A rock hammer marks the Great Unconformity near Las Cruces, New Mexico, where Cambrian-age sandstones deposited in a shallow sea overlie uplifted and eroded grantic rocks of the continental crust that were exposed to the earth's surface during the Cambrian period, about 500 million years ago. Credit: Robert Gaines
Closest Places to Observe the Great Unconformity
The Grand Canyon is he best place to see it locally. "It's present in the inner gorge, the steep inner canyon, in the central part of the Grand Canyon, near the national park's Grand Canyon Village," reports Gaines. "There is also an interpretive trail at Frenchman Mountain in Las Vegas." http://geoscience.unlv.edu/pub/rowland/Virtual/virtualfm.html
The oceans teemed with life 600 million years ago – simple, soft-bodied creatures that would hardly be recognizable as the ancestors of nearly all of the animals on Earth. Then something incredible happened. Over several tens of millions of years – a relative blink in geologic terms – a burst of evolution led to flurry of diversification and increasing complexity, including the expansion of multicellular organisms and the appearance of the first shells and skeletons.
Why and how this happened has remained a mystery, until now. New research by Robert Gaines and Shanan Peters, published in the April 19 issue of the journal Nature, shows that the answer may lie in a second geological curiosity – the Great Unconformity, a dramatic boundary in the geologic record between ancient igneous and metamorphic rocks, and much younger sediments.
"The Great Unconformity is a very prominent geomorphic surface, and there's nothing else like it in the entire rock record," says Peters, a geoscience professor at the University of Wisconsin–Madison who led the new work.
"We realized that its formation must have had profound implications for ocean chemistry at the time when complex life was just proliferating," adds Gaines, a geology professor at Pomona College.
The Great Unconformity consists of "basement" rocks formed billions of years ago, at depths of several kilometers, that were exhumed by erosion, exposed at Earth's surface for a time, and then buried under marine sediments as shallow ancient seas progressively flooded the continents just half a billion years ago, during the Cambrian period. It is a prominent feature of the geologic record that may be traced worldwide.
The Great Unconformity was named in 1869 by explorer and geologist John Wesley Powell during the first documented trip through the Grand Canyon. It has remained a longstanding puzzle – viewed by Charles Darwin, among others, as a huge gap in the rock record and in our understanding of the Earth's history.
In the Nature paper, Peters and Gaines report that the same geological forces that created the Great Unconformity may have also provided the impetus for the burst of biodiversity during the early Cambrian.
"We're proposing a triggering mechanism for the Cambrian explosion," says Peters. "Our hypothesis is that biomineralization evolved as a biogeochemical response to an increased influx of continental weathering products during the last stages in the formation of the Great Unconformity."
Peters and Gaines looked at data from more than 20,000 rock samples from across North America and found multiple clues, such as unusual mineral deposits with distinct geochemistry, that point to a link between the physical, chemical and biological effects.
During the early Cambrian, shallow seas repeatedly advanced and retreated across the North American continent, gradually eroding away surface rock to uncover fresh basement rock from within the crust. Exposed to the surface for the first time, those crustal rocks reacted with air and water in a chemical weathering process that released ions such as calcium, iron, potassium, and silica into the oceans, changing the seawater chemistry.
Those basement rocks were later covered with sedimentary deposits from the Cambrian seas, creating the boundary now recognized as the Great Unconformity.
Evidence of the changes in the seawater chemistry is captured in the rock record by high rates of carbonate mineral formation early in the Cambrian, as well as the occurrence of extensive beds of glauconite, a potassium-, silica-, and iron-rich mineral that is much rarer today.
The influx of ions to the oceans also likely posed a challenge to the organisms living there. "Your body has to keep a balance of these ions in order to function properly," Peters explains. "If you have too much of one you have to get rid of it, and one way to get rid of it is to make a mineral."
The fossil record shows that the three major biominerals – calcium phosphate, now found in bones and teeth; calcium carbonate, in invertebrate shells; and silicon dioxide, in radiolarians – appeared more or less simultaneously around this time and in a diverse array of distantly related organisms.
The time lag between the first appearance of animals and their subsequent acquisition of biominerals in the Cambrian is notable, Peters says. "It's likely biomineralization didn't evolve for something, it evolved in response to something – in this case, changing seawater chemistry during the formation of the Great Unconformity. Then once that happened, evolution took it in another direction." Today those biominerals play essential roles as varied as protection (shells and spines), stability (bones), and predation (teeth and claws).
Together, the results suggest that the formation of the Great Unconformity may have triggered the Cambrian explosion. "This feature explains a lot of lingering questions in different arenas, including the odd occurrences of many types of sedimentary rocks and a very remarkable style of fossil preservation. And we can't help but think this was very influential for early developing life at the time," Gaines says.
Far from being a lack of information, as Darwin thought, the gaps in the rock record may actually record the mechanism as to why the Cambrian explosion occurred in the first place, says Peters.