218
M. Ravaioli et al.
Benthic fauna showed a clear stratification within the sédiment column (Fig.
6). At site A a mean of 72% and at site B a mean of 76% of the total macrobenthos was concentrated in the topmost 5 cm of the core, whereas below 15-cm
depth only occasional living fauna was found (mean 1.1%). The living benthos
at depth in core is represented almost exclusively by some polychaete families,
mainly Capitellidae, Paraonidae, crustacean isopods and tanaids. At site C the
stratification is less évident and the fauna présent in the surficial layer (generally 0-2 cm) was almost as abundant (53%) as in the deeper layers (generally
2-10 cm).
5 Discussion
5.1 Accumulation Rates and Fluxes
At ail sites the 2I0Pbex inventories are higher than the expected (Table 1) by factors
of 3.5, 3 and 4.5 for sédiments A, B and C, respectively. These évidences indicate
that fine biogenic material deposited on banks is resuspended by strong currents
and settles on the bottom of basins, thus contributing to the increase of the 210Pbex
inventory in areas similar to those represented by sites A and B. The same mechanism was observed by Langone et al. [20] in other areas of the Ross Sea. This
observation also confirms the rôle that Anderson et al. [9] and Dunbar et al. [10]
hâve given to the océan circulation as an important controller of the sédiment distribution in the Ross Sea. Sédiment at site C is a residual glacial-marine sand. Due
to the action of strong bottom currents (velocities up to 90 cm s’1 [21]) the sédiment is not accumulating because it is subject to periodic winnowing. This type
of setting would exclude the presence of 210Pbex and the high inventory seems
rather unnatural. The presence of a profile is probably due to the downward
transport of fine particles, rich in 210Pbex, caused by physical mixing. We used this
profile to calculate a mixing coefficient of 0.061 cm2 year1, applying the solution
of the one-dimensional advection-diffusion équation for the case of no sédimentation [22,23],
To calculate the sédiment accumulation rates we used two different models:
the CRS model [24], which assumes 210Pbex constant rate of supply, and the CFCS model [25] based on the assumption of constant flux-constant sédimentation.
The effect of mixing has been considered in that we used, applying the CF-CS
model, the profile below the surficial mixed layer (SML). To calculate the mean
accumulation rate using the CRS model, we can consider the 90% equilibrium
Table 1. Water depth, expected and measured inventories at the three sites
Site
Water depth
Expected inventory
Measured inventory
(m)
(Bq m'2)
(Bq m’2)
A
B
C
810
2700
9505
586
1953
5833
456
1520
6938
M. Ravaioli et al.
Benthic fauna showed a clear stratification within the sédiment column (Fig.
6). At site A a mean of 72% and at site B a mean of 76% of the total macrobenthos was concentrated in the topmost 5 cm of the core, whereas below 15-cm
depth only occasional living fauna was found (mean 1.1%). The living benthos
at depth in core is represented almost exclusively by some polychaete families,
mainly Capitellidae, Paraonidae, crustacean isopods and tanaids. At site C the
stratification is less évident and the fauna présent in the surficial layer (generally 0-2 cm) was almost as abundant (53%) as in the deeper layers (generally
2-10 cm).
5 Discussion
5.1 Accumulation Rates and Fluxes
At ail sites the 2I0Pbex inventories are higher than the expected (Table 1) by factors
of 3.5, 3 and 4.5 for sédiments A, B and C, respectively. These évidences indicate
that fine biogenic material deposited on banks is resuspended by strong currents
and settles on the bottom of basins, thus contributing to the increase of the 210Pbex
inventory in areas similar to those represented by sites A and B. The same mechanism was observed by Langone et al. [20] in other areas of the Ross Sea. This
observation also confirms the rôle that Anderson et al. [9] and Dunbar et al. [10]
hâve given to the océan circulation as an important controller of the sédiment distribution in the Ross Sea. Sédiment at site C is a residual glacial-marine sand. Due
to the action of strong bottom currents (velocities up to 90 cm s’1 [21]) the sédiment is not accumulating because it is subject to periodic winnowing. This type
of setting would exclude the presence of 210Pbex and the high inventory seems
rather unnatural. The presence of a profile is probably due to the downward
transport of fine particles, rich in 210Pbex, caused by physical mixing. We used this
profile to calculate a mixing coefficient of 0.061 cm2 year1, applying the solution
of the one-dimensional advection-diffusion équation for the case of no sédimentation [22,23],
To calculate the sédiment accumulation rates we used two different models:
the CRS model [24], which assumes 210Pbex constant rate of supply, and the CFCS model [25] based on the assumption of constant flux-constant sédimentation.
The effect of mixing has been considered in that we used, applying the CF-CS
model, the profile below the surficial mixed layer (SML). To calculate the mean
accumulation rate using the CRS model, we can consider the 90% equilibrium
Table 1. Water depth, expected and measured inventories at the three sites
Site
Water depth
Expected inventory
Measured inventory
(m)
(Bq m'2)
(Bq m’2)
A
B
C
810
2700
9505
586
1953
5833
456
1520
6938