Jean-Charles Fontes

A Late Pleistocene-Holocene lacustrine record from Lake Manas, Zunggar (northern Xinjiang, western China)

Information on the Late Quaternary palaeoclimate in the Zunggar desert (northern Xinjiang) is deduced from the study of three 5-m long cores taken from Lake Manas. Analyses, including the mineralogical composition of bulk sediments, stable isotope contents of carbonates, organic matter, pollen and diatoms, were performed to help reconstruct the environmental evolution of the area. The chronology is provided by 9 AMS 14C dates done on carbonates or organic matter. The record suggests a Late Pleistocene humid episode radiocarbon-dated at 37,000–32,000 yr B.P. This episode is followed by a period of extreme aridity, attributed to the Last Glacial Maximum. The Early-Mid Holocene record indicates conditions generally wetter than today, as previously established for western China. However, our results show that the Holocene period is rather complex. After a minor wet/warm pulse estimated to be around 12,000 yr B.P., the most important environmental change is recorded at ≈ 10,000 yr B.P. It led to the establishment of a steppic vegetation cover in the catchment, and of a permanent lake which lasted ≈ 4000 years. Fluvial sedimentation took place between about 6000 and 4500 yr B.P., and it coincided with a decrease in moisture availability for vegetation growth. A second lacustrine episode is recorded from about 4500 to 2500 yr B.P., although the pollen record and a sudden influx in detrital organic matter suggest a short-term dry event around 3800-3500 yr B.P. The last 2500 years show climate unstability and the step-wise establishment of hypersaline conditions in Lake Manas. The major post glacial environmental changes appear to be roughly in phase with those observed in Tibet.

Stable isotope disequilibria in travertine from high pH waters: Laboratory investigations and field observations from Oman

Abstract Ultramafic rocks in northern Oman host unusual, hyperalkaline groundwaters (pH > 11.5) which precipitate inorganic travertine by diffusive uptake of atmospheric CO2 after discharge. Stable isotope fractionation during CO2 uptake results in strong depletions of the carbonate phase (δ13C values as low as −27.5‰ and δ18O values of −16.9‰ PDB). Experiments under closed and open system conditions show that a kinetic depletion in the order of 15.5‰ for 13C occurs during hydroxylation of aqueous CO2 (CO2(aq) + OH− reaction), attributed to a lower activation energy for 12C-O vs 13C-O. The observed 18O depletion can be accounted for by CO2 reaction with OH− ( ϵ 18 O OH − -H 2 O ≅ −40‰ ) without subsequent exchange and with no evidence of significant additional kinetic effects. Calculated reaction rates show hydroxylation to be the rate limiting step and that CO2(g)-CO2(aq) exchange at the gas/solution interface achieves isotopic equilibrium.

The evolution of alkaline groundwaters in the continental intercalaire aquifer of the Irhazer Plain, Niger

This paper discusses the hydrochemical evolution of groundwaters in the multilayer artesian aquifer of the Continental Intercalaire (the mainly Jurassic Agadez-Dabla sandstones) aquifer of northern Niger. Isotopic and chemical changes in relation to the geological setting suggest that the increasing HCO3− alkalinity of the groundwaters is caused by interaction of deep CO2 with the aquifer matrix. Alteration of feldspars by CO2 with a δ13C value of −3.0‰ accounts for the observed alkalinity and isotopic and trends. Carbon 14 ages have been estimated for recent and Holocene groundwaters which have a distinct stable isotopic (2H and 18O) signature compared with 14C dead groundwaters from further west on the Irhazer Plain. Recharge temperatures, estimated by analysis of noble gas contents, show that the 14C dead waters were recharged under conditions cooler than present averages in the region, possibly at the beginning of deglaciation (≈16 ka B.P.) or during the period 23–30 ka B.P. Trends in the hydrochemistry of U are related to the deep CO2 alteration process. Radiogenic He contents increase along the flow direction and extremely high values occur in the proximity of U ores. Denitrification in the paleowaters of the Irhazer was assessed by measurements of nitrogen/argon ratios.

PALEORECHARGE BY THE NIGER RIVER (MALI) DEDUCED FROM GROUNDWATER GEOCHEMISTRY

Stable isotopes, major elements, trace elements, and noble gases in groundwater from northern Mali permit a reconstruction of the paleohydrology of the Sahelian and sub-Saharan regions of west Africa. Major floods of the Niger River during the more humid episodes of the Holocene, probably associated with northward migration of the interior river delta (flood zone), were responsible for groundwater recharge. A marine aerosol input, somewhat modified by rock-water interaction, dominates the groundwater chemistry. Stable isotope compositions distinguish the Saharan zone groundwater from waters recharged in the Sahelian zone south of the Azaouad ridge. Carbon 14 dating shows that whereas the latter are modem, the Saharan zone was recharged in the Early to Middle Holocene. Noble gas contents suggest that recharge north of Azaouad occurred under cooler climatic conditions than presently prevail.

Variations hydrologiques et extension des lacs holocenes du Desert Danakil

Abstract During the Holocene period, lake basins existed in some parts of the Danakil Desert. These basins were supplied either by the Awash River or by ground waters flowing through highly fissured basaltic flows. It is possible to reconstruct the areas and fluctuations of lake levels because of the preservation of old shore lines. Calcareous and diatomaceous bottom deposits give information about lacustrine paleo-environments (isotopic analyses, diatoms). It is suggested that the Holocene period started with an extensive lacustrine episode between 11,000 and 7,000 years B.P. At least one other transgressive stage, of lesser importance, took place between about 2,000 and 1,000 years B.P. The lakes mainly supplied with ground waters (Afrera) exhibit evidence of chemical and biochemical sedimentation. The depostion continued after 7,000 years B.P., and indicates a buffering effect owing to the aquifers. Hence, these basins do not have the simple behaviour of a rain gauge recording the local climate fluctuations of the Danakil Desert. The Dobi Graben can be partly related to this kind of basin. Because of the strong turbidity of the flood waters, the lakes with both shallow and underground supplies are quickly filled up in the head of the alluvial plain of the Awash River. The lakes at the downward part of the plain (Gamari and terminal Lake Abhe) have been preserved from the solid supply during the two humid episodes. During the intermediate stage, the lacustrine regression allowed the transport of silts and muds as far as Lake Abhe. The Awash River has not ceased flowing and the progression of the alluvial deposits isolates some area of retention on the margins of the plain (lacustrine relicts at about 4,000 and 5,000 years B.P.). After subtracting the local hydrological influences, by means of comparison with the recent phenomena, this succession fits into the general pattern of the Holocene climatic events of the southern margin of the Sahara.

Exemple d'etude geochimique et isotopique de circulations aquiferes en terrain volcanique sous climat semi-aride (Amurga, Gran Canaria, Iles Canaries)

Abstract A hydrochemical and isotopic study of a groundwater flow system in volcanic media has been carried out in the southern part of Gran Canaria (Canary Islands). Monthly samples of precipitation were collected over a two year period at stations distributed all over the island. Sampling of groundwaters took place mainly from drillholes in the Amurga phonolitic massif but also from wells and springs discharging from the volcanic and related detrital formations. The δ 2 H vs. δ 18 O relationship of the precipitation is close to the “global meteoric water line”. Two distinct altitudinal isotopic gradients are observed. On the warm and dry southern slope, the combination of a contribution of saharan dusts and seasprays and of partial evaporation explains the high content of dissolved salts (Cl − , SO 2− 4 ) in precipitation. Additional processes may be invoked as sources of groundwater mineralization: leaching of silicate minerals during infiltration and/or from hydrolysis during the long residence time of the waters in the volcanic rocks. Salt content is increased through a concentration of the percolating solutions by evaporation, as shown by their stable isotope contents ( 18 O, 2 H). The groundwaters in the phonolitic aquifer are well differentiated from the deeper groundwaters, which are recharged at an altitude of about 1000m above sea level, or even less, if exceptional episodes of rain have a lower heavy isotope content. The interpretation of the 13 C contents of soil CO 2 , sampled throughout the area, and of the 13 C and 14 C contents of total dissolved inorganic carbon (TDIC) leads to the concept of an incorporation of pure biogenic carbon under open-system conditions in the Amurga waters. For these groundwaters, high estimates of residence time (ca. 11000 years) can be calculated from the low 14 C activities. In the neighbouring valleys, the input of deep “dead” volcanic carbon and the occurrence of mixing do not allow an exact quantification of residence time values. However, they seem to be shorter than those measured for the Amurga waters.

Origin of the oil-field brines in the Paris basin

From Br{sup {minus}}/Cl{sup {minus}} ratios coupled with stable-isotope compositions, the components of deep sedimentary brines - namely, primary and secondary brines (from dissolving evaporites) and diluting waters of meteoric origin - can be identified, and in favorable cases, their respective proportions can be quantified. Dilution of primary marine brines (mother liquors of halite) by meteoric waters has occurred in the Dogger and the Keuper aquifers. The presence of highly saline brines (molality of Cl{sup {minus}} = 3.9) in the Rhaetian aquifer may be due to dissolution of primary marine halite by sea water with minor contributions of primary brines. The resulting mixture is diluted by meteoric circulation.

Accelerator mass spectrometry in hydrology

Abstract Applications of 14C, 36C1 and 129I in hydrology using the AMS technique are reviewed. The small amounts of carbon needed for AMS measurement of 14C have facilitated new areas of hydrological research. Uncertainties in the natural fall-out of 36Cl and the dissolution of 36Cl-labelled chloride during groundwater migration are limitations for groundwater dating with this nuclide. Estimation of recharge by measurements of the advance of the 6Cl bomb peak through the unsaturated zone is of increasing importance in hydrology. AMS measurements of 129 I will provide an environmental monitor for nuclear pollution plumes. Some directions for future studies are suggested.