Believe it or not, the geology at the bottom of the Grand Canyon is extraordinarily common. There, layers of sedimentary rock lie flat atop angled layers of significantly more ancient metamorphic rock. The gap there is enormous—if Earth’s rocks constitute a book of the planet’s history, there are about a billion pages missing. The story only picks up again around 540 million years ago in the Cambrian period, with an evolutionary explosion of complex life just as remarkable as the sudden change in the rock.

This gap can be found all around the world, and has picked up the name the Great Unconformity. Cambrian sedimentary rocks rarely rest on anything other than much older metamorphic or igneous rock, implying that whatever rock formed in the intervening time was scrubbed away by something. This erasure of a chunk of geologic history has long been an enticing mystery for geologists.

Have you seen this rock?

A period of intensive global erosion doesn’t seem sufficient to fully explain the pattern of change in the rock. An alternative, that the formation of new rock suddenly accelerated beginning in the Cambrian, doesn’t quite fit the evidence, either. So what gives?

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

To look for that chemical mark, the researchers analyzed an isotope of the element hafnium. This isotope is produced by the (very slow) radioactive decay of element-you-also-forgot-existed lutetium, meaning it is slowly accumulating in the Earth’s mantle. But this is not happening in the Earth’s crust, which means crustal rocks are a little light in the hafnium isotope department.

So what can hafnium tell us? Imagine you’re cooking down a stew, aiming for a slightly thicker and saltier broth. At some point, you worry you’ve gone too far, so you add a little water back in. If you took out a spoonful every few minutes and set them aside in a sort of stew timeline, you could figure out just by taste where in that sequence you added the water.

The idea here is similar. If a lot of continental sediment—containing less of that interesting hafnium isotope—was being eroded and recycled back into the zone where mantle rock is melted, you ought to see a sudden drop in the hafnium numbers in zircons produced by the volcanoes above.

Snowball-driven dump

And that’s exactly what the researchers found. The zircons in the database span nearly the entire history of the Earth, and by far the most noticeable wiggle lines up neatly with the Great Unconformity. When they ran the numbers to see how much erosion would be required to explain a wiggle of that size, they found that it would be something in the neighborhood of 3 kilometers (or 2 miles) of rock shaved off all the world’s continents and dumped on the ocean floor.

Erosion alone can’t explain all the details of this episode, and you need something that affects the entire globe. Is there anything else that can wipe a few kilometers of rock off the Earth’s face? The authors propose that three periods of epic cold snaps in the 180 million years leading up to the start of the Cambrian—sometimes referred to as “Snowball Earth” periods—could be the key.

The first two of these episodes, in particular, are thought to have seen huge ice sheets draped over every continent for millions of years. There are still big questions about how these events played out, but glaciers are often pretty potent agents of erosion. If temperatures drop low enough, glaciers will freeze to the ground like the tongue of an unfortunate child stuck on a flag pole. But it doesn’t take much for normal geothermal warmth to keep that base thawed, and sliding ice will grind up a lot of bedrock.

On top of that, the growth of ice sheets on land comes with a lowering of global sea level, exposing vast areas of former seafloor to erosion. That also lowers the base level that glaciers and streams flow to, giving them a little more downhill energy.

Modeling erasure

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 with sea level lower, all that eroded material would get deposited on the deep seafloor—ready to ride the tectonic plate into the recycler.

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

Because several remarkable events—the great glacial periods, the gap in the rock record, and the rise of multicellular life—happened around the same time, researchers have long wondered about connections among them. The geological changes, for example, may well have produced chemical changes in the oceans that enabled interesting evolutionary responses. The huge swings in climate, too, may have had something to do with the timing of the evolutionary explosion. This new study builds up the idea that all three were linked. So the history book is not just missing pages—some of them were used to write the chapter that followed.

PNAS, 2018. DOI: 10.1073/pnas.180435011  (About DOIs).

https://arstechnica.com/?p=1434295