沿岸化学がメタンのモデリングを改善(Coastal chemistry improves methane modeling)

2024-01-25 オークリッジ国立研究所(ORNL)

＜関連情報＞

• https://www.ornl.gov/news/coastal-chemistry-improves-methane-modeling
• https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023MS003762

不均質なミシシッピ川デルタ湿地における地下の酸化還元相互作用が排出性メタンフラックスを制御する Subsurface Redox Interactions Regulate Ebullitive Methane Flux in Heterogeneous Mississippi River Deltaic Wetland

Jiaze Wang, Theresa O’Meara, Sophie LaFond-Hudson, Songjie He, Kanchan Maiti, Eric J. Ward, Benjamin N. Sulman
Journal of Advances in Modeling Earth Systems  Published: 07 January 2024
DOI:https://doi.org/10.1029/2023MS003762

Abstract

As interfaces connecting terrestrial and ocean ecosystems, coastal wetlands develop temporally and spatially complex redox conditions, which drive uncertainties in greenhouse gas emission as well as the total carbon budget of the coastal ecosystem. To evaluate the role of complex redox reactions in methane emission from coastal wetlands, a coupled reactive-transport model was configured to represent subsurface biogeochemical cycles of carbon, nitrogen, and sulfur, along with production and transport of multiple gas species through diffusion and ebullition. This model study was conducted at multiple sites along a salinity gradient in the Barataria Basin at the Mississippi River Deltaic Plain. Over a freshwater to saline gradient, simulated total flux of methane was primarily controlled by its subsurface production and consumption, which were determined by redox reactions directly (e.g., methanogenesis, methanotrophy) and indirectly (e.g., competition with sulfate reduction) under aerobic and/or anaerobic conditions. At fine spatiotemporal scales, surface methane fluxes were also strongly dependent on transport processes, with episodic ebullitive fluxes leading to higher spatial and temporal variability compared to the gradient-driven diffusion flux. Ebullitive methane fluxes were determined by methane fraction in total ebullitive gas and the frequency of ebullitive events, both of which varied with subsurface methane concentrations and other gas species. Although ebullition thresholds are constrained by local physical factors, this study indicates that redox interactions not only determine gas composition in ebullitive fluxes but can also regulate ebullition frequency through gas production.

Key Points

• Subsurface methane productivity declines with increasing salinity gradient
• Ebullitive methane flux is episodic and highly variable in coastal wetland system
• Subsurface redox interactions regulate the frequency and gas composition in ebullitive flux

Plain Language Summary

Coastal wetlands store a large amount of carbon from the atmosphere and oceans, which is usually referred as blue carbon. Buried carbon in coastal wetlands can be decomposed into methane (CH₄) whose global warming potential over a century is about 30 times higher than carbon dioxide (CO₂), and this decomposition process may increase under a warming climate. Surface methane fluxes are controlled by multiple factors that influence subsurface methane production, consumption, and transport from subsurface to surface. We built a model to simulate how important chemical reactions in the subsurface (e.g., nitrification, denitrification, sulfate reduction) influence the relationships between surface methane fluxes and abiotic drivers including temperature and water level in a river deltaic wetland system. Our model results show that methane fluxes decrease from freshwater marsh to salt marsh, which is due to a combination of lower methane production and higher methane consumption in the subsurface of wetlands that are more strongly influenced by saltwater. Rates of methane emission over short time scales depend on episodic gas escaping events, which are related to subsurface redox interactions and drive large variations in surface flux rates.