Harappan collapse: Prof. Peter Clift
The fall of the Harappan Civilization has been associated with
rapid weakening of summer monsoon rains. New work now shows that
changing river patterns may also have played an important part
in their demise. Peter Clift* reports.
Geoscientist 19.9 September 2009
Throughout history human societies have prospered or failed,
not only because of their relationships to other cultures, but
also because of environmental conditions affecting a range of
key issues, including agriculture and drinking water supply. Periods
of rapid climate change are particularly dangerous, as existing
communities struggle to adjust to new conditions. Studying cultural
decline in periods of climate change past, should help us plan
for our own uncertain future.
No period is better for illustrating the interrelationship of
environment and culture than the Late Neolithic, when the Indus,
Akkadian, and Longshan civilisations all appear to have experienced
a major shift in the way they lived. The Indus Valley, or â€Harappanâ€
civilisation (see Box) was one of the earliest advanced urban
cultures known to archaeology - and appears to have collapsed
around 2000 BCE.Â
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Earlier palaeoclimate work has suggested a link between the end
of settlement in major urban centres and a rapid weakening of
summer monsoon rains. However, life may prosper in arid environments
as long as it can be sustained by large river systems. The Leverhulme
Trust has therefore funded a new study involving a diverse international
group of scientists to explore the role that drainage reorganisation
in the Indus Valley may have had on societal change at that time.
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Our campaign of trenching and drilling across the flood plain
of the Indus River system in western India and Pakistan is beginning
to quantify, for the first time, how the Indus River and its major
tributaries have changed over the last 8000 years - a period when
summer monsoon rains were stronger than they are today. Although
sedimentation continues to be active in the lower reaches of the
river system, the new data show a cessation in sediment deposition
in the north as the monsoon weakened and the supply of sediment
from the Himalaya reduced. Provisional age data now show that
between 2000 and 3000 BCE, flow along a presently dried-up course
known as the Ghaggur-Hakkra River ceased, probably driven by the
weakening monsoon and possibly also because of headwater capture
into the adjacent Yamuna and Sutlej Rivers.
The possible impact of drainage reorganisation on early cultures
in South Asia has long been a matter of debate, but has been consistently
hampered by a lack of hard data. Major river reorganisation causes
many problems for civilisations - as can be recognised in the
repeated changes in course of the Yellow River in China over the
past 1000 years, and the subsequent displacement of populations.
More recently, the Kosi River floods of Nepal and India in summer
2008, caused massive disruption.
Abandoned former courses of the River Indus have also long been
recognised, in the form of dried-up river channels along the edge
of the Thar Desert. These were observed as long ago as the 1920s
and 30s, in the work of Sir Marc Aurel Stein. More recently, they
have been mapped in great detail using aerial and satellite images,
and it has been possible to delineate the course of a now defunct
â€Ghaggur-Hakkra†River, which
once ran from the Himalayas, between the Sutlej and the Yamuna
Rivers. This palaeo-river was well positioned to have sustained
the Harappan civilisation; though the age of water flow, and the
patterns of interconnection between channels (and to the Indus
itself) have remained speculative.
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Drilling
Following drilling in the Indus Delta by myself, Liviu Giosan
(Woods Hole Oceanographic Institution) and Ali Tabrez (Pakistani
National Institute of Oceanography) it has become clear that since
the Last Glacial Maximum (around 20,000 years ago) the Indus experienced
great changes in the composition and volume of sediment flowing
through its channels. These changes appear to be linked to the
changing strength of the summer monsoon rains.
Building on this earlier study, Clift and Giosan, together with
Mark Macklin (Abersytwyth University) have initiated a new project
to constrain how the river has evolved since the middle Holocene,
~5000 BCE. In 2008 Anwar Alizai and Sam VanLaningham (University
of Aberdeen) undertook the first field excavations on the floodplain
in the Pakistani state of Punjab, where the supposed Ghaggur-Hakkra
River used to flow.
Using mechanical diggers (and local workmen where these were
not available) they dug trenches into the deposits of the Holocene
outwash plain. Targeting the channels themselves was hard, even
with the aid of high-resolution satellite images. But in the end
they were able to sample the flood plains of the palaeo-rivers,
which allowed the team to start narrowing down when the river
was flowing, and where its sediments were coming from.
Dates of sedimentation were obtained by radiocarbon dating freshwater
gastropod shells and woody material recovered from the pits. Together,
these showed that active river-flow along the Ghaggur-Hakkra had
finished before 2000 BCE, at least in that region. New optically
stimulated luminescence (OSL) ages, which measure the time since
sediment was last exposed to sunlight (produced by Geoff Duller,
Helen Roberts and Julie Durcan at Aberystwyth) support this general
scenario.
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Today, the Ghaggur in India is a very small river, within a modest
mountain catchment. How could it once have been a much larger
stream? We believe it is possible that the river was once swelled
by other headwater catchments that are now diverted into other
directions. The neighbouring Yamuna and Sutlej Rivers are the
most likely candidates for this, and could well have been captured
from the Ghaggur during the Holocene. If either or both of these
streams formerly flowed into the Ghaggur channel then the river
could have been very much larger than it appears today.
It is clear that the Indus has experienced major changes since
the mid Holocene, when the whole system appears to have been in
active deposition. However, since that time the northern reaches
of the Indus and its various major tributaries have incised river
valleys 20â€30 m deep. What caused this change
in behaviour? A number of possibilities are presently in contention.
While delta drilling proved that the early Holocene (11,000 â€
8000ka) was a time of very rapid sediment flux, probably driven
by fast erosion under the influence of a strong summer monsoon,
the period since 8000 years ago has been one of weakening rains
and reduced sediment flux, as established from lake sediment records
in India and in cave records from Oman. In this case, the river
may be â€cannibalising†itself
in its upper reaches, reworking the older floodplain sediments
over which it is now flowing.
In order to reconstruct what the river system looked like at
any given time in the past we have to know the provenance of the
sediments in its overbank deposits. This can tell us how each
tributary was connected to its neighbours and indeed to the trunk
stream itself. In this respect the Indus is a great system for
geologists because it receives sediment from several sources,
with each characterised by quite different geochemical characteristics
and ages.
Zircon to the rescue
The western Himalaya are especially heterogeneous with respect
to the U-Pb age of zircon grains. Grains from the Lesser and Greater
Himalayan Range display old zircon ages of around 400 Ma, 1000
Ma and 1800 Ma and older, whereas the Karakoram and Kohistan typically
display ages younger than 250Ma - representing Andean-style magmatism
along the southern edge of Asia, prior to its collision with India.
This makes changes in large-scale drainage or erosion patterns
easy to spot.
New technology (see Box) now allows large numbers of single grains
to be analysed quite rapidly. U-Pb dating of zircon grains tells
us when each grain cooled below 750?C (i.e. the age of its crystallisation
from its source magma). The Laser Ablation Inductively Coupled
Plasma Mass Spectrometer (LA-ICP-MS) at University College, London,
now allows around 100 such grains to be dated every day. Such
large numbers are needed for each sample in order to generate
statistically reliable data sets.
Andrew Carter who runs this operation has shown that grains younger
than ~250 Ma are unique to the main Indus River, whereas the major
tributaries of the Indus that join from the east are dominated
by much older grains sourced from the Greater and Lesser Himalaya.
Thus zircon grains have the potential to show us whether sands
were deposited only from the Ghaggur-Hakkra River, or also had
contributions from the main Indus River too.
(For more information on the science of Single Grain Provenance,Â
click here) http://www.geolsoc.org.uk/webdav/site/GSL/shared/images/geoscientist/Geoscientist%2019.9/Figure%208resized.JPG
Initial analyses of the sands sampled in pits on the course of
the Ghaggur-Hakkra River at Fort Abbas have shown a significant
number of grains with young U-Pb age signatures. At first sight
this would seem to require a huge swing in the Indus River, since
they were deposited more than 5000 years ago. Although it is possible
that the Indus flowed this far east (c. 200km further east than
its present course) it seems more likely that the sands found
have been reworked from the sand dunes of the Thar Desert, which
directly abut the river valley.
Nonetheless, this observation is important. If the Ghaggur-Hakkra
River did flow through this channel and connected with the Indus
5000 years ago, then it appears that the river must have ceased
to flow before the analysed sands were blown by wind into its
channel. What might have caused this cessation in river flow?
Although the headwaters of the ancient river may have been lost
by capture it is also possible that the river simply died out
because its supply of rainwater fell. Other radiocarbon ages from
farther south (around the enigmatic Nara River valley) suggest
that the sand dunes of the Thar Desert expanded in that region
at 5000â€6000 years ago. Not only is that conclusion
consistent, it is also corroborated by other climate indicators,
suggesting a steady decrease in summer monsoon rains.
It now seems that the river system in this region was indeed
responding to climate change taking place during the mid to late
Holocene. At present, our age control is insufficient to allow
conclusive links to be made with the evolution of human societies.
Early signs are encouraging, however, that we shall be able to
build intriguing links between climate and cultural development
in SW Asia in the not too distant future.
This is of more than academic interest at a time of accelerating
climate change. A more detailed understanding of how the Indus
valley river system has responded to climate change during the
Holocene will allow for better planning for anticipated changes
driven by global warming.
Author affiliation
*Prof. Peter Clift, University of Aberdeen, is the leader of
the Harappan investigation.Â
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