Paleontologists spent 20 years carbon-dating thousands of marine fossils, then used them to decode a process fundamental to Earth’s history
Key points
GAINESVILLE, Fla. --- In productive marine environments, a square meter of seafloor can be perforated by hundreds to thousands of isolated and interconnected tunnels through which crawl and writhe a cornucopia of clams, shrimp, sea stars, sand dollars, snails, worms and other animals. All that excavation mixes up the sediment, along with any shells and other skeletal remains that happen to be there. This temporal smearing is a problem for paleontologists, because when that piece of seafloor is buried and becomes part of the fossil record, it’s difficult and expensive to figure out how much mixing took place.
“What continually amazes me is just how much time a bunch of fossils collected from a single sediment layer can represent. In some cases, well-preserved fossil organisms that are found next to each other might have lived hundreds or thousands of years apart,” wrote Rafal Nawrot, a paleontologist at the University of Vienna.
The mixing of fossils that lived at different times but are preserved together is called time averaging.
According to Daniele Scarponi, a colleague of Nawrot’s and an associate professor at the University of Bologna, time averaging dictates the types of questions paleontologists can ask.
“Before interpreting a fossil assemblage, we need to know the interval it represents. Some fossil assemblages are like the ruins of Pompeii — buried rapidly and thus providing a snapshot of past communities frozen in time. Others are more akin to a prehistoric graveyard used continuously over centuries, in which human remains from different generations are slowly accumulating over time. Both types can provide valuable insights into the past, but the kinds of data we can extract from them will be different in each case,” Scarponi wrote.
In addition to burrowing animals, several other factors influence time averaging, including:
Through a project that was 20 years in the making, members of an international consortium of scientists say they have determined which of these factors is the most important for time averaging and thus primarily controls the temporal resolution of paleontological data.
“Our results demonstrate that 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: The faster individual shells or bones are buried below the sediment surface, the less likely it is that remains from multiple generations of organisms will accumulate and be preserved together,” Nawrot wrote.
Sedimentation rates have long been anticipated to be an important component of time averaging, but gathering data needed to rigorously and comprehensively assess this issue is difficult, time-consuming and very expensive.
By integrating multiple projects, the authors analyzed more than 7,500 fossils, which were dated using radiocarbon and other methods and collected from a variety of oceanic environments around the world, from shallow coastal settings to the edges of continental shelves.
The various research groups involved in the project — which includes scientists based in Australia, Austria, the Bahamas, Brazil, Italy, Germany, Slovakia and the United States — separately collected, studied and published papers on the fossils over a period of two decades. When they learned of each other’s work, they decided to join forces and share data.
“Nothing of this scale has ever been attempted before because it’s simply not feasible to do so, 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,” said the study’s co-lead author, Michal Kowalewski, the Thompson chair of invertebrate paleontology at the Florida Museum of Natural History.
Radiometric dating, one of the primary methods the authors used, takes advantage of the fact that radioactive atoms always decay into more stable, non-radioactive atoms at a steady, predictable rate. This allows scientists to estimate the age of minerals and fossils.
Many animals have skeletons that contain a type of radioactive isotope called carbon-14. Plants absorb carbon-14 during photosynthesis and use it to make more of themselves. Herbivores get carbon-14 secondhand by eating plants, carnivores get it from herbivores, and decomposers get it from all of the above. This list includes humans. Any part of your body that contains carbon — which is every part of your body — is radioactive. Fortunately, carbon-14 emits radiation in the form of electrons, which for us is kind of like receiving a constant but imperceptibly low-level electric shock — not at all like the cell-shredding gamma rays emitted by uranium.
Carbon-14 has a half-life – the amount of time it takes for half of any given number of radioactive atoms to decay — of around 5,730 years. That meant the authors were restricted to the most recent fossil record, up to 55,000 years old, which is about the cutoff when any remaining carbon-14 in a fossil can be reliably measured.
The researchers also used a technique known as amino-acid racemization, which uses ratios of amino acids. As in the case of carbon isotopes, the ratio of different forms of a given amino acid also changes through time in a predictable way.
The reason no one has attempted dating on such a grand scale before is primarily due to the high cost of radiocarbon and amino-acid dating. Most research groups can afford to obtain data for only a few dozen specimens, but thousands of specimens are needed to fully evaluate the scale and drivers of time averaging. Distributing the cost across multiple labs over two decades helped significantly reduce this barrier, as did recent technological advances in radiometric dating that lowered the cost and made it possible to use much smaller samples than was previously possible.
Through this unique collaboration, Kowalewski and his colleagues have what is possibly the largest collection of fossil carbon dates ever compiled, which can now be used on a variety of research topics that would have been intractable otherwise.
“The dataset is incredibly powerful. We’re now working on multiple follow-up projects that explore various aspects of time averaging and related processes. You can use it to answer a lot of questions, but of course, we started with the big one,” he said.
After compiling the carbon dates from their fossil specimens, the authors simulated age distributions by varying the rates of bioturbation (mixing caused by burrowing animals), sedimentation and fossil destruction. Then they compared the real age distribution of carbon-dated fossils with the different simulated distributions to see which of the models most closely matched the actual patterns observed in the data.
The results were unambiguous.
“Sometimes life turns out to be more exciting than you thought,” Kowalewski said. “In this case, the outcome is beyond any dreams we may have had when we started.”
Knowing that the rate of sedimentation is the single most important factor in determining the extent to which fossils of different ages become mixed will unlock research avenues that were previously restricted. And assuming the same pattern holds true for oceans further back in time, the results can be extended to fossils that are much older than the ones that still contain residual amounts of carbon-14.
The authors published their results in the journal Proceedings of the National Academy of Sciences.
Additional co-authors of the study are: Adam Tomašových of the Slovak Academy of Sciences; Martin Zuschin, Bettina Bachmann, Michaela Berensmeier and Jan Steger of the University of Vienna; Paolo Albano of the Stazione Zoologica Anton Dohrn; Quan Hua of the Australian Nuclear Science and Technology Organisation; Darrell Kaufman of Northern Arizona University; Susan Kidwell of the University of Chicago; Matias Ritter of the Universidade Federal do Rio Grande do Sul; Marcello Simões of the Universidade Estadual Paulista; Luis Torres Jr. of the Florida Museum of Natural History; Lukas Schweigl of the University of Bologna; Troy Dexter of the University of The Bahamas; Ivo Gallmetzer of the Natural History Museum Vienna; Claudio Pellegrini of the National Research Council of Italy; and Matthew Kosnik of Macquarie University.
Proceedings of the National Academy of Sciences
Sediment accumulation rate predicts the temporal resolution of marine fossil assemblages
29-Jul-2026