In a first for geology, researchers in Japan used fatty acids from photosynthetic plankton to carbon-date sediments from one of the lakes around Mount Fuji, revealing a more accurate timeline of eruptions than was possible using previous dating methods.

The study, published April 18 in Geochemistry Geophysics, Geosystems, is the first to use fatty acids to date lake sediments, which are layers of material at the bottom of lakes. These layers contain the remains of living organisms and are like a continuous record of past eruptions and their effects on the environment.

"On land, sediments are often lost and destroyed by weathering and erosion after deposition," said first author Shinya Yamamoto, a researcher at Mount Fuji Research Institute. "In contrast, lake sediments are continuously deposited and preserved for a long time, providing a more complete record of past volcanic activity."

Mount Fuji is the largest volcano in Japan. And while the last major eruption of Mount Fuji occurred in 1707, it remains active to this day. The volcano and the surrounding area also attract millions of visitors each year, meaning that a future eruption could be extremely deadly.

"It has also been pointed out that volcanic ash may reach the metropolitan area of Tokyo in future eruptions," Yamamoto said. "It is necessary to reveal its eruption history in detail for volcanic disaster mitigation."

The source of carbon used for dating lake sediment is usually the remains of plants, which absorb carbon from the atmosphere. But the poor availability of plant material in lake sediments around Mount Fuji makes accurate carbon dating difficult.

"In the lakes around Mount Fuji, a number of volcanic ejecta from Mount Fuji were layered and preserved in the sediment," Yamamoto said. "However, the successive eruptions destroyed the vegetation, so plant fossils rarely occurred for radiocarbon dating, which makes it difficult to determine the age of the eruption."

To solve this problem, the researchers had to find a source of carbon in the volcanic lake sediments, focusing on Lake Kawaguchi, the second-largest of the Fuji Five Lakes. Using high-performance liquid chromatography, the team was able to extract organic compounds from the lake sediment, including palmitic acid, a fatty acid that originates in photosynthetic plankton. 

The researchers then used carbon dating to determine the age of the compounds and compared this to the age of plant leaves from the same sedimentary horizon, or wide area of sediment, finding that the age of the various layers of sediment "matched surprisingly well," with the plant-based measurements.

"This convinced us that the age of the lake sediments can be determined using organic compounds from phytoplankton," Yamamoto said.

Though the age matched leaf-based estimates, the team found some discrepancies in the timing of eruptions, which could have implications for predicting future catastrophes.

"The eruption called Om, which was previously thought to [occur] about 3,200 years ago, was actually about 270 years younger, about 2,940 years before the present," Yamamoto said. "This is the third-largest eruption of Mount Fuji in the past 5,600 years."

The team is interested in applying its methods to the other four lakes around Mount Fuji to understand the history of the volcano in more detail. More broadly, the researchers hope that fatty acids will prove useful for studying volcanoes around the globe, which the team considers essential for predicting and mitigating future eruptions.

"Lake sediments are distributed all over the world and are important archives for past environmental studies," Yamamoto said. The method proposed in our study will provide an alternative option to determine the age of sediments, especially in volcanic and/or arid regions."

The study, "Dating lake sediments using compound‐specific  14C analysis of C16 fatty acid: A case study from the Mount Fuji volcanic region, Japan," published April 18 in Geochemistry, Geophysics, Geosystems, was authored by Shinya Yamamoto and Mitsuhiro Yoshimoto, Mount Fuji Research Institute; Fumikatsu Nishizawa, Kanagawa Prefectural Museum of Natural History; Yosuke Miyairi and Yusuke Yokoyama, University of Tokyo; and Hisami Suga and Naohiko Ohkouchi, Japan Agency for Marine-Earth Science and Technology.