So, Uh, Where Did a Billion Years of the Geologic Record Go?
A new study suggests Earth's surface was eroded in pieces long before the Great Unconformity.
Scientists have wondered if Snowball Earth caused the Unconformity and the Cambrian explosion.
By studying helium signatures in layers of rock, researchers pieced together its erosive history.
Scientists are floating a new way to explain a geological feature called the Great Unconformity, where the record of a billion years is just missing from between the layers of rock.
Or, to be more specific, they say they’ve disproved one of the most popular existing theories—and in scientific inquiry, that can be just as valuable.
In Earth’s distant, primordial past, scientists believe the entire planet froze over into something they call Snowball Earth. This sounds adorable, but it’s one extreme end of the spectrum of what has happened to Earth over billions of years. And for those who study the geological record of the Earth, it’s an effective bookend to the time periods before and after.
This is the key piece of what researchers say they’ve disproven. As they write in Proceedings of the National Academy of Sciences:
“Erosion below the Great Unconformity has been interpreted as a global phenomenon associated with Snowball Earth. We suggest that there are multiple, regionally diachronous Great Unconformities that are tectonic in origin.”
So instead of the Chekhov’s gun explanation that of course a global ice-over must singly explain the great anomaly found beneath it, the reality could be a coincidental combination of several different causes.
During Snowball Earth, the world would have been covered by glaciers and subject to continuous glacial erosion. This has been a natural explanation for the way a large portion of geological record was “erased” in order to create the Great Unconformity: Glaciers sanded a billion years worth of accumulated rock off of the Earth’s surface and continuously redistributed it in sediment form.
Researcher Francis Macdonald said in a UC Santa Barbara statement that he found arguments for the “Snowball Earth mass erosion” theory to have a whiff of, well, over-convenience. To examine this assumption, scientists from several American universities combined efforts to study the visible Great Unconformity in the Grand Canyon.
Studying the very oldest Earth is difficult for many overlapping reasons. First, while the Grand Canyon and other exceptional surface features reveal the geological record dating back millions of years, these naturally occurring sample gouges are rare and dissimilar; it’s hard to study a dozen enormous canyons around the world and draw a unifying conclusion. And second, the very way Earth exists ends up destroying much of the evidence.
That’s because Earth’s surface is a solid crust made of overlapping pieces, and as the pieces move around, the edges are constantly being shoved under, melted, and recirculated as new surface area. (Deep beneath the Atlantic Ocean, between 0.5 and 4 inches of new seafloor emerge each year.) But in this research, scientists have harnessed that temperature signature as a way to help them better understand geological events.
By studying the thermochronology of rock layers in the Grand Canyon, scientists were able to extract data using amounts and kinds of helium present in the rock. This is because rocks at different temperature levels beneath the Earth’s surface degrade in specific ways such that the helium level encodes the maximum temperature the rock experienced. And that means scientists can reverse engineer where in the geological record each section of rock “began” its journey.
Was everything hunky dory until Snowball Earth after all? Using helium signatures, these scientists have demonstrated that the levels of rock under the Great Unconformity had already been eroded, changed, and moved around.
“Our results provide evidence that most erosion below the Great Unconformity in Colorado occurred before the first Neoproterozoic Snowball Earth and therefore cannot be a product of glacial erosion,” the scientists conclude.
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