Missions

This postdoctoral project funded by the ANR NILAFAR aims to:
Assess the hydrological changes in the Afar depression, specifically from the lakes of the Abhe sedimentary basin in Ethiopia and Djibouti, to enhance our comprehension of the mechanisms driving aridification episodes over the last 20,000 years. This study employs high-resolution analysis of recent hydrological changes to investigate the co-evolution of climate, human activity, and the environment.

Activités

The post-doctoral student will be in charge of
- the characterization of the organic signatures of the sedimentary sequence ABH-23. These analyses include the composition and abundance of organic biomarkers, including GDGTs, fatty acids, n-alkanes, triterpenes, HBI, and molecular isotopy (δ13C and δ2H).
- the integration of the organic and inorganic components in lacustrine sediments on two scales: i) on a recent timescale, utilizing a sedimentary sequence from Afambo lake (which cover the last 50 years with seasonal resolution), and compares it with instrumental meteorological data to enhance the interpretation of organic tracers within the Afar depression. ii) On a Holocene scale, the research examines fossil biomarkers from the core ABHE23 and soils in the watersheds of Afambo, Gamari, and Abhe lakes to reconstruct hydrological variations, particularly during arid episodes around 4.2 ka and 8.2 ka, aiming to clarify the timing and characteristics of these arid phases. The results will be contextualized at regional scale. The recent research on the expression of events 4.2 and 8.2 in Africa, particularly north of Madagascar, as well as in the Mediterranean, Atlantic, and Indian Oceans, aims to enhance the understanding of the mechanisms governing the African monsoon, particularly in relation to the El Niño phenomenon.

Compétences

- Ph.D. in geosciences, paleoclimatology, geology, or geochemistry, along with relevant experience,
- Demonstrated organizational skills and a high degree of autonomy,
- Capability to adopt a multidisciplinary approach and engage in collaboration with other teams

Contexte de travail

Context and research issues =
Tropical Africa has experienced cyclical precipitation changes during the Quaternary, associated with the African monsoon, and in response to variations in the Earth's orbital dynamics. The last period, known as the African Humid Period (14-6 ka), was characterized by wetter conditions in North Africa, which are gradually diminishing. Beginning 15,000 years ago, this period extends for approximately 7,000 years. This wet period is punctuated by several brief dry episodes of hundreds of years, around 12, 8.2, and 4.2 ka, the mechanisms of which are linked to internal climate variability and remain unexplained. Weaker tropical monsoon episodes appear to correlate with the frequency of Heinrich events, which follow the rapid melting of icebergs and the influx of freshwater into the North Atlantic Ocean. The rapid variations indicate the highly nonlinear characteristics of the climate system, implying unforeseen future climate developments in response to significant greenhouse gas emissions. The greenhouse effect is a consequence of human activities.
The sedimentation of the Abhe paleolake, situated in Ethiopia and Djibouti, reflects hydrological variations influenced by orbital and internal forcings
Initially encompassing 6000 km² at the onset of the Holocene, the lake has since diminished to an area of 300 km² . The reduction has significantly affected surrounding populations; therefore, reconstructing Holocene hydrological variations in the Horn of Africa will assist in addressing critical questions within the archaeological community concerning Neolithization:
- Why did humans adopt farming practices only 4000 years ago, which is 8000 years after the Levant and 3000 years after Western Europe, in the Horn of Africa?
- Did the significant alterations in water availability, influenced by fluctuations in the level of Lake Abhe, contribute to the Neolithic transition from a predation economy (hunter-gatherers) to a production economy (animal and plant domestications), wherein humans produce and manage their food, generate surpluses, innovate (pottery), and store resources?

Equipment and work strategy =
This project focuses on the analysis of two small sedimentary cores collected in the Afambo and Gamari lakes and a 15-meter-long sedimentary sequence from Lake Abhé, which spans the Holocene and documents terrigenous inputs from the Awash rivers and Gobaad basin. This sedimentary dispersion system from the sources of the Awash River from the Ethiopian Highlands to the endemic lakes, is significantly influenced by variations in the intensity of the East African monsoon. The sediments of these lakes provide a unique hydro-climatic record on the watersheds of these three lakes.
A preliminary chrono-stratigraphy of the sedimentary sequences has been established based on radiocarbon, radionuclides and paleomagnetism has been established as well as the sedimentary description of the sequence and the analysis of the core in major elements using X-ray Fluorescence Core scanner (XRF).

Contraintes et risques

The candidate will mainly work at LGLTPE ENS-Lyon (Guillemette Ménot), focusing on analytical organic geochemistry, with frequent missions at LSCE (Jérémy Jacob). This funded postdoctoral research is grounded in the consortium that supports the ANR NILAFAR initiative . This research project focuses on the analysis of the AFA-18 and GEM-18 cores from the CLIMAFAR 2018-20 project, directed by L. Khalidi (CEPAM-CNRS) and M.Revel (GEOAZUR), as well as the long ABH23 core from the ANR NILAFAR, also led by M. Revel. A collaboration is established with Nathalie Vigier (LOV-UMR7093, Villefranche sur Mer) to study the erosion and alteration of lake sediments, and with Pierre Brigode (Geosciences Rennes) for hydrological modeling. Additionally, partnerships are formed with Jessie Cauliez (TRACES) and L. Khalidi (CEPAM), archaeologists focused on the Neolithic transition in the Horn of Africa. A collaboration is established with researchers from the CERD (M. Jalludin) and ORREC (Regional Observatory for Research on Environment and Climate) in Djibouti.