Didier L. Bourles

Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelghazali: Mio-Pliocene hominids from Chad

Ages were determined at two hominid localities from the Chad Basin in the Djurab Desert (Northern Chad). In the Koro Toro fossiliferous area, KT 12 locality (16°00′N, 18°53′E) was the site of discovery of Australopithecus bahrelghazali (Abel) and in the Toros-Menalla fossiliferous area, TM 266 locality (16°15′N, 17°29′E) was the site of discovery of Sahelanthropus tchadensis (Toumaï). At both localities, the evolutive degree of the associated fossil mammal assemblages allowed a biochronological estimation of the hominid remains: early Pliocene (3–3.5 Ma) at KT 12 and late Miocene (≈7 Ma) at TM 266. Atmospheric 10Be, a cosmogenic nuclide, was used to quasicontinuously date these sedimentary units. The authigenic 10Be/9Be dating of a pelite relic within the sedimentary level containing Abel yields an age of 3.58 ± 0.27 Ma that points to the contemporaneity of Australopithecus bahrelghazali (Abel) with Australopithecus afarensis (Lucy). The 28 10Be/9Be ages obtained within the anthracotheriid unit containing Toumaï bracket, by absolute dating, the age of Sahelanthropus tchadensis to lie between 6.8 and 7.2 Ma. This chronological constraint is an important cornerstone both for establishing the earliest stages of hominid evolution and for new calibrations of the molecular clock.

ESTIMATION OF SLIP RATES IN THE SOUTHERN TIEN SHAN USING COSMIC RAY EXPOSURE DATES OF ABANDONED ALLUVIAL FANS

Cosmic ray exposure dates (based on in situ–produced 10 Be), in combination with measured heights of fault scarps that cut three abandoned fans, imply a slip rate of less than ∼2 mm/yr on the bounding thrust fault along a segment of the southern flank of the Tien Shan of central Asia. This rate, somewhat lower than those for the segment farther west and for the northern margin of the belt, implies that the distribution of shortening across the Tien Shan must change markedly along the belt. The date of abandonment of the smallest of the three fans is consistent with fan formation since the last glacial maximum, but the other two fans appear to have been formed and abandoned prior to that time. These and other results suggest that advances and retreats of alpine glaciers may not be synchronized with continental ice sheets and that not all prominent geomorphological features in arid Asian regions have formed since the last glacial maximum. In this study we have assessed the relationship between cosmogenic nuclide content of individual clasts and the time since their deposition on a surface. Cosmogenic nuclide accumulation represents the integrated total surface exposure of rocks in source regions during transport and since deposition. Postdepositional processes (erosion, burial, bioturbation, and cryoturbation), all of which decrease the quantity of cosmogenic nuclides produced in a rock in a given time and hence the apparent exposure age, also cause significant scatter among apparent ages of samples from the surfaces. Very low 10 Be concentrations in material from active streambeds and at depth below one of the fan surfaces indicate that exposure prior to deposition onto fan surfaces was minimal (

DETERMINATION OF PREDEVELOPMENT DENUDATION RATES OF AN AGRICULTURAL WATERSHED (CAYAGUAS RIVER, PUERTO RICO) USING IN-SITU-PRODUCED 10BE IN RIVER-BORNE QUARTZ

Accurate estimates of watershed denudation absent anthropogenic effects are required to develop strategies for mitigating accelerated physical erosion resulting from human activities, to model global geochemical cycles, and to examine interactions among climate, weathering, and uplift. We present a simple approach to estimate predevelopment denudation rates using in-situ-produced cosmogenic 10Be in fluvial sediments. Denudation processes in an agricultural watershed (Cayaguas River Basin, Puerto Rico) and a matched undisturbed watershed (Icacos River Basin) were compared using 10Be concentrations in quartz for various size fractions of bed material. The coarse fractions in both watersheds bear the imprint of long subsurface residence times. Fine material from old shallow soils contributes little, however, to the present-day sediment output of the Cayaguas. This confirms the recent and presumably anthropogenic origin of the modern high denudation rate in the Cayaguas Basin and suggests that pre-agricultural erosional conditions were comparable to those of the present-day Icacos.

Evidence for muon‐induced production of 10Be in near‐surface rocks from the Congo

Distributions of 10Be in vein quartz from a lateritic weathering profile from the Congo provide evidence for significant in situ production of 10Be by reactions other than neutron-induced spallation. After consideration of geomorphological and nuclear processes which might contribute to the observed depth variability, we conclude that cosmic ray muon-induced reactions are the most likely mechanism, and that such reactions lead to 1% to 3% of 10Be production at the surface. This small muon-induced component has only minor implications for the use of 10Be for quantification of exposure histories of surficial rocks. Nevertheless, the muogenic component has the potential to expand the utility of cosmogenic nuclides in examination of surficial processes, overcoming some of the limitations associated with the use of the neutron-produced component.

A high resolution authigenic 10Be/9Be record of geomagnetic moment variations over the last 300 ka from sedimentary cores of the Portuguese margin.

A high resolution study of authigenic Be isotopes (10Be and 9Be) combined with continuous relative paleointensity records has been performed along the same marine sedimentary sequences from the Portuguese margin (N.E. Atlantic) covering the past 300 ka in order to assess relationships between geomagnetic moment variations and 10Be production rate variations. A carefull examination of the various ways of taking into account environmental disturbing effects on the authigenic 10Be concentration leads to the conclusion that the most reliable proxy of cosmonuclide production rates is presently the authigenic 10Be/9Be ratio. Eight intervals of significant authigenic 10Be/9Be enhancement evidence geomagnetic moment drops related to global paleomagnetic excursions, some being already admitted, others being proposed as new geomagnetic features. Since, contrarily to sedimentary magnetic remanence, the authigenic 10Be/9Be records dipole moment variations without significant acquisition delay, it provides better constraints on their timing. Comparison of 10Be/9Be and benthic δ18O records from the same cores suggests that dipole moment lows preferentially occured during or at the end of interglacial episodes, with a quasi-period of 100 ka.

Cosmogenic dating ranging from 20 to 700 ka of a series of alluvial fan surfaces affected by the El Tigre fault, Argentina

It is crucial to date continental landforms to quantify processes involved in terrestrial surface evolution, especially in regions affected by active tectonics. Andean quaternary alluvial fan surfaces affected by the El Tigre strike-slip fault have been studied using combined geomorphic and 10 Be exposure age approaches. Field observations and SPOT (French acronym for “Satellite for Observation of the Earth”) image analysis enable the identification of six alluvial fan units. Measurements of in situ–produced cosmogenic 10 Be concentrations in quartzite boulders exposed on the top of fan surfaces show that the depositional periods ended during successive major interglacial stages. The calculated minimum exposure ages date the abandonments of the alluvial fan surface from 41 000 ± 8500 yr for the youngest to 670 000 ± 140 000 yr for the oldest unit. When linked to the measured maximum cumulative right-lateral displacement of stream channels, the exposure ages yield a horizontal slip rate of about 1 mm/yr on the El Tigre fault. This study shows that for arid regions, where fan surface erosion is minimal, in situ–produced 10 Be can be used to constrain the age of stratigraphically separate alluvial fan surfaces. These fan surface exposure ages can be further used to calculate slip rates on active faults and infer depositional periods correlative with climatic events.

Age and Date for Early Arrival of the Acheulian in Europe (Barranc de la Boella, la Canonja, Spain)

The first arrivals of hominin populations into Eurasia during the Early Pleistocene are currently considered to have occurred as short and poorly dated biological dispersions. Questions as to the tempo and mode of these early prehistoric settlements have given rise to debates concerning the taxonomic significance of the lithic assemblages, as trace fossils, and the geographical distribution of the technological traditions found in the Lower Palaeolithic record. Here, we report on the Barranc de la Boella site which has yielded a lithic assemblage dating to ∼1 million years ago that includes large cutting tools (LCT). We argue that distinct technological traditions coexisted in the Iberian archaeological repertoires of the late Early Pleistocene age in a similar way to the earliest sub-Saharan African artefact assemblages. These differences between stone tool assemblages may be attributed to the different chronologies of hominin dispersal events. The archaeological record of Barranc de la Boella completes the geographical distribution of LCT assemblages across southern Eurasia during the EMPT (Early-Middle Pleistocene Transition, circa 942 to 641 kyr). Up to now, chronology of the earliest European LCT assemblages is based on the abundant Palaeolithic record found in terrace river sequences which have been dated to the end of the EMPT and later. However, the findings at Barranc de la Boella suggest that early LCT lithic assemblages appeared in the SW of Europe during earlier hominin dispersal episodes before the definitive colonization of temperate Eurasia took place.

The development of iron crust lateritic systems in Burkina Faso, West Africa examined with in-situ-produced cosmogenic nuclides

We have investigated the development of iron crust laterites on the stable West African Craton in northern Burkina Faso using cosmogenic radionuclides produced in situ in quartz veins and pebbles. Lateritic soils develop in tectonically stable, slowly eroding, tropical environments and are a major component of the Earths surface. To examine processes affecting laterite formation, we determined 10Be and 26Al in samples of quartz from three sites representing two sequential and connected iron crust laterite systems. Results from outcropping quartz veins suggest that the mean erosion rate in this region is about 3–8 m Myr−1. In addition, data from quartz cobbles and pebbles incorporated in iron crusts demonstrate that depth-dependent distributions of these nuclides may be used to distinguish surfaces undergoing burial from those undergoing erosive loss. Results from sections of the lowland lateritic system are consistent with mean accumulation rates of a few metres per million years. Quartz cobbles, presently at depths of a few metres in a paleochannel filled with rapidly deposited fluvial-colluvial material, have10Be distributions that suggest that the lowland lateritic surface may have formed during an erosive episode, presumably associated with a wetter climate, roughly 300 kyr BP. These results illustrate the practicality and the potential of the use of in-situ-produced cosmogenic nuclides for understanding the history of formation of laterites and for differentiating between systems formed through in-situ chemical weathering and mechanical transport.

Beryllium isotope geochemistry in tropical river basins

Abstract The distributions of beryllium-9 and beryllium-10 in rivers within the Orinoco and Amazon basins have been examined to extend our understanding of their geochemical cycles and to develop their use both in geochronometry, and in studying erosional processes. Beryllium-9, the stable isotope, is weathered from mineral lattices before entering the riverine dissolved or exchangeable phase. The cosmogenic radioisotope 10Be ( t 1 2 = 1.5 Myr ) is produced primarily in the atmosphere and is brought into riverine systems, via rainfall, in dissolved or exchangeable form; it may be used to examine the processes which affect beryllium partitioning between the dissolved and particulate phases. Ancillary data, such as major ion distributions, provide a basis for selecting regions in which riverine chemistry is dominated by a single geochemical process, allowing examination of its effects on Be distributions in isolation. Analyses of 9Be in dissolved and suspended material from rivers with a wide range of chemical compositions indicate that its geochemistry is primarily controlled by two major factors: 1. (1) its abundance in the rocks of the watershed and 2. (2) the extent of its adsorption onto particle surfaces. The relative importance of these parameters in individual rivers is determined by the extent of interaction with flood-plain sediments and the riverine pH. This understanding of 9Be geochemistry forms a basis for examination of the geochemical cycling of 10Be. In rivers which are dominated by interaction with sediments, the riverine concentration of dissolved 10Be is far lower than that in the incoming rainwater, indicating that a substantial proportion of it is retained within the soils of the basin or is adsorbed onto riverine particles. However, in acidic rivers in which the stable dissolved Be concentration is determined by the Be level in the rocks of the drainage basin, dissolved 10Be has essentially the same concentration as in precipitation. These observations imply that the soil column in such regions must be saturated with respect to 10Be, and that the ratio of the inventory to the flux does not represent an age, as may be the case in temperate latitudes, but rather a residence time.

The AMS facility at Gif-sur-Yvette: progress, perturbations and projects

We report on developments at the Gif-sur-Yvette AMS facility since 1990. On the technical side these include: (i) installation of a new dual port injection line based on a rotating electrostatic analyzer designed and constructed by IsoTrace laboratory; (ii) new estimates of 10Be efficiency and background; (iii) the failure of one of the Delrin compression rods on the column, resulting in the rupture of the low energy accelerator tube; (iv) plans (unfortunately not yet fulfilled) to add a second high energy line to permit analysis of ions heavier than mass 30. We also give a brief resume of the research programs currently being carried out using 10Be and 26Al.