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21.
Fluid mud dynamics in the Weser estuary turbidity zone tracked by high-resolution side-scan sonar and parametric sub-bottom profiler 总被引：1，自引：0，他引：1
Kerstin Schrottke Marius Becker Alexander Bartholomä Burghard W. Flemming Dierk Hebbeln 《Geo-Marine Letters》2006,26(3):185-198
Fluid mud in estuarine turbidity maximum zones (TMZ) can pose considerable navigation risks due to potentially substantial reductions in nautical depth, coupled with an inherent difficulty of detection by conventional echo-sounders. Despite intensive research efforts, however, our knowledge about the spatial and temporal dynamics of fluid mud is still not sufficient. In this study, the combined use of a side-scan sonar (Sportscan®, Imagenex) and a parametric sub-bottom profiler (SES-2000®, Innomar Technology GmbH) has proved successful for high-resolution fluid mud detection and volumetric quantification in an estuarine environment. In 2004 and 2005, repeated surveys were conducted in the navigation channel of the upper meso- to lower macrotidal Weser estuary TMZ (German North Sea coast) at different tidal stages and river discharges. Current velocity data were simultaneously collected by 1,200-kHz broadband ADCP (RDInstruments) measurements. Ground-truthing was carried out by means of grab sampling and gravity coring, adapted to fluid mud conditions. It was found that fluid mud occurrence in the Weser estuary is highly variable on time scales of a few hours and spatial scales of several metres. The riverbed is characterised by sand and mud deposits, and a complex morphology including subaqueous dunes and smooth bed deposits intermittently overlain by fluid mud. Thus, a continuous, coherent fluid mud body covering the entire TMZ riverbed was not observed. Rather, spatial distribution was patchy and highly dependent on suspended particulate matter (SPM) concentrations in the water column, as a result of which local fluid mud deposits varied in thickness from centimetres to metres. The formation of fluid mud was largely restricted to slack water, although slack-water conditions were not necessarily associated with large-scale fluid mud appearance. Advective SPM transport of resuspended fluid mud seems to be the most plausible explanation for the high spatial variability observed, even between two successive tides. The amount of fluid mud deposited and resuspended in the course of a tidal cycle can reach several 10s of tons even in small riverbed depressions. 相似文献
22.
The impact of dredge spoil disposal on asymmetrical large-scale dunes has been studied at a disposal site in a shallow subtidal area of the outer Weser Estuary (German Bight, southeastern North Sea). Between June and December 1998, this site was used for the disposal of ~3×106 m3 dredge spoil. Repeated bathymetric surveys with a multibeam echosounder system reveal that the artificial supply of sediment provoked significant morphological changes in the dune field, including the infill of dune troughs and even the complete burial of individual dunes. However, even completely buried dunes began to regenerate within a few months. In addition, slow migration of the dunes toward the open sea was observed, indicating net seaward sediment transport in the survey area. Since the dumped sediment does not appear to have a persistent effect on the bedforms and, in all likelihood, will be exported from the estuarine system on medium- to long-term timescales, the investigated area constitutes a suitable disposal site. 相似文献
23.
Quantification of dune dynamics during a tidal cycle in an inlet channel of the Danish Wadden Sea 总被引：2，自引：3，他引：2
Verner B. Ernstsen Riko Noormets Christian Winter Dierk Hebbeln Alex Bartholomä Burg W. Flemming J. Bartholdy 《Geo-Marine Letters》2006,26(3):151-163
High-resolution swath bathymetry measurements at centimetre-scale precision conducted during a tidal cycle in the Grådyb tidal inlet channel in the Danish Wadden Sea reveal the short-term dynamics of a large, ebb-directed compound dune with superimposed small to medium dunes, all composed of medium sand. Dune dynamics were related to simultaneous measurements of flow using an acoustic Doppler current profiler. Spatially, dune crests displayed greater mobility than did dune troughs, due to higher flow velocities at the crests than in the troughs. Temporally, superimposed lower lee-side dunes migrated more during the flood than the ebb tide, due to higher near-bed trough flow velocities during the flood phase, resulting in varying exposure to flow. Net dune migration was flood-directed over the tidal cycle, despite annual net migration being ebb-directed. Hence, extrapolation of short-term migration rates is not possible in this case. The superimposed dunes reversed direction during each half tidal cycle whereas the compound dune only developed a flood cap during flood tide, i.e. the time required for complete reversal of the compound dune was much longer than that available in a half tidal cycle. Over the tidal cycle, the bed level was stable but significant erosion and accretion occurred during the tidal phases. During the ebb tide, bed material was brought into suspension with accelerating flow and settled with decelerating flow, resulting in an average erosion and accretion of the bed of ~7 cm in each case. During the flood tide, the bed of the compound dune was overall stable, although bed material was eroded from the exposed lower lee side, being partly transported to the crest in bedload and partly brought into suspension. In general, dune height fluctuated during the tidal cycle whereas dune length remained stable. The height of the compound dune responded to changes in water depth, which acts as a limiting factor to dune growth. By contrast, the height of the stoss-side dunes responded to flow velocity, i.e. the stoss-side dunes were water depth-independent. 相似文献
24.
Circulation Variability Reflected in Ice Core and Lake Records of the Southern Tropical Andes 总被引：2，自引：0，他引：2
The circulation mechanisms of climate anomalies in the southern tropical Andes are of particular interest for the January–February core of the precipitation season. With this focus, we evaluate in context upper-air and surface analyses, water level measurements of Lake Titicaca, and records of net balance and 18O from ice cores. Precipitation is more abundant with enhanced and southward expanded easterlies through a deep layer of the troposphere over the southern tropical Andes. Concomitant with this is a southward displaced circulation system over the equatorial Atlantic, entailing reduced interhemispheric gradient of sea surface temperature (SST; cold/warm anomalies in the North/South), more southerly position of the surface wind confluence and Intertropical Convergence Zone, and thus more abundant rainfall in Northeast Brazil. Such ensemble of circulation departures in boreal winter is common to the high phase of the Southern Oscillation.18O in the ice cores from Peru's Quelccaya Icecap, as wellas the cores from Sajama and Ilimani in Bolivia is more negative with more abundant precipitation, both in the same annual cycle and on interannual timescales. The large-scale circulation departures associated with the more negative 18O are in the sense as for anomalously abundant precipitation activity over the southern tropical Andes. The variability of 18O seasonally and interannually appears to be controlled mainly by the fate of the water vapor along its trajectory and over the Andes, rather than by the SST of the South Atlantic source region. 相似文献
25.
26.
Late Quaternary rapid climate change in northern Chile 总被引：2，自引：0，他引：2
Analyses of terrigenous sediments from the Chilean continental slope off the southern border of the Atacama desert (27.5°S), focusing on illite crystallinity and the Fe:Al ratio of the sediments, reveal a high-frequency variability of the position of the Southern Westerlies, which is very similar to the coeval short-term climatic events known from Greenland ice cores and from North Atlantic sediments. Besides showing dominantly precession-driven variability in precipitation over the Andes, these analyses also reveal rapid changes in weathering intensity along the Chilean Coastal Range during the last 80,000 years. These rapid changes occur at much shorter timescales than the 19–100 kyr orbital forcing of the Milankovitch cycles. 相似文献
27.
Jens Weiser Jürgen Titschack Dierk Hebbeln 《Boreas: An International Journal of Quaternary Research》2023,52(3):314-340
Under glacial climates, continental ice sheets such as, e.g., the Greenland Ice Sheet, extended onto the continental shelves and often carved out deep cross-shelf troughs. The sedimentary infill of such troughs commonly is a product of the complex interactions between the ice sheets, largely driving sediment input into the ocean, and the surrounding water masses. Off West Greenland, research has focused on the Disko and Uummannaq troughs, leaving the northerly adjacent Upernavik trough relatively understudied. Hence, neither the chronology of deglaciation nor the details of its postglacial infill are sufficiently well understood. Here, we combine computed tomography image-derived information with geochemical and granulometric data from four sediment cores recovered from the Upernavik trough that point to (i) deglaciation of the mid-shelf probably around 13.4 cal. ka BP that was most likely driven by a northward advection of warmer Atlantic waters during the Bølling–Allerød, (ii) the presence of widespread mass wasting around 8 cal. ka BP on the inner shelf and (iii) the complex interplay between various modes of sediment input, transport and deposition under hemipelagic sedimentation afterwards. While this interplay complicates provenance studies, we identify two major sediment delivery mechanisms that control transport and deposition from four sediment source areas. Through the Early Holocene the relative contributions of sediments from the various sources changed from a predominantly local origin to more southerly sources, mainly driven by decreasing input from the local sources. The integration of relative sediment source contributions with varying sedimentation rates challenges previous studies postulating intensified sediment delivery from the south through a greater influence of the West Greenland Current and highlights the need for the integration of sediment input and transport mechanisms into provenance studies in the area. 相似文献
28.
A high-resolution sediment core (sedimentation rate ~2 mm/year) from the German Bight was analysed for its foraminiferal stable oxygen isotope (18O) composition. These data were correlated with instrumental summer sea-surface temperature and salinity data from the nearby island of Helgoland, reaching back 100 years. Comparing the isotope data with the instrumental records reveals a distinct 18O–salinity relationship (18O=0.34×S–9.36; r=0.86) for the German Bight, where salinity is mainly driven by freshwater input from the Elbe River. Thus, these findings provide the possibility for future regional paleosalinity and paleodischarge reconstructions for times far beyond the instrumental timescale. 相似文献
29.
Riko Noormets Verner B. Ernstsen Alex Bartholomä Burg W. Flemming Dierk Hebbeln 《Geo-Marine Letters》2006,26(3):165-176
The bedforms and the local scour at the base of a cylindrical pile were studied in a tidal inlet in the Wadden Sea, southern North Sea, using high-resolution multibeam bathymetry data from four surveys. The observed changes in scour and bedform dimensions were interpreted in terms of hydraulic forcings varying periodically at different time scales. It appears that bedform orientation reacts to changing flow conditions on a semidiurnal basis, whereas bedform height and steepness reflect the spring-neap cycle as well as seasonal signals. The scour depth carries a strong overprint of the semidiurnal tidal cycle, which is at a maximum during the strongest tidal flow. Subtler variations in scour depth can possibly be attributed to the spring-neap tidal cycle. Based on these data on bedform and scour dimensions, correlation functions were established between scour depth and dune height as well as dune length. In measuring the scour depth under mobile bed conditions, establishing the seabed level based on the trough level of the bedforms nearest to the scour proved useful. These findings suggest that the dimensions of bedforms in dynamic equilibrium with prevailing hydraulic flow conditions can be used to estimate scouring in tidal environments. 相似文献
30.
Environmental changes and growth history of a cold-water carbonate mound (Propeller Mound,Porcupine Seabight) 总被引：4，自引：0，他引：4
Andres Rüggeberg Christian Dullo Boris Dorschel Dierk Hebbeln 《International Journal of Earth Sciences》2007,96(1):57-72
On- and off-mound sediment cores from Propeller Mound (Hovland Mound province, Porcupine Seabight) were analysed to understand
better the evolution of a carbonate mound. The evaluation of benthic foraminiferal assemblages from the off-mound position
helps to determine the changes of the environmental controls on Propeller Mound in glacial and interglacial times. Two different
assemblages describe the Holocene and Marine Isotope Stage (MIS) 2 and late MIS 3 (∼31 kyr BP). The different assemblages
are related to changes in oceanographic conditions, surface productivity and the waxing and waning of the British Irish Ice
Sheet (BIIS) during the last glacial stages. The interglacial assemblage is related to a higher supply of organic material
and stronger current intensities in water depth of recent coral growth. During the last glaciation the benthic faunas showed
high abundances of cassidulinid species, implying cold bottom waters and a reduced availability of organic matter. High sedimentation
rates and the domination of Elphidium excavatum point to shelf erosion related to sea-level lowering (∼50 m) and the progradation of the BIIS onto the shelf. A different
assemblage described for the on-mound core is dominated by Discanomalina coronata, Gavelinopsis translucens, Planulina ariminensis, Cibicides lobatulus and to a lower degree by Hyrrokkin sarcophaga. These species are only found or show significantly higher relative abundances in on-mound samples and their maximum contribution
in the lower part of the record indicates a higher coral growth density on Propeller Mound in an earlier period. They are
less abundant during the Holocene, however. This dataset portrays the boundary conditions of the habitable range for the cold-water
coral Lophelia pertusa, which dominates the deep-water reefal ecosystem on the upper flanks of Propeller Mound. The growth of this ecosystem occurs
during interglacial and interstadial periods, whereas a retreat of corals is documented in the absence of glacial sediments
on-mound. Glacial conditions with cold intermediate waters, a weak current regime and high sedimentation rates provide an
unfavourable environmental setting for Lophelia corals to grow. A Late Pleistocene decrease is observed in the mound growth for Propeller Mound, which might face its complete
burial in the future, as it already happened to the buried mounds of the Magellan Mound province further north. 相似文献