1 dating method, 3 ecosystems, many clues to pace of climate change

Tufa, or stacks of calcium carbonate formed from the mixing of lake and groundwater, are found in 3- to 4-meter pillars around Mono Lake, geoscientist Guleed Ali said. Their ages can be determined using uranium/thorium dating. (Photo courtesy of Guleed Ali)

By Grace Finnell-Gudwien
Medill Reports

Inside caves, lakes and corals, small crystals dot the landscapes and scaffold into majestic pillars up to 4 meters high. These cream- to brown-colored crystal formations not only look beautiful, but they also provide paleoclimate scientists with a time capsule peering into the past — almost as far back as 500,000 years.

To use this crystalline time machine, paleoclimate scientists use uranium-thorium (U/Th) dating to determine the age of the crystals and rocks containing calcium carbonate at the specific level in the cave, lake or coral.

U/Th dating works in two main steps: first by measuring the time-sensitive decay of uranium-238 into uranium-234 in the crystals and then by measuring the decay of the new uranium-234 into thorium-230, said Guleed Ali, a postdoctoral researcher at the Berkeley Geochronology Center and the University of California, Berkeley. The rate of this decay acts as a clock, but the decay occurs slowly so the changes can represent tens of thousands of years. As an added timing bonus, thorium ultimately converts into lead. “

By using this two-step process,” Ali said, “you can measure (the date) precisely.” From this precise date, geoscientists can determine how past climate conditions changed, how these changes impacted ecosystems and the pace of accelerating climate change now in an era of human-driven fossil-fuel emissions. Ali uses U/Th dating on calcium carbonate pillars called tufa at Mono Lake near Lee Vining, California. Tufa forms when bicarbonate and carbonate from lake water and calcium from groundwater mix, so dating these structures can help determine previous water levels of the lake and climates of the region, Ali said.

Ali explained this process in an interview after the Comer Climate Conference, an annual gathering of global climate experts who research climate change over thousands and even millions of years. Their research emphasizes just how quickly climate change is accelerating now.

While using U/Th dating on tufa at Mono Lake, Ali specifically dated Ash 19, an ash bed seen as a distinct line in the walls of the canyons near the lake. This Ash 19 layer represents a transition from a drier to wetter climate in the region, so determining when this transition occurred can help scientists learn more about past climate change. To do this, Ali also used U/Th dating on carbonate deposits from before and after the Ash 19 layer.

This canyon wall on the western side of the Wilson Creek near Mono Lake shows the layers of rock deposits overtime, Ali said. The Ash 19 layer is marked with a green line, separating the deposits from different climates. (Photo courtesy of Guleed Ali)

“We determined that Ash 19 was deposited about 70,000 years ago,” Ali said. “Hence, the shift to wetter climate in the Mono Lake Basin took place about this time.” This time period is the same time as the beginning of the most recent ice age as well, and it lasted until the end of the ice age around 11,000 years ago, Ali said. This time period also correlates with the beginning of a drop in atmospheric carbon dioxide, Ali said. When atmospheric carbon dioxide is lower, the amount of water evaporated is also lower. This reduced evaporation was likely one contributing factor that led to the wetter climate, as shown by the lake sediments above Ash 19, Ali said. In addition to tufa, U/Th dating is used in calcium carbonate formations in caves.

These calcium carbonate crystals at Mono Lake formed similarly to crystals in caves, Ali said. (Photo courtesy of Guleed Ali)

“The U/Th method is the go-to one for this archive,” said Paul Töchterle, a Ph.D. student at the University of Innsbruck who is studying three caves in the United Kingdom. “(It) gives the most confidence.”

In caves, crystals made from calcium carbonate form dripstone deposits that researchers refer to as “speleothems.” Töchterle is interested in a special type of speleothem called Cryogenic Cave Carbonates (CCCs). He researched 10 occurrences of CCCs and took 59 U/Th dates in the three caves he studied.

“What makes them different from other speleothems is that they form in freezing pools of water,” Töchterle said.

Because they form in freezing water, they can only form in freezing temperatures. Thus, whenever CCCs form, the climate must be cold. By using U/Th dating, whenever Töchterle determines the time period a CCC formed, he knows that time period had a cold climate, which would include the ice ages.

The CCCs tested were from between 13,000 and 11,500 years ago, Töchterle said. This means the ground was frozen during that time period. (Photo courtesy of Paul Töchterle)

“This gives us indications of when and where permafrost was present,” Töchterle said. Permafrost, or soil that typically stays frozen, can only exist in cold climates — like the kind where CCC forms.

Using his U/Th dating, Töchterle found that permafrost was present from 13,000 to 11,500 years ago, which matches the end of the wet and cold climate period Ali dated at Mono Lake.

Other climate models, however, do not reflect permafrost being present during this time. Töchterle explained that their dating shows the temperature was cold enough for permafrost, but the snow cover in that period protected the ground from extremely cold temperatures. Essentially, the snow acted as a blanket that kept the ground cool but not as cold as the air temperature in that climate. With his U/Th dating results, though, Töchterle said he can “reverse-engineer” the data to hypothesize what the snow cover would have looked like in that period, which is something he is currently doing.

Paul Töchterle’s work on three caves in the United Kingdom involved lots of adventure and hiking. (Photo courtesy of Paul Töchterle)

Both Töchterle and Ali’s work shows how U/Th dating acts as a time capsule to help scientists look into the past as a way of revealing clues to today’s climate change. By researching how the climate changed in the past, scientists can predict the pace of climate change, how it will impact ecosystems and consider ways to mitigate extreme changes.

“That’s really how I got into paleoclimate,” Ali said, talking about climate change concerns. “We should all want to learn a little bit more.”

Grace Finnell-Gudwien is a Health/Science/Environment graduate student at Medill. You can follow her on Twitter at @GGudwien.