鋇是海洋中的“類營養鹽”元素，其溶解態分布與硅酸鹽類似，表層濃度低，深層濃度高；而次表層水和中層水中往往形成顆粒鋇的極大值，主要與沉降有機顆粒物降解過程中產生的過飽和微環境利于重晶石（barite，BaSO4）的沉淀有關。因此，鋇是現代海洋學和古海洋學中示蹤水團混合、營養鹽循環和生物生產力的重要指標，但其生物地球化學行為的具體控制機制仍不清晰。近年來，質譜分析方法的日趨完善已實現精確測量鋇穩定同位素比值，探討其在海水中的分布和變化有望加深對海洋鋇循環及相關生物地球化學過程的理解，為示蹤海洋學過程提供新指標。然而，迄今為止，鋇穩定同位素在地球化學和海洋學中的實際分析與應用幾為空白，亟待開發和研究。
我實驗室新晉科研人員曹知勉副教授成功建立雙稀釋劑（double spike）多接受器電感耦合等離子體質譜儀（MC-ICP-MS）測量鋇穩定同位素方法，首次分析東海和南海溶解態和顆粒態及世界多條大河（包括長江和珠江）的鋇同位素比值。發現河水溶解態鋇同位素組成（d137BaDBa）整體輕于海水，而不受河流輸入影響的上層海洋d137BaDBa值顯著高于深層水和相應懸浮顆粒物鋇同位素組成（d137BaBaxs），大體呈現類似硅穩定同位素的營養鹽型分餾模式。但是，一一比較鋇和硅同位素數據，卻發現東海和南海表層水硅同位素比值由于硅藻耗盡硅酸鹽最高，其后在真光層內隨深度逐漸降低（Cao et al., 2012, 2015），而鋇同位素組成由表層至100米分布均勻，沒有顯著變化；同時，溶解鋇濃度及其同位素組成也與硝酸鹽和磷酸鹽濃度的分布不耦合。基于此，提出輕質量鋇同位素優先吸附于生源顆粒物，而非被浮游植物直接吸收，是造成上層海洋溶解態鋇同位素組成偏重、顆粒態偏輕的主要機制；指出鋇同位素是研究海洋水團混合、河流輸入的潛力指標，但其是否能夠有效示蹤營養鹽循環或生物生產力仍有待進一步探索。
該研究從全新的穩定同位素視角進行闡釋，顯著提升了對海洋鋇生物地球化學過程的認識，為在GEOTRACES（“痕量元素及其同位素海洋生物地球化學循環”國際合作計劃）大框架下發展新同位素體系和指標奠定基礎，也有望在海洋生物泵、碳循環乃至古海洋、古氣候重建等研究領域中取得突破。
該成果于2015年11月28日在線發表于地學一區期刊《Earth and Planetary Science Letters》上：Cao Z.*, Siebert C., Hathorne E. C., Dai M., Frank M., 2015. Constraining the oceanic barium cycle with stable barium isotopes. Earth and Planetary Science Letters, http://dx.doi.org/10.1016/j.epsl.2015.11.017.
曹知勉博士于2013年獲廈門大學理學博士學位，之后獲洪堡學者基金赴德國亥姆霍茲基爾海洋研究中心從事博士后研究；2015年加盟我實驗室，主要研究方向為非傳統穩定同位素地球化學和海洋碳循環。
Abstract:
The distribution of barium (Ba) concentrations in seawater resembles that of nutrients and Ba has been widely used as a proxy of paleoproductivity. However, the exact mechanisms controlling the nutrient-like behavior, and thus the fundamentals of Ba chemistry in the ocean, have not been fully resolved. Here we present a set of full water column dissolved Ba (DBa) isotope (?137BaDBa) profiles from the South China Sea and the East China Sea that receives large freshwater inputs from the Changjiang ( 
River). We find pronounced and systematic horizontal and depth dependent ?137BaDBa gradients. Beyond the river influence characterized by generally light signatures (0.0 to +0.3‰), the ?137BaDBa values in the upper water column are significantly higher (+0.9‰) than those in the deep waters (+0.5‰). Moreover, signatures are essentially constant in the entire upper 100 m, in which in the very surface waters. Combined with the decoupling of DBa concentrations and ?137BaDBa from the concentrations of nitrate and phosphate this implies that the apparent nutrient-like fractionation of Ba isotopes in seawater is primarily induced by preferential adsorption of the lighter isotopes onto biogenic particles rather than by biological utilization. The ?137BaDBa distribution is dominated by water mass mixing. The application of stable Ba isotopes as a proxy for nutrient cycling should therefore be considered with caution and both biological and physical processes need to be considered. Clearly, however, Ba isotopes show great potential as a new tracer for land-sea s and ocean mixing processes.
Figure. Vertical distributions of dissolved barium (DBa) concentrations and their stable barium isotopic compositions (?137BaDBa) in the East China Sea and the South China Sea. (a) bathymetric map showing the locations of sampling stations; (b) stations PN10, PN04, and DH13; (c) the entire water column at station KK1; (d) the upper 200 m at station KK1 (open circles) including the stable isotopic compositions of excess particulate barium (?137BaBaxs) in the upper 150 m at station A0 (solid circles).
全文鏈接 Link to full text:
http://www.sciencedirect.com/science/article/pii/S0012821X15007219
About the author:
Dr. Zhimian Cao got his PhD from Xiamen University in 2013 and has worked as a Humboldt Postdoctoral Research Fellow at GEOMAR Helmholtz Center for Ocean Research Kiel, Germany from 2013 to 2015. He joined MEL in 2015 and his research interests include nontraditional stable isotope geochemistry and ocean carbon cycling.
延伸閱讀 References:
(1) Cao Z., Frank M., Dai M., Grasse P., Ehlert C., 2012. Silicon isotope constraints on sources and utilization of silicic acid in the northern South China Sea. Geochimica et Cosmochimica Acta 97, 88-104. (Link: http://www.sciencedirect.com/science/article/pii/S0016703712004978)
(2) Cao Z.*, Frank M., Dai M., 2015. Dissolved silicon isotopic compositions in the East China Sea: Water mass mixing vs. biological fractionation. Limnology and Oceanography 60, 1619-1633. (Link: http://onlinelibrary.wiley.com/doi/10.1002/lno.10124/full)
