The Ponto-Caspian basin as a final trap for southeastern Scandinavian Ice-Sheet meltwater

This paper provides new data on the evolution of the Caspian Sea and Black Sea from the Last Glacial Maximum until ca. 12 cal kyr BP. We present new analyses (clay mineralogy, grain-size, Nd isotopes and pollen) applied to sediments from the river terraces in the lower Volga, from the middle Caspian Sea and from the western part of the Black Sea. The results show that during the last deglaciation, the Ponto-Caspian basin collected meltwater and fine-grained sediment from the southern margin of the Scandinavian Ice Sheet (SIS) via the Dniepr and Volga Rivers. It induced the deposition of characteristic red-brownish/chocolate-coloured illite-rich sediments (Red Layers in the Black Sea and Chocolate Clays in the Caspian Sea) that originated from the Baltic Shield area according to Nd data. This general evolution, common to both seas was nevertheless differentiated over time due to the specificities of their catchment areas and due to the movement of the southern margin of the SIS. Our results indicate that in the eastern part of the East European Plain, the meltwater from the SIS margin supplied the Caspian Sea during the deglaciation until ∼13.8 cal kyr BP, and possibly from the LGM. That led to the Early Khvalynian transgressive stage(s) and Chocolate Clays deposition in the now-emerged northern flat part of the Caspian Sea (river terraces in the modern lower Volga) and in its middle basin. In the western part of the East European Plain, our results confirm the release of meltwater from the SIS margin into the Black Sea that occurred between 17.2 and 15.7 cal kyr BP, as previously proposed. Indeed, recent findings concerning the evolution of the southern margin of the SIS and the Black Sea, show that during the last deglaciation, occurred a westward release of meltwater into the North Atlantic (between ca. 20 and 16.7 cal kyr BP), and a southward one into the Black Sea (between 17.2 and 15.7 cal kyr BP). After the Red Layers/Chocolate Clays deposition in both seas and until 12 cal kyr BP, smectite became the dominant clay mineral. The East European Plain is clearly identified as the source for smectite in the Caspian Sea sediments. In the Black Sea, smectite originated either from the East European Plain or from the Danube River catchment. Previous studies consider smectite as being only of Anatolian origin. However, our results highlight both, the European source for smectite and the impact of this source on the depositional environment of the Black Sea during considered period

Isotopic fractionation mechanism: a key to interpretating isotopic compositon of calcite at aquifer outlet as hydrosystem dynamic

International audienceThe isotopic composition of the calcite that precipitates at the level of springs outlets records the hydrodynamic variations of the hydrosystems. However, to be able to decipher this recording, it is necessary to understand the kinetic isotopic fractionations. A mathematical approach has been developed to reproduce the chemical and isotopic partition during calcite precipitation and CO2 degassing. The approach correctly reproduces the evolutions observed in situ. The following step remains the integration of environmental parameters to field constraints on the model (flow, water level…)

ISOSCAPE FOR GROUNDWATER RECHARGE ASSESSMENT OF THE CONTINENTAL TERMINAL SHALLOW AQUIFER OF THE TOGO COASTAL SEDIMENTARY BASIN

International audienceIn the Coastal Sedimentary Basin (CSB) of Togo, the shallow aquifer of the Continental Terminal (CT) is the most stressed water resource despite the continuous degradation of its quality. To be able to discuss the sources of groundwater degradation in this aquifer, knowledge of the origins of the waters that contributes to its renewal is very important. This study proposes to use the stable isotopes of rainwater, surface water and aquifers to understand the recharge of the CT aquifer. Using a d 18 O-d 2 H diagram, the groundwater representative points are aligned around or below the local meteoric line of slope 7.3. This suggests that this aquifer is recharged by precipitation from oceanic origin and evaporated waters. The isoscape map of the CT waters revealed an enrichment of the isotopic signature from south to the north. The enriched signatures of the waters of the northern sector of the CT and those depleted in the southern sector are aligned on straight lines with the same slope of 4.9, thus revealing the same conditions of groundwater recharge with contribution of surface water

Chronology of the Late Pleistocene Caspian Sea hydrologic changes: A review of dates and proposed climate-induced driving mechanisms

International audienceThis paper provides a review of the dates used to define major hydrologic changes in the Caspian Sea during the Late Pleistocene that includes the Early Khvalynian and Hyrcanian highstands and the Atelian lowstand, and it discusses their driving mechanisms. Dates obtained by 14 C (that are the most numerous), by OSL and by U/Th for samples that are well identified in the literature through their geographic location and their laboratory number, are considered. Samples derive from sediments both in the current Caspian Sea coast and in its Middle and Northern basins. Most of dates concern the Early Khvalynian highstand while less abundant ones correspond to the Hyrcanian highstand and Atelian lowstand that preceded the Early Khvalynian. The review of these Late Pleistocene dates provides an overall revision of the hydrologic changes that the Caspian Sea experienced. Accordingly, the Early Khvalynian highstand presents two major phases. The first Early Khvalynian phase (hv1-1) took place between 36.0 and 22.0 cal kyr, and water level reached ca 0 m a.s.l. Its second phase (hv1-2) is characterised by the deposition of the so-called Chocolate Clays especially in the Northern Peri-Caspian and took place between ca 22.0 and 12.5 cal kyr BP with a water level rise until at least 40 m a.s.l. between ca 17 and 13 cal kyr BP. Before the Early Khvalynian highstand, the Caspian water level underwent a lowstand between at least 70.0 and 36.0 kyr, i.e. the Atelian lowstand, which was interrupted by the Hyrcanian highstand. During the late stage of the Atelian lowstand, between 43.5 and 42.9 and 36.0 cal kyr BP, the water level decreased between more than À60 m a.s.l. and less than-90 m a.s.l. at 42.0 cal kyr BP. The importance of the water level drop in the early stage of the Atelian regression between ca 70 and 55.2 kyr could not be estimated but according to seismic profiles, it was much less pronounced than later. The dates for the Hyrcanian highstand range from 55.2 (or even before) to 43.5 e42.9 cal kyr BP and the water level reached at least 0 m a.s.l. in its late phase. Through this synthesis, the origins of water inflows into the Caspian Sea could be suggested for the upper part of the Early Khvalynian transgression: it matches with a very large inflow of meltwater from the southern front of the Scandinavian Ice Sheet; the fluvio-glacial waters from the ice cap discharging to the Caspian Sea via the Volga River. Concerning the Hyrcanian transgression, meltwater inputs from the glaciers of western High Asia can be proposed, the fluvio-glacial waters being transported into the Caspian Sea by the Syr Darya and Amu Darya (via the Aral Sea or bypassing it)

Impacts of changes in groundwater recharge on the isotopic composition and geochemistry of seasonally ice-covered lakes: insights for sustainable management

International audienceLakes are under increasing pressure due to widespread anthropogenic impacts related to rapid development and population growth. Accordingly, many lakes are currently undergoing a systematic decline in water quality. Recent studies have highlighted that global warming and the subsequent changes in water use may further exacerbate eutrophication in lakes. Lake evolution depends strongly on hydrologic balance, and therefore on groundwater connectivity. Groundwater also influences the sensitivity of lacustrine ecosystems to climate and environmental changes, and governs their resilience. Improved characterization of groundwater exchange with lakes is needed today for lake preservation, lake restoration, and sustainable management of lake water quality into the future. In this context, the aim of the present paper is to determine if the future evolution of the climate, the population, and the recharge could modify the geochemistry of lakes (mainly isotopic signature and quality via phosphorous load) and if the isotopic monitoring of lakes could be an efficient tool to highlight the variability of the water budget and quality. 'Small groundwater-connected lakes were chosen to simulate changes in water balance and water quality expected under future climate change scenarios, namely representative concentration pathways (RCPs) 4.5 and 8.5. Contemporary baseline conditions, including isotope mass balance and geochemical characteristics, were determined through an intensive field-based research program prior to the simulations. Results highlight that future lake geochemistry and isotopic composition trends will depend on four main parameters: lo-cation (and therefore climate conditions), lake catchment size (which impacts the intensity of the flux change), lake volume (which impacts the range of variation), and lake G index (i.e., the percentage of groundwater that makes up total lake inflows), the latter being the dominant control on water balance conditions, as revealed by the sensitivity of lake isotopic composition. Based on these model simulations, stable isotopes appear to be especially useful for detecting changes in recharge to lakes with a G index of between 50 and 80 %, but response is non-linear. Simulated monthly trends reveal that evolution of annual lake isotopic composition can be dampened by opposing monthly recharge fluctuations. It is also shown that changes in water quality in groundwater-connected lakes depend significantly on lake location and on the intensity of recharge change

Chronology of the Late Pleistocene Caspian Sea hydrologic changes: A review of dates and proposed climate-induced driving mechanisms

International audienceThis paper provides a review of the dates used to define major hydrologic changes in the Caspian Sea during the Late Pleistocene that includes the Early Khvalynian and Hyrcanian highstands and the Atelian lowstand, and it discusses their driving mechanisms. Dates obtained by 14C (that are the most numerous), by OSL and by U/Th for samples that are well identified in the literature through their geographic location and their laboratory number, are considered. Samples derive from sediments both in the current Caspian Sea coast and in its Middle and Northern basins. Most of dates concern the Early Khvalynian highstand while less abundant ones correspond to the Hyrcanian highstand and Atelian lowstand that preceded the Early Khvalynian. The review of these Late Pleistocene dates provides an overall revision of the hydrologic changes that the Caspian Sea experienced. Accordingly, the Early Khvalynian highstand presents two major phases. The first Early Khvalynian phase (hv1-1) took place between 36.0 and 22.0 cal kyr, and water level reached ca 0 m a.s.l. Its second phase (hv1-2) is characterised by the deposition of the so-called Chocolate Clays especially in the Northern Peri-Caspian and took place between ca 22.0 and 12.5 cal kyr BP with a water level rise until at least 40 m a.s.l. between ca 17 and 13 cal kyr BP. Before the Early Khvalynian highstand, the Caspian water level underwent a lowstand between at least 70.0 and 36.0 kyr, i.e. the Atelian lowstand, which was interrupted by the Hyrcanian highstand. During the late stage of the Atelian lowstand, between 43.5 and 42.9 and 36.0 cal kyr BP, the water level decreased between more than -60 m a.s.l. and less than - 90 m a.s.l. at 42.0 cal kyr BP. The importance of the water level drop in the early stage of the Atelian regression between ca 70 and 55.2 kyr could not be estimated but according to seismic profiles, it was much less pronounced than later. The dates for the Hyrcanian highstand range from 55.2 (or even before) to 43.5-42.9 cal kyr BP and the water level reached at least 0 m a.s.l. in its late phase. Through this synthesis, the origins of water inflows into the Caspian Sea could be suggested for the upper part of the Early Khvalynian transgression: it matches with a very large inflow of meltwater from the southern front of the Scandinavian Ice Sheet; the fluvio-glacial waters from the ice cap discharging to the Caspian Sea via the Volga River. Concerning the Hyrcanian transgression, meltwater inputs from the glaciers of western High Asia can be proposed, the fluvio-glacial waters being transported into the Caspian Sea by the Syr Darya and Amu Darya (via the Aral Sea or bypassing it)

Le SNO Renoir : réseau français d'observation des isotopes dans les précipitations

International audienceLe SNO RENOIR est un réseau de mesures à long terme des teneurs en isotopes de la molécule d'eau (d18O et d2H) dans les précipitations (pluie, neige) à l'échelle de la France métropolitaine et ultramarine. Il est constitué de 33 stations de prélèvement des eaux de pluie réparties sur tout le territoire métropolitain et ultramarin. Cet observatoire national de la composition isotopique des précipitations a pour but de partager des données essentielles sous forme de chroniques isotopiques au pas de temps mensuel avec toute la communauté scientifique

Using 222Rn to quantify wetlands interflow volume and quality discharging to headwater streams

International audienceHeadwater streams are highly dependent on groundwater discharge to maintain low flows during dry periods and to dilute pollutants. Groundwater discharge to streams can have different flow paths, either from groundwater flowing directly to the river through the hyporheic zone or groundwater that emerges at the contact with a riparian wetland and flows mainly on the wetland surface. Differentiating these flows could be useful to assess the contribution of riparian wetlands in protecting stream water quality. The objective of this research was to expand the use of 222Rn as a groundwater tracer for small streams in headwater catchments to distinguish flows received directly from the aquifer and through riparian wetlands. 222Rn activities, phosphate (PO43−) and nitrate (NO3−) concentrations, along with stream flows were used in a mass balance model to establish the proportions of groundwater flow that discharge to a small stream located southwest of the Paris Basin (France). This watershed is typical of headwater catchments in this region because it receives a wastewater treatment plant (WWTP) effluent at its source and its banks are occupied by many small riparian wetlands. To obtain the best accuracy of groundwater flow assessment, the field work was done during low flow conditions, where the stream flow was only 0.079 m3/s at the outlet. The model gives a good estimation of each flow path with 83 % of the stream baseflow originating from riparian wetlands. The large contrast in 222Rn activity between groundwater inflow from the aquifer (mean of 21 200 Bq/m3) and interflows from wetlands (mean of 2310 Bq/m3) renders the mass balance model sensitive to the separation of these two types of groundwater flow paths. At the head of the stream, water is characterized by high concentrations of PO43− and NO3− due to the WWTP effluent into the stream (13 and 21 mg/L respectively). All groundwater flows are PO43− free and contribute to the improvement of stream water quality. The NO3− cycle is more difficult to constrain because of the spatial heterogeneity in groundwater concentrations. Nevertheless, the results of the modeling approach showed that the main part of the evolution of NO3− concentrations along the river can be explained by the dilution of stream flow with interflows. The method developed is considered sufficiently accurate to quantify groundwater inflows for different flow paths in headwater catchments and to estimate the impact of groundwater flow paths on stream water quality

The evolution of the Caspian Sea over the last 20,000 years: mineralogical data

International audienc

Salt wedges and trapped brines of low-latitude endoreic saline lakes as potential modulators of GHG emission

International audienceAbstract Large salt lakes are long-term witnesses to climatic conditions and land use in their basins. The majority are experiencing a drastic drop in water levels due to climate change and human impact. Endoreic Lake Urmia (NW Iran), the sixth largest salt lake worldwide, is a striking example of this decline. Quantification of the relative contributions of natural variability and human impact on the lake's water supply is therefore essential. Here we present isotopic and radiocarbon analyses of surface and groundwater from the Shahr Chay River catchment, entering Lake Urmia on its western shore, and radiocarbon dating of a sedimentary core. Lake Urmia behaves like a large saltwater wedge almost entirely fed by the river and shallow groundwater. This leads to trapping of residual brines and formation of CH 4 and secondary CO 2 greenhouse gases, impacting sediment geochemical records and corresponding time scales for paleoenvironmental reconstructions. We conclude that (1) salt lakes functioning like a saline wedge, allowing organic matter oxidation, could contribute to increasing methane sources or reducing carbon sinks globally, and (2) endoreic basins worldwide need to be monitored before aridification-related salinization leads to the establishment of a saline wedge precluding any possibility of return to an equilibrium state