The modern Northeast Atlantic Radiocarbon Content viewed along a basin transect from 29°- 61°N

The Northeast Atlantic is crucial regarding the northward heat and carbon export into the Arctic Ocean. At the surface and at mid-depth (0 -1000 m), however, most of the water re-circulates through two basin scale gyres, with warm and salty waters in the sub-tropical gyre (STG) and significantly fresher and colder waters in the sub-polar gyre (SPG). In addition, the Azores Front (AF) separates the northeastern branch from the southeastern branch of the STG, which is today positioned around 34.5°N in the east Atlantic. Underneath both gyres newly formed deep waters from the Arctic Ocean and Labrador Sea are spread into the Atlantic basin. Here we investigate, whether these water masses and recirculation patterns reveal distinct histories of carbon uptake and advection in the eastern North Atlantic. Therefore, water samples spanning the entire water column have been collected at 6 stations along a north-south transect at 25°W spanning from 42° to 61°N in 2012 (N/O Thalassa ICE-CTD cruise). In 2018 (RV Meteor M151 ATHENA cruise) samples further south were collected spanning until 29.5°N thus including the present day AF.

The radiocarbon content of Dissolved Inorganic Carbon (DIC) from 8 profiles (N>60, 30° to 61°N, 50-3000m depth) were measured at the AMS facility at CEZA facility in Mannheim following CO₂ extraction from seawater at Heidelberg University. Δ¹⁴C values range from +50 ‰ in the upper layers of the subtropical Atlantic to -100 ‰ in 3000 m depth also in the subtropical Atlantic. Three main feature appear in the radiocarbon distribution. The surface shows a moderate difference between SPG and STG Δ¹⁴C values of <15‰ with a decreasing trend towards the North, hence indicating equilibration with the atmosphere. Underneath, between 100-1000 m depth SPG (46° – 61°N) Δ¹⁴C values of nearly 0‰ are found identical to the modern Northern Hemisphere atmosphere. In contrast, the STG (30°-43°N) reveals up to 50‰ enriched water reflecting limited carbon uptake from the atmosphere. Thus this layer will act as a source of radiocarbon to the polar seas and atmosphere in the near future. Below, between 1000 and 2000 m water masses north of the AF reveal a nearly homogeneous Δ¹⁴C value of -10‰ with a moderate decreasing trend with depth. South of the AF Δ¹⁴C values show a strong decrease with depth from 0 to -75‰, hence water masses remain still little affected by the advection of bomb radiocarbon (thus anthropogenic carbon). Thus, as expected the AF and the mid-depth gyre play a crucial role in distributing carbon throughout the east Atlantic.