The isotopic composition of particulate organic carbon in mountain rivers of Taiwan     

Robert G. Hilton  Albert Galy  Ming-Jame Horng 《Geochimica et cosmochimica acta》2010,74(11):3164-1679

Small rivers draining mountain islands are important in the transfer of terrestrial particulate organic carbon (POC) to the oceans. This input has implications for the geochemical stratigraphic record. We have investigated the stable isotopic composition of POC (δ¹³C_org) in rivers draining the mountains of Taiwan. In 15 rivers, the suspended load has a mean δ¹³C_org that ranges from −28.1±0.8‰ to −22.0±0.2‰ (on average 37 samples per river) over the interval of our study. To investigate this variability we have supplemented suspended load data with measurements of POC in bedrock and river bed materials, and constraints on the composition of the terrestrial biomass. Fossil POC in bedrock has a range in δ¹³C_org from −25.4±1.5‰ to −19.7±2.3‰ between the major geological formations. Using coupled δ¹³C_org and N/C we have found evidence in the suspended load for mixing of fossil POC with non-fossil POC from the biosphere. In two rivers outside the Taiwan Central Range anthropogenic land use appears to influence δ¹³C_org, resulting in more variable and lower values than elsewhere. In all other catchments, we have found that 5‰ variability in δ¹³C_org is not controlled by the variable composition of the biomass, but instead by heterogeneous fossil POC.In order to quantify the fraction of suspended load POC derived from non-fossil sources (F_nf) as well as the isotopic composition of fossil POC (δ¹³C_fossil) carried by rivers, we adapt an end-member mixing model. River suspended sediments and bed sediments indicate that mixing of fossil POC results in a negative trend between N/C and δ¹³C_org that is distinct from the addition of non-fossil POC, collapsing multiple fossil POC end-members onto a single mixing trend. As an independent test of the model, F_nf reproduces the fraction modern (F_mod) in our samples, determined from ¹⁴C measurements, to within 0.09 at the 95% confidence level. Over the sampling period, the mean F_nf of suspended load POC was low (0.29 ± 0.02, n = 459), in agreement with observations from other mountain rivers where physical erosion rates are high and fossil POC enters river channels. The mean δ¹³C_fossil in suspended POC varied between −25.2±0.5‰ and −20.2±0.6‰ from catchment to catchment. This variability is primarily controlled by the distribution of the major geological formations. It also covers entirely the range of δ¹³C_org found in marine sediments which is commonly thought to derive from mixing between marine and terrigenous POC. If land-sourced POC is preserved in marine sediments, then changes in the bulk δ¹³C_org observed offshore Taiwan could instead be explained by changes in the onshore provenance of sediment. The range in δ¹³C_org of fossil organic matter in sedimentary rocks exposed at the surface is large and given the importance of these rocks as a source of clastic sediment to the oceans, care should be taken in accounting for fossil POC in marine deposits supplied by active mountain belts.  相似文献   

The Use of Spatial Cognitive Abilities in Geographical Information Systems: The Map Overlay Operation   总被引：2，自引：0，他引：2  

William S. Albert  & Reginald G. Golledge 《Transactions in GIS》1999,3(1):7-21

Spatial cognitive abilities play an important role in the use of GIS, although they have yet to be examined in a controlled experimental setting. This study aimed to develop an experimental design which measures spatial cognitive abilities in the use of GIS, specifically the map overlay operation. Subjects ( n = 134) received three map overlay tests in which they were given two of the following: input map layers, logical operator(s), or output map layer(s). Subjects were required to select the correct logical operator for Test 1, to select the correct output map layer for Test 2, and to select the correct input map layers for Test 3. Each test contained a total of 16 questions, based on a 4 ('and', 'or', 'xor', 'not' operators) × 2 (one or two polygons per map layer) × 2 (three or five polygon edges) factorial design. Results indicated a significant main effect of logical operators and number of polygons on performance; however, there was no effect of the number of polygon edges on performance. Significant two-way interactions revealed an effect of the number of polygon edges and the number of polygons using various logical operators on performance. In addition, performance was not significantly different between males and females or between GIS users and non-users. Overall, results show that map overlays in which a visual correspondence can be made between the same polygons in the input and output map layers are cognitively less demanding than map overlays in which the shape of the polygons have been radically transformed between the input and output map layers. This study helps further develop our understanding of the spatial cognitive abilities which are required in the use of GIS, and whether certain sub-populations differ in these cognitive abilities. These results may contribute to more effective and efficient GIS teaching and interface design by taking into account individual spatial cognitive abilities.  相似文献   

Sediment characteristics and internal architecture of offshore sand ridges on a tideless continental shelf (western Mediterranean)     

Ruth Durán  Jorge Guillén  Marta Ribó  Gonzalo Simarro  Araceli Muñoz  Albert Palanques  Pere Puig 《地球表面变化过程与地形》2020,45(14):3592-3606

An integrated approach combining swath bathymetry, an extensive dataset of vibrocores and high-resolution seismic reflection data was used to assess the origin and evolution of offshore sand ridges on a tideless continental shelf (Gulf of Valencia, western Mediterranean). The sand ridges are located in the mid-outer shelf at 55–85 m water depth, obliquely oriented to the shoreline. They are 1.5 to 7 m high, with a wavelength between 600 and 1,100 m and a mean height-to-wavelength ratio of 0.004. The sand ridges are composed of well-sorted medium sand and are partially covered by a mud layer, evidencing a moribund stage. They overlie an erosion surface that locally crops out at the seafloor and is interpreted as the Holocene wave-ravinement surface. In the sediment cores, this surface corresponds to an erosional lag composed of coarse sand and gravel with pebbles. Small topographic irregularities on this surface are interpreted as shoreline-associated features that may act as the precursor for ridge development. Their preservation within the sand ridges could be related to the hardness of these features. Internally, the sand ridges display high-angle dipping reflections, indicating ridge migration towards the southeast in the direction of the present-day sediment transport direction. The presence of interbedded mud layers, associated with these reflections, indicates intermittent episodes of mud deposition when active. The internal architecture of some small ridges also provides new insights into their transition from an active to moribund state, as evidenced by a change in the geometry of the internal units from progradational to aggradational, finally being overlain by onlapping finer deposits over the flanks and in the troughs. The Gulf of Valencia sand ridge field constitutes a valuable potential sand resource of 22 million m³ of well-sorted medium and coarse sand with limited mud content, which must be preserved as a strategic sand reservoir. © 2020 John Wiley & Sons, Ltd.  相似文献   

Flux-Corrected Transport with MT3DMS for Positive Solution of Transport with Full-Tensor Dispersion     

Shuo Yan  Albert J. Valocchi 《Ground water》2020,58(3):338-348

Solute transport is usually modeled by the advection-dispersion-reaction equation. In the standard approach, mechanical dispersion is a tensor with principal directions parallel and perpendicular to the flow vector. Since realistic scenarios include nonuniform and unsteady flow fields, the governing equation has full tensor mechanical dispersion. When conventional grid-based numerical methods are used, approximation of the cross terms arising from the off-diagonal terms cause nonphysical solution with oscillations. As an example, for the common scenario of contaminant input into a domain with zero initial concentration, the cross-dispersion terms can result in negative concentrations that can wreak havoc in reactive transport applications. To address this issue, we use the well-known flux-corrected-transport (FCT) technique for a standard finite volume method. Although FCT has most often been used to eliminate oscillations resulting from discretization of the advection term for explicit time stepping, we show that it can be adapted for full-tensor dispersion and implicit time stepping. Unlike other approaches based on new discretization techniques (e.g., mimetic finite difference, nonlinear finite volume), FCT has the advantage of being flexible and widely applicable. Implementation of FCT requires solving an additional system of equations at each time step, using a modified “low order” matrix and a modified right-hand-side vector. To demonstrate the flexibility of FCT, we have modified the well-known and widely used groundwater solute transport simulator, MT3DMS. We apply the new simulator, MT3DMS-FCT, to several benchmark problems that suffer from negative concentrations when using MT3DMS. The new results are mass conservative and strictly nonnegative.  相似文献   

Dependence of sediment sorting on bedload transport phase in a river meander          下载免费PDF全文

Albert Rovira  Francisco Núñez‐González  Carles Ibañez 《地球表面变化过程与地形》2018,43(10):2077-2088

It is widely recognized nowadays that there are at least two different phases of bedload sediment transport in gravel‐bed rivers. However, the transition between these phases is still poorly or subjectively defined, especially at bends in rivers, where cross‐stream sediment transport can strongly influence changes in the texture of the transported sediment. In this paper, we use piecewise models to identify objectively, at two points in the cross‐section of a river bend, the discharge at which the transition between bedload transport phases occurs. Piecewise models were applied to a new bedload data set collected during a wide range of discharges while analysing the associated changes in sediment texture. Results allowed the identification of two well‐differentiated phases of sediment transport (phase I and phase II), with a breakpoint located around bankfull discharge. Associated with each phase there was a change in bedload texture. In phase I there was non‐dominance in the transport of fine or coarse fractions at a particular sampling point; but in phase II bedload texture was strongly linked to the position of the sampling point across the channel. In this phase, fine particles tended to be transported to the inner bank, while coarse sizes were transferred throughout the middle parts of the channel. Moreover, bedload texture at the inner sampling point became bimodal while the transport of pebble‐sized particles was increasing in the central parts of the river channel. It is suggested that this general pattern may be related both to secondary currents, which transfer finer particles from the outer to the inner bank, and to the progressive dismantling of the riverbed surface layer. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

Thermal history and evolution of Mars     

M. Nafi Toksöz  Albert T. Hsui 《Icarus》1978,34(3):537-547

A theoretical thermal evolution model of Mars is constructed, utilizing as constraints the available geophysical and geological data, including those provided by the Viking missions. The calculation includes conduction and subsolidus mantle convection. Calculated models indicate that Martian evolution can be roughly characterized by four different stages. (1) Core formation and crust differentiation: this stage starts from the planet formation to about 1 by thereafter. During this period, Martian core is separated and the initial crust is differentiated. (2) Heating, expansion, and mantle differentiation: this stage begins after the core separation and extends to about 3 by. First, mantle temperatures rise and reach partial melting. Between 2 and 3 by, extensive melting, differentiation, and outgassing occur. Planetary radius increases and extensional features observed at the surface are most likely generated at this stage. (3) Mature phase: after 3 by, the planet reaches maturity. Between 3 and 4 by slow and sustained evolution continues. Lithosphere thickens and partial melt zone deepens. (4) Cooling period: this stage represents the last phase of Martian history. The planet is cooling slowly. The partial melting zone shrinks and volcanic activity tapers off. At present, Martian lithosphere is about 200 km thick and the mantle is convecting slowly. The models suggest that the core is molten, and the calculated surface heat flux is 35 erg cm^?2 sec^?1.  相似文献   

Testing the preservation potential of early diagenetic dolomites as geochemical archives     

Mathias Mueller  Onyedika A. Igbokwe  Benjamin Walter  Chelsea L. Pederson  Sylvia Riechelmann  Detlev K. Richter  Richard Albert  Axel Gerdes  Dieter Buhl  Rolf D. Neuser  Giovanni Bertotti  Adrian Immenhauser 《Sedimentology》2020,67(2):849-881

Early marine diagenetic dolomite is a rather thermodynamically-stable carbonate phase and has potential to act as an archive of marine porewater properties. However, the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity. Here, the archive potential of early marine dolomites exposed to extreme post-depositional processes is tested using various types of analyses, including: petrography, fluid inclusion data, stable δ¹³C and δ¹⁸O isotopes, ⁸⁷Sr/⁸⁶Sr ratios, and U-Pb age dating of various dolomite phases. In this example, a Triassic carbonate platform was dissected and overprinted (diagenetic temperatures of 50 to 430°C) in a strike-slip zone in Southern Spain. Eight episodes of dolomitization, a dolostone cataclasite and late stage meteoric/vadose cementation were recognized. The following processes were found to be diagenetically relevant: (i) protolith deposition and fabric-preservation, and marine dolomitization of precursor aragonite and calcite during the Middle–Late Triassic; (ii) intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a Proto-Atlantic opening stress regime (T ca 250°C) during the Early–Middle Jurassic; (iii) dolomite cement precipitation during early Alpine tectonism, rapid burial to ca 15 km, and high-grade anchizone overprint during Alpine tectonic evolution in the Early Eocene to Early Miocene; (iv) brecciation of dolostones to cataclasite during the onset of the Carboneras Fault Zone activity during the Middle Miocene; and (v) late-stage regression and subsequent meteoric overprint. Data shown here document that, under favourable conditions, early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 10⁸ or more years. Evidence for this preservation includes preserved Late Triassic seawater δ¹³C_DIC values and primary fluid inclusion data. Data also indicate that oversimplified statements based on bulk data from other petrographically-complex dolomite archives must be considered with caution.  相似文献   

Assessing land-use and carbon stock in slash-and-burn ecosystems in tropical mountain of Laos based on time-series satellite images     

Yoshio Inoue  Yoshiyuki Kiyono  Hidetoshi Asai  Yukihito Ochiai  Jiaguo Qi  Albert Olioso  Tatsuhiko Shiraiwa  Takeshi Horie  Kazuki Saito  Linkham Dounagsavanh 《International Journal of Applied Earth Observation and Geoinformation》2010

In the tropical mountains of Southeast Asia, slash-and-burn (S/B) agriculture is a widely practiced and important food production system. The ecosystem carbon stock in this land-use is linked not only to the carbon exchange with the atmosphere but also with food and resource security. The objective of this study was to provide quantitative information on the land-use and ecosystem carbon stock in the region as well as to infer the impacts of alternative land-use and ecosystem management scenarios on the carbon sequestration potential at a regional scale. The study area was selected in a typical slash-and-burn region in the northern part of Laos. The chrono-sequential changes of land-use such as the relative areas of community age and cropping (C) + fallow (F) patterns were derived from the analysis of time-series satellite images. The chrono-sequential analysis showed that a consistent increase of S/B area during the past three decades and a rapid increase after 1990. Approximately 37% of the whole area was with the community age of 1–5 years, whereas 10% for 6–10 years in 2004. The ecosystem carbon stock at a regional scale was estimated by synthesizing the land-use patterns and semi-empirical carbon stock model derived from in situ measurements where the community age was used as a clue to the linkage. The ecosystem carbon stock in the region was strongly affected by the land-use patterns; the temporal average of carbon stock in 1C + 10F cycles, for example, was greater by 33 MgC ha⁻¹ compared to that in 1C + 2F land-use pattern. The amount of carbon lost from the regional ecosystems during 1990–2004 periods was estimated to be 42 MgC ha⁻¹. The study approach proved to be useful especially in such regions with low data-availability and accessibility. This study revealed the dynamic change of land-use and ecosystem carbon stock in the tropical mountain of Laos as affected by land-use. Results suggest the significant potential of carbon sequestration through changing land-use and ecosystem management scenarios. These quantitative estimates would be useful to better understand and manage the land-use and ecosystem carbon stock towards higher sustainability and food security in similar ecosystems.  相似文献   

Evidence of Younger Dryas and Neoglacial cooling in a Late Quaternary palaeotemperature record from a speleothem in eastern Victoria,Australia     

Albert Goede  Frank McDermott  Chris Hawkesworth  John Webb  Brian Finlayson 《第四纪科学杂志》1996,11(1):1-7

A calcitic stalagmite collected from a limestone cave in the Buchan area of eastern Victoria has been dated by three mass-spectrometric uranium series analyses. Two growth phases are represented: the older from 13.4 to 10.6 ka and the younger from 3.2 to 2.1 ka. Oxygen isotope analysis reveals that temperatures were below present-day values at all times, but particularly cool conditions are indicated between 12.3 and 11.4 ka, and Neoglacial conditions occurred at about 3 ka. The older cold climate event is clearly synchronous with the Younger Dryas in Europe and this is the first time that strong evidence for this event has been found in Australia. Carbon isotope variations are interpreted as indicating changes in plant productivity on the surface and are most likely controlled by variations in summer rainfall. They indicate particularly high levels of plant productivity from 11.5 to 11.0 ka. © 1996 John Wiley & Sons, Ltd.  相似文献   

FOREWORD     

Mary R. Albert  Susan Taylor 《水文研究》1996,10(12):1551-1551

  相似文献