Using
Glenn Milne's sea-level model to estimate the age of the sunken
cities of Cambay
Carbon-dates
for artifacts that have been submerged in salt-water for a
long period of time may possibly underestimate the age of
those artifacts. This means that the piece of wood might
possibly be even older than 9,500 years. |
The submerged
cities discovered by India's National
Institute of Ocean Technology
in the Gulf of Cambay are around 40 metres deep. Basic
knowledge of earth sciences leads to the conclusion that, by virtue
of their great depth, the Cambay cities are very probably more than
6,000 years old - and hence stand in contradiction to orthodox views
on the origins of civilisation.
But as part of
our research for Underworld, we wanted to know whether it would be
possible to get more precise lower limits on the dates of submerged
ruins. For this we went to geologist Dr. Glenn Milne at the
University of Durham, who has developed a state-of-the-art model of
sea-level change. Glenn's model takes into account a number of
major processes that affect sea-level and it can date the
submergence of underwater ruins anywhere in the world with
relatively good precision and reliability. Generally speaking,
it can estimate the date of submergence to within 1000 years.
An important exception concerns areas that have been subject to
large-scale vertical tectonic motion. Such motion cannot be
accurately modeled at the global level. Since Cambay is a
tectonically active area, we have to keep tectonics in mind when
using Glenn Milne's sea-level model to estimate the minimum age of
the Cambay cities.
The first
approach we took involved inundation maps. Glenn Milne made
these maps by applying his sea-level model to topographic (or
bathymetric) data for the Gulf of Cambay. By knowing how deep
the gulf is today, and by also knowing how this depth has changed
since the end of the last Ice Age, Glenn Milne can produce graphical
maps showing what land was exposed at various points in time.
These maps suggested that the Gulf of Cambay was only formed between
7,700 years ago and 6,900 years ago. The map for 7,700 years
ago showed the gulf to be dry land, whereas the map for 6,900 years
ago showed it to have been flooded by the Indian Ocean, creating the
gulf that we recognize today. In Underworld and our subsequent
correspondence with the press we have put forward the date of 7,000
years as a very conservative minimum amount of time since
submergence of the Cambay cities.
In recent
weeks, Glenn Milne took an alternative approach to putting a minimum
date on the submergence of the Cambay cities. Instead of using
inundation maps, Glenn Milne ran his model on a specific pin-point
location. Inputting the approximate long/lat co-ordinates of
the cities, followed by the NIOT's empirical measurement of the
depth of those cities (40 metres) produced a time since submergence
of 12,000 years. This result is more compatible with what one
would expect given knowledge of the depth of the site and of late
glacial sea-level rise. However, there is clearly a huge of
difference between this estimate and the earlier one of 7,000 years.
How did this
disparity come about?
And why did we
go with the date of 7,000 years in Underworld even though it is,
arguably, excessively conservative?
The answer to
the first question is simple but unexpected. To create the
inundation maps, Glenn Milne applied his model to a global
electronic topographic dataset. This dataset is called
Terrain-Base. Terrain Base is state-of-the-art and used in top
universities around the world, but since our planet's oceans have
not been fully explored and bathymetric data about them has not been
fully compiled and converted into an electronic format, this does
not make it totally reliable. (The
dataset is public and can be accessed through America's National
Geographic Data Center).
We did not expect the topography to be wildly inaccurate, but we now
know that it is - at least for the region we're interested in here.
TerrainBase tends to underestimate the depth of the Gulf of Cambay
by 15 to 20 metres relative to a 1993 sounding chart made by the US
Defense Agency. So as far as Glenn Milne's model was concerned
when making the inundation maps, the Gulf of Cambay was much
shallower than it actually is and was submerged much later than it
probably was.
There are a
number of reasons why we went with the conservative date of 7,000
years for the submergence of the cities. Firstly, towards the
beginning of our interest in the Gulf of Cambay, we were not
absolutely sure whether the depth of 40metres given in the Indian
press reports was accurate or whether perhaps it was a maximum depth
reached by specific parts of the cities - parts that may have been
too small to show up on the inundation maps. At that time we
were more inclined to trust the Terrain Base bathymetry, and had no
real reason not to. Secondly, even as involvement with the
Cambay story progressed, our interested in it remained primarily
archaeological rather than geological. The cities would still
be pre-Harappan and anomalous (relative to conventional views on the
origins of civilization) regardless of whether they were submerged
7,000, 10,000 or 12,000 years ago. We therefore continued
putting forward the date of 7,000 years while explicitly emphasizing
that it was a conservative
minimum period of time
since submergence. Thirdly, in the absence of direct
information on tectonics in the Gulf of Cambay, we felt that the
date of 7,000 years was perhaps not excessively conservative.
What if some huge (but hypothetical) earthquake had caused the
cities to subside by 15 metres at some time during the last 7,000
years? From the point of view of Milne's model, this would
mean that the cities were only 25 metres deep rather than 40 metres,
because Milne's model does not take tectonic motion into
consideration. Therefore and given our specific archaeological
interests, the possibility of extreme tectonic subsidence at least
partially cancelled out TerrainBase's 15-20metre underestimation of
depth in the Gulf of Cambay.
Given the
location of the cities, it is very probable that they have been
affected by vertical tectonic motion to some extent. However,
there is no direct evidence to show that a large proportion of the
depth of these cities has resulted from tectonic subsidence.
The fact that
the cities are still in tact strongly suggests that tectonic motion
has been neither massive nor abrupt. Certainly, anything more
than ten metres of subsidence in the last 7,000 years would be
highly unlikely and without geological precedent. Ten
metres of tectonic subsidence would still leave 30 metres of depth
to be accounted for by slower eustatic and isostatic processes, such
that a late glacial date for submergence of the cities would still
be most probable. (And after all, the most likely time
for dramatic tectonic activity would not be in the last 7,000 years,
but around the late glacial period when sea-levels were in their
greatest state of flux and causing greatest disruption to the forces
on the earth's crust.)
If tectonic
subsidence has played any role in the subsidence of the cities, then
Glenn Milne's later result of 12,000 years would provide an upper
limit on the time since they were submerged. Combined with our
conservative lower limit of 7,000 years, we would expect the cities
to have been submerged at some point between 10,000 and 5,000 BC.
Indeed, the carbon-dates released recently by the NIOT fit well
within these limits. The NIOT's carbon-dating placed a piece
of cut wood, recovered from a shallow layer on top of the city, to
7,500 BC (calibrated).
Does this mean
that the cities themselves are 9,500 years old? Yes - they are
very unlikely to significantly younger than that. But, for a
number of reasons, we suspect that the cities might turn out to be
even older. Firstly, the cities are enormous and
sophisticated, so they could not have sprung out of nowhere.
This means that there could well be layers of habitation deeper and
older than that which yielded the cut piece of wood that was
carbon-dated. Secondly, though we have no direct evidence on
tectonic motion in the precise location of the cities (and will not
have until core-sampling has been carried out by the NIOT on a
future expedition), we are inclined to suspect that such motion has
contributed a lot less than 10 metres to the depth of the cities,
meaning that submergence may have happened closer to Glenn Milne's
upper limit of 12,000 years ago than to his lower limit of 7,000
years ago. Part of this suspicion arises from an article by
Prasad et al. (Geomorphology,
1998, vol.25, no.3-4, pp.207-223)
which suggests that the land in the region of Lothal (near the Gulf
of Cambay) has actually experienced tectonic uplift since the
incursion of the gulf. Lastly, carbon-dates for artifacts that have
been submerged in salt-water for a long period of time may possibly
underestimate the age of those artifacts. This means that the
piece of wood might possibility be even older than 9,500 years.
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