Late Paleozoic icehouse
The late Paleozoic icehouse, formerly known as the Karoo ice age, was between 360–260 million years ago (Mya) during which large land-based ice-sheets were present on Earth's surface. It was the second major glacial period of the Phanerozoic. It is named after the tillite (Dwyka Group) found in the Karoo Basin of South Africa, where evidence for this ice age was first clearly identified in the 19th century.
The tectonic assembly of the continents of Euramerica (later with the Uralian orogeny, into Laurasia) and Gondwana into Pangaea, in the Hercynian-Alleghany Orogeny, made a major continental land mass within the Antarctic region, and the closure of the Rheic Ocean and Iapetus Ocean saw disruption of warm-water currents in the Panthalassa Ocean and Paleotethys Sea, which led to progressive cooling of summers, and the snowfields accumulating in winters, causing mountainous alpine glaciers to grow, and then spread out of highland areas, making continental glaciers which spread to cover much of Gondwana.
At least two major periods of glaciation have been discovered:
- The first glacial period was associated with the Mississippian subperiod (359.2–318.1 Mya): ice sheets expanded from a core in southern Africa and South America.
- The second glacial period was associated with the Pennsylvanian subperiod (318.1–299 Mya); ice sheets expanded from a core in Australia and India.
Late Paleozoic glaciationsEdit
According to Eyles and Young, "Renewed Late Devonian glaciation is well documented in three large intracratonic basins in Brazil (Solimoes, Amazonas and Paranaiba basins) and in Bolivia. By the Early Carboniferous (c. 350 Ma) glacial strata were beginning to accumulate in sub-andean basins of Bolivia, Argentina and Paraguay. By the mid-Carboniferous glaciation had spread to Antarctica, Australia, southern Africa, the Indian Subcontinent, Asia and the Arabian Peninsula. During the Late Carboniferous glacial accumulation (c. 300 Ma) a very large area of Gondwana land mass was experiencing glacial conditions. The thickest glacial deposits of Permo-Carboniferous age are the Dwyka Formation (1000 m thick) in the Karoo Basin in southern Africa, the Itarare Group of the Parana Basin, Brazil (1400 m) and the Carnarvon Basin in eastern Australia. The Permo-Carboniferous glaciations are significant because of the marked glacio-eustatic changes in sea level that resulted and which are recorded in non-glacial basins. Late Paleozoic glaciation of Gondwana could be explained by the migration of the supercontinent across the South Pole."
The evolution of land plants with the onset of the Devonian Period, began a long-term increase in planetary oxygen levels. Large tree ferns, growing to 20 m high, were secondarily dominant to the large arborescent lycopods (30–40 m high) of the Carboniferous coal forests that flourished in equatorial swamps stretching from Appalachia to Poland, and later on the flanks of the Urals. Oxygen levels reached up to 35%, and global carbon dioxide got below the 300 parts per million level, which is today associated with glacial periods. This reduction in the greenhouse effect was coupled with lignin and cellulose (as tree trunks and other vegetation debris) accumulating and being buried in the great Carboniferous Coal Measures. The reduction of carbon dioxide levels in the atmosphere would be enough to begin the process of changing polar climates, leading to cooler summers which could not melt the previous winter's snow accumulations. The growth in snowfields to 6 m deep would create sufficient pressure to convert the lower levels to ice.
Earth's increased planetary albedo produced by the expanding ice sheets would lead to positive feedback loops, spreading the ice sheets still further, until the process hit limit. Falling global temperatures would eventually limit plant growth, and the rising levels of oxygen would increase the frequency of fire-storms because damp plant matter could burn. Both these effects return carbon dioxide to the atmosphere, reversing the "snowball" effect and forcing greenhouse warming, with CO2 levels rising to 300 ppm in the following Permian period. Over a longer period the evolution of termites, whose stomachs provided an anoxic environment for methanogenic lignin-digesting bacteria, prevented further burial of carbon, returning carbon to the air as the greenhouse gas methane.
Once these factors brought a halt and a small reversal in the spread of ice sheets, the lower planetary albedo resulting from the fall in size of the glaciated areas would have been enough for warmer summers and winters and thus limit the depth of snowfields in areas from which the glaciers expanded. Rising sea levels produced by global warming drowned the large areas of flatland where previously anoxic swamps assisted in burial and removal of carbon (as coal). With a smaller area for deposition of carbon, more carbon dioxide was returned to the atmosphere, further warming the planet. By 250 Mya, planet Earth had returned to a percentage of oxygen similar to that found today.
The rising levels of oxygen during the late Paleozoic icehouse had major effects upon evolution of plants and animals. Higher oxygen concentration (and accompanying higher atmospheric pressure) enabled energetic metabolic processes which encouraged evolution of large land-dwelling vertebrates and flight, with the dragonfly-like Meganeura, an aerial predator, with a wingspan of 60 to 75 cm.
The herbivorous stocky-bodied and armoured millipede-like Arthropleura was 1.8 metres (5.9 ft) long, and the semiterrestrial Hibbertopterid eurypterids were perhaps as large, and some scorpions reached 50 or 70 centimetres (20 or 28 in).
The rising levels of oxygen also led to the evolution of greater fire resistance in vegetation and ultimately to the evolution of flowering plants.
Also during this time, unique sedimentary sequences called cyclothems were deposited. These were produced by the repeated alterations of marine and nonmarine environments.
- Montañez, Isabel P.; Poulsen, Christopher J. (2013-05-30). "The Late Paleozoic Ice Age: An Evolving Paradigm". Annual Review of Earth and Planetary Sciences. 41 (1): 629–656. Bibcode:2013AREPS..41..629M. doi:10.1146/annurev.earth.031208.100118. ISSN 0084-6597."The late Paleozoic icehouse was the longest-lived ice age of the Phanerozoic, and its demise constitutes the only recorded turnover to a greenhouse state."
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