THE "FROSTBITER," A NEW METHOD FOR THE PREPARATION OF PETROGRAPHIC THIN SECTIONS FROM WET, UNCONSOLIDATED LACUSTRINE OR MARINE SEDIMENTS

Making petrographic thin sections from wet, unconsolidated sediments is a time-, mud-, and chemical-intensive process. Several cubic cm of valuable sediment are expended to make a thin section less than 100 microns thick, and even at its most conservative the process uses a relatively large volume of hazardous epoxy and sometimes acetone. A new alternative method uses a small aluminum block filled with liquid nitrogen placed on top of a microslide on the split and smoothed core face, flash-freezing a thin layer of sediment directly to the underside of the slide. The sample is freeze dried and embedded with a small amount of Spurr's (or other) epoxy inside a vacuum chamber. The cured epoxy-impregnated sediments are then treated like rock chips: lapped to a thickness of ~200 microns and polished. The “Frostbiter” was tested on several sediment types, including laminated and unlaminated sediments with high water content, and found to be a major improvement in efficiency: the method uses far less sediment and epoxy, and no acetone. The thin subsample on the slide was also found to be less prone to cracking, parting along bedding planes, and “pingo-ing” than traditional ~cm-thick thick freeze dried slabs when subjected to liquid nitrogen shock. The only new piece of equipment required is the aluminum block, which can be easily and cheaply machined. The technique cannot easily be used on sediments with very low water content, and coarse grains may be plucked in rock shop processing as they may in other types of thin sections. However, for most later-Quaternary lacustrine and marine cores, the Frostbiter represents a significant methodological improvement.