Narragansett Bay Hypoxic Event Characteristics Based on Fixed-Site Monitoring Network Time Series: Intermittency,Geographic Distribution,Spatial Synchronicity,and Interannual Variability     

Daniel L. Codiga  Heather E. Stoffel  Christopher F. Deacutis  Susan Kiernan  Candace A. Oviatt 《Estuaries and Coasts》2009,32(4):621-641

Low dissolved oxygen events were characterized in Narragansett Bay (NB), a moderate-size (370 km²) temperate estuary with a complex passage/embayment geometry, using time series from 2001 to 2006 at nine fixed-site monitoring stations. Metrics for event intensity and severity were the event-mean deficit relative to a threshold (mg O₂ l⁻¹) and the deficit-duration (mg O₂ l⁻¹ day; product of deficit and duration [day]). Hypoxia (threshold 2.9 mg O₂ l⁻¹) typically occurred intermittently from late June through August at most stations, as multiple (two to five per season) events each 2 to 7 days long with deficit-duration 2 to 5 mg O₂ l⁻¹ day. Conditions were more severe to the north and west, a pattern attributed to a north–south nutrient/productivity gradient and east–west structure of residual circulation. Spatial patterns for suboxic and severely hypoxic events (thresholds 4.8 and 1.4 mg O₂ l⁻¹) were similar. The view that different processes govern event variability in different regions, each influenced by local hydrodynamics, is supported by both weak spatial synchronicity (quantified using overlap of event times at different sites) and multiple linear regressions of biological and physical parameters against event severity. Interannual changes were prominent and season-cumulative hypoxia severity correlated with June-mean river runoff and June-mean stratification. Benthic ecological implications for areas experiencing events include: NB hypoxia classifies as periodic/episodic on a near-annual basis; highest direct mortality risk is to sensitive and moderately sensitive sessile species in the northern West Passage and western Greenwich Bay, with some risk to Upper Bay; direct risk to mobile species may be ameliorated by weak spatial synchronicity; and indirect impacts, including reduced growth rates and shifts in predator–prey balances, are very likely throughout the sampled area due to observed suboxic and hypoxic conditions.  相似文献   

Time Scales in the Unstable Atmospheric Surface Layer   总被引：2，自引：2，他引：0  

Meredith Metzger  Heather Holmes 《Boundary-Layer Meteorology》2008,126(1):29-50

Calculation of eddy covariances in the atmospheric surface layer (ASL) requires separating the instantaneous signal into mean and fluctuating components. Since the ASL is not statistically stationary, an inherent ambiguity exists in defining the mean quantities. The present study compares four methods of calculating physically relevant time scales in the unstable ASL that may be used to remove the unsteady mean components of instantaneous time signals, in order to yield local turbulent fluxes that appear to be statistically stationary. The four mean-removal time scales are: (t _c ) based on the location of the maximum in the ogive of the heat flux cospectra, () the location of the zero crossing in the multiresolution decomposition of the heat flux, (t ^*) the ratio of the mixed-layer depth over the convective velocity, and () the convergence time of the vertical velocity and temperature variances. The four time scales are evaluated using high quality, three-dimensional sonic anemometry data acquired at the Surface Layer Turbulence and Environmental Science Test (SLTEST) facility located on the salt flats of Utah’s western desert. Results indicate that and , with t _c achieving values about 2–3 times greater than t ^*. The sensitivity of the eddy covariances to the mean-removal time scale (given a fixed 4-h averaging period during midday) is also demonstrated.  相似文献   

Mineralogical and chemical characteristics of some natural jarosites     

George A. Desborough  Heather A. Lowers  Jane M. Hammarstrom  Reinhard W. Leinz 《Geochimica et cosmochimica acta》2010,74(3):1041-108

This paper presents a detailed study of the mineralogical, microscopic, thermal, and spectral characteristics of jarosite and natrojarosite minerals. Systematic mineralogic and chemical examination of a suite of 32 natural stoichiometric jarosite and natrojarosite samples from diverse supergene and hydrothermal environments indicates that there is only limited solid solution between Na and K at low temperatures, which suggests the presence of a solvus in the jarosite-natrojarosite system at temperatures below about 140 °C. The samples examined in this study consist of either end members or coexisting end-member pairs of jarosite and natrojarosite. Quantitative electron-probe microanalysis data for several natural hydrothermal samples show only end-member compositions for individual grains or zones, and no detectable alkali-site deficiencies, which indicates that there is no hydronium substitution within the analytical uncertainty of the method. In addition, there is no evidence of Fe deficiencies in the natural hydrothermal samples. Hydronium-bearing jarosite was detected in only one relatively young supergene sample suggesting that terrestrial hydronium-bearing jarosites generally are unstable over geologic timescales.Unit-cell parameters of the 20 natural stoichiometric jarosites and 12 natural stoichiometric natrojarosites examined in this study have distinct and narrow ranges in the a- and c-cell dimensions. There is no overlap of these parameters at the 1σ level for the two end-member compositions. Several hydrothermal samples consist of fine-scale (2-10 μm) intimate intergrowths of jarosite and natrojarosite, which could have resulted from solid-state diffusion segregation or growth zoning due to variations in the Na/K activity ratio of hydrothermal solutions.  相似文献   

Devil in the detail; The 1150–1000 Ma magmatic and structural evolution of the Ngaanyatjarra Rift,west Musgrave Province,Central Australia     

Paul M. Evins  R. Hugh Smithies  Heather M. Howard  Christopher L. Kirkland  Michael T.D. Wingate  Simon Bodorkos 《Precambrian Research》2010

Mafic and felsic rocks units of the Musgrave Province originally attributed to the c. 1075 Ma Giles Event of the greater Warakurna Large Igneous Province (LIP) are shown to be part of a complex sequence of magmatic and tectonic events punctuated over a span of at least 50 m.y. New geochronology and mapping resolve a sequence of at least 10 magmatic pulses with hiati of up to 10 m.y. consistent with a long-lived intracontinental rift setting. This rift, here named the Ngaanyatjarra Rift, features giant layered mafic-ultramafic Giles intrusions cut by a 10 km wide mafic-felsic magmatic shear zone. The latter is temporally related to the Warakurna LIP, however it is not clear that the Giles intrusions actually form part of the Warakurna LIP. Macroscopic folding and the formation of the large synmagmatic transpressional shear zone attest to synmagmatic basin inversion in the early stages of the rift. The extensive mafic to felsic volcanic rocks of the Tollu Group (traditionally grouped with the Giles Event) were emplaced 25–50 m.y. later than the c. 1075 Ma Warakurna LIP.  相似文献   

The implications of regional variations in rainfall for reconstructing rainfall patterns using tree rings          下载免费PDF全文

Heather A. Haines  Jon M. Olley 《水文研究》2017,31(16):2951-2960

In Australia, multidecadal periods of floods and droughts have major economic consequences. Due to the short duration of Australian instrumental precipitation records, it is difficult to determine the patterns of these multidecadal periods. Proxy records can be used to create long‐term rainfall reconstructions for regions that are lacking instrumental data. However, the spatial extent over which single‐site proxy records can be applied is poorly understood. Southeast Queensland (SEQ) is an area where tree rings can be used to reconstruct long‐term rainfall patterns, but their regional representation is unknown. In this study, the spatial variability in rainfall across SEQ is investigated from 1908 to 2007 using 140 instrumental rainfall stations. Pearson correlation analysis between stations is used to create groups at the r = 0.80, 0.85, and 0.90 correlation levels, and then annual deviations from the mean are determined. These patterns indicate that rainfall is not uniform across SEQ but can be broken into 2 main spatially consistent groups. Each of these groups is broken down into several subgroups with higher correlation levels. Long‐term streamflow records are found to be correlated to rainfall patterns local to the streamflow stations, indicating that analysis of extreme events should consider spatial precipitation variability. Finally, the only currently available proxy rainfall reconstruction for the region, a 140‐year Toona ciliata tree ring width record from Lamington National Park, is compared to rainfall groups at different correlation levels across all of SEQ. The correlation between the reconstruction and the rainfall station groupings is best for the groups within which the tree‐ring record is spatially located, and this correlation improves as rainfall group correlation increases. Correlation is nearly nonexistent for groupings located at a distance from the tree‐ring site. These results demonstrate the importance of assessing the spatial variability of precipitation so that the spatial applicability of proxy records can be assessed.  相似文献   

Geochemical homogeneity of a long-lived,large silicic system; evidence from the Cerro Galán caldera,NW Argentina     

Chris B. Folkes  Shanaka L. de Silva  Heather M. Wright  Raymond A. F. Cas 《Bulletin of Volcanology》2011,73(10):1455-1486

By applying a number of analytical techniques across a spectrum of spatial scales (centimeter to micrometer) in juvenile components, we show that the Cerro Galán volcanic system has repeatedly erupted magmas with nearly identical geochemistries over >3.5 Myr. The Cerro Galán system produced nine ignimbrites (∼5.6 to 2 Ma) with a cumulative volume of >1,200 km³ (DRE; dense rock equivalent) of calc-alkaline, high-K rhyodacitic magmas (68–71 wt.% SiO₂). The mineralogy is broadly constant throughout the eruptive sequence, comprising plagioclase, quartz, biotite, Fe–Ti oxides, apatite, and titanite. Early ignimbrite magmas also contained amphibole, while the final eruption, the most voluminous Cerro Galán ignimbrite (CGI; 2.08 ± 0.02 Ma) erupted a magma containing rare amphibole, but significant sanidine. Each ignimbrite contains two main juvenile clast types; dominant “white” pumice and ubiquitous but subordinate “grey” pumice. Fe–Ti oxide and amphibole-plagioclase thermometry coupled with amphibole barometry suggest that the grey pumice originated from potentially hotter and deeper magmas (800–840°C, 3–5 kbar) than the more voluminous white pumice (770–810°C, 1.5–2.5 kbar). The grey pumice is interpreted to represent the parental magmas to the Galán system emplaced into the upper crust from a deeper storage zone. Most inter-ignimbrite variations can be accounted for by differences in modal mineralogy and crystal contents that vary from 40 to 55 vol.% on a vesicle-free basis. Geochemical modeling shows that subtle bulk-rock variations in Ta, Y, Nb, Dy, and Yb between the Galán ignimbrites can be reconciled with differences in amounts of crystal fractionation from the “grey” parent magma. The amount of fractionation is inversely correlated with volume; the CGI (∼630 km³) and Real Grande Ignimbrite (∼390 km³) return higher F values (proportion of liquid remaining) than the older Toconquis Group ignimbrites (<50 km³), implying less crystal fractionation took place during the upper-crustal evolution of these larger volume magmas. We attribute this relationship to variations in magma chamber geometry; the younger, largest volume ignimbrites came from flat sill-like magma chambers, reducing the relative proportion of sidewall crystallization and fractionation compared to the older, smaller-volume ignimbrite eruptions. The grey pumice clasts also show evidence of silicic recharge throughout the history of the Cerro Galán system, and recharge days prior to eruption has previously been suggested based on reversely zoned (OH and Cl) apatite phenocrysts. A rare population of plagioclase phenocrysts with thin An-rich rims in juvenile clasts in many ignimbrites supports the importance of recharge in the evolution and potential triggering of eruptions. This study extends the notion that large volumes of nearly identical silicic magmas can be generated repeatedly, producing prolonged geochemical homogeneity from a long-lived magma source in a subduction zone volcanic setting. At Cerro Galán, we propose that there is a zone between mantle magma input and upper crustal chambers, where magmas are geochemically “buffered”, producing the underlying geochemical and isotopic signatures. This produces the same parental magmas that are delivered repeatedly to the upper crust. A lower-crustal MASH (melting, assimilation, storage, and homogenization) zone is proposed to act as this buffer zone. Subsequent upper crustal magmatic processes serve only to slightly modify the geochemistry of the magmas.  相似文献   

The fate and transport of nitrate in shallow groundwater in northwestern Mississippi, USA   总被引：2，自引：0，他引：2  

Heather L. Welch  Christopher T. Green  Richard H. Coupe 《Hydrogeology Journal》2011,19(6):1239-1252

Agricultural contamination of groundwater in northwestern Mississippi, USA, has not been studied extensively, and subsurface fluxes of agricultural chemicals have been presumed minimal. To determine the factors controlling transport of nitrate-N into the Mississippi River Valley alluvial aquifer, a study was conducted from 2006 to 2008 to estimate fluxes of water and solutes for a site in the Bogue Phalia basin (1,250 km²). Water-quality data were collected from a shallow water-table well, a vertical profile of temporary sampling points, and a nearby irrigation well. Nitrate was detected within 4.4 m of the water table but was absent in deeper waters with evidence of reducing conditions and denitrification. Recharge estimates from 6.2 to 10.9 cm/year were quantified using water-table fluctuations, a Cl^– tracer method, and atmospheric age-tracers. A mathematical advection-reaction model predicted similar recharge to the aquifer, and also predicted that 15% of applied nitrogen is leached into the saturated zone. With current denitrification and application rates, the nitrate-N front is expected to remain in shallow groundwater, less than 6–9 m deep. Increasing application rates resulting from intensifying agricultural demands may advance the nitrate-N front to 16–23 m, within the zone of groundwater pumping.  相似文献   

Community perceptions of REDD+: a case study from Papua New Guinea     

Matthew Leggett  Heather Lovell 《Climate Policy》2013,13(1):115-134

REDD projects have received considerable attention for their potential to mitigate the effects of climatic change. However, the existing literature has been slow to assess the impacts of proposed REDD projects on the livelihoods of forest communities in the developing world, or the implications of these local realities for the success of REDD+ initiatives in general. This study presents ethnographic research conducted with communities within the April-Salomei pilot REDD+ Project in Papua New Guinea (PNG). Several cases of institutional biases and uneven power relationships have been exploited by local elites to prevent landowners from making free and informed choices about their involvement in the project, although landowners and local communities are well positioned to capture forthcoming project benefits. By underestimating the scale and impact of traditional shifting cultivation practices, the credibility of the REDD+ project design and the value of any future carbon credits are critically undermined. Based on the actual practices found in PNG, the authors' radical proposal is to call for a halt on REDD development in PNG while institutional enabling conditions are improved, comprehensive landowner consultations conducted, and detailed mapping and genealogical surveys of landowners completed. Without these developments, future REDD+ projects in PNG are unlikely to benefit either the global climate or local development.  相似文献   

A review of solar type III radio bursts     

Hamish Andrew Sinclair Reid  ;Heather Ratcliffe 《中国天文和天体物理学报》2014,(7):773-804

Solar type III radio bursts are an important diagnostic tool in the understanding of solar accelerated electron beams. They are a signature of propagating beams of nonthermal electrons in the solar atmosphere and the solar system. Consequently, they provide information on electron acceleration and transport, and the conditions of the background ambient plasma they travel through. We review the observational properties of type III bursts with an emphasis on recent results and how each property can help identify attributes of electron beams and the ambient background plasma. We also review some of the theoretical aspects of type III radio bursts and cover a number of numerical efforts that simulate electron beam transport through the solar corona and the heliosphere.  相似文献   

Who has become more open to nuclear power because of climate change?     

Heather Barnes Truelove  Michael Greenberg 《Climatic change》2013,116(2):389-409

Even in the face of the March 2011 accident at the Fukushima nuclear power plant in Japan, nuclear power is being promoted in the U.S. as a necessary response to global climate change. Conducted prior to the Fukushima accident, the present study used a nation-wide telephone survey of 2751 U.S. residents to assess the factors that influence whether a person has become more open to nuclear power because of global climate change rather than supportive or opposed to nuclear power. Results showed that belief that climate change is a risk and is human-caused, belief that nuclear energy contributes to climate change, environmental support, cultural worldviews, and selected socio-demographics consistently predicted openness to nuclear power because of climate change. Implications of the current results and avenues for additional research on this topic are discussed.  相似文献