Pleistocene Ice Age Cycles 0-2.65 Ma

Pleistocene Ice Age Cycles 0-2.65 ma • Urey (1947) calculated that the oxygen isotope fractionation between calcium carbonate and water should be temperaturedependent. ⎡ R sample ⎤ δ O = 1000⎢ − 1⎥ ⎣ R s tan dard ⎦ 18 • Epstein (1953) grew molluscs in the laboratory and empirically determined the O18-T relationship: • Emiliani (1955 and other papers) analyzed foraminifera from piston cores from the deep sea, and made temperature estimates: • He found multiple cycles of cold and warm periods during the past ~500,000 years. 2. This work created quite a stir, and quickly was criticized on several grounds: a. It violated the prevailing 4-ice-age theory from continental geology. b. Meteorologists though that the tropical temperature change seemed excessive. c. Micropaleotologists thought that their micropaleontological work (G. menardii stratigraphy) contradicted with O-18 record. d. Biologists (e.g. Bé) argued that foraminiferal ecological shifts may have altered the depth habitat of organisms (and hence temperatures). e. The time scale (based on 230Th/231Pa) was criticized. f. Various statistical errors were pointed out. Rayleigh distillation of oxygen isotopes vapor Vapor pressure = f(T) (Clausius-Clapeyron equation, exponential with increasing T) liquid At 25°C, the vapor pressure of H216O is 0.9% higher than H218O Imagine a 50-50 mixture of liquid H216O and H218O, equilibrated with the vapor phase at 25°C. Separate the vapor from the liquid: δ18O = -9‰ T=25°C 1009 1000 H216O H218O Cool the vapor to 20°C; allow liquid to condense from vapor: δ18O = -11‰ 745 737 H216O H216O 264 H216O R α −1 = f Rayleigh equation: R0 T=20°C 263 H218O R0 = initial isotope ratio R = isotope ratio after cooling f = fraction of water condensed α = isotope fractionation factor Cumulative Rayleigh Isotope Distillation as a function of temperature Fraction Remaining Vapor 1.0 18 δ O (SMOW) -5 0.50 0.75 0.25 0 Condensate 9% -10 -15 11% -20 Vapor -25 -30 20 15 10 Cloud Temperature 18 0 O -20 C δ O in cloud vapor and condensate plotted as a function of the remaining vapor in the cloud for a Rayleigh 18 process. The temperature of the cloud in degrees Celsius is shown on the lower axis. δ O values are relative to SMOW. The increase in fractionation with decreasing temperature is taken into account. After Dansgaard (1964). Observed δ18O - surface temperature relationship 0 Gough Is. Valentia Dublin -10 o 61 N Grennedal 18 75 N Upepnavik -20 Copenhagen Goose Bay, Labrador o δ O (SMOW) Barbados Is. o 71 N Umanak S. Greenland -30 N. Greenland -40 o 85 S Horlick MTNS. South Pole -50 -50 -40 -30 -20 -10 0 10 20 30 Mean Annual Air Temperature C O 18 Mean annual δ O of precipitation as a function of the mean annual air temperature at 18 the earth's surface. Note that δ O values are progressively lighter as the mean annual temperature becomes lower. After Dansgaard (1964). Note: this line is not the relationship predicted by the Rayleigh distillation curve. It includes many other effects: evaporationprecipitation cycles, cloudT / surface-T relationships; multiple sources of water vapor at different temperatures, etc. Because they flow, glaciers are filled from their summits: y I Surface C Flow lines x C I Bed rock Vertical cross section of an ice sheet resting on a horizontal subsurface. Ice particles deposited on the snow surface will follow lines that travel closer to the base the farther inland the site deposition. An ice mass formed around the divide (I-I) will be plastically deformed (thinned) with depth as suggested by the lined areas. The dashed curve along the vertical ice core (C-C) shows the calculated horizontal velocity profile Vx (Weertman 1968b). The horizontal arrows along C-C show the adopted approximation to Vx (Dansgaard et al. 1969). Effect of glaciation on the oxygen isotope composition of the ocean Sea-level estimates from drilling submerged coral terraces Earth’s Orbital Geometry: The Milankovitch Hypothesis & the Pacing of Pleistocene Ice Ages Milankovitch Hypothesis: Historical Perspective What: Astronomical theory of Pleistocene ice ages. How: Varying orbital geometry influences climate by changing seasonal & latitudinal distribution of solar radiation incident at top of atmosphere (insolation). Milestones: Hypothesis • Croll (1864, 1875): Proposed that variations in seasonal influx of energy--the cumulative affect of eccentricity, obliquity & precession--could trigger large climate response. • Milankovitch (1920, 1941): Combined laws of radiation with planetary mechanics to derive insolation curves as function of time (600 kyr) and latitude. Concluded summer insolation at high N. lat. (65°N) critical to growth/decay of ice sheets. "The Milankovitch Hypothesis". The Seasons (edge view) The seasons in an elliptical orbit (oblique view, exaggerated eccentricity) Eccentricity of Present Earth Orbit Around Sun (to Scale) e = c/a = Sqrt(1-a2/b2) a c F1 F2 b Present eccentricity = 0.017 Range: 0 - 0.06 100 & 400 kyr periods Precession of elliptical orbit (with respect to fixed stars) Precession Precession influence on climate: why 23,000 years not 25,800? Obliquity (tilt) Higher obliquity leads to higher summer insolation at high latitudes (and slightly less at low latitudes). Eccentricity amplitude-modulates precession Periodic changes in orbital geometry modulate solar radiation receipts (insolation) Insolation at 65°N, June 21 Note: integrated over a full summercentered half year, effect of obliquity is much stronger Did increasing Northern Hemisphere summer insolation cause the end of the last ice age? Milankovitch Hypothesis: Milestones & Support • Kullenberg (1947): Invented deep-sea piston coring. ÆRecovery of long, continuous climate records possible. • Emiliani (1955): Pioneered use 18O/16O ratio of fossil foraminifera in sediment cores as climate (temp.) proxy. • Olausson (1965); Shackleton (1967): Interpret foram δ18O changes as whole-ocean isotopic shifts caused by ice sheet growth/decay. • 1960's: Recognition of magnetic stripes on ocean floor (geomagnetic field reversals) as global stratigraphic markers. • Johnson (1982); Shackleton et al (1990): Use astronomically-driven insolation variations (E, T, P) to derive timescales for deep-sea cores. ÆAccurately predict age of Brunhes-Matuyama (B/M) magnetic reversal, 780-790 kyr BP. (K/Ar dates for B/M incorrectly placed it at 730 kyr BP.) • Baksi et al (1992): 40 Ar/39Ar date for B/M = 783 kyr BP. • Raymo (1997): Multiple δ18O records on 'simple' timescale supports link between N. Hemisphere summer insolation and glacial terminations. ÆStrong support for astronomical influence on climate. (The magnitude of which remains debated…) Absolute chronology and its importance to testing the Milankovitch Hypothesis • Carbon-14 dating: 0-25 kyr BP (where reliable “initial C14” calibrations exist) • Layer counting in sediments and ice cores: 0-40 kyr BP varves, density bands, annual dust cycles (but do you miss some bands or see two where only one should be?) • 234U -> 230Th ingrowth in corals and speleothems: 0-250,000 kyr BP • 40K -> 40Ar dating of basalts at magnetic reversals How do we estimate a time scale for a marine sediment core or ice core? • We measure depth and assume it is an increasing function of the age of the deposit (stratigraphy). • For sediments <25 kyr BP containing appropriate carbonate or organic) fossils, we can measure the 14C content and determine the age from the atmospheric radicarbon calibration. • We can determine the δ18O of carbonate fossils and correlate the features to sea level events of known age (from 230Th-234U dating). • For sediments with appropriate magnetic minerals, we can measure the magnetic alignment and determine the position of known (40Ar/39Ar dated) magnetic reversals. • In-between these known dates, we must interpolate using some plausible (but unprovable) scheme. Oxygen isotopes compared to summer insolation at 65°N 1. Why is climate response in 100-kyr band so strong? Observation: High correlation of δ18O cycles with astronomically-driven radiation cycles at E, T & P frequencies suggests causal link in all 3 bands. Problem: Amplitude of insolation change (~0.2%) is ~10x smaller than in T,P bands. Possible Solution: E modulates climatic effect of P. High E favors NH glaciation when P causes NH summer to occur at maximum Earth-Sun distance (i.e., Imbrie et al, 1993). 2. Why do glacial cycles switch from 41-kyr to 100-kyr period ~700 kyr BP? Possible solution: L/T cooling trend, perhaps from tectonically-driven decrease in atmospheric CO2, facilitates NH ice sheet growth beyond a critical threshold during insolation minima. These large ice sheets drive climate through feedbacks internal to the climate system (geo-, cryo-, atmo-, hydro-sphere). 3. Why do full glacial Terminations, and ensuing interglacial periods, occur ~430 and ~15 kyr BP when E is very low? -Possible solution: 100-kyr cycle of orbital inclination (Muller and MacDonald, 1995). -Caveat: no obvious mechanism linking climate to inclination. Milankovitch Hypothesis Challenges: 100-kyr Cycle Problems