On Fri, 28 Apr 2000, Neil Campling wrote:
> 1) Terms for periods of 1000's of years: Use the informal SI units of
> Ka, Ma, and Ga, for kilo annum, mega annum, and giga annum. These units
> are defined as years before present with present fixed at 1950 as with
> radiocarbon dates. So Ed's examples would be: Holocene - 10 Ka to
> present, Devensian - 120 Ka to 10 Ka, Age of the Dinosaurs - 320 Ma to
> 65 Ma, etc, etc.
ka, Ma and Ga, in and of themselves do not denote time relative to a
reference point ("present"), they are simply units of time like hours,
minutes amd seconds. Further there is no formal agreement on the
reference point that should be used for BP, except for radiocarbon dates
with their nominal 0 BP of 1950 AD/CE, and that is irrelevant if you want
calendar years anyway. Partly the reference point is hardly worth worrying
about if you are talking Ma, because we haven't the precision to measure
the difference between "before 1950" and "before 2000", and in many cases
the difference between BCE and BP goes the same way. When you are talking
a few ka however the reference point for BP is often an issue, and in
published dates is not always stated. I think the thesaurus could do well
to stick to a well known calendar like AD/CE and BC/BCE for periods where
the reference point does matter (i.e. most of the Holocene, and
increasingly the late Pleistocene). Unfortunately the relevant ISO
standard only covers years AD/CE, so there is not even an international
standard for years BCE!
The Holocene actually started about 9ka BCE or 11 ka ago, which equates to
10,000 radiocarbon years BP (10 ka BP - formally BP *implies* radiocarbon
years, as that is the only internationally agreed definition).
Andrew
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Dr. Andrew Millard [log in to unmask]
Department of Archaeology, University of Durham, Tel: +44 191 374 4757
South Road, Durham. DH1 3LE. United Kingdom. Fax: +44 191 374 3619
http://www.dur.ac.uk/~drk0arm/
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