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

F. Mazier  A. B. Nielsen  A. Broström  S. Sugita  S. Hicks 《第四纪科学杂志》2012,27(6):564-574

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.  相似文献   

Lower crustal intrusions beneath the southern Baikal Rift Zone: Evidence from full-waveform modelling of wide-angle seismic data   总被引：1，自引：0，他引：1  

Christoffer Nielsen  H. Thybo   《Tectonophysics》2009,470(3-4):298-318

The Cenozoic Baikal Rift Zone (BRZ) is situated in south-central Siberia in the suture between the Precambrian Siberian Platform and the Amurian plate. This more than 2000-km long rift zone is composed of several individual basement depressions and half-grabens with the deep Lake Baikal at its centre. The BEST (Baikal Explosion Seismic Transect) project acquired a 360-km long, deep seismic, refraction/wide-angle reflection profile in 2002 across southern Lake Baikal. The data from this project is used for identification of large-scale crustal structures and modelling of the seismic velocities of the crust and uppermost mantle. Previous interpretation and velocity modelling of P-wave arrivals in the BEST data has revealed a multi layered crust with smooth variation in Moho depth between the Siberian Platform (41 km) and the Sayan-Baikal fold belt (46 km). The lower crust exhibits normal seismic velocities around the rift structure, except for beneath the rift axis where a distinct 50–80-km wide high-velocity anomaly (7.4–7.6 ± 0.2 km/s) is observed. Reverberant or “ringing” reflections with strong amplitude and low frequency originate from this zone, whereas the lower crust is non-reflective outside the rift zone. Synthetic full-waveform reflectivity modelling of the high-velocity anomaly suggests the presence of a layered sequence with a typical layer thickness of 300–500 m coinciding with the velocity anomaly. The P-wave velocity of the individual layers is modelled to range between 7.4 km/s and 7.9 km/s. We interpret this feature as resulting from mafic to ultra-mafic intrusions in the form of sills. Petrological interpretation of the velocity values suggests that the intrusions are sorted by fractional crystallization into plagioclase-rich low-velocity layers and pyroxene- and olivine-rich high-velocity layers. The mafic intrusions were probably intruded into the ductile lower crust during the main rift phase in the Late Pliocene. As such, the intrusive material has thickened the lower crust during rifting, which may explain the lack of Moho uplift across southern BRZ.  相似文献   

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

Samuli Helama  Jan Kresten Nielsen 《Journal of Paleolimnology》2008,40(1):247-261

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.  相似文献   

Observations of ionospheric flows and particle precipitation following a Sudden Commencement     

E. Nielsen  F. Honary 《Annales Geophysicae》2000,18(8):908-917

On May 4, 1998, at 0227 UT an interplanetary shock crossed the WIND spacecraft, and half an hour later a Sudden Commencement occurred. Coinciding with the Sudden Commencement a rapid intensification of the flux of particle precipitation into the ionosphere was observed. Evidence is presented that the ionospheric electric fields were influenced by the associated dynamic variations of the ionospheric conductivities. Following the initial phase the ionospheric flow speeds increased rapidly over the next 20 min to more than 2000 m/s, in agreement with an increased effective coupling of the solar wind energy to the magnetosphere following the interplanetary shock that caused the Sudden Commencement. These strong flows were meandering in latitude, a type of plasma flow modulation that has been reported before to occur during Omega band events: a string of alternating field-aligned currents propagating eastward. The riometer absorption was found to be at a minimum in regions associated with outward directed field aligned currents. The riometer absorption regions (the regions of particle precipitation) were drifting with E × B drift speed of the ionospheric electrons.  相似文献   

Instability of reconstruction of the low CMB multipoles     

P. D. Naselsky  O. V. Verkhodanov  M. T. B. Nielsen 《Astrophysical Bulletin》2008,63(3):216-227

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%.  相似文献   

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

Mikael?PedersenEmail author  Jesper?K.?Nielsen  Adrian?J.?Boyce  Anthony?E.?Fallick 《Mineralium Deposita》2003,38(1):108-123

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.  相似文献   

Three-dimensional inverse modelling of the thermal structure and implications for lithospheric strength in Denmark and adjacent areas of Northwest Europe   总被引：1，自引：0，他引：1  

Lykke Gemmer  Søren B. Nielsen 《Geophysical Journal International》2001,147(1):141-154

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Velocity distributions of waves and currents in the combined flow   总被引：1，自引：0，他引：1  

Zai-Jin You  D. L. Wilkinson  P. Nielsen 《Coastal Engineering》1991,15(5-6)

The theoretical model presented here is designed to predict waves and currents velocity profiles in the combined wave and current motion near a fixed bed. It is also valid for the pure wave motion, because the pure wave motion is only a special case of the combined flow. This model has been derived by using the equation of motion and the eddy viscosity assumptions which are somewhat different from those of former authors. Although the model is simple, the predictions are quite good compared with experimental data from Jonsson and Carlsen (1976) No. 1 and No. 2, Van Doorn (1981) V00RA, V10RA and V20RA, Van Doorn (1982) S00RAL, S10RAL and S20RAL, and Jensen (1989) Test-12 and Test-13.  相似文献   

Full-Scale Removal of DNAPL Constituents Using Steam-Enhanced Extraction and Electrical Resistance Heating   总被引：6，自引：1，他引：6  

Gorm Heron  Steven Carroll  Steffen G. Nielsen 《Ground Water Monitoring & Remediation》2005,25(4):92-107

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Photosynthetic responses to Cu exposure are independent of light acclimation and uncoupled from growth inhibition in Fucus serratus (Phaeophyceae)     

Hanne Dalsgaard Nielsen  Sren Laurentius Nielsen 《Marine pollution bulletin》2005,51(8-12):715

We have studied the effect of light acclimation on photosynthetic responses and growth during Cu²⁺ exposure (0–0.84 μM) in the brown seaweed Fucus serratus. Measurements of chlorophyll fluorescence parameters showed that Cu²⁺ exposure amplified ETR, reduced the chlorophyll content at the cellular level and that there was no effect of light adaptation on the Cu²⁺ resistance of the algae. In contrast to the inhibitory effects of Cu²⁺ on chlorophyll fluorescence, O₂ evolution and the total content of chlorophyll and carotenoid of the algae was unaffected by Cu²⁺. We conclude that photoinhibition and perhaps pigment degradation in the meristoderm was compensated for by cells deeper in the thallus with the result that the overall photosynthetic fitness of the algae was maintained. The pronounced inhibitory effects of Cu²⁺ on algae growth was not a consequence of photoinhibition and could be attributed to direct inhibitory effects on the growth process.  相似文献