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Isotopes and Snowball Earth

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Isotope chemistry is a bit of an arcane world for the non-initiated. But variants of elements that differ only in the number of neutrons in their nuclei are common tools of the trade of archaeologists, geologists and climate researchers.

In the past 50 years, a much-improved chronology of past climatic events has evolved through analyses of the oxygen isotope record of marine shells and minerals in deep-sea and lake sediments. But information about the Earth’s deep geological past, in particular concerning the chemical composition of the atmosphere, is still hard to get by.

The debut of a new stable mineral-isotope proxy for ancient atmospheric condition is therefore a remarkable event, the editors of a paper in this week’s Nature note in their summary.

When analysing the triple oxygen isotope composition of ancient sulphate deposits, a team of geophysicists led by Huiming Bao of Louisiana State University found that they exhibit variable negative oxygen-17 anomalies over the past 750 million years. They propose that these small anomalies, first noticed a few years ago in a study unrelated to atmospheric chemistry, reflect those of atmospheric oxygen and carbon dioxide in the past.

The new proxy is hardly sensitive enough to record the relatively subtle variations in atmospheric oxygen and carbon dioxide content during the Pleistocene, the Earth’s recent period of repeated glaciations.

It could be useful, though, when evaluating extreme climates much earlier in our planet’s history. For example, oxygen-17 anomalies in barite sulphates display a negative spike – hinting at an extremely high level of atmospheric carbon dioxide – around 635 million years ago, when the Earth was likely recovering from a period of global glaciation in the Early Cambrian. This finding supports the not undisputed ‘snowball’ Earth hypothesis and/or massive methane release in the aftermath of Neoproterozoic glaciation

Quirin Schiermeier

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