Evidence for hot early oceans? (Reply)

François Robert and Marc Chaussidon
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François Robert: Muséum National d'Histoire Naturelle, Centre National de la Recherche Scientifique Laboratoire d'Etude de la Matière Extraterrestre Nanoanalyses, UMS 2679
Marc Chaussidon: †Centre de Recherches Pétrographiques et Géochimiques, Centre National de la Recherche Scientifique, BP20

Nature, 2007, vol. 447, issue 7140, E1-E2

Abstract: Abstract Robert & Chaussidon The possibility of a change in δ18O values of the oceans has been discussed for the past 30 years1 and the comment by Shields and Kasting2 does not really bring any new insight into this issue. The carbonate δ18O curve3 is indistinguishable from that of cherts and shows a huge scatter caused by local geological processes. This scatter in δ18O (up to 20‰ at a given age) far exceeds the sensitivity of the isotope thermometer (about 2‰ per 10 °C). It therefore prevents any precise additional test of the model and no consensus was ever reached in the literature on this debate from oxygen-isotope data alone. Our approach4, by contrast, was to try to find another isotopic proxy (δ30Si) that could bring new and independent constraints to test the hypothesis of hot oceans in the early Precambrian. Our interpretation of the δ30Si values in terms of temperature is fully coherent not only with the δ18O of cherts but also with that of marine carbonates, and certainly does not overlook these data. In addition, we indicated4 that our sample set has not the time resolution that would be necessary to look for an effect on silicon isotopes of Precambrian global glaciations. No chert samples from such periods were analysed.

Date: 2007
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DOI: 10.1038/nature05831

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