房総半島南東岸における旧汀線の指標としてのヤッコカンザシ

Abstract

In order to determine the height and age of paleo sea level precisely, it is necessary to find suitable indicators. The tube worm Pomatoleios kraussii (BAIRD) was an excellent indicator of sea level stands on the southeast coast of Boso Peninsula. Studies of vertical distribution of P. kraussii, living and emergent at the time of the 1923 Taisho Earthquake, showed that the upper limit of the P. kraussii zone was located at 0.1±0.1m above mean sea level. At Hiraiso, the upper limit of the distribution of fossil P. kraussii emergent at the time of the 1703 Genroku Earthquake was 6.8m. Using the relationship between the upper limit of P. kraussii and mean sea level, paleo sea level prior to the Genroku Earthquake was estimated to be 6.7±0.1m.On dating radiocarbon ages of small samples such as P. kraussii tubes, the alternation of samples might lead to incorrect results. Three out of four radiocarbon ages of fossil P. kraussii tubes emergent at the time of the 1703 Genroku Earthquake were dated to be about 100-250 years older than the time of the earthquake. Observation of these tubes by scanning electron microscopy revealed that the tubes which gave the older ages had been recrystallized, and did not yield reliable ages.