Polarforsch1992 2
Polarforschung 62 (2/3): 129-144, 1992 (erschienen 1994)
Recent and Subrecent Marine Sediments of the N orth- Western
Weddell Sea and the Bransfield Strait, Antarctica
By Georg TrolIt**, Dietmar Matthies*, Alfons Hofstetter** and Wolfgang Skeries***
Summary: The raw material for these investigations are sampies from marine
(sub)smface sediments around the northern part of the Antarctic Peninsula. They
had been sampled in the years 1981 to 1986 during several expeditions of the
research vessels Meteor, Polarstern and Walther Herwig,
83 box core, gravity core and dredge sampies from the area of the Bransfield
Strait, the Powell Basin and the northern Weddell Sea have been examined for
their grain-size distribution, their mineralogieal and petrographieal composition.
Silt prevails and its clay proportions exceed 25% wt, in water depths greater than
2000 m. The granulometrical results reveal some typical sedimentation processes
within the area of investigation. While turbiditic processes together with
sediment input from melting icebergs control the sedimentation in the Weddell
Sea, the South Orkney Island Plateau and the Po weil Basin, the fine grained
material from Bransfield Strait mainly relies on marine currents in the shelf area.
In addition, the direct sediment input of coarse shelf sediments from the
Bransfield Strait into the Powell Basin through submarine canyons eould be
proven. Variations in the grain-size eomposition with sedirnent depth are smalI.
The mineral composition of the clay and fine silt fractions is quite uniform in
all sarnples. There are (in decreasing order): illite, montmorillonite, chlorite,
smectite, mi xe d-Iayers, as weil as detrital quartz and feldspars. A
petrographically based sediment stratigraphy can be established in using the
considerable changes in the chlorite- and Ca-plagioclase portions in samples
from Core 224. For this sedimentation area a mean sedimentation rate of 7 cm
/I 000 a is assumed. Remarkable changes in the portions of amorphous silica
components - diatom skeletons and volcanic glass shards - appear all over the
area of investigation. They contribute between 4-83 % to the clay and fine silt
fraction.
Several provinces according to the heavy mineral assemblages in the fine sand
fraction can be distinguished: (i) a province remarkably influenced by minerals
of volcanic origin south and north of the South Shetland Islands; (ii) a small strip
with sediment dominated by plutonie material along the western coast of the
Antarctic Peninsula and (iii) a sediment eontrolled by metamorphie minerals and
rock fragments in the area of the Weddell Sea and Elephant Island.
While taking the whole grain-size spectrum into account a more comprehensive
interpretation can be given: the accessoric but distinct appearance of tourmaline,
rutile and zircon in the heavy mineral assembly along the northwestern coast
of the Antarctic Peninsula is in agreement with the occurrence of acid volcanic
rock pieces in the coarse fraction of the ice load detritus in this region, In the
vicinity of the South Shetland Islands chlorite appears in remarkable portions
in the clay fraction in combination with leucoxene, sphene and olivine, and
pumice as weil as pyroclastic rocks in the medium and coarse grain fractions,
respectively. Amphiboles and arnphibole-schists are dominant on the South
Orkney Island Plateau. In the sediments of the northwestern Weddell Sea the
heavy mineral phases of red spinei, gamet, kyanite and sillimanite in connection
with medium to highgrade metamorphie rocks especially granulitic gneisses, are
more abundant. A good conformity between the ice rafted rock sampIes and the
rocks in the island outcrops could be proven, especially in the vicinity of offshore
islands nearby. On the continent enrichments of rock societies and groups appear
in spacious outlines: acid effusive rocks in the west of the ice divide on the
Antarctic Peninsula, clastic sedimentites at the tip of the Antarctic Peninsula and
granoblastic gneis ses in central and eastern Antarctica.
Dr. DietmarMatthies, Lehrstuhlfür forstlicheArbeitswissenschaften und angewandte
Informatik,Hohenbachemstrasse 22, D-85354 Freising.
** Prof. Dr.Georg Troll(deceased 5'" September1991)and Dr.AlfonsHofstetter, Institut
für Mineralogie u. Petrographie der Universität, Theresienstr. 41, D-80333 München.
Wolfgang Skeries, Institut für optische Gesteinsbestimmung, Richard-StraußStr. 89, D-81679 Müncheu.
Manuscriptreceived 31 March 1993; accepted 31 January 1994
H* Dr.
Coarse grain detritus with more than I cm of diameter must have been rafted
by icebergs. These rock fragments are classified as rock types, groups and
societies. The spacial distribution of their statistically determined weight
relations evidently shows the paths of the iceberg drift and in nexus with already
known iceberg routes also point to the possible areas of provenance, provided
that the density of sampie locations and the number of rock pieces are sufficient.
Zusammenfassung: Gegenstand dieser Untersuchungen waren marine
Oberflächensedimentproben vom nördlichen Teil der Antarktischen Halbinsel.
Sie wurden während mehrerer Expeditionen der Forschungsschiffe Meteor;
Polarstern und Walther Herwig in den Jahren von 1981 bis 1986 gewonnen.
83 Kastengreifer-, Schwerelot- und Dredgeproben aus dem Gebiet der Bransfield-Straße, dem Powell-Becken und dem nördlichen Weddellmeer wurden
hinsichtlich ihrer Korngrößenzusammensetzung, ihrer mineralogischen und
petrographischen Bestandteile untersucht. Es handelt sich überwiegend um Silte,
deren Tonanteile in Wassertiefen größer als 2000 m auf über 25 Gew.-% ansteigen. Anhand der granulometrisehen Befunde konnten einige typische
Sedimentationsprozesse identifiziert werden. Der Sedimenteintrag in das
Weddellmeer, dem South-Orkney-Inselplateau und dem Powell-Becken geschieht überwiegend durch Turbidite und abschmelzende Eisberge. Feinkörniges Material in der Bransfield-Straße wurde durch Meeresströmungen im
Schelfgebiet eingetragen. Darüber hinaus konnte der direkte Sedimenteintrag
aus der Bransfield-Straße in das Powell-Becken über submarine Canyons nachgewiesen werden.
Die mineralogische Zusammensetzung der Ton- und Feinsiltfraktion aller Proben ist sehr einheitlich. Es handelt sich in abnehmender Reihenfolge um Illit,
Montmorillonit, Chlorit, Smektit, Mixed Layers, sowie Quarz und Feldspat.
Aufgrund deutlich unterschiedlicher Anteile von Chlorit und Ca-Plagioklas kann
eine sedimentstratigraphische Unterteilung des Kemes 224 getroffen werden.
Für dieses Sedimentationsgebiet wird eine mittlere Sedimentationsrate von 7 crnJ
1000 Jahre angenommen. Amorphe Kieselsäurekomponenten wie
Diatomeenskelette und vulkanische Gesteinsgläser machen zwischen 4 und 83
% der Ton- und Feinsiltfraktion aus.
Aufgrund der Schwermineralvergesellschaftungen in der Feinsandfraktion können mehrere Provinzen ausgeschieden werden; (l) südlich und nördlich der
South-Shetland-Inseln eine Vergesellschaftung überwiegend vulkanischen Ursprungs; (2) ein schmaler Streifen plutonisch dornierten Materials entlang der
westlichen Küste der Antarktischen Halbinsel und schließlich (3) überwiegend
metamorphe Minerale im Gebiet der Elephant-Insel und des Weddellmeeres.
Unter Berücksichtigung des gesamten Komgrößenspektrums kann eine umfassende Interpretation erfolgen. Das akzessorische, jedoch deutliche Auftreten von
Turmalin, Rutil und Zirkon in der Schwermineralfraktion entlang der nordwestlichen Küste der antarktischen Halbinsel steht im Einklang mit der Präsenz von
saueren vulkanischen Gesteinsbruchstücken in der Grobkomfraktion des Eisfrachtdetritus in dieser Region. In der Nähe der South-Shetland-Inseln erscheint
in bemerkenswertem Umfange Chlorit in der Tonfraktion in Verbindung mit
Leukoxen, Olivin und Bimsstein sowie pyroklastischen Gesteinen in der Mittel- und Grobkomfraktion. Amphibole und Amphibolschiefer herrschen auf dem
South-Orkney-Island-Plateau vor, In den Sedimenten des nordwestlichen
Weddellmeeres treten häufig die Schwerminerale roter Spinell, Granat, Kyanit
und Sillimanit in Verbindung mit mittel- bis hochmetamorphen Gesteinen, insbesondere granulitisehen Gneisen, auf. Desweiteren besteht eine gute Übereinstimmung zwischen den Gesteinsaufschlüssen auf den Inseln und den eistransportierten Gesteinsproben.
129
Gesteinsfragmente von mehr als I cm Größe, die nur in der Sedimentfracht von
Eisbergen transportiert sein können, wurden in Gesteinstypen, -gruppen und
-gesellschaften unterteilt. Ihre räumliche Verteilung gibt eindeutige Hinweise
auf Eisbergdriftwege und erlaubt im Zusammenhang mit bereits bekannten
Eisbergstraßen Rückschlüsse auf mögliche Liefergebiete.
INTRODUCTION
In modern geosciences Arctic as weIl as Antarctic areas serve
for the investigation of short and long-term changes in the global meteorological history. This mainly relies on the preserving
effect of the polar ice caps and the lack of major anthropogenie
impacts. Beside glacial records also limnetic and marine sediments hold important information, which can help to enlight the
meteorology of the past. The detailed knowledge of the mineralogical composition of the clay and fine silt fraction in space
(lateral) and time (vertical) forms the basis for further interpretations concerning the climatical and sedimentological history
of the Antarctic (GROBE 1986).
This paper gives a comprehensive petrological description of the
marine sediments of the northwestern Weddell Sea, the Powell
Basin, South Orkney Island Plateau and the Bransfield Strait,
respectively. While looking to the total grain-size spectra and
the petrological composition, which are in close relationship to
each other, a reconstruction of the sedimentation conditions as
70
60
well as the stratigraphieal position of the sediment samples becomes possible. Moreover, some aspects ofthe geological setting of the hinterland can be deduced from the coarse grain and
heavy mineral fractions. This is of utmost importance because
of the lack of outcrops in this region. The discussion about the
age of the sediment core sampies from the Bransfield Strait
should contribute to a better understanding of the most recent
climatic and glacial history in this region.
The area of investigation (Fig. 1) extends from 57° to 66° latitude south and 38° to 67° longitude west, and covers an area of
about 2,000,000 km", The sea floor topography is quite well
known and will, therefore, be described only very briefly. From
a geographical point of view the investigated area can be subdivided into three sub-areas, of which the Bransfield Strait forms
the southern part, the Powell Basin the middle, and the Weddell Sea the northern part. The Bransfield Strait has an extension
of about 1000 km and a width between 100 and 150 km. Several
elongated basins subdivide the sea floor. Powell Basin follows
to the East and is a structure which opens towards the Weddell
Sea, while it is limited in the North by a submarine Andine orogenie belt.
The area of investigation is documented by 83 sampling stations,
which cover the shelf as well as the deep-sea area. The sampies
40
50
•
60
60
65
65
pe
pe
70
60
50
o
km
500
40
Fig. 1: Geographical distribution of all sampIes investigated. Dashed line indicates 1000 m bathymetric contourline.
Abb. 1: Verteilung der Probenstationen. Gestrichelte Linie beschreibt die 1000 m Tiefenlinie.
130
were delivered during the cruises of the research vessels Meteor, Polarstern and Walther Herwig, The geographical positions
of the stations, the water depths and the sampling techniques are
reported elsewhere. Fig. I shows the geographical distribution
of the stations.
Fig. 2 depicts the drift of larger icebergs (>60 krn-) within this
area. Data from satellite images (Naval Polar Oceanographic
Center) of the years from 1977 to 1986 form its basis. It is
assumed that the marine catchment area for the icebergs and
their ice rafted detritus lies within 102 0 Wand 0 0 • The terrestrial
catchment area for the ice rafted detritus which envelopes the
Weddell Sea, has an areal extension of about 3,500,000 km-. The
knowledge of these drift paths is of great significance for the
discussion of sedimentation processes and transport paths in
marine Antarctic regions. Information on the drift paths are
given by HOEBER et al. (1987) and LAxoN et al. (1992).
Grain-sire analysis
In order to separate the grain-size fractions greater than 0.063
mm the sampIes were wet sieved and fractionated into the grainsize fractions 0.063-0.2 mm (fine sand), 0.2-0.63 mm (sand),
0.63-2 mm (coarse sand), and 2-6.3 mm (fine gravel). The
ATTERBERG sedimentation method was used for the fractions
smaller than 0.063 mm. Koagulation phenomena were eliminated by the use of Hp dist. A comparative study with a sedigraph
(GROBE 1986) revealed a good agreement with our results obtained by the sedimentation method.
Heavy mineral analysis
MATERIAL AND METHODS
For the heavy mineral analysis the sampIes were wet sieved
through a 0.1 mm screen to separate the fine sand (0.-0.2 mm)
and very fine sand (0.63-0.1 mm) fractions present in each
sampIe.
All sampIes were taken during several cruises of RV Meteor (M
56/3,1981), RV Polarstern (ANT-II13, 1983; ANT IIII2,1984;
ANT III/3,1985; ANT IVI2, 1985), and RV Walter Herwig
(Cruise 68/2,1985). Fine and medium grained material derived
from gravity and box cores, while the coarse detritus was
sampled by means of dredges and box cores, The sampIes were
transported and stared in a cold store.
The very fine sand fraction and the fine sand fraction were chosen as the size intervals to be investigated because they commonly contained the highest percentage of heavy minerals, and
they do not cause any optical problems. The two fractions were
split into heavy and light mineral assemblages by the use of an
aqueous sodium poly-tungstenate solution (3 Na ZW04 x 9 W0
3
X Hp, density 2.95 g/cm'). The heavy fractions of all sampIes
70
60
50
40
60
60
6S
......
\
65
~.""
\1
pe
70
60
50
km
500
pe
40
Fig. 2: Drift paths of larger icebergs (>60 km') in the years between 1977 and 1986 based on satellite data from Naval Polar Oceanographic Center.
Abb. 2: Driftwege größerer Eisberge (>60 km') nach Satellitenaufnahmen des Naval Polar Oceanographic Center aus den Jahren 1977 bis 1986.
131
were mounted on slides using Lakeside (n = 1.54). A polarisation microscope was used to identify and count 200 heavy minerals in each of the two fine sand-size fractions of each
sample.
According to BOENIGK (1983) the reliability of mineral percentages based on a count of 400 grains/sample should vary from
1.7-5.0 %, depending on the percentage of a given mineral constituent in the sample,
Dropstone analysis
The qualitative determination of most of the dropstone specimen is ensured by their megascopic features. Carbonatic rocks
were separated into calcitic and dolomitic types by means of
diluted acid (2-5 %). Some magmatic specimens were stained
selectively for potassium-feldspar. The relative amount of
minerals from some magmatic rocks was determined by point
counting under the microscope.
All quantitative data reparted on rock groups and societies are
given in weigth percentages. Sampling locations in close vicinity to each other (offshore <50 km, in the open sea <150 km)
are treated as one sample,
Petrology
The mineralogical composition of the clay fraction was determined by an X-ray diffractometer (PHILIPS, Cu ka, 40 kV and
20 mA). The detrital minerals in the coarser fraction were microscopicallyexamined.
In order to analyse the amount of amorphous silica in the fine
grain fractions, mainly biogenic opal, a selective solubility procedure was used. This wet chemical method relies on the different solubility resistances of amorphous silica and crystalline
components in highly concentrated sodium solutions (MATIHlES
& TROLL 1987).
RESULTS
Granulometry
The investigated marine surface sediments are mostly silts with
varying amounts of clay and sand (Fig. 3 ). Sampies from watel' depths deeper than 2000 m contain more than 25 % clay.
Beside 34 surface samples from box core two additional gravity cares with 13 (Care 224) and 12 samples (Core 278) were
examined for grain-size composition (Tab. 1). The grain-size
SILT
SILT
CLAY
8
= sandy
si
s i lty
t
clayey
S
sand
Si
silt
T = clay
CLAY
90
75
50
2S
Fig. 3: Grain-size classification system according to FÜCHTBAUER &
Abb. 3: Komgrößenklassifikationsschema nach
132
FÜCHTBAUER
&
MÜLLER
MÜLLER
(1970).
(1970).
10
SAND
sampIe
clay
silt
sand
A
Core 224
17 cm
35 cm
62cm
90cm
193 cm
255 cm
301 cm
334 cm
432cm
485 cm
540cm
582cm
648 cm
42
46
48
50
55
52
50
37
47
54
42
49
48
43
45
38
33
41
44
42
48
39
34
38
34
40
15
9
14
17
4
4
8
15
14
12
20
17
12
mean:
48±5
40±4
12±5
Core 278
66cm
121 cm
183 cm
275 cm
465 cm
523 cm
624 cm
700cm
775 cm
875 cm
982 cm
1103 cm
45
48
46
37
30
45
43
30
45
49
48
40
49
51
51
62
67
54
52
66
51
50
51
59
5
1
3
I
3
I
5
4
4
1
I
I
mean:
42±6
55±6
3±2
Tab. 1: Weight proportions of the clay, silt and sand fractions in samples from
gravity cores 224 and 278 (in wt.-%).
Tab. 1: Gewichtsanteile der Ton-, Silt- und Sandfraktion in Sedimentproben der
Schwerelotkerne 224 und 278 (in Gew.%).
distributions remain quite constant with depth, which leads to
a relatively small standard deviation. This is also confirmed by
the excellent agreement between the mean grain-size distribution of the gravity cores (6-11 m) and the corresponding box
core sampIes (approx. 0.5 m) representing the youngest sediment, only. They vary within the single standard deviation of
the gravity core sampIes (Tab. 2).
Granulometrical characteristics of the four geographic areas are
described as follows (Fig. 4):
Weddell Sea
Of all investigated sampIes the Weddell Sea group (1167-5,
1168-2,1169-1,1171-1,1173-6,1174-2) contains the highest
average amount of clay (40 %). Five samples are poorly sorted
clay and silt and represent pelagic sedimentation, whereas
sampIe 1168-2 exhibits a moderately sorted sediment assemblage from clay to gravel (Fig. 4e). According to its textural parameters sample 1168-2 represents a turbiditic layer.
B
Station 224
clay
silt
40±4
48±5
45
43
sand
12±5
12
Station 278
clay silt
sand
3+2
42±6 55+6
2
42
56
Tab. 2: Comparison of the grain-size composition (wt.%) of gravity core (A)
and box core (B) samp1es.
Tab. 2: Vergleich der Komgrößenzusammensetzungen (Gew.%) von Schwere10t- (A) und Kastenlotproben (B).
South Orkney Island Plateau
SampIes of the South Orkney Island Plateau (1176-3,1176-4,
1177-3,1178-4) show similar grain size distribution characteristics as the Weddell Sea sediments. Only a higher average
amount of the fine sand fraction (26 %) has been observed (Fig.
4d). This can be explained by the presence of glacial sediments,
both from the Weddell Sea pack-ice and from the South Orkney
Island Plateau. Sample 1176-4 represents a turbiditic layer.
Powell Basin
Powell Basin pelagic sediments (sarnples 1180-4,210, 225)
consist of clay, silt, and sand (Fig. 4c). Generally they show a
higher amount of fine sand (30 %) than the above described
sediments.
Bransfield Strait
The samples from Bransfield Strait exhibit considerably varying
granulometrical parameters. Based on these characteristics they
can be divided into two different granulometrical groups. Group
I consists of sampIes (n = 8) with an amount of clay + silt of
<50 % (Fig. 4a), whereas Group 11 sarnples (n = 10) show higher clay- silt portions (Fig. 4b). Group I sediments exhibit a bimodale grain size distribution, which is probably the result of
several different overlapping sedimentation processes (Mattnrss
1986). A further subdivision ofthis sarnple group into samples
of the Bransfield shelf (water depth <850 m, sampIes 1183-4,
1337-4, 1353-4, 1355-4) and basin (water depth >1000 m,
sampIes 1138-4, 1141-2, 1186-3, 1187-3) shows weakly to poorly sorted clays to fine gravels in the latter, and a distinct enrichment of the fine sand fraction (36 %) in the former subgroup.
The basin sampIes represent turbiditic layers.
In contrast to Group I sediments, sampIes of Group 11 (1147-6,
1148-1,1181-2,1182-2,1184-6,1324-1, 1333-2, 1336-5, 13381, 240) are poorly sorted, negative-skewed clays to fine sand,
and represent pelagic sedimentation.
Petrology
Fine grain fractions «63 um)
The petrographical composition of the clay and fine silt fraction
of all sampIes is quite uniform. This is in agreement with the
findings of GOLDBERG & GRIFFIN (1964), BISCAYE (1965),
JACOBS (1974), ANDERSON et al. (1980), ELVERH01 & ROALDSET
(1983), HOLLER (1985), and GROBE (1986). Beside illite as a
major, and chlorite and montmorillonite as a minor constituent,
traces of smectite and other rnixed-Iayers are present. Detrital
quartz and plagioclase are always present in minor proportions.
133
c, 100',.-'("-%-'-)-'---
a, 100 ,.l-(!!.%L)---,----.----.-----,----:o;;or-----p=-:;;".-,
50
50
ES
0,002
b,
0,1)j3 0,1
0,2
20
I
6,3
0,63
.:'
PB
1~,::::<
0,002
(ma)
0,1)j3 0,1
0,2
0,63
6,3 (mm)
100r(%::..:.)_,_--~?=~~~~--,....--,-1
50
50
ES
so
20
10
II
Olli:.:.:::-L._ _- L - - L _ - - ' -_ _. l -_ _
0,1:X;3 0,1
0,2
0,63
6,3 (mm)
e'1001(%-)-----,--
0,002
0,1:X;3 0,1
0,2
0,63
.l-_--J~
6,3 Inm)
~.......- :-......-==-,_-_,171
50
20
ws
10
o L-'-""--'_ _- ' -_ _L-"--'_ _- - ' _ _"--'_ _- - L - - - - l
I
0,002
0,1)j3 0,1
I
0,2
I
0,63
6,3 (mm)
Fig. 4: Cummulative grain-size distribution pattern for marine surface sediments of the investigated area (wt. %); (a)+(b) Bransfield Strait (Group I = BS I Group
11 = BS ll), (c) Powell Basin (PB), (d) South Orkney Island Plateau (SO), (e) Weddell Sea (WS).
Abb, 4: Kornsurrunenkurven der Sedimente des Untersuchungsgebietes; (a)+(b) Bransfield-Straße (Gruppe I =BS I, Gruppe II =BS 11), (c) Powell-Becken (PB),
(d) South-Orkney-Inselplateau (SO), (e) Weddellmeer (WS).
Table 3 shows a semiquantitive comparison of the clay mineral proportions in the clay and fine silt fraction of samples from
the Bransfield Strait and the Weddell Sea, respectively. The
differences in the proportions of chlorite and feldspar become
evident.
In order to verify the temporal variability of the mineralogical
composition samples from two gravity cores were analysed.
Semiquantitative determinations of the integral peak area on air
dried and glycole treated samples reveal the different mineralogical composition of the grain size fractions <211m and 2-6.3
um, but also differences with depths (Tabs. 4 and 5).
Core 224
According to the proportions of illite, chlorite and feldspar in
the clay fraction of samples from Core 224 three sec tors within
the depth profile can be distinguished (Fig. 5). While the content of illite steadily decreases from sector I with a mean proportion of x = 43±2 % (n = 4, 17-90 cm depth) to sector ll with
x = 27±2 % (n = 5, 193-432 cm depth) to sector III with x = 33±3
% (n = 4, 485-648 cm depth), the chlorite content increases with
depth (Tab. 4). There is a significantly higher portion of plagioclase in sector 11 with a mean value of x = 14±2 % compared to the sectors I and III with x = 5±2 and 7±1 %, respective134
ly. The proportions of all the other components remain constant
within their estimated standard error deviation. Illite and feldspar
as well as chlorite and mixed-layers are strictly negatively correlated. Their correlation coefficients are r = -0.72 (n = 13,
P >= 99 % security) for illite vs. feldspar, and r = -0.70 (n = 13,
P >= 95 % security) for chlorite vs. mixed-layers, respectively.
Further constituents of the clay and fine silt fraction are amorphous silica components (as in Tab. 4) predominantly biogenic
opal from diatom skeleton fragments and volcanic glass, respectively. Their proportions vary considerably between the different sedimentation areas. Powell Basin and Weddell Sea
samples contain 4 to 18 % (x = 1O±5 %), Bransfield Strait sediments 21-46 % (x = 33±8 %, n =12), while samples from the
South Orkney Plateau contain between 52-83 % (x = 68+16 %,
n = 3). The portion of volcanic glass shards within the clay fraction of 1-3 % remains relatively constant a11 over the sedimentation areas.
The fine silt fraction (2-6.3 um) does not reveal the three fold
subdivision of the sediment profile. Detritic minerals, mainly
quartz, substitute illite and chlorite to a considerable extent.
Smectite and mixed-layers are of sub ordinate importance. There
is a distinct increase in amorphous silica. This is due to frag-
Bransfield Strait <-> Weddell Sea
MAIN CONSTITUENT
MINOR CONSTlTUENT
illite
chlorite
quartz
feldspar
montmorillonite
smectite
mixed-layers
TRACES
Core 224
<
»
»
<
<
Tab. 3: Semiquantitative comparison of the clay mineral proportions in the clay
and fine silt fractions from sampies of Bransfield Strait and Weddell Sea. (=
equal proportions; < > one proportion prevails; « » one proportion prevails
markably).
Tab. 3: Halbquantitativer Vergleich der Tonmineralanteile in der Ton- und Feinsiltfraktion von Proben der Bransfield-Straße und Weddellmeer. (= gleiche Anteile, < > ein Anteil überwiegt,«» ein Anteil überwiegt stark).
depth
musc.!
illite chlorit. smect. mixed-I. fsp.
grain size fraction <211m
17cm
36
14
4
35 cm
62 cm
90 cm
42
37
37
13
14
21
193 cm
255 cm
301 cm
334 cm
432cm
21
24
27
20
19
19
15
14
29
28
26
32
19
25
17
21
quartz
,ac' sector
15
5
14
12
19
14
4
3
4
11
6
6
9
13
12
8
13
9
5
5
5
5
15
10
12
21
18
9
14
12
13
13
15
14
17
11
15
14
13
13
14
11
6
13
17
13
13
17
14
13
14
5
I
Hlite
Chlorite
Smektite
Mixed-layers
Feldspar
Quartz
34±7
20±4
4±1
17±5
9±4
16±3
485
540
582
648
cm
cm
cm
cm
22
3
4
3
4
3
grain-size fraction 2-6.3 11m
17 cm
19
24
+
4
6
7
4
12
30
11
9
45
35
34
9
4
+
18
+
+
+
3
14
14
17
17
18
15
23
23
21
22
19
18
+
+
+
11
31
6
12
13
13
14
33
11
35
31
+
12
6
3
5
+
9
17
12
+
+
+
+
3
6
6
6
22
15
193 cm
255 cm
301 cm
334cm
432cm
17
17
18
18
21
25
21
18
+
1O±3
14±4
13±4
Tab. 5: Mineralzusammensetzung der Tonfraktionen von Proben der SchwereIotkerne 224 und 278 (in Gew.-%).
ments of diatom skeletons, which derive from excrements of
krill. During the digestion of diatoms their skeletons are broken
to pieces of 1-10 11m in size (Gersonde & WEFER 1985), which
accumulate in the fine silt fraction. (MATTHIES & TROLL 1987).
Core 278
The clay fraction of Core 278 cannot be subdivided into three
seetions like the clay fraction of Core 224. Illite and mixed-layers appear in distinctly smaller quantities than in Core 224, while
the contents of smectite and amorphous components are
increased. Apart from the dependence of chlorite and illite from
[ern]
core 224
rxr
(X]
II
III
500
18
21
21
35 cm
62cm
90cm
485 cm
540cm
582cm
648 cm
12
9
21
10
27±10
25±5
1l±3
Tab. 5: The mineral compositions of the clay fractions from Core 224 and Core
278 (in wt.-%).
core 278
22
Core 278
33
I
C
8
10
II
1000
C
11
9
10
12
39
38
38
36
9
5
7
10
III
Tab. 4: Mineralogical composition of the clay and fine silt fraction of core sampies from Core 224 (+: detected).
Tab. 4: Mineralzusammensetzung der Ton- und Feinsiltfraktion der Proben aus
Kern 224 (+: nachgewiesen).
SMLF Q
SML F Q
Fig. 5: Clay mineralogical composition of sediment sampies from Core 224 and
Core 278. Three sections in the profile of Core 224 can be distinguished:
sector I 17-90 cm depth, sector Ir 193-432 cm depth and sector III 485-648 cm
depth. I = illite; C = chlorite; S = smectite; ML = mixed-Iayers; F = feldspar;
Q = quartz.
Abb. 5: Torunineralogische Zusammensetzung der Kerne 224 und 278. Im Kern
224 können 3 Abschnitte unterschieden werden: Abschnitt I in 17-90 cm, Abschnitt 11193-432 cm, Abschnitt 111485-648 cm. 1= Illit, C = Chlorit, S = Smektit, ML = Mixed Layers, F = Feldspat, Q = Quarz.
135
depth (r = 0.84, n = 12, P >99 % seeurity and r = -0.52, n = 12,
P >95 % security, respeetively) no further interdependeneies
beeome obvious.
Heavy mineral composition oj medium grainfractions
(63-200 fll11)
In 41 sediment sampIes more than 25 heavy mineral speeies
were identified, of whieh only 10 oeeur in appreeiable amounts.
They include garnet, amphibole, augite, hypersthene, titanite,
zoisite, spinel, leueoxene and opaque minerals (magnetite and
ilmenite). Other minerals present but not eommon, include apatite, zireon, tourmaline, diopside, sillirnanite, glaucophane, kyanite, rutile, and olivine. In the following seetions petrographie
eharaeteristies of the eommon heavy minerals are deseribed in
their deereasing order of abundanee.
Gamet
Reddish to pale pink almandines and eolourless grossulares are
observed. Generally they are fresh and unaltered. Grains are
sharply angular to slighly subrounded. Solution pits were found.
Augite
Augite fragments mostly are short-prismatie and show eharacteristie saw-tooth marks. Some augites exhibit a weak pleochroism from pale green to greyish green, and a high extinction angle
of about 45°.
Amphibole
Three types of hornblendes were identified by their colour,
pleochroisrn, and birefringenee. Green hornblende is light or
dark green. The mineral is slightly pleoehroitic (pale green to
greyish green or yellowish green to pale bluish green). Most
mineral fragments are prismatic with angular to subangular
shape. Glaueophane shows pleochroism (colourless, pale green
to bluish violet) with high birefringenee and a small extinetion
angle (4_6°). Brown hornblende varies from greenish brown,
reddish brown to dark brown. Like green hornblende the brown
hornblende is slightly pleochronitic and has a low birefringence.
Glaucophane and brown hornblende are aceessory minerals.
Titanite
Titanite exhibits a range of colours from pale yellow to
greenish yellow. Full extinction is prevented by eharacteristic
dispersion of the mineral. Pleochroism is very weak or not detectable.
60
50
\)=
(j
..
a
•
15
~
Garnet
es
Leukoxene
0
Amphibole
Eillillill
Titanite
arm
Augite
...
CJ
Zoisite
0
Hypersthene
0
Sp ine l
C2D
Epidote
Opaque minerals
km
55
500
!BS·A!
60
/
I
(
WEDDELL
SEA
./
J
70
60
50
Fig. 6: Distribution of heavy minerals in the investigated area (mineral frequency expressed in per cent by number); Bransfield Strait (Group A = BS A, Group B
= BS B), Poweil Basin (PB), South Orkney Island Plateau (SO), Weddell Sea (WS).
Abb, 6: Schwermineralverteilung im Untersuchungsgebiet (Häufigkeit in Prozent Komzahl); Bransfield-Straße (Gruppe A = BS A, Gruppe B = BS B), Po wellBecken (PB), South-Orkney-Inselplateau (SO), Weddellmeer (WS).
136
Leucoxene
Leucoxene consists of mineral aggregates of mostly anatase and
titanite. Under crossed polars bluish green and yellow colours
can be seen. Leucoxene shows high birefringence and no extinction. The aggregate rims often exhibit lower birefringence,
and consist mainly of zoisite and clinozoisite.
Zoisite
Colourless zoisite shows a weak pleochroism and low birefringence. An anomalous bluish interference colour has been observed.
Hypersthene
All grains of arthopyroxene present are hypersthene. It occurs
in prismatic grains with rare saw-tooth termination and a distinct
pleochroism was evident (pale brownish red to pale green).
Epidote
Dark yellow epidote exhibits a weak pleochroism (pale green
to yellowish green) and very high birefringence. The grains are
mostly angular to subrounded.
Spinel
Dark red spinel shows a high index of refraction and is isotropie. The grains are often intersected by several cracks.
Heavy mineral distribution of medium grainfractions
(63-200 flm)
The total amount of heavy minerals in the investigated sediment
sampIes ranges from 10-60 %. On the average sediments from
the Bransfield Strait area contain 37 % heavy minerals in the
fine sand fraction, whereas sediments from the Powell Basin,
the Weddell Sea, and the South Orkney Island Plateau hold
about 25 % heavy minerals. According to the geographie
sampIe locations the following heavy mineral characteristics of
each location are evident (Fig. 6).
Weddell Sea
The six investigated sampIes of the Weddell Sea area are characterized by their high amounts of garnet (45 %, average mineral frequency expressed in percent by number). On the average, opaque minerals comprise 23% of total heavy minerals
followed by green hornblende (11 %), augite (6 %), spineI, zoisite, leucoxene, kyanite, and titanite (all « 5 %).
site, apatite, diopside, epidote, rutile, sillimanite, tourmaline, and
zircon. A minor amount of garnet and a higher amount of opaque minerals distinguish this sampIe group from the Weddell
Sea and South Orkney sediments.
Bransfield Strait
The above described mineral associations are quite different
from those of the Bransfield Strait where the highest amounts
of opaque minerals, augite, and leucoxene occur. Garnet is only
a minar constituent.
The Bransfield sampIes can be divided into two different heavy mineral assemblages. Group A consists of 17 sampIes of the
northeastern part of Bransfield Strait and is characterized by
high amounts of opaque minerals (35 %) and augite (15 %). Less
common minerals include garnet (12 %), leucoxene, titanite,
green hornblende (8 % each), hypersthene (5 %), zoisite (5 %)
and epidote (4 %). Accessary minerals are apatite, diopside,
kyanite, glaucophane, rutile, tourmaline, and zircon. Sampies
(263,240, 1147-6, 1148-1) near Elephant and Clarence Islands
exhibit high "amphibole-epidote-zoisite" concentrations.
Sampie Group B (n = 6) is located at the southwestern end of
Bransfield Strait and is dominated by high amounts of opaque
minerals (37 %) and leucoxene (31 %). Despite less common
amounts of augite (13 %) and titanite (10 %), all other minerals have accessory character (glaucophane, green hornblende,
hypersthene, kyanite, olivine, sillimanite, and zoisite).
Dropstone classification system of coarse sized detritus (» 1 cm)
The rock type, group and society classification system introduced for the purpose of these investigations is not in accordance
with commonly used rock terms and classifications. In addition no chemical standard was used here.
The rock type with narrowly defined features referring to its
mineral content, structure and texture also within extensive populations, is represented mostly as a single specimen. In the
source area it is a homogeneous and distinct rock body. The
conception "rock group" means broader defined limits for the
mineral percentages and far textural and structural features.
Frequently it includes several rock types. The terminus "rock
society" includes several rock groups, which show similar genesis and spacious source areas.
0/ coarse sized detri-
South Orkney Plateau
The mineral association of the South Orkney group (3 sampIes )
is similar to the Weddell Sea area. As in the Weddell Sea area
garnet (38 %) is the predominant heavy mineral. Slightly
higher amounts of opaque minerals (25 %), amphibole (15 %)
and augite (8 %) are shown.
The weight distributions of the rock societies are shown in
Figs. 7 and 8. Starting from the percentage frequencies in
these figures it is intended to look for the origin of the freight.
Powell Basin
The Powell Basin group (4 sampIes ) comprises opaque minerals (33 %) as the main constituent followed by garnet (22 %),
titanite (10 %), green hornblende, augite, and leucoxene (8 %
each). Less common minerals include hypersthene, spineI, zoi-
In addition, rock societies in the dropstone population were
compared with rock societies of outcrops on the Antarctic Peninsula. The area of comparison is shown in Fig. 9. In order to
estimate the square percentages of each rock society outcropping in the separate sectors west and east of the ice divide and
Dropstone composition and distribution
tus (>1 em)
137
70
55
30
5\
60
60
65
65
01
socie.ty t ptutoruc to hypobysslc
~gll~~~:f~s moomotues, cctd
_
soeiety 2: intermediate
beste volccrutes
and
r::==
soeiety 3: stuty pororocks
~ the lower Epizane
~ soctety
l:2::2:J
ot
4: Para- onc ortnorocxs
of lhe higher Epl- . IM Mesoand the Kotozone
pe
§
soctsty S, gronobiostir. gneisses
=
seotmentüss
_
_
soelety 7: ctostlc sedimentites
= 3 society 5: cherntcot secretion
0
.
8 P
.
socrety e: robternoticc
PALMER LAHO
70
30
Fig. 7: Lateral distribution of eight dropstone rock societies (wt.%). Bransfield Strait area excluded.
Abb. 7: Flächenhafte Verteilung von 8 Eisfracht-Gesteinsvergesellschaftungen (Gew. %) ohne Bransfield-Straße.
21
+
+
1
JG'
+
+
+
25
+
23
18
+
19
O[(EPlIGHg
IIL>HO
+
+
.
20
60'00'
Fig. 8: Lateral distribution of eight dropstone rock societies in Bransfield Strait.
Abb. 8: Verteilung von acht Eisfracht-Gesteinsvergesellschaftungen in der Bransfield-Straße,
138
+
+
00'
Peninsula west
of the ice divide
Fig. 9: Areal proportions of 6 rock societies on the Antarctic Peninsula and South
Shetland Islands according to the geological map of the British Antarctic Survey (1979-1982). Broken ring around circular plot represents ice cover in square
percentages. RS 1: magmatites in hyp-abyssic to plutonic solidification; RS 2:
intermediate to basic volcanic rocks; RS 3: slaty pararocks of the lower grcenshist facies; RS 4: para- and orthorocks of intermediate to high grade metamorphism (except granulites). RS 6: chernical secretion sedimentites; RS 7: clastic
sedimentary rocks.
Abb. 9: Flächenanteile von 6 Gesteinsvergesellschaftungen auf der Antarktischen Halbinsel und den Süd-Shetland-Inseln entsprechend der geologischen
Karte des British Antarctic Survey (1979-1982). Der unterbrochene Kreis gibt
die Eisbedeckung in Flächenprozent an. RS I: Magmatite in hyp-abyssischer
bis plutonischer Erstarrung, RS 2: intermediäre bis basische Vulkanite, RS 3:
Paraschiefer der niedrigen Grünschieferfazies. RS 4: Para- und Orthogesteine
höhergradiger Metamorphosestufen (ausgenommen Granulitfazies), RS 6: chemische Sedimente, RS 7: klastische Sedimentgesteine.
the South Shetland Islands, the geologieal maps 1: 500,000 of
the British Antaretie Survey (1979-1982) serve for referenee.
DISCUSSION
The investigated sediments of Bransfield Strait, Po weIl Basin
and northwestern Weddell Sea reveal, from a granulometrieal
point of view, the weIl known grain-size distribution pattern of
a deereasing median with inereasing water depth. Within the
grain-size classification the sediments vary from clayey to sandy silts. The similar proportions of clay, silt and sand in one
surfaee sample (box eore) and its eorresponding gravity eore
profile ean be taken as an indieation of more or less eonstant
sedimentation eonditions during the last 400,000 years maximum.
Freshness, angularity and eomposition of the detrial material ean
be interpreted as a direet sediment input by melting ieebergs,
whieh is in aeeordanee with results reported by ANGINa &
ANDREWS (1968), and WRIGHT & ÄNDERSON (1982). Both studies
suggest that the sediments of the Weddell Sea are mainly
transported as iee-rafted detritus. An additional sediment input
by turbidity eurrents into the abyssal regions of the Weddell Sea
ean be demonstrated by the presenee of a moderately sorted
sediment assemblage from clay to gravel in sample 1168-2. The
inerease in sand pereentages in sediments from the South
Orkney Island Plateau is eaused by the greater relative input of
iee-rafted detritus from the Weddell Sea paek-iee and the South
Orkney Island glaeiers, respeetively. These poorly sorted deposits have the granulometrieal eharaeteristies of "basal tills"
(ANDERSON et al. 1980). Beside these typieal glaeial marine sediments turbiditie layers oeeur in the eontinental slope area of
the South Orkney Island Plateau (see sample 1176-4).
Different sedimentation proeesses are responsible for the
observed sediment distribution in the Bransfield Strait. Our granulometrieal results eorrespond with investigations of EDWARDS
& GOODELL (1969), who deseribed high sand eoneentrations
(> 50 %) in marine sediments of the shelf regions of the Antaretie
Peninsula and the South Shetland Islands. Two strong bottom
counter-currents in the northern and southern part of Bransfield
Strait eoming from the Bellingshausen Sea and the Weddell Sea
(WITTSTOCK & ZENK 1983), respeetively, holding the finer
sediment fraetions in suspension, are responsible for the predominanee of the eoarser fraetions in this region (Group I). In
eontrast to these results a remarkable deerease of sand eoneentrations ean be noted at both ends and in the deeper basins
(>2000 m) of the Strait (Group ll). Marine eurrents had little or
no effeet on these sediments after deposition. An important
feature of the eontinental shelf of the Antaretie Peninsular are
submarine eanyons reaehing into the Bransfield Strait basins.
Through these eanyons remarkable amounts of eoarser size fraetions are transported as turbidity eurrents into the basins. The
eastern part of the Powell Basin does not exhibit any partieular
granulometrieal eharaeteristies probably representing a eombined model ofthe different sedimentary meehanisms ofthe surrounding regions.
In eontrast to terrestrial and lake sediments from the Bransfield
Strait region, the age of the marine sediments is still uneertain.
On the basis of tephrostratigraphieal investigations on limnetie
and marine ash-layers from this region, MATTHIES et al. (1990)
reported marine sedimentation rates of 7 cm/lOOO a
(station 1338-1) and 24 ern/lOOO a (station 1347-1), respeetively.
From this a maximum age of 110,000 a (7 ern/lOOO a) or 27,000
a (24 ern/lOOO a) for Core 224 (lenght 648 em) ean be dedueed.
HOLLER (1985) earried out geoteehnieal investigations on the
same sediment eore and found 2 hiatus of 10 m eaeh within the
sediment sequenee. They are loeated in 17-35 em depth and at
355 em depth, respeetively. Own geoehemieal investigations
(unpublished) eonfirm these findings by distinet ehemieal diseontinuities in the postulated depths. Aeeording to this, Core
224, whieh is 650 em in length, most probably represents three
seetions from an originally 26.5 m thiek sediment sequenee. This
results in the new estimation of the maximum sediment age of
380,000 a (7 ern/lOOO a) or 100,000 a (24 ern/lOOO a) given in
Table 6. These evaluations are based on the assumption of constant sedimentation rates as weIl as of no further hiatus. This ean
be judged as most unlikely, if the following is taken into aeeount.
Earlier results from tephrostratigraphieal investigations on three
other sediment eores from Bransfield Strait (1346-1, 1347-1 and
139
Station 224
without hiatus
Sector I
(17-90 cm)
700-4000
2400-13000
with 2 hiatus
10m
sedimentation
700-45000
2400-156000
24 cm/1000 a
7 cm/1000 a
8000-18000
50000-100000
28000-62000
170000-350000
24 cm/1000 a
7 cm/lOOO a
Sector III
20000-27000
(485-648 cm) 103000-110000
69000-100000
355000-380000
24 cm/lOOO a
7 cm/lOOO a
Sector II
(193-432 cm)
Tab. 6: Sediment ages on the basis of sedimentation rates of 7 and 24 cm/l 000
a for core profile 224.
Tab. 6: Sedimentalter basierend auf Sedimentationsraten von 7 bzw. 24 cm/l 000
a für das Kernprofil 224.
1357 -1) (MATTHIES et al. 1988) indicate severa1 considerable
hiatus, each longer than the entire sediment cores considered.
Therefore, according to the present state ofknowledge it must be
assumed that the sediment sequences in the cores from this
region are in most cases not cornplete. Furthermore, several
glaciation periods took place on the southem hernisphere during
the last approximately 400,000 a. They caused large-scale ice
avalanches and extended pack ice fields accompanied by sea level changes of several hundred meters causing dryness of large
shelf areas, It is for sure that during these climatic changes the sedimentation processes in this region changed several times, too.
While comparing the clay mineralogical findings in Core 224
with the climatic history of the Atlantic sector of Antarctica
(according to ANDERSON 1972) the warm and cold stages correspond fair1y weIl with the sequence of sectors I to III, a sedimentation rate of 7 cml1000 a including two hiatus. Sector I
Cl ,000-45,000 a) represents the last warm stage, sector 11
(50,000-56,000 a) corresponds with the last cold stage, while
sector III (103,000-110,000 a) represents the isotope stage 5, a
wann period. Furthermore, sec tors 11 and III would also agree
with the sedimentological findings of GROBE (1986), who investigated sediment cores from the eastern part of the Weddell Sea,
off Kap Norvegia. According to these findings a sedimentation
rate of ? cml1000 a seems to be most like1y as it also meets the
mean sedimentation rate of 1-2 cm/lOOO a given by GROBE
(1986) for the more eastern area. As a conclusion the cold
stage is characterized by an increased presence of detritic minerals, like quartz and feldspar, while simultaneously illite
decreases, According to this the sequence of 183-465 cm depth
of Core 278 must be attributed to a cold stage. As the sections
in the top and the bottom of this core differ distinct1y from the
mineralogical pattern of sectors land III of Core 224, an assignment to the climatic pattern of ANDERSON (1972) is not done.
Due to the great uncertainties in the sediment age an assignment
to the climatic record of the Antarctic ice shield (LORIUS et al.
1985) is not possib1e, either.
The aeolian sediment input has not been described too much in
the recent Antarctic literature, yet. There are comprehensive investigations on the mode, composition and extent of wind load
over the Atlantic and the Pacific Oceans by DELANY et al. (1967),
PARKIN et al. (1970), CHESTER et al. (1971), LANGE (1982), and
140
BLANK et al. (1985). The grain-size spectra cover the whole clay
and silt fraction. In decreasing order the wind load carries
illite, kaolinite, chlorite, montmorillonite, smectite, quartz,
feldspars, as weIl as diatom frustrules. WINDOM (1969) and LEINEN & HEATH (1981) calculated the aeroso1 proportion of the Pacific and Atlantic sediments to be as much as 75-95 %. Corresponding estimations for the Antarctic marine sediments are rnissing
due to the lack of aeroso1 investigations. However, this factor
should not be neglected when sediments from this region are going
to be interpreted for their climatic and sedimentary reeords.
The heavy mineral distribution in the sediments of the investigated area points to several petrographic provinces. A province
characterized by metamorphic minerals occurs in the sediments
of the Weddell Sea, the South Orkney Island Plateau, and a small
area near Elephant Island, The dominant mineral assemblage
consists of gamet and green hornblende with minor amounts of
epidote, spinel, sillimanite, and kyanite. The Weddell Sea sediments probably originated along the eastern coast of the
Antarctic Peninsula. ADlE (1957) describes almandine-rich silIimanite-garnet-biotite hornfelses at Cape Christmas (72° 20' S,
60° 41' W). As already mentioned a majority ofthese sediments
is transported by icebergs. However, several local sources for
gamet, green hornblende and epidote exist in the metamorphic
rocks of Elephant and Clarence Islands (DALZIEL 1984, LOSKE
et al. 1985) and the South Orkney Islands (THoMsoN 1968,
1974), and in the quartzdiorite and the sandstones of the
northern part of the Antarctic Peninsula (SMELLtE 1987).
A province remarkably characterized by minerals of volcanic
origin can be distinguished south and north of the South Shetland Islands, especially around the still active volcano of
Deception Island, The dominant mineral assemblage consists of
augite and leucoxene with minor amounts of hypersthene and
olivine. This result does not surprise as the South Shetland
Islands are of volcanic origin, The volcanic suite interfingers
with the metamorphic mineral assemblage east of Elephant and
Clarence Islands and the Weddell Sea (EDWARDS & GOODELL
1969).
A small strip of an intrusive mineral assemblage occurs along the
west coast of the northern Antarctic Peninsula. This intrusive
mineral suite is characterized by green hornblende and minor
concentrations of ruffle, tourmaline, and zircon. They originate
in lowgrade metamorphic sediments of the Antarctic Peninsula
(e.g. Mount Bransfield at 63° 17' S, 57° 06' W; ADlE 1957).
Some selected rock types give rise to search for distinct rock
bodies in ice free outcrops. One of those is leuco-mangerite with
light red feldspars, wh ich was found ha1fway from the tip of the
Antarctic Peninsula to the South Orkney Islands. Due to its fabric it might derive from the Red Ridge Granite at Marguerite
Bay, which is described by ADlE (1955). A reddish rhyolith with
fluidal texture found at the same locality, might originate from
the Gambacorta Formation in the Pensacola Mountains (LAIRD
& BRADSHAW 1982). A "dry" diorite with orthopyroxene from
a sample location 150 km south of the South Orkney Islands
probably is derived from the East Antarctic Craton. Some spe-
cimens of acid to intermediate fine grained magmatites with
dispersed inclusions of sulphides were found in sample locations
offshore the Pacific coast. Rocks with similar enrichments in
sulphide occur in outcrops at the west coast and also on some
offshore islands (Cox et al. 1980, ROWLEY & PRIDE 1982,
ROWLEY & WILLlAMS 1982, HOECKER & AMSTUTZ 1986).
Only few outcrops of granob1astic gneisses occur at the western
coast of Graham Land. Within the basement complex ADlE
(1954) describes them in the Cape Calmette gneiss.
Referring to the reconstruction of migration paths of rock groups
and rock societies estimations of origin remain more spacious.
Several groups of acid volcanic rocks are culminating northwest
of the Antarctic Peninsula. Perhaps they moved along a tonguelike path in an acute angle to the coast line and the main surface stream, out into the Drake Passage. A similar distribution
in the northwestern area can be observed with the rock society
of magmatites in plutonic to hypabyssic intrusion levels
(Figs. 7 and 8).
The rock group of dark blue-grey slaty claystone occurs on the
Antarctic Peninsula (ROWLEY & WILLIAMS 1982), in Ellsworth
Land (BUGGISCH & WEBERS 1982) and in the sediment cover of
Dronning Maud Land (sample by M. PETERS). The group of
brown and red sandstones and arcoses can be seen for example
in the western part of the Shackleton Range (Blaiklock Glacier
Group, according to LAIRD & BRADSHAW 1982) and in the
eastern Dronning Maud Land. Both groups point to far more
southern SOUl"Ce areas.
The samples with the highest proportion of intermediate to
basic volcanic rocks (>50 %) are found in the vicinity of Elephant Island. This rock society appears to be composed predominantly of rocks from the volcano islands of the South Shetland Islands (BIRKENMEYER et al. 1991), while volcanic rocks
from the southwest probably Palmer Land, contribute a minor
proportion. According to W ATKINS & SELF (1972) volcanic
detritus is generally enriched at 55° S. This is not generally valid as our results reveal. The findings of ROESSLER-VIANA (1985)
are in good agreement with our results concerning the percentages of volcanic rocks, although the percentage estimations of
ROESSLER-VIANA (1985) seem to be too high for the Bellingshausen Sea, and too low for the area around Astro1abe Island.
Referring to the close vicinity of the northern part of the Antarctic Peninsula (Fig. 9), the dropstone populations show some
relationships between weight percentages and areal proportions
of the terrestial outcrops. For example, the samples from Powell
Basin and the northwestern Weddell Sea partly ressemble the
ones from the sector east of the ice divide. Along the Pacific
coast three offshore sample fields show a good similarity to the
sector west of the ice divide. The sample fields nearby the South
Shetland Islands on1y partly resemble the ones on the islands.
Antarctica (KAMENEV 1980) is hinting at highly metamorphosed
terrains.
CONCLUSIONS
The results from this study lead to the following conclusions:
It is still an open task to search for the distribution path of
pumice. Although pumice usually remains on the water surface,
it is also deposited in surficial sediments of Bransfield Strait.
Summed up together with pyroclastites in the statistical evaluation, it culminates between 10-30 % around Deception Island.
One possibility for its deposition in the marine sediments might
be the sudden release of a major lens of detritus sweeping it
downwards, and burrying it within the dropstone deposit. The
other possibility could be an autochthonous formation by submarine volcanic eruptions.
The society of para- and orthometamorphites of the upper 10wgrade, medium- and highgrade zones can be found nearly everywhere. Their almost exclusive occurrence near the South Orkney
Is1ands obviously indicates their origin from there.
The society of granoblastic gneisses mainly found in the open
northwestern Weddell Sea, is supposed to be a typica1 facies
within the Precambrian of East Antarctica. Mostly, it is represented by pyroxene-granualite (SUWA 1968, GREW 1984,
HARLEY 1985, FITZIMONS & THOST 1992). The most western outcrop of this facies is found in the Kottas Mountains south of the
Weddell Sea (ARNDT et al. 1986, SPAETH & FIELITZ 1986,
JACOBS 1991). The resu1ts of dropstone investigations (OSKIERSKI
1985, 1988, KUHN et al. 1993) give rise to assurne a far more
southern extension of the granulite facies underneath the ice. In
addition, the relatively great thickness of the crust in East
• The sedimentary sequences in the Bransfield Strait basin can
be subdivided into cold and warm stage sediments by their
mineralogical composition. In combination with tephrostratigraphical considerations a sedimentation rate of approximately
7 cm/1 000 a can be established.
• Off the Pacific coast along the Antarctic Peninsula acid volcanie rocks occur together with tourmaline, zircon and rutile. They
also coincide with sulphide impregnated intermediate to acid
magmatites. Their source bodies are located on the Antarctic
Peninsula.
• Near the South Shetland Islands the basic volcanic rocks are
associated with single grains of olivine.
• In the northwestern Weddell Sea pyroxene-granulite dropstones
occur together with garnet, red spinel and orthopyroxene. They
mainly originate from the East Antarctic Craton.
ACKNOWLEDGEMENTS
The authors heartly appreciated the collaboration of colleagues
from the Alfred Wegener Institute, the Universities of Göttingen, Kiel and Hamburg, as well as of the Institute of Marine
Research in Hamburg and the Niedersächsische Landesamt für
141
Bodenkunde in Hannover. They also thank the crews of the
research vesse1s Meteor, Polarstern and Walter Herwig, who
did an excellent job at sea. This investigation was financially
supported by the Deutsche Forschungsgemeinschaft (DFG),
grants Tr 61/28-4, Tr 61/32-1, Tr 61/33-1 and Tr 61/36-1.
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