North European last glacial–interglacial transition (LGIT; 15–9 ka) tephrochronology: extended limits and new events

High‐precision correlation of palaeoclimatic and palaeoenvironmental records is crucial for testing hypotheses of synchronous change. Although radiocarbon is the traditional method for dating late Quaternary sedimentary sequences, particularly during the last glacial–interglacial transition (LGIT; 15–9 ka), there are inherent problems with the method, particularly during periods of climate change which are often accompanied by major perturbations in atmospheric radiocarbon content. An alternative method is the use of tephras that act as time‐parallel marker horizons. Within Europe, numerous volcanic centres are known to have erupted during the LGIT, providing considerable potential for high‐precision correlation independent of past radiocarbon fluctuations. Here we report the first identification of the Vedde Ash and Askja Tephra in Ireland, significantly extending the known provenance of these events. We have also identified two new horizons (the Roddans Port Tephras A and B) and tentatively recognise an additional horizon from Vallensgård Mose (Denmark) that provide crucial additional chronological control for the LGIT. Two phases of the Laacher See Tephra (LST) are reported, the lower Laacher See Tephra (LLST) and probably the C2 phase of the Middle Laacher See Tephra (MLST‐C2) indicating a more northeasterly distribution of this fan than reported previously. Copyright © 2006 John Wiley & Sons, Ltd.     

Composition of brines in halite‐hosted fluid inclusions in the Upper Ordovician,Canning Basin,Western Australia: new data on seawater chemistry

Analyses of primary and early diagenetic fluid inclusions in the halite from the Late Ordovician Mallowa Salt, Canning Basin, Western Australia indicate a Ca‐rich composition and high concentration of parent brines in the basin which were close to sylvite and carnallite precipitation. The salt‐bearing series in the sampled interval was overheated up to 62 °C. The recorded differences in gas compositions result from the input of several gas sources including dispersed organic matter in the salt series and hydrocarbon deposits in the underlying rocks. The high concentration of the brines in fluid inclusions does not allow quantitative reconstruction of the chemical composition of Late Ordovician parent seawater. Using the information from Early Cambrian and Late Silurian basins as a proxy, however, the new data indicate that Late Ordovician seawater was undoubtedly Ca‐rich and, in comparison with modern seawater, had a similar K content, considerably lower Mg content (c. 30%), approximately three times the Ca content and one‐third the SO₄ content.     

WEKU – a GIS-Supported stochastic model of groundwater residence times in upper aquifers for the supraregional groundwater management

 The supraregional GIS-supported stochastical model, WEKU, for the determination of groundwater residence times in the upper aquifers of large groundwater provinces is presented. Using a two-dimensional analytical model of groundwater flow, groundwater residence times are determined within two extreme cases. In the first case, maximal groundwater residence times are calculated, representing the part of groundwater, that is drained by the main surface water of a groundwater catchment area. In the second case, minimal groundwater residence times for drainage into the nearest surface water are determined. Using explicit distribution functions of the input parameters, mean values as well as potential ranges of variations of the groundwater residence times are derived. The WEKU model has been used for the determination of groundwater residence times throughout Germany. The model results – mean values and deviations of the groundwater velocity and the maximal and minimal groundwater residence times in the upper aquifers – are presented by general maps and discussed in detail. It is shown that the groundwater residence times in the upper aquifer vary regionally, differentiated between less than 1 year and more than 2000 years. Using this information, the time scales can be specified, until measures to remediate polluted groundwater resources may lead to a substantial groundwater quality improvement in the different groundwater provinces of Germany. With respect to its supraregional scale of application, the WEKU model may serve as a useful tool for the supraregional groundwater management on a state, federal or international level. Received: 15 August 1995 · Accepted: 15 October 1995     

Radon variations in a Hungarian village

 A steady radon exhalation is assumed in most publications. In a village of North-East Hungary, however, high radon concentrations have been measured, differing strongly in neighbouring houses and varying in time, due to the interplay of geochemical phenomena. Received: 20 November 1995 · Accepted: 18 June 1996     

A technique for calibration of monostatic echosounder systems

A calibration technique has been adapted to render complete system calibrations of high-frequency acoustical instrumentation. This is based on standard targets; specifically, precisely manufactured spheres composed of tungsten carbide with 6% cobalt binder. The use of multiple sphere sizes was found to be advantageous, both as an independent check of the calibrations, and so that resonances in the sphere responses at certain frequencies could be avoided. Complete system gains and beam patterns, which include effects of bandpass filters and finite-pulse lengths, were determined by moving the spheres individually in the transducer far-fields. Use of this procedure ensures control over the acoustical characteristics of transducers, which may change from the time of manufacture and first testing due, for example, to platform mounting. It also provides a direct means of measuring the sampling volume at relatively high and constant signal-to-noise ratios. Implementation of this technique is discussed using a multifrequency sonar system as an example     

Generating interface waves using a freely falling, instrumentedsource

In recent years, interface waves such as the Scholte wave have become important tools in the study of the geoacoustic properties of near-bottom seafloor sediments. Traditionally, these waves have been generated by explosive or pneumatic sources deployed at or near the seafloor and monitored by ocean-bottom seismographs or geophone arrays. While these sources generate the requisite interface waves, they also produce higher frequency compressional waves in the water and sediment that tend to contaminate the surface wave and make inversion of the data difficult in the near field. In this paper, a new source consisting of a freely falling projectile instrumented with an accelerometer is described. When the projectile impacts the bottom, the exact time history of the vertical force applied to the sediment is known and therefore may be convolved with the transfer function of a sediment geoacoustic model to produce accurate synthetic seismograms. Moreover, the vertical force applied to the seafloor is very efficient in generating surface wave motion while producing very little compressional wave energy so that the near-field signals are much more easily analyzed. An example of the use of the new source is presented including inversion of the received signals to obtain shear-wave velocity and attenuation as a function of depth in the near bottom sediments at a shallow-water site     

A 20‐ka climate record from Central Himalayan loess deposits

The southwest monsoon that dominated Central Himalaya has preserved loessic silt deposits preserved in patches that are proximal to periglacial areas. The occurrence of such silts suggests contemporary prevalence of cold and dry northwesterly winds. Field stratigraphy, geochemistry, mineral magnetism, infrared stimulated luminescence (IRSL) and radiocarbon dating has enabled reconstruction of an event chronology during the past 20 ka. Three events of loess accretion could be identified. The first two events of loess deposition occurred betweem 20 and 9 ka and were separated by a phase of moderate weathering. Pedogenesis at the end of this event gave rise to a well‐developed soil that was bracketed around 9 to > 4 ka. This was followed by the third phase of loess accretion that occurred around 4 to > 1 ka. Episodes of loess deposition and soil formation are interpreted in terms of changes in the strength of the Indian southwest monsoon. Copyright © 2005 John Wiley & Sons, Ltd.     

The Preboreal climate reversal and a subsequent solar‐forced climate shift

Accurate chronologies are essential for linking palaeoclimate archives. Carbon‐14 wiggle‐match dating was used to produce an accurate chronology for part of an early Holocene peat sequence from the Borchert (The Netherlands). Following the Younger Dryas–Preboreal transition, two climatic shifts could be inferred. Around 11 400 cal. yr BP the expansion of birch (Betula) forest was interrupted by a dry continental phase with dominantly open grassland vegetation, coeval with the PBO (Preboreal Oscillation), as observed in the GRIP ice core. At 11 250 cal. yr BP a sudden shift to a humid climate occurred. This second change appears to be contemporaneous with: (i) a sharp increase of atmospheric ¹⁴C; (ii) a temporary decline of atmospheric CO₂; and (iii) an increase in the GRIP ¹⁰Be flux. The close correspondence with excursions of cosmogenic nuclides points to a decline in solar activity, which may have forced the changes in climate and vegetation at around 11 250 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd.