Short-lived thorium isotopes (, ) as indicators of POC export and particle cycling in the Ross Sea, Southern Ocean
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Repeated measurements of depth profiles of Th-234 (dissolved, 1-70 and > 70 mum 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). Deficiencies of Th-234 relative to its parent U-238 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 Th-234 data shows that the flux 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 significant Th-234 removal for a longer period than is evident at Orca or Sei. Particulate Th-234 activities and POC are greater in the 1-70 mum size fraction, except late in the summer cruise, when the > 70 mum POC fraction exceeds that of the 1-70 mum fraction. The POC/Th-234 ratio in the > 70 mum fraction exceeds that in the 1-70 mum fraction, likely due in part to the greater availability of surface sites for Th adsorption in the latter. Particulate Th-234 fluxes are converted to POC fluxes by multiplying by the POC/Th-234 ratio of the > 70 mum fraction (assumed to be representative of sinking particles). POC fluxes 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 flux 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 C m(-2) d(-1) at Orca (1/19-2/1/97). The decline in POC inventories (0-100 m) is most rapid in the southern Ross Sea during the austral summer cruise (Smith et al., 2000. The seasonal cycle of phytoplankton biomass and primary productivity in the Ross Sea, Antarctica. Deep-Sea Research 47, 3119-3140; Gardner et al., 2000. Seasonal patterns of water column particulate organic carbon and fluxes in the Ross Sea, Antarctica. Deep-Sea Research II 47, 3423-3449), and the Th-234-derived POC fluxes indicate that the sinking flux of POC is similar to 30-50% of the POC decrease, depending on whether steady-state or non-steady-state Th fluxes are used. Rate constants for particle POC aggregation and disaggregation rates are calculated at station Orca by coupling particulate Th-234 data with Th-228 data on the same samples. Late in the early spring cruise, as well as during the summer cruise, POC aggregation rates are highest in near-surface waters and decrease with depth. POC disaggregation rates during the same time generally increase to a maximum and are low at depth (> 200 m). Subsurface aggregation rates increase to high values late in the summer, while disaggregation rates decrease. This trend helps explain higher values of POC in the > 70 m fraction relative to the 1-70 m fraction late in the summer cruise. Increases in disaggregation rate below 100 m transfer POC from the large to small size fraction and may attenuate the Bur of POC sinking out of the euphotic zone. (C) 2000 Elsevier Science Ltd. Ail rights reserved.