Saturday 20 April 2024

Assessing the importance of plate tectonic for the development of complex life.

Over the past three decades, research has shown that the overwhelming majority of stars in our galaxy have planets. Logically, a proportion of those planets will have the conditions for life, and life will actually arise on some proportion of the planets which can support it. Furthermore, a small proportion of the planets with life are likely to produce intelligent life, capable of radio communication and other activities which we might be able to detect. This works out at a very small proportion of stars hosting civilizations, but a very small proportion of hundreds of billions still works out at a very large number. We have, nevertheless, never detected any sign of intelligent life (of life at all) beyond our own planet, leaving us, as far as we know the only civilization in the universe, something referred to as the Fermi Paradox. A number of possible explanations have been offered for this paradox, most of which revolve around the idea that while life is probably quite common in the galaxy, complex life is probably much rarer. A variety of reasons why this might be the case have also been proposed, with a link to the other phenomenon also as far as well know unique to our planet, plate tectonics, frequently being proposed.

In a paper published in the journal Scientific Reports on 12 April 2024, Robert Stern of the Department of Sustainable Earth Systems Science at the University of Texas at Dallas, and Taras Gerya of the Department of Earth Sciences at ETH-Zurich, re-examine the biological and geological history of the Earth, looking for connections between key stages in the development of life on Earth and changes in the Earth's geological activity.

Life is generally accepted to have first appeared on Earth more than 3800 million years ago, but the first Animals did not appear until less than 1000 million years ago. Various possible explanations have been proposed for this, including lower oxygen levels on the early Earth and a possible lack of key biological innovations until more recent times. The fact that Animals, Plants, and even multicellular Algae did not appear until quite late in the Neoproterozoic suggests that some profound change in the Earth or its biology led to this development, and Stern and Gerva suggest that this change was a shift from single lid to plate tectonics, and the subsequent profound shifts in the Earth's atmospheric oxygen levels.

The timing of the onset of plate tectonics is still a matter of debate among Earth Scientists, with two broad camps, one which argues that plate tectonics began during the Archean, while the other argues that the current plate tectonic regime began in the Neoproterozoic (although there may-or-may-not have been earlier phases of plate tectonics on Earth). The processes of seafloor spreading, subduction, and continental collision lead to the formation of a distinctive set of minerals, rocks, and rock assemblages, some of which appear to be absent before the onset of the Neoproterozoic, although it has been argued that these differences are due to higher mantle temperatures on the early Earth. Similarly, it is possible to track the movements of continents over the Neoproterozoic and Phanerozoic through the traces of magnetic fields left in some types of rocks when they form, but in rocks older than about 1200 million years this method is not reliable, leaving us uncertain about the movement of the continents that long ago. 

In previous work, Robert Stern divided indicators of plate tectonics into three groups, (1) indicators of seafloor spreading and subduction initiation, (2) subduction indicators, and (3) plate collision indicators. All of these indicators are present in rocks from Neoproterozoic and Phanerozoic times, but their presence in older rocks is far less clear. 

In order for plate tectonics to operate, it is necessary for oceanic lithosphere to be subducted at plate margins, and it is likely that this oceanic lithosphere was not sufficiently strong and dense enough for this to happen in a coherent way until the upper mantle had cooled to between 100°C and 150°C above current temperatures, again something which is predicted to have happened in the Neoproterozoic.

If plate tectonics did indeed start, then it is unlikely to have started all over the world at once, but instead should have started in one place, then taken hundreds of millions of years to spread around the globe. This would result in a gradual increase in the presence of Stern's three indicators for the presence of plate tectonics, something which can again be seen in the Neoproterozoic rock record. The first ophiolites, which are indicators of subduction initiation, appear about 870 million years ago, with the first indicators of actual subduction zones appearing about 750 million years ago, and the first indicators of continental collision appearing about 600 million years ago. 

As far as we understand, an active silicate body can have either a single lid or plate tectonic system. This being the case, for plate tectonics to have initiated in the Neoproterozoic, the Earth must have had a single lid system in the Mesoproterozoic. Stern's previous work also identified three potential indicators for single lid systems. These are (1) an elevated thermal regime, (2) an abundance of unusual dry magmas such as A-type granites and anorthosites, and (3) a lack of new continental margins. The Mesoproterozoic shows an absence of Stern's indicators for plate tectonics, but is rich in the three indicators for single lid tectonics. Curiously, indicators for plate tectonics do appear to be present in the Palaeoproterozoic.

Evolution of Earth’s tectonic regime over the past 1.6 billion years; (a) single lid tectonic indicators, (b) plate tectonic indicators cumulative plot, (c) simplified climate history, (d) Simplified biological evolution. Stern & Gerva (2024).

There is a similar shift in the formation of mineral deposits between the Mesoproterozoic and Neoproterozoic, with orogenic gold and porphyry copper deposits, all associated with an active tectonic regime, being common in Neoproterozoic strata but absent in the Mesoproterozoic, while iron oxide copper gold  deposits and iron-titanium-vanadium-phosphorus deposits, both of which are associated with anorthosites are common in the Mesozoic but rare in the Neoproterozoic.

Finally, the palaeomagnetic record does not show any significant movement of continental landmasses during the Mesoproterozoic, while this is common in younger rocks. Particularly noteworthy is the supercontinent of Nuna (or Colombia), which assembled during the Palaeoproterozoic, then appears to have persisted relatively unchanged throughout the Mesoproterozoic.

Plate tectonics requires a global mosaic of plates, which it could be reasonably expected would take hundreds of millions of years to form a a single lid system. Theoretically, following the formation of an initial subduction zone with associated transform systems and divergent plate boundary margins, the system could slowly propagate to form a global mosaic. The rate at which such a system could form would be governed by the rate at which new subduction zones can form and lengthen. In Cretaceous and younger rocks, subductive trenches appear to be able to lengthen at rates of between 100 and 600 km per million years, which would require between 92 and 550 million years to develop a global network of about 55 000 km of convergent margins.

The last 1.6 billion years of Earth’s tectonic history. Stern & Gerva (2024).

The Neoproterozoic is also known to have had major carbon isotope excursions (changes in the proportions of different carbon isotopes laid down in sedimentary deposits) as well as several glacial interludes, major disruptions to the Earth's environment which we associate with plate tectonics. The most notable of these is the Neoproterozoic Snowball Earth, a prolonged phase of near-global glaciation, which may have been triggered by a huge increase in volcanic activity or true polar wander.  The first major carbon isotope event in the Neoproterozoic is the Bitter Springs Event, at about 811 million years before the present, while the youngest is the Shuram Event, about 570 million years ago. If these events bracket a change from a single lid to a plate tectonic system, then that change took about 241 million years, with the Neoproterozoic Snowball Earth, which started at about 720 million years ago and ended about 580 million years ago, in the middle. If the beginning and end of the Snowball Earth mark the transition period then it is shorter, at about 140 million years. The fact that both the carbon isotope excursions and the glaciation events were sporadic suggests that this process was not smooth, bur occurred in a series of episodes. The Palaeoproterozoic is also noted for several isotope excursions, as well as glacial events, and the Great Oxidation Event, during which the Earth first developed an oxygenated atmosphere. All of these events occurred between about 2.50 and 2.05 billion years ago, with a proposed interval of plate tectonics between 2.05 and 1.80 billion years ago, giving an apparent different relationship between these geochemical events and plate tectonics. This Palaeoproterozoic tectonic interval appears to have ended with the formation of the Supercontinent of Nuna, although these ancient events are not well understood and would merit significant further investigation.

Life first appeared on Earth more than 3.8 billion years ago, but appears to have remained fairly simple for the next three billion years, with all terrestrial ecosystems dominated by Prokaryotes (Bacteria and Archaea), simple organisms which lack cell nuclei organelles. All complex multicellular life on Earth is Eukaryotic (i.e. has cells with nuclei and organelles), so Eukaryotic life had to appear before multicellular life-forms. The fossil record shows what appear to be Eukaryotic single-celled organisms dating back to at least the Palaeoproterozoic, suggesting a link between the emergence of the first Eukaryotes and the Great Oxygenation Event, the first case of co-evolution between the evolution of the atmosphere and Eukaryotic life.

The Mesoproterozoic lacks any such major events, making it impossible to split it into any subdivisions, and making its beginning and end points somewhat arbitrary. The interval between 1800 and 800 million years ago (roughly the Mesoproterozoic) has been referred to as the 'Boring Billion' because of this lack of events, with oxygen levels staying roughly constant, geobiological systems apparently remaining unchanged, and constant carbon isotope ratios throughout. The period, which lasted for about 20% of Earth's history, also saw stability in the proportion of sulphur molybdenum, chromium, and strontium isotopes trapped in sediments throughout, and a prolonged low nutrient system.

In contrast, the Neoproterozoic is a period of both climatic instability and rapid biological evolution, during which the Snowball Earth occurred, as well as major shifts in the carbon cycle, the ocean's oxygen content, a major diversification in microscopic Eukaryotes, and the appearance of Metazoans. The era can be split into three periods based upon clear geological differences, the longer and somewhat uneventful Tonian, between 1000 and 720 million years before the present, the Snowball Earth Cryogenian, and the Ediacaran, which saw the first widespread Metazoan fossils. Molecular clocks suggest that the first multicellular organisms appeared during the Tonian, the bilaterian body plan appeared in the Cryogenian, and that the majority of Metazoan phyla diversified during the Late Ediacaran, between 560 and 540 million years ago. All known Animal phyla are believed to have arisen during the Neoproterozoic.

Five conditions are thought to have been needed for this biological shift to have occurred; an increased nutrient supply, increased oxygen levels in both the atmosphere and oceans, an improved climate, an increased rate of habitat formation and destruction, and a sustained evolutionary pressure caused by such shifting environments.

Summary diagram showing how plate tectonics stimulates life and evolution whereas a single lid tectonic style retards life and evolution. Stern & Gerva (2024).

Nutrients are essential for life, and in particular organic carbon (i.e. compounds with bio-available carbon - we cannot, for example, eat diamonds), ammonium (which provides bio-available nitrogen), ferrous iron (again, bio-available iron) and phosphates (bio-available phosphorus). Phosphorus, in particular, plays vital role in biogeochemistry and is considered a global limiting nutrient. A shortage of phosphorus is thought to have been one of the major restrictions on the Mesoproterozoic biosphere. Phosphorus typically becomes available through the erosion of rocks, and its subsequent delivery to the oceans via rivers. This makes it likely that rock weathering was much reduced during the Mesoproterozoic. In the modern world, fresh rocks are constantly exposed at the surface due to tectonic processes, providing new sources of phosphorus and other nutrients, while soil formation covers rocks, inhibiting this supply. The Earth has gone through phases of enhanced nutrient supply associated with major uplift events, such as the Pan-African Orogeny, the Transgondwanan Supermountain Orogeny, and the Circum-Gondwanan Orogens, which all occurred on convergent plate boundaries associated with tectonic transitions. These events greatly increased the rate of erosion, and therefore the delivery of phosphorus into the oceans, with the microbial enhancement of carbon and sulphate acid weathering being an important part of this delivery process. Rising oxygen levels in the atmosphere would have increased the role of microbes in weathering, which in turn would have increased the rates at which organic carbon was buried and phosphorus was delivered to the oceans, resulting in depleted phosphorus depletion in palaeosols (preserved terrestrial soils), something observed during both the Neoproterozoic Oxygenation Event and the Palaeoproterozoic Great Oxygenation Event.

Further evidence for a major onset of uplift, erosion, and weathering during the Ediacaran can be seen in a rise in the proportion of the isotope strontium⁸⁷ within marine sediments. Strontium⁸⁷ is radiogenic, formed by the decay of rubidium within rocks, and can enter the water column either by the erosion of rocks in which this decay has occurred, or by the erosion of older marine sediments. The proportion of this isotope began to rise during the Tonian, and continued to do so throughout the Neoproterozoic, with a significant increase during the Ediacaran, and the highest values recorded in Earth's rock record being found in the Early Palaeozoic. This Neoproterozoic increase in the proportion of strontium⁸⁷ is thought to have been associated with the Pan-African uplifts and the formation of the Transgondwanan Supermountains. These events were caused by continental collisions, with no similar events having apparently happened during the Mesoproterozoic. Thus the low strontium⁸⁷ levels seen in the Mesoproterozoic are another line of evidence supporting a phase of single lid tectonics during this era. The production of phosphorus, iron, and other nutrients by erosion broke the Mesoproterozoic nutrient drought, stimulating biological evolution. 

Free oxygen levels in both the atmosphere and oceans are likely to have been caused by a proliferation of photosynthetic Cyanobacteria, Prokaryotes which have been around since at least the Palaeoproterozoic, combined with a more efficient burial of organic carbon (which will tend to react with free oxygen). This increased oxygen availability enabled the evolution of larger more complicated organisms, such as Animals, something impossible under the low-oxygen conditions of the Mesoproterozoic. Larger, more complicated Animals need higher oxygen levels than smaller, simpler ones, with oxygen levels during the Cambrian thought to have been much lower than today, but Mesoproterozoic oxygen levels are thought to have been lower still, incapable of supporting even simple Animal life. A range of isotopic proxies indicate a significant oxygenation event during the Neoproterozoic, leading to oxygen levels capable of supporting Animal life in most marine ecosystems by the end of the Cryogenian. 

The most likely explanation for this increase in oxygen is that an increase in nutrient supply led to a boom in phytoplankton growth, converting more carbon dioxide into organic matter. This would have allowed the development of more sophisticated Algae with increased photosynthetic abilities, something thought to have happened in the Late Cryogenian. This in turn further boosted oxygen production, as well as transforming the base of the food chain and providing novel food sources for the first Animals. An alternative explanation is increased weathering on land, leading to more nutrients flowing into the oceans, provoking a surge in Cyanobacterial and Algal production, which caused oxygen levels to rise. The common element to all hypothesis is that more phytoplankton were dying and being buried, increasing the  amount of organic carbon sequestered at the same time as sedimentation rates increased in the new rift basins and continental margins of the changing world.

A stable climate is important for Metazoan life. Prokaryotes can thrive at temperatures between 0°C and about 120°C, but most Animal life needs a temperature between about 5°C and about 35°C. Single lid and plate tectonics will provide different climatic regimes. Under a plate tectonic system, the regular release of volcanic gasses can have either a warming or cooling effect; notably mid-ocean ridges produce large amounts of carbon dioxide, tending to warm the climate, while volcanoes on convergent margins produce lots of sulphur dioxide, tending to cool the environment. 

The presence of oceans on the Earth's surface tends to modulate the overall temperature, due to the thermal inertia of water (it takes a lot more energy to warm water than air). This means that the Earth has a more temperate climate when a higher proportion of its surface is covered by water, and a harsher climate when the proportion of the surface covered by water is lower. This means that during a plate tectonic regime, the climate will go through cycles, with a warm greenhouse phase typically arising about 100 million years after a continental breakup event as the oceans widen. It is unclear how the depth and extent of the oceans would have varied under a single lid tectonic system, but it is likely that any change would have been considerably less significant than under a plate tectonic regime.

The process of weathering silicate rocks uses carbon dioxide. This means that the continual exposure and weathering of new silicate rocks, as happens under a plate tectonic system, will consume more carbon dioxide, leading to a reduction in the proportion of this greenhouse gas in the atmosphere, cooling the climate. Thus the enhanced erosion and weathering under a plate tectonic regime will not only release more nutrients, leading to more photosynthesis in the oceans and a subsequent rise in the burial of organic carbon in marine sediments, it also directly removes carbon dioxide from the atmosphere. Under a single lid system, the amount of uplift occurring should be close to zero, leading to a much lower nutrient flux and a lower exposure of silicate rocks to weathering by carbon dioxide.

Plate tectonics also removes large amounts of marine carbonate rocks and buried organic carbon from the Earth's surface systems as they are drawn down into the Earth at subduction zones, further reducing the amount of carbon dioxide in the atmosphere, and leading to further cooling. 

The carbon cycle on planets with single lid tectonic cycles is not well understood, and that of the Mesoproterozoic Earth less so. Two planets in the modern Solar System have single lid tectonic systems, Venus and Mars, and both of these have atmospheres which are more than 95% carbon dioxide, suggesting a poor ability to cycle this gas. However, models of the early Earth suggest that it might have been possible to recycle carbon dioxide reasonably efficiently if volcanic activity was sufficiently high, through the weathering, burial, sinking and delamination of carbonated crust. This fits with the observation that the Mesoproterozoic Earth had a relatively warm climate without any glacial phases, despite the Sun being 5-20% dimmer than today, presumably due to the contribution of greenhouse gasses.

The constant formation and then destruction of new ecosystems is a feature of an active plate tectonic system. This is also something required for the efficient evolution of biological organisms, but is unclear to what extent this would happen under a single lid tectonic system, possibly leading to a system of biological stasis.

The continuous environmental change of a plate tectonic system should present a constant need for biological innovation, with constantly changing nutrient fluxes, topographies, climates, and habitats. This is particularly true along active plate margins, in shallow marine ecosystems which appear to have been hotspots for biological innovation throughout the Earth's recent history. In these environments plate tectonics produces shifting habitats with abundant nutrient and sediment supplies, as well as strong currents and tides, which will tend to distribute these nutrients. 

The switch a plate tectonic system appears to have stimulated the rapid diversification of life, something which may not have been possible at all under a single lid system. The most dramatic environmental shifts encountered under a single lid system are likely to have been mantle plumes, which would cause global warming when they first appeared, due to the production of carbon dioxide, followed by a period of cooling as weathering of basalts leads to carbon oxide levels lowering again. Under this system the oceans would also likely suffer from conditions of anoxia, acidification, and toxic metal-input.

Without the driving force of plate tectonics, the evolution of biological life appears to be an extremely slow process. Under a plate tectonic regime, the evolution and demise of new organisms, groups of organisms, and whole global ecosystems, appears to follow the opening and closing of oceans. This makes it extremely unlikely that complex life would have arisen on Earth without the development of a plate tectonic system.

We know that life appeared in Earth's oceans more than 3.8 billion years ago, and remained within the oceans for more than 3 billion years. Despite this, it is generally accepted that the presence of dry land on Earth was needed for both the origin and evolution of life, since without this all nutrients would eventually be lost from surface systems. It is possible that the first life originated in ancient palaeosols (or more accurately, regalith), but seawater appears to have been a vital environment for much of the history of life, providing a nutrient bath in which primitive organisms could absorb nutrients through their cell membranes, as well as protection from the Sun's harmful ultraviolet radiation. All complex life on Earth is Eukaryotic, and it is generally accepted that Eukaryotic cells first evolved in the sea, where water would provide structural support for these larger cells until they evolved it themselves. This structural support would also have been needed for the first Metazoans, which appear to have been the soft-bodied organisms recorded in the Ediacaran Biotas.

Stern and Gerva reason that while primitive life must evolve in the sea, advanced civilizations need to evolve on dry land. Changing terrestrial ecosystems provide even more varied habitats than the oceans, providing an additional stimulus for biological evolution, and areas around the margins of continental plates tend to produce particularly varied habitats, as can be seen in the circum-Mediterranean, Mesoamerica, Madagascar and Southeast Asia today. The harsher terrestrial environment also stimulates life to produce specialist water retention and gas exchange structures, reproduction by internal fertilization, and movement systems which do not rely on the support of water, all of which lead organisms to become increasingly sophisticated and complex. 

This biological complexity is one of the prerequisites to developing a system for organisms to transfer experiences and knowledge to one-another, which in turn has the potential to lead to abstract thinking, and the development of language, technology, and science.  In particular, an advanced civilization would require the organisms building it to develop a familiarity with both fire and electricity, something more-or-less impossible if they are restricted to water. Thus the development of an advanced civilization on a planet would require the presence of a plate tectonic system, which Stern and Gerva suggest should be added to the Drake Equation.

The Drake Equation, as envisaged by astrobiologist Frank Drake, proposed that the number of potentially detectable  advanced civilizations in the Galaxy would be equal to the average rate of star formation, multiplied by the fraction of stars which host planets, multiplied by the fraction of planets which hold the conditions for life, multiplied by the fraction of planets which hold the conditions for life which actuallt develop life, multiplied by the proportion of planets with life which develop civilizations, multiplied by the proportion of civilizations which produce detectable signals (such as radiowaves etc.), multiplied by the lifetime of such civilizations. 

Based upon this, Drake made an 'educated guess' that between 200 and 50 000 000 detectable civilizations might exist in the Galaxy, with subsequent estimates by other scientists producing figures from below a hundred to several million. Stern and Gerva suggest that the proportion of planets with life that go on to develop civilizations should be considerably lower than in most estimates, due to the additional requirement for these planets to develop plate tectonic systems which operate for several hundred million years.

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