Comments
Clamshells Trump Tree-Rings as a Temperature Proxy?
Nature News reports on a new PNAS paper by Patterson et al that uses clamshells in order to reconstruct temperatures in Northwest Iceland over a 2000 year period up to the year 1660:
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 study1. 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.
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.
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.
“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.”
The data shows the Roman Warm Period, a cooling in the Dark Ages, the Medieval Warm Period, and the Little Ice Age.
All we need now is for ‘Michael Clam’ to use his special methodology in order to create a straightish line, and then add some instrumental data in order to create a ‘hockey stick.’
An extract from the actual paper says:
The interval from ∼230 B:C: to A.D. 40 was one of exceptional warmth in Iceland, coinciding with a period of general warmth and dryness in Europe known as the Roman Warm Period, from ∼200 B:C: to A.D. 400 (23). On the basis of δ18O data,reconstructed water temperatures for the Roman Warm Period in Iceland are higher than any temperatures recorded in modern times.
That should provide some comfort for those who worry about the current Arctic warming and sea ice etc.
The Nature News article concludes:
One can envision a tree-ring-like continuous history, given a lot more effort. 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.
Two millennia of North Atlantic seasonality and implications for Norse colonies
Patterson, W. P., Dietrich, K. A., Holmden, C. & Andrews, J. T. Proc. Natl Acad. Sci. USA doi: 10.1073/pnas.0902522107 (2010).
Abstract:
δ18O values of mollusks recovered from near-shore marine cores in
northwest Iceland quantify significant variation in seasonal
temperature over the period from ∼360 B:C: to ∼A:D: 1660.
Twenty-six aragonitic bivalve specimens were selected to represent
intervals of climatic interest by using core sedimentological characteristics.
Carbonate powder was sequentially micromilled from
shell surfaces concordant with growth banding and analyzed for
stable oxygen (δ18O) and carbon (δ13C) isotope values. Because
δ18O values record subseasonal temperature variation over the lifetime
of the bivalves, these data provide the first 2,000-year secular
record of North Atlantic seasonality from ca. 360 cal yr B.C. to cal yr
A.D. 1660. Notable cold periods (360 B.C. to 240 B.C.; A.D. 410; and
A.D. 1380 to 1420) and warm periods (230 B.C. to A.D. 140 and A.D.
640 to 760) are resolved in terms of contrast between summer
and winter temperatures and seasonal temperature variability.
Literature from the Viking Age (ca. 790 to 1070) during the establishment
of Norse colonies (and later) in Iceland and Greenland
permits comparisons between the δ18O temperature record and
historical records, thereby demonstrating the impact of seasonal
climatic extremes on the establishment, development, and, in some
cases, collapse of societies in the North Atlantic.