Wednesday, March 23, 2016

In Search Of Early Humans In India

I attended a talk yesterday by Prof. S. N Rajguru on the evidence for early hominins in India. This was part of the C. Meenakshi Memorial Lecture series hosted by the Bhandarkar Oriental Research Institute in Pune.

Prof. Rajguru is an acknowledged expert on Quaternary geology and paleoclimates of the Indian subcontinent and the talk followed the contours of his expertise.

Here is the abstract which was given to us:

In Search of an "Early Man" (>2.5 Ma yrs to around 50 Ka yrs) in India

Recent progress in absolute dating methods, in understanding palaeo-landscape of India in light of behavior of monsoonal rainfall in the last 10 million years (Ma), fluctuations in sea level on 7500 km, long coastline of India and the rise of Himalayas and Tibetan plateau from 1000 m during the Late Miocene (around 15 Ma yrs ago) to around 3000 m during early Pleistocene(~1 Ma yrs ago) have added new information on chronology and environment of "Early Man" (hominin) in India.

It is now know that the Indian monsoon is at least 10 Ma old and was fairly strong till 2 Ma yrs. It started fluctuating (strong, moderate, weak) during the Pleistocene (2.5 Ma-10 Ka). The Indian monsoon was relatively strong to moderate during early Pleistocene (2.5 Ma - 0.7 Ma), moderate to weak during middle Pleistocene (0.7 Ma- 130 Ka) and weak during the late Pleistocene (130 Ka- 10 Ka).

Owing to changes in the strength of monsoonal rainfall the landscape of India also responded dramatically. The sea level went down by 100 to 150 m below the present sea level around 18 Ka BP (before present) and was high around 125 Ka BP by 5 to 7 m in tectonically stable part of coastal India. The peninsular rivers also responded to climatic changes in terms of strong erosion and excess deposition. The Himalaya was affected by tectonic movements and by glaical and interglacial climates which were part of global climatic changes during the Pleistocene.

These environmental changes did affect the biological world including 'hominins' in India.  It appears that early man was present in the foothills of the Himalayas in NW India around 2.6 Ma yrs BP and around 1.7 Ma BP in the coastal parts near Chennai in Tamil  Nadu. These two important discoveries in the form of stone tools and bones with cut marks made by early man raise doubts about the 'out of Africa' migration of early man in Asia during the early Pleistocene.

We are not certain who was the maker of stone tools like choppers around 2.6 Ma yrs or little  earlier in the foothills of the Himalayas near Chandigarh. Most likely he belongs to "Australopithicus" group of hominin. On the other hand 'Homo erectus' was responsible for making Acheulian (Lower Paleolithic) artifacts, like handaxes, cleavers, etc., around Chennai during the early Pleistocene (~ 1.7 Ma).

There is a long gap between 'Homo erectus' (~1.7 Ma, with brain capacity of 800 to 1100 cc) and modern man or Homo sapien sapiens (with brain capacity of 1100-1300cc) who had his origin in Africa 200,000 yrs BP.

In Indian context, there are large number of Lower Paleolithic sites well preserved in varieties of environment during the middle Pleistocene (0.7 Ma- 130 Ka). There is a cultural change in the form of stone tools consisting of small size scrapers, points, choppers, etc., made on flakes removed from cryptocrystalline minerals like chert and chalcedony. It is as yet not clear whether the maker of Middle Pleistocene artifacts was 'Modern Man' who arrived in India via West Asia, or he still belongs to advanced form of ' Homo erectus'. Excepting a single fossil skull of hominin, dated to around 250 Ka yrs in the Central Narmada Basin, we do not have any other human fossil data of early late Pleistocene (~ 100 Ka BP).  Thus, the search for early man, though shrouded in scientific controversy, will continue in future in old Himalayas, warm peninsula and in humid coastal strip of India.

One quibble is the comment that evidence of putative 2.6 million year old stone tools from the Siwalik foothills of the Himalaya raises doubt about the 'Out of Africa' migration of early humans into Asia in early Pleistocene. I really don't see how this is so. Unless you are claiming that bipedalism and hominins originated in Asia, a position for which there is not an iota of evidence, all this discovery does (and the finding is still being debated) is deepen the timeline of the earliest migration of hominins from Africa into Asia.
I came across the same argument in a more detailed article about the Siwalik stone tools and scratch marks on bovine bones discovery in Outlook magazine. That article went even further and stated that this 2.6 million year old tools and bone marking may tell us "when we started looking and behaving more like Homo sapiens rather than apes" Huh!... How so? How does an unconfirmed find of possible Early Pleistocene tools in India  signal the evolution  of Homo sapiens from ape like ancestors? The mere presence of these archaic tools can't be a signal. If so, the same reasoning applies to the Pleistocene tool record in Europe and China too. So, such a broad overreaching claim explains nothing.  It is just sensational journalism by Outlook magazine.

I don't want to dwell on this issue too much since it was not the thrust of Prof.  Rajguru's talk. Instead, he gave quite an engrossing presentation with lots of pictures of the various field sites where stone tools ranging in age from 1.7 million years to 50 thousand years or so have been found. He spoke in detail about the paleo-environments and it is clear that humans in India occupied a wide range of landscapes and ecology during the Pleistocene. There were some awesome sites in Ladakh in the Himalayas, a few sites along the Gujarat coast associated with mid-late Pleistocene aeolian carbonate sand made up of Miliolite shells (great cross bedding), and a very interesting site closer to Pune. This was in a laterite cave in a sea cliff overlooking the Arabian sea near the town of Guhaghar. Tools provisionally dated to about a hundred thousand years old or so have been found there.

What did come out of this talk was some of the limitations faced by researchers in India. The first is the lack of a skeletal record of humans. We have just a few skeletal fragments from the Narmada valley. I asked Prof. Rajguru about this lack of bones. He suggested that the African record is richer in bones because of preservation in volcanic ash and fine river muds and sands. In contrast, most of the sites where stone tools have been found in India are surface sites where the preservation potential of skeletal material is poor. That does point the way to a future program of more focused search for bones in the Pleistocene sediments across India. The second problem is establishing absolute chronology. Dating methods are very expensive and until recently Indian researchers could afford to date only the occasional sample. Or, they had to rely on collaboration with western (and Australian) researchers. Hopefully this will change in the future.

Prof. Rajguru is a field geologist. He must be in this seventies now but his enthusiasm for field work is still undiminished. He kept stressing the importance of understanding the geology, stratigraphy and paleo-ecology of human habitation sites to fully understand the significance and variability of human occupation in India in terms of past climates and landscapes. Even without skeletal material one can make a useful contribution toward understanding human evolution....

having said that.. we do need to find more bones!

Tuesday, March 15, 2016

Sedimentation Patterns Bay Of Bengal: How Old Is The River Ganga

...In our legends it is said that the goddess Ganga's descent from the heavens would have split the earth had Lord Shiva not tamed here torrent by tying it into his ash-smeared locks. To hear this story is to see the river in a certain way: as a heavenly braid, for instance, an immense rope of water, unfurling through a wide and thirsty plain. That there is a further twist to the tale becomes apparent only in the final stages of the river's journey - and this part of the story always comes as a surprise, because it is never told and thus never imagined. It is this : there is a point at which the braid comes undone; where Lord Shiva's matted hair is washed apart into a vast knotted tangle. Once past that point the river throws off its bindings and separates into hundreds, maybe thousands of tangled strands....

Amitav Ghosh- The Hungry Tide

A study by K.S Krishna and colleagues published recently in Current Science shows very elegantly using sesimic reflection profiles and sediment isopach maps how the Bay of Bengal has been filling up with sediment since Late Cretaceous times.

The Bay of Bengal (BoB) originated with the rifting of India from Antarctica by early Cretaceous, thus forming the Indian east coast margin. The depression over time evolved into an ocean basin with new oceanic lithosphere forming in the Bay of Bengal at sea floor spreading centres. Its conjugate oceanic crust is probably beneath the Enderby Basin at the margin of East Antarctica. In this depression, an enormous volume of sediment has been deposited from mid-late Cretaceous to recent times. The age ranges of sediment packages, their distribution, geometry and thickness tell us about the changing source regions of these sediments and the influence of the rising Himalayas and the monsoons on sedimentation history.

Scientists involved in this study used seismic  reflection profiles to construct a seismic stratigraphy of the sediment pile in the BoB. The ages ranges of the sesimic sequence were then calibrated using biostratigraphy erected from two deep sediment cores from the vicinity of the seismic lines. Thus, a Cretaceous to recent subdivision of the mega sequence into distinct depositional episodes separated by unconformities could be recognized. The seismic profiles also revealed the geometry and thicknesses of the sedimentary sequences and the topography of the basement.

Several coast perpendicular grabens (linear depressions) were clearly outlined. These are continuations of ancient Archaean and Proterozoic sutures zones and rifts now occupied by the major rivers of Peninsular India, the Cauvery, Krishna, Godavari and the Mahanadi. Sedimentary packages until the late Oligocene are thicker near the east coast and thin out into the deeper shelf areas. From early Miocene onwards the sediment packages are thicker near the Ganges Brahmaputra delta and in the deeper shelf areas to the east and north and thin towards the shallower coastal shelf. This implies a changes in direction of sediment delivery systems, with the Indian craton being the major source in the earlier phase, to the Himalayas and the Indo Burman ranges being the major source in the younger phase.

This is brought out beautifully by sediment isopach maps constructed for several time slices. Isopach maps show the thickness of sediments for a particular time slice. In this case four time slices were used: 1) Basement to Late Cretaceous 2) Late Cretaceous to Oligocene 3) Oligocene top to Late Miocene 4) Late Pleisocene to recent.

The results are shown below and they clearly show changing sediment sources and distribution pathways.

From Cretaceous to the Oligocene, thick sediment wedges coincide with the Peninsular river grabens indicating that sediments derived by erosion of the Indian craton was the major source to the BoB.

 Source: K.S Krishna et al. 2016

From late Oligocene throughout much of the Miocene the sediments are thickest in the Ganges Brahmaputra delta region and thin out southwestwards towards the east coast shelf area.

 Source: K.S Krishna et al. 2016

This indicates that river systems eroding the rising Himalayas were now the major suppliers of sediment to the BoB. Sediment thickness from late Miocene to mid Pleistocene also show enhanced sedimentation from the north and this pattern coincides with an increase in the Asian monsoon. From Pleistocene to recent times, there has been more sediment from the Godavari Krishna system to the Bob, while a strong sediment delivery system from the Ganges-Brahmaputra continues.

This is the end of part 1 of the post, but I had an intriguing question..

How old is the river Ganga of the plains, flowing from the Himalayan front near Haridwar to the Bay of Bengal?  What does the data from the Bengal Basin tell us about paleo-rivers and how do geologists go about collecting and analyzing this data?

Monday, March 7, 2016

Balmy Shores And Icy Wastes- Carbonates Overlying Proterozoic Glacial Deposits

This problem was articulated quite well in a review article more than 20 years ago in 1993: Balmy Shores and Icy Wastes: the paradox of carbonates associated with glacial deposits in Neoproterozoic times. The  thinking has been that the Neoproterozoic must have seen very abrupt changes in climate from a "Snowball" earth with glaciers spreading to low latitudes, to a much warmer climate,  more amenable for widespread carbonate deposition. A variety of explanations for the origin of these carbonates were proposed depending upon their stratigraphic relationships with glacial deposits. There are carbonates interlayered with glacial deposits. Some layers based on their being composed of intraclasts (particles of eroded older limestones) were interpreted as detrital limestones. In other cases of very fine grained rock, it was thought that glaciers grinding and eroding older Proterozoic rock would have delivered fine rock flour to lakes and seas. This  reactive rock flour then recrystallized into a secondary carbonate layer.

In other instances, there is a "cap" carbonate, i.e. a layer overlying a glacial deposit. These cap carbonates vary in the environments of  deposition they represent. They are supratidal  to intertidal  dolomites, or shallow  subtidal limestones to deeper water limestones. They are primary precipitates, meaning the calcium carbonate crystals precipitated out of sea water or in the case of dolomites, an initial aragonite or calcite mineralogy was replaced by the mineral dolomite. Their presence overlying glacial deposits indicate an end to glacial conditions and marking a sudden shift of climate to warmer times. The sea water was thought to be supersaturated in calcium and magnesium ions, but the reason for this has not been well understood.

A new study (T. M. Gernon et al 2016) on these cap carbonates now points the finger to an increase in alkalinity of sea water due to the alteration of volcanic glass being formed at sea floor spreading centers due to the breakup of the Rodinia supercontinent in the Neoproterozic-

During Neoproterozoic Snowball Earth glaciations, the oceans gained massive amounts of alkalinity, culminating in the deposition of massive cap carbonates on deglaciation. Changes in terrestrial runoff associated with both breakup of the Rodinia supercontinent and deglaciation can explain some, but not all of the requisite changes in ocean chemistry. Submarine volcanism along shallow ridges formed during supercontinent breakup results in the formation of large volumes of glassy hyaloclastite, which readily alters to palagonite. Here we estimate fluxes of calcium, magnesium, phosphorus, silica and bicarbonate associated with these shallow-ridge processes, and argue that extensive submarine volcanism during the breakup of Rodinia made an important contribution to changes in ocean chemistry during Snowball Earth glaciations. We use Monte Carlo simulations to show that widespread hyaloclastite alteration under near-global sea-ice cover could lead to Ca2+ and Mg2+ supersaturation over the course of the glaciation that is sufficient to explain the volume of cap carbonates deposited. Furthermore, our conservative estimates of phosphorus release are sufficient to explain the observed P:Fe ratios in sedimentary iron formations from this time. This large phosphorus release may have fuelled primary productivity, which in turn would have contributed to atmospheric O2 rises that followed Snowball Earth episodes.

One broader picture that is emerging is that the time period between 1000 and 550 million years ago was one of the most remarkable times in the history of our planet. On one side, from the origin of the earth, lies 3 billion years of a microbial biosphere, and on the other, the extraordinary diversification of large complex multicellular life. The 400 odd million years of the Neoproterozoic created the conditions for this transformation. The infographic below summarizes the changes in the geo-biosphere during Neoproterozoic  times.

Source: Butterfield N.J. 2015

There were changes in continental configuration due to plate tectonics (breakup of supercontinent Rodinia ~ 850 mya), extreme icehouse conditions in the Cryogenian with two major glacial phases, the Sturtian and the Marinoan, lasting tens of millions of years  and sea water chemical changes with increase in ocean oxygenation to crucial threshold levels needed for metabolically demanding activity, major perturbations in organic and inorganic carbon cycles and widespread deposition of phosphate on the sea floor. A confluence of these events not just led to the formation of geologically unusual deposits but also influenced the evolution of complex multicellular life. That in turn completely changed sea floor sediment fabric and chemistry, further providing new ecologic niches for evolutionary innovations. Engineered by animal activity, the construction of a new world began.