Mechanical controls on methane bubble solute exchange within muddy aquatic sediments and its growth characteristics

Abstract

Methane (CH4) bubbles in muddy aquatic sediments threaten climate sustainability and sediment mechanical stability. Mechanical response of muddy sediment to bubble growth is described by Linear Elastic Fracture Mechanics (LEFM). Minor roles of mechanical sediment characteristics in CH4 bubble solute supply and growth rates, were quantified by preceding studies, compared to biogeochemical controls. We investigate them using a coupled single-bubble mechanical/reaction-transport numerical and analytical models. We demonstrate that inner pressure of the growing bubble at fracturing, concentration at its surface, bubble size and spatial location, are uniquely defined by the Fracture Toughness of muddy sediment. However, a temporal evolution of the bubble inner pressure at expansion between the fracturing events depends on the Young's modulus. Fracture Toughness and Young's modulus thus play complementary, spatial and temporal, roles in the bubble inner pressure evolution. The proportionality suggested by LEFM manages the bubble solute exchange and growth rates. Higher Fracture Toughness controls development of longer flatter bubbles in the deeper sediments. A substantial role of the mechanical muddy sediment characteristics in the CH4 bubble growth dynamics and solute exchange is demonstrated, comparable to the role of the biogeochemical controls. Their contributions to development of no-growth and competitive bubble growth conditions, affecting a gas dynamics at a macro-scale, are discussed.