Mind the gap

If you go for a hike down into the Grand Canyon, you might notice that something is missing.

At the top of the ancient basement rocks that rise from the Colorado River lies a much younger layer of reddish sandstone. That old-to-younger gap represents a billion-year piece of history known as the Great Unconformity.

During that gap, the elements exhumed and denuded basement igneous and metamorphic rock that had formed 1.7 billion to 2.8 billion years ago. Then, a mere 600 million years ago, shallow seas transgressed to further erode and deposit the sandstone-forming sediment.

As the seas advanced and retreated across the continents, newly exposed basement rock reacted quickly to release calcium, iron, potassium, and silica into the Cambrian ocean. And suddenly—within tens of millions of years—the residents of the Cambrian primordial soup changed from simple, soft-bodied creatures to a huge array of multicellular organisms with shells and skeletons. Such features provided competitive and functional advantages and allowed animal evolution to explode.

Great Unconformity, meet the Cambrian Explosion

“Think of the continents [basement rock] as being eroded and developing a scab [during the Great Unconformity],” explains University of Wisconsin–Madison geologist Shanan Peters. The first marine transgression during the Cambrian period picked away that scab and caused a rapid increase of surface mineral flux to the seas from freshly exposed basement rock that had been “charged” with minerals for a long time.

Peters’s database Macrostrat is a compilation of more than 20 000 rock units distributed among 830 regions in North America. It, along with reference samples from the whole world, gave him and fellow geologist Bob Gaines of Pomona College the means by which to determine the areal magnitude of the Great Unconformity. “When we realized that this was a global surface, we realized it would have effects on the chemistry of the oceans,” says Gaines.

Peters and Gaines proposed that the rapid spicing of the primordial soup with calcium phosphate, calcium carbonate, and other ions stimulated animals’ initial acquisition of bones and shells. Such uptake of biominerals provided means for protection, stability, and predation and so allowed evolution to progress. Shells and bones are recorded in Cambrian fossils, whose exceptional preservation is due to a pattern of altered sea level chemistry .

Each of three belts of sedimentary rock provide geochemical evidence for a sustained flux of continental weathering products, most notably carbonate sediments. Seawater trapped in the rocks shows an increase in calcium deposits during the time of the Cambrian explosion. Additionally, glauconite, a potassium-, silica-, and iron-rich mineral, was much more abundant than it is today.

There’s a big gap in the rock record right before the world’s biggest biodiversification event. But that gap has just as much information as the rocks themselves. “You’ve got to interrogate both the gappiness and the rockiness to get at the whole picture,” says Peters. And a beautiful picture it is that captures both a global geological surface and the beginnings of evolution to life as we know it.