‘Meghalayan’: How a North East cave rock formation now defines our Age

IUGS tweeted the latest International Chronostratigraphic Chart/Geologic Time Scale with the new Holocene subdivisions, including Meghalayan, on July 13.

Written by Sowmiya Ashok | New Delhi | Updated: July 20, 2018 7:50:29 am
On July 13, the IUGS tweeted a picture of “the stalagmite from India with the GSSP for the base of the Meghalayan Stage”. On July 13, the IUGS tweeted a picture of “the stalagmite from India with the GSSP for the base of the Meghalayan Stage”.

From late June, the world has “officially” been in the Meghalayan Stage (or Age) of the Holocene Epoch — the present interglacial period that we live in. The Holocene started around 11,700 years ago.

The Meghalayan Age, which is the most recent subdivision of the Holocene Epoch, began about 4,200 years ago, at a time when agricultural societies around the world experienced a very abrupt, critical and significant drought and cooling. Last month, this Age was officially ratified as the most recent unit of the Geologic Time Scale.

Why ‘Meghalayan’?

The three subdivisions of the Holocene Epoch — the Greenlandian, Northgrippian and Meghalayan Ages — are marked out by sediments accumulated on sea floors, lake bottoms, glacial ice and in calcite layers in stalactites and stalagmites across the world. Clues to the Greenlandian and Northgrippian Stages were available at specific levels in Greenland’s ice cores — snow turns into ice, and preserves a record of the climate each year. But this method did not work as well for the younger (newer) part of the Holocene as it did for the older (early) part.

This is where India — and Meghalaya — came into the picture. The Meghalayan Stage has been defined at a specific level in a stalagmite in the Mawmluh caves — India’s longest and deepest — in Cherrapunji, Meghalaya. Professor Ashish Sinha of the Department of Earth Sciences at California State University took a sample of the stalagmite back to his lab, and through uranium-thorium dating, ascertained the record of the climate over the last few thousand years. Both the ice cores and the stalagmite are now defined as “international geostandards”, and have been placed in protected archives that are accessible for further study.

The IUGS also tweeted the latest version of the International Chronostratigraphic Chart, or the famous Geologic Time Scale with new subdivisions in the Holocene Epoch. The Meghalayan Age appeared as the newest subdivision, which started 4,200 years ago. The IUGS also tweeted the latest version of the International Chronostratigraphic Chart, or the famous Geologic Time Scale with new subdivisions in the Holocene Epoch. The Meghalayan Age appeared as the newest subdivision, which started 4,200 years ago.

Why must geological time be divided?

Division provides a means of communication about time periods, Philip Gibbard, professor at the Scott Polar Research Institute at the University of Cambridge told The Indian Express.

“We are living in an interglacial, warm period within the ice ages; there have been many of these within the last two million years. The last interglacial period was 120,000 years ago. It is difficult to divide this present interglacial period using fossils, so we have to use some other means of dividing time. The best way we found is by looking at the way climate has changed throughout this period,” said Prof Gibbard, who is also secretary-general of the International Commission on Stratigraphy (ICS), the largest and oldest constituent scientific body in the International Union of Geological Sciences (IUGS).

This is exactly what Prof Sinha had done, Prof Gibbard said. “He identified that around 4,200-4,300 years ago, there was a significant weakening of the monsoon which then had a significant effect on the amount of precipitation and on human settlements and their food security.” Further geological research marked this as a significant event.

What is special about the Mawmluh caves stalagmite?

The event under examination was represented in the mid and low latitudes, and the record had to be complete, and also had to have what geologists describe as a “higher resolution”. “The group which looked into this looked all around the world for potential sites, (and the) Indian site was the best record,” Prof Gibbard said. “Geological sequences is like a tape recorder, (in) some instances we have a cassette recording, and in some instances we have a studio recording. This is what is high resolution. We needed the best quality recording we could get to really pinpoint where the change occurred, and to identify that particular change point,” he said.

The Meghalayan Age is unique among the many intervals of the Geologic Time Scale in that its beginning coincides with a cultural event produced by a global climatic event, Dr Stanley Finney, professor of geological sciences at California State University, Long Beach, and secretary general of the IUGS, said in a statement. IUGS is an international scientific union in which geologists of 121 countries cooperate in the field of geology.

The first scholarly paper on the Meghalayan Age came about six years ago, Prof Gibbard said. The proposal was discussed and critiqued, and it then went through voting at the Subcommission on Quarternary Stratigraphy (SQS) and the ICS, before being finally accepted by the IUGS, the body that formally ratifies and selects the boundaries. IUGS tweeted the latest International Chronostratigraphic Chart/Geologic Time Scale with the new Holocene subdivisions, including Meghalayan, on July 13.

READING THE CHART

The columns in the chart represent, from left, Eon, Era, System/Period, Series/Epoch, and Stage/Age. Eons are divided into Eras, Eras into Periods, Periods into Epochs, and Epochs into Ages. We are currently in the Holocene Epoch, Meghalayan Age. On the far right is the measure of numerical age of each subdivision of geologic time, in mega-annum, or million years (Ma). The Meghalayan Age extends up to .0042 Ma (or 4,200 years) ago, and comes ahead of the Northgrippian and Greenlandian Ages. Thereafter, Holocene ends and Pleistocene begins. The Cenozoic Era, of which the Neogene and Quarternary Periods are part, has not been shown in full, nor has the Phanerozoic Eon, which is subdivided into the Cenozoic and Mesozoic Eras. The full International Chronostratigraphic Chart extends all the way to the beginning of the Pre-Cambrian Eon, approximately 4.6 billion years ago. The colouring of the chart follows the Commission for the Geological Map of the World (CGMW).

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