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:

More Neutrons=More MASS 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.                                                                                                                    Atoms of the same element with different numbers of neutrons are called isotopes. More Neutrons=More MASS HYDROGEN ISOTOPES Hydrogen Deuterium

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. d18O ‰ = 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 H216O evaporates more readily than H218O H218O precipitates more readily than H216O 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 16O H218O is heavier than H216O

What isotope of oxygen evaporates more readily? O18 or O16? Why?

Oxygen isotopes and paleoclimate Precipitation favors H218O …so cloud water becomes progressively more depleted in H218O as it moves poleward… … and snow and ice are depleted in H218O relative to H216O. Evaporation favors H216O H218O H218O Ice Land H216O, H218O Ocean Carbonate sediments in equilibrium with ocean water record a d18O signal which reflects the d18O 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 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 d18O. Warming yeilds a decrease in d18O of marine carbonates. JOIDES Resolution Scientists examining core from the ocean floor.

Long-term MARINE oxygen isotope record Ice cap begins to form on Antarctica around 35 Ma This may be related to the opening of the Drake passage between Antarctica and S. America From K. K. Turekian, Global Environmental Change, 1996

Drake passage 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?

MARINE O isotopes—the last 900 k.y. Dominant period is ~100,000 yrs during this time Note the “sawtooth” pattern.. after Bassinot et al. 1994

Explain the relationship between MARINE dO18 and temperature.

Global temperature- instrumental record (thermometers) 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 temperature Explain the relashionship.

Temperature reconstructed from Greenland Ice core Temperature reconstructed from Greenland Ice core. When did the last ice age end?

Ice Age Cycles: *This was the dominant period prior to 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

Milutin Milankovitch, Serbian mathematician                        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 NOAA

Before studying Milankovitch cycles, we need to become familiar with the basic characteristics of planetary orbits Much of this was worked out in the 17th century by Johannes Kepler (who observed the planets using telescopes) and Isaac Newton (who invented calculas)

Kepler’s Laws First law: Planets travel around the sun in elliptical orbits with the Sun at one focus r’ r r’ + r = 2a a = semi-major axis (= 1 AU for Earth) a Major axis Minor axis

Ellipse: Combined distances to two fixed points (foci) is fixed r’ r r’ + r = 2a a The Sun is at one focus

Aphelion Point in orbit furthest from the sun Earth (not to scale!) ra ra = aphelion distance

Aphelion Point in orbit furthest from the sun Perihelion Point in orbit closest to the sun Earth rp rp = perihelion distance

Eccentricity e = b/a so, b = ae a = 1/2 major axis (semi-major axis) b = 1/2 distance between foci b a

Kepler’s Second Law Corollary: Planets move fastest when 2nd law: A line joining the Earth to the Sun sweeps out equal areas in equal times Kump et al., The Earth System, Box Fig. 14-1 Corollary: Planets move fastest when they are closest to the 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 

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 http://www.geo.lsa.umich.edu/~crlb/COURSES/205/Lec20/lec20.html

Q: What makes eccentricity vary 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

65o N solar insolation Unfiltered Orbital Element Variations Today 800 kA Today Unfiltered Orbital Element Variations 0.06 65o N solar insolation Imbrie et al., Milankovitch and Climate, Part 1, 1984

Q: What makes the obliquity and precession vary Q: What makes the obliquity and precession vary? A: First, consider a better known example… Example: a top 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 g

Q: What makes the obliquity and precession vary Q: What makes the obliquity and precession vary? A: i) The pull of the Sun and the Moon on Earth’s equatorial bulge N g g Equator The Moon’s torque on the Earth is about twice as strong as the Sun’s S

Q: What makes the obliquity and precession vary Q: What makes the obliquity and precession vary? A: ii) Also, the tilting of Earth’s orbital plane N  N S  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 S

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 Today  N S *Actual precession period is 26,000 yrs, but the orienta- tion of Earth’s orbit is varying, too (precession of perihelion)

Which star is the North Star today? 11,000 yrs ago Today  N S

 Polaris N S Which star was the North Star at the opposite side of the cycle? Polaris 11,000 yrs ago Today  N S

 *Actually, Vega would have been the North Star more Polaris Vega N S 11,000 yrs ago* Today  N S *Actually, Vega would have been the North Star more like 13,000 years ago

65o N solar insolation Unfiltered Orbital Element Variations Today 800 kA Today Unfiltered Orbital Element Variations 0.06 65o N solar insolation Imbrie et al., Milankovitch and Climate, Part 1, 1984

Ref: Imbrie et al., 1984 Eccentricity Obliquity Precession Filtered Orbital Element Variations Today 800 kA

Optimal Conditions for Glaciation: Low obliquity (low seasonal contrast) 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: High obliquity (high seasonal contrast) High eccentricity and NH summers during perihelion (hot summers in the north) Today 11,000 yrs ago  N S N  S Optimal for glaciation Optimal for deglaciation

NH Insolation vs. Time

O isotopes—the last 900,000 yrs Peak NH summertime insolation after Bassinot et al. 1994

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