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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Colourful EBSD

Electron backscatter diffraction is a method that can visualise the orientation of crystals in a mineral, yielding beautiful stained glass-like pictures of biominerals. When it is used on sections of earthworm granules this is the result:

Electron Backscatter Diffraction (EBSD) orientation contrast map of an earthworm granule highlighting its polycrystalline microstructure. Image by Martin Lee, School of Geographical and Earth Sciences, University of Glasgow.

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Earthworms as biomineralisers

Photo: NERC

We all know earthworms and might consider them slimy and scary, beneficial to our garden, or chicken feed. What is not very well known is that many earthworm species produce calcite (the stuff that e.g. oysters are made of) as tiny granules produces in specialised glands. Darwin already described this in his last book, and hypothesised they might have something to do with excretion or digestion, but the reason why these granules are produced is still a mystery.

A SEM picture of an earthworm granule.

Earthworm granules are quite often found in archaeological finds and buried soils. I investigate if their chemical composition can be used to reconstruct past climate and environments, in a similar way as this is done using corals or shells.

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Oxygen isotopes

A lot of palaeoclimate research makes use of stable oxygen isotope compositions of various substances, like ice cores or microfossils. But what are isotopes and how do they end up in our climate records?

The Oxygen-18 isotope has an extra two neutrons, for a total of 10 neutrons and 8 protons, compared to the 8 neutrons and 8 protons in a normal oxygen atom. Picture from NASA.

Isotopes are different versions of atoms of the same element, but with a different mass. Oxygen, for example, has atomic number 8, which means its nucleus contains 8 protons. The amount of neutrons in the nucleus, however, can vary from 8 to 10, resulting in oxygen having three different isotopes known as 16O, 17O and 18O. Some isotopes are unstable and after some time split to form two new elements, a form of radioactive decay. The aforementioned oxygen isotopes are stable and occur naturally in various proportions (99.76, 0.035 and 0.20 %, respectively). Here I will only discuss 16O and 18O, since the amount of 17O in the natural environment is negligible.  Because they’re all oxygen, they generally behave the same in chemical reactions, there are however some differences. A heavier isotope requires more energy to be mobilised in chemical reactions and processes like evaporation, with interesting consequences:

Water (H2O) of course contains a lot of oxygen atoms. When clouds form over the ocean, fewer of the heavy isotopes will evaporate, resulting in clouds that are depleted in 18O. When these clouds than move over land and start to produce rain, the molecules containing the heavy isotope will rain out first. This results in rain containing less and less 18O, the further you go land inwards.

This so called fractionation between light and heavy isotopes is most pronounced at low temperatures, since at high temperatures enough energy is available to mobilise both isotopes. So temperatures can also be reflected in oxygen isotope compositions. This is for example the case in in oxygen isotope records of microfossils from the ocean floor (benthis foraminifera). In the well-know ice cores both processes are at work, and result in an accurate reflection of ice ages and interglacials over the past several millions of years.

(I could not find where this picture comes from originally, so if you happen to know, please let me know.)