Low particulate organic carbon export in the frontal zone of the Southern Ocean (Indian sector) revealed by 234Th

Abstract The water column deficiencies of 234 Th were used to estimate the Particulate Organic Carbon (POC) fluxes in the Indian Sector of the Southern Ocean. Samples were collected in January–February 1999 in a frontal zone from 42°S to 47°S and from 60°E to 66°E during the ANTARES 4 cruise. Beta counting was used to measure the 234 Th activities onboard. The 234 Th export fluxes were estimated from the 234 Th/ 238 U disequilibria using a steady state 234 Th model. A non-steady state model gave results close to the steady-state model in the Subtropical Zone and could not be used in the Subantarctic Zone due to a strong vertical mixing event. Small and large particles analysis indicated that the POC/ 234 Th ratios decreased when the particle size increased. From the POC/ 234 Th ratios on the large filtered particles, it appears that the POC export fluxes exported below 100 m were very low (from 0.10 to 2.53 mmolC m −2  d −1 ) compared to those observed in the Southern Ocean and (Deep Sea Res. II 48 (2001) 4275; Deep Sea Res. II 47 (15–16) (2000) 3451; Deep Sea Res. II 44 (1997) 457) and with a strong zonal variation . It is hypothesized that the low POC export fluxes were related to the low predominance of diatoms, characteristic at the end of a bloom period. In this way, the very low POC export observed in the Subtropical Zone suggests an efficient remineralization process and/or a high bacterial activity. Otherwise, a decoupling between the primary production and the POC export derived from 234 Th could also explain the low POC export.