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Large grains coated by iron oxides

Caption

The photo shows a sample collected in northern China with abundant large grains coated by iron oxides. The grains, deposited in shallow, nearshore settings in the ancient ocean, can capture the chromium isotope composition of river water draining the ancient continents. Those waters, in turn, can record reactions in ancient soil in the presence or absence of appreciable atmospheric oxygen. In this way, the iron-rich grains are something like an oxygen paleobarometer for the ancient atmosphere. Photo credit: X. Chu, Chinese Academy of Sciences.