Misplaced Pages

#434565

74-599: The Tehran Plain is a landscape formation in Iran. It consists of Neogene and Quaternary sediments. The North Tehran Fault thrust up Eocene rocks of the Alborz Mountain range . Geomorphologically three different zones can be described: In the western part, the plain has been deformed by thrusting , as can be seen by a segmented and internally deformed anticline and old and diverted channels on both sides. This process has been dated at possibly after 25,000 years. In

148-537: A daily record of Arctic sea ice cover and the rate of melting compared to an average period and specific past years, showing a continuous decline in sea ice extent. In September 2012, the Arctic ice extent reached a new record minimum. Compared to the average extent (1979–2000), the sea ice had diminished by 49%. Human habitation in the North American polar region goes back at least 17,000–50,000 years, during

222-568: A journey northward in 325 BC, to a land he called " Eschate Thule ", where the Sun only set for three hours each day and the water was replaced by a congealed substance "on which one can neither walk nor sail". He was probably describing loose sea ice known today as " growlers " or "bergy bits"; his "Thule" was probably Norway , though the Faroe Islands or Shetland have also been suggested. Early cartographers were unsure whether to draw

296-629: A pressure system that drives the anticyclonic motion of the Beaufort Gyre. During the summer, this area of high pressure is pushed out closer to its Siberian and Canadian sides. In addition, there is a sea level pressure (SLP) ridge over Greenland that drives strong northerly winds through the Fram Strait, facilitating ice export. In the summer, the SLP contrast is smaller, producing weaker winds. A final example of seasonal pressure system movement

370-559: A series of continental glaciations in the Quaternary Period that followed. In ICS terminology, from upper (later, more recent) to lower (earlier): The Pliocene Epoch is subdivided into two ages: The Miocene Epoch is subdivided into six ages: In different geophysical regions of the world, other regional names are also used for the same or overlapping ages and other timeline subdivisions. The terms Neogene System (formal) and Upper Tertiary System (informal) describe

444-627: Is 45,390 km (28,200 mi) long. It is the only ocean smaller than Russia , which has a land area of 16,377,742 km (6,323,482 sq mi). The Arctic Ocean is surrounded by the land masses of Eurasia (Russia and Norway), North America ( Canada and the U.S. state of Alaska), Greenland, and Iceland . Note: Some parts of the areas listed in the table are located in the Atlantic Ocean . Other consists of Gulfs , Straits , Channels and other parts without specific names and excludes Exclusive Economic Zones . The Arctic Ocean

518-596: Is a key driver in Arctic Ocean circulation and the formation of water masses. With this dependence, the Arctic Ocean experiences variations due to seasonal changes in sea ice cover. Sea ice movement is the result of wind forcing, which is related to a number of meteorological conditions that the Arctic experiences throughout the year. For example, the Beaufort High—an extension of the Siberian High system—is

592-735: Is a water mass referred to as Atlantic Water. Inflow from the North Atlantic Current enters through the Fram Strait, cooling and sinking to form the deepest layer of the halocline, where it circles the Arctic Basin counter-clockwise. This is the highest volumetric inflow to the Arctic Ocean, equalling about 10 times that of the Pacific inflow, and it creates the Arctic Ocean Boundary Current. It flows slowly, at about 0.02 m/s. Atlantic Water has

666-554: Is called Arctic Bottom Water and begins around 900 m (3,000 ft) depth. It is composed of the densest water in the World Ocean and has two main sources: Arctic shelf water and Greenland Sea Deep Water. Water in the shelf region that begins as inflow from the Pacific passes through the narrow Bering Strait at an average rate of 0.8 Sverdrups and reaches the Chukchi Sea . During the winter, cold Alaskan winds blow over

740-416: Is cloudy year-round, with mean cloud cover ranging from 60% in winter to over 80% in summer. The temperature of the surface water of the Arctic Ocean is fairly constant at approximately −1.8 °C (28.8 °F), near the freezing point of seawater . The density of sea water, in contrast to fresh water, increases as it nears the freezing point and thus it tends to sink. It is generally necessary that

814-856: Is connected to the Pacific Ocean by the Bering Strait and to the Atlantic Ocean through the Greenland Sea and Labrador Sea . (The Iceland Sea is sometimes considered part of the Greenland Sea, and sometimes separate.) The largest seas in the Arctic Ocean: Different authorities put various marginal seas in either the Arctic Ocean or the Atlantic Ocean, including: Hudson Bay , Baffin Bay ,

SECTION 10

#1732772757435

888-565: Is contained in a polar climate characterized by persistent cold and relatively narrow annual temperature ranges. Winters are characterized by the polar night , extreme cold, frequent low-level temperature inversions, and stable weather conditions. Cyclones are only common on the Atlantic side. Summers are characterized by continuous daylight ( midnight sun ), and air temperatures can rise slightly above 0 °C (32 °F). Cyclones are more frequent in summer and may bring rain or snow. It

962-481: Is due to the comparatively fine divisibility of time units as time approaches the present, and due to geological preservation that causes the youngest sedimentary geological record to be preserved over a much larger area and to reflect many more environments than the older geological record. By dividing the Cenozoic Era into three (arguably two) periods ( Paleogene , Neogene, Quaternary ) instead of seven epochs,

1036-702: Is often seen as an analogous climate to the projected climate of the near future as a result of anthropogenic global warming . Towards the end of the Pliocene, decreased heat transport towards the Antarctic resulting from a weakening of the Indonesian Throughflow (ITF) cooled the Earth, a process that exacerbated itself in a positive feedback as sea levels dropped and the ITF diminished and further limited

1110-721: Is subject to international territorial claims . The Chukchi Plateau extends from the Chukchi Sea Shelf. An underwater ridge , the Lomonosov Ridge , divides the deep sea North Polar Basin into two oceanic basins : the Eurasian Basin , which is 4,000–4,500 m (13,100–14,800 ft) deep, and the Amerasian Basin (sometimes called the North American or Hyperborean Basin), which is about 4,000 m (13,000 ft) deep. The bathymetry of

1184-541: Is the coldest of the world's oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, although some oceanographers call it the Arctic Mediterranean Sea . It has also been described as an estuary of the Atlantic Ocean . It is also seen as the northernmost part of the all-encompassing world ocean . The Arctic Ocean includes the North Pole region in the middle of

1258-573: Is the low pressure system that exists over the Nordic and Barents Seas. It is an extension of the Icelandic Low , which creates cyclonic ocean circulation in this area. The low shifts to centre over the North Pole in the summer. These variations in the Arctic all contribute to ice drift reaching its weakest point during the summer months. There is also evidence that the drift is associated with

1332-719: Is the second period of the Cenozoic and the eleventh period of the Phanerozoic . The Neogene is sub-divided into two epochs , the earlier Miocene and the later Pliocene . Some geologists assert that the Neogene cannot be clearly delineated from the modern geological period, the Quaternary . The term "Neogene" was coined in 1853 by the Austrian palaeontologist Moritz Hörnes (1815–1868). The earlier term Tertiary Period

1406-506: The Groswater of Labrador and Nunavik . The Dorset culture spread across Arctic North America between 500 BC and AD 1500. The Dorset were the last major Paleo-Eskimo culture in the Arctic before the migration east from present-day Alaska of the Thule people , ancestors of the modern Inuit . The Thule Tradition lasted from about 200 BC to AD 1600, arising around

1480-623: The Jurassic and Triassic periods led to significant sediment deposition, creating many of the reservoirs for current day oil and gas deposits. During the Cretaceous period, the Canadian Basin opened, and tectonic activity due to the assembly of Alaska caused hydrocarbons to migrate toward what is now Prudhoe Bay. At the same time, sediments shed off the rising Canadian Rockies built out the large Mackenzie Delta. The rifting apart of

1554-630: The Northern Hemisphere and extends south to about 60°N . The Arctic Ocean is surrounded by Eurasia and North America, and the borders follow topographic features: the Bering Strait on the Pacific side and the Greenland Scotland Ridge on the Atlantic side. It is mostly covered by sea ice throughout the year and almost completely in winter . The Arctic Ocean's surface temperature and salinity vary seasonally as

SECTION 20

#1732772757435

1628-676: The Norwegian Sea , and Hudson Strait . The main islands and archipelagos in the Arctic Ocean are, from the prime meridian west: There are several ports and harbours on the Arctic Ocean. The ocean's Arctic shelf comprises a number of continental shelves , including the Canadian Arctic shelf, underlying the Canadian Arctic Archipelago , and the Russian continental shelf , which is sometimes called

1702-793: The Paleogene . The Early Miocene was relatively cool; Early Miocene mid-latitude seawater and continental thermal gradients were already very similar to those of the present. During the Middle Miocene , Earth entered a warm phase known as the Middle Miocene Climatic Optimum (MMCO), which was driven by the emplacement of the Columbia River Basalt Group . Around 11 Ma, the Middle Miocene Warm Interval gave way to

1776-911: The Wisconsin glaciation . At this time, falling sea levels allowed people to move across the Bering land bridge that joined Siberia to northwestern North America (Alaska), leading to the Settlement of the Americas . Early Paleo-Eskimo groups included the Pre-Dorset ( c.  3200–850 BC ); the Saqqaq culture of Greenland (2500–800 BC); the Independence I and Independence II cultures of northeastern Canada and Greenland ( c.  2400–1800 BC and c.  800–1 BC ); and

1850-405: The ice cover melts and freezes; its salinity is the lowest on average of the five major oceans, due to low evaporation , heavy fresh water inflow from rivers and streams, and limited connection and outflow to surrounding oceanic waters with higher salinities. The summer shrinking of the ice has been quoted at 50%. The US National Snow and Ice Data Center (NSIDC) uses satellite data to provide

1924-545: The supercontinent Pangea , beginning in the Triassic period, opened the early Atlantic Ocean. Rifting then extended northward, opening the Arctic Ocean as mafic oceanic crust material erupted out of a branch of Mid-Atlantic Ridge. The Amerasia Basin may have opened first, with the Chukchi Borderland moved along to the northeast by transform faults. Additional spreading helped to create the "triple-junction" of

1998-701: The "Arctic Shelf" because it is larger. The Russian continental shelf consists of three separate, smaller shelves: the Barents Shelf, Chukchi Sea Shelf and Siberian Shelf . Of these three, the Siberian Shelf is the largest such shelf in the world; it holds large oil and gas reserves. The Chukchi shelf forms the border between Russian and the United States as stated in the USSR–USA Maritime Boundary Agreement . The whole area

2072-473: The 'perfect storm' conditions of the Great Arctic Cyclone of 2012 . Waters originating in the Pacific and Atlantic both exit through the Fram Strait between Greenland and Svalbard Island, which is about 2,700 m (8,900 ft) deep and 350 km (220 mi) wide. This outflow is about 9 Sv. The width of the Fram Strait is what allows for both inflow and outflow on the Atlantic side of

2146-734: The Alpha and Lomonosov Ridges), Amundsen Basin (between Lomonosov and Gakkel ridges), and Nansen Basin (between the Gakkel Ridge and the continental shelf that includes the Franz Josef Land ). The crystalline basement rocks of mountains around the Arctic Ocean were recrystallized or formed during the Ellesmerian orogeny, the regional phase of the larger Caledonian orogeny in the Paleozoic Era. Regional subsidence in

2220-846: The Alpha-Mendeleev Ridge in the Late Cretaceous epoch . Throughout the Cenozoic Era, the subduction of the Pacific plate, the collision of India with Eurasia, and the continued opening of the North Atlantic created new hydrocarbon traps. The seafloor began spreading from the Gakkel Ridge in the Paleocene Epoch and the Eocene Epoch, causing the Lomonosov Ridge to move farther from land and subside. Because of sea ice and remote conditions,

2294-540: The Arctic Ocean was heavily contested : the Allied commitment to resupply the Soviet Union via its northern ports was opposed by German naval and air forces. Since 1954 commercial airlines have flown over the Arctic Ocean (see Polar route ). The Arctic Ocean occupies a roughly circular basin and covers an area of about 14,056,000 km (5,427,000 sq mi), almost the size of Antarctica . The coastline

Tehran Plain - Misplaced Pages Continue

2368-461: The Arctic Ocean, the top layer (about 50 m [160 ft]) is of lower salinity and lower temperature than the rest. It remains relatively stable because the salinity effect on density is bigger than the temperature effect. It is fed by the freshwater input of the big Siberian and Canadian rivers ( Ob , Yenisei , Lena , Mackenzie ), the water of which quasi floats on the saltier, denser, deeper ocean water. Between this lower salinity layer and

2442-740: The Arctic Ocean. Because of this, it is influenced by the Coriolis force , which concentrates outflow to the East Greenland Current on the western side and inflow to the Norwegian Current on the eastern side. Pacific water also exits along the west coast of Greenland and the Hudson Strait (1–2 Sv), providing nutrients to the Canadian Archipelago. As noted, the process of ice formation and movement

2516-529: The Arctic Ocean. Much of the Arctic ice pack is also covered in snow for about 10 months of the year. The maximum snow cover is in March or April—about 20–50 cm (7.9–19.7 in) over the frozen ocean. The climate of the Arctic region has varied significantly during the Earth's history. During the Paleocene–Eocene Thermal Maximum 55 million years ago, when the global climate underwent

2590-588: The Bering Strait and later encompassing almost the entire Arctic region of North America. The Thule people were the ancestors of the Inuit, who now live in Alaska, Northwest Territories , Nunavut , Nunavik (northern Quebec), Labrador and Greenland. For much of European history , the north polar regions remained largely unexplored and their geography conjectural. Pytheas of Massilia recorded an account of

2664-419: The Chukchi Sea due to inflow from large Canadian and Siberian rivers. The final defined water mass in the Arctic Ocean is called Arctic Surface Water and is found in the depth range of 150–200 m (490–660 ft). The most important feature of this water mass is a section referred to as the sub-surface layer. It is a product of Atlantic water that enters through canyons and is subjected to intense mixing on

2738-429: The Chukchi Sea, freezing the surface water and pushing this newly formed ice out to the Pacific. The speed of the ice drift is roughly 1–4 cm/s. This process leaves dense, salty waters in the sea that sink over the continental shelf into the western Arctic Ocean and create a halocline. This water is met by Greenland Sea Deep Water, which forms during the passage of winter storms. As temperatures cool dramatically in

2812-436: The Gelasian Age, which was formerly considered part of the Neogene Period and Pliocene Epoch. Thus the Neogene Period ends bounding the succeeding Quaternary Period at 2.58 Mya. Arctic Ocean Main five oceans division: Further subdivision: The Arctic Ocean is the smallest and shallowest of the world's five oceanic divisions . It spans an area of approximately 14,060,000 km (5,430,000 sq mi) and

2886-438: The Miocene-Pliocene boundary. The first hominins , the ancestors of humans, may have appeared in southern Europe and migrated into Africa. The first humans (belonging to the species Homo habilis ) appeared in Africa near the end of the period. About 20 million years ago gymnosperms in the form of some conifer and cycad groups started to diversify and produce more species due to the changing conditions. In response to

2960-428: The Neogene and the Pliocene end at 2.58 Ma, that the Gelasian be transferred to the Pleistocene, and the Quaternary be recognized as the third period in the Cenozoic, citing key changes in Earth's climate, oceans, and biota that occurred 2.58 Ma and its correspondence to the Gauss-Matuyama magnetostratigraphic boundary . In 2006 ICS and INQUA reached a compromise that made Quaternary a sub-era, subdividing Cenozoic into

3034-427: The Pliocene Warm Interval (PWI), interrupting the longer-term cooling trend. The Pliocene Thermal Maximum (PTM) occurred between 3.3 and 3.0 Ma. During the Pliocene, Green Sahara phases of wet conditions in North Africa were frequent and occurred about every 21 kyr, being especially intense when Earth's orbit's eccentricity was high. The PWI had similar levels of atmospheric carbon dioxide to contemporary times and

Tehran Plain - Misplaced Pages Continue

3108-404: The Siberian Shelf. As it is entrained, it cools and acts a heat shield for the surface layer on account of weak mixing between layers. However, over the past couple of decades a combination of the warming and the shoaling of Atlantic water are leading to the increasing influence of Atlantic water heat in melting sea ice in the eastern Arctic. The most recent estimates, for 2016–2018, indicate

3182-429: The Tehran Plain dating from the Late Neolithic was described in 2009, suggesting that farmers at Tepe Pardis in Iran irrigated crops in the 6th millennium. This corresponds to findings from Choga Mami in Iraq. Stone tool finds on the Tehran Plain from the late Neolithic through to Early Chalcolithic show that stone blade production was organised around craft specialists and also part-time blade producers within

3256-410: The United States in the 1850s and 1860s, the explorers Elisha Kane and Isaac Israel Hayes both claimed to have seen part of this elusive body of water. Even quite late in the century, the eminent authority Matthew Fontaine Maury included a description of the Open Polar Sea in his textbook The Physical Geography of the Sea (1883). Nevertheless, as all the explorers who travelled closer and closer to

3330-426: The atmosphere, C 4 plants expanded and reached ecological dominance in grasslands during the last 10 million years. Also Asteraceae (daisies) went through a significant adaptive radiation . Eucalyptus fossil leaves occur in the Miocene of New Zealand, where the genus is not native today, but have been introduced from Australia. The Neogene traditionally ended at the end of the Pliocene Epoch, just before

3404-426: The bulk of the ocean lies the so-called halocline , in which both salinity and temperature rise with increasing depth. Because of its relative isolation from other oceans, the Arctic Ocean has a uniquely complex system of water flow. It resembles some hydrological features of the Mediterranean Sea , referring to its deep waters having only limited communication through the Fram Strait with the Atlantic Basin , "where

3478-411: The central part of the plain, which lies below the megacity of Tehran , thrusting and left-lateral strike-slip faulting has caused several uplifted fluvial terraces , reminiscent of a transpressional structure of uncertain age, possibly ~195,000 years, i.e. Pleistocene . In the Eastern part, ramp-structures are ~195,000y-old, of middle Pleistocene . A channel for artificial water management on

3552-399: The circulation is dominated by thermohaline forcing". The Arctic Ocean has a total volume of 18.07 × 10 km , equal to about 1.3% of the World Ocean. Mean surface circulation is predominantly cyclonic on the Eurasian side and anticyclonic in the Canadian Basin . Water enters from both the Pacific and Atlantic Oceans and can be divided into three unique water masses. The deepest water mass

3626-409: The cooler, seasonal climate, tropical plant species gave way to deciduous ones and grasslands replaced many forests. Grasses therefore greatly diversified, and herbivorous mammals evolved alongside it, creating the many grazing animals of today such as horses , antelope , and bison . Ice age mammals like the mammoths and woolly rhinoceros were common in Pliocene . With lower levels of CO 2 in

3700-417: The end of the period. Some continental movements took place, the most significant event being the connection of North and South America at the Isthmus of Panama , late in the Pliocene. This cut off the warm ocean currents from the Pacific to the Atlantic Ocean, leaving only the Gulf Stream to transfer heat to the Arctic Ocean . The global climate cooled considerably throughout the Neogene, culminating in

3774-417: The geology of the Arctic Ocean is still poorly explored. The Arctic Coring Expedition drilling shed some light on the Lomonosov Ridge, which appears to be continental crust separated from the Barents-Kara Shelf in the Paleocene and then starved of sediment. It may contain up to 10 billion barrels of oil. The Gakkel Ridge rift is also poorly understand and may extend into the Laptev Sea. In large parts of

SECTION 50

#1732772757435

3848-403: The heat transported southward by the Leeuwin Current . By the end of the period the first of a series of glaciations of the current Ice Age began. Marine and continental flora and fauna have a modern appearance. The reptile group Choristodera went extinct in the early part of the period, while the amphibians known as Allocaudata disappeared at the end of it. Neogene also marked the end of

3922-436: The household economy and the craft collapsed with increasing use of bronze. Study of a "landlord village" in Qasemabad, Pishva (also spelled Kazemabad) on the plain has allowed understanding the social and economic organisation of much of Iran's rural population as it was rooted in the early Islamic period until the 1963 White Revolution . Groundwater sampling in southern parts of Tehran plain in 2012 found more than half of

3996-443: The maximum in April and minimum in September. The sea ice is affected by wind and ocean currents, which can move and rotate very large areas of ice. Zones of compression also arise, where the ice piles up to form pack ice. Icebergs occasionally break away from northern Ellesmere Island , and icebergs are formed from glaciers in western Greenland and extreme northeastern Canada. Icebergs are not sea ice but may become embedded in

4070-414: The much cooler Late Miocene. The ice caps on both poles began to grow and thicken, a process enhanced by positive feedbacks from increased formation of sea ice. Between 7 and 5.3 Ma, a decrease in global temperatures termed the Late Miocene Cooling (LMC) ensued, driven by decreases in carbon dioxide concentrations. During the Pliocene, from about 5.3 to 2.7 Ma, another warm interval occurred, being known as

4144-419: The northern edge of their charts. The few expeditions to penetrate much beyond the Arctic Circle in that era added only small islands, such as Novaya Zemlya (11th century) and Spitzbergen (1596), though, since these were often surrounded by pack-ice , their northern limits were not so clear. The makers of navigational charts , more conservative than some of the more fanciful cartographers, tended to leave

4218-436: The ocean bottom is marked by fault block ridges, abyssal plains , ocean deeps , and basins. The average depth of the Arctic Ocean is 1,038 m (3,406 ft). The deepest point is Molloy Hole in the Fram Strait , at about 5,550 m (18,210 ft). The two major basins are further subdivided by ridges into the Canada Basin (between Beaufort Shelf of North America and the Alpha Ridge ), Makarov Basin (between

4292-459: The oceanic heat flux to the surface has now overtaken the atmospheric flux in the eastern Eurasian Basin. Over the same period the weakening halocline stratification has coincided with increasing upper ocean currents thought to be associated with declining sea ice, indicate increasing mixing in this region. In contrast direct measurements of mixing in the western Arctic indicate the Atlantic water heat remains isolated at intermediate depths even under

4366-401: The old classical Tertiary and Quaternary, a compromise that was rejected by International Union of Geological Sciences because it split both Neogene and Pliocene in two. Following formal discussions at the 2008 International Geological Congress in Oslo, Norway, the ICS decided in May 2009 to make the Quaternary the youngest period of the Cenozoic Era with its base at 2.58 Mya and including

4440-503: The older definition of the beginning of the Quaternary Period; many time scales show this division. However, there was a movement amongst geologists (particularly marine geologists ) to also include ongoing geological time (Quaternary) in the Neogene, while others (particularly terrestrial geologists) insist the Quaternary to be a separate period of distinctly different record. The somewhat confusing terminology and disagreement amongst geologists on where to draw what hierarchical boundaries

4514-471: The pack ice. Icebergs pose a hazard to ships, of which the Titanic is one of the most famous. The ocean is virtually icelocked from October to June, and the superstructure of ships are subject to icing from October to May. Before the advent of modern icebreakers, ships sailing the Arctic Ocean risked being trapped or crushed by sea ice (although the Baychimo drifted through the Arctic Ocean untended for decades despite these hazards). The Arctic Ocean

SECTION 60

#1732772757435

4588-509: The periods are more closely comparable to the duration of periods in the Mesozoic and Paleozoic Eras. The International Commission on Stratigraphy (ICS) once proposed that the Quaternary be considered a sub-era (sub-erathem) of the Neogene, with a beginning date of 2.58 Ma, namely the start of the Gelasian Stage . In the 2004 proposal of the ICS, the Neogene would have consisted of the Miocene and Pliocene Epochs. The International Union for Quaternary Research (INQUA) counterproposed that

4662-498: The phase of the Arctic Oscillation and Atlantic Multidecadal Oscillation . Much of the Arctic Ocean is covered by sea ice that varies in extent and thickness seasonally. The mean extent of the Arctic sea ice has been continuously decreasing in the last decades, declining at a rate of currently 12.85% per decade since 1980 from the average winter value of 15,600,000 km (6,023,200 sq mi). The seasonal variations are about 7,000,000 km (2,702,700 sq mi), with

4736-403: The pole reported, the polar ice cap is quite thick and persists year-round. Fridtjof Nansen was the first to make a nautical crossing of the Arctic Ocean, in the Fram Expedition from 1893 to 1896. The first surface crossing of the ocean was led by Wally Herbert in 1969, in a dog sled expedition from Alaska to Svalbard , with air support. The first nautical transit of the north pole

4810-445: The region around the North Pole as land (as in Johannes Ruysch 's map of 1507, or Gerardus Mercator 's map of 1595) or water (as with Martin Waldseemüller 's world map of 1507). The fervent desire of European merchants for a northern passage, the Northern Sea Route or the Northwest Passage , to " Cathay " ( China ) caused water to win out, and by 1723 mapmakers such as Johann Homann featured an extensive "Oceanus Septentrionalis" at

4884-420: The region blank, with only fragments of known coastline sketched in. This lack of knowledge of what lay north of the shifting barrier of ice gave rise to a number of conjectures. In England and other European nations, the myth of an " Open Polar Sea " was persistent. John Barrow , longtime Second Secretary of the British Admiralty , promoted exploration of the region from 1818 to 1845 in search of this. In

4958-501: The reptilian genera Langstonia and Barinasuchus , terrestrial predators that were the last surviving members of Sebecosuchia , a group related to crocodiles. The oceans were dominated by large carnivores like megalodons and livyatans , and 19 million years ago about 70% of all pelagic shark species disappeared. Mammals and birds continued to be the dominant terrestrial vertebrates, and took many forms as they adapted to various habitats. An explosive radiation of ursids took place at

5032-503: The rocks deposited during the Neogene Period . The continents in the Neogene were very close to their current positions. The Isthmus of Panama formed, connecting North and South America . The Indian subcontinent continued to collide with Asia , forming the Himalayas . Sea levels fell, creating land bridges between Africa and Eurasia and between Eurasia and North America. The global climate became more seasonal and continued an overall drying and cooling trend which began during

5106-402: The same salinity as Arctic Bottom Water but is much warmer (up to 3 °C [37 °F]). In fact, this water mass is actually warmer than the surface water and remains submerged only due to the role of salinity in density. When water reaches the basin, it is pushed by strong winds into a large circular current called the Beaufort Gyre . Water in the Beaufort Gyre is far less saline than that of

5180-408: The samples unsuitable for drinking and agricultural use because of high levels of salinity. Neogene The Neogene ( / ˈ n iː . ə dʒ iː n / NEE -ə-jeen , ) is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period 23.03 million years ago ( Mya ) to the beginning of the present Quaternary Period 2.58 million years ago. It

5254-419: The upper 100–150 m (330–490 ft) of ocean water cools to the freezing point for sea ice to form. In the winter, the relatively warm ocean water exerts a moderating influence, even when covered by ice. This is one reason why the Arctic does not experience the extreme temperatures seen on the Antarctic continent . There is considerable seasonal variation in how much pack ice of the Arctic ice pack covers

5328-424: The winter, ice forms, and intense vertical convection allows the water to become dense enough to sink below the warm saline water below. Arctic Bottom Water is critically important because of its outflow, which contributes to the formation of Atlantic Deep Water. The overturning of this water plays a key role in global circulation and the moderation of climate. In the depth range of 150–900 m (490–2,950 ft)

5402-464: Was made in 1958 by the submarine USS Nautilus , and the first surface nautical transit occurred in 1977 by the icebreaker NS Arktika . Since 1937, Soviet and Russian manned drifting ice stations have extensively monitored the Arctic Ocean. Scientific settlements were established on the drift ice and carried thousands of kilometres by ice floes . In World War II , the European region of

5476-453: Was used to define the span of time now covered by Paleogene and Neogene and, despite no longer being recognized as a formal stratigraphic term , "Tertiary" still sometimes remains in informal use. During this period, mammals and birds continued to evolve into modern forms, while other groups of life remained relatively unchanged. The first humans ( Homo habilis ) appeared in Africa near

#434565