My PhD research was on the early plate tectonic history on earth. Plate tectonics involves the movement of plates on the surface of the earth. It is thought to be driven by subduction, where one plate dives into the mantle below another plate. Typically this involves oceanic crust, and part of the lithosphere, subducting below continental crust, seen in the image below:
We haven’t yet discovered a planet in our solar system that has plate tectonics like earth does. It seems to be a special property of earth. Yet subduction is a vital process for life on our planet, helping to maintain a supply of elements that life needs to survive.
In brief, what happens is that organisms in the oceans consume elements vital for life — carbon, phosphorous, nitrogen, sulfur — and then they die and sink to the bottom of the oceans where they get buried in sediment. If this process continues unabated then over time, ocean sediment will become a sink that accumulates life-necessary elements. Over time these elements will be segregated from the biosphere, no longer available for living organisms to use and thrive.
Plate Tectonics and Life on Earth
In his book The Wonder of Water: Water’s Profound Fitness for Life on Earth and Mankind, Michael Denton explains why plate tectonics is important for life on earth. He notes this paradox:
The oceans have been continually losing the twenty or so vital essential elements over the entire time span in which marine life has existed, but the replacement supply could not come from land run-off due to erosion, because as mentioned above, the rate of land erosion would denude the continental crustal material completely in a few million years. Why do the oceans have so much nutrient mass to replace every year? … The vast chalk and limestone sediments (CaC03), in many places thousands of meters thick (the result of the raining down of the shells of microorganisms to the ocean floor over millions of years), are ample testimony to the massive loss of the elements in the oceans due to biogeochemical deposition and burial.
The current deposition rate of carbon into oceanic sediments is over twelve million metric tons per year, and the total carbon content of the oceans and atmosphere is over thirty-eight trillion metric tons. Yet despite the size of the carbon pool in the ocean and atmosphere, as the authors of Elements of Physical Oceanography point out, “Three million years will be sufficient to remove all the carbon … thus forcing the atmospheric PCO2 to zero.” In effect, this would sterilize the oceans. Without CO2, which is the carrier of the carbon atom to all life on Earth, there can be no carbon-based life in the oceans or on land. The various biogeochemical processes involved in the loss of minerals from the sea are complex, often involving the transit of a particular element through many organic and inorganic compounds before it is finally trapped in the accumulating sediments on the sea bed. Slow and complex, yes, but also inexorable. Without continual renewal of the mineral content of the oceans, the oceanic ecosystems would grind to a halt in a few million years and the Earth’s oceans would become lifeless. Yes, the oceans receive nutrients from continental runoff, but there is not enough runoff, not enough continental landmass, to keep up with the rate of depletion.
And yet over many hundreds of millions of years, the oceans have not been rendered lifeless, nor the mountains ground into sterile plains. But how could there have been continents and mountains and life on land for 400 million years? And how could there have been life in the seas for four billion years? What mechanisms are continually remaking mountains and replenishing the mineral content of the ocean waters? (pp. 37-39)
Thankfully, there is a solution for this problem on earth, and it’s called plate tectonics — or more specifically, subduction. In plate tectonics, ocean sediment is dragged down through subduction deep into the earth on the surface of the subducting slab. When material on the slab reaches a certain depth, part of the slab melts (especially the sediments on top of the slab), and the elements travel back up to the earth through plumes of magma. There they are finally released back into earth’s surface environment through volcanoes. Again, Denton provides a lucid explanation:
Paradox Resolved: We opened this chapter with a paradox: The mineral constancy of the terrestrial and oceanic hydrosphere is maintained over immense periods of time in the face of continual erosion of continental crust and deposition of minerals from the oceans into sediments on the sea bed. The resolution of the paradox is now apparent: The tectonic recycling of the oceanic and continental crusts holds the key. Because of tectonic recycling, the continental crust is being continually formed and uplifted. This means that the erosion of the mountains can continue to supply the terrestrial hydrosphere with necessary minerals without cessation, as long as Earth exists and has an ocean. And despite the rate of erosion of the mountains, runoff from the land can fertilize the sea waters, not for a limited period of a few million years but for billions of years. And the continual recycling of the oceanic crust, as the sea water interacts with the hot upwelling magma, provides a second ongoing and endless means of mineral input to replenish oceanic waters. So, balanced against the continual and massive loss of minerals to the sea bed, tectonic recycling replenishes the oceans with continental runoff and by the reaction of water with upwelling magma at the mid-ocean ridges. (p. 55)
Recycling Elements for Life
Many scientific papers attest to the fact that subduction and plate tectonics are vital for recycling elements needed for life (all emphases added and internal citations removed):
- “Earth’s rocky outer layer is continuously being recycled into the mantle via subduction, where one tectonic plate descends into the mantle beneath another plate. Water trapped in the subducting plate is released into the mantle at depth, which in turn enhances mantle melting, leading to the development of volcanoes on the overriding plate. In addition to water, essential nutrients such as carbon and sulfur are also carried down into the mantle at subduction zones, and are released back into the atmosphere via volcanic eruptions. Subduction has therefore not only caused destruction, but also provided a critical exchange of life-supporting elements between the biosphere and geosphere over Earth’s history. … The initiation of the first subduction zones on the early Earth likely had major implications for carbon recycling, with consequences for the rise of atmospheric oxygen and thus the development of complex life.” (Nature Communications, 2020)
- “The net flux of carbon between the Earth’s interior and exterior, which is critical for redox evolution and planetary habitability, relies heavily on the extent of carbon subduction. … We suggest that immobilization of organic carbon in subduction zones and deep sequestration in the mantle facilitated the rise (∼103–5 fold) and maintenance of atmospheric oxygen since the Palaeoproterozoic and is causally linked to the Great Oxidation Event.” (Nature Geoscience, 2017)
- “An understanding of fluid circulation in subduction zones is crucial for determining the origin of arc volcanism and to constrain global material recycling. Water bounded in the hydrous minerals in the altered subducting slab is continuously released into the overriding mantle wedge via metamorphic dehydration reactions with increasing pressure and temperature during subduction. The released aqueous fluid can control the partial melting of the mantle wedge because the presence of aqueous fluid effectively decreases the peridotite solidus temperature, which leads to the formation of arc magma. The slab-derived water involved in the arc magma returns to the earth’s surface via volcanic emission, which is assumed to regulate water cycling in the subduction zone.” (Nature Communications, 2019)
Bringing Nutrients into the Earth
It’s also the case that plate tectonics helps bring nutrients deep into the earth to sustain a “deep subsurface biosphere”:
Geological sources of H2 and abiotic CH4 have had a critical role in the evolution of our planet and the development of life and sustainability of the deep subsurface biosphere. Yet the origins of these sources are largely unconstrained. Hydration of mantle rocks, or serpentinization, is widely recognized to produce H2 and favour the abiotic genesis of CH4 in shallow settings. However, deeper sources of H2 and abiotic CH4 are missing from current models, which mainly invoke more oxidized fluids at convergent margins. Here we combine data from exhumed subduction zone high-pressure rocks and thermodynamic modelling to show that deep serpentinization (40–80 km) generates significant amounts of H2 and abiotic CH4, as well as H2S and NH3. Our results suggest that subduction, worldwide, hosts large sources of deep H2 and abiotic CH4, potentially providing energy to the overlying subsurface biosphere in the forearc regions of convergent margins. … Geochemical data from forearc mud volcanos and hydrothermal seeps suggest that life exists as deep as 15 km below the surface at convergent margins and that the essential carbon to sustain deep microbiological habitats in the forearc of convergent plate margins is provided by the metamorphic recycling of subducting slabs. … [O]ur results suggest that high-pressure serpentinization is potentially an important source of reduced volatiles to the deep subsurface biospheres of convergent margins. Considering that low temperature and pressure serpentinization also takes place at subduction zones in the shallow forearc mantle and in obducted ophiolites, we propose that convergent margins may have represented the major source of H2 and abiotic CH4 from different depths to the surface biosphere.Nature Communications, 2020
Plate tectonics and subduction thus appears to be a special parameter of earth that makes it hospitable to life. We might even say that it’s a design parameter.
There’s more that subduction can teach us about intelligent design, as I’ll explore in a post tomorrow.