20-Year Study: Sediment Speed Is Key to Understanding Fossil Age Mixing

A paleontologist opens a fossil layer and sees what looks like a frozen moment—shells and bones scattered together, a snapshot of an ancient seafloor community. But the organisms lying side by side may have lived centuries or even millennia apart. This temporal compression, known as time averaging, has long been one of paleontology's most vexing problems. When you cannot tell whether a fossil assemblage represents a single season or a thousand years of accumulation, the conclusions you draw about past ecosystems become unreliable. For two decades, an international consortium of researchers set out to answer a deceptively simple question: what determines how much time gets squeezed into a single fossil layer?

The answer, they discovered, is sediment speed. How quickly mud and sand bury the seafloor—not the burrowing of clams and worms, not the durability of shells, not the abundance of life—emerges as the dominant force controlling whether fossils from different periods become jumbled together. The finding came from an analysis of more than 7,500 marine fossils collected from shallow coastal waters to continental shelf edges around the world, dated using radiocarbon and amino-acid techniques. Research teams from Australia, Austria, the Bahamas, Brazil, Germany, Italy, Slovakia, and the United States had independently gathered and studied these specimens over roughly twenty years. When they recognized the overlap in their work, they combined their datasets into what may be the largest collection of fossil carbon dates ever assembled.

The scale of the undertaking was possible only because the expense had been distributed across multiple laboratories and decades. Individual research groups can typically afford to date only a few dozen specimens. Evaluating time averaging rigorously requires thousands. Radiocarbon dating and amino-acid racemization are costly procedures, but technological improvements in radiometric methods reduced expenses and allowed researchers to work with smaller samples. Michal Kowalewski, the Thompson chair of invertebrate paleontology at the Florida Museum of Natural History and co-lead author of the study, noted that nothing of this scale had been attempted before. "Thanks to the fact that we had a whole bunch of teams that worked on similar topics and used similar methods, we were able to compile it," he said.

Once the fossil ages were compiled, the researchers created computer simulations that varied the rates of bioturbation—the mixing caused by burrowing animals—sedimentation, and fossil destruction. They then compared these simulated age distributions with the actual ages of the carbon-dated fossils to see which models best matched what they observed. The comparison produced an unambiguous result: sedimentation rate was the overwhelming control on time averaging. When sediment accumulates slowly, skeletal remains from different periods pile up together. When burial happens quickly, remains from multiple generations are far less likely to mix. Rafal Nawrot, a paleontologist at the University of Vienna, explained the implication: "If we know how quickly sediment accumulates—which can be deduced from the environmental context—we can determine how much time is captured by a given fossil assemblage."

Daniele Scarponi, an associate professor at the University of Bologna and Nawrot's colleague, drew a useful distinction. Some fossil assemblages are like the ruins of Pompeii, buried rapidly and providing a snapshot of past communities frozen in time. Others resemble a prehistoric graveyard used continuously over centuries, with human remains from different generations slowly accumulating. Both types can yield valuable insights, but the kinds of data extractable from them differ fundamentally. Before interpreting a fossil assemblage, paleontologists now know they must first determine the interval it represents.

The practical implications are substantial. Paleontologists can now determine more confidently how much time a fossil assemblage actually spans, which allows them to pursue questions that were previously difficult to address. If the same relationship between sedimentation and time averaging holds true for oceans deeper in the geological past, researchers may be able to extend these findings to fossils far older than the roughly 55,000-year limit imposed by measurable carbon-14. The dataset itself has already proven its value beyond the initial question. Kowalewski and his colleagues are pursuing multiple follow-up projects exploring various aspects of time averaging and related processes. "The dataset is incredibly powerful," Kowalewski said. "You can use it to answer a lot of questions, but of course, we started with the big one."