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Isotope source XRF and geochemical palaeolimnology |
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Geochemical analysis of lake sediments is commonly based on readily extractable fractions, because it is these that best reflect the state of the environment at the time of deposition. However, total concentrations of major rock-forming elements are also very important because they provide valuable information about geochemical properties of the sediment, the geology of the catchment, and evolution of the lake system. For this reason, total sediment digestion is necessary to determine the concentrations of lattice-bound components, even when the primary research is based on partial chemical extractions. Although there are many total digestion procedures, all are relatively time consuming and involve the use of highly hazardous chemicals (hydrofluoric and perchloric acids). The necessity for an additional digestion can also be a problem if the amount of sediment available is limited, as is commonly the case with lake sediment cores.
The ideal solution to this problem is a rapid, non-destructive technique for total elemental analysis of the sediment, that can be used in conjunction with other analytical procedures. Energy dispersive radioisotope-source X-ray fluorescence (XRF) analysis meets all of these requirements, but is not widely used in environmental research. The reason for this is partly historical; early instruments were ideal for determination of single elements in materials such as steel or mining products, but did not have sufficient energy-resolution for multi-element analysis. While both detector resolution and signal processing rapidly improved application remained largely confined to simple matrices and to industrial applications. Parallel development of slower, but far more accurate, wave-length dispersive XRF systems may have contributed to the lack of interest shown in the academic research community. The value of isotope source XRF to palaeolimnological research lies in its speed. A single operator can process 70 samples (pre-dried) in an eight hour day. Two operators can double this number, with slightly reduced precision. Thus, the technique is valuable when large sample numbers must be measured in a limited time period. The element concentrations determined by XRF, together with LOI, quantify the major components of the sediment. Enough information is provided to constrain the concentrations of biogenic silica, authigenic iron and manganese, calcium carbonate, sulphur, mineral matter and organic matter. Additionally, in recently deposited lake sediments, the concentration profiles of Pb and Zn can be accurately measured. Under favourable circumstances, these profiles can be used for approximate dating by correlation with expected pollutant profiles. Thus, the XRF provides a considerable amount of environmental information without the use of additional techniques. There are three specific circumstances where an isotope source XRF with gas-filled proportional counters can make a significant contribution to palaeolimnological research. First, it can provide rapid screening of samples. For example, by allowing the rapid elemental analysis of a large number of samples, representative cores can be selected for further analysis. Second, in cases where the sediment composition varies greatly among samples, analytical uncertainty may be far smaller than the between-sample compositional variation. In such cases the advantage of large sample numbers exceeds the disadvantage of reduced accuracy or precision, and results are accurate enough to be used in their own right. Third, where the primary purpose of the research is analysis of a chemically extracted fraction of the sediment, the XRF can provide supplementary total element data to aid interpretation. For example, heavy metal contamination is commonly concentrated in the labile fraction of the sediment, and may be extracted simply and rapidly using relatively safe reagents. This information cannot, however, be fully interpreted without information about the elemental composition of the sediment, which can be readily and rapidly supplied using the XRF.
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