Résultats préliminaires sur la sédimentation pélagique de l'Atlantique tropical au Crétacé et au Tertiaire (plateau de Demerara,Leg ODP 207)     

Taniel Danelian  Laurence Le Callonnec  Jochen Erbacher  David C. Mosher  Mitchell J. Malone  Debora Berti  Karen L. Bice  Helen Bostock  Hans-Jürgen Brumsack  Astrid Forster  Felix Heidersdorf  Jorijntje Henderiks  Thomas J. Janecek  Christopher Junium  Ken MacLeod  Philip A. Meyers  Jörg H. Mutterlose  Hiroshi Nishi  Christine Glatz 《Comptes Rendus Geoscience》2005,337(6):609-616

Five sites located on a bathymetric transect of the distal Demerara Rise were studied by ODP Leg 207. Albian sediments of essentially terrigenous nature (clay, siltstone, sandstone) are the oldest drilled stratigraphic levels and form apparently the top of the synrift sequence. They are overlain by Cenomanian to Santonian finely laminated black shales, rich in organic matter of marine origin, which accumulated on a thermally subsiding ramp. Early Campanian hiatuses are thought to be the result of final disjunction of Demerara Rise (South America) from Africa and the onset of deep water communication between the two Atlantic basins (south and central). The overlying Uppermost Cretaceous–Oligocene chalk includes rich and diversified calcareous plankton assemblages, as well as two radiolarian-rich intervals (Late Campanian and Middle Eocene). A complex erosional surface developed during the Late Oligocene–Early Miocene. Sedimentation was impeded since then on the intermediate and deep sites of Demerara Rise, possibly due to the action of deep submarine currents. To cite this article: T. Danelian et al., C. R. Geoscience 337 (2005).  相似文献   

Climatic and geographic patterns of river runoff formation in Northern Eurasia     

Alexander Onuchin  Heiko Balzter  Helen Borisova  Eleanor Blyth 《Advances in water resources》2006

Siberian rivers are of global importance as they impact on the freshwater budget of the Arctic Ocean, which affects the Thermo-Haline circulation in the North Atlantic Ocean. Siberian rivers, in particular the tributaries to the larger rivers, are under-represented in the international river-regime databases. The runoff of three Russian rivers in the Central Siberian taiga (Kureyka, Karabula and Erba) is modelled to analyse the relative influence of climate. In addition three rivers (Rhine, Maas and Odra) in Western Europe are similarly assessed as a control. The results show that the role of precipitation and autocorrelation as factors in the formation of river runoff is stronger under oceanic climate conditions, increasing from the central regions of Northern Eurasia towards the Arctic Ocean in the North and the Atlantic in the West. At the same time the influence of summer temperatures is weakened. The formation of Northern Eurasian river runoff appears to be influenced by periodically thawing top horizons of permafrost soil. Time served as an indicator for land use change after inclusion of meteorological data in the models. Maas and Erba showed a significant influence of the time factor. For the Erba the onset of agricultural land use in the catchment coincides with a drop in runoff. A similar causal relationship is suggested for the Maas. Land use can change the formation of runoff, which in turn can be used as an environmental indicator for sustainable land use.  相似文献   

The power spectrum of rich clusters of galaxies on large spatial scales     

Helen Tadros  George Efstathiou  & Gavin Dalton 《Monthly notices of the Royal Astronomical Society》1998,296(4):995-1003

We present an analysis of the redshift-space power spectrum, P ( k ), of rich clusters of galaxies based on an automated cluster catalogue selected from the APM Galaxy Survey. We find that P ( k ) can be approximated by a power law, P ( k )∝ kⁿ , with n ≈−1.6 over the wavenumber range 0.04< k <0.1 h Mpc⁻¹. Over this range of wavenumbers, the APM cluster power spectrum has the same shape as the power spectra measured for optical and IRAS galaxies. This is consistent with a simple linear bias model in which different tracers have the same power spectrum as that of the mass distribution, but shifted in amplitude by a constant biasing factor. On larger scales, the power spectrum of APM clusters flattens and appears to turn over on a scale k ∼0.03 h Mpc⁻¹. We compare the power spectra estimated from simulated APM cluster catalogues with those estimated directly from cubical N -body simulation volumes, and find that the APM cluster survey should give reliable estimates of the true power spectrum at wavenumbers k ≳0.02 h Mpc⁻¹. These results suggest that the observed turnover in the power spectrum may be a real feature of the cluster distribution, and that we have detected the transition to a near-scale-invariant power spectrum implied by observations of anisotropies in the cosmic microwave background radiation. The scale of the turnover in the cluster power spectrum is in good agreement with the scale of the turnover observed in the power spectrum of APM galaxies.  相似文献   

The Certainty of Uncertainty: GIS and the Limits of Geographic Knowledge   总被引：4，自引：0，他引：4  

Helen Couclelis 《Transactions in GIS》2003,7(2):165-175

Considerable effort has been devoted over the years to fighting uncertainty in geographic information in its different manifestations. Thus far, research on handling inaccuracy, fuzziness, error and related issues has focused for the most part on problems with spatial data and their direct products, typically representations of spatial objects or fields. This paper seeks to broaden the discussion of uncertainty in the geospatial domain by shifting the focus from information to knowledge. It turns out that there is a surprising number of things that we cannot know (or questions we cannot answer) that are not the result of imperfect information. Forms of not knowing are pervasive in domains as diverse as mathematics, logic, physics, and linguistics, and are apparently irreducible. This being the case it may help to explore how these realms of ignorance may affect our efforts. The paper distinguishes three different modes or forms of geospatial knowledge production, and argues that each of them has built–in imperfections, for reasons of logical principle and not just empirical fact. While much can and needs to be done to manage and resolve uncertainties where possible, I argue for accepting that uncertainty is an intrinsic property of complex knowledge and not just a flaw that needs to be excised.  相似文献   

Solid-state astrochemistry in star-forming regions     

Helen Fraser  David Williams  Ian Sims  Anita Richards  Jeremy Yates 《Astronomy& Geophysics》2003,44(4):4.29-4.33

Surface-catalysed chemical reactions, the molecules they form, and the implications for star formation were key themes in an international workshop on "Solid-state chemistry in star-forming regions" at the Lorentz Centre, Leiden University, the Netherlands from 14–17 April 2003. Helen Fraser, David Williams, Ian Sims, Anita Richards and Jeremy Yates report.  相似文献   

Towards better integration of ecology in palaeoecology: from proxies to indicators,from inference to understanding     

Thomas A. Davidson  Helen Bennion  Michael Reid  Carl D. Sayer  Thomas J. Whitmore 《Journal of Paleolimnology》2018,60(2):109-116

The special issue titled “Putting the Ecology into Palaeoecology” stems from a session with that name that was held at the 2015 International Paleolimnological Association meeting (International Paleolimnology Symposium) in Lanzhou, China. We briefly describe the motivation for the session, and summarise the contributions to this special issue. Additionally, we discuss our perceptions and concerns about the progress, challenges and future directions of palaeolimnology, stressing the importance of meaningful integration of ecological principles and thinking into palaeoapproaches.  相似文献   

Refining the palaeoecology of lacustrine testate amoebae: insights from a plant macrofossil record from a eutrophic Scottish lake     

Stephen V. Prentice  Helen M. Roe  Helen Bennion  Carl D. Sayer  Jorge Salgado 《Journal of Paleolimnology》2018,59(2):189-199

Lakes are common natural systems for which sedimentation is considered to be relatively simple, generally dominated by fluvial processes along the margin and prevailing low-energy settling in the central, deeper parts. However, for many lakes, higher-energy wind-driven processes dominate. As such, a new category of lakes is proposed, herein referred to as wind-driven waterbodies (WWB). WWB display a sedimentation largely dominated by wave related processes and wind-induced lake-scale water circulation evidenced by the construction of beach ridges, spits or cuspate spits along their shorelines, and by sediment drifts, sedimentary shelf progradation and erosional surfaces in their deeper, offshore domains. WWB are observed worldwide, they share a common physiography that favours wind-forced hydrodynamics and related sedimentation patterns. This physiography is expressed by the I_WWB index, a ratio of the maximum representative fetch relative to mean basin depth. It is proposed that an index value greater than three favours the evolution of a lake as a WWB. The WWB concept represents a new end-member in an alternative, sedimentologically-based lake classification that is proposed in this paper.  相似文献   

The age of the meteorite recovery surfaces of Roosevelt County,New Mexico,USA     

Michael E. Zolensky  Helen M. Rendell  Ivan Wilson  Gordon L. Wells 《Meteoritics & planetary science》1992,27(4):460-462

Abstract— We have obtained minimum age estimates for the sand units underlying the two largest meteorite deflation surfaces in Roosevelt County, New Mexico, USA, using thermoluminescence dating techniques. The dates obtained ranged from 53.5 (±5.4) to 95.2 (±9.5) ka, and must be considered lower limits for the terrestrial ages of the meteorites found within these specific deflation surfaces. These ages greatly exceed previous measurements from adjacent meteorite-producing deflation basins. We find that Roosevelt County meteorites are probably terrestrial contemporaries of the meteorites found at most accumulation zones in Antarctica. The apparent high meteorite accumulation rate reported for Roosevelt County by Zolensky et al. (1990) is incorrect, as it used an age of 16 ka for all Roosevelt County recovery surfaces. We conclude that the extreme variability of terrestrial ages of the Roosevelt County deflation surfaces effectively precludes their use for calculations of the meteorite accumulation rate at the Earth's surface.  相似文献   

Using an inverse modelling approach with equifinality control to investigate the dominant controls on snowmelt nutrient export     

Diogo Costa  John Pomeroy  Helen Baulch  Jane Elliott  Howard Wheater 《水文研究》2019,33(23):2958-2977

There is great interest in modelling the export of nitrogen (N) and phosphorus (P) from agricultural fields because of ongoing challenges of eutrophication. However, the use of existing hydrochemistry models can be problematic in cold regions because models frequently employ incomplete or conceptually incorrect representations of the dominant cold regions hydrological processes and are overparameterized, often with insufficient data for validation. Here, a process‐based N model, WINTRA, which is coupled to a physically based cold regions hydrological model, was expanded to simulate P and account for overwinter soil nutrient biochemical cycling. An inverse modelling approach, using this model with consideration of parameter equifinality, was applied to an intensively monitored agricultural basin in Manitoba, Canada, to help identify the main climate, soil, and anthropogenic controls on nutrient export. Consistent with observations, the model results suggest that snow water equivalent, melt rate, snow cover depletion rate, and contributing area for run‐off generation determine the opportunity time and surface area for run‐off–soil interaction. These physical controls have not been addressed in existing models. Results also show that the time lag between the start of snowmelt and the arrival of peak nutrient concentration in run‐off increased with decreasing antecedent soil moisture content, highlighting potential implications of frozen soils on run‐off processes and hydrochemistry. The simulations showed TDP concentration peaks generally arriving earlier than NO₃ but also decreasing faster afterwards, which suggests a significant contribution of plant residue Total dissolved Phosphorus (TDP) to early snowmelt run‐off. Antecedent fall tillage and fertilizer application increased TDP concentrations in spring snowmelt run‐off but did not consistently affect NO₃ run‐off. In this case, the antecedent soil moisture content seemed to have had a dominant effect on overwinter soil N biogeochemical processes such as mineralization, which are often ignored in models. This work demonstrates both the need for better representation of cold regions processes in hydrochemical models and the model improvements that are possible if these are included.  相似文献   

Multi-proxy evidence of postglacial climate and environmental change at Two Frog Lake,central mainland coast of British Columbia,Canada     

Jennifer M. Galloway  R. Timothy Patterson  Christine T. Doherty  Helen M. Roe 《Journal of Paleolimnology》2007,38(4):569-588

Pollen and diatoms preserved in the radiocarbon dated sediments of Two Frog Lake in the Seymour-Belize Inlet Complex of the central mainland coast of British Columbia document postglacial climate change. Two Frog Lake was isolated from the sea prior to 11,040 ± 50 yr BP (13,030 cal. yr BP) when the climate was cool and dry, and open Pinus contorta woodlands covered the landscape. These woodlands were replaced by a mixed conifer forest ca. 10,200 yr BP (ca. 12,300 cal. yr BP) when the climate became moister. A relatively dry and warm early Holocene climate allowed Pseudotsuga menziesii to migrate northward to this site where it grew with Picea, Tsuga heterophylla and Alnus. The climate became cooler and moister at ca. 8,000 yr BP (ca. 9,200 cal. yr BP), approximately 500–1,000 years prior to sites located south of Two Frog Lake and on the Queen Charlotte Islands, but contemporary with sites on the northern mainland coast of British Columbia and south coastal Alaska. Climate heterogeneity in central coastal British Columbia appears to have occurred on a synoptic scale, suggesting that atmospheric dynamics linked to a variable Aleutian Low pressure system may have had an important influence on early Holocene climate change in the Seymour-Belize Inlet Complex. The transition to cooler and moister conditions facilitated the expansion of Cupressaceae and the establishment of a modern-type coastal temperate rainforest dominated by Cupressaceae and T. heterophylla. This was associated with progressive lake acidification. Diatom changes independent of vegetation change during the late Holocene are correlative with the mid-Neoglacial period, when cooler temperatures altered diatom communities.  相似文献