Study offers new evidence to explain giant gap in geological record
A vast gap in the global geological record spanning from 1.4 billion to 1.7 billion years ago to around 500 million years ago has puzzled scientists since the late 1880s, when a member of a John Wesley Powell expedition first spotted it in the rocky layers of the Grand Canyon.
A new study in the journal PNAS supports the hypothesis it was caused by glaciers.
That the gap was subsequently found on continents around the world only added to the mystery. If geologists read Earth's rock layers like an encyclopedia, then the Great Unconformity was like someone threw away books R through W.
"We're missing all the time in between. And what really makes that difficult from a geologic perspective is understanding what happened in terms of the geologic events in that intervening time period," said lead author Kalin McDannell, a postdoctoral researcher at Dartmouth College.
Worse, the missing time intersected with a period of major changes for the Earth, which was experiencing a continental shuffle, a planetwide freeze and an explosion of multicellular life in and around that era.
One hypothesis for the loss of hundreds of millions of the planet's most interesting years blames an upheaval of plate tectonics coinciding with the breakup of the supercontinent Rodinia.
The other credits erosion from widespread glaciation during a "snowball Earth."
"The Earth was mostly covered in ice. There may have been some narrow bands in certain areas that were kind of near the equator that were maybe not completely covered," said McDannell.
Though both may have played a role, researchers found the icy explanation covered more ground.
"Areas along the margins are showing signals that are both in agreement with tectonics and snowball glaciation. They're kind of both happening at the same time. But, when you go into the deep interior of the continent, there's no plate tectonic activity going on at that time," said McDannell. "So really the only thing that can explain these signals at the broad scale is ice sheet glaciation."
The researchers reached that conclusion through a technique called thermochronology, which let them estimate the temperatures key minerals experienced during known time periods — in this case, by looking at diffusion patterns of noble gases like argon in feldspar and helium in zircon or apatite.
"In many cases, they're not dating an actual geologic event; they're dating a point of time, or an interval of time, where you pass through a certain temperature sensitivity," said McDannell.
The geologic record consistently pointed to cooling.
"In the end, the payoff is that you get a much more complete and robust thermal history record. And what's significant about our paper is that we're showing, across a vast area of North America, a fairly consistent and synchronous signal of roughly 100 to 200 degrees C of cooling," said McDannell.
That suggests about 3 to 5 kilometers of erosion occurred on the continent roughly 700 million to 650 million years ago.