Modern and ancient continental shelf anoxia: an overview

TLDR

Mid‑latitude shelf seas are governed by seasonal wind‑driven mixing, which can create shallow, stratified bottom layers that become oxygen‑depleted during extended summer periods. The study proposes standardized oxygenation categories (oxic, dysoxic, suboxic, anoxic) for describing shelf‑sea environments. The authors explain that limited bottom circulation and extended stratification or organic loading can exhaust the thin bottom layer, and they define quantitative oxygen thresholds and corresponding biofacies terms for classifying shelf‑sea oxygenation. The review finds that seasonal dysoxia‑anoxia drives widespread mortalities, favors soft‑bodied benthic faunas, and best explains many ancient black shales, with continuous anoxia being rare except in confined deeper sub‑basins.

Abstract

Abstract The characteristics of mid-latitude shelf seas are primarily controlled by seasonal cyclicity in wind-driven mixing. Seasonal dysoxic-anoxic conditions may occur in summer in salinity-stratified estuarine or pro-delta settings, or more extensively in those open shelf areas where a total depth of ≤60 m and the seasonal thermocline result in a bottom water layer ≤10 m thick. When bottom circulation is limited the oxygen stored in this layer may become periodically exhausted if climatic factors extend the stratified period to seven or more months, or if there is additional organic loading (via flagellate blooms and/or pollution). This causes widespread mortalities and a shift to soft-bodied, non-fossilizing benthic faunas. Our review supports seasonal dysoxia-anoxia as being the best model to account for the key characteristics of many ancient epeiric sea black shales. It would appear that the latter rarely represent true continuous anoxia except locally in more-confined deeper sub-basins. We recommend that the following terms should be applied to environments, facies, or oxygenation levels: oxic (8.0–2.0 ml/l O 2 ), dysoxic (2.0–0.2 ml/l), suboxic (0.2–0.0 ml/l) and anoxic (0.0 ml/l). The corresponding biofacies terms are: aerobic, dysaerobic, quasianaerobic (laminated, without macrofauna, but with in situ benthic microfauna) and anaerobic. Hypoxic and normoxic should only be used with regard to the physiological responses of living organisms.