The exaerobic zone, a new oxygen-deficient marine biofacies

Abstract

Classical biofacies models^1,2 for reconstructing palaeoenvironments of strata deposited in oxygen-deficient marine settings define three principal facies: aerobic (> 1.0 ml 1⁻¹ O₂), dysaerobic (1.0 to 0.1 ml 1⁻¹ O₂) and anaerobic (<0.1 ml 1⁻¹ O₂) zones. These models have postulated a decrease in organism size and degree of calcification as well as a drastic reduction in the relative percentage of fauna possessing calcified skeletons as the dysaerobic/anaerobic boundary is approached. Through use of evidence independent of that provided by body fossils, we demonstrate here that in portions of the Monterey Formation (Miocene; California) the bivalve Anadara montereyana occurs in situ almost exclusively in strata deposited at the dysaerobic/anaerobic boundary. The occurrence of fossils of large well-calcified benthic macroinvertebrates at this redox boundary contradicts classical biofacies models^1,2 and there-fore provides the basis for definition of a new oxygen-related biofacies, the 'exaerobic zone’. The exaerobic zone concept, when adjusted for differences in basin configuration and palaeoceano-graphic conditions, provides a potentially useful model for explain-ing occurrences of shelly benthic fossils within laminated, organic-rich strata of other Phanerozoic marine sequences.