Search

Oxygen isotopes in biogenic silica (δ18OBSi) from lake sediments allow for quantitative reconstruction of past hydroclimate and proxy-model comparison in terrestrial environments. The signals of individual records have been attributed to different factors, such as air temperature (Tair), atmospheric circulation patterns, hydrological changes, and lake evaporation. While every lake has its own local set of drivers of δ18O variability, here we explore the extent to which regional or even global signals emerge from a series of paleoenvironmental records. This study provides a comprehensive compilation and combined statistical evaluation of the existing lake sediment δ18OBSi records, largely missing in other summary publications (i.e. PAGES network). For this purpose, we have identified and compiled 71 down-core records published to date and complemented these datasets with additional lake basin parameters (e.g. lake water residence time and catchment size) to best characterize the signal properties. Records feature widely different temporal coverage and resolution, ranging from decadal-scale records covering the past 150 years to records with multi-millennial-scale resolution spanning glacial–interglacial cycles. The best coverage in number of records (N = 37) and data points (N = 2112) is available for Northern Hemispheric (NH) extratropical regions throughout the Holocene (roughly corresponding to Marine Isotope Stage 1; MIS 1). To address the different variabilities and temporal offsets, records were brought to a common temporal resolution by binning and subsequently filtered for hydrologically open lakes with lake water residence times < 100 years. For mid- to high-latitude (> 45° N) lakes, we find common δ18OBSi patterns among the lake records during both the Holocene and Common Era (CE). These include maxima and minima corresponding to known climate episodes, such as the Holocene Thermal Maximum (HTM), Neoglacial Cooling, Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). These patterns are in line with long-term air temperature changes supported by previously published climate reconstructions from other archives, as well as Holocene summer insolation changes. In conclusion, oxygen isotope records from NH extratropical lake sediments feature a common climate signal at centennial (for CE) and millennial (for Holocene) timescales despite stemming from different lakes in different geographic locations and hence constitute a valuable proxy for past climate reconstructions.

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.

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.

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.

Located at the transition between the Saharan and Sahelian zones, at the center of one of the largest endorheic basins, Lake Chad is ideally located to record regional environmental changes that occurred in the past. However, until now, no Holocene archive was directly cored in this lake. In this paper, we present pollen data from the first sedimentary sequence collected in Lake Chad (13° N; 14° E; Sahel region). Dated between ca. 6700 and ca. 5000 cal yr BP, this record is continuous and encompasses part of the termination of the African Humid Period (AHP). Vegetation reconstructions are based on standard analyses of pollen diagrams and are strengthened by quantitative approaches. Potential biomes are reconstructed using the biomization method and mean annual precipitation (Pann) is estimated using the modern analogues technique. Results show that, between ca. 6700 and ca. 6050 cal yr BP, a vegetation close to humid woodland or humid savanna, including elements currently found further southward, thrived in the vicinity of the Mega-Lake Chad in place of the modern dry woodland, steppe and desert vegetation. At the same time, montane forest populations extended further southward on the Adamawa Plateau. The high abundance of lowland humid pollen taxa, particularly of Uapaca, is interpreted as the result of a northward migration of the corresponding plants during the AHP. This preferential zonal occurrence of these taxa in Lake Chad Basin (LCB) (rather than extrazonal) is driven by more humid local and regional climate conditions at this latitude, as shown by mean Pann estimated values of ca. 800 (−400/+700) mm during this period. However, we cannot rule out that an increase of the Chari–Logone inputs into the Mega-Lake Chad might have also contributed to control the abundance of these taxa. Changes in the structure and floristic composition of the vegetation towards more open and drier formations occurred after ca. 6050 cal yr BP, following a decrease in mean Pann estimates to approximately 600 (−230/+600) mm. But, the constant significant presence of lowland humid taxa until ca. 5000 cal yr BP, contemporaneous with a slight increase in steppic taxa, demonstrates that at this date, the modern vegetation was not yet established in the vicinity of Lake Chad. Our data indicate that between ca. 6700 and ca. 5000 cal yr BP vegetation and climate changes must have occurred progressively, but that century-scale climate variability was superimposed on this long-term mid-Holocene drying trend as observed around ca. 6300 cal yr BP, where pollen data indicate more humid conditions.

Phytoliths are micrometric particles of amorphous silica that form inside or between the cells of higher plant tissues throughout the life of a plant. With plant decay, phytoliths are either incorporated into soils or exported to sediments via regional watersheds. Phytolith morphological assemblages are increasingly used as proxy of grassland diversity and tree cover density in inter-tropical areas. Here, we investigate whether, along altitudinal gradients in northeast Queensland (Australia), changes in the δ18O signature of soil top phytolith assemblages reflect changes in mean annual temperature (MAT) and in the oxygen isotopic composition of precipitation (δ18Oprecipitation), as predicted by equilibrium temperature coefficients previously published for silica. Oxygen isotopic analyses were performed on 16 phytolith samples, after controlled isotopic exchange (CIE), using the IR Laser-Heating Fluorination Technique. Long-term mean annual precipitation (MAP) and MAT values at the sampled sites were calculated by the ANUCLIM software. δ18Oprecipitation estimates were calculated using the Bowen and Wilkinson (2002) model, slightly modified. An empirical temperature-dependant relationship was obtained: δ18Owood phytolith-precipitation (‰ vs. VSMOW) = −0.4 (±0.2) t (°C) + 46 (±3) (R2 = 0.4, p < 0.05; n = 12). Despite the various unknowns introduced when estimating δ18Oprecipitation values and the large uncertainties on δ18Owood phytolith values, the temperature coefficient (−0.4 ± 0.2‰ °C−1) is in the range of values previously obtained for natural quartz, fresh and sedimentary diatoms and harvested grass phytoliths (from −0.2 to −0.5‰ °C−1). The consistency supports the reliability of δ18Owood phytolith signatures for recording relative changes in mean annual δ18Osoil water values (which are assumed to be equivalent to the weighted annual δ18O precipitation values in rainforests environments) and MAT, provided these changes were several ‰ and/or several °C in magnitude.

No abstract available.