Brief oxygenation events in locally anoxic oceans during the Cambrian solves the animal breathing paradox

Abstract

Oxygen is a prerequisite for all large and motile animals. It is a puzzling paradox that fossils of benthic animals are often found in black shales with geochemical evidence for deposition in marine environments with anoxic and sulfidic bottom waters. It is debated whether the geochemical proxies are unreliable, affected by diagenesis, or whether the fossils are transported from afar or perhaps were not benthic. Here, we improved the stratigraphic resolution of marine anoxia records 100–1000 fold using core-scanning X-Ray Fluorescence and established a centennial resolution record of oxygen availability at the seafloor in an epicontinental sea that existed ~501–494 million years ago. The study reveals that anoxic bottom-water conditions, often with toxic hydrogen sulfide present, were interrupted by brief oxygenation events of 600–3000 years duration, corresponding to 1–5 mm stratigraphic thickness. Fossil shells occur in some of these oxygenated intervals suggesting that animals invaded when conditions permitted an aerobic life style at the seafloor. Although the fauna evidently comprised opportunistic species adapted to low oxygen environments, these findings reconcile a long-standing debate between paleontologists and geochemists, and shows the potential of ultra-high resolution analyses for reconstructing redox conditions in past oceans.

A long-term record of early to mid-Paleozoic marine redox change

“…Overall, geochemical data agree with field observations of low/absent bioturbation and a lack of benthic fauna and confirm that the Richardson trough bottom waters were generally anoxic [but with brief intervals of oxygenation (42)] for the vast majority of its history. In terms of connecting this marine redox record with estimates of atmospheric O 2 , most model estimates for the early Paleozoic suggest that atmospheric oxygen levels were lower than modern (43)(44)(45)(46).…”

The Furongian (upper Cambrian) Alum Shale of Scandinavia: revision of zonation

“…The shale is laminated and rich in organic matter, and there is general agreement that the depositional environment was low in oxygen and maybe even euxinic most of the time (see also Dahl et al. ). Olenid trilobites seem to have been adapted to cope with low oxygen conditions (Fortey ; Clarkson & Taylor ), and as part of the survival strategy, the production of larvae was probably high, which at the same time increased their dispersal potential.…”

Molybdenum isotope constraints on the temporal development of sulfidic conditions during Mediterranean sapropel intervals

“…Greater freshwater input resulted in nutrient-rich conditions, enhanced water column stratification, and a decrease in local deep water formation, which ultimately caused the drawdown of dissolved oxygen at depth and the deposition of sapropels (e.g., Rohling et al, 2015). As the environmental boundary conditions during these events are relatively well established, sapropels present excellent analogues for intermittent anoxic and sulfidic conditions in past ocean basins (e.g., Dahl et al, 2019) as well as targets for modelling efforts aiming to address large scale basin deoxygenation, provided that redox conditions are quantitatively constrained.…”