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The 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. Plain Language Summary Climate change is linked to a decline in ocean oxygen levels, impacting fish and other organisms that need oxygen to breathe. Knowledge of past changes in ocean oxygen would help put ongoing deoxygenation trends into context. In this study, we investigated changes in oxygen in the Pacific Ocean over the past 145,000 years. Because low‐oxygen waters are enriched in the metal cobalt, we reconstructed the cobalt abundance of the past oceans as a proxy for oxygen. During the past two Ice Ages, when Earth was colder than today, we find evidence for higher cobalt, and therefore an expansion of oxygen‐poor waters. Key Points The cobalt flux to pelagic sediments reflects heightened sources associated with Oxygen Deficient Zones In Equatorial Pacific sediments, cobalt flux increased by ∼40% during the Last Glacial Period, compared to the Holocene Oxygen Deficient Zones likely expanded during the previous two glacial periods

From June to August 2018, the eruption of Kīlauea volcano on the island of Hawai‘i injected millions of cubic meters of molten lava into the nutrient-poor waters of the North Pacific Subtropical Gyre. The lava-impacted seawater was characterized by high concentrations of metals and nutrients that stimulated phytoplankton growth, resulting in an extensive plume of chlorophyll a that was detectable by satellite. Chemical and molecular evidence revealed that this biological response hinged on unexpectedly high concentrations of nitrate, despite the negligible quantities of nitrogen in basaltic lava. We hypothesize that the high nitrate was caused by buoyant plumes of nutrient-rich deep waters created by the substantial input of lava into the ocean. This large-scale ocean fertilization was therefore a unique perturbation event that revealed how marine ecosystems respond to exogenous inputs of nutrients.

In oligotrophic ocean regions, dissolved organic phosphorus (DOP) plays a prominent role as a source of phosphorus (P) to microorganisms. An important bioavailable component of DOP is phosphonates, organophosphorus compounds with a carbon-phosphorus (C-P) bond, which are ubiquitous in high molecular weight dissolved organic matter (HMWDOM). In addition to being a source of P, the degradation of phosphonates by the bacterial C-P lyase enzymatic pathway causes the release of trace hydrocarbon gases relevant to climate and atmospheric chemistry. In this study, we investigated the roles of phosphate and phosphonate cycling in the production of methane (CH₄) and ethylene (C₂H₄) in the western North Atlantic Ocean, a region that features a transition in phosphate concentrations from coastal to open ocean waters. We observed an inverse relationship between phosphate and the saturation state of CH₄ and C₂H₄ in the water column, and between phosphate and the relative abundance of the C-P lyase marker gene phnJ. In phosphate-depleted waters, methylphosphonate and 2-hydroxyethylphosphonate, the C-P lyase substrates that yield CH₄ and C₂H₄, respectively, were readily degraded in proportions consistent with their abundance and bioavailability in HMWDOM and with the concentrations of CH₄ and C₂H₄ in the water column. We conclude that phosphonate degradation through the C-P lyase pathway is an important source and a common production pathway of CH₄ and C₂H₄ in the phosphate-depleted surface waters of the western North Atlantic Ocean and that phosphate concentration can be an important control on the saturation state of these gases in the upper ocean.