Experiments in waterlogging and sedimentology of charcoal: results and implications

Fossil charcoal has a sporadic occurrence in sedimentary rocks since Devonian time. It is moderately common as a component of the organic material found in a wide variety of facies, but there are also some notable concentrations which occur locally. These occurrences have considerable palaeobotanical and palaeoecological value because the process of charring may result in excellent preservation of plant tissue. If assemblages of charred material are to be used to interpret palaeoenvironments, it is important to understand the behaviour of charcoal during transport from the site of the fire. Charcoal is an unusual sedimentary material because most fresh material floats, but with prolonged immersion becomes waterlogged and sinks. We carried out a series of waterlogging experiments on uncharred material and charcoal from a range of different plant types and tissues. These show considerable variations in the rates of waterlogging of different charred and uncharred plant tissues, and it is suggested that buoyancy contrasts are likely to result in separation during transport and deposition. Furthermore, experiments in a flume tank have shown that the process of deposition of charcoal in sand is controlled by the rate of migration of bedforms, which is in turn determined by depth, flow velocity and sediment supply. Allochthonous assemblages of charred and uncharred plant debris will most probably be biased and not fully representative of the contemporaneous plants or vegetation. The presence of charcoal concentrations may be used as an aid in interpreting palaeohydraulic conditions.