On harbour siltation in the fresh-salt water mixing region

In this paper a novel series of field measurements are presented, which are the first to elucidate the processes influencing siltation in Botlek Harbour. Botlek Harbour is situated at the limit of saline water intrusion in the Rotterdam Waterway. Normally, after the ebb tide fresher river waters are found in the Rotterdam Waterway at the location of Botlek Harbour. On the flooding tide, the tip of the salt wedge is advected along the Rotterdam Waterway towards the mouth of Botlek Harbour. Hence on flood, a lock-exchange mechanism operates between Botlek Harbour and the Rotterdam Waterway. On the flood tide, when there is a supply of suspended particulate matter (SPM) associated with the presence of the estuarine turbidity maximum (ETM) at the mouth of the harbour, the survey data show exchange of SPM into the harbour. This lock-exchange process is found to be the dominant cause for SPM transport into the harbour. This is further substantiated by an analysis of the mass transport mechanisms. In this analysis, the vertical profiles of the instantaneous velocity, salinity and SPM concentration fields, recorded during the surveys, were decomposed into advective and dispersive transport components. The results of this analysis indicate that the correlation between the lock-exchange mechanism on the flood tide with the availability of SPM for exchange and efficient trapping, dominate the total exchange of SPM (97%). Hence, the increase in measured near-bed SPM concentration within the harbour is ascribed to tidal advection of saline water and the ETM along the Rotterdam Waterway. Tidal advection controls the density difference between the estuary and harbour, as well as the availability of SPM for exchange at the entrance to Botlek Harbour. The location of the ETM at the tip of the salt wedge is a key factor in supplying SPM to Botlek Harbour. Consequently the timing of the availability of SPM at the mouth of the harbour needs to be considered in siltation studies. The survey data suggest that Botlek Harbour basin has a 100% trapping efficiency. Analysis of 5 months of data, from a measuring rig located within the harbour, show excursions of the limit of the salt wedge and ETM. These excursions are likely to affect siltation of upstream harbours. Salinity-induced density gradients control the transport and subsequent trapping of SPM in the estuary in close proximity to the harbour entrance, the exchange of SPM between the estuary and harbour, and the trapping of SPM in the harbour basin.     

The local density of matter mapped by Hipparcos

We determine the velocity distribution and space density of a volume-complete sample of A and F stars, using parallaxes and proper motions from the Hipparcos satellite. We use these data to solve for the gravitational potential vertically in the local Galactic disc, by comparing the Hipparcos measured space density with predictions from various disc models. We derive an estimate of the local dynamical mass density of 0.102±0.010 pc⁻³, which may be compared with an estimate of 0.095 M_⊙ pc⁻³ in visible disc matter. Our estimate is found to be in reasonable agreement with other estimates by Crézé et al. and Pham, also based on Hipparcos data. We conclude that there is no compelling evidence for significant amounts of dark matter in the disc.     

Towards bridging the gap between climate change projections and maize producers in South Africa

Theoretical and Applied Climatology - Multi-decadal regional projections of future climate change are introduced into a linear statistical model in order to produce an ensemble of austral...     

Is there a chronological record of atmospheric mercury and lead deposition preserved in the mor layer (O-horizon) of boreal forest soils?

The organic horizon (the mor layer) of podzolized boreal forest soils has accumulated atmospheric fallout of mercury and lead over centuries, resulting in current concentrations close to levels where negative effects on soil biota are thought to occur. To what extent the pollution history is preserved in the stratigraphy of this horizon is not well known. In this study we asses whether the chronology of a large historic pulse of atmospheric pollution emitted from the Rönnskär smelter in northern Sweden, particularly between 1950 and 1980, is preserved within the stratigraphy of the mor layer, which is typically 5-cm thick. Vertical sub-sampling (?5 mm) of five mor profiles sampled along a 100-km pollution gradient away from the smelter are analyzed for mercury and lead concentrations, spheroidal carbonaceous particles from fossil fuel combustion (SCPs) and stable lead isotopes (²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb). Their vertical distribution is compared with the temporal variations in atmospheric inputs reconstructed for the last ∼100 years from analyses of an ombrotrophic peat core and a varved lake sediment core sampled within a distance of 50 km of the smelter. The mor profiles situated ?12 km from the smelter record the pollution history of the smelter. There is a 20 to 40-times enrichment of Hg, Pb and SCP at the transition in the O-horizon from the F- to H-layer compared to the basal part and a distinct peak in the ²⁰⁶Pb/²⁰⁷Pb ratio (∼1.22) in the F-layer. The mor profiles situated outside the historical contamination range of the smelter (80 and 100 km away) record no obvious influence from the Rönnskär smelter, instead their vertical ²⁰⁶Pb/²⁰⁷Pb profiles follow the general regional pollution history in northern Sweden. We conclude that the mor layer preserves a record of atmospheric Hg, Pb and SCP inputs and due to low leaching rates this organic horizon serves as a semi-archive of atmospheric Hg and Pb pollution. We stress the need of including this property in the existing ‘black-box’ models predicting the fate of Hg and Pb within contaminated boreal forest soils.     

Oceanography in northwestern Europe during the last interglacial from intrashell δO ranges in Littorina littorea gastropods

Coastal sea-surface temperature (SST) and sea-surface salinity (SSS), including seasonality, in northwest (NW) Europe during the early phase of the Eemian interglacial ca. 125 ka ago were reconstructed from Littorina littorea (common periwinkle) gastropods. The results were based on intra-annual δ¹⁸O analyses in recent and fossil shells, mainly originating from the sea of Kattegat (Sweden) and the English Channel (United Kingdom), and confined to intertidal settings. The Eemian L. littorea shells indicated annual SSTs in the range 8–18°C for the English Channel and 8–26°C for Kattegat. All specimens from the Eemian sites experienced summer SSTs of ca. 1–3°C above recent conditions. The estimated winter SST in the English Channel during the Eemian was comparable to modern measurements of ca. 8°C. However, the Kattegat region displayed Eemian winter SST approximately 8°C warmer than today, and similar to conditions in the western English Channel. The recent-fossil isotope analogue approach indicated high SSS above 35 practical salinity units (psu) for a channel south of England in full contact with the North Atlantic Ocean during the last interglacial. In addition, the Kattegat shells indicated a SSS of ca. 29 psu, which points out a North Sea affinity for this region during the Eemian.     

Primary alteration—oxidation of marine algal organic matter from oil source rocks of the North Sea and Norwegian Arctic: new findings

The presence of partially oxidized algal organic matter in oil-prone marine source rocks, is the rule rather than the exception. Partially oxidized, algal kerogen can still act as a significant source of liquid hydrocarbons. However, the corresponding peak of C₁₂ + hydrocarbon generation is shifted to a considerably lower maturity level compared with that of the classical Type II kerogen. The extent of primary alteration-oxidation of marine algal kerogen is monitored by means of solid state microfluorescence spectroscopy. A new parameter, the Primary Alteration Factor (PAF) is established, and the relationships between PAF and H/C, O/C, HI, TOC and between PAF and %₀δ¹³C are determined. The present data show large variations in the bulk chemistry of immature marine algal kerogens, and reveal evidence for gradational dehydrogenation/oxidation of the source organic matter. This contrasts with the recently proposed mechanism for kerogen formation. SEM analysis reveals a relationship between the physical breakdown of algal organic matter and the formation of liptodetrinite. FTIR analysis shows that the incorporation of primary oxygen in the kerogen macromolecules is not in the form of carbonyl or carboxyl functionalities. The presence of highly unreactive, stable oxygen, associated with aromatic structures in partially oxidized algal kerogen, is suggested by resistance of the kerogen to graphitization. The FTIR data also suggest the presence of aryl ether oxygen. The present findings raise fundamental questions regarding the mechanisms of kerogen cracking and kerogen formation, and have important implications for petroleum exploration.     

A well-preserved Cambrian impact exposed in Central Sweden

The Lockne impact crater south of Östersund formed in the early Middle Cambrian with a diameter of 7 km. It is identified by its rim wall of crushed Precambrian basement granite, by fragments of impact melt, and by grains of shocked quartz. The exceptional preservation, in particular of the rim wall, is due to a complicated geological history, the first stage of which consisted of burial by marine sediments. This stage lasted until the Middle Ordovician, or over 50 million years. An early Caradoc lowering of the sea-level may have induced debris flows that stripped the rim wall of much of its sedimentary cover. Because normal marine sedimentation recommenced soon after this event, the structure was not seriously damaged, as it was buried again. The Caledonian orogeny emplaced an overthrust nappe as ultimate protection, which was removed by a recent erosion episode from all but the center of the structure.Structures formed by the impact of extraterrestrial bodies are very rare throughout much of Europe, because such structures are neither well preserved nor displayed in young mountain belts or sedimentary basins. However, northern Europe has several ascertained structures of this kind (Svensson &Wickman, 1965;Svensson, 1968;Bruun &Dahlman, 1982;Kala et al., 1984;Flodén et al., 1986;Wickman, 1988). Unfortunately, the hitherto known structures are either poorly preserved or hidden by younger deposits. We are reporting the discovery of a well-preserved exposed and accessible impact structure that has escaped the notice of geologists although important features of it have been described and puzzled over by generations of researchers (Wiman, 1900;Hadding, 1927;Thorslund, 1940;Lindström et al., 1983).The structure is located in the Lockne area to the south of Östersund in central Sweden. It has a diameter of 7 km and its center is near Tramsta on the northwest shore of Lake Locknesjön (Fig. 1). Its middle is covered by folded Lower to Middle Ordovician Orthoceratite Limestone with the sheared Lower Ordovician Töyen Shale at its base. This local expanse of deformed rock is the remainder after erosion of an extensive nappe of overthrust rocks emplaced during the Caledonian orogeny. The good preservation of the impact structure is due to the nappe cover, which had to be removed before erosion could attack the underlying structures.The rim of the impact crater is outlined by a wall of strongly shattered fragments of Proterozoic crystalline rocks (Fig. 2), which formed the local bedrock (Strömberg et al., 1984) at the time of impact. The rim wall is best preserved along the western part of the structure. It was referred to as »arkose-like breccia« byThorslund (1940), who interpreted it as the result of continental weathering, but the components rather show evidence of intense crushing than of weathering (Simon, 1987a). The »arkose-like breccia« does not contain components derived from the lower Palaeozoic deposits of the area.     

Petrophysical parameters and modelling of the Eocene reservoirs in the Qadirpur area,Central Indus Basin,Pakistan: implications from well log analysis

The Eocene rock units of the Qadirpur field, Central Indus Basin (Pakistan), are investigated petrophysically for their detailed reservoir characterization. The different petrophysical parameters determined include the following: true resistivity, shale volume, total porosity, effective porosity, density and neutron porosity, water and hydrocarbon saturation, bulk volume of water, lithology, gas effect, P-wave velocity, movable hydrocarbon index and irreducible water saturation and integrated with different cross-plots. The Eocene reservoirs are excellent with high effective porosity (2–32 %) and hydrocarbon saturation (10–93 %). Among these, the Sui Upper Limestone is an overall a poor reservoir; however, it has some hydrocarbon-rich intervals with high effective porosity and better net pay. All the net pay zones identified show low and variable shale volume (5–30 %). The secondary porosity has added to the total and effective porosities in these reservoirs. The main contributors to the porosity are the chalky, intercrystalline and vuggy/fracture types. The thickness of the reservoirs zones ranges from 4.5 to 62 m. These reservoirs are gas-producing carbonates with almost irreducible water saturation (0.002–0.01) and are likely to produce water-free hydrocarbons. The lower values of moveable hydrocarbon index (0.07–0.9) show that the hydrocarbons are moveable spontaneously to the well bore. The proposed correlation model shows that the reservoirs have an inclined geometry and are a part of an anticlinal trap.