Age discrimination among eruptives of Menengai Caldera,Kenya, using vegetation parameters from satellite imagery

This paper reports an investigation to determine the degree to which digitally processed Landsat TM imagery can be used to discriminate among vegetated lava flows of different ages in the Menengai Caldera, Kenya. Since Landsat data display vegetation parameters well, and plant communities vary with type and depth of soil development, selective digital processing techniques were applied to take advantage of these characteristics for discriminating relative age differences of the underlying volcanics. A selective series of five images, consisting of a color‐coded Landsat 5 classification and four color composites, were compared with geologic maps. These included a color coded, modified, unsupervised classification and contrast enhanced, color composite images using TM bands 3–2–1, 4–3–2 and 7–5–3, and the first 3 Karhunen‐Loeve transformation axes that had been generated using 7 Landsat TM bands.

The most recent of more than 70 post‐caldera flows within the caldera are trachytes, which are variably covered by shrubs and subsidiary grasses. Soil development evolves as a function of time, and as such, supports a changing plant community. Progressively older flows exhibit the increasing dominance of grasses over bushes. It was found that the Landsat images correlated well with geologic maps, but that the two mapped age classes could be further subdivided on the basis of different vegetation communities. It is concluded that field maps can be modified, and in some cases corrected by use of such imagery, and that digitally enhanced Landsat imagery can be a useful aid to field mapping in similar terrains.     

Millennial-scale variations in hydrography and biogeochemistry in the Eastern Equatorial Pacific over the last 100 kyr

In this study, we use records of nitrogen isotope ratios (δ¹⁵N), UK’37 temperature estimates, organic carbon and opal percentages from high-resolution sediment cores located in the eastern equatorial Pacific (EEP) to explore the mechanisms linking millennial-scale changes in low-latitude sea surface temperature, water column denitrification and surface productivity to the timing of northern or southern polar climate during the last 100,000 yr. Our results support a hypothesis that the Southern Hemisphere, and its connection to the low latitudes via shallow subsurface ocean circulation, has a primary influence on the biogeochemistry of the EEP. In addition, our results suggest that, during the last glacial stage, denitrification rates fluctuated on millennial timescales in response to water-column ventilation rather than upstream oxidant demand in intermediate-depth waters.However, due to the poor age constraints available for Marine Isotopic Stage (MIS) 3, the EEP sedimentary data presented here could support two conflicting mechanisms, one driven by enhanced intermediate overturning circulation in the Southern Ocean during Heinrich Events/Antarctic Warm Events, implying that subsurface flow rates control thermocline ventilation, and a second one consistent with more sluggish intermediate circulation during Antarctic Warm Events and giving a central role to the temperature control on oxygen solubility in Southern Ocean surface waters.     

Heterogeneous accretion,composition and core–mantle differentiation of the Earth

A model of core formation is presented that involves the Earth accreting heterogeneously through a series of impacts with smaller differentiated bodies. Each collision results in the impactor's metallic core reacting with a magma ocean before merging with the Earth's proto-core. The bulk compositions of accreting planetesimals are represented by average solar system abundances of non-volatile elements (i.e. CI-chondritic), with 22% enhancement of refractory elements and oxygen contents that are defined mainly by the Fe metal/FeO silicate ratio. Based on an anhydrous bulk chemistry, the compositions of coexisting core-forming metallic liquid and peridotitic silicate liquid are calculated by mass balance using experimentally-determined metal/silicate partition coefficients for the elements Fe, Si, O, Ni, Co, W, Nb, V, Ta and Cr. Oxygen fugacity is fixed by the partitioning of Fe between metal and silicate and depends on temperature, pressure and the oxygen content of the starting composition. Model parameters are determined by fitting the calculated mantle composition to the primitive mantle composition using least squares minimization. Models that involve homogeneous accretion or single-stage core formation do not provide acceptable fits. In the most successful models, involving 24 impacting bodies, the initial 60–70% (by mass) of the Earth accretes from highly-reduced material with the final 30–40% of accreted mass being more oxidised, which is consistent with results of dynamical accretion simulations. In order to obtain satisfactory fits for Ni, Co and W, it is required that the larger (and later) impactor cores fail to equilibrate completely before merging with the Earth's proto-core, as proposed previously on the basis of Hf-W isotopic studies. Estimated equilibration conditions may be consistent with magma oceans extending to the core–mantle boundary, thus making core formation extremely efficient. The model enables the compositional evolution of the Earth's mantle and core to be predicted throughout the course of accretion. The results are consistent with the late accretion of the Earth's water inventory, possibly with a late veneer after core formation was complete. Finally, the core is predicted to contain ~ 5 wt.% Ni, ~ 8 wt.% Si, ~ 2 wt.% S and ~ 0.5 wt.% O.     

Geomechanical and geochemical effects on sandstones caused by the reaction with supercritical CO2: an experimental approach to in situ conditions in deep geological reservoirs

The geochemical and geomechanical behaviour of reservoir rocks from deep saline aquifers during the injection and geological storage of CO₂ is studied in laboratory experiments. A combination of geochemical and geomechanical studies was carried out on various sandstones from the North German Basin. After the mineralogical, geochemical and petrophysical characterization, a set of sandstone samples was exposed to supercritical (sc)CO₂ and brine for 2–4 weeks in an autoclave system. One sample was mineralogically and geochemically characterised and then loaded in a triaxial cell under in situ pressure and temperature conditions to study the changes of the geomechanical rock properties. After treatment in the autoclaves, geochemical alterations mainly in the carbonate, but also in the sheet silicate cements as well as in single minerals of the sandstones were observed, affecting the rocks granular structure. In addition to partial solution effects during the geochemical experiments, small grains of secondary carbonate and other mineral precipitations were observed within the pore space of the treated sandstones. Results of additional geomechanical experiments with untreated samples show that the rock strength is influenced by the saturation degree, the confining pressure, the pore fluid pressure and temperature. The exposure to pure scCO₂ in the autoclave system induces reduced strength parameters, modified elastic deformation behaviour and changes of the effective porosity in comparison to untreated sandstone samples. Experimental results show that the volume of pore fluid fluxing into the pore space of the sandstones clearly depends on the saturation level of the sample.     

Characterization of SNAP-25 gene from marine teleostean, Lateolabrax japonicus

The t-SNARE protein SNAP-25 (synaptosome-associated protein of 25 kDa) plays an essential role in regulating fusion between the vesicle and plasma membranes during exocytosis. To clone and characterize SNAP-25 gene, the first step in the functional study of SNARE proteins in marine teleostean, was to obtain the cDNA of sea perch SNAP-25 (SPsn25) by RT-PCR and RACE-PCR amplification of a Japanese sea perch. The full-length cDNA of 831bp contains a CDS of 615 bp, coding 204 amino acid residues, and a 5′UTR of 219bp. Bioinformatic analysis revealed that SPsn25 corresponds with SNAP-25a isoform and shares 91.1% identity with SNAP-25a of a goldfish and a zebrafish. The SPsn25 expression in both mRNA and protein levels in the Japanese sea perch had been identified through semi-quantitative RT-PCR and Western Blot assay. Together, these data again confirmed the nerve tissue specificity of the fish SNAP-25 gene expression.     

The end of the Pleistocene: a general critique of chronostratigraphic classification

Formal Chronostratigraphic Units are defined in the International Stratigraphic Guide on the basis of boundary stratotypes and then extended globally as isochronous units. They are unrealistic for application to late-Quaternary history, which involves events that are demonstrably time-transgressive owing to measurable lags between the causative force and the geologic response. Natural geologic time units that are diachronous globally are preferred because they represent events identifiable in the stratigraphic record. A similar objection is raised against formal Chronostratigraphy for the pre-Quaternary: recognizable biostratigraphic or lithostratigraphic units on which the geologic time scale has been built are diachronous globally, for they depend on slow evolutionary or tectonic processes. The imposition of Chronostratigraphic Units that are unrecognizable stratigraphically at a distance away from the type locality introduces an unnecessary terminology in the efforts of stratigraphers to econstruct the geologic history.     

On the asteroidal conductivities as inferred from meteorites

Solar wind interactions with planetary bodies without intrinsic magnetic fields depend to a large extent on the electrical conductivities of the objects in question. If the combined (i.e., ionospheric and interior) electrical conductivities are large, as in the case of Venus, the solar wind interaction is strong due to the generation of a large electrical current flow. It is suggested here that a similar interaction may occur at some asteroids, if their interior conductivity can be approximated by the conductivities of carbonaceous or iron-bearing meteorites. This interaction, in turn, can be used as a tool for remote sensing of the asteroidal interior properties in a spacecraft mission to asteroids.     

Hf-W chronology of the accretion and early evolution of asteroids and terrestrial planets

The ¹⁸²Hf-¹⁸²W systematics of meteoritic and planetary samples provide firm constraints on the chronology of the accretion and earliest evolution of asteroids and terrestrial planets and lead to the following succession and duration of events in the earliest solar system. Formation of Ca,Al-rich inclusions (CAIs) at 4568.3 ± 0.7 Ma was followed by the accretion and differentiation of the parent bodies of some magmatic iron meteorites within less than ∼1 Myr. Chondrules from H chondrites formed 1.7 ± 0.7 Myr after CAIs, about contemporaneously with chondrules from L and LL chondrites as shown by their ²⁶Al-²⁶Mg ages. Some magmatism on the parent bodies of angrites, eucrites, and mesosiderites started as soon as ∼3 Myr after CAI formation and may have continued until ∼10 Myr. A similar timescale is obtained for the high-temperature metamorphic evolution of the H chondrite parent body. Thermal modeling combined with these age constraints reveals that the different thermal histories of meteorite parent bodies primarily reflect their initial abundance of ²⁶Al, which is determined by their accretion age. Impact-related processes were important in the subsequent evolution of asteroids but do not appear to have induced large-scale melting. For instance, Hf-W ages for eucrite metals postdate CAI formation by ∼20 Myr and may reflect impact-triggered thermal metamorphism in the crust of the eucrite parent body. Likewise, the Hf-W systematics of some non-magmatic iron meteorites were modified by impact-related processes but the timing of this event(s) remains poorly constrained.The strong fractionation of lithophile Hf from siderophile W during core formation makes the Hf-W system an ideal chronometer for this major differentiation event. However, for larger planets such as the terrestrial planets the calculated Hf-W ages are particularly sensitive to the occurrence of large impacts, the degree to which impactor cores re-equilibrated with the target mantle during large collisions, and changes in the metal-silicate partition coefficients of W due to changing fO₂ in differentiating planetary bodies. Calculated core formation ages for Mars range from 0 to 20 Myr after CAI formation and currently cannot distinguish between scenarios where Mars formed by runaway growth and where its formation was more protracted. Tungsten model ages for core formation in Earth range from ∼30 Myr to >100 Myr after CAIs and hence do not provide a unique age for the formation of Earth. However, the identical ¹⁸²W/¹⁸⁴W ratios of the lunar and terrestrial mantles provide powerful evidence that the Moon-forming giant impact and the final stage of Earth’s core formation occurred after extinction of ¹⁸²Hf (i.e., more than ∼50 Myr after CAIs), unless the Hf/W ratios of the bulk silicate Moon and Earth are identical to within less than ∼10%. Furthermore, the identical ¹⁸²W/¹⁸⁴W of the lunar and terrestrial mantles is difficult to explain unless either the Moon consists predominantly of terrestrial material or the W in the proto-lunar magma disk isotopically equilibrated with the Earth’s mantle.Hafnium-tungsten chronometry also provides constraints on the duration of magma ocean solidification in terrestrial planets. Variations in the ¹⁸²W/¹⁸⁴W ratios of martian meteorites reflect an early differentiation of the martian mantle during the effective lifetime of ¹⁸²Hf. In contrast, no ¹⁸²W variations exist in the lunar mantle, demonstrating magma ocean solidification later than ∼60 Myr, in agreement with ¹⁴⁷Sm-¹⁴³Nd ages for ferroan anorthosites. The Moon-forming giant impact most likely erased any evidence of a prior differentiation of Earth’s mantle, consistent with a ¹⁴⁶Sm-¹⁴²Nd age of 50-200 Myr for the earliest differentiation of Earth’s mantle. However, the Hf-W chronology of the formation of Earth’s core and the Moon-forming impact is difficult to reconcile with the preservation of ¹⁴⁶Sm-¹⁴²Nd evidence for an early (<30 Myr after CAIs) differentiation of a chondritic Earth’s mantle. Instead, the combined ¹⁸²W-¹⁴²Nd evidence suggests that bulk Earth may have superchondritic Sm/Nd and Hf/W ratios, in which case formation of its core must have terminated more than ∼42 Myr after formation of CAIs, consistent with the Hf-W age for the formation of the Moon.     

An improved crustal magnetic field map of Mars from electron reflectometry: Highland volcano magmatic history and the end of the martian dynamo

We apply improved kinetic modeling of electron transport in the martian thermosphere to fit pitch angle distributions measured by the Mars Global Surveyor (MGS) Magnetometer/Electron Reflectometer (MAG/ER), together with appropriate filtering, binning, averaging and error correction techniques, to create the most reliable ER global map to date of crustal magnetic field magnitude at 185 km altitude, with twice the spatial resolution and considerably higher sensitivity to crustal fields than global maps of magnetic field components produced with MAG data alone. This map compares favorably to sparsely sampled dayside MAG data taken at similar altitudes, insofar as a direct comparison is meaningful. Using this map, we present two case studies. The first compares the magnetic signatures of two highland volcanoes, concluding that the comparatively greater thermal demagnetization at Syrtis Major compared with Tyrrhena Patera is likely due to a higher ratio of intruded to extruded magmas. The second uses the map along with topographic data to compare the magnetic signatures and crater retention ages of the demagnetized Hellas impact basin and magnetized Ladon impact basin. From this comparison, we determine that the martian global dynamo magnetic field went from substantial to very weak or nonexistent in the absolute model age time interval 4.15±0.05 to 4.07±0.05 Ga ago.