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

Tais W Dahl,Kurt Kjær,Niels H Schovsbo,Marie-Louise Siggaard-Andersen,Søren Husted,Alexander J Dickson,Iben W Hougård,Arne T Nielsen,Daniel O Persson

doi:10.1038/s41598-019-48123-2

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.

Highlights

Deposition of black shales was widespread in the Cambrian oceans (541–488 Ma)[1]
This problem has led to controversies between paleontologists and geochemists regarding strata where fossils of benthic animals are found in intervals containing geochemical evidence for anoxic bottom waters with poisonous H2S; e.g. the Alum Shale Formation of Scandinavia[8,10]
We used core scanning X-Ray Fluorescence spectroscopy (XRF) to record the sedimentary Mo contents of the Alum Shale to reconstruct bottom-water redox conditions in the late Cambrian Alum Shale Sea, where abundant benthic animal fossils have been found in strata with bulk-rock geochemical evidence for sulfidic-anoxic conditions at the seafloor[8,12]

Summary

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That olenid trilobites had chemoautotrophic symbionts and were adapted to this habitat[11], but this hypothesis has not been widely accepted. The Mo curve shows the same meter-scale variability as recorded in the Andrarum-3 drill core from Scania, Sweden, including a famous Mo decline across the Miaolingian–Furongian boundary followed by an increase from the Olenus Superzone onwards[8] This systematic decrease in Mo enrichment has previously been linked to a global marine Mo drawdown associated with a global expansion of anoxic water masses on the continental shelves, known as the ‘SPICE’ event[8,12]. Fossiliferous intervals are dominated by only 1–2 species that occur in immense numbers, whereas trilobite species known from better ventilated environments elsewhere are exceedingly rare in the Alum Shale[23,24,25] In this way, frequent bottom-water ventilation in oxygen-restricted Cambrian oceans created intermittent habitats for opportunistic animals

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Additional Information