Verlag des Forschungszentrums Jülich
JUEL-3968
Rings, Andreas
Extraktion von Diatomeenschalen mittels SPLITT-Fraktionierung
151 S., 2002
Diatom frustules are an interesting tool for climatic
and ecological research and they have for some
time been employed in isotope research work as a
paleothermometer. Diatom frustules are said to be
very stable over a long period of time, and thus the
stable oxygen isotopes within the frustule' s SiO2
can under certain conditions represent a good proxy
for climatic or ecological reconstructions (Juillet,
1980; Juillet & Labeyrie, 1987; Shemesh et al.,
1992). Especially in those cases where carbonates
are not available for climate research, diatoms seem
to be a very promising alternative. This is for
example the situation in the high resolution
sedimentary sequences of Westeifel Maar lakes.
The sedimentary sequences consist of turbidites,
siderit-laminites and annually laminated
diatomaceous gyttja but mostly lack any carbonates
(Negendank & Brauer & Zolitschka, 1990). The
problem, however, is to get highly purified diatom
material for reliable oxygen isotope investigations.
This implies that minerogenic impurities have to be
separated since they can easily alter the isotope
signal of a diatom layer and, thus, lead to wrong
results. Additionally it is in many cases desirable to
distinguish between diatom fractions of different
sizes, i.e. to distinguish between different diatom
species. It therefore was the aim of this work to
develop a new method for separating quantitatively
diatom frustules from lake sediments for
investigations of the stable oxygen isotope
composition in diatom SiO2. This method uses a
new technique for separating particles depending on
their sinking velocity in a liquid, streaming through
a narrow channel. The technique to be used splits
particles hydrodynamically into different size
classes, a procedure which is called SPLITT -
fractionation (split-flow lateral-transport thin
separation cells) and was first presented by
Giddings (1985).
As with field-flow fractionation (FFF) techniques,
thin flow channels in conjunction with a field
(gravitation) applied perpendicular to the flow are
used for SPLITT-fractionation. A sample
suspension containing the particles to be separated
is continuously introduced through one inlet into
the cell while sediment-free liquid is introduced
into the cell through a second inlet. Within the cell
the two flows merge smoothly, the suspension
overlying the sediment-free carrier flow, resulting
in a laminar flow in which the separation of
particles takes place. Within the laminar current
grains migrate according to their density, shape and
size towards either of two outlets. As a result the
sample will be divided into two fractions around a
cut-off diameter, whicb is controlled by the ratio of
the two outlet flow rates.
As diatoms bebave bydrodynamically different
compared with mineral grains, the described
method provides a tool to separate them by
SPLITT -fractionation. The advantages of SPLITT -
fractionation over other separating methods for
diatoms are reproducibility, high throughput by
continuous flow, minimum losses, fast procedure
and minimum contamination of the sample. The
SPLITT cell used in this study bas a length of
20 cm, a breadth of 4 cm, and a beight of 371µm.
The sample concentration used was below I weight
percent to minimise particle-particle interaction
whicb spoils resolution. Sediment samples of
different origin, age, compaction and diatom
amount were tested. In a first step a method was
establisbed to suspend the material in water with
H2O2 (30% ). In this process also most of the
organic matter was removed, to prevent
agglomeration in the sample suspension.
Afterwards the samples were sieved into fractions
of > 80µm, 20 - 80µm and < 20 µm. The fractions
< 80 µm were used for SPLITT-fractionation. The
fraction 20 - 80 µm was fractionated in one step.
Because of the small amount of grains with
bydrodynamical properties similar to diatom
skeletons, it was relatively easy to get a clean
diatom fraction. The fraction < 20 µm was treated
several times to enricb a sample fraction with
diatoms. Because of the high amount of mineral
grains (especially grains with equal sinking
velocities) it wasn't possible to get an entirely clean
sample of diatom frustules. To avoid problems
related therewith an additional step was introduced
by dividing the corresponding samples with 5 and
10 µm sieves. The resulting fractions do no more
contain minerals with bydrodynamical properties of
diatoms and therefore a separation of diatom
frustules is again possible.
In addition a method was developed for analysing
grain distribution and amount of diatom frustules
using image processing methods. For eacb SPLITT -
fractionation subsamples from the two outlets were
collected and pbotograpbed under the microscope.
The pictures were then evaluated using image
processing software. The pbotograpbs represented
the particle size distribution of the fractions, the
amount of frustules and gave a cut-off diameter at
different in- and outlet flowrates.
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