The formation and environmental significance of calcite rafts in tropical tufa-depositing rivers of northern Australia

Abstract

Spring-fed rivers of the Barkly karst in tropical northern Australia form an array of tufa and related freshwater carbonate deposits. One of these deposits, calcite rafts, is precipitated at the water-air interface principally as a consequence of CO2 degassing and evaporation. Calcite rafts have been reported in cave environments but have not been described in detail from fluvial systems. Observations using scanning electron microscopy coupled with water chemistry data reveal that they form by a combination of physical, chemical and biological processes. They grow downwards into the water column and form a dentate lower surface, while a flat upper surface occurs at the water-air interface. T..