The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Sunday, September 21, 2014

September 21. Triassic Antarctica

We haven’t talked much about Antarctica, for the obvious reasons that it’s remote and largely covered with ice and snow. The big-picture aspects of Antarctica are well known, including its position within the ancient Gondwana continent and its rifting history, which separated it from Australia, India, Africa, and South America as Pangaea was dismembered. But there’s plenty of detail to be filled in.

Despite the ice and snow, there are good outcrops of rock across the continent, especially in the Transantarctic Mountains, a 3500-km chain that was uplifted in the Cretaceous, which we will get to in November. The mountains expose older rocks, including a thick sequence laid down during the Triassic Period. 

You recall that the Triassic was a time of global high temperatures and arid conditions, but don’t think of that as something uniform across the planet. It’s inconceivable that the poles were not at least somewhat cooler than the tropics, but there is no evidence for any glaciation anywhere, even at the poles, during the Triassic. That’s a dramatic shift from the extensive glaciation during Permo-Carboniferous time just 50 or 60 million years before the start of the Triassic.

Within the supercontinent of Pangaea during the Triassic, Antarctica was pretty close to the South Pole, if not quite on top of it. Much of Triassic Antarctica was within the Antarctic Circle, though, and some reliable paleolatitude measurements put parts of the continent as close to the pole as 75° S Latitude. But there were still no glaciers.

Triassic Globe (Antarctica near south pole) by Ron Blakey, NAU Geology, under Creative Commons license (notes by Gibson)

Much of the Triassic that’s exposed in Antarctica is called the Fremouw Formation, a thick package of river-laid sediments including channel gravels, sands and silts, and floodplain mud deposits. The age is estimated to span many millions of years, probably beginning in the latest Permian and continuing well into the Triassic. That’s an unusually long period for a more or less unchanging depositional setting, and perhaps it represents episodic deposition over the long time span rather than continuous deposition, but the package is more than 3,000 feet thick, so it probably represents a long period, no matter how you slice it.

The Fremouw contains lots of amphibian and reptile bones, as well as synapsids such as cynodonts and dicynodonts that are more closely related to mammals than to reptiles. Because the deposit spans the Permian-Triassic extinction event, it’s a valuable source of information about the responses to the extinction. The fact that the same lineages exist in Permian and Middle Triassic rocks means that they must have existed during the early Triassic, right after the extinction event, even if their numbers were reduced. The fossils in the Fremouw Formation include extensive early Triassic species that are absent or poorly represented in most of the rest of the world.

Tree logs, including an intact fossil forest, have also been found in the Fremouw. They lived along the rivers and on the floodplains, together with many other plants including cycads, horsetails, and ferns.

Taking the early Triassic animal fossils together with the evidence for extensive forests leads to the speculation that Triassic Antarctica was a refuge ecosystem where survivors of the Permian extinction rode out the event and the challenging environments of the early Triassic that prevailed in many other parts of the globe. You recall the other day I mentioned a paper that suggested that the tropical Triassic Earth might have been “lethally hot” and that ocean temperatures in the Triassic might have been so hot that life could not survive. The polar regions might have been enough cooler that they provided this refuge for life, which would have expanded across the planet as conditions became more favorable later in the Triassic and Jurassic. A 2010 research report, linked below, focused on one of the dicynodont survivors of the extinction whose fossils are found in the early Triassic rocks of Antarctica. I also have a link below to the paper describing the intact fossil forest.

One thing to keep in mind when you think about the life in Antarctica during the Triassic: even though the polar areas were much, much warmer than they are today, the seasonal change in length of the day would have been about the same. The poles would have experienced a season of 24-hour sunshine as well as a season of 24-hour darkness. It’s conceivable that the need to forage in continuous darkness might have provided evolutionary pressure that helped animals develop their hearing and vision, and that could be an important step for the ancestors to the mammals.

The name Fremouw comes from Fremouw Peak in the Transantarctic Mountains, which in turn was named for Edward J. Fremouw, an American Antarctic aurora researcher.

* * *

Today we mark the birthday of Douglass Houghton, born September 21, 1809 in Fredonia, New York. He’s best known for his investigations of the copper deposits in the Keweenaw Peninsula of Upper Michigan, work that stimulated the first copper boom in the United States – really the first really big mining boom of any sort – in the early 1840s. He died in a boating accident in a storm on Lake Superior in 1845. He was only 36 years old. Houghton, Michigan, in the copper country, is named for him.

—Richard I. Gibson

In situ fossil forest 
An Antarctic Refuge? 
Triassic Globe by Ron Blakey, NAU Geology, under Creative Commons license (notes by Gibson)

No comments:

Post a Comment