Atmospheric Stability Effects on Wind Fields and Scalar Mixing Within and Just Above a Subalpine Forest in Sloping Terrain

Air temperature T _a , specific humidity q, CO₂ mole fraction χ _c , and three-dimensional winds were measured in mountainous terrain from five tall towers within a 1 km region encompassing a wide range of canopy densities. The measurements were sorted by a bulk Richardson number Ri _b . For stable conditions, we found vertical scalar differences developed over a “transition” region between 0.05 < Ri _b < 0.5. For strongly stable conditions (Ri _b > 1), the vertical scalar differences reached a maximum and remained fairly constant with increasing stability. The relationships q and χ _c have with Ri _b are explained by considering their sources and sinks. For winds, the strong momentum absorption in the upper canopy allows the canopy sublayer to be influenced by pressure gradient forces and terrain effects that lead to complex subcanopy flow patterns. At the dense-canopy sites, soil respiration coupled with wind-sheltering resulted in CO₂ near the ground being 5–7 μmol mol⁻¹ larger than aloft, even with strong above-canopy winds (near-neutral conditions). We found Ri _b -binning to be a useful tool for evaluating vertical scalar mixing; however, additional information (e.g., pressure gradients, detailed vegetation/topography, etc.) is needed to fully explain the subcanopy wind patterns. Implications of our results for CO₂ advection over heterogenous, complex terrain are discussed.     

Biotite and muscovite <Superscript>40</Superscript>Ar–<Superscript>39</Superscript>Ar geochronological constraints on the post-Svecofennian tectonothermal evolution,Forsmark site,central Sweden

In order to characterize the post-Svecofennian tectonothermal evolution of the Fennoscandian Shield, ⁴⁰Ar–³⁹Ar biotite and some ⁴⁰Ar–³⁹Ar muscovite geochronological data are reported from a total of 30 surface outcrop and 1,000 m long borehole samples at Forsmark, central Sweden. The 13 surface samples were collected across 3 branches of a major WNW to NW trending system of deformation zones, whereas the boreholes were drilled within a tectonic lens, in between two of these zones. The ⁴⁰Ar–³⁹Ar biotite ages indicate that the present erosion surface, in central Sweden, cooled below c. 300°C at 1.73–1.66 Ga, and that the rocks could have accommodated strain in a brittle manner between 1.8 and 1.7 Ga. The variation in surface ages is suggested to be due to fault along the large WNW to NW trending deformation zones, following the establishment of a sub-Cambrian peneplain. The minor variation of ages within a single crustal block may be due to disturbance along ENE to NNE trending fracture zones. Possible cooling paths, derived from ⁴⁰Ar–³⁹Ar hornblende, muscovite and biotite ages, were calculated for the time interval from 1.80 to 1.67 Ga, when the area cooled from c. 500 to 300°C. Cooling rates of 1.9–4°C/m.y. have been attained. Between 1.68 and 1.64 Ga, uplift rates of c. 22 m/m.y. were calculated from borehole ⁴⁰Ar–³⁹Ar biotite data. Tectonothermal histories, inferred from the combined cooling and uplift rates, are related to simple cooling after the Svecofennian orogeny, to crustal movement in response to far-field effects of c. 1.7 Ga orogenic activities further to the west or to a combination of these possibilities.     

Intertidal mudflat properties, currents and sediment erosion in the partially mixed Tamar Estuary, UK

Results are presented from a 1-year campaign to measure the seasonal variability of some key physical and biological properties of intertidal mudflats over a section of the central Tamar Estuary and to relate these to the physical environment. Seasonal variations in physical mudflat properties, such as grain size, density and moisture content were relatively small. With the exception of the particulate organic carbon content in the upper 0.002 m of surface sediment, biological variations were large. Redox potential exhibited considerable seasonal variation and showed that the sediments were less reduced in winter and more reduced in summer. Chlorophyll a and extracellular polymeric substances (EPS) within the surface 0.002 m of sediment (due to the presence of benthic diatoms) were strongly correlated and exhibited a pronounced seasonal pattern, with smallest values during winter and greatest values during late summer and early autumn. EPS had a dominating influence on sediment erosion, as determined from annular flume measurements. Velocity measurements and velocity modelling indicated that during the flood, and for much of the time during benthic diatom bloom conditions of high chlorophyll a and EPS sediment contents, the stresses exerted by tidal currents were too small to cause significant suspension of sediments over much of the middle and upper mudflats. Suspended fine sediment in the turbidity maximum zone was transported down-estuary and deposited in the main channel at low-water (LW) slack. Some of this sediment, in the form of relatively large aggregates, was subsequently transported onto the mudflats during the flooding tide, where slack currents and fast settling velocities may have enhanced sediment deposition there.Responsible Editor: Hans Burchard     

Geoscience and high-level nuclear waste disposal： the Nordic scene

In Norden, two countries, Sweden and Finland, are actively engaged in site investigations for the location of deep repositories for spent nuclear fuel from powerproducing nuclear reactors. These investigations are being carried out in crystalline rocks of the Fennoscandian Shield. In Sweden, a long history of site selection has led to the identification and investigation of two sites, Forsmark and Laxemar/Simpevarp, based on a strategy of combining favourable bedrock with consent by the local population. Surface-based geoscientific investigations of the two candidates, with extensive deep drilling, are now drawing to a close. A proposal as to which of the sites would be most suitable for the development of a deep repository will be submitted to the governmental regulatory authorities in 2009. In Finland, the site selection process was shorter and less politically controversial, and led to a ＂Decision in Principle＂ by the Finnish parliament, in May 2001, to develop a deep repository at the Olkiluoto site. The access tunnel to an underground rock characterisation facility at 400-500 m depth is at present under construction, accompanied by extensive geoscientific investigations in the subsurface. An application for a construction licence for a deep repository will be submitted in 2012. Although all sites are located in Precambrian crystalline rocks, the Swedish homogeneous granitic rocks, sites both lie in relatively whilst the Finnish site is located in an heterogeneous migmatite complex. The Nordic approach to high-level nuclear waste disposal in crystalline rock will be the theme of a Topical Symposium at the 33rd International Geological Congress at Oslo, in August 2008, and the three sites mentioned above will be the focus of Congress Excursion no. 14.     

Polycrystalline amphibole aggregates (clots) in granites as potential I‐type restite: An ion microprobe study of rare‐earth distributions

Aggregates of polycrystalline grains of amphibole (clots) occur widely in the granodiorites of the Strontian pluton, Scotland. These clots are complex structures with numerous small grains in the interior exhibiting zonation from actinolite cores to hornblende rims. Amphiboles in the outer parts of these clots are indistinguishable from hornblendes that have crystallised from the melt. A rare‐earth element (REE) study of individual amphibole and pyroxene grains using an ion microprobe has also shown a marked difference in REE abundances, with clots generally being depleted in their interiors. Modelling of the compositions shows that the clots are consistent with being derived from pyroxene + plagioclase ± amphibole precursors. These granular precursors are recognised as being consistent with the residual crystalline material encountered in dehydration melting experiments of amphibolitic starting materials. It is suggested that these features could represent restite in I‐type granodiorites and tonalites. Extensive, but incomplete equilibration of the clot material provides an explanation for the infrequent identification of restite (other than grain cores) in I‐type granites, in marked contrast with S‐types.     

The Whitsunday Volcanic Province, Central Queensland, Australia: lithological and stratigraphic investigations of a silicic-dominated large igneous province

Contrary to general belief, not all large igneous provinces (LIPs) are characterised by rocks of basaltic composition. Silicic-dominated LIPs, such as the Whitsunday Volcanic Province of NE Australia, are being increasingly recognised in the rock record. These silicic LIPs are consistent in being: (1) volumetrically dominated by ignimbrite; (2) active over prolonged periods (40–50 m.y.), based on available age data; and (3) spatially and temporally associated with plate break-up. This silicic-dominated LIP, related to the break-up of eastern continental Gondwana, is also significant for being the source of >1.4×10⁶ km³ of coeval volcanogenic sediment preserved in adjacent sedimentary basins of eastern Australia.The Whitsunday Volcanic Province is volumetrically dominated by medium- to high-grade, dacitic to rhyolitic lithic ignimbrites. Individual ignimbrite units are commonly between 10 and 100 m thick, and the ignimbrite-dominated sequences exceed 1 km in thickness. Coarse lithic lag breccias containing clasts up to 6 m diameter are associated with the ignimbrites in proximal sections. Pyroclastic surge and fallout deposits, subordinate basaltic to rhyolitic lavas, phreatomagmatic deposits, and locally significant thicknesses of coarse-grained volcanogenic conglomerate and sandstone are interbedded with the ignimbrites. The volcanic sequences are intruded by gabbro/dolerite to rhyolite dykes (up to 50 m in width), sills and comagmatic granite. Dyke orientations are primarily from NW to NNE.The volcanic sequences are characterised by the interstratification of proximal/near-vent lithofacies such as rhyolite domes and lavas, and basaltic agglomerate, with medial to distal facies of ignimbrite. The burial of these near-vent lithofacies by ignimbrites, coupled with the paucity of mass wastage products such as debris-flow deposits indicates a low-relief depositional environment. Furthermore, the volcanic succession records a temporal change in: (1) eruptive styles; (2) the nature of source vents; and (3) erupted compositions. An early explosive dacitic pyroclastic phase was succeeded by a later mixed pyroclastic-effusive phase producing an essentially bimodal suite of lavas and rhyolitic ignimbrite. From the nature and distribution of volcanic lithofacies, the volcanic sequences are interpreted to record the evolution of a multiple vent, low-relief volcanic region, dominated by several large caldera centres.     

Characteristics of Stable Flows over Southern Greenland

The main characteristic features of stable atmospheric flows over a large mountain plateau are summarised and then compared with mesoscale and synoptic scale numerical simulation, meteorological analysis, satellite imagery, and surface observations for the cases of flows over Southern Greenland for four wind directions. The detailed features are identified using the concepts and scaling of stably stratified flow over large mountains with variations in surface roughness, elevation, and heating. For westerly and easterly winds detached jets form at the southern tip, where coastal jets converge, which propagate large distances across the ocean. Near coasts katabatic winds can combine with barrier jets and wake flows generated by synoptic winds. Note how the approach flow rises/falls over southern Greenland for easterly/westerly winds, leading in both cases to more cloud on the western side. Some conclusions are drawn about the large-scale influences of these flows; detached jets in the atmosphere; air-sea interaction; formation of low pressure systems. For accurate simulations of these flows, mesoscale models are necessary with resolutions of order of 20 km or less.     

Progress in optical dating of guano-rich sediments associated with the Deep Skull, West Mouth of the Great Cave of Niah, Sarawak, Borneo

The West Mouth of the Great Cave of Niah is one of the major archaeological sites in Southeast Asia; the radiocarbon chronology of this site currently places the earliest presence of Homo sapiens (a Deep Skull find) in Southeast Asia at about 45 ka BP. An optical dating programme using sand-sized quartz was initiated in the West Mouth to complement the radiocarbon chronology. This paper reports on the progress in dating a key sample (sample 376r) that was taken from deposits containing the Deep Skull. A somewhat novel procedure was developed to separate and clean quartz grains from the unusual guano-rich deposits. From this, only a small quantity of quartz grains could be recovered and 36 aliquots (1 mm in diameter) were prepared, each consisting of 80 grains. This sample was dated by means of the single-aliquot regenerative-dose (SAR) method using the protocols of Murray and Wintle [2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiation Measurements 32, 57–73], and of Choi et al. [2003. Luminescence dating of well-sorted marine terrace sediments on the southeastern coast of Korea. Quaternary Science Reviews 22, 407–421]. The results showed a wide distribution of equivalent doses; this distribution is inferred to be due to bioturbation, variable bleaching at deposition, and to local variations in the dose rate within the sample site. Dose rates were measured using both field (in-situ γ-ray spectrometry, IGRS) and laboratory (inductively coupled plasma mass spectrometry/atomic emission spectrometry, ICP-MS/AES; and isotope dilution, ID) methods. There was significant disagreement between the concentrations of radioisotopes found using laboratory and field measurements, indicating sediment heterogeneity and also possible disequilibrium in the ²³⁸U decay chain. Future work in the West Mouth should therefore concentrate on taking sediment samples of at least 500–600 cm³, improving the dose rate estimates (e.g. by high-resolution γ-ray spectrometry) and investigating other luminescence signals (e.g. red thermoluminescence) as potential geochronometers.     

Changes in water quality following tidal inundation of coastal lowland acid sulfate soil landscapes

This study examines the remediation of surface water quality in a severely degraded coastal acid sulfate soil landscape. The remediation strategy consisted of partial restoration of marine tidal exchange within estuarine creeks and incremental tidal inundation of acidified soils, plus strategic liming of drainage waters. Time-series water quality and climatic data collected over 5 years were analysed to assess changes in water quality due to this remediation strategy. A time-weighted rainfall function (TWR) was generated from daily rainfall data to integrate the effects of antecedent rainfall on shallow groundwater levels in a way that was relevant to acid export dynamics. Significant increases in mean pH were evident over time at multiple monitoring sites. Regression analysis at multiple sites revealed a temporal progression of change in significant relationships between mean daily electrical conductivity (EC) vs. mean daily pH, and TWR vs. mean daily pH. These data demonstrate a substantial decrease over time in the magnitude of creek acidification per given quantity of antecedent rainfall. Data also show considerable increase in soil pH (2–3 units) in formerly acidified areas subject to tidal inundation. This coincides with a decrease in soil pe, indicating stronger reducing conditions. These observations suggest a fundamental shift has occurred in sediment geochemistry in favour of proton-consuming reductive processes. Combined, these data highlight the potential effectiveness of marine tidal inundation as a landscape-scale acid sulfate soil remediation strategy.