Particle Fluxes at the Edge of the Ross Ice Shelf: the Rôle of Physical Forcing
187
marked decrease of fluxes has already been observed at Bransfield Strait during
the period February-October [25], in the Weddell Sea during May-December [26]
and under the fast ice in Lützow-Holm Bay during May-October [27]. Enhanced
summer biogenic sédiment flux has, moreover, been recorded in various sites of
the Southern Ross Sea [2] and can be considered as a typical feature of Antarctic
settings.
Unfortunately flux measurements hâve been made at only a limited number of
locations throughout the Ross Sea and few data are reported on year-round particle fluxes in this area [2, 4]. Our data are comparable with those observed in a
site located at 74°S, 175°E (annual averaged flux at 200 m-depth = 11.2 mg
m'2 day1; Langone, pers. comm.), and with data from various sites in McMurdo
Sound [24] and elsewhere in the Antarctic Océan [25, 28-30], but appear lower
than fluxes recorded by sédiment traps in other Antarctic areas [2, 4, 24 in the
Ross Sea; 27, 31-33].
Particulate carbon daily flux estimated on an annual basis is in the range of
that recorded in the northwestern Ross Sea at 433 m [4] and, on shorter time
scales, of those reported for some sites in western McMurdo Sound [24]. Biogenic
silica percentage contribution to total mass flux calculated for our samples is
lower than in other sites of the Ross Sea, even if sédiment trap samples relatively
depleted (13-47%) in biogenic silica (and with higher than usual proportions of
organic carbon and calcium carbonate) hâve already been observed in the southcentral Ross Sea (76°30'.336 S, 174°59'.128W) [34].
The percentage of total flux represented by opal in our samples agréés with
those calculated for sédiment trap samples from the Bransfield Strait [25] even if
fluxes expressed in milligrams per square métré per day are lower.
3.3 Transmissometer Record
Owing to the malfunctioning of the stepping motor, no samples are available from
the 230-m sédiment trap, so that transmissometer data (Fig. 7) are the only record
we hâve of particulate material concentrations in the layer above the 423-m trap.
From the moment of deployment (January 27, 1995) to the end of August, background values are around 20% of light atténuation, with a significant number of
records between 45 and 18% and the highest peaks of particle concentration (up
to 84% of light atténuation) occurring in the last week of March. From September
through early December the minimum turbidity levels are recorded, with light
transmission reaching and maintaining values > 95% through the whole period,
while turbidity abruptly increases on June 12, 1995 (64% of light transmission).
After this date and until the end of the recording period (January 21,1996) light
transmission measurements show trend and percentage values similar to those
observed in the period previous to the maximum light transmission interval, with
peaks of up to 50%.
For easier comparison with the sédiment trap collection data we can divide
the transmissometer record into three periods: period A, from the beginning of
the experiment through August, during which significant particle concentrations are recorded at 230 m, with amounts constantly decreasing from the start
to the end of the considered time interval; period B, from September to early
187
marked decrease of fluxes has already been observed at Bransfield Strait during
the period February-October [25], in the Weddell Sea during May-December [26]
and under the fast ice in Lützow-Holm Bay during May-October [27]. Enhanced
summer biogenic sédiment flux has, moreover, been recorded in various sites of
the Southern Ross Sea [2] and can be considered as a typical feature of Antarctic
settings.
Unfortunately flux measurements hâve been made at only a limited number of
locations throughout the Ross Sea and few data are reported on year-round particle fluxes in this area [2, 4]. Our data are comparable with those observed in a
site located at 74°S, 175°E (annual averaged flux at 200 m-depth = 11.2 mg
m'2 day1; Langone, pers. comm.), and with data from various sites in McMurdo
Sound [24] and elsewhere in the Antarctic Océan [25, 28-30], but appear lower
than fluxes recorded by sédiment traps in other Antarctic areas [2, 4, 24 in the
Ross Sea; 27, 31-33].
Particulate carbon daily flux estimated on an annual basis is in the range of
that recorded in the northwestern Ross Sea at 433 m [4] and, on shorter time
scales, of those reported for some sites in western McMurdo Sound [24]. Biogenic
silica percentage contribution to total mass flux calculated for our samples is
lower than in other sites of the Ross Sea, even if sédiment trap samples relatively
depleted (13-47%) in biogenic silica (and with higher than usual proportions of
organic carbon and calcium carbonate) hâve already been observed in the southcentral Ross Sea (76°30'.336 S, 174°59'.128W) [34].
The percentage of total flux represented by opal in our samples agréés with
those calculated for sédiment trap samples from the Bransfield Strait [25] even if
fluxes expressed in milligrams per square métré per day are lower.
3.3 Transmissometer Record
Owing to the malfunctioning of the stepping motor, no samples are available from
the 230-m sédiment trap, so that transmissometer data (Fig. 7) are the only record
we hâve of particulate material concentrations in the layer above the 423-m trap.
From the moment of deployment (January 27, 1995) to the end of August, background values are around 20% of light atténuation, with a significant number of
records between 45 and 18% and the highest peaks of particle concentration (up
to 84% of light atténuation) occurring in the last week of March. From September
through early December the minimum turbidity levels are recorded, with light
transmission reaching and maintaining values > 95% through the whole period,
while turbidity abruptly increases on June 12, 1995 (64% of light transmission).
After this date and until the end of the recording period (January 21,1996) light
transmission measurements show trend and percentage values similar to those
observed in the period previous to the maximum light transmission interval, with
peaks of up to 50%.
For easier comparison with the sédiment trap collection data we can divide
the transmissometer record into three periods: period A, from the beginning of
the experiment through August, during which significant particle concentrations are recorded at 230 m, with amounts constantly decreasing from the start
to the end of the considered time interval; period B, from September to early