Columnar calcites as testimony of diagenetic overprinting at the boundary between Upper Tournaisian dolomites and limestones (Belgium): multiple origins for apparently similar features

In topographic flat areas, sedimentary settings may vary from one outcrop to another. In these settings, calcite precipitates may yield macroscopically similar columnar features, although they are products of different sedimentary or diagenetic processes. Three columnar calcite crystal fabrics, i.e. rosettes, palisade crusts and macro-columnar crystal fans, have been differentiated near and at the contact between Upper Tournaisian dolomites and limestones along the southern margin of the Brabant-Wales Palaeohigh. Their petrographic characteristics, and geochemical and fluid inclusion data provide information on the (dia)genetic processes involved. Rosettes composed of non-luminescent columnar calcite crystal fans (1–5 cm in diameter) developed on top of one another, forming discrete horizons in repetitive sedimentary cycles. The cycles consist of three horizons: (I) a basal horizon with fragments from the underlying horizon, (II) a micrite/microspar horizon with incipient glaebules, (III) an upper horizon consisting of calcite rosettes, with desiccation features. The petrographical features and δ¹⁸O signatures of −10·0 to −5·5‰ and δ¹³C values of −5·5 to −3·2‰ support either evaporative growth, an evaporative pedogenic origin, or overprinting of marine precipitates. Palisade crusts, composed of a few to 10 mm long non-luminescent calcite crystals, coat palaeokarst cavities. Successive palisade growth-stages occur which are separated by thin laminae of micrite or detrital quartz, displaying a geopetal arrangement. Palisade crusts are interpreted as intra-Mississippian speleothems. This interpretation is supported by their petrographic characteristics and isotopic signature (δ¹⁸O = −8·7 to −6·5‰ and δ¹³C = −4·8 to −2·5‰). Macro-columnar crystals, 1–50 cm long, developed mainly perpendicular to cavity walls and dolomite clasts. Crystal growth stages in the macro-columnar crystals are missing. δ¹⁸O values vary between −16·4 and −6·8‰ and δ¹³C values between −5·2 and −0·9‰. These features possibly support a late diagenetic high temperature precipitation in relation to hydrothermal karstification.     

ASSESSING THE PALAEOCLIMATE POTENTIAL OF CAVE GLACIERS: THE EXAMPLE OF THE SČRIŞOARA ICE CAVE (ROMANIA)

ABSTRACT. The ice block in Scarisoara Cave, NW Romania, is preserved due to unusual climate and permafrost conditions within the cave. The air temperature in the cave is governed by the winter cold, the cooling effect of the ice block, and only to a minor extent influenced by summer temperatures. At present, the ice block is slowly thinning, but the present-day climate is sufficiently cold to preserve the permafrost conditions caused by the cold air trapped in the cave. In February 2003 a 22.5 m long ice core was recovered from the ice block. Approximately 200 ice layers have been identified by visual examination. Ice crystallographic analyses indicate a steady growth of ice crystals with depth and there is no sign of deformation. Carbon-14 dates on wood-related samples collected from a natural vertical exposure of the ice block indicate that the ice spans more than 1000 years. Observations on the exposure indicate that a basal melting phase may have occurred in the past.     

DIFFRACTION BY A WEDGE IN AN ACOUSTIC CONSTANT DENSITY MEDIUM1

A new method is presented for solving the 2D problem of diffraction of a plane wave by a wedge of arbitrary angle in a purely acoustic, constant-density medium with different constant compressional wave speeds inside and outside the wedge. The diffraction problem is formulated as integral equations, and a wavenumber–frequency representation of the scattered field is obtained. With the aid of the Cagniard–de Hoop method, exact analytical expressions in the space–time domain are obtained for the different wave constituents, i.e. geometric optical scattered waves and edge diffracted waves including head waves. These expressions can be computed to any degree of accuracy within reasonable computation times on a computer, and the semi-analytical method of solution presented thus constitutes a means of constructing reference solutions for wedge configurations. Such highly accurate reference solutions are of importance for verification of results that include diffraction phenomena modelled by general numerical approximate methods, e.g. finite differences, finite elements and spectral methods. Examples of such applications of the method of solution are given.     

Three-dimensional architecture and development of Danian bryozoan mounds at Limhamn, south-west Sweden, using ground-penetrating radar

Bryozoan mounds from the middle Danian (Lower Palaeocene) of the Danish Basin represent a possibly new class of non‐cemented skeletal mounds. The sedimentology and palaeoecology of the mounds have recently been studied in detail. Three‐dimensional images of middle Danian bryozoan mound structures in the Limhamn limestone quarry, south‐west Sweden, obtained from combined reflected ground‐penetrating radar signals and outcrop analysis provide new information about the architecture and growth development of such mounds. The mounds are composed of bryozoan limestone and dark‐grey to black flint bands which outline mound geometries. Ground‐penetrating radar data sections are collected over a 120 m by 60 m grid of data lines with trace spacing of 0·25 m, providing a depth penetration of 7 to 12 m and a vertical resolution of ca 0·30 m. The ground‐penetrating radar images outline the geometry of the internal layering of the mounds which, typically, have widths and lengths of 30 to 60 m and heights of 5 to 10 m. Mound architecture and growth show great variability in the ground‐penetrating radar images. Small‐scale mound structures with a palaeorelief of only a few metres may constitute the basis for growth of larger mounds. The outermost beds of the individual mounds are commonly characterized by sub‐parallel to parallel reflections which have a circular to slightly oval appearance in map view. The mounds are mainly aggrading and do not show clear signs of pronounced lateral migration during growth, although some mound structures indicate a preferential growth direction towards the south. Growth patterns interpreted from the ground‐penetrating radar images suggest that the palaeocurrents in the study area may have shown great variability, even on a small scale. This observation is in contrast to results from studies of extensive, slightly older early Danian mound complexes exposed in coastal cliffs at Stevns Klint and Karlby Klint located 50 and 200 km away from the study area, respectively. At these locations the mounds show a remarkably uniform development and typically are asymmetrical, clearly showing migration directions towards the south. These differences in mound geometry may be the result of differences in the current systems and water depths that existed during formation of the early and middle Danian mounds, respectively. The mounds at Limhamn were located closer to the basin margin in shallower water than those at Stevns Klint and Karlby Klint. In addition, the difference in mound architecture may be due to the occurrence of non‐layered, irregular coral mounds intercalated with the bryozoan mounds at Limhamn.     

Late Holocene climatic variability reconstructed from incremental data from pines and pearl mussels – a multi‐proxy comparison of air and subsurface temperatures

Helama, S., Läänelaid, A., Tietäväinen, H., Macias Fauria, M., Kukkonen, I. T., Holopainen, J., Nielsen, J. K. & Valovirta, I. 2010: Late Holocene climatic variability reconstructed from incremental data from pines and pearl mussels – a multi‐proxy comparison of air and subsurface temperatures. Boreas, Vol. 39, pp. 734–748. 10.1111/j.1502‐3885.2010.00165.x. ISSN 0300‐9483. Dendrochronological and sclerochronological records are mean series of arboreal and molluscan increments that are correctly aligned in time by rigorous dating. These records of tree rings and annual shell‐growth increments exhibit climate signals that can be used to reconstruct fluctuations and trends in past climates. Here we present a multi‐proxy reconstruction of temperature histories using a combination of dendrochronological and sclerochronological evidence. Regional curve standardization (RCS) was used to remove the non‐climatic variations from dendrochronological and sclerochronological series prior to palaeoclimatic interpretation. Conventional and signal‐free methods of RCS were compared. It was found that the signal‐free methods produced more reliable chronologies and systematically higher climate–proxy correlations. Consequently, the temperature reconstructions were derived using the chronologies constructed by this method. Proxy‐based histories of summer (July–August) temperatures were reconstructed from AD 1767 onwards. The compound use of proxies resulted in reconstructions that were of higher quality than single‐proxy reconstructions. Further improvement of reconstructions was accomplished by the inclusion of lagging increment values in the transfer functions. The final multi‐proxy model explained 58% of the temperature variance over the instrumental period. The multi‐proxy temperature reconstruction correlated well with the long records of instrumental temperatures from Tornedalen, St. Petersburg, Uppsala and Stockholm. Overall, the reconstruction for the past 250 years agreed reasonably well with borehole temperature reconstructions obtained in northern Finland. In general, this study demonstrates the benefits of the compound use of several proxies in reconstructing climate histories. In particular, the study emphasizes the so far largely unexploited advantages of multi‐proxy data sets obtained by rigorously cross‐dated incremental chronologies to produce more robust palaeoclimatic reconstructions.     

The influence of grazing by Hypogastrura viatica (Insecta: Collembola) on microbial activity in decomposing kelp on Spitsbergen

The influence of three different grazing levels (5-20 ind.) by Hypogastrura viatica (Collembola) on microbial activity in a Spitsbergen kelp Laminaria has been evaluated during a 10 day laboratory experiment under oxic conditions.
Respiration has been measured by infra-red gas analysis in the form of CO ₂-emissions. The total and metabolicatly active fungal and bacterial biomass has been estimated by direct microscopical counts and epifluorescence techniques.
Despite the results of other similar experiments, an increase in microbial numbers and biomass was recorded following the introduction of grazing.
Grazing increases the mean respiration between 0.6 to 1.4 times. A corresponding increase in the microbial biomasses was also recorded. Although a trend towards decreased mean bacterial cell size was found, the fungi were more influenced by grazing than the bacteria. Bacteria constituted the predominant part of the microbial biomass in all the experimental vessels.
The results indicate that Collembola plays an important functional role by regulating the microbial activity, probably by nutrient mobilization.     

Glacial history of interior Jameson Land, East Greenland

The plateaus between 400 and 800 m a.s.l. around the water-divides on central and eastern Janieson Land are covered by the 'Jameson Land Drift' up to 50 m thick glacial. placiotluvial and glaciolacustrine deposits. A high content of far-travelled wcsterii rocks indicates the overriding by extensive glaciers channelled from the west through the Scoresby Sund basin. The Jameson Land Drift deposits have bccn lithostratigraphically divided into two groups. each representing the sedimentary successions from one glaciation in the wider sense of the word. sediments from the lower Lollandselv glaciation are upwards delimited by a distinct periglacial surface. TL-dates suggest a prc-Saalian (approximately isotope stages 11–9) age. The following Scoreshby Sund glaciation . when most of the studied Jameson Land Drift sediments were laid down. is of Saalian age (e. isotope stages 8 6). The deposits from the Scoresby Sund glaciation are interpreted as representing a complete glaciation deglaciation succession, including proglacial sandur and glaciolacustrine sediments. followed by till deposition, with an overlying succession of glaciolacustrinc and glaciofluvial sediments. From 200–250 m to c . 400 m a.s.l. there is a driftless area, exposing Jurassic sandstones, probably a result of intensive and long-lasting periglacial erosion. Extensive occurrences of tors and of glaciofluvially (subglacially as well as subaerially) eroded canyons and channels characterize the landscape. A similar. although less well defined. upper driftless zone is found above c 500 m a.s.l. on northern Jameson Land, north of the drift-covered plateaus. During the Wcichsclian (isotope stages 5d 2). thick glacial. fluvial and marine deposits were laid down in a coastal zone below c . 200 m a.s.l., and only cold-based local ice caps seem to have existed on the interior plateaus of Jameson Land. The now driftless areas were characterized by periglacial erosion during this period.     

IDENTIFICATION OF CLIMATE CONTROLS ON THE DYNAMIC BEHAVIOUR OF THE SUBARCTIC GLACIER SALAJEKNA, NORTHERN SCANDINAVIA

ABSTRACT. In this paper we describe the dynamic behaviour of Salajekna, a valley glacier, over the last 200 years using terrestrial observations, in situ measurements, remote sensing observations and glacier reconstructions. The response time of the glacier was calculated using analytical models and field measurements. We were subsequently able to attribute specific dynamic responses to climate trends in the available climate record. The glacier's historical maximum extension was reached around 1880–1910 and was the result of a more continental climate with multi-modal airflows in the late 18th and early 19th centuries. A transition to more maritime conditions in the mid-19th century resulted in a near-continuous 20th century retreat before the glacier adjusted to a near-steady state.     

Holocene palaeoecology and shoreline displacement on the Biskopsmåla Peninsula, southeastern Sweden

High-resolution palaeoecological proxies of pollen, macrofossils and diatoms from an isolation lake provide a long-term record of the Holocene landscape history and shoreline displacement on the Biskopsmåla Peninsula in central Blekinge, SE Sweden. During the Preboreal/Boreal transition, the peninsula was sparsely vegetated by woodlands, along with lateglacial dwarf shrub/steppe communities. The lake basin was isolated from the shallow Yoldia Sea during this time. The regional climate improved from 10 700 cal. BP, evident as progressive expansion of Pinus-dominated mixed forest with deciduous trees. The lake basin was probably connected with the Ancylus Lake during the period 10700–10 100 cal. BP. Subsequently the basin became isolated again, corresponding to the Early Littorina Sea phase. Replacement of freshwater diatoms by those with brackish-water affinity at 8100 cal. BP indicates the initial transgression of the Littorina Sea in this basin. But not until 7500 cal. BP were brackish conditions fully established. Peaks of brackish-marine diatoms and dinoflagellates during 7500–7000 cal. BP indicate increased saltwater inflow to the Baltic Sea in response to global meltwater pulse 3. However, interactive changes in seagrass and stonewort macrofossil concentrations suggest that three minor transgressions during 5900–5300, 5000–4700 and 4400–4000 cal. BP occurred locally, associated with centennial-scale variations in regional wind pattern or coastal storminess. By 3000 cal. BP, the lake basin was finally isolated from the Baltic, and thereafter the landscape on the peninsula became gradually more influenced by human activities.