234Th as a tracer of scavenging and particle export in the Beaufort Sea

Water column measurements of 234Th were made at three stations in the shelf–slope regime and one offshore station in the Beaufort Sea, August–September 1995, to quantify rates of scavenging and particle export. Large-volume water samples (200–600 l) were collected in situ and dissolved and particulate 234Th activities determined at-sea. 234Th/238U disequilibrium was evident at all locations. The distribution of the total 234Th/238U activity ratio reveals a transition, centered at the shelf break, from low values (0.4–0.6) over the shelf to elevated values approaching secular equilibrium offshore. These results further constrain the extent to which shelf scavenging and lateral transport contribute to 234Th/238U disequilibrium in the Arctic interior. Residence times of dissolved 234Th range from ∼25–50 d in the shelf and slope waters to ∼150 d offshore, compared to ∼12–18 d for particulate 234Th. Estimates of POC export from the upper 50 m derived from 234Th range from ∼4–7 mmol C m−2 d−1 over the shelf to ∼1–2 mmol C m−2 d−1 in the slope waters and central Beaufort.