2How 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.
4More Neutrons=More MASS IsotopesAtoms 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 MASSHYDROGEN ISOTOPESHydrogen Deuterium
5Oxygen 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 standard18O/16O of standard 1000
6Oxygen isotopes and paleoclimate Oxygen isotopes are fractionated during evaporation and precipitation of H2OH216O evaporates more readily than H218OH218O precipitates more readily than H216OOxygen 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 16OH218O is heavierthan H216O
7What isotope of oxygen evaporates more readily? O18 or O16? Why?
8Oxygen isotopes and paleoclimate Precipitation favorsH218O…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 favorsH216OH218OH218OIceLandH216O, H218OOceanCarbonate sediments in equilibriumwith ocean water record a d18O signal which reflects the d18O of seawater and the reaction of marine CaCO3 producers to temperature.CaCO3
18Oxygen isotopes and paleoclimate As climate cools, marine carbonates record an increase in d18O.Warming yeilds a decrease in d18O of marine carbonates.JOIDES ResolutionScientists examining core from the ocean floor.
19Long-term MARINE oxygen isotope record Ice cap begins toform on Antarcticaaround 35 MaThis may be relatedto the opening ofthe Drake passagebetween Antarcticaand S. AmericaFrom K. K. Turekian, Global EnvironmentalChange, 1996
20DrakepassageOnce the Drake passage had formed, thecircum-Antarctic current prevented warm oceancurrents from reaching Antarctica
21Marine O isotopes during the last 3 m.y. Kump et al., The Earth System, Fig. 14-4Climatic cooling accelerated during the last 3 m.y.Note that the cyclicity changes around Ma− 41,000 yrs prior to this time− 100,000 yrs after this time
23MARINE O isotopes—the last 900 k.y. Dominant period is ~100,000 yrs during this timeNote the “sawtooth” pattern..after Bassinot et al. 1994
24Explain the relationship between MARINE dO18 and temperature.
25Global temperature- instrumental record (thermometers) Global temperature- instrumental record (thermometers). Why are dO18 proxies are important?
26Glaciers as records of climate Ice cores:Detailed records of temperature, precipitation, volcanic eruptionsGo back hundred of thousands years (400,000 YEARS)
27Methods of Dating Ice Cores Counting of Annual LayersTemperature DependentMarker: ratio of 18O to 16Ofind number of years that the ice-core accumulated overVery time consuming; some errorsUsing volcanic eruptions as MarkersMarker: volcanic ash and chemicals washed out of the atmosphere by precipitationuse recorded volcanic eruptions to calibrate age of the ice-coremust know date of the eruption
30Delta O18 and temperature Temperature affects 18O/16O ratio:colder temperatures more negative values for the delta 18Owarmer temperatures delta 18O values that are less negative (closer to the standard ratio of ocean water)
31ICE Delta 18O and temperature Explain the relashionship.
32Temperature reconstructed from Greenland Ice core Temperature reconstructed from Greenland Ice core. When did the last ice age end?
33Ice Age Cycles: *This was the dominant period prior to 100,000 years between ice agesSmaller cycles also recorded every41,000 years*,19, ,000 years*This was the dominant period prior to900 Ma
34Milutin Milankovitch, Serbian mathematician Milutin Milankovitch,Serbian mathematician1924--he suggested solar energy changes and seasonal contrasts varied with small variations in Earth’s orbitHe proposed these energy and seasonal changes led to climate variationsNOAA
35Before studying Milankovitch cycles, we need to become familiar with the basic characteristicsof planetary orbitsMuch of this was worked out in the 17th centuryby Johannes Kepler (who observed the planetsusing telescopes) and Isaac Newton (whoinvented calculas)
36Kepler’s Laws First law: Planets travel around the sun in elliptical orbitswith the Sun at one focusr’rr’ + r = 2aa = semi-major axis(= 1 AU for Earth)aMajor axisMinor axis
37Ellipse:Combined distances to two fixed points (foci) is fixedr’rr’ + r = 2aaThe Sun is at one focus
38AphelionPoint in orbit furthest from the sunEarth (not to scale!)rara = aphelion distance
39AphelionPoint in orbit furthest from the sunPerihelionPoint in orbit closest to the sunEarthrprp = perihelion distance
40Eccentricitye = b/a so, b = aea = 1/2 major axis (semi-major axis)b = 1/2 distance between fociba
41Kepler’s Second Law Corollary: Planets move fastest when 2nd law: A line joining the Earth to the Sun sweepsout equal areas in equal timesKump et al., The Earth System, Box Fig. 14-1Corollary: Planetsmove fastest whenthey are closest tothe Sun
42Kepler’s Third Law3rd law: The square of a planet’s period, P, is proportional to the cube of its semi-major axis, aPeriod—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., thenP2 = a3
43Other characteristics of Earth’s orbit vary as well. The three factors that affect climate are
45Q: What makes eccentricity vary Q: What makes eccentricity vary? A: The gravitational pull of the other planetsThe pull of anotherplanet is strongestwhen the planetsare close togetherThe net result ofall the mutual inter-actions betweenplanets is to vary theeccentricities of theirorbits
46Eccentricity Variations Current value: 0.017Range:Period(s): ~100,000 yrs~400,000 yrs
4765o N solar insolation Unfiltered Orbital Element Variations Today 800 kATodayUnfilteredOrbitalElementVariations0.0665o NsolarinsolationImbrie et al., Milankovitch and Climate, Part 1, 1984
48Q: What makes the obliquity and precession vary Q: What makes the obliquity and precession vary? A: First, consider a better known example…Example: a topGravity exerts a torque--i.e., a force that actsperpendicular to the spinaxis of the topThis causes the top toprecess and nutateg
49Q: 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 bulgeNggEquatorThe Moon’s torque onthe Earth is about twiceas strong as the Sun’sS
50Q: What makes the obliquity and precession vary Q: What makes the obliquity and precession vary? A: ii) Also, the tilting of Earth’s orbital planeNNSTilting of the orbital plane is likea dinner plate rolling on a tableIf the Earth was perfectly spherical,its spin axis would always point inthe same direction but it would makea different angle with its orbital planeas the plane moved aroundS
51Obliquity Variations Current value: 23.5o Range: 22o-24.5o Period: 41,000 yrs
52Precession Variations Range: 0-360oCurrent value: Perihelion occurs on Jan. 3 North pole is pointed almost directly away from the Sun at perihelionPeriods*: ~19,000 yrs~23,000 yrsTodayNS*Actual precession periodis 26,000 yrs, but the orienta-tion of Earth’s orbit is varying,too (precession of perihelion)
53Which star is the NorthStar today?11,000 yrs agoTodayNS
54 Polaris N S Which star was the North Star at the opposite side of the cycle?Polaris11,000 yrs agoTodayNS
55 *Actually, Vega would have been the North Star more Polaris Vega N S 11,000 yrs ago*TodayNS*Actually, Vega would have been the North Star morelike 13,000 years ago
5665o N solar insolation Unfiltered Orbital Element Variations Today 800 kATodayUnfilteredOrbitalElementVariations0.0665o NsolarinsolationImbrie et al., Milankovitch and Climate, Part 1, 1984
57Ref: Imbrie et al., 1984EccentricityObliquityPrecessionFilteredOrbitalElementVariationsToday800 kA
58Optimal 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.)
59Optimal Conditions for Deglaciation: High obliquity (high seasonal contrast)High eccentricity and NH summers during perihelion (hot summers in the north)Today11,000 yrs agoNSNSOptimal for glaciationOptimal for deglaciation
61O isotopes—the last 900,000 yrs Peak NHsummertimeinsolationafter Bassinot et al. 1994
62Big 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 changeTwo 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