Ecological distribution of scaled-chrysophyte assemblages from the sediments of 54 lakes in Nova Scotia and southern New Brunswick,Canada     

Brian K. Ginn  Michael Rate  Brian F. Cumming  John P. Smol 《Journal of Paleolimnology》2010,43(2):293-308

Chrysophyte scales have been used in several paleolimnological studies to track long-term environmental change, however little data exist for the many lakes in the Maritime provinces of eastern Canada. As part of a multi-disciplinary investigation of acidification and other environmental stressors in the Maritimes, chrysophytes scales were identified and enumerated from the sediments of 52 lakes from Nova Scotia and two lakes from New Brunswick. A total of 25 chrysophyte taxa were identified from the surface sediments, reflecting the modern-day chrysophyte assemblages. The dominant species included Mallamonas duerrschmidtiae and Mallomonas acaroides. Taxa of the genus Synura were present in some lakes, but mainly in the more southern sites. In general, the floras were less diverse than those recorded from similar studies in other temperate regions. This may be related to the fact that the calibration lake set contained only a relatively short limnological gradient, and the assemblages reflect the acidic to circumneutral conditions of these lakes. Synura petersenii, a taxon that has been linked to imparting taste and odor problems to lakes, and had been shown to increase in the recent sediments of many other Canadian lakes, was only rarely present. In contrast to other studies, scaled chrysophytes were very rare in the pre-industrial sediments, with substantial nineteenth century populations only present in four relatively deep (>19 m) lakes. Detailed stratigraphic analyses of eight sediment cores revealed that scaled-chrysophyte assemblages increased dramatically during the latter part of the twentieth century. Limnological changes associated with climate (e.g. increased thermal stratification due to a 1.5°C temperature increase since ~1850) may have influenced chrysophyte distributions in these lakes.  相似文献   

Climate change and the oceans – What does the future hold?     

Jelle Bijma  Hans-O. Pörtner  Chris Yesson  Alex D. Rogers 《Marine pollution bulletin》2013

The ocean has been shielding the earth from the worst effects of rapid climate change by absorbing excess carbon dioxide from the atmosphere. This absorption of CO₂ is driving the ocean along the pH gradient towards more acidic conditions. At the same time ocean warming is having pronounced impacts on the composition, structure and functions of marine ecosystems. Warming, freshening (in some areas) and associated stratification are driving a trend in ocean deoxygenation, which is being enhanced in parts of the coastal zone by upwelling of hypoxic deep water. The combined impact of warming, acidification and deoxygenation are already having a dramatic effect on the flora and fauna of the oceans with significant changes in distribution of populations, and decline of sensitive species. In many cases, the impacts of warming, acidification and deoxygenation are increased by the effects of other human impacts, such as pollution, eutrophication and overfishing.  相似文献   

Mode coupling analysis and differential rotation in a flow driven by a precessing cylindrical container     

Thierry Lehner  Waleed Mouhali  Jacques Leorat  Alex Mahalov 《地球物理与天体物理流体动力学》2013,107(4):369-401

We present a theoretical weakly nonlinear analysis of the dynamics of an inviscid flow submitted to both rotation and precession of an unbounded cylindrical container, by considering the coupling of two Kelvin (inertial) waves. The parametric centrifugal instability known for this system is shown to saturate when one expands the Navier–Stokes equation to higher order in the assumed small precession parameter (ratio of precession to rotation frequencies) with the derivation of two coupled Landau equations suitable to describe the dynamics of the modes. It is shown that an azimuthal mean flow with differential rotation is generated by this modes coupling. The time evolution of the associated dynamical system is studied. These theoretical results can be compared with water experiments and also to some numerical simulations where viscosity and finite length effects cannot be neglected.  相似文献   

A model of secondary upwelling over the shelf break     

Hezi Gildor  Erick Fredj  Alex Kostinski 《地球物理与天体物理流体动力学》2013,107(4):301-308

Motivated by our puzzling high-resolution radar observations of surface vortices in the nearly rectangular Gulf of Eilat/Aqaba, northern Red Sea, we propose and explore the driven cavity approach to this geophysical phenomenon. While the lid-driven cavity has long been considered a benchmark problem in computational fluid dynamics, its oceanographic context has not been considered. Despite the additional effects of rotation and stratification, our modeling demonstrates that when the fluid within a cavity geometrically similar to the Gulf of Eilat is driven by the external current, an interior vortex can develop as in our observations. Furthermore, the Eilat vortices appear only under relatively calm conditions, adding evidence to the intriguing possibility of their simple shear-driven origin.  相似文献   

Assessment of drought over the past two millennia using near-shore sediment cores from a Canadian boreal lake     

Heather A. Haig  Melanie V. Kingsbury  Kathleen R. Laird  Peter R. Leavitt  Robyn Laing  Brian F. Cumming 《Journal of Paleolimnology》2013,50(2):175-190

Lack of long-term studies on drought in the boreal region of northwest Ontario limits our ability to assess the vulnerability of this region to climate change. We investigated changes in diatoms, scaled chrysophytes, and sedimentary pigments in two near-shore cores from Gall Lake to infer limnological and water-level changes over the last two millennia. The two coring locations, at modern water depths of 7.5 and 11.5 m, were selected to contrast inferences for past lake level based on distance from the modern water-depth boundary between predominantly benthic and planktonic diatom assemblages in surface sediments (6.0 m). Diatom-inferred depth inferences were more variable in the core from 7.5-m water depth, consistent with the hypothesis that the greatest changes occurred at water depths closest to the benthic:planktonic boundary. Both sites revealed a pronounced drought from ~AD 950 to 1300, synchronous with the medieval climate anomaly (MCA). This finding suggests a northeast expansion of the arid MCA into northwest Ontario, extending the known spatial extent of this megadrought. Scaled chrysophytes and sedimentary pigments suggest a recent increase in thermal stratification. Our findings indicate this region is more susceptible to changes in moisture than was previously suspected based on the instrumental record for the past century.  相似文献   

Monitoring ground surface deformation over the North China Plain using coherent ALOS PALSAR differential interferograms     

Kui Zhang  Linlin Ge  Xiaojing Li  Alex Hay-Man Ng 《Journal of Geodesy》2013,87(3):253-265

North China Plain (Huabei Plain) is one of the most densely populated regions on earth. Due to excessive underground water extraction, the North China Plain has been experiencing severe ground deformation over the last three decades. Therefore, for the purpose of hazard mitigation, it is necessary to monitor the ground displacement occurred in this region. As an extension of the differential radar interferometry (DInSAR) technique, advanced DInSAR techniques involving multiple images have demonstrated the potential to effectively map ground displacement. Such techniques are able to measure the temporal evolution of ground deformation with millimetre-level accuracy by using a stack of differential interferograms. In this study, the ALOS PALSAR data acquired over the North China Plain, which cover an area of approximately 16,000 km $^2$ , were processed based on the concept of advanced DInSAR techniques. Because of the large size of the PALSAR images, a targeted processing strategy was designed. This strategy is able to reduce required disk storage space and I/O operations, leading to the improvement of the computational efficiency. The resulting mean deformation velocity map demonstrates that a large portion of the area covered by the data was affected by various degrees of ground deformation between January 2007 and April 2010. The ground deformation is mostly distributed in rural areas, while the downtown areas are generally stable.  相似文献   

Use of LIDAR in landslide investigations: a review   总被引：13，自引：0，他引：13  

Michel Jaboyedoff  Thierry Oppikofer  Antonio Abellán  Marc-Henri Derron  Alex Loye  Richard Metzger  Andrea Pedrazzini 《Natural Hazards》2012,61(1):5-28

This paper presents a short history of the appraisal of laser scanner technologies in geosciences used for imaging relief by high-resolution digital elevation models (HRDEMs) or 3D models. A general overview of light detection and ranging (LIDAR) techniques applied to landslides is given, followed by a review of different applications of LIDAR for landslide, rockfall and debris-flow. These applications are classified as: (1) Detection and characterization of mass movements; (2) Hazard assessment and susceptibility mapping; (3) Modelling; (4) Monitoring. This review emphasizes how LIDAR-derived HRDEMs can be used to investigate any type of landslides. It is clear that such HRDEMs are not yet a common tool for landslides investigations, but this technique has opened new domains of applications that still have to be developed.  相似文献   

Middle Eocene diatoms from Whitecliff Bay, Isle of Wight, England: stratigraphy and preservation     

Alex De Jonghe  Malcolm B. Hart  Stephen T. Grimes  Alex G. Mitlehner  Gregory D. Price  Christopher W. Smart 《Proceedings of the Geologists' Association. Geologists' Association》2011,122(3):472-483

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60Fe–60Ni systematics in the eucrite parent body: A case study of Bouvante and Juvinas     

Ghylaine Quitté  Christopher Latkoczy  Maria Schönbächler  Alex N. Halliday  Detlef Günther 《Geochimica et cosmochimica acta》2011,75(23):7698-7706

Eucrites are basaltic meteorites that cooled rapidly but are in many instances thermally metamorphosed and impact brecciated. The exact timing of these events remains unclear. In this study, Ni isotopic compositions and Fe/Ni elemental ratios are presented for two non-cumulate eucrites, Bouvante and Juvinas, including mineral separates from the latter. The samples are characterized by variable, well-resolved ⁶⁰Ni-excesses consistent with the former presence of live ⁶⁰Fe (t_1/2 = 2.62 Ma) at the time of eucrite crystallization. A significant fraction of Ni with a terrestrial-like composition appears to be surface correlated. This Ni may be the product of terrestrial contamination or was introduced by a chondritic impactor during brecciation. Altogether, the data provide evidence for a complex and probably multi-stage history of Fe and/or Ni redistribution, which impedes the interpretation of the chronological data.  相似文献   

Elemental and mineralogical changes in soils due to bioturbation along an earthworm invasion chronosequence in Northern Minnesota   总被引：1，自引：0，他引：1  

Kathryn Resner  Kyungsoo Yoo  Cindy Hale  Anthony Aufdenkampe  Alex Blum  Stephen Sebestyen 《Applied Geochemistry》2011

Minnesota forested soils have evolved without the presence of earthworms since the last glacial retreat. When exotic earthworms arrive, enhanced soil bioturbation often results in dramatic morphological and chemical changes in soils with negative implications for the forests’ sustainability. However, the impacts of earthworm invasion on geochemical processes in soils are not well understood. This study attempts to quantify the role of earthworm invasion in mineral chemical weathering and nutrient dynamics along an earthworm invasion chronosequence in a sugar maple forest in Northern Minnesota. Depth and rates of soil mixing can be tracked with atmospherically derived short lived radioisotopes ²¹⁰Pb and ¹³⁷Cs. Their radioactivities increase in the lower A horizon at the expense of the peak activities near the soil surface, which indicate that soil mixing rate and its depth reach have been enhanced by earthworms. Enhanced soil mixing by earthworms is consistent with the ways that the vertical profiles of elemental and mineralogical compositions were affected by earthworm invasion. Biologically cycled Ca and P have peak concentrations near the soil surface prior to earthworm invasion. However, these peak abundances significantly declined in the earthworm invaded soils presumably due to enhanced soil mixing. It is clear that enhanced soil mixing due to earthworms also profoundly altered the vertical distribution of most mineral species within A horizons. Though the mechanisms are not clear yet, earthworm invasion appears to have contributed to net losses of clay mineral species and opal from the A horizons. As much as earthworms vertically relocated minerals and elements, they also intensify the contacts between organic matter and cations as shown in the increased amount of Ca and Fe in organically complexed and in exchangeable pools. With future studies on soil mixing rates and elemental leaching, this study will quantitatively and mechanically address the role of earthworms in geochemical evolution of soils and forests’ nutrient dynamics.  相似文献