Paleozoic

"Cambrian.

"Cambrian.

Ordo-

J_l_L

Devon-

Carbon

vician

1 ian. 1

Perm-

Mesozoic

Tri-assic_

Jur-

assic_

Ceno-

zoic

Cenozoic

Paleogene

Neogene

Q.

MM

Miocene ill 11111

Plii cen I I I

e ' i

60 50 40 30 20 10

Figure 1.7: The geological time scale. Numbers on the scales represent millions of years before present. The entire span of time before the Phanerozoic is also known as the Precambrian.

geologic time. For this reason, magnesium-calcium paleothermometry has become a crucial tool for probing the past few million years of climate history, and the tool is being extended farther backward as understanding of the longer term evolution of the oceanic Mg to Ca ratio improves.

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