Varnish microlaminations: new insights from focused ion beam preparation

The cross-sectional texture of rock varnish varies considerably with the scale of analysis and technique used to image a sample. Each jump in resolution results in new insight, with the current state-of-the-art resting at the nanoscale. One key to nanoscale analysis involves focused ion beam (FIB) techniques used most frequently in material science and semiconductor failure analysis. FIB preparation remains challenging, however, for samples like rock coatings with heterogeneous density and abundant porosity. A new technique involving multiangle ion thinning and in situ plan-view lift-out facilitated a scanning transmission electron microscopy study of rock varnish from Death Valley. The results reveal variability in lateral continuity of nanometer microlaminae that can be interrupted by post-depositional diagenesis involving leaching of Mn and Fe, and this variability could explain why some of the visual varnish microlaminations (VML) used in paleoclimatic research can sometimes appear discontinuous. Because these breaks result from post-depositional processes, they do not undermine the paleoclimatic interpretations of VML.     

On the distribution of diatoms on cover slips: cause for concern

We investigated the effect of frustule morphology on the distribution of various diatom taxa on microscope cover slips, using an artificial assemblage. Seven diatom morphotypes were processed separately in nitric acid and an eighth morphotype (Chaetoceros muelleri) was processed in Lugol’s. These were mixed into a single assemblage and dried on a cover slip. We then mapped the locations of all 1,664 valves in the debris area, which covered 244.4 mm². The inner half of the debris area (A = 122.2 mm², r ≤ 6.237 mm,), the “Central Region,” contained 1,303 valves (78 % of the total). Each of the eight morphotypes was significantly (p ≤ 0.035) more numerous in the Central Region than in the Marginal Region. In addition, the assemblages in the two Regions were quite distinct: C. muelleri accounted for 19 % of the Central Region diatoms, but 57 % of the Marginal Region ones, whereas Surirella peisonis accounted for 17 and 6 %, respectively. We also considered valve distribution relative to aliquot volume by comparing the number of valves under the proximal 50 μl of the aliquot (r ≤ 4.758 mm) with the number of valves under the distal 50 μl. Four morphotypes were significantly (p < 0.02) more numerous in the Proximal Region. In contrast, valves of C. muelleri were significantly more numerous in the Distal Region (p < 10^?43). Valve distribution may reflect valve morphology, such as spine length and degree of silicification. Also important may be other environmental variables, such as the abundance and size of sediment particles. Although preliminary, these data indicate that some taxa are not distributed randomly on cover slips, especially valves that have long spines, thin walls, or thick walls. Whereas many natural assemblages lack such morphotypes, investigators must be aware of such potential heterogenous distributions when counting.     

A tale of two magmas,Fuego, Guatemala

Fuego volcano in Guatemala erupted in 1974 in a basaltic sub-Plinian event, which has been well documented and studied. In 1999, after a period of quiescence lasting 20 years, Fuego erupted again, this time less violently, but with persistent low-level activity. This study investigates the link between these episodes. Previous melt inclusion studies have shown magma erupted in 1974 to have been a volatile-rich hybrid tapped from a vertically extensive system. By contrast, magma erupted in 1999 and 2003 is similar in composition to that erupted in 1974, but melt inclusions are more evolved. Although melt inclusions from the later period are CO₂ rich (up to ∼1,500 ppm), they have low H₂O concentration (max 1.5 wt.%, compared to ∼6 wt.% in 1974). These melt inclusions have a modified H₂O concentration due to diffusive re-equilibration at shallow pressures. Despite this diffusive exchange, both eruptions show evidence of recent mingling of the same low and higher K melts, one of which was slightly cooler than the other and as a result traversed the amphibole stability field. (²¹⁰Pb/²²⁶Ra) data on selected bulk rock samples from 1974 suggest that whereas the cooler, more evolved end-member may have been degassing since the last major eruption in the 1930s, the warmer end-member intruded at most a decade prior to the 1974 eruption. The two end-members are thus batches of the same magma emplaced shallowly ∼30 years apart during which time the older batch was cooled and differentiated before mixing with the younger influx. The presence of the same two melts in the later eruptions suggests that magma in 1999 and 2003 is partly residual from 1974. The current eruptive activity is clearing the system of this residual magma prior to an expected new magma batch.     

An Extreme Solar Event of 20 January 2005: Properties of the Flare and the Origin of Energetic Particles

The famous extreme solar and particle event of 20 January 2005 is analyzed from two perspectives. Firstly, using multi-spectral data, we study temporal, spectral, and spatial features of the main phase of the flare, when the strongest emissions from microwaves up to 200 MeV gamma-rays were observed. Secondly, we relate our results to a long-standing controversy on the origin of solar energetic particles (SEP) arriving at Earth, i.e., acceleration in flares, or shocks ahead of coronal mass ejections (CMEs). Our analysis shows that all electromagnetic emissions from microwaves up to 2.22 MeV line gamma-rays during the main flare phase originated within a compact structure located just above sunspot umbrae. In particular, a huge (≈ 10⁵ sfu) radio burst with a high frequency maximum at 30 GHz was observed, indicating the presence of a large number of energetic electrons in very strong magnetic fields. Thus, protons and electrons responsible for various flare emissions during its main phase were accelerated within the magnetic field of the active region. The leading, impulsive parts of the ground-level enhancement (GLE), and highest-energy gamma-rays identified with π ⁰-decay emission, are similar and closely correspond in time. The origin of the π ⁰-decay gamma-rays is argued to be the same as that of lower-energy emissions, although this is not proven. On the other hand, we estimate the sky-plane speed of the CME to be 2 000 – 2 600 km s⁻¹, i.e., high, but of the same order as preceding non-GLE-related CMEs from the same active region. Hence, the flare itself rather than the CME appears to determine the extreme nature of this event. We therefore conclude that the acceleration, at least, to sub-relativistic energies, of electrons and protons, responsible for both the major flare emissions and the leading spike of SEP/GLE by 07 UT, are likely to have occurred nearly simultaneously within the flare region. However, our analysis does not rule out a probable contribution from particles accelerated in the CME-driven shock for the leading GLE spike, which seemed to dominate at later stages of the SEP event. S.N. Kuznetsov deceased 17 May 2007.     

The national geophysical data center bathymetric program

The National Geophysical Data Center's (NGDC's) mission is data management in the broadest sense, playing a role in the nation's research into the environment and providing data to a wide group of users. NGDC also operates components of the International Council of Scientific Unions (ICSU) World Data Center A. The Marine Geology and Geophysics Division of NGDC handles bathymetric data acquisition, manipulation, archival, and dissemination and operates the International Hydrographic Organization Data Center for Digital Bathymetry. Four major data bases have been developed to manage the large volume of data received: the Global Marine Geophysical Data Base (geophysical data acquired in a time series); the NOAA National Ocean Service Hydrographic Data Base; the International Hydrographic Data Base (contains bathymetric data, other than NOS surveys, with no time‐tagging); and the Multibeam Bathymetric Data Base. In addition, gridded data sets such as ETOPO5 are available from NGDC. Bathymetric data are acquired by NGDC through data exchange, funded and contract programs, processing of long‐term data holdings, data digitization from hardcopy sources, and national and international linkages. NGDC personnel participate nationally and internationally on numerous committees associated with studies of the seafloor including charting and bathymetric needs.     

Modelling of a Single-Channel Beta–Gamma Coincidence Phoswich Detector Using Geant4 for the Conversion Electron Energy Peak Resolution and Beta–Gamma Coincidence Efficiency Improvement

In this study, an optimized single-channel phoswich well detector design has been proposed and assessed in order to improve beta–gamma coincidence measurement sensitivity of xenon radioisotopes. This newly designed phoswich well detector consists of a plastic beta counting cell (BC404) embedded in a CsI (Tl) crystal coupled to a photomultiplier tube. The BC404 is configured in a cylindrical pipe shape to minimise light collection deterioration. The CsI (Tl) crystal consists of a rectangular part and a semi-cylinder scintillation part as a light reflector to increase light gathering. Compared with a PhosWatch detector, the final optimized detector geometry showed 15 % improvement in the energy resolution of a ^131mXe 129.4 keV conversion electron peak. The predicted beta–gamma coincidence efficiencies of xenon radioisotopes have also been improved accordingly.     

Large in‐stream wood studies: a call for common metrics

During the past decade, research on large in‐stream wood has expanded beyond North America's Pacific Northwest to diverse environments and has shifted toward increasingly holistic perspectives that incorporate processes of wood recruitment, retention, and loss at scales from channel segments to entire watersheds. Syntheses of this rapidly expanding literature can be facilitated by agreement on primary variables and methods of measurement. In this paper we address these issues by listing the variables that we consider fundamental to studies of in‐stream wood, discussing the sources of variability in their measurement, and suggesting more consistency in future studies. We recommend 23 variables for all studies of in‐stream wood, as well as another 12 variables that we suggest for studies with more specific objectives. Each of these variables relates either to the size and characteristics of in‐stream wood, to the geomorphic features of the channel and valley, or to the ecological characteristics of the riparian zone adjacent to the study reach. The variables were derived from an overview of those cited in the literature and from our collective field experiences. Copyright © 2010 John Wiley & Sons, Ltd.     

Groundwater in fractured crystalline rocks, the Clara mine, Black Forest (Germany)

The chemical composition of water sampled in a 700 m deep underground barite-fluorite mine in the crystalline basement of the Black Forest area (SW Germany) varies systematically with depth and the length of flow paths trough, the fracture porosity of the gneiss matrix. Calcium and sulfate increase as a result of a combined sulfide oxidation and plagioclase alteration reaction. The gneiss contains andesine–plagioclase (An₂₀–An₄₀) and is rich in primary sulfide. As an effect of Ca and SO₄ release by the prime water–rock reaction, dissolved oxygen decreases and the waters become more reduced. The waters have Cl/Br mass ratios of about 50, which is very close to that of experimentally leached gneiss powders indicating that the rock matrix is the source of the halogens. The waters are undersaturated with respect to calcite in the upper parts of the mine. With increasing reaction progress, calcite saturation is reached and carbonate forms as a reaction product of the prime reaction that also controls the partial pressure of CO₂ to progressively lower values. The chemical evolution of groundwater in fractured basement of the Clara mine suggests that the partial pressure of CO₂ is an internally buffered parameter rather than a controlling external variable.     

Geology of the Side Crater of the Erebus volcano,Antarctica

The summit cone of the Erebus volcano contains two craters. The Main crater is roughly circular (∼ 500 m diameter) and contains an active persistent phonolite lava lake ∼ 200 m below the summit rim. The Side Crater is adjacent to the southwestern rim of the Main Crater. It is a smaller spoon-shaped Crater (250–350 m diameter, 50–100 m deep) and is inactive. The floor of the Side Crater is covered by snow/ice, volcanic colluvium or weakly developed volcanic soil in geothermal areas (a.k.a. warm ground). But in several places the walls of the Side Crater provide extensive vertical exposure of rock which offers an insight into the recent eruptive history of Erebus. The deposits consist of lava flows with subordinate volcanoclastic lithologies. Four lithostratigraphic units are described: SC 1 is a compound lava with complex internal flow fabrics; SC 2 consists of interbedded vitric lavas, autoclastic and pyroclastic breccias; SC 3 is a thick sequence of thin lavas with minor autoclastic breccias; SC 4 is a pyroclastic fall deposit containing large scoriaceous lava bombs in a matrix composed primarily of juvenile lapilli-sized pyroclasts. Ash-sized pyroclasts from SC 4 consist of two morphologic types, spongy and blocky, indicating a mixed strombolian-phreatomagmatic origin. All of the deposits are phonolitic and contain anorthoclase feldspar.