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

<p>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.</p><p>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, CO<sub>2</sub> concentrations, and drip rates. Ca<sup>2+</sup> concentrations and relative fluorescence intensities to quantify fulvic-acid concentrations were measured monthly from drip-water samples.</p><p>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. CO<sub>2</sub> concentrations in the cave, closed to outside air values from November to March, became greater from April and reached maximum values in September. Ca<sup>2+</sup> 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.</p><p>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 CO<sub>2</sub>, low Ca<sup>2+</sup> 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.</p>

Comparison of annual-layer formation pattern and climate monitoring data: an example of a stalagmite from Gyokusen-do Cave, Okinawa Island, southwest Japan

<p>Stalagmites can provide long, accurate, and continuous palaeoenvironmental records of the Earth’s surface. However, insufficient or biased information on stalagmites has also been derived from some observed data, such as fluorescent annual-layer patterns and cave-climate monitoring data, which indicate sub-annual stalagmite growth rates can change with seasonal cave environments. Observations of stalagmite growth processes compared with cave-climate monitoring data provide an estimate of changes in growth rate. However, this method is considered unreliable as growth rates of normal stalagmites (~ 0.001 – 0.1 mm yr<sup>-1</sup>) cannot provide sufficient data for validation. Many caves developed in uplifted Quaternary coral-limestones of subtropical islands in the Northeastern Pacific region.</p><p>The Gyokusen-do Cave in the southern part of Okinawa Island, southwest Japan, is famous for frequent and massive speleothems and as a tourist destination. This cave has stalagmites with a high growth rate (~ 1 mm yr<sup>-1</sup>) along a pathway laid in 1987. The cave climate (temperature, carbon dioxide concentration, drop rates, and water chemistry) has been monitored since the summer of 2017. Distinctive seasonal changes in the cave environment are apparent in the data. In this study, we sampled sub-annual layer patterns collected in January 2019 from a stalagmite (~ 20 mm in length) on a stone wall in the cave and compared them with the cave-climate monitoring data and climate records near the study site, thus verifying the formation of annual layers. About 31 or 32 years are reflected in the (0.63 – 0.65 mm yr<sup>-1</sup>) in the stalagmite record, because the stone wall was constructed in 1987. From base to top, the stalagmite has about 30 couplets of a transparent layer and a coarsely crystalline zone. The uppermost 5 mm has continuous layers without any hiatus, whereas concave points such as the drop position have thick layers of large crystals still in development. The stalagmite surface is covered with relatively large crystals that developed in the winter of 2018, which suggests that the winter climate produces coarse-grained layers precipitated during the winter season. The cave-climate monitoring data, collected about 150 m from the stalagmite, shows calcium ion concentrations of around 1 – 1.5 mol m<sup>-3</sup>, temperature around 24 – 25 °C, and drastically different carbon dioxide concentrations in summer and winter seasons (around 400 – 500 ppm from the end of October to the beginning of May and around 2500 ppm from the middle of May to the middle of October). Precipitation and drop rates are highest in summer as compared to other seasons. Stalagmite growth simulations based on the monitoring data showed that the growth rate during the summer season was about five times that in winter. These results suggest that alternation between the transparent layer precipitated in summer and the coarse-grained layer precipitated in winter make annual layers that were strongly affected by drop rates and carbon dioxide concentrations. As some seasonal layers have significantly different thicknesses, more precise comparisons with cave-climate data are required to fully understand on the processes that occur in cave environments.</p>

Rapid changes of the ice mass configuration in the dynamic Diablotins ice cave – Fribourg Prealps, Switzerland

The Gouffre des Diablotins is a deep cave system located in the Swiss Prealps. In 1991, the entrance zone of the cave was almost free of ice. Nevertheless ice volume sharply increased in 1994, plugging almost totally the gallery from the lower entrance. The ice cave have also experience flooded period between 1996 and 2007, and very heterogeneous ice surface morphology and textures have formed. Continuous cave climate measurements initiated in 2009 showed the predominant role of winter atmospheric air conditions to drive both the efficiency of chimney-effect circulation and seasonal modifications of the ice mass. Main part of the ice loss is currently due to sublimation in wintertime.