Tracing the triple isotope composition of air by high-precision analyses of meteorites, rocks and fossils

High-precision measurements of the triple oxygen isotope ratios (δ¹⁸O and Δ‘¹⁷O) in terrestrial waters, rocks and minerals opened new and exciting applications in the field of stable oxygen isotope geochemistry. After giving a short overview over measurement techniques and various applications, it will be emphasized on tracing the atmospheric composition in rocks and minerals.

Atmospheric samples from ice cores only date back ~1 Myrs. To obtain information about the atmosphere for the 99.98% of Earth history that is not covered by ice cores, we need to look for rocks. The oxygen isotope composition of the atmosphere younger than 2.4 Gyrs is dominated by molecular oxygen (O₂). Molecular O₂ is one of few components on Earth that has a mass-independent oxygen isotope signature. The anomaly in ¹⁷O provides information about the presence of an ozone layer, the global biosphere primary production, or the atmospheric CO₂ mixing ratio. A few rocks and fossils provide information about the ¹⁷O anomaly of air O₂. Sedimentary sulfates may form by precipitation from SO₄^2- that formed by subaerial oxidation of pyrite. In that process, a part of the oxygen in the sulfate originates from air O₂. Mobilizing of the sulfate oxygen can carry this anomaly over to other minerals like Fe oxides. The isotope signature of fossil tooth enamel also provides information about the atmospheric composition. Air O₂ is inhaled and used to oxidize carbohydrates and fat to (mainly) CO₂ and H₂O, which equilibrate with body water. Tooth apatite then precipitates from body water and inherits an anomaly in ¹⁷O from the inhaled air O₂. Manganese oxides are known to form by oxidation of Mn under participation of O₂. If the isotope composition of dissolved O₂ in the aqueous environment, in which the manganese oxides form is controlled by air, manganese oxides can be used to trace the composition of air O₂. It has been shown that some meteorite impact melts (tektites) have exchanged with ambient air O₂. As result of that exchange, they carry a ¹⁷O anomaly that may be used to trace the composition of air O₂. Also, I-type cosmic spherules have been shown to be indicators for the isotope anomaly of air O₂. These spherules form by aerial oxidation of asteroidal metallic Fe,Ni particles and thus can carry the anomaly of air O₂. Such recent discoveries open insights into the composition of the Earth atmosphere beyond the 1 Myrs limit from the ice core record.