Contourite

A contourite is a sedimentary deposit commonly formed on continental rise to lower slope settings, although they may occur anywhere that is below storm wave base. Countourites are produced by thermohaline-induced deepwater bottom currents and may be influenced by wind or tidal forces.[1][2] The geomorphology of contourite deposits is mainly influenced by the deepwater bottom-current velocity, sediment supply, and seafloor topography.[3]

Definition

The definition of the term contourite has varied throughout the decades. Originally, Heezen et al. (1966)[4] defined the concept, without using the actual word, as a sedimentary deposit on the continental rise derived from thermohaline-induced geostrophic bottom-currents that flow parallel to bathymetric contours. They did this to emphasize the difference between these deposits and turbidites in order to explain the ubiquitous smoothness and lack of irregularities of the continental rise in the Blake-Bahama Basin. Before this, it was thought that only turbidity flows were capable of depositing and reworking sediment at depths greater than the continental slope.[1] Hollister and Heezen (1972)[5] adopted the name contourite for these deposits and provided a list of characteristics that described their sediments. Faugères and Stow (1993)[6] note that as research on the subject developed, the term contourite was used to describe various forms of sedimentary deposits from bottom-currents including those at much shallower depths and even in lacustrine settings. They suggested going back to the original definition of a contourite, that is for deposits at depths greater than 500 m derived from stable thermohaline-induced geostrophic bottom-currents (i.e. deepwater bottom-currents), in order to avoid using the same name when describing sedimentary deposits formed by different processes. They also suggest the umbrella term bottom-current deposit, which includes contourites and deposits generated by other bottom-currents.

Flow conditions

Contourite bottom water flow IODP Gulf of Cadiz
Bottom current flow in the Gulf of Cadiz[7]

Thermohaline circulation is the principal driving force of deepwater bottom-currents. The term refers to the movement of water over large distances as a consequence of global oceanic density gradients. This circulation commonly travels at velocities between 2 – 20 cm/s.[4] Note that at this velocity range, considering the general shape of the Shields diagram[8][9] still holds in these conditions, a flow will only be able to continue transporting finer sediment that is already in suspension but will not be able to erode the same sized sediment once it is deposited. However, flow velocity may be intensified as a consequence of the Coriolis force driving currents west against continental margins or as current squeezes between two ridges.[3]

Periodically, velocities may increase dramatically or even reverse due to atmospheric storms raising the local surface eddy kinetic energy, which gets partially transmitted down to abyssal depths in episodes called benthic storms.[10] These velocities may reach magnitudes well above 40 cm/s and vary significantly depending on the specific location. At the lower continental rise, south of Halifax, Nova Scotia,[10] and at the lower slope around the Faeroe Islands[11] these velocities may reach up to 73 cm/s and 75 cm/s, respectively. Bottom-current flow velocities have been measured as high as 300 cm/s in the Strait of Gibraltar.[12][13] These benthic storms occur only 5 to 10 times per year and usually last between 3 and 5 days,[1] but that is enough to heavily erode benthic sediment and keep the finer grains in suspension even after flow velocities return to normal and the bedload has been deposited.[3][10] During benthic storms, the eroded sediment may be transported over thousands of kilometers and deposited rather quickly (i.e. ~1.5 cm/month) once the storm wanes. However, the net sedimentation rate over thousands of years may be much smaller (i.e. ~5.5 cm/year) due to the intense periods of erosion during benthic storms.[6]

Sediment supply

Contourite bedform diagram Stow2009
Bedform phase diagram for contourites (Stow et al. 2009)[14]

Erosion of the seafloor contributes to the growth of a deepwater nepheloid layer. This layer plays a key role in supplying the sediment for the deposition of contourites under appropriate flow conditions.[3]

Terrigenous sediment supply to the deepwater bottom-currents and to the nepheloid layer primarily depends on climate and tectonics in the continental environment.[3] The rate of tectonic uplift is directly related to the amount of sediment available and variations in sea level will determine the ease with which this sediment is transported basinward. The sediment will most likely reach deepwater in the form of turbidity flows, which travel across bathymetric contours, only to be “blown” parallel to these contours as the finer sediments cross a deepwater bottom-current.[1] Other sources of terrigenous sediment may include airborne and seaborne volcanoclastic debris.[3]

Biogenic deposition from suspension may also supply sediment to these deepwater bottom-currents. The deposition of this material has strong implications for the biology, chemistry and flow conditions at the time. It must occur in areas of high biogenic productivity, during periods of relatively quiet flow and, if calcareous, must also occur at depths above the carbonate compensation depth.[3][6] There is also a contribution to the concentration of suspended sediment by the burrowing activity of benthic organisms.[6]

Geomorphology

The accumulation and geomorphology of contourite deposits are mainly influenced by three factors: intensity of deepwater bottom-currents, seafloor topography, and sediment supply.[3] There are five main types of contourite accumulations: giant elongate drifts, contourite sheets, channel related drifts, confined drifts and modified drift-turbidite systems.[3][15]

Giant elongate drifts

Contourite sparker seismic elongate drift
Sparker seismic line showing elongate drifts in the Gulf of Cadiz [7]

Giant elongate drifts form very large mounded elongated geometries parallel to the deepwater bottom-current flow. They are characterized by a near complete lack of parallel bedding. Mounded drifts are often bounded on one or both sides by non-depositional or erosional channels, sometimes known as moats.[2] These drifts can be “tens to hundreds of kilometers long, tens of kilometers wide, and range from 0.1 to more than 1 km in relief above the surrounding seafloor”.[3] Their length to width ratio ranges from 2:1 to 10:1.[15] They can accumulate to thicknesses greater than 2 km and can form anywhere from the upper slope to the deepest parts of the basin depending on the specific location of the bottom-current.[3][15] Sedimentation rates range from 20 – 100 m/Ma. They tend to be finer-grained with a lot of mud, silt and biogenic material. Coarse-grained contourites are very rare.[3] They may also form detached or separated versions due to seafloor topography and flow conditions.[15] Detached drifts are isolated and migrate downslope while separated drifts typically are asymmetric in shape, tend to form at the base of a slope and migrate up-slope.[2] Large sediment waves have been observed partially covering some giant elongate drifts.[3]

Contourite sheets

Contourite sheets seismic gulf of cadiz
Contourite sheets shown in reflection seismic data off the coast of Portugal[7]

Contourite sheets are broad, low-relief features that extend through very large areas (i.e. ~1,000,000 km2) and are seen covering the abyssal plains or even plastered against the continental margins.[3] They are characteristic of very deep water.[2] They have a relatively constant thickness of up to a few hundred meters with a slight thinning towards the continental margin.[15]

Sediment wave fields are a variety that is generally located near the rise to slope transition. Seismic reflection profiles show that the sediment waves tend to migrate up-slope.[16]

Channel-related drifts

Channel-related drifts form when deepwater bottom-currents are confined to a smaller cross sectional area of flow and therefore their velocity increases substantially. This can happen if the deepwater bottom-current is trapped within a deep channel or within a gateway that connects two basins. Due to the high velocities, it is common to see scours and erosional features as well as different types of deposits at the floor of the channel, the flanks, and the down-current exit of the channel.[3][15]

Flank deposits are usually patchy and small (tens of km2), can be elongate and subparallel to flow direction and may have a sheeted or mounded geometry. At the down-current exit of the channel, flow velocity decreases dramatically and a cone-shaped contourite fan is formed which is much larger than the flank deposits, measuring about 100 km in radius and about 300 m in thickness. Channel floor deposits can be patchy and contain sand, gravel and mud clasts in the form of a channel lag.[15]

Confined drifts

Confined drifts are contourite accumulations that occur within small basins. The basins in which they form tend to be tectonically active in order to allow for topographic confinement of the deposit.[15]

Modified drift-turbidite systems

Modified drift-turbidite systems refer to the interactions of contourite and turbidite deposits. These can be observed as modifications of one another depending on the dominant process at the time. Examples range from asymmetric turbidite channel levees caused by strong deepwater bottom-currents as seen in the Nova Scotian Margin, to alternations in turbidite/debrite and contourite deposits both in time and space as seen in the Hebridean Margin.[15] The Caledonia and Judith Fancy formations in St. Croix were studied by Stanley (1993)[17] in which he found an ancient analog of an alternating turbidite and contourite deposit and generated a stratigraphic model of a continuum from a turbidite dominant environment to a contourite dominant one.

Distinguishing turbidites, contourites, and bottom-current modified turbidite deposits is essential for reconstructing the paleoenvironment in deepwater settings. Traction structures, such as cross-stratification, indicate bottom-current reworking because it is more likely to have avalanching in clear bottom-currents than it is in sediment saturated turbidity flows.[18] Deposition from suspension in turbidity flows do not generate a sharp upper contact as bottom-current reworked deposits show due to the highly oscillating energy conditions. Stanley (1993)[17] proposes that the transition from a turbidite to a contourite involves a continuous transition from a sandy deposit to lenticular bedding passing through wavy bedding.

Occurrence

Present day

Contourite deposition is active in many locations throughout the world, but particularly in areas affected by the thermohaline circulation.

Ancient examples

Identifying contourites in ancient sedimentary sequences is difficult as their distinctive morphology becomes obscured by the effects of later bioturbation, sedimentation, erosion and compaction. Most examples of contourites identified in the geological record come from the Cenozoic but examples have been noted from as far back as the Ediacaran.[19]

See also

References

  1. ^ a b c d Hollister, C.D. (1993). "The concept of deep-sea contourites". Sedimentary Geology. 82: 5–11. Bibcode:1993SedG...82....5H. doi:10.1016/0037-0738(93)90109-I.
  2. ^ a b c d Rebesco, M. & Camerlenghi, A. 2008. Contourites, Elsevier Science, 688pp. ISBN 978-0-444-52998-5
  3. ^ a b c d e f g h i j k l m n o Faugères, J.-C.; Mézerais, M.L.; Stow, D.A.V (1993). "Contourite drift types and their distribution in the North and South Atlantic Ocean basins". Sedimentary Geology. 8: 189–203. Bibcode:1993SedG...82..189F. doi:10.1016/0037-0738(93)90121-k.
  4. ^ a b Heezen, B.C.; Hollister, C.D.; Ruddiman, W.F. (1966). "Shaping of the continental rise by deep geostrophic contour currents". Science. 152: 502–508. Bibcode:1966Sci...152..502H. doi:10.1126/science.152.3721.502. PMID 17815077.
  5. ^ Hollister, C.D.; Heezen, B.C. (1972). "Geologic effects of ocean bottom-currents: western north Atlantic". In: Studies in Physical Oceanography. 2: 37–66.
  6. ^ a b c d Faugères, J.-C.; Stow, D.A.V (1993). "Bottom-current-controlled sedimentation: a synthesis of the contourite problem". Sedimentary Geology. 82: 287–297. Bibcode:1993SedG...82..287F. doi:10.1016/0037-0738(93)90127-Q.
  7. ^ a b c IODP Expedition 339 Scientists (2012). "Mediterranean outflow: environmental significance of the Mediterranean Outflow Water and its global implications". IODP Prel. Rept. 339. doi:10.2204/iodp.pr.339.2012.
  8. ^ Sam Boggs Jr. (2006). "Ch. 2: Transport and Deposition of Siliciclastic Sediment". Principles of Sedimentology and Stratigraphy. Prentice Hall. pp. 30–31. ISBN 0-13-154728-3.
  9. ^ Miller, M.C.; McCave, I.N.; Komar, P.D. (1977). "Threshold of sediment motion under unidirectional currents". Sedimentology. 24: 507–527. Bibcode:1977Sedim..24..507M. doi:10.1111/j.1365-3091.1977.tb00136.x.
  10. ^ a b c Hollister, C.D.; McCave, I.N. (1984). "Sedimentation under deep-sea storms". Nature. 309 (5965): 220–225. Bibcode:1984Natur.309..220H. doi:10.1038/309220a0.
  11. ^ Damuth, J.E.; Olson, H.C. (2001). "Neogene-Quaternary contourite and related deposition on the West Shetland Slope and Faeroe-Shetland Channel revealed by high-resolution seismic studies". Marine Geophysical Researches. 22: 369–399.
  12. ^ G. Shanmugam (2006). "Ch. 4: Deep-water bottom currents". Deepwater Processes and Facies Models: Implications for Sandstone Petroleum Reservoirs. Elsevier Science. pp. 85–139. ISBN 0-444-52161-5.
  13. ^ Gonthier, E.G.; Faugères, J.-C. (1984). "Contourite facies of the Faro Drift, Gulf of Cadiz". In: "Fine-Grained Sediments: Deep-Water Processes and Facies", Geological Society of London Special Publication. 15: 275–292. Bibcode:1984GSLSP..15..275G. doi:10.1144/gsl.sp.1984.015.01.18.
  14. ^ Stow, D.A.V.; Hernandez-Molina, F.J.; Llave, E.; Sayago-Gil, M.; Diaz del Rio, V.; Branson, A. (2009). "Bedform-velocity matrix: The estimation of bottom current velocity from bedform observations". Geology. 37: 327–330. Bibcode:2009Geo....37..327S. doi:10.1130/g25259a.1.
  15. ^ a b c d e f g h i Stow, D.A.V.; Faugères, J.-C.; Pudsey, C.J.; Viana, A.R. (2002). "Bottom currents, contourites and deep-sea sediment drifts: current state-of-the-art". In: "Deep-Water Contourite Systems: Modern Drifts and Ancient Series, Seismic and Sedimentary Characteristics", Geological Society of London, Memoirs. 22: 7–20. doi:10.1144/gsl.mem.2002.022.01.02.
  16. ^ Damuth, J.E.; Olson, H.C. (2001). "Neogene-Quaternary contourite and related deposition on the West Shetland Slope and Faeroe-Shetland Channel revealed by high-resolution seismic studies". Marine Geophysical Researches. 22: 363–398. Bibcode:2001MarGR..22..369D. doi:10.1023/A:1016395515456.
  17. ^ a b Stanley, D.J. (1993). "Model for turbidite-to-contourite continuum and multiple process transport in deep marine settings: examples in the rock record". Sedimentary Geology. 82: 241–255. Bibcode:1993SedG...82..241S. doi:10.1016/0037-0738(93)90124-N.
  18. ^ Shanmugam, G. (1993). "Traction structures in deep-marine, bottom-cuurent reworked sands in the Pliocene and Pleistocene, Gulf of Mexico". Geology. 21: 929–932. Bibcode:1993Geo....21..929S. doi:10.1130/0091-7613(1993)021<0929:TSIDMB>2.3.CO;2.
  19. ^ Dalrymple, R.W.; Narbonne, G.M. (1996). "Continental slope sedimentation in the Sheepbed Formation (Neoproterozoic, Windermere Supergroup), Mackenzie Mountains, N.W.T.". Canadian Journal of Earth Sciences. 33: 848–862. Bibcode:1996CaJES..33..848D. doi:10.1139/e96-064.
Anna Wåhlin

Anna Wåhlin is a Swedish researcher on the Antarctic and the polar seas. She is a professor of physical oceanography at the University of Gothenburg and co-chair of the Southern Ocean Observing System (SOOS).

Azores–Gibraltar Transform Fault

The Azores–Gibraltar Transform Fault (AGFZ), also called a fault zone and a fracture zone, is a major seismic fault in the Central Atlantic Ocean west of the Strait of Gibraltar. It is the product of the complex interaction between the African, Eurasian, and Iberian plates.

The AGFZ produced the large-magnitude 1755 Lisbon and 1969 Horseshoe earthquakes and, consequently, a number of large tsunamis.

Bahama Banks

The Bahama Banks are the submerged carbonate platforms that make up much of the Bahama Archipelago. The term is usually applied in referring to either the Great Bahama Bank around Andros Island, or the Little Bahama Bank of Grand Bahama Island and Great Abaco, which are the largest of the platforms, and the Cay Sal Bank north of Cuba. The islands of these banks are politically part of the Bahamas. Other banks are the three banks of the Turks and Caicos Islands, namely the Caicos Bank of the Caicos Islands, the bank of the Turks Islands, and wholly submerged Mouchoir Bank. Further southeast are the equally wholly submerged Silver Bank and Navidad Bank north of the Dominican Republic.

Canadian Arctic Rift System

The Canadian Arctic Rift System is a major North American geological structure extending from the Labrador Sea in the southeast through Davis Strait, Baffin Bay and the Arctic Archipelago in the northwest. It consists of a series of interconnected rifts that formed during the Paleozoic, Mesozoic and Cenozoic eras. Extensional stresses along the entire length of the rift system have resulted in a variety of tectonic features, including grabens, half-grabens, basins and faults.

Development of the Canadian Arctic Rift System was accompanied by two plate tectonic episodes that originated on opposite sides of the North American Plate and were propagated toward each other. Both were strongly controlled by pre-existing structures, which either guided the propagating faults or impeded their growth. The rift system is now inactive except for minor adjustments that are indicated by occasional earthquakes in Baffin Bay and the Labrador Sea.

Carbonate platform

A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms (usually microbes) which induce carbonate precipitation through their metabolism. Therefore, carbonate platforms can not grow up everywhere: they are not present in places where limiting factors to the life of reef-building organisms exist. Such limiting factors are, among others: light, water temperature, transparency and pH-Value. For example, carbonate sedimentation along the Atlantic South American coasts takes place everywhere but at the mouth of the Amazon River, because of the intense turbidity of the water there. Spectacular examples of present-day carbonate platforms are the Bahama Banks under which the platform is roughly 8 km thick, the Yucatan Peninsula which is up to 2 km thick, the Florida platform, the platform on which the Great Barrier Reef is growing, and the Maldive atolls. All these carbonate platforms and their associated reefs are confined to tropical latitudes. Today's reefs are built mainly by scleractinian corals, but in the distant past other organisms, like archaeocyatha (during the Cambrian) or extinct cnidaria (tabulata and rugosa) were important reef builders.

List of submarine volcanoes

A list of active and extinct submarine volcanoes and seamounts located under the world's oceans. There are estimated to be 40,000 to 55,000 seamounts in the global oceans. Almost all are not well-mapped and many may not have been identified at all. Most are unnamed and unexplored. This list is therefore confined to seamounts that are notable enough to have been named and/or explored.

Location hypotheses of Atlantis

Location hypotheses of Atlantis are various proposed real-world settings for the fictional island of Atlantis, described as a lost civilization mentioned in Plato's dialogues Timaeus and Critias, written about 360 B.C. In these dialogues, a character named Critias claims that an island called Atlantis was swallowed by the sea about 9,200 years previously. According to the dialogues, this story was passed down to him through his grandfather, also named Critias, who in turn got it from his father, Dropides, who had got it from Solon, the famous Athenian lawmaker, who had got the story from an Egyptian sanctuary. Plato's dialogues locate the island in the Atlantic Pelagos "Atlantic Sea", "in front of" the Pillars of Hercules (Στήλες του Ηρακλή) and facing a district called modern Gades or Gadira (Gadiron), a location that some modern Atlantis researchers associate with modern Gibraltar; however various locations have been proposed.

Oceanic plateau

An oceanic or submarine plateau is a large, relatively flat elevation that is higher than the surrounding relief with one or more relatively steep sides.There are 184 oceanic plateaus covering an area of 18,486,600 km2 (7,137,700 sq mi), or about 5.11% of the oceans. The South Pacific region around Australia and New Zealand contains the greatest number of oceanic plateaus (see map).

Oceanic plateaus produced by large igneous provinces are often associated with hotspots, mantle plumes, and volcanic islands — such as Iceland, Hawaii, Cape Verde, and Kerguelen. The three largest plateaus, the Caribbean, Ontong Java, and Mid-Pacific Mountains, are located on thermal swells. Other oceanic plateaus, however, are made of rifted continental crust, for example Falkland Plateau, Lord Howe Rise, and parts of Kerguelen, Seychelles, and Arctic ridges.

Plateaus formed by large igneous provinces were formed by the equivalent of continental flood basalts such as the Deccan Traps in India and the Snake River Plain in the United States.

In contrast to continental flood basalts, most igneous oceanic plateaus erupt through young and thin (6–7 km (3.7–4.3 mi)) mafic or ultra-mafic crust and are therefore uncontaminated by felsic crust and representative for their mantle sources.

These plateaus often rise 2–3 km (1.2–1.9 mi) above the surrounding ocean floor and are more buoyant than oceanic crust. They therefore tend to withstand subduction, more-so when thick and when reaching subduction zones shortly after their formations. As a consequence, they tend to "dock" to continental margins and be preserved as accreted terranes. Such terranes are often better preserved than the exposed parts of continental flood basalts and are therefore a better record of large-scale volcanic eruptions throughout Earth's history. This "docking" also means that oceanic plateaus are important contributors to the growth of continental crust. Their formations often had a dramatic impact on global climate, such as the most recent plateaus formed, the three, large, Cretaceous oceanic plateaus in the Pacific and Indian Ocean: Ontong Java, Kerguelen, and Caribbean.

Outline of oceanography

The following outline is provided as an overview of and introduction to Oceanography.

Physical oceanography

Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.

Physical oceanography is one of several sub-domains into which oceanography is divided. Others include biological, chemical and geological oceanography.

Physical oceanography may be subdivided into descriptive and dynamical physical oceanography.Descriptive physical oceanography seeks to research the ocean through observations and complex numerical models, which describe the fluid motions as precisely as possible.

Dynamical physical oceanography focuses primarily upon the processes that govern the motion of fluids with emphasis upon theoretical research and numerical models. These are part of the large field of Geophysical Fluid Dynamics (GFD) that is shared together with meteorology. GFD is a sub field of Fluid dynamics describing flows occurring on spatial and temporal scales that are greatly influenced by the Coriolis force.

Southeast Indian Ridge

The Southeast Indian Ridge (SEIR) is a mid-ocean ridge in the southern Indian Ocean. A divergent tectonic plate boundary stretching almost 6,000 km (3,700 mi) between the Rodrigues Triple Junction (25°S 70°E) in the Indian Ocean and the Macquarie Triple Junction (63°S 165°E) in the Pacific Ocean, the SEIR forms the plate boundary between the Australian and Antarctic plates since the Oligocene (anomaly 13).The SEIR is the spreading centre closest to the Kerguelen and Amsterdam–Saint-Paul hotspot.

The SEIR has an intermediate full spreading rate of 65 mm/yr, and, because Antarctica is virtually stationary, this results in a northward ridge migration of half that rate.

Spreading rates along the SEIR varies from 69 mm/yr near 88°E to 75 mm/yr near 120°E.

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. 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) upwell in the North Pacific. Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system. The water in these circuits transport both energy (in the form of heat) and mass (dissolved solids 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. Moreover, temperature and salinity gradients can also lead to circulation effects that are not included in the MOC itself.

Turbidite

A turbidite is the geologic deposit of a turbidity current, which is a type of sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean.

Undersea mountain range

Undersea mountain ranges are mountain ranges that are mostly or entirely underwater, and specifically under the surface of an ocean. If originated from current tectonic forces, they are often referred to as a mid-ocean ridge. In contrast, if formed by past above-water volcanism, they are known as a seamount chain. The largest and best known undersea mountain range is a mid-ocean ridge, the Mid-Atlantic Ridge. It has been observed that, "similar to those on land, the undersea mountain ranges are the loci of frequent volcanic and earthquake activity".

Wave base

The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. For water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion above.

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