The role of hydrothermal fluids in sedimentation in saline alkaline lakes: Evidence from Nasikie Engida,Kenya Rift Valley     

Robin W. Renaut  R. Bernhart Owen  Tim K. Lowenstein  Gijs De Cort  Emma Mcnulty  Jennifer J. Scott  Anthony Mbuthia 《Sedimentology》2021,68(1):108-134

Saline alkaline lakes that precipitate sodium carbonate evaporites are most common in volcanic terrains in semi‐arid environments. Processes that lead to trona precipitation are poorly understood compared to those in sulphate‐dominated and chloride‐dominated lake brines. Nasikie Engida (Little Magadi) in the southern Kenya Rift shows the initial stages of soda evaporite formation. This small shallow (<2 m deep; 7 km long) lake is recharged by alkaline hot springs and seasonal runoff but unlike neighbouring Lake Magadi is perennial. This study aims to understand modern sedimentary and geochemical processes in Nasikie Engida and to assess the importance of geothermal fluids in evaporite formation. Perennial hot‐spring inflow waters along the northern shoreline evaporate and become saturated with respect to nahcolite and trona, which precipitate in the southern part of the lake, up to 6 km from the hot springs. Nahcolite (NaHCO₃) forms bladed crystals that nucleate on the lake floor. Trona (Na₂CO₃·NaHCO₃·2H₂O) precipitates from more concentrated brines as rafts and as bottom‐nucleated shrubs of acicular crystals that coalesce laterally to form bedded trona. Many processes modify the fluid composition as it evolves. Silica is removed as gels and by early diagenetic reactions and diatoms. Sulphate is depleted by bacterial reduction. Potassium and chloride, of moderate concentration, remain conservative in the brine. Clastic sedimentation is relatively minor because of the predominant hydrothermal inflow. Nahcolite precipitates when and where pCO₂ is high, notably near sublacustrine spring discharge. Results from Nasikie Engida show that hot spring discharge has maintained the lake for at least 2 kyr, and that the evaporite formation is strongly influenced by local discharge of carbon dioxide. Brine evolution and evaporite deposition at Nasikie Engida help to explain conditions under which ancient sodium carbonate evaporites formed, including those in other East African rift basins, the Eocene Green River Formation (western USA), and elsewhere.  相似文献   

Detailing the impact of the Storegga Tsunami at Montrose,Scotland     

Mark D. Bateman  Tim C. Kinnaird  Jon Hill  Robert A. Ashurst  Jenna Mohan  Rebecca B. I. Bateman  Ruth Robinson 《Boreas: An International Journal of Quaternary Research》2021,50(4):1059-1078

The Storegga tsunami, dated in Norway to 8150±30 cal. years BP, hit many countries bordering the North Sea. Run-ups of >30 m occurred and 1000s of kilometres of coast were impacted. Whilst recent modelling successfully generated a tsunami wave train, the wave heights and velocities, it under-estimated wave run-ups. Work presented here used luminescence to directly date the Storegga tsunami deposits at the type site of Maryton, Aberdeenshire in Scotland. It also undertook sedimentological characterization to establish provenance, and number and relative power of the tsunami waves. Tsunami model refinement used this to better understand coastal inundation. Luminescence ages successfully date Scottish Storegga tsunami deposits to 8100±250 years. Sedimentology showed that at Montrose, three tsunami waves came from the northeast or east, over-ran pre-existing marine sands and weathered igneous bedrock on the coastal plain. Incorporation of an inundation model predicts well a tsunami impacting on the Montrose Basin in terms of replicate direction and sediment size. However, under-estimation of run-up persisted requiring further consideration of palaeotopography and palaeo-near-shore bathymetry for it to agree with sedimentary evidence. Future model evolution incorporating this will be better able to inform on the hazard risk and potential impacts for future high-magnitude submarine generated tsunami events.  相似文献   

Unveiling buried aeolian landscapes: reconstructing a late Holocene dune environment using 3D ground-penetrating radar     

Luis Rees-Hughes  Natasha L. M. Barlow  Adam D. Booth  Landis J. West  George Tuckwell  Tim Grossey 《第四纪科学杂志》2021,36(3):377-390

Across the UK, sandy beaches and dunes protect coastal infrastructure from waves and extreme water levels during large-scale storms, while providing important habitats and recreational opportunities. Understanding their long-term evolution is vital in managing their condition in a changing climate. Recently, ground-penetrating radar (GPR) methods have grown in popularity in geomorphological applications, yielding centimetre-scale resolution images of near-surface stratigraphy and structure, thus allowing landscape evolution to be reconstructed. Additionally, abrupt changes in palaeo-environments can be visualized in three dimensions. Although often complemented by core data, GPR allows interpretations to be extended into areas with minimal ground-truth control. Nonetheless, GPR data interpretation can be non-intuitive and ambiguous, and radargrams may not initially resemble the expected subsurface geometry. Interpretation can be made yet more onerous when handling the large 3D data volumes that are facilitated with modern GPR technology. Here we describe the development of novel semi-automated GPR feature-extraction tools, based on ‘edge detection’ and ‘thresholding’ methods, which detect regions of increased GPR reflectivity which can be applied to aid in the reconstruction of a range Quaternary landscapes. Since reflectivity can be related to lithological and/or pore fluid changes, the 3D architecture of the palaeo-landscape can be reconstructed from the features extracted from a geophysical dataset. We present 500 MHz GPR data collected over a buried Holocene coastal dune system in North Wales, UK, now reclaimed for use as an airfield. Core data from the site, reaching a maximum depth 2 m, suggest rapid vertical changes from sand to silty-organic units, and GPR profiles suggest similar lateral complexity. By applying thresholding methods to GPR depth slices, these lateral complexities are effectively and automatically mapped. Furthermore, automatic extraction of the local reflection power yields a strong correlation with the depth variation of organic content, suggesting it is a cause of reflectivity contrast. GPR-interpolated analyses away from core control thus offer a powerful proxy for parameters derived from invasive core logging. The GPR data collected at Llanbedr airfield highlight a complex dune system to a depth of 2.8 m, probably deposited in several phases over ~700 years, similar to elsewhere in North Wales.  相似文献   

Approaches to knowledge sharing and capacity building: The role of local information systems in marine and coastal management     

Tim Stojanovic  David R. Green  Graham Lymbery 《Ocean & Coastal Management》2010,53(12):805-815

This paper explores how coastal data and information can be mobilised in information systems and applied in coastal management. The concept of an Information System is defined and described, and the potential role of Local Information System (LIS) in integrated coastal management (ICM) is considered. Three examples, from the Severn Estuary, UK the coast of the North West of England and North Wales, and the Fal and Helford estuaries, are used to demonstrate the requirements of a LIS for coastal areas. The role of GIS as part of the solution is considered in detail. The paper demonstrates how ideas from the disciplines of information systems and information science can be practically applied in coastal areas. The findings promote a holistic approach for those involved in the development of technologies and dealing with data and information about coasts and oceans.  相似文献   

Wildfire preparedness,community cohesion and social–ecological systems     

Tim Prior  Christine Eriksen 《Global Environmental Change》2013,23(6):1575-1586

The consequences of wildfires are felt in susceptible communities around the globe on an annual basis. Climate change predictions in places like the south-east of Australia and western United States suggest that wildfires may become more frequent and more intense with global climate change. Compounding this issue is progressive urban development at the peri-urban fringe (wildland–urban interface), where continued infrastructure development and demographic changes are likely to expose more people and property to this potentially disastrous natural hazard. Preparing well in advance of the wildfire season is seen as a fundamental behaviour that can both reduce community wildfire vulnerability and increase hazard resilience – it is an important element of adaptive capacity that allows people to coexist with the hazardous environment in which they live. We use household interviews and surveys to build and test a substantive model that illustrates how social cohesion influences the decision to prepare for wildfire. We demonstrate that social cohesion, particularly community characteristics like ‘sense of community’ and ‘collective problem solving’, are community-based resources that support both the adoption of mechanical preparations, and the development of cognitive abilities and capacities that reduce vulnerability and enhance resilience to wildfire. We use the results of this work to highlight opportunities to transfer techniques and approaches from natural hazards research to climate change adaptation research to explore how the impacts attributed to the social components of social–ecological systems can be mitigated more effectively.  相似文献   

Subduction and back-arc activity at the Hikurangi convergent Margin,New Zealand     

Euan G. C. Smith  Tim Stern  Martin Reyners 《Pure and Applied Geophysics》1989,129(1-2):203-231

The Hikurangi Margin is a region of oblique subduction with northwest-dipping intermediate depth seismicity extending southwest from the Kermadec system to about 42°S. The current episode of subduction is at least 16–20 Ma old. The plate convergence rate varies along the margin from about 60 mm/a at the south end of the Kermadec Trench to about 45 mm/a at 42°S. The age of the Pacific lithosphere adjacent to the Hikurangi Trench is not known.The margin divides at about latitude 39°S into two quite dissimilar parts. The northern part has experienced andesitic volcanism for about 18 Ma, and back-arc extension in the last 4 Ma that has produced a back-arc basin onshore with high heaflow, thin crust and low upper-mantle seismic velocities. The extension appears to have arisen from a seawards migration of the Hikurangi Trench north of 39°S. Here the plate interface is thought to be currently uncoupled, as geodetic data indicate extension of the fore-arc basin, and historic earthquakes have not exceededM _s=7.South of 39°S there is no volcanism and a back-arc basin has been produced by downward flexure of the lithosphere due to strong coupling with the subducting plate. Heatflow in the basin is normal. Evidence for strong coupling comes from historic earthquakes of up to aboutM _s=8 and high rates of uplift on the southeast coast of the North Island.The reason for this division of the margin is not known but may be related to an inferred increase, from northeast to southwest, in the buoyancy of the Pacific lithosphere.  相似文献   

Calculated mineral equilibria in the greenschist-blueschist-eclogite facies in Na2O−FeO−MgO−Al2O3−SiO2−H2O     

Michel Guiraud  Tim Holland  Roger Powell 《Contributions to Mineralogy and Petrology》1990,104(1):85-98

The new, greatly expanded internally-consistent dataset of Holland and Powell includes thermodynamic data for a wide range of mineral end-members in common rock-forming minerals, in particular, including FeMg_-1 substitutions in glaucophane, garnet, chloritoid and carpholite, and FeMg_-1 and MgSiAl_-1Al_-1 substitutions in talc and chlorite. Moreover, we have the uncertainties and correlations for these data. With the data, we have calculated the full pressure-temperature phase diagram for the system Na₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NFMASH) for quartz (or coesite) and H₂O in excess, in the range 300°–800°C and 5–50 kbars. By solving the set of non-linear equations formed by the equilibrium relationships for an independent set of equations between the end-members in an assemblage in NFMASH, the compositions of the minerals (and PT) can be calculated. Thus the changes in MgSiAl_-1Al_-1 along NMASH reactions, and FeSiAl_-1Al_-1 along NFASH reactions, are calculated, and the changes in FeMg_-1 and MgSiAl_-1Al_-1 along NFMASH reactions are calculated. From this information it is straightforward to generate PT diagrams for specific rock compositions. Mineral assemblages and mineral compositional changes in the phase diagram are discussed in relation to greenschist, blueschist and eclogite facies assemblages in metapelitic rocks. It is found that the correspondence between the predictions of the phase diagrams and the observations on rocks is remarkably good. When semiquantitative extensions of the phase diagram to include Ca(MgFe)_-1, NaSiCa_-1Al_-1, Fe³⁺Al_-1 and KNa_-1 substitutions are taken into account the agreement is essentially complete.  相似文献   

Model for Predicting Virus Movement Through Soils   总被引：3，自引：0，他引：3  

Udoyara S. Tim  Saied Mostaghimi 《Ground water》1991,29(2):251-259

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Alluvial gold in Kalimantan, Indonesia: A colloidal origin?     

John B. Seeley  Tim J. Senden 《Journal of Geochemical Exploration》1994,50(1-3)

Extensive placer gold deposits occur in Quaternary palaeochannels in the Ampalit and Cempaga Buang drainage basins near Kasongan, Central Kalimantan, Indonesia. Broad interfluvial Pleistocene terraces surrounding the drainages also contain significant amounts of placer gold. The “channel” and “terrace” gold deposits together cover an area in excess of 180 km².Exploration for gold at Ampalit followed traditional alluvial methods, and assumed that mechanical and gravitational factors were the principal mechanisms affecting gold accumulation and concentration. Gold was considered to be physically reworked from terraces and redeposited in the present drainage channels or underlying, laterally displaced, palaeochannels. However, a comparison of gold grains from the Ampalit drainage channel and adjacent terraces indicates that gold grains in the drainage channels are possibly of colloidal origin and not mechanically transported to their present domain.The morphology of gold grains from the Ampalit drainage is compared with grains from adjacent terraces, using scanning electron microscopy and atomic force microscopy. The latter procedure allows for nondestructive analysis to give true three dimensional surface topography down to nanometre resolution. Evidence from force microscopy supports a colloidal origin for gold aggregates.The average purity of gold produced from the Ampalit dredging operation is 970 fine (970/1000) and the majority of examined gold grains extracted from beneath the drainage channel have a purity of 998 fine. This unusually high degree of purity suggests a selective dissolution/aggregation mechanism. We propose that gold is transported downward and laterally in groundwater percolating through the terrace sands and gravels to the current drainage as a humic acid-stabilised colloid. As the pregnant groundwater migrates toward the present drainage channels, it encounters a sleep chemical gradient and the colloid aggregates. The aggregation process occurs near clay zones within palaeochannel sands and gravels beneath the present river sediments. Here the colloidal aggregates form small grains (usually <1 mm) of extremely high purity.Today, the inherently unstable gold deposits at Ampalit are in a state of both aggregation and dissolution. Identification of low energy zones, location of point bar lags in palaeo and recent drainage channels, and the extrapolation of areas of low energy are not solely valid as exploration guides for gold accumulation in this environment. Exploration methods that also recognise ground water migration and composition, and the geochemical controls of dissolution and aggregation, will improve the ability to identify ore accumulations, thus contributing to improved economics of mining these deposits.  相似文献   

Determining Paleosol Topography Using Seismic Refraction     

Tim C. Rolph  John Shaw  Edward Derbyshire  An Zhisheng 《Quaternary Research》1994,42(3)

The seismic refraction reversed profiling technique has been used to investigate the topography of the last interglacial soil (paleosol S1) within the central Chinese Loess Plateau near Xifeng. The results suggest an essentially flat-lying soil at a depth which varies by only a few meters over an area of more than 10 km². In addition, the results indicate a high-velocity layer at 50-60 m depth which is thought to coincide with a layer of carbonate concretions at the base of paleosol S5. The results agree well with the local loess-paleosol stratigraphy for this area and indicate that the seismic refraction method is a rapid technique for investigating paleotopography.  相似文献