Recent scientific research uncovered evidence of a tsunami that occurred over 100 million years ago in northern Japan. Amber deposits found in the Shimonakagawa quarry on Hokkaido Island reveal traces of the ancient event, which may be the oldest known record of a tsunami. The study, conducted by a team of scientists including Aya Kubota, a paleontologist at Chuo University in Tokyo, provided new insights into catastrophic natural events of the distant past.
"Finding concrete evidence of ancient tsunamis can be challenging because tsunamis devastate the coastal landscape, and events from antiquity are difficult to recognize in the geological record," the researchers noted. It is often difficult to differentiate tsunami traces from those left by severe storms, as both can leave similar deposits. However, advancements in science are making it easier to distinguish between storm events and tsunami events.
The research team analyzed amber-rich silica deposits from the Shimonakagawa Quarry, dated between 116 and 114 million years ago. These deposits provided one of the oldest records to date of a tsunami. The amber samples exhibited flame structures, a geological term describing upward-facing deformations that form when a soft material is rapidly buried before having time to solidify. This suggested that the resin was transported by water and buried under marine sediments quickly.
"By combining detailed field observations with the internal structures of amber, we were able to conclude that the most plausible cause was tsunamis," said Kubota. The amber's deformations indicated that trees and plant debris were rapidly swept out to the ocean and sank to the seafloor around 115 million years ago. This rapid deposition was more indicative of a tsunami than a storm, as tsunamis could cause rapid effects that storms could not.
Further evidence supporting the tsunami hypothesis was found in the same geological layers, including signs of landslides triggered by earthquakes and disturbed seabed sediments. The researchers observed tree trunks trapped on the ocean floor, providing additional evidence of the events that occurred during the Early Cretaceous period. "Large chunks of mud were seemingly ripped up by the destruction of the seafloor," the team noted.
Amber, which is fossilized tree resin, offers a snapshot of depositional processes. Normally, amber hardens when it comes into contact with air after oozing from the bark of trees. In this case, the amber was not exposed to air, which would have hardened it, and then sank to the seafloor, where it was covered by a layer of silt and preserved for millions of years.
The study suggested that deposits of amber in deep-sea sediments may reveal other tsunamis at other sites. "Large-scale tsunamis destroy coastal areas and rapidly transport plants and other debris over long distances," the study authors wrote. These findings indicate that a large amount of tree resin was simultaneously washed from land into the open ocean, likely caused by tsunamis that struck the coast between 116 and 114 million years ago.
The deformations in the amber deposits gave paleontologists a new way to identify past tsunamis. The researchers suggested that looking at ocean floor geological and fossil evidence painted a more complete picture of previous tsunamis. Other sediments originating on land and transported to open waters may be useful for investigating ancient events such as tsunamis.
The results of this study were published in the journal Scientific Reports. "The emerging concept of amber sedimentology has exciting potential to provide unique insights into sedimentological processes," concluded Kubota.
Scientists also estimate past tsunamis by looking at abrupt changes in sediment deposits near coastlines. However, traces of ancient tsunamis are hard to identify because the waves can reshape coastlines, and despite their immediate devastation, they don't leave a distinctive or long-lasting trace on the landscape. This makes the identification of such events challenging.
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