Tuesday, May 10, 2022

Field Photos: Dikes And Gneiss

My friends have been traveling across India and sending me pictures of landscapes and rocks. I am doing field work vicariously.

Posting below a few pictures that I have received.

Dikes Intruding Bundelkhand Gneiss. Pictures by Rajesh Sarde.

These two photos were taken at the Ken River gorge in Madhya Pradesh, near a gharial sanctuary. 

The dark rock making up the floor of the gorge is a dike. It has intruded the pink colored gneiss rock. Notice that the gneiss is fractured. Intrusions follow major weak zones in the gneiss.  Being softer than the gneiss, erosion over time has removed much of the dike, forming a narrow valley.

And in this picture, an arm of the dike known as an apophysis can been seen. It is almost at right angles to the gorge.


The Bundelkhand Gneiss ranges in age between 3.2 billion to about 2.5 billion years ago. The mafic dikes, igneous rocks rich in iron and magnesium silicate minerals, intruded later into the granite gneiss. Recent geochronological work on the dikes suggest two distinct events of dike emplacement, an early episode dated to about 2 billion years ago, and a later one at 1.1 billion years ago. Interestingly, the magnetic signatures frozen in these dikes have been used to make inferences about paleogeography. The results indicate that the north and south Indian crustal blocks which had independent origins were in close proximity by about 2.5 billion years ago. 

The magnetic signatures of the 1.1 billion year old dikes throw up a puzzle. They match those preserved in the Upper Vindhyan strata and intrusive rocks, seemingly constraining the age of the Upper Vindhyan sequence to around 1 billion years. However, recent fossil finds which I wrote about in a recent article for Nature India point to the Upper Vindhyans being much younger, about 550 million years old!

Dikes Intruding the Deccan Traps. Pictures by Rajesh Sarde.

These two photos were taken at the base of Tamhini Ghat, west of Pune, near a popular trekking spot known as Plus Valley. The rocks are about 66-65 million years old.

As in the previous example, the dike has eroded away faster than the host rock forming a narrow depression. Notice the closely spaced jointing pattern or cracks in the dike. 


 In this photo, the sharp boundary between the dike and the basalt rock can be clearly seen.


 Tonalite Trondhjemite Gneiss, Palolem Beach, Goa. Picture by Aneeha

These rocks, abbreviated as TTG, are relicts of early continental crust. They are about 3.4-3.2 billion years old. They represent Archaean age magmatism that formed the lighter continental crust. Such TTG's  are found all across India. They are the oldest component of cratons, the nucleus of the first continents. These magmas are generally granodiorites, rich in sodium and calcium feldspars and poor in potassium feldspars. They were deformed and metamorphosed subsequently in to a gneiss, in the process acquiring a characteristic banding. 

Next time hopefully pictures from my own field trips!

Monday, April 25, 2022

Earth Day Geology Conversations

It is really heartening to see students take an initiative in outreach. The Science Paradox is one such science outreach effort.  Do take a moment to browse through their website and subscribe to their newsletter.  

For this Earth Day 2022, they invited me for a conversation about geology. The interview was conducted by Anaya Tiwari and Bhargavi Nerikar.  We talked about my decision to take up geology, the importance of geosciences, the role of outreach, books, and my parallel existence as a sports coach.

The conversation has been broken up into seven short videos. I am embedding the section on Geotourism below. 

The complete set of links to the interview are as posted.

1) What drove you towards pursuing geoscience?

2) Importance of geosciences.

3) How do you think geosciences can be incorporated in our education system?

4) What are  your thoughts on science outreach in reaching the masses and how effective that is?

5) Science outreach through geotourism.

6) Book recommendations for the field of geology.

7) What was the motivation to shift from academia to sports coaching?

 It was fun!

Thursday, April 21, 2022

Neglected Children Of The Sea Floor

Carl Simpson and Jeremy B.C. Jackson, in an essay titled Bryozoan Revelations for Science Advances, sketch out the important biological aspects of bryozoans, a type of marine invertebrate. 

From their article: 

"Bryozoans are neglected children of the sea floor. Google corals and you get nearly 4 billion hits, whereas bryozoans get just 4 million. This disparity reflects the enormity and notable beauty of coral reefs and extraordinary diversity of associated species that have long attracted intense scientific research. Yet for all their grandeur, corals occupy less than a tiny fraction of 1% of the global ocean, whereas bryozoans extend from the equator to the poles and intertidal to abyss. Bryozoans are species-rich. As a group, they have at least 10 times more species than corals. They also have a more extensive and continuous fossil record and have been major components of vast seafloor communities for half a billion years".

The enormous diversity of bryozoan skeletal shapes and their abundance as living communities and as fossils from the late Cambrian onwards make it possible to use them to understand evolutionary questions such as timing of origins, rates of change, convergent evolution, and origin of variable form in animal communities. This succinct essay summarizes these lines of research quite well.

As a sedimentologist my interest in these organisms was the role they played as sedimentary particles. Bryozoans, along with echinoids and brachiopods, were among the most common sea floor inhabitants of Paleozoic shallow marine realms. They made up a large proportions of the sediment that formed by the disintegration of shells and skeletons of sea creatures.  

A typical bryozoan skeleton has a trellis like appearance. Bryozoans are colonial organisms building a lattice like structure with the animal living in chambers called zooids. It is these zooids that was my main interest, or rather what I could see inside them. The animal was long gone, decayed away, but the chamber or cavity was filled with different types  of calcite, archiving the process of the transformation of loose sediment into rock. Through geologic history different types of fluids had entered the open spaces within the skeleton and precipitated different types of calcite. Petrologists can infer the changing geochemical conditions as sediments get buried and importantly trace the changes in open spaces, or porosity, as fluids either dissolve sediment or deposit new minerals, information that is useful to the petroleum industry.

I will share a couple of examples of these diagenetic changes observed inside a zooid.

This thin section of a limestone from the Middle Ordovician strata of the southern Appalachian mountains has been stained using a mixture of Alizarin Red-S and Potassium Ferricyanide. The pink is an iron free calcite. The purple in the center of the zooids is an iron rich calcite. This sequence from an iron free to an iron rich calcite indicates oxygen poor reducing conditions upon burial, a chemical environment in which iron can enter the growing calcite crystal.

And in this thin section the pale bronze colored mineral highlighting the skeletal frame is chert, a variety of silica that has partially replaced the calcite skeleton. The replacement process has been quite delicate, preserving the original structure of the skeleton.

Evolution is not the only story that bryozoans reveal.

Wednesday, April 20, 2022

Kolar Gold Field

Geology and Livelihoods #19

I came across this excellent documentary on the Kolar Gold Field via Twitter. The film is directed by Basav Biradar and produced by Sahapedia.

Email subscribers who are unable to see the embedded video can watch it here- In Search of Gold.


Like many established mining towns, Kolar too saw generations of the same family work in the mines. Son followed father into the dark shafts. Mining provided employment, but it was dangerous back breaking work. The documentary highlights the lives of workers and their struggle for better work conditions. 

Kolar gold is late Archaean in age with mineralization taking place between 2700-2500 million years ago. There are two types of deposits. There is a "stratiform sulphide type", so called because the gold bodies are contained within iron sulphide rich volcanic and sedimentary layers. These deposits formed on the sea floor contemporaneous with volcanism and sedimentation. The second type is a hydrothermal deposit wherein mineralizing fluids mobilized and precipitated gold in veins along N-NE oriented fracture zones. This mineralization event occurred at a later stage when magmatism and metamorphism affected the host terrain.

But do watch this for the many personal stories of the people who worked the mines. 

Friday, March 25, 2022

Campus Memories: Prof. Sohan Modak

I came to know from media reports and Twitter posts that biologist Dr. Sohan Modak passed away on March 23rd. Although I never interacted with him directly I do have some distinct memories of his presence on the Pune University campus when I was completing my Masters in Geology in the late 1980's. On various occasions there were protests and strikes organized by faculty in opposition to some decision regarding University functioning. I don't remember Dr Modak ever making common cause with the rest of the faculty regarding going on a strike. He used to show up to the canteen with large posters which outlined his opinion of the situation. He then used to vociferously appeal to the faculty to get back to teaching. 

On the academic side, many of my college batch mates had opted for a Master's in Zoology. I started hearing a different language from them. There was less emphasis on the rigid divisions between Zoology and Botany. Instead, universal mechanisms underlying all life were being discussed. Gene regulation, chromosome structure, development. All this was new and alien to me, but exciting to hear about none the less. 

Dr. Modak had played a big role in the transformation of biology syllabus I was told. The various tributes now poring in bears this out. 

R.I.P. 

 

Thursday, March 17, 2022

Links: Noisy Soils, First Art, Mars Geology

A few readings for your perusal.

1) Biologists are poking senors into soil to listen to the hum of life. Amazing article by Ute Eberle on what we can learn from acoustic signals given off by animals living within a soil profile.

Life in the soil was thought to be silent. What if it isn’t?

2)  When was the first 'árt' made? Is there a neat sequence from abstract scratches on rock and bone to representational art that adorns the walls of caves? Excellent article by Amy McDermott on this question, bringing together the viewpoints of archeologists and cognitive scientists. 

What was the first “art”? How would we know?

3) One year on, NASA's Mars Perseverance has been drilling into rock, collecting samples and finding some surprises along the way. It will now head towards an ancient delta to look for past life! Alexandra Witze reports on the progress.

A year on Mars: How NASA’s Perseverance hit a geological jackpot.

 

Tuesday, February 22, 2022

Marking Vindhyan Time

About this time last year I wrote a piece for Nature India on the discovery of the fossil Dickinsonia from the uppermost Vindhyan strata exposed at Bhimbetka caves in Central India. Dickinsonia is considered to be an early animal that lived between 560 to 550 million years ago. This finding seems to confirm an Ediacaran age  (635-541 million years ago) for the youngest Vindhyan strata, termed formally as the Bhander Group. Earlier thinking was that these rocks are 1000 to 900 million years old! 

I have been thinking on and off about this discovery, not as much doubting it as trying to understand its implication from a different angle, that of the ways in which sediments accumulate in a basin and how the passage of time is recorded in sedimentary successions. 

The sedimentary rocks of the Vindhyan Basin have been subdivided by field geologists into four units based on characteristic sediment associations. From the oldest to the youngest, these are, the Semri Group, the Kaimur Group, the Rewa Group, and the Bhander Group.  Deposition of the Semri sediments began around 1700 million years ago and ended by 1600 million or so. Tectonic movements then uplifted, tilted, and eroded these rocks. Between 1200 million and 1100 million years ago the basin floor subsided and new sediment was deposited on the inclined layers of the Semri Group. There is thus a marked  'angular unconformity' between Semri and the overlying Kaimur strata, marking two distinct phases of basin history separated by 400 million years.

The age of Kaimur, Rewa and Bhander Groups, which are often referred to as the Upper Vindhyans, has been more difficult to pin down due to a lack of reliable radiogenic dating and age diagnostic biota. Despite this, a combination of magnetic properties, a few Uranium series dates from limestones, and the age ranges from zircon found in Bhander sandstones,  hinted that the youngest Vindhyan sediments are about 900 million years old.

The fossil Dickinsonia has upended this assumption and appears to have expanded the life of the Vindhyan Basin by a whopping 350-400 million years. This revision has been bolstered by two other age criteria. Microbial fossils typical of the Ediacaran Period have been reported from the Bhander. And secondly, in the year 2020, a detrital zircon dated to 548 million years ago was recovered from the Bhander. Zircons form in magmas or during high temperature metamorphic reactions. The mineral is then eroded away from these source rocks and deposited as a sedimentary particle in adjacent basins. The youngest zircon population in a sedimentary layer sets its maximum age,  since the strata cannot be older than the detritus it is made up of. It could be much younger than the contained zircon, but in this case a lack of Cambrian fossils has constrained the age of the Bhander to be older than 541 million years ago.

Let me now dive in to what has been pricking my geological curiosity. The Kaimur, Rewa and the Bhander are thought to be three distinct episodes of sedimentation. Stratigraphers may recognize them as 'depositional sequences' formed when accommodation space for sediment to accumulate is being created either by the basin floor sinking due to tectonic movements or due to eustatic (global) sea level changes. 

Such depositional sequences are building blocks of sedimentary successions in basins of all ages, each episode lasting at most few tens of million of years. V.S. Kale in a discussion note on Indian Purana basins points out that the physical characters of the sediments are not suggestive of slow sedimentation rates, and given their thickness, estimates that these sequences span 10-20 million years each. Even assigning the upper limit, Kaimur, Rewa and Bhander were deposited in about 60 million years or so. If Upper Vindhyan sedimentation began around 1200 -1100 million years ago and ended by 550-540 million years ago, that leaves about 550 million years of unrecorded elapsed time.

Sedimentation is an inherently episodic process with periods of non-deposition alternating with sediment accumulation. See this close up of an outcrop of a cross bedded sandstone from the Badami area in Karnataka.  


These sediments were deposited in a long lasting river system. Surfaces that result from non-deposition are marked as S1, S2.  In this hierarchical scheme, S1 spans perhaps a few minutes to a few hours and is a break in deposition on the lee side of  migrating ripples due to fluctuations in current energy. The S2 surface has developed over the channel sand body and spans months to years and represents seasonal fluctuations in currents or even a long drought which dries up the channel. 

 Now, cast  your eye on this larger outcrop of the Badami sandstone. 

 
It has formed by the stacking of smaller sandstone units, each containing several surfaces of non deposition. The higher order surface S3 in this thicker pile marks a longer period of non-deposition and indicates a shift in course of the river channel. Hundreds, even thousands of years will pass, before the channel migrates back to its original location and deposition resumes at that locale.

Although I may be amiss in my specific interpretation of these surfaces of non-deposition, what I want to convey is that sediment deposition in all types of environments is interrupted by periods of non-deposition ranging from a few minutes, to thousands, even millions of years. The vast majority of elapsed time a sedimentary succession represents may be accounted for by periods of non-deposition and erosion. 

The Kaimur, Rewa and Bhander collectively account for a 60 million year time-span. If the revised age range for the Upper Vindhyans is correct then there were gaps of hundreds of millions of  years between the deposition of these Groups.

But these very long breaks invariably result due to tectonic uplift and cause pronounced erosion and chemical alteration of the exposed sedimentary surface. In outcrop it is recognizable as an undulating surface with the debris of weathering consolidated into hard soils, and in the formation of solution pits in limestone terrains. Such basin wide surfaces of marked erosion have not been found separating the Kaimur and the Rewa, and between the Rewa and the Bhander strata. 

Smaller breaks though do occur. Chandan Chakraborty in a study of sedimentary cycles of the Vindhyan Basin finds an angular discordance between the Kaimur and the Rewa, and the Rewa and the Bhander. However, this is restricted only to the south of the Vindhyan Basin. The discordance between the older and younger sequences dies out towards the north. This stratigraphic relationship  suggests that the tectonic movements were contemporaneous with sedimentation. The basin floor was tilted up on one side interrupting sedimentation and eroding the exposed strata there, while at the other end (north), subsidence and deposition continued. The periods of non-deposition and erosion between these sequences was a more localized event, lasting few millions of years at most. The kind of tectonic upheavals that may halt deposition for hundreds of millions of years across the entire basin are simply not attested to in the stratigraphic record of the Upper Vindhyans. 

It seems to me, and I say this mischievously, that the earlier estimate of 1000-900 million year age of the uppermost Bhander strata and for the end of Vindhyan sedimentation sounds quite reasonable! 

How secure is the finding of Dickinsonia and the assigned Ediacaran age of the Bhander Group? I am in no way trying to rebut this finding. The microbial fossil Arumberia found in the Bhander is considered by experts to be a reliable Late Ediacaran age indicator. However, the identification of Dickinsonia is based on a 3D digital reconstruction of photographs taken at the site of discovery. Due to the protected status of the Bhimbetka Caves, the scientists were unable to excavate the impression and subject it to a physical examination or a chemical test for organic residue. This is the only report of Dickinsonia from the entire Vindhyan Basin.

The young 548 million year date of the detrital zircon is based on one grain. The researchers are confident of their analysis and see no reason to reject this data point, though they admit that further corroboration of this date will be necessary given that previous studies did not report such young zircons from the Bhander strata. Curiously, no source terrain of this young zircon is identified in the paper.  

The news about Dickinsonia received wide coverage in India. That was expected given the long standing uncertainty of the age of the Vindhyans and the importance of the fossil for understanding early animal evolution and Ediacaran ecology and paleogeography.  But these new proposed timelines have thrown up a puzzle about rates of accumulation of sedimentary sequences and magnitudes of intervening breaks. We are quick to appreciate changes in sedimentary layer color, their structures, and their geometry, and interpret these changes in the context of fluctuating depositional conditions. But what about surfaces of non-deposition? Do they have anything valuable to contribute to our understanding of geological processes. 

The paleontologists Niles Eldredge and Stephen Jay Gould once perceptively observed that 'stasis is data'. The unchanging morphology of many fossil species lineages tell us something about the mode and tempo of evolution. Geological surfaces of stasis, when sedimentation has come to a standstill, are richly informative too in their own way. Surfaces representing very long time-spans may develop during convulsions in the earth's crust and periods of mountain building. The soils that often mantle such surfaces archive information about past climate and terrestrial life. Breaks of smaller and smaller time-spans like wise have a variety of drivers such as orbital control on climate and sea level, migration of depositional environments, and even tidal cycles.  

How many more hiatuses of unknown duration lie hidden within the Vindhyan sediment pile?  They may mark phases when no sediment accumulated but they are as much an integral part of basin history as are the iconic sandstone faces of Bhimbetka.


 

Wednesday, February 9, 2022

Saved By A Projection

I just had to share this wonderfully imaginative piece of science fiction from xkcd comics.

Using map projections to alter our perception of geography is an old trick.

Friday, February 4, 2022

Human Impact On Earth's Sediment Cycle

One common type of argument I hear from anthropogenic climate change deniers is that human activity is too insignificant to affect the balance of global natural processes. On one debate a participant claimed that one large volcanic eruption emits more carbon dioxide than that by human activity. The actual amounts contradict this claim. Volcanism on earth emits about 0.13 -0.44 billion tons of CO2 per year. Human activity on the other hand emits about 35-40 billion tons of CO2 per year.

Jaia Syvitski and colleagues have produced a similar eye opening review of the human impact on earth's sediment cycle. The production, mobilization , transport, and deposition of sediment is based on a balance between tectonic processes, climate, erosion, and human activities. Our impact on sediment movement and its sequestration has now become so large that it dwarfs natural processes. 

The paper is open access for a limited time. Earth's sediment cycle during the Anthropocene

It is dense reading, full of numbers on sediment loads and fluxes.

"Human activities have increased fluvial sediment delivery by 215% while simultaneously decreasing the amount of fluvial sediment that reaches the ocean by 49%, and societal consumption of sediment over the same period has increased by more than 2,500%".

or: The Indus River once transported about 270 million tons of sediment to its delta. It presently deposits only about 13 million tons per year. So much of Indus water is siphoned off by canals, that it  often turns dry before reaching the sea.  

 and one more: "Large dams have trapped about 3,200 Gt of sediment since 1950 (ref.123), approximately 74% of which would likely have reached the coastal ocean". (Gt =billion tons)

There are many such stories from around the globe about the staggering amounts of sediment extracted and redirected for human use. Next time, don't shrug off the news you read about unregulated sand mining from our rivers. It is causing serious damage to riverine and coastal ecosystems.

The review ends with a proposal to set up a ‘Earth Sediment Cycle Grand Challenge’, a collaborative effort to better understand the changes to the sediment cycle. Such an initiative we surely need to address the many ongoing and future threats to our rivers and deltas.

Sunday, January 23, 2022

Errata : Old Carbon

 Dear Email Subscribers,

A correction to my last post on ancient human footprints from New Mexico. I mistakenly typed their estimated age as 22,0000 (Two hundred and twenty thousand). The correct assigned age in the paper I referred to is 22,000 (Twenty two thousand). 

Saturday, January 22, 2022

Links: Tonga Volcano, Old Carbon, Indian Palaeontology

 Sharing these readings.

1) My first impression of the massive Volcanic eruption in Tonga was the mushrooming ash plume seen in a satellite imagery. Over the days more sensors have captured additional information about this event. When it is safe, geologists will travel to the site to sample the volcanic debris and subject it to detailed textural and geochemical analysis to piece together the journey of magma from its source to its explosive entry on the surface.

Scientific American has a good summary - Why the Tonga Eruption Was So Violent, And What to expect next

2) Debate is an integral part of scientific progress. One common platform to engage in a critique and discussion with your colleagues is the Comment and Reply section in scientific journals. You can submit a note explaining the issues you have about a paper, and it is published along with the author’s response. I am across a good example of this in the journal Science. The topic was the recent announcement of roughly 22,000 year old human footprints from Lake Otero, New Mexico. These dates suggest that humans  were present in North America during the Last Glacial Maximum, a few thousand years earlier than what other data has indicated.

The debate revolves around the accuracy of dating these footprints. Of particular interest here is the problems one can encounter with carbon dating a sample. Living beings have amounts of the radioactive isotope carbon 14 in them in equilibrium with the atmosphere. After death, the amount of this isotope in organic tissue starts decreasing due to natural radioactive decay of carbon 14. Knowing the decay rate and measuring its proportion in the organic material gives us estimates of how old the sample is. But what if the source of carbon is old? For example,  it is from very old groundwater or from the bottom of a lake which is not exchanging gases with the atmosphere? The carbon 14 values in such a réservoir will be very low due to ongoing decay and no replenishment of newly formed carbon 14 from the atmosphere. If organisms consume carbon from such a reservoir (which contains carbon 12 and carbon 13 too) and then are sampled , they will be estimated to be older than they really are. 

The Comment and Reply focuses on this problematic aspect of recognising and correcting for the ‘reservoir age’ of carbon 14. Of pointed importance too is the context and location of collected samples. 

A very informative debate on the nuances of sampling and assigning ages. 

Comment- Évidence of humans in North America during the last glacial maximum

Reply- Evidence of humans in North America during the last glacial maximum

3) I have posted about this topic before. Thé challenges and triumphs of Indian palaeontology very well described by Kamala Thiagarajan in this recent article. 

Why India’s Fossil Wealth Has Remained Hidden

Previously, Sreelatha Menon had written about the lack of importance palaeontology is accorded in the earth sciences and the devastation this neglect is inflicting to palaeontology education, awareness, and research. Her essay is worth reading too; What do you do when palaeontology is itself endangered in India?