Silent witnesses

How geochemistry tells about climate and environments

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Freshwater mussels record droughts

To make a long story short, the freshwater mussels that grew in the monitoring experiment for over a year, indeed nicely recorded the environment. In this case that means their shell oxygen isotopic composition (δ18O values) reflected the δ18O values and temperature of the water they were living in:

water δ18O + temperature → shell δ18O

relationship between discharge (Q) and water oxygen isotope composition (δ18Ow) in the river Meuse (The Netherlands).

The next thing to find out was if this could be used to reconstruct floods or droughts in a river. In the river Meuse, there is a logarithmic relationship between discharge and water δ18O values.

By analysing shells from a wet time interval (1912-1918) and an extremely dry time interval (1969-1977) we found out that droughts with a very low river discharge are readily recorded by the shells. This is logical if you look at the graph. The water δ18O composition does not change very much between average and high discharges, but values become much higher when discharges (Q) are very low (at the left side of the graph).

This means that when we are going to analyse old shells, from archaeological finds or palaeogeographical samples in the Meuse River area, we will be able to recognise droughts.

If you’re interested in reading the full story, it can be found here (open access – free of charge).

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Freshwater mussels

At first sight, unionid freshwater mussels neither sound nor look very interesting. However, studying them for my PhD, I quickly became fascinated with this group of animals. Actually, when you have a proper look at these large-shelled bivalves, they are quite beautiful. In Europe we only have a handful of species, but in North America their variety is immense.

Some species can live very long, up to 250 years, and they can even produce pearls. Unfortunately many species are also critically endangered because of habitat loss and introduction of invading species that overgrow our out-compete native species.

My favourite freshwater mussel fact is that they have larvae (glochidia) that are parasitic on the gills of fish. Many species have specific host species, and some have evolved very elaborate methods to target their hosts. For example, in some species the glochidia form a “lure” that closely resembles the prey of the host fish, gets eaten, and the glochidiae end up on the gills.

The most amazing behaviour is displayed by the “Snuffbox”. It actually catches the host fish between the valves of the shell and “pumps” the glochidia into its gills. A video can be found here. Highly recommended!