Abstract: |
Stalagmites are an increasingly important archive of paleoenvironmental change for timescales ranging from the Late Holocene to the Permian, with the most significant contribution to date being Late Quaternary records of climate variability. The rate of annual growth rates of stalagmites is recorded in changes of calcite fabric, annual fluxes of fluorescent organic matter or annual variations in trace element composition. The determining processes governing stalagmite growth are increasingly well understood and modeled.
At the scale of chemical processes, the physical controls of stalagmite growth are the flux of water, the CO2 saturation of drip water relative to the cave atmosphere, and the temperature. The processes determining all three are complex and inter-related. Therefore, although past climates are recorded in the growth laminates, the climatic signal is perturbed by a noise component related to local hydrologic factors.
In order to separate local from global factors, we analysed with geostatistical tools annual growth rate data from 14 stalagmites located on 4 different continents. The records range from 200 to 2500 years before present. Detailed variographic analyses showed that the temporal correlation of growth rates is of a very specific type in all 14 stalagmites, which has never been observed before. The growth derivative is highly anticorrelated at a lag of 1 year, meaning that an increase in growth rate tends to be systematically followed by a decrease in growth rate. We call this behaviour a "flickering" growth. Flickering cannot be explained by climatic factors that tend to vary on larger time scales, and therefore must be related to changes in local hydrologic conditions. We show that the intensity of flickering fluctuates in the last millennia, giving insights in the temporal scale of variability of hydrologic systems under natural conditions. |
