Wyoming's plant fossils clues to ancient climate

Fossil Butte National Monument, Wyo. » Fifty million years ago, a system of lakes stretched across what is now southwest Wyoming, once habitat for alligators, turtles and fish basking in a warm, humid climate so different from the high sagebrush desert it is today.

Fossilized animal bones make it clear Fossil Lake stewed in a subtropical climate, even though it was nowhere near such low latitudes. But how warm and wet was the area really? Using the basin's rich but under-appreciated record of plant fossils, scientists at Fossil Butte National Monument are answering these questions.

For a contemporary parallel, think Louisiana, said monument paleontologist Arvid Aase, who is

poised to publish a study of the ancient lake based on a five-year analysis of fossilized leaves.

Fossil Butte, the National Park Service property closest to Salt Lake City after Timpanogos Cave and Golden Spike, offers a memorable lesson in paleontology. At 8,200 acres, the monument preserves less than 1.3 percent of the basin's 930 square miles of lake sediments, now uplifted and eroded into buttes and valleys around Kemmerer, Wyo. It is visited by 17,000 to 20,000 people a year.

The region's 250-foot-deep Green River Formation preserves a fossil snapshot of the Eocene, the geologic epoch spanning 55 to 34 million years ago when mammal life boomed after dinosaurs disappeared.

"While these plants were growing you

have the first deer-like animals, the first horses, the first bats," says Aase, a Brigham Young University graduate and the 2006 recipient of the Freeman Tilden Award, the National Park Service's highest honor for excellence in interpretation. The atmosphere's carbon load was five to 11 times what it is today and the Earth was substantially warmer.

Fossil Lake's alkaline chemistry and fine sedimentation were ideal for preserving ancient life forms in stone. Its fish fossils have been quarried by the thousands and hang on walls all over the world.

"Instead of floating up and rotting, they sink in this mud layer and that keeps them from being eaten by scavengers," naturalist and park ranger John Collins says. "The leaves tend to be closer to the shoreline, introduced by river deltas. They get washed in the lake en masse in flooding events."

The science Aase is using to reconstruct the region's climate is paleobotany, the study of plant fossils to understand an ancient landscape's ecology and climate. The field owes much to the Russian climatologist Wladimir Koppen, who developed a climate classification system a century ago based on plant life. He theorized that native vegetation was the best predictor of an area's precipitation and temperature.

Paleobotanists take Koppen's concept, which has survived for more than a century with modest modifications, and apply it to the fossil record, Aase says. This way, scientists can make informed guesses about a region's ancient climate, but only if they have fossils of enough different species of woody flowering plants. Herbaceous plants do not reveal much about climate, but dicotyledon trees and shrubs do, Aase says.

Dicot leaf size speaks to annual precipitation and leaf margins speak to temperature. Bigger leaves mean wetter climate, while serrated edges indicate higher temperatures. Koppen's theories have helped paleobotanists develop precise formulas for translating leaf types

into annual mean precipitation and temperature.

Scientists need fossil leaves from at least 25 different species of ancient dicots associated with the region and time they are studying. Until last year, Aase was working with 146 ancient species, most of them from private quarries and collections. But that was before University of Utah geologists told him about a recent donation of nearly 1,000 Green River Formation leaves.

Salt Lake City orthopedic physician Lonnie Paulos spent decades collecting the fossils, then gave them to U. when he moved to Florida. Marjorie Chan, who chairs the department of geology and geophysics, invited Aase to borrow any well-preserved leaves that would help his study.

At the time, designers John and Lee Diamond were arranging dozens of the leaf fossils into a panel to decorate the U.'s new geology building. To be useful to paleobotany, the fossilized leaves' vein networks must be preserved down to the third largest vein. Aase found 254 such fossils in the Paulos collection and the U. had those shipped to Fossil Butte. Scientists are studying them to determine how many represent previously unknown species and then use these new species to bolster the climate study.

The Diamonds completed the leaf panel, which now hangs near the entry to the new Frederick Albert Sutton building.

Before the Paulos collection was added to the study, Aase says, his data suggested that the Fossil Lake climate was like modern-day Baton Rouge, La., which has a mean annual temperature of near 70 degrees Fahrenheit and precipitation ranging from 45 to 50 inches.

The study's next step is determining how the basin's climate changed during the 1 to 5 million years that Fossil Lake existed. To do that, Aase needs to know the precise layer where particular leaf fossils were recovered, data that commercial quarries did not often record. The Green River Formation's benchmark layer is called K-spar Tuff, a 10-inch blanket of volcanic ash that has been pegged to an eruption 51.7 million years ago.

Monument scientists are working with quarriers to document locations of leaf recoveries, as well as Massachusetts Institute of Technology geologists to date other Green River tuff layers.

bmaffly@sltrib.com