ALBERT

Description: 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.

Description: Direct measurements of magnitude of the northward flow of the Malvinas (Falkland) Current have recently been made with two types of Lagrangian platforms: ALACE floats which cycled between 750-m depth and the sea surface, and 100-m drogued surface drifters. Each data set clearly delineates the path of the Malvinas Current, and the vertical shears inferred from them are commensurate with historical geostrophic shears. Velocities from the surface drifters are used here to adjust geostrophic shears from historical measurements, and the results confirm a large transport of the current, as previously implied by numerical models and a regional inverse calculation. At 42°S, the northward transport of the Malvinas Current in the upper 3000 m appears to be about 70 Sv, several times larger than estimates obtained by adjusting geostrophic shears to assumed levels of no motion. This large barotropie component may have significance in the cross-frontal transfer of intermediate and deep waters from the circumpolar current to the adjacent flow regimes in the South Atlantic, and thus on the inter-basin exchange of water masses.