摘要:

AbstractWe present the results of magnetic measurements (low field susceptibility, anhysteretic and isothermal remanences) on samples of sediment from 11 cores from the Potomac River between Washington DC and its mouth. The magnetic properties indicate that at sites in the upstream reaches especially, there has been a shift to surface‐soil‐derived sediments in recent times. No such shift is indicated at the mouth of the Estuary. Comparison with pollen,210Pb and trace metal analyses, where available, suggests that the shift to soil‐derived material dates from the early to mid‐19th century and coincides with a major land use change previously documented by Brushet al.(1982). Inventories of unsupported (excess)210Pb activity in the cores increase upstream, are positively correlated with total sedimentation rates, and, in the upper reaches where the magnetic properties indicate soil inwash, greatly exceed values expected from direct atmospheric deposition alone. We infer that in these cores, most of the unsupported210Pb is deposited from the surfaces of the catchment as a result of soil

ISSN:0267-8179    

DOI:10.1002/jqs.3390040301

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractRadiocarbon dating of the organic‐rich sediments of Lake Illisarvik in the outer Mackenzie Delta indicates that formation of the lake occurred approximately 9500yr BP, with maximum expansion around 6000 yr BP. Sedimentation rates have remained relatively constant at an average of 0.3mm/yr.13C results on biogenic and inorganic carbonates and organics indicate a change from dominantly terrestrial organics (−27 to −28%0) to submerged aquatic vegetation or plankton (−18 to −23%0) upon formation of the lake (9500yr BP), and a dramatic return to dominantly terrestrial organics at 5800yr BP (δ13C = −27 to −30%0). This latter shift is accompanied by a drastic reduction in the macroflora and fauna populations.18O results suggest that a warmer climate than today existed prior to the shif

ISSN:0267-8179    

DOI:10.1002/jqs.3390040302

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractA late Quaternary deep‐water stratigraphic framework has been established for the deep‐water areas (>450m) of the northern Rockall Trough and Faeroe‐Shetland Channel. Four stratigraphic units (1–4) are identified; these are primarily biostratigraphic units based on dinoflagellate cyst evidence. Unit 1 represents the late Weichselian glacial (pre‐13 000 yr BP); unit 2 the Late Glacial Interstadial (11 000‐13 000 yr BP); unit 3 is of Younger Dryas age (10 000‐11 000 yr BP); and unit 4 represents the Holocene interglacial (post‐10 000 yr BP). This stratigraphy is supported by the discovery of the mixed Vedde Ash (10 600 yr BP) and North Atlantic Ash zone 1, and the Saksunarvatn Ash (9000–9100 yr BP), concentrated in units 3 and 4 respectively. The sedimentology indicates that the oceanographic regime underwent a major change between the glacial and interglacial stages. This is marked by the onset of strong bottom current activity, allied to the restoration of overflow of the Norwegian Sea Deep Water into the North Atlantic, towards the end of the Younger Dryas Stadial. Despite intense bioturbation and bottom‐current reworking the basic stratigraphic framework is maintained. Recognition of two volcanic ash markers enables correlation with established onshore and offshore sequences of marine and non

ISSN:0267-8179    

DOI:10.1002/jqs.3390040303

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractPalynological results from Liastemmen indicate a tripartite division of the Late Weichselian. In the pleniglacial period, from deglaciation ca. 14000 BP to ca. 13000 BP,Artemisia‐dominated pioneer vegetation on disturbed, mineral‐soil was strongly influenced by cold winters and katabatic winds. The Late Weichselian Interstadial (ca. 13000 BP‐ca. 11000 BP) comprises aSalix‐shrub consolidation phase, and from ca. 12700 BP a tree‐birch phase. In the last 500 years of this period July and January means are estimated to about 16°C and between −2°C and −6°C, respectively. In the Younger Dryas Stadial (ca. 11000 BP‐ca. 10200 BP)Artemisia‐dominated vegetation returns. Three brief climatic deteriorations (ca. 12 250 BP, 11 700 BP, and 11 300 BP), unfavourable to woody vegetation on humus soils, are demonstrated within the interstadial. Critical climatic factors include cool winters and strong winds, exposing vegetation and soil to frost, drought, and erosion. The oldest and strongest oscillation, probably involving local deforestation, is correlated with the ‘Older Dryas deterioration’. Boreal‐circumpolar, eurasiatic, and arctic‐alpine plants dominated the late‐glacial flora. For the majority of the late‐glacial taxa a northward migration is demonstrated. This may also apply forPapaver radicatum, Pinguicula alpina, andPrimula scandinavica, all with bicentr

ISSN:0267-8179    

DOI:10.1002/jqs.3390040304

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractMoraines in six cirques in Northern Scotland are shown to be asymmetrically developed, being larger below north‐ or east‐facing valley sides. Moraine asymmetry is strongly correlated with the distribution of free faces in the valleys, a relationship that is interpreted as the result of variation in slope retreat rates. Analysis of clast form and roundness demonstrates that following initial release from bedrock, debris entered both passive high‐level glacial transport and tractive transport at the ice‐bed interface. The importance of glacier velocities and medial moraines on debris flux is discussed. Consideration is also given to the implications of process asymmetry to long term landform deve

ISSN:0267-8179    

DOI:10.1002/jqs.3390040305

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractBuried channels cut to a depth of over 100m into the underlying substratum of North Germany are almost exclusively of Elsterian age. Evaluation of geophysical well logs in the Hamburg area for the first time allows the construction of detailed longitudinal and transverse profiles. It can be shown that an apparently chaotic infill known from borehole records in some cases is actually a regular sedimentary sequence, fining from proximal to distal. Channel formation seems to have been the result of catastrophic meltwater release in the marginal parts of the ice sheet. The channel infill reflects these episodic outbursts of meltwater. During deglaciation, the channel remnants became infilled with fine sediments. The channels ceased to exist as morphologically detectable forms at the end of the Holsteinian Stage.

ISSN:0267-8179    

DOI:10.1002/jqs.3390040306

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractTectonic and soil mechanics investigations of a strongly plastic clay (Miocene ‘Mica Clay’) led to the detection of strain phenomena induced in connection with the formation of Elsterian buried channels. A microjoint system with randomly distributed fissures is thought to result from early Elsterian dessiccation and frost crack formation. It occurs frequently in the upper part of the clay horizon. A younger joint system, which is encountered throughout the entire clay body, comprises macrojoints arranged in three joint sets. The evidence strongly suggests that they were formed by gravitational and kinematic loading of the clay body by overriding ice. The macrojoints display various secondary alterations such as slickensides, escarpments, shear zones, erosive and pressure zones, the formation of which is seen in context with the channel incision. Investigations into the genesis of these alterations indicate a strong horizontally‐orientated stress induction lateral to the channels, which resulted in stiffness modulus anisotropy. It has also caused a horizontal compression of the clay body, which at that time was already consolidated through ice overburdening. Horizontal compression of the clay led to evorsion of pore water eroding clay particles from the joint walls. Lower water content of the clay, in combination with increased dry unit weights, are found adjacent to the channels

ISSN:0267-8179    

DOI:10.1002/jqs.3390040307

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

ISSN:0267-8179    

DOI:10.1002/jqs.3390040308

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

ISSN:0267-8179    

DOI:10.1002/jqs.3390040309

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

ISSN:0267-8179    

DOI:10.1002/jqs.3390040310

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY