Late Holocene (ca. AD 370-1210) ecosystem changes inferred from a stalagmite from northwestern Madagascar: the role of the ITCZ and human activity

Abstract:

High-resolution stable isotope records combined with petrographic examination of stalagmite MA3 from Anjohibe Cave provide a better understanding of the climatically- and anthropogenically- induced ecosystem change in NW Madagascar between ca. AD 370 and 1210. The record suggests strong climatic control prior to ca. 700 and strong influence of human activities after ca. 920.

Prior to 700, monsoonal rainfall and ecosystem change seem to respond to the change in the relative position of the ITCZ. This is inferred from the positive correlation between δ¹³C and δ¹⁸O and T anomalies in the Northern Hemisphere. A cooler NH pushed the ITCZ southward, thus favoring longer visits in northwestern Madagascar and strengthening the Malagasy monsoon. This offered favorable conditions to vegetation cover growth (the opposite was possible during warmer NH). The period between ca. 675 and 700 is the driest and might represent a period of abrupt change in vegetation cover as suggested by the stalagmite's petrography, smaller layer specific width, and greater values of both δ¹³C and δ¹⁸O. This drier period is followed by slight change in hydrology and a gradual recovery in vegetation in NW Madagascar, which lasted until ca. 920. This change is reflected in the decrease of stable isotope values and an increase in the layer specific width.

Since ca. 920, the records suggest that the ecosystem change around the cave was controlled by something other than climate. The coincidence in time with human settlement and the gradual shift from lower to higher δ¹³C would suggest a strong involvement of human activities, particularly the practice of "tavy" or slash-and burn activities. This finding is important because Madagascar has experienced major ecological changes since the late Holocene, the cause of which has raised two hypotheses as to whether caused by severe climate change or by human activities. Deciphering the difference between human-induced and abrupt climatic changes has been challenged by the difficulty in recognizing cause-and-effect relationships in the paleorecords. This study has therefore helped in better understanding the main controls of ecosystem changes in Madagascar between ca. AD 370 and 1210, and we conclude that the strong evidence of human influence on the ecosystem change did not start until ca. 920.