Th measured particle export from surface waters in north-western Mediterranean: comparison of spring and autumn periods

Abstract. ²³⁴Th was used to quantify the short-term variability of particle dynamics and of POC export during transition periods in surface waters over the north-western Mediterranean. As a part of DYNAPROC I and II cruises, two intensive time-series of ²³⁴Th were carried out near the DYFAMED station (43° 25´N–7°51´E) during late spring (May, 1995), when the system changes towards oligotrophy, and during autumn (October, 2004), when the stratification is disturbed by wind. Particulate fluxes derived from ²³⁴Th measured in the upper water column and in drifting sediment trap showed large differences between the two situations: the flux decreased from high to low values during late spring, at the difference of the autumnal situation where the fluxes were always low. ²³⁴Th-derived POC fluxes, calculated from the ²³⁴Th²³⁸U disequilibrium in the water column and POC/²³⁴Th ratio on trapped material, and export ratios (ThE: ratio of ²³⁴Th-derived POC export to primary production) showed a large range, from 8 to 110 mgC m⁻² d⁻¹ and 3–55%; the lowest values were observed at the end of the productive period (end May) and in automn. The ²³⁴Th-derived information is in agreement with the annual variations in Mediterranean Sea productivity. From these experiments during transition periods, it is not obvious that renewal of nutrients by wind events is strong enough to sustain significant export after the end of the productive period or to initiate significant export in autumn.