Bioturbation by symbiont-bearing annelids in near-anoxic sediments: Implications for biofacies models and paleo-oxygen assessments

Anoxic or nearly anoxic conditions (< 4 μM O₂) have long been associated with the absence of bioturbation and animal traces. This premise has guided interpretation of paleoceanographic conditions from rocks and sediments. We recently observed a high-density, living assemblage of highly mobile, symbiont-bearing, burrowing, phallodrilinid oligochaetes within a nearly anoxic basin (<1 μM O₂ [0.02-0.03 ml l^-1]) on the Peru margin (305 m). These observations were made during the most intense part of the 1997-98 El Niño when there may have been slight oxygenation of an otherwise anoxic basin, but oligochaete presence prior to this event is likely. The occurrence of symbiont-bearing gutless oligochaetes mainly within the upper 5 cm of the sediment column coincided with a bioturbated zone overlying distinctly laminated sediments. Our observations redefine the lower oxygen limit of macrofaunal bioturbation to ≪2 μM, and indicate a need to modify currently accepted ideas about the relationship between bioturbation and paleo-oxygen concentration. These results also address an ongoing debate about the lifestyles of bioturbating organisms in oxygen-poor settings.