20-Year Global Study of 7,500 Fossils Reveals Key Driver of Time Averaging

An international team of researchers has spent 20 years carbon-dating more than 7,500 marine fossils from ocean beds around the world to identify what drives “time averaging” — the mixing of fossils from different time periods into a single rock layer. A fossil layer can look like a snapshot of one ancient community, even though organisms preserved side by side may have lived hundreds or thousands of years apart.

The mixing happens naturally on the seafloor, where burrowing animals such as clams, shrimp, sea stars, snails and worms disturb sediment and redistribute shells and other skeletal remains. “What continually amazes me is just how much time a bunch of fossils collected from a single sediment layer can represent,” wrote Rafal Nawrot, a paleontologist at the University of Vienna. Daniele Scarponi, an associate professor at the University of Bologna, compared some fossil layers to the ruins of Pompeii, buried quickly, and others to a graveyard used for centuries, mixing remains from many generations.

The researchers found that sedimentation rate — how quickly sediment builds up on the seafloor — is the primary factor controlling time averaging, ahead of how well remains survive or how many organisms lived in an area. “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.

The study’s scale was possible only by combining datasets from research groups in Australia, Austria, the Bahamas, Brazil, Germany, Italy, Slovakia and the United States, which had independently collected and dated fossils over roughly two decades before realizing their work overlapped. “Nothing of this scale has ever been attempted before,” said co-lead author Michal Kowalewski, the Thompson chair of invertebrate paleontology.

The more than 7,500 specimens came from marine environments ranging from shallow coastal waters to the edges of continental shelves and were dated using radiocarbon and other techniques.