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.