Macrofossils from the Early Ediacaran Brachina Sequence of the central Flinders Ranges, South Australia.

Fossils of soft-bodied organisms were first discovered in the Ediacaran Hills, part of the Flinders Ranges of South Australia in 1946. At the time, these fossils were presumed to be Medusae (Jellyfish) of Early Cambrian age, since it was believed that there were no Precambrian fossils. In the 1950s, fossils were found in reliably-dated Precambrian rocks in Charnwood Forest, England, showing that this presumed Cambrian appearance of life was incorrect. Furthermore, the Charnwood fossils closely resembled fossils previously described from Namibia (then Southwest Africa) in the 1930s, suggesting that these were of similar age.

Eventually, all of these fossils were grouped together as the 'Ediacaran Fauna', found at numerous locations in the world, and eventually split into three separate assemblages, each with its own distinct fossils, laid down in different environments at different times; the Avalon Assemblage, which appeared about 578 million years ago, and persisted to about 555 million years ago, the White Sea Assemblage, which included the Ediacaran Hills fossils, and which appeared about 560 million years ago, and persisted to about 551 million years ago, and the Nama Assemblage, which appeared about 555 million years ago, and disapeared 539 million years ago (around the onset of the Cambrian).

The earliest of these Ediacaran fossil assemblages seemed to have appeared slightly after the Gaskiers Glaciation, between 580.9 and 579.2 million years ago, taken as a middle point for the Ediacaran Period, leading to the assumption that the appearance of large Metazoans post-dated this event. However, a number of locations have subsequently produced Ediacaran-type fossils that apparently predate the Gaskiers Glaciation. Notably, the Charnwood Forest fossils can be dated to 603 million years before the present (Early Ediacaran), while the Lantian Biota of Anhui Province, China, has been dated to  605 million years ago. 

Since the discovery of the original Ediacaran Hills fossils, sporadic attempts have been made to find older fossils within the Flinders Ranges, although an absence of obvious fossils, combined with a perception that they were unlikely to exist, has tended to limit such searches. In 2021, Philip Plumber of the Department of Earth Sciences at the University of Adelaide, reported finding macro-fossils in the approximately 700 million years old (Cryogenian) Areyonga Formation, and 970–950 million years old (Tonian) Heavitree Formation of Central Australia, opening the possibility that pre-Middle Ediacaran fossils might be more widespread in Australia.

In a paper published in the journal Transactions of the Royal Society of South Australia on 2 September 2024, Philip Plumber reports macrofossils from the Early Ediacaran Brachina Sequence of the Flinders Ranges. 

The Brachina Sequence spans the interval between the end of the end of the Marinoan Glaciation, 635 million years before the present, which marks the boundary point between the Cryogenian and the Ediacaran periods, and the Acraman Asteroid Impact, 580 million years ago, which is coeval with the onset of the Gaskiers Glaciation. The Brachina Sequence begins with the Nuccaleena Dolostones, which overlay the Marinoan glacial deposits, above which lies the purple Moolooloo Siltstone, made up of clastic deposits brought into a shallow marine basin by turbid bottom currents. Around 620 million years ago, the tectonic situation changed, causing a delta to spread across the basin from the southwest. These delta deposits form the ABC Range Quartzite, formed in a shallow, wave-dominated environment, while other parts of the basin were covered by a tidal flat environment, recorded as the Moorillah Siltstone. 

Stratigraphic column (not to chronostratigraphic scale) showing positions of the fossiliferous Ediacara and Nilpena members (Pound Subgroup) and Moorillah Siltstone (Brachina sequence) within the Ediacaran succession. Plumber (2024).

The first fossil described by Plumber was first recorded in 1969 by palaeontologist Martin Glaessner, who identified it as a trace fossil, Bunyerichnus dalgarnoi, apparently made by a 'bilaterally symmetrical animal which used rhythmic muscular contractions rather than discrete appendages for propulsion'. The exact stratigraphic position where this fossil originated is unclear, but it was found on a surface bedding plane on a partly cross-laminated dark purplish micaceous siltstone, at the entrance to Bunyeroo Gorge in the central Flinders Ranges, which would imply it came from either the upper Moolooloo Siltstone or the lower Moorillah Siltstone.

Subsequent to this discovery, other intepretations of Bunyerichnus have been put forward. The curving shape of the fossil led to the suggestion that it might be a portion of a Medusa, but this did not explain why the specimen appeared to taper to one end. An alternative suggestion is that the specimen might be a trace left by a Rangeomorph (frond-like) Ediacaran sweeping over the sediment in a shallow setting. Plumber notes that Rangeomorph fronds were described from the base of the ABC Range Quartzite in 1985 (when the age of these deposits was unknown, although they were recognised as being stratigraphically significantly lower) by Ian Dyson of Flinders University, and that these would have been of approximately the same age as Bunyerichnus.

(a) Partial fossil of the Medusoid Paramedusium showing radial marking across its outer ring compared to (b) arcuate Bunyerichnus from the lower to mid Brachina sequence, Bunyeroo Gorge, Flinders Ranges. (c) Impression of the rangeomorph Akrophyllas (South Australian museum specimen SAM P24593) and (d) sketch of the rangeomorph Pteridinium showing transverse ridges extending from a central stem similar to Bunyerichnus. Plumber (2024).

Plumber also notes a number of circular features 0.5 to 1.0 cm in diameter from the base of the Moorillah Siltstone about 22 km southeast of Bunyeroo Gorge. These were first described by Plumber in 1980, when he interpreted them as inorganic fluid escape structures. However, subsequent examination of the specimens by Jim Gehling of the South Australian Museum led them being re-interpreted as examples of Aspidella, a Rangeomorph holdfast impression, with a fallen frond-lying next to the largest example. Plumber dates the horizon from which these fossils were recovered to about 620 million years before the present, firmly within the Early Ediacaran, older than the Charnwood Forest fossils, and about 60 million years older than the Ediacaran Hills biota.

(a) Several circular Aspidella on the bedding plane of a fine sandstone, near the base of the Moorillah Siltstone (lower Brachina Sequence), southeast of Wilpena Pound, Flinders Ranges (South Australian Museum specimen SAM P59911). (b) Enlargement of fallen frond (outlined) compared to (inset) the type specimen of the Ediacaran fossil Charniodiscus arboreus (South Australian Museum specimen SAM P19690a). Plumber 2024.

The Moorillah Siltstone of the Brachina Sequence has been dated to between 620 and 605 million years before the present. Philip Plumber reports the presence of frond-like Rangeomorph fossils near the base of the Moorillah Siltstone, suggesting that these must therefore be close to 620 million years in age. Such fossils are roughly coeval with the Lantian Biota of South China, and at least 40 million years older than the global Avalon Assemblage. These fossils therefore contribute to growing body of evidence for the emergence of Metazoan life before the Gaskiers Glaciation in the Middle Ediacaran Period.

See also...

Tubular fossils from the Terminal Ediacaran La Ciénega Formation of Sonora State, Mexico.

The first fossils of organisms with mineralized skeletons appear in strata from the Terminal Ediacaran, between about 550 and 538 million years ago. This appearance coincides with the decline of the organisms of the 'classical' Ediacaran White Sea Fauna. One of the most abundant of these early mineralized fossils is Cloudina, a diverse group of fossils with a cone-in-cone or funnel-in-funnel structure, although the extent to which Cloudina was mineralized appears to have been variable, with some forms heavily mineralized, some only lightly so, and others apparently having skeletons made from tough organic materials. Cloudina was first described from the Terminal Ediacaran Nama Formation of Namibia, but has since been found in Brazil, Spain, China, Oman, and the United States, and is considered to be a useful index-fossil for the Terminal Ediacaran. 

While Clodina is often the dominant fossil in Terminal Ediacaran assemblages, it is often found alongside a variety of other tubular fossils, such as Namacalathus, Namapoikia, and Sinotubulites. Sinotubulites fossils have a tube-in-tube structure and often have annular or longitudinal ridges, as well as both circular and polygonal cross-sections. Originally described from South China, Sinotubulites has also been described from Mexico, the United States, Brazil, Spain, and Namibia. Interestingly, which specimens of Cloudina have been discovered in many places with what-appear to be drill-holes, possibly the oldest known example of predation by boring, this has not been observed in specimens of Sinotubulites, even when the two are found together, potentially representing the oldest example of prey-selection.

Ediacaran fossils in the Terminal Ediacaran La Ciénega Formation of Sonora State, Mexico, were first described in the mid 1980s. The initial descriptions suggested a variety of tubular fossils were present, including Sinotubulites but not Cloudina. However, subsequent studies of the material led to the conclusion that these 'Sinotubulites' fossils were in fact specimens of Cloudina which had undergone taphonomic alteration, causing them to develop compactional folds which were mis-interpreted as the longitudinal striae of Sinotubulites.

In a paper published in the Journal of Paleontology on 10 October 2024, James Schiffbauer of the Department of Geological Sciences and X-ray Microanalysis Laboratory at University of Missouri, Clara Wong also of the Department of Geological Sciences at University of Missouri, and of the Department of Geosciences at Smith College, Cassidy Davis, also of the Department of Geological Sciences at University of Missouri, Tara Selly, again of the Department of Geological Sciences and X-ray Microanalysis Laboratory at University of Missouri, Lyle Nelson of the Department of Earth Sciences at Carleton University, and Sara Pruss, also of the Department of Geosciences at Smith College, re-examine the La Ciénega Formation fossil assemblage, using modern methodologies to provide new insights into the community structure preserved there.

The Caborca Block in Sonora comprises a series of Late Neoproterozoic and Early Palaeozoic strata laid down in shallow-marine environments on the edge of first a rift zone and then the southern margin of the continent of Laurentia. Here the early Ediacaran Period is represented by the Clemente Formation, which is followed by the Late Ediacaran La Ciénega Formation, and the Cambrian Cerro Rajón Formation. Uranium/lead analysis of zircons from the uppermost layer of the La Ciénega Formation places the age of this at 539.4 million years, the latest part of the Ediacaran. Zircon is a volcanic mineral that forms as liquid magma slowly cools to form solid rock. As zircon forms it can incorporate a variety of different elements into its crystal matrix, including uranium but not lead. This is useful as over time uranium decays to form lead, so any lead in a zircon mineral must be the result of the decay of uranium. Since the decay of uranium to lead occurs at a steady rate, it is possible to determine the age of zircons by measuring the ratio of uranium to lead within them.

Locality map and stratigraphy of the Cerro Clemente section: (1) Map indicating position of Caborca localities (yellow star) in northern Mexico, and plausibly correlative fossiliferous units (grey stars) in the southwestern USA. (2) Satellite image from Google Earth denoting the topography of the Cerro Clemente section with longitude and latitude markers. (3) Geologic map corresponding to the same map view in (2). (4) Photograph of collected coquina block. (5) Stratigraphic section with carbon isotope chemostratigraphy showing position of uranium-lead radiometric date (green star) and sampled fossil horizon (yellow star). Schiffbauer et al. (2024).

Schiffbauer et al. obtained a block from a silicified coquina bed within the La Ciénega Formation at the Cerro Clemente section measuring 15 cm x 10 cm x 10 cm. This was partitioned into a number of fragments, which were then treated to different analysis regimes, including dissolution with acetic acid to allow the collection of fossil residues, scanning electron microscope analysis, elemental analysis using an energy dispersive X-ray spectrometer, and thin sectioning for visual microscopy.

These methods enabled them to identify five distinct forms of tubes. Firstly, there were tube-in-tube structures with annular ridges and possible laminae, interpreted as Sinotubulites. Secondly, there were tubes made up of a series of tightly-fitting funnel-in-funnel structures lacking rims, interpreted as Cloudina. Thirdly, there were funnel-in-funnel structures with thickened and/or pronounced rims, interpreted as the Cloudinomorph Saarina. Fourthly, simple straight tubes, and fifthly, simple curved tubes.

Morphological groupings of fossils (SEM). (1, 2) Form 1, Sinotubulitids in lateral (1) and cross sectional (2) views: (1) lateral view showing diagnostic transverse corrugations; (2) cross section illustrating multiple tube-in-tube construction, with substantial silica overgrowth. (3), (4) Forms 2 and 3, Cloudinomorphs, Cloudina sp. indet. (3) and cf. Saarina sp. indet. (4): (3) Cloudina with two nested funnel units and no thickened apertural rims; (4) the other Cloudinomorph form showing thickened apertural rims with observable drooping imbrication. Also note slight change in growth direction or plastic deformation at tube midpoint, along with slight tubular compression. (5), (6) Forms 4 and 5, smooth tubes that are either straight (5) or sinuous (6). Scale bars are 1 mm (1), (3), (4), and (6), 500 μm (2), and (5).  Schiffbauer et al. (2024).

Of the identifiable individual specimens, 33 were tube-in-tube structures, 20 were non-rimmed funnel-in-funnel structures, 23 were rimmed funnel-in-funnel structures, five were straight smooth tubes, and ten were curved or sinuous tubes. Thus, Cloudinomorphs were the most abundant group, making up about 40% of the sample, with Sinotubulitids comprising about 31% of the sample, and unidentified tubes making up 28%.

The Cloudinomorphs ranged from 0.88 to 2.61 mm in diameter, with an average of 1.51. The rimless forms were on average slightly larger, ranging from 1.13 to 2.61 mm in diameter, with an average of 1.65 mm., while the rimmed forms ranged from 0.88 to 2.26 mm in diameter, with an average of 1.43 mm. The Sinotubulitids were generally larger than the Cloudinomorphs, ranging from 1.11 to 5.23 mm in diameter, with an average diameter of 2.41 mm. The indeterminate tubular fossils ranged from 0.36 to 1.84 mm in diameter, with an average of 1.21, and little difference between the two forms (the straight forms average 1.22 mm in diameter, the sinuous forms 1.17 mm). The length of the fossils was much harder to estimate, as fossils of this size are prone to fragmentation; the longest Cloudinomorph found was 4.49 mm long, the longest Sinotubulitid 5.82 mm, and the longest smooth tube 5.59 mm. 

Seen in thin section, much of the block was made up of densely packed tubular fossils. The majority of these were calcareous in nature, with only a minority of examples being silicified. However, the acid-extraction method produced only silicified specimens, with the calcarious fossils apparently lost from the portion of rock treated this way. This is likely to have given a distorted view of the nature of the total assemblage. Furthermore, the silicified fossils within the block cut into thin sections often had very fine skeletal walls, sometimes as little as 2–3 μm thick, while all of those extracted by acid etching were much courser, with the thinnest being 0.15–0.32 mm thick, suggesting that finer silicious fossils had also been lost. 

Petrographic thin section photomicrographs. (1) Silicified tube examples (brighter white material) in transverse section (left) and longitudinal section (right). (2) Silicified funnel-in-funnel tube in longitudinal section, non-orthogonal to the length of the tube. Note blocky calcareous infilling and potential fine layering in the tube wall. (3) Transverse plane of non-silicified tube, with apparent fine layering and blocky calcareous infill. (4) Longitudinal plane of non-silicified tubular fossil with fine layering and micritic infill. Scale bars are 1 mm. Schiffbauer et al. (2024).

The calcitic tubes visible in the thin sections appeared to be made from a fine micrite, with courser carbonate and even dolomite crystals separating them. This could also be seen in scanning electron microscopy images, where it could also be seen that while the fine structures of the fossils were preserved, they were altered by the development of courser crystals around them, with many Cloudinomorph fossils having an elliptical cross-section (interpreted as distortion of an original circular shape), whole Sinotubulitids showed flattening, irregular cross-sections, and corrugation. One smooth-walled tube appears to have been helically twisted along its length. This makes it unlikely that the original surface structure of any of the fossils was preserved.

Surface and deformative features of silicified fossils (scanning electron microscope images). (1) Sagittally flattened Sinotubulitid specimen. (2) Imbricated funnel rims (dashed white lines to guide orientation) of cf. Saarina specimen with little-to-no flattening. (3) Torted funnel (left dashed curve), broken funnel wall (arrow), and intact funnel aperture (right dashed curve) of Cloudina sp. indet. specimen. (4) Ovoid puncture (arrow) in smooth (curved) tube. (5) Subcircular puncture (arrow) in Cloudina sp. indet. funnel (infilled). Scale bars are 1 mm. Schiffbauer et al. (2024).

Backscattered electron imaging and energy-dispersive X-ray spectroscope mapping of the thin sections suggested that there were many 'ghost tubes' present, which could not be visually observed, which were very nearly identical in composition to the host rock. There were detectable primarily by a lack of iron, an element present in the dolomite matrix. 

Scanning electron microscope imaging and energy-dispersive X-ray spectroscope elemental maps of fossils in polished slab. (1) Overview giga-macro photomosaic of a portion of polished thick section; labelled rectangles correspond to scanning electron microscope imaging and energy-dispersive X-ray spectroscope elemental maps image regions as indicated. (2), (3) Silicified fossil in transverse section: (2) backscattered electron (z-contrast) image, with (3) corresponding overlain elemental maps for calcium, silicon, and iron. (4), (5) Calcareous fossil in transverse section: (4) Backscattered electron (z-contrast) image, with (5) corresponding overlain elemental mabs for calcium, silicon, and iron. Scale bars are 5 mm (1), and 500 μm (2), (5). Schiffbauer et al. (2024).

The La Ciénega fauna was first described in 1985 by palaeontologist Mark McMenamin of Mount Hollyoak College, who assigned the fossils to a group of morphotypes rather than trying to assign them to taxa, and believed the sediments to be Early Cambrian in age, at least in part because PreCambrian fossils were thought to be extremely rare (if accepted at all) at this time. Subsequently, some of the fossils were identified as Sinotubulites, an exclusively PreCambrian taxa (the Ediacaran Period had not been named as such at this time), although this led to the postulation that this taxon extended into the Cambrian, as this was the data assigned to the La Ciénega Formation. It was not until the recognition of Cloudina, also exclusively PreCambrian, and later an index fossil for the Terminal Ediacaran, that the La Ciénega Formation was recognised as PreCambrian in origin, with geochemical dating methods later confirming an End Ediacaran age.

Schiffbauer et al. interpret the La Ciénega fauna as a multi-taxa, Terminal Ediacaran assemblage, including Sinotubulites, Cloudina, and other Cloudinomorphs, probably Saarina or Conotubus, as well as other, unidentified tubular fossils. The preservation present is a mixture of silicification, which produces fossils recoverable by acid etching, and calcification, which does not. Many of the silicified fossils show plastic deformation, which is taken to indicate that they were not silicified in life, but rather as the result of some taphonomic process. Examined in thin section, the calcified specimens appear to show finer organic structures, supporting this hypothesis. This means that identification of the fossils from the silicified material only is unreliable. However, the preservation of what appear to be drill holes in some of these fossils, and the fact that these drill holes only appear to be present in examples of Cloudina and not Sinotubulites is likely to be significant, as this repeats a pattern seen in deposits of a similar age in Shaanxi Province, China, increasing the possibility that this represents some form of early predator-prey interaction, with predators able to select certain tubes as suitable targets for predation.

See also...

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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Estimating the phosphorus content of the Ediacaran seas.

The availability of phosphorus is considered to be a major limiting factor on biological productivity, and its availability in the oceans over geological timescales is thought to have been a significant control on the evolution of life. Most phosphorus found in the modern oceans derives from the weathering of continental rocks, and needs to be constantly replenished as it is taken up by living organisms in the photic zone, sinks, and is buried, as well as to a lesser extent being absorbed by iron (oxyhydr)oxide minerals. 

Buried organisms typically break down releasing their phosphorus as they are degraded by microbes, and iron (oxyhydr)oxide minerals often dissolve in reducing subsurface environments, also releasing their phosphorus. This can be reabsorbed by other mineral phases at the sediment surface, including apatite and vivianite, as well as more iron (oxyhydr)oxide minerals, or may be recycled back into the water column, promoting further biological productivity. Such recycling of phosphorus into the water column is thought to be promoted by euxinic conditions (low oxygen, high sulphur) but inhibited by ferruginous conditions (low oxygen, high iron), due to the high capacity for iron mineral formation, although conditions in the sediment are probably more important than in the water column; sulphide generation in shallow sediments is likely to lead to the release of phosphorus.

Calculating the amount of bioavailable phosphorus in ancient oceans is notoriously difficult. One method that has produced results has been to compare the ration of phosphorus to iron in iron-rich sediments, which can give some idea of the proportion of phosphorus in the water column. However, this is complicated by a number of factors, such as the proportion of dissolved silica in the water, which is known to have an influence on the uptake of phosphorus by iron (oxyhydr)oxide minerals. Siliceous Phytoplankton appeared in the oceans in the Cambrian, and are presumed to have lowered the amount of silica in the water column. Thus the Precambrian oceans should have had higher silica contents that those of the Phanerozoic. However, the rock record suggests that the silica content of the Neoproterozoic oceans was probably lower than was the case for Palaeoproterozoic and Mesoproterozoic oceans. Despite these uncertainties, it is generally accepted that the phosphorus content of the world's oceans increased significantly during the global glaciations of the Cryogenian Period.

Iron formations are not ubiquitous in the geological record, and there are long periods of time for which no such deposits are known, making it difficult to reconstruct the phosphorus content of the oceans using this method. Notably, there are few useful iron formations available for the Ediacaran Period, leaving researchers unclear about phosphorus concentrations in the post-Cryogenian oceans in which multicellular Animals first began to diversify. The oceans of the Ediacaran are thought to have undergone some severe redox fluctuations, with oxygen reaching the deep ocean in the places where the distinctive Ediacaran fauna first appeared. Phosphorus can also be measured in siliclastic rocks, which have a nearly unbroken record dating back to the Palaeoproterozoic. However, while some studies of the phosphorus content of these rocks suggests that the amount of phosphorus being incorporated into shales increased between about 800 million years ago and 635 million years ago, as more studies have been carried out, they have produced a picture in which the average amount of phosphorus in shales varies little over the Neoproterozoic and early Palaeozoic.

Depite this, calculations have suggested that the phosphorus content of the oceans increased significantly during the Ediacaran, due to rising sulphate concentrations in sediments, and the release of phosphorus by sulphate-reducing Bacteria. It has been argued that for much of the Proterozoic primary production, and therefore oxygen production, was supressed by limited recycling and consequently high rates of burial of phosphorus, leading to the oxygen-poor oceans seen over much of this time. In the Neoproterozoic, rising sulphate levels are thought to have helped phosphorus recycling, leading to more fertile seas and a rise in marine oxygen levels, although direct evidence for this model has yet to be found.

In a paper published in the journal Communications Earth & Environment on 19 January 2024, Xiuqing Yang of the School of Earth Science and Resources at Chang’an University, and the School of Earth and Environment at the University of Leeds, Jingwen Mao, also of the School of Earth Science and Resources at Chang’an University, and of the Key Laboratory for Exploration Theory & Technology of Critical Mineral Resources at the China University of Geosciences,  Fred Bowyer of the School of GeoSciences at the University of Edinburgh, Changzhi Wu, Rongxi Li, Chao Zhao, and Guowei Yang, again of the School of Earth Science and Resources at Chang’an University, and Simon Poulton, also of the School of Earth and Environment at the University of Leeds, document a newly discovered Ediacaran iron formation within the North Qilian Orogenic Belt of northwest China.

Present distribution of Neoproterozoic iron formations. Yang et al. (2024).

The iron formations of the North Qilian Orogenic Belt have only been very lightly metamorphosed, and comprise largely haematite and jasper, giving it good potential for the study of phosphorus cycling. Yang et al. conducted high-resolution petrographic, mineralogical and geochemical studies on the North Qilian iron formations, and compared these to other datasets from around the world, in order to create a scheme of phosphorus bioavailability across the crucial Ediacaran interval in the evolution of Earth's life.

The Kawa, Jiapigou and Xiaoliugou iron formations of the lower Zhulongguan Group of the Qilian Orogenic Belt have been dated to about 600 million years ago. They comprise mostly haematite and jasper, with smaller amounts of clay minerals,  magnetite, carbonate minerals and apatite. Well-defined banding is rare, but where present comprises separate bands of haematite-rich and jasper-rich laminae, between 0.5 mm and 5 mm in width. All samples were taken from open pit mines, with care being taken to avoid collecting where there were signs of weathering or late-stage hydrothermal alteration.

(a) Schematic tectonic map of China. (b) Simplified geological map of the Qilian Orogen Belt. (c) Geological map of the western segment of the North Qilian area, China. Yang et al. (2024).

In places where banding could be found, phosphorus rich grains were present in both the haematite and jasper layers. Notably, the haematite layers were dusty or microplaty, suggesting that they have retained their original mineralogy. Some courser haematite grains are present, probably as a result of late-stage diagenesis or low-grade metamorphism. Phosphorus is primarily concentrated within apatite grains, which also have a high calcium content. Phosphorus and calcium levels also correlate in analysis of bulk samples. Energy dispersive spectroscopy analysis of apatite grains suggests that these are predominantly carbonate fluorapatite, mostly less than 5 μm in diameter, though some, rare, larger grains may reach 50 μm. Bulk samples were found to have high phosphorus/iron ratios, but low organic 

Photomicrographs of Ediacaran iron formations and carbonate fluorapatite. (a) Thin section of iron formations with a typical banded structure, where the red laminae are jasper and the grey laminae are hematite (sample JPG-30, Banded Iron Formation). (b) False-colour scanning electron microscope mineral map of (a), where the red-violet colour shows carbonate fluorapatite, which is mainly distributed in jasper-rich laminae. (c) Scanning electron microscope images of fine-grained haematite particles (sample KW-6, Iron Formation). (d) Disseminated carbonate fluorapatite grains with a subhedral shape (sample KW-33, Iron Formation). (e) Euhedral carbonate fluorapatite grains (sample JPG-30, Banded Iron Formation). (f) Rare coarse-grained carbonate fluorapatite (sample JPG-28, Banded Iron Formation). (g) Haematite inclusions in a carbonate fluorapatite particle (sample JPG-26, Banded Iron Formation). (h) Energy dispersive spectroscopy spectrum from an apatite particle shown in (e), using a gold-plated thin section. Peaks for carbon, oxygen, fluorine, phosphorus and calcium confirm the mineral is carbonate fluorapatite. Abbreviations: Hem, Haematite; Qtz, Quartz. Yang et al. (2024).

Howthe phosphorus cycle works under ferruginous conditions is poorly understood, as is how this relates to iron formation deposition. Ferruginous oceans were prevalent for much of the Precambrian, and phosphorus levels typically low. It has generally been assumed that these phenomena are connected, with the low phosphorus levels being due to an iron trap, in which phosphorus atoms are bound into iron minerals and taken out of ocean circulation.

If phosphorus was in fact largely being bound into carbonate fluorapatite minerals in Archaean-Mesoproterozoic iron formations and ironstones, then it is possible that much of this phosphorus was being remineralized from biological sources. However, this is difficult to reconcile with the low organic carbon content of these Precambrian iron deposits. An alternative is possibility would be that the early oceans in fact had mush higher phosphorus contents that has previously been supposed, and that the binding of phosphorus into carbonate fluorapatite is simply a consequence of iron silicate precipitation under these conditions.

Stratigraphic column of Ediacaran iron formations in the North Qilian area, China. Yang et al. (2024).

Yang et al.'s study demonstrates that carbonate fluorapatite was the main sink for phosphorus in the Ediacaran iron formations of North Qilian, China, but does not provide any information on the process by which the phosphorus was bound in this way. Nevertheless, Yang et al. do feel able to make some inferences from the data. The lack of an association between phosphorus and aluminium suggests that the phosphorus was not being deposited as detrital particles (i.e. bound to clay minerals, which have high aluminium contents), which in turn suggests that this phosphorus was not derived directly from a terrestrial source. This does not preclude the phosphorus having been originally derived from terrestrial weathering, simply that any such phosphorus must have been dissolved in the ocean, where it could be scavenged by iron minerals, rather than being deposited in a particulate form with aluminium minerals.

The remineralization of phosphorus from organic matter to carbonate fluorapatite cannot be ruled out, however this would have been likely to lead to the formation of the precipitation of iron minerals such as magnetite and siderite as the organic carbon was oxidised. The dominance of haematite in the North Qilian iron formations, combined with the rareness of magnetite, makes this scenario improbable. Thus, the high phosphorus content of the North Qilian deposits compared to earlier iron formations is unlikely to be related to the remineralization of organic material. 

Field and microphotographs of iron formations from North Qilian. (a) Iron formations with haematite-rich laminae and jasper-rich laminae. (b) Iron formations with a jasper lens. (c) and (d) Microphotographs of iron formations; (c) was taken under reflected light, and (d) is a scanning electron microscope image. Abbreviations: Hem, Haematite; Qtz, Quartz. Yang et al. (2024).

This does not, however, imply that the main reason for the high phosphorus levels seen in the North Qilian iron formations was drawdown by iron minerals. The dominance of haematite in these formations implies that iron was being precipitated from the water column as a form of hydrated ferric oxide, probably when ferruginous waters were oxygenated during upwellings. Phosphorus could potentially have been absorbed during this process, but would have been released within the sediment as the ferrihydrite remineralized into more stable haematite. In modern hydrothermal deposits a correlation can be seen between iron and phosphorus because phosphorus adsorbs onto iron (oxyhydr)oxides, but there is no evidence for this happening in the North Qilian deposits. This suggests that the concentration of phosphorus in porewaters was above the saturation point for carbonate fluorapatites, due to phosphorus being released by the remineralization of ferrihydrates into haematite, which would have led to carbonate fluorapatite deposition. This would also help to explain the presence of haematite inclusions within carbonate fluorapatite grains.

Yang et al. also note that where banding is present, carbonate fluorapatite grains are found in both haematite and jasper laminae, but are more common within jasper. This is also consistent with the release of phosphorus during the remineralization of ferrihydrates. Studies of Mesoproterozoic iron deposits suggest that phosphorus was precipitated into carbonate fluorapatite, despite the water being supersaturated for the iron phosphate mineral vivianite, because iron ions were being removed from the water by the formation of iron silicates. In the North Qilian deposits, these iron ions were probably only ever present within pore water, again favouring the deposition of carbonate fluorapatite.

Images of banded iron formation samples JPG-30 (a), (c), (e), (g), (i), (k) and JPG-28 (b), (d), (f), (h), (j), (l) from the Jiapigou section. (a), (b) Thin section photographs; (c), (d) Scanning electron microscope images. (e)–(j) False-colour scanning electron microscope mineral map of haematite, quartz and carbonate fluorapatite. (k), (l) Energy dispersive spectroscopy elemental map of phosphorus. Yang et al. (2024).

Despite the difficulties associated with the determination of ocean phosphorus concentrations from ancient iron formations, Yang et al. believe that they can detect a significant change in phosphorus levels in the Ediacaran compared to earlier deposits. Experiments have determined that no more than about 10% of phosphorus present in iron formations when they form is likely to be subsequently lost due to post-depositional processes; far lower than the determined difference between the Ediacaran and earlier deposits. Studies of the rock record have determined four phases with their own distinctive iron/phosphorus ratios (with some gaps). The oldest of these covers the Archaean and Palaeoprotorezoic, the next the Cryogenian, then the Cambrian to the Jurassic, and finally the Cretaceous to Quaternary. The phosphorus/iron ratio observed in the North Qilian Formation are consistent with those from Ediacaran iron formations in Iran, and much higher than those observed in Archaean to Palaeoprotorezoic, Tonian, or Cryogenian deposits, and indeed much higher than is observed in Cambrian to Jurassic iron formations, falling closest to the levels seen in Cretaceous to Quaternary strata.

This large jump in phosphorus levels in Ediacaran iron deposits seems highly suggestive of elevated phosphorus levels in the Ediacaran oceans, to the extent that phosphorus was probably being adsorbed onto iron (oxyhydr)oxide minerals in preference to ions such as calcium or magnesium. The North Qilian iron formations are interlayered with dolostones and sandstones, which implies that they were being laid down in a shallow marine setting, making it unlikely that the phosphorus levels recorded were something restricted to deep ocean basins. Although the levels of dissolved calcium and magnesium in the Ediacaran seas is poorly understood, the high levels of phosphorus recorded in Ediacaran iron formations compared to Palaeozoic and Mesozoic examples implies that dissolved phosphorus levels were high in the Ediacaran, despite the higher levels of dissolved silica likely to have been present.

If this is correct, then the increase in the proportion of phosphorus in the Ediacaran seas was one of the largest seen in Earth's history. This is consistent with the average proportion of phosphorus in Ediacaran shales (which are about 0.34 % phosphorus by weight) compared to Tonian (0.09 % phosphorus by weight) or Cryogenian (0.13 % phosphorus by weight) shales. Tonian iron formations also show very low levels of phosphorus, supporting the idea that Tonian seas had very low phosphorus levels. This, while the precise reasons for and timing of phosphorus increases in Neoproterozoic oceans is unclear, multiple lines of evidence suggest that this increase was not a phenomenon restricted to the Cryogenian glacial phases, but rather something which intensified during the following Ediacaran Period.

Schematic representation of the evolution of marine redox state through the Neoproterozoic to Cambrian based on compiled palaeoredox proxy records. Yang et al. (2024).

High levels of terrestrial erosion have been proposed as a mechanism for the increase in phosphorus in the Cryogenian, though why this would continue significantly into the Ediacaran is unclear. An alternative is that recycling of phosphorus back into the water column may have played a significant role in keeping phosphorus levels high. Yang et al.'s findings suggest phosphates were mobilized during the remineralization of iron (oxyhydr)oxide minerals to haematite close to the sediment-water interface suggests a degree of phosphorus recycling was likely, but iron formations were relatively rare in the Ediacaran, making it unlikely that this system was having a major impact on the global environment.

Both the Ediacaran and Cryogenian oceans are thought to have been redox-stratified. Ferruginous conditions were probably widespread, but increased sulphate levels in the oceans may have been more important for phosphorus recycling, as sulphate-reducing Bacteria would have increased the rate at which organic phosphorus was remineralized. This would have been a marked difference between Ediacaran and earlier Neoproterozoic oceans, where ferruginous conditions were prevalent, but sulphate levels low.

Most phosphorus entering shallow-marine waters today does so in upwelling zones, where currents bring water upwards from the deep ocean. This source brings about 60 times more phosphorus into shallow  marine waters than all of the world's river systems combined. If this was also the case in the Ediacaran seas, then upwelling currents would have been the major source of the phosphorus which fuelled primary production in these seas, and therefore the Ediacaran rise in oxygen content, as well as the nutrients which fed the diversifying Eucaryotes of the Period; the phosphate-rich iron formations of North Qilian have been dated to approximately 600 million years ago, slightly younger that the Lantian assemblage of South China, which at 602 million years old is the ealiest known example of the macroscopic Ediacaran Fauna.

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