Site Introduction 

Beppu Bay is on the northeast coast of Kyushu, one of Japan’s five main islands. It opens onto the Seto Inland Sea, which sits between Kyushu and two other Japanese islands, Honshū and Shikoku, and is connected to the Pacific Ocean via two channels. The bay was formed in a tectonic depression between two faults around 7 million years ago and today is 475 square kilometers with an average water depth of thirty-six meters. It is bordered by two cities: Beppu, with a population of around 120,000, and Oita, with around 470,000. Several rivers run through the cities before draining into the bay, bringing runoff from urban areas and industrial zones that include several steel, petrochemical, and electronics plants. Kyushu is home to Japan’s most active volcano, Mount Aso, and the geothermal and tectonic activity manifests in the numerous hot springs in Beppu, which make the city a popular tourist attraction. 

Hominids have inhabited the area since the Pleistocene, although until around 12,000 years ago what is now the Japanese archipelago was connected to mainland Asia. Kyushu is the site of the oldest known Jômon pottery fragments dating from 12,700 years ago, and the first rice paddy farming by the Yayoi people around 10,000 years ago. After the rapid economic growth in Japan in the mid-twentieth century, the population and urban area of Oita increased abruptly, while Beppu saw growth owing to the rapid development of spa facilities. Along with a substantial increase in fruit orchards around Beppu Bay, this industrialization and urbanization led to a rise in the quantity of pollutants and fertilizers entering the bay, which caused unprecedented eutrophication11An excessive growth of algae and other plants feeding on increased input of nutrients, which in turn causes a depletion of oxygen and reduction in animal life. of the water from the 1960s.

Location of the Core 

The core was extracted from the deepest point of the Beppu Bay basin, at a depth of seventy meters. There is a sill at the mouth of the bay (at fifty meters deep) that constrains tidal mixing and results in anoxic22Refers to an environment with low or no oxygen. water below this depth from spring to late fall. The undisturbed, anoxic water enables subannual sediment layers, or varves,33Annual or sub-annual layers of sediment that reflect seasonal deposition of lake sediments, recording changes in chemistry, biology, and sediment inputs and thereby providing a chronology of past environments. to form. Counting these layers allows scientists to establish a calendar-year chronology for the core, which is checked against lead-210 dating. The slopes around the deepest part of the basin are very steep and collapse during earthquakes and floods, producing turbidites44Turbidites are sedimentary deposits caused by the disturbance of sediments on a slope by strong wave action or earthquake shock. in the sediment layers, which—owing to records of historical flood events in the Oita and Ōno Rivers—can be used as an additional time control. Luckily, the Holocene-Anthropocene transition period has not been disturbed by such events.

Sedimentation rates are relatively high (around eight millimeters per year), which provides a substantial amount of material for analysis and creates a high-resolution record for study. Continuous deposition of sediment in Beppu Bay has occurred for at least the Holocene and is likely to continue into the far future, so that this site constitutes a very long-running record. 

Beppu Bay is also close to the main spawning grounds of the Japanese anchovy (Engraulis japonicus) and Japanese sardine (Sardinops melanostictus), which means that many scales from these species end up in the sediments. The chemical composition of fish scales provides important information about local ecosystem shifts that occur in response to anthropogenic impacts.

The Core and Results

The Beppu Bay core is 94.2 centimeters long and holds records that extend back 100 years and includes local and global signals of anthropogenic impacts. There is a sharp increase in spheroidal carbonaceous particles (SCPs)55A spheroid-shaped form of black carbon particle produced during high temperature oil and coal combustion. SCPs are a distinct marker of human influence on the planet, and can now be found in environmental archives on every continent, including very remote areas such as the high Arctic and Antarctica. starting in 1965, which corresponds to industrial developments in Oita—including the establishment of a petroleum plant in 1964 and a metal smelting factory in 1972. The 1960s also sees peaks in DDT66Dichlorodiphenyltrichloroethane (DDT) is a highly toxic chemical compound first synthesized in 1874 by the Austrian chemist Othmar Zeidler. After its insecticidal properties were discovered in 1939, it was widely used as an agricultural and household pesticide. Rachel Carson’s 1962 book Silent Spring documented its destructive environmental impacts and sparked opposition to its use. It was banned in the US in 1972 and a worldwide ban on agricultural use has been in force since 2004. and HCH77Hexachlorocyclohexane (HCH) is any one of a group of organic compounds with a six-carbon ring with one chlorine and one hydrogen attached to each carbon. HCHs have poisonous, pesticidal, and persistent organic pollutant properties. derivatives as a result of increased pesticide use; a rise in polychlorinated biphenyls (PCBs)88Polychlorinated biphenyls (PCBs) are organic chlorine compounds that are highly toxic to humans and other animals. They have been banned in the US since 1978 and by the Stockholm Convention on Persistent Organic Pollutants in 2001. Because PCBs are non-flammable, chemically stable, and have electrical insulating properties, they were used in many products including electrical equipment, paints, plastics, pigments, and dyes. due to industrial use; and a sharp increase in microplastics. Furthermore, the late 1960s sees various biotic changes as a result of eutrophication in the area: an increased concentration of diatom frustules,99A diatom is a microscopic single-celled algae with a shell-like cell wall (called a frustule) made of silica. They live in nearly all bodies of salt and freshwater. an increase in phytoplankton productivity, a notable change in phytoplankton communities, and a change in pollen diversity. These variations correspond to an increase of the local human population and land-use changes that led to higher levels of nutrients, pollutants, and fertilizers ending up in the bay. By the late 1960s, eutrophication1010An excessive growth of algae and other plants feeding on increased input of nutrients, which in turn causes a depletion of oxygen and reduction in animal life. also led to high sulfur concentrations due to hypoxia1111Depletion of dissolved oxygen in water and sediments. in the deepest layer of water.

Industrialization is also recorded in Japanese anchovy scales found in the core. Increased atmospheric carbon dioxide from fossil fuel combustion is evident through a decrease in carbon-13 in the scales after 1952, while local inputs of sewage, chemical waste, and nitrogen fertilizers are visible as an increase in nitrogen isotopes in the scales after 1952. Both values are unprecedented in the 300 years before 1950.

Mid-twentieth-century nuclear bomb testing is evidenced with a slight rise in plutonium-239 and plutonium-240 at 1952, a sharp increase at 1958, and a peak at 1962–66. The uranium isotope signatures show a similar trend, with an initial rise around 1951, a sharp increase at 1957, and a peak at 1961–66. There is also a regional signature from the Marshall Islands bomb tests, with a peak in the plutonium isotope ratio around 1958 that can be correlated with signals in coral cores from close to the islands. The caesium-137 fallout maximum is visible in 1955, with a second peak in 2011 that corresponds to the Fukushima nuclear disaster. Meanwhile, carbon-14 in fish scales increases rapidly at 1963, which shows that radioactive contamination from nuclear bomb tests had by this point reached the higher levels of the marine ecosystem food chain.

Collection and Analysis 

Cores were collected from the deepest part of Beppu Bay in June 2021, using an Ashura multicorer with three 120-centimeter-long polycarbonate pipes. The multicorer is lowered to the seafloor to extract the cores, and a handmade protection system was produced and installed on the base of the pipes to prevent sample material falling out as the cores were hauled back to the sea surface.

The cores were subjected to the following analyses: radionuclides (plutonium-239, plutonium-240, lead-210, lead-214, caesium-137, uranium-236, uranium-238, and carbon-14 in fish scales); biotic markers (chlorophyll a, diatoms, algal pigments, biogeochemical indices, total sulfur, and palynomorphs); sediment geochemistry; lead isotopes; organic compounds (PCBs, DDTs, and brominated flame retardants); microplastics; SCPs; stable carbon and nitrogen isotopes; and varve chronology via diatoms in seasonal laminae.

The Research Team 

The research team formed following the annual meeting of the Japan Association for Quaternary Research on August 24, 2019, during which Michinobu Kuwae presented the Beppu Bay sediments as a potential GSSP of the Anthropocene. Yoshiki Saito , a member of the Anthropocene Working Group, encouraged him to establish a Beppu Bay GSSP research team. 

Geological studies in Beppu Bay sediments started with anchovy and sardine fossil scales that were discovered in the sediments in November 2004. Since then, a diverse range of studies (paleoecology, paleotemperature, volcanic ashes, pollen, paleomagnetics, paleoseisemology, event sedimentology, mineral compositions, organic and inorganic geochemistry, sedimentary DNA, and radionuclides) have been conducted by the Beppu Bay research group, using seven 4-meter-long piston cores, nine 10-meter-long piston cores, two 20-meter-long piston cores, twenty-four 1- or 0.6-meter-long multiple cores, and twenty-four 1-meter-long gravity cores. 

The extended team includes Michinobu Kuwae, Yoshiki Saito, Narumi Tsugeki, Peter R. Leavitt, Atsuko Amano, Tetsuro Agusa, Yoshiaki Suzuki, Ken Ikehara, Yusuke Yokoyama, Stephen Tims, Michaela Froehlich, L. Keith Fifield, Takahiro Aze, Takayuki Omori, Bruce P. Finney, Jun Inoue, Hirofumi Hinata, Aya Sakaguchi, Kazumi Matsuoka, Shin Takahashi, Daisuke Ueno, Masanobu Yamamoto, Hikaru Takahara, Tsuyoshi Haraguchi, Keitaro Yamada, Akira Hayashida, Tomohisa Irino, and Keiji Takemura.

Principal investigators (listed alphabetically):

Michinobu Kuwae, Ehime University
Yoshiki Saito, Shimane University

Contributing Scientists/Researchers (listed alphabetically):

Tetsuro Agusa, Prefectural University of Kumamoto, Heavy metals, lead isotopes
Atsuko Amano, National Institute of Advanced Industrial Science and Technology, Heavy metal geochemistry
Takahiro Aze, The University of Tokyo, Plutonium isotopes analysis
L. Keith Fifield, The Australian National University, Plutonium isotopes analysis
Bruce P. Finney, Idaho State University, Carbon and nitrogen stable isotope analysis
Michaela Froehlich, The Australian National University, Plutonium isotopes analysis
Akira Hayashida, Doshisha University, Magnetostratigraphy
Tsuyoshi Haraguchi, Osaka City University, Bathymetry, Tectonics
Hirofumi Hinata, Ehime University, Microplastic analysis
Ken Ikehara, National Institute of Advanced Industrial Science and Technology, Event stratigraphy
Jun Inoue, Osaka City University, Fly ash (SCP) analysis
Tomohisa Irino, Hokkaido University, Sedimentology
Peter R. Leavitt, University of Regina, Pigment analysis
Kazumi Matsuoka, Nagasaki University, Palynomorph analysis
Takayuki Omori, The University of Tokyo, Radiocarbon analysis
Aya Sakaguchi, University of Tsukuba, Actinoides/Iodine analysis
Yoshiaki Suzuki, National Institute of Advanced Industrial Science and Technology, Varve analysis
Hikaru Takahara, Kyoto Prefectural University, Pollen analysis
Shin Takahashi, Ehime University, Organic compound analysis
Keiji Takemura, Kyoto University, Tectonics
Stephen Tims, The Australian National University, Plutonium isotopes analysis
Narumi K. Tsugeki, Matsuyama University, Biotic markers
Daisuke Ueno, Saga University, Organic compound analysis
Keitaro Yamada, Ritsumeikan University, Stratigraphic analysis
Masanobu Yamamoto, Hokkaido University, Environmental DNA
Yusuke Yokoyama, The University of Tokyo, Plutonium isotopes analysis