STALAGMITE δ 13C CHANGES AND VEGETATION COVER CHANGE IN NORTHWESTERN MADAGASCAR

Stalagmites have been considered one of the most potential paleoclimate archives because they contain several proxies that are directly or indirectly influenced by climate. By far, the data most commonly collected and applied from stalagmites have been δ¹⁸O, and the paleoclimate history was established in combination with U-Th chronology. In the application of stalagmites to paleoclimatology, δ¹³C are always measured with δ¹⁸O, but they are rarely discussed. This is because stalagmites δ¹³C are often difficult to interpret and the evaluation of the δ¹³C records is challenging because it is highly variable, and this variability is dependent on several factors. These factors include the CO₂ in the atmosphere, the CO₂ of the soil, the host rock CaCO₃, the CO₂ of the cave air, and other internal of external factors that could influence the δ¹³C of the CO₂ inside and outside the cave. Here stalagmite δ¹³C data from Anjohibe Cave, in northwestern Madagascar, have been closely investigated. In combination with other available paleoenvironmental records and a thorough literature review on Madagascar’s inhabitation over the last ca. 2000 year, the stalagmite δ¹³C records suggest a remarkable change in vegetation around AD 800–1000 from C₃–plants to C₄–plants. With a better understanding of the nature of land colonization, the nature of crops being introduced to Madagascar (Asian crops), and the main land practice in this island (slash-and-burn or ‘tavy’), the C₃ dominated vegetation around the 8^th-10^th centuries must have been Asian rice and mung beans, as documented by Crowther et al. (2016). As ‘tavy’ consists of burning former vegetation in favor of planting various crops, like dry-land rice or “vary an-tanety”, for a few years until soil nutrients are exhausted, the abandonment of land leaves secondary grasslands and other fire-prone open vegetation ecosystems, named “savoka”, to grow. The new vegetation cover has become dominated by C₄–plants, and this transformation is observed in northwestern Madagascar since ca. AD 800–1000. The change significantly altered the δ¹³C signals in stalagmite, and among the factors that could influence stalagmite δ¹³C, the changes in δ¹³C in northwestern Madagascar since ca. AD 800–1000 must have been controlled by the changes in vegetation cover.