Fossil watch

Geoscientist Susan Kidwell finds decades of environmental history written in the remnants of dead seashells.

By Lydialyle Gibson

Photography by Bill Denison

Susan Kidwell likes to say that her whole career—mostly spent on the faculty of Chicago’s geophysical-sciences department—has been one long digression. “I started out as a perfectly normal geologist with a rock hammer,” she jokes. At the College of William and Mary, Kidwell majored in geology, graduating with highest honors in 1976. But as a grad student at Yale University, she switched to paleobiology. “I still wanted to be a geologist,” she says, “but I didn’t want to be naive about the paleobiologic record.” The history of rocks is intertwined with the history of life.

When she got to Yale, she began to think twice about the fossil data used to support established paleobiologic patterns. How could scientists be sure the fossils they were studying gave an accurate picture of the past? “So I thought, ‘I’d better first look at the quality of the fossil record as a record, as an archive of information.’ That’s how I got into postmortem processes”: tracking organisms’ path from corpse to fossil, calibrating how much can be learned from what remains behind.

Photo: Orit Bashkin In Washing­ton, DC, Kidwell and postdoc Adam Tomasovych plumb the Smithsonian’s mollusk­shell archives.

To that end, Kidwell, the William Rainey Harper professor of geophysical sciences, has built a pair of massive databases over the past decade that give a time-lapse view of humans’ recent environmental influence. And although she still gets out her rock hammer now and then, Kidwell focuses most of her research on seafloor environments around the world and the remains of mollusks—clams, oysters, sea snails, and myriad other species that inhabit calcareous shells. “Mollusks say a lot about an ecosystem’s health,” she notes. “And as filter feeders they’re very important for sustaining water quality.” The animals get rid of excess plankton and algae blooms; they help keep chemical levels in balance. Mollusks are resilient, “but if conditions become too unhealthy—if the oxygen content crashes or toxic metals go up—then the mollusks will die,” along with other species that may not leave as durable a record.

To look for changes in mollusk populations, Kidwell built a database of “live-dead studies,” surveys in which scientists scoop samples of seafloor (“about two gallons of mud,” she estimates) onto a sieve. They sort living organisms from the dead and record the identity and abundance of each species. “Then you compare the two,” Kidwell says, “and quantify how well the death assemblage captures the composition of the local living community.”

What Kidwell found, analyzing data from locations as disparate as the San Francisco Bay, the Gulf of St. Lawrence, the Baltic Sea, and the North Adriatic Sea, was that “live-dead agreement”—how closely the fossil record matched the living population—varied widely, depending largely on one factor: human activity. “There are gobs of ways in which we put stress on these systems,” she says: sediment runoff from building and road construction, thermal pollution from power plants, nutrient input from cities and farms, channel-dredging, bottom-trawling, harbor expansions that change the temperature and salinity of bays and lagoons. In the United States, urbanization exploded after World War II, she notes, and the seafloor fossil record shows it.

Studies conducted in relatively pristine areas with no nearby industry or city, Kidwell says, “have very good live-dead agreement.” She cites Cancún, Mexico. “You say that doesn’t sound pristine.

But in 1971, a paleontologist from Utah did his master’s thesis down there. There were no hotels, hardly anyone living there. He camped in a hammock for two weeks, sampling the seafloor.” The 1971 survey is one of 150 in Kidwell’s still-growing live-dead database. When she started it ten years ago, making sure the data was reliable enough to use became another digression.

Paleontologists have long been skeptical of live-dead studies, partly, Kidwell says, because of “all the horrible things that can go wrong postmortem” to skew the picture. Shells can be destroyed or disrupted by currents, dissolved by acidic pore waters, carried off by other animals. Plus, each sample of the living offers only a snapshot of a habitat that may change seasonally or annually. “So live-dead agreement is not going to be one-to-one,” Kidwell says. “From a practical standpoint, my question was, how good does it have to be to still be usable?”

The benchmark, she decided, would be only as high as the agreement between two living samples from the same place at different times. A few years ago she started building what she calls a “live-live” database to weigh against the live-dead. “What we found,” she says, “was that live-dead agreement was actually extremely good, not only in capturing the species, but also their relative abundance.” But only in pristine areas. “Where there’s been degradation, the death assemblage retains a memory that no longer matches the living community.” In any death assemblage, some shells may be hundreds or even thousands of years old, but most, Kidwell says, “reflect relatively recent deaths.” So a mismatch pinpoints recent degradation.

Because most detailed ecological surveys are less than 50 years old, Kidwell has widened her “shelf trawls”—which yield data sets she digitizes by hand—to include not only academic studies but also government-agency reports, international fishery and agricultural surveys, and local developmental histories. “I’m doing a lot of data rescue now,” she says, which means chasing down information on disused floppy disks or dot-matrix printouts or jars of death-assemblage shells sampled years ago. “I’ve been calling retired professors and asking, ‘Do you still have your raw data from ten, 20, 30, 40 years ago?’”

Kidwell “hit the jackpot” a few years ago when she connected with biologists from Southern California wastewater departments. Since the 1960s, those agencies have surveyed the continental shelf from Malibu to San Diego several times and sampled the sites of their wastewater outfall pipes as often as twice a year. “Amazing trove,” Kidwell says. On sabbatical this year, she and her postdoc, Adam Tomasovych, are scouring the Smithsonian library and archives in Washington, DC, for more raw data to add to her databases.

“It’s extremely important to find and stabilize these data,” Kidwell says, “because you cannot go back and sample something after it’s changed.” When scholars retire and clean out their offices or museums and agencies run out of shelf space, “these data just evaporate.”

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