Water column distribution of 230Th and 232Th in the Black Sea

Abstract

Profiles of 230Th and 232Th at a station in the western central Black Sea were determined using a highly sensitive mass spectrometry method. Compared with most open ocean and coastal waters, concentrations of “dissolved” (<0.4 μm) 230Th and 232Th in the Black Sea are significantly higher, primarily due to lower scavenging rate in this predominantly anoxic environment. Above the anoxic zone (0–95 m) about 42% of 230Th and 57% of 232Th are in the particulate form, compared with 9 and 21%, respectively, in the anoxic zone. The distribution indicates that a sizable fraction of particulate Th is associated with Mn-containing particles at the O 2H 2S interface just above the anoxic zone, which is released into solution when such particles are transported across the redox boundary and are dissolved in the anoxic zone. From the partitioning and isotopic composition of Th between dissolved and particulate phases, it is estimated that approximately 20–40% of the dissolved 230Th in the water column is terrigenous. The residence time dissolved Th in the Black Sea water column is 43–48 years, compared with 6–20 years for the same depth range elsewhere in the world oceans.