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Oocyte
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Oocyte
Identifiers
MeSH
D009865
Anatomical terminology
[edit on Wikidata]
An oocyte (UK: /ˈoʊəsaɪt/, US: /ˈoʊoʊ-/), oöcyte, ovocyte, or rarely ocyte[citation needed], is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell (PGC), which then undergoes mitosis, forming oogonia. During oogenesis, the oogonia become primary oocytes. An oocyte is a form of genetic material that can be collected for cryoconservation. Cryoconservation of animal genetic resources have been put into action as a means of conserving traditional livestock.
Contents
[hide]
1 Formation
2 Characteristics
2.1 Cytoplasm
2.2 Nucleus
2.3 Nest
3 Maternal contributions
3.1 Avoidance of damage to germ-line DNA
3.2 mRNAs and proteins
3.3 Mitochondria
3.4 Nucleolus
3.5 Ribosomes
4 Paternal contributions
5 Abnormalities
6 See also
7 References
8 Sources
9 External links
Formation[edit]
Diagram showing the reduction in number of the chromosomes in the process of maturation of the ovum; the process is known as meiosis.
Main article: Oogenesis
The formation of an oocyte is called oocytogenesis, which is a part of oogenesis.[1] Oogenesis results in the formation of both primary oocytes during fetal period, and of secondary oocytes after it as part of ovulation.
Cell type ploidy/chromosomes chromatids Process Time of completion
Oogonium diploid/46(2N) 2C Oocytogenesis (mitosis) third trimester
primary Oocyte diploid/46(2N) 4C Ootidogenesis (meiosis I) (Folliculogenesis) Dictyate in prophase I for up to 50 years
secondary Oocyte haploid/23(1N) 2C Ootidogenesis (meiosis II) Halted in metaphase II until fertilization
Ootid haploid/23(1N) 1C Ootidogenesis (meiosis II) Minutes after fertilization
Ovum haploid/23(1N) 1C
Characteristics[edit]
Cytoplasm[edit]
Oocytes are rich in cytoplasm, which contains yolk granules to nourish the cell early in development.
Nucleus[edit]
During the primary oocyte stage of oogenesis, the nucleus is called a germinal vesicle.[2]
The only normal human type of secondary oocyte has the 23rd (sex) chromosome as 23,X (female-determining), whereas sperm can have 23,X (female-determining) or 23,Y (male-determining).
Nest[edit]
The space within an ovum or immature ovum is located is the cell-nest.[3]
Maternal contributions[edit]
Oocyte poles
Because the fate of an oocyte is to become fertilized and ultimately grow into a fully functioning organism, it must be ready to regulate multiple cellular and developmental processes. The oocyte, a large and complex cell, must be supplied with numerous molecules that will direct the growth of the embryo and control cellular activities. As the oocyte is a product of female gametogenesis, the maternal contribution to the oocyte and consequently the newly fertilized egg is enormous. There are many types of molecules that are maternally supplied to the oocyte, which will direct various activities within the growing zygote.
Avoidance of damage to germ-line DNA[edit]
The DNA of a cell is vulnerable to the damaging effect of oxidative free radicals produced as byproducts of cellular metabolism. DNA damage occurring in oocytes, if not repaired, can be lethal and result in reduced fecundity and loss of potential progeny. Oocytes are substantially larger than the average somatic cell, and thus considerable metabolic activity is necessary for their provisioning. If this metabolic activity were carried out by the oocyte’s own metabolic machinery, the oocyte genome would be exposed to the reactive oxidative by-products generated. Thus it appears that a process evolved to avoid this vulnerability of germ line DNA. It was proposed that, in order to avoid damage to the DNA genome of the oocytes, the metabolism contributing to the synthesis of much of the oocyte’s constituents was shifted to other maternal cells that then transferred these constituents to oocytes.[4][5] Thus, oocytes of many organisms are protected from oxidative DNA damage while storing up a large mass of substances to nurture the zygote in its initial embryonic growth.
mRNAs and proteins[edit]
During the growth of the oocyte, a variety of maternally transcribed messenger RNAs, or mRNAs, are supplied by maternal cells. These mRNAs can be stored in mRNP (message ribonucleoprotein) complexes and be translated at specific time points, they can be localized within a specific region of the cytoplasm, or they can be homogeneously dispersed within the cytoplasm of the entire oocyte.[6] Maternally loaded proteins can also be localized or ubiquitous throughout the cytoplasm. The translated products of the mRNAs and the loaded proteins have multiple functions; from regulation of cellular "house-keeping" such as cell cycle progression and cellular metabolism, to regulation of developmental processes such as fertilization, activation of zygotic transcription, and formation of body axes.[6] Below are some examples of maternally inherited mRNAs and proteins found in the oocytes of the African clawed frog.
Name
Type of maternal molecule
Localization
Function
VegT[7] mRNA Vegetal hemisphere Transcription factor
Vg1[8] mRNA Vegetal hemisphere Transcription factor
XXBP-1[9] mRNA Not known Transcription factor
CREB[10] Protein Ubiquitous Transcription factor
FoxH1[11] mRNA Ubiquitous Transcription factor
p53[12] Protein Ubiquitous Transcription Factor
Lef/Tcf[13] mRNA Ubiquitous Transcription factor
FGF2[14] Protein Nucleus Not known
FGF2, 4, 9 FGFR1[13] mRNA Not known FGF signaling
Ectodermin[15] Protein Animal hemisphere Ubiquitin ligase
PACE4[16] mRNA Vegetal hemisphere Proprotein convertase
Coco[17] Protein Not known BMP inhibitor
Twisted gastrulation[13] Protein Not known BMP/Chordin binding protein
fatvg[18] mRNA Vegetal hemisphere Germ cell formation and cortical rotation
Maternal determinants in Xenopus laevis oocyte
Mitochondria[edit]
The oocyte receives mitochondria from maternal cells, which will go on to control embryonic metabolism and apoptotic events.[6] The partitioning of mitochondria is carried out by a system of microtubules that will localize mitochondria throughout the oocyte. In certain organisms, such as mammals, paternal mitochondria brought to the oocyte by the spermatozoon are degraded through the attachment of ubiquitinated proteins. The destruction of paternal mitochondria ensures the strictly maternal inheritance of mitochondria and mitochondrial DNA or mtDNA.[6]
Nucleolus[edit]
In mammals, the nucleolus of the oocyte is derived solely from maternal cells.[19] The nucleolus, a structure found within the nucleus, is the location where rRNA is transcribed and assembled into ribosomes. While the nucleolus is dense and inactive in a mature oocyte, it is required for proper development of the embryo.[19]
Ribosomes[edit]
Maternal cells also synthesize and contribute a store of ribosomes that are required for the translation of proteins before the zygotic genome is activated. In mammalian oocytes, maternally derived ribosomes and some mRNAs are stored in a structure called cytoplasmic lattices. These cytoplasmic lattices, a network of fibrils, protein, and RNAs, have been observed to increase in density as the number of ribosomes decrease within a growing oocyte.[20]
Paternal contributions[edit]
The spermatozoon that fertilizes an oocyte will contribute its pronucleus, the other half of the zygotic genome. In some species, the spermatozoon will also contribute a centriole, which will help make up the zygotic centrosome required for the first division. However, in some species, such as in the mouse, the entire centrosome is acquired maternally.[21] Currently under investigation is the possibility of other cytoplasmic contributions made to the embryo by the spermatozoon.
During fertilization, the sperm provides three essential parts to the oocyte: (1) a signalling or activating factor, which causes the metabolically dormant oocyte to activate; (2) the haploid paternal genome; (3) the centrosome, which is responsible for maintaining the microtubule system. See anatomy of sperm
Abnormalities[edit]
nondisjunction—a failure of proper homolog separation in meiosis I, or sister chromatid separation in meiosis II can lead to aneuploidy, in which the oocyte has the wrong number of chromosomes, for example 22,X or 24,X. This is the cause of conditions like Down syndrome and Edwards syndrome in humans. It is more likely with advanced maternal age.
Some oocytes have multiple nuclei, although it is thought they never mature.
See also[edit]
Cortical granule
Cryoconservation of animal genetic resources
Folliculogenesis
Oocyte maturation inhibitor
Polar body
Symmetry breaking and cortical rotation
References[edit]
Jump up ^ answers.com
Jump up ^ Biology-online
Jump up ^ Grier HJ, Uribe MC, Parenti LR (April 2007). "Germinal epithelium, folliculogenesis, and postovulatory follicles in ovaries of rainbow trout, Oncorhynchus mykiss (Walbaum, 1792) (Teleostei, protacanthopterygii, salmoniformes)". J. Morphol. 268 (4): 293–310. doi:10.1002/jmor.10518. PMID 17309079.
Jump up ^ Bernstein C. (1993). Sex as a response to oxidative DNA damage. Chapter 10 (see pages 204-205) in “DNA and Free Radicals” (editors: Barry Halliwell, Okezie I Aruoma). Ellis Horwood Limited (publisher), Great Gritain ISBN 0-13-222035-0
Jump up ^ Bernstein, C. (1998). Sex as a response to oxidative DNA damage. Chapter 4, see pages 112-113. In “DNA and Free Radicals: Techniques, Mecchanisms & Applications” (editors: Okezie I Aruoma, Barry Halliwell). OICA International (publisher), Saint Lucia and London ISBN 976-8056169
^ Jump up to: a b c d Mtango N.R., Potireddy S., Latham K.E. (2008). Oocyte quality and maternal control of development. Int. Rev. Cell Mol. Biol. 268, 223-290.
Jump up ^ Zhang J., King M.L. (1996). Xenopus VegT RNA is localized to the vegetal cortex during oogenesis and encodes a novel T-box transcription factor involved in mesodermal patterning. Development. 12, 4119–29.
Jump up ^ Heasman J., Wessely O., Langland R., Craig E.J., Kessler D.S. (2001). Vegetal localization of maternal mRNAs is disrupted by VegT depletion. Dev Biol. 240, 377–386.
Jump up ^ Zhao H., Cao Y., Grunz H. (2003). Xenopus X-box binding protein 1, a leucine zipper transcription factor, is involved in the BMP signaling pathway. Dev Biol. 257, 278–291.
Jump up ^ Sundaram N., Tao Q., Wylie C., Heasman J. (2003). The role of maternal CREB in early embryogenesis of Xenopus laevis. Dev Biol. 261, 337–352.
Jump up ^ Kofron M., Puck H., Standley H., Wylie C., Old R., Whitman M., et al. (2004). New roles for FoxH1 in patterning the early embryo. Development. 131, 5065–5078.
Jump up ^ Takebayashi-Suzuki K., Funami J., Tokumori D., Saito A., Watabe T., Miyazono K., et al. (2003). Interplay between the tumor suppressor p53 and TGF beta signaling shapes embryonic body axes in Xenopus. Development. 130, 3929–3939.
^ Jump up to: a b c Heasman, J. (2006). Maternal determinants of embryonic cell fate. Semin. Cell Dev. Biol. 17, 93-98.
Jump up ^ Song J., Slack J.M. (1994). Spatial and temporal expression of basic fibroblast growth factor (FGF-2) mRNA and protein in early Xenopus development. Mech Dev. 48, 141–151.
Jump up ^ Dupont S., Zacchigna L., Cordenonsi M., Soligo S., Adorno M., Rugge M., et al. (2005). Germ-layer specification and control of cell growth by Ectodermin, a Smad4 ubiquitin ligase. Cell. 121, 87–99.
Jump up ^ Birsoy B., Berg L., Williams P.H., Smith J.C., Wylie C.C., Christian J.L., et al. (2005). XPACE4 is a localized pro-protein convertase required for mesoderm induction and the cleavage of specific TGFbeta proteins in Xenopus development. Development. 132, 591–602.
Jump up ^ Bell E., Munoz-Sanjuan I., Altmann C.R., Vonica A., Brivanlou A.H. (2003). Cell fate specification and competence by Coco, a maternal BMP, TGFbeta and Wnt inhibitor. Development. 130, 1381–1389.
Jump up ^ Chan A. P., Kloc M., Larabell C. A., LeGros M., Etkin L.D. (2007). The maternally localized RNA fatvg is required for cortical rotation and germ cell formation. Mech Dev. 124, 350-363.
^ Jump up to: a b Ogushi S., et al. (2008). The maternal nucleolus is essential for early embryonic development in mammals. Science. 319, 613-616
Jump up ^ Vitale A.M., Yurttas P., Fitzhenry R.J., Cohen-Gould, L., Wu W., Gossen J.A., Coonrod S.A. (2009). Role for PADI6 and the CPLs in ribosomal storage in oocytes and translation in the early embryo. Development. 135, 2627-2636.
Jump up ^ Sutovsky P., Schatten G. (2000). Paternal contributions to the mammalian zygote: fertilization after sperm-egg fusion. Int. Rev. Cytol. 195, 1-65.
Sources[edit]
William K. Purves, Gordon H. Orians, David Sadava, H. Craig Heller, Craig Heller (2003). Life: The Science of Biology (7th ed.), pp. 823–824.
External links[edit]
Micrograph of a primary oocyte and follicle of a monkey
Preceded by
None Stages of human development
Sperm + Oocyte Succeeded by
Zygote
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Thermohaline circulation
From Wikipedia, the free encyclopedia
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A summary of the path of the thermohaline circulation. Blue paths represent deep-water currents, while red paths represent surface currents.
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Thermohaline circulation
Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes.[1][2] The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents (such as the Gulf Stream) travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes (forming North Atlantic Deep Water). This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters (with a transit time of around 1000 years)[3] upwell in the North Pacific.[4] Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system. On their journey, the water masses transport both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe. As such, the state of the circulation has a large impact on the climate of the Earth.
The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt. On occasion, it is used to refer to the meridional overturning circulation (often abbreviated as MOC). The term MOC is more accurate and well defined, as it is difficult to separate the part of the circulation which is driven by temperature and salinity alone as opposed to other factors such as the wind and tidal forces.[5] Moreover, temperature and salinity gradients can also lead to circulation effects that are not included in the MOC itself.
Contents
[hide]
1 Overview
1.1 Formation of deep water masses
1.2 Movement of deep water masses
1.3 Quantitative estimation
2 Gulf Stream
3 Upwelling
4 Effects on global climate
4.1 Shutdown of thermohaline circulation
5 See also
6 References
7 Other sources
8 External links
Overview[edit]
The global conveyor belt on a continuous-ocean map
The movement of surface currents pushed by the wind is fairly intuitive. For example, the wind easily produces ripples on the surface of a pond. Thus the deep ocean—devoid of wind—was assumed to be perfectly static by early oceanographers. However, modern instrumentation shows that current velocities in deep water masses can be significant (although much less than surface speeds). In general, ocean water velocities range from fractions of centimeters per second (in the depth of the oceans) to sometimes more than 1 m/s in surface currents like the Gulf Stream and Kuroshio.
In the deep ocean, the predominant driving force is differences in density, caused by salinity and temperature variations (increasing salinity and lowering the temperature of a fluid both increase its density). There is often confusion over the components of the circulation that are wind and density driven.[6][7] Note that ocean currents due to tides are also significant in many places; most prominent in relatively shallow coastal areas, tidal currents can also be significant in the deep ocean. There they are currently thought to facilitate mixing processes, especially diapycnal mixing.[8]
The density of ocean water is not globally homogeneous, but varies significantly and discretely. Sharply defined boundaries exist between water masses which form at the surface, and subsequently maintain their own identity within the ocean. But these sharp boundaries are not to be imagined spatially but rather in a T-S-diagram[clarification needed] where water masses are distinguished. They position themselves above or below each other according to their density, which depends on both temperature and salinity.
Warm seawater expands and is thus less dense than cooler seawater. Saltier water is denser than fresher water because the dissolved salts fill interstices between water molecules, resulting in more mass per unit volume. Lighter water masses float over denser ones (just as a piece of wood or ice will float on water, see buoyancy). This is known as "stable stratification" as opposed to unstable stratification (see Brunt-Väisälä frequency)[clarification needed] where denser waters are located over less dense waters (see convection or deep convection needed for water mass formation). When dense water masses are first formed, they are not stably stratified, so they seek to locate themselves in the correct vertical position according to their density. This motion is called convection, it orders the stratification by gravitation. Driven by the density gradients this sets up the main driving force behind deep ocean currents like the deep western boundary current (DWBC).
The thermohaline circulation is mainly driven by the formation of deep water masses in the North Atlantic and the Southern Ocean caused by differences in temperature and salinity of the water.
The great quantities of dense water sinking at high latitudes must be offset by equal quantities of water rising elsewhere. Note that cold water in polar zones sink relatively rapidly over a small area, while warm water in temperate and tropical zones rise more gradually across a much larger area. It then slowly returns poleward near the surface to repeat the cycle. The continual diffuse upwelling of deep water maintains the existence of the permanent thermocline found everywhere at low and mid-latitudes. This model was described by Henry Stommel and Arnold B. Arons in 1960 and is known as the Stommel-Arons box model for the MOC.[9] This slow upward movement is approximated to be about 1 centimeter (0.5 inch) per day over most of the ocean. If this rise were to stop, downward movement of heat would cause the thermocline to descend and would reduce its steepness.
Formation of deep water masses[edit]
The dense water masses that sink into the deep basins are formed in quite specific areas of the North Atlantic and the Southern Ocean. In the North Atlantic, seawater at the surface of the ocean is intensely cooled by the wind and low ambient air temperatures. Wind moving over the water also produces a great deal of evaporation, leading to a decrease in temperature, called evaporative cooling related to latent heat. Evaporation removes only water molecules, resulting in an increase in the salinity of the seawater left behind, and thus an increase in the density of the water mass along with the decrease in temperature. In the Norwegian Sea evaporative cooling is predominant, and the sinking water mass, the North Atlantic Deep Water (NADW), fills the basin and spills southwards through crevasses in the submarine sills that connect Greenland, Iceland and Great Britain which are known as the Greenland-Scotland-Ridge. It then flows very slowly into the deep abyssal plains of the Atlantic, always in a southerly direction. Flow from the Arctic Ocean Basin into the Pacific, however, is blocked by the narrow shallows of the Bering Strait.
Diagram showing relation between temperature and salinity for sea water density maximum and sea water freezing temperature.
In the Southern Ocean, strong katabatic winds blowing from the Antarctic continent onto the ice shelves will blow the newly formed sea ice away, opening polynyas along the coast. The ocean, no longer protected by sea ice, suffers a brutal and strong cooling (see polynya). Meanwhile, sea ice starts reforming, so the surface waters also get saltier, hence very dense. In fact, the formation of sea ice contributes to an increase in surface seawater salinity; saltier brine is left behind as the sea ice forms around it (pure water preferentially being frozen). Increasing salinity lowers the freezing point of seawater, so cold liquid brine is formed in inclusions within a honeycomb of ice. The brine progressively melts the ice just beneath it, eventually dripping out of the ice matrix and sinking. This process is known as brine rejection.
The resulting Antarctic Bottom Water (AABW) sinks and flows north and east, but is so dense it actually underflows the NADW. AABW formed in the Weddell Sea will mainly fill the Atlantic and Indian Basins, whereas the AABW formed in the Ross Sea will flow towards the Pacific Ocean.
The dense water masses formed by these processes flow downhill at the bottom of the ocean, like a stream within the surrounding less dense fluid, and fill up the basins of the polar seas. Just as river valleys direct streams and rivers on the continents, the bottom topography constrains the deep and bottom water masses.
Note that, unlike fresh water, seawater does not have a density maximum at 4 °C but gets denser as it cools all the way to its freezing point of approximately −1.8 °C. This freezing point is however a function of salinity and pressure and thus -1.8 °C is not a general freezing temperature for sea water (see diagram to the right).
Movement of deep water masses[edit]
Formation and movement of the deep water masses at the North Atlantic Ocean, creates sinking water masses that fill the basin and flows very slowly into the deep abyssal plains of the Atlantic. This high-latitude cooling and the low-latitude heating drives the movement of the deep water in a polar southward flow. The deep water flows through the Antarctic Ocean Basin around South Africa where it is split into two routes: one into the Indian Ocean and one past Australia into the Pacific.
At the Indian Ocean, some of the cold and salty water from the Atlantic—drawn by the flow of warmer and fresher upper ocean water from the tropical Pacific—causes a vertical exchange of dense, sinking water with lighter water above. It is known as overturning. In the Pacific Ocean, the rest of the cold and salty water from the Atlantic undergoes haline forcing, and becomes warmer and fresher more quickly.
The out-flowing undersea of cold and salty water makes the sea level of the Atlantic slightly lower than the Pacific and salinity or halinity of water at the Atlantic higher than the Pacific. This generates a large but slow flow of warmer and fresher upper ocean water from the tropical Pacific to the Indian Ocean through the Indonesian Archipelago to replace the cold and salty Antarctic Bottom Water. This is also known as 'haline forcing' (net high latitude freshwater gain and low latitude evaporation). This warmer, fresher water from the Pacific flows up through the South Atlantic to Greenland, where it cools off and undergoes evaporative cooling and sinks to the ocean floor, providing a continuous thermohaline circulation.[10]
Hence, a recent and popular name for the thermohaline circulation, emphasizing the vertical nature and pole-to-pole character of this kind of ocean circulation, is the meridional overturning circulation.
Quantitative estimation[edit]
Direct estimates of the strength of the thermohaline circulation have been made at 26.5°N in the North Atlantic since 2004 by the UK-US RAPID programme.[11] By combining direct estimates of ocean transport using current meters and subsea cable measurements with estimates of the geostrophic current from temperature and salinity measurements, the RAPID programme provides continuous, full-depth, basinwide estimates of the thermohaline circulation or, more accurately, the meridional overturning circulation.
The deep water masses that participate in the MOC have chemical, temperature and isotopic ratio signatures and can be traced, their flow rate calculated, and their age determined. These include 231Pa / 230Th ratios.
Gulf Stream[edit]
Main article: Gulf Stream
Benjamin Franklin's map of the Gulf Stream
The Gulf Stream, together with its northern extension towards Europe, the North Atlantic Drift, is a powerful, warm, and swift Atlantic ocean current that originates at the tip of Florida, and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean. The process of western intensification causes the Gulf Stream to be a northward accelerating current off the east coast of North America.[12] At about 40°0′N 30°0′W / 40.000°N 30.000°W / 40.000; -30.000, it splits in two, with the northern stream crossing to northern Europe and the southern stream recirculating off West Africa. The Gulf Stream influences the climate of the east coast of North America from Florida to Newfoundland, and the west coast of Europe. Although there has been recent debate, there is consensus that the climate of Western Europe and Northern Europe is warmer than it would otherwise be due to the North Atlantic drift,[13][14] one of the branches from the tail of the Gulf Stream. It is part of the North Atlantic Gyre. Its presence has led to the development of strong cyclones of all types, both within the atmosphere and within the ocean. The Gulf Stream is also a significant potential source of renewable power generation.[15][16]
Upwelling[edit]
Main article: Upwelling
All these dense water masses sinking into the ocean basins displace the older deep water masses that were made less dense by ocean mixing. To maintain a balance, water must be rising elsewhere. However, because this thermohaline upwelling is so widespread and diffuse, its speeds are very slow even compared to the movement of the bottom water masses. It is therefore difficult to measure where upwelling occurs using current speeds, given all the other wind-driven processes going on in the surface ocean. Deep waters have their own chemical signature, formed from the breakdown of particulate matter falling into them over the course of their long journey at depth. A number of scientists have tried to use these tracers to infer where the upwelling occurs.
Wallace Broecker, using box models, has asserted that the bulk of deep upwelling occurs in the North Pacific, using as evidence the high values of silicon found in these waters. Other investigators have not found such clear evidence. Computer models of ocean circulation increasingly place most of the deep upwelling in the Southern Ocean,[17] associated with the strong winds in the open latitudes between South America and Antarctica. While this picture is consistent with the global observational synthesis of William Schmitz at Woods Hole and with low observed values of diffusion, not all observational syntheses agree. Recent papers by Lynne Talley at the Scripps Institution of Oceanography and Bernadette Sloyan and Stephen Rintoul in Australia suggest that a significant amount of dense deep water must be transformed to light water somewhere north of the Southern Ocean.
Effects on global climate[edit]
The thermohaline circulation plays an important role in supplying heat to the polar regions, and thus in regulating the amount of sea ice in these regions, although poleward heat transport outside the tropics is considerably larger in the atmosphere than in the ocean.[18] Changes in the thermohaline circulation are thought to have significant impacts on the Earth's radiation budget. Insofar as the thermohaline circulation governs the rate at which deep waters are exposed to the surface, it may also play an important role in the concentration of carbon dioxide in the atmosphere. While it is often stated that the thermohaline circulation is the primary reason that Western Europe is so temperate, it has been suggested that this is largely incorrect, and that Europe is warm mostly because it lies downwind of an ocean basin, and because of the effect of atmospheric waves bringing warm air north from the subtropics.[19] However, the underlying assumptions of this particular analysis have likewise been challenged.[20]
Large influxes of low-density meltwater from Lake Agassiz and deglaciation in North America are thought to have led to a shifting of deep water formation and subsidence in the extreme North Atlantic and caused the climate period in Europe known as the Younger Dryas.[21]
Shutdown of thermohaline circulation[edit]
Main article: Shutdown of thermohaline circulation
See also: Deglaciation and Cold blob (North Atlantic)
In 2005, British researchers noticed that the net flow of the northern Gulf Stream had decreased by about 30% since 1957. Coincidentally, scientists at Woods Hole had been measuring the freshening of the North Atlantic as Earth becomes warmer. Their findings suggested that precipitation increases in the high northern latitudes, and polar ice melts as a consequence. By flooding the northern seas with lots of extra fresh water, global warming could, in theory, divert the Gulf Stream waters that usually flow northward, past the British Isles and Norway, and cause them to instead circulate toward the Equator. If this were to happen, Europe's climate would be seriously impacted.[22][23][24]
Downturn of AMOC (Atlantic meridional overturning circulation), has been tied to extreme regional sea level rise.[25]
In 2013, an unexpected significant weakening of the THC led to one of the quietest Atlantic hurricane seasons observed since 1994. The main cause of the inactivity was caused by a continuation of the spring pattern across the Atlantic basin.
See also[edit]
Atlantic Multidecadal Oscillation (AMO) (in regard to "sea surface temperature")
Climate change
Contourite
Downwelling
Halothermal circulation
Hydrothermal circulation
Shutdown of thermohaline circulation
Upwelling
References[edit]
Jump up ^ Rahmstorf, S (2003). "The concept of the thermohaline circulation" (PDF). Nature. 421 (6924): 699. Bibcode:2003Natur.421..699R. doi:10.1038/421699a. PMID 12610602.
Jump up ^ Lappo, SS (1984). "On reason of the northward heat advection across the Equator in the South Pacific and Atlantic ocean". Study of Ocean and Atmosphere Interaction Processes. Moscow Department of Gidrometeoizdat (in Mandarin): 125–9.
Jump up ^ The global ocean conveyor belt is a constantly moving system of deep-ocean circulation driven by temperature and salinity; What is the global ocean conveyor belt?
Jump up ^ Primeau, F (2005). "Characterizing transport between the surface mixed layer and the ocean interior with a forward and adjoint global ocean transport model". Journal of Physical Oceanography. 35 (4): 545–64. Bibcode:2005JPO....35..545P. doi:10.1175/JPO2699.1.
Jump up ^ Wunsch, C (2002). "What is the thermohaline circulation?". Science. 298 (5596): 1179–81. doi:10.1126/science.1079329. PMID 12424356.
Jump up ^ Wyrtki, K (1961). "The thermohaline circulation in relation to the general circulation in the oceans". Deep-Sea Research. 8 (1): 39–64. Bibcode:1961DSR.....8...39W. doi:10.1016/0146-6313(61)90014-4.
Jump up ^ Schmidt, G., 2005, Gulf Stream slowdown?, RealClimate
Jump up ^ Eden, Carsten (2012). Ocean Dynamics. Springer. p. 177. ISBN 978-3-642-23449-1.
Jump up ^ Stommel, H., & Arons, A. B. (1960). On the abyssal circulation of the world ocean. – I. Stationary planetary flow patterns on a sphere. Deep Sea Research (1953), 6, 140-154.
Jump up ^ United Nations Environment Programme / GRID-Arendal, 2006, [1]. Potential Impact of Climate Change
Jump up ^ "RAPID: monitoring the Atlantic Meridional Overturning Circulation at 26.5N since 2004".
Jump up ^ National Environmental Satellite, Data, and Information Service (2009). Investigating the Gulf Stream Archived 3 May 2010 at the Wayback Machine.. North Carolina State University Retrieved 6 May 2009
Jump up ^ Hennessy (1858). Report of the Annual Meeting: On the Influence of the Gulf-stream on the Climate of Ireland. Richard Taylor and William Francis. Retrieved 6 January 2009.
Jump up ^ "Satellites Record Weakening North Atlantic Current Impact". NASA. Retrieved 10 September 2008.
Jump up ^ The Institute for Environmental Research & Eductation. Tidal.pdf Archived 11 October 2010 at the Wayback Machine. Retrieved on 28 July 2010.
Jump up ^ Jeremy Elton Jacquot. Gulf Stream's Tidal Energy Could Provide Up to a Third of Florida's Power Retrieved 21 September 2008
Jump up ^ Marshall, John; Speer, Kevin (26 February 2012). "Closure of the meridional overturning circulation through Southern Ocean upwelling". Nature Geoscience. 5 (3): 171–180. Bibcode:2012NatGe...5..171M. doi:10.1038/ngeo1391.
Jump up ^ Trenberth, K; Caron, J (2001). "Estimates of Meridional Atmosphere and Ocean Heat Transports". Journal of Climate. 14 (16): 3433–43. Bibcode:2001JCli...14.3433T. doi:10.1175/1520-0442(2001)014<3433:EOMAAO>2.0.CO;2.
Jump up ^ Seager,R., 2006, The Source of Europe's Mild Climate, American Scientist
Jump up ^ Rhines and Hakkinen, 2003, Is the Oceanic Heat Transport in the North Atlantic Irrelevant to the Climate in Europe? Archived 16 July 2007 at the Wayback Machine. ASOF Newsletter
Jump up ^ Broecker, WS (2006). "Was the Younger Dryas Triggered by a Flood?". Science. 312 (5777): 1146–8. doi:10.1126/science.1123253. PMID 16728622.
Jump up ^ Garrison, Tom (2009). Oceanography: An Invitation to Marine Science (7th ed.). Cengage Learning. p. 582. ISBN 9780495391937.
Jump up ^ Bryden, H.L.; H.R. Longworth; S.A. Cunningham (2005). "Slowing of the Atlantic meridional overturning circulation at 25° N". Nature. 438 (7068): 655–657. Bibcode:2005Natur.438..655B. doi:10.1038/nature04385. PMID 16319889.
Jump up ^ Curry, R.; C. Mauritzen (2005). "Dilution of the northern North Atlantic in recent decades". Science. 308: 1772–1774. Bibcode:2005Sci...308.1772C. doi:10.1126/science.1109477. PMID 15961666.
Jump up ^ Jianjun Yin; Stephen Griffies (25 March 2015). "Extreme sea level rise event linked to AMOC downturn". CLIVAR.
Other sources[edit]
Apel, JR (1987). Principles of Ocean Physics. Academic Press. ISBN 0-12-058866-8.
Gnanadesikan, A.; R. D. Slater; P. S. Swathi; G. K. Vallis (2005). "The energetics of ocean heat transport". Journal of Climate. 18 (14): 2604–16. Bibcode:2005JCli...18.2604G. doi:10.1175/JCLI3436.1.
Knauss, JA (1996). Introduction to Physical Oceanography. Prentice Hall. ISBN 0-13-238155-9.
"Potential Impact of Climate Change". United Nations Environment Programme / GRID-Arendal. 2006.
Rahmstorf, S (2006). "Thermohaline Ocean Circulation" (PDF). In Elias, S. A. Encyclopedia of Quaternary Sciences. Elsevier Science. ISBN 0-444-52747-8.
External links[edit]
Ocean Conveyor Belt
THOR FP7 projects http://arquivo.pt/wayback/20141126093524/http%3A//www.eu%2Dthor.eu/ investigates on the topic "Thermohaline overturning- at risk?" and the predictability of changes of the THC. THOR is financed by the 7th Framework Programme of the European Commission.
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Hell's Bells (Buffy the Vampire Slayer)
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"Hell's Bells"
Buffy the Vampire Slayer episode
A heartbroken Anya walks back down the aisle after Xander tells her he believes they shouldn't get married
Episode no.
Season 6
Episode 16
Directed by
David Solomon
Written by
Rebecca Rand Kirshner
Production code
6ABB16
Original air date
March 5, 2002
Guest appearance(s)
Casey Sander as Tony Harris
Kali Rocha as Halfrek
Andy Umberger as D'Hoffryn
Lee Garlington as Jessica Harris
Jan Hoag as Cousin Carol
George D. Wallace as Old Xander Harris
Amber Benson as Tara Maclay
Steven Gilborn as Uncle Rory
James C. Leary as Clem
Daniel McFeeley as Warty Demon
Rebecca Jackson as Tarantula
Mel Fair as Tentacle Demon
Nick Kokich as Demon Teen
Robert Noble as Night Manager
Julian Franco as Young Bartender
Susannah L. Brown as Caterer Girl
Joey Hiott as Josh age 10
Abigail Mavity as Sara age 8
Chris Emerson as Josh age 21
Ashleigh Ann Wood as Sara age 18
Megan Vint as Karen
Episode chronology
← Previous
"As You Were" Next →
"Normal Again"
List of Buffy the Vampire Slayer episodes
"Hell's Bells" is the 16th episode of season 6 of the television series Buffy the Vampire Slayer.
Contents
[hide]
1 Plot synopsis
2 Production details
3 Awards and nominations
4 Series continuity
5 Arc significance
6 Quotes
7 References
8 External links
Plot synopsis[edit]
Buffy and Willow criticize their bright green dresses and talk about the rehearsal dinner from the night before. It was explained that Anya's friends are circus people, which explains their very odd appearances and surprisingly, the Harris family bought it. Willow has the honor of best man and Buffy, Dawn, and Tara are bridesmaids. Anya hugs both girls in excitement about their gowns, which she of course loves.
Xander tries to get dressed with his family and Anya's demon friends invading his apartment. Xander's parents arrive and Mrs. Harris rants about not being in the wedding pictures and Mr. Harris bluntly expresses his distaste for the "circus people" on Anya's side. Xander's cousin Carol asks Xander if Anya's demon friend Krevlin would be interested in dating her. Outside on the streets of Sunnydale, an old man appears out of thin air and walks off, a purpose in mind.
Buffy forces Xander into his cummerbund and works on his bow tie while offering her happy wishes to Xander on his special day. Tara and Willow button Anya into her dress while the bride-to-be rehearses her vows, excitedly talking about how happy she is. Xander's Uncle Rory (formerly an unseen character about whom Xander occasionally related anecdotes) shows off his "date" to Dawn, but his date is actually just one of the caterers. D'Hoffryn arrives along with Halfrek and Dawn greets them at the door. D'Hoffryn offers his very alive wedding gift in a box to Dawn.
Dawn continues to mingle through the crowd and encounters Spike with a goth date. Finally ready, Buffy and Xander proceed toward the crowd of mingling guests while reviewing the tasks necessary to keep Xander's parents out of trouble. Xander greets people and is suddenly assaulted by people complaining about problems. Xander's drunken father offers a toast to the waiting wedding attendees and insults the demons on Anya's side of the "family." Clem and another demon talk about how annoying the man is, but before a fight can break out between Mr. Harris and one of the demons, Buffy pulls the drunk man away.
The old man in a trench coat drags Xander away from the others and explains that he is Xander Harris from the future, and the wedding cannot take place. As proof, he shows the younger Xander a crystal ball that will show Xander his future. In a view of the future, Xander sits watching TV, yelling at Anya while their son runs around teasing their daughter, who is clearly part demon. Anya explains that she's going to make money while he sits around wounded and worthless. An argument breaks out as they talk about how he got injured helping Buffy, although that didn't save her life.
In a later scene, Xander, Anya and their two now-teenage kids eat at a restaurant while the teens fight and Anya bitterly notes Xander's drinking. Years later, in their kitchen, Anya yells at Xander for ruining her life and blames him for her misery. He yells back, raising a frying pan - but the vision of the future abruptly ends as he hurls the pan at her. Xander, shocked by the visions, is warned by the old man not to marry Anya.
Buffy finds Spike alone and the two talk about the wedding and Spike's attempt to make her jealous with his date. After he finds that his efforts worked, Spike realizes it's best to just leave and takes his date away.
Willow runs into Xander in the kitchen and offers the final "best man" talk then leaves him to practice his vows. Anya continues to go over her vows in front of Tara, who advises against using the term "sex poodle". As the music begins, Buffy arrives to get Anya, but Willow pulls her out of the room and breaks the news that Xander is gone. Stalling while Willow looks for Xander, Buffy uses the excuse that the minister is also a doctor and the ceremony will be delayed while he performs an emergency c-section.
Anya tries her vows one more time while elsewhere, Xander walks away in the rain. Mr. Harris and Mrs. Harris head back to the bar, complaining about Anya ruining the wedding. Buffy tries to stall the crowd with charades and juggling as Dawn chats outside with a teenage demon and both compare their embarrassing family and friends. Impatient, Anya heads out towards the wedding crowd, determined to get on with the wedding. The news that Xander is gone is accidentally spilled to Anya as Dawn talks to the demon teen and Anya freaks out.
Mr. Harris and other demons begin to argue and then a huge fight breaks out between the two sides of the wedding guests. Tara gets caught up in the battle, but Willow rescues her. Cousin Carol directs Anya to the man in the trench coat and Anya talks to him about what he did to scare Xander off. She finds that he is really a man whom she transformed into a demon many years ago, who now seeks revenge against her. He showed Xander false images about his future to ruin Anya's wedding.
She begins to cry and the demon strikes out at her, prompting Buffy to get involved. Buffy starts to attack the demon and Xander arrives to help save Anya. Anya explains to Xander that he saw only lies in those visions, but Xander isn't exactly relieved. Buffy and Xander finish the demon off and the crowd cheers. Fighting continues, but Anya breaks it up and everyone returns to his or her seats. Xander and Anya talk privately before the ceremony and Xander explains that he can't marry her. Motivated by the fear of turning into an abusive drunk like his father, Xander refuses to allow himself the opportunity to marry Anya and ever hurt her in that way. Anya tries to convince him otherwise, but fails, and sadly, the two part ways.
Buffy, Willow and Dawn sit around at the house and talk about what happened with Xander and Anya. They feel sorry for both of them and wonder where Xander is. Meanwhile, Xander checks into a motel by himself. Devastated, Anya sits alone until D'Hoffryn offers her sympathy, comfort, and her job as a vengeance demon back.
Production details[edit]
Rebecca Rand Kirshner reveals in her DVD commentary of this episode that Anya's reference to herself as a "sex poodle" was a reference to colleague Jane Espenson who had been using the name herself.[1]
Amber Benson said in an interview to Buffy magazine that "I'm the biggest klutz on the set. Remember Emma in that wedding dress? I got that dress off of her at least four times, because I'd be following her and I'd step on the dress and she'd be pulling it back up. But she looked gorgeous in that dress."[2]
Awards and nominations[edit]
This episode was nominated for three Emmy Awards, the most for a year in the show's history:
Outstanding Hairstyling for a Series
Outstanding Makeup Series (Prosthetic)
Outstanding Makeup for a Series (Non-prosthetic)
Series continuity[edit]
Despite being mentioned and heard many times through the series this is the first time that Xander's parents are actually shown (though a silhouette of Xander's father portrayed by a different actor appears in "Restless").
After Willow and Xander see each other in formal wear she jokes that it's a good thing she realized she was gay, referring to a previous occasion ("Homecoming") where she and Xander saw each other in formals and shared an illicit kiss.
Xander's uncle Rory scares a woman at the wedding by informing her that a moose head on the wall is stuffed incorrectly. This is a reference to the Season 2 episode "The Dark Age" in which Xander mentions that Rory is a taxidermist.
As seen in the closing credits, the names of Xander and Anya's children in the vision, are Josh and Sara. They are identified to be 10 and 8 in the first scene, and 21 and 18 in the second.
Although Buffy and Willow express surprise that Xander's family believed the claim that Anya's demon friends are merely circus performers, Sunnydale residents, such as the Harrises, have a long history of ignoring or explaining away supernatural occurrences, as Willow's mother did after "Gingerbread". As recently as the preceding episode, Riley Finn and his wife convinced Sunnydale residents that a clearly visible demon, mere yards away from some of them, was actually a rogue bear.
Arc significance[edit]
Xander jilts Anya at the altar, which has major implications for Anya's character arc. At the end of this episode, D'Hoffryn suggests that she should reprise her role as a vengeance demon.
Quotes[edit]
The Watcher's Guide 3 reveals cut dialogue which explains Giles' absence:
Dawn: "I thought Xander and Anya couldn't afford flowers."
Willow: "Giles sent 'em. Aren't they gorgeous?"
Dawn: "Yeah. I wish Giles was here."
Willow: "Me too. And I'm sure he'd much rather be here than fighting that nasty demon—"
References[edit]
Jump up ^ Writer Rebecca Rand Kirshner's commentary for the episode on the season 6 DVD.
Jump up ^ http://www.buffy.geek.nz/thearchive/amber8.html
External links[edit]
Wikiquote has quotations related to: Hell's Bells
"Hell's Bells" on IMDb
"Hell's Bells" at TV.com
[hide]
v
t
e
Buffy the Vampire Slayer episodes
Season 1
"Welcome to the Hellmouth"
"The Harvest"
"Witch"
"Teacher's Pet"
"Never Kill a Boy on the First Date"
"The Pack"
"Angel"
"I, Robot... You, Jane"
"The Puppet Show"
"Nightmares"
"Out of Mind, Out of Sight"
"Prophecy Girl"
Season 2
"When She Was Bad"
"Some Assembly Required"
"School Hard"
"Inca Mummy Girl"
"Reptile Boy"
"Halloween"
"Lie to Me"
"The Dark Age"
"What's My Line, Parts One and Two"
"Ted"
"Bad Eggs"
"Surprise"
"Innocence"
"Phases"
"Bewitched, Bothered and Bewildered"
"Passion"
"Killed by Death"
"I Only Have Eyes for You"
"Go Fish"
"Becoming, Parts One and Two"
Season 3
"Anne"
"Dead Man's Party"
"Faith, Hope & Trick"
"Beauty and the Beasts"
"Homecoming"
"Band Candy"
"Revelations"
"Lovers Walk"
"The Wish"
"Amends"
"Gingerbread"
"Helpless"
"The Zeppo"
"Bad Girls"
"Consequences"
"Doppelgangland"
"Enemies"
"Earshot"
"Choices"
"The Prom"
"Graduation Day, Parts One and Two"
Season 4
"The Freshman"
"Living Conditions"
"The Harsh Light of Day"
"Fear, Itself"
"Beer Bad"
"Wild at Heart"
"The Initiative"
"Pangs"
"Something Blue"
"Hush"
"Doomed"
"A New Man"
"The I in Team"
"Goodbye Iowa"
"This Year's Girl"
"Who Are You"
"Superstar"
"Where the Wild Things Are"
"New Moon Rising"
"The Yoko Factor"
"Primeval"
"Restless"
Season 5
"Buffy vs. Dracula"
"Real Me"
"The Replacement"
"Out of My Mind"
"No Place Like Home"
"Family"
"Fool for Love"
"Shadow"
"Listening to Fear"
"Into the Woods"
"Triangle"
"Checkpoint"
"Blood Ties"
"Crush"
"I Was Made to Love You"
"The Body"
"Forever"
"Intervention"
"Tough Love"
"Spiral"
"The Weight of the World"
"The Gift"
Season 6
"Bargaining, Parts One and Two"
"After Life"
"Flooded"
"Life Serial"
"All the Way"
"Once More, with Feeling"
"Tabula Rasa"
"Smashed"
"Wrecked"
"Gone"
"Doublemeat Palace"
"Dead Things"
"Older and Far Away"
"As You Were"
"Hell's Bells"
"Normal Again"
"Entropy"
"Seeing Red"
"Villains"
"Two to Go"
"Grave"
Season 7
"Lessons"
"Beneath You"
"Same Time, Same Place"
"Help"
"Selfless"
"Him"
"Conversations with Dead People"
"Sleeper"
"Never Leave Me"
"Bring On the Night"
"Showtime"
"Potential"
"The Killer in Me"
"First Date"
"Get It Done"
"Storyteller"
"Lies My Parents Told Me"
"Dirty Girls"
"Empty Places"
"Touched"
"End of Days"
"Chosen"
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