Ice Ages and Changes in Earth s Orbit. Topic Outline

Transcription

1 Ice Ages and Changes in Earth s Orbit 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 Relationship between orbital changes and variations in ice volume 1

2 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 Relationship between orbital changes and variations in ice volume Geologic Time Scale 2

3 Geologic Time Scale Geologic Time Scale 3

4 The Quaternary Period In the first half of the 19 th century, Louis Agassiz argued that t widespread d glaciation was the explanation for various unusual geologic features in much of North America and Europe. 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 like the blade of a bulldozer, pushing rock and debris in advance. These remnants of glaciation, called terminal moraines, mark the location of maximum ice extent. 4

5 Moraines As a glacier advances, its leading edge acts like the blade of a bulldozer, pushing rock and debris in advance. These remnants of glaciation, called terminal moraines, mark the location of maximum ice extent. Moraines As a glacier advances, its leading edge acts like the blade of a bulldozer, pushing rock and debris in advance. These remnants of glaciation, called terminal moraines, mark the location of maximum ice extent. 5

6 The Surface of the Ice Age Earth LGM Ice Extent in the Northeastern United States Moraines from earlier glaciations are most often destroyed by subsequent glaciations, so moraines are generally evidence of the most recent glacial advance. 6

7 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 Relationship between orbital changes and variations in ice volume Oxygen Isotopes A small fraction of water molecules contain the heavy isotope 18 Oi instead of 16 O. 18 O/ 16 O 1/500 This ratio is not constant, but varies over a range of several percent. Vapor pressure of H 18 2 O is lower than that of H 16 2 O, thus the latter is more easily evaporated. 7

8 δ 18 O As water vapor is transported poleward in the hydrologic cycle, each cycle of evaporation and condensation lowers the ratio of H 18 2 O to H 16 2 O, in a process called fractionation. This ratio is expressed as δ 18 O. 18 δ O = 18 O Osample O Ostd O 16 O std 1000 δ 18 O vs. Temperature As a consequence of fractionation, δ 18 Oin precipitation decreases with decreasing temperature. Ice sheets have very low δ 18 O values. Observed δ 18 O 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) 8

9 δ 18 O and Global Ice Volume As ice sheets grow, the water removed from the ocean has lower δ 18 Oth than the water that remains. Thus the δ 18 O value of sea water in the global ocean is linearly correlated with ice volume (larger δ 18 O larger ice sheets). A time series of global ocean δ 18 O is equivalent to a time series of ice volume. Obtaining a δ 18 O Time Series Microscopic marine organisms called foraminifera incorporate oxygen into their shells in the form of CaCO 3. When these organisms die, their shells fall to the sea floor and are deposited in deep sea sediments. 9

10 Obtaining Sediment Cores As sediments accumulate, the properties of the overlying ocean are recorded sequentially. Sediment cores are obtained by drilling into the sea floor. Obtaining Sediment Cores The sediments are analyzed, using both chemical and visual analysis. To produce a time series of ocean properties, a chronology or age model must be developed. 10

11 A simple age model can be obtained by assuming a constant accumulation rate. Reversals in Earth s magnetic field can be used for benchmarks. Magnetic reversals have been radiometrically dated. Chronology A simple age model can be obtained by assuming a constant accumulation rate. Reversals in Earth s magnetic field can be used for benchmarks. Magnetic reversals have been radiometrically dated. Chronology Brunhes- Matuyama magnetic reversal 11

12 Other Sources of δ 18 O Variation Complicating factor: Changes in ice volume are the largest contributor t to δ 18 O variations, but they are not the only one. Regions of the ocean in which evaporation exceeds precipitation are enriched in δ 18 O, and vice versa. Isotope separation between water oxygen and shell oxygen depends on temperature. Solution Changes in δ 18 O driven by variations in P-E are largest near the ocean surface, so δ 18 O from benthic (i.e., deep dwelling) forams are more representative of global ocean δ 18 O. The Pacific deep ocean temperature is very close to freezing, so it could not have been much colder during glacial periods. 12

13 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 Relationship between orbital changes and variations in ice volume 13

14 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 Relationship between orbital changes and variations in ice volume 14

15 Earth s Orbit Can Vary 15

16 Earth s Orbit Can Vary Earth s Orbit Can Vary 16

17 Eccentricity Eccentricity = (distance from focus to center) / (length of semimajor axis) Eccentricity of Earth s orbit varies from 0 to 0.05, with 100-kyr, 400- kyr and 2 Myr periodicities. Eccentricity 17

18 Obliquity Obliquity (i.e., tilt) of Earth s axis varies from 22 to 24.5, with a 41-kyr periodicity. Obliquity 18

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

20 Astronomical Theory of Ice Ages In 1842, J. Adhémar suggested that slow variations 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 changes in climate. (Colder winters larger snow cover glaciation) Renewed interest in orbital forcing of glacial cycles occurred when M. Milankovitch (1941) computed long-term variations in insolation. Milankovitch believed that cold summers led to glaciation by allowing snow to survive into the next year. Milankovitch 20

21 Three Conceptual Models of Orbital Effects on Glacial Cycles Temporal Variation of Orbital Parameters 0.04 Eccentricity: Relatively Eccentricity 0.03 low for the past 60 kyr Obliquity: Variations 0.01 Obliquity have been quite 24 regular; current value 23 of 23.5 near mean. Precession: Perihelion currently occurs near NH winter solstice. (degrees) (degrees) Longitude of Perihelion AE SS VE WS AE Thousands of years before present 21

22 In the N. Hemisphere, the effects of tilt and distance act in opposite directions, although tilt dominates. In the S. Hemisphere, the effects of tilt and 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. For low summer insolation: Aphelion in summer (esp. with high eccentricity), low obliquity. 22

23 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 Relationship between orbital changes and variations in ice volume Turning Point for Astronomical Theory of Ice Ages Hays, J. D., J. Imbrie, and N. J. Shackleton, 1976: Variations in the Earth s orbit: Pacemaker of the ice ages. Science, 194, It is concluded that changes in the earth s orbital geometry are the fundamental cause of the succession of Quaternary ice ages. 23

24 Peaks in δ 18 O Spectrum Correspond to Orbital Frequencies 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 δ 18 O Record Distinct peaks in ice volume record at orbital frequencies are present. These peaks are robust, even when more powerful spectral methods are used. 24

25 The 100-kyr Problem Model 1: Calder (1974) dv dt = k ( i ) i 0 V = ice volume i = summer insolation at 65 N i 0 = insolation threshold k = k A (accumulation) if i < i 0 k = k M (melting) if i > i 0 25

26 Model 2: Imbrie and Imbrie (1980) Written in dimensionless form (i.e., variables are divided by a scaling value) dv dt V = V τ i V = ice volume V i = equil. ice volume at insolation i i = summer insolation at 65 N τ = τ M if V > i (melting) τ = τ A otherwise Model 3: Paillard (1998) 26

27 Model 3: Paillard (1998) Very good agreement with record, both in time and frequency domain. Weakness: Highly nonlinear, with a 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 (isotopes, dust, acidity) can also be determined from the ice, providing information about temperature, sulfate aerosols, precipitation. 27

28 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 CO 2 concentration are substantial. (But what causes them?) There are uncertainties in time scales. 28