Humid Episodes Research Articles

Long-period Milankovitch cycles from the Late Triassic and Early Jurassic of eastern North America and their implications for the calibration of the Early Mesozoic time–scale and the long–term behaviour of the planets

During the Late Triassic and Early Jurassic the Newark rift basin of the northeastern US accumulated in excess 5 km of continental, mostly lacustrine strata that show a profound cyclicity previousl...

Une révision de la séquence de la Riera (Asturies) et la question du Badegoulien cantabrique

Abstract A new interpretation of the sequence of La Riera (Asturias) is proposed, based on a multidisciplinary analysis including the palaeoenvironment, the patterns of lithic raw material procurement, the knapping process and the lithic and bone industries. After a short summary of the model established by L.G. Straus, the review of the palaeoclimatological framework underlines a very cold context — with some more humid episodes —for the whole stratigraphy, except for the top of the sequence corresponding to the end of the Wiirmian ice age interstadial warming (" Boiling/ Allerod "). Analysis of the raw materials and the knapping process shows a mainly local subsistence economy for the end of the Solutrean and the Badegoulian, coinciding with the Last Glacial Maximum, in contrast with the beginning of the Solutrean and the Magdalenian where circulation along the Cantabrian coast as far as the south of the Aquitanian basin is conceivable. The results of the statistical processing of the stone industries reveal the presence of several industrial fades in stratigraphie sequence, including a new entity : the Cantabrian Badegoulian. Typological analyses demonstrate the existence of continuities and discontinuities between these différents fades. The bone industries underline strong convergences between the Solutrean and the Badegoulian. This structuration is then compared with other data available for the period between 22 000 BP and 16 000 BP in Cantabria. In spite of the age of most research, the sequence of industries fits well with that of La Riera, which allows us to confirm the validity of this new model recognizing the existence of the Cantabrian Badegoulian, in the same cold and damp climatic environment (" Lascaux "). Finally, the authors discuss the problem of the Solutrean/ Badegoulian and Badegoulian/Magdalenian transitions in Cantabria and of the internal structure of the Cantabrian Badegoulian.

Nouvelles données paléoenvironnementales pour le Quaternaire récent en Tunisie centrale (bassin de Maknassy)

Sedimentary deposits outcropping on the Wadi Leben and Wadi Ben Sellam banks in the Maknassy Basin (central Tunisia) have registered paleoclimatic variations, and particularly humid episodes, already known for Upper Pleistocene and Holocene, in present hyperarid areas of northern Sahara. A pluridisciplinary study, including prehistory, sedimentology, mineralogy, ecology and radiochronology ( 14C, uranium-thorium [U-Th]), improves the comprehension of paleoenvironmental interpretations and makes it possible to precisely determine the chronological framework of humid episodes during the Upper Quaternary in Tunisia.

Long-Term vs Short-term Variability of Stable Isotopes in Brachiopods of the Silurian of Gotland (Sweden): Palaeoenvironmental Implications

The Silurian of Gotland consists of 440 m carbonate deposits, spanning the late Llandovery to late Ludlow epochs (431 to 411 m.y.). Repeatedly, uniform sequences of micritic limestones and marls are interrupted by complex-structured reefs and adjacent platform sediments. Previously, the alternation of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses. Ratios of stable carbon and oxygen isotopes in brachiopod shells (more than 370 specimens, esp. Atrypa reticularis) from the Silurian of Gotland, Sweden, have been analysed. Preservation of biological skeletal ultrastructures, observed in SEM-micrographs, and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs. The Silurian sequence of Gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units. Lower values occur in periods dominated by deposition of marly sequences. Higher values are observed in periods dominated by reefs and extended carbonate platforms.The isotope ratios are influenced by local as well as global factors. The oxygen isotope ratios are interpreted to reflect palaeosalinity changes due to varying freshwater input, rather than changes in palaeotemperature. Consequently, the facies distribution of Gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and fresh water runoff rather than by sea level fluctuations. Periods of arid climate and therefore anti-estuarine downwelling of oxygenated surface water appear as short episodes of reef growing (~< 1.5 m.y.) in an epoche characterized by a tropic humid climate, which causes an estuarine circulation and the upwelling of CO2-rich deep water. Carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13Crich surface water (arid episodes) or upwelling of 13C-depleted deep water (humid episodes) of a Silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales. Although the development of 813C and 81SO values have been shown to be parallel in the secular changes, the underlying long-term trends show antiparallel courses. This fact indicates that the factors controlling the long-term trends in isotopes are different from those assumed to control the secular changes. The long-term trends in the Silurian of Gotland interrupt the general increase of both carbon and oxygen isotope values during the Palaeozoic.

Paleoenvironmental changes in the Silurian indicated by stable isotopes in brachiopod shells from Gotland, Sweden

Ratios of stable carbon and oxygen isotopes in brachiopod shells (more than 370 specimens, esp. Atrypa reticularis) from the Silurian of Gotland, Sweden, have been analysed. Preservation of biological skeletal ultrastructures, observed in SEM-micrographs, and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs. The Silurian of Gotland consists of 440 m carbonate deposits, spanning the late Llandovery to late Ludlow epochs (431-411 m.y.). Repeatedly, uniform sequences of micritic limestones and marks are interrupted by complex-structured reefs and adjacent platform sediments. Previously, the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses. The Silurian sequence of Gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units. Lower values occur in periods dominated by deposition of marly sequences. Higher values are observed in periods dominated by reefs and extended carbonate platforms. The isotope ratios are influenced by local as well as global factors. The oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input, rather than changes in paleotemperature. Consequently, the facies distribution of Gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations. Periods of arid climate and, therefore, anti-estuarine downwelling of oxygenated surface water appear as short episodes of reef growing (≤1.5 m.y.) in an epoche characterized by a tropic humid climate, which causes an estuarine circulation and the upwelling of CO 2-rich deep water. Carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13C-rich surface water (arid episodes) or upwelling of 13C-depleted deep water (humid episodes) of a Silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales.

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.

U-Series Dating by the TIMS Technique of Land Snails from Paleosols in the Canary Islands

A sequence of seven superimposed paleosols developed on eolian calcarenites and alluvium was sampled on the island of Lanzarote in order to examine the possibility of dating land snail shells by the U-series method, using a TIMS technique allowing measurement of U and Th isotopes in very small samples. In the lower six units, the fossil shells yielded D-allo/L-isoleucine (A/I) ratios of about 0.5 and apparent AMS 14C ages ranging from 41,000 to 34,000 yr B.P., indicating that most paleosols formed during a relatively short mid-Würm humid episode. The upper unit (paleosol 7) yielded more variable A/I ratios (ranging from 0.6 to 0.2) and a younger 14C age ∼27,000 yr B.P. Most samples contained enough U to allow the calculation of U-series ages, after correction for the presence of a detrital component. In samples containing a few tens of ppb of U (paleosols 1, 2, 3, 6, and 7), the ages are strongly dependent upon the model used for the correction. In samples containing more than 300 ppb of U (paleosols 4 and 5), concordant ages of ∼31,000 ± 1000 yr were obtained regardless of the correction model used. U uptake in these shells occurred during one single early diagenetic phase, soon after burial, since shells of modern snails do not contain any significant amount of U. The arid conditions subsequent to the mid-Würm humid episode have likely ensured since then a fair closure of the radioactive system.

Molluscan assemblages from the loess of North Central China

Although molluscan faunas are the most widespread fossil remains in the Chinese loess. there have been very few attempts to use their potential for climatic reconstruction, or to develop cIimato-stratigraphic schemes. This paper reports assemblages of Mollusca from three sites in the North and South Central parts of the Loess Plateau, and evaluates the uses of molluscan faunas as climatic proxies in the S 1 palaeosol, the Malan Loess and S o palaeosol. The molluscan sequence is compared with the record obtained from pedological and magnetic techniques. The current study shows that major peaks in magnetic susceptibility, thought to be due to pedogenesis under warm, humid conditions, can be matched by levels of high molluscan abundance. The occurrence of a sharp molluscan peak within the Malan Loess, otherwise only indicated by a slight increase in magnetic susceptibility, is held to be due to the existence of a short-lived humid episode and demonstrates the sensitivity of Mollusca as indicators of relatively low magnitude climatic shifts not well indicated by other lines of evidence.

Pollen evidence of late Quaternary vegetation and inferred climate changes in Congo

A detailed palynological analysis of two cores recovered from a swamp in the Southern Batéké Plateaux (Congo), provides information on the botanical history of this region during the last 24,000 yr B.P. Rich and well-diversified pollen counts exhibit changes in the vegetational communities related to hydrological and climatic fluctuations. Around 24,000 yr B.P., the occurrence of hydromorphous forest elements gives evidence of a humid phase. Between 24,000 and around 13,000 yr B.P., swampy herbaceous communities expanded, indicating slightly drier climatic conditions. From approximately 13,000 yr B.P. onward, the beginning of a humid episode is reflected by the development of mesophilous forests. The decrease of these forests is documented since 3000 yr B.P. Grasslands are found to extend locally whereas Elaies guineensis (oil palm) spreads out, indicating both climatic change but also anthropogenic activities.

Rare-earth elements in some environmental matrices

This study establishes for the first time the chronology and limnological history of Lake Amora (Dead Sea basin, Israel), whose deposits (the Amora Formation) comprise one of the longest exposed lacustrine records of the Pleistocene time. The Amora Formation consists of sequences of laminated primary aragonite and silty-detritus, Ca-sulfate minerals, halite and clastic units. This sedimentary sequence was uplifted and tilted by the rising Sedom salt diapir, exposing ∼320 m of sediments on the eastern flanks of Mt. Sedom (the Arubotaim Cave (AC) section).The chronology of the AC section is based on U-disequilibrium dating (230Th–234U and 234U–238U ages) combined with floating δ18O stratigraphy and paleomagnetic constraints. The determination of the 230Th–234U ages required significant corrections to account for detrital Th and U. These corrections were performed on individual samples and on suites of samples from several stratigraphic horizons. The most reliable corrected ages were used to construct an age-elevation model that was further tuned to the oxygen isotope record of east Mediterranean foraminifers (based on the long-term similarity between the sea and lake oxygen isotope archives).The combined U-series-δ18O age-elevation model indicates that the (exposed) Amora sequence was deposited between ∼740 and 70 ka, covering seven glacial–interglacial cycles (Marine Isotope Stages (MIS) 18 to 5).Taking the last glacial Lake Lisan and the Holocene Dead Sea lacustrine systems as analogs of the depositional–limnological environment of Lake Amora, the latter oscillated between wet (glacial) and more arid (interglacial) conditions, represented by sequences of primary evaporites (aragonite and gypsum that require enhanced supply of freshwater to the lakes) and clastic sediments, respectively. The lake evolved from a stage of rapid shifts between high and low-stand conditions during ∼740 to 550 ka to a sabkha-like environment that existed (at the AC site) between 550 and 420 ka. This stage was terminated by a dry spell represented by massive halite deposition at 420 ka (MIS12-11). During MIS10-6 the lake fluctuated between lower and higher stands reaching its highest stand conditions at the late glacial MIS6, after which a significant lake level decline corresponds to the transition to the last interglacial (MIS5) low-stand lake, represented by the uppermost part of the Formation.δ18O values in the primary aragonite range between 6.0 and −1.3‰, shifting cyclically between glacial and interglacial intervals. The lowest δ18O values are observed during interglacial stages and may reflect short and intense humid episodes that intermittently interrupted the overall arid conditions. These humid episodes, expressed also by enhanced deposition of travertines and speleothems, seem to characterize the Negev Desert, and in contrast to the overall dominance of the Atlantic–Mediterranean system of rain patterns in the Dead Sea basin, some humid episodes during interglacials may be traced to southern sources.

Antarctic subtropical humid episode at the Paleocene-Eocene boundary: Clay-mineral evidence

Clay-mineral assemblages from East Antarctica have been analyzed at high stratigraphic resolution (20 to 1 ka) throughout the interval from 55.6 to 55.0 Ma, which includes the terminal Paleocene isotopic excursion in Ocean Drilling Program Site 690B on Maud Rise (lat 65°S) in the Weddell Sea region. Changes in the clay associations reflect a major increase in chemical weathering caused by increased temperature and/or rainfall in at least this sector of East Antarctica for a brief (270 ka) interval in the latest Paleocene. This represents the most intense warming known for the Cenozoic. This high-latitude climatic episode is recorded synchronously by stable isotopes and clay minerals. A progression is evident in the clay assemblages during the latest Paleocene that apparently reflects changing relations between temperature and precipitation. This sequence began with the interval of rapid temperature increase that marks the beginning of the oxygen isotopic excursion. A brief increase in kaolinite at the inception of the excursion suggests a temporary increase in year-round precipitation in Antarctica, in response to an increased continent-to-ocean temperature gradient. This kaolinite spike was followed by almost total dominance by smectite for the remainder of the isotopic excursion (∼120 ka), suggesting that warmer Southern Ocean surface temperatures of 18 to 22 °C were associated with seasonal precipitation (alternating wet and dry seasons). Claymineral variations on Antarctica during the isotopic excursion reflect a tight coupling between oceanic and continental climate change. Immediately following the excursion at ∼55.22 Ma, kaolinite percentages increased to values similar to modern subtropical-tropical areas for ∼150 ka, a remarkable event for the Antarctic. Abundant kaolinite suggests perennial rainfall and minimum soil temperatures of 15 °C during at least part of the year. The kaolinite increased during a time of lower Southern Ocean surface-water temperatures, suggesting increased atmospheric heat transport toward the poles. A temporary change in atmospheric circulation is suggested from dominantly zonal to meridional.

Chronology and paleohydrology of late Quaternary high lake levels in the Manyara basin (Tanzania) from isotopic data ( 18O, 13C, 14C, [formula omitted]) on fossil stromatolites

Superimposed phases of stromatolite buildup are observed on the eastern margin of Lake Manyara and depict a paleoshoreline at about 20 m above modern lake level. Radiocarbon and Th U measurements permitted the dating of the last two phases of stromatolite formation at ca. 90,000 yr and between 27,000 and 23,000 yr B.P., respectively. The Th U chronology is based on the decay of a strong 230Th-excess (over 234U) inherited with the detrital particles cemented into the stromatolites. The various generations of stromatolites show comparable stable carbon and oxygen isotope contents and are located at the same paleolake stabilization levels. This indicates that stringent hydrological conditions are necessary for the development of the encrusting benthic microbial communities responsible for stromatolite formation. A comparison with similar stromatolitic units from the nearby Lake Natron-Lake Magadi basin shows that such conditions occurred during only a few of the late Quaternary humid episodes known in eastern Africa and that they are different in each basin. Stromatolites do not necessarily represent all high lake levels that Lake Manyara experienced during the late Pleistocene and Holocene.

PALHYDAF (Palaeohydrology in Africa) program: objectives, methods, major results

The PALHYDAF program aims to reconstruct the Late Quaternary (130 ka B.P. to Present) environmental and hydrological changes of northwest Africa, and to interpret them in terms of palaeoclimate for testing coupled atmospheric/ocean circulation models. Lacustrine sequences were obtained from closed basins which lie along two meridional transects: southern Tunisia-southern Niger, and Algeria-Mali. Particular attention is given to groundwater storage and periods of recharge of the aquifers. The chronological framework is based on 14C dates (conventional and AMS methods) and/or the Th/U disequilibrium method. In favourable cases (e.g. Mali), correlations can be made between the chronology of surface palaeohydrology and that of aquifer recharge. Various environmental markers (mineralogy, elemental chemistry, stable isotopes, biological remains) are used for palaeoenvironmental purposes. Quantitative estimates of palaeoclimatic variables (e.g. air humidity) may be attempted through the combination of geochemical and palaeobiological data. The major results already obtained are summarized for the following individual sectors: 1 — southern Tunisia. 1′ — northern Algeria. 2 — southern Algeria. 3 — northern Niger. 3′ — Mali. 4 — southern Niger. At the present stage of investigation, the following new inferences can be drown. • - One, or several humid episode(s) occurred in the northern Sahara between ≈ 150 and ≈ 75 ka B.P. Several factors suggest that these humid conditions also affected lower latitudes, although their southern extent cannot be given precisely at present. • - The previously postulated existence of large lakes at ≈20–35 ka B.P. is not confirmed. This time interval seems to coincide with raised water-tables, but not to a major humid phase. In all cases, the 14C chronologies obtained for the Late Pleistocene have to be checked by comparison with other dating methods (e.g. Th/U disequilibrium). • - After the arid period of the Last Glacial Maximum, the return of humid conditions occurred in two major steps, at ≥12 and ≈9.5-9 ka B.P. for both the southern and the northern Sahara.

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.

Étude palynologique du sondage Magadi NF1 (Kenya) implications paléoclimatiques

This paper presents the palynological results obtained from the Late Pleistocene deposits of Lake Magadi, Kenya(1°50′S, 36°18′E, 600 m), today located in the most arid region of East Africa. It documents the vegetational and climatic environment of the lake for two well defined periods: the Pleistocene-Holocene transition before 10 800 ± 120 B.P., and the last glacial episode dated 17 710 ± 220 B.P., Compared with modern pollen data from the region, the microflora show the occurrence of an open vegetation. But the scarcity or absence of arid pollen indicators suggests more humid conditions than today, a conclusion that is confirmed by lithological and sedimentological analysis. The Holocene is missing and the most humid episode is registered between 12 000 and 10 000 B.P., Highest allochtonous pollen input indicates an increased runoff, mostly related to higher rainfall during that period. The palynological results questionned the 17 000 B.P. 14C ages for the bottom of the core.

Holocene evolution of the crater lake at Malha, Northwest Sudan

A reconstruction of the hydrological and environmental evolution of the crater lake at Malha (Northern Darfur, Sudan) resulted from the mineralogical and biological study of a 9.21 m section of lake sediments, representing an uninterrupted sequence of lacustrine deposition since 8 290 14C years BP. Important changes in water supply and conditions of sedimentation are reflected in the nature of the sediments and the morphology and stratigraphical distribution of various salt minerals. Additional information on lake circulation patterns and salinity conditions are obtained from associated benthic paleocommunities, represented by ostracods and dipterid larvae. Combining both lines of evidence, the studied sequence can be divided in six distinct sections, which correspond to six successive periods in the lake's Holocene history. The first three periods, generally characterized by high lake levels, represent three generations of a meromictic lake, two of which have ended with a complete desiccation of the lake basin. Meromixis was stable during Period I, due to wind shelter and pronounced density stratification. In the course of Periods II and III stratification was repeatedly interrupted. During Period II, the disruptions were accompanied by important water budget fluctuations; a superimposed gradual decrease in net water supply eventually resulted in holomictic conditions terminating this period. Evidence of turbulence periodically affecting profundal waters is recorded in the sediments of Period III, suggesting that disruptions of stratification were now initiated by very strong winds. Between Period I and Period III, the littoral mixolimnion gradually evolved from near fresh to mesosaline. In Periods IV to VI, lake level was intermediate to low. The lake was holomictic for most of the time and meso- to hypersaline; during Period V, it repeatedly shrunk to a shallow brine pool. The Holocene evolution of Malha Crater Lake illustrates the progressive increase in aridity over most of North Africa following a well-established, early- to mid-Holocene major humid episode. The uninterrupted sedimentary sequence lends itself for detailed reconstruction of Holocene climatic evolution in arid Northeast Africa, a region where records of continuous lacustrine deposition are extremely scarce. As the chronology of critical events in the lake's history remains as yet unsupported by radiocarbon dates, correlation with other Holocene sequences in the eastern Sahara is highly tentative at this point.

African diatom palaeoecology and biostratigraphy

This paper illustrates the different types of information which can be deduced from African diatoms studies. The first section is devoted to the reconstruction of palaeoenvironments. The accuracy of ecological data (estimates of ecological variables e.g. types of habitats, water chemistry…) depends on the times scale and on the spatial scale considered: short-term environments in a given waterbody, environmental evolution during individual humid episodes considered locally or regionally, long-term changes in diatom flora induced by major climatic tendencies at a regional or continental scale. One then summarizes our present state of knowledge in diatom biostratigraphy in Africa. Several centric taxa, belonging to the genera Aulacoseira, Mesodictyon, Cyclotella, Stephanodiscus and Cyclostephanos appear to have limited stratigraphical ranges and to be good chronological markers.

Fractionation and recycling of U and Th isotopes in a semi-arid endoreic depression of central Syria

Mesozoic and Cenozoic carbonate-rich formations including Late Cretaceous phosphate deposits are exposed in a structural depression which is known as the Palmyra aulacogen. The bottom of this depression has been occasionally filled by water bodies during Quaternary humid episodes. The bottom of the depression now forms a sabkha as a consequence of modern semi-arid climatic conditions. Isotopic activity ratios and concentrations of U and Th were measured in bedrock formations, ground-waters, unsaturated soils, and in Quaternary lacustrine sediments cored in the sabkha in order to investigate the processes governing the fractionation and recycling of U and Th within the endoreic depression. The bedrock samples are close to secular equilibrium with the exception of phosphorites which show a 234U deficit ( 234U 238U = 0.83 ± 0.04 ). Upper soil horizons have a strong excess of 230Th ( 230Th 234U = 1.7 ± 0.2 ) as a consequence of U removal. The U loss occurs without fractionation ( 234U 238U = 1.03 ± 0.01 ) as it is co-dissolved with the CaCO 3 matrix; this is also shown by an inverse relationship between CaCO 3 vs 232Th in soils. Vertical profiles indicate transitional retention of U in the lower unsaturated soil horizons, where deficiency in 230Th ( 230Th 234U as low as 0.37 ± 0.02) and high U concentrations (up to ca. 100 μ · g t̄) characterize “secondary” co-precipitation of U and CaCO 3 which forms in response to downward decreasing pCO 2 during water percolation events. A rate of U removal of ca. 10 −4 to 10 −5 a −1 from the upper soil horizons and a residence time of approximately 40 to 50 ka of soil particles in the U-ablation zone were calculated using a simplified mathematical model based on the assumption that 230Th remains immobile in the soils. The residence time of U corresponds to a rate of erosion of ca. 1 cm for 10 3 a. The activity ratios of the most recent lacustrine sediments of the sabkha (upper 2 m in cores) define a well correlated ( r > 0.97) “dilution curve” of the form: [ 230Th 234U = (1.91 ± 0.03) ( 234U 232Th −1 + 0.15 ±0.01 ]. This indicates that the radioisotopes inherited with the detrital fraction were diluted by an authigenic/early diagenetic phase of U fixation. A residence time for the diluting U (ca. 18,000 a) can be evaluated from the asymptote value (0.15 ± 0.01). This age corresponds to the last humid episode recorded in central Syria (the equivalent of the last glacial maximum at higher latitudes) which was associated with lacustrine sedimentation in the basin. Detrital particles eroded from the surrounding soils were then characterized by 238U/ 232Th and 230Th/ 238U ratios lower than those observed in modern upper soil horizons: i.e., by a longer residence time and a higher U-leaching rate during their pedologic cycle.

Freshwater diatom influx in intertropical Atlantic: Relationships with continental records from Africa

Diatom assemblages from modern West African rivers and from lacustrine sediments subjected to deflation represent the present-day sources of continental diatoms to the sea. Diatom productivity in large rivers is high, especially for the genus Melosira. Windblown diatoms derive mainly from the central and northern Sahara (summer dust plume) where saline chloride-water assemblages are widespread, or from the southern edge of the Sahara (winter dust plume) where Melosira-rich assemblages of dilute water predominate. Freshwater diatom peaks in Atlantic cores may reflect (1) phases of increased river influx, correlated with humid episodes on the continent or (2) phases of enhanced deflation and wind transport during arid episodes (the single hypothesis for fine sediments from mid-ocean sites). Genus Melosira dominates the freshwater assemblages of many modern and fossil marine samples, whatever the transport agent is. Therefore, it is not an accurate paleoclimate indicator by itself, but associated taxa may provide information on the environmental and geographical origin of the displaced diatoms. This tentative approach shows that freshwater diatoms in Atlantic cores may be a good tool for reconstructing paleoclimates and for establishing continent-ocean correlations if species analyses are made and if the continental distribution of the taxa encountered is considered.

Reconstitution d'environnements quaternaires par l'etude de macrorestes vegetaux. (Pays bateke, R.P. du Congo)

Three climatic periods are known in the Congo for the last 40 millennia: the humid Njilian (40,000–30,000 yr B.P.) is followed by the drier Leopoldvillian (30,000–12,000 yr B.P.), and by the humid period of the Kibangian (12,000-recent) which corresponds with the Holocene. Anatomical identification of vegetable macro-fossils found in the Bateke area sand horizons of Congo enables one to determine two important facts for the late Pleistocene (40,000–12,000 yr B.P.) and Holocene: the continuation of a dry season even during the most humid episodes, and continuation of a forest-savanna mosaic of which the variations in favour of one of the other follow the palaeoclimatic changes. Comparison with the results of a littoral pollen sequence shows a close similarity, though the forest-savanna contrast is more marked in coastal than in the continental environment.