Geochemistry of the Upper Neoproterozoic and Lower Cambrian siliciclastic rocks and U-Pb dating on detrital zircons in the Central Iberian Zone,Spain

Sandstones and shales from the Upper Neoproterozoic (UN) succession in the Central Iberian Zone (CIZ) show parallel REE patterns and relatively restricted and similar ranges and average values of some element ratios such as Al₂O₃/TiO₂, Ti/Nb, Eu/Eu*, (La/Yb)n, (Gd/Yb)n and Th/U. This remarkable geochemical homogeneity for related medium- and fine-grained rocks is unusual, and strongly suggests a recycled source area. However, the Lower Cambrian (LC) equivalent rocks are, in general terms, geochemically less mature, more heterogeneous and more fractionated. Their average REE patterns are practically coincident, probably as a consequence of REE redistribution related to the reworking of sediments during a stage of sea level fall in Lower Cambrian times. Ti and Zr abundances, chemical index of alteration (CIA) values and element ratios such as Al₂O₃/TiO₂, K/Rb, Ti/Nb and Rb/Zr can be used for discriminating purposes between the UN and LC siliciclastic rocks. Detrital zircons from a UN and a LC sandstone display morphological differences. However, U-Pb data are discordant and cannot be interpreted in a straightforward manner. Nevertheless, age data are compatible with a model in which the continent of Gondwana would have supplied zircons, showing a bimodal age distribution, in variable proportions to the respective sandstone units.     

The Pindos Fold-and-thrust belt (Greece): inversion kinematics of a passive continental margin

Continuous exposure in the Pindos mountain chain (Greece) and the detailed stratigraphic measurements in the area enable us to construct eight balanced cross sections across the Pindos Fold-and-thrust belt (PFTB) and to approach quantitatively some parameters which controlled foreland evolution. The 160-km-wide passive continental margin of the Apulian continent in Greece was progressively shortened from east to west at rates of 6 mm/year between the Early Oligocene and Late Eocene. From the rear to the frontal part of the wedge, fault-bend folds, duplexes and imbricates were formed, while strain was partitioned into faulting (~34%), layer parallel shortening (~23%) and buckling (~9%). Foreland subsidence and internal deformation of the orogenic wedge are strongly affected by two parameters of equal importance: the thrust load of the overthrusted microcontinents and the rigidity of the underthrusted Apulian passive margin. Changes in the thickness of the pre-orogenic sediments and reactivated transform faults induced salients. During the Lower Miocene, the orogenic wedge in the Peloponnese suffered additional uplift and westward gravitational gliding induced by the intracontinental subduction of the Palaeozoic rift zone of the Phyllite-Quartzite Series, which was reactivated and returned to the earths surface during the Hellenic orogeny.     

Authigenic sericite record of a fossil geothermal system: the Offenburg trough,central Black Forest,Germany

A fossil geothermal area is hosted by the Carboniferous, Permian and Bunter sandstones of the Offenburg intramontane trough in the central Black Forest. The hydrothermal alteration is identified on the basis of newly formed sericites, which appear as pseudomorphs after feldspar and filling of pore spaces. According to K–Ar dating of sericite, serititization occurred about 145 Ma ago (Jurassic). On the basis of ¹⁸O analyses of sericite, sericite composition and vitrinite reflectance, the hydrothermal fluids had temperatures of 150–210 °C. Because their electrolyte content was low, these fluids are assumed to have derived from meteoric water. A second pulse of electrolyte-rich hydrothermal fluids resulted in quartz overgrowths. Fluid mobilization seems to be linked to the disintegration of Pangaea and to reactivated fault systems extending from the crystalline basement into the intramontane sediments.     

Deep structure and kinematics of the Northern Calcareous Alps (TRANSALP Profile)

The Northern Calcareous Alps (NCA) of southern Bavaria and northern Tyrol constitute a carbonate-dominated polyphase fold-thrust wedge; together with its Grauwacken Zone Basement, it is the northernmost part of the far-travelled Upper Austroalpine thrust complex of the Eastern Alps. The present geometry developed in several kinematic stages. Jurassic extensional faults that affected large parts of the NCA and their basement originated when the main part of the NCA was still located southeast of the Central Alpine Ötztal-Silvretta complex. These faults and related facies transitions influenced the later style of detachment of the NCA thrust sheets. Mid to Late Cretaceous detachment, thrust-sheet stacking and motion over the Central Alpine complex are registered in clastic deposits of syntectonic basins. The latest Cretaceous to Cenozoic NNE- to N-directed motion of the NCA towards Europe in front of the Central Alpine complex created another set of significant contraction structures, which at depth overprinted all previous structures. During Cretaceous to Cenozoic deformation, the NCA experienced about 80 km of shortening, i.e., about 73% along the TRANSALP Profile. The European basement and autochthonous Mesozoic cover beneath the allochthonous NCA thrust sheets and flysch complexes seem to have remained relatively undeformed.     

Thermal modeling of Carboniferous to Triassic sediments of the Karawanken Range (Southern Alps) as a tool for paleogeographic reconstructions in the Alpine–Dinaridic–Pannonian realm

Vitrinite reflectance was measured in Late Carboniferous to Triassic shales, siltstones and marls of the Karawanken Range. Thermal models of the central South-Karawanken Range were calibrated on the basis of these data. They suggest an eroded overburden of more than 3,200 m of Jurassic to Cretaceous sediments and a heat flow in the range of 42 to 60 mW m^?2 during the time of maximum subsidence. Because the reconstructed thermal history of the South-Karawanken Range is very similar to the thermal history of the Generoso basin (western Southern Alps), these data provide strong evidence for a deep basinal position of the Southern Karawanken Range during Jurassic to Cretaceous times. A vitrinite reflectance anomaly at the northern margin of the South-Karawanken Range is explained by advective heat transport during the Oligocene. The heat source for the anomalies at the western margin of the Seeberg Rise and in the area between the Periadriatic Lineament and the Donat Fault Zone is unknown. Vitrinite reflectance in Late Triassic sediments indicates the South-Karawanken Range and the South-Zala Unit of the Pannonian basement as exotic blocks in the Sava Composite Unit. This is explained by Miocene displacement of structural units, which were derived from different paleogeographical segments of the Permo-Mesozoic western Tethyan margin.     

Does<Emphasis Type="Italic">Phragmites</Emphasis> expansion alter the structure and function of marsh landscapes? Patterns and processes revisited

We assess the probability and importance of different spatial distributions ofPhragmites australis (Trin Ex Steud) within brackish tidal marshes of the mid-Atlantic United States coast. The comparative impact ofPhragmites expansion on the larger coupled marsh-estuary system may partially be a function of the landscape area dominated byPhragmites, the landscape position occupied byPhragmites, the landscape pattern created byPhragmites expansions, and the resulting impact on tidal drainage networks. We find evidence thatPhragmites establishment can occur at many landscape positions, and thatPhragmites spread within a marsh can occur via colonization (new patches), linear clonal growth (along a preferred axis), or circular clonal growth (non-directional, random spread). Early intervals ofPhragmites spread were dominated by colonization for all sites except for Piermont Marsh (which appeared to be dominated by linear clonal growth) and Lang Tract (which appeared to be dominated by circular clonal growth). Although 46–100% of new patches ofPhragmites occurred within 5 m of drainages, at only one site (Piermont Marsh, New York) didPhragmites populations remain concentrated along creek banks. Except for Iona Island, New York, which appears to be in an early stage ofPhragmites invasion, patch dynamics at all sites showed an increase followed by a decrease in patch number, as independent patches became established, expanded, and coalesced. We also found some evidence for a loss of first order streams at later stages ofPhragmites invasions in several sites (Hog Island, Lang Tract, Silver Run).     

Effect of nutrient loading on Atlantic menhaden (<Emphasis Type="Italic">Brevoortia tyrannus</Emphasis>) growth rate potential in the Patuxent River

We linked a 2-dimensional water quality model of the Patuxent River with a spatially-explicit model of fish growth to simulate how changes in land use in the Patuxent River Basin would affect the growth rate potential (GRP) of Atlantic menhaden (Brevoortia tyrannus). Simulations of three land-use patterns that reflected current nutrient loadings, increased nutrient loadings, and decreased nutrient loadings were used to drive the water quality model. Changes in nutrient loadings caused changes in the timing and intensity of phytoplankton concentrations and the region of hypoxia increased during summer with increased nutrient loading. The spatial distribution of menhaden GRP was highly correlated with phytoplankton concentrations and localized in the middle on third of the Patuxent River. Menhaden growth rate was highest in early June and late summer. During June, menhaden GRP (and phytoplankton concentration) was lowest at the lower nutrient loading simulation. During late summer, mean menhaden growth rates were inversely proportional to nutrient loading rates and menhaden grew best when nutrient loadings were the lowest. Upriver to mid-river phytoplankton patches drove overall mean calculations. Model results suggest that more research is needed on water quality model predictions of phytoplankton levels at a high level of spatial and temporal resolution, menhaden foraging, and menhaden habitat selection.     

Probable causes for cyanobacterial expansion in the Baltic Sea: Role of anoxia and phosphorus retention

This study corroborates the hypothesis that nitrogen-fixing cyanobacteria have probably occurred as an important component of the phytoplankton community in the Baltic Sea at least since brackish water conditions were initiated 8,500¹⁴C yr BP. Pigment analyses indicate that extensive occurrences started prior to a sharp increase in nutrient levels dated to 7,100¹⁴C yr BP. The cyanobacteria could have functioned as a natural trigger for eutrophication in the Baltic Sea by importing nitrogen. This is also verified by a contemporaneous drop in the δ¹⁵N values from 4‰ to around 2‰. We further conclude that the spreading of cyanobacteria was probably caused by a decrease in nitrogen∶phosphorus (N∶P) in the water mass that resulted from the intrusion of oceanic water with high P levels. The fractionation of P in sediments indicated that iron-bound P was efficiently sequestered under anoxic conditions that occurred as a consequence of the establishment of a stable stratification caused by the marine intrusion. This pool only showed minor variations around 3 μmol g⁻¹ at the freshwater-brackish water transition. All P pools except the CaCO₃ fraction showed a distinct increase around 9,300¹⁴C yr BP prior to the transition. We interpreted this increase as a change in preservation of organic matter or in the source of the sediment. Slightly after 4,000¹⁴C yr BP there was a dramatic drop in all P pools without any corresponding decreases in total N and carbon. Total P decreased from around 75 to 25–30 μmol g⁻¹. The most dramatic drop occurred in the organic bound and the detrital apatite fractions, which decreased by a factor of 3–4. We explain this as a preferential regeneration of P, especially organic P, compared to other nutrients due to more prevalent anoxic conditions.     

Circulating subbeam systemsand the physics of pulsar emission

The purpose of this paper is to suggest how detailed single-pulse observations of slow radio pulsars may be utilized to construct an empirical model for their emission. It links the observational synthesis developed in a series of papers by Rankin in the 1980s and 90s to the more recent empirical feedback model of Wright (2003a) by regarding the entire pulsar magnetosphere as a non-steady, non-linear interactive system with a natural built-in delay. It is argued that the enhanced role of the outer gap in such a system indicates an evolutionary link to younger pulsars, in which this region is thought to be highly active, and that pulsar magnetospheres should no longer be seen as being driven by events on the neutron stars polar cap, but as having more in common with planetary magnetospheres and auroral phenomena.Received: 8 May 2003, Published online: 14 November 2003 Correspondence to: Joanna M. Rankin. On leave from: Physics Department, University of Vermont, Burlington, VT 05405, USA     

Environmental controls on clay mineralogy of an Early Jurassic mudrock (Blue Lias Formation,southern England)

We present a dataset including clay mineralogy, gamma-ray spectrometry, organic matter content and magnetic susceptibility of the Hettangian to lowest Sinemurian successions of Dorset and Somerset, southern UK (Blue Lias Formation, Bristol Channel and Wessex basins). In both areas, the clay assemblages comprise predominantly detrital illite, kaolinite and illite/smectite mixed layers. Clays probably originated from the erosion of the Hercynian massifs, the relative proportions of kaolinite and illite being modulated by arid-humid climatic fluctuations. The organic matter (OM) content (types II to IV) ranges up to 12% in both areas. A clear stratigraphical trend in clay mineral assemblages is apparent in Somerset, whereas in Dorset sharp contrasts between adjacent horizons and a greater dilution by carbonate mask the long-term evolution. Correlations between both areas based on similar vertical trends in clay mineral abundance support the suggestion of a hiatus within the angulata Zone of the Dorset succession. As expected, the kaolinite/illite ratio correlates with the Th/K ratio deduced from gamma-ray spectrometry. However, significant departures from the correlation occur in OM-rich intervals, suggesting that Th may be partly adsorbed on to OM particles. Surprisingly, high magnetic susceptibility correlates with abundant kaolinite, not with Fe-rich clays, indicating either that kaolinite is accompanied by a soil-inherited magnetisable phase (possibly iron oxide) or that illite-rich rocks are more strongly diluted by carbonate than are kaolinite-rich strata.