Desert varnish and petroglyphs on sandstone – Geochemical composition and climate changes from Pleistocene to Holocene (Libya)

Desert varnish of pristine sandstone and petroglyph surfaces from Takabart Kabort (Naturalistic Bubaline Art School) and Alamas (Tanzina Art School) can be well classified by their (SiO₂+Al₂O₃):MnO₂, Al₂O₃:SiO₂, and P₂O₅:CaO ratios. Specific ratios are due to the occurrence of clay minerals like illite, kaolinite, smectite, and feldspar, quartz, carbonates like calcite and dolomite, manganese oxyhydroxides, and apatite. Their occurrence corresponds to the local origin and composition of the primary aeolian material.

In general, the analyzed desert varnish shows lamination patterns characterized by alternating MnO₂-rich and -poor layers (25 wt% MnO₂) at rather constant iron oxyhydroxide content (6 wt% Fe₂O₃). Varnish on non-engraved surfaces exhibits three MnO₂-rich layers, whereas varnish-coated petroglyphs reveal minor lamination patterns corresponding to the dating of petroglyphs by rock art. The older Naturalistic Bubaline Art School petroglyphs (about 6–4 ka BP) and the younger Tazina Art School petroglyphs (about 3.8–3 ka BP) contain only two and one MnO₂-rich layer, respectively. It is assumed that the occurrence of such microlaminations is caused by climate changes in North Africa. Three humid periods are discerned from the Terminal Pleistocene to Holocene in the literature. Such periods are suitable to induce manganese accumulation by biotic and abiotic processes. Accordingly, the distinct lamination patterns gained from this study verify the dating of petroglyphs by rock art. From another point of view, classification of the above petroglyphs may be provided by analyses of microlaminations independently on cultural historical aspects.     

Changing compositions in the Iceland plume; Isotopic and elemental constraints from the Paleogene Faroe flood basalts

Elemental and Sr, Nd, Hf and high precision Pb isotopic data are presented from 59 low-Ti and high-Ti lavas from the syn-break up part of the Faroe Flood Basalt Province. The depleted MORB-like low-Ti lavas erupted in the rift zone between the Faroe Islands and central East Greenland around the time of break up of the North Atlantic have isotopic end-member compositions different from the depleted Iceland lavas. We suggest that the main low-Ti mantle component is NAEM (North Atlantic End-Member (Ellam and Stuart, 2000, J. Petrol. 41, 919) and that the ²⁰⁷Pb/²⁰⁴Pb value of the component should be 15.35 and ε_Hf = + 16.5. NAEM is the main depleted component in the early Iceland plume. This is supported by high mantle potential temperatures (up to 1550 °C) calculated for the source of the low-Ti basalts. The unique mantle isotopic composition of NAEM with low ²⁰⁶Pb/²⁰⁴Pb (17.5) and Δ7/4Pb (? 3.8) precludes a derivation from recycled MORB lithosphere. Instead we suggest that NAEM represents a plume component of recycled depleted Archean lithospheric mantle that was further depleted ~ 500 Ma ago, possibly in connection with the recycling process. Two other isotopic end-members are required to explain the variation of the Faroe low-Ti basalts: (1) The Faroe depleted component (FDC), with ⁸⁷Sr/⁸⁶Sr = 0.7025, ε_Nd = + 11, ε_Hf = + 19.5, ²⁰⁶Pb/²⁰⁴Pb = 18.2, ²⁰⁷Pb/²⁰⁴Pb = 15.454 and ²⁰⁸Pb/²⁰⁴Pb = 37.75, which is similar in composition to some Atlantic MORB and is regarded as a local upper mantle source. (2) An enriched EM-type component similar in geochemistry to the Icelandic Öræfajökull lavas. This component is believed to be recycled pelagic sediments in the plume but it can alternatively be a local crustal or lithospheric mantle component. The enriched Faroe high-Ti lavas erupted inland from the rift have isotopic compositions very similar to the enriched Icelandic neo-volcanics and these lava suites apparently share the two enriched plume end-members IE1 and IE2 (Geochim. Cosmochim. Acta 68, 2, 2004). The lack of mixing between high and low-Ti melts at the time of break up, is explained by a zoned plume where only low-Ti sources were present beneath the rift zone surrounded by high-Ti sources on both sides of the rift. The enriched plume components in the high-Ti lava sequences on the Faroe Islands and central East Greenland changed rapidly on a ka-scale which implies, from geophysical modelling, that this area was positioned above the center of the plume, and that the Iceland plume was centered under the Atlantic ridge already from the Paleocene.     

A study of the chemistry of pore fluids and authigenic carbonates in methane seep environments: Kodiak Trench, Hydrate Ridge, Monterey Bay, and Eel River Basin

Analyses of the chemical and isotopic composition of carbonates rocks recovered from methane seepage areas of the Kodiak Trench, Hydrate Ridge, Monterey Bay Clam Flats, and the Eel River Basin, coupled with the studies of the chemistry of the pore fluids, have shown that these carbonates have grown within the sediment column. Geochemical profiles of pore fluids show that, in deep water seeps (Kodiak Trench—4450 m; Monterey Bay—1000 m; Hydrate Ridge—650 m), δ¹³C (DIC) values are low (isotopically light), whereas in the Eel River area ( 350–500 m), δ¹³C (DIC) values are much higher (isotopically heavier). In all cases, the δ¹³C values indicate that processes of methane oxidation, associated with sulfate reduction, are dominant in the shallow sediments. Data on the isotopic composition of authigenic carbonates found at sites in Kodiak Trench, Eel River Basin South, and Eel River Basin North indicate a variable composition and origin in different geochemical environments. Some of the authigenic carbonates from the study sites show a trend in their δ¹³C values similar to those of the pore fluids obtained in their vicinity, suggesting formation at relatively shallow depths, but others indicate formation at greater sediment depths. The latter usually consist of high magnesium calcite or dolomite, which, from their high values of δ¹³C (up to 23‰;) and δ¹⁸O (up to 7.5‰), suggest formation in the deeper horizons of the sediments, in the zone of methanogenesis. These observations are in agreement with observations by other workers at Hydrate Ridge, in Monterey Bay, and in the Eel River Basin.     

Natural tracer profiles across argillaceous formations

Argillaceous formations generally act as aquitards because of their low hydraulic conductivities. This property, together with the large retention capacity of clays for cationic contaminants, has brought argillaceous formations into focus as potential host rocks for the geological disposal of radioactive and other waste. In several countries, programmes are under way to characterise the detailed transport properties of such formations at depth. In this context, the interpretation of profiles of natural tracers in pore waters across the formations can give valuable information about the large-scale and long-term transport behaviour of these formations. Here, tracer-profile data, obtained by various methods of pore-water extraction for nine sites in central Europe, are compiled. Data at each site comprise some or all of the conservative tracers: anions (Cl⁻, Br⁻), water isotopes (δ¹⁸O, δ²H) and noble gases (mainly He). Based on a careful evaluation of the palaeo-hydrogeological evolution at each site, model scenarios are derived for initial and boundary pore-water compositions and an attempt is made to numerically reproduce the observed tracer distributions in a consistent way for all tracers and sites, using transport parameters derived from laboratory or in situ tests. The comprehensive results from this project have been reported in Mazurek et al. (2009). Here the results for three sites are presented in detail, but the conclusions are based on model interpretations of the entire data set. In essentially all cases, the shapes of the profiles can be explained by diffusion acting as the dominant transport process over periods of several thousands to several millions of years and at the length scales of the profiles. Transport by advection has a negligible influence on the observed profiles at most sites, as can be shown by estimating the maximum advection velocities that still give acceptable fits of the model with the data. The advantages and disadvantages of different conservative tracers are also assessed. The anion Cl⁻ is well suited as a natural tracer in aquitards, because its concentration varies considerably in environmental waters. It can easily be measured, although the uncertainty regarding the fraction of the pore space that is accessible to anions in clays remains an issue. The stable water isotopes are also well suited, but they are more difficult to measure and their values generally exhibit a smaller relative range of variation. Chlorine isotopes (δ³⁷Cl) and He are more difficult to interpret because initial and boundary conditions cannot easily be constrained by independent evidence. It is also shown that the existence of perturbing events such as the activation of aquifers due to uplift and erosion, leading to relatively sharp changes of boundary conditions, can be considered as a pre-requisite to obtain well-interpretable tracer signatures. On the other hand, gradual changes of boundary conditions are more difficult to parameterise and so may preclude a clear interpretation.     

P-T data of Ky-St-Grt gneisses hosting HP amphibolites and eclogites from the Austroalpine Polinik complex, Kreuzeck Mountains, Eastern Alps, Austria

This study focuses on metapelites of the Polinik complex in the Kreuzeck Mts. southeast of the Tauern Window, Eastern Alps, where kyanite — staurolite — garnet gneisses host eclogites and high pressure (HP) amphibolites of the Austroalpine basement. The stable mineral assemblage is garnet — staurolite — biotite — kyanite — quartz. Estimated metamorphic conditions from conventional geothermobarometry are 654±30 °C and 0.9±0.08 GPa, and Average P-T values calculated by THERMOCALC, are 665±15 °C at 0.77±0.09 GPa. Formation of the present mineral association in gneisses is related to the exhumation (D2) stage of hosted eclogites/HP amphibolites within a lateral strike-slip zone.     

Iberanaia iberica: The first record of the Trigonioidoidea Bivalvia: Unionoida from the Lower Cretaceous of Teruel, Spain

The first record of the superfamily Trigonioidoidea (Order: Unionoida) from the Lower Cretaceous of Teruel, Spain is described and compared to known taxa from Europe and Asia. This is a new record of the Trigonioidoidea from Europe, following material described from the Barremian of the Isle of Wight (southern England) and Aptian-Barremian from Iberian Range (Spain). The new material, recorded from the Albian of the Escucha Formation (“Barriada” Member) in Utrillas (Teruel, northeastern of Spain), comprises four specimens. Analysis of the main morphological characters including anterior musculature, dentition, and ornamentation indicates that this material constitutes a new genus and species, Iberanaia iberica. The palaeoenvironment in which the bivalves lived corresponds to a fluvial-palustral setting connected to a deltaic system.     

H-bonding scheme and cation partitioning in axinite: a single-crystal neutron diffraction and Mössbauer spectroscopic study

The crystal chemistry of a ferroaxinite from Colebrook Hill, Rosebery district, Tasmania, Australia, was investigated by electron microprobe analysis in wavelength-dispersive mode, inductively coupled plasma–atomic emission spectroscopy (ICP–AES), ⁵⁷Fe Mössbauer spectroscopy and single-crystal neutron diffraction at 293 K. The chemical formula obtained on the basis of the ICP–AES data is the following: \( ^{X1,X2} {\text{Ca}}_{4.03} \,^{Y} \left( {{\text{Mn}}_{0.42} {\text{Mg}}_{0.23} {\text{Fe}}^{2 + }_{1.39} } \right)_{\varSigma 2.04} \,^{Z1,Z2} \left( {{\text{Fe}}^{3 + }_{0.15} {\text{Al}}_{3.55} {\text{Ti}}_{0.12} } \right)_{\varSigma 3.82} \,^{T1,T2,T3,T4} \left( {{\text{Ti}}_{0.03} {\text{Si}}_{7.97} } \right)_{\varSigma 8} \,^{T5} {\text{B}}_{1.96} {\text{O}}_{30} \left( {\text{OH}} \right)_{2.18} \). The ⁵⁷Fe Mössbauer spectrum shows unambiguously the occurrence of Fe²⁺ and Fe³⁺ in octahedral coordination only, with Fe²⁺/Fe³⁺ = 9:1. The neutron structure refinement provides a structure model in general agreement with the previous experimental findings: the tetrahedral T1, T2, T3 and T4 sites are fully occupied by Si, whereas the T5 site is fully occupied by B, with no evidence of Si at the T5, or Al or Fe³⁺ at the T1–T5 sites. The structural and chemical data of this study suggest that the amount of B in ferroaxinite is that expected from the ideal stoichiometry: 2 a.p.f.u. (for 32 O). The atomic distribution among the X1, X2, Y, Z1 and Z2 sites obtained by neutron structure refinement is in good agreement with that based on the ICP–AES data. For the first time, an unambiguous localization of the H site is obtained, which forms a hydroxyl group with the oxygen atom at the O16 site as donor. The H-bonding scheme in axinite structure is now fully described: the O16–H distance (corrected for riding motion effect) is 0.991(1) Å and an asymmetric bifurcated bonding configuration occurs, with O5 and O13 as acceptors [i.e. with O16···O5 = 3.096(1) Å, H···O5 = 2.450(1) Å and O16–H···O5 = 123.9(1)°; O16···O13 = 2.777(1) Å, H···O13 = 1.914(1) Å and O16–H···O13 = 146.9(1)°].     

山东莒南后左山恐龙公园早白垩世恐龙足迹化石初步研究

山东省莒南县后左山恐龙公园内保存了数量众多、形态各异的恐龙足迹化石。该足迹化石群产于早白垩世大盛群田家楼组,主要是兽脚类、鸟脚类恐龙的足迹,尤以前者占优质。最大的一个为椭圆形,大小100cm×70cm,可能系鸟脚类恐龙的足迹。鸟脚类足迹大多为椭圆形,长轴一般30～40cm,常呈幻迹保存。最大的兽脚类足迹为三趾型,长49cm,宽35cm,是大型肉食龙类的足迹;最小的一个长8.5cm,宽4.5cm,可能是虚骨龙类的足迹。另外还发现了少量可能是驰龙的足迹,具有2个很长的趾(Ⅲ和Ⅳ趾),趾间角极小,二者几乎平行。鸟脚类恐龙足迹层位分布局限,但密度较大,产出的层面常发育泥裂、波痕,甚至冰晶痕,与泥岩、粉砂质泥岩等层位相关性较强。     

First Report of Avian Tracks from the Cretaceous of Tibet, China

<正>Objective Despite a large number of bird fossils in the Jehol Biota,Mesozoic bird records from other parts of China are dominated by tracks.Late Cretaceous bird tracks are rarely found in China.Reliable reports so far include a Dongyangornipes and Koreanaornis assemblage of early Late Cretaceous age from Dongyang City,Zhejiang Province,China(Azuma et al.,2013).Buckley et al.     

Low-temperature heat capacity of magnesioferrite (MgFe2O4)

The low-temperature heat capacity of magnesioferrite (MgFe₂O₄) was measured between 1.5 K and 300 K, and thermochemical functions were derived from the results. No heat capacity anomaly was observed. From our data, we suggest a standard entropy (298.15 K) for magnesioferrite of 120.8±0.6 J mol⁻¹ K⁻¹, which is about 2.4 J mol⁻¹ K⁻¹ higher than previously reported calorimetric studies; but is in rough agreement with predictions from sets of internally consistent thermodynamic data.