The most recent age in Earth’s 4.54 billion year history began 4,200 years ago and has officially been designated the Meghalayan Age, after the Indian state.
Earth’s recorded history is divided into eon, era, period, epoch and age, with the age being the smallest unit of geologic time.
Meghalaya is now part of geologic history thanks to a stalagmite found in the Mawmluh cave in the northeastern state. Located at an elevation of 1,290 metres, Mawmluh cave is one of the longest and deepest caves in India, and conditions here were suitable for preserving chemical signs of the transition in ages that an analysis of the stalagmite has now highlighted.
The demarcation is significant for multiple reasons. It is the first formal geological subdivision of the Holocene epoch that began 11,600 years ago and extends to the present, into three ages: Greenlandian, Northgrippian and Meghalayan.
The onset of the Meghalayan is marked by what is known as the 4.2 ka (thousand years) climatic event ,which registers as a severe drought in records from many low latitude regions, and by increases in precipitation in others, especially in the high latitudes, Mike Walker, who heads the working group on the subdivision of the Holocene at the International Commission on Stratigraphy (ICS), the body tasked with defining the geologic time scale, said in an email interview. “It is a period of increasingly unstable climate and reflects a significant reorganisation of ocean and atmosphere circulation patterns.”
To classify a geological period, geologists examine sedimentary deposits, ice cores and deposits below the seafloor for clues to when dramatic changes on earth took place. These changes, which can also be reflected in chemical composition, need to reflect an effect that is unambiguous and global in extent.
In the case of the Meghalayan classification, which was first suggested 7 years ago, this clue was the strongest in a stalagmite found in the Mawmluh cave. It gave a clear indication of a particular way oxygen atoms changed, which Walker said was a proxy for climate, “specifically, precipitation”.
Philip Gibbard, a professor at the University of Cambridge involved in the ratification, described it as “monsoon signal”.”It is a chemical signal of the oxygen that is preserved in the cave formation. There is a substantial drop in the signal that captures a substantial drying of the climate which reflects the weakening of the monsoon,” Gibbard said.
This, the geologists said in the publication, reflected a period of drought that led to the collapse of civilisations in north Africa, the Middle East and Asia.
“...The beginning of this youngest time interval of the Geologic Time Scale corresponds with a major cultural changed driven by a major climatic event,” said Stanley Finney, secretary general of the International Union of Geological Sciences (IUGS) through an email.
This is the first time a geological time scale change has been linked to cultural event – in this case the collapse of civilisations.
The stalagmite has now been tagged a Global Boundary Stratotype Section and Points (GSSP), the first formally ratified marker of a geological time period change in India. These reference points have to be internationally agreed upon based on clear evidence.
To be sure, other scientists have debated the decision to split the Holocene. Some scientists believe a new geologic classification must be made to reflect the influence of humans on the planet – an age tentatively called the Anthropocene — and that the division of Holocene thwarts those efforts.
Scientists believe the division of the Holocene thwarts efforts to carve out the Anthropocene epoch.
“With the ongoing discussions concerning the Anthropocene Epoch many colleagues feel that the Holocene is an outdated term and it should be removed and replaced by the Anthropocene,” Mark Maslin, at the University College London, said. “Hence the three Ages described recently are not required and are purely in place as a political means of trying to save the Holocene.”
However, members of the group that ratified the proposal defend their decision.
“The disagreement seems to have arisen from the fact that this new subdivision is being confused with the Anthropocene,” Walker said, adding that “some seem to think that the designation of the Meghalayan stage compromises moves that are currently underway to establish such a new stage. But this is entirely wrong.”
Walker argued that the subdivision of the Holocene is based on natural and environmental criteria and does not interfere with efforts to designate an epoch that captures human impact on earth.
The other point of contention is that the episodes that are used to mark the start of the stages are not truly global in scope. In the case of the Meghalayan age, Maslin said that “the mega drought only affected civilizations in the Middle East and North Africa, India and parts of China,” and leaves out “the huge civilizations that existed in the Americas and Africa and Northern Europe.”
But that debate does not take away from the fact that the stalagmites in Meghalaya have captured a major event in earth’s history. “It is very special, almost unique because geologists are used to dealing with rock sediments. The cave environment is a special one, it has preserved a very high resolution record,” said Cambridge professor Gibbar. “We hope the local government decides to put a marker there recognising the significance of the cave.”