Crystallographic and magnetic preferred orientation of hematite in banded iron ores

The crystallographic preferred orientation of hematite in banded iron ores and the orientation of both the measured and the calculated principal susceptibility axes are strongly related. The maximum susceptibility is aligned with the lineation and the pole of the foliation coincides with the minimum susceptibility, although there are often distinct differences between the measured and calculated values of the susceptibilities. A wide variety of configurations of c-axis pole figures modeled by varying the parameters of the Bingham distribution and Bingham–Mardia-distribution reveal that quite different c-axis patterns of hematite ores may have the same anisotropy of the magnetic susceptibility (AMS) parameters. Large deviations between calculated and experimental AMS-data should initiate further investigations to resolve a probably unnoticed heterogeneity of the fabric. The present investigations show that the structural analysis of the preferred orientation of hematite ores by means of the rather inexpensive and fast magnetic method must be accompanied by the more expensive but unambiguous determination of preferred orientation by x-ray and neutron diffraction experiments in order to accomplish a complete and sound interpretation.     

Is the “astrometric” plate reduction method rigorous?

It is shown that there is no intrinsic difference between the so-called “astrometric” and “photogrammetric” methods of reducing star plates, and that either, when applied by competent investigators, will yield equivalent results.     

Prebiotic organic synthesis in early Earth and Mars atmospheres: Laboratory experiments with quantitative determination of products formed in a cold plasma flow reactor

The goal of this study was to explore prebiotic chemistry in a range of plausible early Earth and Mars atmospheres. To achieve this laboratory continuous flow plasma irradiation experiments were performed on N₂/H₂/CO/CO₂ gas mixtures chosen to represent mildly reducing early Earth and Mars atmospheres derived from a secondary volcanic outgassing of volatiles in chemical equilibrium with magmas near present day oxidation state. Under mildly reducing conditions (91.79% N₂, 5.89% H₂, 2.21% CO, and 0.11% CO₂), simple nitriles are produced in the gas phase with yield (G in molecules per 100 eV), for the key prebiotic marker molecule HCN at G∼1×¹⁰⁻³ (0.1 nmol J⁻¹). In this atmosphere localized HCN concentrations possibly could approach the 10⁻² M needed for HCN oligomerization. Yields under mildly oxidizing conditions (45.5% N₂, 0.1% H₂, 27.2% CO, 27.2% CO₂) are significantly less as expected, with HCN at G∼3×¹⁰⁻⁵ (). Yields in a Triton atmosphere which can be plausibly extrapolated to represent what might be produced in trace CH₄ conditions (99.9% N₂, 0.1% CH₄) are significant with HCN at G∼1×¹⁰⁻² (1 nmol J⁻¹) and tholins produced. Recently higher methane abundance atmospheres have been examined for their greenhouse warming potential, and higher abundance hydrogen atmospheres have been proposed based on a low early Earth exosphere temperature. A reducing (64.04% N₂, 28.8% H₂, 3.60% CO₂, and 3.56% CH₄), representing a high CH₄ and H₂ abundance early Earth atmosphere had HCN yields of G∼5×¹⁰⁻³ (0.5 nmol J⁻¹). Tholins generated in high methane hydrogen gas mixtures is much less than in a similar mixture without hydrogen. The same mixture with the oxidizing component CO₂ removed (66.43% N₂, 29.88% H₂, 0% CO₂, and 3.69% CH₄) had HCN yields of G∼1×¹⁰⁻³ (0.1 nmol J⁻¹) but more significant tholin yields.     

Reconstruction of the differentiated long-term exhumation history of Fuerteventura, Canary Islands, Spain, through fission-track and (U-Th–Sm)/He data

Miocene Intrusives and Lower Cretaceous siliciclastic sedimentary rocks from the Basal Complex in western-Fuerteventura were analyzed with low-temperature thermochronometric methods such as fission-track, and (U–Th–Sm)/He dating, in order to reveal the evolution of the island’s exhumation history. The obtained thermochronometric data yields a very slow rate of cooling in the order of 1.5–3°C/Myr from ~50 to 20 Ma for the Early Cretaceous siliciclastic rocks. These sedimentary units have never been heated significantly above 240°C after deposition and still record the submarine onset of the island’s formation in the Eocene. Intrusive bodies associated with the early Miocene magmatic activity of the central volcanic complex of the island show rapid initial cooling rates of 50–70°C/Myr from ~20 to 14 Ma. Contemporaneous with the intrusions the cooling rate of the Cretaceous sedimentary units increased to 25–35°C/Myr and it is inferred that this increase is associated with enhanced uplift and erosion of the Central Volcanic Complex. After ~14 Ma rates slowed down to 3–6°C/Myr. Palaeosols overlying the sedimentary units are themselves covered by Pliocene basalt flows and reveal that the sedimentary rocks reached the surface before ~5 Ma. The thermochronometric data obtained in this study for central Fuerteventura is difficult to reconcile with the cooling history derived from previously obtained fission-track and K–Ar data from the north-western part of the island. This inconsistency is likely to indicate that the exhumation history of Fuerteventura is more complex and regionally subdivided than previously believed.     

Stable isotope variation in tooth enamel from Neogene hippopotamids: monitor of meso and global climate and rift dynamics on the Albertine Rift, Uganda

The Neogene was a period of long-term global cooling and increasing climatic variability. Variations in African-Asian monsoon intensity over the last 7 Ma have been deduced from patterns of eolian dust export into the Indian Ocean and Mediterranean Sea as well as from lake level records in the East African Rift System (EARS). However, lake systems not only depend on rainfall patterns, but also on the size and physiography of river catchment areas. This study is based on stable isotope proxy data (¹⁸O/¹⁶O, ¹³C/¹²C) from tooth enamel of hippopotamids (Mammalia) and aims in unravelling long-term climate and watershed dynamics that control the evolution of palaeolake systems in the western branch of the EARS (Lake Albert, Uganda) during the Late Neogene (7.5 Ma to recent). Having no dietary preferences with respect to wooded (C₃) versus grassland (C₄) vegetation, these territorial, water-dependant mammals are particularly useful for palaeoclimate analyses. As inhabitants of lakes and rivers, hippopotamid tooth enamel isotope data document mesoclimates of topographic depressions, such as the rift valleys and, therefore, changes in relative valley depth instead of exclusively global climate changes. Consequently, we ascribe a synchronous maximum in ¹⁸O/¹⁶O and ¹³C/¹²C composition of hippopotamid enamel centred around 1.5–2.5 Ma to maximum aridity and/or maximum hydrological isolation of the rift floor from rift-external river catchment areas in response to the combined effects of rift shoulder uplift and subsidence of the rift valley floor. Structural rearrangements by ~2.5 Ma within the northern segment of the Albertine Rift are well constrained by reversals in river flow, cannibalisation of catchments, biogeographic turnover and uplift of the Rwenzori horst. However, a growing rain shadow is not obvious in ¹⁸O/¹⁶O signatures of the hippopotamid teeth of the Albertine Rift. According to our interpretation, this is the result of the overriding effect of evaporation on ¹⁸O/¹⁶O responding to aridification of the basin floor by a valley air circulation system through relative deepening of the valley. On the other hand, a synchronous arid pulse is not so clearly recorded in palaeosol data and mammalian fauna of the eastern branch of the EARS. This discrepancy indicates that rift mesoclimates may represent an underestimated aspect in previous palaeoclimate reconstructions from rift valley data and represent a clear limitation to attempts at global climate reconstructions. The results of this study also suggest that using ¹⁸O/¹⁶O data as a proxy to rain shadow evolution must take into account relative basin subsidence to properly document mountain range uplift.     

Why the atmosphere became oxygenated: A proposal

The increase in the oxidation state of the atmosphere during Earth history is well documented, but not the reasons for this process. The course of atmospheric evolution has been dominated by the effects of photosynthesis and by the composition of volcanic gases. The H₂/H₂O ratio of volcanic gases has remained essentially constant during most of Earth history. It is here proposed that their CO₂/H₂O and SO₂/H₂O ratios have increased with time. A simple model for the degassing and the recycling of CO₂, SO₂, and H₂O can account for many of the major steps in the oxygenation of the atmosphere.     

Darriwilian (Middle Ordovician) graptolite faunas of the Sandia Region,southern Peru

Ordovician graptolite faunas of Peru are restricted to a short interval in the Middle to basal Upper Ordovician, found in three regions of the country. All Peruvian graptolite faunas are strongly dominated by shallow water elements of the Atlantic Faunal Realm, represented largely by Didymograptus s. str. and Aulograptus, but a number of faunal elements of the pandemic isograptid biofacies have recently been discovered in the Sandia Region of SE Peru. Peruvian graptolite faunas are reviewed and the new records from the Sandia Region are discussed in detail. The faunas from the Purumpata and Iparo members of the San José Formation range in age approximately from the Undulograptus austrodentatus Biozone to the Holmograptus lentus Biozone (early to middle Darriwilian). The faunas provide a better understanding of faunal composition and diversity in this region and help to correlate shallow water and deeper water graptolite faunas from this time interval. Biserial graptolites are rare in most samples and usually indeterminable, but a single identifiable specimen of Undulograptus austrodentatus was found, indicating a level close to the base of the Darriwilian. A number of specimens of the genera Isograptus and Arienigraptus from the Sandia Region represent pandemic graptolite faunas of the isograptid biofacies, described for the first time from this region. Copyright © 2009 John Wiley & Sons, Ltd.     

Noble gases in fossil micrometeorites and meteorites from 470 Myr old sediments from southern Sweden,and new evidence for the L‐chondrite parent body breakup event

Abstract— We present noble gas analyses of sediment‐dispersed extraterrestrial chromite grains recovered from ?470 Myr old sediments from two quarries (Hällekis and Thorsberg) and of relict chromites in a coeval fossil meteorite from the Gullhögen quarry, all located in southern Sweden. Both the sediment‐dispersed grains and the meteorite Gullhögen 001 were generated in the L‐chondrite parent body breakup about 470 Myr ago, which was also the event responsible for the abundant fossil meteorites previously found in the Thorsberg quarry. Trapped solar noble gases in the sediment‐dispersed chromite grains have partly been retained during ?470 Myr of terrestrial residence and despite harsh chemical treatment in the laboratory. This shows that chromite is highly retentive for solar noble gases. The solar noble gases imply that a sizeable fraction of the sediment‐dispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites. The grains in the oldest sediment beds were rapidly delivered to Earth likely by direct injection into an orbital resonance in the inner asteroid belt, whereas grains in younger sediments arrived by orbital decay due to Poynting‐Robertson (P‐R) drag. The fossil meteorite Gullhögen 001 has a low cosmic‐ray exposure age of ?0.9 Myr, based on new He and Ne production rates in chromite determined experimentally. This age is comparable to the ages of the fossil meteorites from Thorsberg, providing additional evidence for very rapid transfer times of material after the L‐chondrite parent body breakup.