Editorial

Abstract

Methane gas bubbles, generated by biochemical processes, are ubiquitous in the organic-rich, muddy sediments of coastal waters and shallow adjacent seas. Seismic surveys have provided considerable information on the spatial distribution of these gassy sediments. The basic biogeochemical processes responsible for methane generation and consumption are well known and models of acoustic and mechanical behavior of gassy sediments have been developed and tested under laboratory conditions. In spite of the considerable past effort, methane bubble distribution and concentration and the resultant sediment behavior have remained unpredictable prior to the studies described herein. This special issue of Continental Shelf Research describes the results of joint US/German led experiments designed to physically characterize and model the effects of benthic boundary layer processes on seafloor structure, properties, and behavior in the gassy sediments of Eckernforde Bay, Baltic Sea. Spatial and temporal distribution of the acoustic turbidity horizon, methane concentration, and the volume, size, shape, and distribution of bubbles are described for the first time. A kinetic model of the complex biochemical interactions of bacterial methane production and consumption, advective and diffusive transport processes, organic supply, and sedimentation rates has successfully been used to predict methane and sulfate concentration profiles, rates of biogeochemical reactions, and gas volumes. The spatial distribution and strength of acoustic turbidity is accurately predicted by these biochemical models, whereas the seasonal migration of the acoustic turbidity horizon correlates with changes in sediment temperature and is modeled using methane solubility. Short-term ebullition of methane from the sediment surface correlates with rapid change in bottom pressure or an increase in hydraulic flow from subbottom aquifers. Fine-scale characterization of bubble volume, shape, and size distribution coupled with concomitant in situ measurement of sound speed, attenuation, and scattering strength has allowed validation of frequency dependent acoustic scattering and propagation models. Eckernforde Bay is without doubt the most studied and well-understood area of gassy sediments and as such provides a 'natural laboratory' for future studies.