Alternative biofacies model for dysaerobic communities

An abundant calcareous fauna has been discovered in the oxygen minimum zone (OMZ; < 0.5 ml/l O₂) off central California at oxygen concentrations considerably less than those predicted by previous ecological models. Analysis of box core samples and bottom photographs has revealed a distinct depth zonation of echinoderms. Asteroids and ophiuroids are most abundant along the upper and lower edges of the OMZ, respectively, whereas echinoids are found near the core of this zone where oxygen levels are as low as 0.3 ml/l O₂. As these heart urchins are very abundant (14/m²), they represent a potentially significant component of the fossil record of OMZ's. Locally these urchins are capable of disturbing over 90% of near surface sediment. The core of the OMZ is inhabited by hermit crabs that actively transport and recycle gastropod shells. This ‘biotransport’ results in an accumulation of potentially preservable, non‐endemic. hard‐bodied organisms which may lead to misinterpretation of paleo‐oxygenation conditions. We propose an alternative to the Rhoads & Morse (1971, Lethaia 4) biofacies model for open‐ocean, dysaerobic environments which consists of: (1) a zone devoid of maeroinvertebrates. characterized by laminated sediments (<0.1 ml/l O₂); (2) a zone dominated by small (1–2 mm) soft‐bodied infauca which exhibits moderate disturbance of laminae due to bioturbation (0.1–0.3 ml/l O₂); and (3) a zone inhabited by an abundant calcareous fauna characterized by highly bioturbated sediments (>0.3 ml/l O₂).