Correlative cathodoluminescence and EDS imaging of the benthic agglutinated foraminifer Liebusella goesi

Abstract

Sedimentary rocks, formed by the accumulation of mineral and organic particles, are important both for studies of the earth’s history as well as for being a source of fossil fuels. Over the course of the last decades, it has been demonstrated that scanning electron microscope (SEM)-based cathodoluminescence (CL) spectros­copy is a valuable technique for the characterisation of sedimentary rocks, complementary to other electron microscopy-based techniques, such as backscattered electron imaging (BSE/EBSD) and energy dispersive x-ray spectroscopy (EDS). Typically the CL yield is high enough for rapid scanning and, in some cases, even video-rate scanning, allowing fast in­spection of relatively large areas. It can be used to quantitatively map the quartz composition of the sample, for example, which enables the rigorous segmentation of granular and cemented material.

Textulariid benthic foraminifers live on and in seafloor sediments and form shells of agglutinated sediment particles. They are very important biostratigraphic markers, and fossil agglutinated foraminifera are important archives for paleoceanographic reconstructions. Furthermore, living textulariids show a strong diversity, populating  a diverse range of marine habitats partly and can reach high living abundances, making them important for benthic ecosystems.

In this work, we show how CL spectroscopy can be employed to study agglutinated foraminifera using the species Liebusella goesi from the Swedish Gullmar Fjord as an example. Fast panchromatic imaging using a photomultiplier tube was performed over a large area of the foraminifera, which revealed textures and contrasts of interest in the shell (test). A high resolution SEM image was acquired simultaneously to provide spatial context. Such a dataset can be valuable in establishing the geological history as well as in identifying the chemical composition of the cement used for the agglutination of sediment particles. Both the composition of the agglutinated particles and the chemical composition of the cement might bear valuable information about the environmental conditions, when the test was formed. EDS measurements were performed, revealing the spatial distribution of elements such as potassium, calcium, sodium, silicon and oxygen, in the sediment particles of the shell. This was useful in indicating the presence of minerals such as quartz and feldspar, and hyperspectral CL imaging was performed to rigorously identify them, and to visualize intragranular features, not visible in the EDS data. Based on the CL spectral data, we were further able to identify different grades/types of quartz and feldspars. These results show that these foraminifera prefer different sediment materials with varying grain sizes, depending on the size of the newly formed chamber, to achieve the highest mechanical stability.