Transcript - Catalyst - University of Washington

CAUSES OF ICE AGES & GLACIATIONS
“Every great scientific truth
goes through three states:
first, people say it conflicts
with the Bible; next they
say it has been discovered
before; lastly, they say they
always believed it”
Louis Agassiz (1807-1873)
Dr. Terry W. Swanson
Department of Earth and Space Sciences and Quaternary Research Center
University of Washington, Seattle WA.
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 onetenth 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.
Proterozoic tillite and
striated bedrock
Late Paleozoic Dwyka tillite,
South Africa
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.
The climate record over the
past 1 million years shows
at least 10 glacialinterglacial cycles as
inferred by ice volume
changes over time
(measured using the
marine oxygen isotope
record).
The cause(s) of glacialinterglacial 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.
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
Present
18 ka
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.
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.
Volcanism and Climate Change
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 earth’s magnetic signature (polarity changes and inclination of the prevailing
magnetic field) is preserved in Fe-bearing minerals. As the magma or lava
crystallizes, the magnetic minerals will be aligned with the prevailing magnetic field
and be preserved in the rock.
Paleomagnetism and
Global Cooling
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?
620 Ma
30 Ma
330 Ma
240 Ma
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
30 Ma
330 Ma
240 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.
Inclination of the earth’s magnetic field varies over latitude. It is
parallel to the surface over the equator and vertical over the
magnetic poles.
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.
Dwyka Tillite,
South Africa
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.
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.
Tethyan Seaway
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.
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
90 Ma
30 Ma
10
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).
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
90 Ma
30 Ma
Drakes Passage
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).
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
30 Ma
Present
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.
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
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?
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.
Proterozoic Ice Age
Enigmas
1. global glaciation (evidence of
continental ice sheet deposits widespread).
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).
Hoffman and Schrag, 1999
5. Carbon isotope (13C/12C ratios)
suggest prolonged drop in biological
activity during glaciation.
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.
Note the decreasing d13C values in ocean water leading up to the
“Snowball Glaciation” followed by increasing values at its cessation.
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 glacialinterglacial cycles must
reconcile the fact that glacialinterglacial periods occur over
time scales of tens of
thousands of years and have
an apparent cyclicity.
Astronomical Theory of GlacialInterglacial 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.
Milutin Milankovitch (1879-1958)
-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.
Eccentricity
Obliquity
Precession
Principal Orbital Parameters:
1.Shape of earth’s orbit (eccentricity)
2.The tilt of the Earth’s axis (obliquity)
3.The season of the year when the earth is closest to the sun.
Astronomical Theory of GlacialInterglacial Cycles
Important Questions to Discuss Regarding
Milankovitch Theory
1.What are aspects of the Earth’s orbital
parameters that result in changed patterns at the
top of the atmosphere?
2.What sensitivity does the climate system display
to externally-driven insolation changes? What
about internal feedback mechanisms (can amplify
or attenuate insolation forcing)?
3.What resulting changes occur in the global
climate (how do air temperature, atmospheric and
ocean circulation, ice sheet respond to orbital
forcing that is modified by internal feedback
mechanisms)?
Milutin Milankovitch (1879-1958)
4.How does the timing of the global climaticenvironmental response correspond to orbital
forcing (time-lags or phase shifts may obscure the
causal link between orbital forcing and
environmental change)?
Solar radiation is distributed over a latitudinal gradient at the top of the
atmosphere (ave. 238 W/m2). Heat can be transferred or exchanged
within the different components of the earth’s climate system over
different time scales.
Changes in orbital parameters do not cause any significant changes in
the total amount of energy received at the top of the atmosphere over a
given year.
Orbital parameters do have an impact on the geographical distribution of
solar radiation and the seasonal timing of when that radiation is received.
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% (ave. 246 W/m2) more
solar radiation than the mean during
perihelion and 3.5% (ave. 230 W/m2) 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..
Eccentricity
-Variation in orbital eccentricity are quasi-periodic with an average period length of
~98,000 years over 5 million years. Note a 400,000 “grand cycle”
-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.
Obliquity
-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.
Precession
-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.
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. Coincident cycles of high eccentricity, low obliquity and
perihelion occurring in the northern hemisphere summer.
Astronomical Theory of Glacial-Interglacial Cycles
Coincident orbital cycles of high eccentricity, low obliquity and periheleon
occurring in the northern hemisphere summer promote the growth of ice sheet
build-up in the northern hemisphere and drive glaciation world-wide.
Can you explain why glaciations are not the same magnitude
during each subsequent cycle?
Note that the terrestrial ice sheet record
does not have the same number of