Seismic Properties of Anita Bay Dunite: an Exploratory Study of the Influence of Water

As a pilot study of the role of water in the attenuation ofseismic waves in the Earth's upper mantle, we have performeda series of seismic-frequency torsional forced-oscillation experimentson a natural (Anita Bay) dunite containing accessory hydrousphases, at high temperatures to 1300°C and confining pressure(P_c) of 200 MPa, within a gas-medium high-pressure apparatus.Both oven-dried and pre-fired specimens wrapped in Ni–Fefoil within the (poorly) vented assembly were recovered essentiallydry after 50–100 h of annealing at 1300°C followedby slow staged cooling. The results for those specimens indicatebroadly similar absorption-band viscoelastic behaviour, butwith systematic differences in the frequency dependence of strain-energydissipation Q^–1, attributed to differences in the smallvolume fraction of silicate melt and its spatial distribution.In contrast, it has been demonstrated that a new assembly involvinga welded Pt capsule retains aqueous fluid during prolonged exposureto high temperatures—allowing the first high-temperaturetorsional forced-oscillation measurements under high aqueousfluid pore pressure P_f. At temperatures >1000°C, a markedreduction in shear modulus, without concomitant increase inQ^–1, is attributed to the widespread wetting of grainboundaries resulting from grain-scale hydrofracturing and themaintenance of conditions of low differential pressure P_d =P_c – P_f . Staged cooling from 1000°C is accompaniedby decreasing P_f and progressive restoration of significantlypositive differential pressure resulting in a microstructuralregime in which the fluid on grain boundaries is increasinglyrestricted to arrays of pores. The more pronounced viscoelasticbehaviour observed within this regime for the Pt-encapsulatedspecimen compared with the essentially dry specimens may reflectboth water-enhanced solid-state relaxation and the direct influenceof the fluid phase. The scenario of overpressurized fluids andhydrofracturing in the Pt-encapsulated dunite specimen may havesome relevance to the high Q^–1 and low-velocity zonesobserved in subduction-zone environments. The outcomes of thisexploratory study indicate that the presence of water can havea significant effect on the seismic wave attenuation in theupper mantle and provide the foundation for more detailed studieson the role of water. KEY WORDS: seismic wave attenuation; water; dunite; hydrous mineral; shear modulus; viscoelasticity; olivine; grain-scale hydrofracturing     

Predicting twenty-first century recession of architectural limestone in European cities

Cities of the twenty-first century will expose buildings to environments that are quite different from those experienced over preceding periods. The recent reduction in sulfur dioxide in cities and continued pressure to lower the emissions of combustion generated pollutants creates a potential for climate induced deterioration, by contrast, to be more important. Given that climate will likely change over the next hundred years, recession rates of calcareous stones have been predicted for Oviedo (Spain), Paris and Prague over the period 1981–2099. This can give guidance as to the likely change in balance of future threat. The Lipfert, ICP, and MULTI-ASSESS functions were used to calculate recession from estimates of climate and air quality. It is likely that under a continued decline or stable levels of pollution, recession rates having reached low values will remain largely unchanged over the coming century, despite likely changes in climate. Although the functions adopted disagree in a quantitative sense, there is evidence that they were reasonably concordant in the last decades of the twentieth century. In a cleaner future their different underlying assumptions lead to poorer agreement.     

How effective are plant macrofossils as a proxy for macrophyte presence? The case of <Emphasis Type="Italic">Najas flexilis</Emphasis> in Scotland

Preventing biodiversity loss is a key aim of modern conservation, and paleolimnology can inform conservation strategies for target species and habitats where other data are unavailable. Care must be taken to fully understand the possibilities and limits of such techniques, particularly where they concern single species. This study uses plant and seed distribution data to inform macrofossil reconstructions of the rare macrophyte Najas flexilis (Slender Naiad) in Scotland, UK. It answers three questions: (a) How does the location of N. flexilis seeds in the surface sediments relate to the distribution of N. flexilis plants? (b) How do the numbers of seeds in surface sediments correlate with % cover of N. flexilis plants across lakes with differing N. flexilis abundances? (c) What are the implications of these findings for paleolimnology? Percentage N. flexilis cover and number of N. flexilis seeds in surface sediments were recorded at ~100 sample points at each of three sites; one where the species was abundant, one where it was occasional and one where it was extinct. At all sites, N. flexilis seeds were present in surface sediments across the entire lake. No correlation between % cover N. flexilis and the number of seeds in surface sediments was found within individual sites. The distribution of seeds in these lakes appeared to be related to multiple environmental and ecological variables including latitude and longitude (proxies for water currents). This is attributed to the ability of seed-bearing N. flexilis plants to fragment and float large distances on water. Between sites, there was a significant difference in the mean seed counts, with higher mean seed counts corresponding to higher abundances of N. flexilis plants. It is concluded that N. flexilis is likely to be well represented in sediment cores taken from any location within a basin, but that care should be taken when inferring changes in N. flexilis abundance from changes in the numbers of seeds in sediment samples. This work demonstrates that the reproductive ecology (number of seeds produced and dispersal mechanisms) is an important factor to consider when attempting reconstructions of single aquatic plant populations from macrofossil records.     

Dynamics of the α-β phase transitions in quartz and cristobalite as observed by in-situ high temperature 29Si and 17O NMR

Relaxation times (T₁) and lineshapes were examined as a function of temperature through the - transition for ²⁹Si in a single crystal of amethyst, and for ²⁹Si and ¹⁷O in cristobalite powders. For single crystal quartz, the three ²⁹Si peaks observed at room temperature, representing each of the three differently oriented SiO₄ tetrahedra in the unit cell, coalesce with increasing temperature such that at the - transition only one peak is observed. ²⁹Si T₁'s decrease with increasing temperature up to the transition, above which they remain constant. Although these results are not uniquely interpretable, hopping between the Dauphiné twin related configurations, ₁ and ₂, may be the fluctuations responsible for both effects. This exchange becomes observable up to 150° C below the transition, and persists above the transition, resulting in -quartz being a time and space average of ₁ and ₂. ²⁹Si T₁'s for isotopically enriched powdered cristobalite show much the same behavior as observed for quartz. In addition, ¹⁷O T₁'s decrease slowly up to the - transition at which point there is an abrupt 1.5 order of magnitude drop. Fitting of static powder ¹⁷O spectra for cristobalite gives an asymmetry parameter () of 0.125 at room T, which decreases to <0.040 at=" the=" transition=" temperature.=" the=" electric=" field=" gradient=" (efg)=" and=" chemical=" shift=" anisotropy=" (csa),=" however,=" remain=" the=" same,=" suggesting=" that=" the=" decrease=" in="> is caused by a dynamical rotation of the tetrahedra below the transition. Thus, the mechanisms of the - phase transitions in quartz and cristobalite are similar: there appears to be some fluctuation of the tetrahedra between twin-related orientations below the transition temperature, and the -phase is characterized by a dynamical average of the twin domains on a unit cell scale.     

Development and Extension of An Aggregated Scale Model: Part 2 – Extensions to ASMITA

Whilst much attention has been given to models that describe wave, tide and sediment transport processes in sufficient detail to determine the local changes in bed level over a relatively detailed representation of the bathymetry, far less attention has been given to models that consider the problem at a much larger scale (e.g. that of geomorphological elements such as a tidal flat and tidal channel). Such aggregated or lumped models tend not to represent the processes in detail but rather capture the behaviour at the scale of interest. One such model developed using the concept of an equilibrium concentration is the Aggregated Scale Morphological Interaction between Tidal basin and Adjacent coast (ASMITA). In a companion paper (Part 1), we detail the original model and provide some new insights into the concepts of equilibrium, and horizontal and vertical exchange that are key components of this modelling approach. In this paper, we summarise a range of developments that have been undertaken to extend the original model concept, to illustrate the flexibility and power of the conceptual framework. However, adding detail progressively moves the model in the direction of the more detailed process-based models and we give some consideration to the boundary between the two.     

Multiphase dolomitization of deeply buried Cambrian petroleum reservoirs,Tarim Basin,north‐west China

Cambrian dolostone reservoirs in the Tarim Basin, China, have significant potential for future discoveries of petroleum, although exploration and production planning is hampered by limited understanding of the occurrence and distribution of dolomite in such ancient rocks buried to nearly 8 km. The study herein accessed new drill core samples which provide an opportunity to understand the dolomitization process in deep basins and its impact on Cambrian carbonate reservoirs. This study documents the origin of the dolostone reservoirs using a combination of petrology, fluid‐inclusion microthermometry, and stable and radiogenic‐isotopes of outcrop and core samples. An initial microbial dolomitization event occurred in restricted lagoon environments and is characterized by depleted δ¹³C values. Dolomicrite from lagoonal and sabkha facies, some fabric‐retentive dolomite and fabric‐obliterative dolomite in the peloidal shoal and reef facies show the highest δ¹⁸O values. These dolomites represent relatively early reflux dolomitization. The local occurrence of K‐feldspar in dolomicrite indicates that some radiogenic strontium was contributed via terrigenous input. Most fabric‐retentive dolomite may have precipitated from seawater at slightly elevated temperatures, suggested by petrological and isotopic data. Most fabric‐obliterative dolomite, and medium to coarse dolomite cement, formed between 90°C and 130°C from marine evaporitic brine. Saddle dolomite formed by hydrothermal dolomitization at temperatures up to 170°C, and involved the mixing of connate brines with Sr‐ enriched hydrothermal fluids. Intercrystalline, moldic, and breccia porosities are due to the early stages of dolomitization. Macroscopic, intergranular, vuggy, fracture and dissolution porosity are due to burial‐related dissolution and regional hydrothermal events. This work has shown that old (for example, Cambrian or even Precambrian) sucrosic dolomite with associated anhydrite, buried to as much as 8000 m, can still have a high potential for hosting substantial hydrocarbon resources and should be globally targeted for future exploration.     

Long-term spatio-temporal precipitation variability in arid-zone Australia and implications for groundwater recharge

Quantifying dryland groundwater recharge as a function of climate variability is becoming increasingly important in the face of a globally depleted resource, yet this remains a major challenge due to lack of adequate monitoring and the complexity of processes involved. A previously unpublished and unique dataset of high density and frequency rainfall measurements is presented, from the Fowlers Gap Arid Zone Research Station in western New South Wales (Australia). The dataset confirms extreme spatial and temporal variability in rainfall distribution which has been observed in other dryland areas and is generally explained by the dominance of individual storm cells. Contrary to previous observations, however, this dataset contains only a few localised storm cells despite the variability. The implications of spatiotemporal rainfall variability on the estimation of groundwater recharge is assessed and show that the most likely recharge mechanism is through indirect and localised, rather than direct, recharge. Examples of rainfall and stream gauge height illustrate runoff generation when a spatially averaged threshold of 15–25 mm (depending on the antecedent moisture conditions) is exceeded. Preliminary assessment of groundwater levels illustrates that the regional water table is much deeper than anticipated, especially considering the expected magnitude of indirect and localised recharge. A possible explanation is that pathways for indirect and localised recharge are inhibited by the large quantities of Aeolian dust observed at the site. Runoff readily occurs with water collecting in surface lakes which slowly dry and disappear. Assuming direct groundwater recharge under these conditions will significantly overestimate actual recharge.     

Identifying landslide preconditions in Swedish quick clays—insights from integration of surface geophysical,core sample- and downhole property measurements

Quick-clay landslides are a serious geohazard in Canada, Norway and Sweden. Identification and mapping of quick clays are essential endeavours because the damage caused by an individual landslide can be large and costly, with potentially fatal consequences. We collected geophysical borehole and soil core data from an area prone to quick-clay landslides in southwestern Sweden. Methodologies included in situ and laboratory measurements, providing information about natural gamma radiation, sonic velocities, electrical conductivity, pH, physical grain size, elemental and mineral composition, magnetic properties, cation exchange capacity and fossil content. A stratigraphic thickness of almost 60 m enables us to study quick clays and their host environment in Sweden at unusually high resolution. Results identify the origin and location of reflections in nearby seismic lines and assign physico-chemical properties to the geological units present in the area. We show that coarse-grained layers are sandwiched between marine clays (some of which are quick clays). These layers function as a conduit for relatively fresh water that infiltrates the marine clays and chemically destabilizes them by leaching out their salts. The salinity distribution in the boreholes indicate that the groundwater movement is downwards, through the coarse-grained layer and towards the Göta river. The presence of these materials is important for the development of quick clays, although not a prerequisite. With the help of surface geophysical methods, the location of the coarse-grained layers can be known faster and more economically, which could be relevant for studying the potential for quick-clay landslide occurrence over large areas.