ALBERT

Description: At present, Lake Chad (∼ 13∘′ N, ∼ 14∘ E) is a shallow freshwater lake located in the Sahel/Sahara region of central northern Africa. The lake is primarily fed by the Chari–Logone river system draining a ∼ 600 000 km2 watershed in tropical Africa. Discharge is strongly controlled by the annual passage of the intertropical convergence zone (ITCZ) and monsoon circulation leading to a peak in rainfall during boreal summer. During recent decades, a large number of studies have been carried out in the Lake Chad Basin (LCB). They have mostly focused on a patchwork of exposed lake sediments and outcrops once inhabited by early hominids. A dataset generated from a 673 m long geotechnical borehole drilled in 1973, along with outcrop and seismic reflection studies, reveal several hundred metres of Miocene–Pleistocene lacustrine deposits. CHADRILL aims to recover a sedimentary core spanning the Miocene–Pleistocene sediment succession of Lake Chad through deep drilling. This record will provide significant insights into the modulation of orbitally forced changes in northern African hydroclimate under different climate boundary conditions such as high CO2 and absence of Northern Hemisphere ice sheets. These investigations will also help unravel both the age and the origin of the lake and its current desert surrounding. The LCB is very rich in early hominid fossils (Australopithecus bahrelghazali; Sahelanthropus tchadensis) of Late Miocene age. Thus, retrieving a sediment core from this basin will provide the most continuous climatic and environmental record with which to compare hominid migrations across northern Africa and has major implications for understanding human evolution. Furthermore, due to its dramatic and episodically changing water levels and associated depositional modes, Lake Chad's sediments resemble maybe an analogue for lake systems that were once present on Mars. Consequently, the study of the subsurface biosphere contained in these sediments has the potential to shed light on microbial biodiversity present in this type of depositional environment. We propose to drill a total of ∼ 1800 m of poorly to semi-consolidated lacustrine, fluvial, and eolian sediments down to bedrock at a single on-shore site close to the shoreline of present-day Lake Chad. We propose to locate our drilling operations on-shore close to the site where the geotechnical Bol borehole (13∘28′ N, 14∘44′ E) was drilled in 1973. This is for two main reasons: (1) nowhere else in the Chad Basin do we have such detailed information about the lithologies to be drilled; and (2) the Bol site is close to the depocentre of the Chad Basin and therefore likely to provide the stratigraphically most continuous sequence.

Description: In the Sahelian belt, Lake Chad is a key water body for 13 million people, who live on its resources. It experiences, however, substantial and frequent surface changes. Located at the centre of one of the largest endorheic basins in the world, its waters remain surprisingly fresh. Its low salinity has been attributed to a low infiltration flow whose value remains poorly constrained. Understanding the lake's hydrological behaviour in response to climate variability requires a better constraint of the factors that control its water and chemical balance. Based on the three-pool conceptualization of Lake Chad proposed by Bader et al. (2011), this study aims to quantify the total water outflow from the lake, the respective proportions of evaporation (E), transpiration (T), and infiltration (I), and the associated uncertainties. A Bayesian inversion method based on lake-level data was used, leading to total water loss estimates in each pool (E + T + I =  ETI). Sodium and stable isotope mass balances were then used to separate total water losses into E, T, and I components. Despite the scarcity of representative data available on the lake, the combination of these two geochemical tracers is relevant to assess the relative contribution of these three outflows involved in the control of the hydrological budget. Mean evapotranspiration rates were estimated at 2070 ± 100 and 2270 ± 100 mm yr−1 for the southern and northern pools, respectively. Infiltration represents between 100 and 300 mm yr−1 but most of the water is evapotranspirated in the first few kilometres from the shorelines and does not efficiently recharge the Quaternary aquifer. Transpiration is shown to be significant, around 300 mm yr−1 and reaches 500 mm yr−1 in the vegetated zone of the archipelagos. Hydrological and chemical simulations reproduce the marked hydrological change between the normal lake state that occurred before 1972 and the small lake state after 1972 when the lake surface shrunk to a one-tenth of its size. According to our model, shrinking phases are efficient periods for salt evacuation from the lake towards the phreatic aquifer.

Description: The 4.2kyrs event, used as a marker of holocene stratigraphy, has been described as a rapid climate change in the northern hemisphere triggering droughts in the Mediterranean region. However, the severity and geographical extent of this event are still the subject of investigation considering the small number of palaeoclimatic records for this time period, and the presence of contrasted climatic expressions between areas. At Petit Lake (France, Mediterranean Alps, 2200ma.s.l) a multiproxy study of Holocene lake sediments has revealed major changes in erosion processes and phytoplanktonic assemblages in the lake ecosystem around 4200 cal. BP. According to pollen analysis, deforestation is unlikely to be the main explanation of environmental changes as the watershed was covered by open vegetation for the duration of the study period. To test the implication of climate, our study presents an analysis of oxygen isotopes (δ18O) in diatoms describing hydrological modalities during the 4.2kyrs event in the Mediterranean Alps. The highest values of δ18Odiatom occur from 4400 to 3900cal.BP and are interpreted as an increase in water evaporation and/or a decrease in freshwater inputs to the lake system. Changes in water balance might have been associated with a change in precipitation sources towards a greater influence of precipitation coming from the Mediterranean area. These results are concomitant to an increase in erosion in the watershed and high representation of very low-dispersal pollen in the sediments suggesting the presence of intense runoff. This new isotopic record together with previously-published proxy-data, allows us to describe the 4.2kyrs event at Petit Lake as an increase in Mediterranean climate influences in the region, amounting to a general dry period punctuated by episodes of intense runoff occurring on the catchment slopes.

Description: In the Mediterranean area, the 4.2 ka BP event is recorded with contrasting expressions between regions. In the southern Alps, the high-altitude Lake Petit (Mercantour Massif, France; 2200 m a.s.l.) offers pollen and diatom-rich sediments covering the last 4800 years. A multi-proxy analysis recently revealed a detrital pulse around 4200 cal BP due to increasing erosion in the lake catchment. The involvement of a rapid climate change leading to increasing runoff and soil erosion was proposed. Here, in order to clarify this hypothesis, we measured the oxygen isotope composition of diatom silica frustules (δ18Odiatom) from the same sedimentary core. Diatoms were analysed by laser fluorination isotope ratio mass spectrometry after an inert gas flow dehydration. We additionally enhanced the accuracy of the age–depth model using the Bacon R package. The δ18Odiatom record allows us to identify a 500-year time lapse, from 4400 to 3900 cal BP, where δ18Odiatom reached its highest values (〉31 ‰). δ18Odiatom was about 3 ‰ higher than the modern values and the shifts at 4400 and 3900 cal BP were of similar amplitude as the seasonal δ18Odiatom shifts occurring today. This period of high δ18Odiatom values can be explained by the intensification of 18O-enriched Mediterranean precipitation events feeding the lake during the ice-free season. This agrees with other records from the southern Alps suggesting runoff intensification around 4200 cal BP. Possible changes in other climatic parameters may have played a concomitant role, including a decrease in the contribution of 18O-depleted Atlantic winter precipitation to the lake water due to snow deficit. Data recording the 4.2 ka BP event in the north-western Mediterranean area are still sparse. In the Lake Petit watershed, the 4.2 ka BP event translated into a change in precipitation regime from 4400 to 3900 cal BP. This record contributes to the recent efforts to characterize and investigate the geographical extent of the 4.2 ka BP event in the Mediterranean area.

Description: Complete understanding of the hydrological functioning of large scale intertropical watersheds like the Lake Chad basin, which become a high priority in the prospect of near future climate change and increasing demographic pressure, require integrated studies of all surface and groundwater reservoirs and their quite complex interconnections. In this respect, detailed hydrological studies of secondary peripheral lakes of these large basins may provide us with interesting small scale analogs of the major waterbodies, which can help disentangling the multiple influences of various forcing factors of the water cycle and its evolution. We present here a simple method for estimating the annual mean water balance of sub-sahelian lakes subject to high seasonal contrast, and located in isolated regions with no road access during the rain season, precluding continuous monitoring of in-situ hydrological data. The approach is illustrated by the study of the two lakes Iro and Fitri in the eastern basin of lake Chad, so far unstudied, and also tested on lake Ihotry (Madagascar), extensively studied previously by our group. We combine the isotopic data (δ18O; δ2H) that we measured during the dry season with altimetry data from the SARAL satellite mission, in order to model the seasonal variation of lake volume and isotopic composition. The annual water budget is then estimated from mass balance equations using the Craig and Gordon's model for evaporation. We show that the closed-system behavior (precipitation equal to evaporation) can be confirmed for lake Ihotry, whereas we calculate E/I ratios of 0.6±0.3 and 0.4±0.2 for Iro and Fitri, respectively, in both cases compatible with water fluxes estimated from nearby gauging stations. In the case of Fitri the estimated output flux is contributing to the groundwater recharge, since the lake has no identified surface outlet. Finally, we use our data to discuss possible inferences about the hydro-climatic budget of the catchment basins of those two lakes. We show that the average rainfall isotopic composition monitored by IAEA at NDjamena is slightly offset from the two distinct Local Evaporation Lines that we obtain on the two lake-aquifer systems, and that this slight difference may reflect the impact of vegetation transpiration on the basin water budget. We conclude that, while being broadly consistent with transpiration being on the same order of magnitude as evaporation in those basins, we cannot derive a more precise estimate of the partition between these two fluxes, owing to the large uncertainties of the different end-members in the budget equations.

Description: Complete understanding of the hydrological functioning of large-scale intertropical watersheds such as the Lake Chad basin is becoming a high priority in the context of climate change in the near future and increasing demographic pressure. This requires integrated studies of all surface water and groundwater bodies and of their quite-complex interconnections. We present here a simple method for estimating the annual mean water balance of sub-Sahelian lakes subject to high seasonal contrast and located in isolated regions with no road access during the rainy season, a situation which precludes continuous monitoring of in situ hydrological data. Our study focuses for the first time on two lakes, Iro and Fitri, located in the eastern basin of Lake Chad. We also test the approach on Lake Ihotry in Madagascar, used as a benchmark site that has previously been extensively studied by our group. We combine the δ18O and δ2H data that we measured during the dry season with altimetry data from the SARAL satellite mission in order to model the seasonal variation of lake volume and isotopic composition. The annual water budget is then estimated from mass balance equations using the Craig–Gordon model for evaporation. We first show that the closed-system behavior of Lake Ihotry (i.e., precipitation equal to evaporation) is well simulated by the model. For lakes Iro and Fitri, we calculate evaporation to influx ratios (E∕I) of 0.6±0.3 and 0.4±0.2, respectively. In the case of the endorheic Lake Fitri, the estimated output flux corresponds to the infiltration of surface water toward the surface aquifer that regulates the chemistry of the lake. These results constitute a first-order assessment of the water budget of these lakes, in regions where direct hydrological and meteorological observations are very scarce or altogether lacking. Finally, we discuss the implications of our data on the hydro-climatic budget at the scale of the catchment basins. We observe that the local evaporation lines (LELs) obtained on both lake and aquifer systems are slightly offset from the average rainfall isotopic composition monitored by IAEA at N'Djamena (Chad), and we show that this difference may reflect the impact of vegetation transpiration on the basin water budget. Based on the discussion of the mass balance budget we conclude that, while being broadly consistent with the idea that transpiration is on the same order of magnitude as evaporation in those basins, we cannot derive a more precise estimate of the partition between these two fluxes, owing to the large uncertainties of the different end-members in the budget equations.