Stable Isotopes in Paleoclimatology

1. Stable Isotopes in Paleoclimatology Lecture 37

2. Water-Carbonate Fractionation • Urey calculated the temperature dependence of the water-carbonate δ18O fractionation and pointed out it could be used as a paleothermometer by solving for T: • T (˚C) = 16.9-4.2∆+0.13∆2 • where ∆ is the difference between calcite and the water it precipitated from. • He then had his students perform experiments to verify predictions.

3. Quaternary δ18O Record • Urey’s student, Cesar Emilliani, analyzed δ18O in forams from a variety of deep-sea cores and reported 15 glacial cycles in the last 600,000 years in his 1955 dissertation. • Subsequent work greatly refined this record, leading to a standard δ18O curve in the late 1970’s. • Emilliani had noticed the cyclicity in the curves and concluded that Milankovitich’s theory of climate change was correct: it was caused by changes in the Earth’s orbit and rotation.

4. Deducing Temperature Change • Two factors result in change in δ18O: • Temperature dependence of the fractionation factor - carbonate will be heavier at lower T. • Storage of isotopically light water on continents as glaciers. Consequently, seawater, and also carbonates, will be heavier during glacial periods. • In order to determine temperature changes, one must know how the isotopic composition of water changed. • Deep water temperature changes less, so benthic forams provide some control on this. • Ice volumes can be determined from sealevel change (subsequently constrained by dating coral reefs with U-Th). • In addition, of course, it is necessary to accurately date strata in the cores. • Has evolved from extrapolating 14C dates and magnetostratigraphy to more sophisticated approaches like U-Th and 10Be, etc.

5. Milankovitch Theory • Earth’s orbit and rotation vary regularly in 3 ways: • The obliquity of the rotational axis relative to the orbital plane. • Eccentricity of the orbit • Precession: the direction the Earth’s rotational axis points at perigee and apogee of orbit. • These factors influence the distribution of solar energy (insolation) in time and space over the course of a year, but do not change global annual insolation. • ‘Milankovitich parameters’ are well determined from astronomical observations (have been known for a very long time).

6. Imbrie, Hayes and others model • Imbrie and colleagues (1976, 1985) applied Fourier analysis to the standardized δ18O curve (CLIMAP project) to deduce the primary frequencies (dividing into two parts, <400ka and >400ka). • They then build a model where each Milankovitch frequency influenced climate with a different phase and gain. • The model accounted for r2= 0.77 of the observed variance in δ18O. • This kind of model has, of course, been greatly subsequently enhanced with better data, GCM’s, ocean circulation models, etc.

7. The Antarctic Ice Record • Much subsequent paleoclimate effort has focused on δD in ice cores from Antarctica and Greenland. • The Vostok core from Antarctica went back 400 ka. Subsequent work shifted to the EPICA core which went back >800 ka. • Complications in interpretation arise here too because of changes in δD of the oceans and changes in atmospheric circulation result in complex relationship between T and δD, but temperatures can be worked out. • Overall, agreement between the marine and Antarctic records is excellent, but shows some differences between Antarctic and global climate change.

8. Greenland Ice Record • Ice records from Greenland are not as long, but provide finer details of the last glacial cycle. • Greenland is ‘ground zero’ of glaciation. • They reveal extremely variable climate in the last ice age -Dansgaard-Oeschager events - likely related to iceberg events documented in deep-sea cores.

9. Feedback Factors • Milankovitch variations provide only a weak climate signal that has been apparently greatly amplified in the Quaternary by feedback factors. • June insolation at 60˚N appears to be the key sensitivity. • Feedbacks include: • Albedo • Shift of CO2 from atmosphere to oceans with consequent change in greenhouse effect • Changes in ocean circulation, particularly with delivery of heat to the North Atlantic (ground zero for continental ice sheets). • The role of CO2 is well documented by CO2 concentrations in bubbles in Antarctic ice. Figure 12.45

10. The Next Ice Age? From Marcott et al. (2013) Science, 339: 1198

11. Soil Paleoclimate Proxies • Hydrogen and Oxygen isotopes in soil clays reflect (with fractionation), the isotopic composition of meteoric water. • This allows reconstruction of paleoprecipitation patterns - Cretaceous precipitation in N. America in this figure.

12. Pedogenic Carbonate • δ18O in pedogenic carbonate also reflects composition of meteoric water (with fractionation). • In Pakistan, δ18O in paleosol carbonates record the evolution of the monsoons.