Snowball Earth:
Superglacials of the Neoproterozoic Era (590 Ma)

Linda Sohl and Mark Chandler

Introduction

With the debate over global warming capturing the attention of many, it is not surprising that a great deal of current climate research is aimed at understanding the causes and effects of warmer climates. However, despite the likelihood that the 21st century will be an exceptionally warm century we are technically still in the midst of an ice age "The Pleistocene" that has persisted for nearly two million years. The Pleistocene ice age has been the focus of many climate studies that have helped us to better understand not only cold climates, but the Earth's climate system in general. However, just as there have been periods in Earth history that were warmer even than what we expect from global warming, the Earth has experienced ice ages that were far colder than the Pleistocene.

Background

Between 543 million and 1 billion years ago, during the Neoproterozoic Era, the Earth twice dipped into deep freezes that most geologists consider to have been among the coldest climates in the history of our planet. A variety of evidence suggests that Earth experienced two broad intervals of widespread glaciation: the first around 750 million years ago (the Sturtian glaciation) and another at approximately 590 million years before present (the Varanger or Marinoan glaciation; see Figure 1). One of the more remarkable features of these glaciations is the determination, based on the characteristics and distribution of certain sedimentary rocks and climate model results, that continent-scale ice sheets existed at sea level within 10 degrees of the equator, equivalent to the modern-day latitude of Costa Rica. Because the extreme cold conditions are thought to have produced snow and ice cover over much of the Earth's surface, the Sturtian and Varanger glaciations have become known as "snowball Earth" intervals.

The possible occurrence of ice sheets in the tropics was a controversial topic until fairly recently, when data clearly supporting their existence came to light (Sohl et al., 1999). Still controversial is the question of whether the tropical oceans were also totally covered with sea ice during these extreme glacial intervals. Proponents of total or near-total freeze-over of the tropical oceans have argued that such occurrences had an enormous impact of the subsequent evolution of multicellular life on Earth, and in fact may have been the trigger for the "Cambrian explosion" of life forms that were the ancestors of much of modern multicellular life. The broader ramifications of the glaciations' effects have thus provoked a great deal of interest in understanding just what climatic conditions might have been like during the glacial intervals, and what climate forcings may have been reponsible for producing such extremely cold conditions.

Snowball Earth: Evolution of a Hypothesis

The controversy over the possible occurrence of extreme glaciation in the Neoproterozoic has a long history. Brian Harland (1964) was the first scientist to propose a "worldwide glaciation" nearly 40 years ago, by combining observations about the widespread distribution of glacial deposits with paleomagnetic data that suggested that at least some of the glacial deposits were laid down in seas at low latitudes. Harland's proposal was the subject of much debate over the years, but basically remained unchanged until Joseph Kirschvink of Caltech developed a more elaborate "snowball Earth" hypothesis (Kirschvink, 1992). According to Kirschvink, the bizarre existence of continental-scale ice sheets at low latitudes could be accounted for if large areas of land were preferentially located in mid- to low latitudes; so from his perspective, the ice sheets simply grew where there was an available surface with sufficient snow accumulation. A concentration of land area in the tropics might also have enhanced global cooling by reflecting a greater amount of incoming solar radiation back into space, as land albedo values would have been higher than the ocean's. The mid- to high latitude oceans would be covered by pack ice, which Kirschvink suggested would reduce evaporation from the sea and cut off oceanic currents from wind patterns, inhibiting exchange of oxygen between the ocean and atmosphere and causing the ocean to stagnate. In this way, the "snowball Earth" hypothesis could also account for the renewed appearance of iron formations in the geologic record: with time, the stagnant ocean bottoms would become anoxic, accumulating reduced iron until ocean circulation was re-established and oxidized iron could be deposited in.

In 1998, Paul Hoffman of Harvard University and his colleagues extended the snowball Earth hypothesis to explain some unusual carbon isotope values in post-glacial cap carbonates associated with glacial deposits in Namibia (Hoffman et al., 1998). Taking the hypothesis considerably further than Kirschvink did, Hoffman et al. envisioned an ocean that was completely frozen over, cutting off any exchange of gases between the atmosphere and ocean. They suggested that recovery from this deep-frozen state occurred once atmospheric CO2, released into the atmosphere through volcanic outgassing, built up to high enough levels that permitted the greenhouse effect of the CO2 to overcome the albedo effect of the extensive snow and ice cover. The highly elevated levels of CO2 in the atmosphere would then briefly fuel a very hot climate in the aftermath of the snowball Earth, until the global carbon cycle could once again fall into equilibrium. (See Figure 2 through 5 for an illustration of the four stages of the Hoffman and Schrag snowball Earth hypothesis.)

The properties of the ocean and atmosphere described by the snowball Earth hypothesis are not a part of the geologic record; they can only be inferred from a particular interpretation of the sedimentary deposits left behind. For difficult problems such as understanding the events associated with a snowball Earth state, climate simulations using a GCM offer the best way to evaluate whether a scenario is possible, given what we know of climate dynamics.

EdGCM Simulations of Snowball Earth Intervals

Paleoclimate modeling does present some unique challenges not posed by climate studies of the near future. For simulations of the Varanger snowball Earth interval, we needed to take into account a sun that shone roughly 4-6% less brightly than at present, as well as a radically different continental configuration (see Figure 1). There are also little or no data to constrain other major forcings we wished to examine, such as atmospheric CO2 levels; the values we selected for these forcings are arbitrary, but are reasonable estimates based upon other climatological or geological considerations. Key boundary conditions altered are as follows:

* Solar Luminosity. As mentioned previously, models of stellar evolution have suggested that a G-type yellow star, such as the Sun, should have been less luminous earlier in Earth's history. By the Neoproterozoic, the Sun would have increased its energy output but would still have been between 4% and 6% less luminous than today; we opted for a 4% reduction in luminosity for our Varanger experiments. Solar radiation is reduced in the model by decreasing the total amount of shortwave radiation entering the top of the atmosphere, and is proportionally reduced at all wavelengths.

* Paleogeographic Distribution. We developed paleocontinental reconstructions for the Varanger glacial interval based upon the available paleomagetic data and other geologic constraints (see Figure 3). Such reconstructions are necessarily tentative, as reliable paleomagnetic data are not abundant and age constraints on the relevant rocks are not well defined. However, the radically different continental confirguration for the Varanger interval, as compared with modern geography, provides an opportunity to test the possible effects of varying land distribution on climate.

* Atmospheric CO2 . The extremity of the Neoproterozoic ice ages suggests that these particular periods were times of CO2 drawdown. Therefore, we ran simulations with atmospheric CO2 levels set to 315 ppm, 140 ppm, and 40 ppm. The value of 315 ppm is the approximate amount of CO2 in the atmosphere in 1958 (the first year for which direct measurements are available) and is the amount used in the GISS current climate control simulations; 140 ppm is one-half of the accepted preindustrial value; and 40 ppm was chosen as an extreme example.

* Ocean Heat Transports. The transport of heat by ocean circulation is critical to the distribution of temperatures on the planet. Today the oceans transport heat, on an annually averaged global basis, away from the tropics and subtropics and into the middle and high latitudes. We have simulated the potential effects of both increased and decreased poleward ocean heat transports, using ocean heat fluxes that yield zonally-averaged, meridional transports comparable to modern values. (The transports are necessarily modified since the Neoproterozoic ocean basin configurations are much different than modern.) The decreased and increased ocean heat transport scenarios use ocean transports that are one-half (OHT0.5) and 1.5 times (OHT1.5) the modern global value.

Summary of Results

The plausibility of forming continental ice sheets at low latitudes depends largely on the existing temperature and moisture regime over low-latitude land areas; generally, the climate must be conducive to annual snow accumulations that withstand summertime melting. Glacial initiation requires that at least some continental regions exist where snow accumulation exceeds ablation. The climate variables of greatest significance, therefore, include the surface air temperature and precipitation, particularly that which falls as snow.

Sea ice growth also generates strong feedbacks that can lead to further cooling. Expanding ice forms a platform for the accumulation of more high-albedo snow. Furthermore, sea ice insulates the atmosphere from the ocean, limiting heat and moisture exchange between the two. Given the dominance of ocean area over land in the low latitudes during the Varanger glacial interval, extensive sea ice coverage was probably essential to the ability to form ice sheets on the continents in the low latitudes. There may be a limit, however, since total coverage of the oceans by ice would eliminate the only significant source of atmospheric moisture for feeding growing glaciers.

We found that none of the individual forcings examined, including a solar luminosity decrease as great as 6%, reduction of CO2 to 40 ppm, or increased or decreased ocean heat transports, could yield coinciding surface air temperature and snow fall rates that would allow snow to accumulate on low-latitude continents. The lower solar luminosity and 40 ppm CO2 simulations do, however, show that snow and ice accumulations on land in mid-latitudes could have occurred in during a snowball Earth interval if either of those conditions actually existed. Pairing climate forcings together, while not necessarily an additive process, does create further cooling of the planet and extends the annual average freeze line, as well as snow and ice accumulations, into the marginal tropics.

As noted above, only the most extreme scenarios in our study yielded tropical conditions cold enough to permit glaciation. Yet, even then, sea ice never completely covered the tropical ocean - as much as 30% of the ocean waters remained ice-free, even with atmospheric CO2 , solar luminosity and ocean heat transports all reduced (see Figure 4). Our results suggest, therefore, that low-latitude continental glaciation may be easier to accommodate with a combination of climate forcing mechanisms than global ocean ice cover might be leading us to a sort of "slushball" Earth, rather than the "hard snowball" Earth of Hoffman and Schrag.