Sedimentation Rate Emerges as Key Factor in Marine Fossil Dating

A paleontologist digging through a single layer of seafloor sediment might pull out two shells that appear to have been neighbors for millions of years. In reality, they could have been separated by half a million years of Earth's history. This temporal confusion—what scientists call time averaging—has long frustrated researchers trying to read the fossil record with precision. Now an international team has identified which environmental factor matters most in creating this temporal blur, and the answer reshapes how paleontologists can use marine fossils to understand the past.

The study, published in the Proceedings of the National Academy of Sciences, analyzed more than 7,500 marine fossils collected from oceans around the world, from shallow coastal zones to the edges of continental shelves. Researchers from Australia, Austria, the Bahamas, Brazil, Italy, Germany, Slovakia, and the United States pooled two decades of separate work, each team having independently collected and studied fossils using similar methods. When they discovered the overlap in their research, they combined their datasets into something unprecedented in scale.

The question driving the work was straightforward but consequential: when fossils of different ages end up preserved together in the same sediment layer, which environmental factors are responsible? Three candidates stood out. Bioturbation—the churning of seafloor sediment by burrowing animals—seemed an obvious culprit. In productive marine environments, a single square meter of seafloor can be riddled with hundreds or thousands of tunnels carved by clams, shrimp, sea stars, sand dollars, snails, worms, and other creatures. All that excavation physically mixes the sediment and anything embedded in it, including shells and skeletal remains. The durability of fossils themselves also seemed important; some organisms decompose or get scavenged before fossilization, while others persist. And then there was sedimentation rate—how quickly new material accumulates on the seafloor, which varies across ocean basins and through time.

The researchers dated their fossil samples using radiocarbon and amino-acid analysis, then built computer simulations that varied each of the three factors independently. They compared the real age distribution of their dated fossils against these test distributions to see which simulation most closely matched what they actually found. The results were unambiguous: sedimentation rate dominated. The speed at which sediment accumulated on the seafloor was far more important than the amount of burrowing or the resistance of shells to decay.

Dr. Quan Hua, a radiocarbon dating specialist at ANSTO, contributed crucial calibration work to the project. Because the level of carbon-14 in the ocean surface varies by region due to currents, upwelling, and differences in air-sea gas exchange, he reconstructed regional marine post-bomb radiocarbon curves for each study area, connecting modern measurements to pre-bomb baseline data. This regional precision was essential for accurate dating.

The implications are substantial. In places where sediment accumulates slowly, fossils from widely separated time periods can be buried and mixed together, creating a compressed temporal record. In places where sediment piles up quickly, the fossil record preserves a tighter chronological sequence. Understanding this relationship means paleontologists can now make more informed decisions about which research questions their fossil samples can actually answer. A fossil bed in a slow-sedimentation zone might be useless for studying short-term ecological change but valuable for understanding long-term trends. The reverse is true for fast-sedimentation zones.

Michal Kowalewski, the study's co-lead author and Thompson Chair of Invertebrate Paleontology at the Florida Museum of Natural History, noted that nothing of this scale had been attempted before. "It's simply not feasible to do so," he said, "but 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 added that the outcome exceeded expectations: "Sometimes life turns out to be more exciting than you thought."

Perhaps most significantly, the findings may extend far backward in time. If sedimentation rate remains the dominant factor in time averaging for much older fossils—those too ancient to contain measurable carbon-14—paleontologists can apply these principles to reshape understanding of Earth's deep history. The work opens research avenues that were previously closed, constrained by uncertainty about how much temporal mixing had occurred in any given fossil assemblage.