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Chapter 14
Pleistocene Glaciations
Pleistocene Glaciations
I.
Geologic evidence
1. glacial deposits, etc.
2. The Oxygen Isotope Record (1970s)
II. Explanation of the glacial-interglacial periods
The Milankovitch Theory (1920s)
III. Glacial climate feedbacks
Theories for the inconsistency between The Milankovitch
Theory and the Oxygen Isotope Record
The Pleistocene Epoch
Pleistocene Epoch: 1.8 million to 10,000 years ago
Holocene Epoch: since 10,000 years ago
The Quaternary Period includes the Pleistocene and Holocene
ice sheets have grown and shrunk many times since 2.5mya
Definitions (confusing):
Glacial Period: When discussing climate change
over millions of years, periods during which the
Northern Hemisphere had ice sheets. Very few
such periods. Includes the Pleistocene
and Holocene, including current
Glacial Interval / Ice Age: time during the
Pleistocene when more than just
Greenland/Antarctica have ice sheets
Interglacial Period/Interval: time during the
Pleistocene when only Greenland/Antarctica have
ice sheets (the Holocence, includes current)
Maximum extent of the Pleistocene ice sheet
The Pleistocene Epoch
Geologic Evidence of Pleistocene Glaciations (discussed previously in this class):
1. Glacial striations
2. Glacial moraines
3. Till
4. Loess
There have been many glaciations, but evidence of previous ice ages gets destroyed
when the ice advances over the same region.
There have been four ice ages known since the 19th century,
in order from oldest to most recent:
Gunz, Mindel, Riss, and Wurm (Europe)
Nebraskan, Kansan, Illinoian, and Wisconsin (N. America)
In Central Park
Rouches Moutonnees
rock hills shaped by
the passage of ice
to give a smooth
up-ice side and a
rough, plucked
and cliffed surface
on the down-ice
side. The
upstream surface
is often marked
with striations
http://www.fettes.com/central%20park/
In Central Park
Glacial Erratic
boulder transported
and deposited by
a glacier having a
lithology different
than the bedrock
upon which it is
sitting
http://www.fettes.com/central%20park/
In Central Park
1 Blue sky over Umpire Rock
2 Erratic
3 Folds
4 Groove
5 Groove
6 Groove
7 Meeting Place
8 Outcrop or boulder?
9 Outcrop or boulder?
10 Students
11 Students
12 Striations
13 Talus or erratic?
14 Talus or erratic?
15 Erratic
16 Asymmetric hill
17 Dike in raised relief
18 X-dike: How geologists argue
http://www.fettes.com/central%20park/
The Pleistocene Epoch - Explanation of Ice Ages
Milankovitch (1920s)
Serbian mathematician Milankovitch
theorized that the waxing/waning of
ice sheets should be related to the amount,
and seasonal timing, of insolation reaching
higher latitudes. He calculated, based on the
physics of gravitational attraction between
earth, other planets, the sun, and moon, that
characteristics of the earth’s orbit, and therefore
insolation and variations in the amount of ice cover on earth,
varies over thousands of years, with three primary cyclicities:
19-23 thousand year cycle of precession (combined effects of precession 25.7 kyr cycle,
precession of perihelion, and influence of Venus and Jupiter)
41
thousand year cycle of obliquity
100 thousand year cycle of eccentricity
With not much solid geological evidence to back it up, this theory went nowhere
The Pleistocene Epoch - Explanation of Ice Ages
Milankovitch (1920s)
Precession of Perihelion
Milankovictch Cycles: periodical changes in the Earth’s climate system due to
(1) changes in the orbit around the Sun; 100,00 years period, (2) changes in the
inclination of the rotation axis; 41,000 years period, and (3) changes in the
orientation of the rotation axis with respect of the orbit; 25,700 years period.
Obliquity and contrast in insolation between the seasons
less contrast
between seasons
more contrast
between seasons
Glacial-Interglacial as Equilibrium States
Combination of various orbital
forcing causes earth’s climate to
oscillate between two states:
glacial and interglacial.
Equilibrium states can be
represented as below, with the
glacial state (longer periods) in a
deeper valley than the
interglacial state. Orbital forcing
periodically and continually send
the system from one valley to the
other, back and forth.
Glaciations
Sun is in one of the foci of the ellipse, at perihelion earth is closer to the sun than
at aphelion – more energy at perihelion than aphelion – perihelion when
Northern Hemisphere (NH) is in the middle of winter – earth moves faster in its
orbit around the sun at perihelion than at aphelion – therefore NH winters are
milder and shorter than Southern Hemisphere (SH) winters and NH summers
tend to be longer and milder than SH summers
Obliquity creates contrast between the seasons, no net change in the amount of
sunlight received by earth – the eccentricity of earth’s orbit causes this contrast to
be slightly greater in the SH than in the NH
Precession (related to direction of the spin axis) modifies the relationship
between the seasons and the distance from the sun – every half precession cycle
the hemisphere with the greatest degree of seasonal contrast switches between
north and south: when the SH has mild summers and winters, the NH has hot
summers and cold winters and vice versa. Glaciations are promoted with
northern summer at aphelion and thus low seasonal contrast (as today).
In Summary: when eccentricity is large, NH glaciation is especially favored
when precession causes the NH summer to occur at aphelion.
The Pleistocene Epoch
The Oxygen Isotope (18O) Record (1970s)
1.
The skeletons of marine
organisms record the 18O
content in the water (sediments)
2.
Marine organisms incorporate
18O preferentially in cold water
3.
Ice sheets preferentially
incorporate water with 16O,
leaving the water remaining in
the ocean enriched in 18O
4.
So, local climate variations
(water Temp), and the global
scale advance and retreat of ice
sheets, are recorded in the
18O/16O ratios in ocean
sediments
As a response to climate
changes ice sheets advance over
the continents and these changes
are recorded in the chemical
changes that take place in the
ocean. O2 isotopes analysis is
used to determine the record of
glacial-interglacial oscillations.
Net transfer of 16O from
oceans to ice sheets,
leaving behind ocean
water rich in 18O
The Pleistocene Epoch
The Oxygen Isotope (18O) Record (1970s)
What does this record tell you about climate
variations over the past few million years?
Deep-sea sediment record of the δ18O of seawater during the Pleistocene – analysis
on two genera, sediments from mid-latitude North Atlantic.
The Pleistocene Epoch
The Oxygen Isotope (18O) Record (1970s)
Fourier Analysis: mathematical procedure designed to identify the strength of different
cycles in a time series.
Fourier analysis of the oxygen isotope record shows the same periods predicted by
Milankovitch!
The Pleistocene Epoch
The Oxygen Isotope
(18O) Record (1970s)
How is the precession
effect modulated by
eccentricity?
Why is the 100K yr
cycle stronger in the
climate than in the
“forcing”?
• ε is obliquity (axial tilt).
• e is eccentricity. ϖ is
longitude of perihelion.
• esin(ϖ) is the precession
index, which together with
obliquity, controls the
seasonal cycle of insolation.
• Q is the calculated dailyaveraged insolation at the top
of the atmosphere, on the day
of the summer solstice at 65
N latitude.
•Benthic forams and Vostok
ice core show two distinct
proxies for past global sea
level and temperature, from
ocean sediment and Antarctic
ice respectively.
• Vertical gray line is current
conditions, at 2 ky A.D.
Glacial Climate Feedbacks
Theories to explain the inconsistency between the Milankovitch
prediction and observed 18O record
Why is the 100k-year cycle so pronounced in the actual climate,
when the solar forcing is so muted? There must be one or more
positive feedbacks – to amplify the initial change.
1. Ice-Albedo Feedbacks
2. Evidence for Feedbacks affecting atmospheric CO2 on Glacial
Time Scales – possible amplifying mechanisms (a, b, c)
Vostok Ice Core – link between global climate change and
variations of GHGs’ amount in the atmosphere
3. Cloud-Albedo Feedbacks
4. Changes in Terrestrial Biomass (negative feedback!)
Theories to explain the inconsistency between the
Milankovitch prediction and observed 18O record
1. Ice albedo feedbacks: definitely happens, but can not explain
global nature of the results, must be other mechanisms at work
Theories to explain the inconsistency between the
Milankovitch prediction and observed 18O record
2. Evidence for feedbacks affecting atmospheric CO2 on glacial
time scales
THE VOSTOK ICE-CORE DATA
C02, CH4, and temperature time series for the last 400,000 years
How are they derived: isotope analysis (O and H)
These co-varying changes are interpreted as evidence of system-like behavior
Theories to explain the inconsistency between the
Milankovitch prediction and observed 18O record
2a. Possible amplifying mechanism
THE BIOLOGICAL PUMP
Why should the biological pump
respond to glacial changes?
We know that sea level drops
during glacial periods, could this
lead to phosphorus fertilization?
Theories to explain the inconsistency between the
Milankovitch prediction and observed 18O record
2a. Possible amplifying mechanism
THE BIOLOGICAL PUMP
phosphorus fertilization: not
consistent with other evidence
Theories to explain the inconsistency between the
Milankovitch prediction and observed 18O record
2b. Possible amplifying mechanism
IRON FERTILIZATION
Glacial periods are believed to have
been dry and dustier, as shown in ice
cores (more dust in the core).
Theories to explain the inconsistency between the
Milankovitch prediction and observed 18O record
2c. Possible amplifying mechanism
CORAL REEF
GROWTH/DECAY
We know that sea level drops
during glacial periods, could this
lead to a coral reef feedback?
Theories to explain the inconsistency between the
Milankovitch prediction and observed 18O record
2c. Possible amplifying mechanism
CORAL REEF
GROWTH/DECAY
We know that sea level drops
during glacial periods, could this
lead to a coral reef feedback?
Theories to explain the inconsistency between the
Milankovitch prediction and observed 18O record
3. Possible amplifying mechanism – not CO2 related
Cloud albedo feedback /
ocean sulfur cycle
The Production of DMS by
phytoplankton lead to cooling of
climate via 2 mechanisms –
sulfate aerosol and cloud seeding
MSA content of Antarctic ice (bottom) and relative local temperature (top)
from hydrogen isotopic composition of the ice.
Theories to explain the inconsistency between the
Milankovitch prediction and observed 18O record
3. Possible amplifying mechanism – not CO2 related
Cloud albedo feedback /
ocean sulfur cycle
The Production of DMS by
phytoplankton lead to cooling of
climate via 2 mechanisms –
sulfate aerosol and cloud seeding
Is this link + or - ?
Theories to explain the inconsistency between the
Milankovitch prediction and observed 18O record
4. Possible negative feedback: change in forest cover,
related to atmospheric CO2
Difference in vegetation between the last glacial maximum and present day potential:
(ignoring human impact on forests).
Theories to explain the inconsistency between the
Milankovitch prediction and observed 18O record
4. Possible negative feedback: change in forest cover,
related to atmospheric CO2

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