International Study Reveals Sedimentation as Key to Understanding Marine Fossil Age Mixing

Paleontologists have long faced a vexing problem: when they excavate a layer of seafloor sediment that has turned to stone, the fossils inside it rarely tell a simple story. A clam shell and a snail shell lying next to each other might have lived hundreds of thousands of years apart, their remains stirred together by the churning of burrowing creatures, by currents, by the slow rain of organic matter from above. This temporal mixing—what scientists call time averaging—has made it difficult and expensive to know what any single fossil layer actually represents. Now, after analyzing more than 7,500 marine fossils collected from oceans around the world, an international team has identified which factor matters most in creating this confusion.

The answer is sedimentation. Not the burrowing of clams and shrimp, not the durability of shells, not the productivity of the water column above—but the rate at which sediment accumulates on the seafloor. This finding, published in the Proceedings of the National Academy of Sciences, emerged from an unusual collaboration. Scientists from Australia, Austria, the Bahamas, Brazil, Italy, Germany, Slovakia, and the United States had spent two decades separately collecting and studying marine fossils, publishing their own papers on regional samples. When they discovered each other's work, they realized they could combine their datasets into something unprecedented: a global analysis of fossil age distributions.

The mechanism of time averaging is straightforward in principle. In productive marine environments, the seafloor is honeycomb with tunnels—hundreds to thousands of them per square meter—carved by clams, shrimp, sea stars, sand dollars, worms, and other burrowing animals. As these creatures excavate, they churn up the sediment and everything in it: shells, skeletal fragments, the accumulated debris of death. This bioturbation mixes organisms that died at different times into a single layer. But bioturbation is not the whole story. Shells decompose at different rates. Some sediment accumulates quickly; some slowly. The number of organisms available to be fossilized varies with the productivity of the water above. All these factors contribute to the age spread of fossils in any given layer.

To determine which factor dominates, the researchers used radiocarbon and amino-acid analysis to date their 7,500 samples, collected from shallow coastal settings to the edges of continental shelves. They then ran simulations, varying the rates of bioturbation, sedimentation, and fossil destruction, and compared the simulated age distributions to what they actually observed in the field. The results were unambiguous: sedimentation rate emerged as the primary control on time averaging. The other factors mattered, but sedimentation mattered most.

Dr. Quan Hua, a radiocarbon specialist at ANSTO, contributed to the study by reconstructing regional marine post-bomb radiocarbon curves for age calibration. Because the level of carbon-14 in the surface ocean varies by region—due to ocean circulation, currents, upwelling, and differences in air-sea gas exchange—Hua constructed calibration curves for each study region using published radiocarbon data and known-age specimens from museum collections. This regional precision was essential to dating the fossils accurately.

The implications are significant. Paleontologists have long been constrained in the kinds of questions they could ask about marine fossils, because the uncertainty around time averaging made it hard to know whether a change in fossil composition reflected a real environmental shift or simply the mixing of organisms from different time periods. Knowing that sedimentation rate is the dominant factor opens new research avenues. Scientists can now focus on understanding sedimentation patterns in their study regions and use that knowledge to interpret their fossil records more confidently. And if the pattern holds for older fossils—those too ancient to contain measurable carbon-14—the methodology can extend paleontological inquiry backward through Earth's history.

Michal Kowalewski, the Thompson Chair of Invertebrate Paleontology at the Florida Museum of Natural History and co-lead author of the study, reflected on the collaboration's unexpected power. "Nothing of this scale has ever been attempted before," he said, "because it's simply not feasible to do so." But because multiple teams had worked on similar topics using similar methods, they could pool their data. "Sometimes life turns out to be more exciting than you thought," Kowalewski added. "In this case, the outcome is beyond any dreams we may have had when we started."