Isotopes are normally fractionated in direct proportion to the differences in their masses. For example, fractionations between ¹⁷O and ¹⁶O are most often half as large exactly as those between ¹⁸O and ¹⁶O. For this reason, ¹⁷O/¹⁶O ratios are rarely measured because ¹⁷O is less abundant in nature than ¹⁸O, the ¹⁸O/¹⁶O ratio being larger is easier to measure, and both ratios usually yield the same information. In the early 1990's, however, small deviations from strict mass-dependent fractionation were discovered in the three isotopes of atmospheric oxygen. They result from photochemical reactions in the stratosphere involving O₃ (ozone), CO₂ and O₂. Experiments indicate that the magnitude of the mass-independent isotope fractionations increases with CO₂ concentration, but are limited in the atmosphere by photosynthesis and respiration. Accordingly, large mass-independent isotope anomalies should have existed at the end of a snowball glaciation because pCO₂ would have been greatly elevated and photosynthesis-respiration severely limited. If only we could obtain a sample of air from those times!

When sulfate (SO₄^2-) is formed by the oxidative weathering of pyrite (FeS₂) in crustal rocks, some of the O in the sulfate comes from rainwater or groundwater, and some from O₂ in the air. Once bound into the sulfate lattice, the O does not exchange readily with waters in which it dissolves. The oxygen isotope composition of sulfate dissolved in seawater, for example, is distinctly different from that in the seawater itself. Might sulfate minerals in ancient sediments preserve a record of mass-independent oxygen-isotope anomalies, which are by convention expressed in terms of ?¹⁷O? Huiming Bao and colleagues¹ measured the triple oxygen isotope compositions of marine evaporites (gypsum, CaSO₄.2H₂O and anhydrite, CaSO₄) and barites (BaSO₄) of Neoproterozoic, Cambrian, Late Paleozoic-Early Mesozoic and Cenozoic-Recent age. They observe small but significant negative ?¹⁷O anomalies (less than -0.20‰) in most Late Paleozoic and younger samples (analytical uncertainty is ±0.05‰). However, larger negative anomalies (less than -0.30‰) are found in the Cambrian and mid-Neoproterozoic (Shaler Group) samples, indicating higher CO₂ levels at those times, as expected because of pCO₂ adjustment to reduced Solar luminosity by means of silicate-weathering feedback².

The most extreme anomalies (as low as -0.69‰) occur in barites from the 635-Ma "cap" dolostone associated with the Nantuo glacial deposits in South China and from the presumed correlative "cap" dolostone atop the Jbéliat tillites in Mauritania, West Africa¹. Huiming Bao and colleagues¹ interpret this finding as indicating an anomalously high level of atmospheric CO₂ in the glacial aftermath, presumably due to volcanogenic CO₂ buildup during a snowball-type glaciation when CO₂ consumption would have been limited2, or to oxidation of methane released from permafrost by deglaciation³. The pCO₂ required to produce the observed ?¹⁷O anomaly is difficult to estimate, primarily because of uncertainty over the percentage of O in the sulfate that is derived from atmospheric O₂. Huiming Bao and colleagues¹ suggest that pCO₂ was ~12,000 p.p.m, assuming a 10mole% O₂ signature in sulfate.

Triple oxygen isotopes now join the list of proxies indicating high levels of CO₂ in the 635-Ma glacial aftermath, a list that includes boron isotopes⁴ and temperature-dependent carbon isotope fractionation^5,6 in syndeglacial "cap" dolostones.

¹Bao, Huiming, Lyons, J.R. & Zhou, Chuanming, 2008. Triple oxygen isotope evidence for elevated CO₂ levels after a Neoproterozoic glaciation. Nature 453, 504-506, doi:10.1038/nature06959. get PDF

²Walker, J.C.G., Hays, P.B. & Kasting, J.F., 1981. A negative feedback mechanism for the long-term stabilization of Earth’s surface temperature. Journal of Geophysical Research 86(C10), 9776-9782.

³Kennedy, M.J., Christie-Blick, N. & Sohl, L.E., 2001. Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth’s coldest intervals? Geology 29, 443-446.

⁴Kasemann, S.A., Hawkesworth, C.J., Prave, A.R., Fallick, A.E., & Pearson, P.N., 2005. Boron and calcium isotope composition in Neoproterozoic carbonate rocks from Namibia: evidence for extreme environmental change. Earth and Planetary Science Letters 231, 73-86.

⁵Higgins, J.A. & Schrag, D.P., 2003. Aftermath of a snowball Earth. Geophysics, Geochemistry, Geosystems 4, 10.1029/2002GC000403.

⁶Hoffman, P.F., Halverson, G.P., Domack, E.W., Husson, J.M., Higgins, J.A., Schrag, D.P., 2007. Are basal Ediacaran (635 Ma) post-glacial "cap dolostones" diachronous? Earth and Planetary Science Letters 258, 114-131.