The Anthropocene is increasingly recognized as a chronostratigraphic unit; however, its definition remains unclear. A Global boundary Stratotype Section and Point (GSSP) that geologically defines its beginning is required to define it. Sediment from Beppu Bay contains evidence characterizing the onset of the Anthropocene, and the 1953 layer has been identified as a suitable GSSP. The birth of the Anthropocene boundary provides scientific evidence for the Anthropocene, while also serving as a warning to humanity about the critical state of the Earth's environment.

The global environmental change caused by human activities since the Industrial Revolution, such as global warming, is one of the most significant, even in the context of Earth's long history. Due to the facts of such large-scale global environmental changes in recent years, experts have proposed the idea of the advent of the Anthropocene, which marks the transition from the Holocene to a new chronostratigraphic unit. Since its proposal by Nobel laureate Dr. Paul Crutzen in 2000, the concept of Anthropocene has been used across fields such as science, sociology, politics, religion, and culture. However, it has not yet been defined geologically. To be recognized as a chronostratigraphic unit, a GSSP, which would mark the beginning of the unit, must be established somewhere on the Earth strata.
The Anthropocene GSSP is now entering a new phase, with the Anthropocene Working Group, a subcommittee of the International Union of Geological Sciences (IUGS), which is the organization that determines the International Chronostratigraphic Chart, formally working on its establishment. As of March 2023, 12 GSSP candidate locations have been proposed, including lake sediments in Canada and China, corals in the Caribbean, and ice in Antarctica, and the selection process to narrow down the candidates is steadily progressing. If all goes well, the GSSP will be decided at the 2024 IUGS meeting, meaning the official birth of the Anthropocene as a chronostratigraphic unit is imminent.

Among the 12 GSSP candidate locations, sediments from Beppu Bay in Oita Prefecture, Japan, are being considered as a strong candidate. Beppu Bay contains a rare varved sediment structure characterized by a striped depositional structure. These layers, composed of seasonally varying colors and densities, can be identified for annual layers and their dates. These layers serve as time capsules, preserving information about the marine and terrestrial environments and the organisms that inhabited them when they were deposited. Analyzing the marine organisms and radioactive chemicals contained in these time capsules has revealed that the varved sediments record in detail when humans began to significantly impact the Earth's environment and ecosystems. Since forming the Beppu Bay Anthropocene GSSP Research Team in 2019, the author at the Center for Marine Environmental Studies at Ehime University and others at 23 institutes, confident in the potential of the sediments in Beppu Bay for geologically defining the beginning of the Anthropocene, have explored various anthropogenic markers in the sediment layers that characterize the onset of the Anthropocene in sediments in Beppu Bay.
The results we have obtained so far have been very interesting. Starting from the 1953 layer, which begins at 64 cm below the seafloor, significant changes can be seen in various indicators of human influence. Traces of human-induced environmental pollution, such as radioactive isotopes from nuclear tests such as plutonium, uranium and cesium, spherical carbon particles from fossil fuel combustion, heavy metals such as lead and mercury, persistent organic pollutants such as PCBs and DDT and microplastics can be detected for the first time from around this period.
Decreased carbon isotope ratios reflecting increased carbon dioxide concentrations from fossil fuel combustion and traces of excessive supply of nitrogen oxides (NOx) and nitrogen fertilizers in the environment reflecting an increase in the nitrogen stable isotope ratio were detected from the scales of Japanese anchovy found in the 1953 layer. The change in nitrogen isotope ratios captures what experts believe is the most substantial change in Earth's nitrogen cycle in the past 2.5 billion years. Ecosystem changes, unprecedented in scale, are also observed from this period, such as the increase in the remains of dinoflagellates that feed on red tide-forming species and significant changes in marine phytoplankton communities, suggesting the degradation of coastal marine ecosystems. Analyzing these changes together with other indicators, it becomes apparent that fingerprints of human influence dramatically increased around 1953 (see figure). This coincides with the acceleration of various indicators of human activity such as historical GDP. Thus, there are numerous pieces of stratigraphic evidence in sediment from Beppu Bay that characterize the beginning of the Anthropocene, and the 1953 layer has been identified as a suitable GSSP for the Anthropocene.

The birth of the Anthropocene as a chronostratigraphic unit signifies the first public recognition of 'the arrival of a new geological era brought about by humanity' based on scientific evidence. The Anthropocene boundary, named after the Greek word for "human," is likely one of the most significant boundaries between chronostratigraphic units in the Earth’s history. The establishment of the Anthropocene boundary goes beyond merely defining a chronostratigraphic unit. The Earth currently has the possibility to exceed the so-called planetary boundary, the limit at which the Earth’s system can tolerate irreversible changes. Without addressing global warming, a runaway effect could begin in as little as seven years, potentially leading to a 7-meter sea-level rise that would submerge many areas where populations and critical infrastructure are concentrated. The rate of species extinction is also unusually fast, already exceeding the 'planetary boundary,' and it is said that a sixth mass extinction event, comparable to those at the boundaries between chronostratigraphic units in Earth's history, could occur within a few hundred years if current trends continue. The Anthropocene is an era in which such crises are unfolding in exchange for human prosperity. In this sense, the Anthropocene boundary serves as an alarm bell to humanity, warning modern society of the signs of the drastic and irreversible changes that are about to take place in the Earth's environment.
The Anthropocene hypothesis, linked to the concept of planetary boundaries, has provided scientific evidence for the United Nations' Sustainable Development Goals (SDGs), particularly Goals 13 (climate action), 14 (life below water), and 15 (life on land), as well as related goals such as 3 (good health and well-being), 6 (clean water and sanitation), 7 (affordable and clean energy), and 11 (sustainable cities and communities). The author hopes that formalizing the Anthropocene as a chronostratigraphic unit will further boost the momentum behind the SDGs. (End)