Triple oxygen isotope variations in sedimentary rocks

Abstract

Relatively large (⩾0.2‰) ^(17)O anomalies in the geologic record have been used to recognize atmospheric processes such as photochemical reactions and to trace changes in the partial pressures of O_2 and CO_2 in Earth's atmosphere through time. However, recent oxygen isotope measurements of terrestrial rocks, minerals and waters also reveal common, smaller (but statistically significant) deviations from a single mass-dependent fractionation line. These subtle anomalies have been explained through differences in mass-dependent isotopic fractionations for various equilibrium and kinetic mechanisms. Here we present triple oxygen isotope data on sedimentary silica and oxides, including Archean and Phanerozoic cherts, and iron formations. The distribution of data reflects the mass fractionation laws of low-temperature precipitation reactions during growth of authigenic minerals, variation in Δ^(17)O of the waters from which sedimentary minerals precipitate, and equilibrium exchange after initial authigenic formation. We use these results to illustrate the potential for small, mass-dependent variations in Δ^(17)O values of sedimentary rocks to provide constraints on the environmental and climatic conditions in which they formed.

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