Anomalous sound velocities in near-surface, organic-rich, gassy sediments in the central Argentine Basin

Methane production during diagenesis of organic-rich sediments within the Argentine Basin (water depths 5000-5600 m) has changed the velocity and density of the upper 50 m of sediments. A near-surface unit of normal-amplitude returns and normal sediment velocities (termed the N unit) is underlain by a unit of low-amplitude returns having low velocities (the L unit, velocities of 1.35-1.42 km s^-1). Where sampled, the L unit sediments contain biogenic methane which forms large gas voids in the recovered cores. Beneath the L unit is a unit of high-amplitude returns and high sediment velocities (the H unit, velocities of 1.8-3.0 km s^-1). The interfaces between the N, L, and H units mimic the sea floor and cut across dipping reflectors which are interpreted as buried mudwave surfaces. Cross-wave changes in the depth of the N, L, and H units may be related to the internal mudwave structure formed as a result of bedform migration. The N/L interface probably marks the depth of maximum downward sulphate penetration since methane production occurs below this depth. The L/H interface probably divides a region of discrete hydrate flakes (the L unit) from a region where the hydrate structure is more continuous (the H unit). Similar reflecting sequences are present in other ocean basins suggesting that the phenomena observed here are not restricted to the Argentine Basin.