Clay Minerals (1983) 18, 65-75
CLAY MINERALS
AS I N D I C A T O R S
OF WIND
AND CURRENT
CONTRIBUTION
TO
POST-GLACIAL
SEDIMENTATION
ON THE
A Z O R E S/ICELAND RIDGE
F. G R O U S S E T ,
C. L A T O U C H E
AND N. M A I L L E T
Institute de G~ologie du Bassin d'Aquitaine, Universit~ de Bordeaux L 351 Cours de la Liberation,
33405 Talence-Cedex, France
(Received 29 September 1982)
A B S T R A C T: Clay mineral and trace element data indicate that sediments in the vicinity of
the North Mid-Atlantic Ridge derivefrom material of both Icelandic and Canadian origin. These
sources agree with the marine and atmospheric circulation patterns observed in this area.
INTRODUCTION
Many papers dealing with the clay mineralogy of present-day saline environments have
shown that the sediments are, in general, continentally-derived (Biscaye, 1965; Griffin et
al., 1968). This also appears to be the case for older deposits (see discussion in Chamley,
1979; 1981). Because of this continental provenance, clays have commonly been used for
reconstructing various oceanic paleoenvironments. In the North Atlantic, for instance,
where terrigenous material fluxes into the ocean are predominant and hydrodynamic
interchanges between different parts of the Basin particularly active, this approach has
enabled paleographic/paleohydrological reconstructions based on clay inputs from
different ages such as the Quaternary (Hoffert, 1973; Latouche & Parra, 1976, 1978;
Alvinerie et al., 1978), Cenozoic (Latouche, 1979; Latouche & Mailtet, 1980) and the
Mesozoic (Chamley, 1979).
In certain environments, use of clay data for sedimentological reconstruction purposes
becomes difficult, e.g. adjacent to volcanoes where various authigenic end-products,
originating in both aerial or sub-marine environments, appear (Bonatti, 1967). Among the
derived minerals, clays are mostly represented by smectites (Grim & Guven, 1978;
Hoffert, 1980) and palygorskites (Bonatti & Joensuu, 1968; Hathaway, 1972; Lomova,
1975). In this type of environment, volcanic products and their by-products mix with
terrigenous inputs, making it difficult to differentiate material of these two origins.
Furthermore, minerals from environments around basaltic regions are often poorly
crystalline (Latouche, 1975).
The Mid-Atlantic Ridge between Iceland and the Azores Islands represents a region
where sedimentation is subject to volcanic influence (Grousset et al., 1982). In fact,
Icelandic erosion products are primarily the result of volcanism. Furthermore, despite
moderate ridge activity, materials of Ridge provenance may contribute to adjacent
sediments (Cronan, 1972). Under these circumstances, the sedimentological and
9 1983 The Mineralogical Society
66
F. Grousset et al.
hydrodynamic processes occurring in this area are difficult to reconstruct on the basis of
mineralogical criteria alone. In this paper, results based on mineralogical criteria are
compared with those derived from distributions of certain trace elements in the post-glacial
sediments which are relatively 'immobile' with respect to secondary alteration processes.
SAMPLING;
GENERAL
CHARACTERISTICS
POST-GLACIAL
SEDIMENTATION
OF RIDGE
Some 40 cores were recovered from the Azores/Iceland Ridge by piston and box cores
(Fig. 1). The average thickness of the post-glacial deposits was ~40 em and these were
sampled every 10 cm, giving 4-5 samples per core. Results were averaged on each
core throughout the post-glacial levels. Values obtained represent the entire post-glacial
period, i.e. approximately 10 000 yr. Deposits consist of silty muds and foraminiferous
ooze (Grousset et al., 1978, 1981, 1982).
The impact of sedimentation on the inorganic fraction has been evaluated in terms of
flux flow. These fluxes represent 13 g/cm2/103 yr in the vicinity of Iceland, decrease
towards the south, and attain a value of ~ 1 g/cm2/103 yr close to the Azores Islands. The
30 ~
I__
- -
20*
I
65~f -
IO0
I
')
(
Basin
60~
/}
/
~.om
/ .,//
"
R ,l~,.- - j ~ w
0 5 S. . /
2
9 I
\ /~
~,J~ ~)
Faraday
Mou~ts
21 ~ 1 9 1 8 2 7
23
.~
.55
~'"~
Fracture
14
Islands 9
/Ba.k/f q
/
55=,
ell
ul~,~/)
28
"50=
'~
45,~
/~~15 ~./-16 ~1tl7
Chaucer
Bank
40 ~.
Biscoy
/ / ........
Azores
M.A.~.
"~,
l
30 ~
" ~
abyssal plain
0 Fa~os ~
~ Fae~as I
:.
I
20~
i
--
FIG. 1. Core samplelocation during the Faegas II (O) and Faegas III (*) cruises.
67
Clay minerals in North Atlantic sediments
latter flux is similar to that of 1.1 g/cm2/103 yr quoted for the North Atlantic by Turekian
(1965).
COMPOSITION
AND DISTRIBUTION
OF CLAY
ORIGIN; HYPOTHESES
PHASES;
The <2 /~m size fraction of sediments from the Reykjanes Ridge was ~15% but this
comprised 40% of the sediments in the Chaucer-Faraday area (Fig. 1). X-ray diffraction
analysis showed that the constituents of the clay assemblage were mainly smectite, illite,
chlorite and kaolinite; traces of palygorskite were noted in post-glacial and interglacial
sediments and minor amounts of clinoptilolite were also observed. In the Icelandic Basin,
close to the Gardar Drift, trace amounts of quartz, feldspars and amphibole were identified
in the <2 ~ a fractions. Although no difficulties were experienced in identifying the various
phases from X-ray diffraction traces, the diagnostic peaks were rarely sharply defined or
prominent and, in particular, smectite often showed broad peaks indicative of poor
crystallinity. The diffraction traces also showed high backgrounds, probably due to the
presence of appreciable amounts of amorphous materials.
Relative amounts of each clay species were determined from the X-ray diffraction
traces. Fig. 2 shows the geographic distribution of smectite. A north-south gradient is
apparent; near Iceland the mineral comprises all the < 2 / t m fraction of the post-glacial
sediments but only 10% of those at a latitude of 40~ This gradient shows that Iceland
and the Faeroe Islands constitute a major source of both smectite and post-glacial deposits
(see also Yeroshchev-Shav, 1964). Person (1976) has described beidellite, nontronite and
meta halloysite as alteration products of Icelandic rocks. Latouche (1975) and Rutherford
& Debenham (1981) have also described expandable clay minerals in Faeroe soils. Thus,
according to marine smectite distribution and previous observations regarding recent
alteration products, the Faeroe Islands area also appears to be a source of these materials
(Grousset & Parra, 1982). Consequently, sub-marine contributions from the active
Ridge--where smectite neoformations were observed by Siever & Kastner (1967), Murray
(1970), Copeland et al. (1971) and Person (1976)--appear to be only of minor importance.
The geographical distribution of chlorite (Fig. 3) shows a 'high' between 40 and 50~
A W - E gradient is apparent across the Ridge. Although chlorite neoformations have been
located by Copeland et al. (1971) in the rift towards 23~ this process is not capable of
explaining this gradient, the direction of which suggests a westerly origin for the mineral.
The same applies to illite, which shows a similar distribution. The illite/chlorite assemblage
may, then, be linked to eastern Canada. In suspended material from the St-Laurent, illites
and chlorites are the main constituents (D'Anglejan & Smith, 1973). They result from
reworking of secondary products from the Canadian shield, such as soils, post-glacial
morainic and lacustrine deposits (Piper & Slatt, 1977).
It must be noted that the distribution patterns of smectite, as well as illite and chlorite,
coincide with the general distribution patterns of minerals encountered in the Atlantic and
described in previous works (Biscaye, 1965; Griffin et al., 1968; Rateev et al., 1969).
The (illite)/(smectite) relationship is shown in Fig. 4. Amounts of these two minerals mix
in a straight fine, which is compatible with a mixture of two detrital clay assemblages, one
smectite of Icelandic origin and the other illite of Canadian provenance. The mixing
F. Grousset et al.
68
_o
,
,~..,
~ .
~%.g
o~-.... ~
~
~
_~
.
:
,.., <
~...,~.,~.,.,~(:_...~ ~
..
.
~,~
~
~-
~
o
t ~ ,~1~o~' ~ : -~'
_~~i~.-.~
~ ~ t / ~ .~ |~F, ~,-~.~.~/-~.
.7"
~v
0
I
I
|
~----,,,-,-,~~ ~ I ~ - ' ~ "-.~ i \
', . . . . _ . . . . . . .
,, o
'~
~i ~o/
I
I
I
a
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i
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,~
,g7
~3
-a
o
~:~
~I ~,~
.
.
o
c,i
o4
Clay minerals in North Atlantic sediments
69
NORTH
-
s%"t
i
i I1 I
ICANADA
t
9 ~
9
9
40-
30-
ZO-
IO-
ICEL.ANO
~
'
8'o
'
•
'
~
'
2'o
'
G
FiG. 4. Mixingline for illite and smectite.
p r o c e s s - - a s is shown by the distribution of points with regard to latitude--is constant and
progressive from NE to SW.
DISCUSSION;
RESULTS; COMPARISON
CRITERIA
WITH
OTHER
The clay minerals have thus been linked to two well-defined sources. The next problem is
to determine whether a relationship exists between transport and settling behaviour and to
specify the exact role of the ridge with regard to the latter. Other minerals or specific
chemical elements from each source exhibit similar settling behaviour; 'immobile' elements,
in particular, should prove useful in environments unsuitable for the approach just
described.
Potential vectors in the North Atlantic
Bottom currents (Fig 5b). Bottom currents of Norwegian origin flow along the
Icelandic flank, cross the basin to the Gibbs Fracture, and from there spread out
(Worthington, 1970). It has been shown by Ruddiman & Bowles (1976) that Icelandic
erosion products--transported into the basins by gravity currents--are cut off by
Norwegian bottom currents and then transported towards the south as a fine-grained
suspension.
Deep-sea circulation patterns show an anticyclonic gyre west of the Mid-Atlantic Ridge
and a cyclonic gyre to the east, which induces, on the Ridge between 40 ~ N and 50 ~ N, a
mean N - S component. It is therefore possible to determine continuity with part of the
bottom current movement processes from the Norwegian Sea. Additionally, fine-grained
particles of Icelandic origin can reach the Ridge towards 4 5 ~
pattern which may
explain the smectite distribution, but not that of illite or chlorite, which are scattered from
west to east.
F. Grousset et al.
7O
I~
I
I
I
I
FIG. 5. (a) Surface current pattern in the North Atlantic showing the Gutf Stream and its
northern continuation, the North Atlantic Drift (after Dietrich, 1960). Area investigated
outlined. (b) Bottom current pattern in the North Atlantic. The Iceland/Azores Ridge is crossed
by N-S currents of low strength (after Worthington, 1970; Vogt, 1972).
Surface currents (Fig. 5a). These are present mainly in the Gulf Stream and its northern
continuation, the SW--NE directional North Atlantic drift. Flowing along the east coast of
the North American continent, the surface currents open out on to the Azores-Iceland
Ridge. This vector is unlikely to be applicable in any discussion of long-distance transport
processes.
Wind (Fig. 6). The present atmospheric circulation pattern (Lamb, 1971) indicates that
the Azores-Iceland Ridge region is subject to an average S W - N E wind regime originating
in the eastern regions of North America. These winds could therefore deposit Canadian
illite and chlorite on the ridge. Features of the sediments which could support this
interpretation are the abundance of quartz in <2 $tm fractions and the presence of
wind-worn, round, unpolished quartz in <37 tim fractions; a decrease in amount of
particles of continental origin in marine aerosol from west (Newfoundland) to east
(Europe) has also been recorded by Folger (1970). Wind direction cannot, however,
account for the scatter in smectite distribution from NE to SW.
GraviO; currents. Although the major transporting agents for Icelandic particles
(especially smectite) into bottom currents, gravity currents do not appear to be responsible
for transporting illite and chlorite onto the Canadian slope. Itlite and chlorite values show
'highs" to the east of the Ridge axis--2000 m above the American abyssal plain--which
would preclude the possibility of any bottom input/transport. Consequently transport
vectors over the ridge must exist.
7"rends of certain 'immobile' chemical elements
Rubidium (Fig,. 7). Only small amounts (0.2-30 ppm) of this element occur in basic
rocks such as those of Iceland (Shaw et al., 1976: Wood, 1978), whereas it is relatively
abundant (110-170 ppm) in acidic rocks such as those of the Canadian shield (Shaw et
al.. 1976) or the Scandinavian shield (Heir & Compston, 1969). The geographical
distribution of rubidium in the vicinity of the Mid-Atlantic Ridge would suggest that the
source is primarily on the Canadian shield. Biscaye & Dasch (1971) reported that
Clay minerals in North Atlantic sediments
71
FIc. 6. Present atmospheric circulation during summer in the North Atlantic (after Lamb,
1971). The zone investigatedis sweptby a wind mass from North America.
amounts Of Rb in deep-sea sediments increased with decrease in particle size, the highest
contents being in the clay-size fractions. This element probably occupies crystalline sites in
illite (Horstman, 1957).
The illite/chlorite/Rb assemblage must therefore be of Canadian origin, having been
initially transported by wind and later by surface currents, thus explaining the Ridge
overflowing.
Rare earth elements. The shale-normalized rare earth distribution pattern in Icelandic
Tertiary basalts is characteristic of enriched light rare earth elements (Fig. 8). In addition,
a positive europium anomaly is observed. Sampled sediments around the island show the
same type of pattern. Amounts decrease progressively by drifting towards the south
because of dilution by acidic shield material with a negative europium anomaly (Haskin &
Haskin, 1966). A mixture of 30--50% Icelandic rare earth elements with 50-70%
Canadian rare earth elements (Shaw et al., 1976) would explain the rare earth element
patterns observed on the Ridge at 45~ (Grousset et al., 1982).
Tantalum (Fig. 9). In igneous rocks, this element is especially associated with pyroxenes
and thus it is scarce in acid rocks of the Canadian shield (Rankama, 1944) but relatively
abundant in the basalt rocks of Iceland (Grousset et al., 1982). Its geographical
distribution on the Ridge shows a gradient centered on Iceland similar to that exhibited by
smectite (Fig. 2). The element is strongly associated with the silt fraction, suggesting a
relatively high bottom current competence capable of transporting the Ta-bearing particles
up to 2000 kin.
L a - T a - T h relationships (Fig. 10). These three elements show constant and welldifferentiated proportions in different magma types (Joron et al., 1978). Thus, the Th/Th
ratio is generally lower than 1 in spreading magmas (e.g. Iceland) and higher than 1 in the
shield-type magmas (e.g. Canada, Scandinavia). Furthermore, these characteristic ratios
are preserved in sedimentary particles, thus permitting mixing lines to be determined.
All Ridge samples show ratios scattered along a straight line joining the Icelandic and
Canadian values, which compare closely with that based on the illite vs. smectite plot
(Fig. 4). The similarity of these plots substantiates the hypothesis based on simple testing
of clay distribution.
F. Grousset et al.
72
.-1
<2o
b.
o
<~.
0
..~
.~..-_"~'.-~
~
~" b , ' - ~
0
.=.
I
i
I
,"
I
I
/
..
.h
",
""
"\
,/
o~
.~
Om
0
0
cJ
"..
-"
.------'~'.
0
,~".
~o
L
."
"~".., o
i
-"
/ i
i
~'~--
m
~ 0 ~-"
I ~I~L_~
i
J
,,
_~
~i-~ i I
0
J
0
m
73
Clay minerals in North Atlantic sediments
[C]sample
[c]
shales
2.-
I.-
0.5-
/
~--~
~fM.A.R.
/
0.2-
~
4
5
O
N
47"N
0.1I
I
I
I
I
I
I
Lo Ce
Sm Eu
Tb
Yb Lu
FIG. 8. Rare earth distribution patterns showing a mixture of Icelandic and shale materials at
4 5 o N latitude.
CANADIAN
SHIELD
Lo/To
60-
503O-/ 9
IO-~o
Defoe
Th/To
;
,;
,~
2'~
FIo. 10. L a / T a vs T h / T a plot for sediments showing mixing line between Icelandic and
Canadian Shield materials.
ACKNOWLEDGMENTS
This study was funded by the C.N.R.S. (A.T.P. framework). We wish to thank M. Treuil and J. L. Joron (P.
Sue Laboratory, Saclay) for helping us with trace element analysis by neutron activation. Their technical and
scientific advice has been most useful. We are very grateful to H. Chamley and M. Hoffert for critically
reviewing the manuscript and for making valuable suggestions for improvement.
74
F. G r o u s s e t et al.
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R E S U M E: Les donn(~es concernant les min~raux argileux et les 61~ments traces indiquent
que les s6diments situes pr6s de la dorsale medio-atlantique nord d6rivent de mat~riaux ~. la
lois d'origine islandique et canadienne. Ces sources sont en accord avec les reseaux de circulation
marine et atmosph~rique observes dans cette r~gion.
K U R Z R E F E R A T : Daten fiber die Tonmineralogie und die Spurenelementgehalte weisen
darauf hin, dab die Sedimente in der N~ihe des N6rdlichen Mittelatlantischen Riickens sowohl
aus dem isliindischen als auch aus dem kanadischen Raum stammen. Dies stimmt mit den
atmosph~irischen und marinen Zirkulationen dieses Bereiches fiberein.
R E S U M E N : Los resultados de los analisis de minerales de la arcilla y de elementos trazas
en sedimentos cercanos al dorsal centro atlfintico septentrional, indican que estos sedimentos
derivan de materiales procedentes de Islandia y Canada.. Este origen estb, de acuerdo con los
esquemas de circulacion marina y atmosf,3rica observados en esta zona.