Contrasting punctuated zircon growth in two syn-erupted rhyolite magmas from Tarawera volcano: Insights to crystal diversity in magmatic systems     

Sonja Storm  Phil Shane  Axel K. Schmitt  Jan M. Lindsay 《Earth and Planetary Science Letters》2011,301(3-4):511-520

Two mineralogically and chemically distinct rhyolite magmas (T1 and T3) were syn-erupted from the same conduit system during the 21.9 ka basalt intrusion-triggered Okareka eruption from Tarawera volcano, New Zealand. High spatial resolution U–Th disequilibrium dating of zircon crystals at the ~ 3–5 μm scale reveals a protracted yet discontinuous zircon crystallization history within the magmatic system. Both magma types contain zircon whose interiors predate the eruption by up to 200 ka. The dominant age peak in the T1 magma is ~ 30 ka with subordinate peaks at ~ 45, ~ 75, and ~ 100 ka, whereas the T3 magma has a dominant zircon interior age peak at ~ 90 ka with smaller modes at ~ 35 and ~ 150 ka. These patterns are consistent with isolated pockets of crystallization throughout the evolution of the system. Crystal rim analyses yield ages ranging from within error of the eruption age to at least ~ 90 ka prior to eruption, highlighting that zircon crystallization frequently stalled long before the eruption. Continuous depth profiling from crystal rims inward demonstrates protracted growth histories for individual crystals (up to ~ 100 ka) that were punctuated by asynchronous hiatuses of up to 30 ka in duration. Disparate zircon growth histories can result from localized thermal perturbations caused by mafic intrusions into a silicic reservoir. The crystal age heterogeneity at hand-sample scale requires considerable crystal transport and mixing. We propose that crystal mixing was achieved through buoyancy instabilities caused by mafic magma flow through crystal mush. A terminal pre-eruptive rejuvenation event was capable of mobilizing voluminous melts that erupted, but was too short (< 10²–10³ years) to result in extensive zircon growth. The contrasting, punctuated zircon histories argue against closed-system fractional crystallization models for silicic magmatism that require protracted cooling times following a mostly liquid starting condition.  相似文献   

Spatial modeling of environmental vulnerability of marine finfish aquaculture using GIS-based neuro-fuzzy techniques     

Navas JM  Telfer TC  Ross LG 《Marine pollution bulletin》2011,62(8):1786-1799

Combining GIS with neuro-fuzzy modeling has the advantage that expert scientific knowledge in coastal aquaculture activities can be incorporated into a geospatial model to classify areas particularly vulnerable to pollutants. Data on the physical environment and its suitability for aquaculture in an Irish fjard, which is host to a number of different aquaculture activities, were derived from a three-dimensional hydrodynamic and GIS models. Subsequent incorporation into environmental vulnerability models, based on neuro-fuzzy techniques, highlighted localities particularly vulnerable to aquaculture development.The models produced an overall classification accuracy of 85.71%, with a Kappa coefficient of agreement of 81%, and were sensitive to different input parameters. A statistical comparison between vulnerability scores and nitrogen concentrations in sediment associated with salmon cages showed good correlation.Neuro-fuzzy techniques within GIS modeling classify vulnerability of coastal regions appropriately and have a role in policy decisions for aquaculture site selection.  相似文献   

Use of single-grain geochemistry of cryptic tuffs and volcaniclastic sandstones improves the tephrostratigraphic framework of Olduvai Gorge,Tanzania     

Lindsay J. McHenry  Harald Stollhofen  Ian G. Stanistreet 《Quaternary Research》2013

Single-grain geochemical composition of volcaniclastic sandstones can be a potential tool to improve correlations of mixed pyroclastic/epiclastic deposits. To test this, trachytic tuffs of the paleoanthropologically important FLK, FLK N, and FLK NN sites of Pleistocene Olduvai Gorge Bed I (Tanzania) are used as an established tephrostratigraphic framework against which to test volcaniclastic sandstone correlations. Fluvio-lacustrine sandstones and tuff samples were collected from eight archeological trenches between Tuffs IB and ID across a 500-m transect, including Leakey's famous Zinjanthropus (FLK) and OH 7/OH 8 (FLK NN) sites. A previously unknown, thin, fine, mineralogically unique, black trachyandesitic fallout ash was discovered below Tuff IC. Compositions of individual augite, feldspar and titanomagnetite grains from sandstones between Tuffs IB and IC reveal some IB-equivalent material, and a new compositional assemblage distinct from the sandwiching marker tuffs. Mineral compositions of the “tripartite” volcaniclastic sandstone between Tuffs IC and ID are similar to ID. Volcaniclastic sandstone grain fingerprints further refine correlations between fluvio-lacustrine sections within the area, providing support for proposed high-resolution stratigraphic reconstruction of the Zinjanthropus and OH 7/OH 8 land surfaces. This method might be applied to other sections where pyroclastic particles are admixed but distinct tuffs are not preserved.  相似文献   

Controls on microbial activity and tidal flat evolution in Shark Bay,Western Australia     

Ricardo J. Jahnert  Lindsay B. Collins 《Sedimentology》2013,60(4):1071-1099

Microbial deposits at Shark Bay constitute a diverse living microbial carbonate system, developed in a semi‐arid, highly evaporative marine setting. Three tidal flats located in different embayments within the World Heritage area were investigated in order to compare microbial deposits and their Holocene evolution. The stressing conditions in the intertidal–subtidal environment have produced a microbial ecosystem that is trapping, binding and biologically inducing CaCO₃ precipitation, producing laminated stromatolites (tufted, smooth and colloform), non‐laminated thrombolitic forms (pustular) and cryptomicrobial non‐laminated forms (microbial pavement). A general shallowing‐upwards sedimentary cycle was recognized and correlated with Holocene sea‐level variations, where microbial deposits constitute the younger (2360 years bp ) and shallower sedimentary veneer. In addition, sediments have been documented with evidence of exposure during the Holocene, from 1040 to 940 ¹⁴C years bp , when sea‐level was apparently lower than present. Filamentous bacteria constitute the dominant group in the blister, tufted and smooth mat types, and coccus bacteria dominate the pustular, colloform and microbial pavement deposit types. In the subtidal environment within colloform and pavement structures, microbial communities coexist with organisms such as bivalves, serpulids, diatoms, green algae (Acetabularia), crustaceans, foraminifera and micro‐gastropods, which are responsible for exoskeleton supply and extensive bioturbation. The internal fabric of the microbial deposits is laminated, sub‐laminar, scalloped, irregular or clotted, depending on the amount of fine‐grained carbonate and the natural ability of microbial communities to trap and bind particles or induce carbonate precipitation. Nilemah tidal flat contains the thickest (1·3 m) and best‐developed microbial sedimentary system; its deposition pre‐dated the Rocky Point and Garden Point tidal flats, with the most positive isotope values for δ¹³C and δ¹⁸O, reflecting strong microbial activity in a highly evaporative environment. There is an evolutionary series preserved within the tidal flats reflecting relative ages and degree of salinity elevation.  相似文献   

Soil CO2 flux baseline in an urban monogenetic volcanic field: the Auckland Volcanic Field, New Zealand     

Agnès Mazot  Elaine R. Smid  Luitgard Schwendenmann  Hugo Delgado-Granados  Jan Lindsay 《Bulletin of Volcanology》2013,75(11):1-9

The Auckland Volcanic Field (AVF) is a dormant monogenetic basaltic field located in Auckland, New Zealand. Though soil gas CO₂ fluxes are routinely used to monitor volcanic regions, there have been no published studies of soil CO₂ flux or soil gas CO₂ concentrations in the AVF to date or many other monogenetic fields worldwide. We measured soil gas CO₂ fluxes and soil gas CO₂ concentrations in 2010 and 2012 in varying settings, seasons, and times of day to establish a baseline soil CO₂ flux and to determine the major sources of and controlling influences on Auckland's soil CO₂ flux. Soil CO₂ flux measurements varied from 0 to 203 g m^?2 day^?1, with an average of 27.1 g m^?2 day^?1. Higher fluxes were attributed to varying land use properties (e.g., landfill). Using a graphical statistical approach, two populations of CO₂ fluxes were identified. Isotope analyses of δ¹³CO₂ confirmed that the source of CO₂ in the AVF is biogenic with no volcanic component. These data may be used to assist with eruption forecasting in the event of precursory activity in the AVF, and highlight the importance of knowing land use history when assessing soil gas CO₂ fluxes in urban environments.  相似文献   

Coarse clast ridge sequences as suitable archives for past storm events? Case study on the Houtman Abrolhos,Western Australia     

Anja M. Scheffers  Sander R. Scheffers  Dieter H. Kelletat  Peter Squire  Lindsay Collins  Yuexing Feng  Jian‐Xin Zhao  Renaud Joannes‐Boyau  Simon Matthias May  Gerhard Schellmann  Heather Freeman 《第四纪科学杂志》2012,27(7):713-724

Prehistoric storm records are relatively scarce in most parts of the world. This article presents storm records derived from coral rubble‐based geological archives of the Houtman Abrolhos Archipelago located off the west coast of Australia, where the southernmost coral reefs of the Indian Ocean are found. Winter storm swell from the circum‐Antarctic ‘Brave Westerlies’, as well as tropical cyclone waves, have left numerous ridge systems on dozens of islands of the archipelago, all composed of coral rubble from adjacent reefs. At three islands, seven ridge systems were dated by three different methods: U‐series (68 dates), radiocarbon (64 dates), electron spin resonance (7 dates); 139 radiometric dates span the last 5500 years of the Holocene. In contrast to the geomorphological interpretation, the age sequences show ‘inversions’, hiatuses and different ages for the same ridge, all pointing to complicated ridge formation processes. Time gaps, some exceeding 1000 years, are interpreted as phases of erosion and not as phases without storm activity. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Modelling the effects of permafrost loss on discharge from a wetland‐dominated,discontinuous permafrost basin     

Lindsay E. Stone  Xing Fang  Kristine M. Haynes  Manuel Helbig  John W. Pomeroy  Oliver Sonnentag  William L. Quinton 《水文研究》2019,33(20):2607-2626

Permafrost degradation in the peat‐rich southern fringe of the discontinuous permafrost zone is catalysing substantial changes to land cover with expansion of permafrost‐free wetlands (bogs and fens) and shrinkage of forest‐dominated permafrost peat plateaux. Predicting discharge from headwater basins in this region depends upon understanding and numerically representing the interactions between storage and discharge within and between the major land cover types and how these interactions are changing. To better understand the implications of advanced permafrost thaw‐induced land cover change on wetland discharge, with all landscape features capable of contributing to drainage networks, the hydrological behaviour of a channel fen sub‐basin in the headwaters of Scotty Creek, Northwest Territories, Canada, dominated by peat plateau–bog complexes, was modelled using the Cold Regions Hydrological Modelling platform for the period of 2009 to 2015. The model construction was based on field water balance observations, and performance was deemed adequate when evaluated against measured water balance components. A sensitivity analysis was conducted to assess the impact of progressive permafrost loss on discharge from the sub‐basin, in which all units of the sub‐basin have the potential to contribute to the drainage network, by incrementally reducing the ratio of wetland to plateau in the modelled sub‐basin. Simulated reductions in permafrost extent decreased total annual discharge from the channel fen by 2.5% for every 10% decrease in permafrost area due to increased surface storage capacity, reduced run‐off efficiency, and increased landscape evapotranspiration. Runoff ratios for the fen hydrological response unit dropped from 0.54 to 0.48 after the simulated 50% permafrost area loss with a substantial reduction of 0.47 to 0.31 during the snowmelt season. The reduction in peat plateau area resulted in decreased seasonal variability in discharge due to changes in the flow path routing, with amplified low flows associated with small increases in subsurface discharge, and decreased peak discharge with large reductions in surface run‐off.  相似文献   

A comparison between manganese nodules and cobalt crust economics in a scenario of mutual exclusivity     

Simone Martino  Lindsay M. Parson 《Marine Policy》2012,36(3):790-800

The past 20 years have been characterised by limited interest in the economic viability of deep seabed mining with the exception of those mineral ores rich in precious metals such as polymetallic sulphides. This paper goes against the tide. After reviewing the most significant literature, it compares the economic feasibility of mining polymetallic manganese nodules and cobalt-rich ferromanganese crusts in a scenario of mutual exclusivity. It uses a new indicator, the cobalt-nickel price ratio, whose aim is to verify at what metal prices the net present value of the two mining projects equalises. Previous research has shown that the turning margin between manganese nodules and cobalt crusts is a cobalt price of $50/kg. The present paper revises this by showing that the choice between mining crust and nodules depends on fluctuations in the price of nickel. A Monte Carlo simulation proves that cobalt crust mining could be reasonably preferred to manganese nodules at cobalt prices of $40-60/kg if the price of nickel fell below $11/kg. Within this condition, investors would receive the minimum acceptable internal rate of return of 15%, which is at a level of risk closer to the land-based mining. However, when the price of nickel passes the $11/kg threshold, the equilibrium between the two ventures can be reached only at a cobalt price greater than $100/kg, causing cobalt crust to become uncompetitive. Finally, the paper, recognising that prices are not unique drivers, introduces legal, political, technological and environmental concerns to show that the final choice between the two mining ventures cannot be merely driven by economic issues.  相似文献   

Detecting mass substructure in galaxy clusters: an aperture mass statistic for gravitational flexion     

Adrienne Leonard  Lindsay J. King  Stephen M. Wilkins 《Monthly notices of the Royal Astronomical Society》2009,395(3):1438-1448

Gravitational flexion has been introduced as a technique by which one can map out and study substructure in clusters of galaxies. Previous analyses involving flexion have measured the individual galaxy–galaxy flexion signal, or used either parametric techniques or a Kaiser, Squires and Broadhurst (KSB)-type inversion to reconstruct the mass distribution in Abell 1689. In this paper, we present an aperture mass statistic for flexion, and apply it to the lensed images of background galaxies obtained by ray-tracing simulations through a simple analytic mass distribution and through a galaxy cluster from the Millennium Simulation. We show that this method is effective at detecting and accurately tracing structure within clusters of galaxies on subarcminute scales with high signal to noise even using a moderate background source number density and image resolution. In addition, the method provides much more information about both the overall shape and the small-scale structure of a cluster of galaxies than can be achieved through a weak lensing mass reconstruction using gravitational shear data. Lastly, we discuss how the zero-points of the aperture mass might be used to infer the masses of structures identified using this method.  相似文献   

Habitat use and longitudinal distribution patterns of native fish from a near pristine Stewart Island,New Zealand,stream     

W. Lindsay Chadderton  Richard M. Allibone 《新西兰海洋与淡水研究杂志》2013,47(3):487-499

Distribution, abundance, and habitat preferences of native fish were investigated down the length of a relatively pristine, medium‐sized, fourth‐order stream on Stewart Island, New Zealand. A limited fish fauna was recorded (six species), including three threatened large galaxiid species (Galaxias argenteus, Galaxias fasciatus, and Galaxias brevipinnis), which have restricted distributions on mainland New Zealand. Results indicated that these large galaxiids occupy diverse habitats including pools and backwaters within the mainstems of this stream. Their extensive distributions and wide habitat usage were attributed to factors such as the intact catchment vegetation, unmodified stream channel, and the absence of introduced fishes, particularly salmonids. Results suggest that some native species have been excluded from mainstem habitats elsewhere in New Zealand. It was also evident that interactions among the native species influenced habitat use; G. brevipinnis avoided backwaters, runs, and pools in reaches occupied by G. fasciatus and Anguilla dieffenbachii; whereas G. fasciatus appeared to avoid habitats occupied by G. argenteus and A dieffenbachii. Maximum densities and biomass of galaxiids and eels occurred in deepwater habitats (>0.75 m). Furthermore the fish inhabiting these deeper waters were larger and more likely to be female. These findings have significant implications for the design and application of sampling protocols for native New Zealand fishes and for the protection of their habitats. If deeper waters are not sampled then species, sex, and size biased data may result.  相似文献