A new theory on one of science's biggest mysteries unveils groundbreaking findings on what caused three ancient mass extinctions. Prolonged depletion of selenium, tiny molecules caused by geological inactivity, led to the collapse of food chains and lack of oxygen that resulted in the death of most species during these periods.
Researchers from Flinders University and the University of Tasmania published their study entitled "Severe selenium depletion in the Phanerozoic oceans as a factor in three global mass extinction events" in the journal Gondwana Research. They found out that the depletion of the essential element selenium led to quiet volcanoes and stagnant tectonic plates starving global food chains and ecosystems of the nutrients they need to survive.
When the researchers studied ancient sedimentary rocks from the sea floor, they measured the levels of trace selenium along with other molecules. They also plotted the rise and fall of selenium levels in the past 600 million years.
Based on their findings, when there is an extreme depletion of selenium levels, a mass extinction event happens. Movement of tectonic plates and erosion of Earth's crust supply the ocean with essential elements and nutrients vital for life.
When there is prolonged inactivity of tectonic plates or an absence of earthquakes, levels of vital elements including selenium deplete to critical levels.
"Plate tectonics and evolution both operate on the same timescale of millions of years, and it seems logical that they could be causally related," wrote Professor John Long, palaeontologist from Flinders University, in his article.
In the last 600 million years, there were five reported mass extinction events that almost wiped the Earth of life. Some of these were caused by known disasters such as the killer asteroid at the end of the Cretaceous period that wiped out dinosaurs.
However, the three major extinctions that took place near the termination of the Ordovician Period, the Devonian Period and the Permian Period were caused by still unknown factors. Many speculate that widespread anoxia and climate change led to these events. The Permian period extinction was the worst, killing almost 96 percent of marine creatures and 70 percent of all species on the planet.
When one part of the food chain dies, everyone - from phytoplankton to dinosaurs - would suffer devastating results. Critically low levels of selenium in the oceans during those times affected the survival of plankton, which could have led to the collapse of food chains. Thus, mass extinction occured.
Normally, modern oceans contain around 155 parts per trillion of selenium, which is good for survival of marine animals. During those times of extinction events, the scientists recorded selenium levels at below critical threshold.
They also found out that depletion of selenium happened around 10 million years before anoxia (lack of oxygen) occurs in oceans. This means that prolonged selenium depletion may have caused low levels of oxygen that eventually led to collapse of food chains and mass extinction.
Today, selenium depletion poses health problems in many nations. Recent outbreak of diseases like Ebola, SARS (Severe Acute Respiratory Syndrome), AIDS (Acquired Immunodeficiency Syndrome) and the Avian flu in Africa and China were also linked to low levels of selenium.
"One issue with our preliminary study is the dating of our samples does not always match the precise timing of the extinction events. This is purely an artifact of the sampling. Additional samples dated closer to extinction times are currently being analysed for the follow up study," Prof. Long added.