Short-lived thorium isotopes (234Th, 228Th) as indicators of POC export and particle cycling in the Ross Sea, Southern Ocean

Repeated measurements of depth pro"les of 234Th (dissolved, 1}70 and ’70 lm particulate) at three stations (Orca, Minke, Sei) in the Ross Sea have been used to estimate the export of Th and particulate organic carbon (POC) from the euphotic zone. Sampling was carried out on three JGOFS cruises covering the period from October 1996 (austral early spring) to April 1997 (austral fall). De"ciencies of 234Th relative to its parent 238U in the upper 100 m are small during the early spring cruise, increase to maximum values during the summer, and decrease over the course of the fall. Application of a non-steady-state model to the 234Th data shows that the #ux of Th from the euphotic zone occurs principally during the summer cruise and in the interval between summer and fall. Station Minke in the southwestern Ross Sea appears to sustain signi"cant 234Th removal for a longer period than is evident at Orca or Sei. Particulate 234Th activities and POC are greater in the 1}70 lm size fraction, except late in the summer cruise, when the ’70 lm POC fraction exceeds that of the 1}70 lm fraction. The POC/234Th ratio in the ’70 lm fraction exceeds that in the 1}70 lm fraction, likely due in part to the greater availability of surface sites for Th adsorption in the latter. Particulate 234Th #uxes are converted to POC #uxes by multiplying by the POC/234Th ratio of the ’70 lm fraction (assumed to be representative of sinking particles). POC #uxes calculated from a steady-state Th scavenging model range from 7 to 91 mmol C m~2 d~1 during late January}early February, with the greatest #ux observed at station Minke late in the cruise. Fluxes estimated with a non-steady-state Th model are 85 mmol C m~2 d~1 at Minke (1/13}2/1/97) and 50 mmol