The Boring Billion, otherwise known as the Mid-Proterozoic and Earth's Middle Ages, is an informal geological time period between 1.8 and 0.8 billion years ago (Ga) during the middle Proterozoic eon spanning from the Statherian to the Tonian periods, characterized by more or less tectonic stability, climatic stasis and slow biological evolution. Although it is bordered by two different oxygenation events (the Great Oxygenation Event and Neoproterozoic Oxygenation Event) and two global glacial events (the Huronian and Cryogenian glaciations), the Boring Billion period itself actually had very low oxygen levels and no geological evidence of glaciations.
The oceans during the Boring Billion may have been oxygen-poor, sulfidic and nutrient-deficient (i.e. euxinic), populated mainly by purple bacteria, a type of bacteriochlorophyll-based photosynthetic bacteria which uses hydrogen sulfide (H2S) for carbon fixation instead of water and produces sulfur as a byproduct instead of oxygen. This is known as a Canfield ocean, and such composition may have caused the oceans to be colored black-and-milky-turquoise instead of blue or green as later. (By contrast, during the much earlier "Purple Earth" phase of the Archean eon, primitive anoxygenic photosynthesis was performed mostly by archaeal colonies using retinal-based proton pumps that absorb green light to directly synthesize adenosine triphosphate, and the oceans would have been magenta-purple.)
Despite such adverse conditions, eukaryotes may have evolved via symbiogenesis around the beginning of the Boring Billion, developed several novel adaptations such as various organelles, multicellularity and possibly sexual reproduction, and diversified into algae, fungi, protists and the early ancestors of animals at the end of this time interval. Such advances may have been important precursors to the large-scale evolutionary radiation of macroscopic complex life later in the Ediacaran Avalon Explosion and the subsequent Phanerozoic Cambrian Explosion. Nonetheless, prokaryotic cyanobacteria were the dominant autotrophic lifeforms during this time period, and likely supported an energy-poor food-web with a small number of protists at the apex level. The land was likely inhabited at the coastal and riparian margins by washed-ashore cyanobacteria and eukaryotic proto-lichens, the latter being more successful on dry land probably due to greater robustness against arid environment and lethal solar irradiation (as then the protective ozone layer had not yet formed).
Description In 1995, geologists Roger Buick, Davis Des Marais, and Andrew Knoll reviewed the apparent lack of major biological, geological, and climatic events during the Mesoproterozoic era 1.6 to 1 billion years ago (Ga), and, thus, described it as "the dullest time in Earth's history". The term "Boring Billion" was coined by paleontologist Martin Brasier to refer to the time between about 2 and 1 Ga, which was characterized by geochemical stasis and glacial stagnation. In 2013, geochemist Grant Young used the term "Barren Billion" to refer to a period of apparent glacial stagnation and lack of carbon isotope excursions from 1.8 to 0.8 Ga. In 2014, geologists Peter Cawood and Chris Hawkesworth called the time between 1.7 and 0.75 Ga "Earth's Middle Ages" due to a lack of evidence of tectonic movement.
The Boring Billion is now largely cited as spanning about 1.8 to 0.8 Ga, contained within the Proterozoic eon, mainly the Mesoproterozoic. However, others have argued that it should instead span from about 2.0 to 1.0 Ga and be tied to variability in the carbon isotope record. The Boring Billion is characterized by geological, climatic, and by-and-large evolutionary stasis, with low nutrient abundance.
In the time leading up to the Boring Billion, Earth experienced the Great Oxygenation Event due to the evolution of oxygenic photosynthetic cyanobacteria, and the resultant Huronian glaciation (Snowball Earth), formation of the UV-blocking ozone layer, and oxidation of several metals. Oxygen levels during the Boring Billion are thought to have been markedly lower than during the Great Oxidation Event, perhaps 0.1% to 10% of modern levels. It was ended by the breakup of the supercontinent Rodinia during the Tonian (1000–720 Ma) period, a second oxygenation event, and another Snowball Earth in the Cryogenian period.
Tectonic stasis
The evolution of Earth's biosphere, atmosphere, and hydrosphere has long been linked to the supercontinent cycle, where the continents aggregate and then drift apart. The Boring Billion saw the evolution of two supercontinents: Columbia (or Nuna) and Rodinia.
The supercontinent Columbia formed between 2.0 and 1.7 Ga and remained intact until at least 1.3 Ga. Geological and paleomagnetic evidence suggest that Columbia underwent only minor changes to form the supercontinent Rodinia from 1.1 to 0.9 Ga. Paleogeographic reconstructions suggest that the supercontinent assemblage was located in equatorial and temperate climate zones, and there is little or no evidence for continental fragments in polar regions.
Due to the lack of evidence of sediment build-up (on passive margins) which would occur as a result of rifting, the supercontinent probably did not break up, and rather was simply an assemblage of juxtaposed proto-continents and cratons. There is no evidence of rifting until the formation of Rodinia, 1.25 Ga in North Laurentia, and 1 Ga in East Baltica and South Siberia. Breakup did not occur until 0.75 Ga, marking the end of the Boring Billion. This tectonic stasis may have been related in ocean and atmospheric chemistry.
It is possible the asthenosphere—the semi-solid layer of Earth's mantle that tectonic plates essentially float and move around upon—was too hot to sustain modern plate tectonics at this time. Instead of vigorous plate recycling at subduction zones, plates were linked together for billions of years until the mantle cooled off sufficiently. The onset of this component of plate tectonics may have been aided by the cooling and thickening of the crust that, once initiated, made plate subduction anomalously strong, occurring at the end of the Boring Billion.
Nonetheless, major magmatic events still occurred, such as the formation (via magma plume) of the central Australian Musgrave Province from 1.22 to 1.12 Ga, and the Canadian Mackenzie Large Igneous Province 1.27 Ga. Plate tectonics were still active enough to build mountains, with several orogenies, including the Grenville orogeny, occurring at the time.
Climatic stability
There is little evidence of significant climatic variability during this time period. Climate was likely not primarily dictated by solar luminosity because the Sun was 5–18% less luminous than it is today, but there is no evidence that Earth's climate was significantly cooler. In fact, the Boring Billion seems to lack any evidence of prolonged glaciations, which can be observed at regular periodicity in other parts of Earth's geologic history. High CO2 could not have been a main driver for warming because levels would have needed to be 30 to 100 times greater than pre-industrial levels and produced substantial ocean acidification to prevent ice formation, which also did not occur. Mesoproterozoic CO2 levels may have been comparable to those of the Phanerozoic eon, perhaps 7 to 10 times higher than modern levels. The first record of ice from this time period was reported in 2020 from the 1 Ga Scottish Diabaig Formation in the Torridon Group, where dropstone formations were likely formed by debris from ice rafting; the area, then situated between 35–50°S, was a (possibly upland) lake which is thought to have frozen over in the winter and melted in the summer, rafting occurring in the spring melt.
A higher abundance of other greenhouse gases, namely methane produced by prokaryotes, may have compensated for the low CO2 levels; a largely ice-free world achieved by an atmospheric methane concentration of 140 parts per million (ppm). Methanogenic prokaryotes could not have produced so much methane, implying some other greenhouse gas, probably nitrous oxide (laughing gas), was elevated, perhaps to 3 ppm (10 times today's levels). Based on presumed greenhouse gas concentrations, equatorial temperatures during the Mesoproterozoic may have been about , in the tropics , at 60° , and the poles ; and the global average temperature about , which is 4 °C (7.2 °F) warmer than today. Temperatures at the poles dropped below freezing in winter, allowing for temporary sea ice formation and snowfall, but there were likely no permanent ice sheets.
It has also been proposed that, because the intensity of cosmic rays has been shown to be positively correlated to cloud cover, and cloud cover reflects light into space and reduces global temperatures, lower rates of bombardment during this time due to reduced star formation in the galaxy caused less cloud cover and prevented glaciation events, maintaining a warm climate. Also, some combination of weathering intensity which would have reduced CO2 levels by oxidation of exposed metals, cooling of the mantle and reduced geothermal heat and volcanism, and increasing solar intensity and solar heat may have reached an equilibrium, barring ice formation.
Conversely, glacial movements over a billion years ago may not have left many remnants today, and an apparent lack of evidence could be due to the incompleteness of the fossil record rather than absence. Further, the low oxygen and solar intensity levels may have prevented the formation of the ozone layer, preventing greenhouse gasses from being trapped in the atmosphere and heating the Earth via the greenhouse effect, which would have caused glaciation. Though not much oxygen is necessary to sustain the ozone layer, and levels during the Boring Billion may have been high enough for it, the Earth may have been more heavily bombarded by UV radiation than today.
Oceanic composition The oceans seem to have had low concentrations of key nutrients thought to be necessary for complex life, namely molybdenum, iron, nitrogen, and phosphorus, in large part due to a lack of oxygen and resultant oxidation necessary for these geochemical cycles. Nutrients could have been more abundant in terrestrial environments, such as lakes or nearshore environments closer to continental runoff.
In general, the oceans may have had an oxygenated surface layer, a sulfidic middle layer, and suboxic bottom layer. The predominantly sulfidic composition may have caused the oceans to be a black-and milky-turquoise color instead of blue.
Oxygen Earth's geologic record indicates two events associated with significant increases in oxygen levels on Earth, with one occurring between 2.4 and 2.1 Ga, known as the Great Oxidation Event (GOE), and the second occurring an approximate 0.8 Ga, known as the Neoproterozoic Oxygenation Event (NOE). The canonical interpretation is that it represents an intermediary period where the Boring Billion is thought to have had low oxygen levels (with minor fluctuations), leading to widespread anoxic waters. More recent work has highlighted that the evidence for a large drop in oxygen levels after the GOE remains inconclusive and thus the boring billion may instead represent an extension of stabilized GOE oxygen levels.
The oceans may have been distinctly stratified, with surface water being oxygenated and deep water being suboxic (less than 1 μM oxygen), the latter possibly maintained by lower levels of hydrogen (H2) and H2S output by deep sea hydrothermal vents which otherwise would have been chemically reduced by the oxygen, i.e., euxinic waters. Even in the shallowest waters, significant quantities of oxygen may have been restricted mainly to areas near the coast. The decomposition of sinking organic matter would have also leached oxygen from deep waters.