Theory of Snowball Earth

@robot

Published in

Nature

· 13 Feb 2024

Glacial Deposits and Carbonate Caps

The first evidence was presented in 1960 by Brian Harland, a geologist from the University of Cambridge. He recognized glacial deposits and "dropstones" in Neoproterozoic sedimentary layers.

Above the Neoproterozoic glacial deposits, there are hundreds of meters of carbonate rock. In cold water, the carbonates are soluble; in warm water, they precipitate out of the water.

Figure showing Rocky cliffs along Namibia's Skeleton Coast.

Paleomagnetic studies

Paleomagnetism uses the alignment of magnetic minerals in rock deposits (termed natural remnant magnetization) to determine where the deposits were formed. If the magnetic minerals are aligned horizontally, the deposit formed at the equator. If they are vertical, they formed at one of the poles.

Paleomagnetism was initially used by Harland to estimate glaciation latitudes, and later advocated by Kirschvink to explain the occurrence of glaciations near the equator. Harland discovered shallow inclinations of magnetic minerals at various study sites in glacial deposits for the Neoproterozoic. These nearly horizontal alignments of magnetic minerals indicate that the deposits were formed near the equator. The evidence for low latitude glaciation was later confirmed by Linda Sohl, Dennis Kent, and Nicholas Christie-Blick in their paleomagnetic studies of the Australian Neoproterozoic Elatina Formation. This supports the snowball theory, which propounds that the entire surface of the earth (even the equator) was covered with ice.

Banded Iron Formations and Manganese Deposits

The presence of banded iron formations and manganese deposits in the Kalahari Manganese Field in southern Africa was another piece of evidence used by Kirshvink et al. (2000) to argue and support the snowball theory. In the manganese field, a layer of banded iron formations were deposited between a different kind of iron formation (known as the jaspilitic iron formation) with occasional glacially deposited drop stones, sandy layers and a layer of manganese. This type of deposit is found only twice in the geologic record, once in the Paleoproterozoic and once in the Neoproterozoic. Kirshvink's argument was that the reduced interaction between the ocean and atmosphere due to global glaciation aided in the build up of reduced iron and manganese in great concentrations in the oceans. The iron was likely generated from mid-ocean ridges or leached from the ocean bottom sediments. Kirshvink et al. (2000) propose that a photosynthetic bloom at the end of a snowball event lead to the precipitation of an iron/manganese mix. However, The manganese could have been reduced by the iron and put back into solution while the iron was deposited. However, once the iron was precipitated, manganese oxides were deposited to form the manganese-rich carbonate that is found in the Kalahari Manganese Field.

Figure showing a dropstone imbedded in a banded iron formations in Canada's Mackenzie Mountains.

Isotope analysis

Isotope measurements are typically used in two ways for geologic studies: radioactive dating and geochemical fingerprinting. Radiogenic isotopes such as carbon-14 and potassium-40 are unstable and decay into either an unstable or stable isotope. Stable isotopes, such as carbon-12 and carbon-13, do not decay and thus are great for looking at geochemical fingerprints.

Researchers collecting data for the snowball earth are using stable carbon isotopes to try to understand the environmental processes that occurred during the Neoproterozoic. The natural abundance of carbon-12 and carbon-13 is 98.89 % to 1.11% respectively. Through environmental processes like photosynthesis, respiration, decomposition, and evaporation, the ratio of these carbons is modified. This term is called isotopic fractionation.

Plants use carbon during photosynthesis, which tends to increase carbon-12 and deplete carbon-13 in plants. Specifically, they use a lighter isotope (carbon-12) over the heavier isotope (carbon-13). Organic matter stored in sediments can reflect the abundance of these two isotopes. If plants are doing well, there will be an abundance of the heavier isotope carbon-13; if they are doing poorly, there will be an abundance of carbon-12. The rock record during the late Proterozoic shows a shift of highly productive activity to a productivity of near zero.

Plate Tectonics, Carbon Dioxide Emissions, and the Reversal of Ice Albedo Feedback

Even though ice covered the surface of the earth, the continental plates were not frozen in place. They were still moving and creating volcanoes, which released large amounts of water and CO₂ into the atmosphere. When enough CO₂, a greenhouse gas, was released into the air, it began to heat up the earth. As the ice and snow melted, there was less reflective surface on the Earth, causing the glaciers to recede rapidly.

The questions are how muchCO₂was needed and how long did it take to build up this muchCO₂?

Caldeira and Kastings make an estimation of about 4 million years after the beginning of snowball Earth. Their two main assumptions in calculating this rate were that the rates of carbon dioxide emission from volcanoes were the same as modern rates and that carbon was not actively being stored in any sinks.

This second assumption is based on the understanding that the erosion of silicate rocks is a major carbon sink. Today, rain carries carbon dioxide from the atmosphere to the earth. Once there, it chemically erodes rocks into bicarbonate, which is then washed into the oceans. However, during snowball Earth, it was too cold to have liquid water in the atmosphere and there weren't any rivers running into the ocean.

Proterozoic Biostratigraphy

Biostratigraphy is another tool that is being used by geologists to produce data that is both paleobiologically, and stratigraphically useful. It has become increasingly successful in helping subdivide time periods to help understand the evolution of life over the years. A short paper by Zang and Walter in Nature, announced the discovery of acanthomorphic acritarchs in terminal Proterozoic shales from Central Australia, leading to a new round of paleontological exploration on biostratigraphically coherent assemblages. Assemblages found in Australia, China, and India occur above the Marinoan tillites, and below levels containing diverse Ediacaran body fossils. These discoveries have led to the use of acritarchs as potential dating tools to identify age ranges of rocks. Grey had recognized several assemblage zones of large acanthomorphs in Australia, enhancing regional, and potential global correlations of terminal Proterozoic rocks. These acritarchs appear to be restricted to lower parts of terminal Proterozoic successions, disappearing at approximately the time when diverse Ediacaran fossils appear.

Extremophiles

How is it possible that live could exist during snowball Earth? The discovery of organisms in toxic sea vents shows that life can exist in harsh environments.

But what about extreme cold, like the conditions of the late Proterozoic? Evidence was presented by W.F. Vincent et al. in Naturwissenschaften. They examine the Ward Hunt Ice Shelf of Canada and the McMurdo Ice Shelf of Antarctica. They found microbial mat communities in meltpools and sediment depressions on the glacial ice shelves. They believe that these photosynthetic mats protect the microbes from freeze-thaw cycles, short wavelength radiation, and desiccation. In addition to cyanobacteria, many other microorganisms were found including ciliates, flagellates, nematodes, and rotifers. These ecosystems can remain frozen throughout the year except for a short time (days or weeks) when meltwater is produced. During these times, microbes can resume metabolic activities. The presence and diversity of microorganisms on the ice shelves shows that opportunities existed for life to persist through snowball Earth.

McKay looked at arctic lakes and used energy balanced models to predict the thickness of ice during the snowball Earth. He found that arctic lakes with ice 5 meters thick could sustain life. He stated that ice near the equator was limited in thickness due to sunlight and latent heat released by freezing. With experiments McKay found that light can be transmitted through ice that is up to 30 meters thick. He concludes his paper with discussion on the likelihood of thin ice sheets that would allow photosynthesis to occur and life to persist.

Theory

From the information we compiled, we will present the following arguments for Snowball Earth theory:

Ice-albedo feedback causing greater cooling
Ice formation beyond critical latitude causes surface temperatures to plummet
Survival of organisms during glaciation
Extent of Neoproterozoic glacial deposits (using paleomagnetic inclination)
Reversal of ice albedo feedback through increasing amounts of carbon dioxide
Natural remnant magnetization (glaciation occurred at low latitudes)
Calcium carbonate formation (i.e. "Cap" dolostones)
Snowball led to evolution of multicellular organisms. . . also fills void explaining existence of animals that were tiny floating organisms leaving no fossil record
Probability of future occurrence of snowball Earth???

The arguments against Snowball Earth: