The impact of global warming on the ocean’s oxygen-deficient zones (ODZs) is uncertain, partly because of a lack of data on past changes. We report monthly resolved records of coral skeleton–bound nitrogen isotopes (CS-δ15N) to reconstruct … The impact of global warming on the ocean’s oxygen-deficient zones (ODZs) is uncertain, partly because of a lack of data on past changes. We report monthly resolved records of coral skeleton–bound nitrogen isotopes (CS-δ15N) to reconstruct … Science