Death Metal

Heavy metals often have adverse biological effects. These are
compounded when they are distributed in bodies of water

Extinction has shaped the course of life on the planet. Species die so that others may prevail; selection is reliant on this process. Yet on occasion, the cause of extinction has been taken beyond the differential survival of species. Abiotic factors such as climate and environment have conspired to cause the mass destruction of biodiversity.

The history of life is littered with small extinction events, but on at least five occasions mass extinctions have rocked the biosphere, radically altering the course of evolution.

The causes are well documented, even if their relative contributions to the various events are disputed. Yet recently, researchers led by Thijs Vandenbroucke from the French CNRS and Poul Emsbo from the US Geological Survey, have found that heavy metal toxicity may have contributed to extinction events occurring during the Ordovician and Silurian periods. Heavy metals often have adverse biological effects, impairing metabolic function and developmental pathways in a wide range of organisms.

Malformed plankton on the left compared to the normal form on the right

Plankton was examined from the Ordovician and Silurian. Extensive and frequent morphological abnormalities suggested that something in the environment had caused developmental defects. Previous studies showed changes in the carbon, sulphur and oxygen isotope record during the extinction events.

The changes in the isotopes correlates strongly with the chemical behaviour of heavy metals. It suggests that widespread oxygen deprivation at the time led to the release of heavy metals into the water, resulting in the morphological defects seen in the plankton.

The low oxygen conditions, combined with the propagation and concentration of toxic heavy metals up food webs would have resulted in widespread biotic decline and eventually the Ordovician and Silurian extinction events. Once a number of key species in any ecosystem begin to decline in number or vanish entirely, other ecological components will begin to collapse. This effect may then propagate, spreading the destruction to produce an extinction. All these events, however, require a trigger and for the Ordovician and Silurian extinctions, anoxia and heavy metal toxicity may have been just that.