Christiane Causse

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.

A discontinuous climatic record from 187 to 74 ka from a speleothem of the Clamouse Cave (south of France)

This study presents a continental paleoclimatic record in the south of France,based on N 18 O, N 13 C and the 234 U/ 238 U activity ratio,dated by the 230 Th/ 234 U disequilibrium method with thermal ionization mass spectrometry. A stalagmite (Cla4) from the Clamouse Cave offers a discontinuous stable isotopic record between 189 ka and 74 ka which covers marine isotope stages (MIS) 7,6.4,5.5,5.3 and 5.1. The growth phases of the Cla4 stalagmite correspond to high sea level stages,except during MIS 6.4 (169^162 ka). All the growth phases of Cla4 correspond to humid periods,corresponding to the sapropel events observed in the eastern Mediterranean basin. Thus,the influence of a strong hydrological activity in the eastern Mediterranean basin during these periods prevailed as far west as the south of France. Because the karstic system studied strongly buffers the isotopic composition of water,isotopic variations of the calcite deposited in the cave represent mainly global and large-scale environmental variations when isotopic equilibrium conditions prevailed for calcite crystallization. Sub-stage 5.3,the end of 5.5 and MIS 7 were colder by about 4^6‡C (calculated temperature) compared to present-day temperature while the growth phases of substages 5.1 and the beginning of 5.5 reflect environmental conditions close to present ones. The N 18 O and N 13 C values of cave deposits of the sub-stage 6.4 are: (1) strongly marked by kinetic fractionation processes such as evaporation due to moisture deficit within the cave atmosphere during the first step of this growth phase and (2) related to higher humidity due to a second period of enhanced rainfall during the second step of growth. This study shows that even if calcite has not been deposited at isotopic equilibrium,its isotopic composition can give insights into the environmental conditions at the time of deposition. > 2002 Elsevier Science B.V. All rights reserved.

Cross dating (Th/U-14C) of calcite covering prehistoric paintings in Borneo

Abstract We present the first application of cross-dating (Th/U measured by thermo-ionization mass spectrometry (TIMS) and 14C measured by accelerator mass spectrometry (AMS)) of calcite covering prehistoric paintings. Th/U age estimates of cave drapery range from 9800 to 27,300 yr B.P. while conventional 14C age is estimated between 9900 and 7610 yr B.P. depending on the dead carbon correction. The age discrepancy is attributed to a disturbance of Th/U and/or 14C geochemical systems, showing the limits of the geochronological approach applied to this kind of material. For the Th/U system, the poor consistency of U data (U content, 234U/238U activity ratios) and apparent ages argue for open system conditions. For 14C system, variation of the dead carbon fraction (dcf) and a possible mixing of successive generations of calcite could account for age discrepancy. Nevertheless, one sample shows concordant ages for the two methods. Compatible ages through corrections for open system conditions are assumed for other samples. Then, the cross-dating suggests 9900 yr as the minimum age of the piece of drapery; the underlying painting must be older. This study of rock art demonstrates the presence of a Pleistocene population before 9900 yr in the southeast of Borneo, whereas previously the only population in evidence in this area was of Austronesian type from ∼5000 to 6000 yrs ago.

Fossil water in large stalagmite voids as a tool for paleoprecipitation stable isotope composition reconstitution and paleotemperature calculation

The deuterium content of macroscopic fluid inclusions, several millimetres large, observed on two stalagmite polished sections has been measured with high accuracy (<F0.5x). Although several recent studies have already focused on the stable isotope content of microscopic fluid inclusions, this is the first time that such measurements have been made on large inclusions by direct injection of the water in the spectrometer, eliminating technical problems due to the extraction of water from microscopic inclusions (crushing and heating). The first results from four fluid inclusions of two stalagmites of South Western and Southern France demonstrate that: (1) for the same stalagmite and the same age, the deuterium content is similar; (2) the fluid inclusion deuterium content difference between the sites is in agreement with the present day dripping water; (3) calculated paleotemperatures at 100 ka are in the range of those derived from pollen, marine and ice core records. Despite the fact that macroscopic fluid inclusions have rarely been observed, it appears that they can give precious data for paleoclimatologists on the stable isotope content (d 18 O, dD and deuterium excess d) of the past cave dripping water, which is a proxy of the rainfall water, and that it can be accurately dated by the TIMS U/Th method on the host calcite up to 500 ka. D 2002 Elsevier Science B.V. All rights reserved.

Humidity changes in southern Tunisia during the Late Pleistocene inferred from U–Th dating of mollusc shells

Abstract Calcareous deposits, mainly consisting of mollusc shell accumulations, which have been dated by the U/Th disequilibrium method, mark the shorelines of paleolake highstands in the Great Chotts Area of Southern Tunisia. The 5 sites studied consist of discontinuous accumulations of fossils of marine-like organisms e.g.: Cerastoderma glaucum, Melania tuberculata, Melanopsis praemorsa, Cerithium rupestre. U/Th isochron plots and age frequency histograms for 39 shell samples are reported here. Limited variations for U content and 234U/238U activity ratios (AR) of shells support the hypothesis of closure of the geochemical system with respect to this element. It is remarkable that 234U/238U AR of shells collected in Chott Fejej or Chott Jerid are clustered around different values, reflecting probably different groundwater recharge from the Continental Intercalaire (CI) or Complexe Terminal (CT) aquifers. Furthermore waters collected near Wadi el Akarit show 234U/238U AR values comparable to those observed for shells. 14C determinations made on aliquots of some of these samples suggested an age distribution between 18 and 34 ka BP. The U/Th data of these 39 shell samples imply that 4 distinct flood episodes of these lakes occurred at about 30, 95–100, 130–150 and 180–200 ka. For the episode centred around 30 ka, the frequency histogram of ages shows a multimodal age group that could represent the existence of several humid pulses rather than a unique event. Moreover, the comparison of δ13C and δ18O with those of older humid Pleistocene phases, when very large palaeolakes have been recorded, suggests that these young carbonate shells are not related to a true highstand lake. It is suggested that they represent a period of less humid climatic conditions with carbonate accumulation in minor water ponds in which intensive biological activity could have taken place. It should be noted that this period was less arid than the present.

The late pleistocene Laurentide glacier: ThU dating of its major fluctuations and δ18O range of the ice

Abstract Direct information on the isotopic composition and timing of the now-vanished Laurentide glacier can be found in subglacial calcite or in early diagenetic concretions from glaciolacustrine deposits. Th U isotopic measurements on early diagenetic concretions frome Lake Deschaillons varves (St. Lawrence Lowland, Quebec) date the early Wisconsinan major ice advance at ca. 80,000 yr (isotopic stage 5a/4 transition). Significant retreat occurred at ca. 46,000 yr (isotopic stage 3) as indicated by Th U isochrones on early diagenetic concretions from Lake Gayhurst varves in the Appalachian foothills of Quebec. During the maximum extension of the ice, which is dated at ca. 22,000 yr by Th U measurements on subglacial calcite at Cantley (northern margin of the St. Lawrence Lowland), δ18O values (vs PDB) ranging from −16 −19 ‰ (surficial ice) to ca. −30‰ (basal ice) are inferred for the Laurentide ice isotopic composition. Superimposition of two phases of carbonate precipitation does not prevent the establishment of their respective age and isotopic composition through correlation of independent parameters and calculation of mixing lines.

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

Abstract 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 ( 14 C, 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.

A petrological and geochemical study of the calcareous sandstone of West African marine Pleistocene (Cap des Biches, Senegal)

Abstract In Senegal, the marine Pleistocene is represented by several outcrops of calcareous sandstone slabs which underly the recent sandy shoreline south of Dakar. These outcrops show dissolution and recrystallization phenomena increasingly evident as one moves south. They disappear at Toubab Dialao, i.e. approximately 40 km from Dakar. The outcrop at Cap des Biches has been studied mainly because of the exceptional preservation of some Ostrea calcitic tests which allowed isotopic U Th measurements. This outcrop of metric thickness was subjected to petrographic and mineralogical examination and to several geochemical analyses (trace elements, stable isotopes, amino acids) which lead to the conclusion that the base of the deposit has been preserved. Two U Th apparent ages obtained (178 ka and > 240 ka) are older than the only age of the isotopic stage 5c obtained in the area. A post-depositional mobility of U cannot be rejected, and ages may not be reliable. However, a minimal age, considerably older than finite 14C ages previously published, may be proposed (Eemian Interglacial, i.e. isotopic stage 5). In these environments, geochemical analyses show high Pleistocene sea levels of either warmer and/or more diluted waters than the present levels, i.e. a probable Northern extension of Guinean waters.