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.
《Earth》2007,80(1-2):75-109
The soil's resistance to concentrated flow erosion is an important factor for predicting rill and (ephemeral) gully erosion rates. While it is often treated as a calibration parameter in process-based soil erosion models, global change studies require the estimation of erosion resistance from measurable soil properties. Several laboratory and field experiments have been conducted to determine the erosion resistance of various types of soils, but no attempts have been made hitherto to summarize all these data and to explore them for general trends. In this study, all available data on the resistance of topsoils to concentrated flow erosion in terms of channel erodibility (Kc) and critical shear stress (τcr) has been collected together with all soil and environmental properties reported in literature to affect the soil erosion resistance. Reported Kc values for cropland topsoils range between 0.002 10− 3 s m− 1 and 250 10− 3 s m− 1 (n = 470), whereas τcr values range between 0 and 15 Pa (n = 522). It is demonstrated that so far, the heterogeneity of measurement methods, the lack of standardized definitions and the shortcomings of the flow shear stress model hamper the comparability of soil erosion resistance values from different datasets. Nevertheless, combining Kc and τcr data from different datasets, a general soil erosion resistance ranking for different soil textures can be proposed. The compiled dataset also reveals that tillage practices clearly affect Kc (Kc for conventional tillage > Kc for reduced tillage > Kc for no tillage) but not τcr.It was concluded that Kc and τcr are not related to each other and that soil and macro-environmental properties affecting the foremost do not necessarily affect the latter as well and vise versa. Often Kc seems to be a more appropriate parameter than τcr to represent the differences in soil erosion resistance under various soil and environmental conditions (e.g. bulk density, moisture content, consolidation, tillage). The two parameters represent different quantities and are therefore both needed to characterize the soil's resistance to concentrated flow erosion.  相似文献   

2.
Pore waters of natural clays, which are investigated as potential host rock formations for high-level nuclear waste, are known to contain large amounts of low-molecular weight organic compounds. These small organic ligands might impact the aqueous geochemistry of the stored radionuclides and, thus, their migration behavior. In the present work, the complexation of Cm(III) with formate in aqueous NaCl solution is investigated by time-resolved laser fluorescence spectroscopy (TRLFS) as a function of the ionic strength (0.5–3.0 mol/kg), the ligand concentration (0–0.2 mol/kg) and the temperature (20–90 °C). The Cm(III) speciation is determined by deconvolution of the emission spectra. The obtained distribution of Cm(III) species is used to calculate the conditional stability constants (log K′(T)) at a given temperature and ionic strength which are extrapolated to zero ionic strength by using the specific ion interaction theory (SIT). Thus, the thermodynamic log K0n(T) values for the formation of [Cm(Form)n](3−n)+ (n = 1, 2) and the ion interaction coefficients (ε(i,k)) for [Cm(Form)n](3−n)+ (n = 1, 2) with Cl− are obtained. The log K01(T) (2.11 (20 °C)–2.49 (90 °C)) and log K02(T) values (1.17 (30 °C–2.01 (90 °C)) increase continuously with increasing temperature. The log K0n(T) values are used to derive the standard reaction enthalpies and entropies (ΔrH0m, ΔrS0m) of the respective complexation reactions according to the Van’t Hoff equation. In all cases, positive ΔrH0m and ΔrS0m values are obtained. Thus, both complexation steps are endothermic and entropy-driven.  相似文献   

3.
Geochronological, geochemical and whole-rock Sr–Nd isotopic analyses have been completed on a suite of alkaline ultramafic dykes from southwest (SW) Guizhou Province, China with the aim of characterising their petrogenesis. The Baiceng ultramafic dykes have a LA-ICP-MS zircon 206Pb/238U age of 88.1 ± 1.1 Ma (n = 8), whereas two phlogopites studied by 40Ar/39Ar dating methods give emplacement ages of 85.25 ± 0.57 Ma and 87.51 ± 0.45 Ma for ultramafic dykes from Yinhe and Lurong, respectively. In terms of composition, these Late Mesozoic ultramafic dykes belong to the alkaline magma series due to their high K2O (3.31–5.04 wt.%) contents. The dykes are characterised by enrichment of light rare earth element (LREE) and large-ion lithosphile elements (LILEs) (Rb and Ba), negative anomalies in high field strength elements (HFSEs), such as, Nb, Ta and Ti relative to primitive mantle, low initial 87Sr/86Sr ratios (0.7060–0.7063) and positive εNd(t) values (0.3–0.4). Such features suggest derivation from low degree (< 1%) partial melting of depleted asthenospheric mantle (garnet-lherzolite), and contamination to various degrees (～ 10%) by interaction with upper crustal materials.  相似文献   

4.
The Anle Zn–Pb deposit, hosted by Upper Cambrian dolostone, is located in the southern Songpan–Ganzi Block in southwest China. In this deposit, ore bodies occur as stratiform lenses and consist of galena, sphalerite and pyrite as ore minerals, and quartz, dolomite and calcite as gangue minerals. The mineralization shows mainly vein, banded and brecciated structures. Four ore bodies have been found in the Anle deposit, with a combined 2.0 million tonnes (Mt) of sulfide ores at average grades of 1.64 wt.% Pb, 6.64 wt.% Zn and 45 g/t Ag. Brown, brownish-yellow and yellow sphalerite samples have δ66Zn values ranging from + 0.08 to + 0.10‰ (average + 0.09‰, n = 3), + 0.12 to + 0.38‰ (average + 0.24‰, n = 8) and + 0.40 to + 0.50‰ (average + 0.46‰, n = 3), respectively. We interpret the progressively heavier Zn isotopes from brown to yellow sphalerite as being led by kinetic Raleigh fractional crystallization. Calcite samples have δ13CPDB and δ18OSMOW values ranging from − 4.8 to − 0.2‰ (average − 1.7‰, n = 7) and + 17.9 to + 21.4‰ (average + 19.6‰, n = 7), respectively. Whole-rock δ13CPDB and δ18OSMOW values of the Cambrian ore-hosting dolostone range from + 0.1 to + 1.1‰ (average + 0.6‰, n = 3) and + 23.2 to + 24.1‰ (average + 23.6‰, n = 3), respectively. This suggests that carbon in the ore-forming fluids was provided by the host dolostone through carbonate dissolution. δ34SCDT values of sulfide samples range between − 1.3‰ and + 17.8‰ with an average value of + 6.3‰ (n = 25), lower than evaporites (such as barite + 19.8‰) in the overlaying Lower Ordovician sedimentary strata. The data suggest that sulfur in the hydrothermal fluids were derived from evaporites by thermo-chemical sulfate reduction (TSR). 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios for sulfide minerals range from 17.63 to 17.86, 15.58 to 15.69 and 37.62 to 37.95, respectively. The data are similar to those of the age-corrected Cambrian ore-hosting dolostone (206Pb/204Pb = 17.70–17.98, 207Pb/204Pb = 15.58–15.65 and 208Pb/204Pb = 37.67–38.06), but lower than those of age-corrected Ordovician sandstone and slate (206Pb/204Pb = 18.54–19.58, 207Pb/204Pb = 15.73–15.81 and 208Pb/204Pb = 38.44–39.60). This indicates that ore Pb was most likely to be derived from the Cambrian ore-hosting dolostone. Therefore, our new geological and isotopic evidence suggests that the Anle Zn–Pb deposit is best classified to be an epigenetic carbonate-hosted Mississippi Valley-type (MVT) deposit.  相似文献   

5.
  相似文献   

6.
We report the structure of a novel sunscreen based on the scytoneman skeleton. This pigment, scytonemin-3a-imine, was isolated from cultures of the cyanobacterium, Scytonema hoffmani, only when grown under high to intense (300–1500+ μmol quanta m−2 s−1) light conditions, with or without added UVR (ultraviolet radiation). It was also isolated from samples of natural cyanobacterial mats growing in shallow/short hydroperiod fresh water (Florida Everglades), soils (Loxahatchee, Florida) and saline cyanobacterial mats (Eleuthera, The Bahamas). These natural samples were all growing under intense (e.g. > 1500 μmol quanta m−2 s−1) light conditions. Scytonemin-3a-imine may eventually become a biomarker for such biota.Characterization included UV/Vis spectrophotometry, Fourier transform infrared (FTIR) spectroscopy, matrix assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) mass spectrometry (MS and MSn), and both 1- and 2-D 1H nuclear magnetic resonance (NMR) spectrometry. Derivatization (NaBH4 reduction, acetylation, deuterium exchange) were utilized to confirm structural features. The UV/Vis spectrum had maxima at 237, 366, 437 and 564 nm. The absorption of UVR is in line with other scytoneman-based pigments. The strong visible absorption bands (437 and 564 nm) indicate an alternate or additional physiological role for the pigment. We propose that it may protect both the chlorophyll reaction centers and the cytochromes from excess radiation. We speculate that excitation of the cytochromes, potentially leading to excess electron transport, or photo-oxidative destruction of the cytochromes, may enhance reactive oxygen species (ROS) generation.  相似文献   

7.
Three models were examined to predict C aromaticity (fa) of biochars based on either their elemental composition (C, H, N and O) or fixed C (FC) content. Values of fa from solid state 13C nuclear magnetic resonance (NMR) analysis with Bloch-decay (BD) or direct polarisation (DP) techniques, concentrations of total C, H, N, and organic O, and contents of FC of 60 biochars were either compiled from the literature (dataset 1, n = 52) or generated in this study (dataset 2, n = 8). Models were first calibrated with dataset 1 and then validated with dataset 2. All models were able to fit dataset 1 when atomic H to C ratio (H/C) < 1 (except two ash rich biochars) and to estimate fa of HF treated biochars (H/C < 1). Model 1, which was based on values of H/C only and calibrated with a root mean square of error (RMSE) of 0.04 fa-unit (n = 41), could predict the experimental data with a RMSE = 0.02 fa-unit (n = 6). Model 2, which was based on biochar elemental composition data, showed the most accurate prediction, with a RMSE of 0.03 fa-unit (n = 41) for the calibration data, and of 0.02 fa-unit (n = 6, H/C < 1) for the validation data. Model 3, which was based on contents of FC and C, and modified with a correction factor of 0.96, displayed the highest RMSE (0.06 fa-unit, n = 19) among the three models. Models 1 and 2 did not work properly for samples having either an H/C ratio > 1, high concentrations of carbonate or high inorganic H. These models need to be further tested with a wider range of biochars before they can be recommended for classification of biochar stability.  相似文献   

8.
Four dense Scandinavian limestones were analyzed to determine their mechanical properties. The generation of dust (? 10 μm) and fines (? 90 μm) during a closed circuit vertical roller mill comminution process was correlated with the calcite crystal size distributions of each limestone. Thin sections were analyzed and by means of stereology the calcite crystal size distributions for each limestone was measured. The dust generation of limestones is governed by a surface abrasive mechanism (R2 = 0.99) and the production of fines is governed by the mechanical strength of limestones (R2 = 0.99). The overall limestone degradation mechanism is predominantly controlled by the calcite cleavage planes which reduce the power consumption during the comminution process. This study is the first step in determining the influence of limestone texture on the wear rates in heterogenous raw mixes used in closed circuit comminution equipment.  相似文献   

9.
We have studied the hydrolytic behavior of Y3 + and trivalent ions of rare earth elements in aqueous solutions at 25 ºC. The stepwise stability constants of hydroxide complexes were measured by spectrophotometry, using m-cresol purple and 1-(2-pyridylazo)-2-naphthol as pH indicators at an ionic strength no more than 0.0005. The results showed that at pH ranging between 6.0 and 11.0 in freshly prepared solutions of REE trichlorides, lanthanides are presented as Ln3 +, Ln(OH)2 +, Ln(OH)2, and Ln(OH)3. The plots of the formation constants of + 0 monohydroxo complexes of 4f n ions M3 + versus atomic number Z deviate from smooth ones and consist of four convex curves. This phenomenon is also observed in normalized spectra of REE concentrations in natural objects and is known as the tetrad effect. The obtained data give an insight into the relationship between REE complex formation and REE fractionation in geochemical processes and can be used for physicochemical modeling of geochemical systems.  相似文献   

10.
Major, trace element and isotopic (Sr, Nd, Pb) data and unspiked K–Ar ages are presented for Quaternary (0.90–0.95 Ma old) basalts from the Hayyabley volcano, Djibouti. These basalts are LREE-depleted (Lan/Smn = 0.76–0.83), with 87Sr/86Sr ratios ranging from 0.70369 to 0.70376, and rather homogeneous 143Nd/144Nd (εNd = + 5.9–+ 7.3) and Pb isotopic compositions (206Pb/204Pb = 18.47–18.55, 207Pb/204Pb = 15.52–15.57, 208Pb/204Pb = 38.62–38.77). They are very different from the underlying enriched Tadjoura Gulf basalts, and from the N-MORB erupted from the nascent oceanic ridges of the Red Sea and Gulf of Aden. Their compositions closely resemble those of (1) depleted Quaternary Manda Hararo basalts from the Afar depression in Ethiopia and (2) one Oligocene basalt from the Ethiopian Plateau trap series. Their trace element and Sr, Nd, Pb isotope systematics suggest the involvement of a discrete but minor LREE-depleted component, which is probably an intrinsic part of the Afar plume.  相似文献   

11.
Narraguinnep Reservoir in southwestern Colorado is one of several water bodies in Colorado with a mercury (Hg) advisory as Hg in fish tissue exceed the 0.3 μg/g guideline to protect human health recommended by the State of Colorado. Concentrations of Hg and methyl-Hg were measured in reservoir bottom sediment and pore water extracted from this sediment. Rates of Hg methylation and methyl-Hg demethylation were also measured in reservoir bottom sediment. The objective of this study was to evaluate potential sources of Hg in the region and evaluate the potential of reservoir sediment to generate methyl-Hg, a human neurotoxin and the dominant form of Hg in fish. Concentrations of Hg (ranged from 1.1 to 5.8 ng/L, n = 15) and methyl-Hg (ranged from 0.05 to 0.14 ng/L, n = 15) in pore water generally were highest at the sediment/water interface, and overall, Hg correlated with methyl-Hg in pore water (R2 = 0.60, p = 0007, n = 15). Net Hg methylation flux in the top 3 cm of reservoir bottom sediment varied from 0.08 to 0.56 ng/m2/day (mean = 0.28 ng/m2/day, n = 5), which corresponded to an overall methyl-Hg production for the entire reservoir of 0.53 g/year. No significant point sources of Hg contamination are known to this reservoir or its supply waters, although several coal-fired power plants in the region emit Hg-bearing particulates. Narraguinnep Reservoir is located about 80 km downwind from two of the largest power plants, which together emit about 950 kg-Hg/year. Magnetic minerals separated from reservoir sediment contained spherical magnetite-bearing particles characteristic of coal-fired electric power plant fly ash. The presence of fly-ash magnetite in post-1970 sediment from Narraguinnep Reservoir indicates that the likely source of Hg to the catchment basin for this reservoir has been from airborne emissions from power plants, most of which began operation in the late-1960s and early 1970s in this region.  相似文献   

12.
Ice sheets flowing across a sedimentary bed usually produce a landscape of blister-like landforms streamlined in the direction of the ice flow and with each bump of the order of 102 to 103 m in length and 101 m in relief. Such landforms, known as drumlins, have mystified investigators for over a hundred years. A satisfactory explanation for their formation, and thus an appreciation of their glaciological significance, has remained elusive. A recent advance has been in numerical modelling of the land-forming process. In anticipation of future modelling endeavours, this paper is motivated by the requirement for robust data on drumlin size and shape for model testing.From a systematic programme of drumlin mapping from digital elevation models and satellite images of Britain and Ireland, we used a geographic information system to compile a range of statistics on length L, width W, and elongation ratio E (where E = L/W) for a large sample. Mean L, is found to be 629 m (n = 58,983), mean W is 209 m and mean E is 2.9 (n = 37,043). Most drumlins are between 250 and 1000 metres in length; between 120 and 300 metres in width; and between 1.7 and 4.1 times as long as they are wide. Analysis of such data and plots of drumlin width against length reveals some new insights. All frequency distributions are unimodal from which we infer that the geomorphological label of ‘drumlin’ is fair in that this is a true single population of landforms, rather than an amalgam of different landform types. Drumlin size shows a clear minimum bound of around 100 m (horizontal). Maybe drumlins are generated at many scales and this is the minimum, or this value may be an indication of the fundamental scale of bump generation (‘proto-drumlins’) prior to them growing and elongating. A relationship between drumlin width and length is found (with r2 = 0.48) and that is approximately W = 7 L 1/2 when measured in metres. A surprising and sharply-defined line bounds the data cloud plotted in E–W–L space, and records a scale-dependent maximum elongation limit (approximated by Emax = L1/3, when L measured in metres). For a given length, for some reason as yet unknown, drumlins do not exceed the elongation ratio defined by this scaling law. We also report and compare our statistics to an amalgamated sample (25,907 drumlins) of measures derived from around 50 published investigations. Any theory must be able to explain the drumlin statistics and fundamental scaling properties reported herein and they thus provide powerful tests for drumlin modelling.  相似文献   

13.
Hydrogen isotopic composition of n-alkanes was measured in sediments from an excavated profile of the Early Cretaceous Yixian Formation in Liaoning Province, NE China, aiming to assess the significance of the δD value of n-alkanes in ancient lacustrine sediments as the indicator for determining the source inputs of organic matters and paleoclimatic conditions. The δD values of n-alkanes are in the range of − 250‰ to − 85‰ and display an obvious three-stage variation pattern through the profile, which is consistent with the distribution of the dominated n-alkanes and the profile of their δ13C values. The δD and δ13C values of n-alkanes suggest that short-chain n-alkanes are primarily derived from photosynthetic bacteria and algae; n-C29 and n-C31 are mainly originated from terrestrial higher plants; n-C28 and n-C30 may be derived from the same precursor but via the different biological mechanism of hydrogen isotopic fractionation; while the source inputs of medium-chain n-alkanes are more complicated, with n-C23 being derived from some specific algae or biosynthesized by various aquatic organisms. The paleoclimatic conditions are reconstructed via two approaches. The reconstructed hydrogen isotopic values of lake water and meteoric water (expressed as δDLW and δDMW, respectively) were at the intervals of − 51.8‰ to 17.0‰ and − 118.1‰ to − 43.5‰, respectively, indicating a general climate transition from semi-arid to arid. The calculated ΔδDLW-MW values vary from 37.0‰ to 89.1‰ and display a similar but a significant large-scale variation trend with the ΔδDC23 ‐ long (− 28.8‰ to 85.0‰; long represents long-chain n-alkanes) and ΔδDmid-long (− 15.4‰ to 43.4‰; mid represents medium-chain n-alkanes) values. The discrepancy may be attributed to the source input overlap for n-alkanes and the uncertainties of εwater/lipid values. The coupling of ΔδDC23 ‐ long, ΔδDmid-long and ΔδDLW-MW values with the paleoclimatic evidence indicates that the δD values of n-alkanes could be more sensitive to the change of paleoclimatic conditions.  相似文献   

14.
The distribution of glycerol dialkyl glycerol tetraethers (GDGTs) can reflect continental environmental changes. Recently, the distribution of branched GDGTs (bGDGTs) has been proposed as a novel tool for paleoelevation reconstructions. Here we report the variation in TEX86 (tetraether index of 86 carbon atoms) of isoprenoidal GDGTs (iGDGTs) and MBT (methylation of branched tetraether index) of bGDGTs along an altitudinal transect on Mt. Xiangpi, NE Qinghai-Tibetan Plateau. Both TEX86 and MBT values of surface soils showed significant linear decreases with altitude (TEX86: R2 = 0.65; n = 50; MBT: R2 = 0.69; n = 24). We suggest that the apparent relationships between the two indices and altitude may be related to temperature. Our preliminary investigation suggests that the TEX86 index can potentially be applied as a paleoelevation indicator in addition to the MBT index on the Qinghai-Tibetan Plateau.  相似文献   

15.
146Sm–142Nd and 147Sm–143Nd systematics were investigated in garnet inclusions in diamonds from Finsch (S. Africa) and Hadean zircons from Jack Hills (W. Australia) to assess the potential of these systems as recorders of early Earth evolution. The study of Finsch inclusions was conducted on a composite sample of 50 peridotitic pyropes with a Nd model age of 3.3 Ga. Analysis of the Jack Hills zircons was performed on 790 grains with ion microprobe 207Pb/206Pb spot ages from 3.95 to 4.19 Ga. Finsch pyropes yield 100 × ?142Nd = ? 6 ± 12 ppm, ?143Nd = ? 32.5, and 147Sm/144Nd = 0.1150. These results do not confirm previous claims for a 30 ppm 142Nd excess in South African cratonic mantle. The lack of a 142Nd anomaly in these inclusions suggests that isotopic heterogeneities created by early mantle differentiation were remixed at a very fine scale prior to isolation of the South African lithosphere. Alternatively, this result may indicate that only a fraction of the mantle experienced depletion during the first 400 Myr of its history. Analysis of the Jack Hills zircon composite yielded 100 × ?142Nd = 8 ± 10 ppm, ?143Nd = 45 ± 1, and 147Sm/144Nd = 0.5891. Back-calculation of this present-day ?143Nd yields an unrealistic estimate for the initial ?143Nd of ? 160 ?-units, clearly indicating post-crystallization disturbance of the 147Sm–143Nd system. Examination of 146,147Sm–142,143Nd data reveals that the Nd budget of the Jack Hills sample is dominated by non-radiogenic Nd, possibly contained in recrystallized zircon rims or secondary subsurface minerals. This secondary material is characterized by highly discordant U–Pb ages. Although the mass fraction of altered zircon is unlikely to exceed 5–10% of total sample, its high LREE content precludes a reliable evaluation of 146Sm–142Nd systematics in Jack Hills zircons.  相似文献   

16.
Small angle neutron scattering techniques have been applied to investigate the phase behavior of CO2 injected into coal and possible changes in the coal pore structure that may result from this injection. Three coals were selected for this study: the Seelyville coal from the Illinois Basin (Ro = 0.53%), Baralaba coal from the Bowen Basin (Ro = 0.67%), and Bulli 4 coal from the Sydney Basin (Ro = 1.42%). The coals were selected from different depths to represent the range of the underground CO2 conditions (from subcritical to supercritical) which may be realized in the deep subsurface environment. The experiments were conducted in a high pressure cell and CO2 was injected under a range of pressure conditions, including those corresponding to in-situ hydrostatic subsurface conditions for each coal. Our experiments indicate that the porous matrix of all coals remains essentially unchanged after exposure to CO2 at pressures up to 200 bar (1 bar = 105 Pa). Each coal responds differently to the CO2 exposure and this response appears to be different in pores of various sizes within the same coal. For the Seelyville coal at reservoir conditions (16 °C, 50 bar), CO2 condenses from a gas into liquid, which leads to increased average fluid density in the pores (ρpore) with sizes (r) 1 × 105 ≥ r ≥ 1 × 104 Å (ρpore ≈ 0.489 g/cm3) as well as in small pores with size between 30 and 300 Å (ρpore ≈ 0.671 g/cm3). These values are by a factor of three to four higher than the density of bulk CO2 (ρCO2) under similar thermodynamic conditions (ρCO2 ≈ 0.15 g/cm3). At the same time, in the intermediate size pores with r ≈ 1000 Å the average fluid density is similar to the density of bulk fluid, which indicates that adsorption does not occur in these pores. At in situ conditions for the Baralaba coal (35 OC, 100 bar), the average fluid density of CO2 in all pores is lower than that of the bulk fluid (ρpore / ρCO2 ≈ 0.6). Neutron scattering from the Bulli 4 coal did not show any significant variation with pressure, a phenomenon which we assign to the extremely small amount of porosity of this coal in the pore size range between 35 and 100,000 Å.  相似文献   

17.
Precise indices based on n-alkane signatures were developed in order to determine the sources and composition of sedimentary organic matter (SOM) in coastal systems. The Arcachon Bay (France), a well-studied temperate lagoon, was used as an example of a complex coastal system sheltering a wide diversity of OM sources. Three main groups of sources were well discriminated from their n-alkane signatures: seagrass (Zostera sp.) produced mainly n-C17, n-C19, n-C21, n-C23 and n-C25 alkanes, algae (Rhodophyta, Chlorophyta) produced n-C15 and n-C17 and the terrigenous input [Quercus sp., Spartina sp. and river suspended particulate OM (SPOM)] was characterized by n-C25, n-C27, n-C29, n-C31 and n-C33. From the above and literature n-alkane fingerprints, we developed a set of indices (n-alkane ratios) to quantify the contribution of these three major sources of the SOM. At the Arcachon Bay scale, they indicated that SOM was composed mainly of seagrass (ca. 53 ± 19%) and terrestrial (ca. 41 ± 17%) material, followed by algae (ca. 6 ± 9%). Moreover, the new n-alkane indices exhibited more relevant spatial patterns than classical ones – the TAR (C27 + C29 + C31/C15 + C17 + C19; terrestrial to aquatic ratio) and the Paq (C23 + C25/C23 + C25 + C29 + C31; aquatic plant %) – with a greater contribution from marine sources in the central part of the lagoon where a high density of Zostera seagrass was observed. Therefore, the development of precise indices adapted to the local diversity of OM sources is needed when using n-alkanes for quantifying the source composition of SOM in complex coastal systems.  相似文献   

18.
《Chemical Geology》2006,225(1-2):16-29
Pyrite oxidation rates were examined at various concentrations of dissolved oxygen (DO) in the presence of the sulfur and iron oxidizer Acidithiobacillus ferrooxidans. Five different batch experiments were performed at room temperature for 75 days under various DO levels (273, 129, 64.8, 13.2, and ≤ 0.006 μM), containing pyrite grains (particle size 63–250 μm) and a modified 9K nutrient medium at pH 3. The reactors were inoculated with A. ferrooxidans. In all experiments, pH decreased with time and sulfur and iron were released to the solution, indicating pyrite oxidation at all DO levels. Pyrite oxidation rates (ca. 5 × 10− 10 mol m− 2 s− 1 at 273 μM DO) from all experiments showed positive correlation with DO, Fe(III), and bacterial concentration. These rates were significantly slower than rates presented in other published studies, but this is probably due to the significantly greater Fe(III) concentration at lower pH in these previous studies. The results obtained in this study suggest that ferric iron reduction at the pyrite surface is the primarily mechanism for microbial pyrite oxidation in the presence of DO. The results from our study support the indirect mechanism of sulfide oxidation, where A. ferrooxidans oxidizes ferrous iron in the presence of DO, which then oxidizes pyrite.  相似文献   

19.
This study seeks to quantify the rate and timing of regolith generation in the Critical Zone at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). Meteoric 10Be depth profiles were determined using measurements from 30 hillslope soil and bedrock core samples in an effort to constrain 10Be inventories. The SSHO is located in the temperate climate zone of central Pennsylvania and comprises a first-order watershed developed entirely on a Fe-rich, organic-poor, Silurian-aged shale. Two major perturbations to the landscape have occurred at SSHO in the geologically recent past, including significant and sustained periglacial activity until after the retreat of the Laurentide ice sheet (～21 ka) and deforestation during early colonial land-use. Bulk soil samples (n = 16) were collected at three locations along a planar hillslope on the southern ridge of the catchment, representing the ridge top, mid-slope and valley floor. Rock chip samples (n = 14) were also collected from a 24 m deep core drilled into the northern ridge top. All meteoric 10Be concentration profiles show a declining trend with depth, with most of the 10Be retained in the uppermost decimeters of the soil. Meteoric 10Be inventories are higher at the mid-slope and valley floor sample sites, at 3.71 ± 0.02 × 1010 at/cm2 and 3.69 ± 0.02 × 1010 at/cm2, than at the ridge top site (1.90 ± 0.01 × 1010 at/cm2). The 10Be inventory at the convex ridge top site implies a minimum residence time of ～10.6 ka, or if erosion is steady, an erosion rate of 19.4 ± 0.2 m/My.  相似文献   

20.
We report the first study of the Re-Os systematics of cobaltite (CoAsS) using disseminated grains and massive sulfides from samples of two breccia-type and two stratabound deposits in the Co-Cu-Au Idaho cobalt belt (ICB), Lemhi subbasin to the Belt-Purcell Basin, Idaho, USA. Using a 185Re + 190Os spike solution, magnetic and non-magnetic fractions of cobaltite mineral separates give reproducible Re-Os analytical data for aliquot sizes of 150 to 200 mg. Cobaltite from the ICB has highly radiogenic 187Os/188Os ratios (17–45) and high 187Re/188Os ratios (600–1800) but low Re and total Os contents (ca. 0.4–4 ppb and 14–64 ppt, respectively). Containing 30 to 74% radiogenic 187Os, cobaltite from the ICB is amenable to Re-Os age determination using the isochron regression approach.Re-Os data for disseminated cobaltite mineralization in a quartz-tourmaline breccia from the Haynes-Stellite deposit yield a Model 1 isochron age of 1349 ± 76 Ma (2σ, n = 4, mean squared weighted deviation MSWD = 2.1, initial 187Os/188Os ratio = 4.7 ± 2.2). This middle Mesoproterozoic age is preserved despite a possible metamorphic overprint or a pulse of metamorphic-hydrothermal remobilization of pre-existing cobaltite that formed along fold cleavages during the ca. 1190–1006 Ma Grenvillian orogeny. This phase of remobilization is tentatively identified by a Model 3 isochron age of 1132 ± 240 Ma (2σ, n = 7, MSWD = 9.3, initial 187Os/188Os ratio of 9.0 ± 2.9) for cobaltite in the quartz-tourmaline breccia from the Idaho zone in the Blackbird mine.All Mesoproterozoic cobaltite mineralization in the district was affected by greenschist- to lower amphibolite-facies (garnet zone) metamorphism during the Late Jurassic to Late Cretaceous Cordilleran orogeny. However, the fine- to coarse-grained massive cobaltite mineralization from the shear zone-hosted Chicago zone, Blackbird mine, is the only studied deposit that has severely disturbed Re-Os systematics with evidence for a linear trend of mixing with (metamorphic?) fluids.The new Re-Os ages and extremely high initial 187Os/188Os ratios of cobaltite reported here favor a magmatic-hydrothermal genetic model for a multi-stage REE-Y-Co-Cu-Au mineralization occurring at ca. 1370 to 1349 Ma, and related to the emplacement of the Big Deer Creek granite pluton at ca. 1377 Ma. In our model, deposition of paragenetically early xenotime and gadolinite was followed by an influx of Mesoproterozoic evaporitic brines and magmatic-hydrothermal fluids containing metals and reduced sulfur derived from mafic and oceanic island-arc Archean to Paleoproterozoic rocks in the Laurentian basement. Cobaltite mineralization occurred upon cooling of these fluids at an inferred temperature of 300 °C or below.  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