UA Geoscientist Studies Clam Shells for Clues on Past Climate

Rene Siqueiros
Oct. 17, 2000

If you think sclerochronology is just a fancy medical term for a nasty case of the flu, think again.

Bernd Schoene, research associate in the geosciences department at the University of Arizona, and his colleagues are working to reconstruct Earth's environmental history with help from clams. Schoene's research is in a young science called "sclerochronology," which analyzes growth bands on bivalve mollusks. A bivalve mollusk is a marine animal characterized by a shell divided into two valves hinged at one side. Clams, oysters, mussels, cockles, and scallops are all bivalve mollusks.

Some clams, for example, grow at a rate of one increment, or band, per lunar day, which is about 24 hours and 50 minutes. As clams grow, they use calcium and carbonate ions from the seawater to form calcium carbonate skeletons. The increments, or bands, along the surface of the shells are produced in varying widths and colors.

Environmental factors such as temperature, salinity of the water the shells grow in, nutrient availability, and physiological processes such as aging, determine the width and chemistry of the increments.

Bivalve mollusks are environmental recorders, much like most other organisms that add skeletal material to form their frame periodically, such as trees and corals. By measuring the growth increments on the shells, scientists can not only determine the growth rate of bivalve mollusks, but also the environmental conditions of the water in which the shells grew.

For instance, if scientists analyzed a clam that was 20 years old and knew when the clam died, they could determine what the temperature conditions in the water probably were during its 20-year life. By comparing growth curves from many of the same specimens whose life spans overlap, scientists are also able to construct longer growth series using cross-dating, a method used in dendrochronology, the study of tree rings.

The data collected from analyzing growth rings on bivalve mollusks can prove invaluable in attempting to predict future weather patterns on Earth.

"If we put together all data sets from other forms of interpreting growth bands, such as in tree rings and coral rings, alongside clam rings, we can get a better picture of the climate history of Earth," Schoene said. "We can predict, for instance, how carbon dioxide enrichment in the atmosphere will affect our climate in the future."

Schoene emphasized the importance of sclerochronology as a tool to complement other proxy records of the environment, like tree rings, because it does have some limitations. One drawback to clam ring research is that because clams don't live long, only brief growth chronologies can be compiled.

UA scientists have been working since 1992 with species of clams in the Chione genus, a genus that lives for no more than 20 years, Schoene estimates.

"If we would have longer chronologies or time series which cover hundreds of years as in trees, we could easily reconstruct regional or global temperature courses for longer periods of time," he said.

Currently, one of Schoene's main goals is to combine individual annual growth increment chronologies to establish longer master chronologies.

Schoene does see one major benefit to sclerochronology lacking in other environmental records.

"One of the main advantages of bivalve mollusks is that they occur worldwide in the aquatic ecosystems. Corals grow mostly in tropical seas, and trees only on land. But bivalve mollusks essentially cover the whole world from the oceans in the tropics, to the far north and south poles," he said.

Scientists plan future studies of Arctica islandica, shells from the North Sea. This species of bivalve mollusks can live for more than 250 years. That life span is long enough to build bivalve mollusk master chronologies needed to reconstruct a few centuries of environmental history, Schoene said.


Resources for the media