Quaternary glaciation

The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began 2.58 Ma (million years ago) and is ongoing. Although geologists describe the entire time period up to the present as an "

ice age

", in popular culture the term "ice age" is usually associated with just the most recent glacial period during the Pleistocene or the Pleistocene epoch in general. Since planet

Earth

still has ice sheets, geologists consider the Quaternary glaciation to be ongoing, with the Earth now experiencing an interglacial period.

During the Quaternary glaciation, ice sheets appeared. During glacial periods they expanded, and during interglacial periods they contracted. Since the end of the last glacial period, the only surviving ice sheets are the Antarctic and

Greenland ice sheet

s. Other ice sheets, such as the Laurentide Ice Sheet, formed during glacial periods, had completely melted and disappeared during interglacials. The major effects of the Quaternary glaciation have been the

erosion

of land and the deposition of material, both over large parts of the continents; the modification of river systems; the creation of millions of

lake

s, including the development of pluvial lakes far from the ice margins; changes in

sea level

; the

isostatic adjustment

of the Earth's crust; flooding; and abnormal winds. The ice sheets themselves, by raising the

albedo

(the extent to which the radiant energy of the Sun is reflected from Earth) created significant

feedback

to further cool the

climate

. These effects have shaped entire environments on land and in the oceans, and in their associated biological communities.

Before the Quaternary glaciation, land-based ice appeared, and then disappeared, during at least four other ice ages.

Discovery

Evidence for the quaternary glaciation was first understood in the 18th and 19th centuries as part of the

scientific revolution

.

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,

moraine

s, striations, and glacial stream channels in order to reveal the extent of the

ice sheet

s, the direction of their flow, and the locations of systems of meltwater channels. They also 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.

Description

To geologists, an

ice age

is marked by the presence of large amounts of land-based ice. Prior to the Quaternary glaciation, land-based ice formed during at least four earlier geologic periods: the

Karoo

(360–260 Ma), Andean-Saharan (450–420 Ma), Cryogenian (720–635 Ma) and Huronian (2,400–2,100 Ma).

Within the Quaternary Period, or ice age, there were also periodic fluctuations of the total volume of land ice, the sea level, and global temperatures. During the colder episodes (referred to as glacial periods, or simply glacials) large ice sheets at least thick at their maximum existed in

Europe

,

North America

, and

Siberia

. The shorter and warmer intervals between glacials, when continental glaciers retreated, are referred to as interglacials. These are evidenced by buried soil profiles, peat beds, and lake and stream deposits separating the unsorted, unstratified deposits of glacial debris.

Initially the fluctuation period was about 41,000 years, but following the Mid-Pleistocene Transition it has slowed to about 100,000 years, 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—the Antarctic ice sheet—has existed continuously. There is uncertainty over how much of

Greenland

was covered by ice during each interglacial.

Currently, Earth is in an interglacial period, which marked the beginning of the Holocene epoch. The current interglacial began between 15,000 and 10,000 years ago; this 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 some mountainous regions.

During the glacial periods, the present (i.e. interglacial) 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 about lower than present.

Causes

Earth's history of glaciation is a product of the ''internal variability'' of Earth's climate system (e.g., ocean currents,

carbon cycle

), interacting with ''external forcing'' by phenomena outside the climate system (e.g., changes in earth's orbit, volcanism, and changes in

solar output

).

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 that these irregularities in Earth's orbit could cause the climatic cycles now 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.Why do glaciations occur?
/ref> The

inclination

, or tilt, of Earth's axis varies periodically between 22° and 24.5° in a cycle 41,000 years long. The tilt of Earth's axis is responsible for the

season

s; the greater the tilt, the greater the contrast between summer and winter temperatures. Precession of the equinoxes, or wobbles of Earth's rotation axis, have a periodicity of 26,000 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 overall amount of solar heat Earth receives. The result is less ice melting than accumulating, and

glacier

s build up.

Milankovitch worked out the ideas of climatic cycles in the 1920s and 1930s, but it was not until the 1970s that a sufficiently long and detailed chronology of the Quaternary temperature changes was 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 these astronomical cycles have been in existence for many millions of years, but glaciation is a rare occurrence. Astronomical cycles correlate with glacial and interglacial periods, and their transitions, ''within'' a long-term ice age but do not initiate these long-term ice ages.

Atmospheric composition

One theory holds that decreases in atmospheric , an important greenhouse gas, started the long-term cooling trend that eventually led to glaciation. Geological evidence indicates a decrease of more than 90% in atmospheric since the middle of the Mesozoic Era. An analysis of reconstructions from alkenone records shows that in the atmosphere declined before and during Antarctic glaciation, and supports a substantial decrease as the primary cause of Antarctic glaciation.

levels also play an important role in the transitions between interglacials and glacials. High contents correspond to warm interglacial periods, and low to glacial periods. However, studies indicate that may not be the primary cause of the interglacial-glacial transitions, but instead acts as a

feedback

. The explanation for this observed variation "remains a difficult attribution problem".

Plate tectonics and ocean currents

An important component in the development of long-term ice ages is the positions of the continents. These can control the circulation of the oceans and the atmosphere, affecting how ocean currents carry heat to high latitudes. Throughout most of

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. This produced mild, uniform climates that persisted throughout most of geologic time.

But during the

Cenozoic Era

, the large

North America

n and

South America

n continental plates drifted westward from the

Eurasia

n plate. This interlocked with the development of the

Atlantic Ocean

, running north–south, with the North Pole in the small, nearly landlocked basin of the

Arctic Ocean

. The Drake passage opened 33.9 million years ago (the Eocene-Oligocene transition), severing

Antarctica

from

South America

. The

Antarctic Circumpolar Current

could then flow through it, isolating

Antarctica

from warm waters and triggering the formation of its huge

ice sheet

s. The Isthmus of Panama developed at a convergent plate margin about 2.6 million years ago, and further separated oceanic circulation, closing the last strait, outside the polar regions, that had connected the

Pacific

and Atlantic Oceans. This increased poleward salt and heat transport, strengthening the North Atlantic

thermohaline circulation

, which supplied enough moisture to arctic latitudes to create the northern glaciation.

Rise of mountains

The elevation of continents surface, often in the form of mountain formation, is thought to have contributed to cause the Quaternary glaciation. Modern glaciers correlate often to mountainous areas. The gradual movement of the bulk of Earth's landmasses away from the

Tropics

in connection with increased mountain formation in the Late Cenozoic meant more surfaces at high altitude and latitudes favouring the formation of glaciers. For example, the

Greenland Ice Sheet

formed in connection to the uplift of the West Greenland and East Greenland uplands. The Western and Eastern Greenland mountains constitute

passive continental margins

that were uplifted in two phases, 10 and 5 million years ago, in the Miocene epoch. Computer modelling shows that the uplift would have enabled glaciation by producing increased orographic precipitation and cooling the surface temperatures. For the

Andes

it is known that the Principal Cordillera had risen to heights that allowed for the development of valley glaciers about 1 million years ago.

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 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 many 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 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

Baltic Sea

and the

Great Lakes

of North America were formed primarily in this way.

The numerous lakes of the

Canadian Shield

, Sweden, and

Finland

are thought to have originated at least partly from glaciers' selective erosion of weathered

bedrock

.

Pluvial lakes

The climatic conditions that cause glaciation had an indirect effect on arid and semiarid regions far removed from the large

ice sheet

s. The increased precipitation that fed the

glacier

s 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 leading to the sea. Instead, stream runoff 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, with less rain, 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 (modern) 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

earthquake

s 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 (called "elastic"), 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 (wind-blown silt), forming irregular blankets over much of the Missouri River valley, central Europe, and northern China.

Sand

dune

s 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 . 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, which blocked the passage of ocean water and affected ocean currents. In addition to these direct effects, it also caused feedback effects, as ocean currents contribute to global heat transfer.

Gold deposits

Moraines and till deposited by Quaternary glaciers have contributed to the formation of valuable placer deposits of gold. This is the case of southernmost Chile where reworking of Quaternary moraines have concentrated gold offshore.

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 (300 to 200 Ma) and the late Precambrian (i.e. the Neoproterozoic Era, 800 to 600 Ma). 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.

One of the best documented records 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

The warming trend following the Last Glacial Maximum, since about 20,000 years ago, has resulted in a Early Holocene sea level rise, sea level rise by about .
This warming trend subsided about 6,000 years ago, and sea level has been comparatively stable since the Neolithic.
The present interglacial period (the Holocene climatic optimum) has been fairly stable and warm, but the previous one was interrupted by numerous cold spells lasting hundreds of years. If the previous period was more typical than the present one, the period of stable climate, which allowed the Neolithic Revolution and by extension human civilization,
may have been possible only because of a highly unusual period of stable temperature.

Based on Milankovitch cycles, orbital models, the cooling trend initiated about 6,000 years ago will continue for another 23,000 years.
Slight changes in the Earth's orbital parameters may, however, indicate that, even without any human contribution, there will not be another glacial period for the next 50,000 years. "Berger and Loutre argue in their Perspective that with or without human perturbations, the current warm climate may last another 50,000 years. The reason is a minimum in the eccentricity of Earth's orbit around the Sun."
It is possible that the current cooling trend may be interrupted by an stadial, interstadial phase (a warmer period) in about 60,000 years, with the next glacial maximum reached only in about 100,000 years.

Based on past estimates for interglacial durations of about 10,000 years, in the 1970s there was some concern that global cooling, the next glacial period would be imminent. However, slight changes in the eccentricity of Earth's orbit around the Sun suggest a lengthy interglacial period lasting about another 50,000 years.
Additionally, Human impact on the environment, human impact is now seen as possibly extending what would already be an unusually long warm period.
Projection of the timeline for the next glacial maximum depend crucially on Carbon dioxide in Earth's atmosphere, the amount of in the atmosphere.
Models assuming increased levels at 750 parts per million (Parts per million, ppm; current levels are at 407 ppm) have estimated the persistence of the current interglacial period for another 50,000 years. However, more recent studies concluded that the amount of heat trapping gases emitted into Earth's oceans and atmosphere will prevent the next glacial (ice age), which otherwise would begin in around 50,000 years, and likely more glacial cycles.

References

External links

Glaciers and Glaciation

*
*


* (the last 2 million years)
IPCC's Palaeoclimate(pdf)
;Causes
Milutin Milankovitch and Milankovitch cycles

{{DEFAULTSORT:Quaternary Glaciation
Ice ages
Pleistocene
Pleistocene events
Quaternary events, Glaciation