Distribution of manganese nodules
with depth in sediments of the
South Georgia Basin, Falkland
(Malvinas) Plateau and adjacent
areas
GEOFFREY P. GLASBY
ment surface, and there is no evidence that these nodules are
associated with sediment unconformities.
Similar results were obtained during ARA Islas Orcadas
cruise 11 (Kaharoeddin, 1978; Sclater et al., 1977), during
which 6 more cores (cores 16, 39, 71, 74, 76, 85), containing a
total of 21 horizons of nodule occurrences, were recovered to
the east of the region described earlier during a transect from
Buenos Aires to Cape Town. In each case, the nodules were
associated with diatomaceous sediment reflecting the
southerly latitudes of the stations and again were randomly
distributed with depth down each core, in the range 4-706
70W60o50
New Zealand Oceanographic Institute, DSIR
P. 0. Box 12-346
Wellington North, New Zealand
".ww
DENNIS S. CASSIDY
Antarctic Research Facility
Department of Geology
Florida State University
Tallahassee, Florida 32306
Although there are considerable statistical problems in
studying the distribution of manganese nodules with depth in
the sediment column (Glasby, 1978; Glasby and Read, 1976),
the consensus is that deep-sea nodules are found predomi nantly at the sediment-water interface, with a fairly regular
distribution below that depth (at least in the upper few meters
of sediment). This is well illustrated in histograms of the
depth distribution of nodules in sediment cores (Cronan and
Tooms, 1967; Goodell et al., 1971; Horn et al., 1972). A more
extensive survey of nodule distribution in 370 Deep Sea Drilling Project (DSDP) cores has revealed that 42 percent of
nodules are present in Pleistocene sediments (Glasby, 1978).
During ARA Islas Orcadas cruise 7 (Cassidy et al., 1977;
Ciesielski and Wise, 1977b; Warnke et al., 1976), 14 piston
cores containing manganese nodules were recovered from the
South Georgia Basin, Falkland (Malvinas) Plateau and adjacent areas (figure 1). Nodule occurrences (see figure 2 for
further discussion concerning nodules and nodule occurrences) in one of these cores (PC 0775-17) are at 16 different
horizons. A histogram of the depth distribution of nodules in
these cores (figure 2) reveals that of the 69 nodule occurrences only 22 (32 percent) are in the upper 1 meter of sediment; and of these only 5 (core PC 0775-50, 4 occurrences;
core PC 0775-56, 1 occurrence) could be considered to be
from the sediment surface. Only 7 percent of the nodules
recovered, therefore, occur at the sediment surface in cores
up to 17 meters in length. The nodules predominantly are
associated with siliceous oozes (68 percent) and red clay (26
percent), with a much smaller proportion occurring in
glauconitic sediment (6 percent). According to Tarney and
Donnellan (1976), there are considerable variations in
lithology and geochemistry (related in part to bathymetry) of
sediments throughout the area.
This study indicates not only the high abundance of
nodules in the South Georgia Basin-Falkland (Malvinas)
Plateau region (approximately five nodule occurrences per
core in cores in which nodules occur), but also their
widespread distribution with depth in the sediment core.
Clearly, the nodules do not occur predominantly at the sedi110
Figure 1. Location of piston cores containing manganese
nodules collected during ARA Islas Orcadas cruise 7. (Depth
contours in fathoms.)
I
Depth of nodules in cores (cm)
Figure 2. Histogram of the depth distribution of manganese
nodules In piston cores collected during ARA Islas Orcadas
cruise 7. Note: In the core descriptions (Cassidy et al., 1977),
the occurrence of manganese nodules is represented In the
lithologic log by the circled symbol, Mn. The symbol represents either a single large nodule or a cluster of small
nodules that are macroscopically observable. Small nodules
not seen may be burled In the sediment. Use of the available
data in this manner does not affect the findings In this paper.
Research that will provide more exact count and sizes of
nodules has begun.
ANTARCTIC JOURNAL
centimeters. Dating of the basal sediments of these cores in
which nodules occur (Ciesielski et at., 1978) reveals the base
of the cores to be late Miocene to Quaternary. Age-date determinations for basal sediments of the nodule-bearing cruise 7
cores (Ciesielski and Wise, 1977a) range from Eocene to
Quaternary, with the nodule occurrence horizons ranging
from the core tops to a depth of 1,039 centimeters.
In a previous paper, Glasby (1978) states that "only DSDP
in
the Argentine Basin
hole 328 taken at a depth of 5,105
has the sort of distribution of nodules in the sediment column
from Oligocene to Pleistocene in age that one would expect if
nodules had been forming at their present level of abundance
throughout the world's ocean since the Mesozoic." This
survey strongly confirms Glasby's suggestion that the formation of abundant nodules on the seafloor is related to the
onset of high bottom-current velocities, which can scour away
sediment, creating low sedimentation conditions favorable for
nodule growth (Barker et at., 1976; Ciesielski and Wise,
1977b). According to Glasby, the strong bottom currents have
been operating in this region over a much longer time than in
the Pacific because of the early opening of the Drake Passage.
The central role of the Drake Passage in establishing high
bottom-current velocities (and therefore high nodule abundances) in the vicinity of the Argentine Basin and South
Georgia Basin over a long geological time-scale is therefore
emphasized. The importance of these bottom currents in controlling the distribution of manganese nodules and crusts in
the Verna Channel to the north recently has been emphasized
by Debrabant and Maillot (1978), Ledbetter et at. (1978), and
Melguen et at. (in press).
Core descriptions were partially supported by National
Science Foundation contract C-1059.
References
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Modern zoogeography of antarctic
planktonic microfossils
J . P. KENNETT
Graduate School of Oceanography
University of Rhode Island
Kingston, Rhode Island 02881
B iogeograph ically the southern ocean, particularly those
antarctic waters south of the Antarctic Convergence, is one of
the most highly distinctive marine ecosystems on earth: it is
one of the most biologically productive marine systems and it
possesses a distinctive endemic assemblage. I have reviewed
southern ocean distributions of the microfossil zooplankton
groups as a whole, namely the planktonic foraminifera,
radiolaria, and the pteropods (Kennett, in press). Several
contributions have described and discussed the southern
ocean distribution patterns of each of these microplankton
111