Paul Hoffman on the Snowball Earth Hypothesis
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Paul Hoffman is Emeritus Professor of Geology at Harvard University. His research on the sedimentary rocks of Namibia revealed compelling evidence of glaciation at sea level in the tropics about 650 million years ago. In the podcast he explains what convinced him that the Earth was almost completely glaciated twice in its history. Here, Paul Hoffman (right) and Dan Schrag point to the contact between the end-snowball glacial deposits with dropstones and the 635-million-year-old dolomite that caps the glacial deposits in Namibia. Dan Schrag is a geochemical oceanographer who reconciled the cap carbonates with the Snowball Earth hypothesis.
Courtesy of Gabrielle Walker
Podcast Illustrations
All illustrations courtesy of Paul Hoffman unless otherwise indicated.
Snowball Earth
The Snowball Earth is a climate state in which the oceans are covered by a continuous ice shelf and continents are mostly buried by ice sheets. In the ablation zone, ice sublimates directly into the atmosphere. Dust or volcanic ash accumulate on the glacier as dark ice-dust (called cryoconite).
Sea glacier on a Snowball planet with a sublimation zone (in red) where dark ice-dust collects and a layer of compressed snow (meteoric ice) accumulates. Sublimation of meteoric ice and melting of marine ice at low latitudes are balanced by an accumulation and freeze-on outside the inner tropics. At the ELL (Equilibrium Line Latitude) the sublimation and deposition are equal.
Glacial epochs in Earth history from 3.5 billion years ago to the present day. The Sturtian and the Marinoan are the only two periods in which the ice sheets extended from pole to pole.
The Geochemical Carbon Cycle and Global Climatic Stability
The geochemical carbon cycle, showing major sources and sinks of carbon dioxide to the ocean and atmosphere. As carbon dioxide accumulates and the planet warms, the weathering of rocks containing silicate minerals increases, which increases the rate of removal of carbon dioxide from the atmosphere, which counteracts the warming. This negative feedback adjusts the amount of carbon dioxide in the atmosphere so as to balance the carbon dioxide sources and sinks, maintaining a steady state.
The geochemical cycle on a Snowball Earth. Volcanic and metamorphic carbon dioxide sources continue unaffected, but removal of carbon dioxide from the atmosphere is limited by the absence of rainfall. In addition, silicate weathering is reduced by ice cover and cold ground temperatures.
Hypothetical depiction of the Sturtian Snowball Earth scenario in terms of global mean surface temperature and ice cover (pale blue) on the Earth with the continents of 750 million years ago. The temperature plot shows that the onset (purple dot) and termination (blue dot) of glaciation near the equator were very abrupt. It also shows a hot aftermath caused by high carbon dioxide levels.
The Evidence for a Snowball Earth
Global Distribution
The present global distribution of Cryogenian glacial formations corresponding to the Marinoan glaciation (651-635 million years ago) and the Sturtian glaciation (717-661 million years ago).
Distinctive Glacial Features
Tillite deposited by the Marinoan glaciation with rounded and polished boulders dispersed in an unstratified silty claystone matrix. Glaciers erode by abrasion and quarrying, and deposit unsorted debris when they melt or sublime. As the glacier flows, boulders are rounded and polished by milling caused by shearing of the substrate below the glacier and the sediment carried by the ice. Hammer (circled) is 32 cm long.
Moonight Valley Tillite (Marinoan), East Kimberley Region, Western Australia
Grooves in a sandstone were produced by northwest-directed (arrow) glacial flow. This glacial pavement is overlain by a moraine composed of glacial till (upper right), onlapped and buried by shallow-marine clastics, implying a former tidewater ice margin or grounding line.
Smalfjord Formation (Marinoan), Bigganjar’ga, Varangerfjord, Finnmark, NE Norway
The diagram shows how a distinctive wedge of sediment is deposited at a grounding line, which is where a grounded ice sheet becomes a floating ice shelf. Massive diamictite (a sedimentary rock with a wide range of particle sizes) is deposited on the landward side of the grounding line (brown), and a stratified diamictite is deposited on the seaward side of the grounding line (orange). The stratified diamictite combines material falling out of a plume of meltwater, ice-rafted debris, and deposits from density flows and bottom currents.
An ice-rafted dropstone impacted the underlying stratified deposits (carbonate turbidites, debrites and plume fallout), which were punctured and deformed by the impact. Below the 2-cm coin (top right) is a doubly-folded sediment flap that was ejected by the impact site. Ice-rafting is a key criterion for glacial action in marine paleoenvironments.
Ghaub Formation (Marinoan), Fransfontein Ridge, NW Namibia
A graded debrite (debris-flow deposit) in the proglacial sediments deposited within a Marinoan grounding-zone wedge in northern Namibia. Coarse debris flows can be triggered by excess debris pile-up at the ice grounding line and by oscillatory movement of the grounding-line ice-front itself.
Ghaub Formation (Marinoan), Fransfontein Ridge, NW Namibia
Carbonate Platforms
Distribution of recent marine carbonate reefs, platforms (e.g., Great Bahama Bank), and continental shelves. Most marine carbonate production occurs in shallow waters and latitudes less than ~35° because of carbonate saturation chemistry. Calcium carbonate is more soluble in colder and deeper water than in warm, shallow water. Certain organisms precipitate skeletal carbonate in colder, deeper water by controlling carbonate saturation in intracellular fluid, but skeletal carbonate is absent in Cryogenian deposits. On tropical coasts near major deltas (e.g., Amazon, Congo), carbonate production is inhibited or diluted by terrigenous sediment input.
Modified after Rodgers (1957) in Le Blanc RJ et al. eds., Regional Aspects of Carbonate Deposition. Sp. Publ. 5, SEPM (Society for Sedimentary Research)
The strata underlying the glacial deposits represent ambient ocean conditions at or close to the time of glacial onset. The figure shows carbonates underlying glacial deposits in several locations (e.g., North and South Namibia), which provide evidence of glaciation at sea level in the tropics. In other locations, the glaciers formed at mid-latitudes where carbonate is uncommon, or were formed in the tropics but near deltas where carbonate production was swamped by detritus from land erosion (terrigenous), such as in the present-day Amazon or Congo River deltas.
Generalized stratigraphy and rock types of carbonate-dominated platform successions of Neoproterozoic age in northern Namibia and western Mongolia. Glaciomarine formations and postglacial cap carbonates of Sturtian and Marinoan age are well developed and exposed in both areas. They indicate glaciation at sea level in the warmest zones and in the absence of mountains. If the warmest areas were glaciated, colder areas must have been frozen as well. This was the rationale for the Cryogenian Snowball Earth hypothesis (Kirschvink 1992).
The Rapitan Group (716-663 million years old) is a glacial sequence sandwiched between carbonate units in the Mackenzie Mountains of northwest Canada. The rocks get younger from right to left.
The Marinoan Wilsonbreen Formation (W) in Spitsbergen (Svalbard) is a 135-meter-thick glacial sequence bracketed by carbonate units (S and D). The much more recent Carboniferous-Permian limestone (CP) is visible in the far distance.
Paleomagnetic Evidence
The right side of the figure shows how the inclination of the magnetic field frozen into a rock at the time it formed tells us its latitude at that time (paleolatitude).
The left side of the figure shows histograms of the paleolatitudes of glacial deposits for various periods of geological history. It indicates that in the Neoproterozoic there was an anomalously high number of glacial deposits in equatorial regions.
Glacial Meltdowns and the Cap Carbonates
Contact between the younger (Marinoan) Cryogenian cap carbonate (CD) that overlies ice-rafted debris (IRD) and glacial debris flows (DF) in Namibia.
Marinoan cap carbonate (dolostone) and underlying Stelfox glacial diamictite in the northwestern Mackenzie Mountains, Northwest Territories, Canada. The Stelfox diamictite is ~40 m thick and disconformably overlies a carbonate-dominated marine shelf sequence (Keele Fm). The cap dolostone is 12 m thick and is conformably overlain by deeper-water marls (Hayhook Fm) and organic-rich black shale (Sheepbed Fm) that yielded a radiometric age of 632 Ma. The pale color of Marinoan cap dolostones is globally distinctive, and those in the Mackenzie Mountains feature giant wave ripples indicating enhanced trade winds during snowball deglaciation.
10-m-thick cap dolostone conformably overlies the Marinoan Storeelv diamictite-rich formation in Tillite Canyon on Kap Weber near the head of Keiser Franz Joseph Fjord, central East Greenland. The cap dolostone is conformably overlain by deeper-water multicolored argillite and marlstone (Canyon Fm) of the postglacial maximum flooding stage. The base of the cap dolostone defines the Cryogenian−Ediacaran boundary. Glacial deposits were first recognized and described at this location by Christian Poulsen during Lauge Koch’s first ship-based expedition to East Greenland in 1929.
Post-Marinoan cap-carbonate sequence on the Otavi/Swakop Group carbonate platform in NW Namibia. The anomalous thickness of cap-carbonate sequences in many areas was an early indication of glacial longevity before radiometric ages were available.
Maieberg Formation (early Ediacaran), upper Hoanib River, NW Namibia
An unusually well-developed post-Sturtian cap dolostone sharply overlies the terminal siltstone of a Sturtian tillite. The siltstone contains ice-rafted dropstones (inset). In this area, the post-Sturtian cap dolostone is better developed than its post-Marinoan equivalent.
Tapley Hill Formation (Cryogenian), Kingsmill Creek, Arkaroola Wilderness Sanctuary, northern Flinders Ranges, South Australia
Phylogenetic Evidence
The time scales of fossil records over three eons. Though most of the pre-snowball fossil cyanobacteria are found in marine formations, the mapping of habitats onto molecular phylogenies indicates that they originally evolved in freshwater habitats before Cryogenian time. pO2 = partial pressure of oxygen in the atmosphere.
An example of habitat mapping onto a phylogenetic tree for living cyanobacteria and archaeplastida (red, green, and glaucophyte algae, and derived plants). Orange and blue dots on the branching points give the relative proportions of freshwater and marine correlates respectively. The dots are mostly orange (freshwater) before the Cryogenian snowballs.
Sanchez-Baracaldo et al. (2017), PNAS 114, E7737
Table summarizing polar-alpine habitat diversity for phototrophs (organisms that obtain their energy from sunlight) in terms of temperature, sunlight intensity, nutrient availability, and redox state. The salt-stratified (meromictic) lake habitat refers to deep waters (over 40 m) that have dim light but are warm (+24 C in Antarctica) and relatively relatively nutrient-rich.
Why Were There Just Two Snowball Earth Glaciations?
Rifting Rodinia
Reconstruction of the Rodinia supercontinent at 720 (bottom) and 635 (top) million years ago. The presence of a supercontinent at equatorial latitudes and its subsequent break-up may have brought higher precipitation to previously arid regions, increasing silicate weathering, and thus drawing down atmospheric carbon dioxide, which in turn would have cooled the Earth.
Large Igneous Province
The 720 Ma reconstruction also shows the large igneous province (Franklin LIP) that erupted at the equator at the start of the Sturtian glaciation. According to one theory, the magma erupted through sulphur-rich deposits, lofting sun-reflecting sulphur particles into the stratosphere, which increased the Earth’s albedo. According to another theory, the large area of freshly exposed lava at equatorial latitudes increased silicate weathering, drawing down atmospheric carbon dioxide. Either mechanism would cause cooling. Glaciation that is rapid enough to overcome the geochemical carbon negative feedback is easier to envisage if the Earth is already cool when the glaciation begins.
Further Reading
Abbot, D.S., Voigt, A., Li Dawei, Le Hir, G., Pierrehumbert, R.T., Branson, M., Pollard, D. & Koll, D.D.B. (2013) Robust elements of Snowball Earth atmospheric circulation and oases for life. Journal of Geophysical Research: Atmospheres 118, 6017−6027.
Budyko, M.I. (1969) The effect of solar radiation variations on the climate of the Earth. Tellus 21, 611−619.
Goddéris, Y., Le Hir, G. & Donnadieu, Y. (2011) Modelling the snowball Earth. In: Arnaud, E., Halverson G.P. & Shields-Zhou, G. (eds) The Geological Record of Neoproterozoic Glaciations. Geological Society, London, Memoir 36, pp. 151−161.
Harland, W.B. & Rudwick, M.J.S. (1964) The great infra-Cambrian ice age. Scientific American 211(2), 28−36.
Hoffman, P.F. (2009) Pan-glacial—a third state in the climate system. Geology Today 25, 107−114.
Hoffman, P.F. & Schrag, D.P. (2002) The snowball Earth hypothesis: testing the limits of global change. Terra Nova 14, 129−155.
Kirschvink, J.L. (1992) Late Proterozoic low-latitude glaciation: the snowball Earth. In: Schopf, J.W. & Klein, C. (eds) The Proterozoic Biosphere. Cambridge University Press, Cambridge, UK, pp. 51−52.
Pierrehumbert, R.T., Abbot, D.S., Voigt, A. & Koll, D. (2011) Climate of the Neoproterozoic. Annual Reviews of Earth and Planetary Sciences 39, 417−460.
Rooney, A.D., Strauss, J.V., Brandon, A.D. & Macdonald, F.A. (2015) A Cryogenian chronology: Two long-lasting synchronous Neoproterozoic glaciations. Geology 43, 459−462.
Sellers, W.D. (1969) A global climatic model based on the energy balance of the Earth–atmosphere system. Journal of Applied Meteorology 8, 392−400.
Vincent, W.F., Gibson, J.A.E., Pienitz, R., Villeneuve, V., Broady, P.A., Hamilton, P.B. & Howard-Williams, C. (2000) Ice shelf microbial ecosystems in the High Arctic and implications for life on snowball Earth. Naturwissenschaften 87, 137−141.