New mineral activity–composition relations for thermodynamic calculations in metapelitic systems     

R. W. White  R. Powell  T. J. B. Holland  T. E. Johnson  E. C. R. Green 《Journal of Metamorphic Geology》2014,32(3):261-286

New activity–composition (a–x) relations for minerals commonly occurring in metapelites are presented for use with the internally consistent thermodynamic dataset of Holland & Powell ( 2011 , Journal of Metamorphic Geology, 29 , 333–383). The a–x relations include a broader consideration of Fe₂O₃ in minerals, changes to the formalism of several phases and order–disorder in all ferromagnesian minerals where Fe–Mg mixing occurs on multiple sites. The a–x relations for chlorite, biotite, garnet, chloritoid, staurolite, cordierite, orthopyroxene, muscovite, paragonite and margarite have been substantially reparameterized using the approach outlined in the companion paper in this issue. For the first time, the entire set of a–x relations for the common ferromagnesian minerals in metapelitic rocks is parameterized simultaneously, with attention paid to ensuring that they can be used together to calculate phase diagrams of geologically appropriate topology. The a–x relations developed are for use in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂ (NCKFMASHTO) system for both subsolidus and suprasolidus conditions. Petrogenetic grids in KFMASH and KFMASHTO are similar in topology to those produced with earlier end‐member datasets and a–x relations, but with some notable differences. In particular, in subsolidus equilibria, the FeO/(FeO + MgO) of garnet is now greater than in coexisting staurolite, bringing a number of key staurolite‐bearing equilibria into better agreement with inferences from field and petrographic observations. Furthermore, the addition of Fe³⁺ and Ti to a number of silicate phases allows more plausible equilibria to be calculated in relevant systems. Pseudosections calculated with the new a–x relations are also topologically similar to equivalent diagrams using earlier a–x relations, although with many low variance fields shifting in P–T space to somewhat lower pressure conditions.  相似文献   

Equatorial Pacific gravity lineaments: interpretations with basement topography along seismic reflection lines     

Neil C. Mitchell  Huw Davies 《Marine Geophysical Researches》2018,39(4):551-565

The central equatorial Pacific is interesting for studying clues to upper mantle processes, as the region lacks complicating effects of continental remnants or major volcanic plateaus. In particular, the most recently produced maps of the free-air gravity field from satellite altimetry show in greater detail the previously reported lineaments west of the East Pacific Rise (EPR) that are aligned with plate motion over the mantle and originally suggested to have formed from mantle convection rolls. In contrast, the gravity field 600 km or farther west of the EPR reveals lineaments with varied orientations. Some are also parallel with plate motion over the mantle but others are sub-parallel with fracture zones or have other orientations. This region is covered by pelagic sediments reaching ~?500–600 m thickness so bathymetry is not so useful for seeking evidence for plate deformation across the lineaments. We instead use depth to basement from three seismic reflection cruises. In some segments of these seismic data crossing the lineaments, we find that the co-variation between gravity and basement depth is roughly compatible with typical densities of basement rocks (basalt, gabbro or mantle), as expected for some explanations for the lineaments (e.g., mantle convection rolls, viscous asthenospheric inter-fingering or extensional deformation). However, some other lineaments are associated with major changes in basement depth with only subtle changes in the gravity field, suggesting topography that is locally supported by varied crustal thickness. Overall, the multiple gravity lineament orientations suggest that they have multiple origins. In particular, we propose that a further asthenospheric inter-fingering instability mechanism could occur from pressure variations in the asthenosphere arising from regional topography and such a mechanism may explain some obliquely oriented gravity lineaments that have no other obvious origin.  相似文献   

MoisturEC: A New R Program for Moisture Content Estimation from Electrical Conductivity Data   总被引：1，自引：0，他引：1       下载免费PDF全文

Neil Terry  Frederick D. Day‐Lewis  Dale Werkema  John W. Lane Jr 《Ground water》2018,56(5):823-831

Noninvasive geophysical estimation of soil moisture has potential to improve understanding of flow in the unsaturated zone for problems involving agricultural management, aquifer recharge, and optimization of landfill design and operations. In principle, several geophysical techniques (e.g., electrical resistivity, electromagnetic induction, and nuclear magnetic resonance) offer insight into soil moisture, but data‐analysis tools are needed to “translate” geophysical results into estimates of soil moisture, consistent with (1) the uncertainty of this translation and (2) direct measurements of moisture. Although geostatistical frameworks exist for this purpose, straightforward and user‐friendly tools are required to fully capitalize on the potential of geophysical information for soil‐moisture estimation. Here, we present MoisturEC, a simple R program with a graphical user interface to convert measurements or images of electrical conductivity (EC) to soil moisture. Input includes EC values, point moisture estimates, and definition of either Archie parameters (based on experimental or literature values) or empirical data of moisture vs. EC. The program produces two‐ and three‐dimensional images of moisture based on available EC and direct measurements of moisture, interpolating between measurement locations using a Tikhonov regularization approach.  相似文献   

Comparing threshold definition techniques for rainfall‐induced landslides: A national assessment using radar rainfall          下载免费PDF全文

Benjamin Postance  John Hillier  Tom Dijkstra  Neil Dixon 《地球表面变化过程与地形》2018,43(2):553-560

Translational landslides and debris flows are often initiated during intense or prolonged rainfall. Empirical thresholds aim to classify the rain conditions that are commonly associated with landslide occurrence and therefore improve understating of these hazards and predictive ability. Objective techniques that are used to determine these thresholds are likely to be affected by the length of the rain record used, yet this is not routinely considered. Moreover, remotely sensed spatially continuous rainfall observations are under‐exploited. This study compares and evaluates the effect of rain record length on two objective threshold selection techniques in a national assessment of Scotland using weather radar data. Thresholds selected by ‘threat score’ are sensitive to rain record length whereas, in a first application to landslides, ‘optimal point’ (OP) thresholds prove relatively consistent. OP thresholds increase landslide detection and may therefore be applicable in early‐warning systems. Thresholds combining 1‐ and 12‐day antecedence variables best distinguish landslide initiation conditions and indicate that Scottish landslides may be initiated by lower rain accumulation and intensities than previously thought. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   

Tidal Habitats Support Large Numbers of Invasive Blue Catfish in a Chesapeake Bay Subestuary     

Mary C. Fabrizio  Troy D. Tuckey  Robert J. Latour  Gary C. White  Alicia J. Norris 《Estuaries and Coasts》2018,41(3):827-840

The introduction of a non-native freshwater fish, blue catfish Ictalurus furcatus, in tributaries of Chesapeake Bay resulted in the establishment of fisheries and in the expansion of the population into brackish habitats. Blue catfish are an invasive species in the Chesapeake Bay region, and efforts are underway to limit their impacts on native communities. Key characteristics of the population (population size, survival rates) are unknown, but such knowledge is useful in understanding the impact of blue catfish in estuarine systems. We estimated population size and survival rates of blue catfish in tidal habitats of the James River subestuary. We tagged 34,252 blue catfish during July–August 2012 and 2013; information from live recaptures (n = 1177) and dead recoveries (n = 279) were used to estimate annual survival rates and population size using Barker’s Model in a Robust Design and allowing for heterogeneity in detection probabilities. The blue catfish population in the 12-km study area was estimated to be 1.6 million fish in 2013 (95% confidence interval [CI] adjusted for overdispersion: 926,307–2,914,208 fish). Annual apparent survival rate estimates were low: 0.16 (95% CI 0.10–0.24) in 2012–2013 and 0.44 (95% CI 0.31–0.58) in 2013–2014 and represent losses from the population through mortality, permanent emigration, or both. The tagged fish included individuals that were large enough to exhibit piscivory and represented size classes that are likely to colonize estuarine habitats. The large population size that we estimated was unexpected for a freshwater fish in tidal habitats and highlights the need to effectively manage such species.  相似文献   

Tropical cyclone contribution to extreme rainfall over southwest Pacific Island nations     

Deo  Anil  Chand  Savin S.  Ramsay  Hamish  Holbrook  Neil J.  McGree  Simon  Magee  Andrew  Bell  Samuel  Titimaea  Mulipola  Haruhiru  Alick  Malsale  Philip  Mulitalo  Silipa  Daphne  Arieta  Prakash  Bipen  Vainikolo  Vaiola  Koshiba  Shirley 《Climate Dynamics》2021,56(11):3967-3993

Southwest Pacific nations are among some of the worst impacted and most vulnerable globally in terms of tropical cyclone (TC)-induced flooding and accompanying risks. This study objectively quantifies the fractional contribution of TCs to extreme rainfall (hereafter, TC contributions) in the context of climate variability and change. We show that TC contributions to extreme rainfall are substantially enhanced during active phases of the Madden–Julian Oscillation and by El Niño conditions (particularly over the eastern southwest Pacific region); this enhancement is primarily attributed to increased TC activity during these event periods. There are also indications of increasing intensities of TC-induced extreme rainfall events over the past few decades. A key part of this work involves development of sophisticated Bayesian regression models for individual island nations in order to better understand the synergistic relationships between TC-induced extreme rainfall and combinations of various climatic drivers that modulate the relationship. Such models are found to be very useful for not only assessing probabilities of TC- and non-TC induced extreme rainfall events but also evaluating probabilities of extreme rainfall for cases with different underlying climatic conditions. For example, TC-induced extreme rainfall probability over Samoa can vary from ~ 95 to ~ 75% during a La Niña period, if it coincides with an active or inactive phase of the MJO, and can be reduced to ~ 30% during a combination of El Niño period and inactive phase of the MJO. Several other such cases have been assessed for different island nations, providing information that have potentially important implications for planning and preparing for TC risks in vulnerable Pacific Island nations.

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New Validity Conditions for the Multivariate Matérn Coregionalization Model,with an Application to Exploration Geochemistry     

Emery  Xavier  Porcu  Emilio  White  Philip 《Mathematical Geosciences》2022,54(6):1043-1068

Mathematical Geosciences - This paper addresses the problem of finding parametric constraints that ensure the validity of the multivariate Matérn covariance for modeling the spatial...  相似文献   

Pattern,style and timing of British–Irish Ice Sheet advance and retreat over the last 45 000 years: evidence from NW Scotland and the adjacent continental shelf     

Tom Bradwell  Derek Fabel  Chris D. Clark  Richard C. Chiverrell  David Small  Rachel K. Smedley  Margot H. Saher  Steven G. Moreton  Dayton Dove  S. Louise Callard  Geoff A. T. Duller  Alicia Medialdea  Mark D. Bateman  Matthew J. Burke  Neil McDonald  Sean Gilgannon  Sally Morgan  David H. Roberts  Colm ó Cofaigh 《第四纪科学杂志》2021,36(5):871-933

Predicting the future response of ice sheets to climate warming and rising global sea level is important but difficult. This is especially so when fast-flowing glaciers or ice streams, buffered by ice shelves, are grounded on beds below sea level. What happens when these ice shelves are removed? And how do the ice stream and the surrounding ice sheet respond to the abruptly altered boundary conditions? To address these questions and others we present new geological, geomorphological, geophysical and geochronological data from the ice-stream-dominated NW sector of the last British–Irish Ice Sheet (BIIS). The study area covers around 45 000 km² of NW Scotland and the surrounding continental shelf. Alongside seabed geomorphological mapping and Quaternary sediment analysis, we use a suite of over 100 new absolute ages (including cosmogenic-nuclide exposure ages, optically stimulated luminescence ages and radiocarbon dates) collected from onshore and offshore, to build a sector-wide ice-sheet reconstruction combining all available evidence with Bayesian chronosequence modelling. Using this information we present a detailed assessment of ice-sheet advance/retreat history, and the glaciological connections between different areas of the NW BIIS sector, at different times during the last glacial cycle. The results show a highly dynamic, partly marine, partly terrestrial, ice-sheet sector undergoing large size variations in response to sub-millennial-scale climatic (Dansgaard–Oeschger) cycles over the last 45 000 years. Superimposed on these trends we identify internally driven instabilities, operating at higher frequency, conditioned by local topographic factors, tidewater dynamics and glaciological feedbacks during deglaciation. Specifically, our new evidence indicates extensive marine-terminating ice-sheet glaciation of the NW BIIS sector during Greenland Stadials 12 to 9 – prior to the main ‘Late Weichselian’ ice-sheet glaciation. After a period of restricted glaciation, in Greenland Interstadials 8 to 6, we find good evidence for rapid renewed ice-sheet build-up in NW Scotland, with the Minch ice-stream terminus reaching the continental shelf edge in Greenland Stadial 5, perhaps only briefly. Deglaciation of the NW sector took place in numerous stages. Several grounding-zone wedges and moraines on the mid- and inner continental shelf attest to significant stabilizations of the ice-sheet grounding line, or ice margin, during overall retreat in Greenland Stadials 3 and 2, and to the development of ice shelves. NW Lewis was the first substantial present-day land area to deglaciate, in the first half of Greenland Stadial 3 at a time of globally reduced sea-level c. 26 kabp , followed by Cape Wrath at c. 24 kabp. The topographic confinement of the Minch straits probably promoted ice-shelf development in early Greenland Stadial 2, providing the ice stream with additional support and buffering it somewhat from external drivers. However, c. 20–19 kabp , as the grounding-line migrated into shoreward deepening water, coinciding with a marked change in marine geology and bed strength, the ice stream became unstable. We find that, once underway, grounding-line retreat proceeded in an uninterrupted fashion with the rapid loss of fronting ice shelves – first in the west, then the east troughs – before eventual glacier stabilization at fjord mouths in NW Scotland by ~17 kabp. Around the same time, ~19–17 kabp , ice-sheet lobes readvanced into the East Minch – possibly a glaciological response to the marine-instability-triggered loss of adjacent ice stream (and/or ice shelf) support in the Minch trough. An independent ice cap on Lewis also experienced margin oscillations during mid-Greenland Stadial 2, with an ice-accumulation centre in West Lewis existing into the latter part of Heinrich Stadial 1. Final ice-sheet deglaciation of NW mainland Scotland was punctuated by at least one other coherent readvance at c. 15.5 kabp , before significant ice-mass losses thereafter. At the glacial termination, c. 14.5 kabp , glaciers fed outwash sediment to now-abandoned coastal deltas in NW mainland Scotland around the time of global Meltwater Pulse 1A. Overall, this work on the BIIS NW sector reconstructs a highly dynamic ice-sheet oscillating in extent and volume for much of the last 45 000 years. Periods of expansive ice-sheet glaciation dominated by ice-streaming were interspersed with periods of much more restricted ice-cap or tidewater/fjordic glaciation. Finally, this work indicates that the role of ice streams in ice-sheet evolution is complex but mechanistically important throughout the lifetime of an ice sheet – with ice streams contributing to the regulation of ice-sheet health but also to the acceleration of ice-sheet demise via marine ice-sheet instabilities.  相似文献   

Inter‐laboratory Characterisation of Apatite Reference Materials for Chlorine Isotope Analysis     

Alicja Wudarska  Ewa S&#x;aby  Michael Wiedenbeck  Jaime D. Barnes  Magali Bonifacie  Neil C. Sturchio  Grard Bardoux  Frdric Couffignal  Johannes Glodny  Linnea Heraty  Timm John  Christof Kusebauch  Sathish Mayanna  Franziska D. H. Wilke  Ewa Deput 《Geostandards and Geoanalytical Research》2021,45(1):121-142

Here we report on a set of six apatite reference materials (chlorapatites MGMH#133648, TUBAF#38 and fluorapatites MGMH#128441A, TUBAF#37, 40, 50) which we have characterised for their chlorine isotope ratios; these RMs span a range of Cl mass fractions within the apatite Ca₁₀(PO₄)₆(F,Cl,OH)₂ solid solution series. Numerous apatite specimens, obtained from mineralogical collections, were initially screened for ³⁷Cl/³⁵Cl homogeneity using SIMS followed by δ³⁷Cl characterisation by gas source mass spectrometry using both dual‐inlet and continuous‐flow modes. We also report major and key trace element compositions as determined by EPMA. The repeatability of our SIMS results was better than ± 0.10‰ (1s) for the five samples with > 0.5 % m/m Cl and ± 0.19‰ (1s) for the low Cl abundance material (0.27% m/m). We also observed a small, but significant crystal orientation effect of 0.38‰ between the mean ³⁷Cl/³⁵Cl ratios measured on three oriented apatite fragments. Furthermore, the results of GS‐IRMS analyses show small but systematic offset of δ³⁷Cl_SMOC values between the three laboratories. Nonetheless, all studied samples have comparable chlorine isotope compositions, with mean 10³δ³⁷Cl_SMOC values between +0.09 and +0.42 and in all cases with 1s ≤ ± 0.25.  相似文献   

Representation and participation in formulating Nepal’s REDD+ approach     

Poshendra Satyal  Esteve Corbera  Neil Dawson  Hari Dhungana  Gyanu Maskey 《Climate Policy》2019,19(6):S8-S22

ABSTRACT

REDD+ is an international policy aimed at incentivizing forest conservation and management and improving forest governance. In this article, we interrogate how newly articulated REDD+ governance processes established to guide the formulation of Nepal’s REDD+ approach address issues of participation for different social groups. Specifically, we analyse available forums of participation for different social groups, as well as the nature of their representation and degree of participation during the country’s REDD+ preparedness phase. We find that spaces for participation and decision-making in REDD+ have been to date defined and dominated by government actors and influential civil society groups, whereas the influence of other actors, particularly marginalized groups such as Dalits and women’s organizations, have remained limited. REDD+ has also resulted in a reduction of influence for some hitherto powerful actors (e.g. community forestry activists) and constrained their critical voice. These governance weaknesses related to misrepresentation and uneven power relations in Nepal cast doubt on the extent to which procedural justice has been promoted through REDD+ and imply that implementation may, as a consequence, lack the required social legitimacy and support. We discuss possible ways to address these shortcomings, such as granting greater prominence to neglected civil society forums within the REDD+ process, allowing for an increase in their influence on policy design, enhancing capacity and leadership of marginalized groups and institutionalizing participation through continued forest governance reform.

Key policy insights

• Participation is a critical asset in public policy design.

• Ensuring wide and meaningful participation can enhance policy legitimacy and thus its endorsement and potential effective implementation.

• Fostering inclusive processes through dedicated forums such as multi-stakeholder groups can help overcome power dynamics.

• While REDD+ is open to participation by different actors through a variety of formal means, many countries lack a clear framework for participation in national policy processes.

• Nepal’s experience with representation and participation of non-state actors in its REDD+ preparedness programme provides useful insights for similar social and policy contexts.

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