review article Indian script

Tue Aug 27 04:48:58 UTC 1996

On Fri, 23 Aug 1996, Harry Falk wrote:

> both of them and they firmly believe in their C14 dates. But if you
> check the latest article in Cambridge Archaeological Journal 6,1 (1996),
> 73-97, you have to realise that their material is rather scanty: 8 sherds
> with 2 to 4 characters each. Their ideas about which character looks
> old and which does not displays a not too deep acquaintance with that
> script at all. They make a whole set of conclusions depend on
> their dating and their interpretation of the sequence of their sherds.
> There is something I cannot evaluate. On page 79 they state that
> "no Southern Hemisphere correction was used as the validity of such
> a correction in the latitude of Sri Lanka has yet to be
> established". What does that mean? Would such a correction change
> the dates to something earlier or later? What does that
> correction mean in those areas where it has been established?
> To make it short: I have severe doubts about the conclusions reached

Radiocarbon dating comes from measuring the residual radioactivity of C14 
in samples. Carbon occurs naturally as three isotopes, C12, (the most widely 
found), C13 and C14. C14 is a radio-isotope, which means that 
it undergoes radioactive decay, with a characteristic rate. The half-life 
of a radio-isotope is the time required for the amount of the 
radio-isotope to decrease to half its initial value. According to the 
laws governing radioactive decay, the half-life has a characteristic 
value for each isotope, and is independent of the actual amounts of 
radio-isotopes present. 

Radiocarbon dating is useful for measuring the ages of fossils and other 
organic matter, which have a high carbon content. The technique is based 
on the fact that material buried in deeper layers is not in equilibrium 
with the relative levels of different carbon isotopes in the earth's 
atmosphere. As a result, the level of C14 in a sample from an 
archeological excavation would be depleted. This gives a measure of the 
age of the sample. Ideally, the geographical location of a find should 
not affect its radiocarbon date. In reality, that is rarely the case. 
There could be wide variations in the relative levels of carbon isotopes 
depending both on the actual age of a sample and its geographical origin.  

It turns out that the southern hemisphere correction for radiocarbon 
dating is a result of the fact that the southern hemisphere has a much 
larger proportion of its surface covered by oceans. Due to a difference 
in the rates of absorption of atmospheric carbon dioxide and 
equilibration with dissolved bicarbonate in the ocean, material found in the 
southern hemisphere is usually dated with some corrections to dates 
obtained in the northern hemisphere. Generally, the offset is of the order of 
40 years, so that after the correction is made, radiocarbon dates from the 
southern hemisphere will typically be some 40 years older. This 
correction applies to terrestrial samples. Marine samples would have a 
larger correction offset. One can imagine that a similar correction might be 
significant if we are dealing with archeological samples from a small 
northern hemisphere island where such oceanic exchange processes could be 
important. Note that the size of this correction would depend upon the size 
and the geological history of the island.  

I checked up on this with Dr. Tom Higham from a radiocarbon dating 
laboratory in New Zealand. His email to me is forwarded below. The website 
mentioned in Dr. Higham's signature file has a very good collection of 
resources for radiocarbon dating, including an informative page on standard 
corrections to radiocarbon dating.

So it turns out that not applying the southern hemisphere correction to 
the radiocarbon dating of Sri Lankan samples errs on the side of 
caution, if at all. 

S. Vidyasankar

ps. As an aside, it seems to be standard practice to date organic 
residues found associated with inorganic finds from an archeological 
excavation. This has some bearing on one of my earlier posts to this 
list, where I discussed a review of the problem of "Vasishtha's head" 
that was published in a Feb '96 issue of Frontline, an Indian newsmagazine. 

--------------Begin Forwarded Mail--------------

Date: Tue, 27 Aug 1996 14:14:16 +1200
From: Tom Higham <thigham at waikato.ac.nz>
To: Vidyasankar Sundaresan <vidya at cco.caltech.edu>
Subject: Re: Southern Hemisphere Corrections to Radiocarbon dates

Dear Sundaresan,

There is a 40 year offset in radiocarbon ages for terrestrial samples
between N and S hemisphere such that SH radiocarbon dates of trees,
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
charcoal, bone etc will be 40 years older on average. The reason, it is
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
thought is due as you say to the larger oceanic exchange process in the SH.
The 40 year offset is really only a consideration for calibrating C14 dates
(or converting c14 dates to calendar dates). If you are calibrating c14
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
dates you must use the calibration curves developed in the northern
hemisphere and prior to calibration you must take off 40 years from the
conventional radiocarbon age.

In terms of pottery, it depends on what it is you are dating. If you are
dating small organic residues of carbon within the pot then it will be a
necessary to account for the 40 yr offset when calibrating if the pot comes
from the SH.

>Also, I presume that such a reservoir
>correction would be significant for samples from small islands or near
>the coastlines of big islands. How significant would it be for the
>the interiors of large islands?

The problem at the moment is that the 40 yr offset is an average generated
from measurements of trees which grew in the 1800s in South Africa and
Holland. The exact size of the offset may in fact vary. Recent research has
shown that there is location dependent variability within the northern
hemisphere as well, so it may well be more complex than it seems, with the
possibility that you will need to develop local or regional calibration
curves to correct c14 dates to calendar dates.

>Additionally, is there a need to take into account such a correction for
>Sri Lanka, which is not technically in the southern hemisphere?

No, I wouldn't have thought so.

>In
>connection with the samples excavated in Sri Lanka, there was
>some mention that the necessity of the southern hemisphere correction has
>not been demonstrated for the latitudes in which Sri Lanka is situated.

I would agree with this. I doubt if Sri Lanka would have to have an offset
for calibration but the only way to be certain would be to measure the
radiocarbon activity of tree rings of known age from there and compare them
with trees of the same age from other places.

Hope this helps,

-------------------------------------------------------------------------
Dr Thomas Higham,                     *  Email: Thigham at waikato.ac.nz
Radiocarbon Dating Laboratory,   *  Phone: +(64) 07 838 4278
University of Waikato,                  *  Fax:   +(64) 7 838 4192
Hamilton,                            *  WWW: Radiocarbon WEB-info:
NEW ZEALAND.         http://www2.waikato.ac.nz/c14/webinfo/index.html