Elisabeth Gibert

Holocene environmental changes in Lake Bangong basin (Western Tibet). Part 1: Chronology and stable isotopes of carbonates of a Holocene lacustrine core

Abstract A 12.4 m core collected from the eastern part of Lake Bangong is analyzed for mineralogy, radiometric chronology and stable isotope contents of authigenic, inorganic carbonate. Isotopic compositions of the modern lake waters and of its major tributaries are first presented. Radiocarbon activity of these waters show that equilibrium with the atmospheric CO2 is not reached, because of admixture of dead carbon in solution in the system. A model is thus developed to correct the radiocarbon ages of fossil material of aquatic origin for this ageing. The core provides a continuous Holocene record. Changes in mineralogy of bulk sediments, and in δ13C and δ18O contents of authigenic carbonates reveal extremely large changes in environmental conditions, induced by climate changes and/or local hydrological factors (e.g. closure/opening of the lake system controlled by the altitude of the outflow sill). The eastern basin of Lake Bangong was closed before ≈ 9.6 and after 1.2 ka B.P., in response to aridity on the catchment. Wet/warm conditions, attributed to the monsoon influence, were established suddenly at ≈ 9.6 ka B.P. and maintained until ≈ 6.2 ka B.P., although minor reversal events occurred between ≈ 8.6 and 7.7 ka B.P. The return towards aridity is non linear. Dry spells at ≈ 6.2, ≈ 3.9-3.2, and ≈ 1.3 ka B.P. are alternated with short-term periods with positive precipitation-evaporation balance.

Stable isotope and radiocarbon balances of two Tibetan lakes (Sumxi Co, Longmu Co) from 13,000 BP

Hydrologic and climatic changes in western Tibet since 13,000 BP are inferred from mineralogical and isotopic analyses of lacustrine sediments from the Sumxi-Longmu Co basin. The interpretation is based on isotope data from the modern system. Carbon isotope chemistry is complex because of the presence of old limestones on the watershed. Sediments contain detrital and authigenic calcites, the proportions of which are quantitatively estimated by X-ray diffractometry. Stable isotope contents of the authigenic calcite are reconstructed from mass-balance equations. The high 13C contents (up to +6‰) are attributed to an enhancement of calcite precipitation from water surface during freezing periods. The 18O content is related to residence time and evaporation. The validity of 14C dates obtained from different types of material is discussed. Our record, in good agreement with paleobiological data from the same material, shows the sudden establishment of wet/warm conditions at ≈10,000 BP which culminated at ≈7500-6000 BP after a brief reversal event at ≈7900 BP. Maximum aridity for the whole Holocene is recorded at ≈4300 BP.

The sensitivity of a Tanzanian crater lake to catastrophic tephra input and four millennia of climate change

Diatom genera in many large East African lakes change little throughout the Holocene period suggesting relatively stable ecological conditions and some resilience to environmental change. Ecosystem stability is less common in smaller, more sensitive lakes, such as those within volcanic craters, where external impacts can cause abrupt and rapid fluctuations. A 4100-year diatom and cyanobacteria pigment record from Lake Massoko, a volcanic crater lake in southern Tanzania, is used to illustrate important switches in resource ratios following tephra deposition 1190 years ago. It is hypothesized that the tephra reduced the rate of P diffusion from the sediments and increased the Si:P ratio in the lake. A period of acute change in planktonic diatom communities resulted from the tephra impact and lasted c. 110 years. The magnitude of the change shown by the diatoms and their slow recovery from the tephra may be due in part to a coincident fall in lake level caused by a reduction in regional rainfall. The statistical significance of the tephra impact relative to that of catchment and climate change has been tested using variance partitioning and rate-of-change analysis. Multiproxy indicators show an important period of positive water balance 1700 ago and a relatively dry episode persisting between 1000 and 400 years ago. The lake ecosystem is shown to be highly sensitive to both climate change and tephra deposition.

Hydrochemical and isotopic characterisation of the Bathonian and Bajocian coastal aquifer of the Caen area (northern France)

Abstract This paper describes the geochemical evolution of groundwater in the Bathonian and Bajocian aquifer along its flowpath. Since this aquifer represents one of the main sources of fresh water supply in the Caen area and has been subjected to a Holocene marine intrusion, its management requires a sound knowledge of (1) the primary conditions and (2) the potential influence of either natural or anthropogenic pressures. Groundwater vertical sampling validity is discussed with the contribution of high resolution temperature logging. The main processes of geochemical evolution along a groundwater flow line and the sea-water intrusion characteristics are discussed using ionic concentrations (Br − , F − and major elements) and isotopes (water δ 2 H and δ 18 O, TDIC δ 13 C and A 14 C, sulphate δ 18 O and δ 34 S). As the 13 C content of TDIC is used as a tracer of water-rock interaction, it shows evidence of specific chemical and isotopic evolutions of groundwater within the aquifer, both related to water-rock interaction and mineral equilibria in groundwater. All the above-mentioned tracers evolve downflow: cation concentrations are modified by exchange with clay minerals allowing a high F − concentration in groundwater, whereas Br − and SO 2− 4 concentrations appear to be redox condition dependant. Superimposed on these geochemical patterns, δ 18 O and δ 2 H compositions indicate that aquifer recharge has varied significantly through time. The chemical evolution of groundwater is locally affected by a salty water intrusion that is characterised by mixing between Flandrian fresh water and sea-water which has interacted with peat as evidenced by a high Br − /Cl − ratio and SO 2− 4 reduction.

Two high levels of continental waters in the southern Tunisian chotts at about 90 and 150 ka

Uranium and thorium content and activity ratios were measured on 20 samples (19 molluscs and 1 calcareous concretion) from the Great Chotts in southern Tunisia. Results were studied by the isochron method and by age frequency histograms. They suggest that two major flood episodes took place about 150 and 90 ka. Uranium activity ratios indicate a ground-water supply of continental origin, in agreement with biological indicators, which show large variations in salinity from fresh to marine-like conditions. The existence of a lacustrine phase, radiocarbon dated to 17-40 ka, as previously suggested for the northern Sahara, is highly questionable.

Environmental changes in a tropical lake (Lake Abiyata, Ethiopia) during recent centuries

Lake Abiyata is a small, closed, saline–alkaline lake located in the central part of the Ethiopian Rift Valley, East Africa. A multi-proxy study of a sediment core, 116 cm long and with undisturbed mud–water interface, was performed to test the sensitivity of the lake system and of different proxies to the changes in climate and human activities that occurred in the catchment during the past few centuries. The 210Pb analyses suggest that the upper 80 cm of the core represent the past 200 years. This study complements millennial-scale environmental records available in the region. The main freshwater-climatic and biological features of the modern lake system and their variations over the past decades, as known from observations, are first summarised. Results derived from individual proxies analysed along the core are then presented (successively, major physical and chemical properties of bulk sediments, diatoms and pollen). Uncertainties on the chronological framework are discussed. Major limnological stages are finally identified based on the multi-proxy interpretation of our record. Our record shows large variations in the lake water and salt balances, in the sediment sources, and in the vegetation distribution in the basin. Using our 210Pb chronology, major changes observed in the core are tentatively compared with environmental events known from instrumental and historical records. The upper 41 cm of the core (210Pb age: 1940–1998 AD) reveal several fluctuations in diatom-inferred water depth and salinity which seem to be consistent with known changes in water level. Human impact on vegetation clearly appears since about 30 years. The interval 85–41 cm suggests a period of overall water deficit. Lake Abiyata experienced episodes shallower and more saline than over the past decades, especially around 68–66 cm, 210Pb dated at ca. 1890 AD. This level may coincide with one of the worst droughts known in the Ethiopian history during 1888–1892. The lower part of the core includes a stage (108–85 cm) of lake level much higher than today and which ended before 1800 AD. Although its base is undated so far, this stage suggests that conditions much wetter than today have prevailed in the region during at least part of the 18th century. Lake Abiyata appears to be a suitable site for a detailed environmental reconstruction over the recent past, although further work is needed to reduce the uncertainties on our record, as discussed in the conclusions.

Comparing carbonate and organic AMS- (super 14) C ages in Lake Abiyata sediments (Ethiopia); hydrochemistry and paleoenvironmental implications.

We studied a 12.6-m-long sequence from Lake Abiyata (Central Ethiopia) to establish a reliable and accurate chronology for use in global paleoclimatic reconstructions. The 26 accelerator mass spectrometry radiocarbon (AMS (super 14) C) ages, performed on carbonates and organic matter, define 2 parallel chronologies, representing the complete Holocene period. However, these chronologies show a significant discrepancy from 500 to 900 BP in depth; ages obtained on carbonates were always older than those on organic matter. The hydrogeological and geochemical behavior of the Lake Abiyata basin has shed light on this discrepancy. We found that the carbonate crystallization is due mainly to the mixing of lake waters with ground-waters from the multi-layered aquifer contained in the 600-m-thick basement of the lake. The (super 14) C activity of total dissolved inorganic carbon (TDIC) measured by AMS from bottom and surface lake waters (111.4 and 111.8 pMC, respectively) confirms that the mixing occurs at the water-sediment interface. This evidence of groundwater participation in the carbonate crystallization calls into question the current paleoclimatic reconstructions based on inorganic carbonates in lakes. Specific attention should thus be given to the respective proportions of each end-member in the mixing for the quantitative estimation of the groundwater input. This will help to validate the paleoenvironmental reconstructions and to highlight an eventual diagenetical evolution of inorganic carbonates during burial, via the study of pore waters.

AMS-14C chronology of a lacustrine sequence from Lake Langano (main Ethiopian rift): Correction and validation steps in relation with volcanism, lake water and carbon balances

Located in the Ziway-Shala Basin of the Main Ethiopian Rift, Lake Langano is part of an asymmetric half-graben, defined by a series of north-northeast-trending faults in the tectonically active zone of the rift. A 15-m deep succession of organic homogeneous muds, silts, bioclastic sands, and pyroclastic layers was cored in 1994. The definition of a certified radiocarbon chronology on these deposits required the indispensable establishment of modern hydrological and geochemical balances. The isotopic contents of the total dissolved inorganic carbon (TDIC) of surface water clearly show the influence of a deep CO (sub 2) rising along the main fault crossing the lake basin. The 5.8 pMC disequilibrium existing in 1994 with the atmosphere likely produces the aging of authigenic materials developing at the lake surface. However, with a mean residence time of approximately 15 years, this apparent (super 14) C aging of Lake Langano water still integrates the (super 14) C produced by the nuclear tests in the 1960s. Reconstructing the natural (super 14) C activity of the lake TDIC allows for the quantification of the deep CO (sub 2) influence, and for the correction of AMS- (super 14) C datings performed along the core. The correction of the AMS- (super 14) C chronology defined on Lake Langano allows for a better understanding of paleohydrological changes at a regional scale for at least the last 12,700 cal BP.

Identification et caractérisation d'un biseau salin dans l'aquifère côtier du Bajocien-Bathonien de la région de Caen (France)*

Abstract The Bajocian-Bathonian aquifier in the Caen region is largely exploited for drinking water and agriculture. When this coastal carbonated aquifer becomes confined, groundwater presents an increasing TDS both vertically and along the flow path. This is explained by water-carbonate interaction and by a mixing with marine water (up to 4 %). Radiocarbon dating, after correction of the initial activity according to different models, shows a wide range of groundwater ‘ages’, from 40 000 years B.P. to the present. The marine transgression has thus been estimated as having occurred during the Early Holocene period.

Erratum to: “The Ziway–Shala lake basin system, Main Ethiopian Rift: Influence of volcanism, tectonics, and climate forcing on basin formation and sedimentation” [Palaeogeography, Palaeoclimatology, Palaeoecology 150 (1999) 135–177]

a UMR CNRS 6538 ‘‘Domaines Oceaniques’’, Institut Universaire Europeen de la Mer, Universite de Bretagne Occidentale, place Nicolas Copernic, 29280 Plouzane, France b EP 1748 CNRS, Equipe Hydrologie, Paleohydrologie et Paleoenvironnement, Universite Paris-Sud, Bâtiment 504, 91405 Orsay Cedex, France c Laboratoire d’Hydrogeologie, Universite d’Avignon et des Pays du Vaucluse, 33 rue Louis Pasteur, 8400 Avignon Cedex, France d 3 Rue des Ajoncs, 64160 Morlaas, France e CEREGE, B.P. 80, 13545 Aix-en-Provence Cedex 04, France f Laboratoire ‘‘Environnements sedimentaires et Stratigraphie’’, Universite de Perpignan, 52 avenue de Villeneuve, 66860 Perpignan, France g Department of Geology and Geophysics, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia h Royal Belgian Institute of Natural Sciences, Freshwater Biology, Vautierstraat 29, B-1000 Brussels, Belgium i Department of Earth Sciences, Via Universita 4, 41100 Modena, Italy