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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Mussel power

musselsweb479589_28559Mussel power – ocean shells can help predict rise in sea levels

Ocean mussels could be key to helping scientists predict more accurately the rise in sea levels caused by the melting of the Greenland Ice Sheet.

A University of Reading study has shown that mussel shells accurately record the amount of meltwater running off the ice sheet during the summer. The researchers believe analysing shells hundreds to thousands of years old will reveal how much Greenland meltwater, caused by the natural climate variability of the Earth, contributed to previous sea level fluctuations.

This information can then be fed into climate models allowing scientists to better predict the behaviour of the Ice Sheet during future variations in climate. Crucially this will also tell us how much melting is due to human influence.

Due to global warming the sheet melted at an unprecedented rate during 2012 with the thawed ice area jumping from 40% to 97% in just four days during July. Melting is predicted to raise global sea levels during the 21st century by up to 10 cm and if the sheet was to completely melt, which would take centuries, sea levels would rise by about seven metres. This could threaten low-lying countries like the Netherlands and less developed countries such as Bangladesh.

Dr Emma Versteegh, from the University of Reading’s Department of Geography and Environmental Sciences, who conducted the research with co-workers from Greenland and Denmark, said: “The Earth’s climate naturally varies over long timescales and there have been warm intervals before during which the amount the Greenland Ice Sheet melted might have varied considerably. Examples include the Holocene Climatic Optimum (roughly 9000 to 5000 years BC) and the Medieval Warm Period (~ AD 950 to 1250).

“Very little is known about the dynamics of Greenland Ice Sheet melting. In order to understand its behaviour and predict future melting, information on meltwater amounts during different climatic regimes is hugely important. However instrumental data only cover the last few decades, so other methods are needed to look further into the past.”

Using blue mussels found in different areas off the coast of West Greenland, Dr Versteegh found that the shells revealed the amount of meltwater running off the ice sheet during the summer. Mussels and other shells have been used before to reconstruct past climate, but this is the first time they have been used to indicate past meltwater amounts.

Dr Versteegh continued said: “Over several years we collected shells at different locations in Godthåbsfjord and also measured water composition and temperature. Meltwater is characterised by a very different oxygen isotopes composition than seawater. By analysing these isotopes in the mussels’ annual growth bands we will be able to reconstruct the melting of the Greenland Ice Sheet during several warm periods over the past 10,000 years. This will give climate scientists vital information on how the Earth’s natural variations in climate affect the Ice Sheet, and in turn sea levels, which they can use to further improve predictions of sea level rises stemming from climate change.”

Oxygen isotope ratios in the shell of Mytilus edulis: archives of glacier meltwater in Greenland is published on the Biogeosciences websites http://www.biogeosciences.net/

ENDS

For all media enquiries please contact James Barr, University of Reading Press Officer on 0118 378 7115 or by email on j.w.barr@reading.ac.uk

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Notes for Editors

The University of Reading is ranked among the top 1% of universities in the world (THE World University Rankings 2012) and is one of the UK’s top research-intensive universities.

The University of Reading’s Department of Geography and Environmental Science has a strong research environment which focuses on key issues at home and abroad, from the social and economic impact of developments in the UK, to studies of Brazilian rainforests, Caribbean towns and livelihoods in Africa.  The Department works with business and industry, charities and non-governmental organisations, and has links with other departments whose expertise provide a diverse and stimulating research environment.

Funders

  • Greenland Climate Research Centre
  • Danish Ministry of the Environment (Dancea);
  • Aarhus University;
  • Commission for Scientific Research in Greenland, Ministry of Education;
  • Research and Nordic cooperation (IIN);
  • Aage V Jensen Charity Foundation;
  • Canada Excellence Research Chair Program (University of Manitoba, Canada)


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Climate research with shells

This video is unfortunately in Dutch. It shows the work of two secondary school students whom I co-supervised during my PhD. They went on an expedition to Svalbard with their teacher and brought back some mussel shells, which they sampled, analysed for stable isotopes, and compared with shells from the Netherlands.


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Donax

In July 2007 was the first International Sclerochronology Conference in St. Petersburg, Florida. It was great to meet people who do such similar research, turned out they’re really nice as well. And shell researchers like to eat their subjects, so oyster bars were frequently visited. Even more fun is to put a bunch of these people on a beach. On this specific one (St. Pete’s Beach) there were small coquinas (Donax variabilis), which live in the surf en bury themselves in the sand after each wave. Here’s a video showing some. You can hear some very excited scientists in the background.


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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.