Microplastics are not a reliable indicator of the Anthropocene

It is now clear that humans are dramatically modifying the planet's geology and ecosystems. To the point that the definition of a new geological epoch, the Anthropocene, succeeding the Holocene, has become under debate. The idea spread at the end of the twentieth century^H In the twentieth century, especially by the meteorologist and chemist Paul Crutzen, winner of the Nobel Prize in Chemistry in 1995. But when does the Anthropocene begin? If some believe that we should begin this era with the industrial revolution at the end of the eighteenth century^H century, the definition is still under debate. The final proposal will have to be ratified by the International Union of Geological Sciences.

A geological interval is defined by a threshold that must correspond to a major event on a global scale that is recorded in sediments. When the definition is adopted, it is embodied by a golden nail embedded in a representative location (the global class point). For example, the first phase of the Holocene is defined at a site in Greenland and dates back to 11,700 years ago (this definition was ratified in 2018).

Which label should be chosen for the Anthropocene? An element associated with the global biological crisis with loss of biodiversity, or with changing geology and geochemistry? One proposed path is global and permanent pollution such as microplastics, heavy metals, or even radionuclides resulting from thermonuclear weapons testing.

Microplastics seem to be a good candidate. They are found everywhere on the planet, even in areas far from human activities: from deserts to the bottom of the oceans, including mountaintops or Antarctica. When the rivers do not carry it, it is carried by the wind and falls with rainfall. It is a very clear sign of human activity.

In a recent study, Inta Dimanti-Dimantovica, of the Institute of Aquatic Ecology in Riga, Latvia, and her colleagues showed that the first traces of microplastics in sediments are not a reliable criterion. By analyzing sedimentary layers at the bottom of several Latvian lakes, researchers found that the smallest plastic particles migrated into deeper sedimentary layers, and were therefore older than the invention of plastic. Dating these layers with microplastics becomes ambiguous and does not allow us to determine the beginning of the Anthropocene.

This difficulty in defining the Anthropocene is not without consequences. At the beginning of March, the International Subcommittee on Quaternary Stratigraphy (SQS) held a vote and rejected, by 12 votes to 4, the proposal to establish the Anthropocene. In principle, this vote is considered final, but two members requested that the vote be annulled, because the procedure did not respect the Statutes of the International Commission on Stratigraphy.

If the Anthropocene has not yet established itself as a strictly defined epoch, the idea has already gained popularity among the general public and what it represents has already been widely adopted. For some SQS members who voted against the proposal, the problem is that the Anthropocene is much more than just an event in the planet's geological history, but a much more global phenomenon.