Digging for shells with geologist Susan Kidwell

MemberCentral website for the American Association for the Advancement of Science 10/23/12

By Delia O’Hara

AAAS Fellow Susan Kidwell spent a week in September on the research voyage of a lifetime, her team working around the clock collecting samples of shell-filled sediment from six sites off the coast of Southern California between Malibu and San Diego.

Kidwell is hoping that the shells she brought back to the University of Chicago, where she is William Rainey Harper Professor of Geophysical Sciences, will help answer the question, “To what extent is the very young part of the sedimentary record able to detect and faithfully record events that have occurred there?”

It is a question that has turned Kidwell, who began her career as a “rock-hammer-carrying geologist,” she said, into an accidental seafarer. After all, the ocean is where mollusks, which Kidwell called a common and “especially informative” part of the fossil record, live, and grow their durable shells.

People may think of the fossil record as a collage of “little Pompeiis,” as Kidwell put it — sudden burials of organisms from particular times and places. However, the fossil record as a whole is probably not that precise, she said.

When mollusks die, they sink to the bottom of the ocean, where they lie in “death assemblages,” collections of creatures’ remains — including many shells, in the ocean — that have ended up in the same place.

Death assemblages do not lie on the ocean floor in neat layers, though, Kidwell noted. Rather, they may be heaved by storms (in shallow areas), mixed up by burrowing animals, or otherwise disrupted. As a result, the surface of the ocean floor might contain shells whose former occupants were alive yesterday, a decade ago, 300 years ago, and even 2,000 years ago.

Kidwell reasons that if scientists can figure out “what’s going on with the death assemblage,” not only will they be able to assess the fossil record more astutely, but they will also be able to “figure out baselines” for restoring living ecosystems that have been damaged by pollution, climate change and other stressors.

“What I do is at the interface of several different kinds of science,” she said, including “sedimentary geology, ecology, biogeochemistry and conservation biology.”

The ocean off Southern California is a good place to look at the fossil record, Kidwell said. The area has only been under water for about 20,000 years. The area’s cities were largely developed during the 20th century. Offshore marine ecosystems there have been affected by fishing, agricultural runoff, industrial waste and other pollutants, but have also benefitted from major cleanup efforts in recent decades, Kidwell said. Local governments have collected voluminous data on the area, some of which she has been able to consult.

“You could never ethically go out and perturb a system to see what would happen,” Kidwell said. However, scientists can take advantage of the area’s history of varying conditions to study how those conditions affected the organisms that lived through them — and how accurately the shell record reflects all those changes.

Collecting samples is an expensive, difficult business, though. Kidwell wanted to go deep enough into the sediment to sample 300 to 400 years’ worth of death assemblages — far back enough to look at those “pre-urban centuries.”

During a four-year-long grant-proposal process that led to funding by the National Science Foundation, Kidwell determined she would need two- to six-meter-long cores, so deep they would likely go into sand. That “ups the type of gear you have to use,” Kidwell said.

So in September, her team set out on an eight-day cruise on the Melville, an ocean-going research vessel from the Scripps Institution of Oceanography at the University of California at San Diego, heavy enough to field the equipment the team needed to get its samples at depths of 50 to 75 meters (164 to 246 feet) of ocean.

In addition to its crew of 23, the Melville can accommodate 38 researchers. Kidwell filled 35 of the berths not only with her own team members, including graduate and undergraduate students, but also with people engaged in other kinds of science.

For example, Maureen Coleman and Jacob Waldbauer, both biogeochemists from the University of Chicago, came along to work on their study of microbes living in near-surface waters. They brought a laboratory technician and a graduate student. And Christopher Hintz, a chemical oceanographer and coordinator of the Marine Sciences Program at Savannah State University in Savannah, Georgia, gathered water samples to test new analytical methods he is developing for monitoring ocean acidification.

“We have videos that make the deck look like an ant colony,” Kidwell said.

Kidwell’s co-principle investigators on the cruise were Clark Alexander, Professor of Geology at the Skidaway Institute of Oceanography in Savannah, Georgia, and her former post-doctoral research fellow Adam Tomasovych, now a researcher with the Geological Institute at the Slovak Academy of Science in Bratislava, Slovakia.

Alexander was impressed with Kidwell. “Her level of energy and attention to detail were greater than anyone else I have ever worked with. She wore me out constantly,” he said.

Kidwell’s team took onto the Melville about a ton of mud, which yielded nearly 500 feet of cored sediment, 3 inches in diameter, as well as dozens of porewater samples, and “several hundred small Ziplocks of mollusk shells” the group sieved from cores while still at sea, she said.

“We came back with everything we wanted to get,” Kidwell said, adding that working with the specimens will keep her and her graduate students busy for years to come.