Photo Credit: UCSD

A scientist removes a sample from the wall of a snow pit during field work at the South Pole. Particles from the upper atmosphere trapped in that deep pile of Antarctic snow hold clear chemical traces of global meteorological events.

In the pits

Scientists dig through South Pole snow for climate clues

Snow retrieved from a pit dug at the South Pole is helping scientists reconstruct past climactic cycles.

Particles from the upper atmosphere trapped in a deep pile of Antarctic snow hold clear chemical traces of global meteorological events, scientists reported in the early online edition of the Proceedings of the National Academy of Sciences published the week of Feb. 25.

Anomalies in oxygen found in sulfate particles coincide with several episodes of the world-wide disruption of weather known as El Niño and can be distinguished from similar signals left by the eruption of huge volcanoes, according to researchers.

“Our ability to link reliable chemical signatures to well-known events will make it possible to reconstruct similar short-term fluctuations in atmospheric conditions from the paleohistory preserved in polar ice,” said Mark Thiemens , co-author on the paper and professor of chemistry and biochemistry at the University of California, San Diego , who directed the research and dug up much of the snow.

Thiemens, graduate student Justin McCabe and colleague Joel Savarino of Laboratoire de Glaciologie et Géophysique de l'Environnment in Grenoble, France, excavated a pit six meters deep in the snow near the South Pole, with shovels.

“At an elevation of 10,000 feet and 55 degrees below zero, this was quite a task,” Thiemens said in a UCSD press release . Their efforts exposed a 22-year record of snowfall, a pileup of individual flakes, some of which crystallized around particles of sulfate that formed in the tropics.

Atmospheric sulfates form when sulfur dioxide — one sulfur and two oxygen molecules — mixes with air and gains two more oxygen molecules. This can happen a number of different ways, some of which favor the addition of variant forms of oxygen, or isotopes, with an extra neutron or two, previous work by Thiemens’ group has shown.

Unlike polar ice, which compresses months of precipitation so tightly that resolution is measured in years, relatively fluffy snow allowed the team to resolve this record of atmospheric chemistry on a much finer scale.

“That was key,” said Robina Shaheen, a project scientist in Thiemen’s research group who led the chemical analysis and was lead author on the paper. “This record was every six months. That high resolution made it clear we can trace a seasonal event such as ENSO.”

El Niño is an abnormal warming of surface ocean waters in the eastern tropical Pacific. The Southern Oscillation is the seesaw pattern of reversing surface air pressure between the eastern and western tropical Pacific. When the surface pressure is high in the eastern tropical Pacific, it is low in the western tropical Pacific, and vice-versa. Because the ocean warming and pressure reversals are, for the most part, simultaneous, scientists call this phenomenon the El Nino/Southern Oscillation or ENSO for short.

ENSO is often blamed for extreme weather events around the world, from droughts to floods.

The warmed air above the sea surface lifts sulfur dioxide high into the stratosphere, a layer of atmosphere above the troposphere, where it’s oxidized by ozone, which imparts a distinctly different, anomalous pattern of oxygen variants to the resulting sulfate particles.

In the Antarctic snow samples, the chemists found traces of these oxygen anomalies in sulfates trapped within layers of snow that fell during strong El Niño seasons.

Volcanoes too can shoot sulfur compounds high into the atmosphere where they react with ozone to produce sulfates with oxygen anomalies. Three large volcanoes, El Chichón, Pinatubo and Cerro Hudson, erupted over the course of this time sample, which stretched from 1980 to 2002 and encompassed three ENSO events as well.

NSF funded research in this story: Mark Thiemens and Joel Savarino, University of California, San Diego, Award No. 0125761 ; and Mark Thiemens and Robina Shaheen, University of California, San Diego, Award No. 0960594 .