Transcript
Resources
• http://www.giss.nasa.gov/research/briefs/schmidt_01/
• http://earthobservatory.nasa.gov/Features/Paleoclimatology_OxygenBalance/oxygen_balance.php
• http://www.globalchange.umich.edu/globalchange1/current/lectures/kling/paleoclimate/index.html
• http://www.geo.utexas.edu/courses/302C/LABS/posted_lab6.pdf
How do we know how warm it was millions of years ago?
•1. Ice cores: bubbles contain samples of the atmosphere that existed when the ice formed. (oxygen isotopes and pCO2)
•2. Marine Sediments : oxygen isotopes in carbonate sediments from the deep ocean preserve a record of temperature.
• The records indicate that glaciations advanced and retreated and that they did so frequently and in regular cycles.
ATOM:
Isotopes
Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons.
Hydrogen Deuterium
Atoms of the same element with different numbers of neutrons are called isotopes.
More Neutrons=More MASSHYDROGEN ISOTOPES
Oxygen isotopes and paleoclimate
• Oxygen has three stable isotopes: 16O, 17O, and 18O. (We only care about 16O and 18O.)
• 18O is heavier than 16O.
• The amount of 18O compared to 16O is expressed using delta notation:
• Fractionation: Natural processes tend to preferentially take up the lighter isotope, and preferentially leave behind the heavier isotope.
18O ‰ = 18O/16O of sample -18O/16O of standard
18O/16O of standard
1000
Oxygen isotopes and paleoclimate
• Oxygen isotopes are fractionated during evaporation and precipitation of H2O– H2
16O evaporates more readily than H218O
– H218O precipitates more readily than H2
16O
• Oxygen isotopes are also fractionated by marine organisms that secrete CaCO3 shells. The organisms preferentially take up more 16O as temperature increases.
18O is heavier than 16OH2
18O is heavierthan H2
16O
What isotope of oxygen evaporates more readily? O18
or O16? Why?
Oxygen isotopes and paleoclimate
OceanH216O, H2
18O
Evaporation favorsH2
16O H218O
Precipitation favorsH2
18O…so cloud water becomes progressively more depleted in H2
18O as it moves poleward…
H218O
… and snow and ice are depleted in H2
18O relative to H2
16O.
Land
Ice
Carbonate sediments in equilibriumwith ocean water record a 18O signal which reflects the 18O of seawater and the reaction of marine CaCO3 producers to temperature.
CaCO3
Precipitation dO18
At the poles; what will the precipitation be? High in O18 or low in O18?
What isotope of oxygen will ocean water be enriched in if
precipitation is stored in the ice sheets (during cold periods)?
O18 or O16? Why?
If the temperatures are cooler, will more or less dO18 be
evaporated? Why?
What isotope of oxygen will precipitation be enriched in
during cool periods then? O18 or O16? Why?
What will the ice be enriched in during cold periods? Why?
ICE BANK
• During a glacial period, where will the O16 be stored?
• Then what will the ocean’s be enriched in?
HOW DO WE FIND Isotope ratios? Drilling Ocean Sediments ODP
Oceanic Sediments: Forams CaCO3
Oxygen isotopes and paleoclimate• As climate cools,
marine carbonates record an increase in 18O.
• Warming yeilds a decrease in 18O of marine carbonates.
JOIDES Resolution
Scientists examining core from the ocean floor.
Long-termMARINE oxygen
isotoperecord
Ice cap begins toform on Antarcticaaround 35 Ma
This may be relatedto the opening ofthe Drake passagebetween Antarcticaand S. America
From K. K. Turekian, Global EnvironmentalChange, 1996
Drakepassage
• Once the Drake passage had formed, the circum-Antarctic current prevented warm ocean currents from reaching Antarctica
Marine O isotopes during the last 3 m.y.
Kump et al., The Earth System, Fig. 14-4
• Climatic cooling accelerated during the last 3 m.y. • Note that the cyclicity changes around 0.8-0.9 Ma
− 41,000 yrs prior to this time− 100,000 yrs after this time
Do climate temperatures change?
after Bassinot et al. 1994
MARINE O isotopes—the last 900 k.y.
• Dominant period is ~100,000 yrs during this time• Note the “sawtooth” pattern..
Explain the relationship between MARINE dO18 and
temperature.
Global temperature- instrumental record (thermometers).
Why are dO18 proxies are important?
Glaciers as records of climate
• Ice cores: – Detailed records of
temperature, precipitation, volcanic eruptions
– Go back hundred of thousands years (400,000 YEARS)
Methods of Dating Ice Cores
• Counting of Annual Layers – Temperature Dependent – Marker: ratio of 18O to 16O– find number of years that the ice-core
accumulated over– Very time consuming; some errors
• Using volcanic eruptions as Markers – Marker: volcanic ash and chemicals washed out
of the atmosphere by precipitation – use recorded volcanic eruptions to calibrate age
of the ice-core– must know date of the eruption
Delta O18 and temperature
• Temperature affects 18O/16O ratio:
– colder temperatures more negative values for the delta 18O
– warmer temperatures delta 18O values that are less negative (closer to the standard ratio of ocean water)
ICE Delta 18O and temperatureExplain the relashionship.
Temperature reconstructed from Greenland Ice core.
When did the last ice age end?
Ice Age Cycles:
100,000 years between ice ages
Smaller cycles also recorded every
41,000 years*,
19,000 - 23,000 years
*This was the dominant period prior to 900 Ma
NOAA
Milutin Milankovitch, Serbian mathematician
1924--he suggested solar energy changes and seasonal contrasts varied with small variations in Earth’s orbit
He proposed these energy and seasonal changes led to climate variations
Before studying Milankovitch cycles, we needto become familiar with the basic characteristicsof planetary orbits
Much of this was worked out in the 17th centuryby Johannes Kepler (who observed the planetsusing telescopes) and Isaac Newton (who invented calculas)
r’
a
rr’ + r = 2a
a = semi-major axis (= 1 AU for Earth)
First law:
Planets travel around the sun in elliptical orbitswith the Sun at one focus
Kepler’s Laws
Minor axis
Major axis
Ellipse:
Combined distances to two fixed points (foci) is fixed
r’
a
r
r’ + r = 2a
• The Sun is at one focus
Aphelion
Point in orbit furthest from the sun
ra
ra = aphelion distance
Earth (not to scale!)
Aphelion
Point in orbit furthest from the sun
Perihelion
Point in orbit closest to the sun
rp
rp = perihelion distance
Earth
Eccentricity
e = b/a so, b = ae
a = 1/2 major axis (semi-major axis)b = 1/2 distance between foci
a
b
Kepler’s Second Law
2nd law: A line joining the Earth to the Sun sweepsout equal areas in equal times
Kump et al., The Earth System, Box Fig. 14-1
Corollary: Planetsmove fastest whenthey are closest tothe Sun
Kepler’s Third Law
• 3rd law: The square of a planet’s period, P, is proportional to the cube of its semi-major axis, a
• Period—the time it takes for the planet to go around the Sun (i.e., the planet’s year)
• If P is in Earth years and a is in A.U., then
P2 = a3
Other characteristics of Earth’s orbit vary as well.The three factors that affect climate are
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Eccentricity (orbit shape) 100,000 yrs&400,000 yrs
Obliquity (tilt--21.5 to 24.5o) 41,000 yrs
Precession (wobble) 19,000 yrs& 23,000 yrs
Q: What makes eccentricity vary?A: The gravitational pull of the other planets
• The pull of another planet is strongest when the planets are close together
• The net result of all the mutual inter- actions between planets is to vary the eccentricities of their orbits
Eccentricity Variations
• Current value: 0.017
• Range: 0-0.06
• Period(s): ~100,000 yrs
~400,000 yrs
800 kAToday
65o Nsolar
insolation
Imbrie et al., Milankovitch and Climate, Part 1, 1984
UnfilteredOrbital
ElementVariations
0.06
Q: What makes the obliquity and precession vary?A: First, consider a better known example…
g
• Gravity exerts a torque --i.e., a force that acts perpendicular to the spin axis of the top
• This causes the top to precess and nutate
Example: a top
Q: What makes the obliquity and precession vary?A: i) The pull of the Sun and the Moon on Earth’s equatorial bulge
g
Equator
N
S
g
• The Moon’s torque on the Earth is about twice as strong as the Sun’s
Q: What makes the obliquity and precession vary?A: ii) Also, the tilting of Earth’s orbital plane
N
S
S
N
• Tilting of the orbital plane is like a dinner plate rolling on a table• If the Earth was perfectly spherical, its spin axis would always point in the same direction but it would make a different angle with its orbital plane as the plane moved around
Obliquity Variations
• Current value: 23.5o
• Range: 22o-24.5o
• Period: 41,000 yrs
Precession Variations
• Range: 0-360o
• Current value: Perihelion occurs on Jan. 3 North pole is pointed almost directly away from the Sun at perihelion
• Periods*: ~19,000 yrs ~23,000 yrs
N
S
*Actual precession period is 26,000 yrs, but the orienta- tion of Earth’s orbit is varying, too (precession of perihelion)
Today
11,000 yrs agoToday
N
S
N
S
Which star is the NorthStar today?
11,000 yrs agoToday
N
S
N
S
Polaris
Which star was the North Star atthe opposite side of the cycle?
11,000 yrs ago*Today
N
S
N
S
Polaris Vega
*Actually, Vega would have been the North Star more like 13,000 years ago
800 kAToday
65o Nsolar
insolation
Imbrie et al., Milankovitch and Climate, Part 1, 1984
UnfilteredOrbital
ElementVariations
0.06
Eccentricity
Obliquity
Precession
800 kAToday
FilteredOrbital
ElementVariations
Ref: Imbrie et al., 1984
Optimal Conditions for Glaciation:
1. Low obliquity (low seasonal contrast)
2. High eccentricity and NH summers during aphelion (cold summers in the north)
Milankovitch’s key insight:
Ice and snow are not completely melted during very cold summers.
(Most land is in the Northern Hemisphere.)
Optimal Conditions for Deglaciation:
1. High obliquity (high seasonal contrast)
2. High eccentricity and NH summers during perihelion (hot summers in the north)
11,000 yrs agoToday
N
S
N
S
Optimal for glaciation Optimal for deglaciation
NH Insolation vs. Time
after Bassinot et al. 1994
O isotopes—the last 900,000 yrs
Peak NHsummertime
insolation
Big Mystery of the ice ages:
Why is the eccentricity cycle so prominent?
The change in annual average solar insolation is small (~0.5%), but this cycle records by far the largest climate change
Two possible explanations:
1) The eccentricity cycle modulates the effects of precession (no change in insolation when e = 0)2) Some process or processes amplify the temperature change. This could take place by a positive feedback loop
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