Organic carbon isotope ratios (δ13C) of Arctic Amerasian Continental shelf sediments     

A. S. Naidu  L. W. Cooper  B. P. Finney  R. W. Macdonald  C. Alexander  I. P. Semiletov 《International Journal of Earth Sciences》2000,89(3):522-532

Organic matter origins are inferred from carbon isotope ratios ('¹³C) in recent continental shelf sediments and major rivers from 465 locations from the north Bering-Chukchi-East Siberian-Beaufort Sea, Arctic Amerasia. Generally, there is a cross-shelf increase in '¹³C, which is due to progressive increased contribution seaward of marine-derived organic carbon to surface sediments. This conclusion is supported by the correlations between sediment '¹³C, OC/N, and '¹⁵N. The sources of total organic carbon (TOC) to the Amerasian margin sediments are primarily from marine water-column phytoplankton and terrigenous C₃ plants constituted of tundra taiga and angiosperms. In contrast to more temperate regions, the source of TOC from terrigenous C₄ and CAM plants to the study area is probably insignificant because these plants do not exist in the northern high latitudes. The input of carbon to the northern Alaskan shelf sediments from nearshore kelp community (Laminaria solidungula) is generally insignificant as indicated by the absence of high sediment '¹³C values (-16.5 to -13.6‰) which are typical of the macrophytes. Our study suggests that the isotopic composition of sediment TOC has potential application in reconstructing temporal changes in delivery and accumulation of organic matter resulting from glacial-interglacial changes in sea level and environments. Furthermore, recycling and advection of the extensive deposits of terrestrially derived organic matter from land, or the wide Amerasian margin, could be a mechanism for elevating total CO₂ and pCO₂ in the Arctic Basin halocline.  相似文献   

Sea ice,hydrological, and biological processes in the Churchill River estuary region,Hudson Bay     

Z.A. Kuzyk  R.W. Macdonald  M.A. Granskog  R.K. Scharien  R.J. Galley  C. Michel  D. Barber  G. Stern 《Estuarine, Coastal and Shelf Science》2008

A conceptual scheme for the transition from winter to spring is developed for a small Arctic estuary (Churchill River, Hudson Bay) using hydrological, meteorological and oceanographic data together with models of the landfast ice. Observations within the Churchill River estuary and away from the direct influence of the river plume (Button Bay), between March and May 2005, show that both sea ice (production and melt) and river water influence the region's freshwater budget. In Button Bay, ice production in the flaw lead or polynya of NW Hudson Bay result in salinization through winter until the end of March, followed by a gradual freshening of the water column through April–May. In the Churchill Estuary, conditions varied abruptly throughout winter–spring depending on the physical interaction among river discharge, the seasonal landfast ice, and the rubble zone along the seaward margin of the landfast ice. Until late May, the rubble zone partially impounded river discharge, influencing the surface salinity, stratification, flushing time, and distribution and abundance of nutrients in the estuary. The river discharge, in turn, advanced and enhanced sea ice ablation in the estuary by delivering sensible heat. Weak stratification, the supply of riverine nitrogen and silicate, and a relatively long flushing time (∼ 6 days) in the period preceding melt may have briefly favoured phytoplankton production in the estuary when conditions were still poor in the surrounding coastal environment. However, in late May, the peak flow and breakdown of the ice-rubble zone around the estuary brought abrupt changes, including increased stratification and turbidity, reduced marine and freshwater nutrient supply, a shorter flushing time, and the release of the freshwater pool into the interior ocean. These conditions suppressed phytoplankton productivity while enhancing the inventory of particulate organic matter delivered by the river. The physical and biological changes observed in this study highlight the variability and instability of small frozen estuaries during winter–spring transition, which implies sensitivity to climate change.  相似文献   

Assessing the performance of a physically based hydrological model using a proxy‐catchment approach in an agricultural environment     

Veronica Escobar‐Ruiz  Hugh G. Smith  William H. Blake  Neil Macdonald 《水文研究》2019,33(24):3119-3137

Physically based models are useful frameworks for testing intervention strategies designed to reduce elevated sediment loads in agricultural catchments. Evaluating the success of these strategies depends on model accuracy, generally established by a calibration and evaluation process. In this contribution, the physically based SHETRAN model was assessed in two similar U.K. agricultural catchments. The model was calibrated on the Blackwater catchment (18 km²) and evaluated in the adjacent Kit Brook catchment (22 km²) using 4 years of 15 min discharge and suspended sediment flux data. Model sensitivity to changes in single and multiple combinations of parameters and sensitivity to changes in digital elevation model resolution were assessed. Model flow performance was reasonably accurate with a Nash–Sutcliffe efficiency coefficient of 0.78 in Blackwater and 0.60 in Kit Brook. In terms of event prediction, the mean of the absolute percentage of difference (μAbs_diff) between measured and simulated flow volume (Qv), peak discharge (Qp), sediment yield (Sy), and peak sediment flux (Sp) showed larger values in Kit Brook (48% [Qv], 66% [Qp], 298% [Sy], and 438% [Sp]) compared with the Blackwater catchment (30% [Qv], 41% [Qp], 106% [Sy], and 86% [Sp]). Results indicate that SHETRAN can produce reasonable flow prediction but performs less well in estimation of sediment flux, despite reasonably similar hydrosedimentary behaviour between catchments. The sensitivity index showed flow volume sensitive to saturated hydraulic conductivity and peak discharge to the Strickler coefficient; sediment yield was sensitive to the overland flow erodibility coefficient and peak sediment flux to raindrop/leaf soil erodibility coefficient. The multiparameter sensitivity analysis showed that different combinations of parameters produced similar model responses. Model sensitivity to grid resolution presented similar flow volumes for different digital elevation model resolutions, whereas event peak and duration (for both flow and sediment flux) were highly sensitive to changes in grid size.  相似文献   

A Transit of Venus Possibly Misinterpreted as an Unaided-Eye Sunspot Observation in China on 9 December 1874     

Hayakawa  Hisashi  Sôma  Mitsuru  Tanikawa  Kiyotaka  Willis  David M.  Wild  Matthew N.  Macdonald  Lee T.  Imada  Shinsuke  Hattori  Kentaro  Richard Stephenson  F. 《Solar physics》2019,294(9):1-13

Solar Physics - Large sunspots can be observed with the unaided eye under suitable atmospheric seeing conditions. Such observations are of particular value because the frequency of their appearance...  相似文献   

Permits to burn: weeds,slow violence,and the extractive future of northern Australia     

Timothy Neale  Jennifer Mairi Macdonald 《The Australian geographer》2019,50(4):417-433

ABSTRACT

This essay narrates the ‘slow violence’, or creeping environmental harms taking place within contemporary environmental governance. It centres on a tall, dense and highly flammable introduced pasture species Gamba grass (Andropogon gayanus), which was listed as a weed across north Australian jurisdictions in 2008. Since this time, it has continued to expand its reach across the Northern Territory (NT). With a potential invasion range of over 380,000?sqkm², this grass is a serious threat to many more-than-human worlds in the north, including Indigenous-led and Indigenous-owned environmental service economies and multimillion-dollar projects engaged in savanna fire management for carbon credits. Drawing upon fieldwork and interviews with a range of public servants, landholders and researchers in the NT between 2015 and 2018, this essay demonstrates how environmental governance is being undermined through specific institutions and practices. Through an ethnographic reading of weed management documents, including several legal permits to grow Gamba grass within the NT’s ‘eradication zone’, this essay narrates the diverse threads of a pressing ‘slow’ disaster. The unfolding story of Gamba grass, we suggest, is instructive for those seeking to understand the present and future of resource extraction or ‘extractivism’ in Australia and elsewhere.  相似文献   

Evidence for extreme fractionation of peralkaline silicic magmas, the Boseti volcanic complex, Main Ethiopian Rift     

Ray Macdonald  Bogus?aw Bagiński  Fiorenzo Ronga  Piotr Dzier?anowski  Michele Lustrino  Andrea Marzoli  Leone Melluso 《Mineralogy and Petrology》2012,104(3-4):163-175

Matrix glass and melt inclusions in phenocrysts from pantellerite lavas of the Boseti volcanic complex, Ethiopia, record extreme fractionation of peralkaline silicic magma, with Al₂O₃ contents as low as 2.3?wt.%, FeO* contents up to 17?wt.% and SiO₂ contents ~65?wt.%. The new data, and published data for natural and experimental glasses, suggest that the effective minimum composition for peralkaline silicic magmas has ~5?wt.% Al₂O₃, 13?wt.% FeO* and 66?±?2?wt.% SiO₂. The dominant fractionating assemblage is alkali feldspar?+?fayalite?+?hedenbergite?+?oxides?±?quartz. Feldspar – melt relationships indicate that the feldspar is close to the minimum on the albite-orthoclase solid solution loop through the entire crystallization history. There is petrographic, mineralogical and geochemical evidence that magma mixing may have been a common process in the Boseti rhyolites.  相似文献   

Manganese Sources and Sinks in the Arctic Ocean with Reference to Periodic Enrichments in Basin Sediments   总被引：2，自引：0，他引：2  

Robie W. Macdonald  Charles Gobeil 《Aquatic Geochemistry》2012,18(6):565-591

Between 1990 and 2007, twenty-nine box cores were recovered within the Arctic Ocean spanning shelf, slope and basin locations, and analyzed for aluminum (Al), manganese (Mn), other inorganic components and organic carbon (C_Org). Using these core data together with literature values, we have constructed budgets for Al and Mn in the Arctic Ocean. Most of the Al and Mn entering the Arctic comes from rivers or coastal erosion, and almost all of these two elements is trapped within the Arctic. Total Mn distributions in sediments reflect the recycling and loss of much of the Mn from shelf sediments with ultimate burial over the slopes and in basins. Mn enrichments observed as bands in long cores from the basins appear to co-occur with inter-glacial periods. Our Mn budget suggests that change in sea level associated with the accumulation and melting of glaciers is a likely cause for the banding. The Arctic Ocean, which presently contains as much as 50% shelf area, loses most of that when global sea level falls by?~120?m during glacial maxima. With lower sea level, Mn input from rivers and coastal erosion declines, and inputs become stored in permafrost on the sub-aerial shelves or at the shelf margin. Sea-level rise re-establishes coastal erosion and large riverine inputs at the margin and initiates the remobilization of Mn stored on shelves by turning on algal productivity, which provides the C_Org required to reduce sedimentary Mn oxyhydroxides.  相似文献   

Towards environmental management of water turbidity within open coastal waters of the Great Barrier Reef     

Rachael K. Macdonald  Peter V. Ridd  James C. Whinney  Piers Larcombe  David T. Neil 《Marine pollution bulletin》2013

Water turbidity and suspended sediment concentration (SSC) are commonly used as part of marine monitoring and water quality plans. Current management plans utilise threshold SSC values derived from mean-annual turbidity concentrations. Little published work documents typical ranges of turbidity for reefs within open coastal waters. Here, time-series turbidity measurements from 61 sites in the Great Barrier Reef (GBR) and Moreton Bay, Australia, are presented as turbidity exceedance curves and derivatives. This contributes to the understanding of turbidity and SSC in the context of environmental management in open-coastal reef environments. Exceedance results indicate strong spatial and temporal variability in water turbidity across inter/intraregional scales. The highest turbidity across 61 sites, at 50% exceedance (T₅₀) is 15.3 NTU and at 90% exceedance (T₉₀) 4.1 NTU. Mean/median turbidity comparisons show strong differences between the two, consistent with a strongly skewed turbidity regime. Results may contribute towards promoting refinement of water quality management protocols.  相似文献   

On the origin of fore-arc basins: new evidence of formation by rifting from the Jurassic of Alexander Island, Antarctica     

Macdonald  Leat  Doubleday  & Kelly 《地学学报》1999,11(4):186-193

The Middle Jurassic–Lower Cretaceous Fossil Bluff Group of Alexander Island, Antarctica represents the fill of a fore-arc basin unconformably overlying an accretionary complex. Like most fore-arc basins, this example had been considered to have a passive origin, as a topographic hollow between the arc and the trench-slope break. Recent discoveries of igneous rock coeval with sedimentation have altered this view. Oxfordian–Kimmeridgian basaltic and rhyolitic sills and lava flows are found in a restricted area at the north of the basin, within a single formation. Chemically, most basalts are high-Nb types, which cannot have originated in a supra-subduction zone setting. Since the age of emplacement of these rocks coincides with a gap in the record of plutonism in the Antarctic Peninsula volcanic arc, it is concluded that a late Jurassic pause in subduction led to active rifting to form the fore-arc basin.  相似文献   

Comment on “Rivers, runoff, and reefs” by McLaughlin et al. [Global Planetary Change 39 (2003) 191–199]     

I.A. Macdonald  C.T. Perry  P. Larcombe 《Global and Planetary Change》2005,45(4):333-337

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