Saturday, 12 March 2022

Dating the Lantian Biota.

Fossils from the Ediacaran Period record a remarkable transition from a world in which there were almost no multicellular organisms to one in which such organisms dominated almost all marine ecosystems. Deposits from the Lower Ediacaran are dominated by the Acritarchs, a diverse group of single-celled marine Algae which may-or-may-not represent a single taxonomic group. Deposits from the Upper Ediacaran produce a range of complex fossils, apparently of soft-bodied organisms of uncertain affinities. One of the earliest macro-fossil producing deposits is the Lantian Formation of South China, a deep-marine basinal black shale which produces a series of fossils of increasing complexity, assumed to have affinities to Algae and Animals, providing insight into the earliest stages in the development of these multicellular organisms.

Macrofossils from the Lantian biota, Anhui Province, China. Specimens are housed in the collections of the Nanjing Institute of Geology and Palaeontology. (A) Lantianella laevis (at left; NIGP163377), and a larger conical form. (B) Lantianella annularis, NIGP163384. Dunn & Liu (2017).

The Lantian Biota is potentially the oldest macrofossil assemblage of the Ediacaran, and therefore is of great interest to palaeontologists, but obtaining a precise date for these deposits has proven problematic. The Lantian Formation lacks any volcanic ash beds, which would allow dating via uranium/lead ratios in zircons. Zircons are minerals formed by the crystallisation of cooling igneous melts. When they form they often contain trace amounts of uranium, which decays into (amongst other things) lead at a known rate. Since lead will not have been present in the original crystal, it is possible to calculate the age of a zircon crystal from the ratio between these elements.

An alternative method of dating sedimentary rocks utilises rhenium/osmium ratios. Rhenium is one of the rarest elements on Earth and the second highest stable element on the periodic table, after hafnium. There are two naturally occurring isotopes of rhenium, the stable rhenium¹⁸⁵, and the 'unstable' rhenium¹⁸⁷. Unusually, the 'unstable' form of rhenium is the more common; this is because it has an extremely long half-life, about 4160 million years, which means that a significant proportion of the rhenium¹⁸⁷ present during the formation of the Earth is still in that form (like all elements heavier than iron, rhenium is formed during supernova events at the end of the life-cycle of massive stars). When rhenium¹⁸⁷ does decay it does so into the stable radiogenic isotope osmium¹⁸⁷. Osmium also has two stable isotopes, the non-radiogenic osmium¹⁸⁸, and the radiogenic osmium¹⁸⁷. This means that over time the amount of rhenium¹⁸⁷ in a rock sample will fall at a predictable rate, while the amount of osmium¹⁸⁷ rises at the same rate, and the amount of osmium¹⁸⁸ remains constant, making it possible to date the rock by comparing the ratios of these elements. However, the exposed deposits of the Lantian Formation all show varying degrees of metamorphism, which prevents useful application of the rhenium/osmium method, as the method relies on elements present at very low levels in the total rock (rather than working on individual mineral grains), with the upshot that any loss of either element will interfere with the result.

Based upon approximate lithostratigraphic correlation between the Lantian or south Anhui Province and the Doushantuo Formations of the Yangtze Gorges area, it has been estimated that the Lantian Formation is between 635 and 551 million years old, but the long distances (the two locations are about 700 km apart) and variable geology between the two sites make even this very approximate estimate unreliable. 

A detrital zircon retrieved from Member II the Lantian Formation (i.e. an individual grain from a sedimentary bed, not part of a volcanic ash layer) yielded an age of 590 million years, giving a possible maximum age for that layer, and a low-yttrium monazite (a mineral likely to have been formed diagetically within the deposit soon after it was laid down yielded a lead/uranium date of 612 million years, which has been argued to be a minimum age for the formation. However, as well as being contradictory, both these dates are poorly constrained, making it hard to assess the actual age of the Lantian Formation, and how the age of the fossils it contains relate to other Ediacaran fossils.

In a paper published in the journal Geology on Chuan Yang of the State Key Laboratory of Lithospheric Evolution at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, and the Geochronology and Tracers Facility at the British Geological Survey, Yang Li, also of the State Key Laboratory of Lithospheric Evolution at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, David Selby of the Department of Earth Sciences at Durham University, Bin Wan, Chengguo Guan, and Chuanming Zhou, of the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, and Xian-Hua Li, once again of the State Key Laboratory of Lithospheric Evolution at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, and the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, present the results of a study which aimed to find a rhenium/osmium date for the Lantian Formation based upon the best-preserved, biota-bearing, organic-rich black shale of Member II of that formation.

Geological maps and stratigraphic column for the Lantian Formation (China). (A) Generalized palaeogeographic map of the Yangtze Block during the early–middle Ediacaran Period showing the approximate location of shelf, slope, and basinal facies. Numbered triangles indicate locations of areas/sections mentioned in the text, with the Lantian area highlighted in red. (B) Geological map of the Lantian area showing the location of the Lantian drill core. (C) Litho- and chemo-stratigraphy of the Lantian drill core highlights the horizon that was sampled for rhenium/osmium geochronology. Yang et al. (2022).

The Laintian Formation is found on the Yangtze Block of the South China craton, which in the Ediacaran comprised a continental shelf to the northwest and a deep basin to the southeast, with a gradation of slope strata in between. This produced a series of sedimentary blocks with exceptionally well-preserved Ediacaran fossils, including the Lantian biota in Anhui and the Weng’an biota in Guizhou, thought to be the two oldest known examples of Ediacaran biotas. The Lantian Basin was laid down in a basin-slope environment, overlying the Leigongwu Formation, a terminal Cryogenian diamictite. It is in turn overlain by the Ediacaran–Cambrian transitional Piyuancun Formation.

Yang et al.'s work is based upon a drill core extracted near Lantian village in Xiuning County, Anhui Province. Within this core, the Lantian Formation is present as four members, with Member I represented by a 4 m-thick unit of light-grey siliceous cap dolostone, Member II comprising a 97 m thick grey, calcareous siltstone interbedded with argillaceous limestone and an upper subunit of black shale with rare argillaceous limestone interbeds, Member III is a 73 m thick interbedded grey, argillaceous dolostone and black shale, followed by about 50 m of grey limestone, and Member IV is a 10 m section of black shale. The fossils of the Langtian Formation are found almost continuously in the upper subunit of Member II, comprising both carbonaceous compression macrofossils and pyritized material. 

Yang et al. extracted a number of black shale samples for rhenium/osmium analysis from a 50 cm interval 48 m above the base of the Formation (i.e. 44 m above the base on Member II).

In order to be able to get a reliable rhenium/osmium date, Yang et al. needed to find patches of shale with the least post-depositional isotope exchange. To do this they selected core samples without post-formation veining and weathering, which were further imaged by X-ray computed tomography and X-ray fluorescence, which revealed well-preserved sedimentary lamination and relatively homogenous elemental patterns within the 50 cm interval used in the study, indicating a stable depositional environment with no evidence of post-depositional chemical weathering. Yang et al took nine samples from within a roughly 25 cm section within this interval for rhenium/osmium analysis, and calculated the abundances of rhenium and osmium within these samples, gaining measurements of 4-12 parts per billion for rhenium and 155-490 parts per trillion for osmium, from which they calculate the deposits have a probable age of 602 million years,

Yang et al.'s date of 602 million years for a layer in the middle part of Member II of the Lantian Formation is the first well constrained date for the Lantian Biota. The fossils of the Lantian Biota extend to slightly below the top of Member II, and is probably below the Shuram Event Interval, a sudden depletion in the ratio of carbon¹³ to carbon¹² recorded in rocks around the world, and which may have been linked to a rise in the amount of oxygen in the Earth's atmosphere, as well as possibly below another negative carbon¹³ excursion recorded at the boundary between Member II and Member III of the Doushantuo Formation in the Yangtze Gorges area, which is thought to be connected to the Gaskiers Glaciation at about 580 million years before the present. Based upon this, Yang et al. suggest that the Lantian Biota is more than 580 million years old. Since the base of the Lantian Formation has been dated to 635 million years ago, and the Lantian Fauna appears in Member II of the formation some way above this, Yang et al. also suggest that the fauna is likely to be younger than 615 million years old.

The Weng'an Biota is a rich microfossil assemblage consisting mainly of reworked and redeposited Acanthomorphic Acritarchs, multicellular algae, tubular microfossils, putative animals, and animal embryos, found in the shelf-facies phosphorite of the Doushantuo Formation in the Weng’an area. Similar microfossils have been found in chert nodules from Member II of the Doushantuo Formation, which have been estimated to be about 632 million years old. This would suggest that the macrofossils of the Lantian Formation appeared after the microfossils of the Doushantuo Formation, but that there is a considerable temporal overlap between the two fossil groups. The Lantian Biota also appears before the soft-bodied fossils of the classical 'Ediacaran Fauna' which first appear in rocks dated to 574 million years ago in Newfoundland and northwestern Canada.

Integrated carbon isotopic profile (A), fossil ranges (B), and initial osmium isotopic data (C) of the Ediacaran Period. Carbonate proportion of carbon¹³ data from the Lantian Formation are highlighted in red. CIE, carbon isotope excursion. Yang et al. (2022).

Five putative Animal species have been described from the Lantian Biota. All are centimetre-scale in size, and show morphological complexity and structural differentiation. If this is correct, then it indicates that Metazoan Animals had appeared by the early–middle Ediacaran, something which has been predicted from molecular clock data, which suggests that modern Animal phyla diverged in the Ediacaran, and by the presence of putative Animal embryos in the Weng'an Biota.

A putative Animal from the Lantian Biota: Lantianella laevis (A)–(B) NIGP163375, part and counterpart, holotype, showing globose holdfast (white arrow) and tentacle-like structures (black arrows). (C)–(D) NIGP163376, part and counterpart, showing globose holdfast (white arrows) and longitudinal filaments (black arrows). (E)–(F) NIGP163377 and NIGP163378, respectively, showing flexible (white arrow in (E)) or straight (white arrow in (F)) tentacle-like structures and longitudinal filaments (black arrows). (G)–(H) NIGP163379 and NIGP163380, respectively, showing rounded (white arrow in (G)) and scalloped (white arrow in (H)) distal margin of conical body, and poorly preserved longitudinal filaments (black arrows in (H)). (I)–(J) and (K)–(L) NIGP163381 and NIGP163382, respectively, part and counterpart, showing sharp contrast between the lighter-coloured tentacular crown and darker-coloured conical body, globose holdfast (white arrows), and patch of diffuse organic mass associated with globose holdfast (black arrows). Millimetre-sized circular and elliptical carbonaceous compressions in the background are specimens of the Acritarch Chuaria circularis, which are best seen in (A)–(B). All scale bars represent 5 mm. Bin et al. (2016).

As well as the putative Animals, 13 species of possible Macroalgae (Seaweed) have been described from the Lantian Biota. Further Macroalgae have been described from the Weng'an Biota, and Acanthomorphic Acritarchs are abundant in deposits of similar ages around the world. Taken together, this implies a major episode of radiation and diversification of Algae in the early–middle Ediacaran. Such an episode would likely have raised oxygen levels within the Ediacaran oceans, facilitating the rise of the Metazoa. Several of the Algal taxa found in the Lantian Biota are found in later Ediacaran Fauna-bearing deposits, with one, Flabelophyton, showing a marked increase in size over the Ediacaran Period. This continuation of related Algal fossils from the Lantian and Weng'an biotas into the deposits bearing the classical Ediacaran Fauna supports the idea that there is an evolutionary continuation between these biotas.

At the beginning of the Ediacaran, benthic marine communities were comprised entirely of mat-forming micro-organisms. By the end of the period, modern, Animal-dominated benthic marine communities had appeared and become more-or-less ubiquitous. The Lantian Biota records a stage in this transition, with most species recorded, Animal and Algae, being erect, sessile, epibenthic forms. The date of 602 million years ago provided by Yang et al. confirms that this macrofossil-dominated assemblage was present in the early-middle Ediacaran.

Yang et al. obtained a ratio of osmium¹⁸⁷/osmium¹⁸⁸ at the 602 million years isochron in the Lantian fauna of 1.14, while a layer of the Old Fort Point Formation in western Canada dated to 607 million years ago produced an osmium¹⁸⁷/osmium¹⁸⁸ ratio of 0.62, a significant difference which implies a regional variation in the control of osmium isotope ratios. This is curious, as the main source of osmium in the Ediacaran oceans is thought to have been radiogenic metal weathered from rocks on land. It is likely that the rise of the Ediacaran Biota was linked to an increase in terrestrial erosion, increasing the amount of mineral nutrients available to marine organisms, but how this related to the variation in the isotopic makeup of osmium is unclear.

Geochemical analysis of the shales of Member II of the Lantian Formation has suggested a persistent euxinic (high sulphur, low oxygen) environment. However, the presence of fossil Algae and Metazoans suggest that the area clearly had at least periodic episodes of oxygenation. The environment is interpreted as having been below the storm base (level to which the influence of storm events penetrates) but within the photic zone (area of the sea where there is sufficient light for photosynthesis). This probably reflects the difference between the geological timescale recorded by the chemical evidence and the ecological timescale recorded by the fossil evidence, but also records that a stable thermal environment (deep water environments typically do not have fluctuating temperatures) was probably more important to the emergence of the Metazoans than a stable oxygen environment.

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