Stable carbon and oxygen isotopic composition in carbonate nodules from alluvial soils and their implications for paleoclimate in Taiwan

Carbonate nodules are pedogenic carbonates commonly found in regions of high evapotranspiration, such as arid and semiarid areas. The stable carbon isotopes in these nodules are influenced by the type of vegetation (C3 or C4 plants), while the stable oxygen isotopes are controlled by soil water and temperature. Carbonate nodules persist in the soil, and their isotopic signatures can reflect the paleoclimatic conditions under which they formed. Carbonate nodules are distributed across the alluvial plains of southwestern Taiwan; however, the present humid conditions may not be favorable for their formation. The objective of this study was to evaluate the climatic conditions and vegetation types that influenced the formation of carbonate nodules using stable carbon and oxygen isotopes. Four pedons with different ages of soil formation were sampled in accordance with marine transgression and regression phases, corresponding to 10,000 years before present (yr BP), 8,000 yr BP, 5,000 yr BP, and 3,000 yr BP. Carbonate nodules were collected from the pedons, and their stable carbon and oxygen isotopes were analyzed using an automated carbonate preparation device. The δ¹³C values indicated a mixed C3/C4 vegetation, with a predominance of C4 plants (68.0 to 98%). The mean annual temperature (12.3-14.0°C), calculated using the climofunction of temperature and δ¹⁸O, was lower than the present (24.7°C). The mean annual precipitation (1036 to 1342 mm yr^-1), calculated from the geochemical climofunction, was also lower than the present (1829 mm yr^-1). The radiocarbon ages of the carbonate nodules ranged from 4063 yr BP to 690 yr BP, implying that climatic conditions may have been drier and cooler than present during this time frame. This may be due to a weaker East Asian Summer Monsoon, which favored calcification in the soils. These climatic conditions are consistent with the formation environment of carbonate nodules.