signature=1b8b230a6322310c1629e544ba5dcb63,Ra-226 and Ra-226/Ba Signatures of Intermediate and Deep ...

Abstract

The Labrador sea plays an important role in the formation of intermediate North Atlantic water, (cf. Labrador Sea Water -LSW) and subsequently, in the general thermohaline circulation of the ocean. In addition, water masses originating from the Norwegian and Greenland seas are present below the LSW, in the deeper part of the basin. Investigations on the geochemistry of radium have been undertaken as a mean to document these deep and intermediate circulation processes. The vertical distribution of Ra-226 and Ba contents has been established for 8 stations located along 2 sections from 48°44 N; 52°58W to 52°23 N 45°17W and from 53.59°N to 60.34°N, 48.13°W. Thermo-Ionization Mass Spectrometry coupled with a new radium extraction and purification technique permitted the measurement of Ra-226 using 200 ml samples (i.e., appr. 10 femtograms of Ra). In parallel, Ba contents were determined using M-C-ICP-MS in order to normalize Ra-226 concentrations, since Ba is often seen as a subrogate stable isotope of Ra, due to their almost similar geochemical behaviour. Large variations in Ra-226 concentrations are observed. They range from 51 fg/g in intermediate and deep water samples to 31 fg/g in surface waters. When normalised to a salinity of 35 or to the Ba contents large discrepancies are still observed. They suggest a specific Ra-signature for each of the major water masses present in the basin. Particular attention has been paid to Ra-226 contents in the LSW in relation to its production rate as well as to the influence of the Western Boundary Under Current (WBUC), which carries the North Atlantic Deep water masses into their gyre along the lower slopes of the Labrador sea. Based on the presently available data set, a peak in Ra-226 and Ba contents seems to characterise the high velocity axis of the WBUC. These data suggest moreover a lower Ra-226 content for the LSW than for the deeper water masses.