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Buried treasure

Posted on 11-09-2018 by Becky Desjardins

Becky is museum preparator. Many different tasks fall under this umbrella: cleaning whale bones, preparing study skins of birds and animals, and repairing broken geological samples, for example. However, of all the tasks I do, the most relaxing is washing fossil mollusc samples.  This involves separating shells from mud and sand through a sieve. It is a meditative task involving swishing water around in a pan, and the fun part is you never know what will turn up. The buried treasure I am looking for is not gold, but beautiful shells!

Currently I am working on a series of hard balls of clay that were collected this past spring. The work is done in a series of steps. The clay balls are so solid that they have to be soaked in water for a few hours first.  This causes the clay to break down and become soft mud, which is easier to process. Once it is soft, the mud gets put into a sieve with a mesh size of 0.5mm. I add a bit of mud to the sieve and put it under running water in a special sink designed to handle sand and mud going down the drain. The water goes through the sieve, taking the clay with it and leaving the fossil shells behind. But...it is not totally straightforward; the mud is filled with small pellets of clay  not yet saturated by water so I have to do a bit of turning the sieve and using the water to move the clay bits around in the bottom to encourage them to break up. This washing quite relaxing! After this, there are still small bits of clay, but I have found that a second overnight in water soak softens them up so during the next day’s washing they disappear.

My favorite are the tiny fossils of molluscs...if you look at them up close they are truly beautiful

The fun part, of course, is seeing what turns up out of each muddy sample. To the naked eye it appears to be mostly shell fragments, through a closer look through a microscope reveals lots microscopic small gastropods.  Once we found is what appears to be the end of a crab claw. My favorite are the tiny fossils of molluscs, usually itty bitty gastropods. They float in the top of the water in the sieve, and they look like white floating grains of sand. But if you look at them up close they are truly beautiful.

This particular batch of specimens came from Texel, the southernmost of the Wadden Sea islands.  A Naturalis researcher and the collection manager of fossil mollusca went on an expedition and collected these dry clay balls from the beaches there. These balls appear after a beach has been renourished.  They are dredged up from the bottom of the North Sea along with sand designated for an eroded beach. When the sand is pumped onto the beach the clay balls are too.

These clay balls from Texel contain fossils of molluscs from a Late Pleistocene temperate period called the Eemian, which occurred between 116,000 and 126,000 years ago. Though individual shells wash up on the beach all the time, when observing the contents of a few clay balls, you see shells from a whole suite of species, including very fragile ones, and can get an idea of what animals lived in the sea and what the Eemian environment was like. In addition, some of the clay balls collected had an imprint of seagrass on them (these we will not wash).  This, combined with the fine grain of the mud, indicates that they came from a warm, shallow, quiet area of water. Quite different than the North Sea today! These samples are a peek back in time to see not only what type of ecosystem the North Sea once was, but also what types of organisms lived there.

These fossil shells will go into the collection after washing, where they will be available for researchers to use.  But first, more relaxing work of sieving and searching for fossil gold.

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