My research has focused on geochemistry, petro-graphy, and mineralogy of stalagmites to reconstruct past climate and environmental changes in the Southern Hemisphere, particularly in Madagascar and Southern Africa. Stalagmites, pillars-like secondary cave deposits, are powerful tool to reconstruct paleo-climate and paleo-environment for two main reasons: (1) they can be accurately dated by uranium series techniques, and (2) high-resolution records have made them more useful in paleoclimate/paleo-environmental studies.
If one is unfamiliar with stalagmites, tree rings are their terrestrial analogs, except the formers are rocks that grow upward from the floor of a cave and the latter are trees that grow on the Earth's surface. To the right is a generalized sketch showing the processes of stalagmite formation (from dissolution, degassing and/or evaporation of the dripwater, to CaCO3 precipitation to form stalagmite).
Stalagmites form when cave dripwater is supersaturated with Ca2+ and CO32- and degasses CO2 to precipitate CaCO3 (see equation below):
While my study primarily aims at reconstructing paleoclimate and paleoenvironmental conditions at regional scale, I am also ambitious putting my data on a larger-scale perspective to test climate models. I specifically want to understand the dynamics of the Inter-Tropical Convergence Zone (ITCZ) and the corresponding climatic responses in Southern Africa and Madagascar. Following climate models of Chiang and Bitz (2005) and Broccoli et al. (2006), I hypothesized that when the ITCZ migrates southward/northward, regions in the Southern Hemisphere, such as Madagascar and Namibia, become wetter/drier. This migration is coeval with marked cooling/warming conditions in the Northern Hemisphere.
The data collected from my PhD research agree with these climate models. A multi-proxy approach using d18O, d13C, mineralogy, and petrography from a U-Th dated Stalagmite DP1 in Dante Cave, Namibia suggest that wet and dry intervals in NE Namibia were linked to latitudinal shifts of ITCZ and changing solar activity from AD 1400 to 1950 (Voarintsoa et al., 2017a, The Holocene). Analyses of data collected from Anjohibe and Anjokipoty Caves in NW Madagascar have also proved interesting, and further demonstrate the usefulness of this multi-proxy approach (e.g. Voarintsoa et al., 2017c, Climate of the Past). We found distinct early-, mid-, and late-Holocene climatic regimes in NW Madagascar. Our datasets suggest that the mid-Holocene period was relatively drier compared to the early and the late Holocene. This implies linkages between climate in NW Madagascar and the latitudinal migration of the ITCZ, which itself responded to interhemispheric difference in temperature and, at some specific time intervals, to changes in the deep ocean circulation.
Simplified models portraying Holocene climate change in NW Madagascar and the possible climatic conditions linked to the ITCZ (from Voarintsoa et al., 2017c). a) Wetter conditions during the early Holocene with the ITCZ further south (prior to c 7.8 ka BP). b) Periodic dry conditions during the mid-Holocene (between c. 7.8 and 1.6 ka BP) with the ITCZ further north. c) Wetter conditions during the late Holocene (after c. 1.6 ka BP) with the ITCZ further south. Drawings are not to scale. The bottom figures are from NASA Earth Observatory, and they are only used here to give a perspective of the possible position of the ITCZ during the early, mid, and late Holocene. Madagascar is indicated with a red ellipse.
The records from northwestern Madagascar have additionally helped us better understand anthropogenic imprints on the landscape (Voarintsoa et al., 2017b, Palaeo3). In combination with d18O, mineralogy, changes in the sample’s layer-specific width, and macroholes (large cavities) distribution in Stalagmite MA3 from Anjohibe Cave, we found that shifts in d13C imply landscape changes in the same study area. These records together combine to suggest a climatically-induced vegetation change prior to ca. 800CE and a non-climatically-induced vegetation change thereafter. The d13C shift may represent increased land use, mainly the practice of swidden agriculture, in Madagascar. Our findings seemed to agree with records and inferences from lake sediment cores from Lake Mitsinjo (Matsumoto and Burney, 1994).
Carbon stable isotope profile of Stalagmites MA3 (Voarintsoa et al., 2017b) put in context with a compiled set of archaeological and paleoenvironmental evidences from surrounding locations. The shift in d13C may primarily reflect changes in vegetation cover, which could also represents anthropogenic imprints on the landscape. Paleoenvironmental inferences are from Matsumoto and Burney (1994), Wright et al. (1996), Burney et al. (2003), and Crowley and Samonds (2013). Archaeological evidences are from Wright et al. (1996), Radimilahy (1998), Crowther et al. (2016), and Burney et al. (2003, 2004). Burns et al. (2016) also reported similar isotopic evidence from the same cave.
2016: John Montagne Award, best proposal in the field of Quaternary Geology & Geomorphology
2016: International Association of Sedimentologists Post-Graduate Research Grant