AbstractThe expansion of oxygen deficient zones (ODZs) within the ocean's interior is anticipated to be a major consequence of anthropogenic climate change, but past changes in ODZs are poorly defined. Recent mapping efforts have revealed plumes of the redox‐active metal cobalt within ODZs, driving a basin‐scale correlation between high cobalt and low O₂. Here, we investigate the cobalt flux to Equatorial Pacific sediments along the Line Islands Ridge as a novel record of basin‐scale fluctuations in ODZ extent. After accounting for remobilization by diagenesis, we document a ∼40% increase in cobalt accumulation over the last glacial period, with a more pronounced peak during the Last Glacial Maximum, indicative of larger ODZs compared to the Holocene. Our results link ODZ expansion with colder climates and lend support to model‐based assertions that ongoing deoxygenation may reflect a transient response to warming.