珊瑚骨骼中月解析度鉛同位素測量及其應用

Abstract

珊瑚骨骼中的微量元素與同位素分析，可作為地質科學、古海洋學、大氣科學與環境科學研究的有利工具。其中，鉛同位素訊號，可用來重建海洋與大氣中的鉛通量，計算出地區性甚至全球性的鉛傳輸系統模型。但是，受限於低鉛含量(10s-100s ng/g)，在過去研究，鉛同位素紀錄僅能被應用於多年至十年尺度之水文變化與人為污染的研究議題上。 此研究精進化學分析步驟與儀器測量方法，建立多頻道感應耦合電漿質譜儀(MC-ICP-MS, Thermo Electron NEPTUNE) 之高精度皮克級鉛同位素(204Pb, 206Pb, 207Pb, and 208Pb)分析。離子交換樹脂分離方法可從低鉛含量樣本中萃取出90%以上的鉛，化學空白值僅2 ± 3 pg。重複分析300s-1000s pg的國際鉛標準品，精準度可達 ± 2‰ (2σ)；此分析量相當於5-10 mg的珊瑚樣本，也就是說，現在我們可以測量月解析度的珊瑚鉛同位素變化紀錄。 本研究分析了一株採自越南中部Son Tra Island (16°12'59.4”, 108°1'57.1”)的微孔珊瑚。1978-1982年間的鉛濃度變化由35至70 ppb；鉛同位素資料範圍為：208Pb/207Pb = 2.4262-2.4813，206Pb/207Pb = 1.1226-1.1745。在時間序列上展現的特性如下：(1) 鉛同位素顯示出明顯的季節性變化。 (2) 季節性的鉛同位素變化指出兩個主要的來源端成份在乾季與濕季的不同混合結果，分別為河流沉積物與鄰近開放海域的海水。 (3) 鉛濃度紀錄呈現年間逐步上升的趨勢。 (4) 1980年夏秋期間的208Pb/207Pb和206Pb/207Pb紀錄出現異常低值，推測為亞洲夏季季風和颱風過後之西南氣流引進馬來西亞、印尼與泰國之氣溶膠，致使鉛同位素比值大幅降低。研究結果證明月解析的鉛濃度變化與鉛同位素紀錄可作為環境指標與來源示蹤劑，幫助我們了解歷史中人為活動造成的影響，過去自然水文變化及特殊事件的發生，更可作為未來永續發展政策的參考。

Pb concentrations and isotopic compositions in coral skeletons have been used to understand the annual to decadal trends of natural hydrological evolution, ocean circulation, and anthropogenic pollution. However, the low Pb content in coral skeletons, commonly of 10s-100s ng/g, has hindered determination of monthly variation of the coral Pb isotopic compositions. Consequently, the applications of Pb isotopic study have been limited. In this study, the chemical separation of Pb and instrumental technique were improved. A reliable procedure was developed for measuring the isotopic compositions (204Pb, 206Pb, 207Pb, and 208Pb) of pictogram Pb in coral skeletons. The isotopic analysis was conducted on a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), Thermo Electron NEPTUNE. The overall procedural Pb blank was only 2 ± 3 pg. Replicate measurements on a standard NIST SRM 981 showed that a precision of ± 2‰ (2σ) with 300-1000s pg of Pb could be achieved. This quantity is equivalent to 5-10 mg of coral sample. A sample size of 1.5 ng of Pb is recommended for routine analysis with acceptable external precision. The monthly-resolution Pb isotopic records during 1978-1982 of a coral Porites from the Son Tra Island, central Vietnam (16°12'59.4”, 108°1'57.1”) were reconstructed. The Pb concentrations vary from 35 to 70 ppb. The Pb isotopic ratios are 2.4262-2.4813 for 208Pb/207Pb, and 1.1226-1.1745 for 206Pb/207Pb. The results yield the following important points: (1) A clear seasonal-to-annual variation in monthly Pb isotopic composition was observed; which had never been reported in the literature. (2) The isotopic compositions 208Pb/207Pb vs. 206Pb/207Pb ratios suggest a mixing between fluvial sediments and adjacent open-ocean seawater from the South China Sea. (3) The Pb concentrations time series shows an interannual gradual increasing trend from 1978 to 1982. (4) The anomalously low 208Pb/207Pb (2.426-2.433) and 206Pb/207Pb (1.122-1.128) ratios in the summer and autumn of 1980 could be due to the contribution of aerosols from Malaysia, Indonesia and Thailand. The aerosols were likely delivered by the Asian summer monsoon and southwesterly flow after typhoon. The present study indicates that monthly resolved coral Pb isotopic sequence can be used as a proxy and tracer for the monthly-seasonal environmental change and the impact of anthropogenic perturbation. The coral Pb isotopic records may help understand the nature of regional hydrological dynamics, planning of resource management, and making of sustainability policy.