Geophys. J. Int. (1998) 133, 499–509
Mineral magnetic study of Late Quaternary South Caspian Sea
sediments: palaeoenvironmental implications
A. Jelinowska,1 P. Tucholka,1 F. Guichard,2 I. Lefèvre,2 D. Badaut-Trauth,3
F. Chalié,4,* F. Gasse,4,* N. Tribovillard5,† and A. Desprairies5
1 L aboratoire de Physique de la T erre et des Planètes, Université Paris Sud, URA 1369-CNRS, Bât. 504, F-91405 Orsay Cedex, France.
E-mail: [email protected]
2 Centre des Faibles Radioactivités, L ab. Mixte CNRS/CEA, Domaine de la T errasse, F-91198 Gif s/Y vette Cedex, France
3 L aboratoire de Géologie du Museum National d’Histoire Naturelle Paris, 43, rue BuVon, F-75005 Paris, France
4 L aboratoire d’Hydrologie et de Géochimie Isotopique, Université Paris Sud, URA 723-CNRS Bât. 504, F-91405 Orsay Cedex, France
5 L aboratoire de Géochimie des Roches Sédimentaires Université Paris Sud, URA 723-CNRS Bât. 504, F-91405 Orsay Cedex, France
Accepted 1997 December 15. Received 1997 November 20; in original form 1997 May 26
SU MM A RY
Magnetic properties of sediments from a core (10 m long) in the southern basin of the
Caspian Sea have been investigated. Varying concentrations of greigite (Fe S ) dominate
3 4
the magnetic fraction in Late Pleistocene sediments. The synsedimentary formation of
greigite indicates that the Late Pleistocene Caspian Sea was a brackish or fresh-water,
poorly ventilated basin and suggests a water level higher than at the present. The
variation in magnetic parameters, with the detrital magnetite-bearing fraction remaining
constant, is interpreted in terms of greigite grain-size variation and related to the slight
variation in water salinity. The Holocene sediments are characterized by detrital
magnetite. This indicates better ventilation of the basin and suggests lower water levels
than in the Late Pleistocene. The gradual change in magnetic properties of the sediments
between 90 and #60 cm depth, with decreasing quantities of greigite, indicates stepwise
establishment of oxic conditions in the Holocene.
Key words: Caspian Sea, Quaternary, rock magnetism, palaeoenvironment.
I NT R O DU C TI O N
The present-day fast rise of the water level in the Caspian
Sea (#2 m since 1977, Rodionov 1994) is an ecological and
economic catastrophe for the five surrounding countries and
has contributed to an increasing interest in the world’s largest
inland water body (66 100 km3), which lies 27 m below sea
level. Without connection to the world’s oceans, the Caspian
Sea is an intracontinental reservoir of saline water (its average
water salinity of 13‰ is about one-third that of the oceans)
and is sensitive to climatic changes in its catchment area
(3.5×106 km2) extending northwards to the central part of
the East European Plain (over 15° of latitude). The Caspian
Sea is divided into three parts that cover roughly equal
areas, but that differ in depth and volume, the north (water
depth <15 m), middle (<900 m) and south (<1020 m) basins,
which represent 1 per cent, one-third and two-thirds of the
total volume of water, respectively.
* Now at: CEREGE, Europole Mediterraneen de l’Arbois, BP 80,
F-13545 Aix-en-Provence Cedex 04, France.
† Now at: Sédimentologie et Géodynamique, UFR de Sciences de la
Terre, Université Lille 1, F-59655 Villeneuve d’Ascq, France.
© 1998 RAS
The Caspian Sea formed in the Middle Pliocene (at about
3 Ma), after its separation from the Black and Pannonian seas
( Varuschenko et al. 1987). Since this time, the Caspian Sea
has experienced numerous transgressions and regressions with
water-level fluctuations of several tens of metres (Varuschenko
et al. 1987). According to many authors (Chepalyga 1984;
Varuschenko et al. 1987; Velichko et al. 1987; Karpytchev
1993), the fluctuations in sea level of the Caspian Sea during
the past 125 kyr are related to climatic changes in the Northern
Hemisphere, although the relationship between glaciations
and Caspian sea-level fluctuations is not well established.
Some authors claim that the Caspian Sea transgressions
and Northern Hemisphere glaciations developed nearly
simultaneously (Chepalyga 1984; Varuschenko et al. 1987;
Karpytchev 1993), while other workers suggest the opposite
( Velichko et al. 1987).
Mineral magnetic and palaeomagnetic measurements are
an important component of an ongoing multidisciplinary
study of the palaeolimnology of the Caspian Sea conducted
by a large Russian–French team, which has presented evidence of a major change in the Caspian Sea system during
the Pleistocene–Holocene transition (Chalié et al. 1997). In
this paper, we focus on the results of mineral magnetic
499
500
A. Jelinowska et al.
measurements of cored sediments from the south basin of the
Caspian Sea.
CO R E CO L LEC TI ON , S ED IM E NT
D ES CR I PT IO N A ND N O N -M A G NE TI C
A N A LY SE S
The 998 cm long sediment core SR-9402 GS05 (Museum
number SR01-GS9405) was collected from the south basin (lat.
38°45∞39◊N, long. 51°32∞16◊E, depth 518 m) with a Kullenberg
corer during a French–Russian cruise in August 1994 (Fig. 1).
Sediments from the core belong to two major units
(Fig. 2). The lower unit (998–90 cm, lithological unit 2) consists
of detrital-rich and carbonate-poor silty deposits, devoid of
endogenic biological remains. These deposits show a regular
alternation (on a millimetre scale) of black and grey layers
(Fig. 3), which rapidly disappears under oxidizing conditions.
The laminations are finer from 120 to 103 cm and the sediment
consists of pale silts. Oblique laminations suggest slumping in
the interval from 130 to 127 cm. A change occurs in the
uppermost 90 cm of the core ( lithological unit 1), with a
stepwise dissipation of laminations (over #30 cm) and the
appearance of homogeneous dark beige carbonaceous mud.
The major element content linked with the detrital supply
of sediments was determined using a scanning electron microprobe fitted with an energy-dispersive spectrometer (SEM/EDS).
Due to the weight deficit to 100, often unrelated to the water
content, the results obtained were recalculated to 100 per cent
(Si, Al, Fe, Ti oxide values in percentage of dry weight). These
elements, together with the Ca/Mg carbonate (calcimetry),
TOC (total organic carbon) (LECO pyrolysis procedure and
Rock Eval) and sulphur (microprobe and LECO) contents
were measured every 10 cm on bulk samples (Figs 2a–c). All
measured parameters clearly distinguish two intervals corresponding to the two lithological units. Above 90 cm depth, the
carbonate content is high (about 50–60 per cent), whereas
below it is much lower (about 20–25 per cent) and remains
constant. Values of sulphur and TOC are low throughout the
core (Chalié et al. 1997), with higher values above 90 cm depth
(S: 0.4–0.8 per cent; TOC: 0.5–0.2 per cent) and lower but
constant values below (S: ~0.2 per cent; TOC: ~0.2 per cent).
The parameters indicating detrital input show variations that
are clearly the opposite of those of the Ca/Mg carbonate
content, the carbonates ‘diluting’ the terrigenous components
of the uppermost (above 90 cm depth) sediments. Below this
section, constant values of all detrital parameters and almost
constant (only small, erratic variation) Si/Al, Fe/Al and Ti/Al
ratios (Fig. 2d), reflecting changes (if any) in composition of
the detrital fraction, indicate a constant detrital input.
The components of the carbonates were determined in detail
using SEM/EDS and X-ray diffraction (XRD). In this way,
besides shell debris, two fractions were identified: (1) authigenic
Ca/Mg carbonates consisting of #10 mm spindle-shaped
particles; and (2) detrital carbonates of non-calibrated grains,
Figure 1. Map of the Caspian Sea area showing the location of the studied core. Maximum extent of last glaciation after Arkhipov et al. (1995).
© 1998 RAS, GJI 133, 499–509
L ate Quaternary S. Caspian Sea sediments
501
Figure 2. Down-core variations in (a) total Si and Al contents (shown as oxides), ( b) total Ti and Fe contents (shown as oxides), (c) Ca/Mg
carbonate and total organic matter contents, (d) Si/Al and Ti/Al ratios (shown as oxides), (e) magnetic low-field susceptibility, (f ) intensity of NRM
after AF demagnetization at 20 mT, (g) intensity of ARM, and ( h) intensity of IRM at 0.5T (SIRM). Simplified lithofacies: 2=detrital-rich and
carbonate-poor deposits with laminations (millimetric alternations of black and grey layers); 1=poorly laminated sediment (lower part: 90−#60 cm)
and homogeneous dark beige carbonaceous mud (#60–0 cm). 14C corrected ages (see text).
without clearly defined shapes, of which calcite predominates,
while dolomite represents only 1–4 per cent of the total
sediment. These two fractions are therefore physically (by
sieving, microsampling, etc.) totally inseparable. The XRD
method enabled us to quantify amounts of authigenic versus
detrital calcite in the upper 120 cm (see below). No other
carbonates (siderite, etc.) were detected.
Apart from reworked shell debris of Mesozoic age, ostracod
shells are rare and siliceous microfossils are totally absent,
probably dissolved, from the lower 900 cm of the sequence.
Diatom valves appear at 95 cm depth, and remain rare, fragmented and partly dissolved until the top of the sequence.
R A D IO C A R B O N C H RO N O LO G Y
Organic remains are not preserved throughout the sequence.
The TOC content is very low: in the upper unit it is <0.5 per
cent and in the lower part unit <0.25 per cent. Because of the
preservation conditions in the field of the cored material, we
felt that this organic material was not suitable for dating.
Moreover, such low contents favour a sensitivity that is too
high to the lesser pollution phenomena. Similar difficulties
were encountered and described in full detail for Lake Baikal
sediments (Colman et al. 1997), for which the field conditions
and TOC contents were close to ours. We therefore did not
undertake 14C dating on the organic carbon.
The 14C dating was undertaken on carbonates. Authigenic
and detrital carbonate ratios may be determined by XRD,
according to the Mg content; the procedure is described in
full by Fontes et al. (1993). The Mg content of the detrital
phase increases gradually downwards so that in the lower
part of the sequence (up to 120 cm depth) the Mg contents of
both authigenic and detrital components were too close and
© 1998 RAS, GJI 133, 499–509
did not enable the two fractions to be distinguished. Such
discrimination was possible only in the upper 120 cm of the
sequence.
In this upper part, the 14C activity of the bulk carbonates
(hence apparent age) was measured, since authigenic grains cannot be separated from detrital grains (Escudié et al. 1997). The
14C activity of the authigenic calcite was calculated using massbalance equations (Fontes et al. 1993), integrating corrections
for the proportion of detrital carbonate, assuming that the
detrital phase is 14C-free. Moreover, we also integrated a
correction for the 14C activity of the total dissolved inorganic
carbon of modern sea surface water. Corrected ages are in
sequence, ranging from #12.2 (120 cm depth) to #4.0 (10 cm
depth) kyr BP; they indicate that the youngest part of the
Holocene is absent.
M IN ER A L M A G NE TIC S A M P LI NG AN D
M EA S UR E M EN TS
The core, cut into seven sections and preserved in plastic tubes
of 12 cm diameter, was sliced longitudinally into two parts
(one was kept as an archive and the second was used for
sampling). Parallel U-channels (transparent plastic tubes) and
several discrete samples were picked from the sediment core.
Tight sealing of the U-channels enabled good preservation
of the sediment (with laminations in the lower part of the
sequence) and measurements were performed soon after
sampling and therefore on fresh material. Discrete samples
were divided into two parts: the first part was stored before
analysis in a vacuum of 10–1 bar (for 2 days after sampling) to
limit oxidation and thus preserve fresh sediment properties,
and the second part was stored in air before analysis.
502
A. Jelinowska et al.
Figure 3. (a) Lithological unit 1, depth 31–60 cm. ( b) Lithological unit 2, from section 5, depth 689–717 cm. Photographs taken by M. Destarac,
Department of Geology, Museum d’Histoire Naturelle, Paris.
Low-field magnetic susceptibility ( x) was measured on
U-channels (measurements at 1 cm intervals) with a Bartington
MS-2 susceptibility meter. All remanent magnetization measurements (at 1 cm intervals) were made on U-channels with a horizontal 2G Enterprises pass-through cryogenic magnetometer.
Natural remanent magnetization (NRM) was demagnetized
with an alternating magnetic field up to 50 mT using the in-line
mounted alternating field (AF) demagnetizer. Anhysteretic
remanent magnetization (ARM) was applied by the superimposition of a 50 mT bias field on an alternating field that
decreases smoothly from a peak of 70 mT (it was impossible
to obtain higher AF values due to the limits of the apparatus
used). Isothermal remanent magnetization (IRM) was applied
using 0.3 and 0.5 T fields. Although saturation was generally
obtained at 0.3 T, 0.5 T was used to ensure the saturation of
IRM (SIRM). Magnetic hysteresis measurements were made
on discrete samples (#5–15 mg of dry, bulk sediment) with
an alternating gradient force magnetometer (AGFM 2900).
A peak applied field of 0.5–1 T was used for the hysteresis
measurements. These measurements were corrected for the
slope at high field ( x ) (above 0.375 T), which represents the
HF
contribution of paramagnetic and diamagnetic minerals. After
removal of this contribution, saturation magnetization (M ),
s
saturation remanent magnetization (M ) and coercivity (B )
rs
c
were obtained from the hysteresis loop. Remanent coercivity
(B ) was obtained by stepwise application of a backfield isocr
thermal remanence to remove the saturation remanence at 1 T.
The saturation of remanence was verified on the acquisition
curve. Transition electron microscope (TEM) fitted with an
energy-dispersive spectrometer (EDS), SEM/EDS and XRD
measurements were performed on magnetic extracts obtained
using a similar method to that of Papamarinopoulos et al.
(1982). TEM/EDS and SEM/EDS analyses were performed
on particles placed on supports, and results obtained for
TEM were processed using the RTS (ratio thin section)
program. The thermomagnetic behaviour of the magnetic
components of the sediment was determined on a horizontal
force translation balance in air.
© 1998 RAS, GJI 133, 499–509
L ate Quaternary S. Caspian Sea sediments
M I NE R A L M AG N ETI C R ECO R D
Variations of x, NRM, ARM and SIRM with depth define two
different zones of magnetic properties, which correspond to
the two lithological units (Figs 2e–h).
Late Pleistocene laminated sediment (lithological unit 2,
998–90 cm depth) is characterized by a high magnetic
grain concentration, with an M /mass value from 20 to
s
540×10−3 Am2 kg−1. The S-ratio measured on U-channels
(IRM
/SIRM=1) indicates the presence of ferrimagnetic
(0.3T)
minerals such as magnetite (Fe O ), maghemite (cFe O )
3 4
2 3
or greigite (Fe S ). This is confirmed by the back-field
3 4
remanence curves, which show saturation before 0.3 T (Fig. 4).
Observation of magnetic extracts through binocular and
optical microscopes show clay mineral flakes with small black
dots (Fig. 5a). SEM/EDS and TEM/EDS analyses (Figs 6a–c)
were performed on these black particles and also on their clay
support in order to correct the Fe content due to the clay
mineral. Some isolated black grains were analysed too. These
grains (Figs 5b–c) are composed of Fe and S. The Fe/S ratio
varies from 0.8 to 1 and indicates iron monosulphides. A few
isolated grains have an Fe/S ratio=1/2, which is characteristic
of pyrite. The XRD data identify the monosulphides as greigite
(Fig. 6d). These experiments also provide evidence of the
presence of small amounts of magnetite. The large size of the
magnetite grains (#10 mm), which contain small amounts of Ti,
indicates their detrital origin. The thermomagnetic behaviour
of the bulk fresh sediment samples at various depths (100, 200,
295, 400, 500, 580, 600, 700, 810, 890 cm) when heated in air
shows a major decrease in magnetization below 400 °C, which
is characteristic of greigite (Kobayashi & Nomura 1972;
Snowball & Thompson 1990a; Snowball 1991; Tric et al. 1991;
Hoffman 1992; Roberts & Turner 1993; Reynolds et al. 1994;
Jelinowska et al. 1995), and a second decrease to zero at
580 °C, which is characteristic of magnetite, this being the
503
product of greigite oxidation (Figs 7a–c). These sediment
samples show clearly that the remanence is carried by greigite,
with a major part of the total magnetization removed below
400 °C. Therefore, magnetite recognized in SEM analyses
represents a minor part of the magnetic particles and is not a
carrier of the magnetization.
Magnetic hysteresis parameters M /M and B /B of the
rs s
cr c
fresh sediment samples vary from M /M values of 0.52 to
rs s
0.22 and from B /B values of 1.37 to 2.62. These data plotted
cr c
on a bilogarithmic plot cluster along a mixing line near singledomain- (SD) and pseudosingle-domain- (PSD) like values
( black circles in Fig. 8a). Extrapolation of the mixing line of
Fig. 8(a) to B /B =1 gives a maximum M /M value of 0.73,
cr c
rs s
close to the value 0.75 obtained for a selection of natural
greigite-bearing samples by Roberts (1995). The hysteresis
loops for samples with PSD-like values do not show waspwaisted shapes (Fig. 8b), which are characteristic of a mixture
of SD and superparamagnetic (SP) grains, as indicated by
Roberts et al. (1995). This suggests that in our samples the SP
fraction is not significant and that these samples contain
particles rather larger than SD. It is also possible that in these
samples the PSD-like behaviour is due to the slightly increased
quantity of magnetite. The contents of paramagnetic and
diamagnetic fractions reflected by x measurements remain
HF
constant.
As demonstrated by other authors (Snowball & Thompson
1990a), greigite is very sensitive to oxidation. We present here
some of the measurements made on the fresh and oxidized
sediment samples from the same depths. The thermomagnetic
behaviour of oxidized samples does not show an increase of
the magnetization after 400 °C, characteristic of the transformation of greigite into magnetite during the experiment
(Figs 7d–e). Values of the magnetic hysteresis parameters B ,
cr
B , M /mass and M /mass decrease after oxidation of the
c
s
rs
sediment (Table 1).
Figure 4. Saturation remanence for samples from (a) 390 cm, (b) 590 cm and (c) 980 cm. B values are shown in circles on a logarithmic scale.
cr
Figure 5. (a) Optical photomicrograph of clay mineral flake with small black dots. TEM photomicrographs of (b) black particles of iron
monosulphides in a clay support and (c) isolated black grain of iron monosulphide. From H. Jacot des Combes (unpublished data), depth 787 cm.
© 1998 RAS, GJI 133, 499–509
504
A. Jelinowska et al.
Figure 6. Typical TEM/EDS spectra of magnetic minerals and their non-magnetic supports (clay minerals) extracted from laminated Late
Pleistocene sediments: (a) clay mineral, ( b) magnetic particle on clay support, (c) isolated magnetic particle (Cu is from sample support), and
(d) XRD pattern obtained for magnetic extract from oxidized sediment at a depth of 500 cm; G=greigite, Q=quartz, M=magnetite.
Figure 7. Thermomagnetic curves of bulk sediment samples heated at 200 °C/10 min in air atmosphere and in a magnetic field of 0.5 T: (a) 100 cm,
( b) 580 cm, (c) 700 cm—fresh sediment; (d) 580 cm, (e) 700 cm—the same levels as (b) and (c) but from oxidized material—and (f ) 14 cm depth.
The difference between the heating and cooling curves is due to the oxidation of the magnetic fraction. Sample (a), at 100 cm depth, is the only
one showing a significant presence of magnetite at around 400 °C; all other samples from the lower unit behave as samples ( b) and (c), that is the
magnetic moment around 400 °C is lower than 10 per cent.
© 1998 RAS, GJI 133, 499–509
L ate Quaternary S. Caspian Sea sediments
Figure 8. (a) Classification of magnetic minerals in terms of magnetization and coercivity ratios after Day et al. (1977) presented as a
bilogarithmic plot. Black circles: greigite-bearing unit 2; squares:
magnetite from the upper part of unit 1; open circles: the lower part
of unit 1. (b) Typical hysteresis loop shape for samples from laminated
lithological unit 2.
The concentration of greigite found in the Caspian Sea
varies throughout the Late Pleistocene laminated sediment
column. This explains the scatter of x, NRM, ARM and SIRM
values, and could also explain the PSD-like behaviour of some
discrete samples (a slight decrease in greigite corresponds to a
relative increase in magnetite). The varying distribution of greigite
was also found at laminae scale. The scattered occurrence of
505
greigite was also observed by others authors (Snowball &
Thompson 1990a). These authors suggested that it could be
a means by which the greigite- and magnetite- (often with a
homogeneous distribution) dominated magnetic mineralogy
may be distinguished. Long-period changes in our sediments
are superimposed on high-frequency changes of x, NRM, ARM
and SIRM; this is shown best by the ARM intensity record,
with a decrease in ARM values between 500 and 740 cm depth
(Fig. 2g). In this Late Pleistocene laminated part of the core
(1) the magnetic fraction is dominated by greigite, (2) there
are constant amounts of detrital elements such as Si, Ti and
Al (Figs 2a–b), and (3) the Si/Al, Fe/Al and Ti/Al ratios,
reflecting changes in composition of the detrital fraction, if
any, are constant (Fig. 2d). These observations suggest homogeneous distribution of detrital magnetite at the scale of longperiod ARM intensity changes. If this distribution of detrital
magnetite is constant, we conclude that the long-period ARM
variations are due to greigite variations. This is consistent with
the observation that in mixed magnetic assemblages, authigenic greigite dominates the ARM and SIRM/x only when
its concentration is relatively high (Snowball 1997). The first
possibility for explaining ARM changes in our core is a
variation of the greigite content. In this case the decrease in
ARM intensity between 500 and 740 cm depth, related to a
decrease in the greigite content, would have the effect of
decreasing the susceptibility, while magnetite and paramagnetic
contents remain constant. The second possibility is greigite
grain-size variation. A robust framework for the use of
magnetic properties in estimating the grain size of greigite has
yet to be established. Some authors (Roberts 1995; Housen &
Musgrave 1996) claim that the SIRM/x ratio is largely a
measure of magnetic grain size. Snowball (1997) showed that
SD grains of greigite exhibit certain parameters (ARM/SIRM)
similar to that of MD magnetite. Therefore, only the relative
variation of the size of greigite particles in our sediment may
be reasonably estimated using the ARM/x ratio, as has been
proposed for magnetite by King et al. (1982). Lower values of
the ARM/x parameter correspond to larger grains. Within the
Late Pleistocene part of the sequence, ARM/x (Fig. 9a) and
ARM versus x (Fig. 10) show a marked increase in grain
size between 500 and 740 cm depth. The ARM/x ratio shows
large variations between 90 and 500 cm (341.8±59.3 A m−1),
less dispersed and lower values between 500 and 740 cm
(253.9±24.4 A m−1) and again higher values below 740 cm
Table 1. Magnetic hysteresis parameters of fresh and oxidized bulk sediment samples from various levels.
fresh
ox
fresh
ox
fresh
ox
fresh
ox
fresh
ox
fresh
ox
fresh
ox
depth
cm
B
c
mT
B
cr
mT
M /M
rs s
B /B
cr c
M /mas
rs
10−3 Am2 kg−1
M /mass
s
10−3 Am2 kg−1
240
39.5
26.6
35.9
24.4
41.4
26.5
38.2
23.7
41.4
25.6
35.6
19.1
32.5
21.0
58.8
54.9
58.0
55.1
56.8
51.6
56.1
58.9
59.5
57.9
57.6
53.0
61.6
49.9
0.47
0.27
0.39
0.24
0.52
0.28
0.44
0.23
0.47
0.25
0.39
0.23
0.32
0.23
1.49
2.06
1.62
2.26
1.37
1.95
1.47
2.49
1.44
2.26
1.62
2.77
1.90
2.39
160.05
4.89
15.62
4.74
128.84
8.73
17.44
4.18
37.02
4.73
12.44
3.58
9.02
4.82
341.27
18.17
40.50
20.03
249.52
31.04
39.55
18.23
78.71
18.74
31.76
15.71
28.42
21.22
275
280
327
340
577
760
© 1998 RAS, GJI 133, 499–509
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A. Jelinowska et al.
Figure 9. Down-core variations in (a) ARM/susc, ( b) ARM (after AF demagnetization at 40 mT)/ARM, (c) SIRM/susc. Shaded interval corresponds
to the relatively larger greigite grain sizes (see text).
Figure 10. ARM versus magnetic susceptibility. Unit 1: open circles 2–65 cm depth; white squares 66–89 cm depth; unit 2: dots 90–500 and
741–980 cm depth; black squares 501–740 cm depth.
© 1998 RAS, GJI 133, 499–509
L ate Quaternary S. Caspian Sea sediments
(315.0±29.1 A m−1). The stepwise demagnetization of ARM
at 10, 20, 30 and 40 mT (Fig. 9b) also shows slightly lower
coercivities between 500 and 740 cm depth. The SIRM/x values
show a similar variation (Fig. 9c). SIRM/x of natural greigite
has been reported as having high values [#40–110 kA m−1,
Horng et al. (1992); 70–80 kA m−1, Snowball & Thompson
(1990a); #40 kA m−1, Snowball & Thompson (1990b);
19–24 kA m−1, Hilton (1990); 50–70 kA m−1, Reynolds et al.
(1994); >70 kA m−1, Fassbinder & Stanjek (1994)]. Synthetic
greigite shows values of about 40–75 kA m−1 (Dekkers &
Schoonen 1996). Most of these natural samples contained
small-grained greigite. An arbitrary value of 40 kA m−1 was
used to distinguish greigite from magnetite by Snowball
& Thompson (1990b). In our record few high-frequency
spikes fall below this value. The SIRM/x ratio shows higher
values (#81±30 kA m−1) from 90 to 500 cm, lower values
(#55±16 kA m−1) between 500 and 740 cm and higher values
again (#70±25 kA m−1) below 740 cm. It is likely that
variation in the SIRM/x ratio is inversely related to the grain
size, and that lower values correspond to larger grains in
our case.
Holocene poorly laminated (from 90 to #60 cm) and nonlaminated (#60–0 cm) homogeneous dark beige sediments
( lithological unit 1) are characterized by a low concentration
of magnetic minerals (6<M /mass<10×10−3Am2 kg−1).
s
The S-ratio indicates that all the grains are saturated below
0.3 T, as in the lower unit. The thermomagnetic behaviour in
air of the bulk sediment from a depth 14 cm clearly indicates
the presence of magnetite as the dominant magnetic mineral
(Fig. 7f ). SEM/EDS analysis of the magnetic extract (#14 cm),
shows Fe, occasionally with small amounts of Ti. This confirms
the presence of magnetite and indicates its detrital origin.
M /M versus B /B of samples from the homogeneous dark
rs s
cr c
beige sediment (#60–0 cm) shows scattered points in the PSD
range (squares in Fig. 8a), as is commonly found in detrital
material. Other parameters such as SIRM/x, ARM versus x
and ARM AF demagnetization (Figs 9b–c and 10) clearly
show the difference between the mineral population in the
lower Late Pleistocene laminated part of the sequence and this
upper part of the sequence. Magnetite dominates the magnetic
fraction in this lithological unit but the smooth passage
between the units with intermediate values from 90 to #60 cm
in Fig. 10 suggests the existence of a mixture of magnetite and
a decaying quantity of greigite in this part of the sequence.
The presence of greigite is also suggested here by the small
scatter of points around the mixing line of greigite (open circles
in Fig. 8a).
D IS C U S S IO N
Iron sulphides: formation and significance
The formation of authigenic iron sulphides, with pyrite (FeS ),
2
which is the most stable iron sulphide, is possible during early
diagenesis and is related to the bacterial degradation of organic
matter in anoxic environments with sufficient quantities of
H S and dissolved iron (Berner 1971, 1980, 1981; Curtis 1987;
2
Roberts & Turner 1993). In environments with a low organic
matter content and/or low sulphate conditions ( low salinity
and/or fresh water), iron sulphide formation will be limited
to cryptocrystalline FeS or greigite (Berner 1981), which
persists upon burial. Therefore, the presence of greigite in such
© 1998 RAS, GJI 133, 499–509
507
sediments with a constant iron content may be a useful palaeosalinity indicator (Berner 1971, 1981; Berner et al. 1979). Greigite
could also be produced by several species of magnetotactic
bacteria. In this case, however, intracellular magnetosomes
show an excellent sorting of single-domain grains (Fassbinder
& Stanjek 1994). The distribution of grain sizes of greigite in
the Caspian Sea sediments and the absence of chains of greigite
magnetosomes suggest an inorganic origin. Because of the low
and constant organic matter content and the constant detrital
iron input to the Caspian Sea recorded in sediments from
the lower laminated part of the core (Figs 2a–d), the greigite
present here indicates the fresh-water or low-salinity character
of the water body in the Late Pleistocene.
The iron sulphide can form directly from sulphidic bottom
waters as well as from interstitial waters in basins with
restricted water circulation. The best evidence of an anoxic
depositional environment is the presence of finely laminated
sediment, resulting from a lack of bioturbational mixing by
benthic organisms (Berner 1981). In the Caspian Sea sediments, laminae are not destroyed by bottom dwellers and they
disappear in the open air within a few hours. This lamination
seems to be related only to the varying amounts of labile iron
monosulphides (variations too small to be detected with
TEM/EDS analyses). This allows us to rule out a depositional
origin for the lamination, which would thus be due to an
early diagenetic process (related to the authigenesis of iron
sulphides), reflecting subtle variations in the depositional
environment. This diagenetic origin of lamination is also
indicated in Russian literature (Maiev et al. 1989; Kuprin &
Bagirov 1971), where black laminae are attributed to increased
contents of iron monosulphide (called hydrotroillite in the
Russian literature). If these laminae are of early diagenetic
origin, the creation of iron sulphides depends on the sediment
pore-water composition. In this way, the changes of greigite
concentration reflect pore-water fluctuations in the sediment
via diffusive exchange with corresponding fluctuations in the
overlying water. These rapid changes at the laminae scale are
possible only near sediment–water interfaces, and not deeper
in the sediment, where differences in water composition are
damped with depth due to diffusion. Moreover, laminae are
laterally continuous and horizontal, suggesting formation near
the water–sediment interface.
Thus, the presence of laminae and greigite provides evidence
of anoxic conditions in the South Caspian Sea near the
sediment–water interface during the Late Pleistocene. It also
indicates the authigenesis of iron sulphides near the sediment
surface, rather than at depth after burial. The dominance of
iron monosulphides indicates, as discussed above, a brackish
or fresh-water body. The constant values of TOC and Fe, and
the increase in greigite grain size (and/or conversion of greigite
to pyrite) suggested by the magnetic parameters between #500
and 740 cm depth, could be related (Housen & Musgrave
1996) to an increase in the water salinity in the Caspian Sea
basin during this period. Nevertheless, the water remained low
salinity because greigite was growing and did not completely
transform to pyrite.
Environmental implications in the South Caspian Sea area
The change in magnetic mineralogy at #90 cm depth reveals
an important change in the operation of the Caspian Sea
system which marks the Late Pleistocene–Holocene transition.
508
A. Jelinowska et al.
The presence of greigite indicates brackish or fresh-water
conditions in the Late Pleistocene. It also provides evidence
of anoxic conditions within near-bottom sediments, which
implies poor ventilation of the basin. During this period, other
complementary proxies indicate the important and constant
input of detrital material into the basin, the low contribution
of endogenic Ca/Mg carbonates (Figs 2a–d) and a lower
evaporation rate than today (Chalié et al. 1997). These results
most probably reflect the higher level of the Caspian Sea
during the Late Pleistocene than at present and a relatively
high erosion rate in its catchment area. The relationship
between ventilation of the deep sea waters and the water level
suggested here was observed over the past century: deep
water ventilation has significantly decreased during phases of
relatively higher water levels (Kosarev & Yablonskaya 1994).
The change of the magnetic parameters between 500 and
740 cm depth, interpreted as the increase in greigite grain sizes,
can be related to the increased salinity of waters due to the
lower water level, while the basin is still poorly ventilated. The
simplest explanation of this would be that corresponding
regression was less marked than that in the Holocene. The
transition to the Holocene is marked by the gradual disappearance of greigite and the appearance of detrital magnetite. This
implies the progressive establishment of good ventilation and
the total oxygenation of the water and the bottom sediments
in the Holocene. This change occurs simultaneously with an
increase in the abundance of endogenic Ca/Mg carbonates
and an important decrease in the detrital fraction proportion.
This suggests a decrease in the erosion and/or a reduction of
the catchment area and a decrease in the Caspian Sea level to
its present elevation.
Changes of the Caspian sea water level can also be deduced
from the salinity variation of the waters: because the Caspian
Sea is a closed basin, lower salinity is related to higher water
levels. The salinity of the pore waters measured in sediments
from our core was lower in the Late Pleistocene than today
(Chalié et al. 1997). This change in the salinity of the water,
also found by Russian authors (Loiatina et al. 1996), confirms
our interpretation of the mineral magnetic record.
The anoxic bottom conditions, similar to those in the Late
Pleistocene in our study, are also known from Black Sea
sediments. In the Black Sea sediments, the black FeS minerals
are completely converted to pyrite (resulting in grey sediments)
due to the high contribution of sea-water sulphate (Berner
1970). However, in older buried sediments of the Black Sea,
the black FeS-rich sediments are interbedded with grey pyritic
clay without FeS minerals. The black clay represents an
arrested stage of diagenesis caused by a low concentration of
sulphates in the bottom waters at the time of deposition. This
is due to the desalinization of the Black Sea during the last
glaciation, when the worldwide sea level was lowered below
the Bosphorus and the input of sea water into the Black
Sea stopped.
CON CLU SION S
(1) Caspian Sea sediments over the Late Pleistocene and
Holocene periods present two different magnetic-mineraldominating assemblages: authigenic greigite in the former and
detrital magnetite in the later.
(2) A major difference in the conditions of circulation and
ventilation of this water body therefore has to be considered:
poorly oxygenated conditions in the Late Pleistocene and
well-ventilated oxic conditions in the Holocene. This difference
may be explained by a difference of the total area and volume
of the Caspian Sea, which was much larger for the Late
Pleistocene interval covered in this study than in the Holocene.
(3) The presence of greigite indicates brackish or freshwater conditions in the Late Pleistocene. Biological indicators
(diatom flora) of water salinity are absent from these sediments,
therefore greigite appears to be an important indicator of
this factor.
(4) Variations of the magnetic parameters, interpreted in
terms of greigite grain size within the Late Pleistocene, are
related to climatic change: an increase in greigite grain size
probably reflects increased salinity of the South Caspian Sea.
This may indicate a regression, but probably only to a water
level higher than in the Holocene.
(5) This interpretation has to be supported by other
independent proxies, and the correlation of the Caspian Sea
water-level fluctuations with climatic global changes can be
established only when absolute ages are obtained.
(6) The next step of the study is the establishment of the
reasons for black and grey laminae and the understanding of
the cyclicity of this phenomenon.
A CKN O W LE DG M ENT S
This work is part of the INSU-DYTEC Caspian program
and INCO-COPERNICUS European Union funded project
‘Understanding the Caspian Sea erratic fluctuations’ No.
IC15-CT96–0112. We thank IFREMER-Genavir and CEA for
their support during the sea cruise. This cruise would not have
been possible without the support of the Russian Academy of
Sciences, numerous Russian institutions and the Russian Navy,
who allowed us to use a military ship. The crew of the ship
and the whole scientific French–Russian shipboard party
contributed to the collection of cores. We profoundly thank
C. Kissel for access to the equipment of the Palaeomagnetic
Laboratory in CFR (Gif s/Yvette) and discussions on this
subject. The French and Russian teams working in the Caspian
Sea project are acknowledged for discussions on this subject.
AJ was supported by a grant from the Société de Secours des
Amis des Sciences. The authors are greatly indebted to two
anonymous reviewers for extended comments on and corrections
to an earlier version of this manuscript.
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