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 quaternary periodالعصر الجليدى

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مُساهمةquaternary periodالعصر الجليدى



The Quaternary Period




The Quaternary is the most recent geological period of time
in Earth’s history, spanning the last two million years and extending up to the
present day. The Quaternary period is subdivided into the Pleistocene (“Ice
Age”) and the Holocene (present warm interval) epochs, with the Pleistocene
spanning most of the Quaternary and the Holocene covering the past 10 000 years.
The Quaternary period is characterized by a series of large-scale environmental
changes that have profoundly affected and shaped both landscapes and life on
Earth. One of the most distinctive features of the Quaternary has been the
periodic build-up of major continental ice sheets and mountain ice caps in many
parts of the world during long lasting glacial stages, divided by warm episodes
(interglacials) of shorter duration, when temperatures were similar to or higher
than today. During long periods of these climatic
cycles, perhaps 8/10th of the time, temperatures were cool or cold.
The number of Quaternary interglacial-glacial cycles
is probably in the order of 30-50.



Oxygen isotope record for the past 2,6 million years. Peaks

represent warm Earth, troughs a cold Earth


There have been shifts in the frequency of climate
oscillations and amplitude of temperatures and glaciations through the
Quaternary. At the onset of the Quaternary, many arctic areas were comparatively
warm, with trees and bushes growing far north of the present treeline. Prior to
about 800 000 years ago each interglacial-glacial cycle lasted for about 40 000
years, but after that the periodicity shifted to a prevailing rhythm of about
100 000 years. Prior to this shift in frequency there was a repeated build-up of
relatively small-to-moderate sized ice sheets at high northern latitudes. After
c. 800 000 years ago there occurred a major intensification of glaciations, with
repeated growth of continental-scale ice sheets reaching mid-latitudes and with
ice volumes much larger than during the earlier Quaternary glaciations. There
have occurred 8-10 major glaciations during the past 800 000 years. Two of the
largest Northern Hemisphere glaciations are the last one (called the Weichselian/Wisconsin
glaciation, at its maximum about 20 000 years ago) and the one occurring prior
to the last interglacial (called the Saalian/Illinoian glaciation, occurring
prior to c. 130 000 years ago). During the peak of both glaciations, ice sheets
covered extensive areas north of 40-50oN in both Eurasia and N
America. The Saalian glaciation was particularly extensive in the high Eurasian
north, covering vast areas of N Russia, coastal Arctic Ocean and Siberia
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رد: quaternary periodالعصر الجليدى
مُساهمة في الجمعة أكتوبر 07, 2011 6:19 pm من طرف المدير العام




Global view of the Last Glacial Maximum, 18.000
years ago. From http://www.scotese.com/lastice.htm


The effects of the Quaternary climate oscillations were not
only repeated expansion of glaciers at mid- and high latitudes, but mid-latitude
areas were repeatedly subject to cold climate and permafrost, forcing plant and
animal populations to migrate or adapt to changed environmental conditions – or
become extinct. At lower latitudes, forested areas, deserts and savannahs
shifted through several degrees of latitude as climate zones responded to higher
latitude cooling. Global patterns of wind and energy transfer by ocean currents
changed, causing large-scale shifts in the pattern of aridity and precipitation
around the world. Rates of weathering and erosion changed globally in response
to changes in temperature and precipitation, and river regimes fluctuated
considerably. During peak glaciations in the Eurasian north, the large rivers of
N Russia and Siberia entering the Arctic Ocean were dammed by the huge ice
sheets and forced to flow southwards. When huge volumes of water were trapped in
ice sheets during peak glaciations, global sea level fell up to 150 m. This
caused vast continental shelf areas to become dry land, particularly the shallow
shelf areas bordering the Arctic Ocean. Land bridges formed across sounds and
between islands, in turn affecting ocean surface currents, shallow-sea life and
productivity and opening and closing routes of migration for plants and animals.
The Bering land bridge, existing due to lowering of sea level during the last
glaciation, made possible the spread of humans from Asia to N America.




Mammoths and American Lions in the late Pleistocene of
Alaska


The frequent and rapid Quaternary environmental changes
stimulated rapid evolution and the rise of large mammals, or megafauna.
The Pleistocene megafauna included woolly rhinoceros, woolly mammoths and large
wolves that were well adapted to cold climates. The major type of ecosystem
covering the European, Asian and North American continents south of the ice
sheets was a type of grass steppe that has been called the "mammoth steppe". It
differed from the modern tundra environment in having higher biomass, much
higher productivity and a reduced snow cover in winter. The changing
precipitation patterns at the end of the last glaciation probably caused the
collapse of the mammoth steppe. Since many animals were dependent on the grass
steppe, they became highly vulnerable to extinction when the ecosystem
collapsed. This, together with hunting by humans, has probably been the root
cause of many of the megafaunal extinctions at the end of the Pleistocene. The
last mammoths, lingering on the Siberian islands, became extinct 4000 years ago.
Other mammals that evolved during the Pleistocene, like the caribou, the musk ox
and the polar bear, continue to be an important part of the arctic fauna. It is
also during the Pleistocene that humans evolve and develop the use of
technology, language, art and religion. Earliest signs of human occupation in
the Russian arctic are 30 000-40 000 years old. Much of the arctic flora and
fauna, including native peoples of the arctic, have, however, during the past 10
000-15 000 years migrated from lower latitudes to the arctic latitudes.




Fluted surface in front of Brúarjökull, Iceland. Photo:
Ólafur Ingólfsson 2004
رد: quaternary periodالعصر الجليدى
مُساهمة في الجمعة أكتوبر 07, 2011 6:21 pm من طرف المدير العام

Quaternary
paleogeography




Definitions -
Paleogeography deals with reconstructing the physical geography of past
geological times, where the focus is on physical features such as the shifting
locations of shorelines, rivers and drainage systems, tectonics and
mountain-building, paleolatitude and continental drift, location in time and
space of continental shelf areas and other sedimentary basins. The field of
Quaternary paleogeography broadly includes all aspects of paleo-map
reconstructions through the Quaternary Period; ice sheet and sea-level
fluctuations in time and space; the delineation of past topographic or
bathymetric contours; the compilation of biologic, morphologic or
lithostratigraphic data that can be presented on time slices, such as
paleovegetation maps or distribution of loess basins and fossil permafrost. A
Quaternary paleogeographic dataset will cover a number of time intervals,
showing e.g. deviations from the present day values of the winter, summer and
annual mean temperatures and annual precipitation for the time slices, the
values of albedo, sea surface temperatures, sea-ice distribution, zones of
permafrost, mountain glaciation and inland ice, geomorphic processes and loess
formation, natural vegetation and landscape types for the different time
intervals. Paleogeo­graphical maps and reconstructions are used as base
information for studies of e.g. past fossil distributions, past climatic
changes, evolution of vegetational or oceanographic patterns and for computer
modelling studies.




Oceanic Circulation Patterns (Source:
Office of Naval Research. Oceanography (http://www.onr.navy.mil/focus/ocean))


Background -
The frame for major global environmental changes is set by
large-scale tectonics and position and configuration of the continental
landmasses. These affect the paths of ocean currents and air masses and in turn
decide the global energy distribution. The steady northward drift of Europe,
Asia and North America through the Tertiary Period (65-2 Ma (million years) BP
(before present)) caused the gradual tectonic closing of the connection between
the Pacific Ocean and the Arctic Ocean and reduced the previously efficient
ocean heat transport from equatorial regions toward the North Pole. The northern
hemisphere thereby experienced increased cooling. The ice sheet in Greenland
presumably first formed about 7 Ma BP in response to this cooling. The build-up
of huge mountain chains (The Himalayas, The Alps, The Rocky Mountains and The
Andes) as well as closing of Equatorial ocean pathways also greatly affected
global circulation patterns. The onset of glaciation in Antarctica can be traced
back to late Eocene times, 35-40 Ma BP, reflecting the drift of Antarctica over
the South Pole and the establishment of the circum-Antarctic pattern of oceanic
and atmospheric circulation that inhibits energy transfer to high Southern
Hemisphere latitudes. There is strong evidence of large ice sheets in Antarctica
during the Miocene, after ca. 24 Ma BP, and since then Antarctica has functioned
as a heat sink in the global climate system and strongly influenced the global
energy budget and climate.




An example of Paleogeographical reconstruction: The Late
Weichselian Barents Sea Ice Sheet (from Forman et al. 2004)
رد: quaternary periodالعصر الجليدى
مُساهمة في الجمعة أكتوبر 07, 2011 6:22 pm من طرف المدير العام



Most Quaternary paleogeographic
reconstructions focus on time slices through the past ca. 130 ka (kilo-years).
That is for the simple reason that there is ample geological and biological
evidence preserved in the geological record with resolution high enough to allow
for reasonably detailed reconstructions, whereas evidences of earlier
large-scale Quaternary environmental changes usually are fragmentary. During the
past 130 ka the climate has changed from interglacial to glacial and then back
to the present-day interglacial, i.e. fluctuated between end members in the
climate-environmental system. It is assumed that environmental changes through
the last interglacial-glacial cycle have occurred repeatedly through earlier
glacial cycles. The climate-environmental system is an interactive system
consisting of five major components: the atmosphere, the hydrosphere, the
cryosphere, the land surface and the biosphere, forced or influenced by various
external forcing mechanisms, the most important of which is the Sun. There is a
general consensus among Quaternary scientists that changes in earth's orbital
parameters that influence amount and distribution of energy from the Sun (the
tilt of the earth's rotational axis, the eccentricity of the earth's orbit about
the sun, and season of perihelion) are very important for explaining the
fundamental timing of interglacial and glacial events. However, the phasing and
the amplitude of the climate response to orbital changes are non-linear and
involve atmosphere, ocean, ice sheet, land and vegetation feedbacks. It is one
purpose of paleogeographic reconstructions to highlight spatial and temporal
differences in these physical parameters as expressed in the geological
archives, for better understanding the underlying processes and dynamics.





The following reconstructions will
focus on paleoenvironments in Eurasia and Beringia, with brief references to
Arctic Canada, Greenland and Svalbard, during the Pliocene and three widely
defined time periods through the last 130 ka: (a) The last interglacial, the
Eemian/Sangamon/marine oxygen isotope stage (MIS) 5e, 130-115 ka BP; (b) The
Early-Middle Weichselian/Wisconsin/MIS 5d-4, ca. 115-50 ka BP; (c) The Last
Glacial Maximum (LGM)/MIS 2, 20-18 ka BP. The reconstructions are based on a
range of proxy data, from terrestrial macrofossil and pollen data to marine and
ice-core data.





Paleogeographic
reconstruction of the Pliocene (5.4-1.8 Ma) in the Arctic



The Arctic is not a uniform environment today. Different
geological histories, large differences in topography and proximity to the
Arctic Ocean between regions, as well as varying weather patterns, bring
diversity to the present Arctic environment and has done so through time.
Climatically, the Arctic today is often defined as the area north of the 10°C
July isotherm, i.e. north of a line or region that has a mean July temperature
of 10°C. In some areas the treeline roughly coincides with the 10°C July
isotherm and defines the southern boundary of the Arctic. The treeline defines a
transition zone where continuous forest gives way to tundra with sporadic stands
of trees and finally to treeless tundra. The Arctic is thus by definition
primarily a treeless area with low summer temperatures. But it has not always
been so.




During the Pliocene, global temperatures, particularly at
high latitudes, are believed to have been significantly warmer than today.
(Source: http://www.giss.nasa.gov/research/paleo/pliocene)


Generally, the Pliocene world was warmer than at present. The
ancient distribution of warm-climate ocean plankton, and of animal and plant
fossils on land, shows that globally the greatest warming relative to the
present situation was in the Arctic and cool-temperate latitudes of the Northern
Hemisphere. There, summer and annual mean temperatures were often warm enough to
allow species of animals and plants to exist hundreds of kilometers north of the
ranges of their nearest present-day relatives. In the Arctic, boreal-type
forests dominated all the way to the present Arctic Ocean where tundra exists
today. This has been verified by finds of fossil wood at a number of sites in
northern Greenland and Arctic Canada. Fossil wood logs that have been identified
include Larix, Pinus and Picea. Fossil mammalian remains
include the extinct rabbit Hypolagus, and fossil insects and marine
mollusks from a number of sites around the Arctic confirm with a considerably
warmer-than-present environment prior to the onset of Pleistocene cooling and
expanding Arctic glaciers. Paleogeographical reconstructions for the Pliocene in
the Arctic suggest that summer sea surface temperatures (SST) in the Arctic
Ocean were at least 1-3oC higher than today, and sea ice cover was
considerably reduced or even absent during long periods of time. There was
considerably more rainfall over the Arctic, originating over the warmer Arctic
Ocean, and permafrost was probably restricted to higher terrain. Because there
were less ice volumes at high latitudes, global sea level may have been as much
as 30m higher than at present during the warmest intervals. The peak phases of
warmth during the Pliocene were mostly during the interval 3-4 Ma (the
mid-Pliocene), although almost all of the Pliocene was warmer than today's
world.



The Pliocene warmth in the Arctic has been enigmatic for our
understanding of what controls the Quaternary development of climate and
glaciations, since the present continental configuration was largely in place in
Pliocene. The Arctic then as now experienced a polar night north of the polar
circle. The causes of the generally warmer climate of the Pliocene are something
of a mystery. The warmth may have been related to changes in ocean and
atmospheric circulation patterns, perhaps combined with higher-than-present
concentrations of greenhouse gases in the atmosphere. Temperature estimates
derived from paleodata reveal that when the global temperature warms, changes at
higher latitudes, and in the Polar Regions in particular, are systematically
larger than nearer the equator. In general, climate models do a better job of
estimating global temperature changes through time than regional changes. This
is because the energy budget of the entire planet is affected. Regional changes
reflect the response of the atmosphere and ocean circulation to changes in the
total energy budget, and as a result, are more difficult to model and
understand. One of the challenges for Pliocene paleogeographical reconstructions
in the Arctic is to provide an understanding, in the perspective of the geologic
record, of possible environmental responses to a future greenhouse situation.

رد: quaternary periodالعصر الجليدى
مُساهمة في الجمعة أكتوبر 07, 2011 6:23 pm من طرف المدير العام
The Eemian/Sangamon
interglacial, 130-115 ka BP
- The beginning of
the last interglacial is reflected in the marine records by abrupt shift to
lighter isotope values. The preceding Saalian/Illinoian glaciation was extremely
extensive at both high and middle latitudes, and the onset of the Eemian/Sangamon
interglacial is marked at many Arctic locations by marine transgression across
isostatically depressed coastal areas. Deposits from this marine transgression
are particularly pronounced along the northern Russian and Siberian coastal
lowlands. A range of proxy data suggests that the Eemian/Sangamon climate
optimum summer temperatures were considerably (2-4oC) warmer than
that of the present day, and that vegetation zones on the continents migrated
northwards. Regional SST zones also migrated, and sub-tropical warm water was
pushed northwards in the North Atlantic. Estimates of SST suggest considerably
warmer waters than present in coastal Arctic waters, and even the Arctic Ocean
may have been ice-free some summers. Glacier extent throughout the Arctic was
probably significantly more restricted than present during the Eemian/Sangamon
interglacial, with the Greenland Inland Ice considerably reduced. Global
eustatic sea level was 4-6 m higher than today as a result of extensive melting
of glaciers on the continents and thermal expansion of ocean water.







The figure, pictured above, demonstrates the difference
between modern sea surface temperature and estimated February sea surface
temperature (in °C) at the last interglaciation, some 120 ka ago. Negative
values mean that the last interglacial ocean was colder than today. Note that
most SST values are similar to present. Samples with more than one estimate
reflect use of more than one proxy source (F = foram, R = radiolaria, C =
coccolith).
Figure
from CLIMAP, 1984
.





In northern Russia and western to
central Siberia, Eemian marine and estuarine sediments are widely exposed in
river sections from the Kola Peninsula in the west to the Taymyr Peninsula in
the east. Their fossil content of warm boreal benthic faunas, in areas that
today have arctic waters lacking boreal species, easily identifies them.
Fennoscandia was an island, with water passage between the North Sea and the
White Sea across the Baltic Sea and Finland. Finds of fossil marine mammals,
such as Narwhales, in marine sediments on the Siberian high arctic islands
suggest at least seasonally reduced sea-ice cover compared to the present. The
warm Eemian climate in the Eurasian north is also evidenced by more northerly
tree line limits than present, with boreal forests spreading all the way to the
Arctic Ocean in northern Russia. Summer temperature estimates suggest 2-8oC
warmer temperatures than present in the Eurasian north, depending on site and
proximity to the Arctic Ocean.





Studies of marine and terrestrial
deposits of the last interglacial in Beringia suggest that it was warmer than
present conditions. There is evidence for warmer-than-present marine conditions
offshore Alaska during the Eemian/Sangamon interglacial, and the winter sea-ice
limit in the Bering Strait was at least 800 km further to the north than
present. At the same time, the treeline was more than 600 km further north in
places, displacing the tundra. A compilation of last interglacial localities
indicates that boreal forest was much extended beyond its present range in
Alaska and Yukon Territory and probably extended to higher elevation sites now
occupied by tundra in the interior. The treeline on Chukotka Peninsula,
easternmost Siberia, was more than 600 km further north than today and
displacing the tundra all the way to the Arctic Ocean. Summer temperature
reconstructions for Beringia vary considerably, from showing values similar to
modern to considerably warmer summers. Studies of fossil beetle assemblages
estimate that Eemian/Sangamon interglacial July temperatures at certain sites in
Beringia may have been about 5oC warmer than modern.





Investigations on East Greenland
have revealed that during the Eemian/Sangamon interglacial dwarf-shrub heaths
with a diverse insect fauna and tree birch and alder growing in sheltered
localities dominated the terrestrial environment in areas around 70oN,
which today are polar deserts. This suggests that summer temperatures were at
least 3-4oC warmer than present. To the contrary, fossil molluscs
from proposed Eemian deposits on Svalbard suggest SST similar to modern, but not
as warm as during the Holocene climate optimum (see below). There are numerous
collections of fossil shells and some of terrestrial plant materials from arctic
NW Canada that are thought to correlate to the Eemian/Sangamon interglacial.
Most fossil mollusc species are representative of arctic conditions, but few
finds of sub-arctic species suggest the marine climate may have been somewhat
warmer than present.






The Early-Middle Weichselian/Wisconsin,
115-50 ka BP
- In a Northern Hemisphere and
global perspective, this time interval represents a transition from interglacial
to glacial conditions, with successively falling global sea level as continental
ice volumes increased. Recent research has, however, increasingly shown that ice
sheets in the high arctic probably reached their maximum extent and volume
during the early stages of ice build-up, during the Early-Middle Weichselian/Wisconsin.




A reconstruction of the Eurasian ice sheet during the Early
Weichselian glacial maximum (90-80 ka BP). Figure from Svendsen et al. 2004
رد: quaternary periodالعصر الجليدى
مُساهمة في الجمعة أكتوبر 07, 2011 6:24 pm من طرف المدير العام
In the Eurasian north, west of the
Taymyr Peninsula, the limits of the Eurasian ice sheets have been reconstructed
for two Early-MiddleWeichselian/Wisconsin glaciations. The Late Quaternary
glacial maximum in the Eurasian Arctic occurred around 90 ka BP, in strong
contrast to the ice sheets over Scandinavia and North America, which at that
time were much smaller than during the LGM. During the 90 ka BP glaciation an
ice sheet centred in the Barents Sea-Kara Sea area expanded far onto the Russian
continent and blocked the northbound drainage of rivers towards the Arctic Ocean
(Fig. 1). A re-growth of the ice sheet occurred 60-50 ka BP. The Barents-Kara
ice sheet expanded well onto the continent in N Russia and covered the
northwestern rim of the Taymyr Peninsula, also leading to blockage of rivers
draining to the north and the formation of huge, ice-dammed lakes. Siberia, east
of Taymyr Peninsula, was ice-free throughout the last interglacial-glacial
cycle, and constituted an enormous steppe environment. It has been called “the
mammoth steppe” because of the characteristic presence of mammoths in its
ecosystem, but supported a diverse herbivorous fauna including mammoths,
caribou, musk ox, bison and horses. It differed from the modern tundra
environment in having higher biomass, much higher productivity and a reduced
snow cover in winter. The floral composition of the mammoth steppe may have its
closest modern analogue with the Central Asian grass steppe, where grasses form
the base of the nutritional chain although brushes and trees have occurred in
sheltered and wet locations.





There is evidence from both
Svalbard and East Greenland of two Early-Middle Weichselian/Wisconsin
glaciations, with ice extent and volumes similar to or smaller than the LGM
glaciation. In Beringia, glacial mapping, soil/loess profiles and chronological
data suggest that during Early-Middle Weichselian/Wisconsin Alaskan glaciers
were considerably expanded. In southwestern Alaska glaciers broadly extended
beyond the present coast, while further north the glacial expansion was more
limited. This was probably due to differences in proximity to moisture sources.
Early-Middle Weichselian/Wisconsin glaciers in Alaska defined a considerably
more extensive glaciation than the following LGM glaciation. The glacial record
on Chukotka Peninsula likewise suggests that Early-Middle Weichselian/Wisconsin
glaciers were more extensive than during the very limited LGM glaciation there.
Because much of Beringia was not glaciated throughout the last glacial cycle,
vast areas remained open to active eolian deposition, with resultant loess/yeodoma
dune fields and blanketing of the landscape.
رد: quaternary periodالعصر الجليدى
مُساهمة في الجمعة أكتوبر 07, 2011 6:24 pm من طرف المدير العام
The Last Glacial Maximum (LGM),
MIS 2, 20-18 ka BP
-
LGM is defined as the
maximum global ice volume as seen in marine oxygen isotope records and
coinciding with the maximum extension of middle latitudes Northern Hemisphere
ice sheets during the last glacial cycle. It is generally thought to have
occurred around 20-18 ka BP, but it is, however, acknowledged that the timing,
duration and extent of ice cover at LGM differed considerably in different
regions of the Arctic.




A recent (Svendsen et al. 2004) reconstruction of the
extent of the Eurasian ice sheet at the Last Glacial Maximum, 20-18 ka BP.


Recent interpretations of the
northern Eurasian glacial record suggest that most of the mainland of N Russia
and Siberia remained ice-free during the LGM. A huge LGM ice sheet built-up in
the Barents Sea area and extended over Svalbard and Franz Josef Land. It
probably coalesced with an ice sheet over Novaya Zemlya as well as with the
Scandinavian inland ice sheet. The Greenland inland ice expanded considerably,
filling many outer shelf basins and extending out on the shelf areas. Major
ice-streams probably developed in many Greenland fjords, feeding extensive ice
shelves fringing the ice sheet. The LGM ice cover over northern Greenland was
thin and probably cold-based except in the fjords where fast moving outlet
glaciers and ice streams terminated or fed ice shelves. In northwestern
Greenland, the ice coalesced with the Innuitian ice sheet over Ellesmere Island.
The Innuitian ice sheet probably covered most of the islands in the northeastern
Canadian Arctic during LGM. With the exception of a few nunataks on Ellesmere
Island, the margin of the Innuitian ice sheet at LGM lay offshore. West of the
Innuitian ice sheet and north of the Laurentide ice sheet, some islands (e.g.
Banks Island, Prince Patrick Island and Melville Island) experienced limited
glaciation or were ice free at LGM. Baffin Island was heavily glaciated and
partly overrun by ice of the Laurentide ice sheet advancing from the Foxe Basin
to the west. The ice drained through ice streams developing in the major fjord
systems on southeastern and eastern Baffin Island. It has been suggested that
some coastal nunataks on eastern Baffin Island remained ice-free during the LGM.
Climatic conditions in Beringia during the LGM are generally believed to have
been cold and dry. Glaciers grew in regional mountain ranges, but reached the
lowlands only south of the Alaska Range. The environment was largely a mosaic of
steppe-tundra landscapes. The lowest parts of the Bering Land Bridge were
covered with shrub tundra. The Bering Land Bridge was flooded by the sea about
11 ka BP, and closed migration routes for plants and animals between N America
and Asia.




References
and suggested further reading
:



Andersen, B.G. & Borns,
H.W.Jr., The Ice Age World, Oslo, Scandinavian University Press, 1994.



Clark, P.U. & Mix, A.
(eds.), Ice Sheets and Sea Level of the Last Glacial Maximum. Quaternary
Science Reviews,
21(1-3), 2002.



CLIMAP 1984: Difference between modern sea surface temperature
and estimated February sea surface temperature (in °C) at the last
interglaciation 120,000 years ago." Image taken from text. Crowley, T.J. and
North, G.R. Paleoclimatology - Oxford Monographs on Geology and Geophysics,
18. Figure 6.10, p.118. Oxford University Press, Inc. New York, NY. 1991.



Elias, S.A. & Brigham-Grette,
J. (eds.), Beringian Paleoenvironments. Festschrift in Honour of D.M.
Hopkins. Quaternary Science Reviews
, 20(1-3), 2001.



Elverhøi, A. (ed.),
Glacial and Oceanic History of the Polar
North
Atlantic Margins
, Quaternary Science Reviews,
17(1-3), 1998.



Forman, S.L. et al. 2004: A review of postglacial
emergence on Svalbard, Franz Josef Land and Novaya Zemlya, northern Eurasia. Quaternary Science Reviews 23, 1391–1434




Frenzel, B., Pécsi, M. &
Velichko, A. A. (eds.), Atlas of paleoclimates and paleoenvironments of the
Northern Hemisphere: Late Pleistocene – Holocene
.
Budapest,
Geographical Research Institute, Hungarian Academy of Science,

1992.




Manley, W.F., 2002, Postglacial Flooding of the Bering Land Bridge: A Geospatial
Animation: INSTAAR, University of Colorado, v1,

http://instaar.colorado.edu/QGISL/bering_land_bridge



Svendsen, J.I. et al. 2004: Late Quaternary ice sheet
history of northern Eurasia. Quaternary Science Reviews, 23,
1229-1271.



Thiede, J. & Bauch, H.A.
(eds.), The Late Quaternary Stratigraphy and Environments of
Northern Eurasia and the Adjaent Arctic Seas – New
Contributions from Queen. Global and Planetary Change
,
31(1-4), 2001.
رد: quaternary periodالعصر الجليدى
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Fossil periglacial phenomena




A general definition of periglacial
environments refers to conditions where frost-action and permafrost related
processes dominate the physical environment. Common to all periglacial
environments are circles of freezing and thawing of the ground and the presence
of permafrost, or perennially frozen ground. Presently these environments
primarily occur at high latitudes in the Arctic and Antarctic and at high
elevations in mountainous areas at mid-latitudes. About 25% of the Earth’s land
surface currently experiences periglacial conditions.




Permafrost in the Northern Hemisphere. Illustration credits:
www.solcomhouse.com/Permafrost.htm


Certain processes and geological
products are unique to the periglacial environment. These include the formation
of permafrost and wedge and injection ice, development of thermal contraction
cracks, and the formation of thermokarsts due to thawing of permafrost. Other
processes, such as frost heaving, soil creep, solifluction and wind action
processes acting on barren soils are also important in the periglacial
environment. Recognizing and interpreting fossil periglacial phenomena is an
integrated part of reconstructing Quaternary climate development. Fossil
periglacial phenomena commonly occur at mid-latitudes in Eurasia and N America;
areas that experienced periglacial conditions during cold spells of the
Pleistocene but which today have temperate climates. Pleistocene periglacial
conditions were not restricted to mid-latitude locations. Extensive areas in the
central and eastern Siberian Arctic, as well as parts of the Beringian area and
north-western Canadian Arctic remained ice-free through long periods in the
Pleistocene and were subject to intensive periglacial activity.




A number of phenomena are
indicative of frozen ground and intense frost action, and can be used for
paleoclimate reconstructions. These include:




Patterned ground, Thule, Greenland. Photo: Ólafur
Ingólfsson 1986.


Frost
fissures
. These are wedge-shaped structures
interpreted to be casts of thermal contraction cracks. Since the development
of frost fissures only occurs under permafrost conditions and intense cooling
(-15o to –20oC) these are first order indicators of
periglacial environments. Fossil frost fissures, in the form of frost fissure
polygons and ice- and sand wedge casts, have been described from extensive
areas in northern and central Europe and N America, and have been mapped for
providing evidence on distribution of Pleistocene permafrost.





Rock glacier at
Qivitut, Diskofjord, Disko Island, Greenland. Photo: Ole Humlum



Blockfields, screes and rock glaciers
. These
indicate frost action and weathering. Extensive accumulations of angular
boulders blanketing mountain plateaux and talus scree accumulations along
mountain slopes are thought to have formed primarily by frost wedging and
cracking of bedrock. In both Europe, N America and Asia, blockfields talus and
frost-shattered debris occur on uplands and mountains outside present day
distribution of permafrost, and are taken to indicate occurrences of
Pleistocene permafrost. Rock glaciers form in the periglacial zone of
mountains, and are unique permafrost landforms. They are often fed by taluses
formed upslope by frost shattering of bedrock. Relict (inactive) rock
glaciers, occurring below the periglacial zone or below the treeline, have
been reported from many mountainous areas in the world. In the Alps many of
those turned inactive by the end of the Pleistocene.




Open system pingo in
upper Eskerdalen, 35 km east of Longyearbyen, Svalbard. Photo: Hanne
Christiansen


Frost-creep and frost-disturbed deposits
. In
permafrost areas with frequent freeze-thaw cycles, frost heaving of the
surface layers can lead to down-slope movement of the material by frost creep.
This is a process that probably was more active at mid-latitudes than high
latitudes during the Pleistocene, since mid-latitudes experienced more
freeze-thaw cycles than colder arctic environments. Frost-disturbed deposits,
or cryoturbated deposits as they also are referred to, very frequently occur
in Pleistocene soils at both high and mid-latitude sites. They form by
repeated frost heaving and turbation in the active layer, as well as by
gravity loading and water-saturation in connection with thermokarst
degradation.




Tors in Öxnadalur, northern Iceland. Photo: unknown
photographer
رد: quaternary periodالعصر الجليدى
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Tors.
Hillslope and summit tors commonly occur in both high and mid-latitude uplands
and mountain areas. Tors are frequent in the alpine landscapes of e.g. western
Spitsbergen today, occurring on mountain ridges and arêtes between glaciated
cirques and valleys. Their formation has been attributed to intense frost
shattering at non-glaciated sites, although an alternative explanation for
some tors suggests they are mainly the result of chemical weathering and thus
not indicative of frost action.




Blockfield on
Amsterdamöya, Spitsbergen, Svalbard. Photo: Ólafur Ingólfsson 2001.



Fossil
permafrost
. Much of the thick permafrost in
Siberia is in disequilibria with the present climate and is largely a relict
of Pleistocene climate conditions. The present occurrence of sub-sea
continental shelf permafrost in the Arctic Ocean developed during periods of
low global sea levels during the last glacial maximum, illustrates the
preservation potential of relict permafrost. The history of permafrost in the
Russian Arctic has been traced some 2-1.5 million years back in time and it is
presumed that much of it might have started to form in the Middle Pleistocene
(>500 thousand years ago).




Massive ground ice along Tuktoyaktuk Coast, Arctic Canada.
Photo credits: S.R. Dallimore, Geological Survey of Canada


Massive ground ice.
Thick bodies of massive ground ice have been described from
northern Alaska, the western Canadian Arctic, China
and western Siberia. A favoured genesis is that most bodies of
massive ground ice formed through ice segregation and injection processes,
where excess pore water froze within the sediments. An alternative explanation
is that massive ground ice is buried glacier ice and remnants of Pleistocene
ice sheets. Other theories explaining geneses of massive ground ice include
buried lake, river or sea ice, and buried snow bank ice. It has been pointed
out that massive ground ice in the Arctic primarily occurs within the limits
of formerly glaciated areas One explanation for this
relationship is that most of these ice bodies might be relict glacier ice.



Duststorm in the highlands of Iceland, north of
Vatnajökull. Photo: Ólafur Ingólfsson 2004

Aeolian
sediments and ventifacts
. It has long been
recognized that wind action was particularly intense in the Pleistocene
periglacial environment. Huge, non-vegetated outwash plains dried out during
late summer and fall, and strong winds generated extensive dust clouds. Sand
dunes, cover sands and loess deposits, which occur in belts outside formerly
glaciated areas, mainly at mid-latitudes, are the geological products of the
periglacial wind action. Pleistocene ventifacts – or stones facetted by
dust-laden wind abrasion – are very common in mid-latitude regions of Europe
and N America, and have been used to infer prevailing wind directions at the
time of their deposition/abrasion.


The interpretation of fossil
periglacial phenomena is not always straight forward, and some structures,
sediments and forms may develop under non-periglacial conditions as well. As the
understanding of the present periglacial environment increases, there will be
improved understanding of how fossil periglacial phenomena relate to and provide
insight into climates of the past.



References and
further reading:




Black, R.F. 1976: Periglacial features indicative of permafrost: ice and soil
wedges. Quaternary Research, 6, 3-26.



French, H. M. 1996: The Periglacial Environment. Harlow, Longman, 341 pp.



Guoqing, Q. & Guodong, C. 1995: Permafrost in China, past and present.
Permafrost and Periglacial Processes
, 6, 3-14.



Ingólfsson, Ó. & Lokrantz, H. 2003:
Massive ground
ice body of glacial origin at Yugorski Peninsula, arctic Russia. Permafrost
and Periglacial Processes 14
,
199 - 215



Kondratjeva, K.A., Krutsky, S.F. & Romanovski, N.N. 1993: Changes in the extent
of permafrost during the Late Quaternary Period in the territory of the former
Soviet Union. Permafrost and Periglacial Processes, 4, 113-119.


Péwé,
T.L. The periglacial environment in North America during Wisconsinan time. In
Porter, S.C. (ed.), The Late Pleistocene. Late Quaternary environments of
the United States
, Vol. 1. Minneapolis, University of Minnesota Press,
157-189.



Vandenberghe, J. & Pissart, A. 1993: Permafrost changes in Europe during the
last glacial. Permafrost and Periglacial Processes, 4, 121-135.



Washburn, A.L. 1980: Permafrost features as evidence of climatic change.
Earth Science Reviews
, 15, 327-402

 

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