A NEW TECHNIQUE FOR ASSESSING TUBIFICID BURROWING ACTIVITIES, AND RECOGNITION OF BIOGENIC GRADING FORMED BY THESE OLIGOCHAETES

Tubificids are important conveyor-belt feeders in freshwater settings because dense populations can rapidly rework bottom deposits through selective ingestion of silt and clay. The rate at which these organisms redistribute sedimentary particles is the focus of this research in which a new method is presented to address specific disadvantages of previous studies documenting tubificid bioadvection. The new approach incorporates an aquarium inoculated with sediment and worms in which the sediment surface is photographed through time as tubificids produce fecal mounds. Employing computer software, mounds are traced, and using a known pixel length, the program calculates the traced area, which is converted to volume of upturned sediment by assuming an idealized conical shape. This method resulted in a particle redistribution rate for a population of Limnodrilus and Tubifex at 0.042-0.139 cm/d/100,000 individuals/m(2) at 21 degrees C. During sediment reworking, segregation of silt and clay forms biogenic graded bedding defined by a poorly sorted bed with an overall decrease in mean, modal, and median grain sizes upward. This tubificid-formed graded bedding could be recognized in the rock record through careful analyses of grain-size distributions that distinguish biogenic reworking from physically graded beds. Ichnologically, this occurrence corresponds to the broad conditions defined by the Mermia Ichnofacies, but is yet unrecognized and may actually obliterate typical traces associated with this ichnofacies. Identification of ancient tubificid-formed graded beds has the potential to enhance interpretations of environmental conditions (sedimentation rate and current velocities), provide evidence for a previously unrecognized paleobiomass, and broaden the definition of the Mermia Ichnofacies.