It’s difficult to pin the destruction of a tsunami to a tsunami — that is, unless someone was around to witness the devastation. But a new study shows that there are some surprising geological sources that scientists can consult as an archive of ancient tsunami occurrences, many millions of years after they occur.
Turning to Hokkaido Island in Japan, the Scientific Reports study suggests that deposits of amber in deep-sea sediments on the island may reveal tsunamis that occurred there between 116 and 114 million years ago. At that time, the study authors say, one or more tsunamis may have swept this fossilized tree resin — then still soft — from the island’s forests to the ocean floor, where it settled and solidified, preserving its particular method of deposition within its structure.
“We describe extraordinarily rich amber concentrations in Early Cretaceous deep-sea deposits,” the study authors write in their study. “The most plausible cause for the presence of this enigmatic amber in a deep-sea setting is large-scale tsunamis.”
The Ancient Traces of Tsunamis
Despite their immediate devastation, tsunamis don’t leave a distinctive or long-lasting trace on the landscape.
“Large-scale tsunamis destroy coastal areas and rapidly transport huge amounts of plants and other debris over long distances,” the study authors write. “However, due to their poor preservation potential and the lack of unequivocal identifying features, tsunami deposits are rarely recognized in the geological record.”
One part of the problem is that the traces of ancient tsunamis are deposited along the coast, a dynamic landscape that’s constantly changing due to the crashing of continuous waves. Another is that distinguishing between tsunami deposits and storm deposits is incredibly difficult, with tsunamis leaving some of the same signs as hurricanes, cyclones, and typhoons.
The new study suggests, however, that records of ancient tsunamis may be found in the form of amber and other terrestrial material deposited in deep-sea sediments. Indeed, studying sediments that were once buried beneath the open ocean on Hokkaido, the study authors found that soft amber was moved, in water, so rapidly from land to sea between 116 and 114 million years ago, that it was probably transported by a tsunami.
Read More: 5 Worst Tsunamis in History
Amber Archives
To arrive at their results, the study authors focused on amber-filled silica deposits from Hokkaido’s Shimonakagawa Quarry, which were deposited when the area was still covered with water. Fluorescence imaging found that “flame structures,” or distinctive flame-shaped deformations, were preserved within the deposits. Forming in amber that is still soft at the time of its deposition, these distortions suggest that the fossilized resin on Hokkaido traveled from land to open ocean in water, without popping up for air, only to sink to the ocean floor and solidify.
“These resin deformations occurred underwater, implying their direct transport,” the study authors write. “Such rapid and direct transport of terrestrial materials from land to ocean could be driven by a tsunami.”
According to the study authors, there are signs that landslides (and, thus, earthquakes) may have occurred on Hokkaido at around the same time in the Early Cretaceous, adding additional support to the tsunami theory. There are also displaced masses of mud, tree trunks, and other plant material that appear to have been deposited rapidly within the sediment between 116 and 114 million years ago — a deposition that is also more indicative of a tsunami than a storm.
Similar deposits of amber in deep-sea sediments may reveal other tsunamis at other sites, the study authors say. And these archives aren’t limited to amber alone, as other material that starts on land and travels to open ocean may also signal the occurrence of ancient tsunamis, many millions and millions of years ago.
“Large-scale tsunamis may not be recorded in coastal areas due to the destruction of such settings,” the study authors write, but deep-sea sediments and the terrestrial deposits within can still “serve as significant archives for large-scale destructive events.”
Read More: Here’s How Earthquakes Cause Tsunamis
Article Sources
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Scientific Reports. Amber in the Cretaceous Deep Sea Deposits Reveals Large-Scale Tsunamis
Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.
