ESR Dating of Fossil Shells from Maya-Wharf Boring Core in Hyogo Prefecture

Twelve fragments of fossil shells were sampled from the Ma8, MalO, and Ma 12 marine clay bedsof the May a-Wharf boring cores in Hyogo Prefecture, western Japan, and dated by ESR. Total dosesof the fossil samples were determined by an additive dose method with exponential fitting .Annualdose rates were calculated based on the contents of 238U, 232Th, and 40K in the sediments around thefossils. Because of disequilibrium between 238U and 226Rn and excess of 210Pb, annual dose rateswere recalculated for individual samples considering 222Rn-loss and contribution of the 210Pb excess,in addition to correction of water content of the sediments and of thickness of the fossil shells.The ESR dates thus obtained were 92 ± 15 to 127 ± 20 ka for the Mai2, 216 ± 45 to 322 ± 79 kafor the MalO, and 227 ± 65 to 246 ± 41 ka for the Ma8. The former two dates are consistent withthe previously estimated ages of the Ma 12 and MalO, while the latter dates are much younger thanages of the Ma8 estimated based on the fission-track age of an intercalated tuff and correlation withmarine oxygen isotope stages.The younger ESR dates for the Ma8 are considered to arise from the following reasons:underestimations of total doses due to extension of ESR-signals by the higher-rate radioisotopeaddition to the saturated fossil shell samples, and overestimation of annual doses based on limitedsediment samples. Although such problems remain, our results suggest the possibility of systematicESR dating of fossil shells in long boring core sediments.