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.

See also...

Looking for a conection between the length of the Earth's days and the development of an oxygen-rich atmosphere.

A day on Earth (i.e. the period between one sunrise and the next) lasts for 24 hours, but four billion years ago it may have been as short as six hours. Thus, the length of the day, and the length of time for which any given part of the Earth's surface is exposed to sunlight during each cycle has increased threefold over the history of the planet. The rate of photosynthesis (i.e. the rate at which oxygen is produced by Plants, Algae, and Cyanobacteria exposed to sunlight) is determined by instantaneous photon flux, and should not be affected by the length of the day, as long as the total amount of sunlight over any given period remains the same. However, the net rate of oxygen is influenced by both how much oxygen is produced, and the rate at which organic material (and the bio-available carbon it contains) is buried, and this burial rate is potentially influenced by the length of the day. Thus the net production of oxygen by benthic ecosystems will be influenced by changes in the length of the day, due to changes in the availability of metabolites, the import, export and accumulation of which can be sensitive to daylength.

In a paper published in the journal Nature Geoscience on 2 August 2021, Judith Klatt of the Microsensor Group at the Max Planck Institute for Marine Microbiology, the Department of Earth & Environmental Sciences at the University of Michigan, Arjun Chennu, also of the Microsensor Group at the Max Planck Institute for Marine Microbiology, and of Data Science and Technology at the Leibniz Centre for Tropical Marine Research, Brian Arbic, also of the Department of Earth & Environmental Sciences at the University of Michigan, Bopaiah Biddanda of the Annis Water Resources Institute at Grand Valley State University, and Gregory Dick, again of the Department of Earth & Environmental Sciences at the University of Michigan, present a model which aims to explore this interaction, and how it would influence the export of oxygen into the atmosphere.

Klatt et al. modelled benthic ecosystems as systems in which oxygen photosynthesis, anoxygenic photosynthesis, aerobic respiration, anaerobic respiration (by sulphate reduction or without sulphate reduction), and the oxidation of sulphides by non-biological means, could all occur. The simplest model, with only oxygen photosynthesis and aerobic respiration, showed the amount of oxygen exported to the water column increased with increasing day length, as oxygen will not move from the mats to the water column until it builds up to a certain concentration, which takes time at the beginning of each daylight period. As this build up time is fixed, it represents a larger proportion of the day when the day is shorter, and a smaller proportion when the day is longer. Thus, with a longer day, the amount of oxygen exported from the mats will increase. If the mats are thin, then this will not just apply to oxygen being exported into the water column, but also to oxygen being exported into the substrate, which may create a distinctive weathering pattern that could be detected. This build up of oxygen would also increase the amount of bio-available oxygen within the mats, enabling the export of more organically bound carbon from the mats, therefore influencing the overall efficiency of the mats, and increasing the amount of organic carbon being buried, which in turn influences the level of oxygen in the atmosphere. Thus, although the rate of photosynthesis is independent of the length of the day, increasing the length of the day increases the amount of organic carbon burrial, and therefore the amount of oxyen build up in the water column and atmosphere.

 

Schematic of the global sinks and sources of oxygen with net release vs uptake of reductant by mats. The daylength-driven changes in organic carbon burial from benthic or terrestrial mats (mB; flux arrows not to scale) cause quasi steady-state transitions of global atmospheric oxygen pressure. Offsets in oxygen pressure between such steady states are conceptualised here as atmospheric oxygen (aO). The diel mat processes (inset box) produce organic carbon burial fluxes (mB), which along with burial from the pelagic domain (pB) comprise the global oxygen source. Both oxygen (mO) and reductant (mR) export from mats are controlled by the interaction between mass transfer and mat-intrinsic process rates (oxygenic photosynthesis, OP; anoxygenic photosynthesis, AP; aerobic respiration, Raero; sulphate reduction, Ranaero; aerobic hydrogen sulphide oxidation, SOX), and hence are sensitive to daylength changes. For the global oxygen sinks, Klatt et al. considered that some of the surplus oxygen released from the terrestrial or marine realm was consumed directly in the atmosphere (atmR) by volcanism- and metamorphism-derived gases (vR). Surplus reductant released from mats (mR in (a)) will increase atmospheric reductants (atmR). Surplus reductant consumed by mats (mR in (b)) will decrease atmospheric reductants, and add to source strength organic carbon burial. Thus, mat organic carbon burial is the sum of oxygen export and reductant import, and also sensitive to daylength. Note that volcanic reductant fluxes (vR) are equal to pelagic organic carbon burial (pB) and the equivalent pelagic oxygen export (pO) to illustrate that reductant uptake by mats influence the global availability of reductant. This influences the consumed fraction of oxygen pressure by atmospheric reductants. As a result, organic carbon burial is equal to water exported into the water column (wO), that is the oxygen that escapes reduction by atmospheric reductants. The sink for oxygen in the water column is erosional weathering (WEATH), and the emergent oxygen pressure for a reference weathering level is (wO/(0.95 × tB + uB)), where (uB) describes the size of the global organic carbon reservoir, uplift forcing and a weathering constant, was chosen based on a mid-Proterozoic oxygen pressure of 0.01 or 0.1 and was set constant over Earth age. To account for the direct erosion of terrestrial mats, WEATH was set to interact with 95% of terrestrial organic carbon burial rates (tB; a fraction of total mat burial mB). While this makes WEATH also sensitive to daylength and produces a buffering effect through increased weathering strength, atmospheric oxygenation (aO) still increases with daylength. Klatt et al. (2021).

For a more realistic scenario, Klatt et al. also included the consumption of organic carbon by anaerobic respiration (i.e. through the sulphate reduction process). In theory, both sulphate (the oxidised form) and sulphite (the reduced form) can be reacted with any other redox pair (such as ferrous and ferric iron), but Klatt et al. concentrated on the reduction of sulphate to sulphide, as this is thought to have evolved in Microbes early in life's history, and sulphide is known to have been abundant in many Precambrian coastal sediments. Klatt et al. therefore began with a model in which anaerobic sulphate reduction occurred at a fixed rate, in order to assess the impact of day length changes on the export of sulphites from the mats. As with the export of oxygen, the export of sulphites is determined by diffusion rates, anf the extent to which the produced sulphite is consumed within the mats. The consumption of sulphites within the mats also uses oxygen, competing with aerobic respiration for the available supply. Any organic carbon that is exported from the mats must escape being used for either aerobic or anaerobic respiration, and, therefore, the rate at which this is produced relates directly to the amount of oxygen and the amount of sulphite being exported. As the day length increases, the rate of both aerobic and anaerobic respiration increases, and the rate of sulphur oxidisation decreased. Thus, the rate of aerobic respiration is less sensitive to the effect of day length when sulphate reduction is introduced to the model than it was previously, and the rate at which oxygen is exported from the mats is lowered, but not eliminated, by the inclusion of sulphite production. Modern mat-dwelling sulphate-reducing Bacteria are inhibited by the presence of oxygen; when Klatt et al. introduced this to their model, the rate at which organic carbon was buried increased with increasing daylength.

Since the redox environment on Earth is known to have changed over time (just as the day length has), Klatt et al. examined the relationship between the (daylight driven) rate of organic carbon burrial and the available oxygen in the water column. Increasing the day length was found to increase both the rate of organic carbon burial and the rate at amount of available oxygen in the water column under all circumstances. Increasing the available oxygen in the water in turn led to an increase in the rate of aerobic respiration. However, the influence of available oxygen had a more complicated affect on the rate of organic carbon burial, as organic carbon can be produced both aerobically and anaerobically, dependent on whether or not the anaerobic respiration method was sensitive to the presence of oxygen. When Klatt et al. assumed that anaerobic respiration was inhibited by the presence of oxygen, as is the case with most anaerobic respirating microbes today, then increasing the day length resulted in a higher rate of organic carbon burial.

Next Klatt et al. considered the possibility that anaerobic photosythesisers, using hydrogen sulphide as an electron donor, might be present in the mats. This resulted in a longer time-period before the mats began to export oxygen each day, during which time the anaerobic microbes were depleting the hydrogen sulphite to a level at which oxygen photosynthesis could occur. Thus, although the amount organic carbon produced by photosynthesis remains fairly constant, increasing the day length both increases the amount of oxygen being exported and lowers the rate of anaerobic respiration (due to the presence of inhibiting oxygen). Unexpectedly, this also lowers the rate of aerobic respiration, due to the presence of sulphur oxides, which compete with the respirators for the available oxygen. Thus, when modelled with a wide range of oxygen and sulphur levels in the water column, Klatt et al. found that in all cases the oxygen and sulphur model made organic carbon buial more closely tied to day length than was the case when only oxygen was considered. Thus the range of metabolic pathways available has more influence on the relationship between daylength and organic carbon burial than the redox state of the water column.

Klatt et al. were able to demonstrate that the rate of organic carbon burial, which is thought to be closely linked to the accumulation of oxygen in the atmosphere, would increase with increases in daylength under a wide range of conditions, without assuming a decreasing oxygen sink (i.e. a supply of substances on the Earth's surface which would react with oxygen, thereby preventing it from building up in the atmosphere), or an increase in the global rate of photosynthesis. It is, however, likely that the global rate of photosynthesis did change over this period, as new, more efficient metabolic pathways evolved, and redox and phosphate levels in the oceans changed due to the weathering of rocks on land during what has been turned the 'boring billion' years of the Mesoproterozoic. The accumulation of oxygen in the atmosphere is still expected to be driven largely by the global rate of production by photosynthesis, but the length of the day clearly has a major impact on the burial of organic carbon, and there is a predicted link between the extent of benthic environments in which photosynthesis can occur and the extent to which day length is able ro affect the system.

As a further test of this model, Klatt et al. measured the rate of photosynthesis and oxygen export by Cyanobacterial mats from the Middle Island Sinkhole in Michigan, USA, which are considered to be a good model for Proterozoic microbial mats under low oxygen conditions. They found that the mats only exported oxygen after they had been exposed to light for some time. White Sulphur-oxidising Bacteria grew on top of the mats during the night and early morning, and these reduced the light available for photosynthesis. Light levels did not become high enough for photosynthesis until the early afternoon, at which point the White Sulphur-oxidising Bacteria migrated downwards through the mats, possibly in response to the depletion of sulphides by the oxygen produced by the Cyanobacteria. After this, the oxygen produced needed to react with any sulphide in the water column before free oxygen began to build up, creating an additional lag of 1-8 hours. Increasing the strength of the light to the mats increased the speed at which oxygen was produced and exported into the water column, with the oxygen production remaining high once sulphides were depleted even if light levels fell.

The presence of White Sulphur-oxidising Bacteria at the top of the mats lowered light availability for the Cyanobacteria, delaying the onset of oxygen production. This implies that day length has a strong effect on overall oxygen production in communities where photosynthetic and chemosynthetic microbes are competing. Klatt et al. further tested this model by exposing samples of the mats to 'days' of different length using artificial lighting in a laboratory. They found that when the total day length was less than twelve hours then the mats produced non oxygen, instead becoming a sink for any oxygen in the water column. When the day length reached sixteen hours (predicted for the Late Archaean), then the mats became net exporters of oxygen, with the amount of oxygen produced at a day length of twenty one hours (predicted for the Late Proterozoic) doubling that produced at sixteen hours, and the amount of oxygen produced at a day length of twenty four hours being three times that of the sixteen hour day.

Similar interactions have been seen in other microbial mats at other locations, although it is unclear which of them, if any, provides the best model for life in the Precambrian. However, several different sets of conditions can be shown to produce less oxygen with a shorted day length, largely to fluctuations in the redox state within the mats over the course of  the day. Many types of microbes also have the ability to alter their metabolic pathways in response to environmental conditions, which may lead to further delays in the onset of oxygen production in the presence of sulphides. 

This conceptual model, in which the length of the day effects the burial rate of organic carbon, fits well with observations of modern sediments, in which it has been shown that exposure to oxygen decreases burial efficiency. As the days get longer, aerobic respiration takes over from anaerobic respiration, and the extent to which oxygen penetrates into the mat decreases. This means that layers of the mats which are not dominated by photosynthetic organisms (i.e. those lower down) actually recieve less oxygen over the course of a day. This would enhance the burial of organic carbon. The rate at which mats accumulate must also be taken into account when looking at the burial of organic carbon. Modern mats can accumulate at rates of 0.5 to 5 mm per year, with estimates of ancient mat growth having a slightly higher range, perhaps 0.5 to 15 mm per year. If lengthening days decreased the oxygen availability within the mats, and increased the burial of organic carbon, then it is likely that the accretion rate of the mats would also have increased, potentially making a more pronounced shift in productivity and carbon burial than was observed in the modern mats.

This phenomenon, in which the lengthening day caused changes in the production and export of oxygen and organic carbon from microbial mats would have gone on as long as the 'matworld' existed. Unfortunately, it is hard to quantify the rate at which this would have occurred, as we are uncertain of the rate at which the Earth's days have increased, other than that this has not proceeded at a steady rate over the history of the Earth. If we extrapolate backwards, using the current rate at which our days are lengthening and the related rate at which the Moon is moving away from the Earth, we get a scenario in which the Moon would have collided with the Earth 1.5 billion years ago; something for which there is no evidence. Some recent models have tried to take into account the influence of the position of the continents on the Earth's rotational deceleration, and have come to the conclusion that the lengthening of the days would have been at its lowest in the Middle Proterezoic. Another hypothesis suggests that there may have been periods in which the days did not lengthen at all and the Earth-Moon system remained stable due to a resonant atmospheric thermal tide. In this scenario on the length of the day would have remained stable throughout the 'boring billion' years of the Mesoproterozoic, then started to lengthen at around the time of the Neoproterozoic Oxygenation Event (between 800 and 540 million years ago), which in turn suggests there might be a link between the two events.

In order to model the impacts of a link between daylength, oxygen production, and organic carbon burial on a global scale, Klatt et al. used a recent model which suggests that the length of the days increased steadily until about 2.2 billion years ago, when they stabilized, due to a resonant stability with the Moon, with a resumption in lengthening occurring about 650 million years ago. In this model Klatt et al. combined their findings on organic carbon burial and oxygen production with other events likely to have influenced the composition of the atmosphere, notably the reducing influence on the atmosphere caused by large scale metamorphism, the production of volcanic gasses, and weathering of exposed rocks on the Earth's surface. They found that this model could account for the change in atmospheric composition associated with the Great Oxidation Event (between 2.4 and 2.0 billion years ago) without the need to invoke any change in the global rate of photosynthesis, or any other redox change in the atmosphere. This scenario starts with an 18-hour day in the Archaean, which results in zero burial of organic carbon, increasing to a 21 hour day in the mid-Proterozoic, when about 50% of the current burial rate of organic carbon is achieved, although these mats would only cover 3.7% of the area of the modern oceans (for comparison the modern shallow water zone (where photosynthesis is possible for benthic organisms) covers about 7.5% of the ocean surface. This day lengthening would allow atmospheric oxygen to reach 28% of current levels by about 550 million years ago (the beginning of the Cambrian), which is consistent with a Neoproterozoic Oxygenation Event, between 800 and 540 million years ago, and a later Palaeozoic Oxygenation Event, possibly coincident with the Ordovician Biodiversity Event, at about 400 million years ago.

 

Weathering and organic carbon burial rates over time and corresponding examples for proxies in the geological record. Increases in the latter two parameters indicate enhanced weathering fluxes. All rates were derived from the modelled scenario that include aerobic and anaerobic respiration and exclusive oxygenic photosynthesis. Shaded areas represent the range of rates dependent on 1.5–3.7% modern oceanic coverage by benthic coastal mats (corresponding to 20–50% of global marine organic carbon burial during the mid-Proterozoic) and a continental coverage of 5% by terrestrial mats. Changes in global coastal benthic and terrestrial organic carbon burial fluxes are driven by changes in daylength and are shaped by feedback effects of increasing oxygen pressure on aerobic respiration. Pelagic burial, atmospheric reduction by volcanism- and metamorphism-derived gases and weathering were parameterized for a reference oxygen pressure of 0.1 in the mid-Proterozoic. The rate of atmospheric reduction was assumed to be constant and determined by the flux of reduced gases. In contrast, the rate of erosional weathering increases with daylength as it depends on oxygen pressure and organic carbon burial by terrestrial mats. Klatt et al. (2021).

Klatt et al. suggest that isotope excursions in the Precambrian rock record associated with oxygenation events are actually signals of the increased burial rate for organic carbon, which is in turn caused by increases in the global photosynthesis rate and a drop in remineralisation as the increased available oxygen caused by the increasing day length used up available oxygen-reactive minerals. It is difficult to connect isotope signatures directly to microbial mats, as limited isotope fractionation occurs within them, but the model of atmospheric oxygen enhancement and increased burial of organic carbon driven by increasing day length does match the signatures seen in the rock record. An increase in weathering in terrestrial rocks with rising oxygen levels, which would release more phosphorous into the system, resulting in a further boost in microbial productivity has been previously predicted. Klatt et al. have not included this in their models as they believe any such effect would be transitory in nature, with oxygen levels quickly returning to a steady state. The availability of nutrients could shape the rate of global photosynthesis, and therefore the possible range of oxygen pressures that could be achieved, but Klatt et al. believe the length of the day and burial rate for organic carbon would have actually driven oxygenation rates, at least until the Palaeozoic Oxygenation Event.

Klatt et al.'s models indicate that the length of the day is a consistent driver of oxygenation levels across a range of possible metabolic parameters. The possibility that the Earth and Moon could have gone through periods of resonance locking, with abrupt changes in day length as these phases are entered or escaped from, serves as a possible trigger for the distance oxygenation events seen in the rock record. In this sense, day length changes can be considered to be of similar significance to events such as the opening of major tectonic rifting zones and the formation of supercontinents, which are also thought to have had profound impacts on the composition of the atmosphere, although there is no known link between the Earth's orbital parameters and these events. Even of a simple gradual increase in day length is assumed, Klatt et al.'s model produces an increase in oxygen production and decrease in oxygen sinks that could produce relatively rapid shifts in the atmospheric oxygen level. The exact extent to which the length of days contributed to shifts in atmospheric oxygen levels is impossible to know, nut Klatt et al.'s model provides a remarkably good reconstruction of the shifts in the oxygen content of the Early Earth's atmosphere without the need to invoke other factors, which strongly suggests that the orbital dynamics of the Earth-Moon system played a role in the evolution of the Earth's atmosphere during the Proterozoic Eon.

See also...

Online courses in Palaeontology. 

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