Seawater temperature and water mass provenance changes over the last century in the North Atlantic Ocean reconstructed from cold-water coral geochemistry

The aragonite skeletons of cold-water corals (CWCs) offer critical insights into the physico-chemical changes and dynamics of intermediate-depth water masses at high temporal resolution (e.g. sub-decadal). Previous studies have shown that variations in seawater temperature, water ventilation age, and water mass provenance can be reconstructed from measurements of skeletal Li/Mg ratios, paired U/Th and ¹⁴C, and neodymium isotopes, respectively. Notably, the solitary azooxanthellate scleractinian coral species Desmophyllum dianthus is particularly valuable due to its broad distribution, century-long lifespan, and layered skeletal growth, which facilitates the use of geochemical tracers at sub-decadal intervals.

In this study, we analysed several Desmophyllum dianthus samples collected in 2012 from the Northern Iceland Basin at depths of 570-700 m during the ICE-CTD R/V Thalassa expedition, using the Remotely Operated Vehicle Victor 6000 operated by IFREMER. Sub-samples of the coral skeletons collected along the growth axis were analysed for Li/Mg, stable isotopes (δ¹¹B, δ¹⁸O, δ¹³C), U/Th, ¹⁴C and Nd isotopes, with the aim to reconstruct the physico-chemical changes of the North Atlantic intermediate water masses, specifically the Iceland-Scotland Overflow Water, Sub-Arctic Intermediate Water and Western North Atlantic Central Water, and assess how their contributions have shifted over recent decades. The Li/Mg ratios provided sub-decadal temperature records, showing variations between ~2 to ~6 °C, closely linked to changes in Nd isotopic compositions. These findings can be explained by decadal fluctuations in the North Atlantic Oscillation and East Atlantic atmospheric patterns, which influence the strength of the Atlantic Subpolar Gyre, leading to changes in the amount of warmer Subtropical Gyre-sourced water or colder Subpolar Gyre-sourced water. Additionally, our results suggest a significant reduction, by about half over the past ~70 years, of the ISOW, pointing to an increased northward transport of warm subtropical waters in recent decades. This shift may have contributed to the recent warming in the Arctic region and a notable multi-decadal weaking of the Nordic Sea overflow currents.