Evolution: Iron-ing out the creases in global kill mechanisms

Iron-rich, oxygen-starved oceans delayed the recovery of life following the Earth’s greatest mass extinction 252 million years ago, according to a study published in Nature Communications this week. The findings shed light on the ancient ocean chemistry responsible for the longest recovery period following a mass extinction in Earth's history.

Toxic, sulfur-rich oceans have long been considered the global kill mechanism responsible for the loss of 90% of the planet’s biodiversity at the end of the Permian period (252 million years ago), and the protracted 5 million-year recovery that followed. However, the global nature of this extreme oceanic state is based on limited geochemical tools and is rather poorly constrained.

Matthew Clarkson and colleagues use highly precise chemical techniques (known as iron-speciation techniques) to analyse ancient ocean chemistry preserved in rocks laid down as sediments in the Neo-Tethyan Ocean (modern day Oman) millions of years ago. Chemical data from six sampling sites, spanning the ancient shallows to the deeper ocean, reveal iron-rich, oxygen-poor conditions, punctuated by oxygen-rich pockets, rather than the widespread toxic sulphur-rich conditions previously predicted.

Although the precise cause of this extended recovery period remains unclear, the authors believe enhanced chemical weathering on land (due to high temperatures) led to increases in the flux of iron to the oceans, which would have sustained the iron-rich, low-oxygen state, and suppressed biotic recovery.

doi: 10.1038/ncomms12236

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