252
Andrew CLARKE
Sediment composition and sedimentary processes
Many of the early expeditions to the Southern Ocean
took samples of the seabed (Philippi, 1910; Pirie, 1913;
W¨ ust, 1933; Douglas and Campbell-Smith, 1930), and
142 samples were taken as part of the Discovery
investigations (Neaverson, 1934). More recent data
have come from bottom samples and photographs
acquired by USNS Eltanin, RV Conrad and USCGC
Glacier, mainly under the auspices of Operation Deep
Freeze (Bullivant, 1959, 1967; Anderson, 1990), and
also by Russian (Lisitzin, 1962, 1970) and French
(Dangeard et al., 1977) expeditions.
In common with abyssal plains elsewhere, those
around Antarctica are composed primarily of soft
sediments. They differ from sediments in most deepsea areas in two primary ways: the low temperatures
of the surface waters mean that these sediments are
primarily siliceous rather than the carbonates typical of
lower latitudes, and there is a strong influence of glacial
processes.
Close to the Antarctic continent the sediments
contain an abundant silt fraction comprised of rock
flour with coarse, poorly sorted debris, and contain
little calcite or biogenic debris. These sediments
were termed glacial-marine by the Deutsche S¨ udpolarExpedition (Philippi, 1910) and they form a wide
circumpolar band around Antarctica. Goodell et al.
(1973) proposed a more rigorous definition of glacialmarine sediments, and distinguished four more or
less concentric zones around Antarctica, distinguished
primarily on textural grounds (Fig. 8.7). In general
there is a decrease in the proportion of coarse material
with increasing distance from the continental source,
and the outermost of the four zones corresponds to the
pelagic clays of the abyssal plain. The northernmost
limit of glacial-marine sediments is related to the
0ºC surface isotherm, since this influences the rate of
iceberg melting.
In the Southern Ocean icebergs are the major mechanisms of ice-rafted transport of sediment. In contrast
to the Arctic, sea-ice is of relatively little importance
for transport of terrigenous material, largely because
the extensive development of ice-shelves effectively
precludes the capture of sediments by sea-ice. Iceshelves also greatly reduce the importance of riverine
and aeolian input to the Southern Ocean compared
with the Arctic. Nevertheless the input of sediment
from Antarctica to the southern Pacific is substantially
greater than from other nearby continents (Edwards,
1968; in Anderson, 1990).
Beyond the limits of significant ice-rafted input,
the glacial-marine sediments merge gradually into
biogenic oozes. The low temperatures of surface waters
in the Southern Ocean mean that coccolithophorids
are absent, and primary production is dominated by
diatoms. Since dilution by non-biogenic material is
almost non-existent, and the low temperature and the
depth of the abyssal plain tend to induce dissolution of
carbonates, the biogenic oozes of the Southern Ocean
are almost exclusively siliceous. The boundary between
siliceous oozes and the carbonate oozes formed in
warmer sub-polar surface waters is dictated largely by
the position of the Antarctic Polar Front, and previous
positions of the Polar Front can be inferred from the
switch between siliceous and carbonate sediments in
cores.
The lack of substantial riverine or aeolian input
means that rates of abyssal sediment accumulation
around Antarctica can be very slow, often less than
1 cm per thousand years (Osmond et al., 1971).
Vertical flux has been measured in the Southern Ocean
by the use of sediment traps (reviewed by Honjo,
1990). These measurements reveal a strong seasonal
component, with important contributions from grazing
by zooplankton as well as aggregation processes.
Almost all such measurements have, however, been
over the continental shelf, where rates of primary
production, and hence sedimentation, are considerably
greater than in the pelagic areas over the abyssal
plains.
Terrigenous material may also be delivered to the
abyssal plain by turbidity currents flowing down the
continental slope or along submarine canyons. This
process appears to have been especially important in
the Amundsen and Bellingshausen Seas, where there
are large deep-sea sediment fans (Dangeard et al.,
1977; Wright et al., 1983).
Drop-stones
Although much of the terrigenous material transported
away from Antarctica is in the form of fine to
coarse sediment, icebergs can also carry larger clasts
and boulders. Large ice-rafted boulders (drop-stones)
are important in providing isolated patches of hard
substratum on the otherwise soft abyssal plain of the
Southern Ocean. These drop-stones are released once
the icebergs encounter water warm enough to induce
Andrew CLARKE
Sediment composition and sedimentary processes
Many of the early expeditions to the Southern Ocean
took samples of the seabed (Philippi, 1910; Pirie, 1913;
W¨ ust, 1933; Douglas and Campbell-Smith, 1930), and
142 samples were taken as part of the Discovery
investigations (Neaverson, 1934). More recent data
have come from bottom samples and photographs
acquired by USNS Eltanin, RV Conrad and USCGC
Glacier, mainly under the auspices of Operation Deep
Freeze (Bullivant, 1959, 1967; Anderson, 1990), and
also by Russian (Lisitzin, 1962, 1970) and French
(Dangeard et al., 1977) expeditions.
In common with abyssal plains elsewhere, those
around Antarctica are composed primarily of soft
sediments. They differ from sediments in most deepsea areas in two primary ways: the low temperatures
of the surface waters mean that these sediments are
primarily siliceous rather than the carbonates typical of
lower latitudes, and there is a strong influence of glacial
processes.
Close to the Antarctic continent the sediments
contain an abundant silt fraction comprised of rock
flour with coarse, poorly sorted debris, and contain
little calcite or biogenic debris. These sediments
were termed glacial-marine by the Deutsche S¨ udpolarExpedition (Philippi, 1910) and they form a wide
circumpolar band around Antarctica. Goodell et al.
(1973) proposed a more rigorous definition of glacialmarine sediments, and distinguished four more or
less concentric zones around Antarctica, distinguished
primarily on textural grounds (Fig. 8.7). In general
there is a decrease in the proportion of coarse material
with increasing distance from the continental source,
and the outermost of the four zones corresponds to the
pelagic clays of the abyssal plain. The northernmost
limit of glacial-marine sediments is related to the
0ºC surface isotherm, since this influences the rate of
iceberg melting.
In the Southern Ocean icebergs are the major mechanisms of ice-rafted transport of sediment. In contrast
to the Arctic, sea-ice is of relatively little importance
for transport of terrigenous material, largely because
the extensive development of ice-shelves effectively
precludes the capture of sediments by sea-ice. Iceshelves also greatly reduce the importance of riverine
and aeolian input to the Southern Ocean compared
with the Arctic. Nevertheless the input of sediment
from Antarctica to the southern Pacific is substantially
greater than from other nearby continents (Edwards,
1968; in Anderson, 1990).
Beyond the limits of significant ice-rafted input,
the glacial-marine sediments merge gradually into
biogenic oozes. The low temperatures of surface waters
in the Southern Ocean mean that coccolithophorids
are absent, and primary production is dominated by
diatoms. Since dilution by non-biogenic material is
almost non-existent, and the low temperature and the
depth of the abyssal plain tend to induce dissolution of
carbonates, the biogenic oozes of the Southern Ocean
are almost exclusively siliceous. The boundary between
siliceous oozes and the carbonate oozes formed in
warmer sub-polar surface waters is dictated largely by
the position of the Antarctic Polar Front, and previous
positions of the Polar Front can be inferred from the
switch between siliceous and carbonate sediments in
cores.
The lack of substantial riverine or aeolian input
means that rates of abyssal sediment accumulation
around Antarctica can be very slow, often less than
1 cm per thousand years (Osmond et al., 1971).
Vertical flux has been measured in the Southern Ocean
by the use of sediment traps (reviewed by Honjo,
1990). These measurements reveal a strong seasonal
component, with important contributions from grazing
by zooplankton as well as aggregation processes.
Almost all such measurements have, however, been
over the continental shelf, where rates of primary
production, and hence sedimentation, are considerably
greater than in the pelagic areas over the abyssal
plains.
Terrigenous material may also be delivered to the
abyssal plain by turbidity currents flowing down the
continental slope or along submarine canyons. This
process appears to have been especially important in
the Amundsen and Bellingshausen Seas, where there
are large deep-sea sediment fans (Dangeard et al.,
1977; Wright et al., 1983).
Drop-stones
Although much of the terrigenous material transported
away from Antarctica is in the form of fine to
coarse sediment, icebergs can also carry larger clasts
and boulders. Large ice-rafted boulders (drop-stones)
are important in providing isolated patches of hard
substratum on the otherwise soft abyssal plain of the
Southern Ocean. These drop-stones are released once
the icebergs encounter water warm enough to induce