Geological time is organized into a hierarchy of divisions: Eons, Eras, Periods, Epochs, and Ages. These divisions are based on recognizable, global changes recorded in the Earth’s rock layers. An Epoch is a subdivision of a Period, defined by significant shifts in climate, geology, or life forms preserved in the geological record. The current official designation for the time period we inhabit is the Holocene Epoch.
Defining Our Current Geological Epoch
The Holocene Epoch began approximately 11,700 years ago. It is the most recent epoch of the Quaternary Period, falling within the Cenozoic Era. The name Holocene means “entirely new,” reflecting that it followed the last major Ice Age.
The Holocene is defined by the relatively warm and stable climate that followed the last glacial retreat. This long interglacial period replaced the cold, unstable conditions of the preceding Pleistocene Epoch. This climatic stability allowed for the widespread development of human civilization and agriculture.
The International Commission on Stratigraphy (ICS) divides the Holocene into three distinct ages: the Greenlandian, Northgrippian, and the Meghalayan Age. The Meghalayan began about 4,200 years ago, defined by a global drought event that affected human settlements.
The official recognition of the Holocene provides the baseline for measuring the planet’s natural variability. Its stability is often contrasted with the rapid changes observed in more recent history.
The Scientific Basis for Time Division
Geological time is defined by the International Commission on Stratigraphy (ICS). This international body oversees the International Chronostratigraphic Chart, the global standard for Earth’s history. Time divisions are based on physical evidence found in rock strata, not fixed calendar dates.
The start of a new geological unit is defined by a specific, globally recognizable marker called a Global Boundary Stratotype Section and Point (GSSP). Colloquially known as a “Golden Spike,” a GSSP is a physical point in a rock section representing a permanent change in the geological record. This marker must be synchronous, meaning the event occurred worldwide simultaneously.
For a location to be designated a GSSP, the rock layers must be well-preserved and show a clear change in physical or chemical properties. The primary marker is often accompanied by secondary markers, such as chemical signatures, which help correlate the boundary globally.
The Proposed Anthropocene Epoch
The concept of the Anthropocene, or the “Age of Humans,” was first proposed in 2000 by Paul Crutzen and Eugene Stoermer. They suggested that human activity has altered Earth’s systems—including climate, biodiversity, and geology—so significantly that the Holocene designation is no longer appropriate. The term acknowledges humanity’s role as a geological force.
The Anthropocene Working Group (AWG), under the ICS, investigated whether human impact left a distinct, globally synchronous signal warranting a new epoch. Arguments for the Anthropocene center on the scale and speed of environmental change, which far exceed the natural variability of the Holocene.
A new epoch would signify a deep-time break, meaning the geological layer representing the Anthropocene would be distinct millions of years in the future. The AWG proposed the new epoch should begin around the mid-twentieth century, known as the “Great Acceleration.” This period marked an exponential surge in population and industrialization after World War II.
The debate over formalization is intense because it requires applying traditional stratigraphy rules to a geologically instantaneous event. Although the AWG presented a formal proposal, the Anthropocene has not been officially adopted as a unit of geological time. Discussion continues regarding the precise, globally-correlatable marker for its beginning.
Physical Evidence Used to Define a New Age
To formalize the Anthropocene, a GSSP must be identified—a physical, globally distributed signal in the rock or sediment record. Evidence supporting a mid-20th-century start date coincides with the Great Acceleration. One of the clearest synchronous signals is the “bomb spike,” a sharp increase in artificial radionuclides like plutonium-239.
This plutonium signature was dispersed globally by the atmospheric testing of thermonuclear weapons between 1952 and 1963. These radioactive particles settled worldwide, becoming incorporated into sediments, ice, and coral skeletons. This signal provides a precise date that fulfills the requirement for a synchronous boundary.
Other evidence proposed to define the new age includes technofossils and chemical changes:
- The pervasive presence of microplastics in marine and terrestrial sediments.
- The sheer volume of synthetic polymers, aluminum, and concrete particles, representing a new type of sedimentary material.
- A sharp drop in stable carbon isotopes due to the massive burning of fossil fuels.
- A rapid increase in atmospheric carbon dioxide and methane concentrations, alongside a reorganization of the biosphere through species transfer and extinction.
The combination of these signals provides a multi-layered, globally distributed fingerprint. Sediments found in locations like Crawford Lake, Canada, represent the physical evidence for the proposed new geological epoch.