Research Motivation

 

Climate and environmental changes are among important challenges facing our society today, yet rising debates among politicians whether they are "hoaxes" or not. I see them collectively as a "cancer" that can progressively destroy our planet Earth if preventative measures are not taken. This motivates me to focus more on paleo-reconstructions. Why? Because our knowledge of what has happened in the past will help us better prepare for the future.

 

The causes of climate and environmental changes are diverse but they can be generally grouped into three main categories: (1) natural, (2) anthropogenic, and (3) the combination of 1 and 2. 

 

Paleoclimate and paleoenvironmental studies are among fundamental scientific disciplines that can help us understand the causes and the nature of the changes. As a geologist and paleoclimate scientists, my job can be understood as follow "a doctor investigating the health states of a patient, checking the patient's level of pain, and prescribing medicines or specific treatments to minimize the pain". Analogically, I investigate a range of proxies from stalagmites (=upward growing mounds of sediments in caves), including stable isotopes, petrography, and mineralogy to reconstruct the changes and to understand the magnitude of the changes. Along with other available paleo-records, I use my datasets to test climate models. With my current Marie Curie Fellowship, I also investigate lake sediments and perform monitoring at caves and lakes environments to calibrate paleoclimate and paleoenvironment proxies.

 

I wish to communicate the results of my scientific investigations to bring awarness to a broader–lay audience. I have written few blogs, participated in film documentary, and have founded a non-profit association to help me reach that goal. Public awarness is a primary antidote to the negative impacts of climate and environmental changes, and convincing policy-makers to take preventative actions would minimize the risks associated with them.

 

Research Overview

My research has focused on geochemistry, petrography, and mineralogy of stalagmites to reconstruct past climate and environ-mental changes in the Southern Hemisphere, particularly in Madagascar and Southern Africa. Recently, I have been involved in the study of stalagmites from Rodrigues to provide a climatic context for the megafaunal extinctions in Madagascar and Mascarene Islands. I have also expanded the type of geological archives I investigate by adding lake sediments in the list.

 

In terms of proxies, I have been and be using a range of stable isotopes (d18O, d13C, d15N, dH, D47, and D17O) along with trace elements and other physical features of each geological archives to provide a comprehensive unders-tanding of what has happened in the past.

 

Lake sediments are more generally defined as materials accumulated at the bottom of a lake in an orderly and undisturbed manner (Birks et al., 2012). They can be dated using a variety of methods, including radiocarbon dating, and  they preserve a  myriad information about its surrounding environment either chemically, that is detectable via changes in isotopic signals, or physically, that is observable by the change of color, texture, and structure of the sediment core.

 

Stalagmites are  pillars-like secondary cave deposits (Fairchild and  Baker, 2012). They are powerful tool to reconstruct paleoclimate and paleoenviron-ment 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/paleoenviron-mental studies. 

 

If one is unfamiliar with stalag-mites, 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. 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, particularly the monsoonal responses to the migration. 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 previous 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). Another multi-proxy climate record from a northwestern Botswana stalagmite also suggests wetness late in the Little Ice Age (1810-1820 CE) and drying thereafter in response to changing migration of the tropical rain belt (Railsback et al., 2018). 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, and it was interrupted with several episodes of dry intervals (Wang et al., 2019). Some of the most striking aspects of the Holocene speleothem records from Anjohibe Cave during the early Holocene interval is the preservation of the 8.2 ka event (Voarintsoa et al., 2019Wang et al., 2019).

These finding implies linkages between climate in NW Madagascar, NE Namibia, and NW Botswana and the latitudinal migration of the tropical rainbelt, 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.

 

 

Research Awards

 

2019-2021: Marie Skłodowska-Curie Fellowship

2016: John Montagne Award, best proposal in the field of Quaternary Geology & Geomorphology

2016: International Association of Sedimentologists Post-Graduate Research Grant

 

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© Ny Riavo Voarintsoa 2015-2020