Interpretations of the LPIA vary, with someresearchers arguing it represented one continuous glacial event and others concluding that as many as twenty-five separate ice sheets across Gondwana developed, waxed, and waned independently and diachronously over the course of the Carboniferous and Permian,[5][6][7] with the distribution of ice centres shifting as Gondwana drifted and its position relative to theSouth Pole changed.[8] At the beginning of the LPIA, ice centres were concentrated in western South America; they later shifted eastward across Africa and by the end of the ice age were concentrated in Australia.[9] Evidence from sedimentary basins suggests individual ice centres lasted for approximately 10 million years, with their peaks alternating with periods of low or absent permanent ice coverage.[10]
The first glacial episodes of the LPIA occurred during the lateFamennian[4][11] and theTournaisian,[12][13] withδ15N evidence showing that the transition from greenhouse to icehouse was a stepwise process and not an immediate change.[14] These Early Mississippian glaciations weretransient and minor,[12] with them sometimes being considered discrete glaciations separate from and preceding the LPIA proper.[15] Between 335 and 330 Mya, or sometime between the middleViséan and earliestSerpukhovian, the LPIA proper began.[16][15] A start in glacioeustatic sea level changes is recorded from Idaho at around this time.[17] The first majorglacial period occurred from the Serpukhovian to theMoscovian: ice sheets expanded from a core in southern Africa and South America.[2] During theBashkirian, a global eustatic sea level drop occurred, signifying the first major glacial maximum of the LPIA.[7] The Lhasa terrane became glaciated during this stage of the Carboniferous.[18] A relatively warm interglacial interval spanning the Kasimovian and Gzhelian, coinciding with the Alykaevo Climatic Optimum, occurred between this first major glacial period and the later second major glacial period.[19] TheParaná Basin nonetheless experienced its final glaciation during the early Gzhelian.[20] The second glacial period occurred from the lateGzhelian across the Carboniferous-Permian boundary to the early Sakmarian; ice sheets expanded from a core inAustralia andIndia.[2] This was the most intense interval of glaciation of the LPIA;[16][15] in Australia, it is known as P1.[21] An exceptionally intense cooling event occurred at 300 Ma.[22] From the late Sakmarian onward, and especially following the Artinskian Warming Event (AWE),[23] these ice sheets declined, as indicated by a negativeδ18O excursion.[7] Ice sheets retreated southward across Central Africa and in the Karoo Basin. A regional glaciation spanning the latest Sakmarian and the Artinskian, known as P2, occurred in Australia amidst this global pulse of net warming and deglaciation.[24] This massive deglaciation during the late Sakmarian and Artinskian is sometimes considered to be the end of the LPIA proper,[16] with the Artinskian-Kungurian boundary[2] and the associated Kungurian Carbon Isotopic Excursion used as the boundary demarcating the ice age's end.[25][26][27] Nonetheless, ice caps of a much lower volume and area remained in Australia. Another long regional interval also limited to Australia from the middle Kungurian to the earlyCapitanian, known as P3,[28] though unlike the previous glaciations, this one and the following P4 glaciation was largely limited to alpine glaciation.[29] A final regional Australian interval lasted from the middle Capitanian to the lateWuchiapingian, known as P4.[28] As with P3, P4's ice sheets were primarily high altitude glaciers.[29] This glacial period was interrupted by a rapid warming interval corresponding to a surge in activity from the Emeishan Traps and correspondingCapitanian mass extinction event.[30][31] The final alpine glaciers of the LPIA melted in what is now eastern Australia around 255 Mya, during the late Wuchiapingian.[3]
The time intervals here referred to as glacial and interglacial periods represented intervals of several million years corresponding to colder and warmer icehouse intervals, respectively, were influenced by long term variations in palaeogeography, greenhouse gas levels, and geological processes such as rates of volcanism and of silicate weathering and should not be confused with shorter term cycles of glacials and interglacials that are driven by astronomical forcing caused by Milankovitch cycles.[32]
According to Eyles and Young, "Renewed Late Devonian glaciation is well documented in three large intracratonic basins inBrazil (Solimoes, Amazonas and Paranaiba basins) and in Bolivia. By the Early Carboniferous (c. 350Ma) glacialstrata were beginning to accumulate in sub-Andean basins ofBolivia,Argentina andParaguay. By the mid-Carboniferous glaciation had spread to Antarctica, Australia, southern Africa, theIndian Subcontinent,Asia and theArabian 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 theDwyka Formation (1000 m thick) in theKaroo Basin in southern Africa, the Itararé Group of theParaná Basin, Brazil (1400 m) and theCarnarvon 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."[33]
In northernEthiopia glacial landforms likestriations,rôche moutonnées andchatter marks can be found buried beneath Late Carboniferous-Early Permian glacial deposits (Edaga Arbi Glacials).[34] Glaciofluvial sandstones, moraines, boulder beds, glacially striated pavements, and other glacially derived geologic structures and beds are also known throughout the southern part of the Arabian Peninsula.[35]
In southern Victoria Land, Antarctica, the Metschel Tillite, made up of reworked DevonianBeacon Supergroup sedimentary strata along with Cambrian and Ordovician granitoids and some Neoproterozoic metamorphic rocks, preserves glacial sediments indicating the presence of major ice sheets. Northern Victoria Land and Tasmania hosted a distinct ice sheet from the one in southern Victoria Land that flowed west-northwestward.[36]
TheSydney Basin of eastern Australia lay at a palaeolatitude of around 60°S to 70°S during the Early and Middle Permian, and its sedimentary successions preserve at least four phases of glaciation throughout this time.[37]
Debate exists as to whether theNorthern Hemisphere experienced glaciation like theSouthern Hemisphere did, with most palaeoclimate models suggesting that ice sheets did exist in Northern Pangaea but that they were very negligible involume. Diamictites from the Atkan Formation ofMagadan Oblast,Russia have been interpreted as being glacigenic, although recent analyses have challenged this interpretation, suggesting that thesediamictites formed during a Capitanian integrlacial interval as a result of volcanogenicdebris flows associated with the formation of the Okhotsk–Taigonos Volcanic Arc.[38][39]
The tropics experienced a cyclicity between wetter and drier periods that may have been related to changes between cold glacials and warm interglacials. In the Midland Basin ofTexas, increased aeolian sedimentation reflective of heightened aridity occurred during warmer intervals,[40] as it did in theParadox Basin ofUtah.[41]
Theevolution of plants following theSilurian-Devonian Terrestrial Revolution and the subsequentadaptive radiation ofvascular plants on land began a long-term increase in planetaryoxygen levels. Largetree ferns, growing to 20 m (66 ft) high, were secondarily dominant to the large arborescentlycopods (30–40 m high) of theCarboniferouscoal forests that flourished in equatorialswamps stretching fromAppalachia toPoland, and later on the flanks of theUrals. The enhancedcarbon sequestration raised the atmospheric oxygen levels to a peak of 35%,[42] and loweredcarbon dioxide level below the 300 parts per million (ppm),[43] possibly as low as 180 ppm during theKasimovian,[44] which is today associated withglacial periods.[43] This reduction in thegreenhouse effect was coupled with burial of organic carbon as charcoal or coal, withlignin andcellulose (as tree trunks and other vegetation debris) accumulating and being buried in the great Carboniferouscoal measures.[45] 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. Research indicates that changing carbon dioxide concentrations were the dominant driver of changes between colder and warmer intervals during the Early and Middle Permian portions of the LPIA.[21]
Thetectonic assembly of the continents ofEuramerica andGondwana intoPangaea, in theHercynian-AlleghanyOrogeny, made a major continental land mass within the Antarctic region and an increase in carbon sequestration viasilicate weathering, which led to progressive cooling of summers, and the snowfields accumulating in winters, which caused mountainous alpineglaciers to grow, and then spread out of highland areas. That madecontinental glaciers, which spread to cover much of Gondwana.[46] Modelling evidence points to tectonically inducedcarbon dioxide removal via silicate weathering to have been sufficient to generate the ice age.[47] The closure of theRheic Ocean andIapetus Ocean saw disruption of warm-water currents in thePanthalassa Ocean andPaleotethys Sea, which may have also been a factor in the development of the LPIA.[46]
The capture of CO2 through weathering of large igneous provinces emplaced during the Kungurian brought about the P3 glaciation.[48]
The Mississippian witnessed major uplift in southwestern Gondwana, where the earliest glaciations of the LPIA began. The uplift, driven by mantle dynamics rather than by crustal tectonic processes, is evidenced by the increase in temperature of the southwestern Gondwanan crust as shown by changing compositions of granites formed at this time.[49]
The LPIA, like the presentQuaternary glaciation, saw glacial-interglacial cycles governed byMilankovitch cycles acting on timescales of tens of thousands to millions of years. Periods of low obliquity, which decreased annual insolation at the poles, were associated with high moisture flux from low latitudes and glacial expansion at high latitudes, while periods of high obliquity corresponded to warmer, interglacial periods.[50] Data from Serpukhovian and Moscovian marine strata of South China point to glacioeustasy being driven primarily by long-period eccentricity, with a cyclicity of about 0.405 million years, and the modulation of the amplitude of Earth's obliquity, with a cyclicity of approximately 1.2 million years. This is most similar to the early part of the Late Cenozoic Ice Age, from theOligocene to thePliocene, before the formation of theArctic ice cap, suggesting the climate of this episode of time was relatively warm for an icehouse period.[51] Evidence from the Middle Permian Lucaogou Formation ofXinjiang, China indicates that the climate of the time was particularly sensitive to the 1.2 million year long-period modulation cycle of obliquity. It also suggests that palaeolakes such as those found in theJunggar Basin likely played an important role as acarbon sink during the later stages of the LPIA, with their absorption and release of carbon dioxide acting as powerful feedback loops duringMilankovitch cycle driven glacial and interglacial transitions.[52] Also during this time, unique sedimentary sequences calledcyclothems were deposited. These were produced by the repeated alterations of marine and nonmarine environments resulting from glacioeustatic rises and falls of sea levels linked to Milankovitch cycles.[53]
The development of high-frequency, high-amplitude glacioeustasy, which resulted in sea level changes of up to 120 metres between warmer and colder intervals,[32] during the beginning of the LPIA, combined with the increased geographic separation of marine ecoregions and decrease in ocean circulation it caused in conjunction with closure of the Rheic Ocean, has been hypothesised to have been the cause of theCarboniferous-Earliest Permian Biodiversification Event.[16][54][55] Milankovitch cycles profound impacts on marine life at the height of the LPIA, with high-latitude species being more strongly affected by glacial-interglacial cycles than low-latitude species.[56]
At the beginning of the LPIA, the transition from a greenhouse to an icehouse climate, in conjunction with increases in atmospheric oxygen concentrations, reduced thermal stratification and increased the vertical extent of themixed layer, which promoted higher rates of microbialnitrification as revealed by an increase in δ15Nbulk values.[57]
The rising levels of oxygen during the late Paleozoic icehouse had major effects uponevolution of plants and animals. Higher oxygen concentration (and accompanying higher atmospheric pressure) enabled energetic metabolic processes which encouraged evolution of large land-dwelling arthropods and flight, with the dragonfly-likeMeganeura, an aerial predator, with a wingspan of 60 to 75 cm. The herbivorous stocky-bodied and armoured millipede-likeArthropleura was 1.8 metres (5.9 ft) long, and the semiterrestrialHibbertopterideurypterids were perhaps as large, and somescorpions reached 50 or 70 centimetres (20 or 28 in).
Earth's increased planetaryalbedo produced by the expanding ice sheets would lead topositive feedback loops, spreading the ice sheets still further, until the process hit a 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 thegreenhouse effect, with CO2 levels rising to 300 ppm in the followingPermian period. From a record low 298 million years ago, the CO2-level in the atmosphere rose abruptly to 4-fold 294 million years ago.[58]
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 (ascoal). With a smaller area for deposition of carbon, more carbon dioxide was returned to the atmosphere, further warming the planet. Over the course of the Early and Middle Permian, glacial periods became progressively shorter while warm interglacials became longer, gradually transitioning the world from an icehouse to a greenhouse as the Permian progressed.[59] Obliquity nodes that triggered glacial expansion and increased tropical precipitation before 285.1 Mya became linked to intervals of marine anoxia and increased terrestrial aridification after this point, a turning point signifying the icehouse-greenhouse transition.[60] Increased lacustrine methane emissions acted as a positive feedback enhancing warming.[61] The LPIA finally ended for good around 255 Ma.[3]
^Abbate, Ernesto; Bruni, Piero; Sagri, Mario (2015). "Geology of Ethiopia: A Review and Geomorphological Perspectives". In Billi, Paolo (ed.).Landscapes and Landforms of Ethiopia. World Geomorphological Landscapes. pp. 33–64.doi:10.1007/978-94-017-8026-1_2.ISBN978-94-017-8026-1.
