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Antarctica today.
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Believe it or not, the geology at the bottom of the Grand Canyon is extremely ordinary. There, layers of sedimentary rock lie flat on top of oblique layers of significantly older metamorphic rock. The gap there is huge — if Earth’s rocks make up a book about the planet’s history, about a billion pages are missing. The story picks up again only about 540 million years ago in the Cambrian period, with an evolutionary explosion of complex life as remarkable as the sudden change in the rock.

This canyon can be found all over the world and has been given the name The Great Unconformity. Cambrian sedimentary rocks rarely rest on anything but much older metamorphic or igneous rocks, implying that the rock that formed in the intervening time has been swept away by something. This erasure of geological history has long been an enticing mystery to geologists.

Have you seen this rock?

A period of intense global erosion does not seem sufficient to fully explain the pattern of change in the bedrock. An alternative, which suddenly accelerated the formation of new rock from the Cambrian, also doesn’t quite fit the evidence. So what gives?

To dig into this, a team led by University of California, Berkeley’s C. Brenhin Keller turned to a database of nearly 30,000 zircon crystals. Zircons are most commonly found in the igneous rocks of volcanic arcs along tectonic plate subduction zones, where one plate plunges beneath another (think the Pacific Ring of Fire). If a huge amount of continental rock eroded away, it would have ended up in the ocean, where it could take a ride to the tectonic recycler in these subduction zones — possibly leaving a chemical mark in the magma-feeding volcanoes.

To find that chemical mark, the researchers analyzed an isotope of the element hafnium. This isotope is produced by the (terribly slow) radioactive decay of the element-you-to-forget-existing lutetium, meaning it slowly accumulates in the Earth’s mantle. But this is not happens in the Earth’s crust, meaning crustal rocks are a little light in the hafnium isotope division.

So what can hafnium tell us? Imagine cooking a stew and aiming for a slightly thicker and saltier stock. At some point, you worry that you’ve gone too far, so you add a little water again. If you take out a spoonful every few minutes and set them aside in some sort of stew timeline, you can taste where in that order you added the water.

The idea here is similar. If a lot of continental sediment — containing less of that interesting hafnium isotope — was eroded and returned to the mantle rock melting zone, you should see a sudden drop in the hafnium numbers in zircons produced by the volcanoes above.

Snowball powered dump

And that’s exactly what the researchers found. The zircons in the database cover nearly all of Earth’s history, and by far the most notable wobble neatly matches the Great Discordance. When they sifted through the numbers to see how much erosion it would take to account for a wobble of that magnitude, they found it was something close to 3 kilometers (or 2 miles) rock that has been shaved off all the continents of the world and dumped on the ocean floor.

Erosion alone cannot explain all the details of this episode, and you need something that affects the whole world. Is there anything else that can sweep a few miles of rock from the face of the earth? The authors propose that three periods of epic cold spells in the 180 million years leading up to the beginning of the Cambrian — sometimes referred to as “Snowball Earth” periods — could be key.

Especially in the first two of these episodes, huge ice caps are thought to have been draped over it each continent for millions of years. There are still big questions about how these events played out, but glaciers are often quite powerful means of erosion. If temperatures drop low enough, glaciers will freeze to the ground like the tongue of an unlucky child stuck to a flagpole. But it doesn’t take much for normal geothermal heat to keep that base thawed, and sliding ice will grind up a lot of rock.

In addition, the growth of land-based ice sheets is accompanied by a lowering of global sea levels, exposing large areas of the former seafloor to erosion. That also lowers the base level where glaciers and streams flow, giving them a little more downhill energy.

Clear modeling

The researchers played with a simple numerical model to see how this might work. Using reasonable estimates of glacial erosion rates from modern times, the model has no problem eroding the right amount of rock. And if the sea level were lower, all that eroded material would end up on the deep seafloor — ready to drive the tectonic plate into the recycler.

The really interesting result in the model is that all the glacial erosion creates low patches (especially along the coast) that are ready to hold a new blanket of sediment as the ice melts away. As sea levels rise, more and more coastal sediment can accumulate there, eventually forming the sedimentary rocks that begin the next intact chapter of Earth’s history book. So the snowball glacier is not only a possible source of erosion, but is also responsible for some additional rock formation afterward — a neat “little column-a-small-column-B” explanation.

Because several notable events — the great ice ages, the bedrock rift, and the emergence of multicellular life — occurred around the same time, researchers have long wondered what the links are. For example, the geological changes have very well led to chemical changes in the oceans that have allowed for interesting evolutionary reactions. Also, the huge fluctuations in climate may have had something to do with the timing of the evolutionary explosion. This new study builds on the idea that all three were linked. So the history book isn’t just missing pages – some were used to write the next chapter.

PNAS2018. DOI: 10.1073/pnas.1804350116 (About DOIs).

By akfire1

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