Evidence From Dissolved O₂ Isotopes in North Atlantic Deep Water for a Recent Climatic Shift

Anthropogenic-induced variations of the Atlantic Meridional Overturning Circulation (AMOC) and the associated Deep-Water Formation (DWF) are a major concern. Using measurements of triple oxygen isotopes in the deep North Atlantic, we present novel evidence for a dramatic decadal to centennial shift in ocean conditions at the source region of DWF. These measurements suggest a recent decrease in the percentage of photosynthetic O₂ derived from the source regions of AMOC in the Nordic Seas compared to the Little Ice Age. 1-D model simulations suggest that a reduction in photosynthetic O₂ production can explain the observed decrease. Alternatively, it may indicate a substantial decrease in sea-ice cover and thus increased air-sea gas exchange, bringing the isotopic composition of O₂ closer to equilibrium with the atmosphere. Our novel data can serve as a benchmark for climate models.