Stalagmites are a chemical precipitate found in caves that form from drip waters and grow from the floor toward the ceiling. Various environmental and climatic factors influence how they grow and develop layers. A recent article in Reviews of Geophysics analyzed a global dataset of laminated stalagmites. Here, the lead author explains what influences laminated stalagmites in different locations and what the analysis revealed.
What are stalagmites and how do they grow?
Stalagmites commonly accumulate in a vertical direction and typically have an associated stalactite, which forms on the cave ceiling and grows downward.
They are built by layers of calcite crystals, which may be perfectly stacked one on top of the other if nothing disturbs the growth; however, there are many disturbances in caves.
For example, if tiny particles (known as colloids) are transported from the soil onto the stalagmite, this disturbs the stacking, and may create pores between growing crystals or even slightly change their shape. The addition of certain trace elements also disturbs the growth because they may influence the morphology of the growing crystals, known as “fabric.”
Where there is a seasonal climate, these changes in fabric can occur seasonally, producing layers which we call “annually laminated stalagmites.” Annual layers can also be observed in the trace element, tiny particles, and organic material composition of stalagmites.
Does the presence of annual lamination seen in stalagmites vary from place to place? If so, what are the main influencing factors?
We analyzed a global database to find out about the geographic distribution of laminated stalagmites. We found that they occur in caves that developed within a wide range of host rock geological ages (from Ordovician to the present-day) and lithologies (limestone, dolomite, and aeolianites). Geographically, laminated stalagmites have been reported from between 35 °South and 66 °North. They have also been reported from a very wide range of mean annual temperature (from 1.4 to 26 °C) and total annual precipitation (from 74 to 2052 millimeters).
The main requirement for the presence of laminated stalagmites is the seasonality of rainfall. Laminated stalagmites are predominantly found where precipitation has a definite wet or dry season, and no laminated stalagmites have been reported for sites with no precipitation seasonality. Laminated stalagmites are rare in semi-arid climates, and we interpret this to be due to insufficient water availability for continuous stalagmite deposition. Laminated stalagmites from environments with very high rainfall (where rainfall was more than double evaporation) are also rare, which we interpret as due to the continuous nature of recharge.
What do the characteristics of the annual growth layers reveal?
At a global scale, the database reveals how fast stalagmites grow (or more technically, the rate of vertical extension). The mean annual growth rate is 0.163 millimeters, and the median is 0.093 millimeters. In other words, the “global average stalagmite” will have increased in height by about 1 meter over the last 11,000 years.
Our analysis also revealed that the rate of stalagmite growth in a particular year is similar to that of the previous years. However, there is an interesting property related to the rate of change in growth rate from one year to the next, where high growth rate years tend to be followed by low growth years. This is known as “flickering.” The combination of high autocorrelation over several years, and the short-term flickering between years, appears to be a near-universal phenomena generated by water movement and storage in karstified rocks.
What did you set out to learn through your analysis of multiple studies of annually laminated stalagmites from around the world?
We were particularly interested to understand if there were common properties of laminated stalagmites. Before this analysis, we did not have evidence that they are only found in regions with seasonal precipitation. Nor was it obvious that stalagmite accumulation rate is relatively unchanging over time and that this is a ubiquitous property.
What we have learned is that for an environmental signal to be preserved in stalagmite laminae thickness variations requires a large perturbation, such as wet or dry years associated with the El Niño – Southern Oscillation phenomenon or mega-droughts, which can override the buffering effect of the overlying water reservoir. However, in regions where there is a seasonality of precipitation, the long-term constant growth rate of laminated stalagmites provides an unparalleled capacity for precise chronology building.
What are the most significant similarities and differences you found between growth patterns in different locations?
The biggest difference we observed is between the growth rate and temperature. At a global scale, we observe a strong, positive correlation between annual growth rate and mean annual temperature, and therefore also a negative correlation with latitude.
The biggest similarity is the linearity of growth rate. For the majority of samples, and irrespective of depositional environment, we show that the age-depth relationship is almost exactly linear over the timescale of tens to thousands of years. We interpret this as due to the buffering effect of a well-mixed karst water store that is necessary to ensure a continuous stalagmite deposition. This approximation to linearity can be used for geochronological applications, such improving the age-depth models necessary for attributing precise ages to geochemical proxies such as oxygen and carbon isotopes.
What do you think future studies on annually laminated stalagmites should focus on?
We still have limited understanding on how crystals grow within each lamina, so future studies could investigate the internal structure of the laminae and the crystal growth mechanisms involved.
More research could be conducted in climatically sensitive regions where laminated stalagmites are expected to be ubiquitous (such as Ethiopia), and there should be more focus on 2D elemental mapping techniques, as we believe they will reveal widespread chemical laminae in regions with seasonal rainfall (for example, elemental mapping of trace element composition such as strontium using synchrotron or micro- X-ray fluorescence).
Future studies should also work on continuing to expand the database. In particular, this analysis would not have been possible without the community databases such as the SISAL database of speleothem oxygen and carbon isotope composition, which contained numerous annually laminated stalagmites.
—Andy Baker ([email protected], 0000-0002-1552-6166), UNSW Sydney, Australia; Gregoire Mariethoz ( 0000-0002-8820-2808), University of Lausanne, Switzerland; Laia Comas‐Bru ( 0000-0002-7882-4996), University of Reading, UK; Andreas Hartmann ( 0000-0003-0407-742X), Albert‐Ludwigs‐University of Freiburg, Germany and UNSW Sydney, Australia; Silvia Frisia ( 0000-0001-6568-2696), The University of Newcastle, Australia; Andrea Borsato ( 0000-0003-3858-4462), The University of Newcastle, Australia; Pauline C. Treble ( 0000-0002-1969-8555), UNSW Sydney, Australia; and Asfawossen Asrat, Addis Ababa University, Ethiopia