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1.
Late Quaternary glacier fluctuations in the Macha Khola valley (Gorkha Himal, Nepal) were reconstructed using relative and absolute dating techniques. Our results indicate that younger moraine complexes were left by Late Holocene (<1.7 cal. ka BP), mid-Holocene (ca 3 cal. ka BP), and Lateglacial (ca 13 cal. ka BP) ice advances. Older Late Quaternary glacier advances occurred during Marine Oxygen Isotope Stages (MIS) 2 and 3–4. No relics of Middle or Early Pleistocene glaciations could be found. During MIS 3–4, glaciers advanced down to an altitude of at least 2150 m a.s.l., corresponding to an ELA depression of approximately 1300 m. At about 3500 m a.s.l., the MIS 2 Macha Khola glacier reached almost the thickness of the former MIS 3–4 glacier and retreated some time before 17.9 cal. ka BP. The Lateglacial glacier advanced again several times to altitudes between 2450 and 3400 m a.s.l. The mid-Holocene glaciers extended much farther down-valley than the Late Holocene ones. Dendrochronological data of Abies spectabilis suggested several periods of unfavourable growth conditions especially at the beginning of the 19th (1820) and 20th (1905) centuries.  相似文献   

2.
Atle Nesje   《Quaternary Science Reviews》2009,28(21-22):2119-2136
During the early Holocene abrupt, decadal to centennial-scale climate variations caused significant glacier variations in Norway. Increased freshwater inflow to the North Atlantic and Arctic Oceans has been suggested as one of the most likely mechanisms to explain the abrupt and significant Lateglacial and early Holocene climatic events in NW Europe. The largest early Holocene glacier readvances occurred 11,200, 10,500, 10,100, 9700, 9200 and 8400–8000 cal. yr BP. The studied Norwegian glaciers apparently melted away at least once during the early/mid-Holocene. The period with the most contracted glaciers in Scandinavia was between 6600 and 6000 cal. yr BP. Subsequent to 6000 cal. yr BP the glaciers started to advance and the most extensive glaciers existed at about 5600, 4400, 3300, 2300, 1600 cal. yr BP, and during the ‘Little Ice Age’. Times with overall less glacier activity were apparently around 5000, 4000, 3000, 2000, and 1200 cal. yr BP. It has been proposed that several glacier advances occurred in Scandinavia (including northern Sweden) at 8500–7900, 7400–7200, 6300–6100, 5900–5800, 5600–5300, 5100–4800, 4600–4200, 3400–3200, 3000–2800, 2700–2000, 1900–1600, 1200–1000, and 700–200 cal. yr BP. Glaciers in northern Sweden probably reached their greatest ‘Little Ice Age’ extent between the 17th and the beginning of the 18th centuries. Evidence for early Holocene glacier advances in northern Scandinavia, however, has been questioned by more recent, multi-disciplinary studies. The early to mid-Holocene glacier episodes in northern Sweden may therefore be questioned.Most Norwegian glaciers attained their maximum ‘Little Ice Age’ extent during the mid-18th century. Cumulative glacier length variations in southern Norway, based on marginal moraines dated by lichenometry and historic evidence, show an overall retreat from the mid-18th century until the 1930s–40s. Subsequently, most Norwegian glaciers retreated significantly. Maritime outlet glaciers with short frontal time lags (<10–15 years) started to advance in the mid-1950s, whereas long outlet glaciers with longer frontal time lags (>15–20 years) continued their retreat to the 1970s and 1980s. However, maritime glaciers started to advance as a response to higher winter accumulation during the first part of the 1990s. After 2000 several of the observed glaciers have retreated remarkably fast (annual frontal retreat > 100 m) mainly due to high summer temperatures. The general glacier retreat during the early Holocene and the Neoglacial advances after 6000 cal. yr BP are in line with orbital forcing, due to the decrease of Northern Hemisphere summer solar insolation and the increase in winter insolation. In addition, regional weather modes, such as the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO), play a significant role with respect to decadal and multi-decadal climate variability.  相似文献   

3.
The global climate changings at the end of Pleistocene led to extinction of the typical representatives of Mammoth fauna–mammoth, woolly rhinoceros, wild horse, bison, muskox, cave lion, etc.–on the huge territories of Northern Eurasia. Undoubtedly the Mammoth fauna underwent pressure from the Upper Paleolithic Man, whose hunting activity also could play the role in decreasing the number of mammoths and other representatives of megafauna (large mammals). Archaeological data testify that the typical representatives of Mammoth fauna were the Man's hunting objects only till the end of the Pleistocene. Their bone remains are not usually found on the settlements of Mesolithic Man. Formerly it was supposed that the megafauna of ‘Mammoth complex’ was extinct by the beginning of Holocene. Nevertheless the latest data testify that the global extinction of the Mammoth fauna was sufficiently delayed in the north of Eastern Siberia. In the 1990s some radiocarbon data testified that the mammoths on the Wrangel Island existed for a long time during the Holocene from 8000 till 3700 y. BP. The present radiocarbon data show that wild horses inhabited the north of Eastern Siberia (the lower stream of the Enissey river, the Novosibirskie Islands, the East Siberian sea-shore) 3000–2000 y. BP. Musk-oxen lived on the Taimyr Peninsula and the Lena River delta about 3000 y. BP. Some bison remains from Eastern Siberia belong to the Holocene. The following circumstances could promote the process of preservation of the Mammoth fauna representatives. The cool and dry climate of this region promotes the maintenance of steppe associations – habitats of those mammals. The Late Paleolithic and Mesolithic settlements are not found in the Arctic zone of Eastern Siberia from the Taimyr Peninsula to a lower stream of the Yana River; they are very rare in the basins of the Indigirka and Kolyma Rivers. So, the small number of the Stone Age hunting tribes on the North of Eastern Siberia was another factor in the long-term preservation of some Mammoth fauna representatives.  相似文献   

4.
Palaeoenvironmental studies combining 14C dating, palaeobotanical and archaeological data provide information about the human reaction to Holocene environmental changes registered in the surroundings of Biržulis Lake in northwest Lithuania.Responding to water regression, early Mesolithic communities were established on the lower lake terraces, which were overgrown by predominantly birch and pine forest. The formation of a mixed forest with Ulmus (immigrated at 8100–7500 cal yr BC), Corylus (7600–7200 cal yr BC) and Alnus (7300–6900 cal yr BC) provided plenty of natural resources, which led to the increase in population during the late Mesolithic. The expansion of Tilia (6400–5900 cal yr BC) and Quercus (5900–5700 cal yr BC), as well as the subsequent flourishing of broad-leaved forest, provided inhabitants with suitable living conditions.The reduction of broad-leaved woodland and the expansion of Picea (4400–3700 cal yr BC), which suggest changing temperature and moisture conditions as well as increasing erosion activity, could have negatively influenced the early-middle Neolithic population, as evidenced by the partial abandoning of the land. The lowering of the water level and thinning of the forest structure possibly related to some dry episode, positively influenced late Neolithic groups, as intensive exploitation of the area, including the earliest attempts at agriculture, has been registered. Since 1770–1490 cal yr BC, when intensive bogging began, evidence of periodic inhabitation around the lake has been registered.  相似文献   

5.
The Menderes Massif experienced polyphase deformation, but distinguishing Pan-African events from Alpine tectono-metamorphic evolution, and discriminating Eocene–Oligocene shortening from recent extension remain controversial. To address this, monazite in garnet-bearing rocks from the massifs Gordes, Central, and Cine sections were dated in thin section (in situ) using the Th–Pb ion microprobe method. Cambro–Ordovician monazite inclusions in Cine and Central Menderes Massif garnets are ~450 m.y. older than matrix grains. Monazites in reaction with allanite from the Kuzey Detachment, which bounds the northern edge of the Central Menderes Massif, are 17±5 Ma and 4.5±1.0 Ma. The Pliocene result shows that dating of monazite can record the time of extension. The Kuzey Detachment might have exhumed rocks a lateral distance of ~53 km at a rapid rate of ~12 mm/year assuming the present ~20° ramp dip, Pliocene monazite crystallization at ~450°C, and a geothermal gradient of ~25°C/km. Assuming an angle of 60°, the rate decreases to ~5 mm/year, with the detachment surface at ~21 km depth in the Pliocene. Two Gordes Massif monazites show a similar allanite–monazite reaction relationship and are 29.6±1.1 Ma and 27.9±1.0 Ma, suggesting that the Cenozoic extension in the Gordes Massif, and possibly the entire Menderes Massif, might have begun in the Late Oligocene.  相似文献   

6.
The Wolyu mine is one of the largest vein-type gold-silver-bearing epithermal systems in the Youngdong district and is the first gold-silver deposit in Korea found to contain significant germanium, in the form of argyrodite (Ag8GeS6). Mineralized veins (78.9 ± 1.2 Ma) crosscutting Late Cretaceous hostrock tuff and quartz porphyry (81.5 ± 1.8 Ma) consist of three stages of quartz and carbonates, the first of which contains pyrite, basemetal sulfides and Au-Ag-minerals. Stage I Au-Ag-Ge-mineralized veins show a systematic variation of mineral assemblage with time: (1) quartz + pyrite; (2) quartz + pyrite + sphalerite + electrum + argentite; (3) carbonate + quartz + sphalerite + electrum + argentite; (4) carbonate + native silver + argentite + Ag-sulfosalts + argyrodite + sphalerite. Calculated values of temperature and sulfur activity are: assemblage (1), 360-280°C and 10–7-10–10; (2), 280-210°C and 10–10-10–14; (3), 210-180°C and 10–14-10–16; (4), 180-155°C and 10–17-10–18. These data, the frequent association of gold with sulfides, and the abundance of pyrite in alteration zones indicate that decreasing sulfur activity and cooling were important in triggering gold deposition. Hydrogen and oxygen isotope compositions of ore fluids display a systematic variation with increasing time. Within the main Ag-Au-Ge mineralization, D and 18O values decrease with the transition from quartz to carbonate deposition (from -78 and –2.8% to –90 and –8.7%., respectively), indicating increasing involvement (mixing) of less evolved meteoric water which resulted in progressive cooling and dilution of ore fluids in the shallow ( 370–600 m) Wolyu epithermal system.  相似文献   

7.
Studies were conducted on 16 sections of buried organic matter (pre-Ancylus Lake and pre-Litorina Sea) and associated Stone Age cultural layers in the Pärnu area, southwestern Estonia. Buried organic beds are each part of a sedimentary sequence, which is repeated, forming two overlying sets of an orderly succession of five layers. The organic sedimentation of the lower set (set 1) occurred about 10,800–10,200 years BP, and that of the upper set (set 2) about 9450–7800 years BP. Associated with set 1 is the Early-Mesolithic settlement of Pulli, and with set 2 are the Stone Age cultural layers at Sindi-Lodja. The Early- and Middle-Mesolithic sites in Estonia are concentrated on shores of rivers and lakes to utilise of a variability of resources. The hunters and fishermen followed the ancient Pärnu River downstream to the receding shoreline of the Yoldia Sea. After about 10,700 years BP, they were forced to retreat inland in front of the transgressive Ancylus Lake shore which first inundated the Paikuse area about 10,400 years BP, and Pulli and higher sites about 10,200 years BP. The total amplitude of the transgression preceded 11 m and reached up to 14 m a.s.l. in the area. The Litorina Sea transgression reached 7 m a.s.l. after 8000–7800 years BP. The Mesolithic, Neolithic and modern sites on top of each other in the Pärnu area may suggest that, although years apart, they were inhabited by the same group of people who stayed in the area and moved back and forth together with the shifting shoreline of the Baltic Sea.  相似文献   

8.
A combined analytical and numerical evaluation of the uncertainties in P-T paths is made for three assemblages that propagates errors in the parameters: initial pressure, initial temperature, initial composition, change of composition of monitor parameters, endmember entropy, and endmember volume. Propagated errors along an isobaric heating path (T=77°C) for assemblage 1 (Grt-Bt-Pl-Qtz-Ms-Chl-H2O), using as monitor parameters the mole fractions of almandine, spessartine, grossular, and anorthite, and ignoring uncertainties in thermodynamic properties, are approximately ±320 bars (1) and ±8.3°C (1) if rim compositional uncertainties of 5% in major cations are assumed, or ±2.5°C (1) and ±50 bars (1) if electron microprobe analytical uncertainties are assumed for compositions. The largest source of uncertainty is from the errors in the monitor parameters, and P-T path uncertainties can depend critically on which monitor parameters are used. If the mole fraction of annite is used as a monitor parameter in place of the anorthite content of plagioclase, then propagated uncertainties are worse than ±29°C (1) and ±5800 bars (1). P-T path uncertainties also depend on assemblage. 1 precisions in assemblages 2 (Grt–Bt–Pl–Qtz–Ms–Sil–H2O) and 3 (Grt–Bt–Qtz–Kfs–Sil–H2O) using as monitor parameters the mole fractions of almandine, spessartine, grossular, and anorthite are calculated to be ±267 bars and ±43.4°C and ±372 bars and ±8.2°C respectively. Estimates of the accuracies in P-T paths that include potential errors in endmember entropy of ±1 J/mol·K and in endmember volume of ±1 cm3/mol are: ±324 bars and ±8.5°C (assemblage 1), ±341 bars and ±48.6°C (assemblage 2), and ±388 bars and ±9.7°C (assemblage 3). Use of different garnet, plagioclase, and muscovite activity models can change the length of a P-T path by as much as 15%, but does not typically change directions in P-T space significantly. Models that incorporate changes of fluid composition shorten P-T paths in assemblages 1 and 3 but do not change trajectories significantly. Assemblage 2 is virtually unaffected by fluid phase models. For the mineral assemblages considered here and using appropriate monitor parameters, propagated errors are small compared to the total path length, suggesting that the differential thermodynamic approach is a precise and accurate method for determining amounts of heating or thickening during metamorphism, and hence for interpreting orogenic processes.  相似文献   

9.
A total of 400 samples (33 sites) were collected from the earliest Cretaceous to early Late Cretaceous sandstones of the Khorat Group in the Indochina block for paleomagnetic study to unravel the tectonic evolution of the region. The sites were adopted from 3 traverses located in the northern edge of the Khorat Plateau, northeastern Thailand. Results indicate that almost all the sandstones exhibit similar magnetic values with an average declination (D) = 31.7°, inclination (I) = 30.3°, λ = 59.7°,  = 190.9°, K = 54.4, and A95 = 3.7 at reference point 17°30′N and 103°30′E. The calculated paleolatitude points are inferred to deviate from the present latitude point by 1.2 ± 2.3°. Only the lowermost part of the Cretaceous sandstones can pass a positive fold test at 95% confidence level. The relationship between the virtual geomagnetic poles (VGPs) of Cretaceous rocks of the Indochina plate in Thailand and those of the South China plate advocate that there is a major displacement of Indochina along the northwest-trending Red River and associated faults by about 950 ± 150 km with a 16.0–17.0° clockwise rotation relative to the South China plate during earliest Cretaceous times. Paleomagnetic results of the early Late Cretaceous Indochina plate point to a 20–25° clockwise rotation relative to the present occurring since very Late Cretaceous (65 Myrs)–Early Neogene times which may be due to the collision between India and Asia.  相似文献   

10.
The first laser-induced 40Ar-39Ar incremental-heating mineral age data for the Timok Massif of the Banatitic Magmatic and Metallogenetic Belt (BMMB) demonstrate that the main mineralization stage at the giant Majdanpek Cu–Au(–Mo) porphyry deposit took place at 83.6–84.0±0.6 Ma (2), coinciding with the later, 83±1–89.0±0.6 Ma, stages of the first cycle of andesitic volcanism. The earliest-Campanian age for hydrothermal activity precludes an association with the later history of the BMMB, and discounts the metallogenetic role of Laramide plutonism for the dated deposits.Editorial handling: A. Cheilletz  相似文献   

11.
辽宁海城小孤山遗址是我国北方地区重要的更新世末期-全新世人类遗址,记载着从更新世末期到全新世人类的体质及文化特征的演化历史。对1983年在此遗址发现的距今约5000年左右的、新石器时代晚期的人类头骨进行了研究,该头骨是迄今在辽宁地区发现的惟一较为完整的新石器人类遗骸,为探索早期人类体质特征的变异提供了重要的研究材料。研究显示,小孤山人类遗骸为30岁左右的男性个体。通过与更新世晚期、新石器时代及青铜铁器时代人类头骨的比较,发现小孤山人具有蒙古人种的形态特点,大部分特征与现代人相同,少数特征如颅长和颅容量,位于现代人变异范围的上限,而与更新世晚期人类接近。小孤山人非常低矮的上面部和鼻部,使其具有独特的头骨形态特点而差异于其他全新世头骨。15组头骨样本的主成分分析结果显示,新石器时代人类体质特征的变异非常大,难以将小孤山人的体质类型归入南方或北方类型,提示有必要对我国古代人类体质特征的分型问题作进一步探讨。  相似文献   

12.
A Rb-Sr analysis of suites of samples from a small intrusion of cordierite-bearing alkali granite into the peridotite of the Sierra Bermeja (Serrania de Ronda) yields an age of 22± 4 Ma ( = 1.42×10–11 a–1): Late Oligocene/Early Miocene. It is believed that the intrusion was derived from contact-anatectic melts produced along the hot ultramafic mass during and/or directly following its tangential, tectonic dislocation from a mantle diapir. Its age can thus be taken as dating the termination of the hot emplacement of the ultramafic masses. K-Ar dates of biotites and Rb-Sr dates of biotite/whole-rock pairs in contact-metamorphic wall rocks along the ultramafics mostly lie between 19.5 and 18.5 Ma. This probably indicates that about 19 Ma ago the contact-zones of the ultramafic masses had cooled down to the blocking temperature of biotite to Rb-Sr and K-Ar.  相似文献   

13.
The Archean Eye Dashwa Lakes pluton (2672±24 Ma) has domains of mineralogically fresh isotropic granite, domains that have undergone bulk hydrothermal alteration, and at least eleven sets of sequential fracture arrays, each with distinctive mineral assemblages. Fresh granite is characterized by whole rock 18O=8.1 to 8.6 and primary magmatic quartz-feldspar (+1.3), quartz-biotite (5.2 to 5.4) and quartz-magnetite (+9.8) fractionations. Magmatic fluids had a calculated isotopic composition of 18O=7.9±0.5, and D=–80±5. These isotropic volumes of the granite have not experienced significant incursion of external thermal waters. Pegmatites, quartz-molybdenite veins, and phlogopite-muscovite coated fractures are sporadically distributed in the granite, and were precipitated from high-temperature magmatic fluids where 18O=8.0 to 10.3 and D=–80±5.The most abundant variety of fracture filling assemblage is epidote-quartz-chlorite±muscovite: fractures are bounded by domains of mineralogically similar bulk hydrothermal alteration of the granite. These minerals formed at 160 to 280° C, in the presence of NaCl, and NaCl-MgCl2 brines (up to 25 wt% NaCl equivalent) of probable evolved marine water origin ( 18O=+0.4 to +3.8, D=–10 to –35) undergoing transient boiling. Upper plateau 40Ar/39Ar ages for the muscovite are 2650±15 Ma. Sequentially in the chronology of fracture-infiltration events, calcite-fluorite veins were deposited from boiling fluids at 340 to 390° C, isotopically characterized by 18O=4.7 and 13C=–5; and rare prehnite-chlorite lined fractures formed at 250 to 290° C. A generation of adularia-bearing veins precipitated at 140 to 230° C, from CaCl2-NaCl brines, where 18O=0 to –6.5 and D=–10 to –30. Incremental 40Ar/39Ar age spectra on the K-feldspar yield an upper plateau of 1100 Ma. Subsequently, hematite developed during reactivation of earlier fractures, at 140 to 210° C in the presence of fluids characterized by 18O=–0.4 to –5.4 and D=–15 to –25. Arrays of open fractures partially occupied by gypsum and goethite reflect a fluid infiltration event at temperatures <50° C. Many of the earlier generations of fracture minerals have transgranular fracture infillings which record the presence of low temperature (88–190° C), hypersaline CaCl2-NaCl brines. Narrow fractures lined with clays±calcite are sites for seepage of modern ground-waters. The isotopic signature of clay ( 18O=12 to 20, D=–80±5) plots near the line for modern kaolinites, confirming its formation in the presence of recent surface waters. Calcites coexisting with the clay minerals, and in fractured pegmatite show a common isotopic signature ( 18O=23±0.5, 13C=–13.6), indicating precipitation from modern groundwaters, where reactivated fractures have acted as conduits for infiltration of surface waters to depths of 200 m. Intermittent fracture-infiltration has occurred over 2.7 Ga. The early sequences of fracture-related fluid flow are interpreted in terms of devolatilization of the granite, followed by thermal contraction fracturing, incursion of marine water and convective cooling in the Archean. Hematite and adularia fracture fillings correspond to a stage when meteoric water infiltrated the volcanicplutonic terrain during Proterozoic and later times. Episodic fracture-fluid expulsion events may have been driven by seismic pumping, in response to magmatically and tectonically induced stresses within the Shield, with surface waters penetrating to depths of 15 km in the crust.  相似文献   

14.
The Pillara Zn–Pb deposit is the largest of several known Mississippi Valley-type (MVT) deposits in the Lennard Shelf of the Canning Basin. Paleomagnetic and rock magnetic measurements are reported for 294 specimens from 23 sites in mineralization and its carbonate host rocks from the deposit as well as on 15 artificial specimens of zinc and lead concentrate and of tailings. Pyrrhotite carries the characteristic remanent magnetization (ChRM) in nearly all specimens. The ChRM postdates most faulting as shown by breccia tests and most minor regional tilting as shown by the degraded fit on tilt correction. The mean ChRM direction for all sites is D=20.6°, I=–27.5° (N=23, 95=5.3°, k=34.1), yielding an age of 358±5 Ma (2) that is similar to the comparable age of 354±8 Ma (2) for the Kapok MVT deposit. Host rock diagenesis with attendant secondary remagnetization yields an age of 361±5 Ma (1) and the MVT mineralization with a primary chemical remanent magnetization gives an age of 356±3 Ma (1), co-eval with a published Rb–Sr sphalerite age of 357±3 Ma. Interpretation of this temporal data suggests that the MVT deposits of the southeastern Lennard Shelf originated during extension, probably in response to rift-related topography-driven fluid flow.Editorial handling: C. Brauhart  相似文献   

15.
The Glen Eden Mo-Sn-W deposit in north-eastern New South Wales, Australia, is an example of a leucogranite-related, low-grade, large-tonnage hydrothermal system. It occurs in the southern part of the New England Orogen and is hosted within Permian felsic volcanic rocks, intruded at depth by dykes of porphyritic microleucogranite (Glen Eden Granite). The deposit is hosted within a pipe-like quartz-rich greisen breccia body about 500 m in diameter, surrounded by a greisen zone several hundred metres across, zoning out into altered volcanic rocks. The dominant ore minerals, largely hosted as open space fillings and disseminations in quartz and quartz-rich greisen, are molybdenite, wolframite and cassiterite; they are accompanied by minor to trace amounts of muscovite, fluorite, topaz, siderite, pyrrhotite, arsenopyrite, chalcopyrite, sphalerite, bismuth, bismuthinite, joseite A, cosalite, galenobismutite, beryl, anatase and late-stage dickite and kaolinite. Two types of breccia are recognised: (1) greisenised volcanic rock fragments (quartz + muscovite), cemented by hydrothermal quartz ± K-feldspar ± ore minerals, and (2) fragments of hydrothermal quartz ± cassiterite ± wolframite enclosed in quartz ± clay. In both types of breccia and in stockwork veins, there is evidence of early precipitation of Mo-Sn-W phases, followed by Bi minerals and base metal sulfides (± fluorite, siderite).Breccia formation and associated hydrothermal alteration (greisen, potassic, argillic, propylitic) are interpreted to be related to devolatilisation of the highly fractionated Glen Eden Granite of early Triassic age (240±1 Ma based on 40Ar/39Ar geochronology of greisen muscovite) as well as to fluid mixing with meteoric waters. The breccia pipe could have formed in part by rock dissolution and collapse, as well as by explosive degassing of boiling fluids. Fluid inclusion evidence is consistent with boiling, with breccia pipe formation and mineralisation having mainly occurred at 250–350 °C from fluids with salinity of 0.4–9 wt% NaCl equivalent in the dilute types and 30–47 wt% NaCl equivalent in the hypersaline types. Stable isotopic evidence (O, D, C, S) indicates a strong magmatic contribution to the hydrothermal fluids and metals in the breccia. The 18O values of quartz decrease outward from the breccia pipe (10.6–12.3 in the pipe to 3.4–8.7 in the peripheral quartz) indicating that there has been mixing with isotopically light (high latitude) meteoric fluids, mainly after formation of the breccia pipe.  相似文献   

16.
We explore the hypothesis that the abrupt drainage of Laurentide lakes and associated rapid switch of the North Atlantic thermohaline circulation 8200 yr ago had a catastrophic influence on Neolithic civilisation in large parts of southeastern Europe, Anatolia, Cyprus, and the Near East. The event at 8200 cal yr BP is observed in a large number of high-resolution climate proxies in the Northern Hemisphere, and in many cases corresponds to markedly cold and arid conditions. We identify the relevant archaeological levels of major Neolithic settlements in Central Anatolia, Cyprus, Greece and Bulgaria, and examine published stratigraphic, architectural, cultural and geoarchaeological studies for these sites. The specific archaeological events and processes we observe at a number of these sites during the study interval 8400–8000 cal yr BP lead us to refine some previously established Neolithisation models. The introduction of farming to South-East Europe occurs in all study regions (Thrace, Macedonia, Thessaly, Bulgaria) near 8200 cal yr BP. We observe major disruptions of Neolithic cultures in the Levant, North Syria, South-East Anatolia, Central Anatolia and Cyprus, at the same time. We conclude that the 8200 cal yr BP aridity event triggered the spread of early farmers, by different routes, out of West Asia and the Near East into Greece and Bulgaria.  相似文献   

17.
A lake sediment core spanning 9900 years, collected from a small lake on western Victoria Island, NWT, Canada, provides a high-resolution record of diatom community dynamics over the Holocene. Ten radiocarbon dates and 210Pb dating provided the core chronology. Loss-on-ignition (LOI), magnetic susceptibility, biogenic silica content, and diatom concentrations provided information on changes in the sedimentary environment. LOI gradually increased over the Holocene whereas magnetic susceptibility showed an inverse trend. Biogenic silica content showed three distinct peaks spaced approximately 3000 years apart. Major shifts in diatom assemblages occurred at 8100–8000, 5800–5700, and 3800–3500 cal yr BP. There is evidence of diatom community response to centennial-scale variations such as the ‘Medieval Warm Period’ (1000–700 cal yr BP), ‘Little Ice Age’ (800–150 cal yr BP) and recent warming. Although recent changes in diatom community composition, productivity, and species richness are apparent they were surpassed at other periods throughout the Holocene. Variations of the taxa within the genera Staurosira, Pseudostaurosira, Fragilaria, and Staurosirella, usually combined into one genus in Arctic lake sediment studies, suggest these taxa provide useful insight into paleonvironmental questions.  相似文献   

18.
High-grade iron mineralisation (>65%Fe) in the North Deposit occurs as an E-W trending synclinal sheet within banded iron formation (BIF) of the Early Proterozoic Dales Gorge Member and consists of martite-microplaty hematite ore. Three hypogene alteration zones between unmineralised BIF and high-grade iron ore are observed: (1) distal magnetite-siderite-iron silicate, (2) intermediate hematite-ankerite-magnetite, and (3) proximal martite-microplaty hematite-apatite alteration zones. Fluid inclusions trapped in ankerite within ankerite-hematite veins in the hematite-ankerite-magnetite alteration zone revealed mostly H2O–CaCl2 pseudosecondary and secondary inclusions with salinities of 23.9±1.5 (1, n=38) and 24.4±1.5 (1, n=66) eq.wt.% CaCl2, respectively. Pseudosecondary inclusions homogenised at 253±59.9°C (1, n=34) and secondary inclusions at 117±10.0°C (1, n=66). The decrepitation of pseudosecondary inclusions above 350°C suggests that their trapping temperatures are likely to be higher (i.e. 400°C). Hypogene siderite and ankerite from magnetite-siderite-iron silicate and hematite-ankerite-magnetite alteration zones have similar oxygen isotope compositions, but increasingly enriched carbon isotopes from magnetite-siderite-iron silicate alteration (–8.8±0.7, 1, n=17) to hematite-ankerite-magnetite alteration zones (–4.9±2.2, 1, n=17) when compared to the dolomite in the Wittenoom Formation (0.9±0.7, 1, n=15) that underlies the deposit. A two-stage hydrothermal-supergene model is proposed for the formation of the North Deposit. Early 1a hypogene alteration involved the upward movement of hydrothermal, CaCl2-rich brines (150–250°C), likely from the carbonate-rich Wittenoom Formation (13C signature of 0.9±0.7, 1, n=15), within large-scale folds of the Dales Gorge Member. Fluid rock reactions transformed unmineralised BIF to magnetite siderite-iron silicate BIF, with subsequent desilicification of the chert bands. Stage 1b hypogene alteration is characterised by an increase in temperature (possibly to 400°C), depleted 13C signature of –4.9±2.2 (1, n=17), and the formation of hematite-ankerite-magnetite alteration and finally the crystallisation of microplaty hematite. Late Stage 1c hypogene alteration involved the interaction of low temperature (~120°C) basinal brines with the hematite-ankerite-magnetite hydrothermal assemblage leaving a porous martite-microplaty hematite-apatite mineral assemblage. Stage 2 supergene enrichment in the Tertiary resulted in the removal of residual ankerite and apatite and the weathering of the shale bands to clay.Editorial handling: B. Lehmann  相似文献   

19.
The Trinity peridotite was emplaced over metabasalts and metasedimentary rocks of the central metamorphic belt along the Devonian Trinity thrust zone. Three metamorphic events can be recognized in the Trinity peridotite: (1) antigorite (D= –63 to –65%.) formation related to regional underthrusting of the central metamorphic belt; (2) contact metamorphism associated with Mesozoic dioritic plutons; and (3) late-stage formation of lizardite ± brucite and chrysotile (D= –127 to –175%.) due to infiltration of meteoric waters. Abundant relict phases indicate incomplete reactions and strongly suggest that the availability of H2O was a controlling factor during serpentinization.Antigorite (event 1) formed as a result of infiltration into the Trinity peridotite of mixed H2O-CO2 fluids derived from the underlying central metamorphic belt. Foliation defined by magnetite veins and shear zones within antigorite serpentinites are subparallel to the Trinity thrust. The assemblage Fo + Atg + Chl + Mag ± Tr ± Carb reflects partial hydration of peridotite at 425–570° C. Talc-rich serpentinite formed along the thrust as a result of the infiltration of silica-bearing fluids. Metasomatic mass-balance calculations based on silica solubilities and the extent of antigorite serpentinization suggest that 80–175 volumes of fluid have passed through a given volume of original peridotite at the Trinity thrust.The Trinity thrust probably represents a Devonian subduction zone. Thermodynamic calculations suggest that hydration reactions account for 30–35% of the total heat released by the cooling Trinity peridotite. By analogy, similar hydration reactions are to be expected in the overlying mantle wedge of a subduction zone which act to retard cooling of the hanging wall, just as dehydration reactions delay heating of the downgoing slab. Metasomatic zones formed in peridotite at the Trinity thrust may reflect similar metasomatic processes to those proposed to occur in the mantle wedge above a subducting slab.  相似文献   

20.
High resolution pollen, plant macrofossil, charcoal, mineral magnetic and sedimentary analyses, combined with AMS 14C measurements, were performed on multiple sediment sequences along a transect through the former crater lake Preluca iganului in northwestern Romania in order to reconstruct the climatic and environmental changes during the early part of the Last Termination. Lake sediments started to accumulate at 14,700 cal yr BP. Initially the upland vegetation consisted of an open forest with mainly Betula and Salix and few Pinus sp., but from 14,500 cal yr BP onwards, Pinus mugo, P. sylvestris and Populus and later on also Larix became established around the lake. Between 14,150 and 13,950 cal yr BP, Pinus cembra seems to have replaced P. mugo and P. sylvestris. At 13,950 cal yr BP the tree cover increased and Picea appeared for the first time, together with Pinus cembra, P. mugo and Larix. From 13,750 cal yr BP onwards, a Picea forest developed around the site. Based on the combined proxy data the following climatic development may be inferred: 14,700–14,500 cal yr BP, cooler and wet/humid; 14,500–14,400 cal yr BP: gradually warmer temperatures, wet/humid with dry summers; 14,400–14,320 cal yr BP: warm and dry; 14,320–14,150 cal yr BP: cooler and wet/humid; 14,150–14,100 cal yr BP: warm and dry; 14,100–13,850 cal yr BP: warmer and wet/humid; <13,850 cal yr BP: warm and dry. The tentative correlation of this development with the North Atlantic region assumes that the period >14,700 cal yr could correspond to GS-2a, the time span between 14,700 and 14,320 to GI-1e, the phase between 14,320 and 14,150 cal yr BP to GI-1d and the time frame between 14,150 and 13,600 cal yr BP to the lower part of GI-1c.  相似文献   

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