Vocabulary and Concepts Chapters 8, 9 and 10
A. How do we know there was an Ice Age?
1. Evidence of alpine glaciation
Glacial deposition
Till: Unsorted mass of clay, silt, sand, and boulders
Moraines: Linear or arcuate masses of till that outlines the former ice
margin.
Erratics: Stones deposited by ice (Usually from a distant source)
Loess: Wind-blown glacial dust
Glacial erosion
U-shaped valleys
Striations (scratches), grooves
Agassiz popularized the idea of a recent ice age. There were three points in the glacial
theory:
a. Alpine glaciers: Modern glaciers in the Swiss Alps and Norway once extended
farther down their valleys than today (historical evidence)
b. Mountain ice caps: Glacier cover in the Swiss Alps and Norway had once been
much thicker (or mountains higher) so that ice caps formed and spread into
adjacent regions.
c. Continental ice sheets: The most radical aspect was the idea that vast sheets of
ice spread from the Arctic and covered all of Europe as far south as the
Mediterranean, as well as most of Canada and well into northeastern and
midwestern United States.
2. Evidence of continental glaciation?
glacial erosion
glacial deposition
3. How many glaciations?
The limits of the terrestrial record
The advantage of marine archives
Marine archives indicate more than 20 glacial cycles in the Quaternary.
There is a change in the character is ice volume changes between 0 and
0.9 Myr, compared to 0.9 to 2.7 Myr. Frequencies of ice volume changes
18
as preserved in
O in marine archives.
0-900 ka: main power is at 100 ka
900-2500 ka: main power is at about 40 ka
Throughout all intervals there is also a periodicity of about 20 ka.
Periods: 100 ka, 40 ka, 20 ka.
B. Insolation variations in the Quaternary.
For continental ice sheets to grow over North America and Eurasia in the
Quaternary, conditions must have been different from present.
Insolation: The amount of solar radiation received per unit area (e.g. watts/m2)
Could changes in solar radiation (=temperature) have caused ice ages?
1. Earth’s orbit and the distribution of solar radiation.
a. Tilt of Earth’s axis. Tilt causes the seasons. Without tilt there would be no
seasons.
b. Earth’s eccentric orbit. Not a perfect circular orbit. Perihelion (closest) = 153
M km; Aphelion (farthest) = 158 M km. 3% difference. Controls the insolation/unit area
striking Earth. Elliptical orbit amplifies seasonal contrasts.
2. Irregularities in Earth’s Orbit: Tilt, Precession and Eccentricity.
a. Tilt: varies between 22.2° and 24.5°
Complete cycle lasts 41,000 years.
Cycles are regular in magnitude.
Changes are in phase in both N and S hemisphere
b. Precession of the equinoxes: Cycles are about 21,000 years, but are opposite
phase between N and S Hemispheres
c. At low latitudes PRECESSION influences dominate the changes in insolation
At high latitudes AXIAL TILT dominates the changes in insolation.
C. Insolation control of Monsoons (Ch 9)
1. Monsoon circulation
Driven by difference in heat capacity of land and sea. Accentuated by the fueling of
latent heat release as water evaporated from the adjacent ocean condenses as it rises over
land. Strongest where there are large land masses, warm oceans, high evaporation, latent
heat release. Monsoons are stronger where there are high elevations close to the sea.
2. The African Monsoon: Twice the area of the US, all in the tropics/subtropics.
Warm ocean nearby. Present-day distribution of vegetation is strongly controlled by
limits of monsoon moisture.
3. Orbital controls on summer monsoons.
Because modern monsoons are a clear response to seasonal changes in insolation,
Quaternary changes caused by Milankovitch variations should have similarly impacted
the intensity of monsoon circulation. At its peak, summer insolation in the subtropical
Northern Hemisphere was about 12% higher than at present. Winter insolation
necessarily was 12% less.
4. Evidence of orbital control of monsoons
What kind of evidence should we look for?
Archives? Proxies?
Lake level records in closed-basin lakes.
Pollen records of vegetation.
Flooding of rivers.
Winter wind-blown dust plumes.
Possibly changes in sea surface conditions of adjacent oceans.
What about timing?
Milankovitch variations suggest the most recent time of maximum summer insolation
occurred about 10,000 years ago (+8%). 125,000 years ago it was +12%.
4a. Closed-basin lakes in Sahara, and sub-Saharan Africa as a type example.
Bir Tarfawi and Bir Sahara: A Case Study. In historic times Bedouin traders have used
the well into the Nubian Sandstone to water their camels during trading safaris from
Sudan to the Nile Valley. Have humans actually lived here in times past? What is the
evidence for this, and what does it mean if they did? Cross-section through the Bir
Tarfawi depression shows how wind deflation has slowly lowered the basin. Oldest lakes
are the highest, and youngest are the lowest.
Bir Tarfawi and Bir Sahara show clear evidence for intervals of permanent lakes
between about 125,000 and 80,000 years ago. They also show evidence of wetter than
present conditions between 11,000 and 6000 years ago. In the lakeshore deposits of
100,000 years ago there is evidence that humans made permanent settlements and hunted
lions, hippos, zebras as well as smaller game. To support these large animals requires
about 600 mm of rainfall each year, about twice what Boulder receives! There is no
evidence of permanent lakes in the hyper-arid Sahara during the Holocene (the last
10,000 years), but there is evidence that humans lived in the region and hunted
grazing animals. This requires more rainfall than present (which is about zero).
The proxies for strong monsoon circulation from the hyper-arid Sahara are consistent
with a dominant Milankovitch control. But are they exactly coincident with insolation
forcing? Why does monsoon intensity lag behind insolation forcing?
Other proxy records that show monsoon changes tracking insolation.
The flow of the Nile, exhibits strong variations in phase with the comings and goings of
the African Summer Monsoon. Satellite sensors that can penetrate dry sand reveal
ancient stream channels buried beneath Saharan dunes that were occupied during times of
strong summer monsoon activity.
What happens when a closed basin lake that carries permanent water during an
interval of strong summer monsoon activity dries out in a subsequent arid phase?
Deflation. Wind easily erodes some of the dry fluffy lake muds. Where can we find an
archive to record this deflation? What direction does the wind blow?
Do the changes in wind impact adjacent sea surface conditions? During weak
monsoons the trades are vigorous, causing upwelling, greater NPP, shallow
thermoclineDuring strong monsoons the trades are diverted, so less vigorous, less NPP,
deeper thermocline, warmer surface watersThis contrast in SST should be seen in
changes in foraminfera assemblages (proxies for SST) in marine archives.
Based on the proxy records for the African Summer Monsoon (both strong times
and weak times) preserved in archives on land and adjacent oceans, there is a strong
correlation of strong summer monsoon with maximum summer insolation at 30 °N.
Adjacent oceans have less upwelling, deeper thermocline, warmer surface waters during
peak monsoon flow, and receive sediment and biology from wind-deflated lake basins
during times of weak summer monsoons.
4b. Milankovitch control on the Australian Summer Monsoon: A case study
Tracking the Australian Summer Monsoon on Milankovitch timescales: What is the
geologic evidence we will be looking for?
a. Changes in levels for closed-basin lakes
b. Changes in river flow
c. Wind deflation (erosion) of dry lake beds
d. Changes in vegetation (with different moisture requirements)
e. Changes in animals dependent on veg.
f. Human occupation more common in wet times
Lake Eyre, Arid central Australia.
The Lake Eyre catchment reaches into the region of the Australian Summer Monsoon,
but only in exceptional years is their enough rainfall to fill the streams with enough water
that they deliver runoff into the lake. Lake Eyre catchment is monsoonal, with a clear
summer precipitation maximum. Lake Eyre is normally a dry salt playa, with a thick crust
of salt.
Observation: A hole through the salt crust reveals an even thicker buried salt layer, about
a meter below the surface. It is separated from the upper crust by about 60 cm of lake
muds. The muds started to accumulate about 10,000 yr ago; in the past 5000 years little
mud has accumulated. More lake muds are beneath the lower crust; the age of the mud
just below the lower crust is >30,000 years.
Fact: Salt dissolves very readily in water. In fact, during every historic flooding event
the entire salt crust has dissolved, and then re-precipitated, as the lake dried.
Interpretation: (time runs forward, so we start at the bottom)
1) Until 30,000 yr ago, at least intermittant floods, delivered mud to the basin.
2) Very dry conditions between 30 and 10 ka caused salt to harden. No monsoon.
3) Monsoon re-invigorated about 10 ka, but so quickly that the newly formed lake could
not dissolve all of the old salt crust before burying the deeper parts of it with mud washed
in by the newly flowing streams. Relatively strong monsoon.
4) Lack of much mud younger than 5 ka indicates weakening of the monsoon in the late
Holocene.
Evidence of wetter times, evidence of arid times…..deflation = erosion by wind of driedup lake basins. The sand dunes are currently stable, so it was drier when they formed.
Taking all of the evidence for wet and arid phases, and the dates on each, allows
development of a paleo-monsoon intensity record. The timing of the peak Australian
Summer Monsoon intervals is in-phase with minimum northern hemisphere January
insolation (peak NH winter monsoon forcing) and out-of-phase with southern hemisphere
insolation forcing. This is contrary to a simple first-principled argument of how a
monsoon might operate over Australia.
What other variables should we be thinking about on these time scales that might
influence monsoon intensity?
a. Size of the continent
b. Are there mountains to enhance monsoon circulation?
c. Sea surface temperatures
d. Sea level
Summary of the Australia example
Times of intensification of the Australian Summer Monsoon can be identified from
specific traces left in the environment, related to old shorelines, evidence of greater stream
discharge, the presence of plants and animals requiring greater moisture. Evidence for
arid phases includes dried-up lake basins and deflation of those basins into longitudinal
dunes. Dating of these traces indicates peak monsoon rainfall occurred at times of weakest
insolation forcing, not as predicted from first principles. Instead, because Australia is
small, and flat, it is overwhelmed by its large northern neighbor, Asia, and the Asian
Winter Monsoon, paces the intensity of the Australian Summer Monsoon, modulated by
sea level and SST.
D. Insolation control of Ice Sheets (Ch. 10)
1. Irregularities in Earth’s orbit, Tilt, Precession and Eccentricity, alter the
seasonal and geographic distribution of insolation across the planet.
2. The deep-sea record of ice volume changes 18O of sea water as recorded
by carbonate fossils [foraminifera] in deep sea sediments) show regular oscillations at
Milankovitch frequencies.
3. How might Milankovitch variations control the growth and decay of
continental ice sheets?
How do we define ice sheet “health”?
How do ice sheets grow and disappear
An ice sheet in equilibrium (Antarctica, Greenland today) requires a balance between the
snow that falls in winter (accumulation) and the snow and ice that melts in summer
(ablation).
At the higher elevations, snowfall exceeds melt (accumulation zone); at lower
elevations, ablation exceeds snowfall (ablation zone). In the ablation zone the ice sheet
is maintained only by the constant flow of ice from the accumulations zone.
Accumulation: all by direct precipitation
Ablation:
Melting caused by sunshine
Melting caused by warm air masses originating elsewhere (even rain)
Calving
Antarctica loses almost all of its mass by calving
Boundary between accumulation and ablation zones is the equilibrium line. At this
elevation, exactly as much snow melts in summer as falls in winter
Typically, two thirds of an ice sheet or a glacier is in the accumulation zone, and only one
third is in the ablation zone. The asymmetry of melting and snowfall tell us that
temperature is the dominant control on ice sheet health.
If accumulation exceeds ablation (positive mass balance), the ice sheet will
grow. Conversely, if ablation exceeds accumulation (negative mass balance), the ice
sheet will shrink.
Because precipitation on large ice sheets tends to get less at higher
elevations (colder temperatures), whereas melting gets dramatically faster at lower
elevations, there is a non-linear relationship between the amount of area in the ablation
and accumulation zones.
Temperature is closely correlated with insolation. Consequently, periods of
greater summer insolation in the regions where continental ice sheets form, will result in
negative mass balance, and conversely periods of low summer insolation will be periods
of ice sheet growth. Milankovitch theory proposes that when NH summer insolation is
strong, ablation is greater and either ice sheets melt or do not form. When NH summer
insolation is low, lower melt allows ice sheets to form or to expand.
Minimum tilt, summer at aphelion (farthest from sun) favors NH ice sheet growth.
Maximum tilt, summer at perihelion bad for NH ice sheets
Summer insolation at 60 °N: maximum tilt and precession seasonality melt ice sheets;
min tilt and precession seasonality grow or expand ice sheets. Summer insolation at 60
°N was at a minimum 20 ka ago, the peak of the last glacial maximum. Summer
insolation at 60 °N was at a maximum 10 ka ago, long before the ice sheet disappeared
(about 5 ka ago).
4. Complications in the Milankovitch theory of Ice Ages
a. Response times of ice sheets to climate: Ice sheets are inherently sluggish ….
Lots of mass, lots of cold. … They take a long time to grow and a long time to melt.
They have response times between 10,000 and 20,000 years.
b. Ice-elevation feedbacks. When ice sheets are higher, more of their surface lies
above the Equilibrium Line with the same climate forcing.
c. Isostasy feedbacks
Isostasy: When two objects of different density are combined, they will adjust their
heights to reflect the differences in density. Because the density of ice (~1 g/cm3) is about
1/3 of rock (3.3 g/cm3), the amount of isostatic depression under an ice load is about 1/3
of its total thickness.
Plastic deformation of rock in the asthenosphere is required to explain the
depression of the crust. It takes more than 10,000 years to reach isostatic equilibrium.
The half-life is about 3000 yr. Around the center of the former ice sheet, the land is still
rising. Around Hudson Bay it is rising about 1 m/century, even though the ice sheet
melted away 8000 years ago.
A 3-km-thick ice sheet sinks ~1 km due to isostatic depression of the underlying
crust. The lowering of the ice surface by 1 km is equivalent to a warming of 6 °C. Free
air adiabatic lapse rate: 0.6 °C / 1OO m vertical rise in elevation. The delay in isostatic
compensation keeps the growing ice sheet at high elevations (where it is cold): Positive
feedback for ice sheet growth. Isostasy will eventually (10 to 20 ka) lower ice sheet
height, making it more vulnerable to melt at next insolation max. Positive feedback on
ice sheet melt.
5. Speeding up the ice sheet flow: slip and calve
a) Frozen = slow ice deformation. If the glacier is frozen to its bed (cold glaciers)
all flow is by internal deformation of the ice. This is SLOW. Typical ice
velocities from deformation are a few meters per year.
b) If the ice is not frozen to its bed, it can slide. This is faster, because sliding
reduces the frictional coupling of the ice to its bed. Tens of meters per year.
c) If there is enough water at the base of the ice, the ice can actually float on a thin
film of water and clip right along. Frictional resistance is reduced to near zero…
speeds of up to 5 to 10 kilometers (5000 to 6000 m) per year. These areas are
called Ice Streams or Surging Glaciers.
d) Calving: Most ice streams reach the sea where they calve icebergs into the
ocean. Difficult to predict calving, but it can remove VERY LARGE amounts of
an ice sheet by transferring unmelted ice directly into the ocean where it floats
away to parts unknown to melt.
d) Calving = Allows fast glacier flow to reduce the size of an ice sheet very
rapidly.
6. Anatomy of a full glacial cycle
Individual glacial cycles can be explained by Milankovitch variations of insolation. But
how does an Ice Age (the longer term period made up of many glacial cycles) start?
We know the world has been in an Ice Age for about the past 2.5 million years.
But Milankovitch variations have been going of for much longer times at about the same
level of change. Why was there no glaciation before 2.5 Myr? The Earth must have been
too warm to allow Milankovitch changes to reach the threshold for glaciers to grow.
Slow global cooling since the age of the dinosaurs (60 Myr), probably due to slow
reduction in greenhouse gases, lowered the equilibrium line globally, until finally reached
the glaciation threshold about 2.5 Myr ago.
We can see from the global 18O record that the nature of glacial cycles has
evolved through the Quaternary, even though Milankovitch variations have remained the
same. Both the magnitude and frequency of glaciations have changed. Prior to 0.9 Myr,
glacial cycles lasted about 41 ka, but after 0.9 Myr ago, 100,000 yr cycles began to
dominate. There is little explanation for this in Milankovitch.
a. The explanation for the start of an ice age is very different from the explanation
for the variations within a single glacial cycle.
b. Within the Quaternary Ice Age (the last 2.5 Myr), fundamental change from
dominantly 41 ka cycles until 0.9 Myr ago, when cycles shifted to 100 ka duration. Not
explained by Milankovitch.
d. Tilt- and to a lesser extent precession-driven changes in insolation pace the
timing and magnitude of ice sheet growth and decay.
7. Confirming estimated ice volumes.
We use 18O in marine carbonate fossils to estimate global ice volumes. We can test
these estimate by dating fossil coral reefs on stable oceanic islands. Corals incorporate
Uranium into their carbonate structure (all Uranium is radioactive) but not Thorium.
Uranium decays to Thorium, so by measuring the proportion of Uranium and Thorium in
a fossil coral we can calculate when it was alive back to about 500 ka ago. Certain coral
species always grow within 2 m of sea surface
Archive: Fossil coral reef
Proxy: Upper surface of certain coral species represent sea level (within 2 m).
Dating: Uranium decay.
a. Stable islands, like Bermuda and Hawaii, have a prominent fossil coral reef
about 5 m above present sea level dated by uranium decay to about 125 ka ago. The
global ice volume curve from 18O in marine carbonate fossils indicates the last time ice
volume was less than present was about 125 ka ago. This reinforces the 18O marine
record.
We can further test ice volume estimates by dating fossil coral reefs on small
oceanic islands that are steadily rising as a consequence of Plate Tectonics. This allows
both high-stands and low-stands of the sea to be dated. These results support the marine
18O record as a proxy for ice volume.
8. The need for Feedback.
Dominant power in the ice-volume signal is at the tilt frequency (41 ka); there is also
power at precession frequencies (ca. 20 ka) but it is less dominant despite strong
Milankovitch forcing. Strong 100 ka power observed in the ice volume signal is
virtually absent in Milankovitch.
Although much of the pacing of glacial cycles is at the same frequencies as
Milankovitch tilt and precession (41 and 20 ka), documenting its overall control of ice
sheets, the actual change in energy associated with the changes in insolation alone is
insufficient to account for the magnitude of glacial-age temperature changes. To explain
the large magnitude of global changes and the 100 ka cycles requires us to look for
feedbacks in Earths climate system (ocean-atmosphere-ice sheet interactions). = Ch 11.