Biological alteration of physically structured flood deposits on the Eel margin, northern California

Sediment profile and surface images collected in December 1995 were used to examine sediments from the mid-continental shelf off northern California. The Eel margin, as the area is known, has been subjected to episodic flood deposition associated with high discharge events of the nearby Eel and Mad Rivers. Sediments in the deposit region consisted of stratified silty clays which were highly bioturbated. Three regions were defined within water depths of 28–83 m, near the S transect of the STRATAFORM study, based upon image data and information from other studies. They were inshore shelf sands, a transitional region where sands transported by storms alternate with flood beds, and mid-shelf flood deposits. Infaunal bioturbational features were measured along a cross-shelf transect. The number and type of biogenic features changed with changing sedimentary region. The degree of bioturbation was highest in the deposit region where burrows, surface unconsolidated layer, active and relict feeding voids, and animals were more prevalent. The uppermost 5–10 mm in the deposit region were highly porous due to an abundance of 0.3 mm diameter polychaete burrows. In the transitional zone, there was evidence for both shallow and deep infaunal activity, but also apparently less utilization of the storm sand layer. Sediment laminae and the positions of bioturbational features suggest that the infaunal community has adapted to the combined influence of deposition and storm transport in the transitional region. Whereas the deeper infauna appear to be exploiting the resources within the flood deposit sediments, they in turn appear to support surface-dwelling fauna. A hypothetical model is proposed to explain the organism–sediment interactions for this area.