Thursday, July 21, 2016

Conversations: Tempo Of Deccan Traps Eruptions

My friend V.V Robin who is an evolutionary biologist and researches the genetics of bird species along the Western Ghats also takes a keen interest in geology. He pointed me to this new work on the tempo of Deccan Volcanism;

Tempo of the Deccan Traps eruptions in relation to events at the Cretaceous-Paleogene boundary - Renne, Paul et al 2016; Presented at the meeting of the EGU General Assembly 2016, held 17-22 April, 2016 in Vienna Austria

Robin wrote- I found this quite cool - that all the volcanism was possibly within one million time frame! Must have also had a huge impact on the organisms living then.

But it was not clear from this where the epicentre was - "Poladpur or uppermost Bushe Fm., near the base of the laterally extensive Wai Subgroup"

Right and this is very important... this refers to when the style of magmatism changed in relation to the K-Pg boundary event. Within the enormous pile of lava, in the upper parts of the section subdivided based of geochemical signatures into the Bushe (older), Poladpur and Ambenali (youngest) formations lies the K-Pg boundary. See figure to the left (Source: Earth Magazine; credit K. Cantner AGI) which shows the chemical stratigraphy of the Deccan Basalts as defined primarily by sampling the Western Ghat sections. The change in magmatism style is somewhere near the top of the Bushe Formation. Lava of the entire Wai Subgroup may have erupted in just a few hundred thousand years.  It is key to know at what stratigraphic level the K-Pg boundary is. If the boundary lies say near the base of Bushe, then this would strengthen the hypothesis that the Chicxulub impact triggered a more effusive phase of Deccan volcanism. In this scenario, since it would have taken hundreds to thousands of years before volcanic rates increased, the more effusive phase of volcanism may not have caused the mass extinction, but played a role in stifling recovery of ecosystems for hundreds of thousands of years after. On the other hand if the boundary lies higher up, say somewhere in upper part of Poladpur or Ambenali, then the implication would be that the more effusive phase of volcansim started before the impact and that would mean a more direct causal link between volcanism and mass extinction.

Robin though had read the abstract a little differently and he answered back -

So, they dont really say much about WHERE the epicentre was, as suggested in the initial part of the abstract. Maybe that's ongoing research. Very exciting stuff! This would certainly be interesting to folks studying older taxa - centipedes, frogs etc

He wanted to know where the volcanic epicentre was geographically. He was referring to these sentences from the abstract -

 40Ar/39Ar geochronologic study is providing the first indications of variable time-averaged eruption rates in the important Western Ghats region, in addition to providing the first precise location of the KPB within the Deccan pile

" radioisotope geochronology has failed to clarify the tempo of the eruptions or to delineate where the KPB age-equivalent horizon occurs within the eruptive sequence"


 Well, in the language of stratigraphy the authors are not referring to a geographic place but where in the stratigraphic sequence.  They are referring to its temporal position .

Geographically, the K-PG age equivalent horizon, meaning a lava flow which erupted 66.04 million years ago (coincident with the Chixculub impact) could be preserved anywhere. Near Lonvala, near Mahabaleshwar, in southern Konkan or Goa or all of these places. We haven't found it yet, but it could be a matter of time, with better radiometric resolution now allowing finer and finer sampling.

Off course it is a perfectly reasonable question to ask if there was an geographic epicenter of Deccan volcanism. Here is a cross section of the Western Ghat with formation boundaries and structural features.



Source: M Widdowson  and K.G Cox 1996

Note that there is a large vertical exaggeration (40X) to the profile which is about 650 km in length. The long wavelength fold and dips are not apparent at outcrop scale and have been revealed through accurate measurements of topographic levels of formation boundaries. What you do see is that the older formations (Kalsubai Subgroup)  are to the north and the younger to the south (Wai Subgroup). Is this due to a southerly migration of volcanism or is this due to deeper erosion of the lava pile to the north which has stripped away younger lavas and exposed the older section of the lava pile?
 
Likely there was not one epicentre of volcanism in the Deccan volcanic province. Rather, there would have been many many eruptive centers spread out along the N-S oriented western rifted margin and the E-W oriented Narmada Tapi rift zone and more in the central parts of the plateau too. Within the Western Ghat section there have been some suggestions that the lava thickness and laterite geochemistry might be consistent with the idea of a south moving focus of volcanism during the eruption of the Wai Subgroup. But on a larger scale, taking into account the entirety of the province it is hard to make a case of any one epicentre.

Take a look at this map with plotted ages of Deccan basalts. Younger and older basalt ages are scattered all over.


Source: Hethu Sheth - The Deccan Beyond The Plume Hypothesis

The Western Ghats have been the most intensely studied and sampled region of the province and hence its age distribution is now better constrained. But the volcanic province is spread over large areas in Madhya Pradesh, Gujarat, eastern Maharashtra and importantly a large area under the Arabian sea covered by hundreds of feet of Cenozoic sediment.

Here is a map which shows the distribution of selected Deccan lava sections beyond the Western Ghats. Many sections to the north, northeast and the east contain the Bushe, Poladpur and Ambenali Formation lavas which are thought to span the K-Pg boundary. Geochemical work has identified dykes feeding these lavas, and they are distributed over wide regions. A few have been identified in the Narmada rift region which may have erupted upper formation lavas present in the north and northeast of the province. There is also a cluster between Nasik and Pune and between the coast to Sangamner on the plateau to the east of the Western Ghats also. Some of these feeder dykes likely gave rise to the bulk of the Western Ghat upper formations. So, eruptive centers of lavas spanning the mass extinction was occurring in regions well away from the Western Ghat region too, suggesting there wasn't any one epicenter of volcanism during this time.


 Source: The Feeder System of the Deccan Traps (India): Insights from Dike Geochemistry

..to be continued as more questions come in!

Wednesday, July 13, 2016

Neoproterozoic Atmospheric Oxygen

Captured from air bubbles trapped in 815 million year old Halite crystals:

We present a new and innovative way of determining the oxygen level of Earth's past atmosphere by directly measuring inclusion gases trapped in halite. After intensive screening using multiple depositional, textural/fabric, and geochemical parameters, we determined that tectonically undisturbed cumulate, chevron, and cornet halite inclusions may retain atmospheric gas during crystallization from shallow saline, lagoonal, and/or saltpan brine. These are the first measurements of inclusion gas for the Neoproterozoic obtained from 815 ± 15–m.y.–old Browne Formation chevron halite of the Officer Basin, southwest Australia. The 31 gas measurements afford us a direct glimpse of the composition of the mid- to late Neoproterozoic atmosphere and register an average oxygen content of 10.9%. The measured pO2 puts oxygenation of Earth's paleoatmosphere ∼100–200 m.y. ahead of current models and proxy studies. It also puts oxygenation of the Neoproterozoic atmosphere in agreement with time of diversification of eukaryotes and in advance of the emergence of marine animal life

Oxygen is tied to the evolution of complex life on earth. Early earth contained little free oxygen in the atmosphere and the oceans. Life was made up of two divisions of prokaryote cell types, the Archaea and the Bacteria. The activities of one type of aerobic bacteria, Cyanobacterial photosynthesis, eventually triggered the Great Oxygenation Event by 2.5 to 2 billion years ago and began raising the levels of free oxygen in the atmosphere and the oceans. Cyanobacteria are cells capable of burning fuel in this free oxygen.  Between 2 billion and 1.5 billion years ago this type of oxygen utilizing cell merged with an Archaea cell to form a large complex cell type known as the eukaryote cell. The oxygen utilizing cell evolved into the mitochondria. The other partner evolved into the larger host which contains our genome and undertakes other physiological functions.  All complex multicellular life forms are descendants of this symbiotic cell type.

Another boost to oxygen levels was needed to ratchet up the evolution of more diverse and large life forms. That happened by the Neoproterozoic beginning about 1000 million  years ago. This study pins down the oxygen content of the atmosphere to about 10%-11% by 815 million years ago, about 200 million years before fossil evidence of multicellular animals (Ediacaran biota) first appears.

Still, there may not be a simple and direct causal link between enhanced oxygen levels and the evolution of multicellular animals.  Two good reviews, on the Neoproterozoic by Nicholas Butterfield and on the Cambrian Explosion by Derek Briggs make a case that the transformation of the biosphere from the Cryogenic to the Cambrian, from simpler eukaryote to the diversification of multicellular marine animal life, was engineered partly by life itself. For example, pumping of sea water due to filter feeding action of sponges would have dispersed oxygen more efficiently through the water column, ventilating the marine shelf environment where physiologically demanding predator prey evolutionary arms races began to be played out. The evolution of grazing invertebrates destroyed the microbial mats that covered large areas of the shallow sea floor. This gave access to the sediment column to the burrowing activity of the earliest bilaterans which would have likewise allowed oxygen to reach deeper into the sediment pile, thus opening up new ecosystems where an infaunal biota evolved. The activities of animals thus created new ecological opportunities for other creatures.

This finding though aligns well with the thinking that threshold levels of oxygen required for multicellular animal life were already present in the atmosphere and the oceans. With the right climatic (ending of the Cryogenic Period and the warming of the earth) and tectonic (breakup of Rodinia resulting in formation of wide shallow shelf areas) triggers, evolutionary opportunism took over.

Open Access
 

Monday, July 4, 2016

Quote: Alfred Wallace On Human Driven Extinction

He writes in The Malay Archipelago (1869):

It seems sad that on one hand such exquisite creatures should live out their lives and exhibit their charms only in these wild, inhospitable regions, doomed for ages yet to come to hopeless barbarism; while on the other hand, should civilized man ever reach these distant lands,  and bring moral, intellectual, and physical light into the recesses of these virgin forests, we may be sure that he will so disturb the nicely-balanced relations of organic and inorganic nature as to cause the disappearance, and finally extinction, of these very beings whose wonderful structure and beauty he alone is fitted to appreciate and enjoy. This consideration must surely tell us that all living things were not made for man. 

Via - The Song of the Dodo: Island Biogeography In An Age Of Extinction.

This was apparently a refutation of the argument made by the Duke of Argyll, that beauty in nature is evidence of God's handiwork. Wallace though was also clearly worried that European expansion and demand for natural resources would put these ecosystems at grave risk.

You can read The Malay Archipelago at Wallace Online.

Monday, June 20, 2016

Nepal Earthquake Aftermath- What Is Happening Underneath Kathmandu?

We continue to learn more about the buildup and release of strain along the master detachment fault underneath the Himalaya. The master detachment fault known as the Main Himalayan Thrust is the surface along which the Indian plate is sliding underneath Asia.

This data using GPS records of surface motion in the area affected by the 2015 Gorkha (Nepal) earthquake:

Himalayan strain reservoir inferred from limited afterslip following the Gorkha earthquake -David Mencin et al 2016 (behind paywall)

The magnitude 7.8 Gorkha earthquake in April 2015 ruptured a 150-km-long section of the Himalayan d├ęcollement terminating close to Kathmandu. The earthquake failed to rupture the surface Himalayan frontal thrusts and raised concern that a future Mw ≤ 7.3 earthquake could break the unruptured region to the south and west of Kathmandu. Here we use GPS records of surface motions to show that no aseismic slip occurred on the ruptured fault plane in the six months immediately following the earthquake. We find that although 70 mm of afterslip occurred locally north of the rupture, fewer than 25 mm of afterslip occurred in a narrow zone to the south. Rapid initial afterslip north of the rupture was largely complete in six months, releasing aseismic-moment equivalent to a Mw 7.1 earthquake. Historical earthquakes in 1803, 1833, 1905 and 1947 also failed to rupture the Himalayan frontal faults, and were not followed by large earthquakes to their south. This implies that significant relict heterogeneous strain prevails throughout the Main Himalayan Thrust. The considerable slip during great Himalayan earthquakes may be due in part to great earthquakes tapping reservoirs of residual strain inherited from former partial ruptures of the Main Himalayan Thrust.

The Himalaya cross section below shows seismicity along the interface of the Indian plate boundary faults.  MFT is the Main Frontal Thrust, MBT is the Main Boundary Thrust and MCT is the Main Central Thrust. These south younging sequence of thrust faults are thought to absorb the crustal shortening in the Himalayas. The youngest, the Main Frontal Thrust is thought to be active today, i.e. great earthquakes that nucleate underneath the Himalaya, rupture the surface along this fault system. All these thrusts faults are inferred to be splays of the Main Himalayan Thrust. Red and Orange dots are instrumentally recorded earthquakes.


Source: Bollinger et al 2014: Estimating the return times of great Himalayan earthquakes in eastern Nepal: Evidence from the Patu and Bardibas strands of the Main Frontal Thrust

Monday, June 13, 2016

The Unconformity At Rathivade

My friend Pradeep Sarkar died last Tuesday June 7th. He got up that morning and complained of shortness of breath. In a few minutes it was all over. He leaves behind a shattered family, a wife and a son, and a large number of stunned friends, colleagues and students.

I met Pradeep when he joined the faculty at Fergusson College, Pune, during my second year B.Sc. He had a way with students and his enthusiasm for geology rubbed on to others. A boring summer vacation was looming ahead.  Having already decided to major in geology, Anil Lalla, Nalin Nair and myself asked him if he was going for fieldwork to the Konkan coast. As it happened he was and to our delight he invited us to go along with him.

We traveled to the little town of Malvan in the Sindhudurg district in southern Maharashtra. His research involved mapping the Precambrian geology with emphasis on understanding sedimentary structures and environments of deposition of the mid-late Proterozoic Kaladgi Group. Peninsular Gneiss, Greenstone schist belt, deformed conglomerate, trough cross bedding, fining up sequence. We had read and heard about these magical geological features in class. Pradeep made them come alive in the field in the week of our arriving at Malvan. He taught us how to read a toposheet and how to overlay the geology on it. He taught us how to measure and describe lithological sections. He taught us how to scan the horizon and see beyond.

In the evenings after fieldwork we used to go to the rocky  Malvan beach where the Kaladgi Group sediments were exposed. He used to point out to us ripple marks and bedforms made by the action of waves on a sandy sea floor more than a billion years ago. Sometimes we walked a little out of the way, outside town, to one of his favorite spots. A bridge over a small estuary; there we stood watching the rise or ebb of tides, taking in the salty breeze, waiting for the Arabian Sea sunset. Malvan would remain very close to his heart long after he finished his PhD work. He ended up marrying a local girl, the daughter of the lodge he always stayed in.

We remained good friends throughout. After I left India to pursue graduate studies we struck up a lively correspondence. I used to mail him research papers. His interests were moving toward studying calcrete, calcium carbonate deposits found as nodules and veins in the semi arid eastern Deccan plateau. On my return to India a few years back we renewed our meetings. During short chai sessions at the local tapri he used to hold court, talking excitedly of his research and teaching commitments. He wanted to see some of my PhD samples of Ordovician carbonates and maybe hold a lab session with graduate students. I agreed to meet him in his lab. Alas, that day never came. His work load was such that he just never got around organizing it.

Last week on June 10th I attended a sad but memorable condolence meeting for Pradeep. Friends and colleagues talked touchingly on the many memories they had of time spent with him. He leaves behind a legacy of 3 decades of excellence in teaching and of inspiring countless students to take up geology as a career.

When I heard the news of his passing on June 7th, my mind skipped back to that field trip and I found myself thinking about the unconformity near the village of Rathivade some distance away from Malvan. Amidst monsoon showers and sunny interludes we walked through the Konkan countryside. In a stream bed we came across something he had badly wanted us to see- The Great Eparchaean Unconformity. There scattered along were patches of Archean gneiss. Along the stream bank overlying the gneiss were mid Proterozoic Kaladgi sediments. By the time I had walked to the stream edge, Pradeep had rushed in and was already standing knee deep in water. It is an important feature of Indian geology he explained.... this unconformity. Etched on the surface of Peninsular India, it speaks of a change from a hot tectonically active Archean earth in which was formed the gneiss, to a cooler more stable Proterozoic world, where, on large cratonic basins in shallow seas, thick sequences of sandstones and limestones accumulated. This unconformity- he told us-  marks a long period of quiescence between these two phases of formation of the earth's crust, maybe a 300 -500 million year break.

With amazement we stared at him. 

Since then, at different times and places, I met and was influenced by other good teachers and mentors who kept my interest in geology alive. But it all began during that late summer in June 1986 when the generosity of a young PhD scholar opened the eyes of three geology enthusiasts to the wonders of this earth.

 Pradeep Sarkar 1960-2016