Stalactites and stalagmites are the most common speleothems, the morphology of which is basically controlled by dripping. Stalactites are centimeter to meter in scale, hanging from the ceiling and growing toward the cave floor. Stalagmites grow from the cave floor upward and are commonly fed by water dripping from an overheard stalactite. When a stalagmite and overhanging stalactite merge, they form a column.
Most stalactites and stalagmites are composed of calcite, a few of aragonite and phases of calcium carbonate. Rare stalactites and stalagmites consisting of huntite, halite, gypsum and even opal have been found.
Most speleothems that have been extensively studied date from the Quaternary, and the genesis of these is commonly driven by the process of degassing, which occurs when drip waters having a high carbon dioxide concentration interact with the cave atmosphere that has a relatively low concentration.Image: Researches Andrea Borsato, Adam Hartland and Mohammadali Faraji (current PhD candidate, 2019) in Nurau Cave, Atiu Island. Cook Islands. Photo by Garry K Smith.
Speleothems are used to reconstruct palaeoclimate and palaeoenvironmental changes using proxy data.
Changes in the morphology and mineralogy of stalagmites clearly respond to climate-related phenomena, such as drip rate variability and temperature-modulated cave ventilation. The physical and chemical properties of stalagmites therefore provides information on past climate and environmental changes down to the annual to seasonal temporal resolution.
Petrographical, geochemical and radiometric analysis are all methods in reconstructing climate and environmental changes. Microscopy and X-Ray Fluorescence (XRF) elemental mapping are also used to determine the geochemistry of the speleothems.
In order to gain an insight into the environment and climate conditions at the time of stalagmite formation, we can study the fabric of stalagmites and how these vary.
Image: Valentina Vanghi preparing a speleothem sample. Valentina completed her PhD at The University of Newcastle in 2018, under the supervision of A/Prof Silvia Frisia
Image: Speleothem sample preparation.
Stalagmite fabric variates due to changing environmental conditions, outside and inside the cave, and these changes can be tracked using a number of techniques. Information gained by analysing stalagmites includes internal structure, element composition and dating. To look at the internal structure, a stalagmite is cut in half, using a saw followed by polishing until the sample surface is smooth and clear. Using a micro drill on the polished sample, small holes can be made and the powder from these holes are collected and analysed using a spectrophotometer.
The information gained from spectrophotometry includes isotope geochemistry and trace element geochemistry of the stalagmite as well as knowing how old the stalagmite is through uranium-thorium and uranium-lead dating. Thin section preparations can be viewed under the microscope and are also be used to study the petrography of stalagmites.
Studying the composition of stalagmites leads to a better understanding of how the climate outside of the cave variates. As stalagmites are formed from rain entering the cave through seepage or cracks in rocks, these differences in drip rate are reflected in the compositional makeup of the stalagmites. Each stalagmite profile is unique varying greatly from cave to cave, and even between stalagmites within the same cave.