W. Bengal Bangladesh- Geologic Controls On Arsenic Distribution In Ground Water

Science writer  Priyanka Pulla has written an excellent article exploring the geologic, socio-economic and technological issues related to the widespread arsenic contamination of groundwater in W. Bengal. Sadly, the government response to this crisis has been slow.

I thought I would elaborate on the geological question -  Why are Arsenic (As) levels much higher in shallower Holocene age aquifers and lower in the deeper Pleistocene age aquifer? The answer encompasses mineralogy, climate change, sea level changes and bacteria.

The ultimate source of As are high Himalayan rocks and Indo-Burman ranges with additional contributions from the Precambrian terrains of Peninsular India and the Siwalik hills.  Minerals like biotite, magnetite, illmenite, olivine, pyroxene, amphiboles contain As. These minerals release As when they undergo weathering in catchment areas and deposits of the alluvial plains. This As is absorbed on secondary minerals like Fe hydroxides like goethite. Such Fe hydroxides are authigenic, i.e. they grow in the shallow buried sediments of the alluvial plains. Under oxidizing conditions, As is immobile, sequestered in Fe hydroxides. However,  conditions may change, and these sediments may get overlain by or be redeposited in environments rich in organic material. Certain bacterial species living on this organic material break down these Fe hydroxides, using the oxygen for their metabolism, and releasing Fe and As into the groundwater. This is known as reductive dissolution of Fe hydroxides and is the principal mechanism for As entering the groundwater in the alluvial plains of Bangladesh and West Bengal.

During the Pleistocene.. 1) the high Himalaya was glaciated. Therefore, important sources of As like the Fe-Mg rich rocks of the Indus ophiolite belt (slices of oceanic crust that existed between India and Asia which have been thrust up during continental collision) and high grade metamorphic rocks such as schists and gneisses were covered in ice and not releasing sediment. Indian cratonic areas, the Siwalik foothills  and the Indo-Burman ranges were being eroded, but overall less As was making its way on to alluvial plains. 2) Since climate was cooler and drier, there was less organic material accumulating in sediment of alluvial plains. Conditions were oxidizing and As remained sequestered in Fe hydroxide minerals. 3) Sea level was much lower then. Almost the entire continental shelf was dry land. Ganga and Brahmaputra met the sea much to the south of present shoreline. Reducing environments like delta front marshes, ponds, estuaries, existed much to the south.

Sedimentary conditions changed by 12-15 thousand years ago. Glacial melt exposed As bearing rocks in high Himalaya. As a result, more As made its way on to alluvial plains. Importantly, sea level rose and flooded the continental shelf. The Pleistocene delta front reducing environments were drowned. Shorelines shifted northwards. The climate was warmer, encouraging vegetation growth. Reducing delta front environments like swamps, coastal marshes and lakes developed on previous alluvial plain sediments.

The map below shows the position of shorelines between 7 thousand and 4 thousand years ago along with the location of wells with high levels of As. This study focuses on Bangladesh but similar conditions existed in West Bengal as well. The sea has receded 2- 3 meters to its present location since 4 thousand years ago.  The delta front and shoreline belt that existed 4-7 thousand years ago is now a densely inhabited region .

 Source: Quaternary shoreline shifting and hydrogeologic influence on the distribution of groundwater arsenic in aquifers of the Bengal Basin- M. Shamsudduha, Ashraf Uddin 2007

Notice clustering of wells with high As along the past shorelines. Here, organic rich delta marshes and swamps developed. Bacterial reduction of Fe hydroxides released As in to groundwater.

As distribution also shows correlation with topography. This map shows high As levels in groundwater coinciding with topographic lows. Such low lying areas accumulate more fine sediment and organic material. Again, this will apply also to W. Bengal.

 Source: Quaternary shoreline shifting and hydrogeologic influence on the distribution of groundwater arsenic in aquifers of the Bengal Basin- M. Shamsudduha, Ashraf Uddin 2007

So, a change in climate and shifts in sedimentary environments in response to changing sea level from Pleistocene to Holocene exerted a strong control on As distribution in the alluvial plains of Bangladesh and W. Bengal. 

Some Thoughts On The Middle Paleolithic Stone Tools From India Story

Early Middle Palaeolithic culture in India around 385–172 ka reframes Out of Africa models - Kumar Akhilesh, Shanti Pappu, Haresh M. Rajapara, Yanni Gunnell, Anil D. Shukla and Ashok K. Singhvi

From a site in Tamil Nadu, South India, stone tool types named Levallois were dated to be 385 ka -172 ka (ka-thousand years). Levallois tools are made by striking a stone core to produce smaller flakes which are then put to various uses.  They were more versatile than the older clunkier hand axes. Previous estimates for the arrival of this technology in India was thought to be around 125 ka or later, introduced by migrating Homo sapiens.

So, who made these older Levallois tools? The recent finding from Morocco of Homo sapiens like fossils dated to be older than 300 ka has prompted many to interpret this finding as evidence of an early arrival of Homo sapiens into south Asia.

Some thoughts-

1) this discovery has put a rare spotlight on the Indian hominin record. The paucity of hominin skeletal fossils and a lack of a rigorous chronology for deposits and tools have meant that the Indian record, if not ignored, has received less attention. This study has established a robust chronological framework of the sedimentary sequence in which these tools are found. A change from older Acheulean style tools to Levallois styles is documented within this dated sequence. Finding a trend, something changing or being replaced by something else, at one locality and within one sedimentary sequence is rare at hominin sites across the world. I think this make it more a compelling story than an isolated find of some stone tools.

2) I've noticed that some media article headlines and discussions in social media are suggesting that the "Out of Africa" theory needs to be reassessed. Well, what exactly do you mean by 'Out of Africa'? The original and popular Out Of Africa theory proposes that Homo sapiens originated in Africa around 200 ka. Then, 60ka-50ka ago these modern Africans migrated and settled the globe, replacing earlier archaic human populations. But, it is not news that there have been many 'Out of Africa's'.  By that I mean there have been many dispersals of humans out of Africa. Early archaic Homo dispersals occurred by 1.8 mya. The ancestors of Neanderthals and Denisovans left Africa by 1 mya to 700 ka ago.  There is genetic, fossil and archaeological evidence for a Homo sapiens migration around 65-50 ka ago. There is also evidence of an earlier migration of Homo sapiens (dated to ~120 ka) into the Levant and possibly into south Asia as well. An older Homo sapiens fossil dated to 185 ka has been found recently in Israel. Now, this discovery of  advanced tool technology has been interpreted by some to indicate an even earlier migration of Homo sapiens into India.

To me, the bigger evolving story is that Homo sapiens are getting older and older, originating earlier that 385 ka! That they might have dispersed into Asia soon after is less of a surprise. Migrations out of Africa seem to have occurred again and again, and so another at around 385 ka doesn't seem to be an extraordinary event. There would have been back migrations into Africa as well. I suspect that the recent finding of anatomically modern humans in Morocco dated to more than 300 ka has shaped the media narrative of this stone tool finding into an 'early migration into India' story.

How would have these stone tools been interpreted without supportive fossil evidence that Homo sapiens existing by 385 ka?  There always was an alternative hypothesis that proposed that evolution of complex behavior and associated advances in tool technology took place independently in disparate human populations residing in Africa, Europe and Asia.  Aspects of 'modern' anatomy and behavior might have developed multiple times in different places.  In this context, the hominin skull dated to around 236 ka found in the Narmada valley, Central India, is intriguing. Given its antiquity, the reasonable interpretation is that it represents a population descended from an earlier Homo erectus migration into India. Yet, it has a mix of archaic and derived (modern) features. It's estimated cranial capacity is comparable to modern humans.  Is it an example of  parallel evolution of 'modern' traits outside Africa?  Or, does it represent a hybrid population formed by the mixing of archaic hominins and Homo sapiens?

Skeptics like Michael Petraglia have pointed out that the technological transition seen in India may be a local invention of technology and not due to migration of a new population carrying advanced tools. Changing environmental conditions may have spurred similar inventions in different parts of the world.

3) One point to note is whether these earlier archaics and Homo sapiens living outside Africa contributed ancestry to today's people. Some recent genetic analyses suggests that all non-Africans are descended from Homo sapiens that dispersed from Africa around 80 ka -50 ka ago. These people did mix with archaic hominins in Europe and Asia but the degree of admixture is low. We contain about 2-4% Neanderthal and/or Denisovan genes.  If this is true, if earlier people migrating from Africa did not leave much of a genetic legacy in us, then the original 'Out of Africa' model still has some relevance.

Geology And Homo Sapiens Habitats Pleistocene Indian Subcontinent

Came across an interesting passage from this review paper:

Environments and Cultural Change in the Indian Subcontinent: Implications for the Dispersal of Homo sapiens in the Late Pleistocene - by James Blinkhorn and Michael D. Petraglia

Yet beyond relief, the geological structure of the Indian subcontinent plays another important role in patterns of habitability in the region. The analysis of the structure of geological basins within the Indian subcontinent led Korisettar (2007) to the conclusion that the Purana basins exerted a strong influence on hominin dispersals and occupation history. Although direct precipitation within the Purana basins is lower than other regions of the subcontinent, perennial supplies of freshwater are available because of spring activity from aquifers that deliver water resources from regions that receivemuch higher monsoonal precipitation.As a result of reliable water resources and abundant raw materials for stone tool manufacture, these geological basins are thought to have acted as refugia not only for hominin populations but also for varied flora and fauna (Korisettar 2007).

The importance of such Purana basins for providing refugia is well exemplified by the recent study of fauna from the Billasurgum caves, located within the Cuddapah Basin. Here, excavations revealed the first stratified sequence to document patterns of faunal occupation spanning the late Middle Pleistocene to Late Pleistocene (Roberts et al. 2014). This study illustrated the long-term continuity of large-bodied fauna within South Asia with only a single taxon of twenty-four identified as having gone extinct across the subcontinent (Roberts et al. 2014).

The "Purana" basins are Proterozoic in age. They are scattered all over Peninsular India. A common lithology is silica cemented sandstone or quartzite which forms prominent hill ranges, ridges and escarpments with ledges, overhangs and caves. These hard quartzites would have been one source of raw material for stone tools.  The rocks are also fractured and networks of pervasive cracks allow the storage and movement of groundwater.

The map (from a different paper) below shows the distribution of Middle Paleolithic sites (red dots) in India, Arabia and Eastern Africa. I have outlined in black (very approximate!) the location of three Purana basins. V stands for Vindhyan, C for Cuddapah and B&K for Bhima and Kaladgi.

 Modified from Huw S. Groucutt et.al. 2015

This paper have lots of information about climate change, ecology and stone tool record found in India. The authors discuss the Late Acheulean (130k - 100 K) ,  Middle  Paleolithic (94k - 34 K) and the Late Paleolithic ( < 45 K). These terms refer to particular styles of stone tool manufacture.

The India skeletal fossil record is very poor. However, based on comparisons with Middle Paleolithic of Africa and Homo sapiens fossils and tool associations in SE Asia and Australia, the authors are in favor of a wave of  Homo sapiens migrating into India as early or perhaps a little earlier than 100 k ago. This was followed by a later wave around 50 k years ago.  Do changes in cultural style and tool use point to changing populations.. with an intrusive population replacing an earlier one?.. that is an intriguing question. Some recent genetic work suggests that people from these earlier migrations died out without leaving a genetic legacy in us. All non African humans have descended from migrants who left Africa between 50K-80K years ago.  I had summarized these results in an earlier post on human population continuity in India.

See also other papers from this special volume of Current Anthropology on Human Colonization of Asia In the Late Pleistocene. 

Open Access.