Metal pollution triggered world's largest extinction events

Metal poisoning caused the malformation observed in fossil remains of marine plankton from the late Silurian (415 million years ago) and may have contributed to their extinction and that of many other species, new research suggests.

Several Palaeozoic mass extinction events during the Ordovician and Silurian periods (ca. 485 to 420 million years ago) shaped the evolution of life on our planet.

Although some of these short-lived, periodic events were responsible for eradication of up to 85 percent of marine species, the exact kill-mechanism responsible for these crises remains poorly understood.

An international team led by Thijs Vandenbroucke, researcher at the French CNRS, and Poul Emsbo from US Geological Survey initiated a study to investigate a little known association between 'teratological' or 'malformed' fossil plankton assemblages coincident with the initial stages of these extinction events.

They said that malformed fossil remains of marine plankton from the late Silurian contain highly elevated concentrations of heavy metals, such as iron, lead, and arsenic.

These are well-known toxins that cause morphologic abnormalities in modern aquatic organisms.

This led the authors to conclude that metal poisoning caused the malformation observed in these ancient organisms and may have contributed to their extinction and that of many other species.

Documented chemical behaviour of these metals, which correlates with previously observed disturbances in oceanic carbon, oxygen and sulphur signatures, strongly suggests that these metal increases resulted from reductions of ocean oxygenation.

Thus, metal toxicity, and its expressions in fossilised malformations, could provide the 'missing link' that relates organism extinctions to widespread ocean anoxia.

The mobilisation of metals in spreading anoxic waters may identify the early phase of the kill-mechanism that culminated in these catastrophic events, researchers said.

The recurring correlation between fossil malformations and Ordovician-Silurian extinction events raises the provocative prospect that toxic metal contamination may be a previously unrecognised contributing agent to many, if not all, extinction events in the ancient oceans.

The study was published in the journal Nature Communications.