only part of the earlier supercontinents.

Over the next 200 ma many of the small pieces came together to form another large continent called Laurasia. Laurasia and Gondwana joined approximately 275 ma ago to form the supercontinent of Pangea. The breakup of Pangea is still going on today and contributes in the formation of the Atlantic Ocean. Eventually a new supercontinent will form and then it will break apart and so on.Source: https://scienceline.ucsb.edu/getkey.php?key=22

The northern margin of Gondwana was the locus of active extension after 480 Ma and extension-related uplift of the Gondwanan Highlands may have triggered polar Saharan glaciation after 440 Ma. Outlying ice masses lay on the proto Andes and in southern Africa where they reached sea level (Cancanari Formation and Pakhuis Formation respectively). The Saharan ice sheet was short lived and disappeared by the Early Silurian but ice remained over the uplifted active margin of South America into the Devonian of Brazil and Bolivia but not over the pole (see text). When Gondwana collided with Laurentia to form Pangea beginning in the mid-Carboniferous this remnant ice would expand to form an extensive Gondwanan ice complex

Palaeogeography of Late Ordovician Saharan glacio-epoch: c. 440 Ma.

The Late Cenozoic glacio-epoch after 55 Ma.The breakup of Pangea moved large landmasses into higher latitudes, isolated Antarctica and changed the configuration and bathymetry of ocean basins.

Palaeogeography of Late Ordovician Saharan glacio-epoch: c. 440 Ma.

Geometry of continental extension (after Ebinger et al., 2002) as occurred during the Paleoproterozicand Neoproterozoic glacio-epochsThe most extensive uplifts are created where crust is old, thick and thus flexurally rigid.

Simplified diagram illustrating principal differences between glacio epochs resulting from uplift resulting from continental collision (A1 and A2) and resulting from continental extension (B1, B2).

Palaeogeography of Late Ordovician Saharan glacio-epoch: c. 440 Ma.

Conclusions:• There is a close relationship between glacio-epochs and times of

enhanced crustal extension during the Proterozoic and Phanerozoic;• Most of Earth's glacial record appears to be preserved in extensional

basins. Tectonically generated topography produced by crustal extension may be an important control on cooling in conjunction with increased availability of moisture.

• There are times in earth history of rifting with no ice, and ice with no rifting but the marked association between the two for most ancient glacio-epochs cannot be simply coincidental.

• Having recognised the importance of tectonic preconditions under which glacio-epochs develop and glacial deposits are preserved, detailed consideration of the role of tectonics in influencing climate and controlling water depths, sediment supply and the age of sedimentary successions, is essential in future basin investigations and climate models.

Thank You !

Key References:Eyles, N. (2008): Glacio-epochs and the supercontinental cycle after 3.0 Ga: tectonic boundary conditions for glaciation. Palaeogeography, Palaeoclimatology, Palaeoecology, 258, 89–129.Li, Z.-X., Evans, D.A.D., Halverson, G.P. (2013): Neoproterozoic glaciations in a revised global palaeogeography from the breakup of Rodinia to the assembly of Gondwanaland. Sedimentary Geology, 294, 219–232.Hoffmann, P.F., Schrag, D.P. (2002): The Snowball Earth hypothesis: testing the limits of global change. Terra Nova, 14, 129–155.

Disclaimer: Most of the Images, tables and charts used in this presentation are either from the references above or from other sources as cited in respective slides. Some other images are extracted from unspecified online sources, featured under “Creative Commons licenses”.

QUICK NOTE:

The earliest known glaciation (mid Archean ∼ 2.9 Ga) is recorded in the marine Mozaan Group of South Africa deposited along the passive margin of the Kapvaal Craton then part of the early continent Ur.

It remains unclear whether the passive-margin related Kaapvaal glaciation represents a glacio-epoch or a short-lived event.

A long Paleo-Mesoproterozoic non-glacial interval (c. 2.3 Ga to 750 Ma?) coincides with continental collisions and high standing Himalayan-scale orogenic belts marking the suturing of

supercontinents Nena-Columbia and Rodinia. A near absence of glacial deposits other than at 1.8 Ga, may reflect lack of preservation.

The anomaly of the lack of a glacial record during the Paleo-Mesoproterozoic growth of Nena-Columbia is clearly evident though Williams (2005) reports evidence of glaciation at 1.8 Ga. The sedimentary record of most glacio-epochs occurs in the geodynamic context of intracratonic rifting, crustal extension and the formation of passive margins.

The timing and number of glacial events in the Neoproterozoic (3a, b, c) is uncertain. Paleoproterozoic (c.2.5 Ga) and Neoproterozoic glacio-epochs (c. 750–580 Ma) occurred during the breakup of Kenorland and Rodinia respectively. It is also possible that extension along high latitude continental margins and consequent uplift also played a role in triggering Ordovician glaciation at c. 440 Ma (when terranes rifted off Gondwana; see text). Most of the Gondwanan glacio-epoch deposits are stored in rift basins even though glaciation was initiated during the compressional growth phases of Gondwana.

The extensive and prolonged Neoproterozoic glacio-epoch records either diachronous glaciations or discrete pulses of cooling between ∼ 750 and ∼ 580 Ma, and is overwhelmingly recorded by substantial thicknesses (1 km+) of glacially influenced marine strata stored in rift basins. These formed on the mid to low latitude (< 30°) oceanic margins of western (Panthalassa: Australia, China, Western North America) and eastern (Iapetus: Northwest Europe) margins of a disintegrating Rodinia. The youngest glacially influenced deposits formed about 580 Ma along the compressional Cadomian Belt exterior to Rodinia (Gaskiers Formation) possibly correlative with the classic passive margin Marinoan deposits of South Australia.

Tectonics played a major role in Cenozoic cooling after 55 Ma culminating in continental scale Northern hemisphere ice sheets only after 3.5 Ma.

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Primary glacial sediment is extensively reworked by mass flow processes and terrestrial glacial facies are seldom preserved. Sedimentation is markedly diachronous as a consequence of propagating faults and the non-synchronous formation and filling of different sub-basins. The fill of any one sub-basin comprises a tectonostratigraphic succession recording changing relationship between subsidence and sediment supply. Marked intrabasinal variability in the timing of rifting and the sedimentation response prohibits correlations of like facies (e.g., diamictites) and also wider extrapolation of age dates on any one stratigraphic horizon to other basins worldwide.

The well-developed relationship between Paleoproterozic rifting and glacial deposits suggests either a causal relationship between rift-related uplift and climatic cooling orselective preservation of glaciated rift deposits. Deposits are dominantly submarine debrites associated with thick turbidites. They are part of very thick marine tectono-stratigraphic successions, often associated with volcanics, recording the changing interplay of subsidence rates and sediment supply as rifting progresses. An association between glacials and banded iron formations may reflect deposition in semi-enclosed basins with incipient spreading centres.

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The bulk of the Neoproterozoic glacial record is stored within thick marine debrite-turbidite successions that accumulated within rift basins. Terrestrial ‘tillites’ and associated deposits are poorly represented. Neoproterozoic glaciers were wet based and produced abundant meltwater and sediment incompatible with catastrophically cold conditions of a hard Snowball Earth. The breakup of Rodinia took place over a 200 million year period and by analogy with other episodes of rifting there was significant along-strike diachroneity in the timing of rifting, basin formation and glacially influenced sedimentation (Kendall et al., 2006). Large-scale rearrangement of landmasses and oceanic

configurations created by an evolving disintegrating supercontinent may have played a key role in climate change. There is growing recognition that Neoproterozoic glaciations were initiated as regional ice centres (Halverson et al., 2005, p. 1198) whose growth was diachronous (op cit., p. 1198) countering the longstanding use of glacial deposits as precise global time markers. Earlier ideas of ‘instant glaciation’ involving notional albedo-feedback mechanisms and runaway refrigeration are now underplayed (see Halverson et al., 2005).

In the light of the substantial gaps in knowledge identified above, and the emerging theme of diachroneity of Neoproterozoic glaciation, it is profitable to revisit the conceptual underpinning of current efforts to subdivide Proterozoic time using Global Stratotype Sections and Points (GSSP). Knoll et al. (2006, p. 14) believe that ‘the great ice ages that wracked the later Neoproterozoic world… were global in impact, and because they are associated with carbon isotopic excursions larger than any recorded in Phanerozoic rocks, the glaciations offer what are undoubtedly our best opportunities for the sub-division of Neoproterozoic time’. It can be argued in fact that the geologic consensus is moving away from catastrophic global freeze events and instantaneous deglaciations. In contrasts to ‘wracking’ the world, the Neoproterozoic rock record informs us that glaciers were wet-based and may have been part of diachronous events as tectonotopography evolved during the dispersal of crustal blocks.

A superplume occurs when a large mantle upwelling is convected to the Earth's surface. ... Although similar, a superplume forms at the mantle-core boundary while a hot-spot occurs at the mantle-crust layer. Superplumes create cataclysmic events that affect the whole world when they explode.