老於60萬年洞穴石筍鈾釷定年開放系統行為之探討

Abstract

鈾釷定年是一項在岩石礦物中強大的定年工具，尤其被廣泛應用在碳酸鹽類的古氣候與古環境重建。鈾釷年紀是在封閉系統下由鈾系衰變當中之鈾-238、鈾-234及釷-230這三種核種的相對豐度而求得。然而，在核反彈效應(alpha-recoil)的過程中，鈾釷核種的遷移可能會使真實年紀有所偏差，尤其是對於老於60萬年的碳酸鹽標本來說。本研究採集了沉積時間介於60-90萬年前間位於義大利接近地中海北部，總長為84公分的一株石筍；29個位於不同層位的石筍樣本做了鈾釷含量與同位素組成的定量分析。我根據 (1)鈾釷定年理論、(2)利用海洋岩芯紀錄賦予石筍年紀的調整方法以及 (3)提出基於鈾釷核種在毫米尺度碳酸鈣晶格中擴散行為之新的假設，來推演鈾-234與釷-230的可能遷移。微米尺度的礦物分析也被用來觀察是否有後期沉積的碳酸鈣形成進而造成年紀的偏差。結果顯示這些倒序的年紀受到鈾-234與/或 釷-230的開放系統行為之影響。經由掃描式電子顯微鏡與光學顯微鏡的觀察發現晶格缺陷中孔洞愈多則核種遷移的程度愈大而造成年紀的偏差，在洞穴碳酸鹽晶格裂隙中的再結晶與高鎂質方解石礦脈也是導致鈾釷定年系統開放的原因。本研究指出我們在提供可信的鈾釷年紀之前，最重要的事情就是進行詳細的礦物觀察。

U-Th dating has been one of powerful chronological tools on rocks and minerals. Especially, it was widely applied to carbonate-inferred paleoclimate and paleoenvironment reconstructions. U-Th dates are generally derived with relative abundances of three nuclides of 238U, 234U, and 230Th of the 238U decay series under a close system. However, movement of U-Th nuclides during alpha-recoil processes can bias the true ages, especially for carbonates with ages larger than 600 thousand years ago (ka). Here, I collected a 840-mm-long stalagmite with a deposition interval from 600 to 900 ka from Italy nearby the northern Mediterranean region. U-Th isotopic compositions and contents were determined at 29 depths. The determined U-Th ages are out of stratigraphic order and are not allowed to construct a robust age model. I addressed the possible migrations of nuclides of 230Th and 234U at depths based on (1) U-Th dating theory, (2) an age tuning method by matching stalagmite and marine proxy records, and (3) one hypothesis of similar diffusion behaviors for each nuclide on millimeter scale in carbonate lattice. Micro-domain mineral analyses were also conducted to evaluate age bias by the post-deposition formation of carbonate. Results show that reversal ages can be caused by open-system behaviors of nuclides 230Th and/or 234U. Observations by scanning electron microscopic and optical microscopic techniques show that more pores in the lattice defect can bring in more significant nuclide migrations and bias U-Th ages. The speleothem recrystallization and Mg-rich calcite veins in the lattice fractures are also the factors causing the U-Th dating open system. This study indicates that careful mineral inspection is the first step required to offer the fidelity of determined U-Th ages.