排序方式： 共有12条查询结果，搜索用时 15 毫秒
1.
A New Reference Material for Tritium Organic Molecules in Sediment: Results of an International Intercomparison Exercise 
下载免费PDF全文
下载免费PDF全文 Phillip E. Warwick Ian W. Croudace Richard M. Marsh Nicolas Baglan Sang Bog Kim 《Geostandards and Geoanalytical Research》2018,42(2):253-262
Solid matrix 3H reference materials are challenging to prepare given the volatile nature of 3H and are often unrepresentative of the range of 3H forms that may be encountered during routine analysis. As a result, few 3H reference materials are currently available, undermining verification of analytical techniques for environmental 3H measurement. To address this, an International Working Group on Organically‐Bound Tritium Analysis determined to produce a tritium natural matrix reference material (NMRM). The reference material comprises marine sediment blended with sewage sludge contaminated with 3H‐organic species arising from authorised discharges from a radiopharmaceutical manufacturing site. Previous studies have demonstrated that the 3H species have persisted in the environment over three decades providing valuable supporting data to underpin the characterisation of the NMRM. The preparation and characterisation of the NMRM are described along with the subsequent application of the reference material in an international intercomparison exercise involving nineteen laboratories from nine countries. A reference value of 168 ± 22 Bq kg?1 was derived from the data arising from the proficiency test. 相似文献
2.
Ian W. Croudace 《Geochimica et cosmochimica acta》1982,46(4):609-621
The Lleyn Peninsula of North Wales is penetrated by at least twenty stocks varying from microtonalite through to microgranite and these are closely associated with Caradocian volcanic rocks which include andesitic lavas.Major and trace element analyses show that the sample population may be separated into a peralkaline group and a sub-alkaline group. This latter group may be sub-divided into three compositional series which range from andesite to granite. The andesites are represented by the Caradocian lavas of the Moel y Penmaen district and these plot at one end of major and trace element variation diagrams. Their coherent relationships to the granitoids demonstrate the latter to be Ordovician and therefore ends a longstanding controversy concerning their age. The distinctive geochemistry of the andesites indicates the strong probability that they have evolved from transitional tholeiitic magma by low-pressure crystal fractionation. Apatite fractionation has strongly controlled the rare earth element abundances in the granitoids and for most compositions from andesite through granite their bulk distribution coefficient is greater than unity.The association of peralkaline granites, transitional tholeiites and other geological and geophysical evidence suggests that the area was a region of ensialic crustal tension during the mid-Ordovician. 相似文献
3.
4.
P.G. Langdon C.J. Caseldine I.W. Croudace S. Jarvis S. Wastegrd T.C. Crowford 《Quaternary Research》2011,75(3):451-460
Few studies currently exist that aim to validate a proxy chironomid-temperature reconstruction with instrumental temperature measurements. We used a reconstruction from a chironomid percentage abundance data set to produce quantitative summer temperature estimates since AD 1650 for NW Iceland through a transfer function approach, and validated the record against instrumental temperature measurements from Stykkishólmur in western Iceland. The core was dated through Pb-210, Cs-137 and tephra analyses (Hekla 1693) which produced a well-constrained dating model across the whole study period. Little catchment disturbance, as shown through geochemical (Itrax) and loss-on-ignition data, throughout the period further reinforce the premise that the chironomids were responding to temperature and not other catchment or within-lake variables. Particularly cold phases were identified between AD 1683–1710, AD 1765–1780 and AD 1890–1917, with relative drops in summer temperatures in the order of 1.5–2°C. The timing of these cold phases agree well with other evidence of cooler temperatures, notably increased extent of Little Ice Age (LIA) glaciers. Our evidence suggests that the magnitude of summer temperature cooling (1.5–2°C) was enough to force LIA Icelandic glaciers into their maximum Holocene extent, which is in accordance with previous modelling experiments for an Icelandic ice cap (Langjökull). 相似文献
5.
The Timbarra Tablelands pluton is an extensive (～550 km2) complexly zoned intrusion forming one of many predominantly monzogranite I‐type plutons, which constitute the Moonbi Supersuite in northern New South Wales, Australia. It comprises an outer rim of Rocky River monzogranite (Zones 1–3), an intermediate zone of Sandy Creek syenogranite (Zones 4A–4C), surrounding a core of Surface Hill syenogranite (Zones 5–7). The suite is calc‐alkaline, high‐K, and varies from mildly metaluminous to weakly peraluminous with increasing fractionation. Average Rb/Sr ratios range from 0.4 in the least evolved very coarse‐grained monzogranite (Zone 3) to 46 in the most evolved very fine‐grained biotite microgranite (Zone 6). Trace‐element modelling indicates that the observed compositional variation could have been produced by crystal fractionation. New bulk rock major‐ and trace‐element data for 71 samples are presented, and indicate that a compositional continuum exists that varies between 63 and 78 wt% SiO2. Importantly, there is no systematic chemical variation with spatial distribution of samples from the core of the pluton to its margin, requiring multiple separate pulses of an evolving magma to explain compositional discontinuities. The pluton is interpreted to have been emplaced at mesozonal levels (～180 ± 60 MPa, 5–10 km depth) and crystallised at temperatures between 620 and 820°C under moderately oxidising conditions (log fO2 = ‐11.5 to ‐19). The association of gold‐molybdenite mineralisation at Timbarra with moderately oxidised I‐type magmas is consistent with fractionation‐redox controls on ore‐element behaviour in magmatic systems in other studies. 相似文献
6.
This paper examines the evolution of Pagham Harbour, southern England following storm-induced breaching of a protective barrier in AD 1910. Stratigraphic studies of sediments collected from intertidal areas show the presence of a distinct stratigraphic horizon in the northeast of Pagham Harbour at ca. 0 to +1.0 m ordnance datum (OD). Radiometric data indicate that this horizon is a ‘reclamation surface’ formed after land claim in AD 1846. Following marine flooding in AD 1910, sediment has accreted relatively rapidly (at a broadly constant rate of between 4 and 8 mm a−1), wave and/or tidal energy have decreased and extensive marshes have developed. An asymptotic reduction in sediment accretion rate through time, as predicted in various theoretical models of salt marsh accretion, is not observed. Over the entire Pagham Harbour area, the period between AD 1948 and AD 1986 has seen an average marsh loss of 0.0087 km2 a−1, which is relatively small in comparison with other more exposed sites in the local area. Historically breached sites such as Pagham Harbour are common around European coasts, and these provide important natural laboratories within which the medium-term (decadal to centennial) coastal response to barrier breaching, and to managed-realignment coastal protection schemes, can be assessed. 相似文献
7.
A novel manganese dioxide (MnO2) resin is suitable for the determination and decontamination of lead isotopes (208Pb, 207Pb, 206Pb) in aqueous solutions at trace concentrations. This is desirable due to the toxic nature and high abundance of lead in the environment. Current techniques are both time consuming, expensive and not suitable for low-level lead decontamination. The MnO2 resin has been demonstrated to pre-concentrate with extraction efficiency above 90% for a range of pH values, and with a mean extraction of 92.5% from fresh waters at a flow rate of 100?ml?min?1. The lead distribution coefficient is 1.3?×?104 (pH 7) with tolerance to calcium and sodium. Adsorption isotherms have been investigated and the resin shown to follow the Langmuir adsorption isotherm with a saturation point of 41.5?mg per g of MnO2 resin. 相似文献
8.
Single cores from two salt marshes in the United Kingdom located near different nuclear facilities were investigated to compare chronostratigraphic estimates derived from the natural radionuclide 210Pbexcess with estimates from the known times of introduction of artificial radionuclides to the environment. Both cores selected had clear visual indications of redox zonation, and evidence for diagenetic redox perturbation of the radionuclide records was also sought. In the core from Beaulieu Marsh on the south coast of England, the redox zonation was revealed by the profiles of the redox-sensitive elements Mn + I, Fe + P + As, and S, and the records of nuclear discharges were entirely contained within oxidized conditions in the upper 40 cm. The constant flux/constant sedimentation 210Pbexcess accumulation estimate was 76% of that derived from the 1963 fallout 137Cs level (0.35 vs. 0.46 g cm−2 yr−1 dry mass), but the constant flux 210Pbexcess method indicated that accumulation rates were lower at Beaulieu before ∼1950. On any timescale, 137Cs appears earlier in the sediment record than its introduction to the environment, but although downward diffusion of 137Cs relative to 241Am has clearly occurred, the 137Cs peak still appeared in place and there was negligible penetration of 137Cs into underlying reduced conditions. This core also contained a peak of the 60Co discharges from either or both the Winfrith and La Hague nuclear plants that peaked in 1980 and 1984, respectively. The sediments in the core from Wyre Marsh on the eastern coast of the Irish Sea had accumulated more rapidly than those at Beaulieu, and in this case the redox zonation could be established only from Mn and S profiles. Here, the constant initial activity 210Pbexcess accumulation rate estimate was 125% of that derived from the 137Cs peak correlated with the 1975 Sellafield discharge maximum (0.79 vs. 0.64 g cm−2 yr−1). Sellafield discharge 137Cs swamps fallout or Chernobyl 137Cs signals in this core, but the 137Cs and 241Am sediment records are well separated and remain consistent with the slightly different discharge patterns over time. This is so despite the fact that the maximum activity levels of both isotopes are now located well into reducing conditions out of which Mn must have migrated. The 210Pb profile appeared similarly unaffected by the oxidized/reduced boundary in this case. This core was too short to define the limits of any downward 137Cs migration. 210Pbexcess accumulation rate estimates for salt marshes should be viewed with some caution because of the steady-state assumptions inherent in all 210Pbexcess methods and the potential for fluctuating accumulation conditions and open system behavior in salt marshes. 相似文献
9.
R.C. Chiverrell D.A. Sear J. Warburton N. Macdonald D.N. Schillereff J.A. Dearing I.W. Croudace J. Brown J. Bradley 《地球表面变化过程与地形》2019,44(12):2366-2376
We present the first quantitative reconstruction of palaeofloods using lake sediments for the UK and show that for a large catchment in NW England the cluster of devastating floods from 1990 to present is without precedent in this 558-year palaeo-record. Our approach augments conventional flood magnitude and frequency (FMF) analyses with continuous lake sedimentary data to provide a longer-term perspective on flood magnitude recurrence probabilities. The 2009 flood, the largest in >558 years, had a recurrence interval larger (1:2,200 year) than revealed by conventional flood estimation using shorter duration gauged single station records (1:1,700 year). Flood-rich periods are non-stationary in their correlation with climate indices, but the 1990-2018 cluster is associated with warmer Northern Hemisphere Temperatures and positive Atlantic Multidecadal Oscillation. Monitored records rarely capture the largest floods and our palaeoflood series shows, for this catchment, such omissions undermine evaluations of future risk. Our approach provides an exemplar of how to derive centennial palaeoflood reconstructions from lakes coupled well with their catchments around the world. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd. 相似文献
10.
Robert B. Watts Shanaka L. de Silva Guillermina Jimenez de Rios Ian Croudace 《Bulletin of Volcanology》1999,61(4):241-264
The Cerro Chascon-Runtu Jarita Complex is a group of ten Late Pleistocene (∼85 ka) lava domes located in the Andean Central
Volcanic Zone of Bolivia. These domes display considerable macroscopic and microscopic evidence of magma mixing. Two groups
of domes are defined chemically and geographically. A northern group, the Chascon, consists of four lava bodies of dominantly
rhyodacite composition. These bodies contain 43–48% phenocrysts of plagioclase, quartz, sanidine, biotite, and amphibole in
a microlite-poor, rhyolitic glass. Rare mafic enclaves and selvages are present. Mineral equilibria yield temperatures from
640 to 750 °C and log ƒO2 of –16. Geochemical data indicate that the pre-eruption magma chamber was zoned from a dominant volume of 68% to minor amounts
of 76% SiO2. This zonation is best explained by fractional crystallization and some mixing between rhyodacite and more evolved compositions.
The mafic enclaves represent magma that intruded but did not chemically interact much with the evolved magmas. A southern
group, the Runtu Jarita, is a linear chain of six small domes (<1 km3 total volume) that probably is the surface expression of a dike. The five most northerly domes are composites of dacitic
and rhyolitic compositions. The southernmost dome is dominantly rhyolite with rare mafic enclaves. The composite domes have
lower flanks of porphyritic dacite with ∼35 vol.% phenocrysts of plagioclase, orthopyroxene, and hornblende in a microlite-rich,
rhyodacitic glass. Sieve-textured plagioclase, mixed populations of disequilibrium plagioclase compositions, xenocrystic quartz,
and sanidine with ternary composition reaction rims indicate that the dacite is a hybrid. The central cores of the composite
domes are rhyolitic and contain up to 48 vol.% phenocrysts of plagioclase, quartz, sanidine, biotite, and amphibole. This
is separated from the dacitic flanks by a banded zone of mingled lava. Macroscopic, microscopic, and petrologic evidence suggest
scavenging of phenocrysts from the silicic lava. Mineral equilibria yield temperatures of 625–727 °C and log ƒO2 of –16 for the rhyolite and 926–1000 °C and log ƒO2 of –9.5 for the dacite. The rhyolite is zoned from 73 to 76% SiO2, and fractionation within the rhyolite composition produced this variation. Most of the 63–73% SiO2 compositional range of the lava in this group is the result of mixing between the hybrid dacite and the rhyolite. Eruption
of both groups of lavas apparently was triggered by mafic recharge. A paucity of explosive activity suggests that volatile
and thermal exchanges between reservoir and recharge magmas were less important than volume increase and the lubricating effects
of recharge by mafic magmas. For the Runtu Jarita group, the eruption is best explained by intrusion of a dike of dacite into
a chamber of crystal-rich rhyolite close to its solidus. The rhyolite was encapsulated and transported to the surface by the
less-viscous dacite magma, which also acted as a lubricant. Simultaneous effusion of the lavas produced the composite domes,
and their zonation reflects the subsurface zonation. The role of recharge by hotter, more fluid mafic magma appears to be
critical to the eruption of some highly viscous silicic magmas.
Received: 23 August 1998 / Accepted: 10 March 1999 相似文献