The Meghalayan Age and the Mawmluh Cave

Meghalaya, one of the states in North East India, has become part of the earth’s geological history. The study of a stalagmite discovered in a Mawmluh cave, attributed to its claim to fame in the earth’s chronology; adding “The Meghalayan Age” to the geological timescale.  The Mawmluh cave is located at an elevation of 1290 meters. It is one of the longest and deepest caves in India and it is located about 4 kilometers southwest of Sohra.

Geologically, in terms of rocks, Meghalaya is home to a variety of rocks. This ranges from the oldest, the Precambrian gneissic complex (basement gneiss) to the youngest type of rocks, the recent alluvium formations. These rocks are the storehouses of many valuable mineral deposits like limestone, coal, uranium, sillimanite, iron ore, etc. 

Climatically, Meghalaya has the reputation of having the world’s rainiest place at Sohra. This region receives more than 11000 mm of rainfall annually. The geological formations and very high rainfall produce wide-ranging landforms such as karst topography, structural platforms, mesas, canyons, etc. in the southern slopes of the state, such give rise to the occurrence of numerous caves and cave systems.

Mawmluh Caves

Formation of the caves in Meghalaya

As is the case of Meghalaya, limestone and rainwater formed the caves. In other words, rainwater seeps through tiny pores that are present in the limestone or by dribbling through larger cracks. The limestone will then begin to dissolve because rainwater is slightly acidic. Raindrops pick up acidity from chemicals in the atmosphere as they hurtle earthward. Although the atmosphere’s chemical composition varies, generally rainwater has a pH of around 5. But rock and rainwater are not enough to dissolve it; there’s one more catalyst that plays an important role in cave formation: plants.

Randall Orndorff, a geologist with the U.S. Geological Survey (USGS), said that wetter regions on the planet also tend to have denser vegetation. This means thick mats of organic material accumulate. What can be seen in many places of Mustoh and Nongkem areas is the evidence of such accumulation of organic materials. Plants die and decompose in a process that produces carbon. “So rainwater, when it hits, starts seeping through the soil,” Orndorff said. He added “and as it does it starts picking up all that carbon in the decaying organic material. The rainwater itself turns into carbonic acid.” That carbonic acid slowly eats through the rock. It takes tens of thousands, even hundreds of thousands of years later, to form a cave.

Speleothems

Speleothems

A cave doesn’t mean just an opening inside the surface of the earth but there are these formations, termed “Speleothemsformed by the deposition of minerals. Moreover, the deposition of minerals formed these caves features. The word speleothem is an amalgamation of the Greek words spelaion meaning “cave” and thema meaning “deposit”. The speleothems with which most people are familiar are stalactites and stalagmites. Stalactites grow down from the cave ceiling, while stalagmites grow up from the cave floor. 

Speleothems actually form because of water. Rainwater seeps through cracks in the rock. As it passes through organic material, it picks up carbon dioxide gas, creating carbonic acid. This weak acid passes through joints and cracks in the limestone. The mineral calcite is dissolved from the limestone rock forming a cave. When this water that now holds the dissolved rock, is exposed to the air in the cave, , much like when a bottle of soda is opened. As the carbon dioxide is released, calcite is precipitated (redeposited) on cave walls, ceilings, and floors. As the redeposited minerals build up after countless water drops, a stalactite is formed. If the water that drops to the floor of the cave still has some dissolved calcite in it, it can deposit more dissolved calcite there, forming a stalagmite.

Factors that determine the rate of growth of Speleothems

In addition, Speleothems form at varying rates as calcite crystals build up. Several factors can determine the rate of growth. One of the factors is the temperature outside (which affects the rate of decay of plants and animals, hence the amount of carbon dioxide in the soil). Another factor is the amount of rainfall. The acidity of the water that enters the caves and the way it flows determines the shapes of speleothems (by dripping, seeping, or splashing). 

Most scientists believe that the mineral content determines the color of speleothems. Pure calcite is white and almost colorless. Iron and other minerals, as well as acids from surface vegetation, combine with calcite crystals to add shades of red, orange, and black to the color of speleothems. Others believe that humic and fulvic acids in the soil may also contribute to speleothem coloration.

Based on this premise, the study of a stalagmite buried for millennia deep inside the Mawmluh cave led to the culmination of the naming of the earth’s most recent age – the Meghalayan age.

What is an Age in Earth’s history?

The earth’s recorded history is divided into the eon, era, period, epoch, and age, with the age being the smallest unit of geologic time. The most recent age in Earth’s 4.54 billion year history began 4,200 years ago. Meghalayan Age has become its officially designated name after the Indian state. The demarcation of the earth’s history is significant for many reasons. It is the first formal geological subdivision of the Holocene epoch that began 11,600 years ago that covers the present time. The three ages are Greenlandian, Northgrippian, and Meghalayan.

The 4.2 ka (thousand years) climatic event marks the onset of the Meghalayan Age . This 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 reorganization of ocean and atmosphere circulation patterns.”

To classify a geological period, geologists examine sedimentary deposits, ice cores, speleothems, 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.

How did the geologists figure out the start of the Meghalayan Age?

In the case of the Meghalayan classification, which was first suggested 7 years ago. The stalagmite found in the Mawmluh cave gave strong evidence of this clue. Moreover, it gave a clear indication of a particular way oxygen atoms changed. Walker said this was a proxy for climate, “specifically, precipitation”.

Philip Gibbard, a professor at the University of Cambridge involved in the ratification, described it as a “monsoon signal”. “Preserved in the formations as a chemical signal” 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 civilizations 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 change 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 that there is a link between a geological time scale cultural events. In this case, the collapse of civilizations.

Points of Contention of The Meghalayan Age

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 to reflect the influence of humans on the planet, a new geologic classification must be made.  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.”

Consequently, 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,”. He added it leaves out “the huge civilizations that existed in the Americas and Africa and Northern Europe.”

Points of Defense

However, members of the group that validated 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 “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.”

In addition, Walker argued that the subdivision of the Holocene is based on natural and environmental criteria. In addition, it does not interfere with efforts to designate an epoch that captures human impact on the earth.

Final Words

But that debate does not take away from the fact that the stalagmites in Meghalaya have captured a major event in earth’s history. In conclusion “because geologists are used to dealing with rock sediments, it is very special, almost unique. 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 recognizing the significance of the cave.”

Compiled by

Dr. Dondor Giri Nongkhlaw


Bibliography

Malavika Vyawahare,                    “Meghalayan Age” makes the state  a part of Geologic history                                                                    

                                                                                      Hindustan Times, New Delhi, July 18, 2018 

Andrea Mustain                      How Do Caves Form?                        LiveScience October 07, 2010

 

Previously from the Barking Deer Blog