Seafloor amber may hold hints of a tsunami 115 million years ago

Wavelike patterns in 115-million-year-old amber suggest that a long-ago tsunami inundated what is now northern Japan, researchers report May 15 in Scientific Reports.

Tsunamis can be destructive and, to anything alive nearby, often terrifying. But the physical damage wrought by these giant waves eventually erodes away, typically leaving behind little evidence of their passage. As a result, there’s scant records of tsunamis stretching back beyond the current geologic epoch, which began roughly 12,000 years ago.

That’s where amber comes in. The tree resin is notoriously durable stuff, and much has been learned from ancient samples of the substance and the critters unfortunate enough to become entombed in it.

Scientists recently analyzed sediments quarried from a sand mine on Japan’s northernmost island of Hokkaido. That field site, which was at the bottom of the Pacific Ocean roughly 115 million years ago, yielded an unexpected discovery, says Aya Kubota, a geologist at the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan. “We found a weird form of amber.”

Layers of amber, with its characteristic orange hue, lay interspersed with darker layers of sandstone. Analyses with both visual and ultraviolet light revealed that the amber interlocked with the sandstone in a curious way. In particular, the fossilized resin formed shapes known as “flame structures.”

These shapes, which resemble flames or cresting waves, form as material of different densities settles vertically. “Generally, they will form when a denser layer gets deposited on top of a softer layer,” says Carrie Garrison-Laney, a geologist at Washington Sea Grant in Seattle and a liaison at the National Oceanic and Atmospheric Administration’s Center for Tsunami Research who was not involved in the new work.

Spotting such flame structures suggests the amber must have been soft when it was deposited on the seafloor so that it interlocked with the sandstone, the team concluded. And since tree resin forms on land and solidifies in just about a week when exposed to air, something must have rapidly transported that fresh resin out to sea and sent it to the bottom of the ocean, where it remained soft enough to form the flame structures before eventually fossilizing.

That realization, paired with the team’s simultaneous discovery of fossilized plant debris and wood over a meter long in the same sediments, led Kubota and her collaborators to hypothesize that they were seeing the aftermath of several ancient tsunamis, each of which deposited a layer of resin-rich wood on the seafloor. The researchers ruled out a smaller event like a flood since the analyzed sediments did not exhibit the characteristic flood signature of larger particles on top of smaller particles.

It’s novel to think about using amber as an indicator of ancient tsunamis, says Garrison-Laney. But more evidence is needed before this case is closed, she says. For starters, a tsunami would have impacted a broad area, so examining more of this amber deposit is important. “What does it look like 100 meters away?” Garrison-Laney is also skeptical of the idea that tree resin would remain soft when exposed to the cold waters of the deep ocean. “That seems like a stretch to me,” she says.