WATER COLUMN DISTRIBUTION OF TH-230 AND TH-232 IN THE BLACK-SEA

Profiles of Th-230 and Th-232 at a station in the western central Blac k Sea were determined using a highly sensitive mass spectrometry metho d. Compared with most open ocean and coastal waters, concentrations of ''dissolved'' (<0.4 mum) Th-230 and Th-232 in the Black Sea are signi ficantly higher, primarily due to lower scavenging rate in this predom inantly anoxic environment. Above the anoxic zone (0-95 m) about 42% o f Th-230 and 57% of Th-232 are in the particulate form, compared with 9 and 21%, respectively, in the anoxic zone. The distribution indicate s that a sizable fraction of particulate Th is associated with Mn-cont aining particles at the O2-H2S interface just above the anoxic zone, w hich is released into solution when such particles are transported acr oss the redox boundary and are dissolved in the anoxic zone. From the partitioning and isotopic composition of Th between dissolved and part iculate phases, it is estimated that approximately 20-40% of the disso lved Th-230 in the water column is terrigenous. The residence time of dissolved Th in the Black Sea water column is 43-48 years, compared wi th 6-20 years for the same depth range elsewhere in the world oceans.