Geotimes - October 2003 - Ancient glaciers near L.A.

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Paleoclimate
Ancient glaciers near L.A.

The small alpine glaciers that once existed 120 kilometers east of Los Angeles, Calif., were, according to a new study, present as recently as 5,000 years ago, a time when regions to the north had already become ice-free. The research may thus upend some thinking about the paleoclimate of California.

To determine when the ice receded, researchers at the University of California, Riverside (UCR), and Lawrence Livermore National Laboratory used cosmogenic radionuclide dating, also called surface exposure dating, to date boulders from moraines on San Gorgonio Mountain in the San Bernardino range. The moraines mark the extreme southwestern limit of glaciation in the United States and represent several episodes of late Quaternary glaciation.

Lewis Owen, pictured here in the Sierra Nevada at the southern end of the western cordillera, and colleagues have dated boulders from nearby moraines at ages younger than expected — perhaps reshaping understanding of California’s paleoclimate. Photo courtesy of University of California, Riverside.

Geomorphologist Lewis Owen and colleagues report in the August Geology that the moraines’ ages clustered to indicate four stages of glacial advance: 20,000 to 18,000 years ago, 16,000 to 15,000 years ago, 13,000 to 12,000 years ago and 9,000 to 5,000 years ago.

Earlier studies based on the relative amount of weathering and degree of soil formation had found the oldest moraines to be up to 80,000 years old. And although some of the new ages for the moraines are consistent with other more recent studies, the youngest stage is not consistent with the glacial record just 250 kilometers to the north, says Alan Gillespie, a Quaternary geologist at the University of Washington.

“The Sierra Nevada is thought to have been ice-free during the mid-Holocene, at the same time Owen concludes glaciers were advancing on San Gorgonio,” Gillespie says. “If the claimed mid-Holocene advance at San Gorgonio can be confirmed, it does imply that the paleoclimate in California, and elsewhere, may be more spatially variable than we have previously been able to model or document.”

The authors acknowledge that the exposure ages of boulders in the youngest moraine are not as tightly grouped as the older moraines. However, they say that the dates are nonetheless consistent with a Holocene cold snap called the Neoglacial that occurred 4,000 to 5,000 years ago, as well as a cooling around 8,500 years ago brought on by the collapse of the Laurentide ice sheet.

Surface exposure dating, which uses ratios of beryllium-10, has recently become a popular means of establishing ages for glacial landforms. In rocks exposed at Earth’s surface, cosmic rays interact with the silicon and oxygen in quartz to produce beryllium-10. The more beryllium-10 that has accumulated, the longer the rock has been exposed at the surface.

However, “one has to be aware of the limitations,” says co-author Robert Finkel of Lawrence Livermore. With surface exposure dating, the younger the sample is, the higher the degree of uncertainty because there are fewer nuclides available to measure. This limitation may also help explain the wide age scatter of boulders in the youngest moraine.

Owen’s team also notes that the discrepancy with the Sierra Nevada shows that a local characteristic of San Gorgonio Mountain — blowing snow — can be an important, though often overlooked, factor in glacier formation.
“The lateral transport of snow and high winds during storms resulted in much heavier deposition of snow into these cirques,” says co-author Richard Minnich, an ecologist at UCR, “and that was the support of these glaciers.”

The timing of glacial advance is important because small mountain glaciers are particularly sensitive to climate change and, with accurate dating, can be used as proxies in climate models. In Southern California, well-dated moraines could help paleoclimatologists reconstruct the past locations of the jet stream.

The study has another implication, for modern times. Owen and his colleagues say the recent glacial advance shows that under certain conditions, snowpack can become perennial even at relatively low latitudes and low altitudes — meaning, they say, that small glaciers could exist there again with just a slight change in climate.

“We can already demonstrate that permanent perennial snow still occurs on this mountain in about one-third of the years,” Minnich says. “So a slight increase in the annual precipitation here in the West could, in fact, produce permanent snow and ice — enough to make moraines, even in the modern Holocene climate.”

Sara Pratt
Geotimes contributing writer

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