Growth mechanism of stalagmites under different vegetation cover: a case study of stalagmites in Minami-Daito Island, Japan

Speleothems formation depends on geological, hydrological, water chemistry, and climatic factors. Therefore, the δ13C, δ18O, and growth rate of stalagmites are known as a good indicator of paleoenvironment. Minami-Daito... [ view full abstract ]

Speleothems formation depends on geological, hydrological, water chemistry, and climatic factors. Therefore, the δ13C, δ18O, and growth rate of stalagmites are known as a good indicator of paleoenvironment.
Minami-Daito Island had been an uninhabited island until 1900 A.D. By the cultivation after 1900 A.D., the vegetation had changed from sub-tropical forests (C3 plant) to sugarcane field (C4 plant). Our previous study revealed that the growth rate and δ13CStalagmite of the IM-2 stalagmite under sugarcane field became drastically higher after 1900 A.D., although those of the YS-4 stalagmite under forests have not significantly changed. To clarify the growth mechanism under different vegetation, we measured the soil and cave environmental indexes and also water chemistry of the drip waters (DW) for each season from August 2013 to December 2015.
Majority of saturation index for calcite (SIc) in DW are positive and higher in dry season (winter). The SIc under sugarcane field are higher compared to those under forests. The δ13C of the soil air (δ13CSoil) shows distinctive differences between both vegetations (Sugarcane field, -11.40〜-15.54‰; Forests, -17.40〜-24.31‰). The δ13C of dissolved inorganic carbon in DW (δ13CDW-DIC) show highest value in winter (Sugarcane field, -5.58〜-10.71‰; Forests, -10.13〜-13.25‰). The amounts of DW are relatively constant under forests, while change drastically responding to rainfall under sugarcane field.
These results suggest a following model for the growth mechanism of stalagmites in Minami-Daito Island; At beginning, ground water has high dissolved CO2 concentration due to percolating through soil with high CO2 concentration, and dissolves underlying carbonate rocks. With the ground water flowing downward, degassing of the dissolved CO2 with evaporation leads to increase the SIc value. Especially, the SIc become higher by evaporation increased in in dry season. At that time, as light CO2 is selectively degassed, δ13CDW-DIC become heavier. The evaporation is more active in sugarcane field than in forests due to characteristic of soil and protection from sunshine by tree. Consequently, the SIc of DW and growth rate of stalagmite under sugarcane field is higher. The δ13CStalagmite is attributed to the δ13CDW-DIC which is controlled by the δ13CSoil. Therefore, it is concluded that the drastic changes of the growth rate and δ13CStalagmite under sugarcane field after 1900 A.D. are due to the replacement from forests to the sugarcane and aridification with the cultivation.