Stable Isotopes in

PaleoclimatologyLecture 37

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∆2o where ∆ is the difference between

calcite and the water it precipitated from.

• He then had his students perform experiments to verify predictions.

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.

Deducing Temperature Change

• Two factors result in change in δ18O:o Temperature dependence of the fractionation

factor - carbonate will be heavier at lower T.o 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.o Deep water temperature changes less, so benthic

forams provide some control on this.o 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.o Has evolved from extrapolating 14C dates and

magnetostratigraphy to more sophisticated approaches like U-Th and 10Be, etc.

Milankovitch Theory• Earth’s orbit and rotation vary

regularly in 3 ways:o The obliquity of the rotational axis relative

to the orbital plane.o Eccentricity of the orbito 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).

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.

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.

Greenland Ice Record• Ice records from

Greenland are not as long, but provide finer details of the last glacial cycle.o 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.

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:o Albedoo Shift of CO2 from atmosphere to oceans

with consequent change in greenhouse effect

o 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

The Next Ice Age?

From Marcott et al. (2013) Science, 339: 1198

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.

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.