THE NEW MEGHALAYAN AGE:
WHAT IT MEANS AND WHY IS IT NECESSARY?
The human race has divided time into different units since it culturally stumbled upon the abstract concept of time. We have broken time into variable slices like millennia, century, decade, hour, minute, second and so on. The ‘duration’ or ‘length’ of each of this division is finite. Century (100 years) as a time domain is fixed and so is hour (60 minutes). The distinction of time, as we understand it, is to have non-ambiguity in our dealings with respect to time.
The notion of time has been applied to compile digital history of our cosmos, and more specifically, our very own planet earth. The need to put a ‘number’ to a geological event has many scientific and academic benefits. These are not too obvious at first glance to the non-specialists. But, the serious students of earth’s history and evolution know the importance of assigning a discrete number to an event. Earth has undergone a range of dynamical physical and chemical processes that has shaped and re-shaped the surface and its innards. There have been cataclysmic events like volcanic eruptions, earthquakes, splitting and migration of crustal blocks, building up of mountains and generation of valleys. Celestial entities like meteorites have also impacted the course of earth’s history in no small measure. All these events and impacts have left an indelible mark on the surface of our earth and preservation of fossils in its subsurface realms. Geologists have taken these ‘game changing’ events to codify our earth’s history into different time compartments.
Earth is almost 4.6 billion years old and it came about through a very ‘heated birthing’. 4.6 billion is a very staggering number. It is huge. It is a very long piece of time, with many defining events spread unevenly throughout the time capsule. Just like the British era or Mughal era, the longer time frame, in geology or more precisely stratigraphy, is cut down into smaller pieces. The biggest chunk of geological time is an Eon, which is further broken down to an Era. The Era is then subdivided into a Period or System and this is then lacerated into an Epoch or Series.
Some of the terms that are part of Era are Palaeozoic (544-248 million years ago), meaning ‘old life’ in Greek and Cenozoic (65 million years ago) means ‘age of recent life’ in Greek.
The Period or System (broken down constituents of Era) and the Epoch (smaller slices of Period) are named after the place or geographical location in which the rocks or sediments defining a certain episode occur, and were meticulously studied. Accordingly, Cambrian (544-505 million years ago), the earliest period of the Palaeozoic Era, is named after ‘Cambria’, the Roman name for Wales, where rocks of this age were first studied. The Devonian (410-360 million years ago) is named after Devonshire, England, and Permian (286-248 million years ago) after the province of Perm, found in Russia; Jurassic (213-145 million years ago) the Jurra mountains found between France and Switzerland, and Cretaceous (145-65 million years ago) is the name derived from the Latin word ‘creta’ for chalk, which are found in the white cliffs along the English channel between England and France. These are just a few examples, but the rest of the Period entities are named along the same lines.
Against this backdrop we need to look at the new ‘Meghalayan age’ that has just been adopted by the stratigraphical community. Along with Meghalaya, two more names have also been included in the list that forks the Holocene (spanning from 11700 years ago till now). These two terms are Greenlandian and Grippian. The stratigraphic records that define Holocene contain evidences of sea level changes, changes in surface of earth leading to changes in the course of rivers and streams, evolution of different vegetational forms and animal migrations. They also showcase archaeological artefacts revealing the course of human civilizational development. Since these records are relatively (geologically speaking) new and well preserved, and the tools to study them are continuously improving, a lot many new ‘path breaking’ episodes are shining a light on themselves. The International Union on Geological Sciences (IUGS), an NGO founded in 1961 by a group of geologists, whose headquarter is in Trondheim, Norway, is a body entrusted with the responsibility to officially carry out the naming and renaming the different units of the geological timescale. They felt the need to revisit the evidences that have accumulated over the years to see what sort of changes need to be made or if they need to be made at all. It appointed a few committees to collect all the relevant data towards fulfilling this task.
Since the 1950s geological investigations have been on the rise all around the globe. In the recent past, records from marine sequences were studied in great detail because of which the bias between land and marine record has reduced considerably. Earlier, it was heavily loaded in favor of continental investigations, since there were very sparse oceanic studies. The scrutiny of countless published papers on Holocene reveals the Holocene has been arbitrarily divided into ‘early, middle and late’. There was no clear cut, and agreed upon, demarcation between these units. The early, middle and late Holocene of different workers coalesced into one another. This allowed creeping in of a few tiny inaccuracies while correlating some of the dynamical events. It did not, however, create a wide difference in the time frame of a certain ‘happening’ that occurred in widely separated places. The handle provided by radiometric dates is quite accurate. However, the individual scientist referring to his own and that of others’ work could inadvertently assign an age to a particular occurrence may not coincide with others’ demarcation of early, middle or late Holocene. To some, early Holocene might end at 9000 years, but others may think it ended 1000 years early or later. This uncertainty is another reason why the Holocene had to be broken up into accurate numbers coinciding with the ‘loose’ terminology already in use. There is thus a practical value for Holocene subdivision. It will also put an end to any slight confusion that exists in published literature of serious kind.
In the 1970s, the Pleistocene-Holocene boundary was demarcated using conventional stratigraphic procedures. The prototype unit for this defining boundary was a core drilled from Greenland with the age 11.7 ka coinciding at a spot placed at 1492.45 m depth/length of the core. The committee assigned the task to bifurcate Holocene, decided to adopt this approach.
For this they turned to the Greenland ice core. The Greenland ice core project was launched by the European Science Foundation comprising 8 nations from the European Union. The project was in force from 1989 to 1995. The drilling was carried out from 1990 to 1992 wherein they recovered 3029 metre ice core from central Greenland. The study of different constituents of the core, like isotopes and different atmospheric gases, revealed climatic variation history of more than one lakh years. The climate enshrined in the core during the timeframe comprising the Holocene was quite stable, but abrupt variations were observed at some depths of the core.
The Greenland ice core records, the now famous, 8200 year old event. This is a major, but very short-lived, cooling episode that is captured not just by the Greenland ice core but by other entities scattered all over the globe as well. This cooling was brought about by the stoppage of North Atlantic Deepwater formation, which is associated with the transfer of heat towards north. The cessation of this important deep water formation was because of the anomalously high influx of melt water from Laurentide ice sheet covering Canada and northern United States. The 8200 event is reflected in two more ice cores from Greenland. The signatures have also been obtained from pollen, lake sediment, lake isotopes, cave speleothems, marine foraminifera and many other proxy studies have also yielded this 8200 year event from North Atlantic Ocean, Oman, Yemen, China, Brazil, Africa, Tibetan plateau, Mediterranean, east Africa, Siberia, northwest Pacific, South Atlantic, New Zealand and many other places.
The 8200 year old climate change event greatly impacted the flora and fauna, including humans. The cultural effects have been recorded in North Africa, south and southeastern Europe, and the near east localities. These places grew dry because of the climatic effect unleashed by cooler north Atlantic surface waters. The 8200 year event in Mediterranean and Europe is seen to coincide with the Mesolithic to Neolithic transition. This, in other words, is the switching of communities from foraging to agriculture. The dry conditions must have made scarce the natural resources on which the communities subsisted. The ‘event’ forced them to shed their lackadaisical attitude of hunting in the wild for food to becoming more responsible by storing food from agriculture and cultivation. The same transition was seen in southeastern Europe, Anatolia, Cyprus and the Near East (comprising countries of the Arabian Peninsula, Cyprus, Egypt, Iran, Iraq, Israel, Jordan, Lebanon, Palestine, Syria and Turkey). This event brought social and cultural changes in coastal hunter-gatherer communities of Finland.
The 8200 event was first unraveled from Greenland ice core, and later, many other studies identified the same episode occurred throughout the globe. The IUGS accepted this event as the boundary between Early-Middle Holocene and the prototype locality is accepted to be Greenland from where the ice core was raised. So, we have a new Greenlandian age from 11700 years to 8200 years.
Just as the 8200 event, scientists have likewise identified another global major 4200 year event. This was an event that ‘dried’ up the environment and the signatures of it are found in North America, Middle East, China, India, Africa, South America and Antarctica. This aridity is more evident in mid latitude regions. This diagnostic event was probably caused by the migration of Inter-Tropical Convergence Zone (related to the occurrence of monsoon) that led to increase in strength of the westerlies over the North Atlantic, increased precipitation and consequent glacier advancement in western North America. The 4200 arid event coincides with a degree or two cooling ofNorth Atlantic surface waters, whereas the tropical ‘deep’ waters of Pacific cooled sufficientlytriggering a switch-on of the modern El Nin˜o SouthernOscillation (ENSO) regime. The effects of this switch are observed in low- and high latitude regions, but are morepronounced in the mid-latitude regions. It must be noted that an overactive El Nin˜o inhibits and weakens the Asian monsoon. The weakening of monsoon around 4000 year,and later, is reflected in many Pacific and Asian proxyrecords that led to widespread drought. Whatever may be the cause of this episode, the fact of the matter is that this idiosyncrasy is registered in many geomorphological,stratigraphical and archaeological records from many parts ofthe world. Hence, it constitutes an ideal time marker for the Middle–Late Holocene.
The 4200 year weakened monsoon had far reaching consequences on human cultural activities in North Africa, Middle East and Asia. The Mesopotamian Akkadian Empire collapse, Old Kingdom collapse in Egypt, and the transition from urban to rural society of Harappa civilization are all related to failed monsoon. The abandonment of city and town also came about this time in places like Iraq, Syria and Palestine. During the same period, in some parts of China, pastoral lifestyle replaced agriculture-basedculture.
All these evidences make it easy to demarcate the boundary between the Middle and Late Holocene at 4200year ago event. Because this phenomenon is conspicuous by its presence at mid- and low-latitude regions, the prototype location should ideally be from this geographic extent. Such a location was identified in India within the confines of its Meghalaya state at a place called Mawmluh cave near Cherrapunji. A well-developed calcite speleothem from Mawmluh cave was dissected to understand the changes in oxygen 18 isotope. The increased or decreased monsoon in the region regulates their concentration or content in the calcite waters that drip down to the ground from the roof of the caves. To understand these changes 1128 isotopic measurements were carried out on this speleothem containing record from 3500 to 12000 years. Therefore, we are now living in a Meghalayan age which starts at 4200 years and continues till date.
The sedate intervening time between Greenlandian and Meghalayan is the Grippian age and is inferred from the Greenland ice core.
We are now officially living in the Meghalayan age and all Indians should be proud of this unique achievement that has catapulted our country permanently into the geological timescale. This timescale is referred by all the geologists from all the countries of the world. In that sense Mawmluh cave stalagmite from Meghalaya has served a very important scientific purpose. It has quietly, and accurately, frozen within its realm the 8500 year long signatures of monsoonal variations. This cave is indeed a natural marvel!
Praveen B. Gawali