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

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^-1) cannot provide sufficient data for validation. Many caves developed in uplifted Quaternary coral-limestones of subtropical islands in the Northeastern Pacific region.

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^-1) 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^-1) 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^-3, 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.