Triple oxygen isotopes are powerful tracers of hydrological processes, yet their variability in atmospheric water vapor and the underlying controls remain poorly understood. We present a one-year record of triple oxygen and hydrogen isotopes of atmospheric water vapor (delta 18OV, d-excessV, 17O-excessV) measured below, within and above a downy oak forest canopy at the AnaEE platform O3HP in the French Mediterranean. This vapor dataset is complemented by isotopic data of precipitation (delta 18OP, d-excessP, 17O-excessP) and groundwater, as well as monthly observations of stomatal conductance and transpiration. Seasonal variations in 17O-excessV and d-excessV likely reflect changing evaporative conditions at oceanic moisture sources. d-excessP and 17O-excessP show a similar seasonal pattern, enhanced by summer rain re-evaporation. However, no clear isotopic differences were observed in vapor or precipitation derived from different oceanic source regions and weather regimes, likely due to frequent mixing of multiple moisture sources. Diurnal variations in 17O-excessV and d-excessV reflect a combination of vegetation-related processes, including local evapotranspiration. However, the impact of evapotranspiration was not evident at daily timescales. Although precipitation often deviates from isotopic equilibrium with near-surface atmospheric water vapor at the event scale due to incomplete equilibration and rain re-evaporation, equilibrium water vapor reliably approximates the near-surface isotopic composition of atmospheric water vapor at annual scale. Our results highlight the potential of 17O-excess for understanding water exchange between the land and the atmosphere, regardless of climatic and vegetation conditions. They enhance the mechanistic interpretation of precipitation isotopes, which is essential for reliable paleoclimate reconstructions.

During the early and mid-Holocene, the Sahara and Sahel experienced a humid phase, the so-called African Humid Period (AHP)1. The AHP started around 14.8 thousand years before present (kyr BP), peaked between 9.0 kyr BP and 6.0 kyr BP and experienced short-lived droughts of as yet poorly constrained age and duration2,3. Here we show that the AHP was punctuated by two droughts of decadal-scale duration, at about 9.3 kyr BP and 8.2 kyr BP, and another more tentatively identified drought at 6.3 kyr BP. Our findings arise from a multiproxy time series from the annually layered (varved) sedimentary archive of Lake Yoa in Chad, which covers the past 10.25 kyr continuously. During the more prominent drought at 8.2 kyr BP, pollen and diatom data, along with leaf-wax isotopes and geochemical source area indicators, imply that a reduction in local precipitation and fluvial supply to Lake Yoa caused a lake-level drop accompanied by an expansion of reed belts along the shore. The proxy data, together with our climate simulations, suggest that the 8.2 kyr BP drought event was a direct and rapid response to a potential weakening of the Atlantic Meridional Overturning Circulation (AMOC) owing to sudden freshwater input into the North Atlantic. The results underline the need for improved decadal predictions4 to better anticipate such drought risks in the future.

Diatoms are microscopic yellow-brown algae widely distributed in aquatic environments, where they play a key role in trophic networks and primary productivity. This study aims to better characterize the current diatom assemblages of Lake Iro, located in southern Chad, a region previously unexplored from a limnological perspective. A taxonomic inventory was conducted based on seven samples collected from different substrates (sediments, plankton, macrophytes), complemented by in situ measurements of hydrochemical parameters (temperature, pH, conductivity, and dissolved oxygen). The results reveal low species diversity, dominated by Aulacoseira granulata, a typical freshwater species that thrives in alkaline, well-mixed waters. The highest diversity indices were observed in samples collected from macrophytes. As the first study of Lake Iro, this work highlights the need for additional, especially seasonal, investigations to better understand the ecological dynamics of this Sahelian lake system.

The Sahara and the Sahel are currently experiencing an increase in rainfall during the rainy season, possibly related to the strengthening of the hydrological cycle induced by global warming. However, the slowdown of the Atlantic Meridional Overturning Circulation (AMOC) might be able to counteract this trend, decreasing the North African monsoon rainfall in a timely, but highly uncertain, manner. During the early and mid-Holocene, the Sahara and the Sahel experienced a wet phase that offers a good analogue for testing the impact of an AMOC perturbation on the climate of this area. Here, we document this wet phase based on the sediment record of Lake Yoa from the central Sahara spanning the last 10,800 years at annual to decadal resolution. This record shows abrupt droughts at 8.2 kyr BP and at 6.3 kyr BP, which interrupted the humid period for a few decades. By modelling precipitation changes in the Lake Yoa area over the Holocene, we support the idea that these droughts were triggered by a slowdown of the AMOC. These results suggest that ongoing AMOC changes could severely weaken the African monsoon on decadal time scales. This process will reduce or even counteract the observed precipitation increase in these areas, highlighting the crucial need for reducing uncertainty in the rapidity of future AMOC weakening.

The climate of the African Holocene Humid Period (AHHP) is reconstructed in the Tibesti Volcanic Massif (TVM) in the central Sahara from well-preserved diatomaceous deposits in the two crater palaeolakes of Trou au Natron at Pic Toussidé and Era Kohor at Emi Koussi. The two records cover the period from ∼9500 to 4500 cal yr BP. Climate and palaeoenvironmental changes during this period were inferred from diatom assemblages, interpretation of variations in their oxygen isotope composition (δ18Odiatom), reconstruction of lake water conductivity from diatom-based transfer functions, and estimation of the lake water balance (Evaporation/Inflow ratio, E/I). Our findings provide evidence for two distinct lacustrine episodes. During the early to mid-Holocene transition, low δ18Odiatom values, high percentages of planktonic diatoms, low lake water conductivity and a positive water balance (E/I < 1) suggest wet conditions, which were likely related to the optimum of the AHHP. From the mid-to late Holocene transition, an aridification trend is revealed by increasing δ18Odiatom values, high percentages of benthic diatoms and a negative water budget (E/I > 1), occurring as early as 6500 cal yr BP and intensifying after 5300 cal yr BP. Moreover, our data show on average a decrease in precipitation amounts of ∼35% between the peak and the end of the AHHP in the Tibesti region. This timing of the AHHP in the mountainous Tibesti is consistent with the aridification of the central Sahara recorded at lowland sites, which has mainly been related to the southward retreat of the Intertropical Convergence Zone (ITCZ) and the associated African monsoonal rainfall belt, following the gradually declining summer insolation that led to the termination of the AHHP. Our results prove the existence of Holocene lakes in the TVM craters that developed contemporaneously with the lakes of the Chadian basin and the Libyan Sahara. On a broader scale, our data share similar hydroclimatic patterns with studies from the eastern and northern Sahara.

For decades, diatoms have been recognized as powerful bio-indicators of modern water quality. They have also been utilized in the design of transfer functions, which can be applied to diatom assemblages in lake sediment cores to infer aspects of past lake hydrochemistry and estimate variables that can be incorporated into paleohydrology models. The Ounianga lakes, in the heart of the Chadian Sahara, possess unique and well-preserved sediment records that extend back beyond the middle Holocene. Today, the lakes display a range of hydrochemical conditions, from fresh to hypersaline. Mainly fed by fossil groundwater that originates in the Nubian Sandstone Aquifer System, measured conductivity across these lakes varies from 217 to 352,000 µS cm−1, values that are influenced by factors such as hydrology, local geomorphology (e.g., depth and area), and aquatic vegetation. Although these lakes have been on the UNESCO World Heritage List since 2012, they have never been studied in detail because they are located on the fringes of the Chadian Sahara. The distribution of diatom taxa in the lakes today is closely linked to water-column physical and chemical conditions, especially conductivity. Whereas each lake has particular features that influence its diatom flora, diatoms across a conductivity gradient enabled identification of three distinct waterbody types, freshwater lakes, meso-saline to hyper-saline lakes, and freshwater springs. Relationships between diatom species distributions and environmental variables were examined using multivariate analysis, which revealed that conductivity is the variable that explains most of the variance in the diatom flora. We used modern diatom assemblages from the lakes to develop a predictive model (transfer function) for conductivity, using the weighted averaging method. Our conductivity prediction model is strong, with a coefficient of determination (R2) of 0.89 between estimated and measured values, and a value of 0.78 using jackknife estimates of prediction. This study better constrained conductivity optima and tolerance values for diatom species found in the Ounianga lakes, thereby enabling development of a model that will yield better inferences for past conductivity, using diatoms from lake sediment records in the region.

Epiphytism on benthic motile diatoms is not common and can be considered a phoretic habitat. Nitzschia sigmoidea is a very long diatom which often exhibits epiphytes on its frustule. It was found abundantly in a small regulated stream in Provence and about half of the individuals had a variable number of adnate epiphytes on them. Examination of fresh material under a scanning electron microscope revealed that the epiphytes were diatoms belonging to the species Fallacia helensis. This species had not been reported as epiphytes on other diatoms before. More research is needed to understand the relationship between the two species and, more generally, to investigate phoretic epiphytism as an original biotic interaction between diatoms.

The termination of the African Humid Period in northern Africa has been described as abrupt, occurring within centuries, as well as gradual, in response to incremental decreases in summer insolation. This study examined the rapidity of the change in diatom assemblages over the period from 6.5 to 4.5 cal ka BP, in a core studied previously at a coarser resolution. This transition was characterized by high variability of assemblages, which could be related, in part, to changes in water conductivity, and potentially enhanced by a site-specific hydrological threshold or ecological salinity threshold. We hypothesize that the variations in diatom assemblages reflect climate fluctuations, which may have been an early warning signal of an impending climate regime shift.

The response of large water-bodies to global change in terms of ecosystem services and economical value is a major concern.The Caspian Sea, the world's largest enclosed water-body, has a poorly-known water-level history, but observed changes are a hundred times faster than recent global sea-level rise.This ancient lake, characterised by brackish waters, is rich in endemic species; some of them have spread to similar environments worldwide.However, the ecology of Pontocaspian species remains poorly understood and must be studied in their original habitat.This work aims at improving the capacity to reconstruct Quaternary environments of the Pontocaspian region and to provide a benchmark for biodiversity turnover studies.A transect of surface sediment across a wide shelf was subjected to multidisciplinary analyses: stable isotopes, pollen, dinocysts, diatoms, foraminifers, ostracods and molluscs and vertical oceanographic profiles.Three depositional environments with characteristic communities were found: shore face, shelf and slope.Invasion impact was strongly felt by the molluscs.All biota groups, except diatoms, reflected high endemism.The radiocarbon reservoir effect is highlighted in differential 14 C ages for different groups.Understanding such discrepancies require detailed insight into reworking processes.Tephra presence in the sediment shows a potential for tephrochronology.Stable isotope ratios in ostracods appear to reflect temperature depth gradients.Our results provide a baseline for calibrating proxy data to the present Pontocaspian environment.

Continental atmospheric relative humidity (RH) is a key climate parameter. Combined with atmospheric temperature, it allows us to estimate the concentration of atmospheric water vapor, which is one of the main components of the global water cycle and the most important gas contributing to the natural greenhouse effect. However, there is a lack of proxies suitable for reconstructing, in a quantitative way, past changes of continental atmospheric humidity. This reduces the possibility of making model–data comparisons necessary for the implementation of climate models. Over the past 10 years, analytical developments have enabled a few laboratories to reach sufficient precision for measuring the triple oxygen isotopes, expressed by the 17O-excess (17O-excess = ln (δ17O + 1) – 0.528 × ln (δ18O + 1)), in water, water vapor and minerals. The 17O-excess represents an alternative to deuterium-excess for investigating relative humidity conditions that prevail during water evaporation. Phytoliths are micrometric amorphous silica particles that form continuously in living plants. Phytolith morphological assemblages from soils and sediments are commonly used as past vegetation and hydrous stress indicators. In the present study, we examine whether changes in atmospheric RH imprint the 17O-excess of phytoliths in a measurable way and whether this imprint offers a potential for reconstructing past RH. For that purpose, we first monitored the 17O-excess evolution of soil water, grass leaf water and grass phytoliths in response to changes in RH (from 40 to 100 %) in a growth chamber experiment where transpiration reached a steady state. Decreasing RH from 80 to 40 % decreases the 17O-excess of phytoliths by 4.1 per meg/% as a result of kinetic fractionation of the leaf water subject to evaporation. In order to model with accuracy the triple oxygen isotope fractionation in play in plant water and in phytoliths we recommend direct and continuous measurements of the triple isotope composition of water vapor. Then, we measured the 17O-excess of 57 phytolith assemblages collected from top soils along a RH and vegetation transect in inter-tropical West and Central Africa. Although scattered, the 17O-excess of phytoliths decreases with RH by 3.4 per meg/%. The similarity of the trends observed in the growth chamber and nature supports that RH is an important control of 17O-excess of phytoliths in the natural environment. However, other parameters such as changes in the triple isotope composition of the soil water or phytolith origin in the plant may come into play. Assessment of these parameters through additional growth chambers experiments and field campaigns will bring us closer to an accurate proxy of changes in relative humidity.

The species Thalassiosira faurii (Gasse) Hasle was found dominating the planktonic diatom community during summer 2012 in Lake Langano which is part of the Ziway-Shala alkaline lake system in Ethiopia. In a previous study, the species had been recorded in high abundance in 13.5-12 cal ka old sediments from the neighbor Lake Abiyata. Using a transfer function for conductivity, peaks in abundance had been interpreted as dry spells at the Pleistocene-Holocene transition since the species was considered strictly halophilous.In this study, a strain of Thalassiosira faurii was isolated from Lake Langano and grown in the laboratory. The observation of its morphology and reproduction, and comparison between modem and fossil populations helped to improve the taxonomic definition of the species. The autecology of the species was also investigated through a salinity tolerance test. The results revealed that the species was much more sensitive to salinity than previously believed. Anew conductivity optimum was derived from the experiment and incorporated into the transfer function. The reassessment of lake conductivity variations along the core of Lake Abiyata showed that the intensity of the dry periods preceding the onset of the Holocene had been probably over-estimated.The reconsideration of the species relationship to conductivity may have important consequences for paleoclimatic reconstructions in tropical Africa where Thalassiosira faurii has been reported in abundance. The use of experimentation on local diatom strains allows paleoecologists to solve the problems of low representation of some species in modem dataset and of regional autecological variability within morphotypes. (C) 2014 Elsevier B.V. All rights reserved.