For two decades, an international team of paleontologists has been assembling what may be the largest collection of fossil carbon dates ever compiled—and the results reveal that the speed at which seafloor sediment accumulates is the strongest control on how precisely scientists can reconstruct ancient marine life. By radiocarbon-dating and analyzing more than 7,500 marine fossils collected from ocean environments around the world, the researchers have shown that sedimentation rate plays a greater role in fossil “time averaging” than burrowing animals, fossil durability or biological productivity. Their findings, published in the Proceedings of the National Academy of Sciences, could change how scientists interpret fossil beds and determine what questions those ancient remains can reliably answer.
A fossil assemblage may look like a snapshot of a prehistoric ecosystem, but it often represents something much more complicated. Shells, bones and other skeletal remains can accumulate on the seafloor over decades, centuries or even thousands of years before being buried deeply enough to enter the fossil record. During that interval, animals continually disturb the sediment. Clams, shrimp, worms, sea stars, snails, sand dollars and countless other organisms tunnel through the seabed, mixing sediment and the remains embedded within it. As a result, fossils found side by side may belong to organisms that lived at dramatically different times. This process, known as time averaging, creates a kind of temporal blur in the geological record.
The extent of that blur determines how paleontologists interpret ancient communities. A fossil bed buried rapidly may preserve a relatively narrow interval, similar to an archaeological site sealed by a sudden disaster. Another deposit may resemble a graveyard that accumulated gradually over many generations, combining remains from organisms that never lived alongside one another. Both types of assemblages can be scientifically valuable, but they reveal different kinds of information. A rapidly buried deposit may preserve evidence of community structure at a particular moment, while a time-averaged deposit may provide a broader picture of ecological change, species persistence or long-term environmental conditions. Without knowing the duration represented by a fossil bed, researchers risk treating a composite record as if it were a single biological event.
“Sometimes, 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 and one of the study’s contributors. Daniele Scarponi, an associate professor at the University of Bologna, emphasized that recognizing the time interval represented by a fossil assemblage is essential before drawing conclusions from it. The same fossil collection could be used to study population abundance, biodiversity or environmental change, but the appropriate interpretation depends on whether the fossils were deposited together or gradually mixed across time.
Several processes can influence time averaging. The durability of biological material is one obvious factor: soft tissues usually decay or are consumed before fossilization, while shells, teeth and bones are more likely to survive. Yet even hard skeletal material can break apart if it remains exposed on the seafloor for too long. Biological productivity also matters because a region populated by abundant organisms has a greater potential to generate fossils. Bioturbation—the physical mixing of sediment by burrowing animals—can move remains vertically and horizontally through the seabed. Scientists have long suspected that sedimentation, however, might be the decisive factor. When sediment accumulates rapidly, shells and bones are buried quickly, reducing the opportunity for remains from multiple generations to collect together. When sedimentation is slow, fossils remain exposed near the surface for longer periods, allowing time averaging to intensify.
Testing that idea on a global scale required an extraordinary amount of data. The researchers combined results from multiple projects conducted independently over approximately 20 years. The fossils came from a wide range of marine settings, including shallow coastal environments and areas near the edges of continental shelves. Participating scientists were based in Australia, Austria, the Bahamas, Brazil, Italy, Germany, Slovakia and the United States. Each group collected and analyzed fossils using comparable approaches, and the teams later joined forces after recognizing that their datasets could answer a much larger question when examined together. The resulting compilation contained more than 7,500 specimens dated through radiocarbon analysis, amino-acid racemization and related methods.
Radiocarbon dating works because carbon-14, a radioactive isotope of carbon, decays at a predictable rate. Living organisms continuously exchange carbon with their environment, incorporating carbon-14 through food webs. After an organism dies, that exchange stops, and the carbon-14 in its tissues and skeletal material gradually decays into more stable forms. Because carbon-14 has a half-life of approximately 5,730 years, scientists can estimate the time since death by measuring how much remains. The method is most useful for relatively recent fossils, generally up to about 55,000 years old, beyond which the residual carbon-14 becomes too scarce for dependable measurement. Amino-acid racemization provided an additional dating tool. In many biological molecules, amino acids gradually shift between mirror-image forms after death, and the ratio between those forms can serve as a time-dependent chemical signal.
The team then used the dated fossils to test competing explanations for the observed age distributions. Researchers created simulations in which they varied the rate of sediment accumulation, the intensity of bioturbation and the rate at which fossils were destroyed. They compared those model distributions with the actual ages measured in the fossil samples. If burrowing activity were the primary driver, differences in bioturbation should have produced the strongest match between the simulations and observations. If fossil durability or biological productivity dominated, those variables should have explained most of the variation. Instead, the results consistently pointed to sedimentation rate. The faster sediment accumulated, the narrower the time interval represented by individual fossil assemblages tended to be. The slower the burial, the greater the mixture of fossils from different generations.
“This is the first time we have been able to evaluate the major drivers of time averaging across such a large and geographically diverse dataset,” said Michal Kowalewski, the study’s co-lead author and the Thompson chair of invertebrate paleontology at the Florida Museum of Natural History. The conclusion gives paleontologists a practical way to estimate the temporal resolution of fossil deposits. If scientists can determine how quickly sediment accumulated in an ancient environment—through sedimentary structures, depositional context or comparisons with modern systems—they may be able to estimate how much time is compressed into a single fossil bed. That information can guide interpretations of ancient food webs, biodiversity changes, extinction patterns and ecological recovery.
The implications extend far beyond the relatively young fossils that can be dated with carbon-14. Although the study focused on marine deposits containing material suitable for radiocarbon and amino-acid analysis, the authors argue that the relationship between sediment accumulation and time averaging may also apply to much older rocks. In ancient environments where direct dating is difficult, sedimentological evidence could help researchers assess whether a fossil assemblage represents a brief ecological moment or a long interval of accumulation. The new dataset may also support follow-up studies of how fossil destruction, biological activity and environmental change interact over time. By identifying sedimentation as the dominant control, the researchers have provided a new framework for reading one of Earth’s most incomplete archives—and for understanding when fossils truly lived together and when they only happen to be preserved together.
Subject of Research: Marine fossil assemblages, time averaging and sediment accumulation
Article Title: Sediment accumulation rate predicts the temporal resolution of marine fossil assemblages
Web References: https://doi.org/10.1073/pnas.2615368123
References: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2615368123
Image Credits: Florida Museum photo by Kristen Grace
Keywords: Paleontology, marine fossils, time averaging, sedimentation, fossilization, radiocarbon dating, amino-acid racemization, bioturbation, stratigraphy, marine biology, Earth sciences
