The distribution of 227Ac along the GA01 section in the North Atlantic

Abstract

Actinium-227 is a powerful tool to study vertical mixing in the ocean, and was more recently proposed as a tracer of hydrothermal plumes. However, because 227Ac activities are especially low in the ocean, relatively few studies have been conducted to date, and none has been done on the large scale, as is reported in the present study. Here, we report a section of dissolved 227Ac in the North Atlantic Ocean between Portugal-Greenland-Canada based on nine full-depth profiles obtained in the framework of the international GEOTRACES program (GA01 section - GEOVIDE cruise, May–July 2014). The simultaneous determination of 231Pa, the parent nuclide of 227Ac, along the section allowed us to report a section of excess 227Ac activities (227Acex).Actinium-227 activities were especially low along the section (<0.4 dpm m−3) compared to other ocean basins. In most cases, 227Ac activities reached secular equilibrium with 231Pa in the water column 500 m above the seafloor. Secular equilibrium is considered reached after ∼five half-lives of the daughter. Secular equilibrium therefore suggests no external input over the last ∼100 years. The highest 227Ac activities were often found close to the seafloor, indicating that 227Ac diffuses out of the sediments. However, two different patterns question the traditional one-dimensional vertical mixing model typically applied to 227Ac released by deep-sea sediments: first, significant 227Acex were found in the upper 500 m of the water column at several stations located near the Iberian margin. In the upper 500 m, the high 227Acex activities could indicate lateral advection of waters that interacted with the margins. Second, mid-water peaks of 227Acex were occasionally observed along the transect. At station 44 in the Irminger Basin, a peak of 227Acex activity at 2500 m that was co-located with high dissolved Fe concentrations could be interpreted as the signature of a hydrothermal plume. Near the seafloor, we often observed bottom nepheloid layers and we cannot exclude that these layers impact the 227Ac distributions.