Case study for the application of micro-computed tomography on a Miocene sample of Holstein Erratics to identify and asses included molluscs and foraminifera

The assessment of fossils in sediments and sedimentary rocks often involves the destruction of the sedimentary matrix and even of parts of the fossil assemblage (e.g. via removing and/or dissolution). Therefore, the destruction-free assessment of fossils in sediments (e.g. sediment cores) and sedimentary rocks is of great interest to the geoscience community. In addition, the three-dimensional examination of fossils becomes more and more important to evaluate morphological features and improve morphometrical analyses.

The "Holsteiner Gestein" is a sandstone and glacial erratic which is frequently found at certain outcrops in northern Germany. While the material was transported during the Pleistocene, the original deposition of this sediment took place during the Miocene, perhaps also the upper Oliocene (Schallreuter et al. 1984).

Its fossil content and paleoecology has not been investigated in detail, and since the 1980s, scientific publications on this sediment are rare. This type of material, if analysed at all, is generally subjected to destructive methods to isolate fossils such as marine snails or foraminifers (marine protists), which both comprise taxa with calcareous shells. These fossils support the reconstruction of the paleo-ecosystem and depositional environment.

In the framework of a case study, a piece of glacial erratics – “Holsteiner Gestein” was scanned with a Comet Yxlon FF35 CT system employing the directional beam tube: First, an overview scan of the whole sample (210 kV, 160 µA, 1.0 mm Cu filter, 50.00 µm iso-voxel size) was used to identify a region with high fossil count and potentially interesting fossils. The region of interest was then scanned (210 kV, 160 µA, 1.0 mm Cu filter 7.23 µm iso-voxel size) with higher resolution using a scan trajectory with a flexible rotation center that allowed for maximal resolution by adjusting the position of the sample such that it was located as close as possible to the x-ray tube but a collision was prevented. In addition, a laminography scan (180 kV, 70 µA, 0. 5 mm Cu filter, 5.02x5.02x9.58 µm voxel size) was performed to achieve the maximally possible sharpness and resolution in cross-sectional images. Data were visualized and analysed in the software Amira (version 6.0.1).

Our approach did not only enable us to three-dimensionally assess relatively big snails shells, but also foraminifera of less than 1 mm in size. The scans additionally allow quantifying the number of microfossils inside a certain part of the sample.

The foraminiferal taxa comprise agglutinating foraminifers which closely resemble the genus Entzia. These imply a former salt marsh environment.

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References:

Schallreuter, R., Vinx, R., Lierl, H.J. (1984): Geschiebe in Südholstein. In Degens et al. (eds.): Exkursionsführer Erdgeschichte des Nordsee- und Ostseeraumes, Geologisch-Paläontologisches Institut der Universität Hamburg.