For decades, geologists have been drilling — literally — for clues that would help them understand ancient wholesale changes in Earth’s climate, clues that could shed light on current global warming.
Usually, their efforts have been aimed at sea sediments taken from cores extracted hundreds of feet beneath the ocean floor. But in a more terrestrial project this past summer, an international geological team led by the University of New Hampshire began deep-core drilling at three sites in Wyoming’s Bighorn Basin east of Yellowstone National Park.
These six new core samples from Bighorn, until now better known for the dinosaurs in its past and the natural gas in its future, should make it possible to map ancient climate change at highly precise — in the realm of 1,000 to 10,000 year — intervals.
The 2.5-inch-diameter cores were drilled some 500 feet into the basin sediment in hopes that complex geochemical analyses could better explain the causes and effects of the biggest and best-studied period of known extreme global warming — the so-called Paleocene-Eocene Thermal Maximum.
PETM took place 56 million years ago and involved the release of as much as 7,000 gigatons of carbon in less than 10,000 years. By comparison, if released in a similar manner, all the world's known fossil fuels today would only contribute 5,000 gigatons.
“These hyperthermals are random freak events, but there have been about 40 [of varying intensity] in the last 300 million years,” explains Gregory Retallack, a paleontologist at the University of Oregon in Eugene.
Because these cycles have already been identified in marine cores, these new continental measurements should make it possible to place these ancient regional climate and ecological changes into a global framework. In other words, climatologists will have a much better idea of how these ancient warming events affected the whole of Earth’s climate.
At the end of drilling this past summer, the team sent the six fresh cores to the international core repository at the University of Bremen in Germany. Researchers are meeting this week to split the cores for detailed analyses at their respective institutions.
“The same climate events have been studied in the ocean core records,” said Will Clyde, the project’s lead principal investigator and a geologist at the University of New Hampshire. “But this is the first core record of these events from the North American continental environment.”
As might be expected, the researchers are trying to fathom how much carbon was released into the atmosphere at any specific time during these hyperthermals. A good way to do this using ancient dirt is to measure the isotopic signature of carbon residue in rock sediments, which reveals exactly in what quantities and over which timeframes this outgassing took place.
It’s known that the PETM pushed lots of carbon-12 isotope into Earth’s atmosphere. This isotope, a variation of a normal carbon atom but with a differing number of neutrons in its nucleus that eventually morphed into carbon dioxide, is one we recognize as a greenhouse gas. The result in ancient times was that global temperatures increased by 5 degrees Celsius or more, making the oceans more acidic and less oxygenated.
But what was the source of this massive burp?
This large injection of greenhouse gases may have been caused by the freeing of icy microscopic methane solids trapped in frozen water buried in sediments underneath the oceans' continental shelves. (You may recall these “clathrates” — as the methane solids are called — as the stumbling blocks to closing off the Deepwater Horizon oil spill last year.)
“Destabilizing clathrates would require some initial warming,” said James Zachos, a geologist at the University of California, Santa Cruz. “We know that during extremes in Earth's [orbital] eccentricity, CO2 levels rose and the global climate warmed. One hypothesis posits that the PETM might have been triggered during a maximum phase of eccentricity, pushing the climate system across a tipping point.”
Such eccentricities in Earth’s orbit would have caused our planet to come closer to the sun, and the resulting extra solar heat would have increased enough to warm Earth’s ocean bottoms.
But more likely, erupting underwater volcanoes triggered this carbon release.
“One idea is that you had a lot of rifting and volcanic activity associated with the North Atlantic’s mid-ocean ridge,” said Clyde. “Volcanic activity in the midst of large petroleum deposits would be a way of burning a lot of fossil fuel quickly,” thereby releasing loads of carbon.
The researchers hope that when the final laboratory analyses of the Bighorn cores are completed they will have a much better overall picture of what happened globally during these periods of extreme warming. To date, this anomalous influx of greenhouse gases appears to have been about twice as large, based on ocean core sediments, than on dry-land cores, but then most land cores have been conducted on surface outcrops.
The sediments his team cored, says Clyde, are ancient stream deposits (sandstones and mudstones) that contain evidence of a continental setting during these hyperthermals and thus quite different from the deep sea core records that have been studied in more detail. "For instance, we will recover fossil pollen from the cores in order to determine how the plant communities in the Bighorn Basin changed during these global warming events," he explains, noting that the researchers will also be able to determine how the soil processes and the streams themselves changed.*
Analyzing the Bighorn cores may also help contemporary geo-researchers refine current climate-change models.
As Clyde points out, the worry now is that our reliance on fossil fuel is inducing general warming that could change ocean-bottom temperatures enough to destabilize methane clathrates and release even more greenhouse gas.
“If another hyperthermal event pushed up global temperatures by 5 degrees C, the polar ice caps would melt,” said Retallack. Something similar happened 250 million years ago when a mass release of methane clathrates resulted in large-scale climate change; 97 percent of all marine creatures went extinct.
“Corals and sponges had a hard time, but vertebrates did quite well,” said Retallack. “So humans might survive.”
* This paragraph describing the Bighorn cores was added after the story was initially published. (Back)