My doctoral research focused on the tectonic history of early plates on Earth. Plate tectonics involves the movement of plates on the earth’s surface. It is believed to be driven by subduction, where one plate plunges into the mantle under another plate. Typically, this involves an oceanic crust and part of the lithosphere, subduing under the continental crust, as shown in the image below:
We have yet to discover a planet in our solar system that has plate tectonics like the Earth. It seems to be a special property of the earth. Yet subduction is a vital process for life on our planet, helping to maintain a supply of the elements that life needs to survive.
In short, what happens is that organisms in the oceans consume essential elements for life – carbon, phosphorus, nitrogen, sulfur – then they die and sink to the bottom of the oceans where they are buried in the sediment. If this process continues unabated, over time ocean sediments will become a sink that accumulates the elements necessary for life. Over time, these elements will be separated from the biosphere and can no longer be used by living organisms and thrive.
Plate tectonics and life on Earth
In his book The wonder of water: the deep physical condition of water for life on Earth and humanity, Michael Denton explains why plate tectonics are important to life on earth. He notes this paradox:
The oceans have continuously lost the twenty or so vital elements throughout the lifespan of marine life, but the replacement supply could not come from land runoff due to erosion, because as mentioned above- above, the rate of soil erosion would completely strip material from the continental crust within a few million years. Why do the oceans have so much nutrient mass to replace each year? … The vast limestone and limestone sediments (CaC03), in many places thousands of meters thick (the result of the rain of microorganism shells on the ocean floor for millions of years), amply testify to the massive loss of elements in the oceans due to biogeochemical deposits and landfill.
The current rate of carbon deposition in ocean sediments is over twelve million metric tonnes per year, and the total carbon content of the oceans and the atmosphere is over thirty-eight trillion metric tonnes. 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 enough to remove all carbon … thus forcing the PCO atmospheric2 to zero. Indeed, it would sterilize the oceans. CO free2, which is the carrier of the carbon atom for all life on Earth, there can be no carbon-based life in the oceans or on earth. The different 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 sediments that accumulate on the sea. seabed. Slow and complex, yes, but also inexorable. Without a continuous renewal of the mineral content of the oceans, ocean ecosystems would shut down in a few million years and Earth’s oceans would become lifeless. Yes, the oceans receive nutrients from continental runoff, but there is not enough runoff, not enough land mass to keep pace with depletion.
And yet, over several hundred million years, the oceans have not been rendered lifeless, nor the mountains turned into barren plains. But how could there have been continents, mountains and life on earth for 400 million years? And how could there have been life in the seas for four billion years? What mechanisms continually rebuild mountains and replenish the mineral content of ocean waters? (pp. 37-39)
Fortunately, there is a solution to this problem on Earth, and it’s called plate tectonics – or more precisely, subduction. In plate tectonics, oceanic sediments are driven downward by subduction deep into the earth at the surface of the subduction slab. When the material on the slab reaches a certain depth, part of the slab melts (especially the sediment on top of the slab), and the elements rise back to earth through plumes of magma. There, they are finally released into the earth’s surface environment by volcanoes. Again, Denton provides a lucid explanation:
Paradox solved: 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 the continuous erosion of the continental crust and the deposition of minerals from the oceans in the sediments of the seabed. The resolution of the paradox is now apparent: the tectonic recycling of oceanic and continental crusts holds the key. Due to tectonic recycling, the continental crust is continually forming and lifting. This means that mountain erosion can continue to supply the Earth’s hydrosphere with the necessary minerals without interruption, as long as the Earth exists and has an ocean. And despite the rate of erosion of mountains, runoff from the land can fertilize the waters of the sea, not for a limited period of a few million years but for billion years. And the continuous recycling of the oceanic crust, as seawater interacts with the rising hot magma, provides a second, continuous and endless means of mineral input to replenish ocean waters. Thus, counterbalanced by the continuous and massive loss of minerals to the seabed, tectonic recycling replenishes the oceans with continental runoff and by the reaction of water with the magma rising in the middle of the ocean ridges. (p. 55)
Recycle items for life
Numerous scientific papers attest to the fact that subduction and plate tectonics are vital for recycling the elements necessary for life (all accents added and internal quotes removed):
- “The earth’s rocky outer layer is continually recycled into the mantle by subduction, where one tectonic plate descends into the mantle under another plate. Water trapped in the subductor plate is released into the deep mantle, which in turn improves mantle melting, leading to the development of volcanoes on the dominant plate. In addition to water, essential nutrients such as carbon and sulfur are also transported in the mantle in subduction zones, and are released into the atmosphere by volcanic eruptions. Subduction therefore not only caused destruction, but also provided a critical exchange of vital elements between the biosphere and the geosphere over the course of Earth’s history. … The initiation of the first subduction zones on early Earth probably had major implications for carbon recycling, with consequences for the increase in atmospheric oxygen and therefore the development of complex life.. “(Nature communications, 2020)
- “The net flux of carbon between the interior and exterior of the Earth, which is critical for redox evolution and planetary habitability, strongly depends on the extent of carbon subduction. … We suggest that the immobilization of organic carbon in the subduction zones and the deep sequestration in the mantle facilitated the ascent (∼103–5 pli) and the maintenance of atmospheric oxygen since the Paleoproterozoic and is causally related to the great oxidation event. (Geoscience of nature, 2017)
- “An understanding of the circulation of fluids in subduction zones is crucial to determine the origin of arc volcanism and to force the global recycling of materials. Water bound to hydrated minerals in the modified subduction slab is continuously released into the dominant mantle corner via metamorphic dehydration reactions with increasing pressure and temperature during subduction. The released aqueous fluid can control the partial melting of the mantle corner because the presence of aqueous fluid effectively lowers the temperature of peridotite solidus, which leads to the formation of arc magma. Water derived from the slab involved in the magma arc returns to the earth’s surface via volcanic emission, which is believed to regulate the water cycle in the subduction zone. (Nature communications, 2019)
Bring nutrients to the Earth
It’s also true that plate tectonics help bring nutrients deep into the earth to maintain a “deep underground biosphere”:
Geological sources of H2 and abiotic CH4 have played a critical role in the evolution of our planet and the development of life and the sustainability of the deep underground biosphere. Yet the origins of these sources are largely unconstrained. Mantle rock hydration, or serpentinization, is widely recognized to produce H2 and promote the abiotic genesis of CH4 in shallow environments. However, deeper sources of H2 and abiotic CH4 are absent from current models, which mainly rely on more oxidized fluids at converging margins. Here, we combine data from high pressure rocks of the exhumed subduction zone and thermodynamic modeling 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, around the world, harbors large sources of deep H2 and abiotic CH4, potentially providing energy to the overlying subterranean biosphere in the regions of the forearm converging margins. … Geochemical data from forearm mud volcanoes and hydrothermal seeps suggest that life exists up to 15 km below the surface at converging margins and that the carbon essential to maintain deep microbiological habitats in the forearm arm of the converging plate margins is provided by the metamorphic recycling of the slabs sub-direction. … [O]Our results suggest that high pressure serpentinization is potentially an important source of reduced volatiles for the deep underground biospheres of converging margins. Considering that low temperature and pressure serpentinization also takes place in subduction zones in the shallow mantle of the forearm and in obstructed ophiolites, we propose that the converging margins may have represented the main source of H2 and abiotic CH4 from different depths to the surface biosphere.
Nature communications, 2020
Plate tectonics and subduction therefore appear to be a special parameter of the earth that makes it welcoming to life. You could even say it’s a design parameter.
There is more that subduction can teach us about intelligent design, as I will explain in an article tomorrow.