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Continental shelves/North east American

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This map is a bathymetric or hydrographic map of the North Atlantic ocean floor as it exists today. This map is constructed from U.S. Navy data. The floor of the North Atlantic is elevated along the Mid Atlantic Rift from Iceland to well South of the Azores in the southern Atlantic. The Azores Plateau and the area surrounding it are shown. This is a under water depth map, and it is color coded by depth, brown is approximately 200 m, which would have been near to or above sea level during the last ice age.

This is a bathymetric or hydrographic map of the North Atlantic ocean floor. Credit: U.S. Navy.{{free media}}

Greenland shelves

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Greenland bedrock is shown at current elevation above sea level. Credit: Skew-t.{{free media}}

In 2007 the existence of a new island was announced. Named "Uunartoq Qeqertaq" (English: Warming Island), this island has always been present off the coast of Greenland, but was covered by a glacier discovered in 2002 to be shrinking rapidly, and by 2007 had completely melted away, leaving the exposed island.[1] The island was named Place of the Year by the Oxford Atlas of the World in 2007.[2] Ben Keene, the atlas's editor, commented: "In the last two or three decades, global warming has reduced the size of glaciers throughout the Arctic and earlier this year, news sources confirmed what climate scientists already knew: water, not rock, lay beneath this ice bridge on the east coast of Greenland. More islets are likely to appear as the sheet of frozen water covering the world's largest island continues to melt".[3]

Some controversy surrounds the history of the island, specifically over whether the island might have been revealed during a brief warm period in Greenland during the mid-20th century.[4]

Labrador shelves

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File:Map-labrador-sea.jpg
The Labrador Sea between Greenland, Labrador, and Qikiqtaaluk, especially the continental shelf at about 100 m depth. Credit: University of Goettingen, Germany.{{fairuse}}

Through the Mesozoic and into the Cenozoic, the landscape eroded, shedding sand into the Labrador Shelf, mainly from older metasediments and metavolcanic rocks that reached amphibolite grade on the sequence of metamorphic facies.[5] Close to Makkovik, breccia from the Mesozoic, cut by lamprophyre-carbonatite dikes marks the opening of the Labrador Sea.[6]

The region experienced repeat glaciations during the Pleistocene. Glaciomarine silts and mud records the melting of the glaciers on the Labrador Shelf, with significant deposition around 20,000 years ago. Analysis suggests most of these sediments originated on land in Labrador, although large amounts of limestone (which is completely absent in the Labrador) indicate a second source to the north, likely Paleozoic limestone on the Hudson Strait and Ungava Bay.[7]

Massive iron deposits are found in the Labrador Trough along with copper, uranium and molybdenum. Iron forms in chert from the Ungava Bay to the Grenville Front, over a span of 700 miles, while copper and nickel minerals such as pyrite, pyrrhotite, sphalerite and galena form dispersed deposits or massive bodies in Kaniapiskau Supergroup rocks. The Aillik Group hosts uranium as uraninite and pitchblende dispersed in veins in pegmatite, argillite, granulite and quartzite.[8]

Scotian shelves

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Bathymetry is near the Gulf of Saint Lawrence, including the Scotian Shelf. Credit: NGDC, NOAA.{{free media}}

The Scotian Shelf is a geological formation, part of the Continental shelf, located southwest of Nova Scotia, Canada. It covers an area of 120,000 km²,[9] is 700 km long and ranges in width from 120 to 240 km. It has an average depth of 90 metres.[10] The Scotian Shelf contains the ecologically important Scotian Shelf Large Marine Ecosystem (LME) and the Scotian Shelf Waters (SSW).[11][12][13]

The northeastern boundary is defined by the Laurentian Channel, where it drops off to 400 m. Further south is the continental slope, which sharply drops off to a depth of more than 3,000 m.[9] The southwestern boundary ends at the Northeast Channel, including the Gulf of Maine.[10]

The Scotian Shelf is characterized by shallow, offshore banks 25 m to 100 m under the ocean surface, with deep basins and troughs between that vary in depth from 160 m to 300 m.[9][10] These culminate at Sable Island.[9]

A southwesterly ocean current, (occasionally containing runoff from the Gulf of St Lawrence) flows over the inner shelf. The water flow over the banks is weaker and tends have greater variation.[10] The Scotian Shelf contains a canyon called the "Gully", which is more than 1000 m deep. Currents flow through this canyon southward, mixing offshore waters with the Nova Scotia Current. This causes an increase in biological productivity toward the east, across the Continental Shelf.

The Scotian Shelf is heavily influenced by the Gulf Stream, resulting in a variety of marine species being present which are normally found further south. These appear at regular intervals due to the main current spinning off cores of warm water.[9]

The right whale has a critical habitat in the Roseway Basin, the northeastern part of the Scotian Shelf. Approximately 30 percent of the known population uses this habitat throughout the course of the year.[9]

The northern bottlenose whale also lives in the Scotian Shelf Waters area, in particular, the Gully, with about 230 individual specimens recorded there; the sperm whale and harbour seal, are also found in this region, including the grey seal, which is common on Sable Island.[9]

Eastern United States shelves

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File:M2I22 Shoreline.png
This image shows the hypothetical sea level of about 460 m (1500 ft) in light gray for the Gulf of Mexico. Credit: David Bice.
 
Names show the Seewarte seamounts. Credit: NOAA.{{free media}}

At 37 degrees North the Atlantis seamount located on the Mid-Atlantic Ridge is flat topped at a depth of around 180 fathoms and has a current-rippled sand and or cobbles. Around a ton of limestone cobbles were brought up from the summit a sample of which gave a radiocarbon date of 12,000 ± 900 years. The limestone was lithified in a location above the water and the seamount had once been an island but was submerged in the last 12,000 years.[14]

The top of the Atlantis seamount was around 180 fathoms deep now but was above sea level during the last ice age. 180 fathoms x 1.8288 m/fathom = 330 m. That's one datum for how much sea level has risen after the ice melted. Above the yellow band on the bathymetric maps is the top 1,000 m of the orange-brown band. A depth of 330 m would put a good portion of the Eastern United States continental shelves near or above sea level during the last ice age.

The image on the right shows the 120 m black line for sea level about the last glacial maximum (20,000 years ago). The light gray area corresponds to the 1500 ft or 460 m depth that may have been the sea level for the glacial maximum during the last 50 kyrs.

Georges Bank shelves

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Map of the Gulf of Maine; Georges Bank is the light blue region in the bottom center of the image. Credit: National Oceanographic and Atmospheric Administration and U.S. Geological Survey (USGS) Woods Hole, MA.{{free media}}

Georges Bank is the most westward of the great Atlantic fishing banks. The now-submerged portions of the North American mainland now comprise the continental shelf running from the Grand Banks of Newfoundland to Georges. Georges Bank was part of the North American mainland as recently as 12,000 years ago.[15]

New England shelves

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File:Location of major seamounts off the New England shelf.jpg
Location of major seamounts are off the New England shelf. Credit: NEFSC, NOAA.{{fairuse}}

"Although our major focus is on the continental shelf and adjacent slope regions, it is worth noting the importance of a major seamount chain located off the New England shelf. Seamounts are focal points of biological activity in the deep sea. They are the subject of considerable recent conservation interest because of the unique fauna of these systems. The profile, topography, and the associated complex currents associated with seamounts make them unique habitats, but ones that are difficult to sample and study. Only a small fraction of the estimated 30,000 Pacific and 800+ Atlantic seamounts have been extensively studied. Fundamental questions regarding the diversity of organisms, their abundance, how they colonized the seamounts, and how the seamount community is structured and functions remain unanswered."[16]

"The New England Seamounts (NES; [image on the right]) comprise the longest seamount chain in the North Atlantic, encompassing more than 30 major volcanic peaks extending from Georges Bank southeast for about 1200 km to the eastern end of the Bermuda Rise. Bear Seamount (39° 55’N 67° 30’W) is the most inshore of the New England Seamounts, located inside the U.S. Exclusive Economic Zone (EEZ) south of Georges Bank. It rises from the continental slope at depths of 2000-3000 m to a generally flat summit at 1100 m depth."[16]

Northeast U.S. Continental Shelves

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File:Bathymetry of the NES LME and adjacent offshore waters.png
Bathymetry of the Northeast U.S. Continental Shelf Large Marine Ecosystem (NES LME) and adjacent offshore waters with the names of major features. Credit: NEFSC, NOAA.{{fairuse}}
File:Last Glacial Maximum Susquehanna River drainage.png
Last Glacial Maximum Susquehanna River drainage map shows the locations of the Cinmar site and Ryolite Quarry. Credit: Dennis Stanford, Darrin Lowery, Margaret Jodry, Bruce A. Bradley, Marvin Kay, Thomas W. Stafford and Robert J. Speakman.{{fairuse}}
File:The Cinmar Biface.png
The Cinmar stone tool is a large, thin knife with evidence of well-controlled percussion thinning flake scars on both faces. Credit: Sarah Moore.{{fairuse}}
File:Locations of Chesapeake Bay laurel leaf bifaces.png
Locations of Chesapeake Bay laurel leaf bifaces are red stars. Credit: Dennis Stanford, Darrin Lowery, Margaret Jodry, Bruce A. Bradley, Marvin Kay, Thomas W. Stafford and Robert J. Speakman.{{fairuse}}
File:Laurel leaf bifaces from underwater contexts.png
Laurel leaf bifaces are from underwater contexts. Credit: Sarah Moore and Dennis Stanford, Darrin Lowery, Margaret Jodry, Bruce A. Bradley, Marvin Kay, Thomas W. Stafford and Robert J. Speakman.{{fairuse}}

"The Northeast U.S. Continental Shelf Large Marine Ecosystem (NES LME) encompasses an area of approximately 260,000 km2 from Cape Hatteras in the south to the Gulf of Maine in the north. The shelf is wide off northern New England, extending over 200 km from shore, and relatively narrow off Cape Hatteras where the shelf break is approximately 30 km from shore [image on the right]. The physiography of the Gulf of Maine and its complex shoreline was strongly shaped by glacial activity during the last ice age which ended approximately 12,000 years ago. Similarly, Georges Bank was created with the retreat of the Laurentide ice sheet during the Wisconsinan glaciation event. To the south, in the Middle-Atlantic Bight, the topography is more uniform and the shelf gently slopes to the edge of the continental shelf."[16]

"The Gulf of Maine, a semi-enclosed continental shelf sea, is characterized by an extremely complex physiographic structure. Three major deep basins occur in the Gulf [image on the right]. Georges Basin is the smallest but deepest of the three, covering an area of 4100 km2 with a mean depth of approximately 300 m. Wilkinson and Jordan Basins are similar in average depth (approximately 225 m) with Wilkinson the larger of the two (~7100 km2 vs ~6700 km2). There are over 20 smaller basins located with the Gulf of Maine. Two relatively large ledge-bank systems (Stellwagen and Jeffries Ledges) occur within the Gulf of Maine proper. Four major river systems feed into the Gulf of Maine (the Androscoggin, Penobscot, Merrimack, and Kennebec Rivers), playing an important role in the oceanography of the coastal Gulf of Maine."[16]

"Georges Bank, a broad shallow submarine plateau forming the seaward boundary of the Gulf of Maine, is delineated to the north and east by the Northeast Channel and to the south and west by the Great South Channel [image on the right]. The bank encompasses approximately 42,000 km2 within the 100 m isobath. When water levels were substantially lower at the end of the last ice age, Georges Bank was part of a larger cape, and later with rising sea levels (approximately 14,000 years ago), an island. It was completely submerged approximately 11,500 year ago. The average depth of Georges Bank is now 75 m but is just 30 m at its shallowest on Georges Shoals."[16]

"The seaward margin of Georges Bank on the continental slope is incised with 11 major submarine canyons. Submarine canyons are typically deep, V-shaped valleys cut into the sediments of the continental slope and shelf approximately perpendicular to the depth contours of those structures, [...]. A large number of smaller, unnamed canyons, also called gullies, cross the slope in between the large ones, but do not impinge on the shelf."[16]

"Canyons converge at their lower ends into a smaller number of collection valleys, called canyon channels, that continue down across the continental rise toward the abyssal plain. Like terrestrial canyons, these submarine megafeatures were created by erosion. However, while several of the large canyons are aligned with the now-submerged valleys of rivers that flowed across the shelf during the last ice age when sea level was nearly 100 m lower, none of the submarine canyons were actually exposed; they were not carved by rivers like terrestrial canyons. Rather, they are thought to have formed initially by slumping of accumulated sediments beginning on the slope, which propagated progressively shelfward with time, and were further excavated by sediment flows. Hence the largest canyons are thought to be the oldest. Submarine canyons in the NES LME are not restricted to the Georges Bank region. Major canyons further to the south include Block Canyon off southern New England, Hudson Canyon (part of the Hudson River drainage system, and Norfolk Canyon off the Virginia capes."[16]

"Major influences in the Middle-Atlantic Bight region include several large estuaries including the Hudson, Delaware Bay, and Chesapeake Bay."[16]

"In 1974, Captain Thurston Shawn and the crew of Cinmar, a scallop trawler working 100 km east of the Virginia Capes, were dredging at a depth of 70 m ([image second down on the right]). Just after starting their run, the dredge became very heavy and when reeled in, it contained a mastodon skull. While cleaning the bone from the dredge, a large bifacially flaked rhyolite knife was discovered [drawing on the left]. Shawn carefully plotted the water depth and the exact location of the find on his navigation charts and noted that all of these items were dredged at the same time."[17]

"The find location, designated the Cinmar site, is on the edge of the outer continental shelf, south of the last glacial maximum (LGM) Susquehanna Paleo-River Valley, which is referred to as the Cape Charles channel [image second down on the right]. During the LGM, 19,000–26,500 years ago (Clark et al. 2009), sea stand is estimated to have been 130 m below the present sea level (Milliman and Emery 1968; Belknap and Kraft 1977). The site was on the edge of the LGM James Peninsula, immediately west of a LGM barrier island and channel. This terrestrial landscape, which existed between at least 14,500 years ago and possibly more than 25,000 years ago, would have been 10–14 meters below sea level (mbsl) by the time Paleoindians occupied North America approximately 13,500 years ago (Waters and Stafford 2007)."[17]

"Two sections of the [female mastodon] tusk were sampled to obtain bone collagen for accelerator mass spectrometry, 14C dating. The resulting age was 22,760±90 RCYBP (UCIAMS-53545)."[17]

"Limited data are available for environmental reconstruction of the mid-Atlantic outer continental shelf during the last glacial maximum. Freshwater peat dated to 15,500 years ago was dredged from depths of 64–66 m (210–216 feet) near the Washington Canyon, north of the Cinmar site (Emery et al. 1967). Pollen extracted from the peat suggests that spruce, water lily, sedge, pine, oak, and fir were growing on the continental shelf shortly after the last glacial maximum. Another pollen sample, recently extracted from a soil sample taken from the Miles Point site dated to greater than 25,500 years ago on the Eastern shore of the Chesapeake, and revealed krummholz yellow birch, red spruce, balsam fir, and C3 grasses (Lowery et al. 2010). These data are evidence that the adjacent terrestrial vegetation likely extended as an unbroken biome onto portions of the continental shelf that were dry land during the LGM. The likelihood of abundant freshwater springs and ponds along the margin of the continental shelf (Faure et al. 2002), and the shrubby environment of the adjacent inter barrier island lagoon, as well as a relatively large number of mastodon remains reported from the continental shelf (Whitmore et al. 1967), indicate an ideal environment to support a reasonable mastodon population."[17]

Those laurel leaf bifaces found in underwater contexts are labeled, named (a Mopjake Bay. b Cinmar. c Heavily tumbled biface from Tar Bay. d Taylor’s Island. e Heavily resharpened knife from Mopjack Bay)[17] and arranged by size in the second image down on the left.

The locations "of Chesapeake Bay laurel leaf bifaces: 1 Cinmar site; 2 Hampton, Virginia; 3 Ocean City, Maryland; 4 Gore site; 5 Dauphin County, Pennsylvania; 6 Tar Bay, Maryland; 7 Taylor’s Island, Maryland; 8 and 9 Mopjack, Bay Virginia"[17] are the correspondingly numbered red stars in the image third down on the right.

The "manufacturing technology used to produce the Chesapeake Bay bifaces and the tool types themselves reflect the same technology as that used by the Solutrean people of southwestern Europe during the LGM (Stanford and Bradley 2012)."[17]

Younger Dryas

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The "Alleröd/Younger Dryas transition [occurred] some 11,000 years ago [11,000 b2k]."[18]

In the Southern Hemisphere and some areas of the Northern Hemisphere, such as southeastern North America, there was a slight warming.[19]

Effects of the Younger Dryas were of varying intensity throughout North America.[20]

The Younger Dryas is a period significant to the study of the response of biota to abrupt climate change and to the study of how humans coped with such rapid changes.[21] The effects of sudden cooling in the North Atlantic had strongly regional effects in North America, with some areas experiencing more abrupt changes than others.[22]

New England

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The effects of the Younger Dryas cooling impacted New England and parts of maritime Canada more rapidly than the rest of the United States at the beginning and the end of the Younger Dryas chronozone.[23][24][25][26] Proxy paleoecology indicators show that summer temperature conditions in Maine decreased by up to 7.5oC. Cool summers, combined with cold winters and low precipitation, resulted in a treeless tundra up to the onset of the Holocene, when the boreal forests shifted north.[27]

Southeastern

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Vegetation in the central Appalachian Mountains east towards the Atlantic Ocean was dominated by spruce (Picea spp.) and tamarack (Larix laricina) boreal forests that later changed rapidly to temperate, more broad-leaf tree forest conditions at the end of the Younger Dryas period.[28][29] Conversely, pollen and macrofossil evidence from near Lake Ontario indicates that cool, boreal forests persisted into the early Holocene.[29] West of the Appalachians, in the Ohio River Valley and south to Florida rapid, no-analog vegetation responses seem to have been the result of rapid climate changes, but the area remained generally cool, with hardwood forest dominating.[28] During the Younger Dryas, the Southeastern United States was warmer and wetter than the region had been during the Pleistocene[29][22][30] because of trapped heat from the Caribbean within the North Atlantic Gyre caused by a weakened Atlantic meridional overturning circulation (AMOC).[31]

Central

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There was also a gradient of changing effects from the Great Lakes region south to Texas and Louisiana. Climatic forcing moved cold air into the northern portion of the American interior, much as it did the Northeast.[32][33] Although there was not as abrupt a delineation as seen on the Eastern Seaboard, the Midwest was significantly colder in the northern interior than it was south, towards the warmer climatic influence of the Gulf of Mexico.[22][34] In the north, the Laurentide Ice Sheet re-advanced during the Younger Dryas, depositing a moraine from west Lake Superior to southeast Quebec.[35] Along the southern margins of the Great Lakes, spruce dropped rapidly while pine increased, and herbaceous prairie vegetation decreased in abundance but increased west of the region.[36][33]

Rocky Mountains

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Effects in the Rocky Mountain region were varied.[37][38] In the northern Rockies, a significant increase in pines and firs suggests warmer conditions than before and a shift to subalpine parkland in places.[39][40][41][42] That is hypothesized to be the result of a northward shift in the jet stream, combined with an increase in summer insolation[39][43] as well as a winter snow pack that was higher than today, with prolonged and wetter spring seasons.[44] There were minor re-advancements of glaciers in place, particularly in the northern ranges,[45][46] but several sites in the Rocky Mountain ranges show little to no changes in vegetation during the Younger Dryas.[40] Evidence also indicates an increase in precipitation in New Mexico because of the same Gulf conditions that were influencing Texas.[47]

West

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In western North America, its effects were less intense than in Europe or northeast North America;[48] however, evidence of a glacial re-advance[49] indicates that Younger Dryas cooling occurred in the Pacific Northwest. Speleothems from the Oregon Caves National Monument and Preserve in southern Oregon's Klamath Mountains yield evidence of climatic cooling contemporaneous to the Younger Dryas.[50]

The Pacific Northwest region experienced 2o to 3oC of cooling and an increase in precipitation.[51][30][52][53][54] Glacial re-advancement has been recorded in British Columbia[55][56] as well as in the Cascade Range.[57] An increase of pine pollen indicates cooler winters within the central Cascades.[58] Speleothem records indicate an increase in precipitation in southern Oregon,[54][59] the timing of which coincides with increased sizes of pluvial lakes in the northern Great Basin.[60] A pollen record from the Siskiyou Mountains suggests a lag in timing of the Younger Dryas, indicating a greater influence of warmer Pacific conditions on that range,[61] but the pollen record is less chronologically constrained than the aforementioned speleothem record. The Southwest appears to have seen an increase in precipitation as well, also with an average 2o of cooling.[62]

Allerød Oscillation

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File:Older Dryas.jpg
Comparison of the GRIP ice core with cores from the Cariaco Basin shows the Older Dryas. Credit: Konrad A Hughes, Jonathan T. Overpeck, Larry C. Peterson & Susan Trumbore.{{fairuse}}

"The most negative δ 18O excursions seen in the GRIP record lasted approximately 131 and 21 years for the [inter-Allerød cold period] IACP and [Older Dryas] OD, respectively. The comparable events in the Cariaco basin were of similar duration, 127 and 21 years. In addition to the chronological agreement, there is also considerable similarity in the decade-scale patterns of variability in both records. Given the geographical distance separating central Greenland from the southern Caribbean Sea, the close match of the pattern and duration of decadal events between the two records is striking."[63]

In the figures on the right, especially b, is a detailed "comparison of δ 18O from the GRIP ice core24 with changes in a continuous sequence of light-lamina thickness measurements from core PL07-57PC. Both records are constrained by annual chronologies, although neither record is sampled at annual resolution. The interval of comparison includes the inter-Allerød cold period (12.9-13 cal. kyr BP) and Older Dryas (13.4 cal. kyr BP) events (IACP and OD from a). The durations of the two events, measured independently in both records, are very similar, as is the detailed pattern of variability at the decadal timescale."[63]

Black Mats

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File:Black mat (12.9 ka) Murray Springs Clovis site in Arizona.jpg
The dark line shown is the black mat (12.9 ka) along the arroyo wall of the Murray Springs Clovis site in Arizona. Credit: R. B. Firestone, A. West, J. P. Kennett, L. Becker, T. E. Bunch, Z. S. Revay, P. H. Schultz, T. Belgya, D. J. Kennett, J. M. Erlandson, O. J. Dickenson, A. C. Goodyear, R. S. Harris, G. A. Howard, J. B. Kloosterman, P. Lechler, P. A. Mayewski, J. Montgomery, R. Poreda, T. Darrah, S. S. Que Hee, A. R. Smith, A. Stich, W. Topping, J. H. Wittke, and W. S. Wolbach.{{fairuse}}
File:Map of black mats of Younger Dryas age United States.png
Map of the United States shows 57 locations where one or more sites with black mats of Younger Dryas age occur (filled circles). Credit: C. Vance Haynes, Jr.{{fairuse}}
File:Black mats of Younger Dryas age.jpg
Black mats of Younger Dryas age are from localities in the United States. Credit: C. Vance Haynes, Jr., Glen L. Evans, Vance T. Holliday, Stanley A. Ahler.{{fairuse}}

"A carbon-rich black layer, dating to ≈12.9 ka [in the image on the left], has been previously identified at ≈50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at ≅12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at ≈12.9 ka. This layer also extends throughout at least 15 Carolina Bays [second image down on the left], which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain."[64]

"Most Younger Dryas (YD) age black layers or ‘‘black mats’’ are dark gray to black because of increased organic carbon (0.05–8%) compared with strata above and below (6, 7). Although these layers are not all alike, they all represent relatively moist conditions unlike immediately before or after their time of deposition as a result of higher water tables. In most cases higher water tables, some perched, are indicated by the presence of mollisols and wet-meadow soils (aquolls), algal mats, or pond sediments, including dark gray to black diatomites, at >70 localities in the United States [see map of the United States on the right]. Therefore, black mat is a general term that includes all such deposits, and some YD marls and diatomites are actually white. These latter cases are included in the nearly 30 localities containing strata representing the Pleistocene-Holocene transition (Allerød-Younger Dryas-Holocene) that do not exhibit a black layer because of little or no interaction with ground water or are represented by white to gray diatomaceous strata of YD age [...]. There are both younger and older black layers, but they do not appear to be widely distributed over the continent like the YD black mat, nor are they known to be associated with any major climatic perturbation as was the case with YD cooling."[65]

The map on the right shows "57 locations [...] where one or more sites with black mats of Younger Dryas age occur (filled circles). Open circles are 27 localities with Pleistocene-Holocene transitional sediments but no black mats [...]."[65]

The second image down on the right shows black mats from the following specific locations corresponding to map numbers: "(a) The black mat at the Murray Springs Clovis site in Arizona (locality 1a) is a black algal mat that blankets the Clovis occupation surface. (b) At the Naco Clovis site (locality 1b) the mammoth bones and artifacts are directly overlain by laminated marls and clay bands that are pond facies of the San Pedro Valley black mat. (c) The type Clovis site in Blackwater Draw, New Mexico (locality 5) has a dark-gray diatomite stratum D containing Folsom artifacts and bones of Bison bison antiquus overlying a "brown sand wedge" (stratum C) with Clovis artifacts and mammoth bones over a "gray sand" (stratum B) with Clovis artifacts and bones of mammoth and B. bison antiquus beyond where stratum C pinches out. Photograph courtesy of Glen L. Evans. (d) At the Lubbock Lake site in Texas (locality 6) a black and white diatomite (stratum 2A) [...] contains Folsom artifacts and bones of extinct bison and directly overlies a gray fluvial sand (stratum 1B). Photograph courtesy of Vance T. Holliday. (e) Folsom, Goshen-Plainview, and Agate Basin artifacts are found in situ in the lower portions of the Leonard paleosol at several sites in the Knife River - Lake Ilo region of North Dakota (locality 7). Photograph by Stanley A. Ahler. Published courtesy of the Center for the Study of the First Americans. (f) The Lindenmeier Folsom site in Colorado (locality 8) has Folson artifacts and bison bones covered by a black cumulic mollisol overlying Peoria loess."[65]

Nanodiamonds

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A Younger Dryas impact event, may have occurred in North America about 12,900 calendar years ago, that initiated the Younger Dryas cooling.[66]

Among other things, findings of melt-glass material in sediments in Pennsylvania, South Carolina and Syria, which dates back nearly 13,000 years, was formed at temperatures of 1,700 to 2,200 °C (3,100 to 4,000 °F) as an apparent result of a bolide impact that occurred at the onset of the Younger Dryas.[67]

Most of the results cannot be confirmed.[68][69][70]

Sediments claimed by hypothesis proponents to be deposits resulting from a bolide impact date from much later or much earlier times than the proposed date of the cosmic impact having examined 29 sites commonly referenced to support the impact theory to determine if they can be geologically dated to around 13,000 years ago, but only three of those sites actually date from then.[71]

The distribution of nanodiamonds produced during extraterrestrial collisions: 50 million square kilometers of the Northern Hemisphere at the YDB have the nanodiamonds.[72] Only two layers exist showing these nanodiamonds: the YDB 12,800 calendar years ago and the Cretaceous-Tertiary boundary, 65 million years ago, which, in addition, is marked by mass extinctions.[73]

Earth may have collided with one or more fragments from a larger (over 100-km diameter) disintegrating comet (some remnants of which have persisted within the inner solar system to the present day), which is consistent with large-scale biomass burning (wildfires) following the putative collision, analyses of ice cores, glaciers, lake- and marine-sediment cores, and terrestrial sequences.[74][75]

Carolina Bays

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LIDAR elevation image of 300 square miles (800 km2) of Carolina bays is in Robeson County, N.C. Credit: Swampmerchant.{{free media}}

The second image down on the left is similar to an aerial "photo (U.S. Geological Survey) of a cluster of elliptical and often overlapping Carolina Bays with raised rims in Bladen County, North Carolina. [...] The largest Bays are several kilometers in length, and the overlapping cluster of them in the center is ≈8 km long. Previous researchers have proposed that the Bays are impact-related features."[64]

"The Carolina Bays are a group of »500,000 highly elliptical and often overlapping depressions scattered throughout the Atlantic Coastal Plain from New Jersey to Alabama (see [second image down on the left]). They range from ≈50 m to ≈10 km in length (10) and are up to ≈15 m deep with their parallel long axes oriented predominately to the northwest. The Bays have poorly stratified, sandy, elevated rims (up to 7 m) that often are higher to the southeast. All of the Bay rims examined were found to have, throughout their entire 1.5- to 5-m sandy rims, a typical assemblage of YDB markers (magnetic grains, magnetic microspherules, Ir, charcoal, soot, glass-like carbon, nanodiamonds, carbon spherules, and fullerenes with 3
He). In Howard Bay, markers were concentrated throughout the rim, as well as in a discrete layer (15 cm thick) located 4 m deep at the base of the basin fill and containing peaks in magnetic microspherules and magnetic grains that are enriched in Ir (15 ppb), along with peaks in charcoal, carbon spherules, and glass-like carbon. In two Bay-lakes, Mattamuskeet and Phelps, glass-like carbon and peaks in magnetic grains (16-17 g/kg) were found ≈4 m below the water surface and 3 m deep in sediment that is younger than a marine shell hash that dates to the ocean highstand of the previous interglacial."[64]

Laacher See volcanic eruption

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The Laacher See volcano erupted at approximately the same time as the beginning of the Younger Dryas, is a maar lake, a lake within a broad low-relief volcanic crater about 2 km (1.2 mi) diameter in Rhineland-Palatinate, Germany, about 24 km (15 mi) northwest of Koblenz and 37 km (23 mi) south of Bonn, within the Eifel mountain range, and is part of the East Eifel volcanic field within the larger Vulkan Eifel (Vulkaneifel).[76][77] This eruption was of sufficient size, VEI 6, with over 20 km3 (2.4 cu mi) tephra ejected,[78] to have caused significant temperature change in the Northern Hemisphere.

The timing of the Laacher See Tephra is relative to signs of climate change associate with the Younger Dryas Event within various Central European varved lake deposits.[78][79] The very large eruption of the Laacher See volcano was at 12,880 years BP, coinciding with the initiation of North Atlantic cooling into the Younger Dryas.[80][81]

Although the eruption was about twice size as the 1991 Mount Pinatubo eruption, it contained considerably more sulfur, potentially rivalling the climatologically very significant 1815 eruption of Mount Tambora in terms of amount of sulfur introduced into the atmosphere.[81] Evidence exists that an eruption of this magnitude and sulfur content occurring during deglaciation could trigger a long-term positive feedback involving sea ice and oceanic circulation, resulting in a cascade of climate shifts across the North Atlantic and the globe.[81] Further support for this hypothesis appears as a large volcanogenic sulfur spike within Greenland ice, coincident with both the date of the Laacher See eruption and the beginning of cooling into the Younger Dryas as recorded in Greenland.[81] The mid-latitude westerly winds may have tracked sea ice growth southward across the North Atlantic as the cooling became more pronounced, resulting in time transgressive climate shifts across northern Europe and explaining the lag between the Laacher See Tephra and the clearest (wind-derived) evidence for the Younger Dryas in central European lake sediments.[82][83]

Although the timing of the eruption appears to coincide with the beginning of the Younger Dryas, the amount of sulfur contained would have been enough to result in substantial Northern Hemisphere cooling, evidence exists that a similar feedback following other volcanic eruptions could also have triggered similar long-term cooling events during the last glacial period,[84] the Little Ice Age,[85][86] and the Holocene in general.[87]

It is possible that the Laacher See eruption was triggered by lithospheric unloading related to the removal of ice during the last deglaciation,[88][89] a concept that is supported by the observation that three of the largest eruptions within the East Eifel Volcanic Field occurred during deglaciation.[90] Because of this potential relationship to lithospheric unloading, the Laacher See eruption hypothesis suggests that eruptions such as the 12,880 year BP Laacher See eruption are not isolated in time and space, but instead are a fundamental part of deglaciation, thereby also explaining the presence of Younger Dryas-type events during other glacial terminations.[81][91]

Clovis periods

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"The [Younger Dryas boundary] YDB markers, including magnetic grains and microspherules, iridium, soot, and fullerenes with ET helium, are present in the few centimeters just below the black mat at the top of the underlying sediment. This lithologic break represents the surface at the end of the Clovis period before the formation of the black mat. Clovis artifacts, a fire pit, and an almost fully articulated skeleton of an adult mammoth were recovered at Murray Springs with the black mat draped conformably over them. Excavations by Vance Haynes, Jr., and colleagues also revealed hundreds of mammoth footprints in the sand infilled by black mat sediments. These footprints and the mammoth skeleton appear to have been preserved by rapid burial after the YDB event (1). No in situ Clovis points and extinct megafaunal remains have been recovered from in or above the black mat, indicating that the mammoths (except in isolated cases) and Clovis hunting technology disappeared simultaneously."[64]

The "end-Clovis stratum (the YDB) is well dated at Murray Springs, AZ, (eight dates averaging 10,890 14
C yr or calendar 12.92 ka) and the nearby Lehner site (12 dates averaging 10,940 14
C yr or 12.93 calendar ka). Haynes (2) correlated the base of the black mat (the YDB) with the onset of YD cooling, dated to 12.9 ka in the GISP2 ice core, Greenland [...] (18)."[64]

Older Dryas

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File:Older Dryas.jpg
Comparison of the GRIP ice core with cores from the Cariaco Basin shows the Older Dryas. Credit: Konrad A Hughes, Jonathan T. Overpeck, Larry C. Peterson & Susan Trumbore.{{fairuse}}
 
NGRIP late Weichselian glacial age Bölling-Alleröd-Younger dryas methane amount data is graphed. Credit: Merikanto, M. Baumgartner, A. Schilt, O. Eicher, J. Schmitt, J. Schwander, R. Spahni, H. Fischer, and T. F. Stocker.{{free media}}

"Older Dryas [...] events [occurred about 13,400 b2k]".[63]

"Recent stratigraphical achievements and long time established chronologies exist for the Late Weichselian, i.e. 10-25 ka BP. During this period Denmark experienced the complex Main-Weichselian glaciation from 25 to about 14 ka BP (Jylland stade, Houmark-Nielsen 1989) followed by the Late Glacial climatic amelioration including the interstadial Bølling-Allerød oscillation (13-11 ka BP), finally leading to the interglacial conditions that characterize the Holocene (Hansen 1965)."[92]

"During the Late Weichselian glacial maximum (20-15 ka BP) the overriding of ice streams eventually lead to strong glaciotectonic displacement of Late Pleistocene and pre-Quaternary deposits and to deposition of till."[92]

Bølling Oscillation

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The "intra-Bølling cold period [IBCP is a century-scale cold event and the] Bølling warming [occurs] at 14600 cal [calendar years, ~ b2k] BP (12700 14C BP)".[93]

MIS Boundary 1/2 is at 14 ka.[94]

See also

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References

edit
  1. ↑ McCarthy, Michael (24 April 2007). "An island made by global warming". The Independent. London. Archived from the original on 30 August 2008. Retrieved 4 May 2010.
  2. ↑ "Place of the Year". Blog.oup.com. 3 December 2007. Retrieved 6 September 2010.
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  94. ↑ Lisiecki, L.E., 2005, Ages of MIS boundaries. LR04 Benthic Stack Boston University, Boston, MA
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