Quaternary glaciation, also known as the Pleistocene glaciation or the current
ice age, refers to a series of glacial events separated by interglacial events
during the Quaternary period from 2.58 Ma to present. During this period, ice
sheets expanded, notably from out of Antarctica and Greenland, and
fluctuating ice sheets occurred elsewhere. The major effects of the ice
age are erosion and deposition of material over large parts of the
continents, modification of river systems, creation of millions of lakes,
changes in sea level, development of pluvial lakes far from the ice margins,
isostatic adjustment of the crust, and abnormal winds. It affects oceans,
flooding, and biological communities. The ice sheets themselves, by raising
the albedo, effect a major feedback on climate cooling.
Description The Quaternary glaciation is the last of
five known glaciations during Earth's history. The other four are Huronian
glaciation, Cryogenian, Andean-Saharan glaciation, and Karoo Ice Age.
During the Quaternary Period, the total volume of land ice, sea level, and
global temperature has fluctuated initially on 41,000- and more recently
on 100,000-year time scales, as evidenced most clearly by ice cores for
the past 800,000 years and marine sediment cores for the earlier period.
Over the past 740,000 years there have been eight glacial cycles. The entire
Quaternary Period, starting 2.58 Ma, is referred to as an ice age because at
least one permanent large ice sheet—Antarctica—has existed
continuously. There is uncertainty over how much of Greenland was covered by ice
during the previous and earlier interglacials. During the colder
episodes—referred to as glacial periods—large ice sheets at least 4km
thick at their maximum also existed in Europe, North America, and Siberia. The
shorter and warmer intervals between glacials are referred to as
interglacials. Currently, the earth is in an
interglacial period, which marked the beginning of the Holocene epoch. The
current interglacial began between 10,000 and 15,000 years ago, which
caused the ice sheets from the last glacial period to begin to disappear.
Remnants of these last glaciers, now occupying about 10% of the world's land
surface, still exist in Greenland, Antarctica and mountainous regions. The
retreat of glaciers since 1850 is largely a consequence of anthropogenic
warming of the climate system during the period.
During the glacial periods, the present hydrologic system was completely
interrupted throughout large areas of the world and was considerably modified
in others. Due to the volume of ice on land, sea level was approximately 120
meters lower than present. The evidence of such glaciation in the recent past is
robust. Over the last century, extensive field observations have provided
evidence that continental glaciers covered large parts of Europe, North
America, and Siberia. Maps of glacial features were compiled after many years
of fieldwork by hundreds of geologists who mapped the location and orientation
of drumlins, eskers, moraines, striations, and glacial stream channels.
These maps revealed the extent of the ice sheets, the direction of flow, and
the locations of systems of meltwater channels, and they allowed scientists to
decipher a history of multiple advances and retreats of the ice. Even before the
theory of worldwide glaciation was generally accepted, many observers
recognized that more than a single advance and retreat of the ice had
occurred. Extensive evidence now shows that a number of periods of growth and
retreat of continental glaciers occurred during the ice age, called glacials and
interglacials. The interglacial periods of warm climate are represented by
buried soil profiles, peat beds, and lake and stream deposits separating the
unsorted, unstratified deposits of glacial debris.
Causes No completely satisfactory theory has
been proposed to account for Earth's history of glaciation. The cause of
glaciation may be related to several simultaneously occurring factors, such
as astronomical cycles, atmospheric composition, plate tectonics, and ocean
currents. = Astronomical cycles =
The role of Earth's orbital changes in controlling climate was first advanced
by James Croll in the late 19th century. Later, Milutin Milanković, a Serbian
geophysicist, elaborated on the theory and calculated these irregularities in
Earth's orbit could cause the climatic cycles known as Milankovitch cycles.
They are the result of the additive behavior of several types of cyclical
changes in Earth's orbital properties. Changes in the orbital eccentricity of
Earth occur on a cycle of about 100,000 years. The inclination, or tilt, of
Earth's axis varies periodically between 22° and 24.5°. Changes in the tilt occur
in a cycle 41,000 years long. Precession of the equinoxes, or wobbles on Earth's
spin axis, complete every 21,700 years. According to the Milankovitch theory,
these factors cause a periodic cooling of Earth, with the coldest part in the
cycle occurring about every 40,000 years. The main effect of the
Milankovitch cycles is to change the contrast between the seasons, not the
amount of solar heat Earth receives. These cycles within cycles predict that
during maximum glacial advances, winter and summer temperatures are lower. The
result is less ice melting than accumulating, and glaciers build up.
Milankovitch worked out the ideas of climatic cycles in the 1920s and 1930s,
but it was not until the 1970s that sufficiently long and detailed
chronology of the Quaternary temperature changes were worked out to test the
theory adequately. Studies of deep-sea cores, and the fossils contained in them
indicate that the fluctuation of climate during the last few hundred thousand
years is remarkably close to that predicted by Milankovitch.
A problem with the theory is that the astronomical cycles have been in
existence for billions of years, but glaciation is a rare occurrence.
Astronomical cycles correlate perfectly with glacial and interglacial periods,
and their transitions, inside an ice age. Other factors such as the position
of continents and the effects this has on the earth's oceanic currents, or long
term fluctuations inside the core of the sun must also be involved that caused
Earth's temperature to drop below a critical threshold and thus initiate the
ice age in the first place. Once that occurs, Milankovitch cycles will act to
force the planet in and out of glacial periods.
= Atmospheric composition = One theory holds that decreases in
atmospheric CO 2, an important greenhouse gas, started
the long-term cooling trend that eventually led to glaciation. Recent
studies of the CO 2 content of gas bubbles preserved in
the Greenland ice cores lend support to this idea. The geochemical cycle of
carbon indicates more than a 90% decrease in atmospheric CO
2 since the middle of the Mesozoic Era. An analysis of CO
2 reconstructions from alkenone records shows that CO
2 in the atmosphere declined before and during Antarctic glaciation and supports
a substantial CO 2 decrease as the primary cause of
Antarctic glaciation. CO
2 levels also play an important role in the transitions between interglacials
and glacials. High CO 2 contents correspond to warm
interglacial periods, and low CO 2 to glacial periods. However, studies
indicate that CO 2 may not be the primary cause of the
interglacial-glacial transitions, but instead acts as a feedback. The
explanation for this observed CO 2 variation "remains a difficult
attribution problem." = Plate tectonics and ocean currents =
An important component in the long-term temperature drop may be related to the
positions of the continents, relative to the poles. This relation can control the
circulation of the oceans and the atmosphere, affecting how ocean currents
carry heat to high latitude. Throughout most of the geologic time, the North
Pole appears to have been in a broad, open ocean that allowed major ocean
currents to move unabated. Equatorial waters flowed into the polar regions,
warming them with water from the more temperate latitudes. This unrestricted
circulation produced mild, uniform climates that persisted throughout most
of geologic time. Throughout the Cenozoic Era, the large
North American and South American continental plates moved westward from
the Eurasian plate. This drift culminated in the development of the
Atlantic Ocean, trending north-south, with the North Pole in the small, nearly
landlocked basin of the Arctic Ocean. The Isthmus of Panama developed at a
convergent plate margin about 3 million years ago, and further separated oceanic
circulation and created the Pacific and Atlantic oceans.
Effects The presence of so much ice upon the
continents had a profound effect upon almost every aspect of Earth's
hydrologic system. The most obvious effects are the spectacular mountain
scenery and other continental landscapes fashioned both by glacial erosion and
deposition instead of running water. Entirely new landscapes covering
millions of square kilometers were formed in a relatively short period of
geologic time. In addition, the vast bodies of glacial ice affected the Earth
well beyond the glacier margins. Directly or indirectly, the effects of
glaciation were felt in every part of the world.
= Lakes = The Quaternary glaciation created more
lakes than all other geologic processes combined. The reason is that a
continental glacier completely disrupts the preglacial drainage system. The
surface over which the glacier moved was scoured and eroded by the ice, leaving
myriad closed, undrained depressions in the bedrock. These depressions filled
with water and became lakes. Very large lakes were created along the
glacial margins. The ice on both North America and Europe was about 3,000 m
thick near the centers of maximum accumulation, but it tapered toward the
glacier margins. Ice weight caused crustal subsidence which was greatest
beneath the thickest accumulation of ice. As the ice melted, rebound of the
crust lagged behind, producing a regional slope toward the ice. This
slope formed basins that have lasted for thousands of years. These basins became
lakes or were invaded by the ocean. The Great Lakes and the Baltic Sea of
northern Europe were formed primarily in this way.
Pluvial lakes The climatic conditions that cause
glaciation had an indirect effect on arid and semiarid regions far removed
from the large ice sheets. The increased precipitation that fed the glaciers also
increased the runoff of major rivers and intermittent streams, resulting in the
growth and development of large pluvial lakes. Most pluvial lakes developed in
relatively arid regions where there typically was insufficient rain to
establish a drainage system to the sea. Instead, stream runoff in those areas
flowed into closed basins and formed playa lakes. With increased rainfall,
the playa lakes enlarged and overflowed. Pluvial lakes were most extensive during
glacial periods. During interglacial stages, when less precipitation fell,
the pluvial lakes shrank to form small salt flats.
= Isostatic adjustment = Major isostatic adjustments of the
lithosphere during the Quaternary glaciation were caused by the weight of
the ice, which depressed the continents. In Canada, a large area around Hudson
Bay was depressed below sea level, as was the area in Europe around the Baltic
Sea. The land has been rebounding from these depressions since the ice melted.
Some of these isostatic movements triggered large earthquakes in
Scandinavia about 9,000 years ago. These earthquakes are unique in that they are
not associated with plate tectonics. Studies have shown that the uplift has
taken place in two distinct stages. The initial uplift following deglaciation
was rapid, and took place as the ice was being unloaded. After this "elastic"
phase, uplift proceed by "slow viscous flow" so the rate decreased
exponentially after that. Today, typical uplift rates are of the order of 1 cm
per year or less. In northern Europe, this is clearly shown by the GPS data
obtained by the BIFROST GPS network. Studies suggest that rebound will
continue for about at least another 10,000 years. The total uplift from the
end of deglaciation depends on the local ice load and could be several hundred
meters near the center of rebound. = Winds =
The presence of ice over so much of the continents greatly modified patterns of
atmospheric circulation. Winds near the glacial margins were strong and
persistent because of the abundance of dense, cold air coming off the glacier
fields. These winds picked up and transported large quantities of loose,
fine-grained sediment brought down by the glaciers. This dust accumulated as
loess, forming irregular blankets over much of the Missouri River valley,
central Europe, and northern China. Sand dunes were much more widespread and
active in many areas during the early Quaternary period. A good example is the
Sand Hills region in Nebraska, USA, which covers an area of about 60,000
km2. This region was a large, active dune field during the Pleistocene epoch,
but today is largely stabilized by grass cover.
= Ocean currents = Thick glaciers were heavy enough to
reach the sea bottom in several important areas, thus blocking the
passage of ocean water and thereby affecting ocean currents. In addition to
direct effects, this caused feedback effects as ocean currents contribute to
global heat transfer. Records of prior glaciation
Glaciation has been a rare event in Earth's history, but there is evidence
of widespread glaciation during the late Paleozoic Era and during late
Precambrian. Before the current ice age, which began 2 to 3 Ma, Earth's climate
was typically mild and uniform for long periods of time. This climatic history
is implied by the types of fossil plants and animals and by the characteristics
of sediments preserved in the stratigraphic record. There are,
however, widespread glacial deposits, recording several major periods of
ancient glaciation in various parts of the geologic record. Such evidence
suggests major periods of glaciation prior to the current Quaternary
glaciation. The best documented record of
pre-Quaternary glaciation, called the Karoo Ice Age, is found in the late
Paleozoic rocks in South Africa, India, South America, Antarctica, and
Australia. Exposures of ancient glacial deposits are numerous in these areas.
Deposits of even older glacial sediment exist on every continent except South
America. These indicate that two other periods of widespread glaciation
occurred during the late Precambrian, producing the Snowball Earth during the
Cryogenian Period. Next glacial period
In popular culture, there is often reference to "the next ice age".
Technically, since the Earth is already in an ice age at present, this usually
refers to the next glacial period. In the 1970s many paleoclimatologists
were concerned with the possibility of global cooling, and suggested that the
next glacial could be rapidly approaching. The previous interglacial
periods seemed to have lasted about 10,000 years each; a report in 1972
assuming that the present interglacial period would be equally long concluded,
"it is likely that the present-day warm epoch will terminate relatively soon if
man does not intervene." Since then, our understanding of the climate system has
improved. It is known that not all interglacial periods are of the same
length and that solar heating varies in a non-linear fashion forced by the
Milankovitch orbital cycles. At the same time, it is also known that greenhouse
gases are increasing in concentration with each passing year. Based on the
variations in solar heating and on the amount of CO
2 in the atmosphere, some calculations of future temperatures have been made.
According to these estimates, the interglacial period the Earth is in now
may persist for another 50,000 years if CO
2 levels increase to 750 parts per million (the present atmospheric
concentration of CO 2 is about 398 ppm by volume, but is
rising rapidly as humans continue to burn fossil fuels.) If CO
2 drops instead to 210 ppm, then the next glacial period may only be 15,000
years away. Moreover, studies of seafloor sediments and ice cores from
glaciers around the world, namely Greenland, indicate that climatic change
is not smooth. Studies of isotopic composition of the ice cores indicate
the change from warm to frigid temperatures can occur in a decade or
two. In addition, the ice cores show that an ice age is not uniformly cold,
nor are interglacial periods uniformly warm. Analysis of ice cores of the
entire thickness of the Greenland glacier shows that climate over the last
250,000 years has changed frequently and abruptly. The present interglacial
period has been fairly stable and warm, but the previous one was interrupted by
numerous frigid spells lasting hundreds of years. If the previous period was
more typical than the present one, the period of stable climate in which humans
flourished—inventing agriculture and thus civilization—may have been possible
only because of a highly unusual period of stable temperature.
References External links
Glaciers and Glaciation Pleistocene Glaciation and Diversion of
the Missouri River in Northern Montana Correlation of late Pleistocene
glaciation in the western United States with North Atlantic Heinrich events
After the Ice Age: The Return of Life to Glaciated North America by E.C. Pielou
Alaska's Glacier and Icefields Pleistocene glaciations(the last 2
millions years) IPCC's Palaeoclimate(pdf)
Causes Astronomical Theory of Climate Change
Milutin Milankovitch and Milankovitch cycles