Probable tsunami origin for a Shell and Sand Sheet from marine ponds on Anegada, British Virgin Islands     

Eduard G. Reinhardt  Jessica Pilarczyk  Alyson Brown 《Natural Hazards》2012,63(1):101-117

A distinctive Shell and Sand Sheet found beneath the marine ponds of Anegada, British Virgin Islands, was formed by a post-1650 AD overwash event, but its origin (tsunami or hurricane) was unclear. This study assesses the taphonomic characters of the shell and large clast material (>2?mm) to determine its provenance and origin. Pond-wide stratigraphic units (Shelly Mud, Shell and Sand Sheet, Mud Cap) were analyzed (12 samples) at four sites in Bumber Well and Red Pond along with eight samples from the Shell and Sand Sheet in a 2-km transect of Bumber Well. Mollusks in the pond muds include Anomalocardia spp. and cerithids with no allochthonous shells from the offshore reef-flat. Results show that the shells and clasts (>2?mm) are derived from the erosion and winnowing of the underlying Shelly Mud of the former marine pond, forming a distinctive sheet-like deposit with Homotrema sand. The Shell and Sand Sheet contains articulated Anomalocardia bivalves and moderate numbers of angular fragments (approximately 35%) that are likely from crab predation. Radiocarbon dates of articulated Anomalocardia specimens from the Shell and Sand Sheet range widely (approximately 4000?years), with shell condition (pristine to variably preserved) showing no correlation with age. The articulated condition of the bivalves with the wide-ranging dates suggests erosion and winnowing of the underlying Shelly Mud but minimal transport of the bivalves. The Shell and Sand Sheet has taphonomic characteristics indicative of a widespread tsunami overwash (sheet-like extent and articulated specimens) but lacks allochthonous reef-flat shells. Reef-flat shell material may not have penetrated the pond, as a tsunami would have to cross the reef-flat and overtop high dunes (2.2?m) hindering transport of larger shell material but allowing the Homotrema sand to penetrate. Processes including hurricane overwash, pond wave action, or tidal channel opening and closure are not favoured interpretations as they would not produce extensive sheet-like deposits. Taphonomic analysis is hampered by the limited (400?C500?years BP) depositional history from Anegada??s ponds and the lack of comparative data from other Caribbean locations.  相似文献   

运城盆地地下水同位素年龄特征及其演化过程和可持续利用   总被引：1，自引：0，他引：1  

侯金武 《水文地质工程地质》2012,39(6):1-5

通过分析运城盆地地下水的碳同位素组成,结合水化学特征,揭示了盆地深层承压地下水的补给期为22～3 ka BP （现代碳百分比(a¹⁴C) 6~38 pmC）。浅层地下水（71~89 pmC）由现代水或现代水和老水混合组成。深层地下水氢氧同位素组成特征（δ¹⁸O~-10‰; δ²H~-70‰）表明地下老水在气候较冷的环境下受到补给,而浅层地下水的氢氧同位素组成（δ¹⁸O~-8‰; δ²H～-51‰）特征与现代西安降水组成相似。浅层地下水NO^-₃平均含量（31mg/L）比深层地下水（1.8 mg/L）高,硝酸盐的δ¹⁵N－δ¹⁸O_NO3 组成 (0‰～5‰）揭示了硝酸盐的主要来源为综合肥料。此外,浅层地下水的TDS由于蒸散发、矿物溶解,可达8.5 g/L（平均2.0 g/L）,深层地下老水TDS可达1.8 g/L（平均1.1g/L）水质相对较好。研究区目前主要开采深层地下水,受断裂带影响,浅层地下水已经侵入中深层地下水并与之发生混合,严重影响了中深层地下水的水质。如果发生大规模的浅层地下水与中深层地下水混合,会造成中深层地下老水的NO^-₃、TDS等含量越来越高。  相似文献   

The origins and behaviour of carbon in a major semi-arid river,the Murray River,Australia, as constrained by carbon isotopes and hydrochemistry     

Ian Cartwright 《Applied Geochemistry》2010

δ¹³C values of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate organic C (POC) together with δ¹⁸O and δ²H values of water, δ³⁴S values of dissolved SO₄, and major ion concentrations were measured in the Murray River and its tributaries between November 2005 and April 2007 to constrain the origins and behaviour of riverine C. δ¹³C_DIC values in the Murray River vary between −9.5 and −4.7‰ with a range of <3‰ within any sampling round. δ¹³C_DIC values of the tributaries are −11.0‰ to −5.1‰. DIC concentrations of the Murray River increase from ∼25 mg/L in the middle and upper reaches of the river to 45–55 mg/L in the lower reaches. However, the mass ratio of DIC as a proportion of the total dissolved solids (TDS) decreases from ∼0.6–0.7 in the headwaters to ∼0.2–0.3 in the lower reaches of the river, with similar downstream changes in DIC/Cl ratios. This precludes simple evaporative concentration of DIC and is interpreted as the river evading CO₂; this interpretation is consistent with pCO₂ values that are in the range 550–11,200 ppm volume (ppmv), which are far higher than those in equilibrium with the atmosphere (∼360 ppmv). The δ¹³C_DIC values are similar to those that would be produced by the weathering of marine limestone (δ¹³C ∼ 0‰). However, the lack of marine limestones cropping out in the Murray–Darling Basin and the relatively uniform δ¹³C_DIC values of the Murray River (even in upland reaches where the dominant rock types are metamorphosed silicates and granites) make this unlikely. Rather the high pCO₂ values and δ¹³C_DIC values are best explained by a combination of mineralisation of low δ¹³C organic C and evasion to the atmosphere. The rate of these two processes may attain near steady state and control both DIC concentrations and δ¹³C values.  相似文献   

Giant sandstone pipes record basin-scale liquefaction of buried dune sands in the Middle Jurassic of SE Utah     

Mads Huuse  Simon J. Shoulders  Dennis I. Netoff  Joe Cartwright 《地学学报》2005,17(1):80-85

Giant sandstone pipes crop out over a 20 000 km² area of SE Utah. They range from metres to decametres in diameter and crosscut more than 100 m of continental sediments. New and published observations, encompassing all known pipe localities in SE Utah, suggest that the pipes formed by upward movement of groundwater, sand and sediment blocks in response to liquefaction of buried dune sands. Pipe formation was most likely triggered by seismicity during a phase of Cordilleran shortening and regional tectonic reorganization in the late Middle Jurassic. The depth of liquefaction recorded by the pipes far exceeds depths usually considered in earthquake engineering.  相似文献   

Impact of changing climate and land use on the hydrogeology of southeast Australia ∗     

I. Cartwright  I. Simmonds 《Australian Journal of Earth Sciences》2013,60(8):1009-1021

Anthropogenic climate change is the Earth's most serious large-scale environmental concern. While the projected changes of global temperatures, rainfall and surface water have been modelled in a sophisticated manner, the impact on groundwater resources is much less well constrained. In southeast Australia, the decrease in rainfall amount and an increase in temperature that are predicted by climate models are generally assumed to reduce the amount of recharge to the groundwater systems. However, the increase in recharge that has resulted from clearing of the native vegetation will almost certainly produce a greater impact on the groundwater system, increasing quantity and potentially improving quality. Additionally, the impact on recharge of changes to rainfall frequency rather than just total amount is not well documented. Overall our understanding of the impacts of climate change on groundwater systems is insufficiently advanced to make firm predictions. Indirect impacts of climate change, particularly the projected increased demand for groundwater or surface water to supplement surface water supplies also will have a major impact that may be greater than the direct effect of climate change.  相似文献   

Concentration versus streamflow trends of major ions and tritium in headwater streams as indicators of changing water stores     

Ian Cartwright  Uwe Morgenstern  Harald Hofmann 《水文研究》2020,34(2):485-505

Documenting the processes that control the variations in stream geochemistry at different streamflows is important for being able to use chemical tracers to understand catchment functioning. The concentrations of most solutes, including reactive cations (e.g., Na, Ca, K, and Mg) and anions that are primarily derived from precipitation (Cl and Br), in five headwater streams from southeast Australia vary little with streamflow and are close to being chemostatic. By contrast, NO₃ and SO₄ concentrations are higher at high streamflows. There is also a systematic increase of ³H activities from as low as 1.1 to as high as 2.6 TU with increasing streamflow. The changes in geochemistry cannot be explained solely by increased mineral dissolution at high streamflows or enhanced baseflow driven by hydraulic loading. They are best explained by an increased baseflow input augmented by water mobilized from shallower stores as the catchments wet up. The mean transit times of the water sustaining streamflow varies from 35 to 70 years at low streamflows to <7 years at high streamflows. The use of a range of geochemical tracers, including radioactive isotopes, allows the different possible causes of chemostatic behaviour to be assessed and improves our understanding of catchment functioning.  相似文献   

New‐Build Development and the Gentrification of Oklahoma City's Deep Deuce Neighborhood     

Adam A. Payne  Alyson L. Greiner 《Geographical review》2019,109(1):108-130

New‐build development has become associated with the phase of gentrification that has taken shape since the mid‐1990s. This article examines the gentrification of Deep Deuce, a historically black neighborhood in Oklahoma City. An analysis of property sales identifies the major external agents involved and leads to a discussion of the area's racial turnover. Considering the relational aspects of place, specifically how the identity of Deep Deuce has been constructed in relation to the nearby area of Bricktown, provides new insights on the nature of changes affecting this neighborhood. Supplementing this with an examination of resistance to the gentrification of Deep Deuce shows how city neighborhoods can come to be defined by limited understandings of place, and how historic preservation efforts can generate symbolic capital and facilitate cultural appropriation. This article also contributes to the study of gentrification in smaller metropolitan areas.  相似文献   

Analysing the structure of elongated star clusters     

Annabel Cartwright  Anthony P. Whitworth 《Monthly notices of the Royal Astronomical Society》2009,392(1):341-345

We have previously reported a measure     which both quantifies and distinguishes between a (relatively smooth) large-scale radial density gradient and multiscale (fractal) subclustering. Here, we extend the applicability of     to clusters which deviate significantly from an overall circular shape.
    varies systematically as clusters assume a more elongated shape, and it is therefore possible to correct for the effect, if the elongation of the cluster is also quantified.     therefore remains a useful and robust analytical technique for classifying and quantifying the internal structure of star clusters, even when their overall shape is far from circular.
The corrections required are small for individual clusters which are not extremely elongated (not more than three times longer than they are wide) of the same order as the uncertainty in the value of     for a particular cluster type. We therefore recommend that no correction be applied to the calculation of     for individual clusters, unless they are more than three times longer than their width, but that correction for elongation be applied when     is used for statistical analyses of large numbers of observed or simulated clusters.  相似文献   

The effect of Poisson noise on SPH calculations     

Annabel Cartwright  Dimitris Stamatellos  Anthony P. Whitworth 《Monthly notices of the Royal Astronomical Society》2009,395(4):2373-2380

A simple algorithm is presented which generates a lattice-like, regular spacing of smoothed particle hydrodynamics (SPH) particles in discs, with any prescribed density gradient. Using this for comparison we demonstrate the effect of Poisson noise on SPH estimates of density, pressure and viscous forces when calculated using randomly distributed particles. The standard deviation of density and pressure is typically increased by greater than an order of magnitude. In a disc with a Keplerian velocity profile, the effectiveness of the Balsara switch in reducing the shear component of SPH artificial viscosity is greatly enhanced when the particles are properly spaced, reducing the magnitude of viscosity by two orders of magnitude. Noise problems are exacerbated, not removed, by increasing the numbers of SPH particles, if the number of neighbours used is kept constant. However, comparison of the evolution of a disc created using perfectly spaced particles and a disc with identical parameters but randomly placed particles, reveals very similar results. Although there are subtle differences in the evolution, and the smooth disc takes longer to begin developing structure, in both cases the identical number of objects is created by gravitational collapse. SPH disc simulations do not depend on initial density disturbances to evolve objects by gravitational collapse, which gives added credence to the validity of the results. It also appears that complicated disc settling procedures are unnecessary.  相似文献   

Tidal estimation in the pacific with application to SEASAT altimetry     

Braulio V. Sanchez  David E. Cartwright 《Marine Geodesy》2013,36(2):81-115

Abstract

The techniques for computing the eigenfunctions of the velocity potential (Proudman functions) set out in Sanchez et al. (1986) in relation to the Atlantic‐Indian Ocean are here applied to the Pacific Ocean, using a 6° × 6° grid of 510 points (455 points for the associated stream functions). Normal modes are computed from the first 150 Proudman functions and have natural periods from 43.9 hours downwards. Tidal syntheses are derived from these modes by direct application of the (frictionless) dynamic equations and by least‐squares fitting of Proudman functions to the dynamically interpolated tide‐gauge data of Schwiderski (1983). The modes contributing most energy to the principal harmonic tidal constituents are different in the two computations; their natural periods are typically in the range of 9–16 hours for semidiurnal, 14–43 hours for diurnal tides. The RMS of fit for Proudman functions is in all cases better than the corresponding value for the same number of spherical harmonics.

Before fitting the Proudman functions to the altimetry from the 3‐day repeat cycle of Seasat, the data are processed by novel methods. The geoid component is eliminated by taking collinear differences at a fixed time‐lag of two repeat cycles. Orbit errors are reduced by extracting the 1 rev^?1 component at every ascending node; this component varies slowly and nonlinearly in time. The spatial fitting process includes M₂ and O₁ frequencies, both of which emerge with significant and realistic tidal mapping, but residual noise in the data limits the number of Proudman functions to about 50–60 before showing signs of “over‐fitting.”; Fitting the same data by spherical harmonics gives marginally lower predicted variance for the same number of parameters.  相似文献