The Cryogenian Period was ratified in 1990 by theInternational Commission on Stratigraphy.[9] In contrast to most other time periods, the beginning of the Cryogenian is not linked to a globally observable and documented event. Instead, the base of the period is defined by a fixed rock age, that was originally set at 850 million years,[10] but changed in 2015 to 720 million years.[8]
This could cause ambiguity because estimates of rock age are subject to variable interpretation and laboratory error. For instance, the time scale of theCambrian Period is not reckoned by rock younger than a given age (538.8 million years), but by the appearance of the worldwideTreptichnus pedum diagnostictrace fossil assemblages, which can be recognized in the field without extensive lab testing.[11]
Currently, there is no consensus on what global event is a suitable candidate to mark the start of the Cryogenian Period, but aglobal glaciation would be a likely candidate.[10]
Characteristic glacial deposits indicate thatEarth suffered the most severe ice ages in its history during this period (Sturtian and Marinoan). According to Eyles and Young, "Late Proterozoic glaciogenic deposits are known from all the continents. They provide evidence of the most widespread and long-ranging glaciation on Earth." Several glacial periods are evident, interspersed with periods of relatively warm climate, with glaciers reaching sea level in low paleolatitudes.[12]
Glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.[13]
Diamictite of the Elatina Formation in South Australia, formed during the Marinoan glaciation of the late Cryogenian
The Cryogenian is generally considered to be divisible into at least two major worldwide glaciations. TheSturtian glaciation persisted from 720 to 660 million years ago, and theMarinoan glaciation which ended approximately 635 Ma, at the end of the Cryogenian.[14] The deposits of glacialtillite also occur in places that were at low latitudes during the Cryogenian, a phenomenon which led to the hypothesis of deeply frozen planetary oceans called "Snowball Earth".[15] Between the Sturtian and Marinoan glaciations was a so-called "Cryogenian interglacial period" marked by relatively warm climate andanoxic oceans,[16] along with marine transgression.[17]
Before the start of the Cryogenian, around 750 Ma, thecratons that made up the supercontinentRodinia started to rift apart. The superoceanMirovia began to close while the superoceanPanthalassa began to form. The cratons (possibly) later assembled into another supercontinent calledPannotia, in theEdiacaran.[18]
Eyles and Young state, "Most Neoproterozoic glacial deposits accumulated as glacially influenced marine strata along rifted continental margins or interiors." Worldwide deposition of dolomite might have reduced atmospheric carbon dioxide. The break up along the margins ofLaurentia at about 750Ma occurs at about the same time as the deposition of the Rapitan Group in North America, contemporaneously with the Sturtian in Australia. A similar period of rifting at about 650 Ma occurred with the deposition of the Ice Brook Formation in North America, contemporaneously with the Marinoan in Australia.[12] The Sturtian and Marinoan are local divisions within theAdelaide Rift Complex.[19]
Between the Sturtian and Marinoan glaciations, global biodiversity was very low.[16]
Fossils oftestate amoeba (orArcellinida) first appear during the Cryogenian Period.[20] Since 2009, some researchers have argued that during the Cryogenian Period, potentially the oldest known fossils ofsponges, and thereforeanimals, were formed.[21][22][23] However, it is unclear whether these fossils actually belong to sponges, though the authors do not rule out the possibility of such fossils to represent proto-sponges or complex microbial precursors to sponge-grade organisms.[24] The issue of whether or not biology was impacted by this event has not been settled, for example Porter (2000) suggests that new groups of life evolved during this period, including thered algae andgreen algae,stramenopiles,ciliates,dinoflagellates, and testate amoeba.[25]
^Maloof, Adam C.; Rose, Catherine V.; Beach, Robert; Samuels, Bradley M.; Calmet, Claire C.; Erwin, Douglas H.; Poirier, Gerald R.; Yao, Nan; Simons, Frederik J. (17 August 2010). "Possible animal-body fossils in pre-Marinoan limestones from South Australia".Nature Geoscience.3 (9):653–659.Bibcode:2010NatGe...3..653M.doi:10.1038/ngeo934.
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