Proterozoic Eon (2.50 Ga - 541 Ma)a

Proterozoic means "early life," but this eon is most famous for its equatorial glaciations. Grant Young and George Williams described the Proterozoic Eon in the Encyclopedia of Geology (2021).

"At the start of that vast eon, Earth was an alien world that supported only primitive life forms such as algae and bacteria. During the following 1.96 billion years Earth twice underwent widespread glaciations that mostly affected continents situated at low paleolatitudes. There was repeated assembly and breakup of continents and Earth’s surface was episodically subjected to huge impacts with asteroids and comets. Other more gradual changes included Earth’s slowing rotation rate, cooling of its interior, increasing solar radiation, and changing atmospheric composition."[1]

There were two equatorial glaciation periods, one at the beginning (Paleoproterozoic) and one at the end (Neoproterozoic). The middle Proterozoic is the "boring billion." Scientists debate the cause of equatorial glaciations. There are two primary hypotheses that attempt to explain the equatorial glaciations. The proposed global snowball Earth (SBE) was a hypothesis in which the seas were covered by 1 km of sea ice, and the land was covered by >2.5 km thick land glaciers.[2] The HOLIST hypothesis (High Obliquity, Low-latitude Ice, STrong-seasonality) assumes that Earth's axis had a much higher tilt and was angled toward the sun during the Proterozoic. With a high obliquity, Earth’s summer pole was more angled toward the Sun at summer solstice. With a higher tilt, the equator has a lower average temperature than the poles and would preferentially form glaciers. The single supercontinent of Rodinia was on the equator during the Neoproterozoic (750 Ma, Figure 7-11). While the equatorial belt was glaciated, scientists do not know the conditions at the poles because there was no land at the poles. There is no confirmed evidence of Proterozoic glaciations more than 40 degrees latitude.

Even if ice formed at the winter pole with a high obliquity (HOLIST), it would melt during the following hot summer. Only low latitudes avoid these extremes of climate, possibly allowing ice to be preserved throughout the year.

As described in chapter 6, photosynthesis probably became more efficient at the end of the Archaean Eon. The atmospheric oxygen level rose to 1% near the beginning of the Proterozoic Eon. With respect to life, this Great Oxidation Event might be the important event in Earth’s climate history. It killed most of the archaebacteria near and on Earth's surface, which depended on an anoxic atmosphere, and it enabled the evolution of the next levels of organisms in the food chain, heterotrophs, which use oxygen to consume other organisms. One proposed cause of the rise in oxygen is the evolution of a more efficient photosynthetic process. Another proposed cause is that Earth’s mantle dynamics changed at the beginning the Proterozoic Eon (2.50 Ga – 541 Ma), which caused volcanoes to emit gases that were less reducing. Although the precise timing and levels of oxygen are difficult to determine, atmospheric oxygen might have progressed through five stages in Earth’s history (Figure 7‑12). 1) No atmospheric oxygen prior to the Great Oxidation Event. 2) Rise in oxygen to 1% during early Proterozoic. 3) Oxygen produced by algae gases out of oceans during Proterozoic but land rocks absorb it. 4 and 5). Beginning 0.85 Ga, oxygen is no longer absorbed by land surfaces and builds up in atmosphere to present equilibrium concentration at 0.2.

Because the Great Oxidation Event (2.4 Ga) reduced the carbon dioxide and methane in the atmosphere, it reduced global warming, causing a major glaciation at the beginning of the Proterozoic Eon, (Paleoproterozoic). A recent paper placed this rise in oxygen 16 million years prior to the initiation of the Paleoproterozoic equatorial glaciations, [3] which indicates that the rise in oxygen caused the glaciation, and not vice-versa as some have proposed. The Paleoproterozoic Huronian glaciation period (2.4 - 2.1 Ga) took place soon after the Great Oxidation Event. If there was an orange haze during the Archaean, this might also have been a point when the skies cleared (Figure 7-13). The Cryogenian (Neoproterozoic) glaciations took place at the time that oxygen concentration rose during Stage 4 (Figure 7‑12). The first glaciation during the Cryogenian was the Sturtian glaciation, which took place from 720 - 660 Ma. The second glaciation during the Cryogenian was the Marinoan glaciation, which lasted from 650 - 635 Ma. While solar radiation intensity was increasing over time, each glaciation event took place after a reduction in the global warming potential of the atmosphere. There appears to have been a balance between increasing solar radiation and atmospheric greenhouse gas global warming potential. During the times when the global warming potential dropped below the warming potential of the sun due to changes in greenhouse gas concentration, continent distribution, or other changes, there was a glaciation event.

There were three Huronian glaciations between 2.4 to 2.1 Ga.[4] One location with evidence for this glaciation is near Lake Huron in the Great Lakes of North America, but this area was near the equator during the Huronian. Scientists can determine the positions of continents (paleomagnetic latitudes) based on the alignment of iron (magnetized) particles in ancient rocks. "Paleomagnetism is the study of ancient magnetism preserved in rocks, permits paleolatitudes (former latitudes) to be determined by measuring the direction of magnetism locked in iron-bearing minerals at or soon after the time the rocks were formed." [5] The dots shown on Figure 7-14 represent the palaeolatitude versus age for numerous igneous rocks that intrude the Superior Craton in Canada. The series of dots shows the latitudinal movement of the craton with time, and Huronian glaciation occurring when the craton crossed the palaeoequator. Even though the Huronian glaciation period continued until 2.1 Ga, evidence for glaciation at the Huron site ended when it traveled away from the equator in 2.3 Ga.

In the 1950s and 1960s, researchers first discovered the existence of equatorial glacial deposits in the Neoproterozoic. As Brian Harland thought about the possible cause of such an odd event, he proposed that a global ice sheet covered the entire planet (Figure 7‑15).[6] It seemed to be a logical explanation for glaciations at the equator since glaciers always begin at the poles. It is ironic that the magnetic direction Harland obtained for late Precambrian (Cryogenian) glacial deposits in Greenland that suggested a low paleolatitude were in fact overprints, that is, a magnetic direction acquired by the rocks during later tectonism. Since then, field tests are employed to ascertain the timing of acquisition of magnetism in rocks. [7]

In 1975, George Williams proposed that a greater tilt of Earth's axis allowed for equatorial glaciations without covering the entire planet with ice.[8] The current obliquity of Earth’s axis is 23.4 degrees. Williams proposed that an angle of obliquity greater than 54 degrees would have allowed for equatorial glaciations (Figure 7-15). The Encyclopedia of Geology describes the HOLIST hypothesis as follows:

• high latitudes at times of summer solstices would be too hot for the accumulation of snow to produce ice sheets at high latitudes;

• continental regions at low latitudes (<30) would be cooler, on average, than the poles and would therefore be preferentially glaciated;

• very large seasonal air-temperature ranges (40 C) would prevail on continents, including those in low latitudes;

• complex life forms could not survive because of stresses imposed by extreme seasonal temperature variations. [9]

Snowball Earth proponents and opponents disagree on the interpretation of the glaciation data from the Proterozoic. While the majority of geologists support Snowball Earth, the geological data support for Snowball Earth is weak. For example, geologists who focus on glacial sedimentation data and interpretation generally did not support Snowball Earth. Prof Ian Fairchild and numerous co-authors wrote in Sedimentology 2016, “... most glacial sedimentologists opposed the Snowball theory...” [10], but that is not to say that most supported HOLIST. There is no reliable evidence of glaciations north of 40⁰ latitude or south of 40⁰ S latitude (Figure 7-11). The equator is at 0⁰ latitude and the north and south poles are at 90⁰ latitude. On the other hand, support in principle for the HOLIST hypothesis comes from Mars, which had high obliquity during much of its history. There is evidence of equatorial glaciations on Mars during the period of high obliquity. [11]

The dramatic shift in the understanding of the timing of plant evolution in the last few years might have an impact on the viability of SBE. Although the emergence of land plants is uncertain in the Neoproterozoic, molecular clock analysis indicates an origin of land plants deep in the Proterozoic (Chapter 6). It is conceivable that plants could evolve in the conditions of the high-obliquity Earth in the Proterozoic. It is inconceivable that land plants could evolve in the Snowball Earth scenario; however, if and until there is a discovery of plant fossils in the Proterozoic, the possible evolution of land plants in the Proterozoic will have little impact on the viability of the Snowball Earth scenario.

Whether Snowball Earth or HOLIST equatorial glaciations, the cause of the Cryogenian (Neoproterozoic) Period glaciations may have been the breakup of the continent of Rodinia (Figure 7-11). This breakup triggered flood basalt events, which may have sent sulfur dioxide into the stratosphere, thus causing a long-term cooling effect. In addition, flood basalts consume carbon dioxide from the atmosphere as they cool. With solar radiation 6% lower than the present, sulfur dioxide in the stratosphere, and low carbon dioxide, the balance of incoming and outgoing radiation would have been in favor of colder climates and glaciation. In the SBE scenario, ice began to form in polar regions, reflecting sunlight back into space, and thus cooling the atmosphere and forming more ice. Once glaciers covered the entire planet, the global mean temperature would have been -50 ⁰C because the ice would have reflected most of the sun’s energy back into space. There would have been no clouds to trap heat because water vapor would have frozen out of the atmosphere. Geological evidence indicates that the equatorial glaciers persisted for tens of millions of years, so the Snowball Earth scenario also has glaciers covering the planet for tens of millions of years.

In 1969, Soviet scientist Mikhael Budyko showed through an energy balance model that Earth could plausibly enter an ice-covered state such as SBE but that there would be no way to undo such a scenario and remove the ice.[12] Simulations indicate that the carbon dioxide concentration would have needed to be no higher than 100 to 200 ppm (low global warming potential) to initiate glacier formation and SBE. The idea that the earth with lower solar radiation could enter a snowball earth condition with low carbon dioxide in the atmosphere and at the present angle of obliquity was confirmed by several global climate models.[13]

In 1992, Joseph Kirschvink proposed that elevated carbon dioxide levels in the atmosphere might have formed enough of a greenhouse effect to return Earth from SBE conditions to nonglaciated conditions.[14] Simulations indicate that removal of glaciers by heat retention in the atmosphere would have required CO₂ concentrations in the range of 100,000 to 200,000 ppm (high global warming potential), which is several hundred times higher than the present carbon dioxide concentration and is in the range of oxygen concentration in the modern atmosphere. Proponents of SBE claim that volcanic emissions for tens of millions of years would have increased atmospheric CO₂ to this concentration. However, there is no evidence of such elevated levels of carbon dioxide.[15]

Geologists refer to formations with pebbles, rocks, or boulders embedded in sediments as diamictites. Glacial diamictites are poorly sorted, that is, show a wide range in the size of material ranging from clays and sand to pebbles and boulders, and the larger constituents may be striated through glacial grinding. Diamictites can have several causes, but the most common cause is glaciers transporting rocks across the land surface and then dropping them on another land area or in the sea. In the SBE scenario, glaciers would quickly advance across the sea, causing sea level to fall by 1- to 1.5-km during the glaciation onset and cause glacial advance across land surfaces, leaving behind glacial till and other glacial diamictites (deposited rocks) as well as formation of deeply incised valleys,[16] as observed in formations; however, the diamictites and incised valleys might also have been caused by local glaciations.

The SBE scenario has a rapid (less than 10,000 years) end of glaciation due to the feedback mechanism of ice melting and albedo (reflectance) decreasing along with it. Snowball Earth proponents stated the rapid melting of glaciers would result in sea level rise of 1 to 1.5 km although recent geologic analysis of Cryogenian formations show no evidence of large rises in sea level. Snowball Earth proponents thus revised this estimate downward to the range of 0.2 to 1 km sea level rise.

The most commonly cited evidence of SBE is the Ghaub formation in northwestern Namibia. Based on this formation, estimates of the sea level fall during the Late Cryogenian glaciation include 1500 m [17], 700 m [18] and 200 m. The 1500 m drop in sea level implies that the average thickness of ice sheets was 3.3 km (2 miles).[19] Bechstadt et al. (2018) assessed whether the geologic evidence in Cryogenian strata from the two Namibian locations supports the Snowball Earth scenario.[20] The Ghaub formation evidence of sea level drop during the onset of glaciation might have been due to tectonic uplift, resulting in an what appeared to be sea level drop but was actually a rise in the land surface elevation. The estimates for sea level change referred to a fall of sea level during SBE glaciations (although it would have to be followed by a reciprocal rise in SL). The Ghaub formation is part of the Otavi Mountainland. The southern section of the Otavi Mountainland was a platform sedimentation area in a shallow subtidal zone with modest sea level fall over millions of years. Columnar stromatolite fossils up to a meter in height and evidence of other microbes indicate that the Otavi Mountainland study area included euphotic and photic zones (sunlight, day and night, and photosynthesis) in a shallow tidal area over the entire period of the Cryogenian period. Because there was a slight change in sea level accompanied by moderate land subsidence. This type of stable depositional and biological environment would be impossible if the entire earth was covered by a sheet of ice. There is also direct evidence of tectonic uplift in the Ghaub formation in the form of lateral and vertical inhomogeneities. The overlying cap carbonate in the Ghaub formation has variable clast content, which also indicates a “strongly uplifted local source area.” Thus, Bechstadt et al. found ‘strong evidence against a thick, laterally continuous ice cover over oceans and continents.’

George Williams focused much of his Neoproterozoic research on the Elatina formation in South Australia. This formation was near the paleoequator at the time of the Neoproterozoic glaciations. Although evidence of glaciation in the Elatina formation is widespread, not even the entire Elatina formation was glaciated, and there were several glaciation cycles. In research patterned after the Bechstadt paper, Williams and Gostin assessed three neighboring Cryogenian formations in south Australia. [21] In the Elatina formation, there were as many as six glacial advances and retreats during the Late Cryogenian, which indicates that the ice margin (edge of glaciated area) fluctuated back and forth over the land during this period. There was no evidence of large-scale sea level rise or fall at the beginning or end of the Cryogenian glaciations. Williams and Gostin also noted that the sequence boundary between the Elatina Formation and the cap carbonate is marked by a disconformity to very low-angle unconformity, indicating a major hiatus. As with the intermittent glaciation in the Elatina formation, Condon found a geologic formation a glacial deposit during the Neoproterozoic that was 5,500 m thick that had intermittent nonglacial sediments.[22]

Banded iron formations reappear during the Cryogenian, which might be an argument for Snowball Earth since a global ice cap might have deoxygenated the oceans; however, the banded iron formations are rare and not global, which probably means that they were restricted to inland bodies of water and not world oceans. Grant Young long argued that the Neoproterozoic iron-formations resulted from rifting and submarine volcanism, and recent opinion is swinging towards this concept.[23]

Some modelling suggests that a frozen-over Earth would have seasonal changes of temperature, but these changes would not affect Earth’s surface, which would be insulated beneath a thick ice cover.

The following paragraphs discuss data that contradicts the Snowball Earth scenario, including evidence of glaciers flowing into the sea, tidal rythmites, ice-rafted dropstones, heating and cooling of permafrost in sand wedges, and evidence of glacial movement in rock striations.

"In the SBE scenario, there may have been seasonal variations in atmospheric temperature, but the thick ice cover would have insulated the Earth from the affects of such variations, that is, there would have been no summer thaw and winter freezing to produce annual varves, or winter contraction cracking and summer expansion of permafrost to permit the formation of sand wedges." [26]

Snowball Earth supporters have proposed that algae and other microorganisms might have survived in small pools near volcanic vents or that dust of the surfaces of glaciers might have helped algae survive;[27] however, these claims were made prior to the recent shift of the model of plant evolution into the Proterozoic. They probably don't apply to land plant evolution and survival.

In favor of SBE, Evans stated that evaporites and microbial mats during the mid Proterozoic are in locations and have growth patterns consistent with the present Earth’s angle of obliquity.[28] On the other hand, Williams stated that ‘Microbial and palynofloral life and trophic complexity continued during low-latitude glaciations (Corsetti et al., 2003, 2006; Grey et al., 2003; Dutkiewicz et al., 2006 ).’ [29]

One of the points of disagreement between HOLIST proponents and SBE proponents is the interpretation of “cap carbonates” that are at the top of the Marinoan glaciation (630 Ma) deposits. Snowball Earth proponents claim that cap carbonates were deposited during the rapid melting of the ice sheet and subsequent rapid erosion of land areas over a period of approximately 1,000 years. The erosion of the land areas would have saturated sea water with carbonate and led to carbonate precipitation. Williams argues that polarity reversals between the cap carbonate layers indicate that they were laid down over 100,000 to 1,000,000 years.[30] Nevertheless, proponents still claim that cap carbonates are evidence of the end of Snowball Earth.[31]

Recently, Lang studied Cryogenian glaciation patterns in the Nantuo Formation in China and observed cyclicity in glacial deposits during this period.[32] They also found that deposit of cap carbonates did not immediately follow the deglaciation process, which contradicts the concept that rapid melting and weathering immediately saturated the ocean with carbonates. Because SBE is inconsistent with the cyclicity of glacial deposits,[33] researchers have investigated an alternative, Slushball Earth. In this scenario, the oceans in tropical zones had minimal ice, and there was open water. This would allow for the survival of photosynthetic organisms, allow glaciers to flow into the sea, and not require extremely high concentrations of carbon dioxide in the atmosphere to begin the thawing process.

Climate modelers, Pollard and Kasting, coupled sea glacier flow with a global climate model to evaluate whether an equilibribum “ice line” would form in a Slushball Earth scenario.[34] They could not find any stable situation that would form thick glaciers in regions toward the poles and open water or thin ice in tropical regions. Thus, the conclusion from their simulations was that Slushball Earth was not a viable hypothesis. Slushball earth had been proposed as a cause of sand wedges, but a recent study showed that sand wedges would not form under slushball Earth (waterbelt Earth) but only under a complete Snowball Earth. This was not a validated sand wedge formation model but just based on temperature fluctuation. Slushball (waterbelt) Earth did not have enough temperature fluctuation to cause sand wedges.[35] The formation of periglacial sand wedges requires Earth’s surface to be directly exposed to seasonal cycles, so each winter, wind can blow sand into thermal contraction cracks at the top of exposed permafrost. Even a thin snow cover can inhibit the formation of sand wedges

Climate simulations support Williams’ HOLIST (High Obliquity) hypothesis. Models show that continental ice sheets would have formed at the equators rather than the poles if Earth’s axis angled toward the Sun. Simulations indicate that the pole pointing away from the Sun would have been arid, which means that glaciers would also not have formed at the cold pole during the half year that it was pointing away from the sun. The reason that Williams defines the HOLIST hypothesis as greater than 54⁰ is that glaciers would form at the equator rather than the poles if the angle of obliquity were greater than 54⁰.

The HOLIST hypothesis also provides a solution to the Faint Young Sun paradox. Seas at each pole would have been hot for the half of the year when they pointed toward the sun. Temperatures would have fluctuated between -70 ⁰C in winter and 80 ⁰C in summer at the poles. Thus, there would have been liquid water for half of the year at each pole. This explains why some rocks from the Archaean appear to have formed in hot water.

In the HOLIST hypothesis, there was a shift in obliquity during the Ediacaran Period of the Late Proterozoic and the early Cambrian. Young and Williams (2021, p. 566) stated that the high-obliquity hypothesis “requires that Earth’s obliquity attained a value close to that of today during the Ediacaran Period, between low-paleolatitude glaciation at 635 Ma and early Cambrian (~535 Ma) glaciation at a high paleolatitude.” This time range is based on reliable paleomagnetic data.[36] There are a few other glaciations within this period, but their location on Earth (close to equator or poles) is uncertain.

The Shuram negative carbon excursion was "The largest-known carbon isotope excursion in Earth’s history.” [37] Scientists in 1993 discovered that carbonate sediments in Oman were depleted in ¹³C at unprecedented levels during a relatively short period in the Ediacaran Period.[38] The Shuram negative carbon excursion is measured with the same carbon isotope and delta C 13 measurements (δ¹³C) that Bell used to test for biological evidence in the zircon crystals from the Hadean Eon (Chapter 5-4). Biologically derived organic matter has a δ¹³C _(carb) value between – 25 and – 35 however, the measurements of carbon from rock formations and the mantle is – 6_. Slow changes in δ¹³C_(carb) are common. For example, the δ¹³C_(carb) ratio in ocean sediments slowly rose from +1 to + 3 during the almost 2 billion years of the Proterozoic Eon. Negative excursions generally take place during extreme events such as mass extinctions. The most negative excursion other than the Shuram excursion was – 8. The Shuram excursion (580 – 571 Ma) reached an unprecedented δ¹³C_(carb) = – 12 in all parts of the world.[39]

The Shuram negative carbon excursion might indicate a shift in the angle of obliquity between 580 and 571 Ma. Williams and Schmidt (2018) proposed that this negative carbon excursion was due to a shift from equatorial glaciations to polar glaciations.[40] The hypothesis of Williams and Schmidt argued that with the present-day obliquity, cold, dense water descends at the poles to great depths and rises at the equator, whereas with a high obliquity cold, dense water would descend at the equator to intermediate depths and rise at the poles, with great oceanic depths having little circulation and being anoxic, leading to changes in δ¹³C_(carb), thus providing support for the HOLIST hypothesis.

There are two uncertain glaciations that might eventually hold evidence that indicates that the shift in obliquity took place at the time of the Shuram negative carbon excursion. The Gaskiers glaciation, which holds probable evidence of glaciation near the equator, took place 580 Ma. The next glaciation, the Fauquier glaciation (571 Ma) has ambiguous palaeomagnetic data – different results yield high and low palaeolatitudes.

Williams argued in 2008, “sedimentology, chemostratigraphy, biogeochemistry, micropalaeontology, geochronology and climate modeling argue against (Snowball Earth).” [41] With the recent shift of plant evolution into the Proterozoic (Chapter 6), paleontology should be added to the list. Table 7-1 classifies the geologic data related to SBE and HOLIST as purple, yellow, or green. Purple rows provide evidence against Snowball Earth or provide evidence for a high obliquity axis. Yellow is neutral. Green provides evidence against a high obliquity axis or provides evidence supporting Snowball Earth. There is 1 green, 2 yellow rows, and 16 purple rows. The one green row does not directly support SBE. It merely provides evidence against HOLIST, and that evidence is not strong since HOLIST might also explain the data. Thus, there is really no data specifically in support of SBE over HOLIST. If the geologic data overwhelmingly supports a high obliquity axis in the Archaean and Proterozoic over SBE as an explanation for equatorial glaciations, then the HOLIST hypothesis should be viewed as the most likely hypothesis.

Scientists spent many years searching for a cause of a shift of obliquity but could find none; however, the fact that there is no known cause of a shift in obliquity is not direct geologic evidence against HOLIST. In addition, the scientists who searched for a cause of the shift in obliquity were not experts in celestial mechanics. Until an expert in celestial mechanics makes such an assessment, judgement on the viability of the HOLIST hypothesis should be held in abeyance.

[1] Young, Grant, and George Williams. Paleoclimates/Proterozoic Climates, Encyclopedia of Geology (2nd edition). Elsevier. 2021. 557 - 570.

[2] Williams, George, personal communication, 2/13/2021.

[3] Warke, Matthew R., Tommaso Di Rocco, Aubrey L. Zerkle, Aivo Lepland, Anthony R. Prave, Adam P. Martin, Yuichiro Ueno, Daniel J. Condon, and Mark W. Claire. "The great oxidation event preceded a paleoproterozoic “snowball Earth”." Proceedings of the National Academy of Sciences 117, no. 24 (2020): 13314-13320.

[4] Young and Williams, Encyclopedia of Geology

[5] Young and Williams, Encyclopedia of Geology

[6] W. B. Harland (1964). "Critical evidence for a great infra-Cambrian glaciation". International Journal of Earth Sciences. 54 (1): 45–61. Bibcode:1964GeoRu..54...45H. doi:10.1007/BF01821169

[7] Young and Williams, Encyclopedia of Geology

[8] Williams, George. Geological Magazine , Volume 112 , Issue 5 , September 1975 , pp. 441 - 465

DOI: https://doi.org/10.1017/S0016756800046185

[9] Young and Williams, Encyclopedia of Geology

[10] Fairchild, Ian J., Edward J. Fleming, Huiming Bao, Douglas I. Benn, Ian Boomer, Yuri V. Dublyansky, Galen P. Halverson et al. "Continental carbonate facies of a Neoproterozoic panglaciation, north‐east Svalbard." Sedimentology 63, no. 2 (2016): 443-497.

[11] Young and Williams, Encyclopedia

[12] M.I. Budyko (1969). "Effect of solar radiation variation on climate of Earth". Tellus A. 21 (5): 611–1969. doi:10.1111/j.2153-3490.1969.tb00466.

[13] Pierrehumbert, R. T., D. S. Abbot, Aiko Voigt, and D. Koll. "Climate of the Neoproterozoic." Annual Review of Earth and Planetary Sciences 39 (2011).

[14] Kirschvink, Joseph (1992). "Late Proterozoic low-latitude global glaciation: the Snowball Earth". In J. W. Schopf; C. Klein. The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge University Press.

[15] Michael Schirber (2015) Snowball Earth might have been Slushy. NASA Goddard Institute of Space Studies. https://www.giss.nasa.gov/research/features/201508_slushball

[16] Bechstädt, Thilo, Hartmut Jäger, Andreas Rittersbacher, Bolko Schweisfurth, Guy Spence, Georg Werner, and Maria Boni. "The Cryogenian Ghaub Formation of Namibia–New insights into Neoproterozoic glaciations." Earth-Science Reviews 177 (2018): 678-714.

[17] Hoffman, P.F.. On Cryogenian (Neoproterozoic) ice-sheet dynamics and the limitations of the glacial sedimentary record. South African Journal of Geology, 108 (2005), pp. 557-577

[18] Hoffman, P.F.. Strange bedfellows: glacial diamictite and cap carbonate from the Marinoan (635 Ma) glaciation in Namibia. Sedimentology, 58 (2011), pp. 57-119

[19] Williams, George E., and Victor A. Gostin. "Late Cryogenian glaciation in South Australia: Fluctuating ice margin and no extreme or rapid post-glacial sea-level rise." Geoscience Frontiers 10, no. 4 (2019): 1397-1408.

[20] Bechstädt, Thilo, Hartmut Jäger, Andreas Rittersbacher, Bolko Schweisfurth, Guy Spence, Georg Werner, and Maria Boni. "The Cryogenian Ghaub Formation of Namibia–New insights into Neoproterozoic glaciations." Earth-Science Reviews 177 (2018): 678-714.

[21] Williams, George E., and Victor A. Gostin. "Late Cryogenian glaciation in South Australia: Fluctuating ice margin and no extreme or rapid post-glacial sea-level rise." Geoscience Frontiers 10, no. 4 (2019): 1397-1408.

[22] Condon, D.J.; Prave, A.R.; Benn, D.I. (1 January 2002). "Neoproterozoic glacial-rainout intervals: Observations and implications". Geology. 30 (1): 35–38.

[23] Lei, Ru-Xiong, Kai Zhang, M. N. Muhtar, and Chang-Zhi Wu. "Neoproterozoic non-glaciogenic iron formation: Insights from Fe isotope and elemental geochemistry of the Shalong iron formation from the Central Tianshan block, southern Altaids." Precambrian Research 351 (2020): 105959.

[24] Williams, George E. "Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit." Reviews of Geophysics 38, no. 1 (2000): 37-59.

[25] Pollard, D., J. F. Kasting, and M. E. Zugger (2017), Snowball Earth: Asynchronous coupling of sea-glacier flow with a global climate model, J. Geophys. Res. Atmos., 122, 5157-5171, doi:10.1002/2017JD026621.​

[26] Williams, George, personal communication, 2/13/2021.

[27] Hoffman, P.E. and Schrag, D.P.: Snowball Earth, Sci. Am., 282, 68-75. 2000.

[28] Evans, Neoproterozoic.

[29] Williams, George (2008) Proterozoic (pre-Ediacaran) glaciation and the high obliquity, low-latitude ice, strong seasonality (HOLIST) hypothesis: Principles and Tests. Earth-Science Reviews. 87(3-4): 61-93.

[30] Williams, High obliquity.

[31] Casado, Juan. "Revisiting the Causes of the Neoproterozoic Snowball Earth." Available at SSRN 3586346 (2020).

[32] Lang, Xianguo, Jitao Chen, Huan Cui, Ling Man, Kang-Jun Huang, Yong Fu, Chuanming Zhou, and Bing Shen. "Cyclic cold climate during the Nantuo glaciation: evidence from the Cryogenian Nantuo Formation in the Yangtze block, south China." Precambrian Research 310 (2018): 243-255.

[33] Li, Jian, Cuiguo Hao, Zhihong Wang, Lin Dong, Yiwu Wang, Kang-Jun Huang, Xianguo Lang et al. "Continental weathering intensity during the termination of the Marinoan Snowball Earth: Mg isotope evidence from the basal Doushantuo cap carbonate in South China." Palaeogeography, Palaeoclimatology, Palaeoecology (2020): 109774.

[34] Pollard, Asynchronous

[35] Liu, Yonggang, Jun Yang, Huiming Bao, Bing Shen, and Yongyun Hu. "Large equatorial seasonal cycle during Marinoan snowball Earth." Science Advances 6, no. 23 (2020): eaay2471.

[36] Young and Williams, Encyclopedia of Geology. 2021

[37] Grotzinger, John, David.A. Fike, and Woodward W. Fischer. 2011. Enignmatic origin of the largest-known carbon isotope excursion in Earth’s history. Nature Geoscience, 4, 285-292

[38] Burns, S. J. & Matter, A. Carbon isotopic record of the latest Proterozoic from Oman. Eclogae Geol. Helv. 86, 595–607 (1993).

[39] Grotzinger, Enigmatic.

[40] Williams, George E., and Phillip W. Schmidt. "Shuram–Wonoka carbon isotope excursion: Ediacaran revolution in the world ocean's meridional overturning circulation." Geoscience Frontiers Volume 9, Issue 2, March 2018, Pages 391-40. https://doi.org/10.1016/j.gsf.2017.11.006

[41] Williams, George (2008) Proterozoic (pre-Ediacaran) glaciation and the high obliquity, low-latitude ice, strong seasonality (HOLIST) hypothesis: Principles and Tests. Earth-Science Reviews. 87(3-4): 61-93.