Cenozoic Climate: From the Greenhouse to the Icehouse

The Cretaceous Superplumes

During the Cretaceous Period the Earth was unusually warm (mean temperature 10 to 15 degrees centigrade warmer than today). Forests spread to latitudes that today are tundra or ice covered. There were no polar ice caps. Reptiles, including dinosaurs, lived at high latitudes to the Arctic and Antarctic circles. This period of warmth was coincidentally a time of rapid seafloor spreading as Pangea and Gondwana continued to break up. Sea level rose and flooded vast continental areas, as it had done many times before. Massive flood basalts marked the points of breakup of Pangea and Gondwana. The Earth's magnetic field, which ordinarily reverses at odd intervals, maintained a constant polarity for more than 30 million years.

Under a proposed scenario this was all the result of a major episode of plumes rising from deep in the mantle. For some period prior to the Cretaceous, the lower mantle heated up by heat transferred from the core and from radioactive decay in the mantle. Eventually, the lower mantle got so hot that it was less dense than the mantle above it. So blobs of hot, low density mantle began to rise from the core-mantle boundary region up toward the Earth's surface leaving trails or conduits through which heated mantle material could continue to rise for tens of millions of years. These are called mantle plumes. They leave hot spot tracks, such as the Hawaiian Islands, on the Earth's surface as the plates slowly glide over the plumes.

When the mushroom-shaped plume heads neared the Earth's surface, the great heat in them caused widespread melting and the formation of the flood basalts. The excess heat coming to the surface also induced rapid seafloor spreading. This increase in volcanic activity released additional CO2 from the mantle into the atmosphere which increased the greenhouse effect and caused the climate to warm.

The increased rate of seafloor spreading caused sea level to rise. Increasing the rate of seafloor spreading inflates the ridge. Hot, young lithosphere is forming and moving away from the ridge at a faster rate and moves a greater distance from the ridge before it cools and contracts. So the area of "expanded" hot, young ridge extends much farther out from the ridge axis as compared to times of slower seafloor spreading. The inflated ridge takes up more space in the ocean basin. It takes up space formerly taken up by water. So sea level rises.

[The release of heat from the core-mantle boundary increased the convection in the Earth's outer core, making the Earth's magnetic field more stable, thus accounting for the long period of constant normal polairty.]

Descent to the Cenozoic Icehouse

Rapid seafloor spreading and Pangea breakup and separation in the Cretaceous had tectonic consequences. It resulted in rapid convergence and the subduction of a midocean ridge under western North America during the Cenozoic which resulted in the Laramide Orogeny and building of the Rocky Mountains. The breakup of Gondwana led to the collisions of Africa, Arabia, and India with Eurasia forming the Alpine-Himalayan mountain belt which is still experiencing active uplift. These steep, young mountains are the site of increased weathering and erosion. The principal agent of chemical weathering is carbonic acid which is formed when CO2 dissolves in water. The global increase of chemical weathering resulting from the Cenozoic mountain building decreased the amount of CO2 in the atmosphere. Also during the Cenozoic, seafloor spreading rates decreased and so the rate of outgassing of CO2 also decreased. Eventually, the Cenozoic became much cooler than the Cretaceous.

The Ice Ages Cometh

Two tectonic events led to continental glaciation in Antarctica and the northern hemisphere (see Stanley, Fig 19-18, p. 535). Antarctica was largely ice free during the Cretaceous and early Cenozoic in spite of lying over the south pole. This is because Antarctica was still connected or at least lay close to South America and Australia. warm ocean currents, deflected by South America and Australia bathed the coast of Antarctica. But by around 35 million years ago in the mid Cenozoic the last connections with Australia and South America were severed and deep water passages surrounded Antarctica allowing the formation of a continuous Circum- Antarctic cold current to thermally isolate the now frozen continent. It is at this time that glacial dropstones are first found in deep marine sedimentary strata off the coast of Antarctica. This marks the beginning of glaciation in Antarctica.

Northern Hemisphere glaciation did not begin until around three million years ago. This is around the time that the Isthmus of Panama closed. Prior to closure the surface currents in the Atlantic connected with the Pacific between the Americas. After closure the Atlantic circulation stayed in the Atlantic; the Gulf Stream intensified. One leading hypothesis holds that an intensified Gulf Stream carried warm water (producing warm, moist air over it) to high latitude. This warm, moist air caused an increase in precipitation (snowfall) which resulted in the growth of glaciers. Anothet hypothesis considers that prior to the closure of the Isthmus of Panama the Gulf Stream carried warm water into the Arctic Ocean thereby keeping the Arctic relatively warm. After closure the Gulf Stream was saltier and denser and as it cooled it became dense enough to sink in the North Atlantic forming a deep ocean current (North Atlantic Deep Water), thus depriving the Arctic of a source of heat.

Continental ice sheets have advanced and retreated many times during the past three million years. These advances and retreats can be correlated with variations in the Earth's orbit around the sun. The amount of solar radiation that the northern hemisphere receives in the summer (to melt the winter's snow) varies with changes in 1) the eccentricity (more round to more oval) of the Earth's orbit (100,000 year cycles), 2) the tilt of the axis from 21.5° to 24.5° (41,000 year cycles), and 3) the precession of the Earth's axis like the wobbling of a child's spinning top (23,000 year cycles). With each increase in the polar ice caps comes a decrease in sea level; with each reduction of the ice caps sea level rises. During the maximum advances of the ice sheets the entire continental shelves were exposed, including the Bearing land bridge between North America and Asia. Today, we are in an interglacial period. Sea level has been rising slowly during the past 10,000 years and is still rising at a rate of about 1 mm per year. The glaciers will come back despite the short term climate warming that humans may be causing by burning fossil fuels.

It is interesting to note that humans evolved during the ice ages. Humans first came to the Americas over the Bearing land bridge before the latest continental ice sheets broke up around 10,000 years ago. Great civilizations arose in the Middle East, India, the Far East and the Americas during the current interglacial period.

(see Past Ice Ages)