A thorough understanding of the change in chemistry of Earth’s stratosphere and upper troposphere through time requires direct samples of the upper atmosphere. Ancient samples of the upper atmosphere are captured in the fusion crust of meteorites during their atmospheric entry. Stratospheric O2 is fractionated light, in a mass-independent way, through a series of reactions between ozone , CO2, O, and O2. Pre-industrial stratospheric ozone concentrations more than 30% above current concentrations have been postulated, but not measured due to the absence of a suitable record. Such differences in ozone concentrations should produce a ~0.5‰ decrease in Δ17O in O2 (by mass balance). If a difference in Δ17O between pre-1900 and recent falls can be measured, meteorite fusion crusts would prove to be a useful record of stratospheric gas. This record can possibly extend back millions of years with Antarctic meteorite finds.
For this study, we measured the O isotope composition of fusion crusts from four pre-1900 witnessed iron meteorite falls: Hraschina (type IID, fell 1751), Braunau (IIAB, 1847), Mazapil (IAB-sLL, 1885), Quesa (IAB-ung, 1898), and one recent fall: Ban Rong Du (ungrouped iron, 1993). All samples are from the collection of the National History Museum in Vienna. These meteorites have thin (100-1000 µm) fusion crusts with compositiona consistent with magnetite (measured by SEM-EDX). Using the University of Hawaii Cameca ims 1280 ion microprobe, we measured 16O, 17O, and 18O on Faraday cups using a ~3 nA Cs+ primary beam rastered over 15 µm, and 5500 mass-resolving power to resolve OH from 17O. Results of our analyses are shown in the figure.
The pre-industrial iron meteorite fusion crusts tend to have lower Δ17O values (by ~1‰) than the terrestrial standards, and also lower than the 1993 fusion crust of Ban Rong Du, though only four measurements from this meteorite were taken (the average is shown in the figure), with larger uncertainties due to OH contribution. Our measurements point to a lower Δ17O in stratospheric O2 sampled by the pre-1900 iron meteorite falls, likely due to the increased amount of ozone in the stratosphere at this time. Future analyses of Antarctic meteorites and older finds will expand our understanding of ancient ozone concentrations in the stratosphere.