Seasonal Influence of the Front Area on Sediment Deposition in the Northwestern Adriatic Sea
419
onshore, in the mud belt, far from the front. In
the figures, they are compared to the stations
sampled in Jun97 that were in different seasons
and with different front influences.
Northern Area
The area south of the Po Delta is characterised by
silty clay sediment mainly colonised by
Polychaetes of different species and rarely by
molluscs (Turritella, Corbula). Core stratigraphies show a different degree of bioturbation due
to the activity of the invertebrates that tend to
destroy the primary sedimentary structures represented by flat laminations. Some of the offshore
stations show a different lithology with higher
percentages of sand, up to 50%. with abundant
shell debris and layers rich in mollusc remains.
These sites are located in the transitional area
between the pelitic mud belt and the offshore
relict sands where no net deposition occurs.
The 137CS shows a pattern clearly not dependent on the frontal position (Fig. 2), with values
decreasing from 15-18 to 5-8 Bq/kg, as the distance and the water depth from the delta
increase. This behaviour confirms the predominant influence in this area of the 131 Cs supplied
by the river with respect to the atmospheric.
strictly «marine" delivery, as already recognised
by previous Authors (Albertazzi et a1.1984, 1992;
Frignani and Langone 1991). The surficial activity of excess 234Th (234Th .. ), over the background
in equilibrium with 238U, spans between 0 and
250 Bq/kg with the highest value dose to the
frontal system (summer 97. depth 32 m). In general, the activities are lower in winter than in
summer, but they are greatly influenced by the
distance from the front line. In fact, we can see in
Fig. 2 that higher activities are recorded at the
location immediately behind or on the front line
both in summer and in winter. Furthermore, if
we compare the surficial activities in the two
winter seasons, we record values of around 150170 Bq/kg in the presence of the frontal system
(Feb98). These values decrease to one half (77
Bq/kg) with the frontal system. displaced further
offshore. These results suggest a general strong
front influence on the fast deposition of suspended matter inside the frontal system, which is
also due to the shallow depth of the water column. The influence seems to be even higher in
the warm season that couples a higher concentration of suspended matter with an enhanced
surficial biological activity and a stratification of
the water column that slows down the recycling
of the particulate matter sinking to the bottom.
The 234Th .. profIles in the sediment cores show a
deeper penetration during the warm season
probably due to the enhanced biological activity
on the sea floor linked to the higher input of particulate matter and warmer temperature on the
bottom. Furthermore, if we look at the activity/depth profIles of the two radionudides (Fig.
3), we can often observe an inverse correlation
with relatively low 234Th .. activity corresponding
to high cesium activity and vice versa. The different origin and behaviour of the two radioisotopes in this area could then be used to recognize
in the sedimentary record periods of sediment
deposition with a predominant riverine versus
marine sources. In this area, in fact, 234Th is
derived by the direct decay of 238U dissolved in
the water column. On the contrary, even if small
amounts are directly derived from atmospheric
fallout, 137CS is mainly delivered to the marine
system adsorbed onto the river particulate matter. These particles derive from soil erosion of
the drainage basins, where 137CS has been widely
deposited during the Chernobyl accident in 1986
(Frignani and Langone 1991; Albertazzi et al.
1992).
The distribution of nutrients in surficial sediments, organic C and N, do not show a clear signallinked to the frontal system. However, some
patterns can be highlighted (Fig. 2). Organic
nitrogen shows generally low values, between
0.12 and 0.18% dry weight. The N content is similar in the two seasons but it shows higher percentages in shallower water in summer and lower
in winter, and it seems slightly depleted just
behind the front. The organic carbon has the
same behaviour with concentrations between
0.9-1.3% dry weight. The C/N ratio shows values
always lower than 10 with higher values in wintertime. especially close to the coast. This behaviour suggests a more important contribution of
organic riverborne material to the sediment during wintertime.
Sites A and B
Two sites were sampled in different seasons to
understand the time evolution of the sedimentation at fixed locations. Site A is located offshore
419
onshore, in the mud belt, far from the front. In
the figures, they are compared to the stations
sampled in Jun97 that were in different seasons
and with different front influences.
Northern Area
The area south of the Po Delta is characterised by
silty clay sediment mainly colonised by
Polychaetes of different species and rarely by
molluscs (Turritella, Corbula). Core stratigraphies show a different degree of bioturbation due
to the activity of the invertebrates that tend to
destroy the primary sedimentary structures represented by flat laminations. Some of the offshore
stations show a different lithology with higher
percentages of sand, up to 50%. with abundant
shell debris and layers rich in mollusc remains.
These sites are located in the transitional area
between the pelitic mud belt and the offshore
relict sands where no net deposition occurs.
The 137CS shows a pattern clearly not dependent on the frontal position (Fig. 2), with values
decreasing from 15-18 to 5-8 Bq/kg, as the distance and the water depth from the delta
increase. This behaviour confirms the predominant influence in this area of the 131 Cs supplied
by the river with respect to the atmospheric.
strictly «marine" delivery, as already recognised
by previous Authors (Albertazzi et a1.1984, 1992;
Frignani and Langone 1991). The surficial activity of excess 234Th (234Th .. ), over the background
in equilibrium with 238U, spans between 0 and
250 Bq/kg with the highest value dose to the
frontal system (summer 97. depth 32 m). In general, the activities are lower in winter than in
summer, but they are greatly influenced by the
distance from the front line. In fact, we can see in
Fig. 2 that higher activities are recorded at the
location immediately behind or on the front line
both in summer and in winter. Furthermore, if
we compare the surficial activities in the two
winter seasons, we record values of around 150170 Bq/kg in the presence of the frontal system
(Feb98). These values decrease to one half (77
Bq/kg) with the frontal system. displaced further
offshore. These results suggest a general strong
front influence on the fast deposition of suspended matter inside the frontal system, which is
also due to the shallow depth of the water column. The influence seems to be even higher in
the warm season that couples a higher concentration of suspended matter with an enhanced
surficial biological activity and a stratification of
the water column that slows down the recycling
of the particulate matter sinking to the bottom.
The 234Th .. profIles in the sediment cores show a
deeper penetration during the warm season
probably due to the enhanced biological activity
on the sea floor linked to the higher input of particulate matter and warmer temperature on the
bottom. Furthermore, if we look at the activity/depth profIles of the two radionudides (Fig.
3), we can often observe an inverse correlation
with relatively low 234Th .. activity corresponding
to high cesium activity and vice versa. The different origin and behaviour of the two radioisotopes in this area could then be used to recognize
in the sedimentary record periods of sediment
deposition with a predominant riverine versus
marine sources. In this area, in fact, 234Th is
derived by the direct decay of 238U dissolved in
the water column. On the contrary, even if small
amounts are directly derived from atmospheric
fallout, 137CS is mainly delivered to the marine
system adsorbed onto the river particulate matter. These particles derive from soil erosion of
the drainage basins, where 137CS has been widely
deposited during the Chernobyl accident in 1986
(Frignani and Langone 1991; Albertazzi et al.
1992).
The distribution of nutrients in surficial sediments, organic C and N, do not show a clear signallinked to the frontal system. However, some
patterns can be highlighted (Fig. 2). Organic
nitrogen shows generally low values, between
0.12 and 0.18% dry weight. The N content is similar in the two seasons but it shows higher percentages in shallower water in summer and lower
in winter, and it seems slightly depleted just
behind the front. The organic carbon has the
same behaviour with concentrations between
0.9-1.3% dry weight. The C/N ratio shows values
always lower than 10 with higher values in wintertime. especially close to the coast. This behaviour suggests a more important contribution of
organic riverborne material to the sediment during wintertime.
Sites A and B
Two sites were sampled in different seasons to
understand the time evolution of the sedimentation at fixed locations. Site A is located offshore