April 20, 2012
Cambrian Explosion Trigger Found In Grand Canyon
Millions of years ago, the creatures who would become the ancestors of all life, animals and humans alike, were simple, sometimes composed of individual cells. Evolution had been slow, with very little diversification. Then, as the waters began to shift and separate exposing new areas of land to air and daylight, this slow evolution began to explode into activity. Referred to as the Cambrian Explosion, this diversification is estimated to have taken several million years itself, breeding many new, multicellular organisms and bringing about the first appearance of shells and skeletons.
Fossil records document this explosion, yet it has been largely misunderstood what caused it to happen and at what time. Now, a second geological mystery, known as the Great Unconformity, may provide geologists and scientists with answers about both.Geoscience professor at the University of Wisconsin-Madison, Shana Peters, explains the Great Unconformity this way, “The Great Unconformity is a very prominent geomorphic surface and there´s nothing else like it in the entire rock record.”
The Great Unconformity juxtaposes rocks formed billions of years ago deep within the Earth´s crust with younger rocks formed by deposits left from shallow seas during the Cambrian period. Examples of the Great Unconformity can be seen in the Grand Canyon. Puzzling to geologists and scientists the world over, the Great Unconformity has been seen as a gap in the rock record and even our Earth´s history.
Now, Peters–who lead the new study–says the gap itself could help us understand what caused the Cambrian Explosion.
In the April 19 issue of the journal Nature, Peters and his colleague Robert Gaines of Pomona College report the Great Unconformity and the Cambrian Explosion may have the same geological forces to thank.
“The magnitude of the unconformity is without rival in the rock record,” Gaines says. “When we pieced that together, we realized that its formation must have had profound implications for ocean chemistry at the time when complex life was just proliferating.”
Looking at more than 20,000 rock samples from across North America, Peters and Gaines found clues of a link between the biological, chemical, and physical effects of the two oddities.
It is believed shallow seas in the Cambrian period repeatedly advanced and retreated across the North American continent, eroding away the surface rock. As this erosion occurred, the basement rock below became exposed to the surface for the first time. As these rocks and air reacted with one another, a chemical weather process took place, releasing calcium, iron, potassium, and silica into the oceans. This changed the seawater chemistry and created the Great Unconformity.
The change of seawater chemistry also had an effect on 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.”
Fossil records show these changes happened around the same time, something Peters says is notable.
“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).”
Peters now says the gap in time expressed by the Great Unconformity may actually be less of a gap and more of a starting point for future research.