Processing Backscattered Electron Digital Images of Thin Section

Abstract

Image analysis of sedimentary particles using backscatter electron (BSE) microscopy shows great promise in paleoclimatic and paleoenvironmental studies. Prior to the last few years BSE microscopy has been used primarily for compositional (provenance) studies. Our preliminary work on Paleozoic loessite, as well as previous work on recent sediments (Francus 1998; Francus and Karabanov 2000), suggests that BSE microscopy image analysis is an effective tool for deriving textural data for use as a paleoclimate proxy. Our data on the Paleozoic loessite shows that we are able to document changes in grain size of quartz through several loessite-paleosol couplets. In each case, the quartz was coarser in the loessite facies relative to the overlying paleosol, which is similar to grain size trends observed in the Quaternary Chinese Loess Plateau. Image acquisition is a critical step in this methodology, however, special precautions are needed to make sure that 1) the samples are suitably prepared, 2) the acquisition instrument’s settings are controlled and maintained, and 3) the acquisition system provides an output of suitable resolution. Processing is similar to other types of imagery subjected to image analysis, and includes calibration, filtering, and image segmentation and thresholding. An important component of processing is testing how different filters affect grain boundaries, particularly if grain size or grain shape is to be measured. In terms of image measurements, the magnification is an important consideration, and should be consistent; with standard BSE detectors, grains smaller than approximately 2 μm can not be resolved because of the size of the interaction volume of the backscatter electrons. As the case study illustrates, measurements of grain size or grain perimeter in this methodology do not translate into actual grain size information because of stereological considerations, however, relative changes in grain parameters yield useful information. The two biggest drawbacks of the present methodology are that it is difficult to keep acquisition conditions constant, and that data collection is time consuming. As instruments with BSE capabilities improve with more digital controls, acquisition will become much more stable, and as protocols are developed, it will be possible to semi-automate the procedure, allowing for a much faster rate of data collection.