鈾、硫和鎘同位素在海洋生地化過程之應用

Application of Uranium, Sulfur and Cadmium Isotopes in Marine Biogeochemical Processes

余采倫(Tsai-Luen Yu)

指導教授 ： 陳于高 ； 沈川洲

國立臺灣大學/理學院/地質科學研究所/博士(2018年)

https://doi.org/10.6342/NTU201800801

摘要

在過去的幾十年裡，海洋中元素和穩定同位素（氫、碳、氧、氮和硫）示蹤劑已被廣泛應用於不同的海洋環境（近海、表水與深水環境），以了解全球海洋環流和海洋生物地球化學循環。本研究建立多接收器感應耦合電漿質譜（MC‒ICPMS）分析技術，精確測量鈾（U）、硫（S）和鎘（Cd）的同位素，具體的研究範疇包括：（1）測量東海海域海水234U/238U橫剖面和縱剖面來評估由河流所攜帶進入海洋的鈾總量；（2）測量海底沉積物孔隙水的硫同位素垂直剖面來了解硫同位素分化的機制；（3）探討海洋懸浮顆粒中鎘同位素分化來了解海洋微型浮游生物鎘同位素分化的現象。
東海海域表層海水δ234U的分化範圍從145到164‰，其分析精度為±0.3‰ （2σ）。研究結果顯示在開放性邊緣海環境之鈾混合行為有季節性的差異，且可利用δ234U示蹤沿海及陸棚區域內河流的訊號。鈾在東海海域行為有季節性變化，對於估算河流鈾輸入至海洋的通量具有相當大的影響。本研究係首次在開放性邊緣海觀測到非保守鈾混合。
台灣西南海域冷泉附近沉積物岩芯（長度為474 cm）中孔隙水的硫同位素分化範圍從16-41‰，其分析精度為±0.2‰（2σ）。其結果顯示，在硫酸鹽‒甲烷過渡帶之上（深度為海床下172.5 cm），硫酸鹽（濃度範圍為7‒27 mM）與硫同位素（變化範圍為22‒41‰）的相關性是遵循封閉性瑞利分餾模型（a closed system Rayleigh fractionation model）。在硫酸鹽‒甲烷過渡帶的硫同位素最大值為41‰，隨著深度增加，硫同位素值下降至16-20‰（深度為海床下172.5‒470.0 cm）。研析硫酸鹽-甲烷過渡帶以下硫酸鹽的不完全消耗，可能是硫化物氧化和硫酸鋇溶解所致。甲烷的厭氧氧化和硫酸鹽還原導致硫同位素顯著變化，因此硫同位素分化可作為甲烷的厭氧氧化識別的有效指標。
海洋浮游顆粒樣品中的Cd/P和鎘同位素的變化範圍分別為0.11~0.29 mmol/mol和-4.1~+1.2，其中鎘同位素的分析精度為±0.9（2）。在10-63 μm浮游顆粒中觀察到顯著輕於海水的鎘同位素值，推測是因為浮游植物優先從周圍海水吸收較輕的鎘同位素的緣故，而在大尺寸浮游顆粒（363-20,000 μm）中沒有發現到顯著的鎘同位素變化。此鎘同位素值差異和同位素混合值的特徵反映著食物鏈的轉變。鎘同位素研究可以提供海洋鎘循環的機制，並且可能是古初級生產力識別的有效指標。
本研究選擇鈾、硫和鎘同位素來了解海洋生地化過程，結合以前的相關同位素的研究，明確地指出MC-ICPMS技術可測各種元素的同位素組成且應用於海洋環境。我們可以預料在未來的幾十年裡，它們將被廣泛應用於深入了解海洋生物地球化學過程、海洋環流、古海洋學和古氣候學。

關鍵字

鈾 ； 硫 ； 鎘 ； 同位素 ； 多接收器感應耦合電漿質譜儀

並列摘要

Element,　such as H, C, O, N and S, and their isotopes have been recently used as geochemical tracers to understand biogeochemical cycles and oceanic circulation in different marine territories, which includes coastal/shelf zones, surface and deep-sea oceans,. Here, this study proposed to use isotopic signatures of three different elements, i.e., radiogenic uranium, non-metal sulfur, and nutrient-type cadmium, measured by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS), to decipher the oceanic elemental budget and geochemical pathways. The specific subjects include (1) U budget by measuring the profiles and transects of 234U/238U in the East China Sea, (2) S isotopic fractionation in marine sediments determined from the depth-dependence of S isotopes in marine pore waters, and (3) the exchange of Cd between marine microplankton and seawater, and associated Cd isotopic fractionations in marine suspended particles.
The determination of δ234U in nearshore and offshore surface seawater, with 2-sigma precision of ±0.3‰,reveals seasonal differences of U mixing between river water and East China Sea (ECS) seawater. Isotopic fractionations of δ234U in ECS seawater samples measured in this study range from 145 to 164‰. The distinct seasonal U mixing behavior in the ECS has important implications for U input into the oceans and the total marine U budget. This study also documents for the first time, non-conservative U mixing in the open oceanic area.
The sulfur isotopic composition of pore waters from marine sediment collected from cold seeps off Southwestern Taiwan were determined in nanomole quantities with 2-sigma reproducibility of ±0.2‰. These pore waters were sampled at depths of 0-474 cm below the seafloor. Our results show that the correlation between sulfate contents of 7-27 mM and δ34S values of 22-41‰ follow a closed system Rayleigh fractionation model above the sulfate-methane transition zone (SMTZ), at a depth of 172.5 cm below the sea floor. At the SMTZ, δ34S reaches a summit of 41‰, followed by a decreasing trend to 16-20‰ at depths of 172.5-470.0 cm. The results show that incomplete sulfate consumption occurs below the SMTZ, probably due to sulfide oxidation and barite dissolution. Significant δ34S variation may be caused by anaerobic oxidation of methane (AOM) with sulfate reduction, indicating sulfur isotopic fractionation is an effective indicator for AOM identification.
Cadmium isotopic measurements using standard-sample bracketing give an external precision of ±0.9(2). Cd/P and 114Cd in marine suspended particle samples ranges respectively from 0.11 to 0.29 mmol/mol, and -4.1 to +1.2. The significant lighter-than-seawater 114Cd values of -4.1observed in 10-63 μm plankton indicates that phytoplankton preferentially incorporates lighter Cd isotopes from ambient seawater. No observable Cd isotopic variation is found among the large-size plankton (363-20,000 μm). The distinct Cd isotopic difference and the isotopic mixing-like feature imply the transformation of trophic levels among size-fractionated particles. Cadmium isotopes are confirmed to provide critical insights into the mechanism controlling Cd cycling and may be a useful paleoproxy for ancient primary production.
Combined with previously published researches, the three specific studies given in my dissertation clarify the significant advantages of isotopic studies using MC-ICPMS techniques to increase our understanding of the marine environment. It is expected that in the coming decades, these techniques have potential to find wide use for in-depth studies of marine biogeochemical processes, ocean circulation, paleoceanography and paleoclimate.

並列關鍵字

uranium ； sulfur ； cadmium ； isotopes ； MC-ICP-MS

參考文獻

Abouchami, W., Galer, S.J.G., de Baar, H.J.W., Alderkamp, A.C., Middag, R., Laan, P., Feldmann, H., Andreae, M.O., 2011. Modulation of the Southern Ocean cadmium isotope signature by ocean circulation and primary productivity. Earth and Planetary Science Letters, 305(1-2): 83-91.

Abouchami, W., Galer, S.J.G., Horner, T.J., Rehkamper, M., Wombacher, F., Xue, Z.C., Lambelet, M., Gault-Ringold, M., Stirling, C.H., Schonbachler, M., Shiel, A.E., Weis, D., Holdship, P.F., 2013. A common reference material for cadmium isotope studies: NIST SRM 3108. Geostandards and Geoanalytical Research, 37(1): 5-17.

Abouchami, W., Galer, S.J.G., de Baar, H.J.W., Middag, R., Vance, D., Zhao, Y., Klunder, M., Mezger, K., Feldmann, H., Andreae, M.O., 2014. Biogeochemical cycling of cadmium isotopes in the Southern Ocean along the Zero Meridian. Geochimica Et Cosmochimica Acta, 127: 348-367.

Aharon, P., Fu, B.S., 2003. Sulfur and oxygen isotopes of coeval sulfate-sulfide in pore fluids of cold seep sediments with sharp redox gradients. Chemical Geology, 195(1-4): 201-218.

Ames, L.L., Mcgarrah, J.E., Walker, B.A., 1983a. Sorption of trace constituents from aqueous-solutions onto secondary minerals. 1. uranium. Clays and Clay Minerals, 31(5): 321-334.