Visualizing Surface Morphology of Sedimentary Charred Botanical Particles Using Tangential Flash Macrophotography

Charred fragments of botanical materials in sediments provide important clues as to the historic fire regime of an area. Lacustrine deposits may include a record of many thousands of years, including information about the sources of plant materials. According to previous morphotype syntheses, charred plant materials can be attributed at the biome scale to certain plant groups such as arboreal taxa, monocots, including Poaceae, and at the individual species scale, to certain plant parts such as wood and leaves. Additionally, morphotypes may reflect seasonal timing of burning, fuel moisture, and potentially even taphonomic processes. Particle size has been linked to charcoal dispersal with important implications on area sources for paleoecological reconstructions. High-resolution visualization allows more accurate identification of informative morphotypes for a particular biome as well as more precise morphometrics. This has been achieved for submillimeter fragments (100-500µ) by photography using tangential flash illumination. Sedimentary charcoal samples were fractionated through fine sieves by standard procedures. Individual particles or liquid suspensions were then placed on a highly transparent substrate and allowed to dry in situ. High-resolution photography was performed using a Canon R5 mirrorless camera with a 65mm macro lens set at 5x magnification. This provides a field of view of 7.2 x 4.8 mm at a working distance of about 42 mm. Sample slides are mounted several centimeters above a white background. A dual flash system mounted on the lens creates a tangential illumination of the sample, along with strong back-lighting. Depth of field is very limited so a series of exposures are made at intervals of 100µ and these are then processed by focus-stacking software. The resulting images are sharp through the entire range with the surface texture strongly highlighted. Effective magnification is approximately 400x. The method will be illustrated with samples derived from sediments in the fynbos of South Africa, the tropical Andes of South America, and plant materials charred under controlled burning.