SHORT OXYGENATION EVENTS IN A TOXIC CAMBRIAN OCEAN ALLOWED FOR EARLY ANIMAL INVASIONS

Motile animals require molecular O₂ for living, but anoxic and sulfidic waters, toxic to animals, were still common in the oceans when they first crawled on the seafloor. A central question is how O₂ availability in the marine environment might have challenged the earliest animal ecosystems. A controversy exists where euxinic shales are found with benthic animal fossils, and had led to the hypothesis that some trilobites breathed via chemotrophic symbionts [1]. To explore this further, we investigated the Alum shale formation, where a rich, low diversity, trilobite and brachiopod fauna is found together with bulk rock geochemical evidence for anoxia and poisonous hydrogen sulfide present in the overlying water column. Using Core Scanning X-Ray Fluorescence Spectroscopy (CS-XRF), we have obtained a high-resolution (0.2 mm) geochemical record including 73,863 Mo measurements across four trilobite biozones ~502-498 million years ago that allow us to establish a centennial chemical record of changing redox conditions in the Alum basin. Brief oxygenation events covering only ~2 mm (~1 ky) of stratigraphic thickness are recorded in an otherwise permanently anoxic and sulfidic basin. During oxygenation events the sedimentary Mo content drops systematically, in correlation with sedimentary U contents, independently verified by LA-ICPMS. We infer that the Alum basin was intermittently euxinic with shorter oxygenated periods. In some instances juvenile brachiopod shells were found during oxygenation events. The duration and frequency of events suggest that oxygenation periods were driven by changes in ocean circulation that occasionally ventilated the Alum basin and allowed aerobic organisms to invade the benthos. These observations settle the long-standing debate regarding the redox conditions in the Alum shale basin, and suggest that early animals invaded the basin as soon as the water column was oxygenated and detoxified.

[1] Fortey R, PNAS 97, 12 6574–6578 (2000).