Seasonal Influence of the Front Area on Sediment Deposition in the Northwestern Adriatic Sea
417
are dominated by a cyclonic gyre which causes
an overall transport and deposition of sediment
along the Italian coast. The hydrodynamic of the
area fairly complicated and very dependent on
thermohaline factors. The general circulation of
the Northern Adriatic Sea has two different seasonal regimes (Malanotte Rizzoli and
Bergamasco 1983; Artegiani et al. 1997a, b). In
winter the currents are essentially of cyclonic
type and force southward the Italian river
plumes with a water column that is more or less
homogeneous. In summer these currents are
weaker and delimit small scale eddies, and the
water column is frequently characterized by horizontal stratification (Franco et al. 1982; Orlic et
aL 1992).
The location and the mesoscale seasonal
evolution of the front structure were the object of
collaboration between different subprojects of
Prisma 2. Satellite-derived images were used
together with field data collected during marine
surveys preceding the sampling cruises, to locate
the frontal system. The two most representative
study areas were chosen in relation to the hydrological and biological peculiarities of the front
system in the two seasons. The sampling grids
were defined taking into account the strongest
land-sea gradient of physico-chemical characteristics of the water body.
Figure 1 shows the study area and sampling
locations. These were chosen in order to: (1) spatially follow the frontal area evolution; (2) reconstruct the time evolution of sedimentary input at
fIXed locations. The two areas forming the object
of this study are one immediately south of the Po
Delta and the other the coastal area in front of
Marche Region. The northern area, located close
to the delta, was chosen to evaluate direct and
indirect effects of the frontal system produced by
the Po River plume offshore the Romagna coast,
and the southern one to study the frontal system
formed between the shallow coastal waters of
continental origin and the deeper water of the
Marche coast.
Materials and Methods
Four cruises were carried out in June 96 (Jun96),
February and June 97 (Feb97 and Jun97), and
February 98 (Feb98) onboard of the RN Urania.
Sampling (Fig. 1) was developed along transects
crossing the front line in two different seasons:
June and February were taken as representative
of a summer and a winter situation respectively.
The sampling strategy was based on the collection of two/three cores both landward and seaward with respect to the front line. During all
cruises, some other stations were sampled to
compare different sedimentological environments and in some cases replicates were taken at
the same site to evaluate the spatial variability.
During the first cruise (June 96), all 'stations were
located within the frontal system. The sediment
cores (10-45 em long) were collected using a
multicorer (mod. Midi), equipped with 5.7 and
9.5 em internal diameter Perspex tubes. All cores
were radiographed and then cut longitudinally,
photographed and the lithology was described.
For the first 3 em, 0.5 cm slices were sampled and
dried at 60°C to quantify the water content and to
calculate porosity and bulk dry density according to Berner (1971), assuming a sediment density of 2.5 g cm- 3 . Dry sediment was analysed for
grain size. and slightly desegregated for radiotracers) organic nitrogen and carbon determinations. Grain size analyses were performed by a
pre-treatment with HPz and subsequent wet
sieving at 60 microns to separate the muddy fraction. The sandy fraction was then isolated by dry
sieving at 250 micron to separate the shell debris.
The 234Th and l37Cs surficial activities were measured by gamma spectrometry (Frignani et al.
1991) and then calculated by using the
Gammaplus programme. Total organic carbon
and nitrogen were measured by means of a
Fisons CHN Elemental Analyser after a 2N HC!
treatment to eliminate the carbonate fraction.
Results and Discussion
In this paper, we present the results of radiochemical and organic matter analyses (Figs. 2, 4,
5 and 6) relative to the surface layers (0-0.5 cm),
However, in the case of 234Th , both surficial and
total inventories (first 3 em) ~e discussed. In the
figures, the stations of each transect are shown
along the horizontal axis according to water
depth (i.e. distance from the coast). Front lines
are represented in each diagram by vertical lines.
In the northern area, during the cruise Feb97, the
frontal system was located offshore, where no
recent accumulation occurs and the sediment
consists of relict sands, difficult to sample.
Therefore, at that time, most cores were collected
417
are dominated by a cyclonic gyre which causes
an overall transport and deposition of sediment
along the Italian coast. The hydrodynamic of the
area fairly complicated and very dependent on
thermohaline factors. The general circulation of
the Northern Adriatic Sea has two different seasonal regimes (Malanotte Rizzoli and
Bergamasco 1983; Artegiani et al. 1997a, b). In
winter the currents are essentially of cyclonic
type and force southward the Italian river
plumes with a water column that is more or less
homogeneous. In summer these currents are
weaker and delimit small scale eddies, and the
water column is frequently characterized by horizontal stratification (Franco et al. 1982; Orlic et
aL 1992).
The location and the mesoscale seasonal
evolution of the front structure were the object of
collaboration between different subprojects of
Prisma 2. Satellite-derived images were used
together with field data collected during marine
surveys preceding the sampling cruises, to locate
the frontal system. The two most representative
study areas were chosen in relation to the hydrological and biological peculiarities of the front
system in the two seasons. The sampling grids
were defined taking into account the strongest
land-sea gradient of physico-chemical characteristics of the water body.
Figure 1 shows the study area and sampling
locations. These were chosen in order to: (1) spatially follow the frontal area evolution; (2) reconstruct the time evolution of sedimentary input at
fIXed locations. The two areas forming the object
of this study are one immediately south of the Po
Delta and the other the coastal area in front of
Marche Region. The northern area, located close
to the delta, was chosen to evaluate direct and
indirect effects of the frontal system produced by
the Po River plume offshore the Romagna coast,
and the southern one to study the frontal system
formed between the shallow coastal waters of
continental origin and the deeper water of the
Marche coast.
Materials and Methods
Four cruises were carried out in June 96 (Jun96),
February and June 97 (Feb97 and Jun97), and
February 98 (Feb98) onboard of the RN Urania.
Sampling (Fig. 1) was developed along transects
crossing the front line in two different seasons:
June and February were taken as representative
of a summer and a winter situation respectively.
The sampling strategy was based on the collection of two/three cores both landward and seaward with respect to the front line. During all
cruises, some other stations were sampled to
compare different sedimentological environments and in some cases replicates were taken at
the same site to evaluate the spatial variability.
During the first cruise (June 96), all 'stations were
located within the frontal system. The sediment
cores (10-45 em long) were collected using a
multicorer (mod. Midi), equipped with 5.7 and
9.5 em internal diameter Perspex tubes. All cores
were radiographed and then cut longitudinally,
photographed and the lithology was described.
For the first 3 em, 0.5 cm slices were sampled and
dried at 60°C to quantify the water content and to
calculate porosity and bulk dry density according to Berner (1971), assuming a sediment density of 2.5 g cm- 3 . Dry sediment was analysed for
grain size. and slightly desegregated for radiotracers) organic nitrogen and carbon determinations. Grain size analyses were performed by a
pre-treatment with HPz and subsequent wet
sieving at 60 microns to separate the muddy fraction. The sandy fraction was then isolated by dry
sieving at 250 micron to separate the shell debris.
The 234Th and l37Cs surficial activities were measured by gamma spectrometry (Frignani et al.
1991) and then calculated by using the
Gammaplus programme. Total organic carbon
and nitrogen were measured by means of a
Fisons CHN Elemental Analyser after a 2N HC!
treatment to eliminate the carbonate fraction.
Results and Discussion
In this paper, we present the results of radiochemical and organic matter analyses (Figs. 2, 4,
5 and 6) relative to the surface layers (0-0.5 cm),
However, in the case of 234Th , both surficial and
total inventories (first 3 em) ~e discussed. In the
figures, the stations of each transect are shown
along the horizontal axis according to water
depth (i.e. distance from the coast). Front lines
are represented in each diagram by vertical lines.
In the northern area, during the cruise Feb97, the
frontal system was located offshore, where no
recent accumulation occurs and the sediment
consists of relict sands, difficult to sample.
Therefore, at that time, most cores were collected