Using marine fossils, a large international collaboration decodes a fundamental process in Earth’s history

ANSTO has contributed to a large international study which decodes a fundamental process in Earth’s history and provides guidelines regarding the types of research questions that palaeontologists can investigate using marine fossils.

Dr Quan Hua, an authority on radiocarbon dating, reconstructed regional marine post-bomb radiocarbon curves for age calibration of the fossil samples used in the research.

“The level of radiocarbon or ¹⁴C in the surface ocean is spatially different due to ocean circulation, such as currents and upwelling, and regional variations in air-sea CO₂ gas exchange. Therefore, a marine post-bomb calibration curve connected to the current pre-bomb Marine20 data for each study region was constructed from published ¹⁴C data and the ¹⁴C data on known-age live-collected museum specimens produced by the study,” Dr Hua said.

Scientists used a dataset of thousands of dated marine samples to determine which of several environmental factors contribute most to ‘time averaging’. This mixing phenomenon occurs when organisms that lived in different time periods are mixed and preserved together in the same fossil bed.

The results, published in the prestigious journal PNAS, indicate that sedimentation is more important than other factors, such as the number of burrowing animals in a given area or the durability of fossils.

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 palaeontologists, 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.

The authors analysed more than 7,500 fossils, which were collected from a variety of oceanic environments around the world, from shallow coastal settings to the edges of continental shelves and dated them using radiocarbon and amino-acid analysis methods.

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 dated fossils with the different test distributions to see which of the three biases most closely matched.

The results were unambiguous.

“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 of thousands of years apart,” wrote Rafal Nawrot, a co-author and palaeontologist at the University of Vienna.

In addition to burrowing animals, several other factors influence time averaging, including the durability of organic material, which is important. Most organisms decompose or get picked apart by scavengers before they become fossilised. Another important factor is the rate of sedimentation, which occurs unevenly in different parts of the ocean and changes through time.

Biological productivity is also crucial. The number of fossils palaeontologists can expect to find while digging in one spot is strongly correlated with the number of organisms that were previously around to be fossilised in the first place.

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 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.

“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.”

https://doi.org/10.1073/pnas.2615368123

This content was adapted from a media release provided by the Florida Museum of Natural History