i) Oxygen isotopes and climate /Kepler’s laws Chapter 14—Part 1 i) Oxygen isotopes and climate /Kepler’s laws

How do we know how warm it was millions of years ago? Ice cores: bubbles contain samples of the atmosphere that existed when the ice formed. (ancient pCO2) Marine isotopes: 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.

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

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

Oxygen isotopes and paleoclimate As climate cools, marine carbonates record an increase in d18O. Warming yields a decrease in d18O of marine carbonates. JOIDES Resolution Scientists examining core from the ocean floor.

Long-term 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

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

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

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 (or, more accurately, who analyzed Tycho Brahe’s telescopic observations), and Isaac Newton, who invented calculas, and was thereby able to calculate orbits.

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

Eccentricity e = b/a a = 1/2 major axis b = 1/2 distance between foci Sun-Earth distances Aphelion: a + ae = a(1 + e) Perihelion: a – ae = a(1 – e) b a

Eccentricity e = b/a a = 1/2 major axis b = 1/2 distance between foci Sun-Earth distances Aphelion: a(1 + e) Perihelion: a(1 – e) Today: e = 0.017 Range: 0 to 0.06 Cycles: 100,000 yrs b a

Kepler’s Second Law 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