A special issue honoring Bob Berner     

Don E. Canfield  Timothy W. Lyons  John W. Morse 《Geochimica et cosmochimica acta》2006,70(23):5651-5652

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Lode–gold mineralization in the Tanami region, northern Australia     

David L. Huston  Leon Vandenberg  Andrew S. Wygralak  Terrence P. Mernagh  Leon Bagas  Andrew Crispe  Alexis Lambeck  Andrew Cross  Geoff Fraser  Nick Williams  Kurt Worden  Tony Meixner  Bruce Goleby  Leonie Jones  Pat Lyons  David Maidment 《Mineralium Deposita》2007,42(1-2):175-204

The Tanami region of northern Australia has emerged over the last two decades as the largest gold-producing region in the Northern Territory. Gold is hosted by epigenetic quartz veins in sedimentary and mafic rocks, and by sulfide-rich replacement zones within iron formation. Although limited, geochronological data suggest that most mineralization occurred at about 1,805–1,790 Ma, during a period of extensive granite intrusion, although structural relationships suggest that some deposits predate this period. There are three main goldfields in the Tanami region: the Dead Bullock Soak goldfield, which hosts the world-class Callie deposit; The Granites goldfield; and the Tanami goldfield. In the Dead Bullock Soak goldfield, deposits are hosted by carbonaceous siltstone and iron formation where a late (D₅) structural corridor intersects an early F₁ anticlinorium. In The Granites goldfield, deposits are hosted by highly sheared iron formation and are interpreted to predate D₅. The Tanami goldfield consists of a large number of small, mostly basalt-hosted deposits that probably formed at a high structural level during D₅. The D₅ structures that host most deposits formed in a convergent structural regime with σ ₁ oriented between E–W and ENE–WSW. Structures active during D₅ include NE-trending oblique thrust (dextral) faults and ESE-trending (sinistral) faults that curve into N- to NNW-trending reverse faults localized in supracrustal belts between and around granite complexes. Granite intrusions also locally perturbed the stress field, possibly localizing structures and deposits. Forward modeling and preliminary interpretations of reflection seismic data indicate that all faults extend into the mid-crust. In areas characterized by the N- to NW-trending faults, orebodies also tend to be N- to NW-trending, localized in dilational jogs or in fractured, competent rock units. In areas characterized by ESE-trending faults, the orebodies and veins tend to strike broadly east at an angle consistent with tensional fractures opened during E–W- to ENE–WSW-directed transpression. Many of these deposits are hosted by reactive rock units such as carbonaceous siltstone and iron formation. Ore deposition occurred at depths ranging from 1.5 to 11 km from generally low to moderate salinity carbonic fluids with temperatures from 200 to 430°C, similar to lode–gold fluids elsewhere in the world. These fluids are interpreted as the product of metamorphic dewatering caused by enhanced heat flow, although it is also possible that the fluids were derived from coeval granites. Lead isotope data suggest that lead in the ore fluids had multiple sources. Hydrogen and oxygen isotope data are consistent with both metamorphic and magmatic origins for ore fluids. Gold deposition is interpreted to be caused by fluid unmixing and sulfidation of host rocks. Fluid unmixing is caused by three different processes: (1) CO₂ unmixing caused by interaction of ore fluids with carbonaceous siltstone; (2) depressurization caused by pressure cycling in shear zones; and (3) boiling as ore fluids move to shallow levels. Deposits in the Tanami region may illustrate the continuum model of lode–gold deposition suggested by Groves (Mineralium Deposita 28:366–374, 1993) for Archean districts.  相似文献   

Solar-wind–magnetosphere coupling,including relativistic electron energization,during high-speed streams     

L.R. Lyons  D.-Y. Lee  H.-J. Kim  J.A. Hwang  R.M. Thorne  R.B. Horne  A.J. Smith 《Journal of Atmospheric and Solar》2009,71(10-11):1059-1072

High geomagnetic activity occurs continuously during high-speed solar wind streams, and fluxes of relativistic electrons observed at geosynchronous orbit enhance significantly. High-speed streams are preceded by solar wind compression regions, during which time there are large losses of relativistic electrons from geosynchronous orbit. Weak to moderate geomagnetic storms often occur during the passage of these compression regions; however, we find that the phenomena that occur during the ensuing high-speed streams do not depend on whether or not a preceding storm develops. Large-amplitude Alfvén waves occur within the high-speed solar wind streams, which are expected to lead to intermittent intervals of significantly enhanced magnetospheric convection and to thus also lead to repetitive substorms due to repetitively occurring reductions in the strength of convection. We find that such repetitive substorms are clearly discernible in the LANL geosynchronous energetic particle data during high-speed stream intervals. Global auroral images are found to show unambiguously that these events are indeed classical substorms, leading us to conclude that substorms are an important contributor to the enhanced geomagnetic activity during high-speed streams. We used the onsets of these substorms as indicators of preceding periods of enhanced convection and of reductions in convection, and we have used ground-based chorus observations from the VELOX instrument at Halley station as an indicator of magnetospheric chorus intensities. These data show evidence that it is the periods of enhanced convection that precede substorm expansions, and not the expansions themselves, that lead to the enhanced dawn-side chorus wave intensity that has been postulated to cause the energization of relativistic electrons. If this inference is correct, and if it is chorus that energizes the relativistic electrons, then high-speed solar wind streams lead to relativistic electron flux enhancements because the embedded large-amplitude Alfvén waves give multi-day periods of intermittent significantly enhanced convection.  相似文献   

The informal Australian city     

Kurt Iveson  Craig Lyons  Stephanie Clark  Sara Weir 《The Australian geographer》2019,50(1):11-27

In recent years, a vibrant literature on urban informality has emerged in urban geography. A key claim of this literature is that informality is not only present in cities in the Global South, and that the concept can be usefully applied to understand important dimensions of urbanisation in cities in the Global North. In this paper, we apply insights from that literature to an examination of informality in Australian cities, as a means of deepening our understanding of changes currently underway in urban governance and politics. Drawing on that literature and our own research into informal practices in Australian cities, we examine the forces that help to produce informality, the diverse forms and agents of informality, the forms of authority that they enact, and their relationship to formal economies, regulations and politics. We use examples of informal housing, informal labour, do-it-yourself music venues, graffiti and its regulation, food foraging, and queer community building in Sydney to illustrate our arguments. We conclude by reflecting on the lessons learnt across these diverse cases for our understanding of urban governance and politics in Australia, and offer an agenda for further research on this issue.  相似文献   

The effects of impacts on the cooling rates of iron meteorites     

Richard J. Lyons  Timothy J. Bowling  Fred J. Ciesla  Thomas M. Davison  Gareth S. Collins 《Meteoritics & planetary science》2019,54(7):1604-1618

Iron meteorites provide a record of the thermal evolution of their parent bodies, with cooling rates inferred from the structures observed in the Widmanstätten pattern. Traditional planetesimal thermal models suggest that meteorite samples derived from the same iron core would have identical cooling rates, possibly providing constraints on the sizes and structures of their parent bodies. However, some meteorite groups exhibit a range of cooling rates or point to uncomfortably small parent bodies whose survival is difficult to reconcile with dynamical models. Together, these suggest that some meteorites are indicating a more complicated origin. To date, thermal models have largely ignored the effects that impacts would have on the thermal evolution of the iron meteorite parent bodies. Here we report numerical simulations investigating the effects that impacts at different times have on cooling rates of cores of differentiated planetesimals. We find that impacts that occur when the core is near or above its solidus, but the mantle has largely crystallized can expose iron near the surface of the body, leading to rapid and nonuniform cooling. The time period when a planetesimal can be affected in this way can range between 20 and 70 Myr after formation for a typical 100 km radius planetesimal. Collisions during this time would have been common, and thus played an important role in shaping the properties of iron meteorites.  相似文献   

Knowledge,influence, and firm-level change: A geographic analysis of board membership associated with Canada’s growing and declining businesses     

Murray D. Rice  Sean Tierney  Sean O’Hagan  Donald Lyons  Milford B. Green 《Geoforum》2012,43(5):959-968

Clarification of the factors that drive business expansion and contraction is among the most important topics that can be addressed by research in economic geography and related disciplines. This study analyzes the development of Canadian business by examining the connection between firm-level growth and decline and elite knowledge and influence connections that Canadian businesses use to link themselves to other corporations and cities nationally and globally. The research focuses on the inter-firm and inter-city linkages embodied in the boards of directors of Canada’s leading corporations. Corporate directorships have an important geographic dimension, as firms invite people from a variety of places and backgrounds to serve on their boards. This study finds that board member selection practices among Canadian firms can be linked directly to business growth and decline. Benchmarking board composition against firm expansion and decline in terms of inflation-adjusted annual revenue change over the 1991–2006 period, this study shows that growing Canadian firms are associated with directors from a much broader range of national and international cities than firms that experienced decline over the same period. Firm growth in Canada thus appears to be related to boards encompassing diverse members from national and international locations, while firm decline appears to be associated with boards including more local and regional members. The conclusion interprets the implications of board member diversity and suggests that broadly-based boards can play a role in encouraging firm growth through access to elite knowledge and influence networks.  相似文献   

Eolian deposition of trace elements onto Taylor Valley Antarctic glaciers     

Sarah K. Fortner  W. Berry Lyons  John W. Olesik 《Applied Geochemistry》2011,26(11):1897-1904

The major ion and trace element chemistry in four 60-110 cm deep snow pits was examined from three Antarctic Taylor Valley glaciers (Commonwealth, Canada and Howard), all located within 20 km of the Ross Sea. Taylor Valley (TV) (77°30′S, 163°15′E) is part of the McMurdo Dry Valleys (MDV), the largest ice-free area in Antarctica. Snow chemistry of these glaciers is strongly influenced by cross-valley (SW-NE and NE-SW) winds that deposit eolian materials, including mineral dust and soluble salts. Arsenic, Cd, Cu, Mo, Pb, Sb and Sn in recent TV glacier snow are almost exclusively derived from eolian dust. The relative magnitude of element variations with depth relate to the exposure of snow sites to prevalent winds. Eolian deposition is a primary source of these elements to Canada Glacier snow and supra and proglacial streams. Eolian processes are likely very important to the delivery and availability of nutrients and, potentially toxic elements to dry valley Antarctic ecosystems.  相似文献   

Land cover effects on soil infiltration capacity measured using plot scale rainfall simulation in steep tropical lowlands of Central Panama     

Guy F. Litt  Fred L. Ogden  Alexis Mojica  Jan M. H. Hendrickx  Edward W. Kempema  Christopher B. Gardner  Mario Bretfeld  Jason A. Regina  J. Bruce J. Harrison  Yanyan Cheng  W. Berry Lyons 《水文研究》2020,34(4):878-897

Understanding land use/land cover (LULC) effects on tropical soil infiltration is crucial for maximizing watershed scale hydro-ecosystem services and informing land managers. This paper reports results from a multiyear investigation of LULC effects on soil bulk infiltration in steep, humid tropical, and lowland catchments. A rainfall simulator applied water at measured rates on 2 × 6 m plots producing infiltration through structured, granulated, and macroporous Ferralsols in Panama's central lowlands. Time-lapse electrical resistivity tomography (ERT) helped to visualize infiltration depth and bulk velocity. A space-for-time substitution methodology allowed a land-use history investigation by considering the following: (a) a continuously heavy-grazed cattle pasture, (b) a rotationally grazed traditional cattle pasture, (c) a 4-year-old (y.o.) silvopastoral system with nonnative improved pasture grasses and managed intensive rotational grazing, (d) a 7 y.o. teak (Tectona grandis) plantation, (e) an approximately 10 y.o. secondary succession forest, (f) a 12 y.o. coffee plantation (Coffea canephora), (g) an approximately 30 y.o. secondary succession forest, and (h) a >100 y.o. secondary succession forest. Within a land cover, unique plot sites totalled two at (a), (c), (d), (e), and (g); three at (b); and one at (f) and (h). Our observations confirmed measured infiltration scale dependency by comparing our 12 m² plot-scale measurements against 8.9 cm diameter core-scale measurements collected by others from nearby sites. Preferential flow pathways (PFPs) significantly increased soil infiltration capacity, particularly in forests greater than or equal to 10 y.o. Time-lapse ERT observations revealed shallower rapid bulk infiltration and increased rapid lateral subsurface flow in pasture land covers when compared with forest land covers and highlighted how much subsurface flow pathways can vary within the Ferralsol soil class. Results suggest that LULC effects on PFPs are the dominant mechanism by which LULC affects throughfall partitioning, runoff generation, and flow pathways.  相似文献   

Poker flat radar observations of the magnetosphere–ionosphere coupling electrodynamics of the earthward penetrating plasma sheet following convection enhancements     

L.R. Lyons  S. Zou  C.J. Heinselman  M.J. Nicolls  P.C. Anderson 《Journal of Atmospheric and Solar》2009,71(6-7):717-728

The plasma sheet moves earthward (equatorward in the ionosphere) after enhancements in convection, and the electrodynamics of this response is strongly influenced by Region 2 magnetosphere–ionosphere coupling. We have used Poker Flat Advanced Modular Incoherent Scatter Radar (PFISR) observations associated with two relatively abrupt southward turnings of the IMF to provide an initial evaluation of aspects of this response. The observations show that strong westward sub-auroral polarization streams (SAPS) flow regions moved equatorward as the plasma sheet electron precipitation (the diffuse aurora) penetrated equatorward following the IMF southward turnings. Consistent with our identification of these flows as SAPS, concurrent DMSP particle precipitation measurements show the equatorial boundary of ion precipitation equatorward of the electron precipitation boundary and that westward flows lie within the low-conductivity region between the two boundaries where the plasma sheet ion pressure gradient is expected to drive downward R2 currents. Evidence for these downward currents is seen in the DMSP magnetometer observations. Preliminary examination indicates that the SAPS response seen in the examples presented here may be common. However, detailed analysis will be required for many more events to reliably determine if this is the case. If so, it would imply that SAPS are frequently an important aspect of the inner magnetospheric electric field distribution, and that they are critical for understanding the response of the magnetosphere–ionosphere system to enhancements in convection, including understanding the earthward penetration of the plasma sheet. This earthward penetration is critical to geomagnetic disturbance phenomena such as the substorm growth phase and the formation of the stormtime ring current. Additionally, for one example, a prompt electric field response to the IMF southward turnings is seen within the inner plasma sheet.  相似文献   

PFISR observations of strong azimuthal flow bursts in the ionosphere and their relation to nightside aurora     

S. Zou  L.R. Lyons  M.J. Nicolls  C.J. Heinselman 《Journal of Atmospheric and Solar》2009,71(6-7):729-737

Flow bursts within the ionosphere are the ionospheric signatures of flow bursts in the plasma sheet and have been associated with poleward boundary intensifications (PBIs). Some PBIs extend equatorward from the polar cap boundary, where they can be roughly divided into north–south-aligned and east–west-aligned structures. In this paper, we present two flow burst events observed by the new Poker Flat Advanced Modular Incoherent Scatter Radar (PFISR) in the pre-midnight auroral zone on 28 April 2007, one towards the west and the other towards the east. In both cases, enhanced flows lasted for about 8–10 min with peak velocities exceeding 1500 m/s. The concurrently measured electron density showed that the flow bursts occurred in low conductivity regions. However, near the poleward (equatorward) edge of the westward (eastward) flow burst, strong electron density enhancements were observed in the E region, indicating the presence of discrete auroral arcs. Auroral images from the Polar spacecraft were available at the time of the eastward flow burst and they indicate that this burst was associated with an east–west-aligned auroral structure that connected at later MLT to a north–south structure. In addition, simultaneous precipitating particle energy spectrum measured by the the Defense Meteorological Satellites Program (DMSP) F13 satellite reveals that this auroral structure resulted from mono-energetic electron precipitation associated with a significant field-aligned potential drop. These observations show direct evidence of the relationship between flow bursts, field-aligned currents and auroral intensifications, and suggest that eastward/westward flow bursts are associated with east–west-oriented PBI structures that have extended well within the plasma sheet. This is in contrast to the equatorward-directed flow that has been previously inferred for PBIs near the polar cap boundary and for north–south auroral structures. This paper illustrates the use of the PFISR radar for studying the magnetosphere–ionosphere coupling of flow bursts.  相似文献