| 55821. |
- Yu, Shiyong, et al.
(författare)
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A dinoflagellate cyst record of Holocene climate and hydrological changes along the southeastern Swedish Baltic coast
- 2007
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Ingår i: Quaternary Research. - : Cambridge University Press (CUP). - 0033-5894 .- 1096-0287. ; 67:2, s. 215-224
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Tidskriftsartikel (refereegranskat)abstract
- A high-resolution, well-dated dinoflagellate cyst record from a lagoon of the southeastern Swedish Baltic Sea reveals climate and hydrological changes during the Holocene. Marine dinoflagellate cysts occurred initially at about 8600 cal yr BP, indicating the onset of the Littorina transgression in the southeastern Swedish lowland associated with global sea level rise, and thus the opening of the Danish straits. Both the species diversity and the total accumulation rates of dinoflagellate cysts continued to increase by 7000 cal yr BP and then decreased progressively. This pattern reveals the first-order change in local sea level as a function of ice-volume-equivalent sea level rise versus isostatic land uplift. Superimposed upon this local sea level trend, well-defined fluctuations of the total accumulation rates of dinoflagellate cysts occurred on quasi] 1000- and 500-yr frequency bands particularly between 7500 and 4000 cal yr BP, when the connection between the Baltic basin and the North Atlantic was broader. A close correlation of the total accumulation rates of dinoflagellate cysts with GISP2 ice core sea-salt ions suggests that fluctuations of Baltic surface conditions during the middle Holocene might have been regulated by quasi-periodic variations of the prevailing southwesterly winds, most likely through a system similar to the dipole oscillation of the modem North Atlantic atmosphere.
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| 55822. |
- Yu, Shiyong, et al.
(författare)
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Holocene organic carbon burial rates in the southeastern Swedish Baltic Sea
- 2007
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Ingår i: The Holocene. - : SAGE Publications. - 0959-6836 .- 1477-0911. ; 17:5, s. 673-681
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Tidskriftsartikel (refereegranskat)abstract
- To estimate Holocene changes in organic carbon mass accumulation rates (C-org MARs) in the southeastern Swedish Baltic Sea two sediment cores were studied in combination with biogeochemical modelling. Prior to 11300 cal. BP, C-org MARs were extremely low, indicating low organic matter production on the catchment of the Baltic Ice Lake. Following a brief regression, the Ancylus Lake stage occurred between I I 100 and 9800 cal. BP. C-org MARs increased substantially during this period because of enhanced washing in of terrestrial organic matter, when boreal forests were initially established. The prominent marine stage, known as the Littorina transgression between 8500 and 3000 cal. BP, is marked by a minor increase in Corg MARs. Our modelling reveals a changing terrestrial organic carbon input between 100 and 1000 g/m(2) per yr that accounts for 30-80% of total organic carbon in sediments of the southeastern Swedish Baltic Sea, with maximum values (c. 20 X 10(3)g/m(2) per yr) occurring during the Bronze/lron Age transition at about 3000 cal. BP. C-org MAR in the entire Baltic basin is estimated at 3.01 X 10(6) t/yr during the pre-industrial Holocene, comparable with other large inland water bodies. Regardless of the source of carbon, our data indicate that the Baltic basin is an important sedimentary reservoir for organic carbon storage and thus should be included in accounting for global terrestrial carbon cycling during the pre-industrial Holocene.
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| 55823. |
- Yu, Shi-Yong, et al.
(författare)
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Holocene organic carbon burial rates in the Baltic coast
- 2007
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Ingår i: The Holocene. - 0959-6836. ; 17:5, s. 673-681
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Tidskriftsartikel (refereegranskat)abstract
- To estimate Holocene changes in organic carbon mass accumulation rates (Corg MARs) in the southeastern Swedish Baltic Sea, two sediment cores were studied in combination with biogeochemical modelling. Prior to 11 300 cal. BP, Corg MARs were extremely low, indicating low organic matter production on the catchment of the Baltic Ice Lake. Following a brief regression, the Ancylus Lake stage occurred between 11 100 and 9800 cal. BP. Corg MARs increased substantially during this period because of enhanced washing in of terrestrial organic matter, when boreal forests were initially established. The prominent marine stage, known as the Littorina transgression between 8500 and 3000 cal. BP, is marked by a minor increase in Corg MARs. Our modelling reveals a changing terrestrial organic carbon input between 100 and 1000 g/m2 per yr that accounts for 30–80% of total organic carbon in sediments of the southeastern Swedish Baltic Sea, with maximum values (c. 20 x 103 g/m2 per yr) occurring during the Bronze/Iron Age transition at about 3000 cal. BP. Corg MAR in the entire Baltic basin is estimated at 3.01 x 106 t/yr during the pre-industrial Holocene, comparable with other large inland water bodies. Regardless of the source of carbon, our data indicate that the Baltic basin is an important sedimentary reservoir for organic carbon storage and thus should be included in accounting for global terrestrial carbon cycling during the pre-industrial Holocene.
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| 55824. |
- Yuan, Jinliang, et al.
(författare)
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Analysis of chemical-reaction-coupled mass and heat transport phenomena in a methane reformer duct for PEMFCs
- 2007
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 50:3-4, s. 687-701
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Tidskriftsartikel (refereegranskat)abstract
- Mass, heat and momentum transport processes are coupled with catalytic chemical reactions in a methane steam reforming duct. it is often found that endothermic and exothermic reactions in the ducts are strongly integrated by heat transfer from adjacent catalytic combustion ducts. In this paper, a three-dimensional calculation method is developed to simulate and analyze reforming reactions of methane, and the effects on various transport processes in a steam reforming duct. The reformer conditions such as mass balances associated with the reforming reactions and gas permeation to/from the porous catalyst reforming layer are applied in the analysis. The predicted results are presented and discussed for a composite duct consisting of a porous catalyst reaction layer, the fuel gas flow duct and solid layers. Parametric studies are conducted to reveal the importance of reformer designs and operating conditions. The results show that the variables, such as porous layer configuration, temperature and catalyst loading, have significant effects on the transport processes and reformer performance.
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| 55825. |
- Yuan, Jinliang, et al.
(författare)
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Analysis of Parameter Effects on Transport Phenomena in Conjunction with Chemical Reactions in Ducts Relevant for Methane Reformers
- 2007
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 1879-3487 .- 0360-3199. ; 32:16, s. 3887-3898
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Tidskriftsartikel (refereegranskat)abstract
- Various transport phenomena in conjunction with chemical reactions are strongly affected by reformer configurations and properties of involved porous catalyst layers. The considered composite duct is relevant for a methane steam reformer and consists of a porous layer for the catalytic chemical reactions, the fuel gas flow duct and solid plate. In this paper, a fully three-dimensional calculation method is developed to simulate and analyze reforming reactions of methane, with purpose to reveal the importance of design and operating parameters grouped as three characteristic ratios. The reformer conditions such as mass balances associated with the reforming reactions and gas permeation to/from the porous catalyst reforming layer are applied in the analysis. The results show that the characteristic ratios have significant effects on the transport phenomena and overall reforming reaction performance.
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| 55826. |
- Yuan, Jinliang, et al.
(författare)
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CFD Approach to Analyze Transport Phenomena Coupled Chemical Reactions Relevant for Methane Reformers
- 2007
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Ingår i: Proceedings of the Fifth Internation Conference Hydrogen Economy and Hydrogen Treatment of Materials. ; 1, s. 176-183
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Konferensbidrag (refereegranskat)abstract
- Various transport phenomena in conjunction with chemical reactions are strongly affected by reformer configurations and properties of involved porous catalyst layers. The considered composite duct is relevant for a methane steam reformer and consists of a porous layer for the catalytic chemical reactions, the fuel gas flow duct and solid plate. In this paper, a fully three-dimensional calculation method is developed to simulate and analyze reforming reactions of methane, with purpose to reveal the importance of design and operating parameters. The reformer conditions such as mass balances associated with the reforming reactions and gas permeation to/from the porous catalyst reforming layer are applied in the analysis. The results show that the characteristic parameters have significant effects on the transport phenomena and overall reforming reaction performance.
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| 55827. |
- Yuan, Jinliang, et al.
(författare)
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Simulation of Heat Transfer and Effects on Reaction-Coupled Transport Processes in SOFCs
- 2007
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Ingår i: Proceedings of 2007 ASME-JSME Thermal Engineering Summer Heat Transfer Conference. - 0791842754 ; 2, s. 5-14
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Konferensbidrag (refereegranskat)abstract
- For anode-supported plate solid oxide fuel cells (SOFCs), the size of the porous anode is bigger than that of the fuel gas flow duct in terms of thickness and cross-sectional area. The heat transfer rates are controlled by various operating and design parameters, and have significant effects on chemical reactions and coupled transport processes. In this study, the considered composite duct is relevant for an intermediate temperature (600-800oC) plate design and consists of a porous anode layer for the internal reforming reactions of methane, the fuel gas flow duct and solid plate. A fully three-dimensional calculation method is developed to simulate and analyze heat transfer and combined effects on internal reforming/ electrochemical reactions and the coupled transport processes, with purpose to reveal the importance of various parameters. The results show that the operating temperatures have significant effects on the chemical reactions, fuel gas distribution and overall performance.
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| 55828. |
- Yuan, Jinliang, et al.
(författare)
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Transport Phenomena Coupled by Chemical Reactions in Methane Reforming Ducts
- 2007
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Konferensbidrag (refereegranskat)abstract
- Mass, heat and momentum transport processes are strongly coupled with catalytic chemical reactions in a methane steam reforming duct. In this paper, a threedimensional calculation method is developed to simulate and analyze reforming reactions of methane, and the effects on various transport processes in a steam reforming duct. The results show that the design and operating parameters grouped as characteristic ratios have significant effects on the transport phenomena.
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| 55829. |
- Yurova, Alla, et al.
(författare)
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Carbon storage in the organic layers of boreal forest soils under various moisture conditions: A model study for Northern Sweden sites
- 2007
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Ingår i: Ecological Modelling. - : Elsevier BV. - 0304-3800. ; 204:3-4, s. 475-484
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Tidskriftsartikel (refereegranskat)abstract
- A typical feature of the boreal forest landscape is a gradient from dry to wet sites, with associated increases in the depth of the soil organic layers. In this study, the coupled ecosystem-soil biogeochemistry model GUESS-ROMUL is used to explore how the specific features of soil organic matter decomposition and vegetation dynamics account for an observed difference between the soils formed under contrasting moisture conditions. Two sites, one mesic and one mesic-to-wet, representative of the natural forest in Northern Sweden, are simulated. In addition to the assumptions underlying the GUESS-ROMUL model, it is assumed that the fire frequency was higher at the mesic site. The model shows that with a natural fire regime, the soil organic layers at the mesic-to-wet site store 6.0 kg C m(-2) compared to 3.1 kg C m(-2) at the mesic site. Forty-seven percent of the difference between the sites in this respect is explained by suppressed decomposition under higher moisture conditions, 37% by the decreased litter input into the soil (more frequently disturbed ecosystems have lower productivity) and 16% by direct consumption of the forest floor in fires. It is predicted that due to anthropogenic fire suppression the organic soil layers of mesic sites will, in the future, sequester carbon at an average rate of 0.0103 kg C m(-2) year(-1) and have an equilibrium storage capacity of 5.4 kg C m(-2). For the mesic-to-wet site, the model predicts an extremely slow sequestration rate of 0.0022 kg C m(-2) year(-1). The effect of increased precipitation on the carbon storage at the landscape level is also investigated.
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| 55830. |
- Yurova, Alla
(författare)
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Hydrological aspects of the carbon balance in a boreal catchment: A model study
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- AbstractThe cycling of carbon in its gaseous, solid and dissolved forms is of central importance to ecosystem science, particularly, when the natural carbon cycle has been perturbed by human activity. Mathematical models are commonly used to asses and predict global and regional patterns of terrestrial carbon exchange. At the same time, processes operating on smaller (sub-grid) scales are usually outside the scope of the large-scale models exploring climate?biosphere interactions. However, if a sub-grid feature is found repeatedly in a large number of particular realizations, we may expect that its characteristic dynamics are likely to be important even at larger scales. This work examines some of these phenomena. First, a boreal landscape differentiated into forests and peatlands is investigated. Second, the export of dissolved organic carbon (DOC) in peatland streams is assessed.The work described herein mainly addresses methodology, i.e. how approaches to hydrology and ecosystem modelling can be combined. Specifically, the general ecosystem model GUESS was modified and supplied with new modules to be used for peatland simulations. The model was also designed to reproduce soil moisture and related ecosystem properties along a topographical catena. In addition, a completely new model of DOC production, transport and transformation in peat was developed. The data for model development and validation came from boreal catchments in the Vindelns Forest Research Parks, The Swedish University of Agricultural Sciences, Northern Sweden. A relatively good fit between the modelled and measured NEE at the Degerö Stormyr peatland was demonstrated for 2001-2003. Further, the model was used to demonstrate that the NEE was quite insensitive to changes in water table level within the ranges typical for the dominant mire plant community. The newly constructed model was able to reproduce DOC concentrations measured at the outlet of the Kallkälsmyren peatland for 1993-2001. The model suggested that the main drivers for interannual variability in DOC concentrations were the rates of microbially mediated DOC production and mineralization and the flow intensity in the active surface layer of the peatland. The amount of DOC in the peat water at a particular time was mainly determined by (much larger) amounts of soluble carbon sorbed on the peat matrix. A dynamic, rather than static, description of the adsorption and desorption processes was advocated.The work makes some contribution to filling the gap that currently exists between field observations at the plot scale and large-scale model analysis of the climate?biosphere interaction.
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