Simulation of fluorescent annual-layer patterns in stalagmites based on cave climate monitoring data: an example from Komori-ana Cave, Akiyoshi-dai Plateau, southwest Japan

Fluorescent annual layers with thicknesses of 0.01–0.1 mm occur frequently in stalagmites around the world. Aggradational variations of fluorescence intensity expressing those annual layers have been postulated as being caused by seasonal fluctuations of the supply of fulvic acid from the surface. The variation patterns of fluorescence intensity in annual layers can be classified into symmetric, gradually increasing, and gradually decreasing types. Numerical simulation of fluorescent annual-layer patterns based on the stalagmite-formation model suggests that various patterns of fluorescence intensity in annual layers can form by time lags between a growth season and the fulvic acid supply peak on a stalagmite. However, verification of those fluorescence patterns requires long-term cave climate monitoringin caves. In this study, we simulated fluorescence intensity variations in a modeled stalagmite based on cave climate monitoring data from a cave in a humid-temperature climate and validated annual layer formations.

Cave climate monitoring was performed at point A (40 m inside the entrance), point B (90 m inside the entrance), and other points in Koumori-ana Cave, Mine City, Yamaguchi Prefecture, southwest Japan, from the end of 2016. The monitoring data included cave air temperatures, CO₂ concentrations, and drip rates. Ca²⁺ concentrations and relative fluorescence intensities to quantify fulvic-acid concentrations were measured monthly from drip-water samples.

The monitoring data showed that cave temperatures decrease in winter near the entrance and increase in summer near the upper vent. Drip rates at point A corresponded to rainfall amounts at the meteorological station in Akiyoshi-dai, whereas drip rates at point B were constant throughout the years monitored. CO₂ concentrations in the cave, closed to outside air values from November to March, became greater from April and reached maximum values in September. Ca²⁺ concentration had gradual seasonal variations, showing a maximum in October and a minimum in March. The relative fluorescence intensities, showing fulvic acid concentration, at both points revealed a change range of about four times the minimum.

The stalagmite-growth simulations based on the monitoring data showed different growth patterns at the two monitored points: continuous growth at one and hiatus at the other. The calculated fluorescent annual layer at point A was the symmetric or gradually increasing type, with high concentration of fulvic acid in August. The growth rate varied in the range of 0.45 (Jan–Apr) to 6.2 (May–Oct) µm/month. Because the relative fluorescence intensity of fulvic acid had small variations throughout the years, the simulated fluorescent annual layer at point A is suggested to be affected by the growth rate of stalagmite. At point B, decreased saturation indices of calcite from April to June and September to October suggest no precipitation of calcite. Although the simulated annual thickness of precipitation at point B is around 28 µm, half of the thickness is precipitated in July. Point B stalagmite growth is stopped by a high concentration of CO₂, low Ca²⁺ concentration, and low drip rate. This study suggests that specific seasonal paleoenvironmental changes recorded in stalagmites can be estimated by using fluorescence patterns of annual layers.