Sunday, December 22, 2019

Glacial Saraswati?: New Data On An Old Question

Did a perennial glacial fed river flow through the Indus Civilization region of what is now Haryana and Rajasthan? Previous work on the fluvial history of this region had indicated that a distributary of the glacially sourced Sutlej was flowing through a network of paleo-channels buried under the river now known as the Ghaggar until around 8,000 years ago. The Sutlej distributary system then died out, turning that river course into a smaller monsoon fed channel system.

For a more detailed history of research on this topic you can follow this link - Ghaggar /Saraswati Posts.

Recently, in November 2019, Anirban Chatterjee and colleagues published new data on deposits of grey sand in the subsurface of the Ghaggar channel and adjacent floodplains. The youngest of these deposits are 4, 500 years old. Geochemical fingerprinting points to High Himalayan granites and gneisses as their source. This likely extends the glacial phase of the Ghaggar to more recent times, until about the beginning of the urbanization of the Indus Valley Civilization (IVC).

Here is the abstract:

The legendary river Saraswati of Indian mythology has often been hypothesized to be an ancient perennial channel of the seasonal river Ghaggar that flowed through the heartland of the Bronze Age Harappan civilization in north-western India. Despite the discovery of abundant settlements along a major paleo-channel of the Ghaggar, many believed that the Harappans depended solely on monsoonal rains, because no proof existed for the river’s uninterrupted flow during the zenith of the civilization. Here, we present unequivocal evidence for the Ghaggar’s perennial past by studying temporal changes of sediment provenance along a 300 km stretch of the river basin. This is achieved using 40Ar/39Ar ages of detrital muscovite and Sr-Nd isotopic ratios of siliciclastic sediment in fluvial sequences, dated by radiocarbon and luminescence methods. We establish that during 80-20 ka and 9-4.5 ka the river was perennial and was receiving sediments from the Higher and Lesser Himalayas. The latter phase can be attributed to the reactivation of the river by the distributaries of the Sutlej. This revived perennial condition of the Ghaggar, which can be correlated with the Saraswati, likely facilitated development of the early Harappan settlements along its banks. The timing of the eventual decline of the river, which led to the collapse of the civilization, approximately coincides with the commencement of the Meghalayan Stage.

The geological work looks to be sound. The data on sediment fingerprinting overlaps with what we know about High Himalayan geochemical signatures and present day Sutlej sand composition.

I do want to comment on another sentence from the abstract (emphasis mine)-

"This revived perennial condition of the Ghaggar, which can be correlated with the Saraswati, likely facilitated development of the early Harappan settlements along its banks"

Saraswati is the name given to this river by the Vedic people. Correlation of the river's perennial phase between 9,000-4,500 years ago with Saraswati is valid only if  you can demonstrate that the Vedic people were inhabitants of this region from before 4,500 yrs ago. Geological studies cannot establish this. A combination of archeology, linguistics (cracking the Indus script would be nice!) and genetics will eventually answer that. The other scenario is that the Vedic people could have migrated into this region much later and began venerating a smaller monsoonal Ghaggar as Saraswati. Work by Liviu Giosan and colleagues suggests that stronger monsoons over the Siwaliks sustained sufficient flow in the old channels of the Ghaggar until the late IVC period (~1800-1600 B.C).

When did this river come to be called the Saraswati is still an open question.

Two recent genetics papers using ancient DNA recovered from the IVC site of Rakhigarhi and from Central Asia argue that people from the Pontic-Caspian steppes migrated into South Asia between 2000 -1500 B.C. bringing with them the Indo-Iranian branch of the Indo-European language family. These would presumably be the Vedic people.

Read these papers. They are interesting.

1) On the existence of a perennial river in the Harappan heartland.
2) The formation of human populations in South and Central Asia.
3) An Ancient Harappan Genome Lacks Ancestry from Steppe Pastoralists or Iranian Farmers.

Friday, December 20, 2019

Readings: Erectus SE Asia, Devonian Fossil Forest, Archean Iron Formations

Some selected readings:

1) New dates of Homo erectus from Ngandong Java shows late surviving populations until 117,000 to 108,000 years ago. A short clean summary by Razib Khan on SE Asian hominin diversity.

Southeast Asia during the Eemian was a hominin paradise.

Paper: Last appearance of Homo erectus at Ngandong, Java, 117,000–108,000 years ago.

2) Exquisite preservation of one of the earliest forests from the Mid Devonian ( ~385 million years ago) of New York containing a modern looking root system.

Paper - Mid-Devonian Archaeopteris Roots Signal Revolutionary Change in Earliest Fossil Forests.

Write up : The World’s Oldest Forest Has 385-Million-Year-Old Tree Roots.

3) Before around 2.3 billion years ago there was very little oxygen in the atmosphere. This was a time before the evolutionary invention of oxygenic photosynthesis wherein bacteria harvest electrons from H2O and release oxygen as a byproduct. Instead, during this time another photosynthesis pathway known as photoferrotrophy was prevalent. Here, bacteria use light and ferrous iron (Fe+2) to fix CO2 as biomass, releasing ferric iron (Fe+3) as byproduct. This ferric iron then accumulated to form large iron deposits. But these deposits lack organic matter. How to explain this if the iron was being produced from a biomass? Scientists point to a role of silica. At that time the oceans were saturated in free silica. Experimental work shows that in the presence of free silica cell surfaces repel iron hydroxides, thus creating a source of organic matter free iron deposits. This organic matter then was acted upon by methane producing microbes. The methane released kept the temperature of the earth warmer than it would have been under a dim early sun.

Fascinating story of the feedback between geology and evolution.

Photoferrotrophy, deposition of banded iron formations, and methane production in Archean oceans.