500-year-old clams recorded changes in ocean climate

The quahog clam is the longest-living animal, and its growth rings offer valuable climate insights.

Perhaps best known as an essential ingredient in clam chowder, the longest living animal, the quahog clam, has another important use: providing information about changes in ocean climate. Growth rings on the clams’ shells reveal changes in seawater temperature and salinity over the last 100 years that are unprecedented for the last millennium. Lead author David Reynolds tells us more about the study.

ResearchGate: Please tell us about the quahog clam and how it gets so old

David Reynolds: The ocean quahog is a common type of clam that is found in the coastal waters of northern Europe and the eastern United States and is commonly fished for use in clam chowders, particularly in the eastern United States. It is relatively unknown why they can live for as long as they do, however the cold waters and their sedentary life style are likely contributing factors.

RG: What made you want to study the clam?

Reynolds: We decided to study these shells for several reasons. First, they are extremely common in numerous regions of the North Atlantic. They also lay down growth rings on an annual basis, much like trees. These growth rings allow us to accurately age and measure the annual growth in each of the shells that we studied. In a similar way to trees, all shells that were alive at the same time will respond to the environment in the same way. If conditions are particularly good for growth one year, all the shells experiencing those conditions will respond by laying down a wider growth ring and vice versa. Therefore, the shells will have the same bar code of ring widths.

This means that we can statistically compare the ring widths of living and fossilized shells using these bar codes and generate an absolutely dated history of how these shells have grown that extends far beyond the life span of just one individual. We can then analyze the chemistry of the shell’s growth increments and build a dated record of how ocean chemistry has changed over the last 1000 years. Whilst sediment cores are also able to do this, they lack the absolute dating precision that these clams provide, and it is this dating precision that is essential for understanding the role marine variability plays in the wider climate system.

RG: How did you conduct your study?

Reynolds: To develop the 1000-year oxygen isotope series, we drilled samples of shell material out of the growth increments that had been dated by comparing the patterns of the increment widths. By examining multiple samples from within individual increments we could determine that the shells were growing largely during the summer months, and so the annual samples would tell us about summer oceanic conditions.

Once we had the 1000-year oxygen isotope series, we statistically analyzed it against a suite of oceanographic observational datasets, climate forcing records (solar variability and timing of volcanic eruptions) and contemporary proxy archives of atmospheric surface air temperatures, derived from tree rings and ice cores.

RG: What did you find?

Reynolds: Over the industrial period, the oxygen isotopes contained in the shells shifted significantly. The analysis of the isotope record found that 20th century changes in seawater temperature and salinity on the North Icelandic shelf, especially from 1950-2000, are unprecedented relative to the last millennium.

RG: What does this tell us about the North Atlantic Ocean and the climate?

Reynolds: During the pre-industrial era, the North Atlantic Ocean played an active role in driving atmospheric climate variability. This means that changes in the temperature of the North Atlantic Ocean—rought about by watermass shifts, volcanic activity and solar irradiance changes—are fed back into the atmosphere, influencing air temperatures.

However, during the modern period, the rapid increase in atmospheric surface air temperatures led to a reversal in the coupling between the ocean and atmosphere, with the marine variability now more closely coupled to or lagging atmospheric variability. This is likely due to the more rapid response of the atmosphere to greenhouse gases. However, it is likely that the oceans will continue to influence atmospheric variability.

RG: Can the clams also tell us anything about the ocean’s future?

Reynolds: Studying these clams is important for understanding future climate variability. While the shells don’t directly tell us the future, they help us develop a more robust understanding of the mechanisms that drive the climate. It is with this knowledge that we can help build more robust predictions using numeric climate models.

Featured image courtesy of flickr.