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1 Resources chmidt_01/ chmidt_01/ Paleoclimatology_OxygenBalance/oxygen _balance.php Paleoclimatology_OxygenBalance/oxygen _balance.php change1/current/lectures/kling/paleoclimat e/index.html change1/current/lectures/kling/paleoclimat e/index.html LABS/posted_lab6.pdf

2 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 pCO 2 ) 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.


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

5 Oxygen isotopes and paleoclimate Oxygen has three stable isotopes: 16 O, 17 O, and 18 O. (We only care about 16 O and 18 O.) 18 O is heavier than 16 O. The amount of 18 O compared to 16 O is expressed using delta notation: Fractionation: Natural processes tend to preferentially take up the lighter isotope, and preferentially leave behind the heavier isotope. 18 O = 18 O/ 16 O of sample - 18 O/ 16 O of standard 18 O/ 16 O of standard 1000

6 Oxygen isotopes and paleoclimate Oxygen isotopes are fractionated during evaporation and precipitation of H 2 O –H 2 16 O evaporates more readily than H 2 18 O –H 2 18 O precipitates more readily than H 2 16 O Oxygen isotopes are also fractionated by marine organisms that secrete CaCO 3 shells. The organisms preferentially take up more 16 O as temperature increases. 18 O is heavier than 16 O H 2 18 O is heavier than H 2 16 O

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

8 Oxygen isotopes and paleoclimate Ocean H 2 16 O, H 2 18 O Evaporation favors H 2 16 O H 2 18 O Precipitation favors H 2 18 O …so cloud water becomes progressively more depleted in H 2 18 O as it moves poleward… H 2 18 O … and snow and ice are depleted in H 2 18 O relative to H 2 16 O. Land Ice Carbonate sediments in equilibrium with ocean water record a 18 O signal which reflects the 18 O of seawater and the reaction of marine CaCO 3 producers to temperature. CaCO 3


10 Precipitation dO18 At the poles; what will the precipitatio n be? High in O18 or low in O18?

11 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?

12 If the temperatures are cooler, will more or less dO18 be evaporated? Why?

13 What isotope of oxygen will precipitation be enriched in during cool periods then? O18 or O16? Why?

14 What will the ice be enriched in during cold periods? Why?

15 ICE BANK During a glacial period, where will the O16 be stored? Then what will the oceans be enriched in?

16 HOW DO WE FIND Isotope ratios? Drilling Ocean Sediments ODP

17 Oceanic Sediments: Forams CaCO 3

18 Oxygen isotopes and paleoclimate As climate cools, marine carbonates record an increase in 18 O. Warming yeilds a decrease in 18 O of marine carbonates. JOIDES Resolution Scientists examining core from the ocean floor.

19 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

20 Drake passage Once the Drake passage had formed, the circum-Antarctic current prevented warm ocean currents from reaching Antarctica

21 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

22 Do climate temperatures change?

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

24 Explain the relationship between MARINE dO18 and temperature.

25 Global temperature- instrumental record (thermometers). Why are dO18 proxies are important?

26 Glaciers as records of climate Ice cores: –Detailed records of temperature, precipitation, volcanic eruptions –Go back hundred of thousands years (400,000 YEARS)

27 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



30 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)

31 ICE Delta 18O and temperature Explain the relashionship.

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

33 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

34 NOAA Milutin Milankovitch, Serbian mathematician 1924--he suggested solar energy changes and seasonal contrasts varied with small variations in Earths orbit He proposed these energy and seasonal changes led to climate variations

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

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

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

38 Aphelion Point in orbit furthest from the sun rara r a = aphelion distance Earth (not to scale!)

39 Aphelion Point in orbit furthest from the sun Perihelion Point in orbit closest to the sun rprp r p = perihelion distance Earth

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

41 Keplers Second Law 2 nd 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

42 Keplers Third Law 3 rd law: The square of a planets period, P, is proportional to the cube of its semi-major axis, a Periodthe time it takes for the planet to go around the Sun (i.e., the planets year) If P is in Earth years and a is in A.U., then P 2 = a 3

43 Other characteristics of Earths orbit vary as well. The three factors that affect climate are

44 Eccentricity (orbit shape) 100,000 yrs &400,000 yrs Obliquity (tilt -- 21.5 to 24.5 o ) 41,000 yrs Precession (wobble) 19,000 yrs & 23,000 yrs

45 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

46 Eccentricity Variations Current value: 0.017 Range: 0-0.06 Period(s): ~100,000 yrs ~400,000 yrs

47 800 kA Today 65 o N solar insolation Imbrie et al., Milankovitch and Climate, Part 1, 1984 Unfiltered Orbital Element Variations 0.06

48 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

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

50 Q: What makes the obliquity and precession vary? A: ii) Also, the tilting of Earths 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

51 Obliquity Variations Current value: 23.5 o Range: 22 o -24.5 o Period: 41,000 yrs

52 Precession Variations Range: 0-360 o 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 Earths orbit is varying, too (precession of perihelion) Today

53 11,000 yrs agoToday N S N S Which star is the North Star today?

54 11,000 yrs agoToday N S N S Polaris Which star was the North Star at the opposite side of the cycle?

55 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

56 800 kA Today 65 o N solar insolation Imbrie et al., Milankovitch and Climate, Part 1, 1984 Unfiltered Orbital Element Variations 0.06

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

58 Optimal Conditions for Glaciation: 1.Low obliquity (low seasonal contrast) 2.High eccentricity and NH summers during aphelion (cold summers in the north) Milankovitchs key insight: Ice and snow are not completely melted during very cold summers. (Most land is in the Northern Hemisphere.)

59 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 glaciationOptimal for deglaciation

60 NH Insolation vs. Time

61 after Bassinot et al. 1994 O isotopesthe last 900,000 yrs Peak NH summertime insolation

62 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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