| 1. |
- Bauwens, Maite, et al.
(författare)
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A Nonlinear Multi-Proxy Model Based on Manifold Learning to Reconstruct Water Temperature from High Resolution Trace Element Profiles in Biogenic Carbonates
- 2010
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Ingår i: Geoscientific Model Development. - : Copernicus Publications. - 1991-959X .- 1991-9603. ; 3:3, s. 653-667
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Tidskriftsartikel (refereegranskat)abstract
- A long standing problem in paleoceanography concerns the reconstruction of water temperature from δ18O carbonate, which for freshwater influenced environments is hindered because the isotopic composition of the ambient water (related to salinity) affects the reconstructed temperature. In this paper we argue for the use of a nonlinear multi-proxy method called Weight Determination by Manifold Regularization to develop a temperature reconstruction model that is less sensitive to salinity variations. The motivation for using this type of model is twofold: Firstly, observed nonlinear relations between specific proxies and water temperature motivate the use of nonlinear models. Secondly, the use of multi-proxy models enables salinity related variations of a given temperature proxy to be explained by salinity-related information carried by a separate proxy. Our findings confirm that Mg/Ca is a powerful paleothermometer and highlight that reconstruction performance based on this proxy is improved significantly by combining its information with the information of other trace elements in multi-proxy models. Using Mg/Ca, Sr/Ca, Ba/Ca and Pb/Ca the WDMR model enabled a temperature reconstruction with a root mean squared error of ±2.19 °C for a salinity range between 15 and 32.
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| 2. |
- Bauwens, Maite, et al.
(författare)
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On Climate Reconstruction using Bivalve Shells : Three Methods to interpret the Chemical Signature of a Shell
- 2010
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The chemical composition of a bivalve shell is strongly coupled to the seasonal variations in the environment. The nonlinear nature of this relation however makes it hard to predict, e.g. the temperature, from the chemical composition of a shell. In this paper we compare the ability of three nonlinear system identification methods to reconstruct the temperature from the chemical composition of a shell. The comparison shows that nonlinear multi-proxy approaches are potential tools for climate reconstructions with a preference for manifold based methods that results in smoother and a more precise temperature reconstruction.
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| 3. |
- Bauwens, Maite, et al.
(författare)
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On Climate Reconstruction Using Bivalve Shells : Three Methods To Interpret the Chemical Signature of a Shell
- 2011
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Ingår i: Computer Methods and Programs in Biomedicine. - : Elsevier. - 0169-2607 .- 1872-7565. ; 104:2, s. 104-111
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Tidskriftsartikel (refereegranskat)abstract
- To improve our understanding of the climate process and to assess the human impact on current global warming, past climate reconstruction is essential. The chemical composition of a bivalve shell is strongly coupled to environmental variations and therefore ancient shells are potential climate archives. The nonlinear nature of the relation between environmental condition (e.g. the seawater temperature) and proxy composition makes it hard to predict the former from the latter, however. In this paper we compare the ability of three nonlinear system identification methods to reconstruct the ambient temperature from the chemical composition of a shell. The comparison shows that nonlinear multi-proxy approaches are potentially useful tools for climate reconstructions and that manifold based methods result in smoother and more precise temperature reconstruction.
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| 4. |
- Bauwens, Maite, et al.
(författare)
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On Climate Reconstruction using Bivalve Shells: Three Methods to interpret the Chemical Signature of a Shell
- 2009
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Ingår i: Proceedings of the 7th IFAC Symposium on Modelling and Control in Biomedical Systems (including Biological Systems). - 9783902661494 ; , s. 407-412
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Konferensbidrag (refereegranskat)abstract
- The chemical composition of a bivalve shell is strongly coupled to the seasonal variations in the environment. The nonlinear nature of this relation however makes it hard to predict, e.g. the temperature, from the chemical composition of a shell. In this paper we compare the ability of three nonlinear system identification methods to reconstruct the temperature from the chemical composition of a shell. The comparison shows that nonlinear multi-proxy approaches are potential tools for climate reconstructions with a preference for manifold based methods that results in smoother and a more precise temperature reconstruction.
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| 5. |
- konn, cecile, et al.
(författare)
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Fluids from ultramafic-hosted hydrothermal systems of the Mid Atlantic Ridge - Organics and Life.
- 2007
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Ingår i: EGU general assembly.
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Konferensbidrag (populärvet., debatt m.m.)abstract
- The process of serpentinisation at slow spreading ridges is due to the circulation of seawater in outcropping mantle rocks. The periodites of the mantle are altered via fluid-rocks interaction generating high CH4- and H2-concentrations in the water column. The likely abiogenic origin of the methane has been supported by the isotope-ratio values. This has lead to the idea of abiogenic formation of larger organic compounds such as hydrocarbons or key molecules for the origin of life issue. Furthermore, Fisher-Tropsh type reactions might be a likely pathway to the production of these organic. Both thermodynamics and laboratory work support this idea, whilst field data are currently studied. During the EXOMAR cruise conducted by IFREMER, France; hydrothermal fluids from the MAR have been collected at different hot vent sites presenting highly different features (high/low pH, ultramafic/basaltic petrology, etc.). Innovative and efficient techniques have been used to concentrate and extract compounds from the fluids. As such, SPE (Solid Phase Extraction)-GC-MS and SBSE (Stir Bar Sorptive Extraction)-TD (ThermalDesorption)-GC-MS analyses showed the presence of a whole range of organics in the fluids. Mainly hydrocarbons, but also oxygen- and nitrogen-compounds were clearly identified by comparison of recorded mass spectra with library data. In order to establish whether the compounds were biogenic or abiogenic, carbon isotopic ratio measurements have been performed at the Vrije Universiteit Brussel. The 13C values suggest a mix of abiogenic and biogenic carbon for a great majority of the molecules. However, preliminary results indicate a possible abiogenic origin for a few compounds.This first step allowed us to get an overview of the fluids composition. We are now focusing on specific compounds in an origin of life perspective. Much effort has been put since to optimise the sample conditioning method in order to separate and concentrate targeted compounds. New methods will be tested during the last SERPENTINE mission to be conducted by IFREMER, France. We are expecting especially better results on the fatty acids. We are hoping as well to detect some origin of life key-compounds. This work is carried out partly within the MoMARnet (Monitoring deep sea floor hydrothermal environments on the Mid-Atlantic Ridge: A Marie Curie Research Training network’) framework.
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| 6. |
- Konn, Cécile, et al.
(författare)
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Hydrocarbons and oxidized organic compounds in hydrothermal fluids from Rainbow and Lost City ultramafic-hosted vents
- 2009
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 258, s. 299-314
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Tidskriftsartikel (refereegranskat)abstract
- The first building blocks of life could be produced in ultramafic-hosted hydrothermal systems considering the large amounts of hydrogen and methane generated by serpentinisation and Fischer-Tropsch-Type synthesis, respectively, in those systems. The purpose of this study was to detect and characterise organic molecules in hydrothermal fluids from ultramafic-hosted hydrothermal systems in the Mid-Atlantic Ridge (MAR) region. During the EXOMAR cruise 2005, fluids from the Rainbow (36°14’N) and the Lost City (30°N) hydrothermal fields were collected and treated by Stir Bar Sorptive Extraction (SBSE) and Solid Phase Extraction (SPE). The extracts were analysed by Thermal Desorption - Gas Chromatography - Mass Spectrometry (TD-GC-MS) and GC-MS, respectively. Compared to nearby deep seawater, hydrothermal fluids were clearly enriched in organic compounds, with a more diverse spectrum of molecules. We observed a very similar range of organic compounds in fluids from both sites, with a dominance of aliphatic hydrocarbons (C9-C14), aromatic compounds (C6-C16) and carboxylic acids (C8-C18). The occurrence of these compounds is supported by other field studies on serpentinites and sulfide deposits. Literature on thermodynamic data and experimental work has suggested the possible abiogenic origin of hydrocarbons and organic acids. In addition, it has been shown elsewhere that catalytic reactions producing hydrocarbons likely occur at both Lost City and Rainbow hydrothermal fields as suggested by the evolution of δ13C with increasing C number for methane, ethane, propane and butane. In order to investigate the origin of the organic molecules in the fluids, compound-specific carbon isotope ratio measurements were performed on n-alkanes and carboxylic acids, for which the δ13C values were in the range of -46 to -20‰ (vs. V-PDB). These preliminary data did not allow conclusive support or rejection of an abiogenic origin of the compounds. Indeed, predicting δ13C signatures in hydrothermal systems is likely to be complicated, due to differences in source δ13C signatures (i.e., of the C building blocks), and a variety of, mostly unknown, fractionation steps which may occur along the synthesis pathways. In addition, even though a fraction of the compounds detected in the fluids is likely abiotically produced, a dominance of biogenic sources and/or processes might hide their characteristic signature.
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| 7. |
- konn, cecile, et al.
(författare)
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Organic geochemistry of fluids from 4 ultramafic-hosted hydrothermal systems of the Mid Atlantic Ridge.
- 2007
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Ingår i: Interridge Woods Hole Oceanographic Institute (WHOI) theoretical Institute.
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Konferensbidrag (populärvet., debatt m.m.)abstract
- Ultramafic-hosted hydrothermal sites are characterized by mantle outcrops. On the Mid-Atlantic ridge, the Eurasian and American plates are moving appart resulting in the oceanic crust to tear up and to let the mantle rocks outcrop. Circulation of seawater, along the faults, within the mantle alters the periodites via the serpentinisation process, which produces a large amount of H2. Notably, H2 is a great source of energy for further chemical reactions. Besides, a high CH4 concentration in the water column is associated with ultramafic hydrothermal activity. This methane has been suggested to be abiogenic and to be formed via a Fisher-Tropsh synthesis (3H2 + CO CH4 + H2O). The isotope-ratio values support the later hypothesis. This has lead to the idea of abiogenic formation of larger organic compounds such as hydrocarbons or key molecules for the origin of life issue. Both thermodynamics and laboratory work support this idea, but field data had not been studied yet. During the EXOMAR and the SERPENTINE cruises conducted by IFREMER, France; hydrothermal fluids from the MAR have been collected at different hot vent sites (Rainbow, Lost City, Logachev, Ashadze). Innovative and efficient techniques have been developed, used and improved to concentrate, isolate and extract compounds from the fluids by SPE (Solid Phase Extraction)-GC-MS and SBSE (Stir Bar Sorptive Extraction)-TD (ThermalDesorption)-GC-MS analyses. Mainly hydrocarbons, but also oxygen- and nitrogen-compounds were clearly -identified by comparison of recorded mass spectra with library data. As for the analysis process, a particular attention is now paid to carboxylic acids and hydrocarbons as well as volatiles (C1-C8).This work is carried out partly within the MoMARnet (Monitoring deep sea floor hydrothermal environments on the Mid-Atlantic Ridge: A Marie Curie Research Training network’) framework.
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| 8. |
- konn, cecile, et al.
(författare)
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Organics in hydrothermal fluids from 4 ultramafic-hosted vents of the MAR. Results from the SERPENTINE cruise (2007).
- 2008
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Ingår i: EGU, Vienna.
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Konferensbidrag (populärvet., debatt m.m.)abstract
- Ultramafic-hosted hydrothermal sites are characterized by mantle outcrops. On the Mid-Atlantic ridge, the Eurasian and American plates are moving apart resulting in the oceanic crust to tear up and to let the mantle rocks outcrop. Circulation of seawater, along the faults, within the mantle alters the periodites via the serpentinisation process, which produces a large amount of H2. Notably, H2 is a great source of energy for further chemical reactions. Besides, a high CH4 concentration in the water column is associated with ultramafic hydrothermal activity. This methane has been suggested to be abiogenic and to be formed via a Fischer-Tropsch synthesis (3H2 + CO gives CH4 + H2O). The isotope-ratio values support the later hypothesis. This has lead to the idea of abiogenic formation of larger organic compounds such as hydrocarbons or key molecules for the origin of life issue. Both thermodynamics and laboratory work support this idea, but field data had not been studied yet.During the EXOMAR (2005), SERPENTINE (2007) and MoMARDREAMNAUT (2007) cruises conducted by IFREMER, France; hydrothermal fluids from the MAR have been collected at different hot vent sites (Rainbow, Lost City, Logatchev and Ashadze). Their geological settings vary, leading to largely different fluids features as for T, pH, Cl and H2S concentration. Innovative and efficient techniques have been developed, used and improved (IFREMER, Brest) to concentrate, isolate and extract organic compounds from the fluids: SPE (Solid Phase Extraction)-GC-MS. Mainly hydrocarbons (C8 to C18), carboxylic acids and methyl esters were clearly identified by comparison of recorded mass spectra with library data. δ13C values have been measured by GC-IRMS (VUB Brussels) and were in the range -20‰ to -45‰, which did not allow to conclusively support or reject an abiotic origin.This work is carried out partly within the MoMARnet (Monitoring deep sea floor hydrothermal environments on the Mid-Atlantic Ridge: A Marie Curie Research Training network) framework.
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| 9. |
- Mohammed, Salim, 1950-
(författare)
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Nutrient Dynamics and Exchanges Between a Mangrove Forest and a Coastal Embayment: Chwaka Bay, Zanzibar
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nutrient dynamics and exchanges were studied in the Mapopwe Creek, a tidally dominated mangrove lined waterway, and a shallow lagoon in Chwaka Bay, Zanzibar, in order to assess whether there was any ecological coupling between the mangrove forest and adjacent ecosystems. In this study it was found that seasonal terrestrial input sources and benthic microalgal uptake strongly influenced the concentrations and distribution of dissolved nutrients in the system. The waterway had low inorganic nutrient levels and concentrations showed considerable spatial variability with values decreasing towards the mouth. The low water column concentrations were attributed principally to low input from the sediment. The sediment showed strong porewater gradients of ammonia and soluble reactive phosphorus (SRP) while nitrate plus nitrite (NOx) was limited to the top 1 cm of sediment in low concentrations. Generally, both water column and porewater nutrients showed strong seasonal fluctuations in concentrations. Mapopwe sediment showed complex and inconsistent sediment-water exchange patterns. Generally though, ammonia dominated benthic fluxes and fluxes of ammonia were mainly directed towards the water column. By contrast SRP showed low levels of exchange and the creek sediment acted as a sink for the nutrient. No fluxes of NOx were detected. Unlike the mangrove waterway, there was no significant sediment-water exchange of nutrients in the bay.The sediment in the creek showed low denitrification rates. This was attributed to low water column nitrate concentrations, low rates of in situ nitrification and competition for nitrate between nitrifiers and other benthic microbial populations. Strong spatial differences in the denitrification rates in the creek were attributed to unequal supply of organic matter between sites, and spatial differences in the distribution of bioturbating organisms and microautotrophs in the sediment.The study showed that there was a strong coupling between the forest and adjacent seagrasses and algal communities. Although measurements of tidal exchanges of dissolved nutrients between the mangrove forest and the bay did not show any exchanges between these biotopes, however the study showed that the forest exported a significant amount of particulate organic matter to the bay. This material was however trapped and utilised in production by seagrass beds just outside the mangrove forest; the communities growing farther from the forest were only indirectly affected. Apparently, mangroves may support productions in other systems but not directly. However, the seagrass and algal communities growing adjacent to the forest were vital in utilising and conserving mangrove outputs. Cosnsequently, nutrients are efficiently conserved within the bay and the associated mangrove forest implying that if the forest were to receive an increased nutrient load, they will not be removed and may result in eutrophication of the system.
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| 10. |
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