isotope anomalies in atmospheric sulfates
August 6, 2014

Sulfur Signals In Antarctic Snow Reveal Clues To Climate, Past And Future

Susan Brown, UC San Diego

Anomalous isotopes trace Earth’s ancient atmosphere, past El Niño events and future climate

Sulfur signals in the Antarctic snow have revealed the importance of overlooked atmospheric chemistry for understanding climate, past and future.

Eruptions of huge volcanoes, the disruptive weather pattern known as El Niño, and a fire season from hell each left distinctive chemical marks in layers of snow excavated near the South Pole, researchers from the University of California, San Diego and France report in the Proceedings of the National Academy of Sciences the week of August 4.

Sorting out the chemical reactions that must have led to those traces revealed a process, known but overlooked, that should be included in models of climate, both forecasts of climate to come and those built to understand Earth’s early history.

“We observed huge signals from ENSO driven changes like extreme dry weather and ensuing biomass burning, which surprised me,” said Robina Shaheen, a project scientist in chemistry at UC San Diego and lead author of the report. “The pattern we saw fits signals that have been observed in pre-Cambrian rocks, which prompted us to take another look at which molecules play a role in this chemistry.”

The element sulfur is everywhere and occurs in four stable forms, or isotopes, each with a slightly different mass. Ordinary reactions incorporate sulfur isotopes into molecules according to mass.

But sometimes sulfur divvies up differently so that the relative ratios of the different isotopes is anomalous. Shaheen and her colleagues measured the direction and degree of that anomaly for individual layers of snow representing a single season’s snowfall.

The snow record they analyzed spans the years 1984 through 2001. Two large volcanoes erupted in that time including Pinatubo and Cerro Hudson, both in 1991.

Volcanoes this large inject sulfate into the stratosphere, Earth’s high atmosphere above where clouds and weather normally prevail. This sulfur shows up in the snow, lots of it, and the isotope ratio within it is anomalous.

Experiments have shown this pattern can result from the influence of shortwave UV light, which doesn’t penetrate very far into the atmosphere. Sulfur anomalies in the present-day atmosphere are a sign of reactions that occurred in the stratosphere, above 25 kilometers or 82,000 feet. That is, above Earth’s shield of ozone.

Shaheen and colleagues also saw sulfur anomalies in snow that fell during years of the cyclical weather pattern called El Niño-Southern Oscillation, or ENSO. That snow held no extra sulfur, only an anomalous pattern of isotopes in the sulfate molecules. Because the pattern resembles anomalies originating from volcanic eruptions, it likely results from similar processes: photochemistry of sulfur compounds in the presence of short wave UV light, above the ozone.

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