Immediately after volcanoes around the world spewed carbon dioxide into the atmosphere about 90 million years ago, the Arctic was as warm as present-day Florida, according to fossil evidence discovered by a University of Rochester team in the high Canadian Arctic. The fossils indicate that at least once in Earth's history, high amounts of the greenhouse gas warmed Earth to much higher temperatures than usual.
The find of bones from several crocodile-like beasts known as champsosaurs, along with turtles and fish - champsosaurs' favorite foods - is detailed in the Dec. 18 issue of Science. Analysis of the find was done by the Rochester team in collaboration with researchers from the Royal Tyrrell Museum of Paleontology in Drumheller, Alberta; the Berkeley Geochronology Center; and the Scripps Institution of Oceanography in San Diego.
The highlight of the find are bones that belonged to an eight-foot champsosaur, a now-extinct crocodile-like beast with a long snout and razor-sharp teeth. The team found bones from several champsosaurs, as well as fish and turtles, in rocks scattered over several hundred thousand to a few million years. The reptiles, which were tied to their freshwater environment on Axel Heiberg Island, needed an extended warm period each summer to survive and reproduce. Based on the numbers and sizes of the animals found, the team estimates that the annual mean temperature in the Arctic during the late Cretaceous period, from about 92 million to 86 million years ago, was about 57 degrees Fahrenheit. That means it was rarely if ever freezing during the winter, and summer temperatures consistently reached into the 80s and 90s.
The bones were discovered by a University team of students led by Professor John Tarduno during an expedition in the summer of 1996. Driven off by heavy rain and snow, the team returned in the summer of 1997 to complete the excavation. The bones come from a layer of sediment right on top of 1,000 feet of hardened lava, known as basalt, and below a layer of marine rock common in the Arctic. That dates the fossils to the period immediately after the volcanism ended.
"I had been looking for rocks from this layer for many years - in most places the layer doesn't exist or has worn away," says Tarduno, a geophysicist who studies Earth's magnetic field. "We were walking along a ridge, and we spotted a layer between the brown volcanic rocks and the black marine shale. From a distance we knew that these rocks represented an environment we hadn't seen before. Once we reached them, we realized these were fine-grain sedimentary rocks ideal for preservation of fossils, and I bet the students we'd find fossils. Within five minutes one of my students pulled up a femur and said, "Like this?"
"It was one of those rare instances where you know immediately that what you're looking at has tremendous importance. It was clearly a vertebrate fossil - our guess was a large reptile, which would have required a relatively warm climate to survive."
The fossils locked in a record of what was happening in the Arctic just as extreme volcanism around the planet was winding down. Most of the volcanic activity didn't resemble spectacular eruptions like Mt. Pinatubo. Instead, the eruptions were "basaltic" - tons of lava oozed out, and carbon dioxide floated skyward. Besides huge amounts of lava in the Arctic, where hardened lava rock today measures more than a kilometer thick in some places, magma oozed from volcanoes in the Caribbean, in the Pacific Ocean northeast of Australia, in the Indian Ocean, off the coasts of Madagascar and Brazil, in South Africa and in the Southwestern United States.
Scientists have long considered the Cretaceous period, which lasted from 144 million to 65 million years ago, a warm time period and a possible model of the greenhouse effect, where gases like carbon dioxide collect in the atmosphere and trap in heat, causing global warming. Understanding how warming happened in the past helps scientists predict how our planet might react in the future to the increased CO2 being pumped into the atmosphere from car exhaust, coal plants, and other burning of fossil fuels. That model has come under closer scrutiny recently as some scientists have suggested that the late Cretaceous was actually a cool time. The new evidence indicates that the late Cretaceous may be an even better model of global warming than scientists thought.
"We can't avoid the fact that these fossils are sitting right on top of this extremely large volcanic eruption," says Tarduno. "And if you look around the world, it was an unusually active time, with many eruptions occurring at the same time. It's very reasonable to suggest that so much CO2 was dumped into the atmosphere that it overwhelmed the system, causing global warming."
Scientists have long known that at times in the past the Arctic was much warmer than it is today. One piece of evidence is the many fossil trees in the Arctic; today the nearest living tree is about 1,000 miles away. But no one has found hard evidence before for such warm temperatures.
"This will be a puzzle for people who model climate," says Tarduno, "but the fossils, together with the radiometric dating, provide very hard evidence of extremely warm temperatures in the Arctic."
The bones the team found come from what was likely a freshwater bay on Axel Heiberg Island in the high Canadian Arctic, at 79 degrees latitude. During the Cretaceous the island was a bit south of where it is today but was still well within the Arctic Circle. The two dozen bones the team found include a tibia, a femur, and ribs and vertebrae from both small and large champsosaurs, as well as turtle shells and several bones from fish. All came from a sediment about 12 feet thick running for several hundred feet along the Dragon River.
Analyzing the rocks and fossils were Tarduno; paleontologist Donald Brinkman of the Royal Tyrrell Museum of Paleontology of Canada in Drumheller, Alberta; Paul Renne, director of the Berkeley Geochronology Center, who did the isotope dating of the rocks; and Pat Castillo of Scripps Institute of Oceanography in San Diego, who chose which rocks to date. The project was funded by the National Science Foundation.
Also taking part in the research were several University of Rochester students, including undergraduate Howard Scher and graduate student Rory Cottrell. The find came thanks to the perseverance of the students and Tarduno, who spent six weeks during both summers traipsing through knee-deep snow, huddling in nylon tents flapping in strong winds, and enduring 24 hours of sunshine each day, all to find and excavate just the right rocks. Dubbing their tent city 700 miles from the North Pole the "Polar Hotel" after the tents they slept in, the students were among the first humans to explore some sections of the mountainous, rocky part of the Arctic.
Experiences like this are plentiful at the University, where faculty both conduct world-class research and teach small classes of students. Like Scher, about half the University's undergraduates work closely with professors on independent research projects. The Department of Earth and Environmental Sciences is known nationwide for the dozens of students it equips with extensive field experience and graduates each year.