Clam Shells Record Climate Events Over Past Thousand Years

Brett Smith for – Your Universe Online
Modern climatologists have access to a wide array of technological tools, but an international team looking to study climate events from the past thousand years has decided to utilize something a little more old school.
Researchers led by Alan Wanamaker from Iowa State University have been collecting clam shells from the waters of the North Atlantic because the mollusks act as tiny recorders, storing information about their environment in the growth bans that runs along their shells. As these clams can live in the cold North Atlantic waters for up to 500 years, their shells can tell researchers a lot about the climate during their lifetime.
“In the broadest sense, we´re trying to add to our understanding of oceans over the last several thousand years,” Wanamaker said. “We have a terrestrial record — we can get an excellent chronology from tree rings and there is a climate signal there. But that´s missing 70 percent of the planet.”
According to the team´s report in Nature Communications, they used these shells to examine the climate dynamics during two major recent events: the warmer Medieval Climate Anomaly, from about 950 to 1250, and the Little Ice Age, from around 1550 to 1850.
After collecting the clam shells from 200 feet below the water surface around Greenland and Iceland, the team transported them back to the Stable Isotope Laboratory at Iowa State University where they were cleaned and prepared for carbon dating and microscopic imaging.
To perform the most accurate analysis, the shell´s growth bans are measured in millionths of a meter and microscopes were used during several key steps. Two mass spectrometers also measured shell fragments for different isotopes of carbon and oxygen. Heavier isotopes of oxygen in the shell would indicate that the clams were experiencing colder ocean temperatures.
“Isotopes are just wonderful tracers in nature,” Wanamaker explained.
The analyses performed on the shells allowed researchers to determine if the clams were living in “younger” or “older” water. Younger surface water from the Atlantic and older deep water from the Arctic converge in this part of the world and climate events dictate which type of water dominates the region.
The team´s findings indicate that younger water from the Gulf Stream dominated the region during the warmer Medieval Climate Anomaly. The paper also confirmed through shell data that older, colder water moved into the area during the Little Ice Age.
While the warming effect of the Gulf Stream appears to have been strong in the medieval era, the research showed that it probably weakened during the Little Ice Age and strengthened again around 1940. These fluctuations in Gulf Stream activity likely amplified the relative warmth and coolness of the times.
Wanamaker said he hopes this research on the past millennium allows for more information to be added to the climate debate, possibly assisting policymakers in their decisions.
“Is the natural variability only that, or is it influenced by burning fossil fuels?” he said. “Maybe we can understand what will happen in the next 100 years if we understand oceans over the past 1,000 years.”