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.
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.
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
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.
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.
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.
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.
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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