累積火山灰断面腐植層中の植物珪酸体による年代推定法

(I) 蓄積年率からの推定

抄録

Plant opals (PO) contained in the buried humic soils of volcanic ash profile are known to be mainly derived from grass. Thus, if we assume that PO in the soils is supplied at a constant rate from the grass and estimate the rate, then we can determine the ages of the soils from PO contents divided by the yearly supply or the accumulating rate of PO. The age here used does not mean the time passed since the soil formation but the period needed just for the formation of the soil itself.
1. Two methods for determining PO content in the soil were examined:
(1) Direct isolation of PO from the silt and fine sand fractions of the soil by floatation on a heavy liquid (2.2-2.3 sp. gr.). (2) Counting PO particles in the fine sand fraction under the microscope and changing the count % into the weight % by a previously determined exchange rate.
2. Three methods for determining the accumulation rate of PO were compared:
(1) Direct isolation of PO from plant tissues by wet or dry ashing followed by separation with a sedimentation method. (2) Counting PO particles in a weighed ash under microscope and changing the count % to the weight % of the ash by a preliminarily determined exchange rate. Methods (1) and (2) should be connected with the estimation of the annual production of grass species which can be obtained from ecological studies. (3) Determining the PO content in soil of known age by method 1-(1). The PO content divided by the age gives a mean annual PO accumulation rate.
The values of the accumulation rates deduced by the three methods were concentrated in a narrow range of 0.36-0.47mg/cm²/yr when a few anomalous values were excluded.
3. In two profiles, Yubunebara and Tanukiko at the foot of Mt. Fuji, both being composed of alternation of tephras and buried humic soils, the ages of the soils were determined using the method 1-(1) and accumulation rates of PO, 0.36 and 0.47mg/cm²/yr. If we assume that the ages thus obtained could approximately show the periods during the formation of the soils in the past and that the period of deposition of tephras was extremely shorter than the time needed for the formation of the whole profile, then the sum of the ages of the soils should roughly be equal to the age of the whole profile. Comparison of the former with the latter obtained by ¹⁴C dating showed that the coincidence was good in the period of less than 5, 000-6, 000y.B.P.