Monday, May 24, 2021

Books: Earliest Societies, Early Medieval India

 You didn't think I would stop at just three books, did you? More were delivered few days back.

1) Was the transition from a hunter gatherer lifestyle to agriculture a prerequisite for the formation of complex societies? James C.Scott explores this link between sedentism, domestication and state formation. The Sumerian Ur city state that formed around 3800 B.C is one example of this. New archaeological discoveries are hinting at complex societies of antiquity greater that the agriculture linked complexes that came up in the fertile crescent.  Sites like Gobekli Tepe in Turkey may make us reexamine our assumptions regarding the causes and timing of the formation of early states. Besides this book, I will recommend this essay by Samo Burja - Why Civilization is older than we thought


2) In the past year, I read four fine books on Indian history covering the time span from the 1000's to about the 1750's. India in the Persianate Age 1000-1765 and A Social History of the Deccan, 1300-1761: Eight Indian Lives, both by Richard M. Eaton. The Emergence of the Delhi Sultanate by Sunil Kumar who sadly passed away recently. And the fourth was The Mughal State 1526-1750. This is a collection of essays collated by Muzaffar Alam and Sanjay Subrahmanyam with a long introductory critical essay on Mughal historiography by the two editors. I thought it was time for me to explore the few centuries preceding the arrival of Central Asian Turkic invaders. The Making of Early Medieval India and The Early Medieval in South India look like good introductions to this time period. 

Tuesday, May 18, 2021

Books: Animal Minds, India Language History, India Governance

 New on my book shelf:

1) This came highly recommended from science Twitter. Peter Godfrey-Smith has surveyed a wide section of the animal kingdom and writes about the evolution of sensory experiences in different species. Sponges, corals, worms and octopus all manipulate the environments in specific ways. Disparate evolutionary pathways to be sure, but they all inform us about the origins of our mental capacities. 




2) I had a brief introduction to this book over a chickoo milkshake when the author M. Rajshekhar had visited Pune couple of years ago. He has spent several years traveling across India, surveying both big cities and the rural regions. His Ear to the Ground project resulted in scores of articles on India's everyday economy and the general failure of governance in this country. Its good to see some of his work distilled into this book.




3) Live History India has a really good interview with Peggy Mohan about her new book on India's language history. This is always a fascinating topic, as it tells us so much about population history, their origins, migrations, and intermingling. There is a section on Marathi too, and I'm looking forward to learning about that.

Monday, May 10, 2021

Amorphous Precursors To Calcite Cements

Readers of this blog, I am sure, are familiar with terms like Agate, Jasper, Onyx, and Opal. Out of these, Opal is an amorphous variety of silica, where the silica and oxygen atoms are not attached to each other in a regular repeating geometrical pattern. Agate, Jasper, Onyx are varieties of silica that can show gradations from an amorphous form to being cryptocrystalline i.e. made up of tiny crystals. All these substances originate by hardening of a silica gel that congeals out of a silica supersaturated fluid which has separated from a magma, or from hydrothermal groundwater that has become enriched in silica by reaction with surrounding rock or soil. 

They occur as banded siliceous deposits, either as layers or as discrete nodules, in volcanic and sedimentary rocks. Amorphous silica can even be of biogenic origin. Planktonic creatures like Radiolarians have the ability to extract silica from sea water and use it to build its skeleton.

Posted below is a photomicrograph of a cavity in a sandstone filled with banded amorphous silica (center of picture). Notice the regular growth bands (left in plain polarized light)  and the silica fibers (in crossed nicols) that make up the fabric of the amorphous material. I happen to have this rock thin section in my collection , but unfortunately I don't know its provenance!

Transitions from amorphous to a fully crystalline silica (quartz) often occurs within the same rock cavity. Amorphous silica is quite stable and has been found well preserved in rocks hundreds of millions of years old.

In contrast, amorphous naturally occurring varieties of calcium carbonate seem to be exceedingly rare. In my more than two decades of following literature of sedimentary carbonates I have not come across a report of amorphous calcium carbonate cement or shell material. Until recently that is!

In the December 2020 issue of Geology, Sascha Roest-Ellis, Justin V. Strauss and Nicholas J. Tosca suggest that certain types of microspar cements in Tonian age Neoproterozoic limestones (~650 million years old), formed from an amorphous precursor stage. These microspar cements (fine grained calcite) are quite common in rocks of this time period, yet their mineralogical evolution and the geochemical conditions under which they formed is poorly understood. 

In an effort to understand the origin of these cements, synthetic sea water was prepared of a composition that was similar to that measured from fluid inclusions trapped in Neoproterozoic salt deposits. The finding was that the presence of PO4 above a value of 12 micromoles per liter inhibits the nucleation of crystalline forms of calcite and permits deposition of an amorphous Ca-Mg- Carbonate by production of dense liquid droplets once carbonate supersaturation exceeds a threshold value. Neoproterozoic sea water was rich in PO4 as evidenced by the trapped fluid inclusion composition and by calcium phosphate biomineralizing organisms of that age. 

The texture and chemistry of these microspars also suggest an amorphous precursor. The crystals have spheroidal cores which are likely remnants of immiscible liquid/gel particles that would have initially separated out from carbonate saturated sea water. The grain size distribution points to crystal growth by  Ostwald ripening, a process whereby smaller gel particles or droplets disaggregate and the chemicals are reconstituted into larger growing crystals. Furthermore, these calcites have an enhanced strontium content. Usually that occurs if they have originated from an earlier aragonite phase. But there is no sign of relict aragonite in these cements. An alternate explanation is the incorporation of strontium into an amorphous carbonate which also favors intake of strontium.

The amorphous phase does not exist today in these limestones, having recrystallized to a variety of calcite fairly rapidly, perhaps even within a few days or weeks of it forming. The photomicrograph below shows the microspar calcite hypothesized to have recrystallized from an earlier amorphous phase.

Source: Experimental constraints on nonskeletal CaCO3 precipitation from Proterozoic seawater - Sascha Roest-Ellis, Justin V. Strauss and Nicholas J. Tosca, 2020.

As it happens I've had two strikes in the past month. Subir Sarkar and colleagues in their analyses of the Cretaceous age Garudamangalam Sandstone from Ariyalur in Tamil Nadu mention that some cavity filling calcite cements developed from a gel, by which I assume they mean amorphous calcium carbonate.  But they don't pursue this aspect any further in their study. 

The absence of amorphous calcium carbonate is limestones, both ancient and recent, is likely because it forms under only very restricted chemical conditions where nucleation of aragonite and high magnesium calcite is inhibited by the presence of ions like PO4, and because its high reactivity results in it transforming quickly to crystalline calcite, erasing itself from the rock record. 

Geological discovery relies heavily on direct observation and measurements of rock/mineral material. But what about ephemeral substances? How does one imagine them and tease out their history? This study highlights the importance of experimental work in geology, where careful laboratory reconstruction of past conditions can throw light on mineralization pathways that have left no physical trace behind.

Wednesday, May 5, 2021

Mass Extinction, Peopling Of America, Tale Of The Horse

 Sharing some interesting items:

1) What was the impact of Deccan Volcanism on the end-Cretaceous mass extinction? Improved dating of the timing of volcanism shows that volcanism spanned the mass extinction. But what changes occurred to marine environments because of the outgassing wasn't well documented. A new study uses the oxygen isotope ratios in foraminifera shells to estimate ocean temperature changes before and after the mass extinction. The finding is that the oceans warmed well before the extinction but cooled back again. The warming event doesn't appear to correlate with marine extinctions. Rather the mass extinction coincides with evidence for a meteorite impact. 

Here is a figure from the paper on the estimated temperature changes collated using a variety of proxies:

Source: On impact and volcanism across the Cretaceous-Paleogene boundary

Joshua Sokol has written a good summary of the paper:

A Rapid End Strikes the Dinosaur Extinction Debate.

2) Anthropological geneticist Jennifer Raff has pieced together the genomic story of the peopling of the American continents in this really insightful article. Do read it!

Genomes Reveal Humanity’s Journey into the Americas.

3) And next, onwards to a bit of Indian history. A very interesting conversation between Live History India editor Mini Menon and author Yashaswini Chandra on Ms. Chandra's new book, The Tale of the Horse: A History of India on Horseback. Fascinating story of the horse trade from Central Asia into India and its assimilation as a war animal and into Indian society. 

The Tale of the Horse (video). 

Monday, May 3, 2021

Cretaceous Cauvery Basin Stratigraphy

In the second year of my bachelor's degree course, a few of us friends had gone fossil hunting near the town of Ariyalur in Tamil Nadu. Ariyalur sits on Cretaceous age sediments deposited in a basin that formed as India broke away from Antarctica and Australia. The basin got filled slowly over time, by sediments brought in by rivers, as well as in the marine realm, as the sea episodically kept encroaching on to the continent interior. 

Before leaving for the trip we had approached Dr. V.D.Borkar, a research scientist with the Agarkar Research Institute in Pune, to help us plan the fossil collection. He very generously lent us maps and gave us a detailed idea of the villages to travel to and nearby field locations. 

All in all it was a fun field trip. We roamed the countryside around Ariyalur and collected plenty of fossils. In our collection were plant impressions on clay, ammonoids, belemnites, echinoids, coral fragments, and a variety of bivalves. The non geology highlight was the absolutely delicious vegetarian thali meal served in the canteen next to the town bus station! We used to gorge on it everyday, twice a day.

At that time I didn't have a good understanding of stratigraphy and even sedimentary geology. As it happened I did not grasp the broader implications of the distribution of particular fossils and the arrangement of strata that I was observing in the field. 

Its never too late to update yourself! The past month I have been reading three papers on the Cretaceous outcrops around Ariyalur which focus on basin development and stratigraphic evolution. In simpler language, stratigraphic evolution means the patterns by which basins fill up. A closer look reveals that basins are not made up of uniform continuous layers (layer cake stratigraphy) of one sediment type succeeding another, but rather there is lateral interfingering of different types of sediment, controlled by sediment distribution patterns, water energy, and basin topography.  

There are exogenous influences too. A long term drop in sea level will result in a particular arrangement of strata known as 'progradation', formed for example when deltas build out in to the sea. This may be followed by a long term sea level rise forming an overlay of a different sedimentary pattern, called  'retrogradation'. In this case as the sea encroaches on land, coarser sediments that are deposited closer to the shore get buried under deeper water fine grained sediments  A sedimentary section from base to the top (older to younger) reveals in its sediment characteristics these changing environmental conditions.

Documenting these patterns in not as esoteric an exercise as it may seem to some. Such analysis is very keenly taken up during petroleum exploration.  One may find during outcrop mapping that coarse sand deposits (potential petroleum reservoirs) occur at repeated intervals and are juxtaposed against finer organic rich mud rocks (potential hydrocarbon source rocks). This then may become a guide for optimizing detailed exploration strategy in areas of the basin where strata are buried and can't be observed directly. Just such a situation occurs in the Cretaceous Cauvery Basin. The sediments around Ariyalur is one of the main accessible outcrops. But further to the east, these sedimentary layers continue under the sea bed of the Bay of Bengal. A well documented and well understood outcrop provides an analogue for the unseen portions of the basin.

These three papers clarified to me much of the Cretaceous stratigraphy that I had failed to understand in my college days.

Here are the links:

1) Cretaceous tectonostratigraphy and the development of the Cauvery Basin, southeast India: Matthew P. Watkinson, Malcolm B. Hart and Archana Joshi

A broad study of basin formation by continental rifting and the resulting patterns of basin infilling interpreted in the context of tectonic events, major sea level fluctuation and depositional episodes.

2) Sea level changes in the upper Aptian-lower/middle(?) Turonian sequence of Cauvery Basin, India  An ichnological perspective: Amruta R. Paranjape, Kantimati G. Kulkarni, Anand S. Kale.

Ichnology is the study of trace fossils. These are tracks, trails and burrows made by the movement of  creatures living on the basin floor. Traces differ depending upon the nature of sediment substrate and environmental conditions and can be used along with other sedimentological and fossil data to interpret patterns of sea level change.,

3) Siliciclastic-carbonate mixing modes in the river-mouth bar palaeogeography of the Upper Cretaceous Garudamangalam Sandstone (Ariyalur, India): Subir Sarkar, Nivedita Chakraborty, Anudeb Mandal, Santanu Banerjee, Pradip K. Bose.

The Garudamangalam Sandstone formed during a sea level highstand i.e. at the peak of a sea level change cycle, when the rate of sea level rise finally slows down and stops. Rivers bringing in sediment from the east began building a delta. The exposed Garudamangalam Sandstone is part of this delta complex. This is a very nice analysis of sedimentary processes and products. The various subenvironments in this delta complex are identified and the chemical changes in the sediment after their deposition are documented using various techniques like chemical staining and cathodoluminescence. I really enjoyed reading this one!

On a personal note, the Covid catastrophe unfolding in India is making reading and writing difficult. However, I did find that a few hours of geology time that I am managing to hold on to brings me some comfort.