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he Snowball Earth hypothesis as it currently stands proposes that the Earth was entirely covered by ice in part of the Cryogenian period of the Proterozoic eon, and perhaps at other times in the history of Earth. It was developed to explain sedimentary glacial deposits at tropical latitudes during the Cryogenian period (850 to 630 million years ago) and other enigmatic features of the Cryogenian geological record. After the last big freeze ended, multicellular evolution began to accelerate. Snowball Earth remains controversial, and is contested by various scientists who dispute the geophysical feasibility of a completely frozen ocean, or the geological evidence on which the hypothesis is based.The beginning of a Snowball Earth event could be facilitated by an equatorial continental distribution, which allows rapid, unchecked weathering of continental rocks, absorbing vast quantities of carbon dioxide from the atmosphere. The depletion of this greenhouse gas causes ice accumulation, which further cools the planet by reflecting solar energy back to space. The runaway system would lead a new ice-covered equilibrium with equatorial temperatures similar to modern-day Antarctica.To break out of the frozen condition, huge quantities of greenhouse gases such as carbon dioxide and methane, emitted primarily by volcanic activity, would have to accumulate over millions of years. Once melting began, however, it would be quick, perhaps only 1000 years.Weathering of glacial sediments, by reacting with carbon dioxide, and fertilising oceanic photosynthesisers, may have eventually drawn down enough of the greenhouse gas to instigate another Snowball Earth.Sedimentary features usually formed by glaciers, found in what may have been equatorial locations at the time of deposition, have been taken as evidence implying global ice cover. Many other features of the sedimentary record are easily explained by extensive glacial cover. Geochemical evidence from rocks associated with low-latitude glacial deposits have been interpreted to show a crash in oceanic life during the glacial times, which is consistent with a freezing of the surface oceans.Whilst the presence of glaciers is not disputed, the idea that the entire planet was covered in ice is more contentious, leading some scientists to prefer a "slushball" to a "snowball". In a slushball scenario a band of ice-free, or ice-thin, waters remains around the equator, allowing for a continued hydrologic cycle. This appeals to scientists who believe that certain features of the sedimentary record can only be explained by rapidly moving ice, which would require somewhere ice free to move to, or that observed sedimentary structures could only form below open water. Attempts to construct computer models of a Snowball Earth have also struggled to accommodate global ice cover, without fundamental changes in the laws and constants which govern the planet. Attempts have been made to explain equatorial ice-deposits by claiming Earth's spin axis or magnetic field changed dramatically. Recent research using observed geochemical cyclicity in clastic rocks suggests that the "Snowball" periods were punctuated by warm spells, similar to ice age cycle in recent Earth history.Snowball Earth has profound implications on the history of life on Earth. While many refugia have been postulated, global ice cover would certainly have ravaged ecosystems dependent on sunlight. The melting of the ice may have presented many new opportunities for diversification, and may indeed have driven the rapid evolution which took place directly at the end of the Cryogenian period.The carbon dioxide levels necessary to unfreeze the Earth have been estimated as being 350 times what they are today, about 13% of the atmosphere. Since the Earth was almost completely covered with ice, carbon dioxide could not be withdrawn from the atmosphere by the weathering of siliceous rocks. Over 4-30 million years, enough CO2 and methane, mainly emitted by volcanoes, would accumulate to finally cause enough greenhouse effect to make surface ice melt in the tropics until a band of ice-free land and water developed; this would be darker than the ice, and thus absorb more energy from the sun - initiating a "positive feedback".
The present ice age began 40 million years ago with the growth of an ice sheet in Antarctica. It intensified during the late Pliocene, around 3 million years ago, with the spread of ice sheets in the Northern Hemisphere, and has continued in the Pleistocene. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000- and 100,000-year time scales. The most recent glacial period ended about ten thousand years ago.
The early Paleocene was slightly cooler than the preceding Cretaceous, though temperatures rose again late in the epoch. The climate was warm and humid world-wide, with subtropical vegetation growing in Greenland and Patagonia. The poles were cool and temperate; North America, Europe, Australia and southern South America were warm and temperate; equatorial areas had tropical climates; and north and south of the equatorial areas, climates were hot and arid.
Peat forms when plant material, usually in marshy areas, is inhibited from decaying fully by acidic and anaerobic conditions. It is composed mainly of peat moss or sphagnum, but may also include other marshland vegetation: trees, grasses, fungi, as well as other types of organic remains, such as insects, and animal corpses. Under certain conditions, the decomposition of the latter (in the absence of oxygen) is inhibited, and archaeologists often take advantage of this.Peat layer growth and the degree of decomposition (or humification) depends principally on its composition and on the degree of waterlogging. Peat formed in very wet conditions will grow considerably faster, and be less decomposed, than that in drier places. This allows climatologists to use peat as an indicator of climatic change. The composition of peat can also be used to reconstruct ancient ecologies by examining the types and quantities of its organic elements.Under the right conditions, peat is the earliest stage in the formation of coal. Most modern peat bogs formed in high latitudes after the retreat of the glaciers at the end of the last ice age some 9,000 years ago. They usually grow slowly, at the rate of about a millimetre per year.The peat in the world's peatlands has been forming for 360 million years and contains 550 Gt of carbon.
The size of the oceanic methane clathrate reservoir is poorly known, and estimates of its size have decreased by roughly an order of magnitude per decade since it was first recognized that clathrates could exist in the oceans during the 1960s and 70s. The highest estimates (e.g. 3×1018 m³) were based on the assumption that fully dense clathrates could litter the entire floor of the deep ocean. However, improvements in our understanding of clathrate chemistry and sedimentology have revealed that hydrates only form in a narrow range of depths (continental shelves), only at some locations in the range of depths where they could occur (10-30% of the GHSZ), and typically are found at low concentrations (0.9-1.5% by volume) at sites where they do occur. Recent estimates constrained by direct sampling suggest the global inventory lies between 1×1015 and 5×1015 m³ (1 quadrillion to 5 quadrillion). This estimate, corresponding to 500-2500 gigatonnes carbon (Gt C), is smaller than the 5000 Gt C estimated for all other fossil fuel reserves but substantially larger than the ~230 Gt C estimated for other natural gas sources. The permafrost reservoir has been estimated at about 400 Gt C in the Arctic, but no estimates have been made of possible Antarctic reservoirs. These are large amounts. For comparison the total carbon in the atmosphere is around 700 gigatons.These modern estimates are notably smaller than the 10,000 to 11,000 Gt C (2×1016 m³) proposed by previous workers as a motivation considering clathrates as a fossil fuel resource (MacDonald 1990, Kvenvolden 1998). Lower abundances of clathrates do not rule out their economic potential, but a lower total volume and apparently low concentration at most sites does suggests that only a limited percentage of clathrates deposits may provide an economically viable resource.
Methane is a powerful greenhouse gas which, despite its atmospheric lifetime of around 12 years, nonetheless has a global warming potential of 62 over 20 years and 21 over 100 years (IPCC, 1996; Berner and Berner, 1996; vanLoon and Duffy, 2000). The sudden release of large amounts of natural gas from methane clathrate deposits has been hypothesized as a cause of past and possibly future climate changes. Events possibly linked in this way are the Permian-Triassic extinction event, the Paleocene-Eocene Thermal Maximum.