鐵穩定同位素於淡水河流域之應用

The application of Stable Iron Isotope in Danshui River catchment

林子傑( Zih-Jie Lin)

指導教授 ： 王尚禮

共同指導教授 ： 李德春(Der-Chuen Lee)

國立臺灣大學/生物資源暨農學院/農業化學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201902794

摘要

鐵是生物之必需元素，同時是限制海洋藻類生長一重要因子，對全球氣候與碳循環相當重要。近年來，鐵穩定同位素被應用於追蹤鐵的來源，以及研究鐵的生物地球化學行為。本研究為首次利用鐵穩定同位素探討淡水河流域中鐵的來源與傳輸過程。淡水河流域是台灣北部最大河川，流經人口高度密集之首都台北市與新北市，為大台北地區的重要河川。本研究於2017~2018年間，採集兩個季節的河水水樣並當天過濾，分為溶解態(<0.2μm)與懸浮顆粒態(>0.2μm)，分析樣品之主量元素(例如: 鈉、鉀、鈣、鎂等等)，微量元素(例如: 鐵、鋅、銅、鎳等等) 與鐵穩定同位素。此外，本研究亦分析有機碳，以了解水體有機質是否影響鐵同位素值。
結果顯示，淡水河流域鐵穩定同位素有明顯的空間變化，溶解態( -0.77 ~ 0.89‰)與懸浮顆粒態(-0.26 ~ 0.22‰)皆是，並指出流域中控制鐵同位素變化的兩個主要因子: (1)於人口稠密區，大漢溪有顯著鐵同位素低值 (溶解態= -0.75‰ ; 懸浮顆粒態= -0.24‰)，並與人為重金屬濃度(例如: 鎳、銅、鋅)有高度相關，認為是該區域之人為鐵污染訊號。(2)河口處，溶解態鐵同位素與鹽度呈系統性變化，認為非保守型海水混合有兩階段作用。第一，迅速移除階段，鐵同位素值與鐵濃度呈一指數關係，並造成溶解態與顆粒態之間有0.29‰ 的鐵同位素分化。第二階段，> 90% 溶解態鐵已被移除，並與海水混合進行單純的稀釋作用，鐵同位素值與鐵濃度呈一直線關係。以上初步結果表示，鐵穩定同位素能夠監測人為污染與河口移除作用，並可將此應用於其他河川系統。

關鍵字

鐵穩定同位素 ； 淡水河 ； 人為訊號 ； 河口移除作用

並列摘要

Iron (Fe) is a limiting micronutrient for phytoplankton and is crucial to enhance primary production further to regulate global climate. Fe isotopic composition have been used to trace Fe sources and to study biogeochemical cycling of Fe. This study presents the first Fe isotopic data of suspended particulate matter (SPM) and dissolved fraction to study the biogeochemistry of Fe in the Danshui River catchment. The Danshui River is the biggest river in northern Taiwan, and it flows through both densely populated Taipei and New Taipei city. It is thus important to regularly monitor the properties of SPM and dissolved fraction in the Danshui River. To better constrain the sources and transformation of Fe under the influence of human activities, we also measured organic carbon and the other trace metals.
We observed significant variations of Fe isotopic composition in both SPM and dissolved fraction among sampling sites in the Danshui River (SPM = -0.26 ~ 0.22‰; Dissolved = -0.77 ~ 0.89‰). The variations can be attributed to two major reasons: (1) The anthropogenic Fe. The extremely negative 56Fe values have been found in both dissolved and particulate fraction at the Dahan tributary (Dissolved = -0.75‰; SPM = -0.24‰). The negative values are strongly correlated with the concentration of human-derived trace metals (e.g. Ni, Cu, Zn). The relative light isotopic composition may be originated from the input of anthropogenic Fe with lighter isotopic composition than the natural Fe isotopic signature observed at the up-stream; (2) Seawater mixing. In the estuary, dissolved Fe concentrations show a typical non-conservative mixing pattern and 56FeDiss. values exhibit systemic variations with Fe concentration and salinity. We proposed that the non-conservative mixing process can be separated into two stages. At salinity 0~15‰, heavy Fe isotopes are scavenged by adsorption or/and precipitation, which occurs 56Fe ≈ 0.29‰ between dissolved and particulate fraction. At salinity 15~35‰ (90% of dissolved Fe removed), a clear dilution effect is induced by seawater, since 56FeDiss. values show a linear relationship with salinity, and also with Fe concentrations. The preliminary results indicate that Fe isotopic composition can be used to monitor human activities and Fe scavenging process in the Danshui River, which can be applied to other river systems.

並列關鍵字

Stable iron isotope ； Danshui River ； Anthropogenic signal ； Scavenging process

參考文獻

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

Akerman, A., F. Poitrasson, P. Oliva, S. Audry, J. Prunier and J.-J. Braun. 2014. The isotopic fingerprint of Fe cycling in an equatorial soil–plant–water system: The Nsimi watershed, South Cameroon. Chemical Geology 385: 104-116.

Albarède, F. 1995. Introduction to Geochemical Modeling, Cambridge University Press, Canbridge, US.

Anbar, A. 2004. Iron stable isotopes: Beyond biosignatures. Earth and Planetary Science Letters 217: 223-236.

Bergquist, B.A. and E.A. Boyle. 2006. Iron isotopes in the Amazon River system: Weathering and transport signatures. Earth and Planetary Science Letters 248: 54-68.