Shell secrets: Using freshwater snails to understand past climates

   by Jenni Sherriff (3rd Year PhD Student)

Freshwater gastropod and bivalve shells from the River Thames.

 

A large part of my research involves crushing shells. This is not due to a personal vendetta against freshwater molluscs, but because the chemistry of the shells themselves have the potential to provide important climatic information about the environments in which the molluscs once lived.

My PhD research is mainly focused on understanding climates of past warm-periods, or interglacials, in particular one such interglacial c. 400,000 years ago. This interglacial is fascinating to study because the factors thought to control the structure this interglacial are most similar to the current interglacial in which we are living.

Many deposits from this period are rich in freshwater shells, so by analysing the chemical composition of these shells, you can begin to understand the climatic regime of this period. However, there is one main caveat: in order to use ancient shells to tell us about climate, you need to understand the modern relationship between shell chemistry and environmental variables, such as water chemistry, and more importantly, temperature.

So that is why over the last 18 months I have spent a lot of time standing knee-deep in rivers sieving through large swathes of sediment in search of freshwater shells the size of my fingertip.

The process of collecting and preparing shells for chemical analysis is relatively straightforward. River sediment is collected and carefully sieved on site to remove the really fine grained material (silt-clay sized) and any large cobbles. It is then taken back to the laboratory to be dried, then carefully sieved again, this time picking out whole shells and shell fragments from the residue. Most of the species I am interested in, for example Valvata piscinalis and Bithynia tentaculata are quite small (between 2-8mm) therefore have to be picked under the microscope. These shells are then identified to species level, carefully measured, photographed and described. They are then ready to be prepared for chemical analysis, which, after such careful procedures to maintain an undamaged shell…………..I crush them up in a pestle and mortar.

Collecting shells from the River Thames at Runnymede, Surrey.

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Freshwater Unios from the River Thames

Nonetheless, there are careful stages to this destruction: they are first crushed into large fragments so that any material lurking inside the shell can be removed; and then crushed to a powder, which is chemically treated to remove any organic material that has the potential to modify the results. Once the shell powder has been rinsed and dried, it is ready for chemical analysis.

Small bivalve shells under the low-powered microscope ready to be identified and sorted.

The chemical analysis involves, and what I am most interested in, measuring the stable isotopic composition of the shells. This requires analysing the ratio between heavy oxygen (18O) and light oxygen (16O), which in freshwater shells is controlled mainly by the composition of the water the mollusc lives, and the temperature in which the shell grows. As we know that the isotopic composition of the river water is consistent, we can then predict the 18O/16O ratio of the shells under different over the year. This predicted value is then compared to the actual value derived from the crushed shells. From this you can determine the temperature and the season in which the modern shells have been formed.

Results so far are promising and indicate that many of the modern species I have analysed are growing their shells in line with spring-summer temperatures. Now, I hope to apply this modern relationship to the same shell species found in deposits from 400,000 years ago to understand the temperature regime from this period. If this is successful, it will further help us to understand past climatic regimes of warm periods, and allow us to start to compare with what is happening to our climate currently. Which I think is rather exciting!

 

 

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