Stable isotopic and elemental characteristics of recent tufa from a karstic Krka River (south‐east Slovenia): useful environmental proxies?

Tufa samples from 16 consecutive barrages along a 13 km section of the groundwater‐fed Krka River (Slovenia) were analysed for their petrographical, mineralogical, elemental and stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope composition, to establish their relation to current climatic and hydrological conditions. Waters constantly oversaturated with calcite and the steep morphology of the Krka riverbed stimulate rapid CO₂ degassing and subsequent tufa precipitation. The carbon isotope fractionation (Δ¹³C) between dissolved inorganic carbon and tufa in the Krka River evolves towards isotopic equilibrium being controlled by continuous CO₂ degassing and tufa precipitation rate downstream. The Δ¹³C increased from 1·9 to 2·5‰ (VPDB); however, since tufa precipitation rates remain similar downstream, the major controlling factor of carbon isotope exchange is most probably related to the continuous ¹²CO₂ degassing downstream leaving the carbon pool enriched in ¹³C. In the case of oxygen, the isotope fractionation (Δ¹⁸O) was found to be from 1·0 to 2·3‰ (VSMOW) smaller than reported in the literature. The observed discrepancies are due to different precipitation rates of calcite deposits because Krka tufas on cascades grow relatively faster compared to slowly precipitated calcite deposits in cave or stream pools. Due to non‐equilibrium oxygen isotope exchange between Krka tufa and water, the δ¹⁸O proxy showed from 1·2 to 8·2°C higher calculated water temperatures compared to measured water temperatures, demonstrating that δ¹⁸O proxy‐based temperature equations are not reliable for water temperature calculations of fast‐growing tufa on cascades. Because Mg is bound to the terrigenous dolomite fraction in the Krka tufa samples, the Mg/Ca was also found to be an unreliable temperature proxy yielding over up to 20°C higher calculated water temperatures.     

珠江口盆地Site4B柱状沉积物中脂肪醇分子化合物分布及其来源分析

本研究采用色谱质谱(GC-MS)和色谱-同位素质谱(GC-IRMS)方法,对珠江口盆地Site4B沉积物中脂肪醇分子化合物分布以及单体碳同位素组成进行了研究。结果表明:长链脂肪醇碳优势指数(CPIC26—34:9.41—24.56)、平均碳链长度(ACLC26—34:28.85—29.99)以及n C26—n C30脂肪醇平均碳同位素组成(?29.13‰±0.87‰、?32.98‰±1.28‰和?32.73‰±1.61‰)均揭示了n C26—n C34脂肪醇属于陆源高等植物来源,n C16—n C20和n C24脂肪醇较正的碳同位素组成反映了化合物的海洋细菌来源,而n C22脂肪醇偏负的碳同位素特征(平均?33.26‰±1.51‰)反映了化学自养细菌来源。Site4B沉积物中的陆源输入主要来自南海周边河流,从12.7ka BP至今的沉积时期(0—80cm,文中深度均为埋深)和末次冰盛期(102—120cm),其陆源输入主要来自台湾岛河流,而在19.3—12.7ka BP沉积时期(80—102cm)和更新世沉积时期(120—300cm),台湾岛、吕宋岛和珠江口河流的陆源贡献普遍较低。Site4B沉积物中的海洋生产力主要来自海洋微生物输入的贡献,并主要受到海水温差影响;其中在35—150cm层位,海洋输入和南海暖流的强弱带来的海水温度的高低有关,而在150—300cm层位,则主要和冰期-间冰期的冷暖气候带来的海水温差有关。     

冲绳海槽中部A7孔沉积物地球化学记录及其对古环境变化的响应

对取自冲绳海槽中部的A7孔沉积物进行了元素地球化学的测试与分析,配合高分辨率测年和氧同位素数据,探讨了冲绳海槽中部18000年以来的元素地球化学变化特征及其控制因素。使用R型因子分析方法,将分析的20种常量与微量元素分为4个主要的因子:因子I主要由Al、Ti、Th、Rb、Mg、U和Zn等元素组成,代表了陆源物质有关的元素组合,可作为陆源物质输入的指示因子;因子II主要由Ca、Sr、Ba、P等元素组成,代表了与生物作用有关的元素组合,可作为古生物生产力指示因子。根据各元素及其组合随时间的变化特征,整个A7孔沉积物可明显分为4段:18-15 kaBP(Ⅰ),15-8 kaBP(Ⅱ),8-2 kaBP(Ⅲ)和2 kaBP以来(Ⅳ),前3段中都表现出陆源物质先增加再减少的变化趋势,古生物生产力的变化相对复杂,规律性不强,但总体表现出与陆源物质输入相反的变化趋势;第4段中则表现出2 kaBP年以来陆源物质的输入有一定的加强,而古生产力则没有大的变化。在7.3 kaBP左右发生的火山物质快速堆积事件对柱状沉积物的地球化学特征产生了较大的影响,也使陆源物质输入和古生物生产力急剧降低。自18 kaBP年以来,元素地球化学组成的变化主要受温度、海平面变化、黑潮水的“摆动”的影响,而火山沉积也造成了一定的影响。