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81.
Antoni?CurcóEmail author Carles?Ibà?ez John?W.?Day Narcís?Prat 《Estuaries and Coasts》2002,25(3):309-324
Net primary production was measured in three characteristic salt marshes of the Ebre delta: anArthrocnemum macrostachyum salt marsh,A. macrostachyum-Sarcocornia fruticosa mixed salt marsh andS. fruticosa salt marsh. Above-ground and belowground biomass were harvested every 3 mo for 1 yr. Surface litter was also collected from each plot. Aboveground biomass was estimated from an indirect non-destructive method, based on the relationship between standing biomass and height of the vegetation. Decomposition of aboveground and belowground components was studied by the disappearance of plant material from litter bags in theS. fruticosa plot. Net primary production (aboveground and belowground) was calculated using the Smalley method. Standing biomass, litter, and primary production increased as soil salinity decreased. The annual average total aboveground plus belowground biomass was 872 g m−2 in theA. macrostachyum marsh, 1,198 g m−2 in theA. macrostachyum-S. fruticosa mixed marsh, and 3,766 g m−2 in theS. fruticosa biomass (aboveground plus belowground) was 226, 445, and 1,094 g m−2, respectively. Total aboveground plus below-ground net primary production was 240, 1,172, and 1,531 g m−2 yr−1. There was an exponential loss of weight during decomposition. Woody stems and roots, the most recalcitrant material, had 70% and 83% of the original material remaining after one year. Only 20–22% of leafy stem weight remained after one year. When results from the Mediterranean are compared to other salt marshes dominated by shrubbyChenopodiaceae in Mediterranean-type climates, a number of similarities emerge. There are similar zonation patterns, with elevation and maximum aboveground biomass and primary production occurring in the middle marsh. This is probably because of stress produced by waterlogging in the low marsh and by hypersalinity in the upper marsh. 相似文献
82.
Pattern and process of land loss in the Mississippi Delta: A Spatial and temporal analysis of wetland habitat change 总被引:2,自引:0,他引:2
John W. Day Louis D. Britsch Suzanne R. Hawes Gary P. Shaffer Denise J. Reed Donald Cahoon 《Estuaries and Coasts》2000,23(4):425-438
An earlier investigation (Turner 1997) concluded that most of the coastal wetland loss in Louisiana was caused by the effects of canal dredging, that loss was near zero in the absence of canals, and that land loss had decreased to near zero by the late 1990s. This analysis was based on a 15-min quadrangle (approximately 68,000 ha) scale that is too large to isolate processes responsible for small-scale wetland loss and too small to capture those responsible for large-scale loss. We conducted a further evaluation of the relationship between direct loss due to canal dredging and all other loss from 1933–1990 using a spatial scale of 4,100 ha that accurately captures local land-loss processes. Regressions of other wetland loss on canal area (i.e., direct loss) for the Birdfoot, Terrebonne, and Calcasieu basins were not significant. Positive relationships were found for the Breton (r2=0.675), Barataria (r2=0.47), and Mermentau (r2=0.35) basins, indicating that the extent of canals is significantly related to wetland loss in these basins. A significant negative relationship (r2=0.36) was found for the Atchafalaya coastal basin which had statistically lower loss rates than the other basins as a whole. The Atchafalaya area receives direct inflow of about one third of the Mississippi discharge. When the data were combined for all basins, 9.2% of the variation in other wetland loss was attributable to canals. All significant regressions intercepted the y-axis at positive loss values indicating that some loss occurred in the absence of canals. Wetland loss did not differ significantly from the coast inland or between marsh type. We agree with Turner that canals are an important agent in causing wetland loss in coastal Louisiana, but strongly disagree that they are responsible for the vast majority of this loss. We conclude that wetland loss in the Mississippi delta is an ongoing complex process involving several interacting factors and that efforts to create and restore Louisiana’s coastal wetlands must emphasize riverine inputs of freshwater and sediments. 相似文献
83.
84.
Tarryn Aucamp Geoffrey H. Howarth Chad J. Peel Gelu Costin James M. D. Day Petrus le Roux James M. Scott Ansgar Greshake Rainer Bartoschewitz 《Meteoritics & planetary science》2023,58(11):1654-1676
The Dar al Gani (DaG) olivine-phyric shergottites share mineralogical and geochemical characteristics, which confirm that these meteorites are derived from a single source. Bulk trace elements (La/Yb—0.12), in situ maskelynite 87Sr/86Sr (~0.7014) and redox estimates (FMQ ~ −2) indicate derivation from a depleted, reduced mantle reservoir; identical to all ~470 Ma shergottites ejected at 1.1 Ma. The DaG shergottites have been variably affected by terrestrial alteration, which precipitated carbonate along fractures and modified bulk-rock fluid mobile (e.g., Ba) elements. Nonetheless, sufficient data are available to construct a multi-stage formation model for the DaG shergottites and other 1.1 Ma ejection-paired shergottites that erupted at ~470 Ma. First, partial melting of a depleted mantle source occurred at 1540 ± 20°C and 1.2 ± 0.1 GPa, equivalent to > ~100 km depth. Then, initial crystallization in a staging chamber at ~85 km depth at the crust–mantle boundary took place, followed by magma evolution and variable incorporation of antecrystic olivine ± orthopyroxene. Subsequently, crystallization of olivine phenocrysts and re-equilibration of olivine antecrysts occurred within an ascending magma. Finally, magmas with variable crystal loads erupted at the surface, where varied cooling rates produced a range of groundmass textures. This model is similar to picritic flood basalt magmas erupted on Earth. 相似文献
85.
Sources of knowledge and ignorance in climate research 总被引:1,自引:1,他引:0
Ignorance is an inevitable component of climate change research, and yet it has not been specifically catered for in standard
uncertainty guidance documents for climate assessments. Reports of ignorance in understanding require context to explain how
such ignorance does and does not affect understanding more generally. The focus of this article is on dynamical sources of
ignorance in regional climate change projections. A key source of ignorance in the projections is the resolution-limited treatment
of dynamical instabilities in the ocean component of coupled climate models. A consequence of this limitation is that it is
very difficult to quantify uncertainty in regional projections of climate variables that depend critically upon the details
of the atmospheric flow. The standard methods for quantifying or reducing uncertainty in regional projections are predicated
on the models capturing and representing the key dynamical instabilities, which is doubtful for present coupled models. This
limitation does not apply to all regional projections, nor does it apply to the fundamental findings of greenhouse climate
change. Much of what is known is not highly flow-dependent and follows from well grounded radiative physics and thermodynamic
principles. The growing field of applications of regional climate projections would benefit from a more critical appraisal
of ignorance in these projections. 相似文献
86.
Kirstin Dobbs Jon Day Hilary Skeat John Baldwin Fergus Molloy Laurence McCook Margaret Johnson Bruce Elliot Andrew Skeat Karen Vohland David Wachenfeld Richard Kenchington 《Marine Policy》2011
The Great Barrier Reef Outlook Report 2009 was the first produced in response to a newly legislated requirement for five-yearly reports on the status of and outlook for the Great Barrier Reef. It adopted an ecosystem approach, assessing all habitats and species, ecosystem processes and major uses. By then considering the factors affecting the ecosystem, coupled with an assessment of management effectiveness, it provided a risk-based forward-looking projection for the ecosystem. Rarely has such a comprehensive, ecosystem-based report been produced to guide government action. With no pre-determined path to follow for interpreting the legislative requirements, the Great Barrier Reef Marine Park Authority (GBRMPA) developed a repeatable structure and method for Great Barrier Reef Outlook Reports that impartially and consistently considers the evidence and clearly presents the findings. The GBRMPA worked closely with relevant Australian and Queensland Government agencies as well as researchers, industry representatives and the community while developing the report. That such a report must be produced every five years allows an overview of the effectiveness of management responses to be regularly assessed. It also provides a transparent means of highlighting and tracking emerging risks facing the Great Barrier Reef. 相似文献
87.
Terence R. Smith 《The Professional geographer》1984,36(2):147-158
Artificial Intelligence (AI) may be regarded as an attempt to understand the processes of perception and reasoning that underlie successful problem-solving and to incorporate the results of this research in effective computer programs. At present, AI is largely a collection of sophisticated programming techniques. Many of these techniques are based on the premise that the manner in which knowledge is acquired, organized, accessed and modified in both humans and machines provides the basis of “intelligent” decision-making. The techniques of AI are applicable to a wide variety of geographical problems, including the modeling of individual and aggregate decision-making, and the construction of expert systems and “intelligent” geographical information systems. 相似文献
88.
The Sahel is prone to climate stressors such as droughts, winds and floods. This study employs a systematic review approach to track the frequency with which these stressors are reported in the scientific peer reviewed literature, examining publishing trends to identify which stressors are most reported, documenting the spatial distribution of these stressors from a country and regional perspective, and assessing the role played by climatic and non-climatic drivers in causing the stressors. A total of (n = 388) reports of stressors were documented in (n = 164) peer reviewed articles. From a country perspective, Southern Niger records the highest number of reports on all three stressors (15.97%), followed by Ethiopia (11.85%) and Senegal (10.56%). Regionally, West African Sahel recorded the highest number of reports on all stressors (49.97%) followed by East African Sahel (29.89%) and Central African Sahel (12.11%) respectively. Droughts are observed to be the most frequently reported stressor (n = 219), followed by floods (n = 123) and winds (n = 46). The decade 1975–1985 recorded the highest reports of stressors (n = 207), while the decade 1997–2007 recorded (n = 80) and the decade 1986–1996 recorded (n = 52). While climatic drivers are dominant (52%), there is however an increasing attribution of the drivers of the stressors to non-climatic drivers (47%). The main weakness of this study is that it uses peer reviewed papers dwelling on climate stressors as a proxy for climate stressors in the Sahel and a lot more studies could be hiding in non-peer reviewed studies, underscoring that this work provides a general and baseline overview of the climate stressors in the region. 相似文献
89.
Human activities influence watershed sediment dynamics in profound ways, often resulting in excessive loading of suspended sediment to rivers. One of the primary factors limiting our ability to effectively manage sediment at the watershed scale has been our inability to adequately measure relatively small erosion rates (on the order of millimeters to centimeters per year) over annual and sub‐annual time scales on spatially‐extensive landforms, such as river banks and bluffs. Terrestrial laser scanning (TLS) can be employed to address this need. TLS collects high‐resolution data allowing for more accurate monitoring of erosion rates and processes, and provides a new opportunity to make precise measurements of geomorphic change on vertical landforms like banks and bluffs, but challenges remain. This research highlights challenges and limitations of using TLS for change detection on river banks and bluffs including the presence of vegetation, natural surface crenulations, and difficulties with creating benchmarks, and provides solutions developed to overcome these limitations. Results indicate that data processing algorithms for change detection can have a significant impact on the calculated erosion rates, with different methods producing results that can vary by over 100%. The most accurate change detection technique compares a point cloud to a triangulated irregular network (TIN) along a set of vectors that accommodate bluff curvature. This paper outlines a variety of methods used to measure bluff change via TLS and explains the accompanying error analysis that supports these methods. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
90.
Sarah J. Day Stephen P. Thompson Aneurin Evans Julia E. Parker Leigh D. Connor Chiu C. Tang 《Meteoritics & planetary science》2013,48(8):1459-1471
The structural evolution of sol–gel‐produced amorphous Mg(x)Ca(1–x)SiO3 silicates is investigated. Mid‐IR Fourier transform infrared spectroscopy and synchrotron X‐ray diffraction are used to confirm the amorphous nature of the as‐prepared silicates, while subsequent in situ synchrotron X‐ray powder diffraction measurements are used to study the evolution of crystalline mineral phases as a function of annealing temperature. Multiple silicate phases, including diopside, enstatite, forsterite, and SiO2, are identified, while Rietveld (i.e., structure) refinement of the diffraction data is used to quantify phase change relationships. Investigated as possible analogs for the refractory dust grain materials likely to have been present in the early solar nebula, the likely relevance of these investigations to the observed silicate compositions of chondritic meteorites and cometary bodies and the processing of their precursor materials is discussed. 相似文献