To be able to study the stratigraphic sequences caught in our coring
tubes, plastic liners first need to be opened. We use a specialised core
splitter with two circular saws in order to make parallel, lengthwise incisions,
which can be cut through easily with a knife. Once the liner has been divided
into two halves, the inside sediment still has to be separated. Because surface
deposits from the bottom of Lake Hamana appeared to be extremely runny and
water saturated, we prefer to use metal plates for splitting. Two of these
plates are being pushed through the lengthwise incisions and through the
sediment until hitting the underlying table top, after which they can be pulled
apart. One after the other, the metal plates are slid off the core halves, providing
us with clean sediment surfaces.
Some additional, manual retouches might be necessary for acquiring good
images with the linescan photography setup, which is part of the Ghent
University GEOTEK Multi-Sensor Core Logger (MSCL) (http://www.geotek.co.uk/products/mscl-s-). The MSCL is a tool for
non-destructive, downcore analyses of several geophysical properties of the
sediment, such as density, magnetic susceptibility and colour spectrophotometry.
Density values signify how tightly the matter is packed together and are a good
indicator for porosity and lithology variations. Magnetic susceptibility (MS) is
the degree in which a material magnetises in response to an applied magnetic
field. Positive measurements suggest that the analysed sediment might be
paramagnetic, ferromagnetic, ferromagnetic or antiferromagnetic. MS data can
thus be extremely useful when looking for changes in sedimentary provenance
and/or diagenetic processes. Colour records from the spectrophotometer provide
true colour representations of split core surfaces before oxidation processes
can take place. Therefore, it is of major importance that MSCL logging is done
immediately after core opening. Moreover, sediment colour reflects
compositional elements and mineralogy as well, complementing density and MS
data.
Each of these properties are measured stepwise with contact
point-sensors attached to the MSCL. Logging results in full downcore profiles
of the above properties and can be used for further study. For instance,
stratigraphic sequences will be described macroscopically, paying special attention
to sediment colour, texture, structure, grain size, contacts etc. These
macroscopic observations can then be correlated with the geophysical
properties, hopefully enabling us to recognise some event deposits.
The below (highly speed up) timelapse video shows how split sediment cores move through the linescan setup of the GEOTEK MSCL. Meanwhile, several geophysical properties are measured and acquired data are shown on the computer screen.
