摘要:

IntroductionFerromanganese concretions are sedimentary deposits found in freshwater lakes, mires, in shallow marine and deep-sea environments. They have a complex internal layered structure primarily composed of Fe- and Mn-oxyhydroxides layers. In the early 1960s, Cameron1and Arrhenius2showed the complexity of the layering by means of photomicrographs, Burns and Fuerstenau3later being among the first to demonstrate that chemical variations on a microscopic scale also exist in deep-sea ferromanganese concretions.Winterhalter and Siivola performed the first detailed study of the internal structure of ferromanganese concretions from the Baltic Sea.4They noted a close correlation between P and the Fe-rich layers in the concretions. Suess and Djafari5noted that the outer layers of concretions from Kiel Bay in the Baltic Sea contained anomalously high concentrations of Zn, Pb, Cd and Cu. They concluded that this was an effect of increased anthropogenic input of these elements. However, Ingri and Pontér6showed that even elements with no known pollution source,e.g.Y and La, show enrichment in surface layers in ferromanganese concretions from the Gulf of Bothnia. There exist natural enrichment processes, largely governed by the redox level, which make it difficult to distinguish between natural and man-made contributions of trace metals in concretions.6However,further work in the Kiel Bay7suggests that concretions in this area reflect pollution effects and it has been proposed that laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) could be a useful method to monitor the changes in trace metal incorporation in the concretions.Recently a growth rate estimate on ferromanganese concretions from the Baltic Sea has been performed, by using226Raexcess/Ba ratios in individual layers of the nodules.8It was concluded that it is not yet clear whether ferromanganese concretions can be used as an archive for postglacial variations in Baltic water masses. More data using Nd-, Sr- and Pb-isotope systematics on226Raexcess/Ba dated ferromanganese concretions are needed. Beside these isotope systems, detailed measurements of individual element distributions in the interior of concretions are of utmost importance in order to understand concretion formation and hence evaluate their potential in,e.g.pollution studies.Based on the acid digestion procedure described previously,9it is possible to gain quantitative information on the concentrations of about 60 elements using low mg or even sub-mg amounts of material. However, our experience with the micro-sampling of concretions has shown that the lowest amount of material that can be reliably sampled is in the range of 0.5–0.7 mg. For smaller samples, errors associated with collection and weighing, as well as contamination risks, preclude accurate analyses. As a result, the lowest three-dimensional resolution is on the order of 0.7 mm3, providing about 20–25 sub-samples over the cross-section of a concretion, 20 mm in diameter. As a rule, the growth structure of concretions is in the few µm range, thus only averaged information from many structural segments can be obtained, resulting in only modest spatial information gain compared to whole concretion analysis. Moreover, at least two workingdays are required for complete sampling, digestion and analysis of one concretion (two cross-sections, 40–50 sub-samples).The spatial resolution in the analysis of solid samples can be greatly improved by using LA, and successful applications of this technique for assessing element distributions on the sub-mm scale in a variety of matrices, such as tree rings,10–17corals,18,19mussel shells,20,21fish otoliths,19,22,23teeth,24–26etc., have been reported. Alongside significant time savings (due to the limited sample preparation as well as the absence of uptake and washing times that are inevitable using traditional solution nebulisation), LA offers the additional advantage of reduced spectral interferences caused by oxide and hydroxide species27,28(due to the absence of solvent at dry plasmaconditions). However, quantification by using LA-ICP-MS is much more complicated than for solution nebulisation.29An additional challenge is correction for possible variations in ablation efficiency, which may be caused by a variety of difficult to control parameters27,29including: aging of the laser flashlamp; sample positioning inside the ablation chamber or changing the position of the sampling area relative to the Ar flow during raster ablation; spatial inhomogeneities in the structure, density or colour of the material under investigation. In samples with a finely laminated structure,e.g.shallow-sea concretions,30the latter may, if not corrected adequately, result in false distribution patterns.In this paper, a multielement procedure for high spatial resolution analysis of ferromanganese concretions using LA-ICP-MS is described, with emphasis on the optimisation of measuring parameters, internal standard correction and quantification by external calibration against matrix-matched standards.

ISSN:1467-4866    

DOI:10.1039/b204262m

出版商:RSC    

年代:2002

数据来源: RSC