A pale white dot

“There’s nothing new under the sun” goes a famous saying and these words are very apt when trying to understand Earth’s climate trends. Thanks to numerous discoveries made about Earth’s ancient past, we now know that our climate has never been static. According to geological and palaeontological records, climate change has affected the Earth throughout geologic time.

In this context, this is the second of a series of articles about climate change over geological time. The first (A warming medieval climate supports a revolution in agriculture by Steven Wade Veatch and Cheryl Bibeau) appeared in Issue 48.

To understand climate change today, researchers study past climates and events that affect climates, such as volcanic activity, solar radiation, sunspot activity, astronomical changes and other factors that influence climate. Once we understand the dominoes that have fallen during the past climate change events, we can understand and predict – to some degree – the kind of patterns that may follow current trends. To do this, scientists piece together clues from past climates provided by rock formations. Scientists likewise examine fossil records that yield climate signals from the past. These fossils range from prehistoric pollen to dinosaurs. Putting both geological and fossil records together reconstructs ancient climates and environments. More recent climate change is studied through climate records held in polar ice caps and ice sheets, ice cores, glaciers, isotopes of elements (like oxygen, carbon and sulphurfur), soil sediments and tree rings.
When we think of the term “ice age”, the picture that immediately comes to mind is early Neanderthals or Homo sapiens wrapped in animal fur, hiking endlessly through snow and ice-covered plains, striking fire, hunting mammoths and surviving in nomadic camps. This image stems from the most recent ice age (Pleistocene Epoch), but evidence reveals more severe ice ages before the last one. Scientists know of at least five major glaciation events (see table 1). And it is speculated that some of the ice ages covered the whole Earth in snow and ice.

Table 1: Five Major Continental Glaciations. There have been five episodes of extensive continental glaciation through geologic time. The Cryogenian Glaciation lasted the longest, producing a “Snowball Earth” (Levin, 2013).
Glaciation Time Period Huronian Glaciation (Paleoproterozoic Era) 2.4 – 2.1 Ga
Cryogenian Glaciation (Neoproterozoic Era) 850 – 635 Ma
Andean-Saharan Glaciation (Ordovician-Silurian Period) 460 – 430 Ma
Karoo Glaciation (Carboniferous-Permian Period) 360 – 260 Ma
Pleistocene Glaciation (Pleistocene Epoch) 2.6 Ma to the present.

Tolypothrix_(Cyanobacteria)
Fig. 1. A photomicrograph of Cyanobacteria, Tolypothrix sp. Cyanobacteria produce oxygen as a by-product of photosynthesis and it is thought this process converted Earth’s early, oxygenpoor, reducing atmosphere, into an oxidizing one, causing two major events: (1) the “ Great Oxygenation Event” and (2) the so-called rusting of the Earth. Both events dramatically changed the nature of life forms on Earth and almost led to the extinction of anaerobic organisms. (Image by Matthew Parker, used by permission under Community Commons Licence 3.0.)

Broadly speaking, a number of scientists believe the Earth’s climate, throughout geological time, can be characterised by three climate conditions. First, is that of “Earth as a Greenhouse”, when warm temperatures extend to the poles, eliminating the polar icecaps and all other ice sheets. In some parts of the planet, the climate was like hell in a box. Secondly, is that of “Earth as an Icehouse”, which includes some permanent ice, whose extent varies as glaciers periodically advance and retreat. And lastly, is what is termed “Snowball Earth”, in which the planet’s entire surface is frozen for up to hundreds of millions of years (Walker, 2003). There is credible speculation that there is a fourth state – “Slush House Earth” – in which there is an ice-free zone along the equator (Cowen, 2013). Today’s climate, marked by polar ice caps, is characterised by the second condition, an “Icehouse”. Since primordial times, it has been speculated that the Earth has been cycling between these phases.

The Earth froze completely in defiance to the warmth of the sun between 2.45 and 2.22bya, resulting in Earth’s first Ice Age, known as the Huronian Glaciation (named after Lake Huron in Ontario, Canada). This deep freeze may not have happened just once, but perhaps several times, during the Huronian Glaciation (Levin, 2013).

The cause of this first Snowball Earth event is not known. However, several theories have been proposed, including a decrease in solar output, the Earth passing through so-called space clouds or an extreme cooling caused by a reduction in greenhouse gases (Oceans of Ice: The Snowball Earth Theory of Global Glaciation; see further reading). Some scientists believe a combination of these events could be a reason the Earth became frozen in ice. It seems likely that a sharp drop in carbon dioxide – a greenhouse gas – caused temperatures to plummet. An unimaginably thick, white ice sheet crept down from the poles. Snow, whipped by winds, danced on the crenelated surface of the ice, while the bottom of the ice sheet plucked and ground the rock surface beneath as it crept forward.

BIF large_1
Fig. 2. An exposure of banded iron formations (BIFs) at the Fortescue Falls, Dales Gorge, Karijini National Park, Western Australia. Cyanobacteria contributed oxygen to Earth’s atmosphere. This oxygen, combined with iron in the ocean’s water, caused chemical precipitation of iron oxides and formed dark red bands that alternated with white bands of chert to produce the banded iron formations. (Photo by Graeme Churchard, used by permission under Community Commons Licence 2.0.)

During these frigid times, sunlight, instead of warming the planet, bounced off the ever-spreading ice, in what scientists call the albedo effect, causing temperatures to fall – which created more ice – which bounced more sunlight back into the cold reaches of outer space (Melehzik, 2006). This process repeated in a positive feedback loop until the cooling became unstoppable: the ice marched on, temperatures plunged, and the blue planet became a small white dot – a snowball, surrounded by a riot of stars, orbiting the sun.

Of interest to scientists is that life came to a near biological standstill in the first Snowball Earth event, yet life survived this hyper-freeze phase. Even in an Earth almost entirely covered by ice, volcanoes punched through the ice by melting it while releasing gases, including carbon dioxide, a greenhouse gas. Against these odds and brutal mass extinctions, a handful of tiny organisms, living near volcanic vents on the sea floor, thrived. These organisms were anaerobic bacteria and called methanogens by scientists. The methanogens fed on mineral nutrients like sulphur, iron and manganese from underwater volcanic vents and merrily expelled methane, another greenhouse gas. Oxygen was not present in the Earth’s atmosphere. The methanogens spread and continued to help gas-up an atmosphere that contained methane, nitrogen and few other gasses in trace quantities. The microscopic methanogen’s methane trapped some of the sun’s energy and contributed to the warming of the planet. However, it is believed that carbon dioxide, expelled from volcanoes, played a large role in reheating the planet. Today, scientists think that the methane from the methanogens tipped the scale toward the reheating of Earth.

Following the Huronian Glaciation, the frozen planet thawed (possibly very rapidly), marking one of the greatest periods of transition in our world’s history – the Great Oxygenation Event, one that would change forever the destiny of this planet we call home and which may have been caused by the end of Snowball Earth. Here is what happened.

Soon after Snowball Earth melted, a new kind of bacteria evolved – cyanobacteria, the planet’s first photosynthesizing organisms that made oxygen (Canfield, 2016). There was a slow and episodic enrichment of gaseous oxygen in the atmosphere that continued over millions of years, possibly due to an exponential bloom of the cyanobacteria as mats that rolled and pitched with the waves of the sea. Near the shore, cyanobacteria grew in layered structures known as stromatolites. These were also present in some lakes and in any other shallow aquatic setting where the conditions were favourable.
The rising oxygen levels brought the Great Oxygenation Event – a significant shift in the content of oxygen in the atmosphere (Crowell, 1999). As the cyanobacteria churned out more and more oxygen that bubbled through the water column, the methanogens almost went extinct – oxygen is toxic to them. Those that survived lived in deep ocean water near hydrothermal vents and other places that protected them. In the meantime, due to the higher levels of oxygen resulting from photosynthesis, iron – previously dissolved in the oceans – could no longer stay in solution, leading to an intricate alchemy that brought the “Great Oxidation Event”. This so-called “rusting” event formed rocks known as banded iron formations (BIFs). BIFs are white bands of chemically precipitated quartz or chert, with alternating darker red bands of the iron oxide minerals hematite and magnetite. From this oxidation of iron and the formation of BIFs, we infer that oxygen began to appear in Earth’s atmosphere.

Scientists continue to speculate on the source of the iron that was dissolved in the oceans before the Great Oxygenation Event. One source of the iron likely weathered from iron-bearing rocks on land masses. Another, much larger source of iron spewed out in dark clouds from more active submarine volcanoes and hydrothermal vents on the seafloor.
The BIFs were deposited in a relatively brief geological time between 2.6 and 1.8bya, and occurred in great bodies that exceeded hundreds of meters in thickness and extended thousands of meters laterally (Macdougall, 2004). BIFs are an essential part of our modern industrial complex, as they yield most of the rich iron ore mined today from the massive iron ore deposits of Minnesota, Michigan, Ukraine, Brazil, Labrador and Australia (Levin, 2013).

Despite the frozen conditions of the first Snowball Earth, the period following it was an evolutionary triumph when oxygen became part of Earth’s atmosphere and early life flourished. Oxygen formed the extensive iron ore deposits that are the foundation of modern society. Although we are building a compendium of knowledge about past and present climate change, unanswered questions about Snowball Earth remain, while certain aspects of climate change remain unknown.
An army of scientists, with intellectual fire, continue their work in their search for answers. Even if we do not find some of these unknown factors affecting climate change, those factors will perhaps find us.

References

Canfield, D. (2016). Oxygen: A four billion year history. Princeton: Princeton Univ. Press.
Cowen, R. (2013). History of Life. Oxford: Wiley-Blackwell.
Crowell, J. C. (1999). Pre-Mesozoic Ice Ages: Their Bearing on Understanding the Climate System. Boulder: Geological Society of America.
Levin, R. (2013). The Earth Through Time. Hoboken: John Wiley and Sons.
Macdougal, D. (2004). Frozen Earth: The Once and Future Story of Ice Ages. Berkeley: University of California Press.
Melezhik, V. A. (2006). Multiple causes of Earth’s earliest global glaciation. Terra Nova, 18(2), 130-137.
Oceans of Ice: The Snowball Earth Theory of Global Glaciation. (n.d.). Retrieved from http://dujs.dartmouth.edu/2010/05/oceans-of-ice-the-snowball-earth-theory-of-global-glaciation/.
Walker, G. (2003). Snowball Earth: The Story of the Great Global Catastrophe that Spawned Life as We Know It. New York: Crown Publishers.


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