Abstract
Plant opals (PO) begin to suffer from chemical dissolution immediately after they come into contact with soil material. The dissolution effect appears as pinholes corroded on the surface of PO; these increase with time in size and number, suggesting that they could be useful as an age indicator of the buried soil.“Age”here used means period passed since the formation of the soil.
The degree of corrosion was conveniently classified into one of seven grades (1-7), which could be judged by visual observation of the number and size of the pinholes on the surface of PO in a specimen mounted on a glass slide under the microscope and was designated as weight mean value of the grade numbers. To get a reliable result, the measurement of the degree of corrosion must answer the following basic questions:
(1) How many PO particles should be observed to give a stable mean value?
(2) Are the variances small enough among the values obtained from: (a) plural slide specimens of the identical soil sample, (b) repeated measurements of the identical slide specimens, (c) slide specimens of plural samples taken from similar soil, (d) slide specimens of samples from soil at a similar horizon but at different sites, and (e) measurements by several persons of the identical slide specimen?
As the results of the examinations showed few problems with variances in the measurements, a standard procedure could be established: taking one sample from any soil at any site, preparing one slide specimen from the sample and measuring the degree of corrosion for 200 particles of PO.
Soil samples from several sites at the foot of Mt. Fuji were subjected to measurement according to the standard procedure. The degrees of corrosion of PO obtained had a high positive correlation to the 14C or indirectly estimated ages of the soils at each site. But the regression coefficients varied from site to site, indicating that it is so far difficult to derive absolute ages exclusively from the measurement of the degree of corrosion of PO.
