Pangaean megamonsoon
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The Pangean megamonsoon refers to the theory that the supercontinent
Pangea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
experienced a distinct seasonal reversal of winds, which resulted in extreme transitions between dry and wet periods throughout the year. Pangea was a conglomeration of all the global continental land masses, which lasted from the late
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
to the mid-
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
. The megamonsoon intensified as the continents continued to shift toward one another abd reached its maximum strength in the
Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
, when the continental surface area of Pangea was at its peak. The megamonsoon would have led to immensely arid regions along the interior regions of the continent. Those areas would have been nearly uninhabitable, with extremely hot days and frigid nights. The coasts experienced seasonality, however, and transitioned from rainy weather in the summer to dry conditions during the winter.


Monsoon circulation

Monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscil ...
circulations, defined as a seasonal reversal of winds, exhibit large shifts in precipitation patterns across the impacted region. Monsoons are therefore characterized by two primary seasons: rainy and dry. They are induced by the presence of at least one large land mass and large body of water in close proximity to each other. The most commonly studied present-day monsoon circulation is the East Asian Monsoon.


Discovery

The concept of a Pangean monsoon circulation was first proposed in 1973. The
evaporites An evaporite () is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean ...
in the geologic record suggested vast and extensive regions of persistent dry conditions near the Pangean centre, serving as the initial evidence for the theory’s dissemination. The interior of the supercontinent, especially the eastern portion, would have been extremely dry as the hemispheric pressure systems driving the circulation would have diverted nearly all atmospheric moisture away from the region. The later indication of a monsoon-driven climate was acquired via the examination of coal deposits along the exterior portions of the continent. The presence of both features in the geologic record suggested monsoonal circulations. As the theory of the Pangean megamonsoon began to increase in credibility,
paleoclimatologists Paleoclimatology (American and British English spelling differences, British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the ...
predicted the climatological impacts of the circulation to ascertain whether observations and models supported the hypothesis. The general consensus listed four primary signs that needed to be present to validate the existence of megamonsoon. Firstly, the lithologic indicators of seasonality should span broad distances along the Pangean coasts. Next, evidence depicting a deviation from zonal flow regimes also needed to be identified. Then, records should indicate that the equatorial regions of Pangea would have been plagued by persistent aridity. Finally, models and geologic observations would need to demonstrate that this circulation peaked during the Triassic.


Monsoon climate on Pangea

In the
Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
's
summer Summer is the hottest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, the earliest sunrise and latest sunset occurs, daylight hours are longest and dark hours are shortest, wit ...
, when the earth’s
axial tilt In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbi ...
was directed toward the sun,
Laurasia Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
would have received the most direct solar
insolation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m ...
,which would have yielded a broad area of warm, rising air and low surface pressure over the continent. Models have suggested that this seasonal low was positioned at 35° latitude, relatively near the Tethys Ocean. In
Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...
,
high pressure In science and engineering the study of high pressure examines its effects on materials and the design and construction of devices, such as a diamond anvil cell, which can create high pressure. By ''high pressure'' is usually meant pressures of th ...
would have dominated, as the land would have been receiving less solar radiation, and therefore experiencing cooler temperatures. The
pressure gradient force In fluid mechanics, the pressure-gradient force is the force that results when there is a difference in pressure across a surface. In general, a pressure is a force per unit area, across a surface. A difference in pressure across a surface th ...
dictates that air will travel from regions of high to low pressure. That would have driven the atmospheric flow from the Southern Hemisphere toward Laurasia during which it would cross the
Tethys Ocean The Tethys Ocean ( el, Τηθύς ''Tēthús''), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean that covered most of the Earth during much of the Mesozoic Era and early Cenozoic Era, located between the ancient continents ...
. Water from the Tethys would
evaporate Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
into the
air mass In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to la ...
. Eventually, the air mass would reach the coast of Laurasia and surface
convergence Convergence may refer to: Arts and media Literature *''Convergence'' (book series), edited by Ruth Nanda Anshen * "Convergence" (comics), two separate story lines published by DC Comics: **A four-part crossover storyline that united the four Wei ...
resulted in immense amounts of precipitation. Models estimate the globally-averaged precipitation to equal roughly 1000 mm per year, with coastal regions receiving upwards of 8 mm of rain each day during the rainy season. As the atmospheric flow was directed away from the Gondwana high pressure system, surface winds would have diverged and
subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope move ...
dominated, producing clear and very dry conditions across the Southern Hemisphere. Several studies have indicated that the circulation was so intense during the Triassic, it would have been capable of reversing part of the predominantly-easterly global wind flow and so westerly winds impacted the western coast. That worked to maximize surface convergence and increased seasonality along the western coasts of each continent. During the Northern Hemisphere
winter Winter is the coldest season of the year in polar and temperate climates. It occurs after autumn and before spring. The tilt of Earth's axis causes seasons; winter occurs when a hemisphere is oriented away from the Sun. Different cultures ...
, when the earth’s tilt was directed away from the sun, the circulation reversed as the area of maximum solar insolation shifted toward the Southern Hemisphere. Air then traveled from Laurasia (region of high pressure), across the Tethys Ocean to Gondwana (region of low pressure). Moisture
advection In the field of physics, engineering, and earth sciences, advection is the transport of a substance or quantity by bulk motion of a fluid. The properties of that substance are carried with it. Generally the majority of the advected substance is al ...
toward the Southern Hemisphere would have fueled heavy precipitation along the Gondwana coasts, while Laurasia remained very dry.


East Asian Monsoon compared to Pangean Megamonsoon

There are marked similarities between factors contributing to the East Asian monsoon and those that would have influenced Pangean climate. That supports the theory that Pangean climate was dominated by the monsoon and aids in its study by providing paleoclimatologists with a present-day example to which they can compare their findings. Firstly, the width of the Tethys Ocean is believed to have been roughly the same as that of the
Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...
. It is well-documented that the Indian Ocean can provide onshore-moving air masses with enough moisture to support a monsoon-driven environment. Thus, the Tethys should have been able to as well. Additionally, many paleoclimate models have attempted to recreate climate patterns on Pangea. The models have yielded results that are comparable with the East Asian Monsoon. For instance, one model reported that the seasonal pressure differential (wintertime high pressure – summertime low pressure) over the continent was 25
millibars The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000  Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea leve ...
, while the Asian pressure varies by 36 millibars on average throughout the year. It has also been noted Pangea possessed a mountain range that potentially played a similar role in the megamonsoon like that of the present
Tibetan Plateau The Tibetan Plateau (, also known as the Qinghai–Tibet Plateau or the Qing–Zang Plateau () or as the Himalayan Plateau in India, is a vast elevated plateau located at the intersection of Central, South and East Asia covering most of the Ti ...
in the East Asian Monsoon. Model simulations suggest that without the presence of the mountain range, the monsoon circulation would have been substantially weakened. Higher elevations may have intensified the atmospheric circulation by maximizing the surface heatingand subsequently the
latent heat Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition. Latent heat can be understo ...
release during the summer rainy season. There is still significant uncertainty regarding the extent of the impact that range would have had, however, because mountain elevations are still unknown.


Geologic record


Coal and evaporites

Coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when dea ...
is typically an indicator of moist climates since it needs both plant matter and humid conditions to form. The poleward progression of coal deposits with time suggests that the regions of maximum rainfall shifted away from the equator. Nonetheless, the employment coal as climatic indicator of precipitation is still employed with caution by geologists, as its creation secondarily depends on rainfall amounts. When a significant amount of evaporation occurs, evaporites are formed, which therefore signifies arid conditions.


Loess

Loess Loess (, ; from german: Löss ) is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loess or similar deposits. Loess is a periglacial or aeolian ...
, or windblown dust, can be used as an indicator of past atmospheric circulation patterns. Without the presence of the monsoon, surface winds across the globe would have been primarily zonal and easterly. The geologic record, however, indicates that winds exhibited a meridional cross-equatorial pattern but also that western Pangea experienced westerly flow during the peak period of the megamonsoon.


Paleontological evidence

Fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
dating back to Pangean times also support the claim that a strongly-monsoonal circulation dominated the supercontinent’s climate. For example, tree rings (also called
growth rings Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed. As well as dating them, this can give data for dendroclimatology, the study of climate and atmos ...
) provide convincing proof of distinct changes in annual weather patterns. Trees rooted in areas that do not experience seasonality will not exhibit rings within their trunks as they grow. Fossilized wood excavated from what was once coastal, mid-latitude Pangea, however, display the clear presence of rings. Other paleoflora suggest that a significant portion of the year would have been dominated by a warm, moist season. Large, smooth leaf shapes with thin
cuticles A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...
and symmetric distribution of
stomata In botany, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bor ...
, as well as tropical fern species have been uncovered from those regions. The
invertebrates Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
and
vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
that existed on Pangea offer further evidence of seasonality. For instance, unionid bivalve shells exhibit uniform banding patterns. Unionid bivalves were aquatic organisms that required shallow, oxygen-rich lakes to thrive. During the summer, when rain was persistent, their respiration occurred aerobically and precipitated calcium carbonate to grow their shells. In the winter, when precipitation ceased, the shallow aquatic environments within the Pangean continent began to dry up. Thus, unionid bivalves depleted their environments of oxygen and eventually had to resort to
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...
processes for respiration. The anaerobic respiration yielded acidic waste, which reacted with the calcium carbonate shell, creating a darker ring and marking the presence of a distinct dry season. Once the summer rains returned, aerobic respiration was restored and calcium carbonate was once again produced. The transition from dry winters to rainy summers is therefore recorded in these alternating patterns of light and dark bands on the unionid bivalve shells. It has also been noted that the
lungfish Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
burrowing patterns correlate well with the rise and the fall of the water table. The height of the
water table The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated. T ...
would have increased during the rainy season, but then decreased rapidly as the winds shifted and diverted moisture away from the location, thus initiating the dry season. Additional evidence of seasonality can be observed in the fossilized carcasses of other vertebrate organisms. These show signs of substantial drying, which would have occurred during the winter, before they were buried and preserved by mudflow (resulting from a persistent rainy period).


Evolution


Carboniferous

During much of the
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
, the tropics would have experienced humid conditions, and the high latitudes of Gondwana were covered by glaciers. Still, the first signs of the poleward movement of moisture arose during the late Carboniferous. Geologists have tracked regions of past coal accumulation as they began to be deposited further from the equator with time, evidence of a shift in precipitation patterns from the tropics toward the higher latitudes. Still, land mass distribution remained more heavily concentrated in the Southern Hemisphere. Atmospheric flow, therefore, remained largely zonal, indicating that the monsoon circulation had not yet begun to dominate the climatic pattern.


Permian

By
Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...
times, the monsoon circulation is apparent in the
lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
. Winds with a westerly component (indicative of the summer monsoon, or wet season) are observed for the Early Permian. The continents continued to drift northward. As they did so, the land mass became more evenly distributed across the equator and the megamonsoon continued to intensify. Gondwana’s progression northward also influenced its gradual deglaciation. Climate models indicate that low pressure systems strengthened as planetary ice cover decreased, thus exaggerating the effect of the monsoon. This also acted to magnify the aridity of the tropics. It is therefore suggested that
glacial A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...
-
interglacial An interglacial period (or alternatively interglacial, interglaciation) is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age. The current Holocene in ...
patterns had a significant effect on the Pangean monsoonal circulation. Models have also indicated that worldwide carbon dioxide substantially increased between Carboniferous and Permian times and resulted in increased temperatures.


Triassic

Duying the Triassic, the megamonsoon reached its maximum intensity, which is believed to be a result of the supercontinent attaining its largest surface area during this period from the final addition of present-day
Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part of ...
,
Kazakhstan Kazakhstan, officially the Republic of Kazakhstan, is a transcontinental country located mainly in Central Asia and partly in Eastern Europe. It borders Russia to the north and west, China to the east, Kyrgyzstan to the southeast, Uzbeki ...
, southeast
Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...
, and fragments of
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
. Land mass was also equally distributed between the Northern and Southern Hemispheres, was nearly perfectly bisected by the equator, and extended from 85°N to 90°S. Both the increase in Pangean surface area and the equitable dissemination of the land mass across the hemispheres maximized surface heating during the summer. The stronger the surface heating was, the more extreme the
convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
. By intensifying rising motion, the central pressure of the summertime surface low would have dropped. That, in turn, increased the hemispheric pressure gradient and amplified the cross-equatorial flow. Additionally, the planet was experiencing a
greenhouse A greenhouse (also called a glasshouse, or, if with sufficient heating, a hothouse) is a structure with walls and roof made chiefly of Transparent ceramics, transparent material, such as glass, in which plants requiring regulated climatic condit ...
climate during the Triassic, which resukted in continents completely devoid of ice, including the polar regions. Interglacial periodscorrelate well with an intensification of the monsoon circulation. Records clearly indicate a western component to the wind direction throughout this time period. It is also from that period that the paleontological evidence is most prevalent.


Jurassic

During the early Jurassic, the supercontinent continued to shift northward. The coasts along the Tethys Ocean grew more persistently humid. The monsoon circulation began to weaken through Jurassic time, due to Pangea’s break-up when the continents started to drift apart. Records indicate that large-scale atmospheric flow progressively returned to a primarily zonal pattern. Climatic patterns therefore became less extreme across the continents.


Future work

Today, the presence of the Pangean megamonsoon is generally accepted by the paleoclimate community. There is a substantial amount of evidence, both in the geologic record and model simulations, to support its existence. Nevertheless, a significant amount of uncertainty still remains, particularly from a modeling perspective. One of the greater unknowns paleoclimatologists face is the impact of the Pangean mountain range. Model simulations have suggested that without the presence of the mountain range, the monsoon circulation would have been substantially weakened. Geologists do know that Pangea did possess an extensive mountain range, which was probably comparable to the present-day Andes Mountains. Mountains were located to the north of the Tethys Ocean and resulted from the northward progression and subsequent subduction of the paleo-Tethyan plate. However, the height of these mountains has yet to be quantified. Scientists have acknowledged that approximating their elevation is of “capital importance”. Extremely high mountain ranges (rivaling the Himalayas) would have magnified atmospheric circulation, intensified the low pressure system, accelerated moisture transport to the coasts, and induced a
rain shadow A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side. Evaporated moisture from water bodies (such as oceans and large lakes) is carrie ...
effect, promoting aridity on the
leeward Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...
side of the range. Studies also continue to examine the impact of orbital cycles on the monsoon circulation. The monsoon during the late Triassic appears to have been particularly impacted by
Milankovich cycles Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The term was coined and named after Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he hypoth ...
for a period extending over at least 22 million years.
Orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values betwee ...
seems to have significantly affected precipitation cycles, but further research is required to better understand this correlation. Climate modelers are additionally trying to further understand and account for the surface and deep water circulations of the Panthalassic Ocean. The transport of heat resulting from these circulations significantly alters the simulated monsoon; therefore accurately representing them is of great importance.Peyser, C.E and D.J. Poulsen, 2008: Controls on Permo-Carboniferous precipitation over tropical Pangaea: A GCM sensitivity study. Paleogeography, Paleaoclimatology, Paleoecology, 268, 181-192. Continued research will eventually provide scientists with a much more complete comprehension of the progression and behavior of the megamonsoon that dominated the Pangean climate.


References

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