Particle Fluxes and Sédiment Characteristics at Three Selected Sites
219
Table 2. Apparent mass accumulation rate and burial rates of biogenic components at sites
A and B. The values B*, calculated using both 210Pb„ and 14C data, are the most reliable
Site
Mass
accumulation rate
(mg cm'2 year'1)
Biogenic silica
burial rate
(mg cm'2 year1)
OC burial rate
(mg cm'2 year1)
N burial rate
(mg cm'2 year1)
A
89
22
0.82
0.12
B
104
27
1.13
0.16
B*
38*
9.8*
0.41*
0.06*
depth, i.e. the depth at which the 210Pbex inventory reaches 10% of its value.
Sédiment accumulation rates obtained with the two models are the same for site
B and comparable for site A. According to the CRS model apparent mass accumulation rates are 89 mg cm'2 year ’1 at site A and 104 mg cm'2 year1 at site B,
whereas apparent sédiment accumulation rates are 0.16 cm year '1 for both site A
and site B.
Multiplying the sédiment accumulation rates by the concentrations of biogenic
silica, OC and N at the interface or at depth in the sédiment we obtained apparent
accumulation rates and burial rates for these biogenic components (Table 2). The
highest values characterise site B in the Joides Basin. Labbrozzi et al. [5] discussed
the influence of deep mixing at site B by resolving the advection diffusion équation [4, 23] both for 210Pbex and 14C data. They calculated a sédiment accumulation rate of 0.059 cm year1 (38.4 mg cm'2 year'1, B* in Table 2) and a mixing rate
of 0.53 cm2 year1. This means that fluxes of biogenic species reported in Table 2
are overestimated by a factor of 2.7 for site B and that the value relative to site A
may be affected by an error of the same order.
A comparison between rain rates from sédiment traps is not yet possible,
because the only data presently available are relative to mooring A [6, 26].
Apparent fluxes and burial rates from sédiment data suggest that sites A and B
receive similar food supply, though fluxes at B appear to be a little higher.
5.2 Benthos
The analysis revealed very strong différences in the composition, density and
structure of the benthic communities at the three sites [27],
The above data evidentiate a high degree of consistency between sédiment features and benthic community composition and structure, and confirm that both
are strictly related to the different dynamics of bottom water and sedimentary
régimes. Sites A and B represent areas with high accumulation rates of biogenic
fine sédiments, stable and dominated by infaunal forms, mainly surface and subsurface deposit feeders, that exert a not negligible reworking of the sédiment at
the interface and within the first 5-10 cm of the sédiment column. The organic
input to the sédiment is relatively high with a significant component of vertical
219
Table 2. Apparent mass accumulation rate and burial rates of biogenic components at sites
A and B. The values B*, calculated using both 210Pb„ and 14C data, are the most reliable
Site
Mass
accumulation rate
(mg cm'2 year'1)
Biogenic silica
burial rate
(mg cm'2 year1)
OC burial rate
(mg cm'2 year1)
N burial rate
(mg cm'2 year1)
A
89
22
0.82
0.12
B
104
27
1.13
0.16
B*
38*
9.8*
0.41*
0.06*
depth, i.e. the depth at which the 210Pbex inventory reaches 10% of its value.
Sédiment accumulation rates obtained with the two models are the same for site
B and comparable for site A. According to the CRS model apparent mass accumulation rates are 89 mg cm'2 year ’1 at site A and 104 mg cm'2 year1 at site B,
whereas apparent sédiment accumulation rates are 0.16 cm year '1 for both site A
and site B.
Multiplying the sédiment accumulation rates by the concentrations of biogenic
silica, OC and N at the interface or at depth in the sédiment we obtained apparent
accumulation rates and burial rates for these biogenic components (Table 2). The
highest values characterise site B in the Joides Basin. Labbrozzi et al. [5] discussed
the influence of deep mixing at site B by resolving the advection diffusion équation [4, 23] both for 210Pbex and 14C data. They calculated a sédiment accumulation rate of 0.059 cm year1 (38.4 mg cm'2 year'1, B* in Table 2) and a mixing rate
of 0.53 cm2 year1. This means that fluxes of biogenic species reported in Table 2
are overestimated by a factor of 2.7 for site B and that the value relative to site A
may be affected by an error of the same order.
A comparison between rain rates from sédiment traps is not yet possible,
because the only data presently available are relative to mooring A [6, 26].
Apparent fluxes and burial rates from sédiment data suggest that sites A and B
receive similar food supply, though fluxes at B appear to be a little higher.
5.2 Benthos
The analysis revealed very strong différences in the composition, density and
structure of the benthic communities at the three sites [27],
The above data evidentiate a high degree of consistency between sédiment features and benthic community composition and structure, and confirm that both
are strictly related to the different dynamics of bottom water and sedimentary
régimes. Sites A and B represent areas with high accumulation rates of biogenic
fine sédiments, stable and dominated by infaunal forms, mainly surface and subsurface deposit feeders, that exert a not negligible reworking of the sédiment at
the interface and within the first 5-10 cm of the sédiment column. The organic
input to the sédiment is relatively high with a significant component of vertical