Silent witnesses

How geochemistry tells about climate and environments

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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!

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Sclerochronology – How does it work?

So how do you get a climate record from a shell or a coral? Well, a lot of organisms make growth bands like trees. And when looking at these bands one might see differences in width. Maybe the organism grew faster when temperatures were high, or when there was a lot of rain. So these growth bands are then effectively a record of temperature or rainfall amounts through time. But it goes further. Apart from the width of growth bands, we can look at their chemistry. The chemical composition of shells, or tree rings, or corals, can actually vary with temperature, or with the chemical composition of of the water that they live in.

When organisms become old enough to give a very long record of the past, or when we use shells from archaeological finds, this enables reconstruction of the climate far back in time, when there were no instrumental measurements. And that’s interesting, because it’s fun to find out about the history of our planet, but also because we need that information to understand current climate and model the future.

The oxygen isotope record from a freshwater mussel shell from the river Rhine reflects both water composition and temperature changes over time.