What Geology Tells Us About Climate Change

Set aside an hour and read this article.

Geological Society of London Scientific Statement: what the geological record tells us about our present and future climate.

It really is an excellent explanation. 

Wishing readers a very Happy New Year! 

Readings: Myanmar Geology, Holocene Human Populations, Indian Archaeology

Some interesting readings over the past few weeks:

1) Myanmar Geology- Oblique convergence, where plates converge or collide at an angle, has produced some stunning geological features in Myanmar. Lon Abbot and Terri Cook sail down the Irrawaddy River describing vestiges of volcanic arcs, strike slip faults, en echelon sedimentary basins, and fold mountains, with a fair bit thrown in about the architecture and cultural history of the country.

Sailing Through A Subduction Zone.

2) Genetics And Human Evolution- Razib Khan compiles a nice list of the many aspects of human evolution and especially Holocene population history that has been brought out by recent work in genomics and ancient DNA.

What I'm Thankful To Know About Genetics And History In 2020.

3) Indian Archaeology- A sort of historiography of the field of Indian archaeology from Colonial times to today. Dilip Menon writes about the push and pull of ideas of conquest, politics, and nationalism that influence Indian archaeology research and narratives.

How Archaeology Has Shaped India’s Imagination Of Itself. 

Diego Maradona RIP

I will prefer to remember his godly feet. That liquid motion on the football pitch. His short stocky body in perfect balance, skipping, darting, weaving his way through a maze. A slight feint of his right shoulder, a sudden burst, leaving bewildered defenders in his wake.

He had the footballing world at his feet, we gasped in disbelief, as he made the impossible look utterly simple.

He lived a troubled, imperfect life. But in these troubled times we must find it in us to forgive and keep remembering what makes us happy. And that his magical twinkling feet did, bring us joy unconfined.

On that hot, steamy afternoon in the Azteca Stadium, Mexico City, in 1986, Victor Hugo Morales described one of those unforgettable moments of poetry in motion.

Take a look.

Email subscribers can watch it here- Diego Maradona Goal Against England.

Adios Diego.

Niche: Two Examples From Deep Time

The term 'niche' can very simply mean an ecologic space which a particular type of organism exploits. Scientists are a pedantic lot though. They need more rigorous definitions to work with. This has spawned many different ideas about what a niche means and how it can best be described and measured. There is the environmental niche concept which focuses on the physical and chemical attributes of an available space that may or may not be filled by organisms. In this idea, there may be vacant niches, which opportunistic organisms may come to exploit. On the other hand, there is the population niche concept where the niche itself is an attribute of the population. Organism-specific use of its resources, uniquely shaped by the organism's physiology, community structure, and behavior, defines the niche. 

Similar organisms may co-exist in a particular space. This may result in niche overlap and niche partitioning as different species vie for the available resources. Ideas of competitive exclusion (competition theory) derive from such co-habitation of space.

I am just giving a flavor of the arguments here and not diving into a discussion of the many niche concepts. For the purpose of this short post I will use the term niche to mean the actual utilization of a space and of available resources by a species/population.

Palaios is of my of my favorite science journals. It published papers on themes intersecting palaeontology, ecology, sedimentology and stratigraphy. Unfortunately, most of the papers are behind a paywall, so I have to make do with reading the abstracts and finding the occasional open access paper on Research Gate and Academia. Browsing through it last week, I came across two interesting examples of very specific fossil niches, one from the Cretaceous and another from the earliest Triassic. 

In the March 2020 issue, Alison J. Rowe and colleagues ( Late Cretaceous Methane Seeps As Habitats For Newly Hatched Ammonites) describe a community of ammonites (a type of mollusc) living in the vicinity of cold methane seeps. These fossils are preserved in the sedimentary rocks of the Late Cretaceous Western Interior Seaway, a time when sea levels were high and the mid North American continent was under the sea. Methane seeps often occurred along faults which provided the pathways for the gas to rise from the subsurface and be released on the sea floor. Ancient methane seeps are recognized by the presence of typical communities of clams, fossilized worm tubes, ammonites and bacterial microstructures. There is often the development of cone shaped sediment mounds known as Teppe Buttes, made up of calcium carbonate mud and skeletal remains of organisms. The carbon in the calcium carbonate is enriched in the lighter isotope C12, suggesting its derivation from a hydrocarbon source (the hydrocarbon itself is transformed organic matter which is richer in C12). 

Anaerobic oxidation of methane (bacteria stripped electrons and protons from the hydrogen in the methane to drive respiration) provided the energy to build a food web that formed the base of this chemosynthetic ecosystem. These ammonites were small, implying that they were born in this habitat, and their geochemistry indicated that they were incorporating the carbon released from methane to build their shells. What an utterly fascinating mode of life!

The second example is from the Latest Permian-Earliest Triassic (Dwelling In The Dead Zone- Vertebrate Burrows Immediately Succeeding The End-Permian Extinction Event in Australia), preserved in strata deposited just after the biggest mass extinction in earth history. Stephen McLoughlin and colleagues find and describe burrow structures made by small tetrapods. The sediments which contain these burrows are poor in other organic traces suggesting a terrestrial ecosystem which has been stripped bare due to prolonged debilitating environmental conditions.  Paleogeographic reconstruction indicates that at this time the Sydney Basin  occupied a high paleo-latitude. A burrowing lifestyle, coupled with relatively cooler climate of higher paleo-latitudes may have provided these tetrapods protection from the otherwise harsh post extinction conditions. A nice example of the ecology of post mass extinction survivor fauna.

If you want to explore the history of ideas on the niche concept I can recommend two essays. Niche: Historical Perspectives by James R. Griesmer, and, Niche: A Bifurcation in the Conceptual Lineage of the Term by Robert K. Colwell. Both have been published in the book Keywords in Evolutionary Biology, edited by Evelyn Fox Keller and Elisabeth A. Lloyd.

We have so much more to learn about life.

Landscapes: In The Shadow Of A Giant

Even regular visitors to the Himalaya occasionally come across a sight that makes them stop in their tracks and gape. A few years ago in the Goriganga valley, a couple of days walk north of Munsiyari, Uttarakhand,  I turned a corner along a village path and paused in wonderment.

Photo credit. Saurabh Bhatt

This is Pataun. An absolutely beautiful place.

I was rummaging through my collection and thought I'd share the picture.

Interview: Palaeontologist Prof. Ashok Sahni

This is a rich conversation between paleontologist Prof. Ashok Sahni and Dr. Devapriya Chattopadhaya of the Indian Institute of Science Education and Research, Pune.

Email subscribers who can't see the embedded video can watch it at this link: Interview- Prof. Ashok Sahni.

It was so refreshing to hear Prof. Sahni talk candidly about the state of paleontology research in India,  preserving fossil sites, motivating students, the need for Indian scientists to proactively engage with the public about their work, and the importance of building bridges between research and societal needs.

Prof. Sahni comes from an illustrious line of scientists. His uncle was the paleobotanist Dr. Birbal Sahni after whom the Birbal Sahni Institute of Palaeosciences, Lucknow is named. And his father was also a palaeontologist. His mother was not keen on him taking up geology, admonishing him that there were already too many rocks in the house. However, he persisted. 

Definitely worth your time.

Darwin's Atolls - Beneath The Idyll

 I've expanded on the link I posted last week on Darwin's Atolls. 

A beautiful theory has been undone by ugly facts...well, let's just say facts.

How do coral atolls, those shimmering ring shaped islands set against the blue tropical ocean, form? Charles Darwin had pondered this question on the H.M.S. Beagle as she sailed across the Pacific and Indian Oceans in the early 1830s. He famously reasoned that coral colonies begin growing in the shallow waters surrounding oceanic volcanoes. Eventually the volcanoes sink into the ocean, while the coral keep growing upwards. The central area where the volcano existed becomes a deep lagoon, surrounding by a ring of coral reefs. 

 But he just assumed that the present day corals atolls are growing on a volcanic foundation.

Actually, most are not. Tropical region atolls rest on an earlier generation of coral and limestone. These in turn have grown on an even earlier layer of coral growth and so on through the past few million years.

Darwin at that time didn't know that the climate over the past 2.6 million years had shifted periodically between glacial and inter-glacial phases resulting in sea level changes. The story begins in the Pliocene around 5 million years ago. A long period of sea level stability resulted in reefs and other calcium carbonate sediments accumulating on the shallow waters above sea floor ridges. Thick deposits formed flat topped carbonate banks. The earth's climate began to change around 2.6 million years ago as polar and high latitude ice sheets expanded and then withdrew. As a result, sea level started falling and rising cyclically. 

These repeated sea-level fluctuations amplified around half a million years ago. When sea level fell, rain water dissolved the exposed carbonate flats to form an uneven karst topography comprising a depressed bowl with an elevated rim. Subsequently, when sea level rose, new coral growth began in the optimal water depths above this jagged limestone rim. With every sea level fall and rise, erosion and new coral growth accentuated the relief between the central depression which became the lagoon and the rim which was now made up of towering stacked coral reefs. The modern looking atoll evolved in this manner.

A recent detailed study by Andre Droxler and Stephan Jorry compiling decades of research on the Pacific Ocean and Indian Ocean reefs demonstrates this elegantly.

The image above shows a portion of the Chagos atolls in the Indian Ocean. Notice how the rings of coral reefs have formed on a submerged flat 'mesa' or table land. These are the vestiges of the carbonate banks that formed during the Pliocene and later became the foundation for the modern atolls.

Great scientists get it wrong frequently too. Darwin erred in his too loyal adherence to the principle, 'Present Is The Key To The Past'.

He observed that some volcanic islands have coral colonies growing around them. He extrapolated this condition to the past and theorized that atolls began as reefs fringing volcanic islands. He couldn't imagine how otherwise corals could grow in the middle of the ocean without them having a shallow water foundation to colonize.

Uniformitarianism has been a very helpful paradigm in understanding many aspects of the past, but geologists have learned to apply it with caution. During the long 4.5 billion year history of our planet, critical combinations of atmosphere and ocean compositions and continental configurations coupled with the evolving biosphere have resulted in unique geologic products and processes which have no modern analogue.

Darwin's atolls offer us some lessons about our future too. Ten thousand years of relative climate stability has allowed civilization to flower, but has also lulled us into a dangerous complacency about nature and its permanence. The idyll of these atolls is ephemeral. Their very foundations are testimony to rapid environmental change and great dyings of marine ecosystems. Such changes await us in the future too, now hastened by our own agencies.

Several hundreds of years from now our descendants may well be looking on at a planet where many of Darwin's atolls have disappeared under the sea and our forests and croplands diminished by fire and dust. Will it be possible to modulate this coming change? For that, we must heed the signals from our geological past. Our present behavior will be the key to our future.

A more detailed write up about Andre Droxler and Stephan Jorry's work has been posted on the Rain to Rainforest Media website - If Darwin only knew: His brilliant theory of atoll formation had a fatal flaw.

Readings: Darwin's Atolls, Pre Toba Humans, Herbivore Diets And Ecology

Posting some interesting readings:

1) A beautiful theory has been undone by ugly facts. How do coral atolls, those shimmering ring shaped islands set against the blue ocean, form? Charles Darwin had famously reasoned that coral colonies begin growing on the slopes of volcanoes. Eventually the volcanoes sink into the ocean, while the coral keep growing upwards. The central area where the volcano existed becomes a deep lagoon, surrounding by a ring of coral reefs. But he just assumed that the present day corals atolls are growing on a volcanic foundation. Actually, most are not. Tropical region reefs and atolls rest on an earlier generation of coral and limestone. These in turn have grown on an even earlier layer of coral growth and so on through the past few million years. 

Darwin at that time didn't know that the climate over the past 2-3 million years had shifted periodically between glacial and inter-glacial phases resulting in sea level changes, and how these repeated sea-level fluctuations can create environments where corals grow during a sea level rise or later dissolve during a sea level fall to form a karst landscape. This jagged uneven surface in turn becomes the foundation for a new generation of corals. A new detailed study of the Pacific Ocean and Indian Ocean reefs demonstrates this elegantly.

Paper- The Origin of Modern Atolls: Challenging Darwin's Deeply Ingrained Theory.

Write up - Darwin's theory about coral reef atolls is fatally flawed.

2) Did Homo sapiens enter India prior to the devastating Toba eruption that took place about seventy four thousand years ago or after? This question is of interest in elucidating the timelines and dispersal routes of our species from Africa. Homo sapiens had reached Australia by around 60,000 years ago with India being one obvious migration path.  There are no skeletal human fossils from this time period in India and stone tools have been variously interpreted as belonging either to Homo sapiens or an earlier archaic human. There were few accurately dated sites from the time period of 80,000 years ago to 50,000 years ago. Now, some new work from the Son Valley, Madhya Pradesh, shows long term human occupation in north India from pre Toba eruption times. The layers containing stone tools span from about 79,000 years ago to 65,000 thousand years ago. The tools resemble those from the Middle Stone Age of Africa, Arabia and Australia and are interpreted to have been the handiwork of Homo sapiens. 

Paper- Human occupation of northern India spans the Toba super-eruption ~74,000 years ago.

3) The ecologic context of the evolution of our genus Homo is of great interest. A recent study focuses on using carbon isotopes to tease out dietary shifts in herbivore fauna living in East Africa in the late Pliocene to Early Pleistocene. Analysis of herbivore teeth from 3.6 million years ago to 1.05 million years ago reveals a shift from C3 derived food (woody vegetation) to C4 derived food (grasses), first around 2.7 million years ago and again later around 2.1 million years ago.  Woodlands were giving way to more open savanna, a change that coincides with the evolution of Paranthropus and Homo. 

Paper- Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia.

Write up - Researchers use fossilized teeth to reveal dietary shifts in ancient herbivores and hominins.

 

Himalaya: Critical Wedge

 A while ago a friend commented that she had learned in school geography class that Himalayan faults are the youngest. Now, earthquakes in Bhuj, Gujarat, and Latur,Maharashtra, in recent times tell us that there is active faulting going on elsewhere in India too. But I could understand what she was trying to say, that the Himalaya is a growing mountain chain with active faulting. 

I think a lot of non-geology folks appreciate this point. What is not that well known is that within the Himalaya the locus of active faulting has shifted southwards over geologic time. The Himalaya is the northern margin of the Indian continental crust which has been broken up into blocks or litho-tectonic units and stacked by major faults. From north to south these faults are, South Tibetan Detachment, Main Central Thrust, Main Boundary Thrust, and Main Frontal Thrust. There are plenty of minor faults between these major breaks. 

As the Indian continent collided and underthrust Asia, slices of its crust were pushed up in the following order; The Tethyan Himalaya along the South Tibetan Detachment (45-35 million years ago), the Greater Himalaya along the Main Central Thrust (24-15 million years ago), the Lesser Himalaya by the Main Boundary Thrust (and many subsidiary faults, 11-5 million years ago), and the Siwaliks by the Main Frontal Thrust (and some subsidiary faults, 1 million years ago to recent). All these faults merge at depth with a north dipping (sloping) master fault known as the Main Himalaya Thrust along which the India plate is underthrusting or sliding underneath Tibet. The Himalaya is deformed Indian crust riding atop the MHT.

Why did faulting activity migrate southwards in this growing orogenic (fold and thrust) mountain belt? Geologists give a mechanical explanation of this style of mountain growth using the Critical Wedge Model. 

It will be worth pausing this post to watch a video made by Middlebury Plate Tectonics on Critical Wedge Theory. Email subscribers who cannot see the embedded video may watch it at this link- Critical Wedge Theory- Himalaya.

 

To summarize, the Himalaya may be abstracted as a wedge of crust which is thicker in the north and thinner towards south. The ratio of normal stress to shear stress controls whether a fault can slip. As crust thickens beyond a threshold value of the ratio, increased normal stress can pin down and lock a fault. Subsequently, the locus of active fault slip migrates towards the region with a more favorable stress ratio, which in the case of the evolving Himalaya orogen has been progressively southwards.

This is a very informative video but I know of many geologists who would protest. Their objection will not be that the Critical Wedge model is wrong but that it doesn't explain all of the Himalaya. They argue that the upper structural levels of the Greater Himalaya were extruded by a different mechanism. The growth of compressional mountain belts involves crustal thickening due to folding and thrusting. The Critical Wedge model explains this as taking place by the brittle breakage of slices of underthrusting crust along faults and their continuous accretion to a growing wedge. Rocks of the Greater Himalaya though show signs of ductile deformation. High grade gneisses were partially melted to form migmatites. Pods, lenses and sheets of granite magma was injected along fractures and planar rock fabric (schistosity).

The picture above shows leucogranite sills (white layers) intruding high grade gneisses near the village of Naagling in the Kumaon Himalaya, Uttarakhand. This partial melting and magma injection was contemporaneous with the extrusion of rock from deeper to shallower levels of the crust. 

All this took place beginning about 24 million years ago. Geologists have termed the movement of this hot mushy ductile rock mass as 'channel flow', literally to mean a channel of semi-solid rock that is being squeezed upwards like toothpaste from its container. In this case, the container were two bounding fault systems, the South Tibetan Detachment as the roof, and the Main Central Thrust as the floor. Supporters of channel flow say that the pervasive ductile deformation observed in the Greater Himalaya doesn't support the Critical Wedge mechanism of orogen growth. Instead, they propose that 'channel flow' was a unique phase in Himalaya development, restricted to the Miocene when deeply buried hot crust was being extruded. Over time  shallower levels of the crust were incorporated into the growing orogen where colder temperatures permitted brittle breakage of the crust and critical wedge growth. The Lesser Himalaya and the Siwalik ranges can be more satisfactorily explained by this mechanism of southwards fold and thrust propogation. 

'Critical Wedge' and 'Channel Flow' are statements on how crust with contrasting mechanical properties responds to compressional forces of tectonic origin and/or surface directed pressure gradients generated due to removal of overburden by erosion.

One final point. The video mentions 'out of sequence' thrusting referring to rejuvenation of extinct fault zones in the rear of the wedge. In case of the Himalaya this means renewed faulting in locales much to the north of the Main Frontal Thrust. This out of sequence thrusting manifest by low level earthquakes is taking place near and just southwards of the Main Central Thrust zone and seems to be driven by enhanced erosion stripping away rock, thereby reducing crustal thickness and normal stress.

Interestingly, I came across a paper by Paramjit Singh and colleagues which has used Apatite Fission Track (AFT) to reveal a pattern to this exhumation. Fisson Tracks is a kind of radiation damage in uranium bearing crystals. It is an ongoing process, but the tracks get preserved only below a critical temperature. The density of tracks is correlated to the time since the rock cooled below the healing temperature. In mineral apatite, fission tracks records the time when the rock cooled below 120 deg C. A young AFT date means that the rock was at around 4 km depth at a more recent time and has been exhumed to the surface much rapidly than a rock recording an older AFT date.

AFT dates taken along a north south profile in the Kumaon-Gharwal Himalaya from the Vaikrita Thrust to the Berinag Thrust  show a southward younging of dates, indicating sequential uplift and exhumation from north to south since Pliocene times (<5 million years ago). The 'out of sequence' faulting regime seems to be a second cycle of an 'in-sequence' pattern developing in the footwall (structurally underneath) of the Main Central Thrust zone. Similiar studies done by this team of scientists across nearby transects in the same climatic zone in the High Himalaya show that rocks are following different exhumation patterns. Contrary to what the video depicted, that does not sound like a climate controlled phenomenon. Rather variations in local tectonics may be dictating this style of exhumation. 

Himalaya never cease to be a mystery and a wonder.

Readings: Measuring Sea Levels, Human Evolution, Elements

 Some interesting articles from the past few weeks:

1) What is global mean sea level? What is relative sea level? Is sea level rising or falling along the India coastline? Science writer Shreya Dasgupta explains how scientists measure sea level change with special reference to the Indian coast.

The Surprisingly Difficult Task of Measuring Sea-Level Rise Around India

2) A long thought human ancestor that turns out to be a contemporary.. a cousin perhaps. Multiple human species coexisted across African landscapes in the Mid Pleistocene, around 300,000 to 200,000 years ago, just when skeletal features that we recognize as 'modern' were evolving.  New fossil finds supplemented by genetics is enriching our understanding of human origins. Fine summary article by Katarina Zimmer.

Genetics Steps In to Help Tell the Story of Human Origins

3) What is an element? From Lavoisier to Mendelev to recent times, Philip Ball traces the contentious issue of pinning down what exactly an element is? 

What is an element?

Bihar: Where Many Rivers Meet

I came across this passage in Vipul Singh's interesting book, Speaking Rivers: Environmental History of A Mid-Ganga Flood Country, 1540-1885. 

 "In consequence of the frequent changes which take place in the channels of the principal rivers that intersect the territories immediately to the presidency of Fort William and the shifting of the sands which lie in the beds of those rivers chars or small islands are often thrown up by alluvion in the midst of the stream, or near one of the banks and large portions of land are carried away by an encroachment of the river on one side, whilst accession of land are at the same time, or in subsequent years gained by dereliction of the water on the opposite side;... the lands gained from the rivers or sea by the means above mentioned are a frequent source of contention and affray,and although the law and customs in the country have established rules applicable to such cases these rules not being generally known, The Courts of Justice have sometimes found it difficult to determine the rights of litigant parties claiming chars or other land gained in the manner above described". 

It is a section of the preamble of The Bengal Alluvion and Diluvion Regulation of 1825 which the East India Company passed to assure a regular income from Diara lands. 

Diara land are ephemeral parcels of land that accrete to river banks or emerge in the middle of the river channel by sediment deposition. They can disappear in a decade or so as a major flood cuts away the river bank or erodes an island, only for newer land to appear elsewhere along this meandering fluid riverine landscape.

The satellite imagery covering the region between Patna in the west to Munger in the east forms the heart of this mid -Ganga floodplain.

Diara lands were traditionally farmed by landless peasants for the few months of the year that they were above water and then abandoned during monsoon inundation. There was no concept of ownership of these lands. The East India Company in its quest to maximize land revenue decided to regulate ownership of the Diara. It didn't always work in practice. Zamindars were reluctant to report the ground situation accurately and maps became outdated as topography and landscapes shifted quickly. Diara lands remained and still are a source of dispute.

Fascinating is the struggle outlined in the book of two long lasting empires with this river system. Before the Mughals, the Afghan ruler Sher Shah Suri occupied this region and had adapted to the capricious environment to his advantage. The Mughals though faced a different problem. Their power center was far to the west in Delhi and Agra. Carrying grains and goods from Bihar to Agra-Delhi using pack animals wasn't possible, since the expenses of feeding these animals would have left very little surplus. Being children of the Eurasian steppe, they preferred the horse and land transport, and that meant that they were unable or unwilling to develop a river navigation network on a commercial scale. The direction of river flow also worked against them. Sailing goods-laden boats upstream was challenging, especially as Vipul Singh points out, the Ganga upstream of Varanasi becomes difficult to negotiate. As a result the Mughals could never exploit or control this region fully. 

The British though found the river to their liking. The Grant of Dewani of Bengal, which the East India Company won in 1765, extended to the mid Ganga floodplains. The Company's main port lay downstream in Calcutta and their major markets across the seas in China and Europe. Using their expertise in navigation they soon set up a thriving trade in saltpetre, calico, opium and silk. Slowly, they also began imposing a linear topography on these curving meandering rivers. The building of embankments, canals, and barrages was thought necessary to control the inundation that could lead to a loss of a cropping season. The various Regulations and Acts meant to ensure a permanent and uninterrupted stream of revenue ended up changing the people's interaction with the river. 

These land regulations, beginning with the Permanent Settlement of 1793, entrenched the power of hereditary Zamindars who became Company rent collecting agents. It became especially hard for the landless to eke out a living as they saw even the ephemeral parcels of Diara which they had been farming now being allocated to the nearest Zamindar and his tenants. Large scale flood control projects carried out by Zamindars and encouraged by Company revenue officers began reshaping the ecology of the floodplains with embankments preventing floods in one region but exacerbating them on the opposite side or in downstream areas. Embankments also prevented smaller local streams from draining into the larger rivers, resulting in the water-logging of fields. Praveen Singh in The colonial state, zamindars and the politics of flood control in north Bihar (1850-1945), details how a web of social and economic interests spurred on this construction spree despite warnings from irrigation engineers about the detrimental effects of embankments.

Vipul Singh also emphasizes the linkage between physical processes and cultural evolution.  A unique Bihari regional identity emerged based on the homogeneous ecology, similar agrarian practices and a shared reverence for the Ganga. 

The Ganga of the plains is a turbid river. It transports several hundred million tons of sediment to the Delta. In this section of Bihar, it is joined by the Sone from the south and the Ghaghara, the Gandak and the Kosi from the north. The Kosi and the Gandak are especially sediment rich, carrying a suspended sediment load of 80 million tons per year and 43 million tons per year respectively. All this sediment is what makes this region special. A significant fraction of it gets deposited every year in the river channel and its floodplains. Over time, the Ganga and its tributaries have built vast alluvial deposits, through which the river finds its way, often getting choked on its own sediment, and then breaking free by cutting a new path for itself. This abundance of water and sediment has formed a complex fluvial ecosystem of meandering channels, river islands, abandoned courses, oxbox lakes, ponds, and wetlands. The organic rich silt deposited across floodplains by the rivers during monsoon inundation nourishes multiple crops. Life's daily rhythms became embedded in this ecology and its inhabitants evolved farming practices adapted to the changing tune of the environment.

There was linguistic pride too, not in one common 'Bihari' language, but in the various dialects spoken 'eh /e paar' and 'oh o paar'; this side of the Ganga and on the other side. Bhojpuri was the dialect of the Champaner area north of the Ganga, while to its east on the north side was spoken Maithili. In the Patna region on the south side was Magahi and towards the east near about Munger was Angika. These vernaculars with their common folk tales, poetry, and myths about deities, changing seasons, local plants and animals, and the Ganga, knitted the region together, away from the pull of the Delhi-Agra-Awadhi influence which lay to the west and the Bengali cultural sphere towards the east.

Magh ke garmi, Jeth ke jar
Pahila pani bhar gail tar,
Ghag kahen ham hoban jogi,
Kuan ka pani dhoihen dhobi.

[Heat in Magh (January-February), cold in Jeth (May-June),and the tanks filled with the first fall of rain, are the signs of drought. Ghagh says that I will become a beggar, and the washer-men will wash with well-water.]  

This is a gem of a book. Highly recommended reading! 

Hutton On Erratics

 "There would then have been immense valleys of ice sliding down in all directions towards the lower country, and carrying large blocks of granite to a great distance, where they would be variously deposited, and many of them remain an object of admiration to after ages, conjecturing from whence, or how they came. Such are the great blocks of granite which now repose upon the hills of Saleve".

... James Hutton : Theory of the Earth (1795). 

This passage is quoted in Jamie Woodward's book The Ice Age: A Very Short Introduction, and it is one of the earliest attempts to explain 'erratic boulders'. These are boulders sitting on a surface made up of a different rock type than the boulder, indicating that the boulders have been transported from a far away terrain. The great blocks of granite observed by Hutton were scattered on a limestone landscape that was part of the Jura mountains on the border between France and Switzerland.  Just how these boulders got to their present location was the subject of a lively debate in the late 1700's and the 1800's. Hutton suggested they were brought there by glaciers. Other naturalists, taking inspiring from Scripture, proposed that they were transported by Noah's flood. 

Erratics implied that glaciers must have been much larger in the past. Hutton never developed his thinking about glaciers into a full explanation. That would take several more decades of debate. Erratics found in Alpine regions and on the European and North American temperate plains became part of a growing body of evidence for past climate change.

Closer to home, the picture below shows an 'erratic' made up of a high grade metamorphic rock and sourced from the mountains seen in the background. The location is Darma Valley, Kumaon Himalaya.

Notice To Readers: Comment Moderation

Dear Readers:

If you have left a comment on my blog over the past two years or so and not gotten a reply from me, it is most likely because you posted a comment one week after the publication of my post. After one week, comments are routed to a moderation queue. I am supposed to get a notification by email of pending comments. I noticed today that this email notification setting was turned off! As a result, I have been unaware of the many comments that were languishing in the moderation queue. 

I apologize for my oversight. I have reset the notification settings and I should now be receiving an email regarding any comments pending moderation. You can also email me directly. You can find my email address on the Profile Page. 

As always, a big thank you for your continuing support of my blog.

Books: Speaking Rivers, The Ice Age

 Ordered and received!

Prof. Vipul Singh is with the Dept. of History, University of Delhi, and he writes in the acknowledgments section that environmental history as a formal subject of study in history departments had a late start in India. The focus of the book is the flat lands of Bihar with its annual floods and shifting river channels and how Mughal and later British land use policies transformed the people's relationship with the river system. Looks like a very meaty book with plenty of Notes, Maps and a long Reference section. Will be sharing interesting snippets as I read along.

 

As one blurb says... "Perfect to pop into your pocket for spare moments".  A fine introduction by Jamie Woodward. The recognition that the earth has passed through several glacial and interglacial phases is really a triumph of field geology. Thick sedimentary deposits in Europe and N. America were recognized as being left behind by advancing ice sheets. The stellar role played by geologists in the mid-late 1800's and their debates grounded within the prevailing schools of catastrophism versus uniformitarianism is highlighted. And there are good succinct sections on the many modern theoretical advances in climate science and the techniques that geologists and climate scientists bring to bear upon understanding the mode and tempo of climate change.

 

Happy Reading.

Readings: India Dams, Geology Videos, Parsis in India

 Sharing some readings.

1) Neeraj Wagholikar, Parineeta Dandekar and Himanshu Thakkar weigh in on the dam building epidemic that is afflicting India. These three experts cover issues of environmental governance, destruction of fisheries and livelihoods, and a perspective on their irrigation potential and economic logic.

The deep political drive to push through permissions to build dams is best highlighted by an example of a malign recommendation in a report of the Parliamentary Standing Committee on energy published in January 2019. It seems to view in favor Himachal Pradesh's suggestion to the committee to help declare large hydropower projects as linear projects, thus enabling them to bypass Gram Sabha consent. The statement reads, “If it is done, then, to a large extent, the problem of FRA, which the Secretary also mentioned, will get resolved because the stringent provisions of FRA will get diluted. It is not our purpose to subvert them. Our only purpose is to get them more liberalised.” 

FRA is the Forest Rights Act which gives local forest dwellers a say in the site selection of infrastructure projects. 

Makes you despair and shake in anger, doesn't it?

India, Dammed.

2) Geology fans! I highly recommend Rice University Professor Cin-Ty Lee's YouTube Channel. He has a very informative collection of short videos on rocks and minerals and geologic processes. 

Here is one of my favorites.. Isostacy and what controls the elevation of mountains?

Email subscribers who can't see the embedded video, can view it here - Elevation of Mountains.

3) Like Sugar in Milk.. was the memorable assurance given by the Zoroastrian refugees to the King of Gujarat. We will assimilate in Indian society. And they have in many ways, while maintaining a distinct identity. 

What does genetics tell us? Fine post by Razib Khan.

Endogamy and Assimilation. Parsis in India.

Map: The Deep Geological Cycle of Carbon

When I was a kid not so long ago in geologic time, my understanding of how diamonds are created went something like this.

In forests and swamps, large trees grew and died. The wood got buried under more wood and layers of sand and mud. The wood in the bottom layers under the influence of great pressure and higher temperatures got converted first to coal. As burial to greater depths continued, this coal turned first to graphite and then finally to diamond. The story of woody material turning eventually to coal and then to graphite was roughly correct, but this geological path doesn't lead to diamonds.

Most diamonds form at depths of about 150- 200 kilometers. Rarer varieties known as sublithospheric diamonds form even deeper down. The carbon required to make a diamond is transported from the earth's surface to those depths by a subducting plate. Subduction is the process whereby an oceanic plate made up of dense Mg and Fe rich rocks sinks into the mantle. The Marianna trench for example marks the place where the Pacific plate is sinking underneath the Japan Plate. Why can't coal then move into the mantle this way? It doesn't because coal deposits occur in continental settings where the crust is made up of much lighter rocks richer in Si, Al, Na and Ca. This continental crust is buoyant and does not subduct into the denser mantle. So there is no way for coal that began its journey in ancient river floodplain, bogs, and swamps to get directly transformed into diamonds.

The denser oceanic plate contains carbon from a variety of sources. The lower layers of the oceanic plate is made up of a Mg rich rock known as peridotite. Occasionally, faulting may bring this peridotite to shallower levels where it interacts with sea water and gets transformed into a rock known as serpentinite with calcium carbonate minerals also forming alongside. Carbon is trapped in minerals like calcite and dolomite. The upper layers of the ocean plate is made up of the volcanic rock basalt. It too interacts with sea water, with calcite precipitating in rock cavities. This becomes another source of carbon.

Then there is carbon which is part of the shells and skeletons of planktonic marine creatures. These tiny photosynthesizing organisms which live in the sunlight zones of the ocean precipitate a calcium carbonate skeleton. When the organism dies, these carbon containing skeletons sink and blanket the sea floor. This source of carbon is a relatively late addition in geologic history. Calcareous nanoplankton first appeared in the early Mesozoic, some 225 million years ago. Organic tissue of marine creatures can also get buried, making this another carbon source. And finally, carbon coated sediment washed into the ocean by rivers and then transported into abyssal depths by deep sea currents contribute some carbon to the oceanic plate.

Each oceanic plate has its own selection from this carbon menu, depending on its unique geologic history. For example, calcium carbonate starts dissolving below a depth known as the calcite compensation depth. Sea floor below this depth doesn't retain much skeletal debris. Or, oceanic crust that formed in the Cretaceous contains abundant calcite in veins and vugs, likely because the warmer Mesozoic oceans promoted calcite precipitation on the sea floor.

This beautiful map shows several subduction zones. In the map, SedCarb refers to skeletal carbonate, SedOrgC to organic carbon, AOC Carb to carbonate in altered oceanic crust, SerpCarb to carbonate in serpentinite rocks. The major source of carbon is identified by a particular geochemical signature. Mineral carbonate for example has higher amounts of the heavier isotope of carbon (C13), while carbon that makes up organic tissue is much richer in the lighter isotope (C12).

  Source: Terry Plank & Craig E. Manning: Subducting Carbon

As the oceanic plate subducts carbon begins to get removed from the plate. Some carbon is removed when the sediment and altered oceanic igneous rocks are scraped off and plastered on to the sea floor. Such deposits made from scraped off sea floor are called accretionary prisms. They often poke out above sea level to form island chains. The Andaman Islands is an example of an accretionary prism that is made up of mechanically removed slices of the subducting Indian oceanic plate.

At greater depths, sediments and oceanic crust begins to be metamorphosed under higher temperatures and pressures resulting in the loss of carbon dioxide and water. This carbon dioxide makes its way into the overlying mantle and gets incorporated into magma. The spectacular volcanic eruptions along Japan, Indonesia, Caribbean and the Western North American coastlines are a result of the rising and depressurization of such volatile bearing magma. Some of the carbon in the belched out carbon dioxide has come from the burning of skeletons of marine organisms in the deeply buried plate underneath these volcanic systems.

A quantity of carbon does remain in the downgoing plate and reaches depths of 150-200 kilometer or more. The igneous rocks of the subducted plate gets altered to a dense rock known as eclogite. And at these temperatures and pressures, diamonds may form within these eclogites along carbon dioxide or methane rich domains. The subducting slab is also releasing some trapped carbon along with other volatiles which infiltrate the surrounding mantle. Diamonds can form in such metasomatized or fertile regions of the mantle as well. The main host rock here is peridotite. Subducted carbon is one source of carbon for diamonds.

Geologists think that primordial carbon retained in the mantle from when the earth formed may also be finding its way into diamonds.

Multiple sources perhaps, diamond forming chemical reactions can be summarized simply as driven by the reduction of carbon sourced from either carbon dioxide or methane.

CO2 = C + O2

CH4 + O2 = C + H2O.

From eclogite and peridotite parent rocks, diamonds are transported to the surface by an unusual magma type known as kimberlite and even less commonly by lamproites. These magmas are rich in volatiles like water, carbon dioxide, fluorine and chlorine and are also rich in magnesium. They are generated at the base of thick continental plates generally during episodes of continental fracturing. The volatile rich magma physically disaggregates diamonds from their parent rocks and carry them as they ascend through deep continent penetrating cracks with amazing speed, traveling 200 kilometers in a matter of hours, bringing to the surface its tiny but dazzling prize. The block diagram below summarizes the geological environments of diamond formation and their ascent.

Source: Steven B. Shirey et. al., Diamonds and Geology of Mantle Carbon; modified from Tappert R and Tappert M 2011.

The famous Panna diamonds from Bundelkhand in Central India came to the surface in a kimberlite magma eruption around 1 billion years ago. It is possible that its source carbon was transported from the surface to diamond forming depths hundreds of millions of years earlier, perhaps during the convergence and assembly of an earlier supercontinent.

Diamonds are often older than their host kimberlites by hundreds of millions to billions of years. During diamond growth, other minerals get trapped inside them as micro-inclusions. Their composition is therefore a record of the fluid chemistry of the mantle and the carbon cycle as it existed in deep time, billions of years before present.

Diamonds are one component of the deep geological cycle of carbon. We are familiar with the exchange of carbon between the atmosphere and the biosphere. Carbon is transferred to and fro in this system on a timescale of days to years to hundreds of years, but not much more. Longer geological sequestration of carbon occurs at shallower levels of the crust too. Soil can store carbon for thousands of years. Carbon can get trapped for millions of years in carbonate minerals that make up limestone and also in coal and oil. It is this shallow crustal carbon cycle that we are breaking by burning limestone and fossil fuels.

The deep geological cycle can take carbon from the surface and keep it in the mantle for hundreds of millions of years. The mantle releases it through sustained volcanism thus modulating earth's climate on long time scales. And occasionally as a return gift it throws up a few diamonds as well. 

Infographic: Milestones In Climate Science

Prof. Katharine Hayhoe and Skeptical Science tweeted this infographic showcasing the history of climate science. There is a long article by John Mason on this topic on the Skeptical Science site.

Source: Skeptical Science: The History of Climate Science

Beautifully compiled by John Garrett. Especially telling is the close parallel between rising carbon dioxide levels and rising temperature (the blue and green lines), a fact that the fossil fuel industry has tried mightily to suppress. Don't get taken in by their subversion of this obvious connection.

Himalaya Earthquake Article

Bibek Bhattacharya has written a fine article in liveMint about Himalaya earthquake risk. He describes the geological story fairly accurately and also properly focuses on our woeful preparedness in terms of citizen awareness and in constructing earthquake resilient buildings.

Seismologist Roger Bilham too has been talking and writing about this topic from time to time. He has been quoted in Mr. Bhattacharya's article. There was also an interview with him published in the July 19th edition of Times of India. Apart from reiterating the geologic risk, he says that "Earthquake engineers in South Asia have diligently responded to the need to ensure building codes are implemented".

I have low confidence in this assessment and one that is echoed in Bibek Bhattacharya's article. In India, there is always a wide gap between expert suggestions and proposed guidelines and their ground level implementation over which earthquake engineers have no control. Demonstrations of strong but light weight constructions by various NGOs always remain at a pilot project stage and are not widely realized in the new growth that is taking place. Wherever I have traveled in the Himalaya, especially in Uttarakhand which faces the risk of a magnitude 8 earthquake, I have noticed that mountain towns have become concrete death traps. A congested sea of buildings have mushroomed up willy-nilly with scant regard for safety. The article in liveMint gives one pointed example of this recklessness. The Shimla High Court building is an 11 story structure built on the edge of a hill. Who will regulate the regulators?

The prognosis is grim.

How Old Is Plate Tectonics? Signals From The Mantle

A recent study has used the geochemistry of Archean basalts and komatites to suggest that plate tectonics began by 3.2 billion years ago. The chemical composition of these two volcanic rocks points to active transfer of elements from the crust into the deep mantle by this time. The authors argue that this mass transfer from surface to the interior can most efficiently be achieved only via subduction of plates.

Early in its history the earth differentiated into three chemically distinct shells, the crust, the mantle and the core. There is another type of layering on earth, one that is governed by differences in  rheology or the capacity of materials to flow and deform. The outermost rigid shell of the earth is called the lithosphere. It is made up of the crust and the uppermost part of the mantle. With the exception of earth, all other silicate or rocky planets of our solar system have an unbroken lithosphere shell riding atop a hotter ductile mantle or asthenosphere.

On earth, the lithosphere is broken into independent mobile fragments. New lithosphere is created at mid-oceanic ridges by magmatism and is destroyed at subduction zones where it sinks into the asthenosphere. This is plate tectonics. At places, the lithosphere breaks up and the two pieces drift apart to form rifts and eventually oceanic basins. At other locations, lithospheric slabs sink or collide to form orogenic mountains. The pull exerted by the sinking lithosphere slab is thought to be the major force sustaining plate motion.  This continued motion of plates makes earth a geologically dynamic place.

When did plate tectonics begin on earth? A range of dates have been proposed. Some geologists think that plate tectonics began in the Archean, as early as 3.5 billion years ago. Others insist that while some fragmentation of lithosphere into 'plates' might have occurred in the Archean, it would have been ephemeral. These proponent of late plates say that earth in its early history would have been like other rocky planets with a tectonic style dominated by a 'single lid' or an unbroken lithosphere shell enveloping a hot mantle. During this 'single lid' phase the lithosphere could sink into the mantle by delamination of a denser lower layer or by 'drips' where a dense lithosphere blob detached itself.  However, these processes were localized. A global regime of  a network of mobile plates (plate tectonics) began relatively recently in earth history, about 1 billion years ago.

The figure below summarizes the various styles of tectonics prevalent on the rocky planets of the solar system.

Source: Robert J. Stern: The Evolution of Plate Tectonics

The evidence presented comes mostly from the composition and deformation style of certain types of crustal rocks.  In the early stages of subduction of oceanic lithosphere, rocks are subjected to high pressures but low to moderate temperatures. This results in basaltic rocks that make up the subducting oceanic crust getting transformed into a rather distinctive rock suite known as blue schists, named after the blue tinted mineral glaucophane. Blueschists first appear in the rock record around 1 billion years ago. Their earlier absence has been taken to mean the absence of subduction.

Another distinctive rock type becomes much more common around 1.2 billion years ago or so. These are volcanic rocks known as kimberlites. They are famous for being the main source of diamonds. Kimberlites solidify from explosive magmas rich in carbon dioxide and water. These magmas originate more than 200 kilometers deep in the mantle. The mantle at these depths is generally depleted in water and carbon dioxide and other volatiles. Subducting lithosphere can deliver carbon dioxide and water from the surface to great depths, enriching the mantle in these volatiles. Again, the common appearance of kimberlites after 1.2 billion years ago is seen as a sign of the beginning of subduction and plate tectonics by that date.

Early plate tectonic proponents come up with their own list of indicators. They point out to the presence of 2.2 billion year old eclogites from an ancient terrain in Congo. Eclogites are rocks containing the distinctive minerals omphacite and garnet. They form when a cold oceanic plate subducts and ocean crust basalt is subjected to high pressures and moderate temperatures.

There are hints from Indian terrains too of plate tectonics in operation from before 1 billion years ago. In the Nellore schist belt, between the Eastern Ghat and Dharwar terrains in South India, there are rock types known as ophiolites. They date to between 2.5 billion and 1.8 billion years ago. Ophiolites are slices of oceanic crust that have been scraped off from a subducting plate and thrust up into mountain ranges.

In Central India, south of the Vindhyans and north of the Tapi river lies the Central Indian Tectonic Zone (CITZ). This broad area of crust represents the zone of collision and suturing of the Dharwar continental block with the Bundelkhand continental block. This suturing is thought to have taken place around 1.6 billion years ago based on the age of peak temperature and pressure conditions recorded in metamorphic rocks (Sausar Mobile Belt) of this region. It has been held for some time that there is a distinct pattern to this metamorphism. In the northern areas, rocks of the Sausar Group were metamorphosed under high pressure and high temperature conditions. In southerly regions there are signs of only high temperature and moderate pressure metamorphic conditions. Such 'paired metamorphic belts' are taken to indicate a zone where plates converge and pressure and temperature conditions vary systematically across the terrain.

Other workers though find that within the Sausar Mobile Belt the central and northern belt was metamorphosed around 1.06 to 1.04 billion years ago while the southern belt was impacted by thermal events at 1.6 billion years ago. Another fold belt, the Mahakoshal, at the northern limit of the CITZ, shows peak metamorphism at 1.8 billion years ago. All these have been taken to mark separate tectonic and thermal events and therefore the interpretation of a synchronous development of paired metamorphic belts is wrong.

Geophysical surveys in the CITZ also help in elucidating the deep lithosphere structure. They hint at the presence of dense lithosphere slabs inclined both in a southerly and in the northerly direction. Relating these findings to geochronologic data seems to indicate that these are remnants of ancient plates with southward subduction occurring as early as between 2 billion to 1.8 billion years ago (Mahakoshal), with southerly subduction continuing until 1.6 billion years ago. This was followed by a much later northward directed subduction event in the Neoproterozoic dated to about 1.0 billion years ago. Geologists have interpreted these events in the context of supercontinent formation with the earlier events signalling the assembly and breakup of  supercontinent Columbia and the latter representing the amalgamation of supercontinent Rodinia.

Instead of looking at distinctive rocks and geophysical data, the recent work by Hamed Gamal El Dien and colleagues uses a geochemical approach to time the initiation of global plate tectonics.

The early chemical differentiation of the earth depleted the mantle in elements such as barium, uranium, lead and strontium. These fall under the class known as large ion lithophile elements (LILE). Their large ionic radii means that they fit only uneasily in the atomic frame of the silicate minerals that make up the mantle. During widespread melting of the mantle in Hadean and early Archean (4.5 to 4 billion years ago) they preferentially escaped into the liquid phase resulting in a  chemically stratified earth. The growing continental crust became enriched in these elements. The mantle on the other hand became depleted in these elements and remained so as there was no mechanism to recycle these elements from the surface into the deep interior.

Hamed Gamal El Dien and colleague surveyed the chemistry of Archean age basalts and komatites from a global dataset. Both these rocks are derived from magmas sourced from fairly deep in the mantle. They found that rocks younger that 3.2 billion years show elevated values of LILE. This seems to imply a re- enrichment or a refertilization of the mantle in these elements beginning around 3.2 billion years ago. Since their data set encompassed all the continents, a global change in elemental exchange between the crust and the mantle seems to taken place around this time. This, the scientists argue, was most likely pointing to the start of subduction and global plate tectonics. The ocean floor is blanketed with sediment derived from the erosion of continental crust. As this oceanic crust sinks and dives deeper in a subduction zone, the accompanying sediment heats up and releases volatile elements into the surrounding mantle fertilizing it with a crustal chemical signature.

Another geochemical indicator that is used to distinguish continental crust versus mantle sources is the ratio of Neodymium143/Neodymium144.  Nd143 is derived from radioactive decay of Samarium147, while Nd144 is the stable isotope. During early differentiation Sm147 was preferentially enriched in the mantle. Over time, the mantle has evolved a higher Nd143/Nd144 ratio relative to the crust. Like the temporal trend in LILE, basalts and komatites younger than 3.2 billion years show a much lower Nd143/N144 ratio, suggesting an influx of crustal material into the deep mantle.

There are other mechanisms by which the mantle derived basalts and komatite rocks could inherit a crustal elemental pattern. One way is for the magma to react and get contaminated by continental crust as it ascends. The researchers rule this out by using another chemical discriminant, the ratio of Thorium/Ytterbium which is higher in continental crust than in a depleted mantle. The analysed rocks  have Th/Yb ratios inconsistent to have been derived from magmas mixing with continental crust.  Another pathway for the exchange of materials from the crust to the mantle is the sinking of lower crustal material by 'drip' or by delamination.  However, these processes generally affect only the upper levels of the mantle and would not have altered the composition of the lower mantle which is a source of the komatite magmas. Based on these observations the researchers are fairly confident that only subduction can explain the enriched mantle signals.

The late plate tectonic view has received more direct pushbacks. For example, one counter argument for the absence of older blueschists draws attention to the changing composition of oceanic crust . Archean and Early Proterozoic oceanic crust was richer in magnesium oxide and at the geothermal gradients that prevail in subduction zones this MgO rich crust would have altered to a different type of metamorphic rock known as greenschists, so named after minerals like chlorite and actinolite.

The kimberlite argument too has been countered. There are kimberlites older than 1.2 billion years,  nor do they occur with a uniform frequency through time. Rather, there are pulses centered around 2 billion years ago, 1.2 billion years ago and strikingly between 250 million years and 50 million years. Many geologists see a connection between supercontinent break up and processes in the deep mantle, wherein an influx of carbon dioxide and water initiates melting in cooler mantle beneath thick continents and deep continental fractures provide pathways for the volatile magma to rise quickly to the surface.

These debates will certainly continue for many years to come.

Robert. J Stern, in a review, outlines three conditions that had to be met before global plate tectonics could begin. First, large tracts of lithosphere slightly more dense than the asthenosphere had to form. Second, this lithosphere had to be strong enough to remain intact in a subduction zone and pull along the entire plate without it disintegrating.  And thirdly, zones of lithosphere weakness at least 1000 km long had to develop along which new plate boundaries could form.

At some point between 3 billion and 1 billion years ago, likely as a start and stop process, the earth's tectonic style began diverging from other rocky planets in our solar system, nudged by chemical differentiation and heat flow thresholds which govern the strength,  thickness and buoyancy (or density) of the lithosphere. At a critical combination of these three variables, the outer shell would have begun to regularly break along a global network of fracture zones initiating nascent plate boundaries along which dense lithosphere could sink into the mantle. The presence of copious water would also have been important. Water reacts with the olivine rich rocks of the oceanic lithosphere, transforming them into weaker serpentine rich domains, converting strong lithosphere into long zones of weaknesses that could break and bend more easily. Water released from oceanic sediment also would have acted as a lubricant at subduction interfaces, facilitating the sliding of one plate underneath another.

Plate tectonics has made earth into a restless planet. The great churn within keeps influencing climatic regimes, oscillations in ocean chemistry, and metal enrichment episodes. And it has played a major role in the maintenance of life too. As continents jostle, break apart and collide, new and varied habitats keep forming, profoundly shaping unique trajectories of biological evolution and biodiversity.

Infographic: Reefs Through Geologic History

Academia.edu is always sending me links to papers that I can download. It is a lure to get me to upgrade my membership. I am grateful for the riches that I have discovered on their site.

Last week they sent me a gem-  Palaeoecology: Past, Present and Future by David J. Bottjer. It is a full length book on the field of paleoecology summarizing its scope, its methods, and giving an overview of different ecological domains such as the shallow marine, the pelagic (deep marine), and the terrestrial environments and how they have changed through time. Dr. Bottjer is Professor of Earth Sciences, Biological Sciences and Environmental Studies with the University of Southern California Dornsife, Los Angeles.

One striking aspect of this book is the really informative diagrams that Prof. Bottjer has collated from various papers and books. They really add a lot of value, especially if you want to avoid reading every line of the text!

Here is one which shows the changing composition of reefs through time.

Source: James, N.P. and Wood, R. 2010. Reefs. In James, N.P.,  Dalrymple, R.W., Facies Models 4. Geological Association of Canada, pp. 421–447

They were not always built by corals.

I am going to learn a lot from this book.

Punctuated Equilibrium Is About Small Subtle Changes

The theory of punctuated equilibrium (PE) put forth by paleontologists Niles Eldredge and Stephen Jay Gould proposes that morphological evolution speeds up during lineage splitting events when new species form. In practice, a paleontologist sampling fossil species in a sedimentary section will find that a species over its lifetime does not show any trend in morphological changes. That species eventually goes extinct at a particular interval. Fossils of the inferred descendant species appear abruptly in the sedimentary bed above. A fossil population showing a mix of traits diagnostic of both species is not generally seen. 

Eldredge and Gould argued that this pattern of change observed in fossil species shows that the history of a species is really characterized by long periods of no change, eventually interrupted by rapid evolution to a new species.

A reassessment of one textbook example in bryozoans suggests no evidence of such a punctuated mode. 

A write up in Phys.org describes the new work. 

Revisiting a Landmark Study System: No Evidence for a Punctuated Mode of Evolution in Metrarabdotos - Kjetil Lysne Voje, Emanuela Di Martino, and Arthur Porto

I am not going into the work itself. There are well documented examples of the punctuated equilibrium mode of evolution while others as in this case may see revision from time to time. 

Instead, I wanted to comment on one of my pet peeves about this topic which is the common use of the phrase "major evolutionary change" to describe the actual evolutionary changes.

This idea which never seems to go way that PE involves big or major changes has caused a lot of misunderstanding about the theory.  As the authors of PE took pains to point out, the amount of morphological difference between ancestor and descendant species is subtle, often taking excruciating examination to recognize. 

Here is Niles Eldredge describing his work on Devonian trilobites.

I measured some 50 different lengths and widths- length of the head, distance between the eyes, height of the eyes, length of the tail and so on -on hundreds of specimens. This was tedious. Each specimen had to be cleaned to at least well enough to make all the anatomical landmarks visible. Each had to be mounted on a block of wood, stuck to a blob of plastilene, with the tops of the eyes (a flat surface) in a horizontal plane, perpendicular to my line of sight down the barrel of the microscope. There was a little scale inside the right eyepiece reticle from which to read off the various measurements. After a day of measuring, and until I got used to the microscope, I would see double from the bus window on my way home. 

 from Time Frames: The Evolution of Punctuated Equilibria.

Trilobites have compound eyes, made up of columns of lenses. All this detailed work led to the recognition that a new species had 17 columns of lenses, instead of 18 in the ancestral species found in slightly older strata!

This point about small changes was missed by many observers. Creationists conflated PE with a theory of macro-mutation or saltation, a sudden origin of big morphological change as for example seen in the transition from reptiles to mammals. They claimed that Darwin with his insistence on gradual accumulation of small changes had been wrong all along! 

Not so. The morphological differences observed involved small changes between the ancestor and descendant species.  It takes an expert to recognize this shift in form. 

The real significance of PE is not about the amount of change, but the pattern. PE proposes that very little morphological divergence takes place during the lifetime of a species. It is only when a sub-population of that species gets isolated that it may experience relatively rapid evolution. This genetic isolation results in the formation of a new species. The ancestral species may persist in the main parts of its range, while one of its peripheral populations has evolved into a new species. Biologists term this process of branching or lineage splitting as cladogenesis. 

PE is about explaining these rhythms in the history of lineages. Long periods of stasis (little or no change in form), punctuated by relatively rapid transition to a new form. This transition, though rapid in geologic time,  may take place over hundred of generations and thousand of years.

Why then isn't this transition to a new form preserved in the rock record as a fossil population with a mix of characters? The answer Eldredge and Gould prefer is that reproductive isolation takes place in outlier regions of a species range. Sediments deposited in these isolated basins will be archiving the incremental evolution of a new form, just as Darwin envisaged,  via a mixed or transitional population. However, these outliers containing transitional populations have less chance of getting preserved in the rock record.  The sedimentary sections that paleontologists examine are generally from the main part of the basin. This is not where the change to a new species has taken place.  The sudden appearance of an inferred descendant species in this strata is really recording the migration of the descendant from an isolated part of the species range where it evolved, into the central range of its extinct ancestor. 

Biologists have named this process of formation of a new species through reproductive isolation in a geographically distant area as allopatric speciation. Eldridge and Gould's theory of punctuated equilibrium invokes allopatric speciation and migration to explain the abrupt appearance of new species in the fossil record. In terms of the mechanisms of change, it is not an alternative to Darwinism as is often portrayed. Both the authors accept that even during the period of 'rapid' evolution, changes accumulate through natural selection or genetic drift incrementally across generations. But the vagaries of preservation means that sediments which record this transition are rarely available for study.

Magmas And Mass Extinction: Late Triassic

A new study on the synchronicity of igneous activity and the Late Triassic mass extinction which occurred around 201.5 million years ago.

Large-scale sill emplacement in Brazil as a trigger for the end-Triassic crisis- Thea H. Heimdal, Henrik. H. Svensen, Jahandar Ramezani, Karthik Iyer, Egberto Pereira, René Rodrigues, Morgan T. Jones & Sara Callegaro. The article is open access.

Magma intruded a thick pile of sediments in Brazil. The thermal reactions in the sediment would have resulted in the release of 88 trillion tons of CO2 from the degassing of sediments!

Abstract:

The end-Triassic is characterized by one of the largest mass extinctions in the Phanerozoic, coinciding with major carbon cycle perturbations and global warming. It has been suggested that the environmental crisis is linked to widespread sill intrusions during magmatism associated with the Central Atlantic Magmatic Province (CAMP). Sub-volcanic sills are abundant in two of the largest onshore sedimentary basins in Brazil, the Amazonas and Solimões basins, where they comprise up to 20% of the stratigraphy. These basins contain extensive deposits of carbonate and evaporite, in addition to organic-rich shales and major hydrocarbon reservoirs. Here we show that large scale volatile generation followed sill emplacement in these lithologies. Thermal modeling demonstrates that contact metamorphism in the two basins could have generated 88,000 Gt CO2. In order to constrain the timing of gas generation, zircon from two sills has been dated by the U-Pb CA-ID-TIMS method, resulting in 206Pb/238U dates of 201.477 ± 0.062 Ma and 201.470 ± 0.089 Ma. Our findings demonstrate synchronicity between the intrusive phase and the end-Triassic mass extinction, and provide a quantified degassing scenario for one of the most dramatic time periods in the history of Earth.

This prolonged phase of igneous activity resulted in the formation of the Central Atlantic Magmatic Province. Its connection to the mass extinction was hard to pin down due to a lack of accurate dates of the oldest igneous activity. This and some other work now show that phases of this magmatic episode were synchronous with the mass extinction.

A similar problem of lack of accurate dating of events had limited our understanding of the role of Deccan Volcanism in the mass extinction that took place at 66.04 million year ago. New geochronology work (summarized by Kale et. al. 2019)  is showing that volcanism spanned this mass extinction. Significant amount of lava eruptions took place before the mass extinction and would have played a role in the deterioration of environmental conditions. And volcanism continued well after the mass extinction delaying biotic recovery for hundreds of thousand of years.

Large injections of magma as laterally extensive intrusions (sills) into sediment has also been thought to have been the trigger for the end-Permian mass extinction that took place around 252  million years ago. Interestingly, like the end-Triassic, it was not emissions of carbon dioxide and methane directly from lava eruptions that is thought to be the driver of environmental change. Rather, it was the subsurface emplacement of sills and the thermal reaction (contact metamorphism) in buried sediment in contact with this hot magma that resulted in volumetric degassing from sediments. Limestones when heated this way would have released carbon dioxide upon breakdown of the mineral calcite. And organic matter would have released methane.

The long trajectory of evolution on earth has been disrupted and reoriented many times from deep within.

Readings: Human Evolution, Ice Ages, Brains In Digital Age

Some interesting articles to read:

1) How did Homo sapiens evolve in Africa? Did one population branch off and evolve all the traits of 'modern humans' in isolation or were there several populations spread across the continent which at times evolved in isolation but periodically met and exchanged genes and cultural practices, resulting in a gradual coming together of the constellation of traits we see in us? Recent findings based on the fossil and tool record is pointing to the latter process.

The search for Eden: in pursuit of humanity’s origins by Robin McKie.

2) How do variations in the earth's orbit influence the growth and decline of glacial and interglacial periods? Excellent review article on the factors controlling climate change over the past 2.5 million years.

Tying celestial mechanics to Earth’s ice ages by Mark Maslin.

3) How is the human brain coping with the information deluge of our times?

Extract:

" Humans, of course, forage for data more voraciously than any other animal. And, like most foragers, we follow instinctive strategies for optimizing our search. Behavioral ecologists who study animals seeking nourishment have developed various models to predict their likely course of action. One of these, the marginal value theorem (MVT), applies to foragers in areas where food is found in patches, with resource-poor areas in between. The MVT can predict, for example, when a squirrel will quit gathering acorns in one tree and move on to the next, based on a formula assessing the costs and benefits of staying put — the number of nuts acquired per minute versus the time required for travel, and so on. Gazzaley sees the digital landscape as a similar environment, in which the patches are sources of information — a website, a smartphone, an email program. He believes an MVT-like formula may govern our online foraging: Each data patch provides diminishing returns over time as we use up information available there, or as we start to worry that better data might be available elsewhere.

The call of the next data patch may keep us hopping from Facebook to Twitter to Google to YouTube; it can also interfere with the fulfillment of goals — meeting a work deadline, paying attention in class, connecting face-to-face with a loved one."

There is some sensible advice at the end on how to build healthier habits and manage our dependencies on technology.

How Our Ancient Brains Are Coping in the Age of Digital Distraction by Kenneth Miller.

Books: Tree Story, Rivers of Power

A friend pointed out these two books.

1) Tree Story: The History of the World Written in Rings by Valerie Trouet.

Review: "Trouet, a leading tree-ring scientist, takes us out into the field, from remote African villages to radioactive Russian forests, offering readers an insider's look at tree-ring research, a discipline formally known as dendrochronology. Tracing her own professional journey while exploring dendrochronology's history and applications, Trouet describes the basics of how tell-tale tree cores are collected and dated with ring-by-ring precision, explaining the unexpected and momentous insights we've gained from the resulting samples"...



2) Rivers of Power: How a Natural Force Raised Kingdoms, Destroyed Civilizations, and Shapes Our World by Laurence C. Smith

.."From ancient Egypt to our growing contemporary metropolises, Rivers of Power reveals why rivers matter so profoundly to human civilization, and how they continue to be indispensable to our societies and wellbeing"...

Two other books on rivers that I would recommend are Unruly Waters by Sunil Amrith and The Water Kingdom by Philip Ball.

I also want to read The Unquiet River: A Biography of the Brahmaputra by Arupjyoti Saikia. Hoping to get to it soon.


Happy Reading!