In situ U-series dating by laser-ablation multi-collector ICPMS: new prospects for Quaternary geochronology

The capabilities and potential applications of in situ dating of Quaternary materials using laser ablation-MC-ICPMS are explored. ²³⁴U/²³⁸U and ²³⁰Th/²³⁴U can be measured with precision sufficient for dating at a spatial resolution of 100 μm or better in samples that contain as a little as 1 ppm uranium. Moreover, U and Th concentrations and U-series isotope ratios can be continuously profiled to determine changes in age that occur with sample growth (e.g. in speleothems). These capabilities additionally permit the dating of bones, teeth and possibly molluscs, which are subject to post-mortem open-system behaviour of U-series isotopes, and can be employed to elucidate processes of U-series migration during weathering and diagenesis. A drawback of laser ablation-MC-ICPMS is that it cannot in general provide U-series age estimates with the high precision and accuracy of conventional TIMS or solution MC-ICPMS methods. However, sample preparation is straightforward, the amount of sample consumed negligible, and it can be used to rapidly characterise or screen and select samples from which more precise and accurate dates can be obtained using conventional methods. Given further instrumental developments and the establishment of suitable matrix-matched standards for carbonates and other materials, we foresee that laser ablation-MC-ICPMS will play an increasingly important role in Quaternary dating research.     

Control of estuarine stratification and mixing by wind-induced straining of the estuarine density field

Observations from the York River Estuary, Virginia, demonstrate that the along-channel wind plays a dominant role in governing the estuarine exchange flow and the corresponding increase or decrease in vertical density stratification. Contrary to previous findings that suggest wind stress acts predominantly as a source of energy to mix away estuarine stratification, our results demonstrate that the wind can play a more important role in straining the along-channel estuarine density gradient. Down-estuary winds enhance the tidally-averaged vertical shear, which interacts with the along-channel density gradient to increase vertical stratification. Up-estuary winds tend to reduce, or even reverse the vertical shear, reducing vertical stratification. In two experiments each lasting approximately a month, the estuarine exchange flow was highly correlated with the along-channel component of the wind. The changes in stratification caused by the exchange flow appear to control the amount of vertical mixing as parameterized by the vertical eddy viscosity. The degree of stratification induced by wind straining also appears to play an important role in controlling the effectiveness of wind and tidal mixing.     

Modelling potential impacts of bottom trawl fisheries on soft sediment biogeochemistry in the North Sea

Bottom trawling causes physical disturbance to sediments particularly in shelf areas. The disturbance due to trawling is most significant in deeper areas with softer sediments where levels of natural disturbance due to wave and tidal action are low. In heavily fished areas, trawls may impact the same area of seabed more than four times per year. A single pass of a beam trawl, the heaviest gear routinely used in shelf sea fisheries, can kill 5–65% of the resident fauna and mix the top few cm of sediment. We expect that sediment community function, carbon mineralisation and biogeochemical fluxes will be strongly affected by trawling activity because the physical effects of trawling are equivalent to those of an extreme bioturbator, and yet, unlike bioturbating macrofauna, trawling does not directly contribute to community metabolism. We used an existing box-model of a generalised soft sediment system to examine the effects of trawling disturbance on carbon mineralisation and chemical concentrations. We contrasted the effects of a natural scenario, where bioturbation is a function of macrobenthos biomass, with an anthropogenic impact scenario where physical disturbance results from trawling rather than the action of bioturbating macrofauna. Simulation results suggest that the effects of low levels of trawling disturbance will be similar to those of natural bioturbators but that high levels of trawling disturbance prevent the modelled system from reaching equilibrium due to large carbon fluxes between oxic and anoxic carbon compartments. The presence of macrobenthos in the natural disturbance scenario allowed sediment chemical storage and fluxes to reach equilibrium. This is because the macrobenthos are important carbon consumers in the system whose presence reduces the magnitude of available carbon fluxes. In soft sediment systems, where the level physical disturbance due to waves and tides is low, model results suggest that intensive trawling disturbance could cause large fluctuations in benthic chemical fluxes and storage.     

Spherules and shard-like clasts from the late Proterozoic Acraman impact ejecta horizon,South Australia

Abstract— Spherules and irregular shard-like particles consisting of authigenic mineral phases have been identified in the Acraman impact ejecta horizon preserved within the late Proterozoic shales of the Adelaide Geosyncline, South Australia. The spherules (150 μm to 1 mm diameter) range in shape from near-spherical through ellipsoidal to extended ellipsoidal-dumbbell. The distinctive morphology of the spherules and shard-like particles and their restriction to the ejecta horizon, suggest that they were deposited initially as glassy bodies which subsequently have been pseudomorphed by more stable authigenic phases like calcite, quartz, albite, and barite.     

Efficient hydrologic tracer-test design for tracer-mass estimation and sample-collection frequency. 2. Experimental results

Effective tracer-test design requires that the likely results be predicted in advance of test initiation to ensure tracer-test success. EHTD-predicted breakthrough curves (BTCs) for various hydrological conditions were compared with measured BTCs obtained from actual tracer tests. The hydrological conditions for the tracer tests ranged from flowing streams to porous-media systems. Tracer tests evaluated included flowing streams tracer tests conducted in small and large surface-water streams, a karst solution conduit, and a glacial-meltwater stream and porous-media systems conducted as natural-gradient, forced-gradient, injection-withdrawal, and recirculation tracer tests. Comparisons between the actual tracer tests and the predicted results showed that tracer breakthrough, hydraulic characteristics, and sample-collection frequency may be forecasted sufficiently well in most instances as to facilitate good tracer-test design. Comparisons were generally improved by including tracer decay and/or retardation in the simulations. Inclusion of tracer decay in the simulations also tended to require an increase in set average tracer concentration to facilitate matching peak concentrations in the measured BTCs, however. Both nonreactive tracer and reactive tracer predictions produced recommended sample-collection frequencies that would adequately define the actual BTCs, but estimated tracer-mass estimates were less precise.     

Is laser particle size determination possible for carbonate-rich lake sediments?

This paper examines the potential of a relatively new technique – laser diffraction particle size analysis – for use with carbonate-rich lake sediments, material traditionally difficult to analyse. Three experiments are considered. The first two use a carbonate standard to determine optimal instrument conditions – the choice of optical model, and a suitable transport fluid. The third experiment uses sample material from lake cores and compares four different pre-treatment methods: none, removal of carbon by loss on ignition, removal of carbon by digestion with hydrogen peroxide, and finally removal of carbonates with hydrochloric acid and then removal of organic material by digestion with hydrogen peroxide. Only this final method produces repeatable results. As such, whilst the laser diffraction instrument offers improvements in the speed, required sample volume and repeatability of analysis, it does not overcome the problems inherent in the analysis of carbonates. Any catchment disturbance signal preserved in the carbonate fraction will remain undetected.