Monitoring hydrographic changes in the mid-latitudinal Northeast Atlantic Ocean using seawater stable isotope data

Since 2010 seawater samples for stable carbon and water isotope measurements were collected during various hydrographic cruises in the NE Atlantic between 29° and 60°N and 9° and 33°W, including pluriannual sampling in the Madeira basin, along the western Iberian margin and along the OVIDE/BOCATS (A25) transect between Portugal and Greenland. The samples cover the complete depth range at the respective station. For all samples δ¹⁸O was analyzed, whereas δ²H (δD) measurements were more limited and mainly focused on the wide range of water masses encountered along the OVIDE/BOCATS transect. Carbon isotope values for the dissolved inorganic carbon (δ¹³C-DIC) was monitored over several years at selected stations of the OVIDE/BOCATS transect and in the waters along the Portuguese margin. For some stations along the OVIDE/BOCATS transects, the results of intercomparison measurements between the laboratories at the GeoZentrum Nordbayern (Erlangen) and at LOCEAN (Paris) will also be presented.

The δ¹³C-DIC profiles show a clear signal of anthropogenic carbon entering the water column in the NE Atlantic and leading to lower isotopic values. Whereas data obtained for samples collected in 2010 more or less agree with the data from previous decades compiled in the GLODAP database, shifts to lower values became apparent in the subsurface waters already in 2012. The signal transfer is accelerated in the subsequent years with data from 2016 onwards showing penetration of anthropogenic carbon down to 2000 m and already all the way south to 31°N (Madeira basin). New data from a cruise to the southwestern Portuguese margin in 2022 indicate that the changes now already penetrate down to 2200 m.

Changes in the δ¹⁸O/ δ²H data are less obvious and mostly linked to the subpolar gyre and water mass changes associated with the “North Atlantic cold blob” between winter 2013-2014 and 2016. The presence of surface and subsurface waters with lower isotopic signals clearly tracks the eastward displacement of the subarctic front in 2014 and 2016. Likewise, the front’s subsequent retraction to the west is reflected in the data from 2018 and 2021. Low δ¹⁸O and  δ²H values in depths down to 300 m in the region between the Rockall Plateau and the Reykjanes ridge also clearly distinguish the subpolar mode water formed during the previous “cold blob” winters. On the other hand, and in agreement with the δ¹³C-DIC evidence, hardly any isotope signal changes are observed in the depths of the North Atlantic Deep Water (NADW) and the Northeast Atlantic Bottom Water (NEABW).