Tracing euxinia by molybdenum concentrations in sediments using handheld X-ray fluorescence spectroscopy (HHXRF)
Tracing euxinia by molybdenum concentrations in sediments using handheld X-ray fluorescence spectroscopy (HHXRF)
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DOI：
10.1016/j.chemgeo.2013.10.022
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发表时间：
2013-12-18
期刊：
影响因子：
3.9
通讯作者：
Bjerrum, Christian J.
中科院分区：
文献类型：
--
作者：
Dahl, Tais W.;Ruhl, Micha;Bjerrum, Christian J.
富有机质沉积物中钼（Mo）含量的升高表明其沉积来自缺氧和富硫化物（富氧）水柱。这可以用于从沉积档案中追踪古代海洋过去的缺氧条件。然而，传统的钼含量升高的分析检测是昂贵的，不能直接在现场进行。在这里，我们表明手持式x射线吸收光谱（HHXRF）对岩石粉末和岩石表面都提供了一个强大的工具，用于实验室或现场，Mo含量的地球化学表征和缺氧沉积条件的示踪。我们进一步提出了一个来自全球分布的300多个现代海洋站点的沉积Mo数据库，表明现代富氧海相盆地的特征是相对于平均地壳（类似于1.5 ppm）和河流悬浮物（类似于3 ppm）的钼富集（约为25 ppm）。只有来自含氧深海环境的富锰氧化物沉积物才符合如此高的钼富集程度，但这些沉积物很容易与来自大陆架的富钼沉积物区分开来。因此，过去和现在富含Mo的富有机质沉积物表明（至少是间歇性的）水柱缺氧。采用HHXRF和电感耦合等离子体质谱（ICPMS）对全新世、中新世、三叠纪-侏罗纪、奥陶系和寒武系的粉状和均质样品进行了钼含量分析。我们进一步分析了2个经过认证的NIST标准参考文献（NIST-2781含Mo: 47 ppm和NIST-2702含Mo: 10 ppm）进行分析控制。30、120和300秒测量后的分析精度（1西格玛）分别为4、2和1 ppm，检出限分别为11、5、3 ppm（3西格玛，噪声级）。每天对样品进行校准后，数据精确到给定精度（1西格玛），样品的Mo浓度范围从0到bbb30 ppm。手持式XRF设备还可以在现场和实验室条件下直接对新鲜岩石表面进行Mo测量。寒武纪钻芯的岩石样品与ICPMS从同一岩石的均质粉末中获得的Mo浓度密切匹配。总的来说，我们的数据表明HHXRF是一种快速可靠的方法，可以精确和准确地定量岩石粉末和新鲜岩石表面的钼浓度。HHXRF测量的非破坏性特点与低（每个样品）分析成本、少量样品制备和易于现场应用相结合。(C) 2013 Elsevier b.v.版权所有
Elevated molybdenum (Mo) contents in organic-rich sediments are indicative of deposition from an anoxic and sulfide-rich (euxinic) water-column. This can be used for tracing past euxinic conditions in ancient oceans from sedimentary archives. Conventional analytical detection of elevated molybdenum levels is, however, expensive and cannot be directly performed in the field. Here, we show that handheld x-ray absorption spectroscopy (HHXRF) on both rock-powders and rock surfaces provides a powerful tool for laboratory or field-based, geochemical characterization of Mo content and the tracing of euxinic depositional conditions. We further present a sedimentary Mo database, compiled from over 300 globally distributed modern marine sites, to show that modern euxinic marine basins are characterized by molybdenum enrichment (>25 ppm) relative to average crust (similar to 1.5 ppm) and river suspended matter (similar to 3 ppm). Only Mn-oxide rich deposits from oxygenated deep sea settings match such high Mo enrichments, but these are easily distinguished from Mo-rich sediments from the continental shelves. Hence, past and present organic rich sediments enriched in Mo are indicative of (at least intermittent) water-column euxinia.We analyzed the molybdenum content of powdered and homogenized samples of Holocene, Miocene, Triassic-Jurassic, Ordovician and Cambrian age by HHXRF and Inductively-Coupled-Plasma-Mass-Spectrometry (ICPMS). We further analyzed 2 certified NIST standard references (NIST-2781 with Mo: 47 ppm and NIST-2702 with Mo: 10 ppm) for analytical control. Analytical precision (1 sigma) after 30, 120, and 300 seconds of measuring time was 4, 2, and 1 ppm, with a respective detection limit of 11, 5, 3 ppm (3 sigma, noise level). The data were accurate to within the given precision (1 sigma) after a daily calibration to samples covering a range of Mo concentrations from 0 to >30 ppm.Hand-held XRF equipment also allows Mo measurements directly on fresh rock surfaces, both in the field and under laboratory conditions. Rock-samples from a Cambrian drill core closely match ICPMS obtained Mo concentrations from homogenous powders of the same rocks. Collectively, our data show that HHXRF as a quick and reliable method for the precise and accurate quantification of molybdenum concentrations in both rock powders and at fresh rock surfaces. The non-destructive nature of the HHXRF measurements is combined with low (per sample) analytical costs, minor sample preparation and easy applicability in the field. (C) 2013 Elsevier B. V. All rights reserved.