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<DIV class=formatpublished> </DIV>
<DIV class=formatpublished><FONT size=3>Nature</FONT></DIV>
<DIV class=formatpublished>Published online <ABBR class=published
title=2010-03-08T20:00:00Z>8 March 2010</ABBR></DIV>
<DIV class=formatpublished>doi:10.1038/news.2010.110 </DIV>
<DIV class=formatpublished> </DIV>
<DIV class=type-of-article><FONT size=4>Shellfish could supplant tree-ring
climate data</FONT></DIV>
<DIV class=intro>Temperature records gleaned from clamshells reveal accuracy of
Norse sagas.</DIV>
<DIV class=byline><SPAN class=vcard><SPAN class="author fn">Richard A. Lovett
</SPAN></SPAN></DIV>
<DIV class=byline><SPAN class=vcard><SPAN class="author fn"><A
href="http://www.nature.com/news/2010/100308/full/news.2010.110.html">http://www.nature.com/news/2010/100308/full/news.2010.110.html</A></SPAN></SPAN></DIV>
<DIV class=byline><SPAN class=vcard><SPAN
class="author fn"></SPAN></SPAN> </DIV>
<DIV><SPAN class=cleardiv><!-- --></SPAN>
<DIV class=entry-content sizset="21" sizcache="45">
<P>Oxygen isotopes in clamshells may provide the most detailed record yet of
global climate change, according to a team of scientists who studied a haul of
ancient Icelandic molluscs.</P>
<P sizset="21" sizcache="45">Most measures of palaeoclimate provide data on only
average annual temperatures, says William Patterson, an isotope chemist at the
University of Saskatchewan in Saskatoon, Canada, and lead author of the
study<SUP sizset="21" sizcache="45"><A
href="http://www.nature.com/news/2010/100308/full/news.2010.110.html#B1">1</A></SUP>.
But molluscs grow continually, and the levels of different oxygen isotopes in
their shells vary with the temperature of the water in which they live. The
colder the water, the higher the proportion of the heavy oxygen isotope,
oxygen-18. </P>
<P>The study used 26 shells obtained from sediment cores taken from an Icelandic
bay. Because clams typically live from two to nine years, isotope ratios in each
of these shells provided a two-to-nine-year window onto the environmental
conditions in which they lived.</P>
<P>Patterson's team used a robotic sampling device to shave thin slices from
each layer of the shells' growth bands. These were then fed into a mass
spectrometer, which measured the isotopes in each layer. From those, the
scientists could calculate the conditions under which each layer formed.</P>
<P>"What we're getting to here is palaeoweather," Patterson says. "We can
reconstruct temperatures on a sub-weekly resolution, using these techniques. For
larger clams we could do daily."</P>
<P>It's an important step in palaeoclimatic studies, he says, because it allows
scientists to determine not only changes in average annual temperatures, but
also how these changes affected individual summers and winters. </P>
<P>"We often make the mistake of saying that mean annual temperature is higher
or lower at some period of time," Patterson says. "But that is relatively
meaningless in terms of the changes in seasonality."</P>
<P sizset="22" sizcache="45"><A
href="http://www.nature.com/news/2005/050418/full/050418-11/box/1.html"
sizset="17" sizcache="39"></P>
<DIV class="inline-image left" style="WIDTH: 260px"><IMG
alt="Mollusc temperature record"
src="http://www.nature.com/news/2010/100308/images/260px.chart.2010.Fig3_mussels.jpg"><SPAN
class=imagedescription>Click on image for a larger version.<SPAN
class=imagecredit>Patterson, W. P. et al, PNAS</SPAN></SPAN></DIV>
<P></A></P>
<P>For example, in early Norse Iceland — part of the 2,000-year era spanned by
the study — farmers were dependent on dairy farming and agriculture. "For a
dairy culture, summer is by far the most important," he says. "A one-degree
decrease in summer temperatures in Iceland results in a 15% decrease in
agricultural yield. If that happens two years in a row, your family's wiped
out."</P>
<P>Technically, the molluscs record water temperatures, not air temperatures.
But the two are closely linked — specially close to the shore, where most people
lived. "So, when the water temperatures are up, air temperatures are up. When
water temperatures are down, air temperatures are down," Patterson says. </P>
<H2 class=inlineheading><FONT size=4>Lean times</FONT></H2>
<P>One of Patterson's goals was to verify assertions in historical Icelandic
sagas describing the weather. Because these sagas include dispatches to the king
back in Norway, there was an incentive to exaggerate. "If you're having a bad
year, you're not expected to provide as much in the way of tribute or taxes,"
Patterson explains. </P>
<P>The study's findings suggest that the sagas are reasonably accurate. In the
1000s, for example, the 'Book of Settlements' — a medieval manuscript containing
details of Iceland's settlements — reports a famine so severe "men ate foxes and
ravens" and "the old and helpless were killed and thrown over cliffs", Patterson
says. And according to his shells, it was indeed a difficult era, with summer
water temperatures peaking at only 5–6 °C, down from as high as 7.5–9.5 °C
around 100 years earlier.</P>
<P>Patterson's data also reveal a number of climate changes recorded by
historians, including a Roman-era warming period, a cold snap in the Dark Ages
and a subsequent period of warming, during which the Vikings discovered Iceland.
</P>
<P>But it's not just historians who will be interested. The new data will help
climate modellers to improve their understanding of seasonal effects in the
North Atlantic, Patterson says. "This is a new line of evidence."</P>
<P>Other scientists are impressed. "The technique is fascinating," says
geoscientist Richard Alley of Pennsylvania State University in University Park.
"This represents a huge amount of work, and demonstrates outstanding potential
for revealing past processes in unprecedented detail. One can envision a
tree-ring-like continuous history, given a lot more effort."</P>
<P>If he can find the funding, that is exactly what Patterson would like to
establish next. "We have what may be the world's oldest clam," he says, "that
might give a continuous record going back 400 years." He also wants to push the
study back towards the end of the last ice age. "We have 11,000 years worth of
material," he says.<SPAN class=end-of-item> </SPAN></P>
<UL class=xoxo id=article-refrences sizset="23" sizcache="45">
<LI sizset="23" sizcache="45">
<H2 class=heading>References</H2>
<OL sizset="23" sizcache="45">
<LI id=B1 sizset="23" sizcache="45"><A name=B1><!-- . --></A>Patterson, W.
P., Dietrich, K. A., Holmden, C. & Andrews, J. T. <SPAN
class=journalname>Proc. Natl Acad. Sci. USA</SPAN> doi:
10.1073/pnas.0902522107 (<SPAN class=cite-month-year>2010</SPAN>).
</LI></OL></LI></UL></DIV></FONT></DIV></BODY></HTML>