New paleo-pCO2 proxy confirms elevated greenhouse
forcing during Marinoan snowball meltdown
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 (SO42-) 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 fractionation5,6 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.