Speleothem preservation and diagenesis in South African hominin sites implications for paleoenvironments and geochronology

Plio‐Pleistocene speleothems from australopithecine‐bearing caves of South Africa have the potential to yield paleoenvironmental and geochronological information using isotope geochemistry. Prior to such studies it is important to assess the preservation of geochemical signals within the calcitic and aragonitic speleothems, given the tendency of aragonitic speleothems to recrystallize to calcite. This study documents the geochemical suitability of speleothems from the principal hominin‐bearing deposits of South Africa. We use petrography, together with stable isotope and trace element analysis, to identify the occurrence of primary aragonite, primary calcite, and secondary calcite. This study highlights the presence of diagenetic alteration at many of the sites, often observed as interbedded primary and secondary fabrics. Trace element and stable isotopic values distinguish primary calcite from secondary calcite and offer insights into geochemical aspects of the past cave environment. δ¹³C values of the primary and secondary calcites range from +6 to −9‰ and δ¹⁸O values range from −4 to −6‰. The data are thus typical of meteoric calcites with highly variable δ¹³C and relatively invariant δ¹⁸O. High carbon isotope values in these deposits are associated with the effects of recrystallization and rapid outgassing of CO₂ during precipitation. Mg/Ca and Sr/Ca ratios differ between primary and secondary calcite speleothems, aiding their identification. Carbon and oxygen isotope values in primary calcite reflect the proportion of C₃ and C₄ vegetation in the local environment and the oxygen isotope composition of rainfall. Primary calcite speleothems preserve the pristine geochemical signals vital for ongoing paleoenvironmental and geochronological research. © 2009 Wiley Periodicals, Inc.     

Comparison of risk from pyroclastic density current hazards to critical infrastructure in Mammoth Lakes,California, USA,from a new Inyo craters rhyolite dike eruption versus a dacitic dome eruption on Mammoth Mountain

Renewed volcanic activity near Mammoth Lakes, California, in the form of dome-collapse pyroclastic density currents (PDCs) from either a new eruption at Mammoth Mountain or the Inyo craters would pose a significant hazard to critical infrastructure there. This paper compares the risk from PDC impact hazards upon selected critical infrastructure from: (1) a 100 m tall dacite dome on Mammoth Mountain and (2) three 200 m tall rhyolite domes at the southern end of the Inyo craters. For each scenario, maximum estimated dynamic pressure and velocity from two PDC volumes (10⁶ and 10⁷ m³) are modeled with the EXPLORIS PDC software (Toyos et al. Nat Hazards 41(1):99–112, 2007). Risk to critical infrastructure from Mammoth Mountain PDCs would be much greater than the Inyo PDCs because of both location and the greater kinetic energy of the Mammoth PDC material, providing comparative insight to planners should a real eruption at one location or the other be forthcoming.     

有限元法在滑坡稳定性分析中的应用

对比了滑坡稳定性分析的极限平衡法、数值分析法、概率法。而后,介绍了有限元法滑坡稳定性分析基本原理,并应用有限元法对中里滑坡进行了稳定性分析,得出了中里滑坡的主应力图和稳定系数,并将稳定系数与其他方法的结果进行了比较,得出有限元法是一种可靠的分析方法,其结果可应用于后期治理设计。      

Suspended sediment transport and deposition over a dune: Río Paraná, Argentina

Field data from the Rio Paraná, Argentina, are used to examine patterns of suspended sediment transport over a sand dune. Measurements of three‐dimensional velocity are made with an acoustic Doppler current profiler whilst suspended sediment concentration and particle size have been quantified using a laser in situ sediment scattering transmissometer. Suspended sediment concentration and streamwise and vertical sediment flux are highest close to the bed, with an upward vertical flux over the stoss side of the dune and downward flux over the lee side. Suspended sediment concentrations are higher over the crest compared with the trough and suspended sediment is coarsest near the bed. About 17% of the suspended‐load transported over the crest is deposited in the lee side before it reaches the trough. Most of this deposited sand is coarser sediment that originates close to the bed over the crest, a result consistent with simulations based on the model of Mohrig and Smith (Water Resources Research 1996; 32: 3207–3217) for the excursion lengths of sediment dispersed in the lee side of a dune. Copyright © 2009 John Wiley & Sons, Ltd.     

The value of high-resolution nutrient monitoring: A case study of the River Frome, Dorset, UK

The River Frome was sampled at sub-daily sampling interval, with additional storm sampling, through an annual cycle. Samples were analysed for total phosphorus (TP), soluble reactive phosphorus (SRP), total oxidisable nitrogen (TON) and dissolved reactive silicon (Si). The resulting data set was artificially decimated to mimic sampling frequencies from 12 h to monthly time interval. Monthly sampling interval resulted in significant errors in the estimated annual TP and SRP load of up to 35% and 28% respectively, and the resulting data sets were insufficient to observe peaks in P concentration in response to storm events. Weekly sampling reduced the maximum percentage errors in annual load estimate to 15.4% and 6.5%. TON and silicon concentrations were less variable with changing river flow, and monthly sampling was sufficient to predict annual load estimates to within 10%. However, to investigate within-river nutrient dynamics and behaviour, it is suggested that a weekly sampling interval would be the minimum frequency required for TON and Si studies, and daily sampling would be a minimum requirement to adequately investigate phosphorus dynamics. The loss in nutrient-concentration signal due to reduced sampling interval is presented. Hysteresis in the nutrient concentration/flow relationships for all 32 storm events during the study period were modelled and seasonal patterns discussed to infer nutrient sources and behaviour. The high-resolution monitoring in this study identified, for the first time, major peaks in phosphorus concentration in winter that coincide with sudden falls in air temperature, and was associated with biofilm breakdown. This study has shown that to understand complex catchment nutrient processes, accurately quantify nutrient exports from catchments, and observe changes in water quality as a result of nutrient mitigation efforts over time, it is vital that the newly emerging field-based automated sampler/analyzer technologies begin to be deployed, to allow for routine high-resolution monitoring of our rivers in the future.     

A semi-analytical solution based on a numerical solution of the solute transport of a conservative and nonreactive tracer

In the evaluation of potential risk from ingestion of groundwater near an impacted site, numerical simulation of fate and transport processes of chemicals of concern is often required. If there is potential concern about multiple chemicals, numerical simulation of each chemical separately is often needed. In this paper, a semi-analytical solution is presented based on a numerical solution of the transport of a conservative and nonreactive tracer. When multiple chemicals undergoing sorption and first-order degradation need to be modeled, we can avoid performing individual numerical simulations for each chemical by applying the semi-analytical solution. Numerical test runs were conducted to verify the semi-analytical solution; simulation results reveal that the concentrations derived from the semi-analytical solution are identical to those derived from the individual numerical fate and transport model simulations. The semi-analytical solution requires steady-state flow conditions, no continuing contaminant source, and similar initial source concentration distributions.