From Clam Shells, Echoes of Past Climate

Bryan Black sits at the desk in his office on the fourth floor of the University of Arizona’s Laboratory of Tree-Ring Research. Surrounding him, a dry-erase drawing of a house by his young son Henry, a photo of his 12-year-old Australian shepherd Ochoco, and lots and lots of fish-themed décor and paraphernalia. There’s a coffee mug from OSU’s Marine Science Center, a ceramic starfish paperweight, a framed collage of blue and red fish on raw-edge paper, an anatomical sketch of a cod, a nonfiction book about rockfishes.

On one bookshelf, there’s a slice of a tree trunk.

“I have to have that in here so I don’t get ostracized,” Black jokes.

Black is a forest ecologist gone rogue.

When he was a looking for his first post-doctoral job, he knew nothing about marine science, but he saw a posting for a position to study the growth patterns in fish and he gambled, writing a letter explaining how a tree-ring scientist would approach the research. Since that postdoc work, he’s been using tree-ring science, called dendrochronology, to study climate using the growth increments in bony fishes like salmon and bivalves like saltwater clams. Weirdly, it works very, very well.

His work makes Black a sclerochronologist—a person who studies chemical and physical patterns in skeletal hard parts with incremental growth. The first sclerochronological study that uses a tree-ring approach was published in 1976 by scientists who studied corals to uncover past sea surface temperatures in South Florida’s Biscayne Bay. More studies followed in the early 2000s, with the number of publications increasing dramatically over the past decade. The field is still fledgling, with fewer than 500 sclerochronologists in the world.

Hindcasting Climate, on Land and at Sea

Trees put on annual rings, which serve as recorders of yearly rainfall and temperature. Scientists have used tree rings since the early 20th century to piece together climatic history on land. As it turns out, the scales and shells of aquatic creatures contain within them climate information, too. Each striation—a mingling of calcium carbonate and proteins—on the shell of a geoduck (a clam with an appendage like an anteater’s snout, whose name is pronounced “gooey duck”) represents the passing of a year, and it contains clues about oceanic climate.

Funded by a three-year, $480,000 National Science Foundation (NSF) grant, Black is using geoduck shells from the northeast Pacific Ocean to create a chronological history of climate in these waters.

A Shortcut to the Past

The only long-lived clams in the North Pacific, geoducks can live to be about 180 years old and, according to radiocarbon data, their shells can last in the water for another 2,000 years.

To dive into the faraway past, Black collaborates with professional geoduck divers, Canada’s Department of Fisheries and Oceans, and Washington’s Department of Fish and Wildlife.

“All have been extremely supportive in this sampling effort. They’re quite interested in what we’re turning out, which I greatly appreciate,” Black says.

Professional divers, who commercially fish for geoduck, collect samples for Black, who himself has never eaten a geoduck. The marine scientist has never had a taste for seafood.

These divers collect geoduck shells using an underwater vacuum called a venturi. On their last voyage, the divers hired by Black excavated a nearly seven-foot-deep pit underwater, vacuumed up the sediment within it, filtered out the geoduck shells, and shipped them to Tucson. For their personnel and ship time, Black paid about $5,000 from his NSF grant.

“Hopefully, the deeper the pit, the farther back the record goes,” Black says. “Radiocarbon dating on a subsample of shells indicate some died more than 2,000 years ago.”

This approach to ocean climatology is pretty nifty.

The alternative, periodic sampling of plankton or fisheries, allows scientists to better understand biological processes in the ocean, but it also requires much more time and money and only spans short periods of time.

“For periodic sampling, you need ship time and generations of scientists to do multi-decadal observations of biological processes,” Black says. “And it’s unusual for these records to be more than 20 or 30 years in length.”

And while satellites and ships record oceanic temperatures, the number of observations only really took off around the 1950s, so these, too, are fairly short-term records.

For sclerochronologists like Black, creating chronologies based on growth patterns in fish and clams is cheaper and faster.

“We can get histories that span most of the past century, and that tells us a lot,” Black says. “With these long chronologies, we can establish robust relationships between the biology and the climate.”

Climate Trends Emerge

When the shells arrive at Black’s Sonoran Desert-based lab, members of his group unpack them, slice them, cast them in epoxy, polish the surface of the epoxy, and measure the width of each striation. Then, in a process called crossdating, they establish the exact calendar year each striation was formed.

“Crossdating is the foundation of dendrochronology,” Black says. “As climate varies from year to year, it induces a synchronous growth pattern or ‘bar code’ among all individuals of a given species and site. By matching those ‘bar codes’ among shells, we can determine exactly when each individual lived and extend our record back in time.”

Together, the collection of dated chronologies provides a year-by-year history of the water’s temperature and clues to the ocean’s circulation. Black’s group has been able to record a year-by-year history of Pacific Ocean climate dating back to 1728, the same year Enlightenment writer Voltaire ended his exile in England.

Knowing what the north Pacific Ocean’s climate was like while Voltaire was headed home to France is not only a thrilling feat, but an important one, because it can confirm whether or not the variability we’ve seen in climate of the 20th and 21st centuries is consistent with the past.

Black already has seen some noteworthy trends. Much like with tree-ring chronologies, the geoduck shells have revealed “clear trends in rising temperatures.”

“We see very pronounced warming trends begin in 1880s,” Black says.

The last century, he explains, has been unprecedentedly warm, because the influence of human activity has begun to overwhelm the natural variability seen prior to the Second Industrial Revolution. The warming trend is apparent in the North Pacific waters that Black studies.

Extreme weather events are increasingly common, and fluctuations in weather from year to year are becoming more extreme. In the western United States, for example, climate processes like the El Niño Southern Oscillation are extending their grip across coastal oceans and the adjacent landscape.

Marine and Terrestrial Data Combine

The International Tree-Ring Data Bank contains about 4,000 tree-ring chronologies, some dating back 10,000 years. When integrated with these records, Black explains, marine chronologies have the potential to paint an exquisitely full picture of climate on Earth.

Until now, there hasn’t been a ton of collaboration between sclerochronologists and dendrochronologists. Black intends to change that by working alongside colleagues in the tree-ring lab, integrating his data with theirs.

“The UA’s Laboratory of Tree-Ring Research is a global center for dendrochronology,” he says. “I think having a sclerochronologist here is mutually beneficial because we’re getting to the point where we can start linking this growing network of marine chronologies with terrestrial chronologies from tree rings.”

It’s all in pursuit of that very full picture of climate patterns over time.

“There’s a lot of potential overlap between sclerochronology and dendrochronology,” Black says. “And I’m pleased to be in a position to facilitate more of that.”