TH-230 IN THE EASTERN NORTH-ATLANTIC - THE IMPORTANCE OF WATER MASS VENTILATION IN THE BALANCE OF TH-230

The Th-230 distribution in the water column on a north-south transect in the eastern North Atlantic was measured using thermal ionization ma ss spectrometry (TIMS). Water samples at three locations (L1, 33 degre es N 21.6 degrees W; L2: 47.4 degrees N 19.5W; and L3, 54.4 degrees N 21.1 degrees W) were filtered and the dissolved and particulate fracti ons were analysed for their Th-230 and Th-232 contents. In the upper w ater column (<1000 m water depth), the total Th-230(ex) distribution a t the three locations could be explained by a reversible scavenging mo del. In the deep water column, however, Th-230(ex) concentrations decr ease from south to north with relatively low and constant concentratio ns at stations L2 and L3. The major water mass below 2000 m is North E ast Atlantic Deep Water (NEADW), filling the deep water column at L1. The water column at L2 and L3 is influenced by Labrador Sea Water (LSW ) down to 2000 m, whereas at L3, the deep water is strongly influenced by Iceland-Scotland Overflow Water (ISOW). The anomalous distribution of Th-230(ex) in the deep water column of stations L2 and L3 could be described after a ventilation term was added to the equations of the reversible scavenging model. For L3, a ventilation time (3-12 years) w as found comparable to results derived from other tracers. However, th e ventilation of the deep water (>2000 m) at L2 suggested by our model (8-25 years) is probably too rapid in comparison with previous findin gs based on chlorofluorocarbon data. Enhanced scavenging of Th-230 wou ld be an alternative explanation for the low concentration in NEADW at L2. With the scavenging-mixing model, we calculated an advective expo rt flux of Th-230 Of up to 50% of the production in the water column i n the eastern North Atlantic. Advective loss of Th-230 has to be taken into account when using the Th-230 AUX into sediments for paleoceanog raphic studies in ocean basins where deep water residence time is comp arable to the Th-230 scavenging residence time. (C) 1998 Elsevier Scie nce B.V. All rights reserved.