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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Mussel experiments

To find out if and organism nicely records its environment, you need to do experiments in which variations in the environment are manipulated or closely monitored. In case of freshwater mussels it it not very easy to keep them happy and alive in an aquarium, so I did the experiments in the field. The two field sites were fish ladders in the rivers Rhine and Meuse, that are made for enabling migratory fish to move upriver past a dam or weir.

Freshwater mussels do not attach themselves like blue mussels do, but sit in the mud instead. They can move around quite a bit and muskrats think they’re delicious, so we devised a steel cage for them to live in. I collected some mussels from the river Linge, and bought some other in the pet shop (yes, they sell them there, because people apparently like to have them in their garden ponds). Collection of animals and installation of these experiments involves a lot of work with waders on, and can sometimes be quite cold!

The fish ladder at Lith (Meuse).

Installing the cage at Hagestein.

Collecting mussels from the Linge.