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The researchers used a highly sensitive technique comparing stable forms of chromium — chromium-52 and chromium-53 — to probe the rock for signs of oxygen exposure. Today, chromium-52 is more abundant in land sediments than chromium-53 is because atmospheric oxygen more readily oxidizes, or strips electrons from, chromium-53. When oxidized, chromium-53 dissolves in water, so rivers carry it to the ocean.
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