Short time scale variations of Th-234/U-238 disequilibrium related to mesoscale variability on the continental slope of the Gulf of Lions (France)
Résumé
Th-234/(238) U disequilibrium was studied in the water column during three cruises (March-April 1997) carried out in the framework of the HFF experiment in the Gulf of Lions. The purposes were to investigate short temporal (15-20 days) and spatial (20-50 Km) variations in the water column and to determine export fluxes from the euphotic zone, in relation to environmental features of the study area. Sampling stations were identified in order to evaluate along and across slope transports. Activity-depth profiles show that both temporal and spatial variations occurred at the scale of the experiment. Th-234 deficiencies, with respect to U-238 and indicating removal from surface waters, can be observed at all stations, with higher values measured at the shelf break. Particulate thorium activities do not correlate directly with suspended matter concentration. Dissolved Th-234 values higher than U-238 activities were observed three times in surface waters. They are probably due to desorption processes favoured by vertical mixing and/or lateral transport by the Northern Current and the Rhone river. Fluxes and residence times were obtained using both steady-state (SS) and non-steady-state formalisms (NSS). Lateral transport was calculated to define intra-station influences. Results showed a complex spatial setting with generally higher export at the shelf break (due to enhanced vertical exchange and stronger continental influence), and agreed with the general circulation pattern, characterized by large anti-cyclonic meanders flowing from the Northern Current core area to coastal zones that produce an intense mesoscale activity. Export fluxes from the euphotic zone of particulate organic carbon (P-POC) and suspended particulate matter (P-SPM) were calculated from C/(234) Th-p and SpM/(234) Thp ratios at 165 m depth and thorium fluxes. Under SS conditions, estimates ranged from 11 to 52 mgC m(-2)d(-1) and between 162 and 1244 mg m(-2) d(-1), for P-POC and P-SPM, respectively, and a general agreement was observed with HFF sediment trap results. Export ratios (ThE) varied from typical oceanic settings to early bloom conditions, and decreased after the onset of water column early summer stratification