Topic #5 Causes of Ice Ages/Glaciation I) Ice Age vs Glaciation -Periods of global glaciation

Topic #5 Causes of Ice Ages/Glaciation

I) Ice Age vs Glaciation-Periods of global glaciation-Geographic extent of global glaciation

II) Terrestrial Causes of Ice Age/Glaciation

Volcanism-particle size of ash plume-height of ash plume-water vapor and high acidity gases

The Carbon Cycle-reduction of carbon dioxide content of the atmosphere (Cause or Effect)-methane content in the atmosphere

Magnetic Reversals

Plate Tectonics-origins-tectonic plate movement-Snowball Earth

III) Celestial Causes of Ice Ages

Meteorite Impacts

Solar Input-solar constant-sunspot cycles

Orbital Parameters (Milankovich Forcing)-eccentricity, obliquity, precession of the equinoxes-combined effects on latitudinal distribution of solar insolation

Climatic fluctuations over timescales ranging from 109 to 1 years. Each successive column, from left to right, is an expanded version (by a factor of 10) of one-tenth of the previous column (taken from Kutzbach, 1974).

The higher-frequency climatic variations are “nested” within lower-frequency changes.

Over earth’s climatic history there are at least four protracted cold periods (Ice Age) preserved in the rock/sediment record.

Two of these ice ages occurred during the Precambrian while the more recent occurred during the Late Paleozoic and Cenozoic Eras.

The inferred cause(s) of a protracted Ice Age must correspond to time-scales that occur over millions of years. The inferred causes of shorter term climatic fluctuations

must correspond to similar time-scales.

Proterozoic tillite and striated bedrock

Late Paleozoic Dwyka tillite, South Africa

The climate record over the past 1 million years shows at least 10 glacial-interglacial cycles. The cause(s) of glacial-interglacial cycles within the Cenozoic Ice Age must address this time-scale of the change, as well as its apparent cyclicity.

The earth’s climate system is ultimately driven by solar radiation, but the distribution of this energy is controlled by different components within earth’s atmosphere, hydrosphere, biosphere, cryosphere and lithosphere.

Solar radiation is distributed over a latitudinal gradient at the top of the atmosphere. Heat can be transferred or exchanged within the different components of the earth’s climate system over different time scales.

Atmospheric CO2, water vapor, and CH4 are important greenhouse gases that permit short wave radiation to reach the surface of the earth, but selectively absorb long wave radiation.

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The relationship between global temperature and atmospheric CO2 level is unmistakable as shown by ice core and other paleotemperature proxy data. The question that ultimately needs to be answered regarding atmospheric CO2 content and global cooling is which comes first the “chicken or the egg?”

Feedbacks mechanisms, such as albedo, play a major role in driving climate change. The reflectivity of various surfaces is shown on the chart (right side) above. The seasonal and annual changes in the earth’s northern hemisphere albedo are shown on the image above (100% reflectivity is denoted as 1.0 on the scale). The two images on the lower right are infrared satellite images of Lake

Tahoe, CA taken during winter (left) versus late spring (right).

Albedo of the Earth: Present Conditions vs 18,000 Years Ago

As the Earth’s climate began cool during the last glaciation (25,000 - 15,000 yr ago) it’s albedo increased (shown as white to pale yellow) with increasing snow and ice cover. Note that the northern hemisphere boreal forests (shown as dark green) have as relatively low albedo and were replaced with arctic ice cover and polar vegetation with higher albedo during full glacial time. Also increasing aridification due to relatively lower moisture holding capacity of air masses led to reduced vegetation cover and increasing albedo.

Present

18 ka

Effects of Volcanism on Climate Change

Volcanic gases have been shown to be responsible for global cooling over shorter time scales.

The magnitude and extent of global cooling depends upon the force of the eruption, the amount of particular gases emitted and location of the eruption.

When volcanic gases and fine dust reach the stratosphere, they can produce a wide spread cooling effect.

The cooling effect tends to 1-2° C and last only for a few years.

Emitted SO2 gas combines with H2O to form H2SO4 aerosols in the stratosphere. Aerosols, such as H2SO4, reflect incoming sunlight and reduce the Earth’s surface temperature.

Mean acidity levels in the Greenland ice core strongly correlates with known volcanic eruptions. Increased volcanic activity index (based on acidity record) is inversely related to global temperature.

Volcanic eruptions that occur near the equator, such as Mt. Pinatubo, Phillippines, have a greater impact on global cooling because aerosols are erupted into the atmosphere in both hemispheres. Greatest aerosol

distribution following the Mt. Pinatubo eruption of 1992 is shown in red.

The climate record over the past 1 million years shows at least 10 glacial-interglacial cycles. Based on the data shown to the left, is it a reasonable hypothesis to propose that paleomagnitic fluctuations cause global cooling?

Paleomagnetism and Global Cooling

Plate Tectonics and Global Glaciation

Tectonic plate motions move the continents and determine the form of the ocean basins. Paleoclimatologists have suggested that land must be located over the polar regions in order to provide a site for ice accumulation and corresponding positive feedback mechanism (e.g., increase in albedo or uptake in atmospheric CO2) to promote global glaciation.

620 Ma

240 Ma

330 Ma

30 Ma

620 Ma

240 Ma

330 Ma

30 Ma

Inclination of the earth’s magnetic field varies with latitude. It is parallel to the surface over the equator and vertical over the magnetic poles. Paleomagnetic inclination can be used to infer paleo-latitude of igneous rocks and to reconstruct tectonic plate configurations.

Movement of tectonic plates (including relative movement of continents can explain the apparent wandering of the paleomagnetic pole over geologic time. Without invoking plate motions, paleomagnetic evidence preserved in the rock record would require unique magnetic poles for different aged rock and locations. Apparent motion of the magnetic pole can be used to reconstruct the relative positions of tectonic plates to each other.

Over earth’s climatic history there are at least four protracted cold periods (Ice Age) preserved in the rock/sediment record.

Two of these ice ages occurred during the Precambrian while the more recent occurred during the Late Paleozoic and Cenozoic Eras.

Proterozoic tillite and striated bedrock

Late Paleozoic Dwyka tillite, South Africa

To explain these protracted periods of global glaciation, paleoclimatologists require causal mechanisms that extend over time periods of millions of years.

Lithified till (tillite) deposited during the Late Paleozoic Era (Permo-Carboniferous Period) is preserved in South Africa. Similar Late Paleozoic-aged tillites are preserved in southern South America, India, Australia and Antarctica.

Dwyka Tillite, South Africa

The modern distribution of late Paleozoic glacial deposits can be understood within a context of plate tectonics. Plate configurations during the late Paleozoic placed the present day land masses of southern South America, Africa, India, Australia and Antarctica over the South Pole, where continental ice sheets could develop and expand.

Paleontologic and geologic evidence support the paleomagnetic reconstructions of the plate configurations during the late Paleozoic and early Mesozoic Eras. Following the break-up of the super-continent during the Mesozoic, faunal assemblages underwent divergent evolution and each continent had its own unique fauna. Closer faunal relationships existed between continental land masses that remained contiguous for longer time periods, such as Africa-South America and North America-Eurasia.

About 240 M.Y. ago the super-continent Pangaea began to break-up. Note the latitudinal distribution of the continental land masses and configuration of the ocean basins. An equatorial current existed along the Tethyan Seaway. Increased sea floor spreading added CO2 to the atmosphere directly from volcanism, and indirectly by causing the ocean floor to be more buoyant, which would cause a rise in eustatic sea level. Reduction in land area reduced chemical weathering (uptake of acidified rain) and the uptake of atmospheric CO2. Increasing sea water temperatures further adds CO2 to the atmosphere, as the solubility of CO2 gas is reduced. These positive feedback mechanisms enhanced global warming during the Mesozoic Era.

Tethyan Seaway

Throughout the Mesozoic Era the continental land masses were distributed near the equator and global climatic conditions were much warmer than during the Cenozoic. Note that Australia and Antarctica remained isolated from the other continental land masses over much of Mesozoic and Cenozoic. What impact do you think this had on the evolution of Australian fauna versus the other continents? Also note how the continental and ocean basin configurations changed during the Mesozoic.

By the early Cenozoic (50 M.Y. ago), large continental land masses of North America and Eurasia had migrated to higher latitudes in the northern hemisphere. The Antarctic continent was situated over the South Pole, the Indian sub-continent was impinging upon Asia, and ocean basin configurations were changing, as the Tethyan Seaway closed and the Antarctic Ocean began to form.

During the Cenozoic Era several important tectonic events occurred that had a major impact on the onset of the Cenozoic cooling: 1. Following the break-up of Pangaea North America and Eurasia migrated to high latitudes, 2. Antarctica was situated over the South Pole, 3. The Circum-Antarctic Current formed with opening of Drakes Passage, 4. Collision of the Indian Sub-continent with the Eurasian continent (uplift of the Himalayas and Tibetan Plateau), 5. Miocene marine regression and isolation of the Mediterranean Sea, and 6. Closure of Isthmus of Panama and creation of the Gulf Stream and existing thermo-haline circulation pattern.

Onset of Cenozoic Cooling

Cenozoic surface water temperature reconstruction (using planktonic foraminifera) of the Antarctic Ocean. The relative rapid adjustment of circulation systems that followed tectonic events is the likely cause of the observed, uneven, stepped history of Cenozoic cooling shown on the upper left.

Onset of Cenozoic cooling is likely related to the poleward migration of North America and Eurasia. Antarctica becomes isolated over the South Pole by ~50 Ma BP as Australia moved north. The southern ocean develops and initiation of Antarctic Circumpolar Current (ACC). Think about feedback mechanisms (e.g., albedo, CO2 uptake).

90 Ma

30 Ma

10



India collides with the Eurasian continent ~35 Ma BP causing uplift of Himalayas and Tibetan Plateau. Drakes Passage opens ~30 Ma BP further enhancing the Antarctic Circumpolar Current (ACC). The ACC restricts warm tropical air from reaching Antarctica. Antarctic ice begins to develop (think about feedback mechanisms again with ice development, ocean cooling).

90 Ma

30 MaDrakes Passage



By the Miocene the Antarctic ice sheet is well-developed (ice-rafted sediment observed at sea level). Marine regression (40-50 m) in late Miocene caused isolation of the Mediterranean Sea and Messinian salinity crisis. Water spilling into basin evaporated and major salt deposition occurred. Global ocean salinity fell by 6%. What effect would this have on sea ice production.

30 Ma

Present



By about 3 Ma BP the northern hemisphere ice sheets began to develop (evidence ice-rafted debris in deep sea deposits). This coincident with the timing of the closure of the Isthmus of Panama and development of the Gulf Stream and north Atlantic thermo-haline circulation ,as well as major mountain building in the northern hemisphere. How do you think these events are related?

30 Ma

Present

Proterozoic Ice Age

Enigmas

1. global glaciation (evidence of continental ice sheet deposits wide-spread).

2. paleomagnetic data is not consistent with geological data (i.e., Proterozoic glacial deposits deposited at low latitude locations and near sea level).

3. Fe-rich rock mixed with glacial sediment (no oxygen in oceans during ice sheet development, then rapid oxygenation).

4. Rapid accumulation of warm water carbonates, immediately following deglaciation (cap carbonates overlie glacial deposits).

5. Carbon isotope (13C/12C ratios) suggest prolonged drop in biological activity during glaciation.

Hoffman and Schrag, 1999

Modern distribution of Proterozoic glacial deposits and carbon isotope data.



Plate configurations during the Sturtian Glaciation (730-710 Ma). Annual surface air temperature reconstruction, assuming reduced solar output (-6%)

and reduced atmospheric CO2 content.



Paul Hoffman and Daniel Schrag at the contact between the cap carbonates overlying the glacial diamicton in Namibia.

Carbon isotope data suggest that removal of oceanic carbon by biological activty was shut down during the Proterozoic glacial periods. Modern ocean water is enriched in 13C because 20% of carbon entering ocean waters is removed as organic matter, which is depleted in 13C because of isotopic fractionation.

Absence of banded iron deposits throughout much of the Proterozoic ice cover is consistent with an anaerobic ocean.

Dropstones of carbonates clasts were intermixed with banded iron formation in the MacKenzie Mts., Yukon Territory, Canada.

Hoffman and Schrag, 1999

The extreme climatic events of the Neoproterozoic ice age may have paved the way the for the Cambrian “explosion of life.” The body plans of nearly all living animals appeared in a short interval between 600 and 525 million years ago.

The climate record over the past 1 million years shows at least 10 glacial-interglacial cycles. The inferred cause(s) to explain Quaternary glacial-interglacial cycles must reconcile the fact that glacial-interglacial periods occur over time scales of tens of thousands of years and have an apparent cyclicity.



Astronomical Theory of Glacial-Interglacial Cycles

-James Croll, a Scottish natural historian first proposed the hypothesis that changes in the Earth’s orbital parameters were the ultimate cause of glaciation. The Serbian mathematician, Milutin Milankovitch formalized the theory relating changes in the Earth’s orbital parameters to climate change.

-Milankovitch calculated how the orbital changes of the Earth were related to climate change during the Quaternary.

-While in captivity during WW II Milankovitch (1941) further developed his ideas and determined that particular orbital changes are responsible for promoting the glacial cycles observed in the Quaternary sediment record.

-Although much of Milankovitch’s work has been refined, his contribution to understanding climate change is significant, as he provided the first comprehensive analysis of systematic links between orbital characteristics and global climate.

Milutin Milankovitch (1879-1958)

Elements of the Earth’s Orbital Motion Today

-Earth orbits around the sun in a slightly elipitcal path. Today the earth is closest to the sun (perihelion) around January 3rd and farthest (aphelion) around July 5th. Because of this elliptical orbit the earth receives approximately 3.5% more solar radiation than the mean during perihelion and 3.5% less than the mean during aphelion.

-The earth is tilted on its rotational axis 23.4° from a plane perpendicular to the plane of the ecliptic (orbital plane).

-None of these orbital parameters has remained constant over time because of the gravitational effects of the sun, moon and other planets on the earth..







-Variation in orbital eccentricity are quasi-periodic with an average period length of ~98,000 years over 5 million years.

-the orbit has varied from almost circular to maximum eccentricity when solar radiation receipt (outside the earth’s atmosphere) has varied by 30% between aphelion and perihelion (e.g., 210,000 years ago).

-Eccentricity variation affects the relative intensities of the seasons and has an opposite effect in each hemisphere.

Eccentricity



-Variation in axial tilt are periodic with a mean of ~41,000 years.

-the angle of inclination has varied between 22.1 to 24.5° with the most recent maximum occurring ~100,000 years ago. Inclination angle defines the tropics and the Arctic and Antarctic Circles.

-Obliquity changes have little effect on radiation reception at the equator, but increases towards high latitudes.

-Obliquity changes effect both hemispheres the same.

Obliquity



-Variation in the seasonal timing of perihelion and aphelion results from a wobble in the Earth’s axis of rotation as it moves around the sun. Precessional changes occur with a period of ~21,000 years.

-Precession changes determine timing of perihelion and aphelion relative to the extreme positions the Earth occupies as it revolves around the sun on its elliptical orbit (Precessional effects are minimal when eccentricity is low (I.e., Earth’s orbit is circular).

-Precessional effects on solar radiation reception are opposite for each hemisphere.

Precession

Astronomical Theory of Glacial-Interglacial Cycles

-Milankovitch theory states that changes in the Earth’s orbital parameters result in temporal changes in insolation, which cause significant changes in global climate.

- Much of climatic variability of the Quaternary, particularly glacial- interglacial cycles can be explained by astronomically driven insolation changes at the top of atmosphere.



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Topic #5 Causes of Ice Ages/Glaciation I) Ice Age vs Glaciation -Periods of global glaciation