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500-year-old clams open up history of the oceans

Shell rings reveal influence of climate

- Doyle Rice @usatodaywe­ather USA TODAY

Clams are more than a tasty ingredient in chowder, they’re also important in understand­ing Earth’s history.

The world’s oldest animals, 500-year-old clams have given scientists an unpreceden­ted look at the climate history of the oceans.

By studying growth rings in the shells of quahog clams, scientists have pieced together the history of the North Atlantic Ocean over the past 1,000 years. The method is similar to how tree rings can serve as climate proxies by revealing clues about past weather and climate changes, including droughts.

Quahogs, also known as hard clams or chowder clams, are edible mollusks that live in the North Atlantic Ocean along North America and Europe.

By studying the clams’ shells, scientists from Cardiff University and Bangor University in Wales found that the ocean’s relationsh­ip with the atmosphere drasticall­y changed over the centuries, protbably because of the influence of greenhouse gases such as carbon dioxide and methane that have been pumped into the atmosphere from humanity’s burning of fossil fuels, beginning with the Industrial Revolution.

Although clams have been used as climate proxies through the field called sclerochro­nology since the 1970s, the new study is the first time researcher­s have been able to obtain a 1,000-year record of the ocean with absolute dating precision, according to lead author David Reynolds of Cardiff University.

In the preindustr­ial era, roughly before 1800, the climate was driven by natural factors such as volcanic eruptions and solar activity, he said.

At that time, the ocean influenced the atmosphere. Since then, it’s been the other way around: The atmosphere, with its increasing levels of carbon dioxide and other greenhouse gases, has driven major shifts underwater.

“The changes that we are seeing in ocean chemistry are unpreceden­ted, relative to the last 1,000 years,” Reynolds said.

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