DEEP-SEA SEDIMENTS OF THE INDIAN OCEAN' WOLFGANG SCHOTT
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DEEP-SEA SEDIMENTS OF THE INDIAN OCEAN'
WOLFGANG SCHOTT
Geologische Landesanstalt, Hannover, Germany
ABSTRACT
The different types of sediments in the Indian Ocean and their areal distribution
are described first. Following this their calcium carbonate content is discussed. The
origin of these different deposits depends on the character and amount of the ter-
rigenous detritus, on the nature of the organic substances supplied to the sediment, and
on the physical and chemical properties of the ocean water which influence the car-
bonate content of the deposits. The varied influence of these factors in the individual
ocean regions permits the interpretation of the large-scale distribution of the deep-sea
sediments and of their carbonate content on the bottom of the Indian Ocean. On the
basis of the results of investigations by the "Meteor" Expedition of the sediments of
the equatorial Atlantic Ocean it is possible to explain the stratigraphic relationships of
the pelagic sediments of the Indian Ocean in their major features and to gain some
knowledge of its history back to the last glacial period. This knowledge agrees with ob-
servations in the equatorial Atlantic Ocean, thus giving a wider regional significance to
the conclusions based on the data from the Atlantic Ocean.
TYPES OF SEDIMENTS
Owing to its geographic position, the Indian Ocean contains almost
all known types of recent deep-sea sediments. In the equatorial and tem-
perate latitudes Globigerina ooze, red clay, and radiolarian ooze are
found, as well as hemipelagic deposits with coastal affinitiessuch as the
blue mud, coral mud, and others. On the other hand, in subpolar and
polar regions, diatom ooze and glacial-marine sediments occur.
All these deposits consist dominantly of two types of substances,
mineral and organic. The mineral constituents come from the adjacent
mainland or from islands. They reach the sediment in different ways:
(i) suspended fluvial material, (2) material eroded from the coastline,
(3) glacial débris from icebergs, or () dust from volcanoes and deserts.
Consequently the mineral constituents in individual sedimentary types,
such as red clay, Globigerina ooze, or blue mud, are much the same over
large regions in the ocean. Therefore these types of sediments can not be
distinguished from one another by their mineral constituents. The dis-
tinction depends, on the contrary, mainly on their content of organic
components. The organic constituents depend on the animals and plants
living in the sea water or on the sea bottom. Near the coast and in areas
of shallow water the organic world of the sea bottom is the decisive factor.
'Translated from the German by Marie Siegrist and Parker D. Trask. Manuscript
received, March 21, 1938.
DEEP-SEA SEDIMENTS OF INDIAN OCEAN 397
Thus in the neighborhood of coral reefs, coral ooze and mud originate.
In the open ocean the organic material consists almost exclusively of
those constituents of the planktonic animal and plant world that have
settled to the bottom from the surface waters above. If the calcareous
shells of pelagic foraminifera or the siliceous skeletons of radiolarians pre-
dominate in the sediment, as in the equatorial and temperate latitudes,
we have respectively a Globigerina or a radiolarian ooze. If the siliceous
fragments of the diatoms prevail, we have the diatom ooze, which occurs
principally in polar regions, If the organic components are distinctly
subordinate to the mineral ones, or are completely lacking, in the open
ocean the sediments consist of red clay, but near shore the sediments
consist of hemipelagic deposits, such as blue mud, green mud, or in polar
regions, of glacial marine sediments.
This interpretation of the different sedimentary types occurring in
the Indian Ocean is presented only in its essential features, insofar as is
necessary for the understanding of the deep-sea deposits, and is based on
numerous investigations (I-23). The detailed investigations of the bottom
deposit material from the equatorial Atlantic Ocean collected by the
German Atlantic Expedition on the research boat "Meteor" have materi-
ally helped the proper understanding of the sediments of the Ocean (for
details see Correns (i-6); Leinz (y); Radczewski (ii); and W. Schott
(14-19)).
DISTRIBUTION OF THE SEDIMENTS
For G. Schott's (i.) book on geography of the Indian and Pacific
oceans published in 1935, I drew a map of the bottom deposits of these
two oceans. Figure i which shows the distribution of the deep-sea sedi-
ments in the Indian Ocean is based on this map. However, only the
boundaries between the different types of sediment have been indicated.
On the original map the data for each deep-sea sounding are entered,
which makes it possible to indicate the degree of precision with which the
sedimentary boundaries have been drawn. For details, therefore, refer-
ence is made to that map (13).
The boundaries between the different sediments are not as precise as
shown on the figure. In many places the sediments interfinger so much
that the true boundary may have a jagged, irregular course, as is in-
dicated by Sewell's (20, Pl. 6) map of the boundary between Globigerina
ooze and hemipelagic mud in the Gulf of Bengal. However, owing to
small scale of the map this irregularity of the boundaries can not be
shown on Figure .
398 WOLFGANG SCHOTT
40 60 80 lOO 120
20
120
ri i.. 4_n
o o
I-
4t -
20 20
-(
,_
o\.7:1-..--:
40
o
60 0 20 40 60 80 loo 120 40 60 60
Giobig.rino Onz. over Diatomaceous Os:.
Corals O Globigerina Onz. over Rad Clay
H.mlp.la pis Se4lm.r,ts Globip.rosa Ooze over Glacial Marine Sediments
Globiparina Ooze Oiolamaceouo Ooz, over Glacial Mariol Sedimente
Dlaomoc.ou. 0e:. De finite sedimentary boundaries
RadiolarIos Osi. - Uncertain sedimentary boundaries
Rid Clay -. D. f,nit. Unes of equal lime content
Unc.rlai,, ines ot equal lis,. Snails?
FIG. iDistribution and calcium carbonate content of deep-sea sediments of Indian Ocea
AB, Arabian basin; SB, Somali basin; MB, Madagascar basin; MR, MacQuarie ridge; CaR., Carlsbe
ridge; MiR., Mid-Indian ridge; CR, Crozet ridge. Numbers refer to percentage of calcium carbona
in sediments. Arrows represent direction of Antarctic bottom current.
DEEP-SEA SEDIMENTS OF INDIAN OCEAN
The distribution of the sediments as indicated on Figure i has been
modified from the former map (13) in accordance with more recent
informaticn and interpretation. Red clay in general seems to occupy
larger areas at the expense of the Globigerina ooze than was assumed in
1935.2 Thus the red clay of the east basin between 1100 and 125° East
longitude probably extends southwards as far as the boundaries of the
diatom ooze. In the west basin the area of red clay in the east depression
of Madagascar is larger, and perhaps extends farther than is now known.
Moreover, the bottom of the small basin between Réunion-Mauritius and
Madagascar is probably entirely covered by red clay and radioarian
ooze. According to Wiseman and Sewell (21, p. 223), the entire deeper
part of the Arabian basin is occupied by red clay.
In spite of these changes the regional distribution of the various
sedimentary types remains essentially the Same as it was given in 1935.
The wide deep sea of the tropical and temperate Indian Ocean is occupied
mainly by red clay and Globigerina ooze. Red clay occurs mainly at the
great depths in true deep-sea basins, whereas Globigerina ooze is found
predominantly at relatively less depth, particularly on the deep-sea
ridges.
Sediments of the west part of the Indian Ocean consist almost com-
pletely of Globigerina ooze, except for the red clay area southeast of
Madagascar and in the Arabian basin. As the water deepens from the
Central Indian Ridge toward the east basin, the red clay, which occupies
almost the entire basin, takes the place of the Globigerina ooze. A small
patch of radiolarian ooze lies north of the Kokos Islands within the area
of red clay. The local occurrence of Globigerina ooze in the red clay west
of the Kokos Islands is caused by an elevation on the sea bottom. In
the subpolar region the diatom ooze surrounds the Antarctic continent
in a broad belt and takes the place of the Globigerina ooze and red clay.
In contrast with the distribution of these three eupelagic sediments,
Globigerina ooze, red clay, and diatom ooze,the hemipelagic deposits
are conspicuously scarce. For the most part they merely border the coast
in a narrow band, but in the Arabian Sea and in the Gulf of Bengal they
occupy somewhat larger areas of the sea bottom. Around the south polar
continent these hemipelagic deposits consist of glacial marine sediments
composed essentially of the débris of icebergs and glaciers on Antarctica.
2 The boundary between Globgerina ooze and red clay had to be drawn, in general,
according to the method prevailing at that time, according to which foraminiferal deep-
sea sediments containing more than 30 per cent calcium carbonate were classified as
Globigerina ooze, although a more exact subdivision of the calcareous sediments as
proposed by Correns () would have been preferable.
400 WOLFGANG SCHOTT
The distribution of the main deposits of coral sand and coral mud are
shown in Figure i. They are present in relatively large areas only in the
region of the Laccadive-Maladive Islands, the Seychelles, the Chagos
Islands, Saya da Malha, and on the Nazareth Bank. These two types of
sediments are not as extensively distributed around the individual coral
reefs as was formerly thought, for Globigerina ooze is found very close
to many islands that have coral-reef coasts.
Philippi (io, pp. 523-24) found in the Globigerina ooz earea southwest
of Madagascar far from the African coast the so-called deep-sea sands
whose mode of deposition is still very much debated. On the Carlsberg
Ridge and on the northeast side of the adjacent Arabian basin the Sir
John Murray Expedition encountered several basalt fragments on the
sea bottom. Wiseman and Sewell (21, p. 224), by means of detailed
petrographic examinations of these basalts in connection with various
geophysical and other geologic observations, come to the conclusion that
the tectonic lines of the Carlsberg Ridge system are Tertiary in age.
CALCIUM CARBONATE CONTENT OF THE SEDIMENTS
Murray (8), in 1910, drew up a map of the distribution of calcium
carbonate in the deep-sea deposits of the Indian Ocean, but this map is
now somewhat out of date. An attempt was therefore made to determine
more accurately the distribution of calcium carbonate in the bottom
sediments of the open ocean on the basis of new data (Fig. i). As only
i8o analyses of the carbonate content of the deep-sea deposits of the
Indian Ocean have been made thus far, the present map likewise can be
of only temporary use.3 It is therefore not to be compared with the accurate
maps of carbonate content of the Atlantic Ocean (a).
In the tropical and temperate latitudes the areas of red clay and of
radiolarian ooze are characterized by low calcium carbonate content
(0-30 per cent). Hence the distribution of carbonate in the east part of
the Indian Ocean is portrayed fairly clearly. The few data available
indicate that the red clay of the east basin is for the most part carbonate-
free or poor in carbonate, and only at the boundary with the Globigerina
ooze has it a carbonate content of slightly less than 30 per cent. On the
other hand, the sediments of the west part of the MacQuarie Ridge have
a carbonate content of more than 75 per cent. In the south likewise the
relationships are fairly clear, for there most of the diatom ooze and the
glacial marine sediments are either free of carbonate or they have only
In order not to sacrifice clarity, the individual calcium carbonate analyses are not
indicated on Figure i, desirable as that would have been.
DEEP-SEA SEDIMENTS OF INDIA N OCEAN 401
slightly more than io per cent carbonate. In the west part of the Indian
Ocean, on the other hand, areas of red clay are the only ones that with
certainty can be shown to be poor in calcium carbonate. In order to
indicate roughly the boundaries of the carbonate-rich sediments, it was
in this area more than in other areas necessary to take into consideration
the submarine topography, which indirectly influences the lime content;
for this was the only way to subdivide the deposits. The sediments on
the ridge extending from the Crozet Ridge to South Africa in general
have a carbonate content of more than 75 per cent, and in some places
have as much as 93.9 per cent. The entire Madagascar basin, on the con-
trary, including areas of red clay, seems to have a lime content of less than
50 per cent. The deposits on the entire Central Indian Ridge contain
more than o per cent calcium carbonate and in some places contain more
than 8o per cent. In the east, this ridge of high carbonate content is
separated to a certain extent from the east basin, which is poor in car-
bonate, by the 30 per cent line, but the boundary on the west is still
extremely uncertain.
According to Wiseman and Sewell (21), the south part of the Somali
basin in general is covered entirely by Globigerina ooze, except for a small
area of radiolarian ooze. In view of this and of the depth relations there,
it seems as though the average calcium carbonate content of the sedi-
ments in the entire Somali basin is less that 50 per cent. Information on
this point will surely be yielded by the bottom samples of the Sir John
Murray Expedition.
INTERPRETATION OF THE SEDIMENTS
Although many details still require clarification, it is nevertheless
already possible to interpret, in its main features, the large-scale regional
distribution of the recent deep-sea sediments of the Indian Ocean. The
origin of the individual deposits depends on the nature and quantity of
the mineral constituents supplied; on the types of organismsthe so-
called "biogenic component ;"and on the physical and chemical proper-
ties of the sea water, such as pressure, temperature, and CO2 content,
that influence the calcium carbonate content of the bottom sediments.
The hemipelagic deposits occupy only a narrow band along the African
and Australian coasts, as comparatively little terrigenous material is sup-
plied to the sea either by the streams, which are few in number, or the
coast line, which is smooth and regular. Furthermore, thi, small amount
of material is not deposited immediately, because of the steep gradient
down to the abyssal deeps, which tends to cause the sediment to be
402 WOLFGANG SCHOTT
transported for considerable distance and to be distributed relatively
thinly over a fairly large area before it is laid down.
In the Arabian Sea and in the Gulf of Bengal the opposite conditions
exist. Both of these areas are relatively shallow compared with the open
ocean and several large streams that carry much sediment enter the sea
along the somewhat irregular coasts. Consequently the blue-mud area
extends far out into the ocean. On the south, this area of blue mud is
penetrated by a wedge of Globigerina ooze. Around Antarctica the hemi-
pelagic glacial marine sediments occupy a rather large area of the ocean
bottom. Icebergs, which carry considerable débris, are carried far out
into the ocean by currents, and consequently mineral components of
glacial origin predominate over the biologic componentsthe siliceous
frustules of the Diatomaceaein the deep-sea deposits, even at great
distances from Antarctica. Only where the glacial constituents are ex-
ceeded by the diatom frustules does the diatom ooze begin. The boundary
between diatom ooze and glacial marine sediment therefore depends on
the amount of mineral sediment brought from the Antarctic continent,
whereas the boundary between diatom ooze and Globigerina ooze is
determined by the relative amounts of the organic components.
The pelagic foraminifera, whose shells form the Globigerina ooze of
the sea bottom, occur predominantly in the warm surface waters of
tropical and temperate latitudes and, with the exception of a few species,
avoid cold regions. Therefore Globigerina ooze can be deposited in large
amounts on the sea bottom only in areas of relatively warm water.
On the other hand, the pelagic plant group of the diatoms is most exten-
sively distributed in cool-water areas, and hence in such areas diatom
ooze is found on the deep-sea bottom, underlying areas of cool water.
Locally diatoms are so abundant in the cold surface water that they often
cover the bottom of the collecting net as a greenish brown slime. This
difference in the pelagic life of the surface water is most evident in the
bottom deposits. Consequently it determines the boundary between dia-
tom ooze and Globigerina ooze, which corresponds almost exactly with
the subpolar convergence line of the surface water. In this connection it
is very characteristic that where this convergence line gradually extends
in an east-southeasterly direction some 6° of latitude, from the west part
of the ocean near the Crozet islands to the east part south of Australia,
the northern boundary of the distribution of the diatom ooze changes its
latitude in like manner (Fig. i).
In all oceans along the convergence line, the cold diatom-rich Ant-
arctic surface water, which is called "polar water" by oceanographers, eri-
DEEP-SEA SEDIMENTS OF INDIAN OCEAN 403
counters the foraminifera-rich warm water that is called "mixed water of
the middle latitudes" by the oceanographers. In all three oceans where this
polar water meets the mixed water it sinks to a depth of about 800 to
i,000 meters and moves toward the equator as an intermediate current
(12, Fig. 44; 13, Figs. 46, 49; and 23, Beilage 23-28).
As far as can be determined, the areal distribution of diatom ooze on
the greater part of the floor of the Indian Ocean and also on parts of the
floor of the Atlantic Ocean extends slightly north of the subpolar con-
vergence line. Perhaps the relatively light diatoms are carried towards
the equator by the intermediate current when the cold polar water
sinks (i).
The distribution of the carbonate-rich Globigerina ooze and of the
carbonate-poor to carbonate-free red clay is determined mainly by the
calcium carbonate content of the sediment, The question of carbonate
formation and distribution in recent deep-sea deposits has been consider-
ably clarified by the German Atlantic expedition of the research ship
"Meteor." In addition to investigations by the chemist of the expedition,
H. Wattenberg, and by the oceanographer, G. Wüst, the mineralogist of
the expedition, C. W. Correns, has studied this subject in detail. Accord-
ing to Correns (a), the carbonate content of the deep-sea sediments de-
pends on the CO2 content, temperature, pressure, current velocity of the
sea water, rate of production of the carbonate-yielding material, and on
the supply of noncalcareous constituents. The CO2 content of the ocean
water has a great influence on its ability to dissolve calcium carbonate.
The ability of the water to dissolve calcium carbonate is increased by
lowering the temperature or raising the hydrostatic pressure of the water;
that is to say, ocean water with the same CO2 content dissolves calcium
carbonate more actively on the bottom than at the surface, owing to the
increase in hydrostatic pressure and decrease in temperature. In conse-
quence of the effect of the CO2 content on the solubility of the water for
calcium carbonate, the source of the water in the Antarctic bottom
current near the bottom of the sea in abyssal deeps plays an especially
important rôle.
For the general areal distribution of Globigerina ooze and red clay
the chemical factors seem on the whole to be somewhat more important
than the mechanical factors, such as the velocity of the currents or the
supply of noncalcareous materials. This becomes clear from the close
relationship found by Wüst (23), especially in the Atlantic Ocean, be-
tween the cold, carbonic acid-rich bottom current coming from the Ant-
arctic, and the lime content in the deep-sea sediments.
404 WOLFGANG SCHOTT
Uncertain as the relations may be in detail, the occurrence of red clay
in the Indian Ocean seems to depend largely on the chemical properties
of the Antarctic bottom current. Wüst (22) has already pointed out this
relationship on the basis of Murray's map of the distribution of carbonate
in the sediments, but it is made still clearer by Figure i. The relations are
fairly clear for the east basin, in which the area of red clay coincides
fairly well with the zone of the Antarctic bottom current, and if the
assumed band of red clay in the southeast, extending to the diatom ooze
should prove to be characterized by sediments poor in carbonate, the
predominant dependence of the carbonate content of the sediments on the
chemical factors in the bottom current in this part of the Indian Ocean
would be demonstrated.
In the west basin much is still not clear, but, in general, the influence
of the Antarctic bottom current on the carbonate content of the sediment
seems to be somewhat less than in the east basin, owing perhaps to the
fact that, according to Wüst, the east basin of the Indian Ocean is pre-
ferred by the Antarctic bottom current to the west basin. On the other
hand, the mechanical factors, such as the rate of production of carbonate-
yielding material and the velocity of the currents, may play a larger rôle.
It does, to be sure, seem as though the sediments in the Madagascar
basin, through which the bottom current passes, have less than o per
cent carbonate, whereas those in adjacent parts of the Mid-Indian Ridge
and the Crozet Ridge, which are not touched by the bottom current, have
more than o per cent and 75 per cent, respectively, except where they
lie in the zone of diatom ooze. If the Globigerina ooze of the Somali
basin really has an average carbonate content of less than o per cent,
as is suspected, it would demonstrate an essential influence of the chem-
ical properties of the Antarctic bottom current on the carbonate content
of the sediments in the west basin.
We would then have in the Indian Ocean essentially almost the same
relations as in the Atlantic Ocean, except that in the Indian Ocean the
east basin would be distinguished from the west basin by sediments
poorer in carbonate, just as in the Atlantic Ocean, the central ridge,the
Mid-Indian Ridge,is discernible by a zone of high carbonate content.
Thus it is possible to ascertain to a certain extent those factors in the
Indian Ocean that are important for the origin of the recent deep-sea
deposits, and the essential features of the areal distribution of the differ-
ent sedimentary types are known; nevertheless, many gaps remain and
these must first be filled in before the interpretation of the sediments can
be placed on a surer foundation.
DEEP-SEA SEDIMENTS OF INDIAN OCEAN 405
STRATIGRAPHY OF THE SEDIMENTS
The accurate quantitative determination of the foraminiferal fauna
of the deep-sea deposits of the equatorial Atlantic Ocean both areally and
vertically in the samples collected by the "Meteor" Expedition made it
possible for the first time to establish in deep-sea sediments, on the basis
of the foraminiferal fauna, persistent stratigraphic horizons that are
independent of the type of sediment. Thus it is found that a zone that
contains no Globorolalia mcnardii (d'Orbigny) lies between zones that
contain G. inenardii. The fauna of the layer free from G. menardii in-
dicates that the layer was deposited in the last glacial period; the lower
bed with G. menardii belongs to the youngest interglacial period, whereas
the upper layer with G. menardii has been deposited since the end of the
Pleistocene (for details see W. Schott (ii)).
The information gained from the equatorial Atlantic Ocean, which
can not be discussed in detail here, makes it possible to interpret and
formulate more accurately, in spite of the few samples available, the
stratigraphic problems in the deep-sea sediments of the Indian Ocean with
respect to their areal distribution.
E. Philippi (io, p. i), the geologist of the German South Polar
Expedition, was the first to consider the problem of the stratification of
the deep-sea sediments of the Indian Ocean. On the basis of several bot-
tom samples he recognized an influence of the glacial period, in the super-
position of various sedimentary types, such as Globigerina ooze on red
clay or on diatom ooze, or of diatom ooze on glacial marine sediments.
Furthermore, he observed that in the subsurface layers of certain types
of some sediments, foraminifera were found that now live in water colder
tFan now present at the surface of the ocean in those areas.
Although no quantitative investigations of foraminifera in the de-
posits of the Indian Ocean have yet been made that would permit an
accurate stratigraphic subdivision of the deposits, nevertheless it may be
concluded with certainty, on the basis of results of investigations in the
equatorial Atlantic Ocean, that in the Indian Ocean the same, or at least
very similar, stratigraphic relations have prevailed. These relations are
of similar areal extent. They also represent similar changes in climate
as a result of the influence of the glacial period. Thus we may predict
that in regions of Globigerina ooze, of foraminiferal blue mud, and of
red clay, a layer with predominantly warm-water foraminifera will be
found to underly one with cold-water foraminifera. It should be borne in
mind, however, that within these layers a certain change of the foraminif-
eral fauna will occur according to the geographic latitude from tropic406 WOLFGANG SCHOTT
towards temperate regions. By analogy with the well known deposits of
the equatorial Atlantic Ocean, it is evident that after the deposition of
Globigerina ooze on red claya superposition that corresponds exactly
with the conditions for foraminiferal depositsin this region, like in the
Atlantic Ocean, the areal extent of the red clay in glacial times was
greater than at present and the extent of Globigerina ooze was less. This
is clearly demonstrated, for instance, by two bottom samples from east
and west of the red clay area in the Madagascar basin. Indirectly this
observation points to a greater distribution of the Antarctic bottom cur-
rent during the last glacial period.
In southern latitudes the superposition of Globigerina ooze on diatom
ooze in the general neighborhood of the Crozet Islands (Fig. i) points
to a greater advance of the cold diatom-rich polar surface water during
the glacial period; that is, the convergence line, along which the warmer
"mixed-water of middle latitudes," rich in foraminifera, encountered the
cold polar water, rich in diatoms, lay in lower latitudes during the glacial
period as the result of the wider distribution of cold water masses. The
retreat of the convergence line at the end of the Pleistocene led to the
superposition of Globigerina ooze on diatom ooze. Consequently this
superposition of Globigerina ooze on diatom ooze has a relatively large
areal distribution. The superposition of diatom ooze on glacial marine
sediments is due to the same cause, because during the glacial period
much more glacial material was carried into the sea from the Antarctic
continent than to-day. According to the latest data (Fig. i) glacial ma-
rine sediments extended towards the equator during the last glacial
period to the vicinity of the Heard Islands in 3° South latitude, whereas
today they terminate at about 62° or 63° South. In the boundary region
between red clay and diatom ooze, Globigerina ooze therefore is underlain
respectively by red clay or diatom ooze, and in the south the diatom ooze
in turn is underlain by glacial marine sediments.
On the basis of results obtained in the equatorial Atlantic Ocean we
come, accordingly, in spite of the relatively few observations in the
Indian Ocean, to the same stratigraphic conclusions concerning post-
glacial deep-sea sediments. In the Indian Ocean likewise, the present
distribution of deep-sea sediments is a recent phenomenon evolved only
since the end of the Pleistocene. The Antarctic bottom current already
existed in the last glacial period, but was more strongly developed. The
convergence line in the surface water of southern latitudes likewise
existed but, owing to the prevalence of the masses of colder water, it
lay in lower latitudes than today. Careful consideration of these factsDEEP-SEA SEDIMENTS OF INDIAN OCEAN 407
probably justifies the conclusion that the conditions of currents in the
Indian Ocean have not changed appreciably since the last glacial period.
The thickness of the upper Globorotalia menardii bed, which has been
deposited since the end of the Pleistocene, indicates that the average rate
of sedimentation in the equatorial Atlantic Ocean per x,000 years was
1.78 centimeters for blue mud, 1.2 centimeters for Globigerina ooze, and
less than o.86 centimeter for red clay (W. Schott, is). According to the
few data available, the average rate of sedimentation per i,000 years in
the southern Indian Ocean is 0.59 centimeter for the Globigerina ooze
and 0.54 centimeter for the diatom ooze. This appreciably lower rate of
sedimentation in the Indian Ocean was to be expected, as much more
terrigenous material reaches the relatively narrow deeps of the Atlantic
than reaches the wide abyssal regions of the Indian and Pacific Oceans
(for details see the other article by W. Schott in this Symposium).
It is not surprising, on the contrary it was to be expected, that we
would be led to the same, or very similar, conclusions for the Indian
Ocean as for the equatorial Atlantic; for the stratification in the recent
deep-sea sediments is related to conditions of stratification determined
by climatic factors induced by the glacial periodfactors, which, owing to
widespread distribution of the effects of the glaciation during the Pleisto-
cene, could not be restricted to the equatorial Atlantic Ocean, providing
the factors really had the significance attributed to them.
Fruitful as this knowledge about the deep-sea sediments of the Indian
Ocean may be, it is nevertheless desirable that all these stratigraphic
problems in the Indian Ocean be more accurately answered than is now
possible. The solving of these problems can be facilitated by systematic
regional collecting of bottom samples, such as was done in the Atlantic
Ocean by the "Meteor" Expedition. Unfortunately, however, the collec-
tion of bottom samples is becoming constantly rarer on most oceano-
graphic expeditions, as echo-sounding instead of wire-sounding is being
used more and more for determining ocean depths.
LIST OF REFERENcES
i. CORRENS, C. W., "Bericht über die sedimentpetrographischen Arbeiten, Die
Deutsche Atlantische Expedition. IV. Bericht (Prof. XIIXIV)," Zeit, der
Gesell. für Erdkunde. Nr. 5/6. Berlin, 1927.
2. , "Anzeichen von Beziehungen zwischen Strömungen und Bildung küsten-
ferner (eupelagischer) Sedimente," Neues Jahrbuch für Mineralogie, etc.,"
Beilage Band 7, Abt. A, S. 1109-17. 1927.
"Woraus besteht der Tiefseeboden?" Des Meer in volkstümlichen Dar-
stellungen (Tiefseebuch), Band . Heraasgegeben vom Institut für Meereskunde.
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