Arctic Mackenzie Delta channel planform evolution during 1983–2013 utilising Landsat data and hydrological time series          下载免费PDF全文

Jenni‐Mari Vesakoski  Tua Nylén  Berit Arheimer  David Gustafsson  Kristina Isberg  Markus Holopainen  Juha Hyyppä  Petteri Alho 《水文研究》2017,31(22):3979-3995

Arctic deltas, such as the Mackenzie Delta, are expected to face major climate change and increased human influence in the near future. Deltas are characterised by highly dynamic fluvial processes, and changing climate will cause considerable evolution of the riverine environment. The changes are difficult to predict with existing knowledge and data. This study quantified channel planform change of the Mackenzie Delta (1983–2013), analysing its temporal and spatial patterns. We addressed the main obstacle of research on large remote areas, the lack of data, by developing a unique work flow that utilised Landsat satellite imagery, hydrological time series, remote sensing‐based change analysis, and automatic vectorisation of channels. Our results indicate that the Mackenzie Delta experienced constant evolution but at a highly varying rate over the 30 years. The study demonstrates that the magnitude and duration of flood peaks and the presence of spring ice breakup floods determine the rate of Arctic delta planform change. Changing winter conditions and spring flood magnitudes may therefore affect the stability of Arctic deltas. However, no clear trends towards decreased recurrence or magnitude of spring floods or increased instability of the delta plain have yet been observed in the Mackenzie Delta. The delta plain was most dynamic at the beginning and at the end of the examined period, corresponding to intense flooding, whereas the rates of change were subtle during the low‐flood period 1994–2007. The largest changes have occurred along the wide Middle Channel and in the outermost delta. Relative to their size, however, smaller meandering channels have been highly dynamic. Hotspots of change in the delta plain are located in anastomosing and braiding channel segments and, at the local scale, in point bars and cut‐banks along meandering channels. Our study describes how Landsat satellite data can be utilised for advancing fluvial geomorphological research in remote areas. However, cloudiness in the delta restricts production of dense time series with simultaneous coverage of the whole area and requires manual preprocessing.  相似文献   

No evidence for widespread bird declines in protected South American forests     

Philip C Stouffer  Kristina L. Cockle  Alexandre Aleixo  Juan I. Areta  Juan Mazar Barnett  Alejandro Bodrati  Carlos Daniel Cadena  Adri��n S. Di Giacomo  Sebastian K. Herzog  Peter Hosner  Erik I. Johnson  Luciano N. Naka  C��sar S��nchez 《Climatic change》2011,108(1-2):383-386

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Marine spatial planning in the high seas     

Jeff Ardron  Kristina Gjerde  Sian Pullen  Virginie Tilot   《Marine Policy》2008,32(5):832

Although high seas resources are being exploited, reciprocal legal obligations to protect its environment have not been met. Marine spatial planning (MSP) is clearly a practical way forward, particularly for the high seas, where non-spatial monitoring is difficult, and where data gaps obstruct conventional management approaches. To ensure the effective application of MSP in the high seas, however, some institutional reforms are necessary. This paper outlines the main hurdles, summarizes existing high seas spatial protections, presents an example of a high seas marine protected area that resulted through MSP, identifies three institutional priorities, and suggests three immediate steps.  相似文献   

Effects of precipitation on the eddy exchange in a wind-driven sea     

Kristina B. Katsaros 《Dynamics of Atmospheres and Oceans》1976,1(2):99-126

Effects of precipitation on the temperature and salinity profiles in the top 2 or 3 m of the sea are calculated for four models of turbulent exchange: (1) constant eddy exchange with depth; (2) drift-current eddy exchange; (3) eddy exchange produced by shear in the orbital velocity under a surface wave; and (4) drift current and wave turbulence acting together. The surface temperature and salinity are only weakly affected, but the density gradient can become significantly stabler. The resulting influence on the eddy exchange is modeled with empirical formulas employing the Richardson number. Stability strongly damps drift-current eddy exchange but has much less effect on the wave-induced turbulence.The energy budget shows an increase in turbulent dissipation near the surface for weak stability when the surface stress in assumed to remain constant. Beyond a critical Richardson number of about 0.9 energy consumed by turbulent energy dissipation decreases. This occurs more due to a reduction in mechanical energy production than because of energy being used for the redistribution of mass.  相似文献   

Vegetation and sedimentation in the Lake Edward Basin, Uganda–Congo during the late Pleistocene and early Holocene   总被引：1，自引：1，他引：0  

Kristina R.M. Beuning  James M. Russell 《Journal of Paleolimnology》2004,32(1):1-18

New palynological and sedimentological data obtained from the basal 3 m of core E96-2P from Lake Edward, Uganda–Congo record conditions wetter than present in the Edward basin from 11 000 to 6700 yr BP, in phase with other climate and vegetation records of northern hemispheric East Africa. Dominant pollen taxa include Celtis spp., Alchornea spp., Olea spp., and Moraceae indicating a moist semi-deciduous tropical forest. More xeric indicators such as Amaranthaceae and Asteraceae together with Poaceae comprise less than 5% of the pollen sum throughout this interval as compared with between 44 and 50% during a lake lowstand at 2000 cal yr BP and at the core top (near modern). The differences between these two assemblages suggest a 25 to 60% increase in annual precipitation during the early- to mid-Holocene as compared to modern (1500–2000 vs. 1200 mm/yr today). Early Holocene sediments in E96-2P are composed of laminated diatom oozes with moderately high total sulfur concentrations (2.8–4.7%) and no authigenic calcite, also indicative of conditions wetter than present. Between 9000 and 6700 yr BP, palynological and sedimentary proxies indicate sub-millennial-scale events related to changes in riverine discharge and runoff in the Edward basin. We attribute the variability in runoff, and hence precipitation, to Holocene variability in Indian or Atlantic Ocean SSTs or to shifts in the relative contribution of Indian and Atlantic moisture sources to the western Rift of equatorial Africa.  相似文献   

Geodynamic significance of ultramafic xenoliths from Eastern Serbia: Relics of sub-arc oceanic mantle?     

Vladica Cvetkovi&#x;  Hilary Downes  Dejan Prelevi&#x;  Marina Lazarov  Kristina Resimi&#x;-&#x;ari&#x; 《Journal of Geodynamics》2007,43(4-5):504-527

A suite of highly depleted peridotite xenoliths in East Serbian Palaeogene basanites represents the lithospheric mantle beneath the Balkan Peninsula. The xenoliths are harzburgites, clinopyroxene-poor lherzolites and rare dunites. They contain mostly <5 vol.% of modal clinopyroxene and are characterized by high Mg# in silicates (>91), high Cr# in spinel (mostly 0.5–0.7), and by distinctively low Al₂O₃ contents in orthopyroxene (mostly 1–2 wt.%). They have experienced some mantle metasomatism which has slightly obscured their original composition. Nevertheless, the general characteristics of the xenoliths imply a composition which is significantly more depleted than most non-cratonic sub-continental mantle xenolith suites, as well as orogenic peridotites and abyssal peridotites. Geological and compositional evidence suggests that the xenoliths do not represent Archean mantle. The existence of Proterozoic mantle cannot be entirely excluded, although it is in disagreement with geological evidence. On the other hand, the studied xenoliths are compositionally very similar to peridotites of modern oceanic sub-arc settings. The existence of such a depleted lithospheric mantle segment is also inferred from the presence of rare orthopyroxene-rich xenoliths in the same suite. These are interpreted to have originated as lithospheric precipitates of high-Mg, SiO₂-saturated magmas that require a highly depleted mantle source. Such source is typically required by boninitic-like magmas of intraoceanic suprasubduction settings. A proposed geodynamic model to explain these observations involves accretion or underplating of the lower parts of the Tethyan oceanic lithosphere during the Upper Jurassic closure of the eastern branch of the Vardar ocean.  相似文献   

Adsorption of oxalate and malonate at the water-goethite interface: Molecular surface speciation from IR spectroscopy     

Per Persson  Kristina Axe 《Geochimica et cosmochimica acta》2005,69(3):541-552

The adsorption of oxalate and malonate at the water-goethite interface was studied as a function of pH and total ligand concentrations by means of quantitative adsorption measurements and attenuated total reflectance Fourier transform infrared spectroscopy. The obtained results conclusively showed that oxalate and malonate both form outer-sphere and inner-sphere surface complexes on goethite, and that these complexes coexist over a broad pH interval. The inner-sphere complexes were favored by low pH, while the relative concentrations of the outer-sphere species increase with increasing pH. Based on comparisons with model complexes characterized by Extended X-Ray Adsorption Fine Structure (EXAFS) and results from theoretical frequency calculations, the structures of the inner-sphere complexes of oxalate and malonate were best described as mononuclear five- and six-membered ring chelate structures, respectively. The stability of the inner-sphere complexes followed the trend expected from solutions studies, with the oxalate five-membered ring yielding the more stable complexes compared to the six-membered ring of malonate. The increased stability of the inner-sphere complex of oxalate was manifested in a greater extent of adsorption at acidic pH values. Despite the fact that significant amounts of oxalate and malonate inner-sphere surface complexes were formed, no ligand-promoted dissolution was observed at the experimental conditions in the study.  相似文献   

Assessing the role of North Atlantic freshwater forcing in millennial scale climate variability: a tropical Atlantic perspective     

Kristina A. Dahl  Anthony J. Broccoli  Ronald J. Stouffer 《Climate Dynamics》2005,24(4):325-346

This study analyzes a three-member ensemble of experiments, in which 0.1 Sv of freshwater was applied to the North Atlantic for 100 years in order to address the potential for large freshwater inputs in the North Atlantic to drive abrupt climate change. The model used is the GFDL R30 coupled ocean–atmosphere general circulation model. We focus in particular on the effects of this forcing on the tropical Atlantic region, which has been studied extensively by paleoclimatologists. In response to the freshwater forcing, North Atlantic meridional overturning circulation is reduced to roughly 40% by the end of the 100 year freshwater pulse. Consequently, the North Atlantic region cools by up to 8°C. The extreme cooling of the North Atlantic increases the pole-to-equator temperature gradient and requires more heat be provided to the high latitude Atlantic from the tropical Atlantic. To accommodate the increased heat requirement, the ITCZ shifts southward to allow for greater heat transport across the equator. Accompanying this southward ITCZ shift, the Northeast trade winds strengthen and precipitation patterns throughout the tropical Atlantic are altered. Specifically, precipitation in Northeast Brazil increases, and precipitation in Africa decreases slightly. In addition, we find that surface air temperatures warm over the tropical Atlantic and over Africa, but cool over northern South America. Sea-surface temperatures in the tropical Atlantic warm slightly with larger warm anomalies developing in the thermocline. These responses are robust for each member of the ensemble, and have now been identified by a number of freshwater forcing studies using coupled OAGCMs. The model responses to freshwater forcing are generally smaller in magnitude, but have the same direction, as paleoclimate data from the Younger Dryas suggest. In certain cases, however, the model responses and the paleoclimate data directly contradict one another. Discrepancies between the model simulations and the paleoclimate data could be due to a number of factors, including inaccuracies in the freshwater forcing, inappropriate boundary conditions, and uncertainties in the interpretation of the paleoclimate data. Despite these discrepancies, it is clear from our results that abrupt climate changes in the high latitude North Atlantic have the potential to significantly impact tropical climate. This warrants further model experimentation into the role of freshwater forcing in driving climate change.  相似文献   

There and back again: Epiphany, disillusionment, and rediscovery in political ecology     

Paul Robbins  Kristina Monroe Bishop 《Geoforum》2008,39(2):747-755

This paper reviews the experience of discovering, abandoning, and rediscovering elements of the work of political ecologist Piers Blaikie from the period spanning 1992-2006. In the process, we analyze the cyclic fate of critical ideas in the field of political ecology, including especially the “chain of explanation,” during a tumultuous period where materialist and constructivist theorists encountered one another. The paper concludes with a specific discussion of the power of traditional tools in political ecology for addressing current socio-ecological crises, specifically HIV/AIDS in southern Africa.  相似文献   

Vanishing Horizontal Sea Surface Temperature Gradients at Low Wind Speeds     

Kristina B. Katsaros  Alexander V. Soloviev 《Boundary-Layer Meteorology》2004,112(2):381-396

Sea surface temperature (SST) is a result of multiple interactions in air-sea processes. During days with strong insolation and low wind speed, there may be uneven net heating of the water layer near the surface of the ocean, when there are horizontal temperature gradients at the sea surface. Cooling of the water caused by evaporation, sensible, or longwave radiative, heat loss would be greater from warm water compared to that from relatively cold water. As a result, under low wind speed conditions and clear skies, the horizontal SST discontinuities, occurring at fronts, eddies, or in storm wakes, may diminish or even vanish. This phenomenon is illustrated here with some field and modelling results. The dependence on latitude and mean environmental conditions of the difference in warming on the cold and warm side of SST discontinuities is explored. The time dependence is important for the impact on remote sensing of SST, and it is found to be short enough that substantial masking of SST gradients can occur during the first six hours of the diurnal heating cycle, but the effect would continue to grow if calm and solar heating persist for several subsequent days. An integrated effect of this uneven net heating is seen in the seasonal masking of subsurface temperature gradients in the Gulf of Mexico and Florida Straits.  相似文献