Estimate ofProterozoic glacial periods.[1][2][a] Vertical axis: millions of years ago
TheMarinoan glaciation, sometimes also known as theVaranger glaciation,[3] was a period of worldwideglaciation.[4] Its beginning is poorly constrained, but occurred no earlier than 654.5Ma (million years ago).[5] It ended approximately 632.3 ± 5.9 Ma[4] during theCryogenianperiod. This glaciation possibly covered the entire planet, in an event called theSnowball Earth. The end of the glaciation was caused by volcanic release ofcarbon dioxide and dissolution ofgas hydrates[6] and may have been hastened by the release ofmethane from equatorialpermafrost.[7][8]
The name is derived from thestratigraphic terminology of theAdelaide Geosyncline (Adelaide Rift Complex) inSouth Australia and is taken from theAdelaide suburb ofMarino. The term Marinoan Series was first used in a 1950 paper byDouglas Mawson andReg Sprigg to subdivide theNeoproterozoic rocks of the Adelaide area and encompassed all strata from the top of theBrighton Limestone to the base of theCambrian.[9] The corresponding time period, referred to as the Marinoan Epoch, spanned from the middle Cryogenian to the top of theEdiacaran in modern terminology. Mawson recognised a glacial episode within the Marinoan Epoch which he referred to as the Elatina glaciation after the 'Elatina Tillite' (nowElatina Formation) where he found the evidence.[10] However, the term Marinoan glaciation came into common usage because it was the glaciation that occurred during the Marinoan Epoch, as distinct from the earlier glaciation during the Sturtian Epoch (the time period of deposition of the older Sturtian Series[9]).
The term Marinoan glaciation was later applied globally to any glaciogenic formations assumed (directly or indirectly) to correlate with Mawson's original Elatina glaciation in South Australia.[11] Recently, there has been a move to return to the term Elatina glaciation in South Australia because of uncertainties regarding global correlation and because an Ediacaran glacial episode (Gaskiers) also occurs within the wide-ranging Marinoan Epoch.[12]
Emerging evidence suggests that the Earth underwent a number of glaciations during the Neoproterozoicera.[13] There were three (or possibly four) significant ice ages during the late Neoproterozoic. These periods of nearly complete glaciation of Earth are often referred to as "Snowball Earth", where it is hypothesized that at times the planet was covered by ice 1–2 km (0.62–1.24 mi) thick.[14] Of these glaciations, theSturtian glaciation was the most significant, whereas the Marinoan was a shorter, but still worldwide glaciation. Other Cryogenian glaciations were probably small and not global as compared to the Marinoan or Sturtian glaciations.
During the Marinoan glaciation, characteristic glacial deposits indicate thatEarth suffered one of the most severe ice ages in its history.Glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.[15][16]
The Earth may not have been fully covered in ice, as some computer simulations show an extreme slowdown of the hydrological cycle that inhibited new glacial formation before the Earth was fully ice-covered.[17]
The melting of the Snowball Earth is associated with greenhouse warming due to the accumulation of high levels of carbon dioxide in the atmosphere.[18] Deglaciation likely started in the mid-latitudes, as in the tropics, the intense hydrological cycle replenished snow rapidly. As the mid-latitudes became ice free, dust was blown from them into other regions, lowering albedo and speeding up deglaciation.[19]
Even though much evidence has been lost through geological changes, field investigations show evidence of the Marinoan glaciation in China,Svalbard archipelago and South Australia. InGuizhou Province, China, glacial rocks were found to be underlying and overlying a layer ofvolcanic ashes which containedzircon minerals, which could be dated throughradioisotopes. Glacial deposits in South Australia are approximately the same age (about 630 Ma), confirmed by similar stablecarbon isotopes, mineral deposits (including sedimentarybarite), and other unusual sedimentary structures.[14] Twodiamictite-rich layers in the top 1 km (0.62 mi) of the 7 km (4.3 mi) Neoproterozoic strata of the northeastern Svalbard archipelago represent the first and final phases of the Marinoan glaciation.[20] In Uruguay, evidence of the Marinoan glaciation is known from dropstones, diamictites, rhythmites, clast layers, and varve-like deposits.[3]
According to Eyles and Young, the Marinoan is a second episode ofNeoproterozoic glaciation (680–690Ma) occurring in theAdelaide Geosyncline. According to them, "It is separated from the Sturtian by a thick succession of sedimentary rocks containing no evidence of glaciation. This glacial phase could correspond to the recently described Ice Brookeformation in the northernCordillera."[21]
The survival of benthic macroalgae indicates that there remained areas of suitable habitat for them in the photic zone along the coasts of mid-latitude continents during the Marinoan glaciation.[22]
^Dating of pre-Gaskiers glaciations is uncertain. As for theKaigas, its very existence is doubted by some. TheHuronian glaciation is not shown; there is a lack of any significant evidence for a Snowball Earth during the time period.
^abRooney, Alan D.; Strauss, Justin V.; Brandon, Alan D.; Macdonald, Francis A. (2015). "A Cryogenian chronology: Two long-lasting synchronous Neoproterozoic glaciations".Geology.43 (5):459–462.Bibcode:2015Geo....43..459R.doi:10.1130/G36511.1.
