Subglacial lake
A subglacial lake is a lake under a glacier an ice cap or ice sheet. There are many such lakes, with Lake Vostok in Antarctica being by far the largest known on Earth at present; the water below the ice remains liquid since geothermal heating balances the heat loss at the ice surface. The pressure causes the melting point of water to be below 0 °C; the ceiling of the subglacial lake will be at the level where the pressure melting point of water intersects the temperature gradient. In Lake Vostok the ice over the lake is thus much thicker than the ice sheet around it. Hypersaline lakes remain liquid due to their salt content; the water in the lake can have a floating level much above the level of the ground threshold. In fact, theoretically a sub-glacial lake can exist on the top of a hill, provided that the ice over it is so much thinner that it creates the required hydrostatic seal; the floating level can be thought of as the water level in a hole drilled through the ice into the lake. It is equivalent to the level at which a piece of the ice over it would float if it were a normal ice shelf.
The ceiling can therefore be conceived as an ice shelf, grounded along its entire perimeter, which explains why it has been called a captured ice shelf. As it moves over the lake, it enters the lake at the floating line, it leaves the lake at the grounding line. For the lake to exist there must be a hydrostatic seal along the entire perimeter, if the floating level is higher than the threshold. A hydrostatic seal is created when the ice is so much higher around the lake that the equipotential surface dips down into impermeable ground. Water from underneath this ice rim is pressed back into the lake by the hydrostatic seal; the ice surface is ten times more important than the bed surface in creating the hydrostatic seal. This means that a 1 m rise in the ice surface at the ice rim is as efficient as a 10 m rise in the bed level below it. In Lake Vostok the ice rim has been estimated to a mere 7 m, while the floating level is about 3 km above the lake ceiling. If the hydrostatic seal is penetrated when the floating level is high, the water will start flowing out in a jökulhlaup.
Due to melting of the channel the discharge increases exponentially, unless other processes allow the discharge to increase faster. Due to the high head that can be achieved in subglacial lakes, jökulhlaups may reach high rates of discharge. Russian scientist Peter Kropotkin first proposed the idea of fresh water under Antarctic ice sheets at the end of the 19th century, he theorized that the tremendous pressure exerted by the cumulative mass of thousands of vertical meters of ice could increase the temperature at the lowest portions of the ice sheet to the point where the ice would melt. Kropotkin's theory was further developed by Russian glaciologist I. A Zotikov, who wrote his Ph. D. thesis on this subject in 1967. Andrey Kapitsa used seismic soundings in the region of Vostok Station made during the Soviet Antarctic Expeditions in 1959 and 1964 to measure the thickness of the ice sheet. In cooperation with other scientists, Kapitsa discovered a subglacial lake in this region named Lake Vostok, one of the most remarkable geographical discoveries of the 20th century.
Subglacial lakes in Antarctica were suggested by Oswald and Robin and subsequently confirmed theoretically by Oswald. They are identified in radio-echo sounding data as continuous and specular reflectors which dip against the ice surface at around x10 of the surface slope angle, as this is requirement for hydrostatic stability; the largest lakes are clustered in the Dome C-Vostok area of East Antarctica - due to the thick insulating ice and rugged tectonically influenced subglacial topography. The largest is Lake Vostok with other lakes notable for their size being Lake Concordia and Aurora Lake. Several lakes were delineated by the famous SPRI-NSF-TUD surveys undertaken until the mid-seventies. A compilation by Siegert et al. reported 145 subglacial lakes in Antarctica. Since this original compilation several smaller surveys has discovered many more subglacial lakes throughout Antarctica, notably by Carter et al. who identified a spectrum of subglacial lake types based on their properties in radio-echo sounding datasets.
Gray et al. interpreted ice surface slumping and raising from RADARSAT data as evidence for subglacial lakes filling and emptying - termed "active" lakes. Wingham et al. used radar altimeter data to show coincident uplift and subsidence: implying drainage between lakes. NASA's ICESat satellite was key in developing this concept further and subsequent work demonstrated the pervasiveness of this phenomenon. ICESat ceased measurements in 2007 and the detected "active" lakes were compiled by Smith et al. who identified 124 such lakes. In total around 250-300 Antarctic subglacial lakes are known; the realisation that lakes were interconnected created new contamination concerns for plans to drill into lakes. There are three projects to directly sample subglacial lakes in Antarctica; these are the British led Subglacial Lake Ellsworth project, the U. S. led the Russian led Lake Vostok program. No program has gained access but is expected during the austral summer of 2011-12; the role of subglacial lakes on ice dynamics is unclear - on the Greenland Ice Sheet subglacial water acts to enhance basal ice motion in a complex manner.
The "Recovery Lakes" lie at the head of a major ice stream and may influence the dynamics of the region. A modest speed up of Byrd Glacier may have been influenced by a subglacial drainage event (Stea
Iceberg
An iceberg is a large piece of freshwater ice that has broken off a glacier or an ice shelf and is floating in open water. Another name for iceberg is "ice mountain". Small bits of disintegrating icebergs are called "growlers" or "bergy bits". Icebergs are possible on Earth because the oceans are filled with liquid water, a substance less dense when solid than liquid. Planets with oceans consisting of different substances like methane cannot have icebergs, as their chunks of frozen liquid would sink; because 90 percent of an iceberg is below the surface and not visible, icebergs have been considered a serious maritime hazard since the 1912 loss of the "unsinkable" RMS Titanic, leading to the formation of the International Ice Patrol in 1914. The expression "tip of the iceberg", illustrates a difficulty, only a small, visible part of a larger, complex problem; the largest iceberg reliably recorded was Iceberg B-15A which split off the Ross Ice Shelf in Antarctica in 2000. The word iceberg is a partial loan translation from the Dutch word ijsberg meaning ice mountain, cognate to Danish isbjerg, German Eisberg, Low Saxon Iesbarg and Swedish isberg.
Because the density of pure ice is about 920 kg/m3, that of seawater about 1025 kg/m3 about one-tenth of the volume of an iceberg is above water. The shape of the underwater portion can be difficult to judge by looking at the portion above the surface; the visible "tips" of icebergs range from 1 to 75 metres above sea level and weigh 100,000 to 200,000 metric tons. The largest known iceberg in the North Atlantic was 168 metres above sea level, reported by the USCG icebreaker East Wind in 1958, making it the height of a 55-story building; these icebergs originate from the glaciers of western Greenland and may have interior temperatures of −15 to −20 °C. Winds and currents tend to move icebergs close to coastlines, where they can become frozen into pack ice, or drift into shallow waters, where they can come into contact with the seabed, a phenomenon called seabed gouging; the largest icebergs recorded have been calved, or broken off, from the Ross Ice Shelf of Antarctica. Iceberg B-15, photographed by satellite in 2000, measured 295 by 37 kilometres, with a surface area of 11,000 square kilometres.
The largest iceberg on record was an Antarctic tabular iceberg of over 31,000 square kilometres sighted 150 miles west of Scott Island, in the South Pacific Ocean, by the USS Glacier on November 12, 1956. This iceberg was larger than Belgium. A small iceberg less than 2 meters across that floats with less than 1 meter showing above water is called a growler, is smaller than a bergy bit, less than 5 meters in size. Both are spawned from disintegrating icebergs; as a piece of iceberg ice melts, it produces a fizzing sound called the "Bergie Seltzer". This sound results; as this happens, each bubble bursts. The bubbles contain air trapped in snow layers early in the history of the ice, that got buried to a given depth and pressurized as it transformed into firn to glacial ice. In addition to size classification, icebergs can be classified on the basis of their shape; the two basic types of iceberg forms are non-tabular. Tabular icebergs have steep sides and a flat top, much like a plateau, with a length-to-height ratio of more than 5:1.
This type of iceberg known as an ice island, can be quite large, as in the case of Pobeda Ice Island. Antarctic icebergs formed by breaking off from an ice shelf, such as the Ross Ice Shelf or Filchner-Ronne Ice Shelf, are tabular; the largest icebergs in the world are formed this way. Non-tabular icebergs include: Dome: An iceberg with a rounded top. Pinnacle: An iceberg with one or more spires. Wedge: An iceberg with a steep edge on one side and a slope on the opposite side. Dry-Dock: An iceberg that has eroded to form a slot or channel. Blocky: An iceberg with steep, vertical sides and a flat top, it differs from tabular icebergs in that its aspect ratio, the ratio between its width and height, is small, more like that of a block than a flat sheet. Before the early 1910s, although there had been many fatal sinkings of ships by icebergs, there was no system in place to track icebergs to guard ships against collisions. In 1907, SS Kronprinz Wilhelm, a German liner, had rammed an iceberg and suffered a crushed bow, but was still able to complete her voyage.
The advent of steel ship construction led designers to declare their ships "unsinkable". The April 1912 sinking of the Titanic, which killed 1,518 of its 2,223 passengers and crew, changed all that. For the remainder of the ice season of that year, the United States Navy patrolled the waters and monitored ice flow. In November 1913, the International Conference on the Safety of Life at Sea met in London to devise a more permanent system of observing icebergs. Within three months the participating maritime nations had formed the International Ice Patrol; the goal of the IIP was to collect data on meteorology and oceanography to measure currents, ice-flow, ocean temperature, salinity levels. They monitored iceberg dangers near the Grand Banks of Newfoundland and provided the "limits of all known ice" in that vicinity to the maritime community; the IIP published their first records in 1921, which allowed for a year-by-year comparison of iceberg movement. Aerial surveillance of the seas in the early
Ice
Ice is water frozen into a solid state. Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaque bluish-white color. In the Solar System, ice is abundant and occurs from as close to the Sun as Mercury to as far away as the Oort cloud objects. Beyond the Solar System, it occurs as interstellar ice, it is abundant on Earth's surface – in the polar regions and above the snow line – and, as a common form of precipitation and deposition, plays a key role in Earth's water cycle and climate. It occurs as frost, icicles or ice spikes. Ice molecules can exhibit more different phases that depend on temperature and pressure; when water is cooled up to three different types of amorphous ice can form depending on the history of its pressure and temperature. When cooled correlated proton tunneling occurs below −253.15 °C giving rise to macroscopic quantum phenomena. All the ice on Earth's surface and in its atmosphere is of a hexagonal crystalline structure denoted as ice Ih with minute traces of cubic ice denoted as ice Ic.
The most common phase transition to ice Ih occurs when liquid water is cooled below 0 °C at standard atmospheric pressure. It may be deposited directly by water vapor, as happens in the formation of frost; the transition from ice to water is melting and from ice directly to water vapor is sublimation. Ice is used in a variety including cooling, winter sports and ice sculpture; as a occurring crystalline inorganic solid with an ordered structure, ice is considered to be a mineral. It possesses a regular crystalline structure based on the molecule of water, which consists of a single oxygen atom covalently bonded to two hydrogen atoms, or H–O–H. However, many of the physical properties of water and ice are controlled by the formation of hydrogen bonds between adjacent oxygen and hydrogen atoms. An unusual property of ice frozen at atmospheric pressure is that the solid is 8.3% less dense than liquid water. The density of ice is 0.9167–0.9168 g/cm3 at 0 °C and standard atmospheric pressure, whereas water has a density of 0.9998–0.999863 g/cm3 at the same temperature and pressure.
Liquid water is densest 1.00 g/cm3, at 4 °C and becomes less dense as the water molecules begin to form the hexagonal crystals of ice as the freezing point is reached. This is due to hydrogen bonding dominating the intermolecular forces, which results in a packing of molecules less compact in the solid. Density of ice increases with decreasing temperature and has a value of 0.9340 g/cm3 at −180 °C. When water freezes, it increases in volume; the effect of expansion during freezing can be dramatic, ice expansion is a basic cause of freeze-thaw weathering of rock in nature and damage to building foundations and roadways from frost heaving. It is a common cause of the flooding of houses when water pipes burst due to the pressure of expanding water when it freezes; the result of this process is that ice floats on liquid water, an important feature in Earth's biosphere. It has been argued that without this property, natural bodies of water would freeze, in some cases permanently, from the bottom up, resulting in a loss of bottom-dependent animal and plant life in fresh and sea water.
Sufficiently thin ice sheets allow light to pass through while protecting the underside from short-term weather extremes such as wind chill. This creates a sheltered environment for algal colonies; when sea water freezes, the ice is riddled with brine-filled channels which sustain sympagic organisms such as bacteria, algae and annelids, which in turn provide food for animals such as krill and specialised fish like the bald notothen, fed upon in turn by larger animals such as emperor penguins and minke whales. When ice melts, it absorbs as much energy as it would take to heat an equivalent mass of water by 80 °C. During the melting process, the temperature remains constant at 0 °C. While melting, any energy added breaks the hydrogen bonds between ice molecules. Energy becomes available to increase the thermal energy only after enough hydrogen bonds are broken that the ice can be considered liquid water; the amount of energy consumed in breaking hydrogen bonds in the transition from ice to water is known as the heat of fusion.
As with water, ice absorbs light at the red end of the spectrum preferentially as the result of an overtone of an oxygen–hydrogen bond stretch. Compared with water, this absorption is shifted toward lower energies. Thus, ice appears blue, with a greener tint than liquid water. Since absorption is cumulative, the color effect intensifies with increasing thickness or if internal reflections cause the light to take a longer path through the ice. Other colors can appear in the presence of light absorbing impurities, where the impurity is dictating the color rather than the ice itself. For instance, icebergs containing impurities can appear grey or green. Ice may be any one of the 18 known solid crystalline phases of water, or in an amorphous solid state at various densities. Most liquids under increased pressure freeze at higher temperatures because the pressure helps to hold the molecules together. However, the strong hydrogen bonds in water make it different: For some pressures higher than 1 atm, water freezes at a temperature below
Ice age
An ice age is a long period of reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth is in the Quaternary glaciation, known in popular terminology as the Ice Age. Individual pulses of cold climate are termed "glacial periods", intermittent warm periods are called "interglacials", with both climatic pulses part of the Quaternary or other periods in Earth's history. In the terminology of glaciology, ice age implies the presence of extensive ice sheets in both northern and southern hemispheres. By this definition, we are in an interglacial period—the Holocene; the amount of heat trapping gases emitted into Earth's Oceans and atmosphere will prevent the next ice age, which otherwise would begin in around 50,000 years, more glacial cycles. In 1742, Pierre Martel, an engineer and geographer living in Geneva, visited the valley of Chamonix in the Alps of Savoy. Two years he published an account of his journey.
He reported that the inhabitants of that valley attributed the dispersal of erratic boulders to the glaciers, saying that they had once extended much farther. Similar explanations were reported from other regions of the Alps. In 1815 the carpenter and chamois hunter Jean-Pierre Perraudin explained erratic boulders in the Val de Bagnes in the Swiss canton of Valais as being due to glaciers extending further. An unknown woodcutter from Meiringen in the Bernese Oberland advocated a similar idea in a discussion with the Swiss-German geologist Jean de Charpentier in 1834. Comparable explanations are known from the Val de Ferret in the Valais and the Seeland in western Switzerland and in Goethe's scientific work; such explanations could be found in other parts of the world. When the Bavarian naturalist Ernst von Bibra visited the Chilean Andes in 1849–1850, the natives attributed fossil moraines to the former action of glaciers. Meanwhile, European scholars had begun to wonder. From the middle of the 18th century, some discussed ice as a means of transport.
The Swedish mining expert Daniel Tilas was, in 1742, the first person to suggest drifting sea ice in order to explain the presence of erratic boulders in the Scandinavian and Baltic regions. In 1795, the Scottish philosopher and gentleman naturalist, James Hutton, explained erratic boulders in the Alps by the action of glaciers. Two decades in 1818, the Swedish botanist Göran Wahlenberg published his theory of a glaciation of the Scandinavian peninsula, he regarded glaciation as a regional phenomenon. Only a few years the Danish-Norwegian geologist Jens Esmark argued a sequence of worldwide ice ages. In a paper published in 1824, Esmark proposed changes in climate as the cause of those glaciations, he attempted to show. During the following years, Esmark's ideas were discussed and taken over in parts by Swedish and German scientists. At the University of Edinburgh Robert Jameson seemed to be open to Esmark's ideas, as reviewed by Norwegian professor of glaciology Bjørn G. Andersen. Jameson's remarks about ancient glaciers in Scotland were most prompted by Esmark.
In Germany, Albrecht Reinhard Bernhardi, a geologist and professor of forestry at an academy in Dreissigacker, since incorporated in the southern Thuringian city of Meiningen, adopted Esmark's theory. In a paper published in 1832, Bernhardi speculated about former polar ice caps reaching as far as the temperate zones of the globe. In 1829, independently of these debates, the Swiss civil engineer Ignaz Venetz explained the dispersal of erratic boulders in the Alps, the nearby Jura Mountains, the North German Plain as being due to huge glaciers; when he read his paper before the Schweizerische Naturforschende Gesellschaft, most scientists remained sceptical. Venetz convinced his friend Jean de Charpentier. De Charpentier transformed Venetz's idea into a theory with a glaciation limited to the Alps, his thoughts resembled Wahlenberg's theory. In fact, both men shared the same volcanistic, or in de Charpentier's case rather plutonistic assumptions, about the Earth's history. In 1834, de Charpentier presented his paper before the Schweizerische Naturforschende Gesellschaft.
In the meantime, the German botanist Karl Friedrich Schimper was studying mosses which were growing on erratic boulders in the alpine upland of Bavaria. He began to wonder. During the summer of 1835 he made some excursions to the Bavarian Alps. Schimper came to the conclusion that ice must have been the means of transport for the boulders in the alpine upland. In the winter of 1835 to 1836 he held. Schimper assumed that there must have been global times of obliteration with a cold climate and frozen water. Schimper spent the summer months of 1836 at Devens, near Bex, in the Swiss Alps with his former university friend Louis Agassiz and Jean de Charpentier. Schimper, de Charpentier and Venetz convinced Agassiz that there had been a time of glaciation. During the winter of 1836/37, Agassiz and Schimper developed the theory of a sequence of glaciations, they drew upon the preceding works of Venetz, de Charpentier and on their own fieldwork. Agassiz appears to have been familiar with Bernhardi's paper at that time.
At the beginning of 1837, Schimper coined the term "ice age" for the period of the glaciers. In July 1837 Ag
Fresh water
Fresh water is any occurring water except seawater and brackish water. Fresh water includes water in ice sheets, ice caps, icebergs, ponds, rivers and underground water called groundwater. Fresh water is characterized by having low concentrations of dissolved salts and other total dissolved solids. Though the term excludes seawater and brackish water, it does include mineral-rich waters such as chalybeate springs. Fresh water is not the same as potable water. Much of the earth's fresh water is unsuitable for drinking without some treatment. Fresh water can become polluted by human activities or due to occurring processes, such as erosion. Water is critical to the survival of all living organisms; some organisms can thrive on salt water, but the great majority of higher plants and most mammals need fresh water to live. Fresh water can be defined as water with less than 500 parts per million of dissolved salts. Other sources give higher upper salinity limits for e.g. 1000 ppm or 3000 ppm. Fresh water habitats are classified as either lentic systems, which are the stillwaters including ponds, lakes and mires.
There is, in addition, a zone which bridges between groundwater and lotic systems, the hyporheic zone, which underlies many larger rivers and can contain more water than is seen in the open channel. It may be in direct contact with the underlying underground water; the majority of fresh water on Earth is in ice caps. The source of all fresh water is precipitation from the atmosphere, in the form of mist and snow. Fresh water falling as mist, rain or snow contains materials dissolved from the atmosphere and material from the sea and land over which the rain bearing clouds have traveled. In industrialized areas rain is acidic because of dissolved oxides of sulfur and nitrogen formed from burning of fossil fuels in cars, factories and aircraft and from the atmospheric emissions of industry. In some cases this acid rain results in pollution of rivers. In coastal areas fresh water may contain significant concentrations of salts derived from the sea if windy conditions have lifted drops of seawater into the rain-bearing clouds.
This can give rise to elevated concentrations of sodium, chloride and sulfate as well as many other compounds in smaller concentrations. In desert areas, or areas with impoverished or dusty soils, rain-bearing winds can pick up sand and dust and this can be deposited elsewhere in precipitation and causing the freshwater flow to be measurably contaminated both by insoluble solids but by the soluble components of those soils. Significant quantities of iron may be transported in this way including the well-documented transfer of iron-rich rainfall falling in Brazil derived from sand-storms in the Sahara in north Africa. Saline water in oceans and saline groundwater make up about 97% of all the water on Earth. Only 2.5–2.75% is fresh water, including 1.75–2% frozen in glaciers and snow, 0.5–0.75% as fresh groundwater and soil moisture, less than 0.01% of it as surface water in lakes and rivers. Freshwater lakes contain about 87% of this fresh surface water, including 29% in the African Great Lakes, 22% in Lake Baikal in Russia, 21% in the North American Great Lakes, 14% in other lakes.
Swamps have most of the balance with only a small amount in rivers, most notably the Amazon River. The atmosphere contains 0.04% water. In areas with no fresh water on the ground surface, fresh water derived from precipitation may, because of its lower density, overlie saline ground water in lenses or layers. Most of the world's fresh water is frozen in ice sheets. Many areas suffer from lack of distribution such as deserts. Water is a critical issue for the survival of all living organisms; some can use salt water but many organisms including the great majority of higher plants and most mammals must have access to fresh water to live. Some terrestrial mammals desert rodents, appear to survive without drinking, but they do generate water through the metabolism of cereal seeds, they have mechanisms to conserve water to the maximum degree. Fresh water creates a hypotonic environment for aquatic organisms; this is problematic for some organisms with pervious skins or with gill membranes, whose cell membranes may burst if excess water is not excreted.
Some protists accomplish this using contractile vacuoles, while freshwater fish excrete excess water via the kidney. Although most aquatic organisms have a limited ability to regulate their osmotic balance and therefore can only live within a narrow range of salinity, diadromous fish have the ability to migrate between fresh water and saline water bodies. During these migrations they undergo changes to adapt to the surroundings of the changed salinities; the eel uses the hormone prolactin, while in salmon the hormone cortisol plays a key role during this process. Many sea birds have special glands at the base of the bill; the marine iguanas on the Galápagos Islands excrete excess salt through a nasal gland and they sneeze out a salty excretion. Freshwater molluscs include freshwater snails and freshwater bivalves. Freshwater crustaceans include crayfish. Freshwater biodiversity faces many threats; the World Wide Fund for Nature's Living Planet Index noted an 83% decline in the populations of freshwater vertebrates between 1970 and 2014.
These declines continue to outpace
Cenozoic
The Cenozoic Era meaning "new life", is the current and most recent of the three Phanerozoic geological eras, following the Mesozoic Era and extending from 66 million years ago to the present day. The Cenozoic is known as the Age of Mammals, because the extinction of many groups allowed mammals to diversify so that large mammals dominated it; the continents moved into their current positions during this era. Early in the Cenozoic, following the K-Pg extinction event, most of the fauna was small, included small mammals, birds and amphibians. From a geological perspective, it did not take long for mammals and birds to diversify in the absence of the large reptiles that had dominated during the Mesozoic. A group of avians known as the "terror birds" grew larger than the average human and were formidable predators. Mammals came to occupy every available niche, some grew large, attaining sizes not seen in most of today's mammals; the Earth's climate had begun a drying and cooling trend, culminating in the glaciations of the Pleistocene Epoch, offset by the Paleocene-Eocene Thermal Maximum.
Cenozoic, meaning "new life," is derived from Greek καινός kainós "new," and ζωή zōḗ "life." The era is known as the Cænozoic, Caenozoic, or Cainozoic. The name "Cenozoic" was proposed in 1840 by the British geologist John Phillips; the Cenozoic is divided into three periods: the Paleogene and Quaternary. The Quaternary Period was recognized by the International Commission on Stratigraphy in June 2009, the former term, Tertiary Period, became disused in 2004 due to the need to divide the Cenozoic into periods more like those of the earlier Paleozoic and Mesozoic eras; the common use of epochs during the Cenozoic helps paleontologists better organize and group the many significant events that occurred during this comparatively short interval of time. Knowledge of this era is more detailed than any other era because of the young, well-preserved rocks associated with it; the Paleogene spans from the extinction of non-avian dinosaurs, 66 million years ago, to the dawn of the Neogene, 23.03 million years ago.
It features three epochs: the Paleocene and Oligocene. The Paleocene epoch lasted from 66 million to 56 million years ago. Modern placental mammals originated during this time; the Paleocene is a transitional point between the devastation, the K-T extinction, to the rich jungle environment, the Early Eocene. The Early Paleocene saw the recovery of the earth; the continents began to take their modern shape, but all the continents and the subcontinent of India were separated from each other. Afro-Eurasia was separated by the Tethys Sea, the Americas were separated by the strait of Panama, as the isthmus had not yet formed; this epoch featured a general warming trend, with jungles reaching the poles. The oceans were dominated by sharks. Archaic mammals filled the world such as creodonts; the Eocene Epoch ranged from 56 million years to 33.9 million years ago. In the Early-Eocene, species living in dense forest were unable to evolve into larger forms, as in the Paleocene. There was nothing over the weight of 10 kilograms.
Among them were early primates and horses along with many other early forms of mammals. At the top of the food chains were huge birds, such as Paracrax; the temperature was 30 degrees Celsius with little temperature gradient from pole to pole. In the Mid-Eocene, the Circumpolar-Antarctic current between Australia and Antarctica formed; this disrupted ocean currents worldwide and as a result caused a global cooling effect, shrinking the jungles. This allowed mammals to grow to mammoth proportions, such as whales which, by that time, had become fully aquatic. Mammals like Andrewsarchus were at the top of the food-chain; the Late Eocene saw the rebirth of seasons, which caused the expansion of savanna-like areas, along with the evolution of grass. The end of the Eocene was marked by the Eocene-Oligocene extinction event, the European face of, known as the Grande Coupure; the Oligocene Epoch spans from 33.9 million to 23.03 million years ago. The Oligocene featured the expansion of grass which had led to many new species to evolve, including the first elephants, dogs and many other species still prevalent today.
Many other species of plants evolved in this period too. A cooling period featuring seasonal rains was still in effect. Mammals still continued to grow larger; the Neogene spans from 23.03 million to 2.58 million years ago. It features 2 epochs: the Miocene, the Pliocene; the Miocene epoch spans from 23.03 to 5.333 million years ago and is a period in which grass spread further, dominating a large portion of the world, at the expense of forests. Kelp forests evolved, encouraging the evolution such as sea otters. During this time, perissodactyla thrived, evolved into many different varieties. Apes evolved into 30 species; the Tethys Sea closed with the creation of the Arabian Peninsula, leaving only remnants as the Black, Red and Caspian Seas. This increased aridity. Many new plants evolved: 95% of modern seed plants evolved in the mid-Miocene; the Pliocene epoch lasted from 5.333 to 2.58 million years ago. The Pliocene featured dramatic climactic changes, which led to modern species and plants; the Mediterranean Sea dried up for several million years (because the ice ages reduced sea levels, disconnecting the Atlantic from
Drake Passage
The Drake Passage or Mar de Hoces—Sea of Hoces—is the body of water between South America's Cape Horn and the South Shetland Islands of Antarctica. It connects the southwestern part of the Atlantic Ocean with the southeastern part of the Pacific Ocean and extends into the Southern Ocean; the passage receives its English-language name from the 16th-century English privateer Sir Francis Drake. Drake's only remaining ship, after having passed through the Strait of Magellan, was blown far south in September 1578; this incident implied an open connection between the Pacific oceans. Half a century earlier, after a gale had pushed them south from the entrance of the Strait of Magellan, the crew of the Spanish navigator Francisco de Hoces thought they saw a land's end and inferred this passage in 1525. For this reason, some Spanish and Latin American historians and sources call it Mar de Hoces after Francisco de Hoces; the first recorded voyage through the passage was that of Eendracht, captained by the Dutch navigator Willem Schouten in 1616, naming Cape Horn in the process.
The 800-kilometre wide passage between Cape Horn and Livingston Island is the shortest crossing from Antarctica to any other landmass. The boundary between the Atlantic and Pacific Oceans is sometimes taken to be a line drawn from Cape Horn to Snow Island. Alternatively, the meridian that passes through Cape Horn may be taken as the boundary. Both boundaries lie within the Drake Passage; the other two passages around the extreme southern part of South America, Strait of Magellan and Beagle Channel, are narrow, leaving little room for a ship. They can become icebound, sometimes the wind blows so that no sailing vessel can make headway against it. Hence most sailing ships prefer the Drake Passage, open water for hundreds of miles, despite rough conditions; the small Diego Ramírez Islands lie about 100 kilometres south-southwest of Cape Horn. There is no significant land anywhere around the world at the latitudes of Drake Passage, important to the unimpeded flow of the Antarctic Circumpolar Current which carries a huge volume of water through the Passage and around Antarctica.
Ships in the Passage are good platforms for the sighting of whales and seabirds including giant petrels, other petrels and penguins. The passage is known to have been closed until around 41 million years ago according to a chemical study of fish teeth found in oceanic sedimentary rock. Before the passage opened, the Atlantic and Pacific Oceans were separate, with Antarctica being much warmer and having no ice cap; the joining of the two great oceans started the Antarctic Circumpolar Current and cooled the continent significantly. Elizabeth Island García de Nodal Bransfield Strait Sars Bank Media related to Drake Passage at Wikimedia Commons National Oceanography Centre, Southampton page of the important and complex bathymetry of the Passage A NASA image of an eddy in the Passage Larger-scale images of the passage from the US Navy
