Zahnle, K.J., Catling, D.C. & Claire, M.W.
Chemical Geology, 362, pp 26-34.
The most profound change in Earth’s climate and chemistry over it’s 4.5 billion year history occurred 2.4 billion years ago when the surface oceans and atmosphere became flooded with O2 gas. The scientific community has been considering the question “What caused the rise of Oxygen” for 100 years, and the fundamental answer still remains unclear. This article was written for a commemorative journal issue celebrating the life of Dick Holland, who dedicated his scientific career to this question, as well as mentoring multiple generations (including myself) on the careful use of geochemistry to understand past environments. The paper argues for the primary importance of hydrogen escape in thinking about the evolution of the Earth, which for some reason is generally ignored by Earth scientists.
This paper was written in response to a paper named “What caused the rise of oxygen?” in the same special issue of Chemical Geology, dedicated to Dick Holland. We felt that the provocatively entitled paper didn’t actually answer the question, although it did create a very useful conceptual framework for one way of addressing the question. However, even this framework left out an additional angle to the story that we find important.
As a starting point, consider the starting materials. Most oxygen on Earth is bound up in silicate rocks and these bonds are nearly impossible to break in natural settings. Leaving aside the oxygen bound up in silica (and iron oxides) leaves H2O as the dominant reservoir of oxygen on Earth. This oxygen was delivered to Earth in the form of ice and hydrated minerals. The atmosphere didn’t start with any O2 gas in it, so how do you get O2 gas in the atmosphere? Despite thousands of years of alchemists trying, it turns out that, in chemistry, you can’t get something for nothing. The only way to get O2 in the atmosphere is to split apart the H2 from the O, or more precisely 2H2O -> 2H2 + O2. Left to their own devices, the H2 and the O2 would wind up reforming H2O. In thinking of the evolution of the Earth, it doesn’t really matter if this occurs in minutes or in millions of years – if the H2O reforms, it is a zero-sum game as far as O2 goes. The only way that you can permanently put O2 in the atmosphere is if you find somewhere else to permanently store the H2. So although it can seem quite complicated if you read papers on the subject, the primary aim of all “what caused the rise of O2?” papers is basically trying to find somewhere to store a vast quantity of H2.
There are really only two viable options for permanently removing H2 from Earth’s surface environment – Earth’s mantle or the rest of the Universe. Many Earth scientists, including Dick Holland, focus on the mantle as this reservoir. Somewhat confusingly, the story generally involves a concept called “mantle redox evolution” in which the volcanic gases from the mantle starts out producing more reducing gases (i.e a high H2:H2O ratio) in early times but then calms down with time and starts producing more neutral gases (i.e. a lower H2:H2O ratio). This process certainly could work, and their are many papers explaining how it might work, but they are all complicated by a simple fact. In order to make the mantle gases more oxidised with time, you have to take an already reduced mantle, and reduce some portions of it even more (i.e. sopping up all the H2), but somehow keep that very reduced portion away from the volcanic source regions. There is certainly heterogeneity in the mantle and it is admittedly a huge reservoir that could easily soak up the necessary amount of H2, but we think there is a far simpler option.
The figure shows Earth as imaged in the Lyman alpha filter by NASA’s Dynamics Explorer I satellite. Lyman alpha pictures reveal hydrogen, so all of those red pixels are hydrogen atoms in Earth’s “geosphere” (You can also see the auroral oval prominently on the night side) These hydrogen atoms have all left the gravitational influence of Earth and are swept away by the solar wind to become part of the background H in the solar system, galaxy, and universe. H is escaping the planet as you read this at 3 kilograms (6.6 US pounds) per second. There goes another 3 kg. And another. Although it seems like a lot, when you multiply this escape rate over 4.5 billion years, it’s not enough to account for all the O2 in the atmosphere. However, before there was any O2 in the atmosphere, there was a lot more H2 in the atmosphere. And the escape rate is directly proportional to the amount of H2 in the atmosphere. If you multiply the predicted amount of H2 in the early atmosphere (aided by biogenic CH4 in an interesting aside) by the escape rate, you can account almost perfectly for the amount of free O2 on Earth. We find this a simpler argument than reducing a reduced mantle to make it more oxidising.
This paper is a review, and so provides not much new data or insight. Mostly we just wanted to remind the community of our admittedly minority belief that H escape is important for the rise of O2. H escape is the accepted mechanism for explaining the oxidation state of Venus, Mars, Titan, Ganymede, Io, Europa, and Rhea in our solar system. The hope in writing this paper is that Earth scientists might gain insight from their planetary science colleagues in answering difficult questions about Earth’s evolution.