ice ages and orbital forcing.ppt - rutgers universitybroccoli/mpcc_lectures/ice_ages... · 2013. 2....

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Ice Ages and Changes in Earth’s Orbit

Mechanisms of Past Climate Change (16:107:553)

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Topic Outline

• Introduction to the Quaternary• Oxygen isotopes as an indicator of ice

volume• Temporal variations in ice volume• Periodic changes in Earth’s orbit


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• Relationship between orbital changes and variations in ice volume

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Topic Outline




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Geologic Time ScaleScale


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The Quaternary Period

• In the first half of the 19th century, Louis A i d th t id d l i tiAgassiz argued that widespread glaciation was the explanation for various unusual geologic features in much of North America and Europe.

• A lengthy scientific debate ensued, but the


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A lengthy scientific debate ensued, but the evidence for a number of continental glaciations gradually became accepted.

Moraines• As a glacier advances,

its leading edge acts g glike the blade of a bulldozer, pushing rock and debris in advance.

• These remnants of glaciation, called


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terminal moraines, mark the location of maximum ice extent.

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The Surface of the Ice Age Earth


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LGM Ice Extent in the Northeastern United States


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Moraines from earlier glaciations are most often destroyed by subsequent glaciations, so moraines are generally evidence of the most recent glacial advance.

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Topic Outline




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Oxygen Isotopes

• A small fraction of water molecules contain th h i t 18O i t d f 16Othe heavy isotope 18O instead of 16O.

• 18O/16O ≈ 1/500• This ratio is not constant, but varies over a

range of several percent.Vapor pressure of H 18O is lower than that


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• Vapor pressure of H218O is lower than that

of H216O, thus the latter is more easily

evaporated.

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δ18O

• As water vapor is transported poleward in th h d l i l h l fthe hydrologic cycle, each cycle of evaporation and condensation lowers the ratio of H2

18O to H216O, in a process called

fractionation.• This ratio is expressed as δ18O.


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This ratio is expressed as δ O.

10001618

1618161818 ×

−=

std

stdsample

OOOOOO

Oδ

δ18O vs. Temperature• As a consequence of

fractionation δ18O infractionation, δ18O in precipitation decreases with decreasing temperature.

• Ice sheets have very


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Ice sheets have very low δ18O values.

Observed δ18O in average annual precipitation as a function of mean annual air temperature (Dansgaard 1964). Note that all the points in this graph are for high latitudes (>45°). (From Broecker 2002)

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δ18O and Global Ice Volume

• As ice sheets grow, the water removed f th h l δ18O th thfrom the ocean has lower δ18O than the water that remains.

• Thus the δ18O value of sea water in the global ocean is linearly correlated with ice volume (larger δ18O → larger ice sheets).


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volume (larger δ O larger ice sheets).• A time series of global ocean δ18O is

equivalent to a time series of ice volume.

Obtaining a δ18O Time Series• Microscopic marine

organisms calledorganisms called foraminifera incorporate oxygen into their shells in the form of CaCO3.

• When these organisms die, their shells fall to the


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,sea floor and are deposited in deep sea sediments.

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Obtaining Sediment Cores• As sediments

accumulate theaccumulate, the properties of the overlying ocean are recorded sequentially.

• Sediment cores are obtained by drilling


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obtained by drilling into the sea floor.

Obtaining Sediment Cores• The sediments are

analyzed using bothanalyzed, using both chemical and visual analysis.

• To produce a time series of ocean properties a


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properties, a chronology or “age model” must be developed.

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Chronology• A simple age model

can be obtained bycan be obtained by assuming a constant accumulation rate.

• Reversals in Earth’s magnetic field can be used for benchmarks.M ti l


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• Magnetic reversals have been radiometrically dated.

Chronology• A simple age model

can be obtained by

Brunhes-Matuyamamagneticreversal

can be obtained by assuming a constant accumulation rate.

• Reversals in Earth’s magnetic field can be used for benchmarks.M ti l


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• Magnetic reversals have been radiometrically dated.

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Other Sources of δ18O Variation

• Complicating factor: Changes in ice l th l t t ib t t δ18Ovolume are the largest contributor to δ18O

variations, but they are not the only one.• Regions of the ocean in which evaporation

exceeds precipitation are enriched in δ18O, and vice versa.


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and vice versa.• Isotope separation between water oxygen

and shell oxygen depends on temperature.

Solution

• Changes in δ18O driven by variations inP E l t th fP-E are largest near the ocean surface, so δ18O from benthic (i.e., deep dwelling) forams are more representative of global ocean δ18O.

• The Pacific deep ocean temperature is


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The Pacific deep ocean temperature is very close to freezing, so it could not have been much colder during glacial periods.

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Topic Outline




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Earth’s Orbit Can Vary


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Eccentricity

Eccentricity =(distance from focus to(distance from focus to center) / (length of semimajor axis)

Eccentricity of Earth’s orbit varies from 0 to


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0.05, with 100-kyr, 400-kyr and 2 Myr periodicities.

Eccentricity


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Obliquity


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Obliquity (i.e., tilt) of Earth’s axis varies from 22° to 24.5°, with a 41-kyr periodicity.

Obliquity


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Precession


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The Earth’s axis precesses, or wobbles, with periodicities of 19 kyr and 23 kyr.

Precession


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Astronomical Theory of Ice Ages

• In 1842, J. Adhémar suggested that slow i ti i E th’ bit ld bvariations in Earth’s orbit could be

responsible for climatic changes by altering the lengths of the seasons.

• In 1875, J. Croll hypothesized that orbital variations might lead to substantial


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variations might lead to substantial changes in climate. (Colder winters →larger snow cover → glaciation)

• Renewed interest inorbital forcing of glacialcycles occurred when

Milankovitch

cycles occurred whenM. Milankovitch (1941) computed long-term variations in insolation.

• Milankovitch believed that cold summers led to


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that cold summers led to glaciation by allowing snow to survive into the next year.

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Three Conceptual Models of

Orbital Effects on Glacial

Cycles


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Temporal Variation of Orbital Parameters

• Eccentricity: Relatively 0.04 Eccentricity

low for the past 60 kyr.• Obliquity: Variations

have been quite regular; current value of 23.5° near mean.

0.01

0.02

0.03

24

23

22

(deg

rees

)

Longitude of Perihelion

Obliquity


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• Precession: Perihelioncurrently occurs near NH winter solstice.

22

0

90

180

270

360

0

(deg

rees

)

Longitude of Perihelion

Thousands of years before present

AE

SS

VE

WS

AE20 40 60 80 100 120 140 160

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In the N. Hemisphere, the effects of tilt and distance act in opposite directions although tiltdirections, although tilt dominates.

In the S. Hemisphere, the effects of tilt and distance are in phase,


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distance are in phase, yielding an amplified seasonal cycle of insolation.

Insolation at 65 N• High latitude summer insolation

(June, 65 N) has been regarded as an index of orbital forcing of glaciation. (This is the original Milankovitch hypothesis: Cool summers are beneficial to ice growth.)

• Note that the effects of precession are modulated by eccentricity


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eccentricity.• For low summer insolation:

Aphelion in summer (esp. with high eccentricity), low obliquity.

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Topic Outline




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Turning Point for Astronomical Theory of Ice Ages

• Hays, J. D., J. Imbrie, and N. J. Sh kl t 1976 V i ti i th E th’Shackleton, 1976: Variations in the Earth’s orbit: Pacemaker of the ice ages. Science, 194, 1121-1132.

• “It is concluded that changes in the earth’s orbital geometry are the fundamental


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orbital geometry are the fundamental cause of the succession of Quaternary ice ages.”

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Peaks in δ18OSpectrum

C d tCorrespond to Orbital

Frequencies


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Variance spectra for marine oxygen isotopes for the last 700 kyr (lower curve) compared with spectra for Earth’s orbital parameters (Imbrie,1985). (From Broecker, 2002)

Spectral Analysis of SPECMAP Stacked δ18O Record

• Distinct peaks in ice volume record atvolume record at orbital frequencies are present.

• These peaks are robust, even when more powerful


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more powerful spectral methods are used.

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The 100-kyr Problem


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Model 1: Calder (1974)

( )dV ( )0iikdtdV

−−=

V = ice volumei = summer insolation at 65°Ni0 = insolation thresholdk = kA (accumulation) if i < i0


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A ( ) 0k = kM (melting) if i > i0

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Model 2: Imbrie and Imbrie (1980)

• Written in dimensionless form (i.e., variables are divided by a scaling value)scaling value)

τVV

dtdV i −=

V = ice volume


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Vi = equil. ice volume at insolation ii = summer insolation at 65°Nτ = τ M if V > i (melting)τ = τ A otherwise

Model 3: Paillard (1998)


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Model 3: Paillard (1998)• Very good agreement

with record both inwith record, both in time and frequency domain.

• Weakness: Highly nonlinear, with a number of adjustable


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number of adjustable parameters.

Ice Core Paleoclimatology• As snow falls on very cold

glaciers or ice sheets and gradually is converted to ice, air is trapped in bubbles.

• This “fossil air” can be chemically analyzed to determine past atmospheric composition.

• Other paleoclimatic proxies


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(isotopes, dust, acidity) can also be determined from the ice, providing information about temperature, sulfate aerosols, precipitation.

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Multiproxy Analysis of Glacial Cycles

• Glacial-interglacial cycles are evident in a variety of paleoclimatic and paleoceanographic proxies.

• The shapes of the cycles vary somewhat among the different proxies.

• Glacial-interglacial variations in atmospheric CO2 concentration


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are substantial. (But what causes them?)

• There are uncertainties in time scales.

ice ages and orbital forcing.ppt - rutgers universitybroccoli/mpcc_lectures/ice_ages... · 2013. 2....

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