Mapping the future of microanalysis: application to climate change and palaeoecology research

A project undertaken at the University of Melbourne and supervised by A/Prof Janet Hergt

Despite increasing interest, existing analytical and imaging capabilities for in situ analysis are restrictive. A particular limitation is that existing commercial technologies (e.g., SEM) can only generate compositional maps for elements present in samples at very high levels, and are thus applicable only to a relatively small range of scientific problems. PIXE imaging is also possible, but is again limited to rather high levels of trace elements and cannot determine isotopic compositions. Ideally compositional maps of both trace element and isotope ratios are required but, presently, only rare examples of rudimentary trace element compositional maps exist.

In recent years the development of methods for in situ analysis of trace element and isotopic compositions has produced remarkable advances in a very wide range of scientific disciplines. The next ‘frontier’ will undoubtedly be the ability to construct two dimensional maps of elemental and isotopic compositions, on the micron scale. When coupled to advances in image processing, we aim to develop world-first technologies for producing elemental and isotopic maps of a wide variety of sample types with applications across many scientific disciplines. Once established, these methods will be applied to two pilot studies targeting, respectively, fossil fish otoliths (providing insights into the aboriginal occupation of Lake Mungo region around 40,000 years ago), and stalagmites from beneath the Nullarbor Plain (providing new constraints on climate change in Australia over the past 4 million years).

1. Fossil fish otoliths.
The trace element and Sr-isotope composition of exposed rock-types influences the composition of water from different catchments, and is archived in the daily carbonate record preserved by fish otoliths. In modern fish such variations are used to explore the timing and migration of fish between the sea and fresh water, and even to locate the rivers and lakes in which they have lived (valuable to fisheries management). However fossil otoliths can also be studied in this way, and we propose to apply our new analytical protocols fossil otoliths from the Lake Mungo middens. This will provide new insights into the sources of water replenishment, and the origins of fish mortality which are believed to have been exploited by early Australians at this important archaeological site.
2. Ancient climate records.

Preliminary work on stalagmites sampled from caves beneath the Nullabor have recently provided important constraints on the onset of aridity in Australia. However, palaeo-climate records can often be blurred by crystal growth patterns and alteration—problems which are not always obvious from spot analyses. These samples will be used to illustrate the value of interpreting trace element (and isotope) variations within a well-constrained 2D context, as provided by compositional maps. Armed with this knowledge we will be able to extract more robust records of past temperature and humidity variation, essential to future climate change modelling.

Figures

Fig. 1. Elemental map for a carbonate sample with distinct growth zonation. Colours indicate element concentration: strontium (green), uranium (blue) and iodine (red plus z-axis 'topography'). Map 'A' approximately 20mm in length, 'B' and 'C' represent higher resolution images of successively smaller regions.

Fig. 2. A Sr-isotope line scan across an otolith from a diadramous fish (from Woodhead et al. JAAS 2003) indicating periods of residence in both salt and freshwater. Extension of such results to fossil otoliths would provide new information on environmental conditions no longer available for study. The generation of a 2-D map for such samples would also allow distinction between primary and secondary (alteration) signals that might otherwise compromise the interpretation of the results.

 
Fig. 1. (Click on image to enlarge)

Fig. 2. (Click on image to enlarge)