226Ra and Ba in northeast Atlantic deep water

Abstract

226Ra and Ba data are presented for the northeast Atlantic below 2000 m depth from a section of stations 8°S to 46°N obtained by F.S. Meteor in 1981. The precision of the 226Ra data (±1%) allows a resolution of structures in the 226Ra distribution that is comparable to the resolution for Ba and nutrients. Linear property—property correlations in the upper parts of the depth range appear to be uniform within the observational error along the section for 226Ra, Ba [Ba(nmol kg −1) = 3.88 Ra (dpm 100 kg −1) + 12.3], and alkalinity, whereas the slopes of these properties versus Si increase northwards from about 10 to 25 × 10 −4 dpm Ra μmol Si −1. From this observation, and from studying the property distributions across the Romanche Fracture Zone, it is concluded that CaCO 3 dissolution must be an important process of 226Ra and barium regeneration, while opal dissolution appears to be of minor importance. The water overflowing eastward over the sill of the Romanche Fracture Zone has 17.2 ± 0.3 dpm 100 kg −1 226Ra, 79.5 ± 1.5·nmol kg −1 Ba and 2338 ± 4 μeq kg −1 alkalinity. Within 800–1400 m above the sea floor, excess 226Ra relative to the linear correlation with Ba further up in the water column is generally present, but there is also a reflection of 226Ra decay at intermediate depths at the northern stations of the section. The latter effect appears to amount to only 3 ± 1.5%, but this magnitude is consistent with the 14C distribution reported previously ( Schlitzer et al., Journal of Geophysical Research, 90, 6945–6952, 1985). From the excess 226Ra in the lowest 800–1400 m of the water column, the 226Ra flux from the sediments is estimated to be 7.7 ± 4 × 10 −21 mol Ra (m 2s) −1, a value in accordance with results by Cochran ( Earth and Planetary Science Letters, 49, 381–392, 1980).