Sylvain Gallet

Geochemical characterization of the Luochuan loess-paleosol sequence, China, and paleoclimatic implications

Abstract Chemical (major and trace element) and isotopic (Sr, Nd) analyses of the Luochuan loess-paleosol sequence in China were performed in order to examine the following problems: (1) potential source heterogeneity; (2) element behavior during pedogenesis; (3) paleoclimatic implications; and (4) the use of loess data to determine the average composition of the upper continental crust. New results clearly indicate that Rb, Sr, U and Ce abundances are severely depleted in paleosols relative to their “parental” loess. The behavior of these elements is strongly controlled by the breakdown of primary minerals such as carbonate (Sr) or by alteration processes in highly oxidizing environments (Ce and U). The pattern of element variations through the upper section of the Luochuan sequence mimics the magnetic susceptibility signal both in position and in intensity, thus reinforcing the pedogenesic origin of the enhanced magnetic susceptibility in paleosols. All loess samples display highly uniform REE patterns (except for Ce) characterized by the upper continental crust (UCC) ratios: ( La Yb ) N ≈ 10 and Eu Eu ★ ≈ 0.66. The paleosol REE patterns are similar to those of the loess, with the same LREE/HREE fractionation, but they also show distinct and variable negative Ce anomalies. Nd and Sr isotopic compositions are rather uniform in both loess and paleosols and do not vary with stratigraphic position (ϵNd = −10.5 to −9.2; 87 Sr 86 Sr = 0.715 to 0.719). The isotopic homogeneity in the Luochuan sequence strongly suggests a uniform source region during the entire period of deposition since ∼ 800 ka ago to the present. Several conclusions may be drawn from the present study: 1. (1) No geochemical distinction can be made between loess layers L1 to L7. The sources and the dust storm trajectories must have been essendally the same for the last 800 ka. 2. (2) Some elements, (e.g., Ca, Rb, Sr, U, Ce) are strongly fractionated by pedogenesis between loess and paleosols. Ce mobility in soils is clearly demonstrated but the process responsible for this leaching is still poorly understood. 3. (3) Systematic variations of element abundances and ratios between loess and paleosols can be used as chemical indicators for pedogenesic intensity and so for paleoclimatic change. These chemical indicators serve a function similar to that of oxygen isotopes in deep-sea sediments (pelagic foraminifera), or that of magnetic susceptibility in loess sequences. They are different recorders of paleoclimatic change. 4. (4) The striking uniformity of REE patterns and La Th ratios in the Luochuan loess and paleosols, as well as in loess worldwide, is an excellent starting point for estimating the average composition of the upper continental crust.

Loess geochemistry and its implications for particle origin and composition of the upper continental crust

Abstract Chemical and Nd Sr isotopic compositions of loess samples from Argentina, Europe and Spitsbergen were analyzed to examine the nature of source terrains, the origin of silt-size particles and the suitability of using loess as starting material for estimating the average chemical composition of the upper continental crust. From the relations between Na2O/Al2O3 and K/2O/Al2O3 ratios and CIA values (chemical index of alteration), the loess protoliths must have undergone previous sedimentary differentiation and subjected to moderate chemical weathering. REE patterns are remarkably uniform with (La/Yb)N ≈ 10, which is characteristic of the upper continental crust (UCC). Negative Eu anomalies, expressed in Eu/Eu* ratios, vary from 0.65 for European loess to 0.8 for Pampean loess from Argentina. All loess deposits have nearly constant La/Th or Th/U ratios, which are very similar to those of the average UCC or post-Archean shales. These ratios are not fractionated in size-fractions relative to the whole-rock values. Nd and Sr isotopic compositions clearly distinguish Argentinean loess (87Sr86Sr = 0.706–0.709, eNd(0) = −6to−1.5) from all other loess deposits (87Sr86Sr = 0.712–0.730, eNd(0) = −13to−8). The REE and isotopic results clearly indicate a significant contribution of young Andean volcanics to the Pampean loess deposits, whereas multi-recycled and well-mixed ancient sediments are principal sources for the other deposits. The present results reinforce the earlier conclusion reached by S.R. Taylor, S.M. McLennan and M.T. McCulloch [Geochemistry of loess, continental crustal composition and crustal model ages, Geochim. Cosmochim. Acta 47 (1983) 1897–1905] that the average chemical composition of UCC can be obtained from eolian deposits as well as from fine-grained clastic sediments.

Geochemistry of the Xining, Xifeng and Jixian sections, Loess Plateau of China: eolian dust provenance and paleosol evolution during the last 140 ka

Abstract Loess and paleosol samples from three distant sections (Xining, Xifeng, Jixian) of the Chinese Loess Plateau were analyzed for chemical and Nd–Sr isotopic compositions in order to obtain information about the paleoclimatic variation during the last 140 ka. These three sections represent three different climatic conditions, from arid in the west (Xining) to more humid in the east (Jixian) across the Loess Plateau. Isotopic analyses show that all the three sections display a restricted range of 143 Nd/ 144 Nd ratios (or e Nd ≈−10±0.5), indicating the dominance of relatively young and uniform upper crustal sources for the eolian dust. The loess and paleosol samples have typical upper crustal 87 Sr/ 86 Sr ratios (0.714–0.718), and their variation can partly be attributed to different carbonate/silicate ratios in the samples. REE data from the three sections are indistinguishable from each other; they show typical upper crust patterns characterized by (La/Yb) N ≈10 and Eu/Eu*≈0.65. The high degree of chemical and isotopic homogeneity suggests that the dust source region must have undergone multiple recycling and thorough sedimentary mixing processes. Various chemical indicators have been used to monitor the pedogenetic intensity in these sections. Although results for the Jixian section agree reasonably well with the known climatic conditions on the Loess Plateau, the other two sections yield results not in agreement with some well-accepted proxies like magnetic susceptibility or grain-size distribution. Some chemical proxy indicators established in our previous work on the Luochuan section, such as the systematic U-depletion and negative Ce anomalies in REE patterns in soils, are not clearly recorded in the present case. The discrepancy between different climatic proxies could be related to the effective soil water budget, temperature, as well as the rate of development of each proxy.

Application of U/Th and 40Ar/39Ar dating to Orgnac 3, a Late Acheulean and Early Middle Palaeolithic site in Ardèche, France.

Refined radio-isotopic dating techniques have been applied to Orgnac 3, a Late Acheulean and Early Middle Palaeolithic site in France. Evidence of Levallois core technology appeared in level 4b in the middle of the sequence, became predominant in the upper horizons, and was best represented in uppermost level 1, making the site one of the oldest examples of Levallois technology. In our dating study, fourteen speleothem samples from levels 7, 6 and 5b, were U/Th-dated. Four pure calcite samples from the speleothem PL1 (levels 5b, 6) yield ages between 265 ± 4 (PL1-3) and 312 ± 15 (PL1-6) thousand years ago (ka). Three samples from the top of a second stalagmite, PL2, yield dates ranging from 288 ± 10 ka (PL2-1) to 298 ± 17 ka (PL2-3). Three samples from the base of PL2 (level 7) yield much younger U/Th dates between 267 and 283 ka. These dates show that the speleothems PL1 and PL2 are contemporaneous and formed during marine isotope stage (MIS) 9 and MIS 8. Volcanic minerals in level 2, the upper sequence, were dated by the 40Ar/39Ar method, giving a weighted mean of 302.9 ± 2.5 ka (2σ) and an inverse isochron age of 302.9 ± 5.9 ka (2σ). Both 40Ar/39Ar dating of volcanic sanidines and U/Th dating of relatively pure and dense cave calcites are known to be well established. The first parallel application of the two geochronometers to Orgnac 3 yields generally consistent results, which point to the reliability of the two methods. The difference between their age results is discussed.

New dating evidence of the early presence of hominins in Southern Europe

The first “Out of Africa” migrations represent a seminal event in the history of humankind. At the gates of Europe, the first appearance of Hominins is recorded in Georgia, 1.8 million years ago (Ma); however, the picture of migration across the continent remains incomplete. Vallonnet Cave (France) is a Lower Paleolithic prehistoric site with traces of hominin activities including lithic remains and cut-marks on mammal bones. Here, we apply the uranium-lead (U-Pb) methods to two flowstones to date the intervening archaeological levels. The U-Pb data, coupled with paleomagnetic constraints, provide an age range from 1.2 to 1.1 Ma. The results conclusively demonstrate that Vallonnet Cave is one of the oldest European prehistoric sites in France with early hominin occupations associated with an Epivillafranchian fauna. Combined with data from other archaeological sites, the new precise chronology suggests a widespread occupation the Northern Mediterranean to Southwestern Europe at ~1.2 Ma.

Tectonometamorphic evolution of the Atbashi high-P units (Kyrgyz CAOB, Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction-exhumation of the South Kazakh continental margin

The South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high‐P (HP) rocks along the STS suture, but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (a) a HP tectonic melange including boudins of mafic eclogites in a sedimentary matrix, (b) a large (>100 km long) high‐P metasedimentary unit (HPMU) and (c) a lower blueschist facies accretionary prism. Raman Spectroscopy on carbonaceous material combined with phengite and chlorite multiequilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P–T conditions of 23–25 kbar and 560–570°C along the whole unit. 40Ar/39Ar dating on phengite from the HPMU ranges between 328 and 319 Ma at regional scale. These ages are interpreted as (re‐) crystallization ages of phengite during Tmax conditions at a pressure range of 20–25 kbar. Thermobarometry on samples from the HP tectonic melange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P–T conditions of 5–6 kbar and 290–340°C, highlighting a 17–20 kbar pressure gap between the HPMU‐tectonic melange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340 Ma), HP metamorphic peak (330 Ma), the Tmax (328–319 Ma) and the final exhumation of the HPMU (303–295 Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70–75 km up to 60 km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental‐derived character from the North Tien Shan suggest this unit corresponds to the hyper‐extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (a) N‐dipping subduction below the Kazakh margin of Middle Tien Shan until 390–340 Ma and (b) S‐dipping subduction of remaining Turkestan marginal basins between 340 and 320 Ma.