Time-domain calculation of moored ship motions in nonlinear waves     

Wim van der Molen  Ivo Wenneker 《Coastal Engineering》2008

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Climatic and environmental changes during the Weichselian Lateglacial Interstadial in the Weerterbos region,the Netherlands     

Nelleke Van Asch  Oliver Heiri  Sjoerd J. P. Bohncke  Wim Z. Hoek 《Boreas: An International Journal of Quaternary Research》2013,42(1):123-139

Lake sediment records from the Weerterbos region, in the southern Netherlands, were studied to reconstruct summer temperature and environmental changes during the Weichselian Lateglacial Interstadial. A sediment core obtained from a small lacustrine basin was analysed for multiple proxies, including lithological changes, oxygen isotopes of bulk carbonates, pollen and chironomids. It was found that the oxygen isotope record differed strongly from the other proxies. Based on a comparison with three additional lake sediment records from the same region, it emerged that the oxygen isotope records were strongly affected by local environmental conditions, impeding the distinction of a regional palaeoclimate signal. The chironomid‐inferred July air temperature reconstruction produced inferred interstadial temperatures ranging between ～15° and 18°C, largely consistent with previously published results from the northern part of the Netherlands. A temporary regressive phase in the pollen record, which can be tentatively correlated with the Older Dryas, preceded the expansion of birch woodland. Despite differences between the four pollen records from the Weerterbos region, a comparable regressive vegetation phase that was possibly the result of a shift to drier conditions could be discerned in all of the profiles. In addition, a temporary temperature decline of ～1.5°C was inferred from the chironomid record during this regressive phase. The multi‐proxy approach used here enabled a direct comparison of inferred changes in temperature, vegetation and environmental conditions at an individual site, while the multi‐site approach provided insight into the factors influencing the pollen and isotope records from these small‐scale depressions.  相似文献   

The younger dryas cooling in northeast Germany: summer temperature and environmental changes in the Friedländer Groβe Wiese region     

Nelleke van Asch  Marjan E. Kloos  Oliver Heiri  Pim de Klerk  Wim Z. Hoek 《第四纪科学杂志》2012,27(5):531-543

A lake sediment record from the Friedländer Groβe Wiese in northeast Germany was studied to reconstruct summer temperature changes associated with changes in vegetation development during the Weichselian Lateglacial. The record was analysed for pollen, chironomids, and oxygen and carbon isotopes of lake marl. The combination of radiocarbon dates, the presence of the Laacher See Tephra and correlation of lithological and palynological changes with other records from the region indicated that the record encompassed the Allerød to the early Holocene. Pollen assemblages reflect development of birch and later pine‐dominated forests during the Allerød, comparable to other sites in the region. Chironomid‐inferred mean July air temperatures (C‐IT) for this period range between ～14.0 and 14.8°C. A temporary decrease in C‐IT of ～1°C, a negative shift in the isotope records, and a minor decline of birch may correspond to Greenland Interstadial 1b. Even though the transition to the Younger Dryas appears to be affected by reworking and redeposition processes, a drop in C‐IT to ～11.1°C is reconstructed for the later part of the Younger Dryas, while it appears that pine locally persisted in the region. Comparison with a nearby pollen record further indicates a local expansion of wetland grasses during this period. At the transition to the Holocene, C‐IT increased to ～15.7°C, while birch and pine forests re‐expanded. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Geological constraints on glacio-isostatic adjustment models of relative sea-level change during deglaciation of Prince Gustav Channel,Antarctic Peninsula     

Stephen J. Roberts  Dominic A. Hodgson  Mieke Sterken  Pippa L. Whitehouse  Elie Verleyen  Wim Vyverman  Koen Sabbe  Andrea Balbo  Michael J. Bentley  Steven G. Moreton 《Quaternary Science Reviews》2011,30(25-26):3603-3617

The recent disintegration of Antarctic Peninsula ice shelves, and the associated accelerated discharge and retreat of continental glaciers, has highlighted the necessity of quantifying the current rate of Antarctic ice mass loss and the regional contributions to future sea-level rise. Observations of present day ice mass change need to be corrected for ongoing glacial isostatic adjustment, a process which must be constrained by geological data. However, there are relatively little geological data on the geometry, volume and melt history of the Antarctic Peninsula Ice Sheet (APIS) after Termination 1, and during the Holocene so the glacial isostatic correction remains poorly constrained. To address this we provide field constraints on the timing and rate of APIS deglaciation, and changes in relative sea-level (RSL) for the north-eastern Antarctic Peninsula based on geomorphological evidence of former marine limits, and radiocarbon-dated marine-freshwater transitions from a series of isolation basins at different altitudes on Beak Island. Relative sea-level fell from a maximum of c. 15 m above present at c. 8000 cal yr BP, at a rate of 3.91 mm yr^?1 declining to c. 2.11 mm yr^?1 between c. 6900–2900 cal yr BP, 1.63 mm yr^?1 between c. 2900–1800 cal yr BP, and finally to 0.29 mm yr^?1 during the last c. 1800 years. The new Beak Island RSL curve improves the spatial coverage of RSL data in the Antarctic. It is in broad agreement with some glacio-isostatic adjustment models applied to this location, and with work undertaken elsewhere on the Antarctic Peninsula. These geological and RSL constraints from Beak Island imply significant thinning of the north-eastern APIS by the early Holocene. Further, they provide key data for the glacial isostatic correction required by satellite-derived gravity measurements of contemporary ice mass loss, which can be used to better assess the future contribution of the APIS to rising sea-levels.  相似文献   

Reevaluating the Generality of an Empirical Model for Light-Limited Primary Production in the San Francisco Estuary     

Alexander E. Parker  Wim J. Kimmerer  Ulrika U. Lidstr?m 《Estuaries and Coasts》2012,35(4):930-942

Depth-integrated primary production (??P, in grams of carbon per square meter per day) was measured using ¹⁴C in the northern San Francisco Estuary (SFE) from March through August of 2006 and 2007. Determinations of ??P were then used to calibrate a published light-utilization model that relates ??P to a composite parameter of chlorophyll, solar irradiance, and photic zone depth. The resultant calibration coefficient, ??, varied by a factor of nearly two between 2006 and 2007 and was lower than determined in previous calibrations for the estuary. The now chronically low chlorophyll concentrations in the SFE have resulted in lower predictive power of the light-utilization model. The variation in ?? was likely the result of interannual variation in phytoplankton assimilation number. These results suggest that using a single ?? may yield large errors in estimated estuarine production when applied overbroad spatial and temporal scales. Given the food-limited condition of the SFE, it appears that direct measurements of primary production are necessary for accurately characterizing the base of the estuarine food web.  相似文献   

Do shorebirds graze down zoobenthic biomass at the Banc d'Arguin tidal flats in Mauritania?     

Wim J. Wolff  Hermann Michaelis 《Estuarine, Coastal and Shelf Science》2008,79(3):491-495

The 500 km² of tidal flats at the Banc d'Arguin, Mauritania, are a wintering site for over 2 million shorebirds, making this one of the areas with the highest density of shorebirds in the world. In earlier studies, it was found that these huge numbers of birds are dependent on a rather low biomass of benthic animals. To explain this paradoxical situation, we hypothesized that the earlier assessments of biomass in late winter and early spring do not reflect a year-round situation. Benthic biomass may be high when the birds arrive in autumn to be subsequently grazed down in the course of winter. To test this hypothesis, we have determined benthic biomass in September 1988. The result has been compared with a biomass value obtained in February–April 1986 with the same methods and at the same stations. These two biomass values do not differ significantly; so our hypothesis has to be rejected. It has to be admitted, however, that our experimental design cannot discern between differences due to the year of sampling and those due to the season.  相似文献   

A search for shocked quartz grains in the Allerød‐Younger Dryas boundary layer          下载免费PDF全文

Annelies Van Hoesel  Wim Z. Hoek  Gillian M. Pennock  Knut Kaiser  Oliver Plümper  Michal Jankowski  Maartje F. Hamers  Norbert Schlaak  Mathias Küster  Alexander V. Andronikov  Martyn R. Drury 《Meteoritics & planetary science》2015,50(3):483-498

The Younger Dryas impact hypothesis suggests that multiple airbursts or extraterrestrial impacts occurring at the end of the Allerød interstadial resulted in the Younger Dryas cold period. So far, no reproducible, diagnostic evidence has, however, been reported. Quartz grains containing planar deformation features (known as shocked quartz grains), are considered a reliable indicator for the occurrence of an extraterrestrial impact when found in a geological setting. Although alleged shocked quartz grains have been reported at a possible Allerød‐Younger Dryas boundary layer in Venezuela, the identification of shocked quartz in this layer is ambiguous. To test whether shocked quartz is indeed present in the proposed impact layer, we investigated the quartz fraction of multiple Allerød‐Younger Dryas boundary layers from Europe and North America, where proposed impact markers have been reported. Grains were analyzed using a combination of light and electron microscopy techniques. All samples contained a variable amount of quartz grains with (sub)planar microstructures, often tectonic deformation lamellae. A total of one quartz grain containing planar deformation features was found in our samples. This shocked quartz grain comes from the Usselo palaeosol at Geldrop Aalsterhut, the Netherlands. Scanning electron microscopy cathodoluminescence imaging and transmission electron microscopy imaging, however, show that the planar deformation features in this grain are healed and thus likely to be older than the Allerød‐Younger Dryas boundary. We suggest that this grain was possibly eroded from an older crater or distal ejecta layer and later redeposited in the European sandbelt. The single shocked quartz grain at this moment thus cannot be used to support the Younger Dryas impact hypothesis.  相似文献   

Halo ejection in distant radio galaxies: jet feedback in massive galaxy formation     

Michael A. Dopita  Michiel Reuland  Wil van Breugel  Wim de Vries  S. A. Stanford  Huub Röttgering  George Miley  Bram Venemans  Hy Spinrad  Steve Dawson  Arjun Dey  Mark Lacy  Daniel Stern  Andrew Bunker 《Astrophysics and Space Science》2007,311(1-3):305-309

We present results from a Keck optical and near IR spectroscopic study of the giant emission line halos of the z>3 High Redshift Radio Galaxies (HiZRGs) 4C 41.17, 4C 60.07 and B2 0902+34. The outer regions of these halos show quiet kinematics with typical velocity dispersions of a few hundred km s⁻¹ and velocity shears consistent with rotation. The inner regions contain shocked, clumpy cocoons of gas closely associated with the radio lobes with disturbed kinematics and expansion velocities and/or velocity dispersions >1000 km s⁻¹. We also find evidence for the ejection of chemically enriched material in 4C 41.17 up to a distance of ∼60 kpc along the radio-axis. We infer that these HiZRGs are undergoing a final jet-induced phase of star formation with the ejection of most of their interstellar medium before evolving to become “red and dead” Elliptical galaxies.  相似文献   

Compressibility of molten Apollo 17 orange glass and implications for density crossovers in the lunar mantle     

Mirjam van Kan Parker  Carl B. Agee  Wim van Westrenen 《Geochimica et cosmochimica acta》2011,75(4):1161-1172

We performed density measurements on a synthetic equivalent of lunar Apollo 17 74,220 “orange glass”, containing 9.1 wt% TiO₂, at superliquidus conditions in the pressure range 0.5-8.5 GPa and temperature range 1723-2223 K using the sink/float technique. In the lunar pressure range, two experiments containing pure forsterite (Fo₁₀₀) spheres at 1.0 GPa and 1727 K, and at 1.3 GPa-1739 K, showed neutral buoyancies, indicating that the density of molten orange glass was equal to the density of Fo₁₀₀ at these conditions (3.09 ± 0.02 g cm⁻³). A third tight sink/float bracket using Fo₉₀ spheres corresponds to a melt density of 3.25 ± 0.02 g cm⁻³ at ∼2.8 GPa and ∼1838 K.Our data predict a density crossover for the molten orange glass composition with equilibrium orthopyroxene at ∼2.8 GPa, equivalent to a depth of ∼600 km in the lunar mantle, and a density of ∼3.25 g cm⁻³. This crossover depth is close to the orange glass multiple saturation point, representing its minimum formation depth, at the appropriate oxygen fugacity (2.8-2.9 GPa). A density crossover with equilibrium olivine is predicted to fall outside the lunar pressure range (>4.7 GPa), indicating that molten orange glass is always less dense than its equilibrium olivines in the Moon. Our data therefore suggest that that lunar liquids with orange glass composition are buoyant with respect to their source region at P < ∼2.8 GPa, enabling their initial rise to the surface without the need for additional external driving forces.Fitting the density data to a Birch-Murnaghan equation of state at 2173 K leads to an array of acceptable solutions ranging between 16.1 and 20.3 GPa for the isothermal bulk modulus K₂₁₇₃ and 3.6-8 for its pressure derivative K′, with best-fit values K₂₁₇₃ = 18.8 GPa and K′ = 4.4 when assuming a model 1 bar density value of 2.86 g cm⁻³. When assuming a slightly lower 1 bar density value of 2.84 g cm⁻³ we find a range for K₂₁₇₃ of 14.4-18.0 and K′ 3.7-8.7, with best-fit values of 17.2 GPa and 4.5, respectively.  相似文献   

Late Holocene changes in cyanobacterial community structure in maritime Antarctic lakes     

Rafael Fernandez-Carazo  Elie Verleyen  Dominic A. Hodgson  Stephen J. Roberts  Krzysztof Waleron  Wim Vyverman  Annick Wilmotte 《Journal of Paleolimnology》2013,50(1):15-31

Despite the dominance of cyanobacteria in polar freshwater aquatic ecosystems, little is known about their past biodiversity and response to climate and environmental changes. We explored the use of light microscopy of microfossils, high performance liquid chromatography of the fossil pigment composition and denaturing gradient gel electrophoresis of fossil 16S rRNA genes to study past and present-day differences in cyanobacterial community structure in response to climate changes in two adjacent maritime Antarctic lakes with contrasting depths (4 and 26 m) and light climates. Light microscopy was of limited use because of degradation of cell structures. Fossil cyanobacterial pigment concentrations were below the detection limits of our method in several sediment samples in the deep lake, but abundant and diverse in the sediment core from the shallow pond, probably as a consequence of increased light availability and/or a more diverse and abundant benthic cyanobacterial flora. Total carotenoid and chlorophyll concentrations were highest in both lakes between ca. 2,950 and 1,800 cal yr BP, which coincides with the late Holocene climate optimum recognised elsewhere in maritime Antarctica. Cyanobacterial molecular diversity was higher in the top few centimeters of the sediments in both lakes. In deeper sediments, the taxonomic turnover of cyanobacteria appeared to be relatively small in response to past climate anomalies in both lakes, underscoring the broad tolerance of cyanobacteria to environmental variability. This, however, may in part be explained by the low taxonomic resolution obtained with the relatively conserved 16S rRNA gene and/or the preferential preservation of particular taxa. Our results highlight the potential of fossil DNA in lake sediments to study colonization and succession dynamics of lacustrine cyanobacteria and warrant further investigation of the factors that affect preservation of cyanobacterial DNA.  相似文献