Did the Late Cretaceous host icy conditions in Antarctica? Scientists answer

Let’s imagine it’s the late Cretaceous period, roughly 66 to 100 million years ago. We have land-roaming dinosaurs and strange-looking early species, even though sharks as we know them actually swim in prehistoric oceans that cover 82% of the Earth. Redwoods and other conifers appear for the first time, as well as roses and flowering plants, and with them come bees, termites, and ants. Most of all, it’s warm, volcanically active and humid all around with no ice cap in sight.

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Except, according to a group of scientists from the University of California, Santa Barbara, the University of Oregon and the University of Manitoba, there were icy conditions in the Antarctic region.

“It wasn’t just a single valley glacier, it was probably from several glaciers or a large ice sheet,” said John Cottle, a geologist from the University of California, San Francisco. Contrary to the widely circulated picture of the Late Cretaceous being “hot all over,” he said, there is evidence of polar ice during that period, even at the height of global warming conditions.

The geologists study was published in the journal Nature Communications.

prehistoric puzzle

Fast forward to today. Let’s pretend we’re in Antarctica. It’s cold and barren, and we’re standing near a large group of exposed glass rocks along the Transantarctic Mountains, adjacent to the Ross Ice Shelf, called the Butcher Ridge Igneous Complex (BRIC).

“I actually heard about these rocks when I was a graduate student 20 or so years ago, and they’re really weird,” Cottle said. Distant, even by current Antarctic exploration standards, the Brik area is unusual in that the rock formation and formation are uncharacteristic of nearby rock formations, with, among other things, large amounts of glass and stratification change indicative of significant physical, chemical or environmental events that altered their composition. metallic.

Cottle finally got a chance to sample Brik’s group on a recent expedition, and in the process of analyzing how it formed, he and his team encountered an “unusually large amount of water.”

“So you have a really hot rock that reacts with the water, and when it cools, you embed it into the glass,” he said. “If you look at the composition, you can tell something about where this water came from. It could exist as a hydroxyl, which tells you it probably came from magma, or it could be molecular, which means it might be exogenous.”

What they expected to see was that the change in the rock was caused by the water already in the magma as it cooled. What they found instead is a record of a climate process that was believed to have not existed at the time.

the findings

In their spectroscopic analysis of the samples, the researchers determined that while some of the water did indeed originate from magma as it was rolling upward from the Earth’s interior, as molten rock cooled into glass just below the Earth’s surface, it had also incorporated groundwater.

“We’ve determined that most of the water in these rocks is exogenously derived,” Cottle said. “We then measured the isotopic composition of oxygen and hydrogen in the water and it matched very well with the composition of Antarctic ice and snow.”

Conducting argon chronological geology

To demonstrate the result, Cottle and his team also conducted argon- and argon-geologic chronology to date and alter the rock.

“The problem is that these rocks are from the Jurassic period, so they are about 183 million years old,” he said. “So when you measure change, what you don’t know is when it happened.” They were able to restore the age of the rocks (Jurassic), but they also found a younger tooth (Cretaceous). He continued, “So when these rocks are cooled and changed, the argon isotope is reset as well, and you can match the age of the change to the composition of the change.”

There are other similar volcanic rocks about 700 km north of the Brik group that also have a Cretaceous change age, suggesting that polar glaciation may have been regionally widespread in Antarctica during that period. “What we’d like to do is go to other places in Antarctica and see if we can determine the size of the glacial ice, if we take back the same results we already found,” he said.

Finding evidence of large ice sheets dating back to the Cretaceous period may not change our overall picture of a hot and humid Earth at the time, Cottle said, “but we have to think about the Cretaceous and Antarctica very differently than we do now.”

Research in this study was also conducted by Demian A. Nelson (lead author) from UCSB and Elijah N. (Ani)