ENERGY DISPERSIVE X-RAY MICROANALYSIS OF
LAMINATED SEDIMENTS
FROM LAKE VALKIAJÄRVI, FINLAND
TUOMO A L A P I E T I and MATTI SAARNISTO
ALAPIETI T. AND SAARNISTO M. 1981: Energy dispersive X-raymicroanalysis of laminated sediments from Lake Valkiajärvi, Finland. Bull. Geol. Soc. Finland 53—1, 3—9.
Energy dispersive X-ray spectrometry is applied to the study of
thinly laminated organic sediments of the iron-meromictic Lake
Valkiajärvi, Finland. Si, Fe, Ca, K, S, (and Cl as an artefact) are
present in detectable quantities. It is concluded that the main reason
for the occurrence of such annual laminations (i.e. varves) in this lake
is the seasonal variation in the deposition of mineral matter. Iron
remains low throughout the year and does not play any important
role in varve formation.
Tuomo Alapieti and. Matti Saarnisto, Department
versity of Oulu, SF-90570 Oulu 57, Finland.
Introduction
Sediments f r o m the 25 m deep basin of
Lake Valkiajärvi, Finland, w e r e found to
contain thin laminations each comprising a
light-dark couplet averaging 0.3 m m in
thickness (Fig. 1). The laminations are believed to be annual, i.e. t r u e varves, which
reflect seasonal variations in m i n e r a l m a t t e r
accumulation. It has been suggested (Koivisto
& Saarnisto 1978, Saarnisto 1978, 1979) t h a t
t h e light l a y e r s r e p r e s e n t m a i n l y m i n e r a l
m a t t e r deposited in early s u m m e r a f t e r the
melting of the ice. T h e proportion of m i n e r a l
m a t t e r and its particle size decrease u p w a r d s
t h r o u g h each lamina, while the proportion
of d a r k organic m a t t e r increases t o w a r d s the
winter. T h e b o u n d a r y b e t w e e n the d a r k
w i n t e r layer and the light spring layer is
sharply-defined.
of Geology,
Uni-
Lake V a l k i a j ä r v i is meromictic, i.e. perm a n e n t l y chemically stratified, w i t h a v e r y
high iron content in t h e bottom w a t e r (Meriläinen 1970). The sediments of another iron-
Fig. 1. The laminated sediment of Lake Valkiajärvi as revealed by a microphotograph of a thin
section. Each light-dark couplet represents one
year, i.e. a varve. Average varve thickness ca.
0.3 mm.
4
Tuomo
Alapieti
and Matti
Saarnisto
meromictic lake, the Lake of the Clouds in
Minnesota, also contain thin
dark-light
a n n u a l laminations. A n t h o n y (1977) has suggested t h a t the precipitation of ferric iron
due to t h e partial oxygenation of iron-rich
bottom w a t e r d u r i n g overturns, mainly in
spring, gives rise to the light-coloured lamina,
while the d a r k layer would represent organic
r e m a i n s t h a t settle out d u r i n g t h e summer.
T h u s the precipitation of iron emerges as the
main constituent in the f o r m a t i o n of these
a n n u a l laminations.
The purpose of the present study w a s to
test in the case of L a k e V a l k i a j ä r v i w h e t h e r
the simple mechanism of seasonal variations
in mineral m a t t e r deposition w a s superimposed upon seasonal changes in the p r e cipitation of iron and some other elements.
The use of an energy-dispersive X - r a y microanalysis enabled t h e variations in silicon,
iron, calcium, dhlorine, potassium and s u l p h u r
within t h e laminations to be analysed, and
their role in the formations of the laminations
to be discussed. T h e e l e m e n t analyses w e r e
p e r f o r m e d by the a u t h o r T.A. in t h e Institute
of Electron Optics at t h e University of Oulu.
Lake Valkiajärvi and its sediment
L a k e V a l k i a j ä r v i is situated in the comm u n e of Ruovesi in the L a k e Region of Finland (61°51'N Lat., 23°53'E Long, elevation
110 m a.s.l.), in a granodioritic bedrock basin
(of. Matisto 1961). The lake is deep in comparison to its surface area of only 7.84 hectares, having a mean depth of 8.4 m and a
m a x i m u m d e p t h of 25 m. T h e following brief
listing of its limnological p a r a m e t e r s follows
Meriläinen (1970).
L a k e V a l k i a j ä r v i is meromictic, i.e. p e r m a n e n t l y chemically stratified. The sharply
defined, stable chemocline at 17 m separates
the u p p e r mixolimnion f r o m t h e monimolimnion. The f o r m e r accounts for 93.4 °/o and
the latter 6.6 °/o of the total w a t e r volume of
the lake. T h e mixolimnion undergoes mixing
during spring and a u t u m n o v e r t u r n s (not
complete in L a k e Valkiajärvi), and oxygen
is present in abundance, w h e r e a s the amounts
of dissolved substances are low. T h e monimolimnion is highly stable, with h a r d l y any
mixing. It contains high concentrations of
dissolved substances, b u t is entirely lacking
in oxygen t h r o u g h o u t the year. The high
specific conductivity (max. 1370 juS) indicates
an e n r i c h m e n t of dissolved salts in t h e
monimolimnion. As a consequence of its high
bicarbonate content, the monimolimnion is
also rich in total C0 2 . Similarly the p H rises
somewhat in t h e monimolimnion, although
the reaction is acid t h r o u g h o u t t h e w a t e r
column.
By v i r t u e of its mixolimnion L a k e Valkiaj ä r v i is an unproductive oligotrophic lake.
P r i m a r y production of p h y t o p l a n k t o n is small
and the macrophytic aquatic vegetation is
poorly developed.
The high concentration of iron (max. 380
mg per litre) suggests t h a t this is the most
i m p o r t a n t f a c t o r responsible f o r the p e r m a n e n t chemical stratification of Valkiajärvi.
Manganese (max. 10 mg per litre) and calcium
also contribute m a r k e d l y to t h e stability of
the monimolimnion. Concentrations of calcium, potassium, sodium, silicon and ammonia
in monimolimnion are about 40, 9, 4, 7, and
22 times as great, respectively, as in t h e mixolimnion.
Iron is practically insoluble in the mixolimnion because of the high oxygen concentration, w h e r e a s in the monimolimnion iron
remains in solution in the f o r m of f e r r o u s
bicarbonate because of the lack of oxygen,
the presence of sufficient amounts of carbon
dioxide, a p H a r o u n d 6.5, and organic m a t e rial as a reducing agent f o r f e r r i c hydroxide.
In addition, the concentration of hydrogen
sulphide (H 2 S) m u s t be low. This is the case
in the u n p r o d u c t i v e L a k e Valkiajärvi, as
Energy dispersive X - r a y microanalysis of ...
hydrogen sulphide is largely produced by
decomposing organic matter.
The iron content in t h e meromictic Lake
of the Clouds is higher t h a n in Lake Valkiaj ä r v i ( m a x i m u m 620 mg and 380 mg per litre
respectively), and the chemocline is not so
s h a r p and stable as in Lake Valkiajärvi, and
is situated only about 2 m above t h e bottom
at 31 m. The monimolimnion in the L a k e
of the Clouds is v e r y small in area and
undergoes at least partial oxygenation and
mixing d u r i n g o v e r t u r n s (Anthony 1977).
The sediments of L a k e V a l k i a j ä r v i w e r e
sampled only in the area below the chemocline, w h e r e the organic sediment sequence
is nearly 3 metres in thickness. This rests
on a massive sand layer of varying thickness
(1—10 cm). F u r t h e r down, the sediment is
composed of thinly v a r v e d clay to an u n k n o w n depth. The organic sediment is consistently laminated t h r o u g h o u t its entire
thickness, w i t h an average v a r v e thickness of
0.3 mm, thus representing a sedimentary
record covering nearly 9000 years. T h e varves
are s o m e w h a t thicker n e a r the b o t t o m than
higher up, and s h o r t - t e r m fluctuations in
v a r v e thidkness also occur elsewhere. T h e
thicker varves are normally lighter in colour,
and in some cases they contain vast quantities
of diatoms, w h e r e a s t h e t h i n n e r v a r v e s are
brownish-black. T h e loss-on ignition increases f r o m 5 °/o at t h e bottom u p to 40 to
50 °/o for the m a i n p a r t of the sediment.
Vivianite [Fe 3 (P0 4 ) ä • 8 H a O] is a b u n d a n t in
the lower sediment sequence, its small, w h i t e
nodules (which become blue upon oxidation)
being observable mostly in the dark, h u m u s rich w i n t e r layers.
Laboratory studies
The m a t e r i a l used in the present s t u d y was
sampled w i t h a 90-mm d i a m e t e r P V C - t u b e
piston corer (Core Va-75) f r o m the deepest
5
(25 m) p a r t of the lake. A piece of f r e s h
sediment (surface area 2 X 2 cm) in which the
varves w e r e reasonably clear w a s dried at
room t e m p e r a t u r e and t h e dry sample i m mersed
in
the
impregnating
medium
(EPOFIX) u n d e r a vacuum. Polished thin
sections w e r e m a d e using a p r o c e d u r e
modified f r o m the method of Moreland (1968).
The electrical conductivity necessary f o r
microanalysis w a s achieved b y a thin surface
layer of v a c u u m - e v a p o r a t e d carbon.
Microanalysis
technique
The microanalyses w e r e p e r f o r m e d using a
J E O L JSM-35 scanning electron microscope
(SEM) equipped w i t h P G T DELTA X C E L
energy dispersive spectrometer (EDS). The
main problem in analysing loose, h u m u s - r i c h
material is specimen damage caused b y t h e
electron beam. This effect is much less pronounced w h e n using an SEM + EDS combination, however, t h a n in the case of an
SEM equipped w i t h crystal spectrometers or
in a conventional microprobe, both of which
need a b e a m c u r r e n t of 100 times t h e
m a g n i t u d e of t h e SEM + EDS.
P r e l i m i n a r y qualitative microanalyses w e r e
carried out first, in which a p p r o x i m a t e l y
1 sq. m m of the specimen surface w a s scanned
by the electron beam, and the whole X - r a y
spectrum f r o m about 1 keV to 20 keV was
recorded by the EDS. The elements iron,
calcium, potassium, chlorine, s u l p h u r and
silicon w e r e detected in the spectrum.
These elements w e r e t h e n studied by means
of a digital line profile analysis, in which
the characteristic X - r a y lines of the elements
w e r e simultaneously m a p p e d along a single
line. The electron b e a m of the SEM w a s
moved digitally step b y step across the
sample, and data for each element w e r e collected f r o m exactly the same points on the
scan line. The time for one complete pass
was 1000 seconds and the n u m b e r of points
6
Tuomo
Alapieti
and Matti
Saarnisto
Fig. 2. Backscattered electron image of laminated sediment (A), and digital line scans of Si and Fe (B),
K and Ca (C), and S and Cl (D). The centre line on the backscattered electron image represents the
analysis line across the sample. The original magnification of the scanning electron microscope was 47x.
analysed was 256. The step intervals in the
two-line profiles w e r e 7.2 and 3.7 microns,
respectively.
The m e a s u r e m e n t data stored in the
m e m o r y of the EDS w e r e t h e n t r a n s f e r r e d
to the cathode — ray tube of the SEM, and
the distribution p a t t e r n s of the elements
analysed w e r e photographed. These results
are presented in Figs. 2—3. Semiquantitative
weight percentage scales w e r e calculated, and
are also shown in the figures.
Results
The distribution p a t t e r n s presented in Figs.
2 and 3 suggest t h a t silicon and potassium,
and to some extent also calcium, v a r y in a
broadly similar m a n n e r . This involves a
m a j o r periodicity of the same order as t h a t
in the varves themselves, m a r k e d b y pronounced peaks in concentration, and a minor
periodicity corresponding in f r e q u e n c y to
t h a t observable in the iron content.
The
Energy dispersive X - r a y microanalysis of . . .
7
Fig. 3. An enlarged detail from Fig. 2. The original magnification of the scanning electron microscope was 90x.
concentrations of silicon v a r y in the range
1—15 o/0j i r o n in the range 0.5—5 %> and
chlorine, sulphur, calcium and potassium in
the range 0.1—0.6 %>. Silicon and iron also
show high peaks due to detrital mineral
grains.
The variations in chlorine, which is derived
f r o m the impregnating medium, largely
reflect the porosity of the sample. As a consequence of the relatively high chlorine concentration in this impregnation material, no
indication is obtained of any possible occurrence of chlorine in the sediment itself.
The sulphur present also originates in p a r t
f r o m the impregnating medium, b u t the peak
values f r o m 0.5 °/o up, which coincide w i t h the
m i n i m u m levels of silicon, are higher t h a n
those seen at the margins of the figure, which
represent the impregnating m e d i u m alone,
so t h a t they must denote actual s u l p h u r concentrations present in the sediment.
The concentrations of manganese are so
8
Tuomo
Alapieti
and Matti
Saarnisto
low t h a t they cannot be detected by energydispensive microanalysis, although figures of
0.1—0.3 °/o are obtained using a microprobe
equipped with a crystal spectrometer, and
the values m a y be considerably higher in
certain v e r y n a r r o w peaks which coincide
with peaks in iron concentration.
Conclusions
Silicon, the most a b u n d a n t element in the
sediment of Lake V a l k i a j ä r v i a f t e r organic
matter, shows r h y t h m i c changes comparable
with the suggested v a r v e thickness of 0.3 mm,
while the variation in potassium seems to be
closely connected w i t h that of silicon, possibly indicating t h a t these are p a r t l y derived
f r o m feldspars and micas, and are thus of
detrital origin. Quartz grains are obviously
the main source of silicon, however, although
also originating in p a r t f r o m dissolved diatom
f r u s t u l e s (Meriläinen 1970). Diatom blooms
are associated w i t h a high Si oontent in the
water, and are t h u s superimposed on the
r h y t h m i c changes in the q u a n t i t y of m i n e r ogenic silicon. Large mineral grains produce
erratic, n o n - r h y t h m i c variations in the elem e n t content of some lamina, t h u s disturbing
to some e x t e n t the impression of a highly
regular lamination, as seen in Fig. 1, f o r e x ample.
Iron r e m a i n s low and does not show any
r h y t h m i c changes which could be related to
the visual changes in t h e sediment. Iron in
the sediments of Lake V a l k i a j ä r v i has clearly
smaller values than in light laminae in the
Lake of the Clouds w h e r e it rises to over
20 °/o in places (Anthony 1977). Oxygenation
of the iron-rich bottom w a t e r seems to be a
prerequisite for iron-rich laminae in t h e
sediments, as is the case in the L a k e of t h e
Clouds (see also Renberg 1979). U n d e r the
reducing conditions prevailing in the monimolimnion of Lake Valkiajärvi, only a
p l e n t i f u l supply of H ä S could precipitate f e r rosulphide in the sediments (see description
of the limnology of L a k e Valkiajärvi). Sulp h u r remains low here, however, and reaches
its m a x i m u m values w h e n silicon is low, i.e.
in the h u m u s - r i c h laminae. It m a y t h e r e f o r e
be suggested t h a t the iron in t h e monimolimnion originates f r o m outside the lake
r a t h e r than by dissolving out f r o m the sediment. Iron compounds are highly insoluble
u n d e r the stable conditions prevailing in the
monimolimnion (see discussion in Meriläinen
1970, also K j e n s m o 1967: 294—299).
In addition to iron, m a n g a n e s e and calcium
are a b u n d a n t l y p r e s e n t in the monimolimnion
(Meriläinen 1970). Manganese is h a r d l y detectable in the sediment, however, and t h e
calcium values are also low. The origin of
these in the bottom w a t e r must thus also lie
outside the lake. Calcium together w i t h silicon, iron, and potassium mostly originate
f r o m w e a t h e r i n g products of t h e s u r r o u n d i n g
granodioritic bedrock.
To sum up, the macroscopically observable
thin laminations in L a k e V a l k i a j ä r v i reflect
seasonal variations in mineral m a t t e r deposition, the highest amounts being transported
by spring floods a f t e r the melting of the ice.
The proportion of organic m a t t e r increases as
w i n t e r approaches, the season w h e n the sulp h u r content of the sediment is at its highest.
A scanning electron microscope equipped
with an energy dispersive spectrometer,
as used as routine equipment in mineralogy
and metallurgy, is a fast and r a t h e r accurate
tool f o r investigating loose, h u m u s - r i c h
sediments. Thus equipped, the SEM combines its usual imaging ability with some of
the analytical advantages of an electron
microprobe and is an effective i n s t r u m e n t
also in sedimentology.
Acknowledgements
— We thank Mr. Seppo Sivonen for critically reading part of the manuscript and for providing valuable suggestions,
Energy dispersive X - r a y microanalysis of...
Dr. Gunnar Digerfeldt for reading the manuscript,
Mrs. Ulla Paakkola for making the thin sections,
Mrs. Raija Peura and Mrs. Sinikka Komulainen
9
for their help in the SEM + EDS- instrument
operations, and Mr Malcolm Hicks for checking
the English of the manuscript.
References
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Manuscript received, June 3, 1980