2024-03-28T19:21:37Z
https://nagoya.repo.nii.ac.jp/oai
oai:nagoya.repo.nii.ac.jp:00030348
2023-01-16T04:23:50Z
879:1183:2103:2514
An Attempt to Obtain Accurate Concretion Age of Beachrock Sample (2)
ビーチロック試料の正確な膠結年代の決定方法 (2)
小元, 久仁夫
100776
藁谷, 哲也
100777
塩見, 昌司
100778
大八木, 英夫
100779
横尾, 頼子
100780
池田, 晃子
100781
Omoto, Kunio
100782
Waragai, Tetsuya
100783
Shiomi, Atsushi
100784
Oyagi, Hideo
100785
Yokoo, Yoriko
100786
Ikeda, Akiko
100787
conglomeratic beachrock
AMS-14C dating
pretreatment technique
elemental analysis
2020-06
Formation age of beachrock has been determined based on 14C age of marine carbonate samples (i.e. fossil shell and coral) collected from beachrock, however the age often indicates the dead time and does not indicate correct formation age of beachrock. The formation age of beachrock is the time when the inter-tidal sediments were cemented by physicochemical precipitation of high-Mg calcite and aragonite from seawater or meteoric water as a result of high temperatures, CaCO3 supersaturation, and/or evaporation. Therefore it is necessary and indispensable to measure directly the beachrock cement (CaCO3). We collected 14C dating samples from the coast of Sasebo, Nagasaki prefecture, Zamami Island, Aka Island, Bise and Tsuha coasts of Okinawa prefecture. Aka Island and Zamami Island mainly consist of sandstone and phyllite. The cemented materials attached to the behind of boulders and cobbles consisting conglomeratic beachrock. This means that conglomeratic beachrock never originally include marine calcium carbonate. We used a small electric hand drill with a stainless steel drill edge of 2~3 mm or a stainless steel brush of 1cm in diameter, and drilled or abraded the surface of the cemented material. Then we collected ca. 20~50 mg powder sample for AMS-14C dating and elemental analysis by ICP-MS. Two cemented materials collected from Zamami Island yielded 14C ages of 1040 BP and 1020 BP, while Tridacna squamosa and a coral sample indicated 1600 BP and 1490 BP. Another cemented samples collected from Aka Island yielded 14C ages of 1530 BP and 1340 BP. Shell fragments and two coral samples yielded ages of 1210 BP, 1060 BP and 890 BP, respectively, however Strigatella paupercula sample indicated Modern (103 pMC). If we consider the age of cement material indicates exactly an accurate formation age of beachrock, then the conglomeratic beachrock of Zamami Island was formed during 1040~1020 BP and beachrock of Aka Island was formed 1530 BP and 1340 BP.
ビーチロックの形成年代は,これまで生物化石の年代で決定されてきた。しかしその年代は生物の死亡年代を示すのみであり、ビーチロックの膠結年代を示すものではない。本研究では、ビーチロックの細粒物質間を膠結、または生物化石表面に付着しているセメント物質(炭酸カルシウム)を物理的・化学的に分離する手法を確立し、分離したセメント物質の年代をAMS-14C年代測定により決定する方法の開発を目的としている。本年度は、5地点のビーチロックから試料を採取して、AMS-14C年代測定を行い、沖縄県阿嘉島と座間味島の礫岩質ビーチロックの膠結年代を決定することができた。
departmental bulletin paper
名古屋大学宇宙地球環境研究所
2020-06
名古屋大学年代測定研究
4
9
15
jpn