During the period known as the Last Glacial Maximum some 21,000 years ago, mammoth lakes spread out across large swaths of the West, and scientists have long wondered why the now-dry lakes used to be so big. Mystery solved: A new study finds that the reason has nothing to do with higher precipitation—a long-running theory unraveled by recent analysis that found rain and snowfall levels were relatively low at the time—but rather slower evaporation. Analyzing limestone samples taken from the edges of Lake Surprise, a fossil lake in California, geologists found more amounts of the isotope known as oxygen-18 than oxygen-16, which is slightly lighter. Oxygen-18 water evaporates more slowly, and the ratio of the two isotopes indicates that evaporation rates during the Last Glacial Maximum were a whopping 40% lower than what we experience today.
"Lake Surprise is located in a closed basin," lead author Daniel Ibarra at Stanford tells PhysOrg. "All streams flow into the lake, but there is no outflow. The only way for water to escape is through evaporation." Because precipitation rates in the region appear to have gone up for a brief period 15,000 years ago, right around when Lake Surprise reached its largest size (530 square miles), the giant lakes in the West probably grew large initially due to lower evaporation, and then reached what would be their maximum sizes when rain and snowfall increased several thousand years later. (It may not be the sexiest answer, but at least this mystery can't be blamed on a meteor.)