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Tuesday, November 4, 2014

November 4. Paleogeography and climate

Because it’s so long – 80 million years – it’s definitely not cool to imply anything constant about the distribution of lands and seas nor about the climate of the Cretaceous, but as usual I’m happy to make some generalizations. As we said at the end of the Jurassic, the globe was beginning to become recognizable in terms of the modern continents. We’ll talk about some specifics, like the opening of the South Atlantic Ocean, over the course of the month, but at least by the middle of the Cretaceous, about 100 million years ago, the basic shapes of most of the continents were close to their modern forms.

Cretaceous globe (105 million years ago) by Ron Blakey, used under CC-BY-SA & GDFL

India and Australia were still way south of the Equator, having just separated from Antarctica, and the Tethys Ocean was still between them and the main mass of Asia. Antarctica itself was near or at the South Pole as it is today, and the northern margins of North America and Eurasia were in the north polar region as they are today.

But the climate was much less extreme than it is today, with only a few notable exceptions, throughout most of the 80 million years the Cretaceous lasted. There were no widespread continental glaciers during the Cretaceous, although there were probably mountain glaciers in high latitudes, but even that was probably limited to the very early Cretaceous in what is now Australia. Most of the rest of the period was warm, and the temperature change from poles to equator was not extreme. One probable consequence of the weak temperature gradient would be less extreme winds and perhaps fewer intense storms like modern hurricanes. That, in turn could lead to oceans with poorer circulation – though the general distribution of land masses, at least until the late Cretaceous, might have a greater control on that. This is a possible explanation for the extensive black shales, related to anoxic events in the oceans, which are observed in the Cretaceous rock record.

The equable climate at the poles is reflected in the presence of dinosaurs there, as well as temperate plants. Southernmost Australia today was within the Antarctic Circle in early Cretaceous time, and mud flats that formed there in the rift between Australia and Antarctica have yielded diverse dinosaur bones, and Cretaceous dinosaurs are also known from Antarctica itself.

Temperate forests were present within a few degrees of the poles. Remember that whatever the temperature, the poles still had long seasons of darkness just as they do today. Cretaceous greenhouse conditions allowed plants to grow in polar regions, but it might have been the light and dark cycles that provided evolutionary pressure for plants to evolve. The early Cretaceous forests were much like those of the Jurassic, with cycads, ferns, and conifers. But within 15 or 20 million years, dramatic evolution of the angiosperms, flowering plants, had begun. We’ll talk more about that in a few days.

By late Cretaceous time, average temperatures had risen even more, to perhaps 4 to 6 degrees Centigrade warmer than today. This time is called the Cretaceous Thermal Maximum, and it occurred during the Turonian stage. Exactly what caused it is debated, but two favored factors are both related to rifting. As more and more rifts formed, and oceanic crust was generated, the volcanism associated with rifting could have put more and more CO2 into the atmosphere, enhancing an already strong greenhouse effect. It’s also possible that changing patterns of land and sea allowed for more efficient movement of warm surface water around the globe. There is considerable variation in the patterns of temperature even during the Turonian, so this event is definitely not fully understood.

In North America, the Cretaceous was dominated by the Western Interior Seaway, a long shallow sea that extended from Alaska to Alberta to Colorado to West Texas and into Mexico. We’ll talk more about that when we talk about the mountain-building events that were defining the western margin of North America, later this month.
—Richard I. Gibson

Rifting and Cretaceous Thermal Maximum 

Cretaceous Thermal Maximum

Dinosaur Cove, Australia 

Cretaceous globe (105 million years ago) by Ron Blakey, used under CC-BY-SA & GDFL

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