Völker, Christoph
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Völker, Christoph
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Völker, Christoph
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Item-typ:Veröffentlichung, Uncertainty quantification for ocean biogeochemical models(2024-02-07); ; ; ; Predicting climate change necessitates a thorough understanding of marine biogeochemical (BGC) processes and the coupling between marine ecosystems and the global carbon cycle. Ocean BGC models are tools employed for this purpose. However, current ocean models used to simulate and thus better understand the ocean BGC processes are highly uncertain in their parameterization. This work delves into research to quantify uncertainties that arise in ocean BGC models and obtain improved parameters to reduce those uncertainties utilizing the BGC ocean model Regulated Ecosystem Model Version 2. A Global Sensitivity Analysis (GSA) is performed to identify which parameters most influence the uncertainty of model outputs in a one-dimensional (1-D) configuration at two ocean sites in the North Atlantic (BATS) and the Mediterranean Sea (DYFAMED). This work finds that the grazing parameter, the maximum chlorophyll-to-nitrogen ratio, the photosynthesis parameters, and the chlorophyll degradation rate are significant for BGC simulation. This dissertation uses ensemble data assimilation to estimate the most important BGC process parameters. First, data assimilation experiments are carried out in a 1-D model using an ensemble Kalman Filter to estimate preselected BGC parameters at BATS and DYFAMED stations. Subsequently, the scope and application of experiments are broadened to a global scale 3-D model by incorporating spatial variations in parameter values. Replacing the default parameter values with the optimal values obtained in this work improves the model outcomes in both 1-D and 3-D configurations. This work underscores the importance of spatially varying parameter optimization and highlights the potential benefits of incorporating spatially varying BGC parameters in regional and global 3-D BGC models. Through such rigorous scientific endeavors, we inch closer to a more coherent understanding of the complex interplay between the ocean BGC processes and the carbon cycle.Dissertation429 301 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Mechanistic understanding of the marine biogeochemical proxy d30Si: A modeling approach(2014-03-27); ; ; The stable silicon isotopic composition d30Si of silicic acid and of biogenic opal is used as a proxy for investigating the marine silicon cycle and silicic acid utilization by diatoms both at present and in the geological past. The marine biogeochemical and physical processes involved in determining the modern d30Si distribution have not been fully understood.Hence, the usage of d30Si as a proxy for reconstruction of the marine silicon cycle and paleoproductivity by diatoms is hampered. This work is aimed at providing a comprehensive view and systematic approaches for understanding the oceanic d30Si distribution and its controlling mechanisms under both present and the last glacial maximum (LGM) climate conditions. A coupled ocean (MPI-OM)-biogeochemical (HAMOCC5.1) model is applied to simulate the marine silicon cycle and the silicon isotopic fractionation processes during biogenic opal production and dissolution. In the present-day simulation, the surface d30Si increases along a Rayleigh type distillation curve during the utilization of silicic acid by diatoms, which demonstrates the primary control of biological fractionation on the surface d30Si distribution. The variations between the Rayleigh curves in different ocean basins, on the other hand, show the impact of physical transport of water on determination of the surface d30Si. In the deep ocean, our model captures a significant silicon isotopic gradient between the North Atlantic and the North Pacific. The advection related to the thermohaline circulation is thought to be the essential controlling factor of deep ocean d30Si. The model-data comparison implies that the usage of fractionation during biogenic opal dissolution as explanation to d30Si distribution is still speculative. The modeled silicic acid concentrations and d30Si show good agreement with the observations, when only fractionation during opal production is considered. The capability of the model to reproduce the large-scale modern oceanic d30Si distribution gives us confidence in simulating the d30Si during the LGM. In the LGM simulation, the extension of sea-ice cover in both the Northern and the Southern Hemisphere may cause a reduction of phytoplankton growth due to low light under the ice. The silicic acid utilization by diatoms especially around Antarctica is therefore inhibited, in line with reduced biogenic opal export fluxes. Our preliminary model results of the glacial Si isotopic composition agree with the interpretation from sediment core data that silicic acid utilization by diatoms in the Southern Ocean during the LGM was diminished relative to the present interglacial. The sensitivity of d30Si to glacial-interglacial ocean physical variations such as the strength of overturning circulation and ocean surface mixing is tested, using a simple seven-box model. The results indicate that the changes in global average d30Si due to the glacial-interglacial ocean physical variation may be of a similar magnitude as the total glacial-interglacial d30Si variation. The modeling approach is a valid and powerful tool of promoting a mechanistic understanding of the marine biogeochemical proxy d30Si. One important advantage over common interpretation of local field studies is that models calculate isotopic fractionation without application of Rayleigh or open system approximation. In addition, the sensitivity of d30Si to various biogeochemical and physical factors can be tested systematically.Dissertation370 143 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Coupling and decoupling of biogeochemical cycles in marine ecosystems(2008-12-19); ; ; The biogeochemical cycles of biologically important elements are coupled to each other via the formation of biomass. Many ecosystem models assume this coupling to follow fixed stoichiometric ratios, even though, under certain environmental conditions, the stoichiometric composition of marine phytoplankton can deviate strongly from fixed Redfield ratios. This thesis investigates the effect of variable phytoplankton stoichiometry on large scale biogeochemical fluxes in different marine biological systems. In the first study, an ecosystem model is developed for a shallow coastal tidal basin in the Danish-German Wadden Sea and the adjacent North Sea. The model allows for variations in the cellular quotas of carbon (C), nitrogen (N), and chlorophyll (Chl) of the simulated phytoplankton biomass. The phytoplankton C:N ratio in the tidal basin is found to vary from 5 to 15 between light-limited winter conditions and nitrogen-limited summer growth conditions, respectively. Different water depths between the North Sea and the shallow tidal inlet lead to differences in phytoplankton C:N ratios that can also induce a decoupling of carbon and nitrogen fluxes in the budgeting of the annual tidal transport between the North Sea and the Wadden Sea. The second study extends the parameterization of phytoplankton growth by inclusion of the elements silicon (Si) and iron (Fe) to obtain a parameterization for diatom growth that can be applied in diatom-dominated high-nutrient low-chlorophyll (HNLC) ocean regions like the Southern Ocean. The parameterization considers separate pools of cellular chlorophyll, carbon, nitrogen, and silicon and reproduces the elevated Si:N uptake ratios of diatoms growing under iron-limitation. In the third study, the parameterization of diatom growth is applied to an ecosystem model that is coupled to a global setup of the ocean general circulation model of the Massachussets Institute of Technology (MITgcm). The model is adjusted to the Southern Ocean ecosystem and analysed for the biogeochemical fluxes of silicon and nitrogen in the Southern Ocean. Low iron concentrations in the Antarctic Circumpolar Current (ACC) lead to elevated Si:N uptake ratios of Southern Ocean diatoms and a stronger depletion of dissolved silica over dissolved inorganic nitrogen in northwards flowing surface waters. The northwards flowing surface waters are subducted and involved in the formation of Southern Annular Mode Water (SAMW) which later becomes Antarctic Intermediate Water (AAIW) and supplies the North Atlantic Ocean with nutrients. The stoichiometric signature of SAMW and the supply of nutrients to the North Atlantic thus depends on the decoupling of silicon and nitrogen metabolism in diatoms and its dependence on iron concentrations in Southern Ocean surface waters. The fourth study focuses on scalability and computational costs of a high-resolution model version of the described global biogeochemical model on a multi-processor supercomputer. The models analysed in this thesis present a progression towards the development of a global biogeochemical ocean general circulation model that realistically reproduces nutrient distributions as a basis to make future predictions of global carbon fluxes.Dissertation573 207