Remnants of shooting stars are preserved in form of cosmic spherules in ice and sediments. The extraterrestrial material is heated and melted upon atmospheric entry and is collected at the Earth's surface as cosmic spherules. A fraction of cosmic spherules (I-type cosmic spherules) sources from extraterrestrial Fe,Ni metal. These metal particles melt and become oxidized in the atmosphere. The oxygen in the resulting oxides (magnetite, wüstite) sources entirely from the atmosphere. Here, we demonstrate that I-type cosmic spherules can be used to reconstruct the triple oxygen isotope anomaly of the past atmosphere, which provides information on the gross primary productivity and/or paleo-CO2 levels. We present a calibration of the proxy using Antarctic cosmic spherules. We further introduce Δ′56Fe and demonstrate that triple iron isotopes can be used to obtain information about the underlying fractionation mechanism (e.g., kinetic vs. equilibrium fractionation).
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