Interpretation of potential fields from inclined dikes in the wavenumber domain

The interpretation of the gravity and magnetic fields from inclined dikes has been studied with artifical data contaminated by various noise components: base level, linear trend, and random noise. A Gaussian window was applied to the data prior to transformation to reduce the influence of noise as demonstrated by an analysis of the horizontal cylinder. The case of the dike is more complicated due to the fact that its spectrum has a number of zeroes at wavenumbers which are inversely related to the width of the dike. Around these wavenumbers, especially the random noise distorts the spectrum making interpretation ambiguous.     

Sulfur-isotope and trace element analyses from the sulitjelma ore bodies,Northern Norway

δ ³⁴S-values have been determined in 248 sulfide samples from different profiles through the ore bodies of the Sulitjelma pyrite-copper deposits. The values range for pyrite from ?4,1‰ to +9,2‰, but for an individual orebody the spread is generally much lower. Some profiles exhibit a systematic δ ³⁴S decrease from the footwall to the hanging wall of the ore bodies, whereas laterally the S isotope composition in a distinct layer is rather uniform. Another trend from heavier to lighter sulfur exists from the center towards the margin of large porphyroblastic pyrite crystalls. Among the trace elements only the Co concentration in pyrite appears to be correlated with the δ-values. The sulfur-isotopic fractionation between coexisting sulfides is in agreement with the experience from other deposits, namely δ³⁴S_pyrite>δ³⁴S_pyrrhotite≈δ³⁴S_sphalerite>δ³⁴S_chalcopyite     

The petrology of a melilitolite,melteigite, carbonatite and syenite ring dike system,in the Gardiner complex,East Greenland

The Tertiary Gardiner complex in East Greenland comprises ultramafic cumulates cut by dikes, ring dikes and cone sheets. One major ring dike is of melilite-rick rocks (melilitolites), and shows phase layering and cryptic layering with respect to melilite, perovskite and phologopite. The vertical variation in the assemblages through the ring dike is consistent with established fractionation trends in undersaturated melilite-bearing experimental systems. Related veins and dikes are: a) undersaturated rocks of the melteigite-ijolite-urtite/nepheline syenite suite, b) melanite-calcite veins and søvite dikes and sheets and c) coarse-grained magnetite-phlogopite-diopside-calcite pegmatites and dolomite rocks. It is proposed that a CO₂-bearing Ln-normative nephelinite liquid is parential to both melilitolites and related veins through a combination of fractional crystallization, liquid immiscibility and CO₂-metasomatism of dunite country rocks. Unlike other localities where melilitolites are attributed to metasomatic processes, the Gardiner melilitolites are magmatic.     

Mid-Palaeocene palaeogeography of the eastern North Sea basin: integrating geological evidence and 3D geodynamic modelling

The Mid‐Palaeocene palaeogeography of Denmark and the surrounding areas have been reconstructed on the basis of published geological data integrated with 3D geodynamic modelling. The use of numerical modelling enables quantitative testing of scenarios based on geological input alone and thus helps constrain likely palaeo‐water depths in areas where the geological data are inconclusive or incomplete. The interpretation of large‐scale erosional valleys and small‐scale circular depressions at the Mid‐Palaeocene Top Chalk surface in the Norwegian–Danish basin as either submarine or subaerial features is enigmatic and has strong implications for palaeogeographical reconstructions of the eastern North Sea basin. A 3D thermo‐mechanical model is employed in order to constrain the likely palaeo‐water depths of the eastern North Sea basin during the Palaeocene. The model treats the lithosphere as an elasto‐visco‐plastic continuum and models the lithospheric response to the regional stress field and thermal structure. The model includes the effects of sea‐level change, sedimentation and erosion, from the Mid Cretaceous to the present. Modelling results reproduce to first order geological data such as present sediment isopachs and palaeo‐water depths. It is concluded that the Mid Palaeocene water depths in the Norwegian–Danish basin were about 250 m. The erosional valleys and circular depressions at the top of the Upper Cretaceous‐Danian Chalk Group are thus interpreted to have formed in relatively deep water rather than due to subaerial exposure. Likely interpretations of the structures are therefore submarine valleys and pockmarks.     

Signals of tree volume and temperature in a high‐resolution record of pollen accumulation rates in northern Finland

Pollen accumulation rates (PARs) provide a potential proxy for quantitative tree volume (m³ ha^?1) reconstruction with reliable absolute pollen productivity estimates (APPEs). We obtained APPEs for pine, spruce and birch at their range limits in northern Finland under two temperature periods (‘warm’ and ‘cold’) based on long‐term pollen trap and tree volume records within a 14‐km radius of each trap. APPEs (mean ± SE; × 10⁸ grains m^?3 a^?1) tend to be higher for the ‘warm’ periods (pine 123.8 ± 24.4, birch 528.0 ± 398.4, spruce 434.3 ± 113.7) compared with the ‘cold’ periods (pine 95.5 ± 37.3, birch 317.3 ± 282.6, spruce 119.6 ± 37.6), although the difference is only significant for spruce. Using an independent temperature record and the APPEs obtained, we reconstruct a low‐frequency record of pine volume changes over the last 1000 years at Palomaa mire, where a high‐resolution record of Pinus PARs is available. Five phases are distinguished in the reconstruction: moderate pine volume, AD 1080–1170; high volume, AD 1170–1340; low volume, AD 1340–1630; very low volume, AD 1630–1810; and rising pine volume, AD 1810–1950. These phases do not coincide with periods of high or low June–July–August temperatures, and thus appear to reflect regional variations in tree volume, while high‐frequency changes within each time‐period block show variations in PARs in response to temperature. Copyright © 2012 John Wiley & Sons, Ltd.     

Instability of reconstruction of the low CMB multipoles

We discuss the problem of the bias of the Internal Linear Combination (ILC) CMB map and show that it is closely related to the coefficient of cross-correlation K(?) of the true CMB and the foreground for each multipole ?. We present analysis of the cross-correlation for the WMAP ILC quadrupole and octupole from the first (ILC(I)) and the third (ILC(III)) year data releases and show that these correlations are ～ ?0.52–0.6. Analysing 10⁴ Monte Carlo simulations of the random Gaussian CMB signals, we show that the distribution function for the corresponding coefficient of the cross-correlation has a polynomial shape P(K, ?) ∝ (1 ? K ²)^??1. We show that the most probable value of the cross-correlation coefficient of the ILC and foreground quadrupole has two extrema at K ? ±0.58. Thus, the ILC(III) quadrupole represents the most probable value of the coefficient K. We analyze the problem of debiasing of the ILC CMB and pointed out that reconstruction of the bias seems to be very problematic due to statistical uncertainties. In addition, instability of the debiasing illuminates itself for the quadrupole and octupole components through the flip-effect, when the even ? + m modes can be reconstructed with significant error. This error manifests itself as opposite, in respect to the true sign of even low multipole modes, and leads to significant changes of the coefficient of cross-correlation with the foreground. We show that the CMB realizations, whose the sign of quadrupole (2, 0) component is negative (and the same, as for all the foregrounds), the corresponding probability to get the positive sign after implementation of the ILC method is about 40%.     

Nitrogen Retention in Coastal Marine Sediments—a Field Study of the Relative Importance of Biological and Physical Removal in a Danish Estuary

The aim of this study was to elucidate the relative importance of physical versus biological loss processes for the removal of microphytobenthic (MPB) bound nitrogen in a coastal environment at different times of the year via a dual isotope labeling technique. We used ⁵¹Cr, binding to inorganic sediment particles but not participating in any biological processes, and ¹⁵N–NO₃ ^?, taken up by the MPB and turned over as part of the MPB nitrogen pool. Retention, down-mixing, and export of ¹⁵N were due to both biological and physical processes, so that by comparing retention of the two isotopes, we were able to discern the relative importance of physical and biological processes. The isotope marking was supplemented with measurements of sediment chlorophyll biomass and oxygen fluxes, allowing us to evaluate MPB biomass as well as primary production vs. respiration in the sediment. In spring/early summer, the system was characterized by tight N cycling and high N retention: any remineralized N was immediately taken up and retained in the MPB biomass. In late summer and autumn, the system was still physically stable, but high biological mediated N losses were observed. In early winter, the system was physically dominated due to low MPB biomasses and activity combined with a significant storm event. Our data support the hypothesis that the relative balance between physical and biological processes in determining retention and removal of MPB-bound nitrogen changes seasonally.     

Construction of statistically reliable sclerochronology using subfossil shells of river pearl mussel

Sclerochronology is based on series of skeletal growth records that have been correctly aligned in time. These chronologies have so far been derived from living bivalves and corals. Here we demonstrate the possibility of constructing the sclerochronology from a suite of river pearl mussel (Margaritifera margaritifera (L.)) shells with unknown but at least partly overlapping geological age. Subsequent to measuring of the annual shell growth increments, the sample series were iteratively compared to each other and to the mean series of other samples. This comparison forms the basis of the applied procedure, termed cross-dating, in which more inter-correlative sample series can be progressively added into the mean chronology in step-wise fashion. After cross-dating, the correlation of the 16 sample series (0.52) indicated that the subfossil sclerochronology bore common growth signal that was comparable to previously recorded chronologies from modern samples. The cross-dated sclerochronology represented growth variations over a time window of 91 years. Variability in chronology was found to be statistically reliable, with sample replication in at least six series (this equalled to chronology sub-interval of 78 years). In the wider paleontological context, these results show the possibility of constructing statistically reliable sclerochronologies from fossil specimens at any given locality bearing an abundance of skeletal remains. Such chronologies will be important in their ability to provide paleolimnological records with comparable quality to recent proxy and observational series. Sclerochronologies can be numerically examined for paleolimnological variations using standard time-series analysis tools: a dominant frequency-band at inter-annual and multi-annual time-scales was found in our sample by spectral analysis.     

Timing and genesis of base-metal mineralisation in black shales of the Upper Permian Ravnefjeld Formation,Wegener Halvø, East Greenland

Bituminous mud shales of the Upper Permian Ravnefjeld Formation (Zechstein 1 equivalent) are mineralised with zinc, lead and copper within a ca. 50 km² area on Wegener Halvø in central East Greenland. The occurrence of base-metal sulphides in shale nodules cemented prior to compaction indicates an early commencement of base-metal mineralisation. In other cases, post-compactional sulphide textures are observed. Homogeneous lead isotope signatures of galena and sphalerite from the shales (²⁰⁶Pb/²⁰⁴Pb: 18.440–18.466; ²⁰⁷Pb/²⁰⁴Pb: 16.554–16.586; ²⁰⁸Pb/²⁰⁴Pb: 38.240–38.326) suggest that all base metals were introduced during a single hydrothermal event. Therefore, post-compactional textures are believed to result from recrystallisation of early diagenetic sulphides during deep burial in the Upper Cretaceous to Tertiary. Lead isotope signatures of galena hosted in Upper Permian carbonate build-ups are relatively heterogeneous compared to those of the shale-hosted sulphides. The observed relations indicate a shared lead source for the two types of mineralisation, but different degrees of homogenisation during mineralisation. This suggests that lead was introduced to the carbonate rocks and black shales during two separate events. δ³⁴S of base-metal sulphides in the Ravnefjeld Formation lie between –12 and –4‰, whereas synsedimentary and early diagenetic pyrite in unmineralised shales in general have δ³⁴S between –47 and –16.5‰. Early diagenetic pyrite in the Wegener Halvø area in general has δ³⁴S 15 to 20‰ higher than the same pyrite morphotype in Triaselv in the western part of the basin. This relatively high δ³⁴S can be explained by extensive microbial sulphate reduction within persistent euxinic (super-anoxic) bottom waters under which supply of isotopically light seawater sulphate (and disproportionation of intermediate sulphur compounds) was restricted. The sulphur in the base-metal sulphides is believed to represent sulphide-dominated pore water, enriched in ³⁴S due to preferential removal of ³²S by sulphate-reducing bacteria and precipitation of diagenetic pyrite in the near-seafloor environment. We suggest that the sulphide-dominated pore water was trapped in the shale formation prior to introduction of base-metal-bearing fluids through fractures in the underlying carbonates, and that sulphide precipitation took place when the two fluids met. δ³⁴S values of carbonate-hosted base-metal sulphides fall within the same range as the shale-hosted ones. The relationship between barite and sulphides and evidence for pre-mineralisation entrapment of liquid hydrocarbons in the carbonates suggest that the sulphide in this case is derived by in-situ thermochemical sulphate reduction (TSR). Measured fractionation between sulphide and sulphate ranges from 18.5 to 24.4‰, suggesting temperatures of TSR around 70 to 100 °C. Vitrinite reflectance measurements in mineralised shale samples are all between 1.7 and 2.0%, except for samples taken close to a Tertiary dyke giving ca. 3.0%. Vitrinite reflectance data are comparable to previously published data from unmineralised shale samples in the area and could not be proven to correlate with the degree of mineralisation. This indicates that any early hydrothermal effect has been overprinted later, probably during deep burial in the Late Cretaceous to Early Tertiary as previously proposed.