Seasonal Influence of the Front Area on Sediment Deposition in the Northwestern Adriatic Sea
423
ic matter to the sediment instead of a general
diminishing trend as shown by cesium, organic C
and N. The high activity of thorium during the
warm season could be due to the closest proximity of the frontal system to this location and/or to
the quality of the scavenging material to which
thorium adsorbs.
Site B shows two summer situations and a
winter one (Fig. 5). The 137CS seasonal surfical
activities follow the same trend as at site A. but
with the expected lower surficial values (10-13
Bq/kg) due to the increased distance from the Po
Delta. The 234Th shows different surficial activities (105-187 Bqjkg) in the three seasons, with a
minimum in summer 1997. This behaviour is
opposite to that of site A. But for that location we
do not have the summer 1996 situation. The sediment depth reached by the base of the thorium
profile is higher in both summers (1.5 cm) than
in winter (0.5 em), thus suggesting that the mixing mechanisms are more intense during the
warm season. Excess thorium inventories and
activities show a strong decrease during the period considered and in particular between the two
summer seasons. These results suggest that an
important deposition of particulate matter
occurred in summer 1996 that was much more
productive than summer 1997. This difference
was probably favoured by a higher river runoff as
suggested by the discharge curve of the Po River
recorded at Pontelagoscuro (M.G. Tomasino, personal communication) that supplied more new
organic and inorganic matter to the frontal system (Tomasino 1996). The higher surficial values
of mes, organic carbon and C/N ratio (Fig. 5)
support this hypothesis. The opposite trend,
shown by the organic nitrogen, with percentages
slightly increasing from 0.128 to 0.150, confirms
the higher river contribution of the organic matter during 1996. The 234yh xs values recorded in
summer 1997 suggest a lower input of particles
to the sea floor with respect to 1996 values, probably as a consequence of low river discharge,
reduced productivity and biological activity in
the overlying water.
Southern Area
The lithology of the sediment cores in front of
the Marche Region is characterised by an alternance of silt and day. Together with the presence
of thanatocoenoses, this pattern reflects pulsed
sedimentation with colonisation stages and sudden inputs of sediment that bury the benthic
fauna. The shallower stations show a dear influence of the sandy input due to wave resuspension
of the very dose sandy shores during storm
events, as observed during the winter cruises.
The storm events can be recognized in the sedimentary record until a water depth of 18-20 m.
They are then progressively mixed up by bioturbation with finer sediment in the sedimentary
column.
Unfortunately. we had some analytical problems with two stations of the summer transect
concerning thorium quantification. Thus, the
comparison between the different seasons can
not be very reliable. However, some considerations about thorium behaviour are still possible.
High activities were found at the stations close to
the front (Fig. 6), where the amount of particulate material is high. Behind the front line, thorium activities are lower, due to both the shallow
environment and the sediment grain size. The
maximum values of 234yh activities. characteris'"
tic of the two distal stations, reflect the high
depth of the water column. the enhanced residence time of particles and the very fine grain
size of the sedimentary material. Furthermore,
the high inventories in deep water sediments,
recorded especially in winter, suggest a possible
shift of sediment offshore, due to the enhanced
winter water dynamics (Artegiani et al. 199730 b),
leading to phenomena of resuspension and redeposition on the sea floor. The higher summer
inventories of thorium underneath the front system seem to confirm the presence of lower water
dynamic on the bottom, favoured also by water
column stratification, that permits the vertical
deposition of the suspended material. In this
area, the 137CS distribution in surficial samples
shows a better correlation with the frontal system (Fig. 6), probably related to the fine particle
dynamics and to the lack of an important river
input. Nevertheless, we can observe a decrease of
the activities during the study period that suggests a decreasing riverine discharge. The nutrient behaviour seems to be mainly influenced by
the lithology of the sea floor with lower values
close to the coast and higher values corresponding to the muddy wedge. However, it could be
also an effect of the depletion of nutrients
behind the front line due to the enhanced benthic activity induced by the frontal system itself.
Nutrient concentrations in the three seasons
423
ic matter to the sediment instead of a general
diminishing trend as shown by cesium, organic C
and N. The high activity of thorium during the
warm season could be due to the closest proximity of the frontal system to this location and/or to
the quality of the scavenging material to which
thorium adsorbs.
Site B shows two summer situations and a
winter one (Fig. 5). The 137CS seasonal surfical
activities follow the same trend as at site A. but
with the expected lower surficial values (10-13
Bq/kg) due to the increased distance from the Po
Delta. The 234Th shows different surficial activities (105-187 Bqjkg) in the three seasons, with a
minimum in summer 1997. This behaviour is
opposite to that of site A. But for that location we
do not have the summer 1996 situation. The sediment depth reached by the base of the thorium
profile is higher in both summers (1.5 cm) than
in winter (0.5 em), thus suggesting that the mixing mechanisms are more intense during the
warm season. Excess thorium inventories and
activities show a strong decrease during the period considered and in particular between the two
summer seasons. These results suggest that an
important deposition of particulate matter
occurred in summer 1996 that was much more
productive than summer 1997. This difference
was probably favoured by a higher river runoff as
suggested by the discharge curve of the Po River
recorded at Pontelagoscuro (M.G. Tomasino, personal communication) that supplied more new
organic and inorganic matter to the frontal system (Tomasino 1996). The higher surficial values
of mes, organic carbon and C/N ratio (Fig. 5)
support this hypothesis. The opposite trend,
shown by the organic nitrogen, with percentages
slightly increasing from 0.128 to 0.150, confirms
the higher river contribution of the organic matter during 1996. The 234yh xs values recorded in
summer 1997 suggest a lower input of particles
to the sea floor with respect to 1996 values, probably as a consequence of low river discharge,
reduced productivity and biological activity in
the overlying water.
Southern Area
The lithology of the sediment cores in front of
the Marche Region is characterised by an alternance of silt and day. Together with the presence
of thanatocoenoses, this pattern reflects pulsed
sedimentation with colonisation stages and sudden inputs of sediment that bury the benthic
fauna. The shallower stations show a dear influence of the sandy input due to wave resuspension
of the very dose sandy shores during storm
events, as observed during the winter cruises.
The storm events can be recognized in the sedimentary record until a water depth of 18-20 m.
They are then progressively mixed up by bioturbation with finer sediment in the sedimentary
column.
Unfortunately. we had some analytical problems with two stations of the summer transect
concerning thorium quantification. Thus, the
comparison between the different seasons can
not be very reliable. However, some considerations about thorium behaviour are still possible.
High activities were found at the stations close to
the front (Fig. 6), where the amount of particulate material is high. Behind the front line, thorium activities are lower, due to both the shallow
environment and the sediment grain size. The
maximum values of 234yh activities. characteris'"
tic of the two distal stations, reflect the high
depth of the water column. the enhanced residence time of particles and the very fine grain
size of the sedimentary material. Furthermore,
the high inventories in deep water sediments,
recorded especially in winter, suggest a possible
shift of sediment offshore, due to the enhanced
winter water dynamics (Artegiani et al. 199730 b),
leading to phenomena of resuspension and redeposition on the sea floor. The higher summer
inventories of thorium underneath the front system seem to confirm the presence of lower water
dynamic on the bottom, favoured also by water
column stratification, that permits the vertical
deposition of the suspended material. In this
area, the 137CS distribution in surficial samples
shows a better correlation with the frontal system (Fig. 6), probably related to the fine particle
dynamics and to the lack of an important river
input. Nevertheless, we can observe a decrease of
the activities during the study period that suggests a decreasing riverine discharge. The nutrient behaviour seems to be mainly influenced by
the lithology of the sea floor with lower values
close to the coast and higher values corresponding to the muddy wedge. However, it could be
also an effect of the depletion of nutrients
behind the front line due to the enhanced benthic activity induced by the frontal system itself.
Nutrient concentrations in the three seasons