Trace Elemental Imaging
Figure 5 : Schematic representation of the fossilization pathway in the Djebel Oum Tkout Lagerstätte, showing the acantomorph fi sh Spinocaudichthys oumtkoutensis . During fi sh life, bone apatite composition is controlled by biogenic uptake (1). After death, decay usually removes all soft parts (2), but in exceptional cases, soft tissues may be preserved in apatite minerals, here through microbially mediated phosphatization under a microbial mat that quickly forms around the carcass (3). Some days to weeks after death, the composition of bone apatite and soft tissue authigenic apatite is controlled by local taphonomic conditions (4). A million years after death, apatite appears further enriched or depleted in various elements (5).
non-invasiveness: no delicate sample preparation is needed beyond the preliminary usual cleavage of the sediment blocks on the fi eld, as well as a better elemental detectability than SEM X-ray emission spectroscopy. It should therefore be highly benefi cial for the study of those unique witnesses of ancient traces of life on Earth that are fossils from Lagerstätten, when compression during fossilization makes interpretation harder. T e method can also be informative at the histological scale in higher resolution maps, that is, with a smaller step size between pixels and lower X-ray information depth, that is, in conditions where the XRF signal escapes from a limited depth (such as by collecting signal from lighter elements or lower energy lines).
Insights into local conditions of burial . Trace elements such as strontium, yttrium, and particularly REEs, have provided useful information on the provenance and environment for many geochemical samples such as rocks, waters, and minerals, including bone bioapatite and other apatite group minerals, as they simultaneously refl ect the connectivity of the environmental water network, the local redox, the specifi c surface area of the bioapatite nanocrystals, the physico-chemical conditions, and the properties of substituted apatite [ 11 , 17 – 19 ]. T ese elements were shown to be present in signifi cant quantities in fossil bones and teeth (exceeding 100 ppm), a range straightforwardly detectable with synchrotron XRF techniques, whereas they are encountered in vivo in the lower ppt (parts per trillion) to ppb (parts per billion) range. Indeed, REEs concentration is known to increase by three to four orders of magnitude within thousands of years in fossil bones through intake from the fossilization context. REEs produce trivalent ions that can readily substitute for Ca 2+ isomorphously in apatite minerals. Phosphatized soſt tissues in well-preserved fossils are found in apatite minerals and also
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incorporate trace elements during diagenesis. However, unlike bone apatite, biogenic uptake in soſt tissues is very limited during the life of the animal. In contrast, days to weeks aſt er death, apatite in the bones and in the mineralized soſt tissues incorporate trace elements depending on the local taphonomic conditions. Both will be further enriched or depleted in these elements during the millions of years up to present time ( Figure 5 ). Contrasted elemental signatures therefore evidence diff erences in sorption and/or substitution rates, as well as initial composition, and can therefore provide relevant information on the fossilization and diagenesis processes at the sites. Future directions . Besides morphological information, processed XRF spectra provide semi-quantitative elemental contents. Because it is possible to image numerous trace elements, including most of the lanthanides and some actinides ( Figure 4 ), synchrotron-based XRF mapping can be used to draw local REE fractionation patterns, that is, the relative abundances of REEs normalized by proper reference materials. REE patterns are usually established from point quantifi cation and profi ling, whereas XRF techniques lead generally to limited radiation-induced side eff ects [ 20 ]. Such local quantifi cation of REEs may therefore open new avenues for taphonomic and paleoenvironmental studies.
Conclusion Synchrotron-based XRF mapping appears to be an
effi cient tool for obtaining critical morphological and chemical information at microscale on fl at fossils for taxonomic, phyloge- netic, paleoenvironmental, and taphonomic studies. T ese new developments are expected to provide signifi cantly more accurate description of fossil characters, better understanding of fossilization processes, and gain of paleoenvironmental information from trace elemental fractionation patterns.
www.microscopy-today.com • 2015 May
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