Petrographic features, mineralogy, and stable isotopes from two stalagmites, ANJB-2 and MAJ-5, respectively from Anjohibe and Anjokipoty caves, allow distinction of three intervals of the Holocene in NW Madagascar. The Malagasy early Holocene (between ca. 9.8 and 7.8gka) and late Holocene (after ca. 1.6gka) intervals (MEHI and MLHI, respectively) record evidence of stalagmite deposition. The Malagasy middle Holocene interval (MMHI, between ca. 7.8 and 1.6gka) is marked by a depositional hiatus of ca. 6500 years. Deposition of these stalagmites indicates that the two caves were sufficiently supplied with water to allow stalagmite formation. This suggests that the MEHI and MLHI intervals may have been comparatively wet in NW Madagascar. In contrast, the long-term depositional hiatus during the MMHI implies it was relatively drier than the MEHI and the MLHI.
The alternating wet-dry-wet conditions during the Holocene may have been linked to the long-term migrations of the Intertropical Convergence Zone (ITCZ). When the ITCZ's mean position is farther south, NW Madagascar experiences wetter conditions, such as during the MEHI and MLHI, and when it moves north, NW Madagascar climate becomes drier, such as during the MMHI. A similar wet-dry-wet succession during the Holocene has been reported in neighboring locations, such as southeastern Africa. Beyond these three subdivisions, the records also suggest wet conditions around the cold 8.2gka event, suggesting a causal relationship. However, additional Southern Hemisphere high-resolution data will be needed to confirm this.
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Acknowledgements. This work was supported by grants from (1) the National Natural Science Foundation of China (NSFC 41230524, NBRP 2013CB955902, and NSFC 41472140) to Hai Cheng and Gayatri Kathayat, (2) the Geological Society of America Research Grant (GSA 11166-16) and John Montagne Fund award to Ny Riavo Gilbertinie Voarintsoa, (3) the Miriam Watts-Wheeler Graduate student grant from the Department of Geology at UGA to Ny Riavo Gilbertinie Voarintsoa, and (4) the International Association of Sedimentology postgraduate grant to Ny Riavo Gilbertinie Voarintsoa. We also thank the Schlumberger Foundation for providing additional support to Ny Riavo Gilber-tinie Voarintsoa’s research. We thank the Department of Geology at the University of Antananarivo, in Madagascar, the Ministry of Energy and Mines, the local village, and guides in Mahajanga for easing our research in Madagascar. We specifically thank Voahangy Ratrimo, former department head of the Department of Geology at the University of Antananarivo, for collaborating with us and for giving us permission to conduct a field expedition in Madagascar. We thank Paul Schroeder for giving us access to use the X-ray diffractometer of the Geology Department to conduct analysis on the mineralogy of the two stalagmites. We thank John Shields of the Georgia Electron Microscopy, University of Georgia, for giving Voarintsoa access to use the Zeiss 1450EP (Zeiss Inc., Thornwood, NY) for SEM purposes. We also thank Sally Walker for allowing us to use the microscope of the paleontology lab and for helping us to photograph the stalagmites at very high resolution. We also thank John Chiang of the University of California at Berkeley for sharing his thoughts and guiding us to literature of relevance to this study.
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