The Dolomite Problem - Peeking Under The Hood

A recent paper in Geology addresses one of the most enduring problems in sedimentary geology- the origin of the mineral dolomite. One can extend their findings to answering the origin of massive dolomite carbonate sequences that recur throughout earth history. Massive dolomite means that most of the rock is made up of the mineral dolomite and such dolomite strata can be hundreds of feet thick, representing deposition over millions of years.  The paper  examines one specific mechanism of sedimentary dolomite formation. Microbial activity in shallow water depths has long been suspected to induce dolomite precipitation and the paper brings out the specifics of the bacterial micro-environment that might aid dolomite to precipitate directly from sea water as pore filling cement or replace existing calcite or aragonite.

Such microbial ecosystems exist today in very restricted settings such as hypersaline lakes and supratidal flats and whether such microbial induced dolomite can explain the thick dolomite sequences which were deposited in more varied environments is a question that still needs more attention.

David Bressan on Scientific American blog has written two posts on the history of research on dolomites and in reference to the paper in Geology on the question of the microbial origin of dolomite using the Triassic Dolomite sequences of the Alps as an example. A considerable part of this sequence is made up of thinly laminated strata. The laminae have been interpreted as structures arising from sediment being trapped or precipitated between bacterial sheets. Based on these microtextures there is case being built up that much of the Triassic dolomite, especially that deposited in earlier phase in the Norian stage is likely to be of microbial origin i.e. induced by microbial activity.

First a few SEM images that show a spatial link between microbial filaments and incipient dolomite crystals.

Early Humans In India: Peopling The Planet Special Issue

Ha.. move over Africa. Finally a human evolution mystery that involves India!

Nature has a special issue titled Peopling the Planet. Many of the articles are open access. They deal with what we know now of the migration of Homo sapiens out of Africa  and the peopling of various continents. The essay Eastern Odyssey  is on the controversial question of when Homo sapiens first moved across western Asia and into India.

The stone tool record  typical of modern humans place our species in India by 45 thousand years ago. These tools have been found inland. There may have been an earlier presence but since the first migration routes were likely along coastal areas via Arabia that record is drowned by the Holocene sea level rise. Analysis of mitochondrial DNA suggests a diversification of mitochondrial lineages as early as 60 thousand to 65 thousand years ago, implying modern humans had left Africa and become semi-isolated populations residing in different Asian regions by then.

Now, a group led by Michael Petraglia at the University of Oxford, UK think they have found signs that modern humans may have left Africa and entered Asia and the India subcontinent earlier than 74 thousand years ago, perhaps even 100 thousand years ago. The record of a volcanic eruption at Toba is preserved as an ash deposit in the Jurreru Valley region in Kurnool district , Andhra Pradesh, South India. The eruption is dated to 74 thousand years ago. Dr. Petraglia's team has found tools below this deposit. They have interpreted these tools are belonging to our species, thus implying their presence in India before 74 thousand years ago. Dr. Petraglia thinks that the genetic signal of these first migrant has been lost as populations vanished to be replaced by later migrants.

Other scientists like Paul Mellars are not convinced. They see these tools as the handiwork of another more archaic human species, possibly the descendants of Homo erectus from a much earlier migration.

Confidence that their version of the story is the right one is quite high:

But did these early explorers press on, from Arabia into Iran and India? No, says Mellars. Theirs was a failed expansion, like one documented in Israel, where modern humans were present more than 100,000 years ago but then vanished. When the climate cooled and dried about 75,000 years ago, turning Arabia back into a desert, the Nubian pioneers either died out or retreated to Africa. “There's not a smell, not a whiff [of Nubian technologies] that has ever been detected in India,” says Mellars. “If Mike Petraglia could come to me with one of those Nubian cores and say, 'Look, we found this in India,' I would get down on my knees and say, 'Sorry, Mike, I got it wrong'.”

Anatomically modern humans evolved probably around 200 thousand to 150 thousand years ago in Africa but it is not until 100 thousand years ago that we have an unambiguous record of them outside Africa in the Middle East. Whether these initial migrants went on into India is a mystery.

The story of human evolution in India has not received that much attention until now. One problem is a scarcity of skeletal remains. Why that is so is not clear. Other vertebrate fossil fauna is commonly preserved in fluvial and cave deposits. Maybe human fossils are preserved in very rare contexts. Anthropologist Parth Chauhan suggests that until recently most fossil hunting was being conducted by geologists or archaeologists and not trained physical anthropologists. So we might simply have not recognized skeletal fragments as being of hominin origin. Instead, scientists have had to rely on the tool record which is proving hard to compare across continents and to assign to specific human species.

Fluvial History And The Fortunes Of The Harappan Civilization

 

In a recent issue of PNAS Liviu Giosan et.al. use a combination of high resolution topographic data, geomorphologic analysis and sediment dating to establish a chronology of the evolution of fluvial landforms of the Indus and its tributaries.

The Harappan civilization over a 600 year period from around 4500 B.P (before present) to about 3900 B.P flourished in this region and then went into decline with urban centers abandoned and populations moving eastwards towards the Himalayan foothills and the Gangetic plains. Goisan et.al's work - and there is also independent evidence for this -  shows that this 600 year period was a kind of a Goldilocks period. The region became arid, but not too arid.

Indus and its tributaries get water from two climatic regimes, the summer monsoons from the Arabian sea and winter rains from the northwesterly winter disturbances bringing moisture from the Mediterranian, Caspian and Black sea region. Most of the sediment load of these rivers is generated during the heavy erosion that takes place in the Himalayas during the summer monsoons. When rivers carry and deposit sediment along their course they are said to be in an aggradational mode i.e. the stream bed and the surrounding floodplains get raised as more and more sediment is deposited.

Over much of the earliest part of the Holocene, the Indus and its tributaries were aggradational. Then the monsoons weakened and the sediment load reduced. Winter rains falling as snow though kept river discharge active. Rivers without sediment or less sediment incise or cut into their own deposits. So, by mid Holocene all these rivers had developed a characterized profile of incised valleys and river terraces marking the original river bed and broad surrounding plains that because of  reduced rainfall were less prone to severe flooding.

Giosan et.al suggest that the Harappan people took advantage and adapted to these circumstances. There are Harappan and even pre-Harappan sites within these incised valleys of the Indus and its tributaries like the Beas and the Sutlej. This suggests that incision had occurred in the early Holocene and agriculture evolved to take advantage of manageable floods and a perennial water supply within the valleys and adjoining plains. 

After around 3900 B.P. the aridification intensified. The agricultural heartland of this civilization was along the  Ghaggar/Hakra river, located between the Sutlej and the Yamuna. This river was monsoonal fed and would have been perennial until then. As it slowly dried up, urban population centers could not be sustained and the Harappan civilization went into decline. There was a movement of people to the east towards the wetter Himalayan foothills.

This eastward civilizational shift is captured well in the figure to the left (Source Giosan et.al.). Red dots are Harappan sites. White dots are  Painted Grey Ware sites. Painted Grey Ware is a cultural phase that overlaps with the late Harappan.

This study also lends additional support that the Ghaggar river which has been identified as the mythical Saraswati river mentioned in the Rig Veda was not a glacial river in the Holocene. It has been proposed that the Sutlej and the Yamuna flowed into the Ghaggar and changed course around 4000 B.P to 3900 BP, triggering a water crisis. This study points out that the landforms of incised valleys typical of these glacial rivers are not present across the Ghaggar Hakra region. Sediment dates of river terraces along the Sutlej also indicate that the Sutlej had incised and was flowing along its present course by late Pleistocene.

Work by Clift et.al, using zircon dating to fingerprint sediment and Himalayan catchment areas of these rivers, which I reviewed in an earlier post  indicates that although the Sutlej and Yamuna likely did flow into the Ghaggar in the Pleistocene they changed course towards the Indus and the Ganges by late Pleistocene, thousands of years before civilization was established in this region. And there is some independent isotope studies on ancient water in buried channels along the Ghaggar that also suggests that there were no glacial rivers flowing into the Ghaggar during the Holocene.

Sometime back I got involved in a debate with indologist Michel Danino on my blog. I had pointed out that glacier rivers like Sutlej and the Yamuna have developed a characteristic morphology of a relatively narrow belt of incised valley and terraces. As has been suggested, if such large perennial rivers were flowing across the plains of Punjab in the upper reaches near the Siwaliks and joining the Ghaggar they would have prevented the formation of the alluvial fans systems that have developed in the upper reaches of the interfluve areas around Ghaggar.

Michel though was convinced that the Sutlej had flowed into the Ghaggar until in fact medieval times. I bring this debate up because Michel, who is not a geologist, ultimately came to this conclusion based on his reading of the geological literature. And here, I think, that Indian geologists working on this problem have a lot to answer to.

That the Sutlej or Yamuna may have flowed into the Ghaggar in the past is a perfectly reasonable geological hypothesis. But until these recent studies by Clift et.al and Giosan et. al. there was no convincing evidence for it. Earlier, geological evidence of a glacial Ghaggar/Saraswati has been based primarily on the satellite mapping of dried up channels near the Ghaggar river which indicated that in the past the Ghaggar was a bigger river. But, these channels were never shown to be connected to the present day glacially connected Sutlej and Yamuna basins. They could just as well be interpreted as belonging to a monsoon fed river originating in the lower Himalayan ranges. Nor was there any firm sediment provenance that could identify a specific Himalayan source for these channels or a sediment chronology to establish the long term fluvial history of this region.

Paleobotanical and sedimentological criteria had always indicated that increasing aridification and reduction in monsoon strength better explained the drying of the Ghaggar around 3900 B.P.  Despite all this, the Sutlej or the Yamuna changing course at around 3900 B.P  became the favored explanation for the drying of the Ghaggar. This scenario of a once large Ghaggar neatly fitted the description in the Rig Veda of a mighty Saraswati, a holy river that just like the Ganges was thought to have its source in the high glacial Himalayas. I suspect that the glacial river theory had more emotional appeal and gained acceptance among some geologists.

The strong assertions by geologists that the diversion of glacial rivers from the Ghaggar coincided with the decline of the Harappan civilization was used by archaeologists like Prof. B.B. Lal to place the composers of the Rig Veda on the plains of the Punjab before the Ghaggar dried up, apparently bolstering the theory that the Harappan people and the Vedic people were one and the same. A geological narrative constructed without rigorous evidence has been promoted to support a theory of cultural evolution in northwest India.

Unfortunately, this glacial past of the Saraswati timed to the demise of the Harappan civilization is now enshrined in textbooks written by senior geologists like K.S. Valdiya. They should now be revised or at the very least these geologists  need to admit that their theory has been seriously challenged. If geologists working on this problem still want to stick to the theory of a glacial Saraswati, they will need to come up with a more convincing data driven rebuttal to the work of Clift et.al. and Giosan et. al.

Giosan, L., Clift, P., Macklin, M., Fuller, D., Constantinescu, S., Durcan, J., Stevens, T., Duller, G., Tabrez, A., Gangal, K., Adhikari, R., Alizai, A., Filip, F., VanLaningham, S., & Syvitski, J. (2012). PNAS Plus: Fluvial landscapes of the Harappan civilization Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1112743109

Additional Posts: K.S.Valdiya on the glacial Saraswati in Current Science

Interviewing Young Geologists

Last week I had gone to meet a friend working in groundwater.  When I arrived he was about to interview a geologist for a position in his organization. I was invited to sit in. The applicant handed a six page resume to us. It was littered with references to the applicants expertise in various GIS and geological data manipulation software. He had some programming experience too.

We found him wanting in his understanding of aquifers and hydrogeological mapping.

I could not generalize from this one example, but my friend has been doing this for years. He shrugged his shoulders... Well, that's about how it always is.. he said. Young graduates and even more senior geologists apply for a position with us. They are savvy in technology but poor in the science. I asked him if this was an all India problem. He agreed that this is a big problem not restricted to just Pune or Maharashtra. There are always bright students who shine.. but it appeared to him that they do so despite the system.

I was left wondering whom to blame. Is the quality of students entering the sciences / geology stream that bad? And what do you say about the quality of faculty and teaching if students are graduating without a grasp of basic principles?  And if this is indeed a problem, why is the quality of faculty not good? Is the job reservation policy discouraging a substantial section of interested graduates from applying for faculty jobs? Are bright reservation category candidates being drawn towards the private sector?  My friend had some unkind words to say about the quality of faculty. I think the problem may have become a self perpetuating cycle.

Request To Indiabioscience.org: More Science Please

Why don't Indian scientists working in India write about their research on open platforms? There are scientists who are blogging, but they write about campus life and the sociology of sciences and not about their research. And recently I came across a group blog Indiabioscience.org, which is a platform for biologists to:

The goal of IndianBioscience.org is for Indian biologists (junior and senior, working in India or abroad) to unite as a community- to find and exchange information, to make new contacts and find collaborators, to share perspectives, to attract a new generation of young people to scientific careers, and to welcome back talented Indians who previously might have established their careers abroad.

I went through the collection of posts from the contributors. Most of them are opinions of how to improve science education and the status of science in India, or on their personal experience of setting up research labs in India or negotiating the administrative maze, or advice to young postdocs and PhD candidates. 

All very useful.. but where is the Science? 

One of the stated goals of this initiative is " to attract a new generation of young people to scientific careers". I am not sure that young people will find it engaging to follow this blog and be inspired to take up a career in science if all the scientists on this forum come across as experts in university administration and pedagogy and have little to say about their research.

I find it boring to read post after post on "my views on improving Indian Universities / education / how to deal with various personal problems". 

Instead, why not tell us more about your own work?.. start a conversation with young aspiring students about your day in the lab and the nitty-gritty of science.