25 August 2010, by Adele Rackley
Coastal earthquakes that cause the earth to shift are often dated by the remains of marine creatures that are exposed and killed when the ground lifts out of the sea.
But scientists have discovered that one commonly used shell can produce apparent dates thousands of years older than the event that exposed them.
In AD365 a huge earthquake tilted the whole western end of the Greek island of Crete, exposing up to 10m of previously submerged land along the south and west coasts.
The event was well documented so we can be sure of its date. And it left exposed a ring, like a tide-mark, made from algae that concreted - or set - at sea level when the quake happened. The next ring down is at current sea level, so we know that this ground was lifted up in a single event.
But when a team of scientists from the universities of Cambridge and Oxford dated the remains of two different creatures exposed by the quake they came out with widely differing results.
They carried out their work at the Oxford Radiocarbon Accelerator Unit at the University of Oxford. Here they compared the ratios of different carbon isotopes - different 'weights' of the same carbon element - found in creatures exposed by the quake.
During their lifetimes animals absorb carbon from their surroundings. When they die, the radioactive carbon-14 isotope begins to decay at a known rate, while the amount of the stable carbon-12 isotope stays the same. By comparing the quantities of the two isotopes scientists can work out how many years ago a creature died, and therefore the date of the event that killed it.
The researchers used this method on two marine creatures exposed by the Cretan earthquake: corals, and a mussel-like shell called Lithophaga lithophaga found in the same rocks.
The corals confirmed the documented date of the quake. But the L. Lithophaga samples produced dates up to 2000 years earlier.
L. Lithophaga has a hard shell and bores into its host rock to create a protected cigar-shaped hole that it spends its life in. As a result it has much better chance of preservation than more delicate creatures like corals, which live on the surface.
Both creatures get their carbon from seawater, but while corals live in and feed off open seawater and are unlikely to be contaminated by other carbon sources, 'we thought the Lithophaga could be producing odd results because they had absorbed carbon from the limestone they lived in,' says Professor James Jackson from the University of Cambridge.
'The question was, how could we test the idea?'
The team realised the obvious answer was to use the same dating technique on modern samples of these molluscs. Their results are published in Earth and Planetary Science Letters.
They couldn't use present-day specimens because carbon-14 levels have been affected by nuclear testing since the 1950s. So the researchers located museum specimens of Lithophaga that had been collected in the 1920s. Sure enough, though the creatures had only been dead for a matter of decades, carbon dating produced dates up to 2000 years old.
Clearly the very thing that made these shells so useful for dating - their excellent preservation - was causing the unreliable results in the lab.
'What's important is that we can now understand the limitations of what we can do with these shells,' explains Jackson.
For example there is a similar 'tide-mark' along coast in the Gulf of Corinth in central Greece, in this case 2 to 3m above current sea level. But there are several other rings between it and modern sea level, suggesting the uplift caused a number of events, not just one.
Jackson explains that you would expect the top ring to be around 6000 years old, dating from the end of the last ice age when sea levels rose very quickly then stabilised.
'Lithophaga isn't good for identifying the date of a particular quake - there's too big an error,' he says. 'But it does allow us to demonstrate that in most places sea level stabilised at 6000 years ago, and we can use the preserved 'tide-mark' from that time to see what has happened since then.'
The other problem the researchers identified is that Lithophaga frequently re-colonise old holes left by earlier Lithophaga, meaning several generations may have lived in the same bit of rock. So when these rocks are exposed by a series of coastal uplifts the dates of these shells can suggest an apparent yo-yo effect, dating the uplifts in the wrong order.
'Our work suddenly makes sense of some funny results previous researchers have found, and made it clear what you can and can't do with these sources of dating,' says Jackson.
'Dating modern Lithophaga samples seemed such an obvious approach, we were amazed that no one appears to have done this kind of test before,' he concludes.