Intensified Oxygen Minimum Zone in Response to Both Insolation Maxima and Carbon Dioxide‐Driven Warmth in the Tropical North Atlantic

Abstract

Oxygen (O2) is essential for almost all life on Earth, but O2 concentrations are rapidly declining in the oceans under global warming, stressing marine ecosystems and biogeochemical cycles. Numerical model simulations suggest greatly expanded North Atlantic Oxygen Minimum Zones (OMZs) under warm Pliocene conditions, but sparse data limit validation and evaluation of mechanistic drivers. Here we use fossil tests of Globorotaloides hexagonus, a rare species of planktonic foraminifera associated with OMZs today, to reconstruct past variability in oxygenation in the Eastern Tropical North Atlantic Ocean (ETNA). Our astronomically resolved records from ODP Site 659 cover the warm Pliocene through intensification of North Hemisphere Glaciation (3.45–2.33 Ma) and the last glacial cycle (150–0 ka). We document deoxygenation of the ETNA during both the warm Late Pliocene and marine isotope stage (MIS) 5, followed by re‐oxygenation upon the introduction of colder conditions associated with intensified Northern Hemisphere Glaciation (∼3 Ma) and MIS4 (∼70 ka) respectively. Under deoxygenated conditions, OMZ‐strength fluctuated, paced by precession. We rule out “top‐down” control on deoxygenation by changes in ETNA sea surface temperature, upper ocean stratification, upwelling strength or ocean productivity. Instead, we infer “bottom up” control by changes in the ventilation of intermediate waters supplied to the ETNA. Mechanistic clues emerge from the sign of the relationship that we document: deoxygenation peaks occur during insolation maxima, when tropical rainbelt expansion freshened the Mediterranean Sea and altered its outflow properties to the Atlantic Ocean. We propose that Mediterranean Outflow variability modulated OMZ strength in the ETNA.