Reading Earth history from chemistry

ENVI 1301

Graham H. Edwards, Earth & Environmental Geosciences

Paleoclimate

Reconstructing climates of the past

Earth

2002

Wikimedia

Earth

21 000 years ago

Wikimedia

Earth

105 million years ago

Wikimedia

Earth

690 million years ago

Wikimedia

Earth (& Sun)

4,565 million years ago

NASA

Global temperature over the last 485 million years

Judd+ 2024

How do we figure this out?

(Isotope) Geochemistry

Isotopes

Atoms of the same element with different masses.

particle	charge
proton	+
electron	−
neutron	○

$$^4_2\text{He}$$

Isotopes

$$^3_2\text{He}$$

$$^4_2\text{He}$$

Isotope fractionation records information!

Type	Fractionates by...
Radiogenic	Radioactive decay
Stable	Environmental conditions

To reconstruct climate of the past, we can use isotope ratios of geologic material to infer the state of Earth systems and measure time

Measuring time

Radioactive Decay

$$\frac{dN}{dt} = -\lambda N$$

$$N = N_o e^{-\lambda t}$$

$$n = N(e^{\lambda t} -1)$$

Geochronology in one page

Half-lifes and the decay constant ($\lambda$)
\[\begin{aligned} t_{1/2} &= \frac{ln(2)}{\lambda} \\\\ \lambda &= \frac{ln(2)}{t_{1/2}}\end{aligned}\]

Radioactive Decay

Uranium-238

The uranium decay series

Uranium-lead (U-Pb) and uranium-series chronometers

Mass spectrometry

X62 thermal ionization mass spectrometer — Keck Isotope Facility, University of California Santa Cruz

Wait, this sounds pretty cool!

How did we figure out all of this rad stuff in the first place?

Maria Salomea Skłodowska-Curie

(image credit: Smithsonian)

A long, worrisome complicity of science in war

Wikimedia: Manhattan Project emblem / Trinity shot (J.W. Aeby, 1945) / DOE seal

Measuring climate of the past

Isotopes of oxygen (in water)

$\text{H}_2\text{O}$

Isotopes of oxygen (in water)

$^{16}\text{O}$

99.76%

$^{17}\text{O}$

0.04%

$^{18}\text{O}$

0.2%

Isotopes of oxygen (in water)

$$\frac{^{18}\text{O}}{^{16}\text{O}} = 0.00200520 \pm 0.00000045 $$

VSMOW — Vienna Standard Mean Ocean Water

Isotopes of oxygen (in water)

$$\delta^{18}\text{O} = 1000\times \left(\frac{\frac{^{18}\text{O}}{^{16}\text{O}}_{sample}}{\frac{^{18}\text{O}}{^{16}\text{O}}_{standard}}-1\right)$$

delta oxygen eighteen = dell-oh eighteen

$$\delta^{18}\text{O} \propto \frac{^{18}\text{O}}{^{16}\text{O}}$$