Dynamic Holocene glacial history of St. Jonsfjorden,Svalbard     

Wesley R. Farnsworth  Ólafur Ingólfsson  Riko Noormets  Lis Allaart  Helena Alexanderson  Mona Henriksen  Anders Schomacker 《Boreas: An International Journal of Quaternary Research》2017,46(3):585-603

Evidence of a dynamic Holocene glacial history is preserved in the terrestrial and marine archives of St. Jonsfjorden, a small fjord‐system on the west coast of Spitsbergen, Svalbard. High‐resolution, remotely sensed imagery from marine and terrestrial environments was used to construct geomorphological maps that highlight an intricate glacial history of the entire fjord‐system. The geomorphology and stratigraphy indicate an early Holocene local glacier advance constrained to the Lateglacial–early Holocene transition. Identification and ¹⁴C dating of the thermophilous bivalve mollusc Modiolus modiolus to 10.0±0.12 cal. ka BP suggest a rapid northward migration of the species shortly after deglaciation. Further evidence enhances the understanding of the onset and subsequent climax of the Neoglacial‐Little Ice Age in inner St. Jonsfjorden. The present‐day terminus of Osbornebreen, the dominating glacier system in St. Jonsfjorden, is located over 8.5 km up‐fjord from its Neoglacial maximum extent. Cross‐cutting relationships suggest subsequent advances of all the smaller glaciers in the area following the break‐up of Osbornebreen. Glacial deposits, landforms and their cross‐cutting relationships observed in both terrestrial and marine settings imply a complex and highly dynamic environment through the later part of the Holocene.  相似文献   

Palaeoglaciology of Bayan Har Shan,NE Tibetan Plateau: exposure ages reveal a missing LGM expansion     

Jakob Heyman  Arjen P. Stroeven  Marc W. Caffee  Clas Hättestrand  Jonathan M. Harbor  Yingkui Li  Helena Alexanderson  Liping Zhou  Alun Hubbard 《Quaternary Science Reviews》2011,30(15-16):1988-2001

The Bayan Har Shan, a prominent upland area in the northeastern sector of the Tibetan Plateau, hosts an extensive glacial geological record. To reconstruct its palaeoglaciology we have determined ¹⁰Be exposure ages based on 67 samples from boulders, surface pebbles, and sediment sections in conjunction with studies of the glacial geology (remote sensing and field studies) and numerical glacier modelling. Exposure ages from moraines and glacial sediments in Bayan Har Shan range from 3 ka to 129 ka, with a large disparity in exposure ages for individual sites and within the recognised four morphostratigraphical groups. The exposure age disparity cannot be explained by differences in inheritance without using unrealistic assumptions but it can be explained by differences in post-depositional shielding which produces exposure ages younger than the deglaciation age. We present a palaeoglaciological time-slice reconstruction in which the most restricted glaciation, with glaciers less than 10 km long, occurred before 40–65 ka. More extensive glaciations occurred before 60–100 ka and 95–165 ka. Maximum glaciation is poorly constrained but probably even older. The Bayan Har Shan exposure age dataset indicates that glaciers on the northeastern Tibetan Plateau have remained surprisingly restricted for at least 40 ka, including the global last glacial maximum (LGM). This case of a missing LGM is further supported by high-resolution glacier modelling experiments.  相似文献   

Comment on Frechen et al. “Late Pleistocene fluvial dynamics in the Hochrhein Valle and in the Upper Rhine Graben: chronological frame”     

Frank Preusser  Stéphane Kock  Helena Rodnight 《International Journal of Earth Sciences》2012,101(1):385-387

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Environmental Forcing of Phytoplankton in a Mediterranean Estuary (Guadiana Estuary,South-western Iberia): A Decadal Study of Anthropogenic and Climatic Influences     

Ana B. Barbosa  Rita B. Domingues  Helena M. Galvão 《Estuaries and Coasts》2010,33(2):324-341

Phytoplankton seasonal and interannual variability in the Guadiana upper estuary was analyzed during 1996–2005, a period that encompassed a climatic controlled reduction in river flow that was superimposed on the construction of a dam. Phytoplankton seasonal patterns revealed an alternation between a persistent light limitation and episodic nutrient limitation. Phytoplankton succession, with early spring diatom blooms and summer–early fall cyanobacterial blooms, was apparently driven by changes in nutrients, water temperature, and turbulence, clearly demonstrating the role of river flow and climate variability. Light intensity in the mixed layer was a prevalent driver of phytoplankton interannual variability, and the increased turbidity caused by the Alqueva dam construction was linked to pronounced decreases in chlorophyll a concentration, particularly at the start and end of the phytoplankton growing period. Decreases in annual maximum and average abundances of diatoms, green algae, and cyanobacteria were also detected. Furthermore, chlorophyll a decreases after dam filling and a decrease in turbidity may point to a shift from light limitation towards a more nutrient-limited mode in the near future.  相似文献   

Interactions between the Greenland Ice Sheet and the Liverpool Land coastal ice cap during the last two glaciation cycles     

Lena Adrielsson  Helena Alexanderson 《第四纪科学杂志》2005,20(3):269-283

The sedimentary record from the Ugleelv Valley on central Jameson Land, East Greenland, adds new information about terrestrial palaeoenvironments and glaciations to the glacial history of the Scoresby Sund fjord area. A western extension of a coastal ice cap on Liverpool Land reached eastern Jameson Land during the early Scoresby Sund glaciation (≈the Saalian). During the following glacial maximum the Greenland Ice Sheet inundated the Jameson Land plateau from the west. The Weichselian also starts with an early phase of glacial advance from the Liverpool Land ice cap, while polar desert and ice‐free conditions characterised the subsequent part of the Weichselian on the Jameson Land plateau. The two glaciation cycles show a repeated pattern of interaction between the Greenland Ice Sheet in the west and an ice cap on Liverpool Land in the east. Each cycle starts with extensive glacier growth in the coastal mountains followed by a decline of the coastal glaciation, a change to cold and arid climate and a late stage of maximum extent of the Greenland Ice Sheet. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Assessing characteristics of Mediterranean explosive cyclones for different data resolution     

John Kouroutzoglou  Helena A. Flocas  Ian Simmonds  Kevin Keay  Maria Hatzaki 《Theoretical and Applied Climatology》2011,105(1-2):263-275

A comparison of two objective climatologies of explosive cyclones in the Mediterranean region is performed. The results are derived from two different mean sea-level pressure reanalysis data resolutions, but from the same assimilation model, in order to quantify the pure impact of higher resolution on the identification and characteristics of explosive cyclones, when the assimilation model is the same. The explosive cyclones were identified with the aid of the Melbourne University automatic cyclone finding and tracking scheme over a 40-year period, using the 6-hourly analyses of ERA-40 mean sea-level pressure (MSLP) on: (a) 2.5?×?2.5 and (b) 1?×?1 latitude–longitude grid. The comparison of the two datasets revealed the significant role of the increase in spatial resolution of MSLP data on the identification and tracking process, and the number of the explosive cyclones in the high-resolution dataset is almost four times greater than the respective one in the lower resolution dataset. However, the comparison of explosive cyclone characteristics, including spatial and temporal variations of explosive deepening, revealed differences in the geographical distribution of the location of the maximum explosive deepening and average explosive cyclone Laplacian of the central pressure. These differences are due to the identification in the higher resolution set of smaller scale and secondary explosives along the strongly baroclinic northern Mediterranean boundaries, south of the Alps and the Pyrenees. Explosive deepening appears a bias to the daytime period from 12 to 18 Coordinated Universal Time (UTC) for both datasets, which is more prominent in the LR dataset. Statistically significant difference of pressure tendency between the two datasets appear for the daytime period from 06 to 12 UTC, accounting for better representation of orographic forcing in the HR dataset.  相似文献   

Timing of the HT/LP transpression in the Neoproterozoic Seridó Belt (Borborema Province,Brazil): Constraints from UPb (SHRIMP) geochronology and implications for the connections between NE Brazil and West Africa     

Carlos J. Archanjo  Luiz G.F. Viegas  Maria Helena B.M. Hollanda  Laecio C. Souza  Dunyi Liu 《Gondwana Research》2013,23(2):701-714

The Seridó schist belt is a classical domain of the Borborema Province (NE Brazil) recording crustal melting at HT/LP conditions in a tectonic setting dominated by dextral transpressive deformation. A comparable tectonometamorphic setting has been described in the Pan-African Igarra and Ilesha schist belts of south Nigeria (Africa). The HT‐transpression of the Seridó has previously been imprecisely dated by using a range of isotopic methods on a suite of syntectonic granites. We provide new UPb zircon ages of the Acari granite and Santa Luzia migmatite that constrain the HT/LP metamorphism of the Seridó at c. 575 Ma. Anisotropy of magnetic susceptibility of the Santa Luzia migmatite shows that the magnetic fabric records the NE-trending dextral kinematics similar to those previously described in the Acari pluton. On the other hand, the UPb ages of the Totoró pluton provide evidence for an older, mostly mafic to intermediate magmatism intruding the pelitic schist that precedes the HT/LP metamorphism by approximately 15 m.y. In contrast with the Acari pluton and Santa Luzia migmatite, the magnetic fabrics of Totoró show a concentric steep-dipping structure apparently related to magma ascent. Thermobarometric studies in the Seridó, Igarra and Ilesha schist belts indicate that a large domain in the northern Borborema and south of the Nigeria shields was deformed at a high temperature and at relatively shallow depths. Although the tectonic setting for such a post-collision HT event remains an open issue, it allows us nevertheless to refine the across-Atlantic geological correlation between the continents.  相似文献   

Geographic information science as a multidisciplinary and multiparadigmatic field     

Thomas Blaschke  Helena Merschdorf 《制图学和地理信息科学》2014,41(3):196-213

Despite the fact that the field of GIScience has been around for two decades, there is still little agreement as to the exact contents and boundaries of the field. Many authors have dedicated several publications concerned with defining the field, yet little consensus has been reached. Because of the highly dynamic nature of the discipline, new areas are constantly added and rigid borders may have a constraining influence on the field. On the contrary, one agreed part of the many GIScience definitions is the search for general principles, but principles for what? Can we define one field or discipline or all potential fields using geospatial information? Probably not. Therefore, rather than attempting to demarcate exact boundaries for GIScience as a discipline or a multidisciplinary field in order to prove its respectability, we herein attempt to analyze the contents of such a dynamic field on the basis of scientific literature and to assess the multidisciplinary and multiparadigmatic nature of GIScience. Such a discussion is not purely of academic nature, but also bears implications beyond academic discourse, in terms of external scientific funding and research grants. We question whether there is a “dominant” paradigmatic approach in GIScience and identify a need for adopting a multiparadigmatic view to accommodate the multifaceted nature of space, spatial representations, and the societal implications of geospatial information.  相似文献   

Monitoring Mangrove forest landcover changes in the coastline of Bangladesh from 1976 to 2015     

Md. Monirul Islam  Helena Borgqvist  Lalit Kumar 《国际地球制图》2013,28(13):1458-1476

Abstract

This study used multi-date Landsat images to quantify mangrove cover changes in the whole of Bangladesh from 1976 to 2015. Images were pre-processed with an atmospheric correction using Dark Object Subtraction (DOS) and Relative Radiometric Normalization (RRN) using Pseudo-Invariant Features (PIFs). Land Use/Land Cover (LU/LC) classification map was generated using Maximum Likelihood (MaxLike) algorithm, indicating the areal extent of mangroves increased by 3.1% between 1976 and 2015, where 1.79% of this increase occurred between 2000 and 2015. Though mangrove areas remained almost constant in the Sundarbans, Chakaria Sundarbans has almost disappeared between 1976 and 1989. The overall accuracy of Landsat MSS, TM, ETM+, and L8 OLI classified images were 80%, 80%, 87%, and 97% respectively. The study also found deforestation, shrimp & salt farm, coastal erosion and sedimentation, and mangrove plantation could be responsible for mangrove changes in Bangladesh.  相似文献   

Recalibrating aeolian sand transport models   总被引：1，自引：0，他引：1  

Douglas J. Sherman  Bailiang Li  Jean T. Ellis  Luis Parente Maia  Helena Granja 《地球表面变化过程与地形》2013,38(2):169-178

A quality‐controlled data set comprising measurements of aeolian sand transport rates obtained at three disparate field sites is used to evaluate six commonly employed transport rate models (those of Bagnold, Kawamura, Zingg, Owen, Hsu, and Lettau and Lettau) and to recalibrate the empirical constants in those models. Shear velocity estimates were obtained using the von Kármán constant and an apparent von Kármán parameter. Models were recalibrated using non‐linear regression and non‐linear regression with least‐squares lines forced through axes origins. Recalibration using the apparent von Kármán parameter and forced regression reduced the empirical constants for all models. The disparity between the predictions from the different models is reduced from about an order of magnitude to about a quarter of an order of magnitude. The recalibrated Lettau and Lettau model provided the greatest statistical agreement between observed and predicted transport rates, with a coefficient of determination of 0·77. Evaluation of the results suggests that our estimations of threshold shear velocity may be too slow, causing errors in predicted transport rates. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献