Building California’s Climate Record

Susan Zimmerman

As part of Lawrence Livermore National Laboratory’s efforts to predict future climate change, Susan Zimmerman is spearheading a project to develop high-precision paleoclimate records for use in regional climate models. Current models typically use data for only the last 150 years and, thus, miss wet and dry periods from past millennia.

“Remarkably, over the last century, the West has been relatively wet compared to the average for the last 2,000 years,” says Zimmerman, who received a Ph.D. in earth sciences from Columbia University. “Without more long-term data, predictive modeling is biased toward these anomalously wet conditions.”

With funding from the LDRD Program, Zimmerman is working with researchers across California to analyze lake sediments and develop records that span the last two millennia. “Lakes are long-lived, wet areas where materials are continually deposited over time,” she says. Data from these records will be used to map previous drought patterns in California and help climate modelers more accurately simulate the range of natural climate changes. With this information, state agencies can determine the infrastructure needed to meet future demands for water.

“An important part of my effort is building a network of collaborators who are already working on paleoclimate records in different areas of the state,” says Zimmerman. She and her colleagues collect samples from natural outcrops such as stream cuts or fault scarps or from vertical cores of sediment extracted from a basin. Layers in the core indicate the conditions under which the sediment was deposited.

Zimmerman analyzes samples at the Laboratory’s Center for Accelerator Mass Spectrometry, where she works with Livermore scientists Tom Guilderson, Tom Brown, and Graham Bench, the center’s director. Zimmerman pretreats each sample, which may be charcoal, pine needles, or other macrofossils; combusts it into carbon dioxide gas; and then catalyzes the gas into graphite powder. The powder is pounded into aluminum sample targets, which are arranged with standard targets and blanks in a sample wheel and loaded into the spectrometer. A wheel with up to 50 unknown samples takes 8 to 12 hours to analyze, and the Laboratory’s Natural Carbon Group, which includes Zimmerman, runs 2 to 3 wheels a week. With the highprecision accelerator mass spectrometer, Zimmerman can determine a sample’s radiocarbon age to within 20 years.

Once the radiocarbon dates are calibrated to calendar years, Zimmerman and her colleagues establish a chronology for the paleoclimate records from the original core or outcrop. This record is then compared to other sources of ancient climate information, such as tree rings, to develop a regional picture. Combining well-dated paleoclimate records from statewide sites, Zimmerman will create time-slice maps of wet and dry patterns in California in 100-year intervals. In the last phase of her project, she will analyze spectra of the paleoclimate records to look for influences from mechanisms such as the El Niño climate pattern and the Pacific Decadal Oscillation, the long-term surface fluctuation of the Pacific Ocean.

“The Laboratory offers me the opportunity to work with many people in my field,” says Zimmerman. This relationship with colleagues benefits everyone involved. Not only does Zimmerman get to do field work in new places and handle a variety of samples, but she also provides researchers statewide with data that might otherwise not be available to them. In addition, the data collected in her project will help strengthen the Laboratory’s imate model predictions. With improved models, decision makers can better prepare for what research indicates will be a drier California in the next several decades.

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