Temporal variations of dissolved and particulate Th-234 at a coastal station of the northern Adriatic Sea
Résumé
Water-particle interactions, particle behaviour and short-time scale variability were assessed at a coastal station adjacent to the Emilia Romagna Region (Adriatic Sea) using dissolved and particulate Th-234 analyses. The water column was sampled six times between March and September 1997. Measurements showed that Th-234 is actively scavenged by particles but the dissolved fraction is always prevalent. Changes in hydrological conditions affect to some degree thorium activities and residence times. Dissolved thorium inventories slowly increased from May to July, then decreased in August, and increased again in September. In July, the formation of a sharp pycnocline associated with low productivity led to high dissolved and very low particulate Th-234 activities due to inefficient scavenging. The presence of mucilaginous aggregates, observed in both August and September, may have played a role in scavenging of thorium. In September at 16 m depth, the highest Th-234 particulate activity of the study period was measured, probably due to the presence of mucilage. However, the thorium deficit was scarce, due to the small sinking velocity of these aggregates. Both steady-state and non-steady state models were used to calculate residence times for the whole water column and its topmost part (10 m) obtaining strictly comparable results. Residence times in the whole water column are small, ranging from 15 to 45 and from 0.5 to 24 d for dissolved and particulate thorium, respectively.