Modeling the Dissolution and Transport of Bubbles Emitted From Hydrocarbon Seeps Within the Hydrate Stability Zone of the Oceans

Abstract

Quantifying the vertical distribution of dissolved gases entering the oceans from natural seeps is important to understand biogeochemical cycling of these gases and to constrain their emissions to the atmosphere. The fate and transport of gas bubbles in seawater depend on their rise velocity and their rate of mass exchange with ambient water. In the deep ocean, clathrate hydrates may form as skins on bubbles of natural gases. Although it is known that hydrate skins reduce mass transfer rates, it is unclear how quickly they form and to what extent they may slow mass transfer. In this study, we develop an empirical equation to predict the time scale for hydrates to affect mass transfer, and we apply a Lagrangian particle numerical model to predict the height of rise of natural seep bubbles observed in echo sounder data. We calibrate an equation for hydrate transition time by comparing to field observations in Rehder et al. (2009, https://doi.org/10.1029/2001gl013966), and we validate to other field and laboratory observations. We apply the model to predict the rise heights of bubbles emitted from natural seeps. When comparing to acoustic data, we show that bubbles become acoustically transparent when their sizes fall below a critical size near the resonant frequency of the insonified bubble. Using this insight, our model predicts the rise heights observed in acoustic data of seven natural seeps spanning source depths from 890 to 2,890 m with an $R^2$ of 0.98, bias of 41 m, and absolute relative percentage error of 4.7%.