ALBERT

Description: Massive microbial mats covering up to 4-meter-high carbonate buildups prosper at methane seeps in anoxic waters of the northwestern Black Sea shelf. Strong 13C depletions indicate an incorporation of methane carbon into carbonates, bulk biomass, and specific lipids. The mats mainly consist of densely aggregated archaea (phylogenetic ANME-1 cluster) and sulfate-reducing bacteria (Desulfosarcina/Desulfococcusgroup). If incubated in vitro, these mats perform anaerobic oxidation of methane coupled to sulfate reduction. Obviously, anaerobic microbial consortia can generate both carbonate precipitation and substantial biomass accumulation, which has implications for our understanding of carbon cycling during earlier periods of Earth's history.

Description: Two modes of decrease in load bearing capacity of granular materials are discussed in view of experimental results. Both relate to the fact that frictional materials exhibit nonassociated plastic flow and they undergo considerable volume changes, either contraction or dilation. One mode consists of the instability that may occur in certain regions of stress space and potentially result in liquefaction of the granular material. It is the fact that loading of contracting soil (resulting in large plastic strains) can occur under decreasing stresses that may lead to unstable behavior under undrained conditions. As long as the soil remains drained, it will remain stable in the region of potential instability. The other mode is initiated by localization of plastic strains and subsequent development of shear bands, which in granular materials is followed by a decrease in load bearing capacity. These two modes are mutually exclusive and they occur for different loading and material conditions as discussed here on the basis of experimental observations.

Description: Volcanology has been in the past and in many respects remains a subject dominated by pure research grounded in the earth sciences. Over the past 30 years a paradigm shift has occurred in hazard assessment which has been aided by significant changes in the social theory of natural hazards and the first-hand experience gained in the 1990s by volcanologists working on projects conceived during the International Decade for Natural Disaster Reduction (IDNDR). Today much greater stress is placed on human vulnerability, the potential for marginalisation of disadvantaged individuals and social groups, and the requirement to make applied volcanology sensitive to the characteristics of local demography, economy, culture and politics. During the IDNDR a methodology, broadly similar to environmental impact analysis, has emerged as the preferred method for studying human vulnerability and risk assessment in volcanically active regions. The characteristics of this new methodology are discussed and the progress which has been made in innovating it on the European Union laboratory volcanoes located in western Europe is reviewed. Furnas (São Miguel, Azores) and Vesuvius in Italy are used as detailed case studies.

Description: In order to explain biological zonation, shore height above the ordnance level is frequently used as an indicator of the abiotic gradient in intertidal ecosystems. This is based on the implicit assumption that shore height is directly correlated with inundation frequency and/or duration. Despite the importance of inundation for tidal ecosystems, measurements have rarely been taken directly by measuring inundation at the site of investigation. We measured mean high tide (MHT) and flooding frequency at three sites on the Dutch Barrier Island of Schiermonnikoog. To assess the scale dependence, we compared local measurements with the estimated inundation frequencies based on the official tide gauge (OTG) farther away. Locally measured MHT water levels differed among sites and were consistently higher than estimated MHT water levels. With this data, we subsequently estimated the inundation frequency of vegetation plots from our measurements and correlated it with species distribution. In a logistic regression inundation frequency accounted for twice the variance in explaining the dominance of three salt marsh species than shore height. The discrepancy in annual inundation frequency of the vegetation between sites was ≦300% for a given shore height. Within each site replicated estimates of inundation frequency proved to be consistent (scale 10–50 m). Estimated and measured inundation frequencies thus reliably correlated at a small-scale (tens of metres), but not at a larger scale (hundreds of metres to kilometres). If inundation frequency is used as an explanatory variable, it will therefore be advisable to consider the spatial heterogeneity of the measurements, in particular if different sites are to be compared. We give mean inundation frequencies of three dominant salt marsh species (Elymus athericus, Festuca rubra, Artemisia maritima) measured over 1 year.

Description: A geographically related database called MAGIC has been developed, using GIS (Geographic Information System) technology, for MArine Gas seeps and seep IndiCators. A complementary bibliographic database (GASREF) stores details of related publications. The databases include data relating to natural seabed gas seeps and features such as pockmarks, cold seep communities, and methane-derived carbonates which are known to be found in association with seeps. The databases are compiled from published reports (so far restricted to those written in English), and users are able to interrogate the system for specified features from user-defined areas.

Description: The Catalina Schist (California) contains an amphibolite-grade (0.8–1.1 GPa; 640–750 jC) melange unit consisting of mafic and ultramafic blocks in high-Mg, schistose melange matrix with varying modal proportions of talc, chlorite, anthophyllite, calcic-amphibole, enstatite, and minor phases including zircon, rutile, apatite, spinel, and Fe–Ni sulfides. This melange unit is interpreted as a kilometer-scale zone of tectonic and metasomatic mixing formed within a juvenile subduction zone, the study of which may yield insight into chemical mixing processes at greater depths in subduction zones. Relationships among the major and trace element compositions of the mafic and ultramafic blocks in the melange, the rinds developed at the margins of these blocks, and the surrounding melange matrix are compatible with the evolution of the melange matrix through a complex combination of infiltrative and diffusional metasomatism and a process resembling mechanical mixing. Simple, linear mixing models are compatible with the development of the melange matrix primarily through simple mixture of the ultramafic and mafic rocks, with Cr/Al ratios serving as indicators of the approximate proportions of the two lithologies. This conclusion regarding mafic–ultramafic mixing is consistent with the field observations and chemical trends indicating strong resemblance of large parts of the melange matrix with rinds developed at the margins of mafic and ultramafic blocks. The overall process involved development of metasomatic assemblages through complex fluid-mediated mixing of the blocks and matrix concurrent with deformation of these relatively weak rind materials, which are rich in layer silicates and amphibole. This deformation was sufficiently intense to transpose fabrics, progressively disaggregate more rigid, block-derived materials in weaker chorite- and talc-rich melange, and in some particularly weak lithologies (e.g., chlorite-, talc-, and amphibole-rich materials), intimately juxtapose adjacent lithologies at the (sub-)cm scale (approaching grain scale) sampled by the whole-rock geochemical analyses. Chemical systematics of various elements in the melange matrix can be delineated based on the Cr/Al-based mixing model. Simple mixing relationships exhibited by Al, Cr, Mg, Ni, Fe, and Zr provide a geochemical reference frame for considerations of mass and volume loss and gain within the melange matrix. The compositional patterns of many other elements are explained by either redistribution (local stripping or enrichment) at varying scales within the melange (Ca, Na, K, Ba, and Sr) or massive addition from external sources (Si and H2O), the latter probably in infiltrating H2O-rich fluids that produced the dramatic O and H isotopic shifts in the melange. Melange formation, resulting in the production of high-variance ultramafic assemblages withhigh volatile contents, may aid retention of volatiles (in this case, H2O) to greater depths in subduction zones than in original subducted mafic and sedimentary materials. The presence of such assemblages (i.e., containing minerals such as talc, chlorite, and Mg-rich amphiboles) would impact the rheology of the slab–mantle interface and perhaps contribute to the low-velocity seismic structure observed at/near the slab–mantle interface in some subduction zones. If operative along the slab–mantle interface, complex mixing processes such as these, involving the interplay between fluid-mediated metasomatism and deformation, also could impact slab incompatible trace element and isotopic signatures ultimately observed in arc magmas, producing ‘‘fluids’’ with geochemical signatures inherited from interactions with hybridized rock compositions.

Description: Fluid and melt inclusions in minerals of two native iron-bearing dyke rocks from Disko, Greenland, were investigated using heating and cooling microscopic techniques. The melt inclusions in olivine are homogenized at temperatures of 1170–1200 °C. The melts are characterized by high SiO2, K2O and TiO2 and low MgO contents. The co-existence of combined melt–fluid (gas) inclusions in olivine implies that the olivine phenocrysts crystallized from a heterogeneous magma at low pressure. Gas bubbles were investigated in the glass of the rock. Water and four crystalline phases were observed at room and low temperatures on the walls of the bubbles: serpentine, naphthalene, clathrate, and an unknown phase. The complex composition of trapped volatiles is reflected in considerable ranges of clathrate melting and solidus temperatures (20 to 7.5 °C and −82.6 to −73 °C, respectively). Melt equilibrium with metallic iron at T=1450 K suggests that logfO2=−13.95. The calculated composition of C–O–H fluid in equilibrium with Fe, Fe3C and FeS is characterized by high CH4 and H2 contents. During cooling at the postmagmatic stage, polymerization proceeds, and heavy hydrocarbons are formed inside vesicles. The presence of organic compounds in inclusions trapped at a magmatic stage is a natural consequence of unusual reducing condition during crystallization.

Description: We have developed a model of fluid flow and pressure development in sedimentary basins that incorporates pore volume loss due to mechanical compaction and to chemical diagenesis (quartz cementation, grain contact quartz dissolution and illitization). Mechanical compaction is modeled to be a function of effective stress. In this model, pore volume loss due to mechanical compaction will be retarded when overpressure develops. The diagenetic processes are modeled as being kinetically controlled and the reaction progress depends only on the temperature history. Hence pore volume loss due to chemical compaction is not retarded by overpressure. By including diagenetic effects on overpressure development, the pressure model should be more generally applicable than models that consider mechanical compaction to be the sole process that reduces porosity. To demonstrate the potential importance of chemical compaction in the formation of fluid overpressures in different settings, we calibrated our model with data obtained from the Halten Terrace offshore mid-Norway and from the Gulf of Mexico. In both cases, the diagenetic processes have the potential to control on the timing and magnitude of overpressuring. From 25% and up to 80% of the present-day overpressure may be caused by pore volume loss resulting from diagenetic reactions. Pressure build-up from diagenetic processes also potentially controls the timing of hydraulic fracturing. If diagenetic processes are actively contributing to overpressure generation, then unrealistically low shale permeabilities are not needed to retain overpressures for geologic time periods (〉10 My).

Description: Hydrothermal gases from shallow seafloor vents in the Bay of Plenty, New Zealand contain CO2, CH4, and the higher gaseous hydrocarbons up to i-, n-C4H10. The gases are similar to those discharged at fumaroles on the nearby White Island. Carbon isotope compositions for CO2 fall between −3.4‰ and −5.5‰ PDB and reflect a shallow magmatic carbon source. The δ13C values of CH4 range from −24.6‰ to −28.9‰ PDB and the δD values vary between −122‰ and −135‰ SMOW. The CH4 isotope values and the presence of the higher hydrocarbon compounds such as C2H6 and C3H8 with δ13C values near −20‰ PDB suggest hydrocarbon production by high-temperature maturation of sedimentary organic matter and mixing (∼1:1) of the thermogenic CH4 with abiogenic CH4. Long-chained hydrocarbons occur in dredged samples close to the active vents. Their n-alkane distribution has a high to moderate odd–even predominance and an extensive hopane series, indicative of higher land-plant waxes and prokaryotic membranes in the source. Substantial amounts of unsubstituted polynuclear aromatic hydrocarbons (PAH) mark the transition from aliphatic- to aromatic-dominated bitumens, consistent with extensive source maturation resulting from thermal stress. The bitumens are interpreted as pyrolysates derived from buried near-coastal vegetation and terrestrial detritus under various thermal regimes, mixed with immature seafloor organic matter.

Description: The Hf isotope composition of seawater does not match that expected from dissolution of bulk continental crust. This mismatch is generally considered to be due to retention of unradiogenic Hf in resistant zircons during incomplete weathering of continental crust. During periods of intense glacial weathering, zircons should break down more efficiently, resulting in the release of highly unradiogenic Hf to the oceans. We test this hypothesis by comparing Nd and Hf isotope time series obtained from NW Atlantic ferromanganese crusts. Both isotope systems show a decrease associated with the onset of northern hemisphere glaciation. The observed changes display distinct trajectories in ϵNd–ϵHf space, which differ from previously reported arrays of bulk terrestrial material and seawater. Such patterns are consistent with the release of highly unradiogenic Hf from very old zircons, facilitated by enhanced mechanical weathering