Modern Nereites in the South China Sea-ecological association with redox conditions in the sediment

Nereites ichnofabrics (comprising N. missouriensis) occupy the upper 3-7 cm in brown, oxidized, soft to soupy, uniformly fine-grained sediments that accumulated at a low rate (<5-6 cm / kyr) at water depths >4000 m water in the central South China Sea. Nereites are nearly vertical close to the sediments surface, become increasingly more inclined with depth, and, just above the redox boundary, they show a winding, subhorizontal course. The Nereites producers appear to be guided chemotactically as they maintain a consistent distance relative to the redox boundary, despite variation in depth of penetration between studied sites. The transition from oxic to anoxic conditions is characterized by high concentrations of microbial biomass on which the Nereites producers are inferred to feed. Thus, the Nereites tier in fossil ichnofabrics may reflect the position of the redox boundary. Ash-filled burrows below the 1991 Pinatubo ash layer imply surface-feeding activities of the Nereites-producing animals. However, the small amount of ash in the burrows demonstrates only subordinate use of surface sediments, probably during periods of enhanced particle flux following upwelling. Based on the number of ash-filled burrows and the number of upwelling periods, an average population density of the Nereites producers of 5-6 animals per m 2 is estimated.