Supporting article K: How was it made possible for such a magnitude of variety of organisms to exist on earth?
Go back a bit more than 700 million years and you’d find our planet covered in the mother of all ice ages. But around 630 million years ago things warmed up, the ice receded and there was an explosion of new life, including the earliest animals.
A group of biogeochemists have now proposed a theory to help explain how that happened – more phosphorus. Their Letter, published in this week’s edition of the journal Nature, make this argument: Phosphorus is one of the main rate-limiting factors for the growth of life in the oceans. Phosphorus enters the ocean via rivers following weathering of rocks and minerals on the continents. Extensive glaciers during the Cryogenian may have kicked that process into overdrive.
Most researchers believe the Cryogenian period consisted of two major “Snowball Earth” episodes, between about 750 and 630 million years ago, with a period of lesser ice in between. This wasn’t the ice age of the woolly mammoths, with warmth and life at the equator. This was ice that likely extended from the poles to equator, even at low elevations.
Some scientists, who back the “Hard Ball Earth” theory, think even the oceans were covered in a layer of ice.
But even in this period of extreme glaciations, the ice waxed and waned over tens of millions of years, say Timothy Lyons, a biogeochemist with the Department of Earth Sciences at the University of California, Riverside, and one of the authors on the paper.
The authors argue that the retreating ice ground down the rocks they were on top of, eventually depositing more phosphorus into the oceans. This weathering is known to have happened during our most recent ice ages as well.
As more phosphorus entered the oceans, diverse photosynthetic microorganisms that lived in them “bloomed” much as phosphorus runoff from agricultural fields in the Midwest today causes algae blooms in the Gulf of Mexico. During the Cryogenian, however, the delivery of the life-sustaining nutrient persisted over very long periods and likely at huge levels.
When those tiny organisms grew, they produced oxygen as a byproduct of photosynthesis. When they died, they drifted to the bottom of the oceans, where they were buried. When this happens on a large scale, as stimulated by the phosphorus increase of the Cryogenian, oxygen levels increase in the atmosphere.
Over millions of years this cycle pumped more oxygen into the atmosphere, allowing oxygen-needing animals to evolve. The result was an Earth with much higher levels of oxygen that set the stage for life as we know it—leading ultimately to the great diversity of life on land and the oceans. Team leader, Noah Planavsky, Lyons and their colleagues have, for the first time, tied together a mechanism for the needed increase in atmospheric oxygen, the great Snowball ice age, and the first appearance of animals.
To support their theory, the researchers came up with an ingenious way of measuring how much phosphorous there was in the oceans 750-630 million years ago. They looked at iron-rich deposits formed during this interval around the world. These formations bind with dissolved phosphorous in the ocean in well-understood ways, so that analyses of these ancient deposits today allow estimates of nutrient levels in very ancient sea water.
“The iron-rich rocks behave in a predictable, chemically consistent way that, once you measure the ratio of phosphorus to iron in the rock, allows you to relate that ratio to the amount of dissolved phosphorus in sea water,” says Lyons.
That allowed them to come up with a curve for phosphorous dissolved in sea water through time, which distinctly showed a big peak in high levels during the Cryogenian period.
“There was sustained high phosphorous over this interval, like nothing we’ve seen before or after,” Lyons says.
Lyons cautions that the public shouldn’t take this to mean that the climate change we’re undergoing currently will turn out for the best. The process he and his colleagues describe took place over at least 80 million years. It’s very different from the climate change of today, which is happening in a matter of decades. “Today’s isn’t a natural change, we’ve become geologic agents, and in the process we’ve accelerated the processes of carbon cycling and CO2 release to the atmosphere at previously unknown rates with alarming consequences,” he says.
By Elizabeth Weise