首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Here we present research on previously uninvestigated frost peat mounds occurring on a peat bog in the southern part of Hermansenøya, NW Svalbard. Detailed characteristics are given of the environmental conditions of the peat bog and of the morphological features and surface structure of the frost peat mounds, as well as an analysis of the internal structure of one mound. Three types of frost peat mounds have been distinguished: disc‐shaped mounds (low), mid‐sized mounds with gentle sides, and high mounds with steep sides. Radiocarbon dating of the peat within the frost peat mound performed for the first time on Svalbard and a detailed analysis of the deposits demonstrated that in the high mound (1.3 m) there is an ice‐peat core and peat cover without ice. There are three layers of peat of different ages separated by at least two hiatuses. A generalized history of the development of the peat bog from about 8 ka BP is established. The studied mound displays two development cycles unknown elsewhere. The older relict part of the peat mound was formed during a climatic cooling about 3.0–2.5 ka BP, while the younger part originated during the Little Ice Age (c. AD 1550–1850). Despite certain similarities of these mounds to some palsas, this term should not be applied to the mounds because they are smaller and their cores consist mostly of layers of massive injection ice, the presence of which indicates a pressurized system in their genesis.  相似文献   

2.
Contamination with detrital matter is a well‐known bias in δ18O records from lake carbonates but quantitative information of this effect is yet lacking. Therefore, we developed a new methodological approach combining isotope analyses with microfacies, X‐ray diffraction and micro‐X‐ray fluorescence data and applied this in a case study for the Piànico interglacial lake record in order to provide a quantitative estimate of the effect of detrital carbonate on stable δ18O and δ13C data. The Piànico record contains a long series of distinct and well‐preserved calcite varves and is correlated to Marine Isotope Stage 11. Intercalated in the varve sequence are detrital layers triggered by surface erosion events. These detrital layers are mainly composed of dolomite, thus reflecting the mineralogical signature of the catchment. Microfacies analyses of a 9350 varve year interval allows the identification of detrital layers down to sub‐millimetre scale and a precise selection of three different types of samples for isotope analyses: (1) pure endogenic calcite varves (five varves per sample) without detrital contamination; (2) individual detrital layers; and (3) ‘mixed’ samples including five calcite varves and up to four thin detrital layers. Detrital samples show the isotopic signature of the catchment dolomite and are up to 5.7‰ enriched in δ18O values with respect to endogenic calcite samples. In mixed samples, the degree of isotopic enrichment is directly related to the amount of detrital contamination; δ18O of bulk carbonates is significantly biased when the detrital component amounts to more than 5% of the sample. It is also shown that samples containing detrital material have an influence on the calculation of the covariance between δ13C and δ18O. Covariance is high (r = 0.76) when the correlation coefficient is calculated on the base of all samples, but absent (r = ?0.43) when samples containing detrital dolomite are excluded. It has been demonstrated that microfacies analysis is a quick tool to avoid or reduce detrital contamination in bulk carbonate samples during sample selection. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

3.
Sm–Nd (garnet), U–Pb (monazite) and Rb–Sr (biotite) ages from a composite migmatite sample (Damara orogen, Namibia) constrain the time of high‐grade regional metamorphism and the duration of regional metamorphic events. Sm–Nd garnet whole‐rock ages for a strongly restitic melanosome and an adjacent intrusive leucosome yield ages of 534±5, 528±11 and 539±8 Ma. These results provide substantial evidence for pre‐500 Ma Pan‐African regional metamorphism and melting for this segment of the orogen. Other parts of the migmatite yield younger Sm–Nd ages of 488±9 Ma for melanosome and 496±10, 492±5 and 511±16 Ma for the corresponding leucosomes. Garnet from one xenolith from the leucosomes yields an age of 497±2 Ma. Major element compostions of garnet are different in terms of absolute abundances of pyrope and spessartine components, but the flat shape of the elemental patterns suggests late‐stage retrograde equilibration. Rare earth element compositions of the garnet from the different layers are similar except for garnet from the intrusive leucosome suggesting that they grew in different environments. Monazite from the leucosomes is reversely discordant and records 207Pb/235U ages between 536 and 529 Ma, indicating that this monazite represents incorporated residual material from the first melting event. Monazite from the mesosome MES 2 and the melanosome MEL 3 gives 207Pb/235U ages of 523 and 526 Ma, and 529 and 531 Ma, respectively, which probably indicates another thermal event. Previously published 207Pb/235U monazite data give ages between 525 and 521 Ma for composite migmatites, and 521 and 518 Ma for monazite from neosomes. Monazite from granitic to granodioritic veins indicates another thermal event at 507–505 Ma. These ages are also recorded in 207Pb/235U monazite data of 508 Ma from the metasediment MET 1 from the migmatite and also in the Sm–Nd garnet ages obtained in this study. Taken together, these ages indicate that high‐grade metamorphism started at c. 535 Ma (or earlier) and was followed by thermal events at c. 520 Ma and c. 505 Ma. The latter event is probably connected with the intrusion of a large igneous body (Donkerhoek granite) for which so far only imprecise Rb–Sr whole‐rock data of 520±15 Ma are available. Rb–Sr biotite ages from the different layers of the migmatite are 488, 469 and 473 Ma. These different ages indicate late‐stage disturbance of the Rb–Sr isotopic system on the sub‐sample scale. Nevertheless, these ages are close to the youngest Sm–Nd garnet ages, indicating rapid cooling rates between 13 and 20°C Ma?1 and fast uplift of this segment of the crust. Similar Sm–Nd garnet and U–Pb monazite ages suggest that the closure temperatures for both isotopic systems are not very different in this case and are probably similar or higher than the previously estimated peak metamorphic temperatures of 730±30°C. The preservation of restitic monazite in leucosomes indicates that dissolution of monazite in felsic water‐undersaturated peraluminous melts can be sluggish. This study shows that geochronological data from migmatites can record polymetamorphic episodes in high‐grade terranes that often contain cryptic evidence for the nature and timing of early metamorphic events.  相似文献   

4.
Recent papers (Black et al. 1997; Jagodzinski & Black 1999) reported that SL13‐referenced SHRIMP zircon ages are ca 1.3% younger than QGNG‐referenced ages. It is shown here that these authors overestimated their analytical errors and as a consequence failed to register the presence of large and variable fractions of Pb loss within both QGNG (median 40% of spots) and SL13 (25%). If only the oldest 206Pb/238U age groups within each of the same 16 analytical sessions are selected for comparison, the 206Pb/238U ages of QGNG relative to SL13 agree with the Concordia intersection age by isotope dilution to within 0.4% (standard deviation). This observation eliminates the possibility of systematic bias. Jagodzinski and Black (1999) also overlooked Pb loss in their ages for the Early Devonian Turondale and Merrions Formations, which are corrected here for time‐scale use.  相似文献   

5.
From the Sellevollmyra bog at Andøya, northern Norway, a 440‐cm long peat core covering the last c. 7000 calendar years was examined for humification, loss‐on‐ignition, microfossils, macrofossils and tephra. The age model was based on a Bayesian wiggle‐match of 35 14C dates and two historically anchored tephra layers. Based on changes in lithology and biostratigraphical climate proxies, several climatic changes were identified (periods of the most fundamental changes in italics): 6410–6380, 6230–6050, 5730–5640, 5470–5430, 5340–5310, 5270–5100, 4790–4710, 4890–4820, 4380–4320, 4220–4120, 4000–3810, 3610–3580, 3370–3340 (regionally 2850–2750; in Sellevollmyra a hiatus between 2960–2520), 2330–2220, 1950, 1530–1450, 1150–840, 730? and c. 600? cal. yr BP. Most of these climate changes are known from other investigations of different palaeoclimate proxies in northern and middle Europe. Some volcanic eruptions seemingly coincide with vegetation changes recorded in the peat, e.g. about 5760 cal. yr BP; however, the known climatic deterioration at the time of the Hekla‐4 tephra layer started some decades before the eruption event.  相似文献   

6.
We present a record of peatland development in relation to climate changes and human activities from the Palomaa mire, a remote site in northern Finland. We used fine‐resolution and continuous sampling to analyse several proxies including pollen (for vegetation on and around the mire), testate amoebae (TA; for mire‐wetness changes), oxygen and carbon isotopes from Sphagnum cellulose (δ18O and δ13C; for humidity and temperature changes), peat‐accumulation rates and peat‐colour changes. In spite of an excellent accumulation model (30 14C dates and estimated standard deviation of sample ages <1 year in the most recent part), the potential to determine cause–effect (or lead–lag) relationships between environmental changes and biotic responses is limited by proxy‐specific incorporation processes below the actively growing Sphagnum surface. Nevertheless, what emerges is that mire development was closely related to water‐table changes rather than to summer temperature and that water‐table decreases were associated with increasing peat‐accumulation rates and more abundant mire vegetation. A rapid fen‐to‐bog transition occurred within a few years around AD 1960 when the water table decreased beyond the historical minimum, supporting the notion that mires can rapidly shift into bogs in response to allogenic factors. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

7.
New eclogite localities and new 40Ar/39Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh‐pressure (UHP) domains that are separated by distinctly lower pressure, eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km2; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 × 50 × 5 km. 40Ar/39Ar mica and K‐feldspar ages show that this outcrop pattern is the result of gentle regional‐scale folding younger than 380 Ma, and possibly 335 Ma. The UHP and intervening high‐pressure (HP) domains are composed of eclogite‐bearing orthogneiss basement overlain by eclogite‐bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, calc‐silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15‐cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near‐isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12–17 kbar and c. 750 °C during exhumation from mantle depths, followed by a low‐pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750 °C. New 40Ar/39Ar hornblende ages of 402 Ma document that this decompression from eclogite‐facies conditions at 410–405 Ma to mid‐crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20–30 km. 40Ar/39Ar muscovite ages of 397–380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30 °C Myr?1), perhaps because of continued tectonic unroofing.  相似文献   

8.
The Silurian bituminous sandstones(SBS) in the Tarim Basin, China are important basinwide reservoirs with an estimated area of approximately 249000 km2. We investigated the ages of authigenic illites in the SBS reservoirs and constrained their formation timing by using the ~(40)Ar/~(39)Ar step wise heating method. The age spectra, ~(39)Ar recoil loss and their controlling factors were investigated systematically. The ~(40)Ar/~(39)Ar ages were compared with the conventional K/Ar ages of identical clay fractions. The clay in the SBS reservoirs is dominated by orderly mixed-layer illite/smectite(I/S) with 5%–30% smectite layers. The I/S minerals morphology comprises primarily honeycomb, short filamentous and curved-lath particles, characteristic of authigenic illites. The unencapsulated ~(40)Ar/~(39)Ar total gas ages(UTGA) of the authigenic illites range from 188.56 ± 6.20 Ma to 491.86 ± 27.68 Ma, which are 7% to 103% older than the corresponding K/Ar ages of 124.87 ± 1.11 Ma to 383.45 ± 2.80 Ma, respectively. The K-Ar ages indicate multistage accumulations with distinct distribution patterns in the Tarim Basin: older(late Caledonian-early Hercynian) around the basin margin, younger(late Hercynian) in the basin centre, and the youngest(middle to late Yanshanian) in the Ha-6 well-block, central area of the North Uplift. The age difference is believed to have been caused by the ~(39)Ar recoil loss during the irradiation process. Compared with the K/Ar ages, the estimated ~(39)Ar recoil losses in this study are in the range from 7% to 51%. The ~(39)Ar recoil loss appears to increase not only with the decreasing particle sizes of the I/S, but also with increasing percentage of smectite layers(IR) of the I/S, and smectite layer content(SLC) of the samples. We conclude that due to significant ~(39)Ar recoil losses, UTGA may not offer any meaningful geological ages of the authigenic illite formation in the SBS and thus can not be used to represent the hydrocarbon charge timing. ~(39)Ar recoil losses during ~(40)Ar/~(39)Ar dating can not be neglected when dating fine authigenic illite, especially when the ordered mixed-layer I/S containing small amount of smectite layers(IR30%) in the reservoir formations. Compared with the unencapsulated Ar-Ar method, the conventional K-Ar method is less complicated, more accurate and reliable in dating authigenic illites in petroleum reservoirs.  相似文献   

9.
Thermokarst ponds are widespread in arctic and subarctic regions, but little is known about their temporal evolution prior to human observations. This paper presents a pioneer biostratigraphic study conducted at a subarctic site with limnologically contrasted ponds located on the eastern shore of Hudson Bay, Canada. Fossil diatom and visible near infrared (VNIR) derivative spectral analyses were performed on short sediment cores, confirming the occurrence of three distinct stratigraphic facies as already inferred from an anterior sedimentological study: a lacustrine upper facies (UF) and a marine lower facies (LF), separated by an organic‐rich/peat transitional zone (TZ). Diatoms were almost absent from LF, but increased significantly in both TZ and UF. Identified diatom taxa were mainly benthic species (e.g. genera Fragilaria, Pinnularia), and their down‐core distribution appeared to be related to dissolved organic carbon (DOC) and possibly pH conditions. Diatom‐inferred DOC showed a decreasing trend towards the surface (potentially associated with an increase in pH), inverse to the general trend in this region, suggesting the action of other mechanisms on DOC, such as exhaustion of external inputs from limited catchments and the role of discontinuous peat layers (former surfaces of permafrost mounds) during the initial stages of pond formation. These bryophilous substrates in aerophilic habitats probably controlled diatom community composition. The combination of diatom and VNIR data revealed similar trends between (i) opal (amorphous silica) and diatom abundances; (ii) eukaryotic/prokaryotic algae ratio and anoxia or hypoxia in bottom waters; and (iii) limonite (iron oxide) and redox conditions in surface sediments. These findings indicate that diatom community changes and pond limnological evolution in the recent past were controlled mainly by autogenic processes (e.g. local vegetation/soil development, peat accumulation and erosion), rather than by allogenic forcing mechanisms (e.g. precipitation and temperature, geochemical leaching of the surrounding glaciomarine sediments).  相似文献   

10.
Amesbury, M. J., Barber, K. E. & Hughes, P. D. M. 2010: The methodological basis for fine‐resolution, multi‐proxy reconstructions of ombrotrophic peat bog surface wetness. Boreas, 10.1111/j.1502‐3885.2010.00152.x. ISSN 0300‐9483. The need for Holocene peat‐based palaeoclimatic records of increased temporal resolution has been widely identified in recent research. The often rapid growth rates of ombrotrophic bogs, when combined with fine‐resolution (i.e. millimetre‐scale) sampling, provide an as yet largely unexploited potential to derive sub‐decadal palaeoclimatic data from this proxy‐archive. However, multi‐proxy, fine‐resolution analyses require changes to standard methodologies, and the application of sampling techniques that are new to peat‐based palaeoclimate research. A peat sampler was custom‐built to allow precise and replicable millimetre‐scale subsampling. Subsequent methodological testing revealed that, irrespective of sample thickness (i.e. resolution), halving the standard sample volume used for plant macrofossil (from 4 cm3 to 2 cm3) and testate amoebae (from 2 cm3 to 1 cm3) analyses and the sample weight used for peat humification analysis (from 0.2 g to 0.1 g dried peat) did not affect the interpretation of the results. A contiguous 1‐mm sampling resolution for plant macrofossil analysis was also tested, but it was found that contiguous 5‐mm samples provided a more reliable background record to fine‐resolution testate amoebae and peat humification analyses. Based on these findings, a standardized and systematic methodological approach was developed, using the custom‐built peat slicer to take millimetre‐scale samples that provide enough sample material for both testate amoebae and peat humification analyses to be performed at 1‐mm resolution. This approach will facilitate the testing of the palaeoclimatic reliability of multi‐proxy, fine‐resolution peat‐based records.  相似文献   

11.
Anjar, J., Larsen, N. K., Björck, S., Adrielsson, L. & Filipsson, H. L. 2010: MIS 3 marine and lacustrine sediments at Kriegers Flak, southwestern Baltic Sea. Boreas, 10.1111/j.1502‐3885.2010.00139.x. ISSN 0300‐9483. Sediment cores from the Kriegers Flak area in the southwestern Baltic Sea show a distinct lithological succession, starting with a lower diamict that is overlain by a c. 10 m thick clay unit that contains peat, gyttja and other organic remains. On top follows an upper diamict that is inter‐layered with sorted sediments and overlain by an upward‐coarsening sequence with molluscs. In this paper we focus on the clay unit, which has been subdivided into three subunits: (A) lower clay with benthic foraminifera and with diamict beds in the lower part; (B) thin beds of gyttja and peat, which have been radiocarbon‐dated to 31–35 14C kyr BP (c. 36–41 cal. kyr BP); and (C) upper clay unit. Based on the preliminary results we suggest the following depositional model: fine‐grained sediments interbedded with diamict in the lower part (subunit A) were deposited in a brackish basin during a retreat of the Scandinavian Ice Sheet, probably during the Middle Weichselian. Around 40 kyr BP the area turned into a wetland with small ponds (subunit B). A transgression, possibly caused by the damming of the Baltic Basin during the Kattegat advance at 29 kyr BP, led to the deposition of massive clay (subunit C). The data presented here provide new information about the paleoenvironmental changes occurring in the Baltic Basin following the Middle Weichselian glaciation.  相似文献   

12.
This paper deals with the chronology and the possible correlations among levels of different excavated areas in the Pleistocene vertebrate-bearing deposits at the large San Teodoro Cave (North-Eastern Sicily). Two trenches have been excavated along the eastern side of the cave, located at a distance from the entrance, respectively, of 8 m (α trench) and 28 m (β trench) and at different depths. Lithological features, biometrical data from small mammals and ecological data from molluscs point to similar environmental conditions for the α trench deposits and those located along the eastern wall of the cave in the eastern part of the β trench. The same evidence, and the taphonomic features of large mammals, points to different environmental conditions and perhaps to different ages for the deposits located in the western part of the β trench. The survival of elephants in Sicily up to 32,000 years ago is a new significant result of the 230Th/234U dating carried out from a concretionary stratum from the β trench and represents the youngest elephant survival in the western Mediterranean islands.  相似文献   

13.
Aliphatic des-A-triterpenoids have been tentatively assigned in samples taken from a 260 cm thick peat sequence from the Dajiuhu Basin of southern China using gas chromatography–mass spectrometry (GC–MS). The compounds, possessing the carbon skeletons of oleanane, ursane or lupane, are generally considered as the microbial degradation products of plant triterpenoids under anoxic conditions. Abundant aliphatic des-A-triterpenoids were observed only in peat samples during the transitional period from the last glaciation to the early Holocene (9.5–11.6 cal 14C ka BP). Among these, des-A-lupane is the only saturated ring A-degraded compound, whilst mono- and di-unsaturated des-A-triterpenes are dominated by oleanane and ursane derivatives. The mono- and di-unsaturated des-A-triterpenes show a strong correlation (R2 0.94) in abundance, indicating that they may be derived from the same source or process. Des-A-lupane shows no correlation in abundance with the mono- or di-unsaturated des-A-triterpenes (R2 < 0.25) and may be derived from a different source or process in the catchment. The period with abundant aliphatic des-A-triterpenoids corresponds to the interval with relatively high sedimentation rate (> 0.3 mm per year). These data indicate that the abundance of aliphatic des-A-triterpenoids in the Dajiuhu peat deposit in the early Holocene may result from the prevailing reducing conditions in the water table or surficial sediment and/or rapid preservation of the early diagenetic products. Our study highlights the potential application of des-A-triterpenes in the reconstruction of palaeoenvironments based on peat sequences.  相似文献   

14.
At Lingfeng, (34°17'N, 104°08'E) on the northern slopes of the Qinling Mountains, a stratigraphic survey was made of a 21-m-profile of floodplain sediments on the watershed between the Chang Jiang (Yangtze River) and Huang He (Yellow River) at 2500 m above sea level. The sediments contain <2 m thick layers of peat and detritical plant matter that had been deposited on the floodplain of the Langouhe. If the radiometric dates are reliable, the chronology of the site spans some 80,000 years, covering an accumulation process from the terminal Last Interglacial up to <24 ka BP. The phase >73 ka BP had cool and wet conditions with a coniferous forest vegetation. Between 73 ka and > 40 ka colder and drier conditions were likely. The vegetation changed from aPicea-Pinus forest to aPicea-Abies-Larix forest with a later increase of non-arboreal pollen. Around 40 ka the climate was warm and moist. The accumulation of silt and organic matter as well as the development of a mixed coniferous forest with high rates of thermophilous trees(Quercus, Castanea) characterize the interstadial conditions at this time. Later on, the climate changed to cool and moist conditions, evidenced by silt and peat accumulation and a coniferous forest vegetation up to <24 ka BP. The loess in this area was deposited after 24 ka and erosion by the tributaries of Chang Jiang and Huang He to a depth of at least 40 m created the watershed between the two river systems.  相似文献   

15.
Carbon isotopic composition of Sphagnum macrofossils can potentially be used as a palaeohydrological tool for peat‐based climatic studies since a relationship between Sphagnum δ13C values and peatland surface moisture has been presented in previous studies. In order to verify this hypothesis, modern Sphagnum δ13C values were measured along a moisture (microtopographic) gradient in two boreal peat bogs. Isotopic measurements were performed on bulk material of S. fuscum, S. magellanicum, S. capillifolium and S. pulchrum. Isotopic variations found within and between Sphagnum species along the microtopographic gradient were compared using analysis of variance. A significant positive correlation (P < 0.0001) was found between Sphagnum δ13C values and their position along the surface moisture gradient. Results show that 13C‐depleted values are related to low water table depths (WTD), while 13C‐enriched values correspond to a water table that is close to the peat surface. Although the mechanisms underlying carbon fractionation processes in mosses are not well understood, we demonstrate that water resistance to CO2 diffusion is an important fractionation process that is observed in bulk Sphagnum δ13C measurements, since drier and wetter samples exhibit consistent and very different isotopic signatures. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

16.
A late glacial to early Holocene lacustrine and peat succession, rich in conifer remains and including some palaeolithic flint artefacts, has been investigated in the Palughetto intermorainic basin (Venetian Pre‐Alps). The geomorphological and stratigraphical relationships, 14C dates and pollen analyses allow a reconstruction of the environmental history of the basin and provide significant insights into the reforestation and peopling of the Pre‐Alps. The onset of peat accumulation is dated to 14.4–14.1 kyr cal. BP, coinciding with reforestation at middle altitudes that immediately post‐dates the immigration of Larix decidua and Picea abies subsp. europaea. Plant macrofossils point to the expansion of spruce about 14.3 kyr cal. BP, so far one of the earliest directly dated in the late glacial period of southern Europe. The previous hypothesis of an early Holocene spruce immigration in the Southern Alps from Slovenia needs reconsideration. Organic sedimentation stopped at the end of the Younger Dryas and was followed by the evolution of hydromorphic soils containing lithic artefacts, anthropic structures and wood charcoal. The typological features of the flint implements refer human occupation of the site to the end of the recent Epigravettian. Charcoals yielded dates either consistent with, or younger than, the archaeological chronology, in the early and middle Holocene. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

17.
We report two new eclogite localities (at Kanayamadani and Shinadani) in the high‐P (HP) metamorphic rocks of the Omi area in the western most region of Niigata Prefecture, Japan, which form part of the Hida Gaien Belt, and determine metamorphic conditions and pressure–temperature (P–T) paths. The metamorphic evolution of the eclogites is characterized by a tight hairpin‐shaped P–T path from prograde epidote–blueschist facies to peak eclogite facies and then retrograde blueschist facies. The prograde metamorphic stage is characterized by various amphibole (winchite, barroisite, glaucophane) inclusions in garnet, whereas the peak eclogite facies assemblage is characterized by omphacite, garnet, phengite and rutile. Peak P–T conditions of the eclogites were estimated to be ~600°C and up to 2.0 GPa by conventional cation‐exchange thermobarometry, Ti‐in‐zircon thermometry and quartz inclusion Raman barometry respectively. However, the Raman spectra of carbonaceous material thermometry of metapelites associated with the eclogites gave lower peak temperatures, possibly due to metamorphism at different conditions before being brought together during exhumation. The blueschist facies overprint following the peak of metamorphism is recognized by the abundance of glaucophane in the matrix. Zircon grains in blueschist facies metasedimentary samples from two localities adjacent to the eclogites have distinct oscillatory‐zoned cores and overgrowth rims. Laser ablation inductively coupled plasma mass spectrometry U–Pb ages of the detrital cores yield a wide range between 3,200 and 400 Ma, with a peak at 600–400 Ma. In the early Palaeozoic, proto‐Japan was located along the continental margin of the South China craton, providing the source of the older population of detrital zircon grains (3,200–600 Ma) deposited in the trench‐fill sediments. In addition, subduction‐related magmatism c. 500–400 Ma is recorded in the crust below proto‐Japan, which might have been the source for the younger detrital zircon grains. The peak metamorphic age was constrained by SHRIMP dating of the overgrowth rims, yielding Tournaisian ages of 347 ± 4 Ma, suggesting subduction in the early Carboniferous. Our results provide clear constraints on the initiation of subduction, accretion and the development of an arc‐trench system along the active continental margin of the South China craton and help to unravel the Palaeozoic tectonic history of proto‐Japan.  相似文献   

18.
Previous attempts to radiocarbon date sediments >10 kyr from the high rainfall region of Westland, New Zealand, a critical location for investigation of interhemispheric patterns of climate change, have been problematic. This study, building on recent work by Vandergoes and Prior ( 2003 ), shows that AMS 14C dating of pollen concentrates has potential to provide more reliable ages than other sediment constituents, including plant macrofossils. The method was applied to sediments from three sites containing the 22.6k 14C yr Kawakawa Tephra, which provided an independent test of the 14C ages. Although some minor laboratory contamination was detected in tests on background standards, the modelled relationship between sample mass and measured 14C content permitted an appropriate correction to be determined. Improved pollen concentrations derived by density separation between 1.4 and 1.2 specific gravity and sieving in the range 10–50 μm provided either older ages than other fractions of the same sample or, where in situ contamination was not evident, equivalent ages. Differences in degree of in situ contamination between depositional environments indicated that, in Westland, lake sites may be less susceptible to contamination by younger carbon than peat sites, where this process may be facilitated by root penetration into underlying sediments. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

19.
The South China Sea began to outspread in the Oligocene. A great quantity of terraneous detritus was deposited in the northern continental shelf of the sea, mostly in Pearl River Mouth Basin, which constituted the main paleo-Pearl River Delta. The delta developed for a long geological time and formed a superimposed area. Almost all the oil and gas fields of detrital rock reservoir distribute in this delta. Thirty-three oil sandstone core samples in the Zhujiang Formation, lower Miocene (23–16 Ma), were collected from nine wells. The illite samples with detrital K feldspar (Kfs) separated from these sandstone cores in four sub-structural belts were analysed by the high-precision 40Ar/39Ar laser stepwise heating technique. All 33 illite 40Ar/39Ar data consistently yielded gradually rising age spectra at the low-temperature steps until reaching age plateaus at mid-high temperature steps. The youngest ages corresponding to the beginning steps were interpreted as the hydrocarbon accumulation ages and the plateau ages in mid-high temperature steps as the contributions of the detrital feldspar representing the ages of the granitic parent rocks in the provenances. The ages of the detrital feldspar from the Zhujiang Formation in the four sub-structural belts were different: (1) the late Cretaceous ages in the Lufeng 13 fault structural belt; (2) the late Cretaceous and early Cretaceous-Jurassic ages in the Huizhou 21 buried hill-fault belt; (3) the Jurassic and Triassic ages in the Xijiang 24 buried hill-fault belt; and (4) the early Cretaceous – late Jurassic ages in the Panyu 4 oil area. These detrital feldspar 40Ar/39Ar ages become younger and younger from west to east, corresponding to the age distribution of the granites in the adjacent Guangdong Province, Southern China.  相似文献   

20.
Paleoecological records from two Holocene peat bogs in northern Germany are linked by two microscopic volcanic ash layers, correlated by petrology and geochemistry to explosive volcanism on Iceland. The younger “Microlite tephra” cannot be correlated to any known eruption, while the older tephra layer is identified as a deposit of the Hekla 3 eruption. The tephra layers are dated by an age–depth regression of accelerator mass spectrometry 14C ages that have been calibrated and combined in probability distributions. This procedure gives an age of 730–664 cal yr B.C. for the “Microlite tephra” event and 1087–1006 cal yr B.C. for the Hekla 3 event. Accordingly, the tephra layers were deposited during the late Bronze Age. At this time, human settlement slowly increased pressure on the environment, as indicated by changes in woodland pollen composition at the two bogs. The tephra-marker horizons further show that the palynologically defined transition from the Subboreal to the Subatlantic Period is synchronous in the investigated area. However, the macroscopic visible marker in peat, the change from fibrous to sapric peat, the “Schwarztorf-Weißtorf-Kontakt,” is asynchronous. Bog vegetation did not immediately react in unison to a climatic change at this pollen zone boundary; instead, the timing of vegetation change depended on the location within the bog.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号