While the southwestern United States is primarily desert today, geological records show that 20,000 years ago, huge ancient lakes existed in the area. Geologists used to believe these lakes were created by storms coming off the Pacific Ocean, but new evidence published in the journal Science last month suggests that the storms originated in the tropics.
Through sediment sample collection on both land and water, researchers - including Timothy Herbert P'15, professor and department chair of geological sciences - determined that the ancient lakes in states such as Utah, Colorado and Arizona were not formed by the same Pacific storm systems that determined weather along the California coast.
The findings, which point instead to a tropical storm source that surprised the researchers, were published in Science Sept. 28. The paper was the result of a decade-long research project conducted by Herbert in collaboration with Texas A and M, Columbia, University of California at Santa Cruz, Stanford and the Hokkaido University of Japan.
The original belief that ancient lakes were a result of storm tracks from Pacific weather systems was based on storm tracks that occur today, Herbert said. Winter storm tracks from Alaska bring precipitation down the California coast and into Utah and Arizona, Herbert said.
The belief also stemmed from the fact that "there was a massive ice sheet existing along the border of the United States and Canada, which would have pushed all the winds toward the equator, creating storms in Northern California, Colorado and Utah to make these ancient lakes."
"It was a very logical idea," Herbert said. "But after the work that my colleagues and I did, it just doesn't seem to hold up."
Herbert and his colleagues have been collecting and analyzing sediment core from the Pacific Ocean since the mid-1990s.
"This is the type of work that requires a good decade of data collection," said Debbie Thomas '95, associate professor of geology at Texas A and M University. Thomas prepared and analyzed sediment cores in Herbert's lab during her time at Brown. Thomas said she never thought her undergraduate research would contribute to "solving such a big picture problem."
Sediment analysis performed by Thomas and Herbert allowed the researchers to determine ancient ocean temperatures, which is the "specialty of our lab at Brown," Herbert said. Ocean temperatures are correlated with atmospheric pressure, which helps determine where storms originate.
Researchers can determine ancient ocean temperatures because the chemical composition of algae living in ancient oceans directly corresponds to past ocean temperatures. When the algae die, they fall to the bottom of the ocean "layer by layer," Herbert said. By examining the chemical composition of the layers, the researchers can essentially "map out" the ocean temperature changes overtime, he noted.
"Mud is an archive," Herbert said. The sediment cores are stored in "mud libraries," one of which is housed by Texas A and M, Thomas said.
But ocean temperature wasn't the only factor in determining ancient storm tracks.
"The ocean and the atmosphere talk to each other," Herbert said. So while Herbert was focusing on ocean sediment, collaborators at other institutions were examining pollen content and plant species present in ancient land sediment.
When the researchers shared data, the results were not what they expected.
While the data collected along the California coast clearly correlated with Pacific Ocean temperature changes, the data from the Great Basin region "was the exact opposite," Herbert said. "It's a different system, and that was kind of a surprise."
With the Pacific storm track idea eliminated, researchers looked for another storm track that could have created the ancient lakes and found the Tropics storm track to be the most logical answer. Today, the occasional summer rain in Utah and Arizona comes from Tropic storm systems, Herbert said.
"We eliminated one possibility and are now pointing our finger at the Tropics," Herbert said.
Examining drastic shifts in ancient weather patterns sheds light on the implications of modern climate change.
"Most people don't realize that climate change like temperatures patterns will change things a lot more than just temperature itself. It will change rainfall and ecosystems - water is about as important as it gets," Herbert said. "It's underappreciated that climate change is not just about getting warmer and warmer - it's about changing where it's wet and where its dry."
