From the seep to the surface: the ascent and dissolution of methane bubbles in the ocean

TL;DR

This study models the ascent and dissolution of methane bubbles in shallow oceans to estimate how much methane reaches the atmosphere, aiding understanding of greenhouse gas contributions from oceanic seeps and hydrate mining.

Contribution

A simplified spherical bubble ascent model that estimates methane survival and transfer from ocean seeps to the atmosphere, applied to South China Sea seep sites.

Findings

Bubbles of different sizes have varying survival distances.

A significant percentage of methane can dissolve before reaching the surface.

Results help assess oceanic methane's impact on greenhouse gas levels.

Abstract

Methane, as a strong greenhouse gas, has 21-25 times the warming potential per unit mass than carbon dioxide, and the methane from the oceans can contribute to ~4% of the annual atmosphere methane budget. Large methane bubble plumes have been observed in seep sites globally on shallow continental shelves, and emerging industry of methane hydrates mining causes growing environmental concern on possible disastrous blowout which destabilizes the methane hydrate and releases huge amount of methane gas. To better estimate how much methane in gaseous phase leaked from the seeps can reach the atmosphere, a simplified model is developed to simulate the ascent of a methane bubble from a shallow ocean methane seep, and the methane transfer with the surrounding water. The breakup and coalescence of bubbles are neglected, and the bubble is assumed to remain spherical following a vertical path during the whole rising process. We calculated the survival distance of bubbles with varying initial sizes and depths and the remaining percentage of methane reaching the sea surface, and applied the results to the seep sites in the Shenhu area in the South China Sea. The study can provide insight into the relative significance of different water bodies in contributing to the atmosphere greenhouse gas.