Abrupt Northwest Atlantic deep-sea oxygenation decline preceded the Palaeocene-Eocene Thermal Maximum

Reconstruction of oceanic redox conditions across the Palaeocene-Eocene Thermal Maximum (PETM, ∼55.93 Ma) provides crucial information on potential PETM triggers and biotic turnover. Widespread oceanic deoxygenation has been documented to occur during the PETM, but the exact timing and regional variability is poorly understood. Here we document well-preserved magnetofossils with large variations in magnetic properties and crystal morphologies at International Ocean Discovery Program (IODP) Sites U1409 (∼2913 m palaeodepth) and U1403 (∼4374 m palaeodepth) across the PETM, and use these proxies to trace Northwest Atlantic palaeoredox changes. Our records suggest that deep-sea oxygenation at these sites decreased gradually, starting several hundred thousand years before the PETM, reaching a minimum ∼50 ky before the PETM, followed by a broad increase. This pattern in palaeoredox conditions likely reflects changes in deep-sea circulation and ocean temperature. We thus provide evidence for the existence of precursors to PETM warming, and suggest that ocean circulation changes might have played an important role in triggering carbon release to the ocean-atmosphere system at the PETM onset.