Two decadally resolved records from north‐west European peat bogs show rapid climate changes associated with solar variability during the mid–late Holocene     

Dmitri Mauquoy  Dan Yeloff  Bas Van Geel  Dan J. Charman  Antony Blundell 《第四纪科学杂志》2008,23(8):745-763

Two ¹⁴C accelerator mass spectrometry (AMS) wiggle‐match dated peat sequences from Denmark and northern England record changes in mire surface wetness reconstructed using plant macrofossil and testate amoebae analyses. A number of significant mid–late Holocene climatic deteriorations (wet shifts) associated with declines in solar activity were recorded (at ca. 2150 cal. yr BC, 740 cal. yr BC, cal. yr AD 930, cal. yr AD 1020, cal. yr AD 1280–1300, cal. yr AD 1640 and cal. yr AD 1790–1830). The wet shifts identified from ca. cal. yr AD 930 are concurrent with or lag decreases in solar activity by 10–50 years. These changes are replicated by previous records from these and other sites in the region and the new records provide improved precision for the ages of these changes. The rapidly accumulating (up to 2–3 yr cm^?1, ～1310 yr old, 34 ¹⁴C dates) Danish profile offers an unprecedented high‐resolution record of climate change from a peat bog, and has effectively recorded a number of significant but short‐lived climate change events since ca. cal. yr AD 690. The longer time intervals between samples and the greater length of time resolved by each sample in the British site due to slower peat accumulation rates (up to 11 yr cm^?1, ～5250 yr old, 42 ¹⁴C dates) acted as a natural smoothing filter preventing the clear registration of some of the rapid climate change events. Not all the significant rises in water table registered in the peat bog archives of the British and Danish sites have been caused by solar forcing, and may be the result of other processes such as changes in other external forcing factors, the internal variability of the climate system or raised bog ecosystem. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

Submarine pyroclastic deposits formed during the 20th May 2006 dome collapse of the Soufrière Hills Volcano, Montserrat     

J. Trofimovs  C. Foster  R. S. J. Sparks  S. Loughlin  A. Le Friant  C. Deplus  L. Porritt  T. Christopher  R. Luckett  P. J. Talling  M. R. Palmer  T. Le Bas 《Bulletin of Volcanology》2012,74(2):391-405

The 20th May 2006 lava dome collapse of the Soufrière Hills Volcano, Montserrat, had a total non-dense rock equivalent (non-DRE) collapse volume of approximately 115?×?10⁶?m³. The majority of this volume was deposited into the ocean. The collapse was rapid, 85% of the mobilized volume being removed in just 35?min, giving peak pyroclastic flow flux of 66?×?10³?m³?s^?1. Channel and levee facies on the submarine flanks of the volcano and formation of a thick, steep-sided ridge, suggest that the largest and most dense blocks were transported proximally as a high concentration granular flow. Of the submerged volume, 30% was deposited from the base of this granular flow, forming a linear, high-relief ridge that extends 7?km from shore. The remaining 70% of the submerged volume comprises the finer grain sizes, which were transported at least 40?km by turbidity currents on gradients of <2°. At several localities, the May 2006 distal turbidity currents ran up 200?m of topography and eroded up to 20?cm of underlying substrate. Multiple turbidites are preserved, representing current reflection from the graben margins and deflection around topography. The high energy of the May 2006 collapse resulted in longer submarine run out than the larger (210?×?10⁶?m³) Soufrière Hills dome collapse in July 2003.  相似文献   

A method for tsunami risk assessment: a case study for Kamakura,Japan     

Non Okumura  Sebastiaan N. Jonkman  Miguel Esteban  Bas Hofland  Tomoya Shibayama 《Natural Hazards》2017,88(3):1451-1472

This paper presents a methodology for tsunami risk assessment, which was applied to a case study in Kamakura, Japan. This methodology was developed in order to evaluate the effectiveness of a risk-reducing system against such hazards, also aiming to demonstrate that a risk assessment is possible for these episodic events. The tsunami risk assessment follows these general steps: (1) determination of the probability of flooding, (2) calculation of flood scenarios, (3) assessment of the consequences and (4) integration into a risk number or graph. The probability of flooding was approximated based on the data provided by local institutes, and the flood scenarios were modeled in 1D using the Simulating WAves till SHore model. Results showed that a tsunami in Kamakura can result in thousands of casualties. Interventions such as improvements in evacuation systems, which would directly reduce the number of casualties, would have a large influence in risk reduction. Although this method has its limits and constraints, it illustrates the value it can add to existing tsunami risk management in Japan.  相似文献   

A grassed waterway and earthen dams to control muddy floods from a cultivated catchment of the Belgian loess belt     

Olivier Evrard  Karel Vandaele  Bas van Wesemael  Charles L. Bielders 《Geomorphology》2008,100(3-4):419-428

Muddy floods, i.e. runoff from cultivated areas carrying large quantities of soil, are frequent and widespread in the European loess belt. They are mainly generated in dry zero-order valleys and are nowadays considered as the most likely process transferring material eroded from cultivated hillslopes during the Holocene to the flood plain. The huge costs of muddy flood damages justify the urgent installation of control measures. In the framework of the ‘Soil Erosion Decree’ of the Belgian Flemish region, a 12 ha-grassed waterway and three earthen dams have been installed between 2002–2004 in the thalweg of a 300-ha cultivated dry valley in the Belgian loess belt. The measures served their purpose by preventing any muddy flood in the downstream village, despite the occurrence of several extreme rainfall events (with a maximum return period of 150 years). The catchment has been intensively monitored from 2005–2007 and 39 runoff events were recorded in that period. Peak discharge (per ha) was reduced by 69% between the upstream and the downstream extremities of the grassed waterway (GWW). Furthermore, runoff was buffered for 5–12 h behind the dams, and the lag time at the outlet of the catchment was thereby increased by 75%. Reinfiltration was also observed within the waterway, runoff coefficients decreasing by a mean of 50% between both extremities of the GWW. Sediment discharge was also reduced by 93% between the GWW's inflow and the outlet. Before the installation of the control measures, specific sediment yield (SSY) of the catchment reached 3.5 t ha^− 1 yr^− 1 and an ephemeral gully was observed nearly each year in the catchment. Since the control measures have been installed, no (ephemeral) gully has developed and the SSY of the catchment dropped to a mean of 0.5 t ha^− 1 yr^− 1. Hence, sediment transfer from the cultivated dry valley to the alluvial plain should dramatically decrease. Total cost of the control measures that are built for a 20 year-period is very low (126 € ha^− 1) compared to the mean damage cost associated with muddy floods in the study area (54 € ha^− 1 yr^− 1). Similar measures should therefore be installed to protect other flooded villages of the Belgian loess belt and comparable environments.  相似文献   

U–Pb zircon and monazite age constraints on granulite-facies metamorphism and deformation in the Strangways Metamorphic Complex (central Australia)     

Andreas?M?llerEmail author  Bas?J.?Hensen  Richard?A.?Armstrong  Klaus?Mezger  Michel?Ballèvre 《Contributions to Mineralogy and Petrology》2003,145(4):406-423

The age of Proterozoic granulite facies metamorphism and deformation in the Strangways Metamorphic Complex (SMC) of central Australia is determined on zircon grown in syn-metamorphic and syn-deformational orthopyroxene-bearing, enderbitic, veins. SHRIMP zircon studies suggest that M ₁–M ₂ and the correlated periods of intense deformation (D ₁–D ₂) are part of a single tectonothermal event between 1,717±2 and 1,732±7 Ma. It is considered unlikely that the two metamorphic phases (M ₁, M ₂) suggested by earlier work represent separate events occurring within 10–25 Ma of each other. Previous higher estimates for the age of M ₁ granulite metamorphism in the SMC (Early Strangways event at ca. 1,770 Ma) based on U–Pb zircon dating of granitic, intrusive rocks, are not believed to relate to the metamorphism, but to represent pre-metamorphic intrusion ages. Conventional multi-grain U–Pb monazite analyses on high-grade metasediments from three widely spaced localities in the western SMC yield ²⁰⁷Pb/ ²³⁵U ages between 1,728±11 and 1,712±2 Ma. The age range of the monazites corresponds to the SHRIMP zircon ages in the granulitic veins and is interpreted to record monazite growth (prograde in the metasedimentary rocks). The data imply a maximum time-span of 30 Ma for high-grade metamorphism and deformation in the SMC. There is, thus, no evidence for an extremely long period of continuous high-temperature conditions from 1,770 to ca. 1,720 Ma as previously proposed. The results firmly establish that the SMC has a very different high-grade metamorphic history than the neighbouring Harts Range, where upper amphibolite facies metamorphism in the Palaeozoic caused widespread growth or recrystallization of monazite.  相似文献   

Swath bathymetry with GLORIA     

Le Bas  T.P. Somers  M.L. Campbell  J.M. Beale  R. 《Oceanic Engineering, IEEE Journal of》1996,21(4):545-553

For many years, GLORIA has been producing sonar images of the deep ocean floor. In the mid-1980's, the SeaMARC II system came to prominence producing depth values as well as sonar images. The basic method compares the phases of the signals returning from the seafloor to two rows of transducers. The phase differences are converted into angles of arrival and together with the arrival times converted into range and depth values. This capability has now been added to the GLORIA system. The fact that GLORIA uses a 2s FM pulse means the backscattered reverberation can come from a strip of seafloor up to 1.5 km wide. To accommodate this, overlapping complex FFT's are used to produce a time-frequency matrix for the returning signals. In this matrix, a constant range feature appears as a diagonal. Phases are then calculated using a least-mean-squares estimate along diagonals. The main source of error and bias is due to surface reflection, and this is taken into account. The GLORIA swath bathymetry system was tested on two cruises and it was possible to produce depth contours with a good level of confidence. The total swath width was over eight water depths and would have been greater with a more favorable velocity profile. Comparison with other bathymetry data (such as multibeam systems) showed excellent correlation, having a standard deviation of only 4% of total water depth  相似文献   

New evidence from a calcite-dolomite carbonatite dyke for the magmatic origin of the massive Bayan Obo ore-bearing dolomite marble, Inner Mongolia, China   总被引：7，自引：0，他引：7  

M. J. Le Bas  Yang Xueming  Rex N. Taylor  B. Spiro  J. A. Milton  Zhang Peishan 《Mineralogy and Petrology》2007,90(3-4):223-248

Summary New data on Sr and Nd isotope composition and major and trace element distribution in dolomite-calcite carbonatite dykes at Bayan Obo are provided, and a Mid-Proterozoic age is deduced. The dykes and the neighbouring massive dolomite (H8) body have similar geochemical characteristics, interpreted to indicate a carbonatitic magmatic origin. The occurrence of riebeckite-bearing fenitized quartzites marginal to both dykes and H8 dolomite body, and the presence of xenoliths in the latter, supports this conclusion. Taken together with previously published stable isotope data, these data confirm a mantle-derived origin for the H8 body. The oxygen isotope composition of the dolomite and magnetite in the dykes is lower than that in the fine-grained dolomite. Oxygen data from samples of the coarse-grained dolomite host are either similar to the dykes or to the fine-grained type in agreement with their other geochemical characteristics. The carbonate-magnetite thermometric pairs of the fine-grained dolomite indicate a range of 350–540 °C, which is probably lower than that of the original main magmatic emplacement. This supports the distinction made between the original coarse-grained dolomite marble and dyke composition from the later fine-grained dolomite. Thus the large H8 dolomite is interpreted as a carbonatite intrusion that contains wall-rock xenoliths and caused fenitization of the hanging wall, foot wall and the xenoliths, and that the coarse-grained portions of the H8 marble are those portions that, in the Late Proterozoic to Palaeozoic, escaped recrystallization to fine-grained dolomite and subsequent REE-Fe mineralization. Currently at National Oceanography Centre, University of Southampton, Southampton, UK An erratum to this article is available at .  相似文献   

Influence of tidal regime,diurnal phase,habitat and season on feeding of an intertidal crab     

Claudia Bas  Juan P. Lancia  Tomás Luppi  Agustina Méndez‐Casariego  Marcelo Kittlein  Eduardo Spivak 《Marine Ecology》2014,35(3):319-331

The relationship between diet and feeding activity of intertidal crabs, and environmental cycles (tidal, daily and seasonal), habitat and level of the intertidal zone (high/low) was studied using Neohelice granulata (Brachyura, Varunidae) as a model. This is a semi‐terrestrial burrowing crab occupying different habitats in the Southwestern Atlantic coasts and estuaries from bare low intertidal mudflats to high intertidal salt marshes, and from fine, organic matter rich sediment to very coarse sediment with low content of organic matter. The study was carried out in two contrasting habitats of three sites with diverse sets of physical and biological conditions. Diet and feeding of adult N. granulata were indirectly studied through the proportion of food items and the presence/absence of food in crab stomachs, respectively. This species has a dual mode of feeding: predominantly herbivorous (live plants or plant litter in salt marshes) or deposit feeder (superficial sediment and detritus in mudflats), but the quantity and quality of ingested food varies among habitats and sites. A trend to omnivory (including algae and conspecifics) was detected in relation to low quality of resources. Feeding activity modulated by a complex interaction of factors varied according to spatial and/or temporal changes in some natural cycles. Males and non‐ovigerous females fed preferably after dark and during submersion periods, but also after emersion periods if mudflat sediment remained wet; salt marsh crab feeding is somewhat independent of light and tidal cycles. Ovigerous females almost never fed. Both diet and feeding activity of this crab seem to be flexible traits adapted to different combinations of physical and biological factors.  相似文献