Tuesday, September 6, 2011

Snails help date Britain's last three million years

Scientists have built the most comprehensive timeline yet for working out the exact order in which geological and archaeological events happened in Britain over the last three million years. And they've done it using fossilised snails.

Snail shell showing operculum

The mammoth 11-year project, published online in Nature, is the most comprehensive of its kind and clears up a number of archaeological and geological debates.

It shows that our ancestors lived in Britain during most of the warm periods of the last few million years. But it supports the idea that they were absent in the most recent warm period – or interglacial – 125,000 years ago. During this time, the climate was warm enough for hippopotamuses to have roamed the British Isles.

'It's possible that the warm climate contributed to higher sea levels and people just couldn't get across the Strait of Dover,' says Dr Kirsty Penkman from the University of York, lead author of the study.

The framework also supports the idea that before the warm period we're in now, which scientists call the Holocene, there were four major interglacials in Britain during the last half a million years.

'Opercula work like a dream, because the protein within them is protected so well.'
Dr Kirsty Penkman

The study is a major breakthrough because, until now, no-one had been able to use just one dating technique to figure out the order of terrestrial events going so far back.

'You can use radiocarbon-dating to date objects that are up to around 40,000 years old, but beyond that, it's hard to find one technique that we can use to go back over a million years,' says Penkman.

Ice cores from Antarctica and cores from deep-sea sediments provide scientists with continuous records that show when cold and warm periods happened. 'But the question is, how can we relate the fragmentary records from the land with these master sequences? Terrestrial sediments are most accessible to geologists and they contain virtually all the early archaeology, ' says Dr Richard Preece from the University of Cambridge, co-author of the Nature study.

To answer these questions, Penkman, Preece and their colleagues decided to use a pioneering method developed in the late 1960s. The technique, called amino acid dating, relies on the quirky way that proteins exist in animals, plants and snails.

The building blocks of all proteins, called amino acids, exist in two versions which are mirror images of each other. Only the 'left-hand' version exists in living bodies. But once creatures die, and proteins fossilise, the amino acids slowly degrade into the 'right-hand' version until, over hundreds of thousands – and sometimes millions – of years, there are equal numbers of each.

The higher the ratio of left-hand version to right-hand version, the older the snail fossil is likely to be.

When the researchers applied this method to snail shells sourced from all over Britain, they found problems beyond about 300,000 years. 'The amino acids in snails' shells start to break down at about this time,' explains Penkman.

So, they turned their attention to the tiny trapdoors called opercula that some snails, like Bithynia use to close off the outside world. Opercula are made of a slightly different material to the shell, so are much more stable.

'Opercula work like a dream, because the protein within them is protected so well,' explains Penkman.

They analysed 470 opercula from 74 sites across Britain and the Netherlands, and ranked them in order of relative age, based on the extent of amino acid degradation. The timeline they built extends back over three million years.

To test how well their method worked, the researchers then compared their results with geological and archaeological sites of known age, garnered from various other geological evidence or from palaeontology.

'It was crucial to get samples of Bithynia opercula from sites of known age so that we could be sure that our approach worked,' explains Preece.

The timeline is so complete that any debate over the timing of human occupation in Britain or past geological events should now be dead in the water.

'Bithynia opercula are common in Ice Age sediments found all over the world, so you can extend this technique out across Europe and potentially the rest of the world to create regional timelines,' says Penkman.

For more interesting topics related to archaeology, visit archaeology excavations.

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