US GEOTRACES Northeast Atlantic Zonal Transect: Processes governing Nd isotopes and REE concentrations

Neodymium (Nd) isotopes have been shown to be a valuable proxy for present and past ocean circulation but particulate exchange processes can sometimes impact the quasi-conservative behavior of Nd. Here we focus on samples collected in eastern Atlantic during the US GEOTRACES North Atlantic zonal transect, from Lisbon to the Cape Verde Islands, traversing open ocean and the area directly west of the Sahara. The sampled water masses along this transect include many of the main water masses in the Atlantic, including North and South Atlantic Central Water near the surface, Mediterranean Outflow Water (MOW) and Antarctic Intermediate Water (AAIW) at intermediate depths, and North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) at depth. Within this transect, factors that could potentially influence the Nd isotopes and REE patterns include Saharan dust, an extensive oxygen minimum zone (OMZ), nepheloid layers, and boundary exchange with the African continental margin and the Cape Verde islands. The two open ocean stations south of Iberia show uniform ɛNd ≈ -12 at NADW depths, and increasing [Nd] with depth from 17 to 22.3 pmol/kg at 1500 to 4000 m. The northernmost station shows higher ɛNd ≈ -10.5 at 800 to 1500 m in the presence of MOW, and below 1700 m ɛNd decreases to ≈ -11.5. This station also shows evidence for NACW with ɛNd ≈ -11.8 at 200 to 400 m. The MOW signal ceases by about 750 km to the southwest of the outflow. South of 30°N, samples show ɛNd ≈ -11.6 at both NADW and AAIW water depths. An exception is a station near the Cape Verde Islands, which has a large range of ɛNd from -13 to -10 at these depths. Surface compositions within the Sahara dust plume indicate aerosol dissolution with ɛNd ≈ -12.2 and [Nd] of 28.4 pmol/kg. At these sites, at the chlorophyll max around 60 m, [Nd] sharply decreases to ≈ 18 pmol/kg, but maintains the same isotope values. Below this depth, in the OMZ, [Nd] increases to a local maximum of 23.4 pmol/kg, and ɛNd increases to ≈ -10.6. With increasing depth, [Nd] gradually decreases to ≈ 19 pmol/kg until 2000 m, and then increases to ≈ 24 pmol/kg at the bottom. At the site closest to the Mauritanian margin there is an excursion to significantly lower ɛNd values (-13.4) within a nepheloid layer, likely the result of exchange, but this is not seen in sites farther to the west. At intermediate depths at the sites close to the Mauritanian shelf, ɛNd is slightly lower compared to the nearest open ocean site (-11.8 vs -11.5), which could reflect a small extent of exchange. In addition, intermediate depths at the westernmost station near the Cape Verde islands show more radiogenic values (ɛNd = -9.6) that may reflect influence of local volcanic material. These processes will be further investigated with particle analyses and by pairing isotope data with REE.