Let’s assume it’s the Late Cretaceous, roughly 66 to 100 million years ago. Dinosaurs and strange-looking early bird species roam the earth, although the shark as we know it already swims in the prehistoric oceans that cover 82% of the Earth. Redwoods and other conifers make their debut, as do roses and flowering plants, and with them come bees, termites and ants. Above all, it is warm, volcanically active and everywhere is humid, there is not even an ice sheet in sight.
Except, according to a team of scientists from UC Santa Barbara, the University of Oregon and the University of Manitoba, icy conditions actually existed in the South Pole region. And it wasn’t just a glacier in one valley,” said UCSB geologist John Cottle, “it was probably multiple glaciers or a large ice sheet.” Contrary to our widespread image of the Late Cretaceous as “hot everywhere,” he said. there is evidence that polar ice existed during this period, even at the height of global greenhouse conditions.
Prehistoric puzzle
Fast forward to today. Let’s pretend we’re in Antarctica. It’s cold, it’s barren, and we’re standing near a large cluster of exposed glassy 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 years ago, and they’re just really weird,” Cottle said. Remote, even by today’s Antarctic exploration standards, BRIC is unusual because the composition and rock formations are uncharacteristic of nearby rock formations, among others, with large amounts of glass and layered changes that indicate significant physical, chemical or environmental events that have changed their mineral composition.
Cottle got a chance to finally get a taste of BRIC 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 interacting with the water, and when it cools, it incorporates it into the glass,” he said. “If you look at the composition, you can tell something about where the water came from. It can exist as hydroxyl, which tells you it’s probably magma-derived, or it can be molecular, which means it’s probably external.” They expected to see that the change in rock was caused by water already in the magma as it cooled. Instead, they found a record of a climate process that was thought not to exist at the time.
In their spectroscopic analysis of the samples, the researchers found that while some of the water did indeed come from magma as it rose up from the Earth’s interior as the molten rock cooled to glass just below the Earth’s surface, it also contained groundwater. We found that most of the water in these rocks came from outside,” Cottle said. “We then measured the isotopic composition of the oxygen and hydrogen in the water, and it matches the composition of Antarctic snow and ice very well.”
Conducted argon-argon geochronology
To back up their result, Cottle and team also performed argon-argon geochronology to date the rock and its alteration. The problem is that these rocks are Jurassic, so about 183 million years old,” he said. “So if you measure the change, you don’t know when it happened.” They were able to recover the age of the rock. (Jurassic), but they also found younger age (Cretaceous). “So when these rocks cooled and were altered,” he continued, “it also reset the argon isotope, and you can match the age of the alteration to the composition changes.
About 700 km north of BRIC there are other similar volcanic rocks that are also of Cretaceous alteration age, suggesting that polar glaciation may have been regionally extensive in Antarctica at that time. “We’d like to go to other places in Antarctica and see if we can determine the extent of glaciation if we get the same results we’ve already found,” he said. Finding evidence of large ice sheets dating back to the Cretaceous period might not change our general picture of a hot, wet Earth at the time, Cottle said, “but we would have to think about the Cretaceous and Antarctica very differently than we do now.”
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