Petrologic insights from the spectra of the unbrecciated eucrites: Implications for Vesta and basaltic asteroids

Abstract– We investigate the relationship between the petrology and visible–near infrared spectra of the unbrecciated eucrites and synthetic pyroxene–plagioclase mixtures to determine how spectra obtained by the Dawn mission could distinguish between several models that have been suggested for the petrogenesis of Vesta’s crust (e.g., partial melting and magma ocean). Here, we study the spectra of petrologically characterized unbrecciated eucrites to establish spectral observables, which can be used to yield mineral abundances and compositions consistent with petrologic observations. No information about plagioclase could be extracted from the eucrite spectra. In contrast, pyroxene dominates the spectra of the eucrites and absorption band modeling provides a good estimate of the relative proportions of low‐ and high‐Ca pyroxene present. Cr is a compatible element in eucrite pyroxene and is enriched in samples from primitive melts. An absorption at 0.6 μm resulting from Cr³⁺ in the pyroxene structure can be used to distinguish these primitive eucrites. The spectral differences present among the eucrites may allow Dawn to distinguish between the two main competing models proposed for the petrogenesis of Vesta (magma ocean and partial melting). These models predict different crustal structures and scales of heterogeneity, which can be observed spectrally. The formation of eucrite Allan Hills (ALH) A81001, which is primitive (Cr‐rich) and relatively unmetamorphosed, is hard to explain in the magma ocean model. It could only have been formed as a quench crust. If the magma ocean model is correct, then ALHA81001‐like material should be abundant on the surface of Vesta and the Vestoids.     

A new series of steryl chlorin esters: pheophorbide a steryl esters in an oxic surface sediment

Investigation of chlorins in the oxic surface sediment of a small eutrophic alpine lake (Motte lake) revealed the presence of a new series of steryl chlorin esters containing the pheophorbide a nucleus, together with their pyropheophorbide a steryl ester counterparts previously observed in the anoxic surface sediment of the same lake. Identification of the pheophorbide a steryl esters was based on comparison of spectroscopic, chromatographic and mass spectrometric characteristics of the compounds with those of a synthetic standard and of pyropheophorbide a steryl esters. Combined liquid chromatography-mass spectrometry analysis confirmed the absence of pheophorbide a steryl esters in the anoxic sediment but allowed their detection in traces in the water column, indicating that pheophorbide a steryl esters are, like their pyropheophorbide a analogs, formed in the water column. The distribution of sterols released by hydrolysis of the pheophorbide a steryl esters shows close similarities to that of the free sterols in the water column and of the sterols of the pyropheophorbide a steryl esters. It appears that, like their pyropheophorbide a counterparts, pheophorbide a steryl esters incorporate mainly sterols of phytoplanktonic origin. Their formation probably involves the same mechanism as for pyropheophorbide a steryl ester formation, i.e. metabolism by zooplankton grazing on phytoplankton. The presence of pheophorbide a steryl esters in the oxic sediment and their absence from the anoxic sediment is probably due to a lower stability of compounds containing a carbomethoxy substituent in the anoxic environment.     

History of scoria-cone eruptions on the eastern shoulder of the Kenya–Tanzania Rift revealed in the 250-ka sediment record of Lake Chala near Mount Kilimanjaro

Reconstructions of the timing and frequency of past eruptions are important to assess the propensity for future volcanic activity, yet in volcanic areas such as the East African Rift only piecemeal eruption histories exist. Understanding the volcanic history of scoria-cone fields, where eruptions are often infrequent and deposits strongly weathered, is particularly challenging. Here we reconstruct a history of volcanism from scoria cones situated along the eastern shoulders of the Kenya–Tanzania Rift, using a sequence of tephra (volcanic ash) layers preserved in the ~250-ka sediment record of Lake Chala near Mount Kilimanjaro. Seven visible and two non-visible (crypto-) tephra layers in the Lake Chala sequence are attributed to activity from the Mt Kilimanjaro (northern Tanzania) and the Chyulu Hills (southern Kenya) volcanic fields, on the basis of their glass chemistry, textural characteristics and known eruption chronology. The Lake Chala record of eruptions from scoria cones in the Chyulu Hills volcanic field confirms geological and historical evidence of its recent activity, and provides first-order age estimates for seven previously unknown eruptions. Long and well-resolved sedimentary records such as that of Lake Chala have significant potential for resolving regional eruption chronologies spanning hundreds of thousands of years.     

Tunicate species as marine pollution indicators

Tunicates may serve as marine pollution indicators for monitoring the release of industrial and/or radioactive wastes into the marine environment. Their ability to accumulate certain trace elements from the seawater can be employed in order to define a suitable organism as indicator for some specific pollutants. In this work the concentration of the elements cobalt, zinc, chromium, caesium, silver, iron, rubidium, selenium, scandium and antimony has been determined by instrumental neutron activation analysis and γ-ray spectroscopy, in two tunicates, Microcosmus sulcatus (Pyuridae) and Ciona intestinalis (Cionidae).According to the concentration factors found for these species Ciona intestinalis can be used as indicator for iron, while Microcosmus sulcatus as indicator for the elements selenium, chromium, zinc and cobalt.     

The Eltanin asteroid impact: possible South Pacific palaeomegatsunami footprint and potential implications for the Pliocene–Pleistocene transition

Large asteroid impacts are rare, and those into the deep ocean are rarer still. The Eltanin asteroid impact around 2.51 ± 0.07 Ma occurred at a time of great climatic and geological change associated with the Pliocene–Pleistocene boundary. Numerical models of the event indicate that a megatsunami was generated, although there is debate concerning its magnitude and the region‐wide extent of its influence. We summarise the existing evidence for possible Eltanin megatsunami deposits in Antarctica, Chile and New Zealand, while also examining other potential sites from several locations, mainly around the South Pacific region. In reviewing these data we note that these events were unfolding at the same time as those associated with the Pliocene–Pleistocene boundary and, as such, most of the geological evidence from that time has a climatic interpretation. The potential climatic and geological ramifications of the Eltanin asteroid impact, however, have failed to be considered by most researchers studying this time period. Although we are not advocating that all geological activity at that time is connected with the Eltanin asteroid impact, it raises interesting questions about the role potentially played by such catastrophic events in contributing to or even triggering epochal transitions. Copyright © 2012 John Wiley & Sons, Ltd.     

Photochemical degradation of phenanthrene as a function of natural water variables modeling freshwater to marine environments

Photolysis rates of phenanthrene as a function of ionic strength (salinity), oxygen levels and humic acid concentrations were measured in aqueous solution over the range of conditions found in fresh to marine waters. Photolysis followed first order kinetics, with an estimated photodegradation half-life in sunlight in pure water of 10.3±0.7h, in the mid-range of published results. Photolysis rate constants decreased by a factor of 5 in solutions with humic acid concentrations from 0 to 10 mg C L(-1). This decrease could be modeled entirely based on competitive light absorption effects due to the added humics. No significant ionic strength or oxygen effects were observed, consistent with a direct photolysis mechanism. In the absence of significant solution medium effects, the photodegradation lifetime of phenanthrene will depend only on solar fluxes (i.e. temporal and seasonal changes in sunlight) and not vary with a freshwater to marine environment.     

Earth’s Electromagnetic Environment

The natural spectrum of electromagnetic variations surrounding Earth extends across an enormous frequency range and is controlled by diverse physical processes. Electromagnetic (EM) induction studies make use of external field variations with frequencies ranging from the solar cycle which has been used for geomagnetic depth sounding through the 10\(^{-4}\)–10\(^4\) Hz frequency band widely used for magnetotelluric and audio-magnetotelluric studies. Above 10\(^4\) Hz, the EM spectrum is dominated by man-made signals. This review emphasizes electromagnetic sources at \(\sim\)1 Hz and higher, describing major differences in physical origin and structure of short- and long-period signals. The essential role of Earth’s internal magnetic field in defining the magnetosphere through its interactions with the solar wind and interplanetary magnetic field is briefly outlined. At its lower boundary, the magnetosphere is engaged in two-way interactions with the underlying ionosphere and neutral atmosphere. Extremely low-frequency (3 Hz–3 kHz) electromagnetic signals are generated in the form of sferics, lightning, and whistlers which can extend to frequencies as high as the VLF range (3–30 kHz).The roughly spherical dielectric cavity bounded by the ground and the ionosphere produces the Schumann resonance at around 8 Hz and its harmonics. A transverse resonance also occurs at 1.7–2.0 kHz arising from reflection off the variable height lower boundary of the ionosphere and exhibiting line splitting due to three-dimensional structure. Ground and satellite observations are discussed in the light of their contributions to understanding the global electric circuit and for EM induction studies.     

Verification of short-term runoff forecasts for a small Philippine basin (Marikina)

Storm runoff from the Marikina River Basin frequently causes flood events in the Philippine capital region Metro Manila. This paper presents and evaluates a system to predict short-term runoff from the upper part of that basin (380 km²). It was designed as a possible component of an operational warning system yet to be installed. For the purpose of forecast verification, hindcasts of streamflow were generated for a period of 15 months with a time-continuous, conceptual hydrological model. The latter was fed with real-time observations of rainfall. Both ground observations and weather radar data were tested as rainfall forcings. The radar-based precipitation estimates clearly outperformed the raingauge-based estimates in the hydrological verification. Nevertheless, the quality of the deterministic short-term runoff forecasts was found to be limited. For the radar-based predictions, the reduction of variance for lead times of 1, 2 and 3 hours was 0.61, 0.62 and 0.54, respectively, with reference to a “no-forecast” scenario, i.e. persistence. The probability of detection for major increases in streamflow was typically less than 0.5. Given the significance of flood events in the Marikina Basin, more effort needs to be put into the reduction of forecast errors and the quantification of remaining uncertainties.     

Electrical conductivity of orthopyroxene and plagioclase in the lower crust

The electrical conductivities of lower crustal orthopyroxene and plagioclase, as well as their dependence on water content, were measured at 6–12 kbar and 300–1,000°C on both natural and pre-annealed samples prepared from fresh mafic xenolith granulites. The complex impedance was determined in an end-loaded piston cylinder apparatus by a Solarton-1260 Impedance/Gain Phase analyzer in the frequency range of 0.1–10⁶ Hz. The spectra usually show an arc over the whole frequency range at low temperature and an arc plus a tail in the high and low frequency range, respectively, at high temperature. The arc is due to conduction in the sample interior, while the tails are probably due to electrode effects. Different conduction mechanisms have been identified under dry and hydrous conditions. For the dry orthopyroxene, the activation enthalpy is ~105 kJ/mol, and the conduction is likely due to small polarons, e.g., electrons hopping between Fe²⁺ and Fe³⁺. For the dry plagioclase, the activation enthalpy is ~161 kJ/mol, and the conduction may be related to the mobility of Na⁺. For the hydrous samples, the activation enthalpy is ~81 kJ/mol for orthopyroxene and ~77 kJ/mol for plagioclase, and the electrical conductivity is markedly enhanced, probably due to proton conduction. For each mineral, the conductivity increases with increasing water content, with an exponent of ~1, and the activation enthalpies are nearly independent of water content. Combining these data with our previous work on the conductivity of lower crustal clinopyroxene, the bulk conductivity of lower crustal granulites is modeled, which is usually >~10⁻⁴ S/m in the range of 600–1,000°C. We suggest that the high electrical conductivity in most regions of the lower crust, especially where it consists mostly of granulites, can be explained by the main constitutive minerals, particularly if they contain some water. Contributions from other highly conducting materials such as hydrous fluids, melts, or graphite films are not strictly necessary to explain the observed conductivities.