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´Nâ7°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 3â55%; 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.
Highlights
We present two time-series of 234Th for the DYFAMED station
Our objectives was to 15 compare particle dynamics and the magnitude of particulate carbon export using 234Th and POC data during two contrasted transition periods at the DYFAMED station in the north-western Mediterranean Sea
As part of the JGOFS-France DYFAMED programme, a time-series station was situated in the open central zone of the Ligurian Sea and has been regularly occupied since 1991
Summary
The main application is to examine more in detail the downward flux of particulate material out of the upper mixed layer of the ocean particle sources, in order to better understand the magnitude and the efficiency of the oceanic biologi cal pump. The first experiments on particle export, like the US JGOFS North Atlantic Bloom Experiment, were mainly focused on surface waters in spring (Buesseler et al, 1992; Schmidt et al, 1992). We present two time-series of 234Th (profiles and drifting traps) for the DYFAMED station. Our objectives was to 15 compare particle dynamics and the magnitude of particulate carbon export using 234Th and POC data during two contrasted transition periods at the DYFAMED station in the north-western Mediterranean Sea. The first times-series in May 1995 was previously published (Schmidt et al, 2002) and served as a comparison
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