Characterization of fine grained sediment in the AND-2A drillhole core (southern McMurdo Sound, Antarctica) using QEMSCAN®

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Field, B.D.; Atkins, C. 2012 Characterization of fine grained sediment in the AND-2A drillhole core (southern McMurdo Sound, Antarctica) using QEMSCAN®. Lower Hutt, N.Z.: GNS Science. GNS Science report 2012/42 8 p.

Abstract: AND-2A was drilled in the Southern McMurdo Sound in 2007 to obtain a paleoclimate record for the Miocene of the Ross Sea Region. The drillhole penetrated a thick Early Miocene succession which contained several intervals of cyclic silt-mud laminae. The aim of our study was to obtain grain size distribution curves that could be compared with curves for known aeolian sediments collected from annual sea ice in McMurdo Sound, and to assess whether the silt laminae resulted from settling of aeolian sediment during seasonal ice melt. Induration of the sediments, and the thinness of the laminae meant that grain size determination by conventional means such as laser or pipette were not feasible, and the fineness of the sediment meant thin section grain size characterization would have been imprecise. A technique called QEMSCAN®, using a scanning electron microscope combined with four energy dispersive spectrometers, was used to analyse cut slabs of drillcore within two of the laminated intervals. The scan analysis provided information on the composition and size of individual grains of mudstone and siltstone. Results indicate the laminae studied are predominantly of silt grade, locally with very-fine sand grains of quartz and potassium and plagioclase feldspar, with common, fine silt grade illite/muscovite. Unfortunately, in the two core slabs studied, the presence of salt (probably from pore fluid brines) masked analyses of the clay fraction, and possibly also part of the silt fraction. Cleaning the slab surface improved the results for one sample, revealing additional micro-lamination and probable grading in some laminae. Although approximate grain size distributions were generated, the halite masking combined with suboptimum grain size resolution meant it was difficult to make comprehensive or quantitative comparisons with grain size curves generated by other techniques. Despite the difficulties applying the QEMSCAN® technique to the AND-2A core, it has potential to be useful in other fine-grained sediments provided they are not associated with brines. (auth)