Saturday, April 21, 2018

Everest Summit Limestone

Most people I talk to about geology are aware that the Himalaya formed by the buckling and uplift of crust caught up in the India-Asia collision. But, I do see eyebrows raised when I tell them that the summits of some of the highest peaks are made up of marine sedimentary rocks.

The summit of Mount Everest is a fossil bearing limestone of Ordovician age.

What happened to these sediments as they got caught up in Himalayan mountain building? A recent study published in Lithosphere has teased out the deformation and metamorphic history of this limestone.

Polyphase deformation, dynamic metamorphism, and metasomatism of Mount Everest’s summit limestone, east central Himalaya, Nepal/Tibet - Travis L. Corthouts, David R. Lageson, and Colin A. Shaw

The geologists trained Nepalese Sherpa climbers to recover samples from the Everest summit. The location of the samples and the basic geological divisions of the summit is seen in the annotated photograph posted below

 Source: Travis L. Corthouts, David R. Lageson, and Colin A. Shaw 2018

The Everest region is made up of high grade metamorphic rocks of the Greater Himalayan Sequence. These are intruded by leucogranite dikes and sills. Towards the upper levels, the grade of metamorphism decreases gradationally to upper greenschist facies. The contact between the two metamorphic grades is a shear zone termed the Lhotse Shear Zone. The greenschist faces rocks are termed the Everest Series.  On top of the Everest Series is the 'Yellow Band'. This is a coarse grained marble and calc-schist. The summit limestones (Qomolangma Formation) rests on this Yellow Band. The boundary between them is a fault zone known as the Qomolangma detachment. This fault zone is a strand of the South Tibetan Detachment (STD) that puts the Tethyan Sedimentary Sequence (TSS) on top of the Greater Himalaya Sequence throughout the extent of the Himalaya.

A schematic cross section depicting this stratigraphy is shown below.

Source: Travis L. Corthouts, David R. Lageson, and Colin A. Shaw 2018

Researchers used three types of analysis to figure out the geologic history of the limestone.

a) Microfabric analysis of the samples gave the geologists clues to the deformation and stress regime experienced by the summit limestone. The limestones have been converted into a mylonite. This means that increased temperatures and pressures from faulting resulted in a new textural arrangement in which the original calcite grains of the limestone were recrystallized and deformed. New calcite crystals grew flattened and stretched along one direction, resulting in a foliated (layered) streaky appearance to the rock. This texture forms during ductile deformation in a compressive stress regime. Geologists found that near the vicinity of the Qomolangma fault, a set of dilational fractures indicating extensional forces cut across these ductile deformation textures. This indicates that the summit limestone was subjected to tensile forces and normal faulting at a later stage.

b) Titanium content of quartz and biotite from samples close to the South Summit (EV6) indicated the temperature of metamorphism. This is so because the amount of Ti incorporated in to growing crystals of quartz and biotite increases with increase in temperature of crystallization. Results indicated that the limestones at the base of the Qomolangma Formation experienced temperatures as high as 500 deg C. 

c) The age of metamorphism was estimated by dating muscovite crystals using Ar40/Ar39 technique. Muscovite crystals grew in response to the increased temperature and pressure the limestone was subjected to during Himalayan orogeny. Dates show that there were two phases of mineral growth. The first at 28 million years ago, and a younger phase at about 18 million years ago, indicating separate events of movement and heating along the Qomolangma fault zone.

The leucogranite sills and dikes, which intrude the underlying Greater Himalaya Sequence, also merit a mention. They formed by the partial melting of the crust during Himalaya orogeny.  As this magma intruded and solidified inside the Greater Himalaya Sequence, they expelled fluids with volatile elements which permeated into the overlying limestone. This caused metasomatism and crystallization of secondary minerals in the limestone. Boron, potassium, titanium and H2O were introduced into the limestone and were incorporated into minerals like muscovite, biotite and quartz. This activity is dated to about 28 million years ago based on the age of secondary muscovite in the lower parts of the summit limestone.

The sequence of geologic events is summarized in the graphic below:

Source: Travis L. Corthouts, David R. Lageson, and Colin A. Shaw 2018

And an excerpt of the conclusions from the paper-

The different fabrics and metamorphic temperatures observed between the upper and lower parts of the Qomolangma Formation are the result of distinct events that influenced the summit limestone at different times throughout Himalayan orogenesis. Fabrics seen in summit samples are the result of Eohimalayan deformation and low-grade metamorphism associated with initial thrust faulting, folding, and crustal thickening of the Tethyan Sedimentary Sequence in the Eocene. In contrast, the fabrics and elevated temperatures preserved in South Summit samples are the result of events that occurred in the late Oligocene and early Miocene, including metasomatism associated with Neohimalayan metamorphism and normal faulting on the South Tibetan detachment. This means that several significant tectonic events in Himalayan orogenesis are preserved in the Qomolangma Formation, a succession of deformed Ordovician limestone that now comprises the top of Mount Everest.

Open Access.

Wednesday, April 11, 2018

Dating Rock Art

I love it when science is explained with a well thought out and cleanly drawn illustration.

A commentary in Nature News and Views by David G. Pearce and Adelphine Bonneau presents this diagram on dating rock art.

Two recent studies on dating cave paintings from Spain are discussed.

Who were the artists?

The oldest minimum age for the paintings is 66 ka (thousand  years) leading to speculation that they might have been drawn by Neanderthals. The earliest presence of modern humans in Spain is from  40 ka. This would imply independent evolution of symbolic behavior in Neanderthals.  However, the same painting throws up a range of minimum dates from ~ 60 ka to ~ 3 ka making an exclusive link  to Neanderthals problematic.

Friday, April 6, 2018

Crisis In Indian Palaeontology

This is incredibly sad reading.

Two recent articles highlight the utter state of disarray Indian paleontology finds itself in.

Less offical importance, low budgets, low career prestige, no legal protection for fossil sites, no local fossil repositories to store collections and no national museum with an attached well funded research program.

From the article by Sanjay Kumar in Science:

With few legal protections, sites often fall victim to looting and development. And although funds are scarce for all science in India, the plight of paleontology is particularly acute. Little money is available for excavations and for acquiring and curating specimens, and the country lacks a national institution in which its natural heritage can be studied and preserved.

All of this discourages young people from entering the field. Cash-strapped universities are curtailing or axing paleontology courses, says Ashok Sahni, of Panjab University in Chandigarh, a leading figure in Indian paleontology. Sahni, best known for his finds of dinosaur nesting sites in Jabalpur and insects trapped in amber in Vastan, in Gujarat state, says he has watched waves of colleagues retire—with few young talents stepping in to replace them. "There is no critical mass of researchers left," he says. "Indian paleontology is dying."

..and Sreelatha Menon in The Wire writes about the problems in palaeontology, and more broadly, geology education:

“In well known centres of paleontological teaching and research, such as BHU, Lucknow University, Panjab University, Jadavpur University, etc., the number of palaeontologists has gone down drastically and new, prestigious educational institutions like the IISERs are not showing much interest in hiring palaeontologists,” Prasad said (IISERs: Indian Institutes of Science Education and Research). “So the country has very few palaeontologists working on large invertebrate fossils at present.”

Pratul Saraswati, a micropalaeontologist in the department of earth sciences, IIT Bombay, thinks it’s not about people not being interested in palaeontology. “If you ask me to name some micropalaeontologists other than myself, I won’t be able to give you more than five names. If you ask Prof Sahni to name some vertebrate palaeontologists, he won’t be able to name more than three or four.”

“The problem is with geology departments as a whole across the country. Except in the IITs and central universities, just one or two faculties teach all the subjects coming under geology – and that includes palaeontology,” Saraswati said. “There is no faculty for geology across India. So it is not just palaeontology but all the subjects coming under geology that are taking a beating.”

At the IITs, every subject is taught by a specialist – which is good because, according to Saraswati, “It is difficult for a non-specialist to teach palaeontology.” But in the other universities, “One or two teaching all the subjects in geology is fine till graduation. For post-graduation and research,” that will not be enough.

A couple of weeks ago I visited the Dept. of Geology at Sinhagad College of Science in Pune. One of the faculty there is working on the sequence stratigraphy of the Cretaceous deposits of the Cauvery Basin in Tamil Nadu, South India. She mentioned that many of the famous outcrops and fossil sites are being destroyed as farmlands and small villages and towns expand. This story is repeated elsewhere across India.

That really struck me hard. During my undergraduate years I had visited that area on a field trip. I saw and collected ammonoids, echnoids, molluscs, corals and plant fossils in the field and had come back with a finer appreciation of the stratigraphic and sedimentologic context in which fossils are entombed and preserved. In retrospect, we should not have collected so many fossils. But in those days we weren't taught, and neither did we introspect, about ethical issues regarding fossil collection and outcrop integrity.

India's natural history must be given more importance. It will be a real tragedy if these localities are lost to future generations.

Monday, March 19, 2018

How Old Are The Aravalli Mountains Of Rajasthan?

By the age of a mountain range I mean the time since the formation of significant topography. I don't mean the age of rocks making up the mountains. There are plenty of instances where terrains made up of rocks of a particular age have been rejuvenated and uplifted by earth movements later in time. The most spectacular example in India are the Himalaya. The oldest rocks in the Himalaya are dated to about 1.8 billion years. These, along with rocks ranging in age from more than a billion years to about 50 million years, have up thrust up during mountain building that began about 25 million years ago.

There are other less well known examples from India. The Bababudan hills in Karnataka are made up of rocks as old as 3.5 billion years. The topography though is much younger. This area is a southern extension of the Deccan plateau which has been rejuvenated during the Cenozoic. Another example are the massifs of the Nilgiri Hills in the Western Ghats. These massifs reach about 7500 feet ASL. They are made up of charnockite, a high grade metamorphic rock. This terrain was metamorphosed about 2.5 billion years ago and then again around 550 million years ago. It too has been uplifted in the more recent Cenozoic.

So, how old are the Aravalli mountains?

In a recent article in LiveMint on the ecology, geology and archaeological significance of the Aravalli mountains of Rajasthan, Ananda Banerjee writes-

"These ancient rocks are part of the oldest mountain range in the world—the Aravalli range, or “The Ridge”, as it has been more commonly known in Delhi from the days of British rule".

He quotes author Pranay Lal " “It took two billion years (from a point in time between 3.2 billion years to 1.2 billion years ago) of shoving and pushing of tectonic plates and magma outpourings to create these ancient fold mountains

This statement does not really inform us about the sequence of geological events that took place. Did the Aravalli  mountain building really take two billion years? Was it really initiated 3.2 billion years ago?

The Aravalli fold mountains are made up of layers of sediments interlayered with volcanic rocks. The deposition of these volcano-sedimentary successions took place on the sea floor. This entire pile has been subdivided into the Aravalli Super Group and the Delhi Super Group corresponding to two distinct cycles of sedimentation and orogeny.

The satellite image below shows the folded ridges of the Aravalli mountains west of the city of Udaipur

The geologic story begins, as Lal pointed out, about 3.3. to 3.2 billion years ago (1). At this time prolific 'granitic' magmatism was creating new crust. These magmas go under the name TTG, for tonalites, trondhjemites, and granodiorites. Sediments and interlayered basalt volcanic layers were deposited in contemporaneous basins. Geologists think that the tectonic setting for TTG magmatism would have been similar to a convergent plate margin, where one plate subducts or slides underneath another plate. The setting for volcanic-sedimentary deposits may have been a rifted oceanic basin.

The schematic below shows the evolution of cratons during the Archaean. Early continental nuclei were separated by oceanic basins.

Source: P.A. Cawood, C.J. Hawkesworth, and B. Dhuime - The continental record and the generation of continental crust

 These different terrains slowly sutured and welded together to form larger continental fragments. In this process the TTG  and the volcano-sedimentary deposits got metamorphosed and deformed. The result was a complicated terrain with slices of granite gneiss (metamorphosed TTG) interleaved with low to medium grade metamorphic rocks like chlorite and amphibolite schists (metamorphosed  basalt and other volcanic rocks and sediments).

The cross section below summarizes the complicated structure of the granite-greenstone belts of Rajasthan.

D.B. Guha 2008 - Tectonostratigraphy and Crustal Evolution of the Archaean Greenstone-Granulite Belt of Rajasthan

This granite-greenstone terrain (due to the presence of green colored minerals like chlorite and amphiboles) is called the Banded Gneiss Complex. It is made up of two sub terrains named the Sandamata Complex and the Mangalwar Complex. The formation of the Banded Gneiss Complex was completed by about 2.5 billion years ago when profuse granitic magmatism ended. Geologists call this craton stabilization. Initially, this terrain may have had topography. Hill ranges may have stood out in this area about 2.5 billion years ago. However, this was followed by a long period of erosion wherein the terrain was peneplained. Evidence for deep weathering of this terrain comes from paleosols (soils) which mantle parts of the Banded Gneiss Complex.

This was followed by the sagging of the granite-greenstone crust and the formation of new sedimentary basins.

The Banded Gneiss Complex forms the basement on which the Aravalli Supergroup sediments were deposited. These older rocks therefore were the sea floor at that time. No fold mountain ranges existed in this region around 2 billion years ago.

Galena (Lead Sulphide) which occurs in volcanic rocks interlayered in the lower part of the Aravalli Super Group has been dated to about 2 billion years. This is taken as roughly the start of Aravalli sedimentation. This basin lasted for about 200 million years. Granites intruding the Aravalli Supergroup have been dated to about 1. 85 billion years. These are syn-orogenic granites which form when continental fragments collide, and the deeply buried crust partially melts to generate granitic magma. Geologists think that the tectonic event responsible for this was the collision of the Aravalli craton and the Bundelkhand craton. The Aravalli orogeny and fold belt formation is thus about 1. 8 billion years old.

At this time there would have been a fold mountain range made up of crumpled up Aravalli Supergroup rocks. Subsequently, beginning around 1.7 billion years ago, another basin developed in the north and west of the older Aravalli basin. In this basin were deposited sediments and volcanic material that make up the Delhi Supergroup of rocks. Among these rocks are the resistant quartzites that make up the Delhi Ridge. The Delhi basin closed and the rocks folded and  uplifted by about 1 billion years ago. The tectonic event responsible for the Delhi orogeny is thought to be the collision between the Aravalli-Bundelkhand craton and the Marwar craton to the west. The contact between the two is the Western Margin Fault along which the Phulad Ophiolite rocks lay sandwiched. These are remnants of the oceanic crust that existed between the two continental blocks.

The Aravalli fold belt, made up of the Aravalli Supergroup and the Delhi Supergroup formed over an extended time period in two phases, the first one about 1.8 billion and the second about 1 billion years ago.

The map below shows the different geologic terrains of the Rajasthan craton

Source: Joseph Meert 2010 - Precambrian crustal evolution of Peninsular India: A 3.0 billion year odyssey

Does that mean we can say that the maximum age of the Aravalli mountains is about 1.8 billion years?

This is an intriguing question and it depends on what might seem a rather esoteric question. What is the nature of the contact between the Aravalli Supergroup rocks and the younger Delhi Supergroup rocks? The Aravalli and Delhi rocks have a sheared and faulted contact. This means that the two terrains have been moved along faults from their original positions and juxtaposed against each other. But some work suggests that their original relationship was different. Field relations and inferred contrasting folding histories (2, 3)  implies an angular unconformity between the two. That means that Aravalli Supergroup rocks were folded earlier and then over a time span of 100 million years or so, erosion wore down the Aravalli Supergroup fold mountains to a plain. The crust then sagged, and the Aravalli rocks along with the Banded Gneiss Complex became the basin floor upon which the Delhi Supergroup sediments were deposited.

If this scenario is true, then the Rajasthan fold mountain topography formed during the younger Delhi Supergroup orogeny, that is about 1 billion years ago. The rolling hills and the gentle stream gradients suggest that erosion has been wearing the mountains down and there have not been significant earth movements affecting this part of the crust since.

How does this compare with other ancient mountain ranges. The Barbeton Greenstone Belt, also known as the Makhaonjwa Mountains, on the border of South Africa and Swaziland are thought to be the oldest mountain range in the world. They are made up of 3.5 to 3.2 billion year old rocks. On the web there are top 10/9 lists of the oldest mountain ranges in the world, which include the Hammersley Range in Western Australia (3.4 billion)  and the Waterburg Mountains in South Africa (2.7 billion) (strangely they exclude the Aravallis!) But has the topography existed since the claimed age or has an old peneplain been rejuvenated in more recent times?

That is the billion year(s) question that must be asked when evaluating any "my oldest mountains are older than your oldest mountains" claim.

Saturday, March 17, 2018

Paper: Evaluating The Fossil Record Of Earliest Life

Understanding ancient life: how Martin Brasier changed the way we think about the fossil record - JONATHAN B. ANTCLIFFE, ALEXANDER G. LIU, LATHA R. MENON, DUNCAN MCILROY, NICOLA MCLOUGHLIN and DAVID WACEY

I really enjoyed reading this paper which came out in a special publication issue of the Geological Society, London, in 2017. It is a tribute to the work of Martin Brasier who made significant contributions to our understanding of early life and early animal evolution. In particular, Dr. Brasier argued for a more rigorous approach to analyzing fossils, or claimed fossils, of very early life. He developed detailed criteria for describing and interpreting enigmatic structures as either abiogenic or biogenic, and promoted the use of cutting edge imaging technology to better visualize 'fossil' structures in two and three dimensions.

An excerpt:

Crucial to our understanding of life on Earth is the ability to judge the validity of claims of very ancient fossils. Structures reported from the Apex chert (3.46 Ga) that were interpreted to occur in sedimentary rocks and to be biological in origin (Schopf & Packer 1987; Schopf 1992, 1993) were, for a decade or more, considered compelling candidates for the earliest fossils. Martin Brasier’s most important contribution to this debate was to characterize those structures in great detail and to develop a framework within which claims of the ‘oldest’ or ‘earliest’ life should be couched. In his lectures on this subject, Martin referred to the competitive tendency among palaeontologists working on early life as the MOFAOTYOF principle: My Oldest Fossils Are Older Than Your Oldest Fossils.

In particular, Brasier et al. (2002) made it clear that the burden of proof must fall on those making the claim of ancient life, not those refuting it: Ancient filamentous structures should not be accepted as being of biological origin until all possibilities of their non-biological origin have been exhausted. In particular, it is important to note that complex ‘septate’ carbonaceous structures can result from experimental hydrothermal processes. (Brasier et al. 2002, p. 80) In other words, we should assume that ancient structures resembling fossils, such as those in the Apex chert, are abiological until it can be shown beyond reasonable doubt that they are not, rather than the other way around. Brasier (2015) articulated this concept clearly:

This . . . allows palaeobiologists to set up a hypothesis which will prevail until proved false . . . Any newsworthy, and culturally challenging, interpretation must therefore be tested against a less exciting interpretation. This ‘null hypothesis’ is usually regarded as the ‘most boring explanation’. It is boring precisely because it is thought to have a higher probability of being correct. Brasier (2015, p. 9).

This could be thought of as Brasier’s razor: ‘the most boring answer is probably the correct one’.

This critical approach applies to the problem and controversies surrounding  the fossil record of the earliest animals too. A reassessment led Dr. Brasier  to retract his previous claim about the earliest sponge spicules from the Late Ediacaran ( ~ 560 million to 541 million years ago) age deposits of Mongolia.

Finally, his work on accurately characterizing the scratches, pits, holes, undulations, blobs and globules on and within sedimentary deposits has enormous implications for the search for potential fossils on other planets.

Dr. Martin Brasier died in a car accident in 2014. 

Open Access.

Sunday, March 4, 2018

India Physiography Rendition

I came across this physiography rendition of India via Simon Kuestemacher.

The contrast between the Himalaya and the Indo-Gangetic plain is awesome. But I was struck by other features in the Peninsular region.

1) The E-W oriented linear depression in Central India. This is the Narmada rift valley which accommodates the west flowing Narmada river. The rift is part of the Central Indian Tectonic Zone, a wide zone of continental deformation formed in the early -mid Proterozoic (between 2 - 1 billion years ago) during the collision and eventually suturing of the Dharwar and Bundelkhand cratonic blocks.

2) Distinct mountain belts are seen along the eastern margin of India. These comprise the early -mid Proterozoic Eastern Ghats and the Nallamalai fold and thrust belts. The Eastern Ghats are a granulite grade terrain, with evidence of the original sedimentary basins being caught up in multiple cycles of metamorphism and deformation. The Nallamalai fold and thrust belt was a basin on the eastern extremity of the Cuddapah Basin. This terrain is thrust westwards over the Cuddapah's.

3) The Western Ghats, especially the Deccan Volcanic terrain, are revealed as the edge of the dissected plateau. These are not a distinct orogenic belt like those on the eastern margin. Rather, the edge is a retreated fault scarp. The Deccan volcanic plateau once extended further to the west. The separation of India from Seychelles caused the western margin of India to subside along faults that now lie under the Arabian Sea. Erosion has caused the original cliff to retreat eastwards. And some vertical uplift has accentuated the topography of the region.

Tuesday, February 27, 2018

Himalaya Geology Trek

This summer, from May 26 to June 1,  I will be leading a geological excursion to the Panchchuli Glacier located in the Darma Valley, Kumaon Himalaya, Uttarakhand.

This is the plan:

My blog remains my primary means of geology outreach. But for some time now, my friends have been trying to persuade me to engage more intimately with geology fans and nature lovers by taking them out in the field, up close with rocks and landscapes.

This past December, and in January of this year, I took two groups to the Western Ghat Escarpment and to the rocky west coast and introduced them to the Deccan Volcanic Province. This summer, I will take participants across the core of the Himalaya orogen to the very scenic Panchchuli range. We will walk across the high grade metamorphic rocks of the Greater Himalaya Sequence. There will be thrust faults, folded rock strata, granitic intrusions and migmatites (partially melted rocks) to wonder at. And at the end will be the majestic glacier, an awe inspiring eerie place, source of the river Dhauliganga.

Interested?! Share this with your friends too. The contact info for registering is in the brochure. And you can always connect with me directly for a more detailed conversation about this up coming trip. You can find my email in the Profile page of my blog.

 Panchchuli Range, from village Dantu, Kumaon Himalaya

Tuesday, February 6, 2018

Some Thoughts On The Middle Paleolithic Stone Tools From India Story

Early Middle Palaeolithic culture in India around 385–172 ka reframes Out of Africa models - Kumar Akhilesh, Shanti Pappu, Haresh M. Rajapara, Yanni Gunnell, Anil D. Shukla and Ashok K. Singhvi

From a site in Tamil Nadu, South India, stone tool types named Levallois were dated to be 385 ka -172 ka (ka-thousand years). Levallois tools are made by striking a stone core to produce smaller flakes which are then put to various uses.  They were more versatile than the older clunkier hand axes. Previous estimates for the arrival of this technology in India was thought to be around 125 ka or later, introduced by migrating Homo sapiens.

So, who made these older Levallois tools? The recent finding from Morocco of Homo sapiens like fossils dated to be older than 300 ka has prompted many to interpret this finding as evidence of an early arrival of Homo sapiens into south Asia.

Some thoughts-

1) this discovery has put a rare spotlight on the Indian hominin record. The paucity of hominin skeletal fossils and a lack of a rigorous chronology for deposits and tools have meant that the Indian record, if not ignored, has received less attention. This study has established a robust chronological framework of the sedimentary sequence in which these tools are found. A change from older Acheulean style tools to Levallois styles is documented within this dated sequence. Finding a trend, something changing or being replaced by something else, at one locality and within one sedimentary sequence is rare at hominin sites across the world. I think this make it more a compelling story than an isolated find of some stone tools.

2) I've noticed that some media article headlines and discussions in social media are suggesting that the "Out of Africa" theory needs to be reassessed. Well, what exactly do you mean by 'Out of Africa'? The original and popular Out Of Africa theory proposes that Homo sapiens originated in Africa around 200 ka. Then, 60ka-50ka ago these modern Africans migrated and settled the globe, replacing earlier archaic human populations. But, it is not news that there have been many 'Out of Africa's'.  By that I mean there have been many dispersals of humans out of Africa. Early archaic Homo dispersals occurred by 1.8 mya. The ancestors of Neanderthals and Denisovans left Africa by 1 mya to 700 ka ago.  There is genetic, fossil and archaeological evidence for a Homo sapiens migration around 65-50 ka ago. There is also evidence of an earlier migration of Homo sapiens (dated to ~120 ka) into the Levant and possibly into south Asia as well. An older Homo sapiens fossil dated to 185 ka has been found recently in Israel. Now, this discovery of  advanced tool technology has been interpreted by some to indicate an even earlier migration of Homo sapiens into India.

To me, the bigger evolving story is that Homo sapiens are getting older and older, originating earlier that 385 ka! That they might have dispersed into Asia soon after is less of a surprise. Migrations out of Africa seem to have occurred again and again, and so another at around 385 ka doesn't seem to be an extraordinary event. There would have been back migrations into Africa as well. I suspect that the recent finding of anatomically modern humans in Morocco dated to more than 300 ka has shaped the media narrative of this stone tool finding into an 'early migration into India' story.

How would have these stone tools been interpreted without supportive fossil evidence that Homo sapiens existing by 385 ka?  There always was an alternative hypothesis that proposed that evolution of complex behavior and associated advances in tool technology took place independently in disparate human populations residing in Africa, Europe and Asia.  Aspects of 'modern' anatomy and behavior might have developed multiple times in different places.  In this context, the hominin skull dated to around 236 ka found in the Narmada valley, Central India, is intriguing. Given its antiquity, the reasonable interpretation is that it represents a population descended from an earlier Homo erectus migration into India. Yet, it has a mix of archaic and derived (modern) features. It's estimated cranial capacity is comparable to modern humans.  Is it an example of  parallel evolution of 'modern' traits outside Africa?  Or, does it represent a hybrid population formed by the mixing of archaic hominins and Homo sapiens?

Skeptics like Michael Petraglia have pointed out that the technological transition seen in India may be a local invention of technology and not due to migration of a new population carrying advanced tools. Changing environmental conditions may have spurred similar inventions in different parts of the world.

3) One point to note is whether these earlier archaics and Homo sapiens living outside Africa contributed ancestry to today's people. Some recent genetic analyses suggests that all non-Africans are descended from Homo sapiens that dispersed from Africa around 80 ka -50 ka ago. These people did mix with archaic hominins in Europe and Asia but the degree of admixture is low. We contain about 2-4% Neanderthal and/or Denisovan genes.  If this is true, if earlier people migrating from Africa did not leave much of a genetic legacy in us, then the original 'Out of Africa' model still has some relevance.

Monday, February 5, 2018

Article: Groundwater Worries In Maharashtra

Pune based groundwater researchers Dhaval Joshi and Uma Aslekar write about the need to understand the geology of aquifers and the importance of governance in managing this resource:

Understanding the Triggers of Groundwater Competition in Maharashtra

an excerpt-

The recent vagaries of rainfall and the resultant water scarcity and drought-like situation in Maharashtra has resulted in a series of supply-side programmes being implemented across the state. Be it the promotion of farm ponds or dug-wells through various government programmes, the approach has largely been supply-side interventions. The assumption behind this seems that increasing the number of sources would help resolve the crisis around water. There is a misplaced judgment when it comes to making such assumptions. One, it is perceived, even today, that it is the question of access, and that many of the users still do not have any access to any water source, be it in the form of dug-well and bore-well. etc. Second, it also justifies the understanding that users are efficient in their use of water resources, and that limited supply in itself, is a problem. These two points fuel the approach of supply-side interventions.

They identify these focus areas:

1) Granularity of data
2) Integrating hydrogeological in water security programmes
3) Need for stakeholder participation
4) Effective implementations of legislation on groundwater
5) Larger role for groundwater institutions

Open Access

Tuesday, January 30, 2018

Field Photos: Dikes At Korlai, India West Coast

Sharing a few pics of dikes intruding the Deccan Volcanics at Korlai, a small village south of Mumbai. I had taken a group of nature lovers and science enthusiasts on a traverse from Pune to the   west coast last weekend. We stopped at the Western Ghat escarpment to take in majestic views of massive lava flows and then went to the coast to observe dikes and silica geodes.

Several dikes are exposed along the rocky coastline. Most of them are oriented in a N-S to  NNW-SSE direction.

Here is one with a shape of a serpent

A close up of a dike. Notice the clear contact between the dark colored dike and the brown/grey looking basalt lava flow.

Another large dike with closely spaced fractures.

And this one, intruded along en echelon fractures, showing a sinistral or left handed offset.

A view of the rocky wave cut platform from top of Korlai fort. Arrows point to two dikes.

One interesting feature of these dikes is that many of them contain tiny fragments (2 -20 cm in diameter) of the lower crust, incorporated by the basaltic magma as it ascended. These fragments or 'xenoliths' are composed of granulite, a common rock type formed in the high temperature - high pressure environments of the lower crust. Work done by A.G Dessai and colleagues  suggest that these granulites are present at depths ranging from 15 km to 40 km below the surface. Dike chemistry suggests that the original composition of the parent basaltic magma, formed at even greater depths in the uppermost parts of the mantle, was modified  due to reaction with and assimilation of this lower crustal granulite.

That was a point of great interest to the people who participated in this field trip. They were awestruck that they were looking at solidified sheets of magma that extended to depths of more than 40 km below the surface.

Sunday, January 21, 2018

5700 Year High Resolution Record Of Indian Monsoon From Uttarakhand Cave Deposit

..continuing on the topic of environmental changes and Harappan Civilization. Gayatri Kathayat and colleagues have teased out an intra-decadal record of variability of Indian monsoons from a cave deposit in Uttarakhand.  This they did by measuring the O18/O16 ratio in the mineral calcite (CaCO3) which grew incrementally to form a speleothem. The lighter isotope of oxygen is preferentially retained in the vapour phase. Less and more amounts of rainfall thus results in less or more amounts of O16 in rain and groundwater and eventually in the mineral calcite that precipitates from that groundwater. This is known as the amount effect. A chronology of speleothem growth was established using thorium 230 dating method.

The record for the past 5700 years is summarized in this figure. The time period of the growth and consolidation of urban Harappan society coincides with a period of accentuated monsoons.

Gayatri Kathayat et. al. 2017: The Indian monsoon variability and civilization changes in the Indian subcontinent

Here is their conclusion:

The hydroclimate conditions during the evolution and subsequent decline of the IVC have remained a subject of debate (for example, 9, 14–19). On the basis of the Sahiya d18Orecord, the Early and Mature Phases occurred during a fairly wet/warm and climatically stable period. The Mature Phase began around an abrupt intensification of the ISM at ~4550 yr BP (Fig. 3) and sustained for nearly ~700 years to ~3850 yr BP, corresponding with the late portion of the mid-Holocene Climate Optimum, during which the ISM reached its maximum over the past 5700 years. It is plausible that the optimum (warm/wet) climate might have allowed the civilization to develop a farming system with large and reliant agricultural surpluses, which in turn supports the development of cities.

Previous studies have attributed societal collapses in the Middle East and in the Indus Valley to a climate event, the so-called “4.2 ka BP event” (or ca. ~4.2–3.9 ka BP event) (15–21, 42–45). The 4.2 ka BP event in the Sahiya d18O record manifests as an interval of declining ISM strength, marked by a relatively higher-amplitude d18O variability and a slow speleothem growth rate, rather than as a singular prominent abrupt event (Fig. 2). A lack of an abrupt change in our record around the time is consistentwith the idea that the 4.2 ka event did not influence the Deurbanization Phase (14) in contrast to the more severe societal impact it had on the Old Kingdom in Egypt and the Akkadian Empire in Mesopotamia (42–45). 

Some commentators have already pointed out that this sample is well removed from the Harappan realm and we need to understand regional variation in monsoons before drawing any firm link between monsoon variability and Harappan civilization phases. In that context, let me put up another figure from a study of the Kotla Dahar lake sediments from Haryana. This site falls within the Harappan region. Yama Dixit and colleagues measured oxgyen isotopes of carbonate lake sediment as well as from gastropod (snail) shells. Take a look at the figure below.

Source: Yama Dixit 2014: Abrupt weakening of the summer monsoon in northwest India ~4100 yr ago

The variation in oxygen isotope ratios is due to variation in intensity of evaporation. Greater evaporation during dry phases results in the lake water getting enriched in the heavier isotope (the lighter isotope goes into the vapor phase more readily). Sediment and shells precipitated from this water will therefore get enriched in the heavier isotope during dry phases. Their sampling is coarser than the Sahiya study. But, if you look at the time period from around 5000 BP to around 3800 BP, there is no clear persistent trend towards monsoon intensification (should show up as a centuries long shift towards more negative dO18 values since evaporation will be less during wet phases). A fine resolution local record is needed to fill in the details and explain this apparent contradiction.

Finally, I came across a talk by archaeologist Shereen Ratnagar on environmental changes, river history and the Harappan Civilization. She does not like the theory that climate change was responsible for the decline of the Harappan Civilization. Instead, she prefers a social sciences approach, arguing that factors like the over-extension of empire and social dynamics need to be taken into account. Well, I am not sure that these are mutually exclusive. Civilizations may be in a phase of political and social cohesiveness whereby they could prove resilient against environmental changes. In times where their capacity for collective action is weak for internal reasons of polity and demography, exogenous factors like climate change may trigger disruption and decline.

She spends a lot of time criticizing a study by Liviu Giosan and colleagues on the fluvial history of that region. That paper showed that during Harappan times there were no glacial rivers flowing in the region between Yamuna and the Indus. Ratnagar points out that there might have been tributaries of the Yamuna and overspill of the Sutlej that may have provided water to the channel of the Ghaggar river and so there was no severe water shortage. But Giosan's work has not claimed that! They too point out that higher rainfall in the Siwaliks would have kept the Ghaggar perennial through most of the urban Harappan phase. They find that sedimentation continued through the late Harappan phase as well. The study suggests that continued monsoon decline and drying resulted in migration away from the Ghaggar-Hakra belt. However, they don't argue for a direct link between abrupt climate change and civilization decline across the entire Harappan extent. Anyways, the talk is worth listening too, especially her analysis of the wonderful water management strategies at the Harappan age site of Dholavira in Kutch, Gujarat.

Here is the link to the video

Tuesday, January 9, 2018

Note On The Sutlej Paleochannels

The topographic relief rendition posted below shows beautifully the incised valleys of the glacially sourced Yamuna and Sutlej rivers. This rendition has been derived from the NASA Shuttle Radar Topography Mission (SRTMv3) DEM (Digital Elevation Model) with a 1 arc-second or 30m spatial resolution. The region depicted in the figure is immediately west of the Himalaya frontal ranges covering parts of Punjab and Haryana.

Source: Ajit Singh et. al. 2017 - Counter-intuitive influence of Himalayan river morphodynamics on Indus Civilisation urban settlements.

This incision began during the early Holocene, beginning about 10,000 to 8700 years ago and continuing over the next few thousand years, as proposed in an earlier study by Liviu Giosan and colleagues. A decline in monsoon strength over northwest India resulted in low sediment load carried by the rivers. Under such conditions, starved of sediment, the river starts cutting down or incising into its older deposits. Over time they carve out large valleys as the Yamuna and Sutlej have.

During the mid Holocene, from about 6000 years to 3800 years ago, the region between the Yamuna and the Indus was extensively settled and farmed by the Harappan people. The river Ghaggar flows through this region. One popular theory supported by many geologists was that during Harappan times the river Sutlej flowed into the river Ghaggar, switching to its present course only about 4000 years ago. However, Giosan and colleagues had argued that had that been the case, a large incised valley should have been carved by the Sutlej from the point it exits the Himalaya to the point it joins the Ghaggar. The absence of a wide NE-SW oriented incised valley in the interfluve between the Yamuna and the Indus indicates that the Sutlej did not flow into the Ghaggar during most of the Holocene.

 A recent study led by geologist Sanjeev Gupta (Ajit Singh et. al. 2017)  has validated this scenario using geochemical criteria. They have shown that the Sutlej river once did flow into the Ghaggar but changed course and joined the Indus in the late Pleistocene -early Holocene between 15000- 12,000 years and 8000 years ago. Additional data from Giosan and colleagues shows (SI Text) fluvial deposits of Late Pleistocene-Early Holocene age (latest being 10,000 years old) along the present day Sutlej floodplain. These staggered dates imply that a major channel of the Sutlej avulsed or changed course as early as 15000 to 12000 years ago. A smaller strand of the river continued to flow into the Ghaggar until about 8000 years ago or so.

All this means that the Harappan settlements and agriculture in this region was not sustained by a large perennial glacial fed river. Rather, the Harappans adapted their water usage strategy and farming practices to exploit a smaller and maybe an ephemeral river and more distributed water sources.

The geochemical  work by Gupta and colleagues has been rightly praised and highlighted in many media reports. What did go unnoticed and unappreciated was the relief rendition of the incised channels. They provide a very powerful visual representation of the Holocene fluvial history of this region.

The modified relief rendition below also shows the course of the abandoned Sutlej incised valley. Note that this valley is much narrower than the Sutlej and Yamuna incised valleys. Also, trace these narrower incised valleys upstream and you can see that they originate in the Siwaliks. There are no deep extensive incised valleys along the route I have marked in blue. The Sutlej would have carved a prominent incised valley roughly along the blue route had it been flowing into the Ghaggar during most of the early and mid Holocene. Its absence suggests to me that the valley annotated as the abandoned Sutlej incised valley was really carved out in the earlier part of the Holocene by the smaller Ghaggar river originating in the Siwaliks.

Modified from :  Ajit Singh et. al. 2017 - Counter-intuitive influence of Himalayan river morphodynamics on Indus Civilisation urban settlements

Aside: After Liviu Giosan's paper came out, the archaeologist Shereen Ratnagar asked me whether incised valleys are diagnostic of glacial rivers. She was puzzled because the monsoonal rivers Marakand, Ghaggar and a number of smaller streams which originate in the Siwaliks have also carved incised valleys. The answer is no, they are not. What Giosan's work was pointing out was that wide incised valleys of a particular telltale orientation were absent, thus providing a clue as to when the Sutlej changed its course.