Oxygen and the continents: linked?First, take a deep breath. Now, look outside at the ground below y

Oxygen and the continents: linked?First, take a deep breath. Now, look outside at the ground below you.Unless you’re reading this on a boat or on a plane flying over the ocean, you have just sampled two of the largest changes in the history of the planet Earth. About 2.5 billion years ago, oxygen went from nearly absent to fairly abundant (~1%) in Earth’s atmosphere. At about the same time, the first batches of modern continental crust began to rise permanently out of the oceans. A new hypothesis published by scientist Cin-Ty Lee at Rice University and his group proposes that these two major changes are directly linked, with behavior of the continents making it possible for oxygen to rise in the atmosphere.Organisms figured out how to use the sun’s light for energy as far back as 3 billion years ago (https://tmblr.co/Zyv2Js21P1UXt) but it took hundreds of millions of years for oxygen abundances to rise permanently in the atmosphere. This graph shows oxygen levels over time (red and green curves are high and low estimates) and it even includes a ? at this time since it’s unclear why oxygen wasn’t rising if life was producing it. Dr. Lee’s proposal is that this delay is linked to the chemical properties of the ocean crust.Ocean crust is made of basaltic igneous rock, with about 10% iron and 0.1% sulfur in it. Those two atoms are major sinks for oxygen – if they’re released into ocean water they will use up oxygen. However, the continents are made of felsic igneous rocks; their iron and sulfur contents are about 10x lower than those in oceanic crust.When Earth’s surface was covered entirely by oceanic crust, it would have been constantly releasing large quantities of sulfur and iron to the ocean, chemicals that would use up oxygen. But, when the major change happened that began producing continental crust, perhaps the start of plate tectonics as we know it today, Dr. Lee’s hypothesis is that ocean crust formation slowed down. Suddenly the Earth was making low-iron, low-sulfur continents, and as a consequence the ability of the oceans to absorb the oxygen life was making could have been overwhelmed.This change allowed oxygen levels to rise to about 1% in the atmosphere. In the late Precambrian, oxygen levels jumped again, from those low levels to something close to modern abundance. Dr. Lee also proposes this jump could have related to the growth of continents as well; as the continents grew, more area was available to store carbon in geologic reservoirs. As carbon was progressively locked up on the continents by life, another boost in atmospheric oxygen could have been the result; this jump would stop when the planet reached another steady-state between carbon being deposited on the continents and carbon released through plate tectonics and metamorphism.Geologists are consistently looking to explain how the early Earth developed. This hypothesis now is out there and available to work on, to figure out how to test it.-JBBImage credit: http://bit.ly/1UBiSkvOriginal paper:http://www.nature.com/ngeo/journal/v9/n6/full/ngeo2707.html -- source link

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