4:15 PM - 4:30 PM
[ACG38-09] Modeling of Organic Matter Decomposition Process Considering Redox Properties in Coastal Marine Sediment
Keywords:Sediment model, numerical simulation, organic matter decomposition, redox potential
The carbon cycle in coastal areas is attracting attention as exemplified by blue carbon, one of the adaptation and mitigation measures for climate change (Santos et al, 2021). Organic matter decomposition in seafloor sediments is an important process for carbon sequestration and storage, and sediment models contribute to the quantitative evaluation of carbon sequestration.
In this study, we attempted numerical simulation of the organic matter decomposition process in seafloor sediments in Shizugawa Bay, Miyagi Prefecture, Japan. One-dimensional vertical sediment model with a depth of 20 cm and 101 layers has been created. Concentration changes of both dissolved and solid materials in the sediment are calculated by the mass balance equations including molecular diffusion, bio-diffusion, burial, and net production from 21 chemical reactions. Organic decomposition was modeled using the traditional model in which five oxidants (oxygen, nitrate, manganese dioxide, iron hydroxide, and sulfate ions) are consumed in sequence according to a concentration threshold (Fossing, 2004). Field surveys were conducted to obtain measured data for model inputs and validation. Sediment coring was conducted to analyze particulate organic matter (POM), dissolved organic carbon (DOC), and nutrient concentrations in porewater. Sediment traps were installed on the seafloor and an organic matter deposition flux was obtained. In addition to Shizugawa Bay, field surveys were also conducted in coral reefs in Ishigaki Island, which is considered to have different sediment characteristics. The redox potential (ORP) of sediment cores was measured using a hand-made micro platinum electrode. The redox properties for each study site were discussed from the vertical distribution of ORP.
The model reproduced the distribution of nutrient concentrations, although concentrations of organic matters were overestimated. Comparison of oxidant consumption rates for organic matter decomposition in the model showed that aerobic respiration was dominant in the top 1 cm of sediment, while sulfate reduction was dominant deeper than 1 cm. These redox properties were similar as the tendency of measured ORP distributions, which mean the model seemed to have represented the redox characteristics by primary reaction rates.
In this study, we attempted numerical simulation of the organic matter decomposition process in seafloor sediments in Shizugawa Bay, Miyagi Prefecture, Japan. One-dimensional vertical sediment model with a depth of 20 cm and 101 layers has been created. Concentration changes of both dissolved and solid materials in the sediment are calculated by the mass balance equations including molecular diffusion, bio-diffusion, burial, and net production from 21 chemical reactions. Organic decomposition was modeled using the traditional model in which five oxidants (oxygen, nitrate, manganese dioxide, iron hydroxide, and sulfate ions) are consumed in sequence according to a concentration threshold (Fossing, 2004). Field surveys were conducted to obtain measured data for model inputs and validation. Sediment coring was conducted to analyze particulate organic matter (POM), dissolved organic carbon (DOC), and nutrient concentrations in porewater. Sediment traps were installed on the seafloor and an organic matter deposition flux was obtained. In addition to Shizugawa Bay, field surveys were also conducted in coral reefs in Ishigaki Island, which is considered to have different sediment characteristics. The redox potential (ORP) of sediment cores was measured using a hand-made micro platinum electrode. The redox properties for each study site were discussed from the vertical distribution of ORP.
The model reproduced the distribution of nutrient concentrations, although concentrations of organic matters were overestimated. Comparison of oxidant consumption rates for organic matter decomposition in the model showed that aerobic respiration was dominant in the top 1 cm of sediment, while sulfate reduction was dominant deeper than 1 cm. These redox properties were similar as the tendency of measured ORP distributions, which mean the model seemed to have represented the redox characteristics by primary reaction rates.