Joining forces: Combined biological and geochemical proxies reveal a complex but refined high-resolution palaeo-oxygen history in Devonian epeiric seas

Two Devonian black shale intervals were analysed using a carefully integrated palaeontological–geochemical approach to interpret relative bottom water oxygen levels at a resolution and sensitivity possible only through the combined approach. Biological data presented as both trace and body fossil abundance and size suggest generally low bottom water oxygen conditions. The frequently laminated sediments would be viewed traditionally as recording appreciable intervals of anoxic and/or euxinic (anoxic and H2S-containing) bottom waters. The inclusion of parallel geochemical results, however, refines this perspective.Well-established iron proxies for depositional redox tell us that anoxic and euxinic conditions were intermittent and short-lived, if present at all. Manganese and molybdenum data point to conditions with a likelihood of very low oxygen in bottom waters during intervals with high pore-water concentrations of hydrogen sulphide near the sediment–water interface. These elevated sulphide concentrations under iron limited diagenetic conditions varied with organic content and may have further inhibited benthic colonization, and an active infaunal community more specifically. The combined palaeontological and geochemical data sets point to depleted bottom water oxygen contents with evidence for hydrogen sulphide limited dominantly or exclusively to the pore waters. Anoxia and euxinia, if present, occurred only intermittently and lasted only briefly, perhaps on seasonal time scales tied to primary productivity cycles or climatically induced variation in water column stratification. The time scales and mechanisms of rapid redox variation remain to be studied. The details of such transitional and dynamic settings are best viewed from a multi-proxy, biogeochemical and palaeoecological perspective. As an important byproduct, geochemical methods refined in multi-proxy studies of Phanerozoic shales can then be extended with greater utility to Precambrian strata without trace and body fossils.

Research Highlights
► High resolution palaeontological–geochemical approach increases resolution of palaeo-oxygen.
► Devonian black shales record rapid redox variation.
► Evidence of limited hydrogen sulphide or in exclusively in pore waters with only intermittent anoxia/euxinia.