Caves and Speleothems A Window into Today’s Aquifers and Past Climates
Cave and Speleothem Formation Caves can form when slightly acidic water dissolves a soluble rock, such as limestone. Water that originates as rainfall can drip into a cave, losing dissolved carbon dioxide to the cave air. This forms a layered deposit, called a speleothem, usually composed of the mineral calcite (CaCO3). As they grow (typically less than one hundredth of an inch per year), speleothems encode a history of the cave and of environmental conditions at the surface. Paleoclimate Research Stalagmites are the kind of speleothem that grows upward from the floor of a cave. They can actively grow for hundreds of thousands of years. The shape and chemical composition of a stalagmite depends in part on the environmental conditions above a cave (e.g. temperature, rainfall, and vegetation type). Precise measurement of a growth layer from a stalagmite for isotopes that decay over time, such as carbon-14, uranium-238, and thorium-230, allow for the age of that growth layer to be calculated. These measurements require small samples of the stalagmite calcite to be extracted, either by an automated micromill or by dental drill and a researcher with a very steady hand. A stalagmite cross-section, sampled for uranium, thorium, and other isotopes, can be seen on the right. Glass plates, placed on top of actively growing stalagmites for a month at a time (seen below), allow us to isolate and catalog short intervals of calcite growth in a cave. This helps geoscientists determine how changes at the surface are reflected in the growth and composition of stalagmites. Speleothem Growth
A stalactite is a kind of speleothem that grows from the cave ceiling downward. Soda-straw stalactites are thin tubes that grow downward as water flows through their center. When a droplet of water lands on a solid surface in the cave, degassing of carbon dioxide from the water drives the growth of calcite upward in layered stalagmites. Eventually, a straw may become blocked, forcing water to flow down the outside of the soda straw. This forms a type of stalactite called a “carrot.” Over time, the stalagmite will grow taller, as new calcite is added to the top and sides of the formation. The shape of a stalagmite is dependent on how far the drops of water fall from the cave ceiling, the amount of time between drips, the drip water’s chemical composition, and the composition of the cave air. As water continues to flow, this carrot will become a large stalactite, well-cemented to the ceiling. When a stalactite, growing downwards, meets a stalagmite growing upwards, they begin to form a column. A cut and polished column (right) records a complicated visual history of this union. Can you see the top of the stalagmite at the time the column formed? Can you see the trace of the original soda straw at the center of the stalactite? Vadose or Phreatic?
Calcite can grow above the water table (in the vadose zone) or below the water table (in the phreatic zone). What are the characteristics of each? All stalagmites and stalactites form from dripping water in the vadose zone. “Cave popcorn,” seen below and on the drapery formation in the upper left, form in areas of the cave that undergo evaporation—especially where air is moving. An abundance of cave popcorn might be a sign of a nearby entrance or small passageway! Needle-like acicular crystals (below) and calcite spar (right) are seen growing in all directions, immune to the influence of gravity. These crystals only grow underwater, in the phreatic zone. Identifying where vadose and phreatic features occur can help unravel a region’s geologic history. Imposters! These two “stalagmites” did not grow in a cave. Leaking water from a nearby fountain percolated through the fountain’s cement foundation. This seeping water dissolved calcium oxide (CaO) from the cement, forming a very alkaline calcium hydroxide (CaOH) solution. When this water dripped into a basement room below the fountain, it absorbed carbon dioxide in the air to form these calcite formations. Acknowledgements We would like to thank the following people and organizations for the contributions that made this exhibit possible: Watershed Protection Department of The City of Austin, Water and Environmental Research Institute of the Western Pacifi¬c at the University of Guam, Environmental Science Institute of UT Austin, ACI Consulting, Cambrian Environmental. The owners, managers and staff at Inner Space Cavern, Natural Bridge Caverns, Westcave Preserve, and Cave Without a Name. Design and science by members of the Banner Research Group. Background Photo: Natural Bridge Caverns, Daniel Felan, 2015