Evolution has been driven by huge swings in oceanic nutrient levels, a paper in Gondwana Research argues, and these in turn are driven by cycles of mountain building. The authors matched three of the Earth's mass extinctions to periods of nutrient deficiency, while the periods in which the oceans were richest also saw the greatest flowering of new lifeforms, on land as well as in water.
"Nutrient trace elements such as copper, zinc, phosphorus, cobalt and selenium are vital for life and are critical building blocks for evolutionary change," said the University of Tasmania's Professor Ross Large. When the seas are rich with nutrients, plankton blooms; Large suspects that the extra food supply leads to flowering up the food chain.
"Nutrients in the oceans ultimately come from weathering and erosion of rocks on the continents. Weathering breaks down the minerals in the rocks and releases the nutrient trace elements, which are the key to life and evolutionary change," Large explained. "Thus when weathering and erosion rates increase for extended periods, more nutrients are supplied to the oceans."
Erosion rates peak during, and shortly after, mountain-building events. Mountain ranges large enough to change the oceans result from tectonic collisions.
The Cambrian explosion of multicellular life has been linked to a period of increased erosion previously, but associations with subsequent events have been speculative.
One obstacle to establishing connections, the paper notes, is that, “The nutrient trace element composition of the paleo-ocean cannot be measured directly.” However, a team led by Large collected 4000 pyrites from black shales worldwide. The pyrites hold a record of molybdenum, selenium, cadmium and thallium concentrations when they were laid down.
While not all of these elements are important for life, they correlate with harder-to-measure phosphorus concentration, which certainly is. Based on this data, Large and his coauthors interpreted cycles of nutrient availability over the last 700 million years, peaking first 550–560 million years ago, and then several times thereafter.
The shales can be dated using rhenium-osmium isotope analysis and the layer in which they were found, with more recent deposits dated by fossils at the same level.
“There were times when nutrient levels dropped lower than anyone expected,” Large told IFLS. These coincided with the end-Ordovician, late Devonian and Triassic-Jurassic extinction events. “We don't match the other two, but that is ok because we already have good explanations for those, with the asteroid responsible for killing the dinosaurs and the end-Permian event convincingly linked to massive volcanic eruptions in Siberia.”
Large told IFLS that today “we are at the top of a cycle, the best conditions you can imagine. That is why humans evolved. We are in a phase of collision and rifting, and still feeling the effects of India colliding with Asia to create the Himalayas.”
Large is working with coauthor John Long of Flinders University to see if nutrient deficits can be matched to smaller extinction events, but says that a database of at least 10,000 pyrites will be needed to provide sufficient resolution.
1