Two intriguing pluricolumnals (Crinoidea) from the Lower Palaeozoic of Powys and Shropshire     

Stephen K. Donovan  Anthony Butcher 《Proceedings of the Geologists' Association. Geologists' Association》2021,132(2):170-173

Complete fossils must be preferred to fragments for most palaeontological studies, but disarticulated specimens are nonetheless potential sources of noteworthy data. Two crinoid pluricolumnals are recorded from the lower Palaeozoic; informed discussion shows each is a basis for palaeobiological interpretation. Both are gracile and are probably belong to disparids. Floricrinus (col.) sp. is from the Silurian of Wenlock Edge, Shropshire, either from the Much Wenlock Limestone Formation (Wenlock) or, more likely, the Lower Elton Formation (Ludlow). This is the first crinoid from the Silurian of the British Isles with a pentapetaloid arrangement of the areola, a geometry common in the Middle-Upper Ordovician and higher in the geologic column. Pluricolumnal gen. et sp. indet. is from the Lower Llanvirn of Powys. One end of the otherwise straight specimen is tightly coiled. This is likely the proxistele, the most flexible region of the column, and the coiling occurred after the crown was lost by autotomy in response to an environmental disturbance.  相似文献   

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   

Community-based responses to climate hazards: typology and global analysis     

Paterson  Barbara  Charles  Anthony 《Climatic change》2019,152(3-4):327-343

Climatic Change - The severity and frequency of climate change hazards are increasing around the world. Because the impacts are most acutely felt in local communities, it is critical to improve...  相似文献   

Community involvement in fisheries management: Experiences in the Gulf of Thailand countries   总被引：1，自引：0，他引：1  

Nopparat Nasuchon  Anthony Charles 《Marine Policy》2010

This paper examines the involvement of coastal communities in fisheries management among the countries of the Gulf of Thailand—Malaysia, Vietnam, Cambodia and Thailand. Initiatives to decentralize management to local governing bodies, to utilize traditional management methods and to engage in community agreements to protect local resources are explored. An examination of recent experiences indicates some movement toward more local involvement in management. However, the study also leads to several suggestions for the future: in Vietnam and Cambodia, there is a need for significant legislation to control fisheries operations and greater clarity of the role of communities in management; in Malaysia, there is an overall need for more support to local fisheries management; and in Thailand, the need is for greater support of local-level enforcement and monitoring activities.  相似文献   

Kick‐and‐cook experiments on peridotite: simulating coseismic deformation and post‐seismic creep     

Anthony Druiventak  Agnes Matysiak  Jörg Renner  Claudia A. Trepmann 《地学学报》2012,24(1):62-69

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Effect of chemical affinity on feldspar hydrolysis rates in two natural weathering systems     

Michael Anthony Velbel 《Chemical Geology》1989,78(3-4):245-253

Recent studies of alkali-feldspar hydrolysis kinetics have shown that, in the near-neutral pH range, weathering rates in natural systems are up to three orders of magnitude slower than laboratory rates. It has been hypothesized that decelerated rates may result from lower thermodynamic affinities for the hydrolysis reaction in natural systems than in laboratory systems. However, the chemical affinities for the feldspar hydrolysis reaction in two well-constrained natural systems are significantly higher than the threshold value at which affinity would exert detectable influences on the reaction rates. Thus, the hypothesis is rejected, and closer proximity to thermodynamic equilibrium in natural weathering systems does not account for the observed discrepancy between natural and laboratory rates of feldspar hydrolysis. Differences in feldspar weathering rates between natural and laboratory systems are most likely due to a combination of experimental preparation artifacts, loss of reactive surface to the formation of etch pits in naturally weathered feldspars, and inhomogeneous access of reactive fluids to those surfaces.  相似文献   

Two-dimensional kinematic and rheological modeling of the 1912 pyroclastic flow,Katmai, Alaska     

James E. Beget  Anthony J. Limke 《Bulletin of Volcanology》1988,50(3):148-160

Pyroclastic flow emplacement is strongly influenced by eruption column height. A surface along which kinetic energy is zero theoretically connects the loci of eruption column collapse with all coeval ignimbrite termini. This surface is reconstructed as a two-dimensional energy line for the 1912 Katmai pyroclastic flow in the Valley of Ten Thousand Smokes from mapped flow termini and the runup of the ignimbrite onto obstructions and through passes. Extrapolation of the energy line to the vicinity of the source vent at Novarupta suggests the eruption column which generated the ignimbrite eruption was approximately 425 m high. The 1912 pyroclastic flow travelled about 25 km downvalley. Empirical velocity data calculated from runup elevations and surveyed centrifugal superelevations indicate initial velocities near Novarupta were greater than 79–88 m s^–1. The flow progressively decelerated and was travelling only 2–8 m s^–1 when it crossed a moraine 16 km downvalley. The constant slope of the energy line away from Novarupta suggests the flow was systematically slowed by internal and basal friction. Using a simple physical model to calculate flow velocities and a constant kinetic friction coefficient (Heim coefficient) of 0.04 derived from the reconstructed energy line, the flow is estimated to have decelerated at an average rate of –0.16 m s^–2 and to have taken approximately 9.5 minutes to travel 25 km down the Valley of Ten Thousand Smokes. The shear strength of the flowing ignimbrite at the moraine was approximately 0.5 kPa, and its Bingham viscosity when it crossed the moraine was 3.5 × 10³ P. If the flow was Newtonian, its viscosity was 4.2 × 10³ P. Reynolds and Froude numbers at the moraine were only 41–62 and 0.84–1.04, respectively, indicating laminar, subcritical flow.  相似文献   

Book reviews     

F. Alejandro Nava  Manfred Koch  Andrzej Hanyga  Jacek Leliwa-Kopystyński  Frazer Tabeart  Sueo Veno  Andrzej W. Wernik  Yu. N. Korenkov  Kenji Satake  Carlos Lozano  Aysel Karafistan  Ian N. James  Anthony Dore  Jacek Wałczewski  Krzysztof Haman  Brian F. Farrell 《Pure and Applied Geophysics》1994,143(4):753-780

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The Wavelength-Smoothing Method For Approximating Broad-Band Wave Propagation Through Complicated Velocity Structures     

Anthony Lomax 《Geophysical Journal International》1994,117(2):313-334

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Chemistry of alkali extraction of brown coals—I. Kinetics, characterisation and implications to coalification     

Louis S.K. Pang  Anthony M. Vassallo  Michael A. Wilson   《Organic Geochemistry》1990,16(4-6)

Three Australian brown coals have been separated into humin and humic acid fractions and studied by high resolution solid state ¹³C NMR spectroscopy and Fourier transform IR spectroscopy. The aromatic rings of the humic acids are highly substituted showing that extensive cross linking must have occurred during formation from wood lignin and tannin. However, the humins contain more aliphatic carbon and hydrogen than the corresponding humic acids. This shows that little cross linking has occurred with other components of the brown coal such as resins, waxes cutin and algal detritus, and cross linking has not rendered the aromatics alkali insoluble. The kinetics of extraction are complex and not simple first order. This is reflected in the chemical composition of the humic acid which is extraction temperature dependent. We also observed that there is a conversion of aromatic carbon to aliphatic carbon and gas during extraction, probably by alkaline oxidation, resulting in ring opening. A range of suitable model compounds have been studied to confirm this finding. Such a mechanism may account for the modification of lignin in oxidising environments such as those occurring in the initial stages of coalification (lignite or brown coal formation) and in soils.  相似文献