How many of us take breathing for granted? Our environment consists of about 21% oxygen, which is just right to sustain us. Yet, Earth has not always had the ideal amount of oxygen that we enjoy today. Scientists have learned of a secondary source, called the deep oxygen cycle, that has been “piped in” from Earth’s interior—an elegant process with creation implications.
All animal species on Earth are fundamentally dependent on a high oxygen level in Earth’s atmosphere. Without a whole lot of molecular oxygen in Earth’s atmosphere, there would be no possibility of animals manifesting high metabolic activity.
Most of us were taught that the oxygen we breathe comes from photosynthetic microbes and plants. Much of it does, but not enough by itself
, to give us the oxygen we need to survive. Thankfully, a vital second source is available. That source, the highly fine-tuned deep oxygen cycle, refers to oxygen generated through organic carbon material in Earth’s crust that cycles into Earth’s mantle and then back to the crust.
Chemistry of the Deep Oxygen Cycle
Organic carbon compared to inorganic carbon is depleted in carbon-13 relative to carbon-12. Based on carbon-13 to carbon-12 isotope ratio measurements, at least one-fifth and as much as one-half of the organic carbon is subducted (one tectonic plate descends below another) from the crust into the mantle. It remains there rather than returning to Earth’s surface and crust through volcanic eruptions.1
Then, the portion of the organic carbon subducted into the mantle that is not returned to Earth’s crust and surface undergoes graphitization (conversion to graphite). The chemistry behind graphitization is as follows:2
As an oceanic tectonic plate slides underneath a continental tectonic plate, water and dissolved carbon dioxide in deep seawater and in the oceanic plate gets subducted into the mantle. Some of this water and carbon dioxide react to form formaldehyde and molecular oxygen (H2O + CO2 → CH2O + O2). As the formaldehyde gets subducted deeper into the mantle it decomposes into water and carbon (CH2O → H2O + C). The carbon (C) in the form of graphite gets transformed into diamonds as it descends down to mantle depths of 150–800 kilometers (93–497 miles) where it experiences increasing pressure and temperature.
A similar set of reactions occurs when carbonates (where CO3s take the place of CO2) are subducted into the mantle. One of the outcomes of this chemistry is the production of molecular oxygen (O2).
Tracking the Oxygen Cycle
Evidence for this deep oxygen cycle can be seen in the diamond extraction process. There, scientists observe that volcanic activity brings the diamonds up from the mantle and deposits them into kimberlite and lamproite pipes. These pipes are deep narrow cones of solidified magma that connect partially melted mantle at depths exceeding 150 kilometers with dormant or active volcanoes. The kimberlite pipes are found exclusively in rocks that date back to the Archean eon (4.0–2.5 billion years ago). Lamproite pipes are found in rocks of all ages. Natural diamonds are found only in kimberlite and lamproite rocks.
Laboratory experiments affirm that graphite gets transformed into diamonds under the temperature and pressure conditions that exist at mantle depths ranging from 150 to 800 kilometers.3 At mantle depths greater than 800 kilometers, the same experiments show that diamonds rapidly degrade back into graphite.
Fine-Tuning of the Oxygen Cycle
If not for the efficient subduction of organic carbon from Earth’s oceanic crust into Earth’s mantle at just-right times in just-right amounts in Earth’s history, there would not be sufficient oxygen in Earth’s atmosphere to sustain animal life, especially birds and mammals. For example, the burial of an enormous amount of organic carbon is widely cited as a cause for the rise of oxygen in the Earth’s atmosphere 2.45–2.32 billion years ago, termed the Great Oxidation Event.4 It took this extra injection of oxygen to complement the oxygen being produced by photosynthetic life to overcome the amount of oxygen that was continually absorbed by metals in Earth’s crust that acted as oxygen sinks.
Another event, known as the Great Unconformity, occurred just before the first appearance of animals—the Avalon and Cambrian explosions—and was characterized by crustal erosion and sediment subduction events of unprecedented scale.5 The Great Unconformity is also associated with a set of large global oxygen isotope excursions.6 Like the Great Oxidation Event, the Great Unconformity generated a huge injection of oxygen into the atmosphere through an enormous amount of organic carbon that was efficiently subducted into Earth’s mantle. This event coincided with the time period when the quantity of oxygen in Earth’s atmosphere jumped from 1% or less up to 8% and quickly thereafter up to 10%. The 8% level is the minimum required for the existence of large-bodied animals lacking digestive tracts and internal organs (the Avalon animals). The 10% level is the minimum requirement for animals with complex internal organs (the Cambrian animals).
It is compelling to consider all the “just-rights” needed in the dynamics of Earth’s crust and mantle and the just-right injection amounts of oxygen in Earth’s atmosphere at the just-right times in the history of the Sun and Earth for animals to exist and thrive. Also, when the needed oxygen injections occur animals immediately appear in great diversity. Such an implausible set of just-right events argues powerfully for a Creator’s intelligent activity.
- David J. Des Marais, “Isotopic Evolution of the Biogeochemical Carbon Cycle During the Precambrian,” in Reviews in Mineralogy & Geochemistry: Stable Isotope Geochemistry 43, no. 1, edited by John W. Valley and David R. Cole (Mineralogical Society of America, August 15, 2001): 555–78, doi:10.2138/gsrmg.43.1.555.
- Megan S. Duncan and Rajdeep Dasgupta, “Rise of Earth’s Atmospheric Oxygen Controlled by Efficient Subduction of Organic Carbon,” Nature Geoscience 10 (April 25, 2017): 387–92, doi:10.1038/ngeo2939.
- F. P. Bundy et al., “The Pressure-Temperature Phase and Transformation Diagram for Carbon; Updated Through 1994,” Carbon 34, no. 2 (February 1996): 141–53, doi:10.1016/0008-6223(96)00170-4.
- Heinrich D. Holland, “Volcanic Gases, Black Smokers, and the Great Oxidation Event,” Geochimica et Cosmochimica Acta 66, no. 21 (November 1, 2002): 3811–26, doi:10.1016/S0016-7037(02)00950-x; Timothy W. Lyons, Christopher T. Reinhard, and Noah J. Planavsky, “The Rise of Oxygen in Earth’s Early Ocean and Atmosphere,” Nature 506 (February 19, 2014): 307–315, doi:10.1038/nature13068; Genming Luo et al., “Rapid Oxygenation of Earth’s Atmosphere 2.33 Billion Years Ago,” Science Advances 2, no. 5 (May 13, 2016): id. e1600134, doi:10.1126/sciadv.1600134; Heinrich D. Holland, “Why the Atmosphere Became Oxygenated: A Proposal,” Geochimica et Cosmochimica Acta 73, no. 18 (September 15, 2009): 5241–55, doi:10.1016/j.gca.2009.05.070.
- C. Brenhin Keller et al., “Neoproterozoic Glacial Origin of the Great Unconformity,” Proceedings of the National Academy of Sciences USA 116, no. 4 (January 2019): 1136–45, doi:10.1073/pnas.1804350116; Jon M. Husson and Shanan E. Peters, “Atmospheric Oxygenation Driven by Unsteady Growth of the Continental Sedimentary Reservoir,” Earth and Planetary Science Letters 460 (February 2017): 68–75, doi:10.1016/j.epsl.2016.12.012; Setareh Shahkarami et al., “The Ediacaran-Cambrian Boundary: Evaluating Stratigraphic Completeness and the Great Unconformity,” Precambrian Research 345 (August 2020): id. 105721, doi:10.1016/j.precamres.2020.105721.
- Keller et al., “Neoproterozoic Glacial Origin of the Great Unconformity.”