Clam shells are like tree rings for the ocean

Arctica islandica. (Credit: Karlafg via Wikimedia Commons)

Just as trees have growth rings that scientists can study for clues about past growing conditions, clam shells have annual growth increments that offer information about ocean conditions over time.

Alan Wanamaker, working as a postdoctoral researcher from 2007 to 2009, was asked to compile a 1,000-year record of the marine climate for a spot in the North Atlantic just off the fjords and fishing villages of North Iceland. He was at Bangor University in Wales where there were thousands of clams, each a specimen of Arctica islandica taken from 80 meters of seawater on the North Icelandic Shelf.

Those clams—dead and alive, some able to live up to 500 years in the icy water—were to be the researchers’ sensors under the sea.

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Wanamaker, now an associate professor of geological and atmospheric sciences at Iowa State University, started building the marine archive by processing all those clams—lab work that involved sectioning shells, embedding shell slices in epoxy blocks, measuring down to a millionth of a meter, drilling samples, radiocarbon dating, and determining oxygen isotopes.

Next he eyed cross sections of shells, looking for patterns, matching growth rings, finding overlaps, all with the goal of eventually lining up enough shells to build a master chronology of growth increments covering a millennium. After Wanamaker left Wales, Paul Butler, now a research lecturer at Bangor University, continued with the shell chronology, and David Reynolds, now a postdoctoral research associate at Cardiff University, completed the isotope analysis.

That open-access record, based on 10 years of work and analysis of nearly 1,500 isotope samples from dozens of clams, is now published in the journal Nature Communications.

“Here we report a new 1,048-year precisely dated, annually resolved marine oxygen isotope record that spans the entirety of the last 1,000 years (AD 953-2000),” the researchers write.

2 reasons this work is important

Isotopes are elements with varying numbers of neutrons. Heavier oxygen isotopes in the growth increments of shells are generally associated with colder, denser seawater; lighter isotopes are associated with warmer, less-dense seawater.

The paper is important for two reasons, Wanamaker says.

First, it’s the first time researchers have produced a dated and annual record of ocean conditions that goes back 1,000 years. “That’s incredibly difficult. You need a lot of material to make this kind of chronology. And as you go back in time, there’s less and less to work with.”

Second, analysis of the annual record has allowed the researchers to study how temperature, density, and circulation changes in the North Atlantic have affected changes in the broader climate system.

The record “strongly indicates that ocean variability played an active role in driving the major pre-industrial climate variability of the past 1,000 years,” write the authors. That variability would include the warming of the Medieval Climate Anomaly from about CE 950 to 1250 and the cooling of Europe’s Little Ice Age from about CE 1550 to 1850.

The biggest discovery so far

“The coolest thing that came out of this record is that during the first 800 years of data, ocean changes seemed to be leading atmospheric changes, or the changes were at least simultaneous,” Wanamaker says. “But after 1800 and the industrial revolution, the atmosphere seemed to take complete control.”

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The acceleration in atmospheric change compared to ocean change is probably driven by the faster response of the atmosphere to the warming influence of greenhouse gases, the paper reports. “We haven’t seen anything like that in the last 1,000 years,” Wanamaker says.

Now that the 1,000-year marine record is complete and initial analyses have been made, there’s still work to do.
Wanamaker and colleagues are collecting and analyzing clam shells from waters off Maine and Norway with the goal of building marine climate records for other places in the ocean.

Further, there’s more to learn about the association between oceans and climate.

The shell record of ocean conditions provides a long-term baseline for the state of the North Atlantic coupled climate system and “demonstrates that the role played by the ocean in naturally forced climate variability should be a key focus if we are to realize the societally crucial step forward in near-term climate prediction,” the authors write.

Grants from the United Kingdom’s Natural Environment Research Council and the European Union supported the work.

Source: Iowa State University