Thermoluminescence age determination for chemically selected samples of White Pumice tephra (WP) in the Kamikita plain, northeast Japan

Abstract

Using thermoluminescence (TL) technique (multiple aliquot regeneration method), we dated White Pumice tephra (WP), an unconsolidated tephra on Middle Pleistocene (MIS 7) marine terrace deposits at the Kamikita plain, Aomori Prefecture. We collected 6 and 10 samples from WP on outcrops at Kacchi and Shichihyaku, respectively for evaluation of chemical leaching. Concentration of radiometric elements (U, Th and K2O) was measured for all 16 samples. Among them, five samples were selected for TL age determination. The TL ages of three samples (187±29 ka, 230±23 ka and 229±38 ka, weighted mean : 216±15 ka) well coincide with a reported age of WP (MIS 7b ; 205-210 ka), correlated with δO stratigraphy of deep-sea sediments. The TL ages of other two samples (165±28 ka and 265±33 ka) are beyond the error range to 205-210 ka at mid MIS 7. The chemistry of former three samples falls in the concentrated zone of U-K2O and Th-K2O diagram. Other two samples were plotted far from concentrated zone of the same diagram. For TL age determination of unconsolidated tephra, annual dose evaluation through simple screening of chemical character is critical to obtain reasonable results.