Friday, February 19, 2021

Fossil Dickinsonia in Bhimbetka Sandstones: Nature India Article

 My short piece published in Nature India on the surprising report of Ediacaran age fossil Dickinsonia in the Bhimbetka caves near Bhopal, Central India, and its geological and biological significance.

an excerpt:

"The biological affinity of Dickinsonia is controversial. Most scientists tend to accept it as an early animal. Some like Gregory Retallack, co-discoverer of this fossil, think of it to be a large algae or lichen. He argues that the mainstream view that Dickinsonia was a marine animal is based on weak evidence, while his own detailed work shows that Dickinsonia was a land creature, forming biogenic crusts on soils. Interestingly, the Bhimbetka rocks were deposited in a mostly terrestrial setting, more in alignment with Retallack's interpretation. Importantly for geology, its restricted time span, being found only in rocks between 555-550 million years old, makes it a diagnostic age indicator. So far, no animal fossils have been found in the Vindhyan rocks. This finding may inspire geologists to start searching contemporaneous Indian basins afresh for such subtle clues".

Fossil from dawn of animal life found in India’s famous caves.

Wednesday, February 17, 2021

Readings: Chamoli Debris Flow Disaster, Uttarakhand

The Himalaya are geologically and ecologically fragile. Despite this, the Indian government has repeatedly ignored advice from its scientists and has gone ahead with major infrastructure projects, which have not been assessed rigorously for the inevitable impact they will have on the surrounding environment, people and livelihoods.

The February 7 2021 Chamoli debris flow that destroyed two dams and killed scores of people with hundreds still missing is the latest example of how the natural tendency of Himalayan slopes to fail combines with steel and concrete to cause enormous damage. 

Here is a short list of readings of this event and Himalaya infrastructure projects.

1) Dr. Dave Petley has put together a sequence of events based on crowd sourcing satellite imagery. This was a remarkable example of experts collaborating to pinpoint the location and cause of this debris flow within just a day. 

The catastrophic landslide and flood in Chamoli in Uttarakhand: the sequence of events.

 2) M. Rajshekar explores the messianic drive of the Central and the Uttarakhand government to build dams in the Himalaya. 

Modi said he would revive Ganga but his government is doing the opposite by reviving dams

3) R.Shreedhar, an experienced earth scientist working in the Himalaya writes a fine essay about the neglect of science and the political economy of Himalaya dam building.

The Science and the Political Economy of the Rishi Ganga Flood.

4) The title of this essay by Nivedita Khandekar says it all.. "We have learnt nothing from the 2013 Uttarakhand Disaster".

We have learnt nothing from 2013 Uttarakhand disaster.

 

Sunday, January 31, 2021

Mammalian Evolution, Earth Biosphere, India Geology Outreach

 Sharing these interesting items:

1) Simone Hoffman writes about one of the fundamental transitions in mammalian evolution, the transformation of bones of the lower jaw into those of the middle ear.

Lend an ear to a classic tale of mammalian evolution.

2) How has the earth's evolving biosphere from early microbes to megascopic land plants impacted the biogeochemistry of the earth? A great review article by Noah Planavsky and colleagues. Read this one quickly. It is open access for now, but might go behind a paywall in the next few weeks.

Evolution of the structure and impact of Earth’s biosphere.

3) Live History India anchored by Mini Menon has produced a great geology outreach video. Four geology enthusiasts talk about India's varied geology, how to raise awareness among our citizenry about the importance of geology in our lives, and the urgent need to protect sites of exceptional geological significance. Dr. Pushpendra Ranawat, Bidisha Bayan, Dr. Reddy, and Aliya Babi are the guests. 

Do make the time and watch this. Email subscribers who can't see the embedded video can watch it here - India: What Lies Beneath.

 


Wednesday, January 27, 2021

Fossil Dickinsonia Indicates Ediacaran Age For Upper Vindhyans

 It looks like one of the long running debates in Indian stratigraphy has been resolved. 

The Vindhyan Basin is one of several Proterozoic age basins in Peninsular India. The youngest rocks in this basin have been accepted to be of Neoproterozoic age i.e. less than 1 billion years old. But absolute dating of these rocks is tough. Criteria like youngest detrital zircon ages, which tell us the age of the youngest source rocks from which sediments were derived, were hinting at an age of about 850- 900 million years for the uppermost Vindhyan strata. On the other hand, presence of fossils like Arumberia and Beltanelliformis minuta, both considered microbial organisms, were suggesting of an Ediacaran age (637-541 million years) for youngest Vindhyan rock units, although there wasn't a widespread acceptance of these dates. Then, last year, a paper by Lan and colleagues published new detrital zircon dates of 548 million years from the Maihar Sandstone. They indicated that the uppermost Vindhyans are late Ediacaran in age. Detrital zircon age tells us that the sediment cannot be older than the zircon, i.e. it gives a maximum age for the deposition of the sedimentary layer.

Now, a stunning fossil find seems to confirm this chronology. The Ediacaran age fossil Dickinsonia has been dated to about 555 million years from sites in Russia. This fossil was found as impressions on Maihar Sandstone at the site of Bhimbetka in Madhya Pradesh. Bhimbetka caves are famous for pre-historic cave art. Gregory Rettalack and colleagues report Dickinsonia, in their own words.. " hiding in plain sight, on the roof of the Audi-torium Cave (N22.938402° E77.613504°), the first of several Paleolithic and Mesolithic cave painting and petroglyph sites on the path into Bhimbetka Rock Shelters"..

The fossils remain in place at the site as negative relief impressions and has been documented photographically (pics below).

 

            Source: Retallack G. et.al., 2021: Dickinsonia discovered in India and late Ediacaran biogeography.

The biological affinity of Dickinsonia is contested. Some paleobiologists think it is a representative of an extinct group of early animals. Others think it is likely to be a macro algae. No unequivocal indicators of animal life, either in the form of body or trace fossils, has been found in the Vindhyan Basin so far. Such a recent age for the uppermost Vindhyan strata may renew hope that the earliest stages of animal evolution are preserved in these rocks. 

I had written a long post on the age problem of the upper Vindhyans some time back. In that I had mentioned that physical signals like magnetic signatures and chemical criteria like carbon isotope trends were suggestive of the uppermost Vindhyans being not much younger than 850 million years. But similar magnetic signals and carbon isotope trends may be repeatedly produced under similar physical and chemical conditions.

Fossils are a better guide to age. Evolution produces unique life forms. Species exist over a restricted time range. This is the basis for constructing a relative time scale for global strata. Presence of the same species or same group of organisms in physically disconnected rocks indicate contemporaneity of these units. To ascertain their absolute age they need to be dated using one or more of radiogenic dating methods, usually relying on interlayered volcanic ash which contains the  appropriate minerals. Or, as in this case, using detrital zircon, which gives us the maximum age of the unit.  Global surveys of late Neoproterozoic strata show that Dickinsonia is not found in rocks younger than about 550 million years. This brackets the age range of the Dickinsonia containing unit of the Maihar Sandstone at Bhimbetka to between 555 -550 million years. Detrital zircon ages of 548 million years for Maihar Sandstone indicate that Vindhyan sedimentation continued for some time after.

I find myself compelled to ask one question here. What is the source of the 548 million year old zircons reported from the Maihar Sandstone? I haven't read Lan et. al.'s paper, so I am guessing. The age of the youngest zircons from the Aravalli and Central Indian terrains which were the main source of sediments supplied to the Vindhyan Basin are about 800- 900 million years. The Bastar region could be one source for these even younger zircons. High grade gneiss and granitoids from this craton have yielded 500 million year old zircons likely produced during the collision of the Eastern Ghat terrain with the eastern margin of India. Or perhaps they were sourced from terrains which are now far far way in Antarctica.

There are other Proterozoic basins in Peninsular India where age uncertainties of especially the uppermost units continue to be the subject of lively debate. This fossil find might just inspire geologists to start a fresh search for hitherto elusive age indicators.

Thursday, December 31, 2020