Flat foram

Windy ride
By  Nicole Webster

I had a thin section made of one of my shells, and it came back with a serendipitous hitchhiker. A foraminiferan, you know, those single celled protists that make a gorgeous test (shell) usually out of calcium carbonate? Well it was at just the right place to be beautifully sectioned itself. The section is 30um thick of a 5cm shell, and the foram itself is only 0.5mm wide.

Foram thin section in plane polarized light. Credit N. Webster

Foram thin section in plane polarized light.  The darker spots inside the test are probably dried up tissue, and the little balls underneath the test are probably the glue used to attach the foram to the shell. Credit N. Webster

Foram thin section in cross polarized light. Credit N. Webster

Foram thin section in cross polarized light. Credit N. Webster

Thin sections are typically used in geology to identify different crystals, and thus the rocks that they are made up of. I recommend a quick Google image search, some are really quite pretty. Here I was using it to see how many layers and in what orientation the crystals are layed down in my snails.
A chunk of rock (or shell) is ground down as smoothly as possible to 30um (0.03mm) so that it is transparent. Once under the microscope, a polarizing filter is used to see crystal features better. A cross polarizing filter is used to see interference colours, allowing greater characterization of the mineral. That is why the second image looks a little like something from the 60s. Although the CaCO3 is relatively clear, the glass of the slide refracts the light quite a bit, making a psychodelic pick rainbow. This is a very simplistic view of thin sectioning, please correct me if I’ve misunderstood something.

PS I learned a new word making this post. Arenaceous. It means sandy, or likes sand (for plants), and specifies a geologic grain size ranging from 2 to 0.625mm. In this context it means those foraminifera that don’t grow their own shell, but rather glue bits (read:sediments particles) together to make a shell.

References
1. Rock-Forming Minerals in Thin Section. Steve Dutch, Natural and Applied Sciences, University of Wisconsin. https://www.uwgb.edu/dutchs/Petrology/thinsect.htm
2. Corliss, B. H. 1985. Microhabitats of benthic foraminifera within deep-sea sediments. Nature 314:435-438.