Lacustrine organic fluxes and paleoclimatic variations during the last 15 ka : lac du Bouchet (Massif Central, France)

To assess the influence of climatic changes on organic lacustrine sedimentation, two cores recovered from the centre of the Lac du Bouchet were studied by petrographical (palynofacies) and geochemical methods. Only core LDB H was used for estimation of the organic fluxes. The variation of these fluxes with climo-stratigraphic periods showed : low organic fluxes during the Lateglacial, an increase at the beginning of the Holocene, a minimum at the end of the Atlantic period resulting from the climatic cooling, and a maximum at the end of the Sub-Boreal related to the installation of the present climatic conditions. (Résumé d'auteur

Petrographic and geochemical study of organic matter in surficial laminated sediments from an upwelling system (Mejillones del Sur Bay, Northern Chile).

A study of recent laminated sediments, accumulated in the oxygen minimum zone of Mejillones Bay (Northern Chile), shows that the organic matter is autochthonous and deposited under varying oceanographic conditions. Sedimentary units dominated by light laminations have low values of total organic carbon, total nitrogen and sulphur, and Chaotoceros content, coupled with high values of yellow amorphous organic matter. These were deposited under conditions of low primary productivity, favoring the recycling of the major part of metabolizable organic matter in the water column, and the accumulation only of non-metabolizable organic matter in the bottom sediments. In contrast, a unit rich in dark laminations with high values of total organic carbon, total nitrogen and sulphur, shows high values of brown and black AOM, and Chaetoceros content. This sedimentary unit reflects increased flocculation of organic particles during a period of high productivity and fast transit of organic material through the column water, thus diminishing the recycling of metabolizable organic matter and its accumulation with non-metabolizable organic matter in the sediments. Finally, a unit characterized by presence of both light and dark laminations was formed by abrupt alternations of the two oceanographic states described previously

Mg-Calcites and dolomite in the Brejo do Espinho lagoon, Rio de Janeiro.

The aim of this research is to study the carbonate sedimentation in a hypersaline coastal (Brejo do Espinho) where dolomite precipitation can be expected. The study allowed to make a general characterization of the sediment and to identify different types of carbonate minerals by mineralogical studies and cryo-scanning electron-microscopy

Взаємозв'язок обводнення генеративних бруньок абрикоса й персика та температурних умов їх розвитку після виходу рослин зі стану спокою

Наведено результати досліджень динаміки обводнення генеративних бруньок абрикоса й персика в зимовий період та на початку вегетації, а також темпу накопичення деревами градусо-годин росту, необхідних для початку цвітіння. Встановлено взаємозв’язок між змінами температури повітря, які враховано при визначенні градусо-годин росту, та водним режимом генеративних бруньок.Представлены результаты исследований динамики обводненности генеративных почек абрикоса и персика в зимний период и в начале вегетации, а также темпа накопления деревьями градусо-часов роста, необходимых для начала цветения. Установлена взаимосвязь между изменениями температуры воздуха, которые учтены при определении градусо-часов роста, и водным режимом генеративных почек.The article presents the results of researching the watering dynamics of the apricot and peach generative buds in the period of their development in winter and in the beginning of the vegetation as well as accumulation tempo of the growing degree hours, which trees need to come into flowering. It was established the correlation between environmental temperature changes, taken into consideration while determining the growing degree hours, and water regime of generative buds

Biomarker evidence for recent turf cultivation in Northeast Brazil (Lagoa do Boqueirao, RN State)

The first meter of sediment in Lagoa do Boqueirao [Rio Grande do Norte State (RN), Brazil] is characterized by low sedimentation rates over the period 1000 BC-1500 AD and a high sedimentation rate in the top 20 cm, corresponding to the last 10 years Several pentacyclic triterpene methyl ethers (PTMEs) such as taraxer-14-en-3 alpha-ol ME (crusgallin) and arbor-9(11)-en-3 beta-ol ME (cylindrin) occur in all the samples selected The major change in sedimentation rate recorded at 20 cm is accompanied by a change in PTME concentration and distribution Sediments deposited during the period 1000 BC-1500 AD contain PTMEs in low concentration (1 3 mu g/g sed), which could constitute a geochemical background of the grass that naturally developed in the catchment High PTME concentrations occur during the period 1996-2000. These result mainly from high concentrations of a compound tentatively assigned as arbor-8-en-3 beta ol ME, a potential diagenetic derivative of cylindrin The increase corresponds to the beginning of intensive cultivation of Cynodon dactylon and Zoysia japonica (arundoin and cylindrin producers), for the production of turf to cover Brazilian football stadiums and golf practices The results constitute a novel application of PTMEs to reconstruct land-use changes from lake sediment archives (C) 2009 Elsevier Ltd All rights reserve

Paleohydrology of tropical South America since the Last Glacial Maximum. Insights from the δD of algal and terrestrial molecular markers in lake sediments.

There is renewing evidence that the Tropics might strongly influence climate dynamics on millennial time-scales. Part of this "tropical forcing" is related to changes in the water cycle that impacts atmospheric and oceanic processes on a broad scale. Past variations of the continental hydrology are poorly documented due to the few available records and the lack of quantitative data on climate parameters. The deuterium/hydrogen ratio (δD) of algal derived biomarkers captures the isotopic composition of meteoric waters (Huang et al., 2002; Sachse et al., 2004). In turn, terrestria lly-derived biomarkers captures the isotopic composition of meteoric waters through their δD but with a strong influence of the evapotranspiration. A combination of these two factors should thus allow the quantification of the variations in relative humidity (i.e. precipitation–evaporation; P-E). We applied such an approach to estimate hydrological changes in Northern Brazil during the last 20,000 yr. The δD of algal (n-C18) and land plant (n-C30) fatty acids (FA) have been measured on 30 samples selected on a 6 m long lacustrine sediment core. In a first approximation, we considered a mean hydrogen isotope fractionation of -157 ‰ between the n-C18 FA and water and between the n-C30 FA and water of -128 ‰ (e.g. Sachse et al., 2004). Because the amount of precipitation is the major control on the isotopic composition of meteoric waters in tropical systems, we used the ΔDwat-alg (δD of the water used by algae for lipid synthesis, estimated from the δD of the n-C18 FA) as a proxy of precipitation amount. The δDwat-hp (δD of the water used for land plant lipids synthesis) is measured from the ΔD of the n-C30 FA and used to estimate relative moisture (P-E). The difference in isotope fractionation between land plants and phytoplankton (δDwat-hp - δDwat-alg) is used to quantify evapotranspiration (evaporation from soils and transpiration from plants). These results allowed to distinguish five contrasted periods in our 20,000 yr record (see below). The 20-19 kyr period is characterized by high precipitation associated with high evapotranspiration, i.e. a semi-arid climate. By comparison, from 19 to 17 kyrs, both precipitation and evapotranspiration are both reduced. The resulting climate is nevertheless dryer than during the previous stage. During the Lateglacial interval, i.e. from 17 up to 13.5 kyrs, evapotranspiration is reduced but the precipitation which remains at the same level than before entails a more humid climate. The Younger Dryas interval (YD) shows higher precipitation levels but stronger evapotranspiration, thus a dryer climate than during the Lateglacial but nevertheless more humid than at the end of the Last Glacial Maximum (LGM). Finally, the Holocene shows a decreasing trend in the amount precipitation and with few variations of the evapotranspiration that finally delineate a drying trend. These results are coherent with independent data produced by palynology and biomarker analysis (Ledru et al., 2001; Jacob et al., 2004) and give clue information on the variations in water cycling in the Tropics since the LGM

Southern oscillation signal in South American palaeoclimatic data of the last 7000 years

During strong El Niño events, rainfall anomalies and changes in wind patterns are observed in different regions of South America. Along the central Brazilian coast, during the 1983 El Niño year, the frontal systems were blocked to the south, provoking a reversal of the longshore sand transport. Long-duration reversals of longshore transport were also recorded in Holocene beach-ridge terraces from the Rio Doce coastal plain. This led to the formulation of a model relating these reversals of longshore transport to El Niño-like conditions. El Niño-like conditions are past average climate situations that generate the same perturbations as the strong El Niño events observed during the last decade. They are likely to correspond to the long-duration low phase of the Southern Oscillation. To confirm this hypothesis we compared the Holocene beach-ridge record with other palaeoenvironmental records from regions where strong El Niño events would have a substancial signal as well: (1) water-level fluctuations of Lake Titicaca, (2) a pollen and sediment record in an eastern Amazonian lake, (3) changes of the Rio Xingu discharge in eastern Amazonia, and (4) variations of sand supply at the Rio Piura and Rio Chira outlets in the Sechura Desert. The occurrences of El Niño-like conditions were numerous before 3900-3600 yr B.P., absent between 3900-3600 and 2800-2500 yr B.P., and infrequent after 2800-2500 yr B.P. © 1993 University of Washington