Multiple proxy analyses of a U/Th-dated stalagmite to reconstruct paleoenvironmental changes in northwestern Madagascar between 370 CE and 1300 CE

The timing and causes of paleoenvironmental changes in Madagascar have been debated, specifically in respect to human activity following the settlement in the late Holocene. Here we present δ¹⁸O, δ¹³C, layer-bounding surfaces, layer-specific width, mineralogy, and distribution of macroholes from Stalagmite MA3 from Anjohibe Cave to provide a detailed understanding of the paleoenvironmental changes in northwestern Madagascar between 370 CE and 1300 CE. The stable isotope records of Stalagmite MA3 are compared with stable isotope records of Stalagmites ANJ94-5 and MA2. Detailed examination of the proxies suggests three distinct intervals of changes. (1) Prior to 795 CE, changes in vegetation seem to have responded to changes in monsoonal rainfall linked to the relative position of the ITCZ. The period between ca. 755 CE and 795 was the driest, and the Stalagmite MA3 record is consistent with sediment records (pollen and lithology) from Lake Mitsinjo, northwestern Madagascar, and with sediment records (fossil pollen and charcoal) from Sainte Luce, southeastern Madagascar. (2) Between 795 CE and 870, the environmental conditions became more favorable, when vegetation recovered from the driest interval. The new conditions must have been suitable for community development in the region as suggested by archaeological evidence around Lake Mitsinjo and the Boeny region, and the establishment of the stone town of Mahilaka. (3) After 870 CE, a gradational change in plant communities from C₃ to C₄ marks the record until around 1130 CE, after which vegetation was dominated by C₄ plants. This change cannot be explained by climate alone, as there is no clear relationship in the climate-sensitive proxies. Instead, it could have been caused by “Tavy”, a variety of “swidden” agriculture practiced in the region.