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1.
Seismic stratigraphy, sedimentary facies, pollen stratigraphy, diatom-inferred salinity, stable isotope (δ18O and δ13C), and chemical composition (Sr/Ca and Mg/Ca) of authigenic carbonates from Moon Lake cores provide a congruent Holocene record of effective moisture for the eastern Northern Great Plains. Between 11,700 and 950014C yr B.P., the climate was cool and moist. A gradual decrease in effective moisture occurred between 9500 and 710014C yr B.P. A change at about 710014C yr B.P. inaugurated the most arid period during the Holocene. Between 7100 and 400014C yr B.P., three arid phases occurred at 6600–620014C yr B.P., 5400–520014C yr B.P., and 4800–460014C yr B.P. Effective moisture generally increased after 400014C yr B.P., but periods of low effective moisture occurred between 2900–280014C yr B.P. and 1200–80014C yr B.P. The data also suggest high climatic variability during the last few centuries. Despite the overall congruence, the biological (diatom), sedimentological, isotopic, and chemical proxies were occassionally out of phase. At these times the evaporative process was not the only control of lake-water chemical and isotopic composition.  相似文献   

2.
The Upper Doubs River Valley is a 910-km2watershed feeding into Lake Chaillexon. The lake was formed by a natural rockfall at the end of the Bølling Chronozone (around 14,250 cal yr B.P.) and since then has trapped material eroded from the watershed. The filling process and variations in sediment yield have been investigated by mechanical coring, seismic surveys, and electric soundings. The detrital sediment yield of the upstream watershed can be calculated by quantifying the sedimentary stocks for each climatic stage of the Late-Glacial period and Holocene Epoch and estimating the lake's entrapment capacity. This enables us to determine the intensity of the erosion processes in relation to climate and environmental factors. The Bølling–Allerød Interstade produced the greatest yields with mean values of 19,500 metric tons per calendar year (t/yr). The Younger Dryas Chronozone saw a sharp fall (8900 t/yr) that continued into the Preboreal (2100 t/yr). Clastic supply increased during the Boreal (4500 t/yr) before declining again in the Early Atlantic (2400 t/yr). Since then, yields have risen from 4500 t/yr in the Late Atlantic to 6800 t/yr in the Subboreal and 11,100 t/yr in the Subatlantic. Comparison of quantitative data with the qualitative analysis of the deposits and with the paleohydrologic curve of the watershed based on level fluctuations in lakes around Chaillexon shows that climate was the controlling factor of sediment yield until the Late Atlantic. From the Late Atlantic–Subboreal around 5400 cal yr B.P. (470014C yr B.P.) and especially from the end of the Subboreal Chronozone and during the Subatlantic Chronozone (2770 cal yr B.P./270014C yr B.P.–present) climatic constraints have been compounded by human activity related to forest clearing and land use.  相似文献   

3.
Historic Hg mining in the Cache Creek watershed in the Central California Coast Range has contributed to the downstream transport of Hg to the San Francisco Bay-Delta. Different aspects of Hg mobilization in soils, including pedogenesis, fluvial redistribution of sediment, volatilization and eolian transport were considered. The greatest soil concentrations (>30 mg Hg kg−1) in Cache Creek are associated with mineralized serpentinite, the host rock for Hg deposits. Upland soils with non-mineralized serpentine and sedimentary parent material also had elevated concentrations (0.9–3.7 mg Hg kg−1) relative to the average concentration in the region and throughout the conterminous United States (0.06 mg kg−1). Erosion of soil and destabilized rock and mobilization of tailings and calcines into surrounding streams have contributed to Hg-rich alluvial soil forming in wetlands and floodplains. The concentration of Hg in floodplain sediment shows sediment dispersion from low-order catchments (5.6–9.6 mg Hg kg−1 in Sulphur Creek; 0.5–61 mg Hg kg−1 in Davis Creek) to Cache Creek (0.1–0.4 mg Hg kg−1). These sediments, deposited onto the floodplain during high-flow storm events, yield elevated Hg concentrations (0.2–55 mg Hg kg−1) in alluvial soils in upland watersheds. Alluvial soils within the Cache Creek watershed accumulate Hg from upstream mining areas, with concentrations between 0.06 and 0.22 mg Hg kg−1 measured in soils 90 km downstream from Hg mining areas. Alluvial soils have accumulated Hg released through historic mining activities, remobilizing this Hg to streams as the soils erode.  相似文献   

4.
Isotope ratios of U and Pb were measured in two types of Mn nodules from the Cambrian Timna Formation, Israel. Type A nodules are mainly composed of pyrolusite and hollandite, with Mn, Ba, Pb and U concentrations of 30–60%, 0.2–2.5%, 0.2–1.0% and 500–3500 ppm, respectively, whereas type B nodules were formed by alteration of the former, and contain mainly coronadite, with Mn, Ba, Pb and U concentrations of 7–48%, 0.2–7%, 0.6–5% and 10–160 ppm, respectively. The isotopic composition of U and Pb was measured by MC-ICP-MS on Mn-rich solutions (up to 100 mg/L) without and with chromatographic separation. The values for the 207/206 and 208/206 ratios have been determined with precisions of up to 50 ppm and those of 206/204, 207/204 and 208/204 – up to 200 ppm. The values for the 234/238 ratios have been determined with precisions of 0.4–1%. The results of the separated and unseparated solutions were shown to be equal within the error. Thus there is no significant matrix effect while measuring U and Pb in Mn rich solution using the MC-ICP-MS.The isotopic composition of Pb and U support the distinction between the two types of Mn nodules. Type A nodules have a wide range of 206Pb/204Pb ratios (18.278–19.776), and an almost constant ratio of 208Pb/204Pb. In contrast, type B nodules have almost constant 206Pb/204Pb ratios and a wide range of 208Pb/204Pb ratios (37.986–38.079). Type A nodules form a linear array on a 207Pb/204Pb vs 206Pb/204Pb diagram, while type B nodules form a tight group characterized by lower Pb isotope ratios that slightly deviate from the type A array. The 234U/238U ratio differs between the two types of nodules; type A nodules exhibit a uniform and close to equilibrium 234U/238U ratio while type B nodules show a wide range of 234U/238U ratios above and below the equilibrium value. The isotopic composition of Pb in type A nodules might reflect Pb contributions from plutonic rock weathering, exposed at the time of deposition or later, to the Cambrian sea. These nodules have remained unaffected by processes that occurred since the Cambrian. The higher 208Pb/204Pb values of type B indicate that these nodules were formed from a Th-enriched solution probably during epigenetic processes which occurred also during the last 1 Ma.Thus the two isotopic systems of U and Pb can record formation, leaching and redeposition of Mn ores.  相似文献   

5.
Large glaciers descended western valleys of the Olympic Mountains six times during the last (Wisconsin) glaciation, terminating in the Pacific coastal lowlands. The glaciers constructed extensive landforms and thick stratigraphic sequences, which commonly contain wood and other organic detritus. The organic material, coupled with stratigraphic data, provides a detailed radiocarbon chronology of late Pleistocene ice-margin fluctuations. The early Wisconsin Lyman Rapids advance, which terminated prior to ca. 54,000 14C yr B.P., represented the most extensive ice cover. Subsequent glacier expansions included the Hoh Oxbow 1 advance, which commenced between ca. 42,000 and 35,000 14C yr B.P.; the Hoh Oxbow 2 advance, ca. 30,800 to 26,300 14C yr B.P.; the Hoh Oxbow 3 advance, ca. 22,000–19,300 14C yr B.P.; the Twin Creeks 1 advance, 19,100–18,300 14C yr B.P.; and the subsequent, undated Twin Creeks 2 advance. The Hoh Oxbow 2 advance represents the greatest ice extent of the last 50,000 yr, with the glacier extending 22 km further downvalley than during the Twin Creeks 1 advance, which is correlative with the global last glacial maximum. Local pollen data indicate intensified summer cooling during successive stadial events. Because ice extent was diminished during colder stadial events, precipitation—not summer temperature—influenced the magnitude of glaciation most strongly. Regional aridity, independently documented by extensive pollen evidence, limited ice extent during the last glacial maximum. The timing of glacier advances suggests causal links with North Atlantic Bond cycles and Heinrich events.  相似文献   

6.
Pollen and δ13CTOM data obtained from two contrasting lake sequences (Lakes Kamalété and Nguène), located 200 km apart in the lowland rainforest of Gabon, provide complementary local and regional 1500-yr records of high resolution (15–30 yr) vegetation change. A combination of aquatic, semi-aquatic and terrestrial pollen showed in both records that the tropical rainforest increased during periods of high rainfall and decreased during drought intervals. The strong fluctuations of water balance at decadal scale during the “Medieval Warm Period” ( 1100–800 cal yr BP) coincided with a noticeable increase in shade-intolerant taxa, indicating recurring rainforest canopy disturbance. The δ13CTOM signal showed high-amplitude variations in both records, which positively correlates with the rainforest dynamics and local vegetation changes. The similar trends in both the pollen and the δ13CTOM signals between these sites demonstrate the regional broadly synchronous timing of shifting hydrological conditions. The largely positive co-variation between strong fluctuations of hydrological conditions and changes in rainforest structure and composition indicate that regional climatic change is probably the driving force for major rainforest dynamics in Gabon. Any significant anthropogenic impact on vegetation has not been clearly identified, and this issue still needs to be resolved independently by obtaining detailed archeological records across the interval 1400–800 BP, which currently seem to be extremely rare or not easily available.  相似文献   

7.
Amino acid enantiomeric (D/L) ratios in the mollusk Mercenaria are compared with recently published biostratigraphic and/or U-series solitary coral data from 22 Quaternary localities on the central and southern Atlantic Coastal Plain. In all cases, local relative aminostratigraphic sequences are consistent with relative ages inferred from U-series or biostratigraphic data, although occasionally more depositional events are recognized by aminostratigraphic than biostratigraphic methods. However, if the U-series data are used as age calibrations for the D/L values, latitudinal trends of “isochronous” D/L values are highly variable and conflict with trends expected from the present temperature gradient, which is smooth and nearly linear between 45° and 25° N. Age estimation can be performed independently of the U-series data using a kinetic model that relies on the assumption that Pleistocene temperature gradients have also been smooth functions of latitude, although significantly steeper than the present temperature gradient. Within the uncertainties of this assumption, kinetic model age estimates for localities in the coastal plain fall into the following groups: 70,000–130,000 yr, 200–250,000 yr, 300,000–400,000 yr, 500,000–600,000 yr, 700,000–800,000 yr, and > 1,000,000 yr. Major conflicts between these model age estimates are observed for localities near Charleston, South Carolina and in central Virginia. These conflicts could indicate that the basic temperature assumptions of aminostratigraphy are incorrect, and that apparent local aminostratigraphic sequences (clusters of different D/L values) could be due to factors other than age difference. Alternatively, some of the U-series dates may be only minimum ages for these localities.  相似文献   

8.
The organic deposits derived from the mangrove swamps form reliable stratigraphic markers within the Late Quaternary sequence of Kerala–Konkan Basin. Three generations of such deposits have been identified. The older one is dated to around 43,000–40,000 14C yr B.P., with a few dates beyond the range of radiocarbon. The younger ones date from the Middle Holocene to latest Pleistocene (10,760–4540 14C yr B.P.) and the Late Holocene (<4000 14C yr B.P.). Pollen analyses confirm that the deposits are mostly derived from the mangrove vegetation. Peat accumulation during the period 40,000–28,000 14C yr B.P. can be correlated with the excess rainfall, 40–100% greater than modern values, of the Asian summer monsoon. The low occurrence of mangrove between 22,000 and 18,000 14C yr B.P. can be attributed to the prevailing aridity and/or reduced precipitation associated worldwide with Last Glacial Maximum, because exposure surfaces and ferruginous layers are commonly found in intervals representing this period. The high rainfall of 11,000–4000 14C yr B.P. is found to be the most significant as the mangrove reached an optimum growth around 11,000 14C yr B.P. but with periods of punctuated weaker monsoons. From the present and previous studies, it has been observed that after about 5000 or 4000 14C yr B.P., the monsoons became gradually reduced leading to drying up of many of the marginal marine mangrove ecosystems. A case study of Hadi profile provided an insight to the relevance of magnetic susceptibility (χ) to record the ecological shift in Late Holocene.  相似文献   

9.
The last ice sheet over the British Isles, together with other mid-latitude Pleistocene ice sheets, and in contrast to the modern ice sheets of Greenland and Antarctica, had a relatively low profile, low summit elevation and extensive, elongated lobes at its margin. A thermo-mechanically coupled numerical ice sheet model, driven by a proxy climate, has been used to explore the properties that would have permitted these characteristics to develop. The approach, the key to quantitative palaeoglaciology, is to determine the boundary conditions that permit the simulated ice sheet to mimic the evolution of the real ice sheet through the last glacial cycle. Simulations show how a British ice sheet may have been confluent with a Scandinavian ice sheet during some parts of its history and how unforced periodic and asynchronous oscillations could occur in different parts of its margins. Marginal lobes are a reflection of streaming within the ice sheet. Such streams can be ephemeral, dynamic streams located because of ice sheet properties, or fixed streams whose location is determined by bed properties. The simulations that best satisfy constraints of extent, elevation and relative sea levels are those with major fixed streams that strongly draw down the ice sheet surface. In these, the core upland areas of the ice sheet were cold based at the Last Glacial Maximum, basal streaming velocities were between 500 and 1000 ma−1 compared with surface velocities of 10–50 ma−1 in inter-stream zones, shear stresses were as low as 15–25 kPa in streams compared with 70–110 kPa in upland areas and 60–84% of the ice flux was delivered to the margin via streams.  相似文献   

10.
Two medium to low volatile bituminous rank coals in the Lower Cretaceous Gates Formation (Mannville equivalent), Inner Foothills of Alberta, were cored as part of a coalbed methane exploration program. The target seams (Seam 4 and Seam 10) were intersected at 652 m and 605 m, respectively. The coals were bright banded, relatively competent and reasonably cleated, with cleat spacing between 5–20 mm. The FMI (Formation Micro-Imaging) log identified two primary fracture directions, corresponding to both face and butt cleats, which were developed almost equally in some coal intervals. The amount of shearing was limited, in spite of the presence of numerous thrust faults and fold structures in the corehole vicinity. Total gas content was high, with an average of 17.7 cm3/g (arb; 568.1 scf/t). An adsorption isotherm of the thick Seam 4 showed gas saturation levels of 90% at in-situ reservoir conditions. Methane content was 92–96% and carbon dioxide levels were less than 2%. Isotopic studies on the methane confirmed the thermogenic origin of the gas, as anticipated based on the coal rank. The coal seams were fracture stimulated using 50/50 nitrogen and fresh water along with 9 to 12 tons of 12/20 mesh sand used as a proppant. It is believed that the coals were not stimulated properly because of the small proppant volume and the complex — and often unpredictable — fracture pattern in coals, particularly in the Inner Foothills region that has high stress anisotropy. An injectivity test showed coal absolute permeability to be less than 1 mD, the skin to be −  2 (indicating a slightly damaged coal) and water saturation in the cleats to be 90%. A four-month production test was conducted; gas rates declined from 930 to 310 m3/d (33 to 11 MCFD) and water rates were low (< 5 BWD). Produced water was saline (TDS was 20,000 mg/L) and high in chloride and bicarbonate ions. Production testing was followed by history matching and numerical simulation, which consisted of numerous vertical and horizontal well development scenarios and other parameters. Simulating multiple parallel horizontal wells in the Gates coals resulted in the highest peak gas production rates, cumulative production and recovery efficiencies, in agreement with public data from the Mannville coals in the deeper part of the Alberta Syncline. The positive effect of constructive interference in depressurizing the coal reservoirs and accelerating gas production over short periods of time was demonstrated. Coal quality data from a nearby underground mine shows that drilling horizontal wellbores in the Gates coals would be challenging because of unfavourable geomechanical properties, such as low cohesion and unconfined compressive strength values, and structural complexity.  相似文献   

11.
Groundwaters from crystalline and metamorphic rocks (hardrocks) and from Quaternary deposits, i.e., alluvial and glacial deposits (softrocks) from the counties of Nord-Trøndelag and Sør-Trøndelag were analyzed for major and minor elements and ions including fluoride. The median concentration of F– in water from the hardrock aquifers is 0.28 mg/l (14.7 eq/l) in contrast to water from softrock aquifers in which it is found to be 0.05 mg/1 (2.6 eq/l). More importantly, ca. 15% of the locations where water was abstracted from hardrock wells contain 1.5 mg/l (78.9 eq/l) F– or more. Thus, 15% of all hardrock wells returned F– results that are at or above the maximum recommended value for drinking water. Of the softrock wells, none are above 1 mg/l. Geologists would normally expect higher F–contents in groundwaters derived from acid rocks, e.g., in granitic or gneissic areas. When comparing the host lithology with the observed F–contents, however, no clear relationship between F– content and lithology is visible. The highest observed F– values actually occur in gneissic host rocks. However, wells drilled in amphibolites/greenstones, mica schists, calcareous rocks, and sedimentary rocks all returned some analytical results above 1.5 mg/l F–. These results suggest that all hardrock wells drilled should be tested for F– and the users informed about the results and advised to take any necessary precautions. When applying the recently proposed Norwegian drinking water limits to our data, 51% of all softrock well waters and 56% of all hardrock well waters are unfit for consumption without prior treatment, although we analyzed only for about half of the proposed elements/parameters. This result seriously questions the concept of fixed action levels—many of them with totally unproven health implications—for so many parameters/elements for hardrock groundwaters.  相似文献   

12.
Assimilation of crustal rocks with concomitant fractional crystallisation (AFC) is a well documented phenomenon in many igneous suites, but geochemical evidence from the Tertiary Mull lava succession suggests that in these magmas crustal contamination occurred by a distinctly different mechanism. Lavas from the lower half of the Mull Plateau group (MPG) can be divided into two broad sub-types; high (>8%) MgO basalts with elevated Ba and K; and lower MgO (<8%) basaltic-hawaiites with lower Ba and K. The lower crust and most of the upper crust beneath Mull is probably of Lewisian age. The Sr-, Nd-and Pb-isotope compositions of local Lewisian crustal samples yield the following ranges: 87Sr/86Sr=0.71002–0.72348, 143Nd/144Nd=0.51045–0.51058 and 206Pb/204Pb=14.0–14.6. Ten lavas have also been analysed and yield the following ranges: 87Sr/86Sr=0.7028–0.7042, 143Nd/144Nd=0.51214–0.51230 and 206Pb/204Pb=15.1–17.9. However, within this range, it is predominantly the more primitive mafic compositions, with elevated Mg, Ba and K, that show the lowest Nd- and Pb-, and the highest Sr-isotope values. Modelling of these isotopic results, in conjunction with major and trace element data, show that: (1) contamination by Lewisian lower crustal material does occur; (2) that the process involved was not one of assimilation with concomitant fractional crystallisation (AFC). The proposed contamination process is one whereby the hottest (most MgO rich) magmas have assimilated acidic partial melts of Lewisian lower crust during turbulent ascent (ATA) through thin, poorly connected dyke- and sill-like magma chambers. The chemical composition of the contaminated lavas can be modelled successfully through addition of 5% acidic Lewisian crust to an uncontaminated lava. In contrast, the more evolved magmas — which probably fractionated at sub-crustal levels — were either not hot enough to molt significant amounts of crust, or did not ascend turbulently because of their higher viscosity, and so are less contaminated with crust.  相似文献   

13.
Pollen in Quaternary deposits from the subtropical Hanjiang Delta records three major phases in the local vegetation and climate history during the last 55,000 yr: (1) a prevalent cool-to-temperate and humid climate at ca. 24,000 14C yr B.P. is indicated by abundant pollen of temperate trees including conifers; (2) between 20,000 and 15,000 14C yr B.P., a cold, dry environment was associated with low sea level during the last glaciation, leading to subaerial exposure, weathering, and interruption of sedimentation, as well as departure from the region of Dacrydium and Sonneratia; (3) a short-term expansion of grassland at ca. 10,300 14C yr B.P. reduced the predominant Lauraceae-Fagaceae evergreen forest, possibly corresponding to the Younger Dryas cooling. The combined data indicate a maximum sea-level rise in the mid-Holocene (7500–4000 14C yr B.P.) and a marine influence in the late Pleistocene at 45,000–20,000 14C yr B.P. The Holocene warming, however, did not bring back moisture-sensitive taxa, indicating high seasonal aridity probably caused by renewed monsoon conditions.  相似文献   

14.
Micaceous ultramafic dikes of Jurassic age from Picton and Varty Lake, Ontario, consist mineralogically of olivine — phlogopite — serpentine — calcite-spinel. The rocks are characterized by abundant Ba-rich phlogopite (up to 6.5 wt.% BaO) and spinels with a diagnostic kimberlite trend-1. Compositionally the dikes are characterized by extreme silica-undersaturation (21–30 wt.% SiO2), primitive Mg/(Mg + FeT) ratios (0.75–0.83), large enrichments of volatile components (CO2 and H2O), and relatively high abundances of both incompatible and compatible trace elements. The dikes exhibit pronounced enrichments of light rare earth elements (LREE) (LaN=320–1330) combined with strongly fractionated patterns (LaN/YbN=45–108). Calcite in the dikes is a primary magmatic phase, from textural relations and C-isotopic compositions ( 13C= –4.0 to –8.3). A calcite-rich aphanitic phase of the Picton dike is interpreted to be a late stage magmatic differentiate, which possibly involved olivine fractionation. Although the dikes lack most of the macrocrysts generally considered to be important diagnostic minerals of kimberlite magmatism, the geochemical, mineralogical and C- and O-isotopic characteristics collectively indicate that the dikes are evolved varieties of hypabyssal facies kimerlite.  相似文献   

15.
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   

16.
Marine black shales of the Lower Cambrian Niutitang Formation in southern China host Mo–Ni–platinum group elements (PGE) mineralization confined to a phosphate- and pyrite-rich stratiform body (max. 20-cm thick). The H/C atomic ratio, carbon isotopic composition, FTIR spectra of bulk organic matter, and spectra of extractable part of organic matter indicate similar sources and thermal evolution of organic matter in barren and mineralized black shales.The morphology and relative abundance of organic particles in barren and mineralized shales are different. In barren black shales, organic particles comprise only elongated bodies and laminae 2–10 μm across or elongated larger bodies (> 10 μm) with Rmax = 2.96–5.21% (Type I particles). Mineralized black shales contain Type I particles in rock matrix (90–95 vol%), small veinlets or irregular organic accumulations (Type II particles, 1–5 vol%) that display weak to well developed mosaic texture and a variable reflectance (Rmax = 3.55–8.65%), and small (< 1 to 5 μm) rounded or irregular Type III organic particles (1–4 vol%) distributed within phosphate nodules and sulphide rip-up clasts. Type III particles show similar reflectance as particles of Type I in rock matrix. Type I particles are interpreted as remnants of in situ bacterially reworked organic matter of cyanobacteria/algal type, Type II as solidified products or oil-derived material (migrabitumen), and Type III particles as remnants of original organic matter in phosphatized or sulphidized algal/microbial oncolite-like bodies. Equivalent vitrinite reflectances of Type I and III particles in barren and mineralized rocks are similar and correspond to semi-anthracite and anthracite. Micro-Raman spectra of organic particles in rocks display a wide belt in the area of 1600 cm− 1 (G belt) and approximately the same belt in the area of 1350 cm− 1 (D belt). The ratio of integrated areas of the two belts correlate with Rmax values.The Mo–Ni–PGE mineralized body is interpreted as to represent a remnant of phosphate- and sulphide-rich subaquatic hardground supplied with organic material derived from plankton and benthic communities as well as with algal/microbial oncolite-like bodies that originated in wave-agitated, shallow-water, nearshore environment.  相似文献   

17.
On the south-eastern edge of Russia, the chemical composition of rainwater is controlled by sea salts, terrestrial material, as well as volcanic (Kuril islands volcanic area) and anthropogenic emissions, mostly in the southern part of the area. The predominant major ions of the Primorye, Sakhalin and the Kuril Islands rainwaters were respectively HCO3−–SO42−, Ca–Na, and of Cl–Na. Concentration of trace elements changes within 1–2 orders of magnitude but some difference in the distribution of the elements between continental and island rainwater is found. The concentration of the chemical elements in the particulate fraction varies from < 10% to 90% of the total concentration (dissolved + particulate) with the following distribution: Tl, Na, Ca, Sr, Zn, Cd (< 10%)–Be, Th, Bi, Rb, U, K, Sc (10–20%)–Cu, Mn, Mg, Mo, Se, Ba, Ni, As, Ag, Cs, Co, Y, Ga, V (20–50%)–Sb, Pb, Ge, Cr, Fe, Al (50–90%).The concentration of elements of the particulate fraction of the rainwater usually is significantly different from concentrations in the crust, including both higher and lower concentrations. The terrestrial contribution to dissolved elements was evaluated and follows the decreasing order: Fe > K, Mg, Ca > Ba, Sr > Na (65–1%). Close order was found for total (dissolved and solid) concentrations. Sea salt contribution to dissolved element concentration in the rainwater decrease in the following order: Cl, Mg > K, SO4 > Ca > HCO3−, Ba, Fe (78–0.1%). Calculation of anthropogenic and volcanic inputs for two ions (Cl− and SO42−) shows that anthropogenic inputs for the Vladivostok and Yuzno-Sakhalinsk cities can be evaluated as 15–20% of Cl− and up to 80–90% of SO42−. Volcanic components in the Kuril Islands, where anthropogenic inputs are absent, can reach up to 76% of SO42− and 36% of Cl−.  相似文献   

18.
Stratigraphic analysis of alluvial/colluvial sequences and 14C dating have been used as proxies for Holocene climate changes in the highlands of Tigray (northern Ethiopia). The studied records show alternations of buried soils and peaty–clayey sediments, pointing to wet, stabilization phases, and organic-free colluvium layers resulting from the abrupt occurrence of dry-climate episodes. The 14C dates, mostly unpublished, cluster in the 11,090–9915, 9465–9135, 8450–7330, 6720–3635, 2710–2345, and 1265–790 cal yr B.P. time spans. Evidence of subsequent pedogenesis is lacking in the area, apart from a buried humified horizon dated at 300 ± 60 14C yr B.P. (460–295 cal yr B.P.). Both the timing and the pattern of Tigray paleoclimatic events fit the corresponding framework, based on lake level changes, previously implemented for the Main Rift Valley. These findings give further support for arguing that the forcing mechanisms of the wet/dry fluctuations during the Holocene were effective over a large scale.  相似文献   

19.
Granulite xenoliths within alkali olivine basalts of the Pali-Aike volcanic field, southern Chile, contain the mineral assemblage orthopyroxene + clinopyroxene + plagioclase + olivine + green spinel. These granulites are thought to be accidental inclusions of the lower crust incorporated in the mantle-derived basalt during its rise to the surface. Symplectic intergrowths of pyroxene and spinel developed between olivine and plagioclase imply that the reaction olivine+plagioclase = Al-orthopyroxene + Al-clinopyroxene + spinel (1) occurred during subsolidus cooling and recrystallization of a gabbroic protolith of the granulites.Examination of fluid inclusions in the granulites indicates the ubiquitous presence of an essentially pure CO2 fluid phase. Inclusions of three different parageneses have been recognized: Type I inclusions occur along exsolution lamellae in clinopyroxene and are thought to represent precipitation of structurally-bound C or CO2 during cooling of the gabbro. These are considered the most primary inclusions present. Type II inclusions occur as evenly distributed clusters not associated with any fractures. These inclusions probably represent entrapment of a free fluid phase during recrystallization of the host grains. IIa inclusions are found in granoblastic grains and have densities of 0.68–0.88 g/cm3. Higher density (=0.90–1.02 g/cm3) IIb inclusions occur only in symplectite phases. Secondary Type III CO2+glass inclusions with =0.47–0.78 g/cm3 occur along healed fractures where basalt has penetrated the xenoliths. Type III inclusions appear related to exsolution of CO2 from the host basalt during its ascent to the surface. These data suggest that CO2 is an important constituent of the lower crust under conditions of granulite facies metamorphism, indicated by Type I and II fluid inclusions, and of the mantle, as indicated by Type III inclusions.Correlation of fluid inclusion densities with P-T conditions calculated from both two-pyroxene geothermometry and reation (1) indicate emplacement of a gabbroic pluton at 1,200–1,300° C, 4–6 kb; cooling was accompanied by a slight increase in pressure due to crustal thickening, and symplectite formation occurred at 850±35° C, 5–7 kb. Capture of the xenoliths by the basalt resulted in heating of the granulites, and CO2 from the basalt was continuously entrapped by the xenoliths over the range 1,000–1,200° C, 4–6 kb. Examination of fluid inclusions of different generations can thus be used in conjunction with other petrologic data to place tight constraints on the specific P-T path followed by the granulite suite, in addition to indicating the nature of the fluid phase present at depth.  相似文献   

20.
The geochemistry of dissolved and suspended loads in river catchments of two low mountain ranges in Central Europe allows comparison of pertinent chemical weathering rates. Distinct differences in lithology, i.e. granites prevailing in the Black Forest compared to Palaeozoic sediments in the Rhenish Massif, provide the possibility to examine the influence of lithology on weathering. Here we determine the origin of river water using the stable isotope ratio δ18OH2O and we quantify the geogenic proportions of sulphate from stable isotope ratios δ34SSO4 and δ18OSO4. Particularly in catchments with abundant pyrite, determination of the geogenic amount of sulphate is important, since oxidation of pyrite leads to acidity, which increases weathering. Our results show that spatially averaged silicate weathering rates are higher for the river catchments Acher and Gutach in the Black Forest (10–12 t/km2/yr) compared to the river catchments of the Möhne dam and the Aabach dam in the Rhenish Massif (2–6 t/km2/yr). Correspondingly, the CO2 consumption by silicate weathering in the Black Forest (334–395 × 103 mol/km2/yr) is more than twice as high as in the Rhenish Massif (28–151 × 103 mol/km2/yr). These higher rates for watersheds of the Black Forest are likely due to steeper slopes leading to higher mechanical erosion with respective higher amounts of fresh unweathered rock particulates and due to the fact that the sediments in the Rhenish Massif have already passed through at least one erosion cycle. Carbonate weathering rates vary between 12 and 38 t/km2/yr in the catchments of the Rhenish Massif. The contribution of sulphuric acid to the silicate weathering is higher in the catchments of the Rhenish Massif (9–16%) than in the catchments of the Black Forest (5–7%) due to abundant pyrite in the sediments of the Rhenish Massif. Three times higher long-term erosion rates derived from cosmogenic nuclides compared to short-term erosion rates derived from river loads in Central Europe point to three times higher CO2 consumption during the past 103 to 104 years.  相似文献   

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