The climate 150 million years ago of a large swath of the western United States was more complex than previously known, according to new research from Southern Methodist University, Dallas. It’s been held that the climate during the Jurassic was fairly dry in New Mexico, then gradually transitioned to a wetter climate northward to Montana. But based on new evidence, the theory of a gradual transition from a dry climate to a wetter one during the Jurassic doesn’t tell the whole story, says SMU paleontologist Timothy S. Myers, lead author on the study.
The climate 150 million years ago of a large swath of the western United States was more complex than previously known, according to new research from Southern Methodist University, Dallas.
It’s been held that the climate during the Jurassic was fairly dry in New Mexico, then gradually transitioned to a wetter climate northward to Montana.
But based on new evidence, the theory of a gradual transition from a dry climate to a wetter one during the Jurassic doesn’t tell the whole story, says SMU paleontologist Timothy S. Myers, lead author on the study.
Geochemical analysis of ancient soils, called paleosols, revealed an unexpected and mysterious abrupt transition from dry to wet even though some of the samples came from two nearby locales, Myers said.
Myers discovered the abrupt transition through geochemical analysis of more than 40 ancient soil samples.
He collected the samples from the Morrison Formation, a vast rock unit that has been a major source of significant dinosaur discoveries for more than 100 years.
The Morrison extends from New Mexico to Montana, sprawling across 13 states and Canada, formed from sediments deposited during the Jurassic.
Myers’ study is the first in the Morrison to significantly draw on quantitative data — the geochemistry of the rocks.
The abrupt transition, Myers says, isn’t readily explained.
“I don’t have a good explanation,” he said. “Normally when you see these dramatic differences in climate in areas that are close to one another it’s the result of a stark variation in topography. But in this case, there weren’t any big topographic features like a mountain range that divided these two localities in the Jurassic.”
Surprisingly, paleosols from the sample areas did not reveal marked differences until they were analyzed using geochemical weathering indices.
“It’s sobering to think that by just looking at the paleosols superficially at these localities, they don’t appear incredibly different. We see the same types of ancient soils in both places,” Myers said. “So these are some fairly major climate differences that aren’t reflected in the basic ancient soil types. Yet this is what a lot of scientists, myself included, depend on for a first pass idea of paleoclimate in an area — certain types of soils form in drier environments, others in wetter, others in cooler, that sort of thing.”
That didn’t hold true for the current study.
With the geochemical analysis, Myers estimated the mean average precipitation during the Jurassic for northern Montana was approximately 45 inches, 20 inches for northern Wyoming and 30 inches for New Mexico.
“This changes how we view the distribution of the types of environments in the Morrison,” Myers said. “Too many times we talk about the Morrison as though it was this monolithic unit sprinkled with patchy, but similar, variations. But it’s incredibly large. It spans almost 10 degrees of latitude. So it’s going to encompass a lot of different environments. Regions with broadly similar climates can have internal differences, even over short distances. That’s the take-home.”
He reported his findings, “Multiproxy approach reveals evidence of highly variable paleoprecipitation in the Upper Jurassic Morrison Formation (western United States),” in The Geological Society of America Bulletin.
Continue reading at Southern Methodist University.
Western US landscape image via Shutterstock.
