Lecture 25 – Cenozoic Climate Variations

Q: What is the Cenozoic, and what is it also known as?

The last 66 million years of Earth history, age of mammals.

Q: How is the Cenozoic subdivided?

Paleogene, Neogene (previously Tertiary), and Quaternary.

Q: What broad climatic transition characterizes the Cenozoic?

Greenhouse period to an icehouse period.

Q: What happens to ocean water Mg, SO₄, Ca, and K at times of high sea floor spreading?

Mg and SO₄ are low; Ca and K are high.

Q: What happens to ocean water Mg, SO₄, Ca, and K at times of low sea floor spreading?

Mg and SO₄ are high; Ca and K are low.

Q: How do varying sea floor spreading rates and MOR activity affect carbonate mineralogy?

They shift ocean water chemistry between calcite production (high spreading, low Mg/Ca) and aragonite production (low spreading, high Mg/Ca).

Q: How do sea floor spreading rates relate to atmospheric CO₂ and climate state?

Elevated spreading → elevated CO₂ → greenhouse conditions; low spreading → low CO₂ → icehouse conditions.

Q: What proxy is used to reconstruct Cenozoic climate from marine sediment cores?

δ¹⁸O of carbonate from benthic foraminifera.

Q: If no ice sheets are present, what temperature change does a 1.0‰ shift in δ¹⁸O represent?

A change of 4.2C.

Q: What happened between 66 and 52 Ma in the δ¹⁸O record?

δ¹⁸O decreased by approximately 1‰, a 4C warming that culminated in the Early Eocene Climatic Optimum (warmest Cenozoic interval, 12-15C warmer than today, atmospheric CO₂ 1000-1600 ppm).

Q: What happened between 50 and 34 Ma?

δ¹⁸O increased by 2‰, a cooling trend of 8C.

Q: What happened between 34 and 26 Ma?

δ¹⁸O increased by 1‰, with 2-3C of cooling (the rest is ice volume effect), leading to first permanent Cenozoic glaciation in Antarctica. CO₂ was 500–800 ppm.

Q: What happened between 26 and 15 Ma?

δ¹⁸O decreased by 1‰, a 2C warming and a reduction in Antarctic ice.

Q: What marks 14.7 Ma in the Cenozoic climate record?

The end of the mid-Miocene Climatic Optimum, atmospheric CO₂ likely above 600 ppm.

Q: What happened between 14 and 0.002 Ma?

δ¹⁸O increased by more than 2.5‰, indicating 6C of cooling, massive Antarctic ice growth, first major Northern Hemisphere ice sheets starting at 2.75 Ma, and CO₂ declining to less than 300 ppm.

Q: How has deep ocean temperature changed over the last 50 million years?

Cooled from above 15C to 2C today.

Q: When did glacier ice first appear in Antarctica, and what is the evidence?

No evidence for ice before 35 Ma; the first evidence is dropstones. A large growth of the Antarctic ice sheet occurred at 13 Ma, with another growth period at 7–5 Ma.

Q: When did the first mountain glaciers appear in South America?

In the Andes 7-5 Ma.

Q: When did glacier ice first appear in Greenland and Alaska?

No evidence before 7 Ma. First Greenland dropstones 7 Ma; first mountain glaciers in coastal southern Alaska 5 Ma; first continental ice sheets of significant size 2.75 Ma.

Q: When did mid-latitude mountain glaciers develop in the Northern Hemisphere?

Since 2 Ma.

Q: How did Antarctic vegetation change through the Cenozoic?

Beech trees and ferns were present before 40 Ma but disappeared with the onset of glaciation.

Q: How did Arctic vegetation change through the Cenozoic?

Palm-like and evergreen vegetation (60 Ma) → broadleaf deciduous forests (40–30 Ma) → conifer forests like spruce and larch (20 Ma) → tundra-type vegetation only in the last few million years.

Q: How are fossil leaf margins used as a temperature proxy?

Smooth leaf edges indicate warm-climate species; jagged edges indicate cool-climate species. Fossil leaf assemblages show 12-15C of cooling through the Cenozoic.

Q: Why did the Southern Hemisphere cool earlier than the Northern Hemisphere?

Cold ocean water circulates freely around Antarctica, while in the Northern Hemisphere water was blocked from reaching the pole (a warming effect), and there is more continental land mass in the north.

Q: What additional mechanism helps explain long-term Cenozoic cooling?

Uplift weathering (acting as a CO₂ sink) and decreased sea floor spreading, explain long-term global cooling in the Phanerozoic.