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

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 c overing 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 u pper 100 m are small during the early spring cruise, increase to maximum va lues during the summer, and decrease over the course of the fall. Applicati on 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 Ros s Sea appears to sustain significant Th-234 removal for a longer period tha n is evident at Orca or Sei. Particulate Th-234 activities and POC are grea ter in the 1-70 mum size fraction, except late in the summer cruise, when t he > 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, l ikely due in part to the greater availability of surface sites for Th adsor ption 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 stea dy-state Th scavenging model range from 7 to 91 mmol C m(-2) d(-1) during l ate January-early February, with the greatest flux observed at station Mink e 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., 20 00. 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 flu xes in the Ross Sea, Antarctica. Deep-Sea Research II 47, 3423-3449), and t he Th-234-derived POC fluxes indicate that the sinking flux of POC is simil ar 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 aggregatio n and disaggregation rates are calculated at station Orca by coupling parti culate 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 a re highest in near-surface waters and decrease with depth. POC disaggregati on rates during the same time generally increase to a maximum and are low a t depth (> 200 m). Subsurface aggregation rates increase to high values lat e in the summer, while disaggregation rates decrease. This trend helps expl ain higher values of POC in the > 70 m fraction relative to the 1-70 m frac tion 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.