EXPANDED OCEAN MINIMUM ZONES IN THE OCEANS DURING THE PALEOCENE/EOCENE THERMAL MAXIMUM INDICATED BY I/CA IN PLANKTIC FORAMINIFERA

Forecasting ocean deoxygenation due to global warming is challenging. Information on deoxygenation during past global warming, e.g., the Paleocene Eocene Thermal Maximum (PETM, ~55.5 Ma), may help, but is hampered by a lack of sensitive proxies. The iodine to calcium ratio (I/Ca) in calcite is a proxy for paleo-redox conditions, because oxidized (iodate), but not reduced iodine (iodide) is incorporated. Total iodine (sum of iodate and iodide) in the present oceans is around 0.45 mmol/L, thus iodate and iodide concentration are inversely correlated. The vertical iodate concentration profile in the oceans is influenced by productivity and the presence/absence of an oxygen minimum zone. If there is no OMZ, the iodate concentration increases downwards from the mixed layer. If there is a shallow OMZ, the iodate concentration decreases from the mixed layer down to the upper OMZ. When the top of the OMZ is deeper, the iodate concentration may increase downwards from the mixed layer, decreasing to zero in the OMZ. We use I/Ca in planktic foraminifera and compare our data with modeled oxygen levels. Shallowing OMZs should cause a decrease in planktic foraminiferal I/Ca, with lower values in thermocline than in mixed layer calcifiers. Changes in I/Ca in mixed-layer relative to thermocline dwellers reflect changes in the iodate depth gradient, related to the vertical extent of OMZs. At sites in the open ocean Pacific (865), Weddell Sea (690), SE Atlantic (1262, 1263) and Indian Ocean (738), I/Ca values in monogeneric mixed layer and thermocline dwelling planktic foraminifera decreased over the PETM, probably indicative of widespread deoxygenation in the upper ocean waters, more pronounced in the Atlantic and Indian than in the Pacific Ocean. The mean difference in I/Ca of thermocline and mixed-layer dwellers varies geographically, and is positively correlated to the minimum oxygen levels in the water column estimated from climate modeling. Reconstructed iodate gradients indicate that deoxygenation was widespread in all oceans during the PETM, due to vertical and potentially lateral expansion of Oxygen Minimum Zones, which may have influenced pelagic ecosystems through vertical compression of the zone above the OMZ.