Silent witnesses

How geochemistry tells about climate and environments


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The early bird catches the worm

The "terror bird" Gastornis (http://research.amnh.org/)

The “terror bird” Gastornis (http://research.amnh.org/)

For many of us being an early bird becomes progressively more difficult as conference fatigue takes its toll. I was glad to see a sizeable crowd managed to be there for the start of our session on the role of biominerals in biogeochemical cycling, which happened to be about birds and worms, and some other creatures.

Thomas Tütken showed us that the fearsome Eocene terror bird was neither a predator, nor a worm-catcher, but a gentle herbivorous giant instead! Also, the oxygen isotope composition of the bones of large herbivores forms a remarkably good archive of Cenozoic climate, agreeing well with the famous Zachos curve.

Then it was my turn to talk about worms, who are unexpected biomineralisers, and appear to precipitate atmospheric carbon dioxide in carbonate granules in the soil. Loredana Brinza studied metals in these granules, and demonstrated that they can significantly contribute to zinc immobilisation in polluted soils.

From animals we switched to plants, with two presentations about phytoliths: little chunks of silica produced by plants. Jean-Dominique Meunier and Eric Struyf showed that the contribution of phytoliths to global silicium cycling is much larger than previously thought and is of the same order of magnitude as the marine contribution of diatoms. Humans remove a lot of silicium from the global cycle by harvesting crops, and this likely reduces crop yields.

Going from the terrestrial to the marine realm, Ruth Carmichael spoke about nitrogen in bivalves and their potential for nitrogen sequestration as well as reconstruction of nitrogen cycling before human influences, using shells from middens that are thousands of years old. Adilah Ponnurangam stayed with the bivalves, but looked at rare-earth elements and showed that they reflect seawater composition.

The session ended with Michaël Hermoso presenting some meticulous laboratory culture experiments on coccolithophores, and size-related differences in carbon-isotope fractionation. All-in-all a very interesting morning, and a satisfying debut as a session chair.


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Earthworms could help scientists ‘dig’ into past climates

Lumbricus terrestris

Lumbricus terrestris

A team of UK researchers believe earthworms could provide a window into past climates, allowing scientists to piece together the prevailing weather conditions thousands of years ago.

A laboratory study by researchers from the Universities of Reading and York has demonstrated that balls of calcium carbonate (small lumps of chalk-like material) excreted by the earthworm Lumbricus terrestris – commonly known as lobworms or nightcrawlers – maintain a memory of the temperature at which they were formed.

This, say the researchers, in an article in the journal Geochimica et Cosmochimica Acta, means that calcite granules, commonly recorded at sites of archaeological interest, have the potential to reveal important information about past climates which could be used to enhance and benchmark climate change models.

The study, which also involved English Heritage’s Centre for Archaeology, was funded by the Natural Environment Research Council (NERC).

Lead author Dr Emma Versteegh from the Department of Geography and Environmental Science at the University of Reading, said: “These chalk balls will allow us to reconstruct temperatures for specific time intervals in which they were formed. Reconstructions like this are interesting for archaeologists, because they give a climatic context to their finds. More importantly, climate proxies are the only means we have to study climate beyond the instrumental record, which only goes back about 150 years.

“This knowledge about past climates is of vital importance for developing and benchmarking climate models that make predictions for the future. Many different proxies already exist, but no proxy is perfect, or is available in every location, so it is good to have many different ones.”

The proof of concept study involved keeping modern-day Lumbricus terrestris at different temperatures, then carrying out isotopic testing on the calcite granules excreted. This successfully demonstrated that the granules remembered the temperature at which they were formed.

Principal Investigator Professor Mark Hodson from the University of York’s Environment Department, and formerly of the University of Reading, said: “There are many conflicting theories about why earthworms produce calcite granules, but until now, the small lumps of chalk-like material found in earthworm poo have been seen as little more than a biological curiosity. However, our research shows they may well have an important role to play, offering a window into past climates.”

The researchers are now gathering samples from archaeological sites dating back thousands of years in preparation for isotopic testing.

Dr Stuart Black, from the University of Reading’s Department of Archaeology, added: “We believe this new method of delving into past climates has distinct advantages over other biological proxies. For example, we believe it will work for the full seasonal range of temperatures, whereas methods such as tree rings, do not “record” during winter. In addition, because the chalk balls are found in direct context with archaeological finds, they will reveal temperatures at the same location. At present, links are often attempted with climate proxies many hundreds or even thousands of miles away.”


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Worms in the lab

Last week I set up my third (and for the time being, last) experiment with earthworms. This time I give them different levels of CO2 in the air to see if that has an influence on granule production. The species of earthworms I use is Lumbricus terrestris. One of the most common species you’ll find in your average European garden. I order them at a company that sell them for fishing, and who actually get them from Canada, where they have been introduced by Europeans to improve agriculture. In Canada they even have worm vending machines.

In the lab the worms each get their own bag of moist soil ad horse manure (food) and are kept in darkness and at constant temperature. To achieve the different CO2 levels they are then put in so-called gloveboxes through whcih air with a constant CO2 level is pumped.

Two gloveboxes containing 6 bags-of-soil-with-worm.

And now I wait.


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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.