Geography 40
Global Environmental Change
Fall 2002



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Lecture 20: Pleistocene Glaciations
Read Ch. 11 Kump et al on reserve in E.S.L.
Recap:
Icehouse conditions returned to the Earth around 55 mya and cooling has continued since. While ice sheets have existed on Antarctica for 10s of millions of years, only during the last 2.5 my, has there been continental glaciations in the Nothern hemisphere extending from pole to the midlatitudes.

Pleistocene Epoch: glacial interval, starting around 1.8 my ago ending 10,000 years ago, has been characterized by regular cycles of growth and decay of No. Hemisphere continental icesheets.
As we narrow the focus of study to relatively short timescales, cyclical changes in orbital parameters, to be discussed, become important. These same forcings had effects during the Mesozoic and throughout earth’s history, but magnitude of the forcing is small compared with the changes we discussed on the 100 my time scales.

Geologic evidence of Pleistocene Glaciation

Glacial Erratics – huge boulders, or lumps of rock, that dot the landscape in Northern Europe. These were explained as evidence of the biblical flood.
glacial-erratics

Moraines: ridges of sediment that are deposited at the front and sides of ice sheets, left behind when the ice melts.
matterhorn
Sediment is called till, and is a mixture of material from mud to boulder-sized rocks, and of various composition, reflecting rocks eroded from many different places and all jumbled up and transported by the advancing ice sheet.
glacial-till
Loess is the silt-sized till, which can be picked up and blown very far distances by wind. Loess makes up much of the rich soils of the Midwest U.S. grain belt.
LOESS
Glacial striations: gauges carved out of bedrock by the passing of glaciers. Were explained as evidence of passing carriages.
striation

Oxygen Isotope Record of Glacial Interglacial Oscillations
Deep sea cores have been collected and the sediments studied since the 1950s. Earliest work was looking at the fossil foraminefera assemblages. Forams live in a variety of habitats in the ocean, and the different species that are adapted to different habitats look different and can be distinguished in the record.
Benthic (bottom dwelling species) versus Planktic.(surface dwelling species)
Isotopic evidence from forams: The temperature of the waters surrounding forams affects the isotopic composition of their shells such that: the cooler the water T., the higher the delta 18O.
The amount of polar ice can also be determined by the isotopic composition of the foram shells, evaporation off the ocean preferentially removes 16O from the ocean, and the 18O enriched atmospheric water vapor then falls as snow on ice sheets.

Milankovitch Cycles:
Earth’s climate has been oscillating between glacial and interglacial states with periods of 100 ka and 40 k and the reason appears to be involved with changes in the way Earth orbits the sun:

3 changes that are regular and predictable over thousands of years:
-changes in Earths orbit around the sun elliptical: Eccentricity: today about 1.7%, shift is from 0 to about 0.06.
-change in tilt (obliquity) of Earth’s spin
-change migration of the spin axis and the migration of the elliptical orbit as a whole (Precession)

Milankovitch – around the turn of the 20th century: proposed to calculate the climate of Earth throughout all time and the climates of Mars and Venus also throughout time based on astronomical cycles.

Lecture 21: Pleistocene and Holocene
Ch. 11 Kump et al

Milankovitch Cycles:

Earth – Sun relations today:
earth’s tilt

earth’s orbit around the sun + perihelion/aphelion
Long term changes in Earth’s orbit:
Changes in axial tilt through time (obliquity): if no tilt, then no seasons; if 90 degree tilt, seasonal alterations between day-long darkness and day-long direct overhead sun. In fact, tilt varies between a narrow range, 22.2 to 24.5, and these are on 41,000 year cycles due to pull of other large planets like Jupiter.
cycle of tilt
effect of tilt

Changes in Earth’s eccentric orbit through time: shape of the ellipse changes over long time scales due to pull of other planets. There are 2 major cycles, one at 100K and one at 413K, but really these are a blend of other cycles of varying periods around 100k and 413K
Figures show the change in eccentricity if e = 0 and e = .5

changes in eccentricity
Precession of solstices and equinoxes around earth’s orbit. See fig. 8.2 again. The positions of the solstices and equinoxes in relation to the eccentric orbit are not fixed: they gradually shift position with respect to earth’s orbit and so with respect to the perihelion and aphelion. The distance from earth to the sun has veried with time for each of the seasons, and these changes in distance produce changes in solar radiation received on Earth.
precession of axis
precession of Earth’s axis. Earth’s slow wobbling motion causes its rotational axis to point in different directions through time, but not a smooth cycle: has decelerations and accelerations in the cycle.
precession of ellipse
precession of ellipse. Also not a smooth cycle with acceleration and decelerations.
The two combine to produce a dominant cycle of approximately 23K yrs, but amplitude of that that cycle is modulated by a weaker, 19K year cycle.
combined precession
combined precession produces the precession of the equinoxes.

Looking for orbital scale changes in climate records: many climate records contain 2 or even 3 orbital scale cycles at the same time and it can be difficult to disentangle them.

Orbital scale changes in monsoons:
Monsoon circulation over No. Africa.
monsoon North Africa
Orbital monsoon hypothesis: because modern monsoon circulations are linked to changes in the strength of solar radiation during summer and winter, longer term, orbital scale changes in the strength of summer and winter insolation should have affected the strength of the monsoons in the same manner in the past.
monsoon hyp
Evidence for Orbital monsoon – first, conceptual model of monsoon response to summer insolation:
conceptual
Med stink muds
The stinky muds in Mediterranean sea are black, organic rich mud layers (sapropels) that were produced when strong monsoons occurred, bringing a fresh water lens to the surface of the Mediterranean via the Nile river. This caused oxygen depletion at depth, because reduced circulation, resulting in black muds. Also, the river inflow brought nutrients, which enhanced surface plankton, which then died and added to the oxygen depletion.
Fresh water diatoms in the sediments of tropical Atlantic. These blew from dried lakebeds from No. Africa.
diatoms
Lakes filled by strong monsoon rains, later dried out and exposed to erosion. Evidence of 23k cycle.

Summary:
Earth’s climate has been oscillating between glacial and interglacial states with periods of 100 ka and 40 k and the reason appears to be involved with changes in the orbital parameters of the earth around the sun:

3 changes that are regular and predictable over thousands of years:
- Eccentricity: today about 1.7%, shift is from 0 to about 0.06.
- Obliquity change in tilt of Earth’s spin
- Precession of the spin axis about the vertical (wobble)


Changes that are important to seasonal contrasts: precession of perihelion and aphelion.
Planet’s obliquity: tilt of the axis which creates contrast between the seasons.

Precession of spin axis: precess is time taken to complete one full cycle. About 25,700 years, but there is another precession of perihelion occurring, and is superimposed upon the spin axis, this results in two major cycles, periods of 23K and 19K.
Obliquity variations: wble varies from 22 to 24.5 degrees, with cycle length of about 41k: change does not alter the total amount of sunlight striking earth, but determines the extent of seasonal contrasts.
Eccentricity variations: combined effect of all the planets causes earth’s eccentricity to vary between 0 and 0.06. Two main periods are predicted, about 100K and 400K. This does involve a change in amount of solar energy hitting earth, increasing about .2% more at maximum than at minimum.
Lecture 22: Glacial climate feedbacks

The periodic changes in earth’s orbital parameters during the Pleistocene were subtle. In particular the dominant periodicity of glacial-interglacial fluctuations has been 100K, so some sort of amplifier is needed.

Albedo
Ice-albedo feedback. Positive feedback. Small changes in summer insolation result in large changes in ice-sheet growth and global temp.
Dynamics of glaciation are especially tuned to a frequency of one cycle per 100K years and should respond sensitively to the eccentricity-induced changes. Models show that instabilities develop as ice sheet becomes very large so subtle changes in higher latitudes can lead to catastrophic destruction.

Greehouse effect
Vostok Ice core. Gas bubbles in the ice core have been measured and fluctuations in CO2 measured. Rapidity of changes is remarkable from glacial stage levels (190 ppm) to nearly contemporary levels (240 ppm) occurred over only 4000 years 16K to 12 K bp.
Feedbacks affecting CO2 on glacial timescales
Role of biological pump: surface ocean – photosynthesis, settling and decomposition of organic material at depth. The low CO2 concentrations in the glacial intervals represents greater photosynthesis. Why:
Shelf Nutrient hypothesis – more shelf area exposed for nutrient supply. Positive feedback. Phosphate.
Iron fertilization hypothesis. Trace nutrient blown as surface winds became greater blowing dust further. Positive feedback
Coral Reef hypothesis: Corals grow close to surface between 30 N and 30 S. Production of CaCO3 releases CO2 to the atmosphere (temporary effect, after 10 of 1000s of years the excess is weathered and redeposited as CaCO3). As sea levels rises, the continental shelves are flooded. In tropics, reef growth resumes and releases CO2, amplifying original climate warming. Positive feedback.
Terrestrial biomass. As forests return during warming, more CO2 is taken up in the plant biomass and this is a negative feedback.
Cloud albedo feedback.
During the glaciations, cloud condensation particles (aerosols) were greater, intensifying cooler conditions. The particles in the atmosphere produced by marine algae which grew more rapidly during glaciations because more mixing of deep waters due to more turbulence.

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