Scientists have discovered evidence of an ancient tsunami in Japan — which is hidden in tree amber that dates to the age of the dinosaurs. The amber samples are deformed in a particular way that suggests trees and plant debris were rapidly swept out to the ocean and sank to the seafloor around 115 million years ago, the researchers said, which the team interpreted as evidence of one or more tsunamis. The scientists published their findings today (May 15) in the journal Scientific Reports.
Scientists typically estimate when tsunamis happened in the past using geological evidence such as giant fossilized boulders that were swept away and deposited onto coasts, or by looking at abrupt changes in sediment deposits near coastlines. However, it can be difficult to differentiate tsunami traces in the fossil record from severe storms, which leave similar deposits.
Amber, which is fossilized tree resin — a fluid produced by trees — can also be transported to the ocean when a tsunami sweeps trees and plant debris out to sea, leaving behind a record of the tsunami event.
In the new study, the researchers analyzed amber-rich silica deposits from the Shimonakagawa Quarry in northern Hokkaido, Japan, which were deposited sometime between 116 million and 114 million years ago, during the Early Cretaceous period (145 million to 100 million years ago), when this region was deep seafloor.
The team used fluorescence imaging — a technique that photographs the amber samples while shining ultraviolet light onto them — to observe the amber’s structure.
The amber samples showed a pattern similar to what geologists call “flame structures,” a deformation that happens when soft sediment is deposited somewhere and changes shape before fully hardening — resulting in upward-pointing, flame-shaped tongues between the sediment layers. Amber deposits more commonly form other shapes, as tree resin dries when exposed to air.
The research team interpreted the flame structures to mean the amber was suddenly swept out from the land to the ocean by one or more tsunamis, without being exposed to the air (which would have hardened it), then sank to the seafloor. The amber would have then been covered by a layer of silt and preserved for millions of years.
“Identifying tsunamis is generally challenging,” and it was not immediately apparent that tsunamis were behind the unusual amber samples, study co-author Aya Kubota, a paleontologist at Chuo University in Tokyo, told Live Science in an email. “By combining detailed field observations with the internal structures of amber, we were able to conclude that the most plausible cause was tsunamis.”
Other evidence from the area backed up this hypothesis, including signs of a nearby landslide around the same time that may have been caused by an earthquake; large chunks of mud seemingly ripped up by the destruction of the seafloor; and large tree trunks on what was at the time the seafloor. Severe storm waves would not have affected the seafloor in this way, and if the tree trunks were stacked slowly over time they would have shown evidence of erosion, which these trunks did not — meaning all the evidence points to a huge amount of plant debris being transported quickly and suddenly to the seafloor.
The researchers suggested that looking at ocean floor geological and fossil evidence — that is, beyond just coastal evidence — paints a more complete picture of previous tsunamis, and that examining amber deposits can provide information that helps differentiate tsunamis in the prehistoric record from severe storms.
“Resin offers a rare, time-sensitive snapshot of depositional processes,” Kubota said. Although the study of amber has typically focused on organisms like insects trapped inside samples, “the emerging concept of ‘amber sedimentology’ holds exciting potential to provide unique insights into sedimentological processes,” Kubota added.
