Effects of the Paleocene-Eocene Warming Bowen says previous research has shown that during the Paleocene-Eocene warm period, there was “enhanced storminess in some areas, increased aridity in other places. We see continent-scale migration of animals and plants, ranges are shifting. We see only a little bit of extinction – some groups of deep-sea foraminifera, one-cell organisms that go extinct at the start of this event. Not much else went extinct.” “We see the first wave of modern mammals showing up,” including ancestral primates and hoofed animals,” he adds. Oceans became more acidic, as they are now. “We look through time recorded in those rocks, and this warming event stands out, and everything happens together,” Bowen says. “We can look back in Earth’s history and say this is how this world works, and it’s totally consistent with the expectation that carbon dioxide change today will be associated with these other sorts of change.” The Paleocene-Eocene thermal maximum also points to the possibility of runaway climate change enhanced by feedbacks. “The fact we have two releases may suggest that second one was driven by the first,” perhaps, for example, if the first warming raised sea temperatures enough to melt massive amounts of frozen methane, Bowen says.
Drilling into Earth’s Past The new study is part of a major drilling project to understand the 56-milion-year-old warming episode, which Bowen says first was discovered in 1991. The researchers drilled long, core-shaped sediment samples from two boreholes at Polecat Bench in northern Wyoming’s Bighorn Basin, east of Cody and just north of Powell. “This site has been excavated for well over 100 years by paleontologists studying fossil mammals,” Bowen says. “It documents that transition from the early mammals we see after the extinction of the dinosaurs to Eocene mammals, which are in groups that are familiar today. There is a great stratigraphic sequence of more than 2 kilometers (1.2 miles) of rocks, from 65 million years ago to 52 million years ago.” The Paleocene-Eocene warming is recorded in the banded, flood-deposit tan and rusted red rock and soil layers of the Willwood formation, specifically within round, gray to brown-gray carbonate nodules in those rocks. They are 2 inches to 0.1 inches diameter. By measuring carbon isotope ratios in the nodules, the researchers found that during each 1,500-year carbon release, the ratio of carbon-13 to carbon-12 in the atmosphere declined, indicating two large releases of carbon dioxide or methane, both greenhouse gases from plant material. The decline was three parts per thousand for the first pulse, and 5.7 parts per thousand for the second. Previous evidence from seafloor sediments elsewhere is consistent with two Paleocene-Eocene carbon pulses, which “means we don’t think this is something is unique to northern Wyoming,” Bowen says. “We think it reflects a global signal.”
What Caused the Prehistoric Warming? The double-barreled carbon release at the Paleocene-Eocene time boundary pretty much rules out an asteroid or comet impact because such a catastrophe would have been “too quick” to explain the 1,500-year duration of each carbon pulse, Bowen says. Another theory: oxidation of organic matter – as permafrost thawed, as peaty soils burned or as a seaway dried up – may have caused the Paleocene-Eocene warming. But that would have taken tens of thousands of years, far slower than what the study found, he adds. Volcanoes releasing carbon gases also would have been too slow. Bowen says the two relatively rapid carbon releases (about 1,500 years each) are more consistent with warming oceans or an undersea landslide triggering the melting of frozen methane on the seafloor and large emissions to the atmosphere, where it became carbon dioxide within decades. Another possibility is a massive intrusion of molten rock that heated overlying organic-rich rocks and released a lot of methane, he says.
University of Utah Communications75 Fort Douglas Boulevard, Salt Lake City, UT 84113801-581-6773 fax: 801-585-3350unews.utah.edu
MEDIA CONTACT
Register for reporter access to contact detailsArticle Multimedia
CITATIONS
Nature Geoscience, Dec. 15, 2014; National Science Foundation EAR-0958821,0958622, 0958583,1261312; German Research Foundation