Chromium speciation in marine carbonates and implications on atmospheric oxygenation

Qin and colleagues examined chromium (Cr) valence states in sedimentary carbonates and found that Cr(III) dominates in all samples formed in different geological periods. This finding is in apparent contradiction with the previous assumption that marine carbonates directly incorporate Cr(VI) from seawater, suggesting that some positive Cr isotopic values recorded in these rocks cannot be necessarily linked to atmospheric oxygenation events.