Radiocarbon evidence of a North Atlantic intermediate water reconfiguration between the 1960s and 1980s

The Atlantic Meridional Overturning Circulation (AMOC) is a critical modulating component of the Earth's climate system, therefore, documenting the timing and amplitude of AMOC variability since the Industrial Era is essential for understanding future climate. However, there are very few continuous high-resolution records from this time period in the subsurface ocean which restricts us from exploring the nature of changes in the ocean interior. Here, we present new seawater radiocarbon (14C) records of intermediate depth water (between 1400 m and 2000 m) for the Industrial Era since 1830 CE obtained from the calcitic skeleton of four cold-water bamboo corals from the tropical Atlantic. These samples were collected in 2013 with lifespans ranging from 64 to 161 years. No bomb 14C was detected in the calcitic skeleton, indicating that the tropical intermediate North Atlantic had not yet been invaded by bomb 14C at the time of collection. The data show a relatively constant Δ14C before 1960 CE with a synchronous basin-wide increase in 14C content reached by 1980 CE. The stepwise shift in Δ14C may be explained by the North Atlantic Deep Water (NADW) becoming shallower in the tropical North Atlantic and/or a southward retreat of Antarctic Intermediate Water (AAIW), in either case potentially linked to a weakening of AMOC. Our study indicates synchronous large-scale oceanic reconfiguration occurred throughout the tropical and North Atlantic starting in the 1960s and that there has been no subsequent return to the state of the pre-mid-20th century oceanic interior.