Rapid Changes In Ocean Circulation Patterns And Cumate Change

Some models of climate change show that patterns of ocean circulation can suddenly change and cause global climate conditions to switch from warm to cold, or cold to warm, over periods of a few decades. Understanding how fast climate can shift from a warm period to a cold, or cold to a warm, is controversial. The record of climate indicators is incomplete and difficult to interpret. Only 18,000 years ago the planet was in the midst of a major glacial interval, and since then global average temperatures have risen 16°F (10°C) and are still rising, perhaps at a recently accelerated rate from human contributions to the atmosphere. still, recent climate work is revealing that there are some abrupt transitions in the slow warming, in which there are major shifts in some component of the climate, where the shift may happen on scales of 10 years or less.

One of these abrupt transitions seems to affect the ocean circulation pattern in the North Atlantic Ocean, where the ocean currents formed one of two different stable patterns or modes, with abrupt transitions occurring when one mode switches to the other. In the present pattern the warm waters of the Gulf Stream come out of the Gulf of Mexico and flow along the eastern seaboard of the United states, past the British Isles, to the Norwegian Sea. This warm current is largely responsible for the mild climate of the British Isles and northern Europe. In the second mode, the northern extension of the Gulf Stream is weakened by a reduction in salinity of surface waters from sources at high latitudes in the North Atlantic. The fresher water has a source in increased melting from the polar ice shelf, Greenland, and northern glaciers. With less salt, seawater is less dense, and is less able to sink during normal wintertime cooling.

Studies of past switches in the circulation modes of the North Atlantic reveal that the transition from mode 1 to mode 2 can occur over a period of only five to 10 years. These abrupt transitions are apparently linked to increase in the release of icebergs and freshwater from continental glaciers, which upon melting contribute large volumes of freshwater into the North Atlantic, systematically reducing the salinity. The Gulf Stream presently seems on the verge of failure, or of switching from mode 1 to mode 2, and historical records show that this switch can be very rapid. If this predicted switch occurs, northern Europe and the United Kingdom may experience a significant and dramatic cooling of their climate, instead of the warming many fear.

See also El Niño and the Southern Oscillation (ENSO); energy in the Earth system; hydrosphere; ocean basin; oceanography; pelagic, nektonic, planktonic; thermohaline circulation.

FURTHER READING

Erickson, Jon. Marine Geology; Exploring the New Frontiers of the Ocean. Rev. ed. New York: Facts On File, 2003.

Intergovernmental Panel on Climate Change. Climate Change 2007: The Physical Science Basis. Contributions of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller. Cambridge: Cambridge University Press, 2007.

Intergovernmental Panel on Climate Change home page. Available online. URL: http://www.ipcc.ch/index.htm. Accessed January 29, 2009. Kusky, T. M. Climate Change: Shifting Deserts, Glaciers, and Climate Belts. New York: Facts On File, 2008.

-. The Coast: Hazardous Interactions within the

Coastal Zone. New York: Facts on File, 2008.

oceanic plateau Regions of anomalously thick oceanic crust and topographically high sea-floor are known as oceanic plateaus. Many have oceanic crust that is 12.5-25 miles (20-40 km)

thick, and rise thousands of meters above surrounding oceanic crust of normal thickness. The Caribbean ocean floor represents one of the best examples of an oceanic plateau, with other major examples including the Ontong-Java Plateau, Manihiki Plateau, Hess Rise, Shatsky Rise, and Mid-Pacific Mountains. All of these oceanic plateaus contain thick piles of volcanic and subvolcanic rocks representing huge outpourings of lava; most erupted in a few million years. They typically do not show the magnetic stripes that characterize normal oceanic crust produced at oceanic ridges, and are thought to have formed when mantle plume heads reached the base of the lithosphere, releasing huge amounts of magma. some oceanic plateaus have such large volumes of magma that the total magmatic flux in the plateaus would have been similar to or larger than all of the magma erupted at the mid-ocean ridges during the same interval. The Caribbean sea-floor preserves 5-7.5-mile- (8-21-km-) thick oceanic crust formed before about 85 million years ago in the eastern Pacific ocean. Plate tectonics transported this unusually thick ocean floor eastward, where pieces of the seafloor collided with south America as it passed into the Atlantic ocean. Pieces of the Caribbean oceanic crust are now preserved in Colombia, Ecuador, Panama, Hispaniola, and Cuba, and some scientists estimate that the Caribbean oceanic plateau may have once been twice its present size. An accompanying vast outpouring of lava would have been associated with significant outgassing, with possible consequences for global climate and evolution.

The western Pacific ocean basin contains several large oceanic plateaus, including the 20-mile-

World Map London

Ontong Java

G Infobase Publishing

G Infobase Publishing

Map of the world showing major flood basalt regions, including oceanic plateaus

(32-km-) thick crust of the Ontong-Java Plateau, which is the largest outpouring of volcanic rocks on the planet. The Ontong-Java Plateau is the largest igneous province in the world not associated with the oceanic-ridge spreading-center network, covering an area roughly the size of Alaska (9,300,000 square miles, or 15,000,000 km2). The plateau is located northeast of Papua New Guinea and the Solomon Islands in the southwest Pacific Ocean, centered on the equator at 160°E longitude. Most of the plateau formed about 122 million years ago in the Cretaceous period, probably as a result of a mantle plume rising to the surface and causing massive amounts of volcanism over a geologically short interval, likely lasting only about a million years. Smaller amounts of volcanic material erupted later, at about 90 million years ago. Together, these events formed a lava plateau that is 20 miles (32 km) thick. The amount of volcanic material produced to form the plateau is estimated to be approximately the same as that erupted from the entire global ocean-ridge spread-ing-center system in the same period. Such massive amounts of volcanism cause worldwide changes in climate and ocean temperatures and typically have great impacts on the biosphere. Sea levels rose by more than 30 feet (9 m) in response to this volcanic outpouring. The gases released during these eruptions are estimated to have raised average global temperatures by 23°F (13°C). Perhaps more remarkably, the Ontong-Java Plateau is but one of many Cretaceous oceanic plateaus in the Pacific, suggesting that the Cretaceous was characterized by a long-standing eruption of massive amounts of deeply derived magma. Some geologists have suggested that events like this relate to major mantle overturn events, when plumes dominate heat loss from the Earth instead of oceanic-ridge spreading, as in the present plate mosaic.

The plateau is thought to be composed largely of basalt, based on limited sampling, deep-sea drilling, and seismic velocities. A covering by a thick veneer of sediments, exceeding thousands of feet (a kilometer or more) in most places, presents great difficulties in trying to sample the plateau. The plateau is colliding with the Solomon trench, but thick oceanic plateaus like the Ontong-Java are generally unsub-ductable. When oceanic plateaus are attempted to be subducted,

Ontong Java

they typically get accreted to the continents, leading to continental growth.

See also African geology; large igneous provinces, flood basalt.

FURTHER READING

Kious, Jacquelyne, and Robert I. Tilling. U.S. Geological Survey. This Dynamic Earth: The Story of Plate Tectonics. Available online. URL: http://pubs.usgs. gov/gip/dynamic/dynamic.html. Last modified March 27, 2007.

Kusky, T. M., ed. Precambrian Ophiolites and Related Rocks. Developments in Precambrian Geology Vol. 13. Amsterdam: Elsevier Publishers, 2003. Kusky, Timothy M., and William S. F. Kidd. "Remnants of an Archean Oceanic Plateau, Belingwe Greenstone Belt, Zimbabwe." Geology 20, no.1 (1992): 43-46. Moores, Eldridge M. "Origin and Emplacement of Ophiolites." Review Geophysics 20 (1982): 735-750. Moores, Eldridge M., and Robert Twiss. Tectonics. New York: W. H. Freeman, 1995.

oceanography The study of the physical, chemical, biological, and geological aspects of the ocean basins is oceanography. Oceanographers have begun using an Earth system science approach to study the oceans, with the appreciation that many of the different systems are related, and changes in the biological, chemical, physical, or geological conditions will result in changes in the other systems and also influence other Earth systems such as the atmosphere and climate. The oceans contain important geological systems, since the ocean basins are the places where oceanic crust is both created at mid-ocean ridges and destroyed at deep-sea trenches. Being topographic depressions, they are repositories for many of the sediments eroded from the continents and carried by rivers and the wind to be deposited in submarine settings. Seawater is the host of much of the life on Earth and also holds huge quantities of dissolved gases and chemicals that buffer the atmosphere, keeping global temperatures and climate hospitable for humans. Energy is transferred around the planet in ocean currents and waves, which interact with land, eroding or depositing shoreline environments. Being host to some of the planet's largest and most diverse biota, the oceans may hold the key to feeding the planet. Mineral resources are also abundant on the seafloor, many formed at the interface between hot volcanic fluids and cold seawater, forming potentially economically important reserves of many minerals.

The oceans cover two-thirds of the Earth's surface, yet scientists have explored less of the ocean's depths and mysteries than the surfaces of several nearby planets. The oceans have hindered migration of peoples and biota between distant continents, yet paradoxically now serve as a principal means of transportation. Oceans provide us with incredible mineral wealth and renewable food and energy sources, yet also breed devastating hurricanes. Life may have begun on Earth in environments simulated by hot volcanic events on the seafloor, and marine biologists are exploring the diverse and unique fauna that can still be found living in deep dark waters around similar vents today.

Ocean basins have continually opened and closed on Earth, and the continents have alternately been swept into large single supercontinents and then broken apart by the formation of new ocean

USNS Bowditch, a U.S. Navy T-AGS 60 Class oceanographic survey ship (U.S. Navy photo)

Japanese research vessel Natsushima, support ship for the Shinkai subs (Science Source/Photo Researchers, Inc.)

basins. The appearance, evolution, and extinction of different life-forms is inextricably linked to the opening and closing of ocean basins, partly through the changing environmental conditions associated with the changing distribution of oceans and continents.

Early explorers slowly learned about ocean currents and routes to distant lands, and some dredging operations revealed huge deposits of metals on the seafloor. Tremendous leaps in our understanding of the structure of the ocean basin seafloor were acquired during surveying for the navigation of submarines and detection of enemy submarines during World War II. Magnetometers towed behind ships and accurate depth measurements provided data that led to the formulation of the hypothesis of seafloor spreading, which added the oceanic counterpart to the idea of continental drift. The plate tectonic paradigm later unified these two theories.

ocean circulation is responsible for much of the world's climate. For instance, mild foggy winters in London are caused by warm waters from the Gulf of Mexico flowing across the Atlantic via the Gulf stream to the coast of the British isles. Large variations in ocean and atmospheric circulation patterns in the Pacific lead to alternating wet and dry climate conditions known as El Niño and La Niña. These variations affect Pacific regions most strongly, but are felt throughout the world. other more dramatic movements of water include the sometimes devastating tsunamis initiated by earthquakes, volcanic eruptions, and giant submarine landslides.

one of the most tragic tsunamis in recent history occurred on December 26, 2004, following a magnitude 9.0 earthquake off the coast of northern sumatra in the Indian ocean. The earthquake was the largest since the 1964 magnitude 9.2 event in southern Alaska and released more energy than all the earthquakes on the planet in the last 25 years combined. During this catastrophic earthquake, a segment of the seafloor the size of the state of California, lying above the sumatra subduction zone trench, suddenly moved upward and seaward by more than 30 feet (9 m). The sudden displacement of this volume of undersea floor displaced a huge amount of water and generated the most destructive tsunami known in recorded history. Within minutes of the initial earthquake a mountain of water more than 100 feet (30 m) high was ravaging northern sumatra, sweeping into coastal villages and resort communities with a fury that crushed all in its path, removing buildings and vegetation, and in many cases eroding shoreline areas down to bedrock, leaving no traces of the previous inhabitants or structures. similar scenes of destruction and devastation rapidly moved up the coast of nearby Indonesia, where residents and tourists were enjoying a holiday weekend. Firsthand accounts of the catastrophe reveal similar scenes of horror where unsuspecting tourists and residents were enjoying themselves in beachfront playgrounds, resorts, and villages, and reacted as large breaking waves appeared off the coast. Many moved toward the shore to watch the high surf with interest, then ran in panic as the sea rapidly rose beyond expectations, and walls of water engulfed entire beachfronts, rising above hotel lobbies, and washing through towns with the force of Niagara Falls. In some cases the sea retreated to unprecedented low levels before the waves struck, causing many people to move to the shore to investigate the phenomenon; in other cases, the sea waves simply came crashing inland without warning. Buildings, vehicles, trees, boats and other debris were washed along with the ocean waters, forming projectiles that smashed at speeds of up to 30 miles per hour (50 km/hr) into other structures, leveling all in their paths, and killing more than a quarter million people.

Another tragic tsunami in history was generated by the eruption of the Indonesian volcano Krakatau in 1883. When Krakatau erupted, it blasted out a large part of the center of the volcano, and seawater rushed in to fill the hole. This seawater was immediately heated and it exploded outward in a steam eruption and a huge wave of hot water. The tsunami generated by this eruption reached more than 120 feet (36.5 m) in height and killed an estimated 36,500 people in nearby coastal regions. in 1998 a catastrophic 50-foot- (15-m-) high wave unexpectedly struck Papua New Guinea, killing more than 2,000 people and leaving more than 10,000 homeless.

Rich mineral deposits fill the oceans—oil and gas are on the continental shelves and slopes, and metalliferous deposits form near mid-ocean ridge vents. Much of the world's wealth of manganese, copper, and gold lies on the seafloor. The oceans also yield rich harvests of fish, and care must be taken to not deplete this source. Sea vegetables are growing in popularity, and their use may help alleviate the growing demand for space in fertile farmland. The oceans may offer the world a solution to growing energy and food demands resulting from a rapidly growing population. New life-forms are constantly being discovered in the depth of the oceans, and precautions must be taken to understand these creatures before any changes people make to their environment cause them to perish forever.

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