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1.
Recent sediments in lakes and gulfs are a sensi-tive recorder of the information about environmentalchanges in the catchment areas during recent geologi-cal history. Precise determination of the ages of sedi-ments is the key to deciphering the environmental re-cords. The 210Pb dating technique and the markertechnique based on fallout radionuclide 137Cs havefound wide applications in sedimentation rate on atime scale of several tens to one hundred years, as wellas the varve chron…  相似文献   

2.
Due to their slow growth rates, seamount Co-rich crusts are very difficult to date with high resolution and precision. This paper is to test the use of orbital pacing on the growth profile of crusts to determine high-resolution age and growth rate. Crust CB14 from the central Pacific Ocean was selected for this study. We first examined the growth pattern in detail under a reflected-light microscope and ascertained that the growth environment was stable for the sub-layer 1 (0–3 mm). We then used electron microprobe line-scanning to obtain elemental profiles. The pattern of the power spectrum analysis of the Al-profile revealed that there are significant cycles of 113.9, 87.8, 51.5, 42.2 and 25.8 μm. These cycles correspond to the Milankovitch cycles of 53.1, 41, 24, 19.7 and 12 ka, respectively, and yield the growth rate of about 2.14 mm/Ma and an age of about 1.40 Ma for the boundary between the sub-layer 1 and sub-layer 2. We also used a drilling machine with a numerically controlled drive to obtain high-resolution samples at 0.1mm intervals, and used the 230Thex/232Th method to date the samples. For the uppermost 1.3 mm, the growth rate was about 2.15 mm/Ma, and the age for the layer at the depth of 3 mm was about 1.40 Ma, which coincides perfectly with the results obtained from orbital pacing. Thus, it is considered that orbital pacing is a new and effective method to determine the growth rate of the seamount Co-rich crust. This method is applicable for establishing a high-resolution age frame for the crusts of the world’s oceans. Supported by China Ocean Mineral Resources R & P Association (Grant No. DY105-01-01-08) and National Natural Science Foundation of China (Grant Nos. 40106005, 40476050)  相似文献   

3.
湖泊沉积物储存着长时期的流域环境信息,是百年尺度湖泊环境演变研究的高分辨率记录器。本文通过210Pbex、137Cs组合定年法,分析放射性核素剖面特征,研究沉积速率时空变化规律,并解析沉积物粒度指示的环境信息,揭示百年尺度湖泊沉积环境演变。结果表明:洞庭湖210 Pbex剖面分布呈现多段衰变型、混合—衰变—平衡型、衰变—本底型和无沉积型4种类型。3大湖体200 cm的沉积深度对应的年代跨度为127～162年,构建了洞庭湖沉积物深度—年代关系。洞庭湖平均质量沉积速率为1.47～3.11 g/(cm2·a),整体上呈现南洞庭湖>西洞庭湖>东洞庭湖的空间特征;自1900年后,质量沉积速率增加了3～7倍,呈时序增加趋势。沉积年代上,沉积物粒度特征表明洞庭湖沉积环境呈3个阶段2个过渡期的演变特征,与同时期区域内毁林开荒、水利建设、植树造林等人为活动有较好的对应关系,说明近百年来人为活动是洞庭湖泥沙沉积的重要因素。本研究结果可为强化湖泊及其流域生态环境保护...  相似文献   

4.
To evaluate influence of chemical weathering of the Qinghai-Tibet Plateau (QTP) on seawater 87Sr/86Sr variation, river water and sediment samples were collected, and their Sr concentrations and isotopic compositions analyzed, from the seven large rivers that originated from the QTP. By combining these with the data of the Ganges, Brahmaputra, Indus and Irrawaddy originated in the southern QTP, the total Sr flux of the eleven rivers reaches 3.47×109 mol·a−1, which accounts for 10.2% of the total Sr flux transported by the global rivers. The weighted mean 87Sr/86Sr is 0.71694, higher than the average value of the global rivers. The 87Srex (87Sr flux in excess of the seawater 87Sr/86Sr ratio) of the Chinese seven rivers is 1.55×106 mol·a−1, only accounting for about 6% of the value of the eleven rivers originated from QTP, and the Ganges-Brahmaputra system accounts for 86%. We assume that the QTP rivers have no strontium contributions to the oceans before ∼40 Ma and the Sr fluxes of the global rivers, except the QTP eleven rivers, are constant, then a maximum linear increase in Sr fluxes of the QTP rivers from zero to the modern value in response to tectonic uplift can explain ∼69% increase of seawater 87Sr/86Sr over the past ∼40 Ma and the remainder of 31% is perhaps provided from other factors. Supported by National Natural Science Foundation of China (Grant Nos. 40473009, 40331001, 40873001)  相似文献   

5.
Based on satellite observation data, using dynamics equation, the ionospheric O+ ion’s distribution in the synchronous altitude region for different geomagnetic activity indexK p is studied by theoretical modeling and numerical analyzing, and semi-empirical models for the O+ ion’s density and flux versus longitude in the synchronous altitude region for differentK p are given. The main results show that in the synchronous altitude region: (i) The O+ ion’s density and flux in day-side are larger than those in nightside. (ii) With longitude changing, the higher the geomagnetic activity indexK p is, the higher the O+ ion’s density and flux, and their variation amplitude will be. The O+ ion’s density and flux whenK p ≥ 6 will be about ten times as great as that whenK p = 0. (iii) WhenK p = 0 orK p ≥ 6, the O+ ion’s density reaches maximum at longitudes 120° and 240° respectively, and minimum in the magnetotail. WhenK p = 3−5, the O+ ion’s density gets to maximum at longitude 0°, and minimum in the magnetotail. However, the O+ ion’s flux reaches maximum at longitude 120° and 240° respectively, and minimum in the magnetotail for anyK p value.  相似文献   

6.
Fallout radionuclides, including lead‐210 excess (210Pbex), have been broadly and successfully used to quantify net hillslope sediment transport in agricultural, pastoral and forested landscapes but have only recently been applied in burned terrain. Quantifying post‐fire erosion is important because fires can amplify hillslope erosion, impacting terrestrial and aquatic habitat and water quality. However, we lack a basic understanding of the fate of 210Pbex in fires. To address this knowledge gap, we collected over 400 soil samples from unburned, moderately and severely burned forested sites in central Idaho. We measured soil 210Pbex content at stable reference and eroding sites and in mineral and organic soil components. At all sites, organic matter had the highest concentration of 210Pbex, representing 30% to 73% of the total activity. At the severely and moderately burned sites, 210Pbex reference inventories were lower by 58% and 41%, with about 40% less organic mass, relative to the unburned site. These results indicate that most 210Pbex in our semi‐arid, forested sites was bound to organic matter, and that a substantial portion of this lead was lost due to forest fires. These losses likely occurred through volatilization and wind transport of smoke and ash. In the moderately burned site, 210Pbex losses were more spatially variable, potentially due to spatially uneven fire intensity and effects. Despite equal percent losses of 210Pbex, lower inventories at the burned sites produced lower calculated net erosion rates relative to the unburned site. Thus, given methodological uncertainties, 210Pbex losses due to fire, and the subsequent sensitivity of calculated net erosion rates to these lower 210Pbex inventories, we suggest this method should not be used in burned terrain to calculate absolute net erosion and deposition rates. However, within a given burned site, 210Pbex inventories still provide useful information describing relative soil losses and storage across the landscape. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

7.
Lake Albano (Alban Hills volcanic complex, Central Italy) is located in a densely populated area near Rome. The deep lake waters have significant dissolved CO2 concentrations, probably related to sub-lacustrine fluid discharges fed by a pressurized CO2-rich reservoir. The analytical results of geochemical surveys carried out in 1989–2010 highlight the episodes of CO2 removal from the lake. The total mass of dissolved CO2 decreased from ∼5.8 × 107 kg in 1989 to ∼0.5 × 107 kg in 2010, following an exponential decreasing trend. Calculated values of both dissolved inorganic carbon and CO2 concentrations along the vertical profile of the lake indicate that this decrease is caused by CO2 release from the epilimnion, at depth <9 m, combined with (1) water circulation at depth <95 m and (2) CO2 diffusion from the deeper lake layers. According to this model, Lake Albano was affected by a large CO2 input that coincided with the last important seismic swarm at Alban Hills in 1989, suggesting an intimate relationship between the addition of deep-originated CO2 to the lake and seismic activity. In the case of a CO2 degassing event of an order of magnitude larger than the one that occurred in 1989, the deepest part of Lake Albano would become CO2-saturated, resulting in conditions compatible with the occurrence of a gas outburst. These results reinforce the idea that a sudden CO2 input into the lake may cause the release of a dense gas cloud, presently representing the major volcanic threat for this densely populated area.  相似文献   

8.
Source rock extracts and crude oils from the Songliao Basin were analyzed by high-temperature gas chromatography (HTGC), gas chromatography-mass spectrometry (HTGC-MS) and gas chromatography-isotope ratio-mass spectrometry (GC-IRMS), for high molecular-weight alkanes. The distributions of n-alkanes in the Nenjiang Formation extracts are in the C14―C63 range; a bimodal distribution occurs in the C-21 and C21―40 regions. The C30―C37 n-alkanes are accompanied by C29―C35 hopanes, whereas the high molecular-weight C45―C47 n-alkanes co-occur with abundant isoalkanes, alkylcyclohexanes and alkylcyclopentanes. The high δ 13C values of the n-alkanes and the microscopic maceral compositions indicate a highly diversified organic source input for the Nenjiang Formation source rocks, ranging from aquatic plants, blue alge-bacteria, to land plant material. In contrast, n-alkanes in the rock extracts of the Qingshankou Formation are characterized by a single modal distribution, with relatively low abundances of C29―C35 hopanes, but high molecular-weight isoalkanes, alkylcyclohexanes and alkylcyclopentanes. The relatively low δ 13C values of C22―C44 n-alkanes and organic material compositions indicate that the source rocks in the Qingshankou Formation contain dominantly type I algal organic matter. The relative abundance of C 40 compounds in source rocks changes little at low maturity stage, but decreases drastically at higher maturity levels, with a concurrent reduction in the odd/even carbon predominance. In crude oils, in contrast, the relative abundance of C 40 compounds appears to relate closely with the oil source and oil viscosity.  相似文献   

9.
A large number of sediment cores collected during 2005-2010 from the Taiwan Strait were analyzed for radionuclides (210Pb, 137Cs and 7Be) to elucidate sedimentation dynamics in this all-important gateway linking two largest marginal seas in the western Pacific (namely, the South China Sea and the East China Sea). Apparent sediment accumulation rates derived from 210Pb and 137Cs profiles vary from <0.1 to >2 cm/yr, averaging ∼0.4 cm/yr and showing a spatial pattern closely related to hydrodynamics and sediment source-to-sink pathways. Spatial-temporal variation of 7Be activity in surface sediments off Taiwan’s west coast indicates episodic deposition of flood layers and their mobility from river estuaries toward the north. In conjunction with particle size distribution in surface sediments and the structure of sediment strata revealed by sub-bottom echo images; the radionuclide data can be used to outline three different sediment source-to-sink dispersal systems. Based on sediment loads of surrounding rivers and the distribution of sediment accumulation rates, lateral transport is required to account for the budget and size distribution of sediments in the strait.  相似文献   

10.
The sorption of Eu species onto nano-size silica-water interfaces is investigated at pH range of 1―8.5 and the initial Eu concentrations (CEu) of 2×10−5, 2×10−4 and 2×10−3 M using fluorescence spectroscopy. The sorption rate of Eu is initially low, but significantly increases at pH > 4. For the initial CEu of 2×10−5, 2×10−4 and 2×10−3 M, the dissolved Eu species are completely sorbed onto silica-water interfaces at pH = 4.75, −5.8 and 6.6, respectively, with the respective sorption densities of −1.58×10−8, 1.58×10−7 and 1.58×10−6 mol/m2. The sorbed Eu species at pH < 6 is aquo Eu3+, which is sorbed onto silica-water interfaces as an outer-sphere complex at pH < 5, but may be sorbed as an inner-sphere bidentate complex at 5 < pH < 6, due to the decrease of the NH2O to −6 at pH = 6. At pH = 6 – 8, Eu(OH)2+, Eu(CO3)+and Eu(CO3)2 − form in the solutions, and Eu(CO3)+is dominant at pH = −7.5. These ions may be sorbed onto silica-water interfaces as inner-sphere bidentate complexes or multi-nuclear pre-cipitates.  相似文献   

11.
13C of 367 C3 herbaceous plants was measured in loess area in northern China. Their δ13C values vary between −21.7%. and −30.0%., with a mean of −26.7%.. In the center of Loess Plateau (semimoist area) with annual precipitation of 400–600 mm, the δ13 C values of C3 herbaceous plants range from −24.4%. to −28.5%., with a mean of −27.5%.. In the west of Loess Plateau (semiarid and arid area) with annual precipitation less than 400 mm, they range between −21.7%. and −30.0%., with a mean of −26.2%.. Annual precipitation is the main factor that makes δ13C values of C3 herbaceous plants in the west greater than those in the central Loess Plateau. The composition of δ13C in C3 plants increases with deceasing annual precipitation, and the mean change is −49%./100 mm.  相似文献   

12.
Historical changes in lake sedimentation for the last 150 years and the lifetime of lakes of Ishikari Wetland are reconstructed in this study. The floodplain wetlands surrounding the lakes have gradually converted to farmlands since the 1890s. 210Pb activity in lake profiles was found to have a non-exponential decrease with depth due to increased and variable sediment flux from agricultural activities. 210Pb dates of these lakes were calculated in this study using constant rate of supply model, which was in good agreement with the date based on 137Cs method. The average total sediment mass in the lakes under natural conditions were 54–112 ton/year until 1890s. It was increased to 111–1055 ton/year after 1890s due to conversion of floodplain to agricultural lands. Especially, lakes after the 1940s had the highest sediment accumulations due to sediment influx resulting from intensification of agriculture via increasing farm size and destroying swamp buffers surrounding the lakes. The time forecasted for filling lakes based on post-1940s sediment accumulation mass was about 15 to 23 years. Restoring swamp buffers surrounding the lakes and converting to low intensity agricultural activities are needed to restrict lake shallowing.  相似文献   

13.
We present an uncertainty analysis of ecological process parameters and CO2 flux components (R eco, NEE and gross ecosystem exchange (GEE)) derived from 3 years’ continuous eddy covariance measurements of CO2 fluxes at subtropical evergreen coniferous plantation, Qianyanzhou of ChinaFlux. Daily-differencing approach was used to analyze the random error of CO2 fluxes measurements and bootstrapping method was used to quantify the uncertainties of three CO2 flux components. In addition, we evaluated different models and optimization methods in influencing estimation of key parameters and CO2 flux components. The results show that: (1) Random flux error more closely follows a double-exponential (Laplace), rather than a normal (Gaussian) distribution. (2) Different optimization methods result in different estimates of model parameters. Uncertainties of parameters estimated by the maximum likelihood estimation (MLE) are lower than those derived from ordinary least square method (OLS). (3) The differences between simulated Reco, NEE and GEE derived from MLE and those derived from OLS are 12.18% (176 g C·m−2·a−1), 34.33% (79 g C·m−2·a−1) and 5.4% (92 g C·m−2·a−1). However, for a given parameter optimization method, a temperature-dependent model (T_model) and the models derived from a temperature and water-dependent model (TW_model) are 1.31% (17.8 g C·m−2·a−1), 2.1% (5.7 g C·m−2·a−1), and 0.26% (4.3 g C·m−2·a−1), respectively, which suggested that the optimization methods are more important than the ecological models in influencing uncertainty in estimated carbon fluxes. (4) The relative uncertainty of CO2 flux derived from OLS is higher than that from MLE, and the uncertainty is related to timescale, that is, the larger the timescale, the smaller the uncertainty. The relative uncertainties of Reco, NEE and GEE are 4%−8%, 7%−22% and 2%−4% respectively at annual timescale. Supported by the National Natural Science Foundation of China (Grant No. 30570347), Innovative Research International Partnership Project of the Chinese Academy of Sciences (Grant No. CXTD-Z2005-1) and National Basic Research Program of China (Grant No. 2002CB412502)  相似文献   

14.
Soil H2 and CO2 surveys were carried out along seven active faults and around the aftershock region of the 2000 Tottori-ken Seibu earthquake in Japan. Diffuse CO2 effluxes were also measured along one fault and around the 2000 aftershock region. The results show highly variable H2 concentration in space and time and it seems that the maximum H2 concentration at each active fault correlates with fault activity as exemplified by the time of the latest big earthquakes. Even though observed H2 concentrations in four faults were markedly lower than those collected previously in the latter half of the 1970s, it is evident that the higher H2 concentrations in this study are due to the addition of the fault gases. Comparing the chemical composition of trapped gases (H2: 5–20% and CO2/H2: 0.5–12) in fractured rocks of drill cores bored at the Nojima fault, a soil gas sample with the highest H2 concentration showed large amounts of the trapped fault gas, diluted with atmospheric component. The profile experiment across a fracture zone at the Yamasaki fault showed higher H2 concentrations and lower CO2/H2 ratios as was observed in soil gas from the fracture zone. A few days after the 2000 Tottori-kei Seibu earthquake, no CO2 effluxes related to the occurrence of earthquakes were observed at the aftershock region. However, only above the epicenter zone, relatively high H2 concentrations in soil gases were observed.  相似文献   

15.
Radium-226 (226Ra) activities were measured in the surface water samples collected from the Arctic Ocean and the Bering Sea during the First Chinese National Arctic Research Expedition. The results showed that 226Ra concentrations in the surface water ranged from 0.28 to 1.56 Bq/m3 with an average of 0.76 Bq/m3 in the Arctic Ocean, and from 0.25 to 1.26 Bq/m3 with an average of 0.71 Bq/m3 in the Bering Sea. The values were obviously lower than those from open oceans in middle and low latitudes, indicating that the study area may be partly influenced by sea ice meltwater. In the Bering Sea, 226Ra in the surface water decreased northward, probably as a result of the exchange between the 226Ra-deficient sea ice meltwater and the 226Ra-rich Pacific water. In the Arctic Ocean, 226Ra in the surface water increased northward and eastward. This spatial distribution of 226Ra reflected the variation of the 226Ra-enriched river component in the water mass of the Arctic Ocean. The vertical profiles of 226Ra in the Canadian Basin showed a concentration maximum at 200 m, which could be attributed to the inputs of the Pacific water or/and the bottom shelf water with high 226Ra concentration. This conclusion was consistent with the results from 2H, 18O tracers.  相似文献   

16.
The Italian volcano, Vesuvius, erupted explosively in AD 79. Sanidine from pumice collected at Casti Amanti in Pompeii and Villa Poppea in Oplontis yielded a weighted-mean 40Ar/39Ar age of 1925±66 years in 2004 (1σ uncertainty) from incremental-heating experiments of eight aliquants of sanidine. This is the calendar age of the eruption. Our results together with the work of Renne et al. (1997) and Renne and Min (1998) demonstrate the validity of the 40Ar/39Ar method to reconstruct the recent eruptive history of young, active volcanoes.  相似文献   

17.
The static closed chamber technique is used in the study on the CH4 and N2O fluxes from the soils of primeval Abies fabri forest, the succession Abies fabri forest and the clear-cut areas of mid-aged Abies fabri forest in the Gongga Mountain from May 1998 to September 1999. The results indicate the following: (i) The forest soil serves as the source of atmospheric N2O at the three measurement sites, while the fluxes of CH4 are all negative, and soil is the sink of atmospheric CH4. The comparative relations of N2O emissions between the three sites are expressed as primeval Abies fabri forest < clear-cut areas < succession Abies fabri forest, and those of CH4 consumption fluxes are primeval Abies fabri forest < succession Abies fabri forest < clear-cut areas, (ii) Significant seasonal variations of N2O emission at various sites were observed, and two emission peaks of N2O occurr during summer (July—August) and spring (February—March), whereas N2O emission is relatively low in winter and spring (mid March—April). Seasonal variations of CH4 consumption at each measurement site fluctuate drastically with unclear regularities. Generally, CH4 consumption fluxes of succession Abies fabri forest and clear-cut areas are higher from mid May to late July but lower in the rest of sampling time, while the CH4 flux keeps a relatively high value even up to September in primeval Abies fabri forest. In contrast to primeval Abies fabri forest, the CH4 absorbabilities of succession Abies fabri forest and clear-cut areas of mid-aged Abies fabri forest are weaker. Particularly, the absorbability of the clear-cut areas is even weaker as compared with the other two sites, for the deforestation reduces the soil absorbability of atmospheric CH4. (iii) Evident diurnal variation regularity exists in the N2O emissions of primeval Abies fabri forest, and there is a statistic positive correlation between the fluxes of N2O and air temperature (R=0.95, n=11, α· 0.01), and also the soil temperature of 5-cm layer (R=0.81, n=11, α> 0.01), whereas the CH4 diurnal variation regularities are unclear and have no significant correlation with the soil temperature of 5-cm layer and air temperature.  相似文献   

18.
A simple model for reconstructing the paleomagnetic field intensity with 10Be production rate was used for the first time in Loess 10Be studies of Luochuan profile. Using the LGM (Last Glacial Maxmium) method, the climatic effects and geomagnetic modulation effects on loess 10Be was separated and in turn the 80 ka geomagnetic excursion sequence reconstructed, of which the globally remarkable geomagnetic excursion events such as the Laschamp (42 ka), Mono Lake (32 ka) during the Last Glacial period were revealed and the paleo-geomagnetic intensity curve from Loess 10Be over the past 80 ka was quantitatively reconstructed. The reconstructed paleo-intensity fits well with the paleo-intensity curves (SINT200 and NAPIS75), which indicates the significance of global criterion of the 10Be paleo-intensity curve and the future direction of loess 10Be tracing studies. Results show the irregular variability of the East Asian monsoon precipitation in Loess Plateau is the main cause that has resulted in the ambiguity of the geomagnetic modulation of the 10Be record in the loess, and the intrinsic source component of the loess 10Be and inherited fraction of magnetic susceptibility (SUS) are characterized by the “quasi-homogeneous distribution” manner. Supported by the Key Innovation Project of the Chinese Academy of Sciences (Grant No. KZCX2-YW-118), the National Natural Science Foundation of China (Grant Nos: 40531003, 40121303, 40523002) and State Key Laboratory of Loess and Quaternary Geology in the Institute of Earth Environment of Chinese Academy of Sciences (Grant No. SKLLQG0712)  相似文献   

19.
The radionuclides 210Po and 210Pb were examined to trace the cycling of particulate organic carbon (POC) and particulate organic nitrogen (PON) in the Zhubi coral reef lagoon. The net export flux of POC to the open sea is 14 mgC m−2 d−1. However, the net exchange of PON has not yet been observed. On average, the vertical export fluxes in the lagoon of POC and PON, as derived from 210Po/210Pb disequilibria, are 43 mgC m−2 d−1 and 13.8 mgN m−2 d−1, respectively. The deficit of 210Po relative to 210Pb in particulate matter provides evidence for the degradation of particulate organic matter. According to the mass balance budgets, 310 mgC m−2 d−1 and 121 mgN m−2 d−1 were recycled into dissolved fractions. Based on a first-order kinetics model, the degradation rate constants of POC and PON are 0.28 and 0.30 m−1, respectively. Thus, 210Po and 210Pb can quantify the cycling of carbon and nitrogen in this coral lagoon.  相似文献   

20.
After the severest mass extinction event in the Phanerozoic, biotic recovery from the extinction at the Permian–Triassic boundary required approximately 5 my, which covers the entire Early Triassic. It is important to obtain information on the superocean Panthalassa, which occupied most of the world ocean, to explore paleoenvironmental changes during the Early Triassic at the global scale. In order to establish the continuous lithostratigraphy of pelagic sediments in Panthalassa during the Early Triassic, high‐resolution reconstruction of the Lower Triassic pelagic sequence in Japan was conducted for the first time based on detailed field mapping and lithostratigraphic correlation in the Inuyama area, central Japan. The reconstructed Early Triassic sequence is approximately 9.5 m thick, consists of five rock types, and is divided into eight lithological units. For the reconstructed continuous sequence, measurement of carbon isotopic composition of sedimentary organic matter (δ13Corg) was carried out. Stratigraphic variation of the δ13Corg value shows large‐amplitude fluctuations between ?34.4 and ?21.0‰ throughout the sequence. In order to establish a higher resolution age model for the reconstructed Lower Triassic pelagic sequence, we correlated δ13Corg records in the Inuyama area with high‐resolution isotopic profiles of carbonate carbon (δ13Ccarb) from shallow‐marine carbonate sequences in southern China based on the similarity in general variation patterns with age constraints by radiolarian and conodont biostratigraphy. The result provides a high‐resolution time scale for the pelagic sequence of Panthalassa during the Smithian and Spathian. The age model suggests a drastic increase in sedimentation rate during the late Smithian, which should have been caused by the increase in terrigenous input to this site.  相似文献   

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