4,200 years ago, a sweeping mega-drought devastated agricultural societies across the globe, wiping out civilizations from Ancient Egypt to Mesopotamia, Palestine and the Yangtze River Valley. Now, scientists say the cataclysmic event marks the beginning of a new geologic age: the Late Holocene Meghalayan, which encompasses everything from the start of the drought to the present.
Geologists divide the planet’s 4.54-billion-year history into a series of smaller subdivisions, Laura Geggel writes for Live Science. Earth is currently situated in the Phanerozoic Eon, Cenozoic Era, Quaternary Period, Holocene Epoch and Meghalayan Age.
According to a press release from the International Commission on Stratigraphy, the governing body responsible for delineating these segments of time, the Meghalayan Age is unique amongst intervals in the Geologic Time Scale because its start represents a global cultural event—the rebuilding of decimated agricultural societies—sparked by a global climatic event—the mega-drought, which wreaked havoc over the first 200 years of the stage.
The ICS bases its classifications, as popularly visualized by the colorful International Chronostratigraphic Chart, on markers in the earth's rock record. When an asteroid drove dinosaurs to extinction 66 million years ago, for example, it triggered the transition from the Cretaceous Period to the Palaeogene, leaving traces of the element iridium scattered throughout the planet’s sediment.
Comparatively, Jonathan Amos reports for BBC News, the Meghalayan Age’s “timestamp” is an isotopic shift found in a single stalagmite growing from the floor of the Mawmluh Cave in Meghalaya, India. The change in oxygen atom isotopes indicates the area experienced a 20 to 30 percent decrease in monsoon rainfall during the mega-drought that launched the new geologic age, University of Wales Quaternary science professor Mike Walker tells Amos.
When ICS commission members proposed the boundary in a 2012 paper, they laid out more evidence for the mega-drought: dust found in Peruvian ice cores and lake shoreline deposits that show lakes were drying up in Africa. In the ICS press release, the scientists say that evidence for the Meghalayan Age "has been found on all seven continents."
In addition to introducing the Meghalayan Age, the ICS announced the classification of two earlier Holocene stages dubbed the Greenlandian and the Northgrippian. Newsweek’s Katherine Hignett reports that the former began 11,700 years ago and ended roughly 8,300 years ago, when the Northgrippian began; the Northgrippian lasted until the start of the Meghalayan Age. Both ages are marked by ice cores found in Greenland.
Additional evidence for the three Holocene stages was found in “a wealth of sediment that accumulated worldwide on the sea floor, on lake bottoms, as glacial ice, and as calcite layers in stalactites and stalagmites,” according to the ICS statement.
Although the idea of the Meghalayan Age first arose back in 2012, some scientists think its official classification warrants further discussion. Mark Maslin, a geography professor at University College London, tells BBC News, “After the original paper and going through various committees, they've suddenly announced [the Meghalayan] and stuck it on the diagram. It's official, we're in a new age; who knew?”
Maslin is a central figure in the debate surrounding another geological question: whether to declare a new epoch known as the Anthropocene. This epoch, which has yet to be submitted to or approved by the ICS, would recognize the geological impact humans have had on the planet. Various dates are proposed for the start of the Anthropocene, from the human-caused mega-extinctions of 50,000 years ago to the coal-burning of the Industrial Revolution or the emergence of trace radioactive materials generated by the use of nuclear weapons.
According to ICS secretary general Philip Gibbard, however, the introduction of the Meghalayan Age does not preclude the existence of the Anthropocene.
"Human impacts on the landscape ... and on the environment didn't start at the same time everywhere on Earth,” Gibbard tells Newsweek. “If you live in China these things began 5,000 or more years ago. If you live in North America, they seem to have begun roughly in the 1700s. To draw a single line—which is what geologists have to do—that is the same age everywhere on the Earth's surface, we have to have [a global] event. Either a change in the biology... or some change in the physical environment such as the geochemistry."
A final verdict on the Anthropocene lies somewhere in the future, but in the meantime, humans have another reason to celebrate—the dawn of a new day, or in this case, a 4,200-year-old day. Welcome to the Meghalayan Age.