Tuesday, June 30, 2020

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.



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 Group) of this region. 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.

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. This seems to indicate the remnants of ancient plates which subducted both southwards and northwards as two terrains converged and collided over the long time period from late Archean to the Mesoproterozoic (~2.5 billion to 1.6 billion years ago).

Instead of looking at distinctive rocks, 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.

Monday, June 29, 2020

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.

Friday, June 12, 2020

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.

Monday, May 25, 2020

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.

Sunday, May 3, 2020

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.

Monday, April 27, 2020

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!

Monday, March 30, 2020

An Outing With William Smith

I have started reading Roger Osbourne's The Floating Egg: Episodes in the Making of Geology.

It tells stories set mostly in the 1800's from the Yorkshire region of England of people involved in exploring landscapes, discovering fossils, and slowly constructing a systematic science of geology. The story of James Hutton's insight that the earth was really old based on his observation of the juxtaposition of two sets of strata at Siccar Point on the east coast of Scotland is well known. From that he deduced that the lower set of strata were deposited first, then tilted and eroded, upon which the upper set of strata were deposited. This meant that a vast amount of time separated these events.

In the late 1790's England, the need to transport coal from mines to industry led to a surge in canal building, as transport by barges was cheaper. William Smith was one surveyor in much demand. He began observing consistent patterns of rock and fossil associations through his explorations and realized that he could use these patterns to predict the presence or absence of certain rock types or fossils deep underground, even if the complete association of rocks was not exposed at the surface. Smith went on to make the first detailed geologic map of Britain, a tale best read in Simon Winchester's The Map That Changed The World.

Roger Osbourne imagines a conversation William Smith has with a fellow traveler, a Mr. Palmer of Somerset Canal Company, while on survey near the city of York. Mr Palmer is the narrator.

They climb a high tower which affords a view of the surrounding countryside. William Smith's eyes light up when he sees a line of hills in a distance.

'You are a coal mining man, Mr. Palmer, and for that you have my respect. Now if you were to go to those chalk hills and dig for coal, you would never find it. Never'

Though I had believed I understood Smith's explanations,this seemed a truly fantastic inference.

'But how can you know that Mr. Smith?  Those hills are more that twenty miles away'

'Think on it Mr. Palmer, think on it. You would need to dig through chalk,which is perhaps 500 feet thick, then through shale, limestones and red sand before you even got to the coal measures. You would be miles under the earth'.  He laughed at the thought of it.

'Can this be true?' I asked.

He turned to me, his face and eyes set strangely with a conviction that I had never seen on any man.

'Not "Can it be true?" Mr Palmer. It must be true. These are the laws of nature. They have no variance. What I seek is a philosophy which will embrace the whole earth. It cannot allow any exception'. 

The year is 1794. William Smith was optimizing mineral exploration strategy based on his new found understanding that the package of coal overlain by red sand, overlain by limestone then shale and then chalk will occur unvarying in this region.

A thousand such observations followed by countless inferences ultimately coalesced in to a rigorous science of geological mapping.

This is a lovely book.

Friday, March 20, 2020

Review Papers: Geodynamical Evolution Of India

Episodes, Journal of International Geoscience, has an open access special issue on the geology of the Indian subcontinent.

Excellent source for teachers, researchers, and curious science lovers.

I liked the paper on Deccan Volcanism a lot, especially the emphasis and attention given to the physical properties of the lava flows, and the problems of correlating (establishing their genetic and temporal relationships) lava sections from different parts of the Deccan Volcanic Province.

I don't know much about the Archean to Neoproterozoic age ( > 2500- 542 million years old)  southern granulite terrain, a region where very high temperature high pressure rocks known as granulites and charnockites are exposed. That is a topic I am looking forward to reading and learning about. The famous Anamudi Peak in the Western Ghats  are made up of these rocks. Geologists suspect that their high altitude is partly a result of differential erosion. Charnockites in particular are harder and have resisted being worn down, resulting in them standing out as high domes.

Another cool paper is on the role of microbial colonies on sedimentation patterns in the Proterozoic sedimentary basins of India (2500-542 million years ago). Microbial colonies grew as mats covering sediment surfaces influencing their accumulation and erosional patterns. Such environments became rare since Cambrian times (542 million years ago) when animals which eat and disrupt microbial colonies evolved.

Dive in.

Friday, March 13, 2020

Palghar Earthquake Swarm

My article on Palghar's mystery earthquakes has been published in The Wire Science. The article is an explanation of a recent paper that was published in the journal Tectonophysics. It favors the view that groundwater circulation is causing slippage along faults. According to the scientists involved the earthquakes are due to these very local processes.

One important point is that a link between groundwater and these tremors, even if it does exist here, represents a tipping point in a longer buildup of stress due to tectonic forces. The western margin of India is riddled by large fracture zones and faults. These structures haven't formed by groundwater movement. They are a legacy of earlier and ongoing crustal deformation due to regional and continent wide geological forces.  Groundwater flow or a build up of pore pressure cannot by itself generate enough stress to develop a fault de novo.

Dhundhalwadi is experiencing what is known as an earthquake swarm, a sequence of seismic activity with no clear peak (mainshock), and which is localised to one area. A recent study by researchers around India, including the National Institute of Seismology, has found one potential explanation for the swarm that draws a link between the monsoons, groundwater circulation and rock deformation...

Read more here.

Wednesday, February 26, 2020

Articles: Dehradun, Early Dogs, Warm Blooded Dinos, Louisiana Delta

Sharing some links from the past few days:

1) Dehradun.

A story of the transformation of a beautiful hill town to an ugly unplanned urban center. We shrug resignedly at many such tales from across the country. This one is of Dehradun. Himalaya towns can only be described as disasters in the making. Unscientifically built infrastructure on steep slopes, no garbage management resulting in enormous stray dog and pig populations roaming the streets, and a dwindling water supply. Yet these towns continue to grow, pointing to worsening opportunities for making a livelihood in the Himalayan rural landscapes. The 'smart city' reference is the ultimate insult of all.

Vanishing landscape of ‘smart city’ Dehradun.

2) Early Dogs.

The early stages of dog domestication may have seen a marked behavioral shift appearing before any distinct morphological change. This change in behavior, arising from docile wolves or 'protodogs' living near human camps would have entailed a change in diets.  Scientists have compared wolf and dog like remains from a 28,500 year old site in the Czech Republic. They looked at the dental microwear pattern of these two groups of canids and noticed that the dog like canids show a pattern consistent with eating more hard brittle foods. The wolves show patterns consistent with eating more flesh. 'Throw this dog a bone" wasn't an insult then.

Dental microwear as a behavioral proxy for distinguishing between canids at the Upper Paleolithic (Gravettian) site of Předmostí, Czech Republic

write up: Dog domestication during ice age.

3) Warm Blooded Dinos?

Were Dinosaurs warm blooded like mammals? This debate has raged on for decades. Bone growth patterns have not given any unambiguous evidence of body temperature regulation. A new method known as clumped isotopes may provided a more reliable indicator of estimating body temperatures. Fossilized dinosaur egg shells contain the original calcium carbonate from which these shells were built. A variety of carbon isotopes (C12, C13) may bond with a variety of oxygen isotopes (O16, O17, O18) in the carbonate molecules (CO3). The degree of bonding or clumping of the heavier isotopes i.e. C13 to O18 varies with the temperature during mineral growth. Clumping is more at lower temperatures.

Scientists compared this C13-O18 clumpiness in dinosaur egg shells with C13-O18 clumpiness in the calcium carbonate of mollusc shells from the same fossil bed. Mollusc geochemistry is taken to be a proxy for the ambient conditions. They found out that the egg shells grew at temperatures between 25- 43 deg C, while the molluscs record growth at 25 -30 deg C. This suggests that dinosaurs were capable of maintaining a higher body temperatures than their surroundings.  As a carbonate sedimentologist, I found the details of methods in this paper  of great interest. The researchers used a variety of techniques to make sure that the egg shells had not been altered or subjected to higher temperatures later in their history, which would have made them an unreliable archive of the original temperature during growth. The analyzed egg shells came from Sauropods, Theropods and Ornithischians, a sample across the three main groups of dinosaurs. Very interesting study.

Eggshell geochemistry reveals ancestral metabolic thermoregulation in Dinosauria

write up - Fossil Eggshells Suggest All Dinosaurs May Have Been Warm-Blooded

4)  Eroding Louisiana Coastline.

Over the past several decades, barrages and levees have drastically reduced the amount of sediment that the Mississippi river is carrying to the sea. As a result, the famed Mississippi delta and coastline is eroding away. Efforts are on in a Boston warehouse to figure out a way to reverse this change. An ambitious engineering project which aims at opening up a portion of the levee to funnel sediment into the Barataria Basin south of  New Orleans is being planned. The hope is that the new channel will transport and deposit enough sediment to rebuild part of the endangered delta. A scale model built in a warehouse near Boston is testing the efficacy of this idea.

Fascinating to read the various problems geologists and engineers have to deal with when grappling with modifying nature at this scale.

To Save Louisiana’s Vanishing Coast, Build a Mini Mississippi Near Boston.

Monday, February 17, 2020

Remembrance: Dr V.G. Phansalkar, Palaeontologist

In the February 2020 issue of the Journal of Geological Society of India, Anand Kale has written a very nice tribute to his mentor Dr. V.G. Phansalkar who passed away earlier in December 2019.  It captures very well both the professional aspects of Dr. Phansalkar's career and his endearing personal nature. Dr. Phansalkar was a paleontologist and a stratigrapher who taught with distinction at Banaras Hindu University, University of Pune (now Savitribai Phule Pune University) and Sholapur University ( renamed Punyashlok Ahilyadevi Holkar Solapur University) from 1962 until his retirement in 1999.

Photo credit: Anand Kale. 

I was fortunate to interact with him extensively when I was studying for my masters degree at the University of Pune . He was an original thinker with a penchant for asking that awkward question which you had not thought of or which you were hoping no one will ask. At our department seminars,we students always waited for that moment when Dr. Phansalkar raised his hand to inquire about some aspect of the presentation that he thought could be explored in a new direction. A spirited debate always followed! There was never any malice in his actions, just genuine curiosity and a wish to share his perspective.

During my time as a student in Pune, stratigraphy and sedimentary geology were subjects that were placed in two separate bins and taught as such. Stratigraphy deals with the way strata (sedimentary layers) are laid down and the relationships of bundles of strata across time and between those deposited in different locales at the same time. It is a hugely important subject, providing many of the organizing principles for piecing together the geologic history of a region. Unfortunately, Dr. Phansalkar did not get to teach us this subject. The lecturer who was assigned to teach it did a hack job of it with an insane emphasis on memorizing rock unit names from different parts of the country. Enlightenment came during our paleontology coursework. By interleaving palaeontology and stratigraphy in his lectures Dr. Phansalkar guided us towards understanding stratal layering patterns, sedimentary rock properties, and fossil occurrences as interrelated outcomes of the way sedimentary basins get filled up. I now realize it was an early jargon free introduction to sequence stratigraphy!

Years after my graduation from Pune, on a holiday from my PhD work in the U.S., I paid a visit to my old geology department.  In a conversation with Dr. Phansalkar I happened to mention that I was teaching sedimentary petrology as part of my assistantship duties. He looked at me for just a moment, leaned towards a drawer, and placed a box of limestone thin sections in my hand. Use these to teach he said. They were part of his precious collection of samples from the Cretaceous age sedimentary rocks exposed in the area around Ariyalur in Tamil Nadu. My lab benefited enormously from his thoughtful gift.

His home in Pune is quite close to where I live. In recent years we bumped into each other quite often during his evening walk in the park and on the nearby hill. Long geological stories peppered with humorous anecdotes became a welcome addition to my evening routine. I will miss that now that he is gone.

People often ask me why I took up science outreach. I have no hesitation in saying that it was in no small measure due to educators like Dr. Phansalkar who taught me that knowledge sharing is an immensely fulfilling endeavor to follow.

Friday, February 7, 2020

Sea Water Chemistry Triggers For Evolution Of Biomineralization

Geological Processes and Evolution #20

The bulk of the shells and skeletons of marine creatures are built out of aragonite or high-Mg calcite (> 4 mole% MgCO3) or low-Mg calcite. These three calcium carbonate minerals, along with dolomite (calcium magnesium carbonate), also occur as marine cements, i.e., they are precipitated from sea water as mineral grains in the open spaces between shell particles, resulting in loose sediment getting bound in to hard rock.

I came across this paper by Rachel Wood and colleagues from 2017 on the link between sea water chemistry and the evolution of biomineralization as evidenced in the limestone strata from Siberia. The time period is from 545 million years ago to 500 million years ago, a span in which early animals began secreting calcium carbonate skeletons. What were the main triggers for this evolutionary change?

Abstract:

The trigger for biomineralization of metazoans in the terminal Ediacaran, ca. 550 Ma, has been suggested to be the rise of oxygenation or an increase in seawater Ca concentration, but geochemical and fossil data have not been fully integrated to demonstrate cause and effect. Here we combine the record of macrofossils with early marine carbonate cement distribution within a relative depth framework for terminal Ediacaran to Cambrian successions on the eastern Siberian Platform, Russia, to interrogate the evolution of seawater chemistry and biotic response. Prior to ca. 545 Ma, the presence of early marine ferroan dolomite cement suggests dominantly ferruginous anoxic “aragonite-dolomite seas”, with a very shallow oxic chemocline that supported mainly soft-bodied macrobiota. After ca. 545 Ma, marine cements changed to aragonite and/or high-Mg calcite, and this coincides with the appearance of widespread aragonite and high-Mg calcite skeletal metazoans, suggesting a profound change in seawater chemistry to “aragonite seas” with a deeper chemocline. By early Cambrian Stage 3, the first marine low-Mg calcite cements appear, coincident with the first low-Mg calcite metazoan skeletons, suggesting a further shift to “calcite seas”. We suggest that this evolution of seawater chemistry was caused by enhanced continental denudation that increased the input of Ca into oceans so progressively lowering Mg/Ca, which, combined with more widespread oxic conditions, facilitated the rise of skeletal animals and in turn influenced the evolution of skeletal mineralogy.

Dolomite abundance through geologic time shows a positive correlation with periods of ocean anoxia. One reason could be that sulphate reducing bacteria which thrive in anoxic environments remove dissolved sulphate which interferes with dolomite formation. A 'shallow oxic chemocline' means that only the shallows were oxygen rich, while deeper water were oxygen poor or anoxic. These conditions changed after about 545 million years ago with increasing oxygen in even deeper waters thus increasing habitat suitable for the evolution and spread of oxygen demanding animals. Sponges may have played an important role in the ventilation of the water column by actively removing suspended organic matter during filter feeding, thus making more oxygen available to be transferred to deeper waters.

The terms "aragonite-dolomite seas", "aragonite seas" and "calcite seas" refer to geologic time-bound conditions facilitating the precipitation of marine cements of that mineralogy. Excessive magnesium is a hindrance to formation of calcite and a lowering of Mg/Ca meant a shift from "aragonite seas" to "calcite seas". From Cambrian to recent times, periodic swings in Mg/Ca of sea water has caused either aragonite or calcite to become the dominant marine precipitate.  

It is notable that the mineralogy of skeletons when they first evolve in a particular animal group seems to be determined by the prevailing sea water chemistry. Animal groups like the molluscs which acquired the ability to biomineralize during 'aragonite-high Mg calcite seas' of the late Ediacaran -Early Cambrian (550-520 million years ago) used these minerals to build their skeletons. Later in the Paleozoic, sea water chemistry changed to favor the precipitation of low Mg calcite. Animal groups like the trilobites, echinoderms, brachiopods and tabulate corals that first evolved skeletons during this time period (Early Mid Cambrian to Ordovician, ~520-450 million years ago) began using low-Mg calcite as their shell mineral.

The graphic shows the first appearance of carbonate skeletal groups with their inferred primary mineralogy plotted against the temporal distribution of aragonite and calcite seas (inferred from marine cements).


Source: Susannah M. Porter 2010: Calcite and aragonite seas and the de novo acquisition of carbonate skeletons.

Interestingly, once acquired, animals did not switch their shell mineralogy to match subsequent changes in sea water chemistry. Most aragonite shell secreting animals retained this mineralogy during later 'calcite seas' (e.g. Ordovician to early Permian and Jurassic-Cretaceous) and vice versa ('aragonite seas'- Permian-Triassic, Cenozoic). A wholesale change in skeletal mineralogy may require too many evolutionary steps and would be physiologically demanding. Conserving mineralogy even during changing ambient conditions is likely an evolutionary trade off.

One question remains unanswered. There is evidence as early as 560 million years ago of soft bodied animals making tracks and burrows on the sea floor. If sea water chemistry then was conducive for the precipitation of early dolomite, why didn't at least some early animal groups make skeletons out of dolomite? Perhaps the answer lies in mineral kinetics. Dolomite is slow to precipitate. Its atomic structure is made up of layers of calcium carbonate alternating with layers of magnesium carbonate. This is more difficult to build than the relatively simpler structures of aragonite and calcite which are made up of only calcium carbonate with a few magnesium ions substituting for calcium.

In latest Ediacaran-early Cambrian times, as oxygen levels rose and animal diversity increased, ecologic interactions became more complex. The rise of predators and predator-prey arms races would have favored the evolution of a protective shell that could be assembled rapidly. Faster precipitating minerals like aragonite and calcite became the fixed construction material.

Open Access.

Tuesday, February 4, 2020

Articles: Herculaneum, Magma Ascent, Early Human Migration, Indian Cheetah

Some interesting articles on a variety of topics that I came across in the past few weeks.

1) What Really Happened at Herculaneum?

This off course refers to the violent eruption of Mount Vesuvias in 79 A.D.  A new study analyses the way bone and soft tissue react to extreme heat and proposes that the people found dead at Herculaneum did not vaporize but died of asphyxiation.

2)  The long wait and rapid rise of deep magma.

Magma can reside in deep chambers at the boundary between the crust and mantle for thousands of years before rising to the surface rapidly in a matter of a few days.

3) Neanderthal Genes Hint at Much Earlier Human Migration From Africa.

It was thought that 60,000 years ago modern humans migrated out of Africa and interbred with Neanderthals beginning around 40,000 years ago. As a result all non-Africans carry some Neanderthal DNA. A new DNA analysis technique now suggests that an earlier wave of humans migrated out of Africa some 200,000 years ago and interbred with Neanderthals. Their descendants back migrated to Africa carrying with them the legacy of this earlier mating. As a result, Africans too carry a genetic legacy of Neanderthals.

4) Introduce the cheetah, with caution and guidelines.

There is a proposal to introduce the African cheetah into the Indian landscape. Neha Sinha argues that a grasslands policy needs to be put in place first.
 

Wednesday, January 22, 2020

Sedimentary Structures: Building Stones of Badami, Aihole And Pattadakal Temples

Is this sandstone slab in its original geological orientation (as when the sedimentary layers were deposited) or is it upside down? I'll answer this a little later, but first some background.


I recently visited the Chalukya style temples and rock cut monuments at Aihole, Pattadakal and Badami (6th -8th CE) in northern Karnataka and noticed some great sedimentary structures in the building stones. The term sedimentary structures refers to the shape and form sedimentary layers get sculpted into by the action of waves, currents, tides and wind during deposition of the sediment. The size of the deposited sedimentary particles and the orientation of layers are a reflection of both the vigor of the currents and waves and the direction of flow of water or wind.  


These monuments are made up of Neoproterozoic age (900-800 million year old) sandstones. Geologists have recognized using detailed sedimentological analysis that the sandstones formed mostly in a large braided river system that flowed in a northwesterly direction.

Between  roughly 1800 -800 million years ago, over the course of a billion years, the Indian continental crust sagged due to various tectonic forces to form several long lasting sedimentary basins. The Kaladgi Basin in which the Badami area sandstones were deposited is one such basin. The paleogeographic reconstruction below shows the position of the Indian continent at about one billion years ago and the location of the various sedimentary basins within it.


Source: Shilpa Patil Pillai, Kanchan Pande and Vivek S Kale: 2018: Implications of new 40Ar/39Ar age of Mallapur Intrusives on the chronology and evolution of the Kaladgi Basin, Dharwar Craton, India.

Much of this deposition took place in inland or epeiric seas that flooded the Indian continent. During intervals of sea level fall, rivers carved valleys and deposited coarse sediment. The Badami Cave sandstones are river deposits of the Kaladgi Basin. The stratigraphic column shows various sedimentary deposits of the Kaladgi Basin and their inferred environments of deposition.

Source: Shilpa Patil Pillai, Kanchan Pande and Vivek S Kale: 2018: Implications of new 40Ar/39Ar age of Mallapur Intrusives on the chronology and evolution of the Kaladgi Basin, Dharwar Craton, India.

The Badami braided river system was receiving sediment eroded from Archean age (>2.5 billion year old) rocks situated SE of the basin. These were granites, granodiorites, and low to medium grade metamorphic  rocks of the Dharwar craton (a large block of stable old continental crust). 

Land plants did not exist then. Weathered debris was moved quickly by surface flow into streams. Large sediment load, moving by traction i.e. by rolling and sliding on the stream bed, repeatedly chocked the channels, forcing bifurcation of streams and formation of braids. Very broad braided rivers formed since there were no plants to stabilize banks.  The Badami sandstones (Cave Temple Formation) are technically known as arenites. This term indicates that the rock is made up of mostly coarse sand with very little finer sized mud. Accumulation of mostly coarser sand size and pebbly particles reflects a locale of repeated high discharges and vigorous currents which winnowed away the finer sized mud.  The braided river shown below as an example is from the Canterbury Plains of New Zealand.


 Source: Braided Rivers: What's the Story?

The Badami rocks preserve a record of  various subenvironments of this paleo-river. Picture shows channel and bar deposits in outcrop.


Source: Mukhopadhyay et. al. 2018; Stratigraphic Evolution and Architecture of the Terrestrial Succession at the Base of the Neoproterozoic Badami Group, Karnataka, India.

As river channels episodically migrated sideways and the basin floor subsided to accommodate more sediment, channel deposits and adjacent sand bars got stacked to form thick 'multi-story' sandstones. Each bed tells a story of a discrete depositional episode.


The arrangement of sand layers within each bed tells us about the subenvironments in which it formed and the energy and direction of water flow during deposition. I came across many types of these internal structures. I recognized tabular cross beds, trough cross beds, planar lamination and rippled beds. Water (or wind) can move & shape sand into piles or waves. Sand grains roll along the direction of flow, then avalanche down the steeper side (lee side) of the wave forming a layer inclined (cross) to the orientation of the main sand body. Successive avalanches form a set of cross beds. The graphic shows the formation of a set of cross beds.

 Source: Dr. Diane M Burns in Teaching Sedimentary Geology in the 21st Century.

Here is an example of cross beds from near the town of Badami.


And this one is from a building stone from Pattadakal temple.
 

Such cross beds were built by sediment avalanching on the lee side of migrating sand bars during high flow.

This picture show trough cross bedding from near the Badami cave complex. These represent the internal structure of migrating sinuous sand dunes on a channel floor. 


See this elegant explanation by Dawn Sumner, a sedimentologist at the University of California at Davis,  of how trough cross beds form.



Email subscribers who may not be able to see the embedded video, click on this link: Trough Cross Bedding Video.

And here is a beautiful example of trough cross bedding found in a Pattadakal temple building stone.


This is planar lamination on a slab at Pattadakal. The bed is constructed of parallel layers of coarse sand. It is interpreted to have been deposited in a high flow regime from sheets of water flowing over mid channel sand bars.
 

Ripples on a slab at Pattadakal. This is a rare preservation of a bedding surface showing rippled sand. Erosion usually cuts off the wavy upper part. These ripples indicate migration of small sand waves in a quieter flow regime on the channel floor.


Remember, cross beds are the inclined layers that form on the lee side of a ripple or wave or dune. Here are small cross sets on the floor of Aihole rock cut temple! These represent the cross beds formed by migration of small ripples. The ripples themselves have been eroded away. Arrows indicate the direction of water flow and cross bed accretion as ripples migrated.


Okay, let's go back to my first question. Is the slab I showed in the picture geologically upside down?

Yes it is. But how to tell?

As sand avalanches down the lee slope it forms a tail at the toe of the slope resulting in cross beds which become tangential to the floor. In picture the cross beds are tangential towards the top of slab i.e. that is actually the base.


Lets see at how the cross bed contact with the top and bottom bedding plane looks in an outcrop. Here is the original depositional orientation of cross beds manifest in this outcrop near Badami caves. They show a tail or tangential contact of the cross beds with the base. Since top of cross beds are not usually preserved they show a high angle contact truncated by upper bedding plane.


This slab is upside down too! Notice again the tangential contact of the cross beds (white arrow) is towards the top, which means that must have been the base. Yellow arrow points to high angle contact with the upper bedding surface. 



Towards the top of the exposed section of sandstone around Badami I came across some truly impressive examples of cross bedding. These particular exposures were on the crags opposite the four main Badami temples. There is a narrow passage past the archaeological museum and a short climb to the top. Take a look at these beauties!


These large cross beds reminded me of the inclined beds of wind blown sand dunes. Is it possible that abandoned sand bars were sculpted by wind in to big dunes? Or does this upper level sandstone represent, as a recent study suggests, the beginning of a marine incursion in to the basin? In this scenario, deposition of sand took place in high-energy shallow waters near the shore. These cross beds represent large migrating sand waves which were eventually shaped in to beach ridges and tidal bars.

The outcrops and building stones of these monuments mostly record the processes within the Badami braided paleo-river. 900 million yrs ago a complex of channels and bars, quieter pools and rippled sand beds existed where these temples stand today.





Do visit Aihole, Pattadakal and Badami and gaze at its splendid architecture and sculptures. But spare some time to appreciate the magnificent record of our natural history that these monuments preserve. 





Quiz- Is this slab upside down or in its true depositional orientation? 😉





Until next time....