Boring Billion
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.
(By contrast, during the much earlier Purple Earth phase during the Archean, photosynthesis was performed mostly by archaeal colonies using retinal-based proton pumps that absorb green light, and the oceans would be magenta-purple.)
Despite such adverse conditions, eukaryotes may have evolved around the beginning of the Boring Billion, and adopted several novel adaptations, such as various organelles, multicellularity and possibly sexual reproduction, and diversified into algae, fungi and early animals at the end of this time interval.
Nonetheless, prokaryotic cyanobacteria were the dominant autotrophic lifeforms during this time, and likely supported an energy-poor food-web with a small number of protists at the apex level.
The land was likely inhabited by prokaryotic cyanobacteria and eukaryotic proto-lichens, the latter more successful here probably due to the greater availability of nutrients than in offshore ocean waters.
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".
[2] 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.
[3] 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.[4] 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.
[12] Due to the lack of evidence of sediment build-up (on passive margins) which would occur as a result of rifting,[13] the supercontinent probably did not break up, and rather was simply an assemblage of juxtaposed proto-continents and cratons.
[5] Nonetheless, major magmatic events still occurred, such as the formation (via magma plume) of the 220,000 km2 (85,000 sq mi) central Australian Musgrave Province from 1.22 to 1.12 Ga,[14] and the 2,700,000 km2 (1,000,000 sq mi) Canadian Mackenzie Large Igneous Province 1.27 Ga.[15] Plate tectonics were still active enough to build mountains, with several orogenies, including the Grenville orogeny,[16] occurring at the time.
[18][19] 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.
[20] 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.
[20][18] Methanogenic prokaryotes could not have produced so much methane, implying some other greenhouse gas, probably nitrous oxide, was elevated, perhaps to 3 ppm (10 times today's levels).
[4] 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.
[24][25][26] Though not much oxygen is necessary to sustain the ozone layer, and levels during the Boring Billion may have been high enough for it,[27] the Earth may have been more heavily bombarded by UV radiation than today.
[28] 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.
[39] The intermediary period, during the Boring Billion, is thought to have had low oxygen levels (with minor fluctuations), leading to widespread anoxic waters.
[55] More systematic geochemical study of the Mid-Proterozoic indicates that the oceans were largely ferruginous with a thin surface layer of weakly oxygenated waters,[56] and euxinia may have occurred over relatively small areas, perhaps less than 7% of the seafloor.
[60] Low nutrient abundance may have facilitated photosymbiosis—where one organism is capable of photosynthesis and the other metabolizes the waste product—among prokaryotes (bacteria and archaea), and the emergence of eukaryotes.
[68] Following this, eukaryotic evolution was rather slow,[9] possibly due to the euxinic conditions of the Canfield ocean and a lack of key nutrients and metals[5][1] which prevented large, complex life with high energy requirements from evolving.
Fossils from the late Palaeoproterozoic and early Mesoproterozoic of the Vindhyan sedimentary basin of India,[74] the Ruyang Group of North China,[75][76][77] and the Kotuikan Formation of the Anabar Shield of Siberia,[78] among other places, indicate high rates (by pre-Ediacaran standards) of eukaryotic diversification between 1.7 and 1.4 Ga,[79] although much of this diversity is represented by previously unknown, no longer existing clades of eukaryotes.
[88][86] Given the evolutionary landmarks achieved by eukaryotes, this time period could be considered an important precursor to the Cambrian explosion about 0.54 Ga, and the evolution of relatively large, complex life.
[94][95][96][97] Dust would have supplied an abundance of nutrients and a means of dispersal for surface-dwelling microbes, though microbial communities could have also formed in caves and freshwater lakes and rivers.
