The onset of widespread marine red beds and the evolution of ferruginous oceans

Hexose phosphorylation for a non-enzymatic glycolysis and pentose phosphate pathway on early Earth

Glycolysis and pentose phosphate pathways play essential roles in cellular processes and are assumed to be among the most ancient metabolic pathways. Non-enzymatic metabolism-like reactions might have occurred on the prebiotic Earth and been inherited by the biological reactions. Previous research has identified a part of the non-enzymatic glycolysis and the non-enzymatic pentose phosphate pathway from glucose 6-phosphate and 6-phosphogluconate, which are intermediates of these reactions. However, how these phosphorylated molecules were formed on the prebiotic Earth remains unclear. Herein, we demonstrate the synthesis of glucose and gluconate from simple aldehydes in alkaline solutions and the formation of glucose 6-phosphate and 6-phosphogluconate with borate using thermal evaporation. These results imply that the initial stages of glycolysis-like and pentose phosphate pathway-like reactions were achieved in borate-rich evaporative environments on prebiotic Earth, suggesting that non-enzymatic metabolism provided biomolecules and their precursors on prebiotic Earth.

Evolution of iron and oxygen biogeochemical cycles during the Precambrian

Iron (Fe) is an essential element for life, and its geochemical cycle is intimately linked to the coupled history of life and Earth's environment. The accumulated geologic records indicate that ferruginous waters existed in the Precambrian oceans not only before the first major rise of atmospheric O2 levels (Great Oxidation Event; GOE) during the Paleoproterozoic, but also during the rest of the Proterozoic. However, the interactive evolution of the biogeochemical cycles of O2 and Fe during the Archean-Proterozoic remains ambiguous. Here, we develop a biogeochemical model to investigate the coupled biogeochemical evolution of Fe-O2 -P-C cycles across the GOE. Our model demonstrates that the marine Fe cycle was less sensitive to changes in the production rate of O2 before the GOE (atmospheric pO2  < 10-6 PAL; present atmospheric level). When the P supply rate to the ocean exceeds a certain threshold, the GOE occurs and atmospheric pO2 rises to ~10-3 -10-1 PAL. After the GOE, the marine Fe(II) concentration is highly sensitive to atmospheric pO2 , suggesting that the marine redox landscape during the Proterozoic may have fluctuated between ferruginous conditions and anoxic non-ferruginous conditions with sulfidic water masses around continental margins. At a certain threshold value of atmospheric pO2 of ~0.3% PAL, the primary oxidation pathway of Fe(II) shifts from the activity of Fe(II)-utilizing anoxygenic photoautotrophs in sunlit surface waters to abiotic process in the deep ocean. This is accompanied by a shift in the primary deposition site of Fe(III) hydroxides from the surface ocean to the deep sea, providing a plausible mechanistic explanation for the observed cessation of iron formations during the Proterozoic.

Extreme accumulation of ammonia on electroreduced mackinawite: An abiotic ammonia storage mechanism in early ocean hydrothermal systems

An increasing amount of evidence suggests that early ocean hydrothermal systems were sustained sources of ammonia, an essential nitrogen species for prebiotic synthesis of life's building blocks. However, it remains a riddle how the abiotically generated ammonia was retained at the vent-ocean interface for the subsequent chemical evolution. Here, we demonstrate that, under simulated geoelectrochemical conditions in early ocean hydrothermal systems ([Formula: see text][Formula: see text] V versus the standard hydrogen electrode), mackinawite gradually reduces to zero-valent iron ([Formula: see text]), generating interlayer [Formula: see text] sites. This reductive conversion leads to an up to 55-fold increase in the solid/liquid partition coefficient for ammonia, enabling over 90% adsorption of 1 mM ammonia in 1 M NaCl at neutral pH. A coordinative binding of ammonia on the interlayer [Formula: see text] sites was computed to be the major mechanism of selective ammonia adsorption. Mackinawite is a ubiquitous sulfide precipitate in submarine hydrothermal systems. Given its reported catalytic function in amination, the extreme accumulation of ammonia on electroreduced mackinawite should have been a crucial initial step for prebiotic nitrogen assimilation, paving the way to the origin of life.

Evidence for the early Toarcian Carbon Isotope Excursion (T-CIE) from the shallow marine siliciclastic red beds of Arabia

The Toarcian Oceanic Anoxic Event (T-OAE) and its corresponding Carbon Isotope Excursion (CIE) have been reported widely across the Tethyan region and globally. In Arabia, and based on ammonite dating, the time window of the T-OAE coincided with the deposition of the reddish siliciclastic unit of the Marrat Formation. However, no evidence of the T-OAE/CIE was ever reported from Arabia because these red beds were previously interpreted as continental deposits. Recently, these red beds have been recognized as shallow marine deposits which opened an opportunity to assess the occurrence and expression of T-OAE-CIE in Arabia. In this study, a multiproxy geochemical characterization was performed on the Toarcian Marrat Formation to infer the chemistry of the paleowater column and identify intervals of possible T-OAE/CIE in Arabia. While the low concentrations of redox-sensitive elements (Mo, U, V, Cr) may indicate a shallow oxic marine settings, the coupled negative δ¹³C_organic excursion and apparent increase in the chemical weathering suggests that the deposition of Marrat red beds coincided with the development of T-CIE and possibly time-equivalent to the T-OAE globally. The origin of reddening is interpreted to have occurred during the middle Marrat deposition due to the stabilization of unstable hydrous iron oxides to hematite under oxic marine conditions. The proposed model further indicates the possible development of source rocks in the deep, anoxic environment counterpart where the T-OAE may be expressed. Since our study documents the first record of the T-CIE and discuss the origin of shallow marine siliciclastic red beds in the Arabian Plate, this will have significant implications for the overall understanding of the T-CIE globally and for hydrocarbon exploration through realizations of potential new source rocks associated with the OAEs in the Toarcian and other time intervals.

A new index for quantifying the ornamentational complexity of animals with shells

Morphological complexity reflects the biological structure of an organism and is closely linked to its associated functions and phylogenetics. In animals with shells, ornamentation is an important characteristic of morphological complexity, and it has various functions. However, because of the variations in type, shape, density, and strength of ornamentation, a universal quantitative measure of morphological complexity for shelled animals is lacking. We propose an ornamentation index (OI) derived from 3D scanning technology and a virtual model for quantifying ornamentation complexity. This index is designed to measure the extent of folding associated with ornamentation, regardless of shape and size. Ornamentation indices were measured for 15 ammonite specimens from the Permian to Cretaceous, 2 modern bivalves, 2 gastropods from the Pliocene to the present, and a modern echinoid. Compared with other measurements, such as the fractal dimension, rugosity, and surface-volume ratio, the OI displayed superiority in quantifying ornamentational complexity. The present study demonstrates that the OI is suitable for accurately characterizing and quantifying ornamentation complexity, regardless of shape and size. Therefore, the OI is potentially useful for comparing the ornamentational complexity of various organisms and can be exploited to provide further insight into the evolution of conchs. Ultimately, the OI can enhance our understanding of morphological evolution of shelled organisms, for example, whether shell ornaments simplify under ocean acidification or extinction, and how predation pressure is reflected in ornamentation complexity.

Continental configuration controls ocean oxygenation during the Phanerozoic

The early evolutionary and much of the extinction history of marine animals is thought to be driven by changes in dissolved oxygen concentrations ([O₂]) in the ocean^1-3. In turn, [O₂] is widely assumed to be dominated by the geological history of atmospheric oxygen (pO₂)^4,5. Here, by contrast, we show by means of a series of Earth system model experiments how continental rearrangement during the Phanerozoic Eon drives profound variations in ocean oxygenation and induces a fundamental decoupling in time between upper-ocean and benthic [O₂]. We further identify the presence of state transitions in the global ocean circulation, which lead to extensive deep-ocean anoxia developing in the early Phanerozoic even under modern pO₂. Our finding that ocean oxygenation oscillates over stable thousand-year (kyr) periods also provides a causal mechanism that might explain elevated rates of metazoan radiation and extinction during the early Palaeozoic Era⁶. The absence, in our modelling, of any simple correlation between global climate and ocean ventilation, and the occurrence of profound variations in ocean oxygenation independent of atmospheric pO₂, presents a challenge to the interpretation of marine redox proxies, but also points to a hitherto unrecognized role for continental configuration in the evolution of the biosphere.

Testing Abiotic Reduction of NAD+ Directly Mediated by Iron/Sulfur Minerals

Life emerged in a geochemical context, possibly in the midst of mineral substrates. However, it is not known to what extent minerals and dissolved inorganic ions could have facilitated the evolution of biochemical reactions. Herein, we have experimentally shown that iron sulfide minerals can act as electron transfer agents for the reduction of the ubiquitous biological protein cofactor nicotinamide adenine dinucleotide (NAD⁺) under anaerobic prebiotic conditions, observing the NAD⁺/NADH redox transition by using ultraviolet-visible spectroscopy and ¹H nuclear magnetic resonance. This reaction was mediated with iron sulfide minerals, which were likely abundant on early Earth in seafloor and hydrothermal settings; and the NAD⁺/NADH redox reaction occurred in the absence of UV light, peptide ligand(s), or dissolved mediators. To better understand this reaction, thermodynamic modeling was also performed. The ability of an iron sulfide mineral to transfer electrons to a biochemical cofactor that is found in every living cell demonstrates how geologic materials could have played a direct role in the evolution of certain cofactor-driven metabolic pathways.

Microbial helpers allow cyanobacteria to thrive in ferruginous waters

The Great Oxidation Event (GOE) was a rapid accumulation of oxygen in the atmosphere as a result of the photosynthetic activity of cyanobacteria. This accumulation reflected the pervasiveness of O₂ on the planet's surface, indicating that cyanobacteria had become ecologically successful in Archean oceans. Micromolar concentrations of Fe²⁺ in Archean oceans would have reacted with hydrogen peroxide, a byproduct of oxygenic photosynthesis, to produce hydroxyl radicals, which cause cellular damage. Yet, cyanobacteria colonized Archean oceans extensively enough to oxygenate the atmosphere, which likely required protection mechanisms against the negative impacts of hydroxyl radical production in Fe²⁺ -rich seas. We identify several factors that could have acted to protect early cyanobacteria from the impacts of hydroxyl radical production and hypothesize that microbial cooperation may have played an important role in protecting cyanobacteria from Fe²⁺ toxicity before the GOE. We found that several strains of facultative anaerobic heterotrophic bacteria (Shewanella) with ROS defence mechanisms increase the fitness of cyanobacteria (Synechococcus) in ferruginous waters. Shewanella species with manganese transporters provided the most protection. Our results suggest that a tightly regulated response to prevent Fe²⁺ toxicity could have been important for the colonization of ancient ferruginous oceans, particularly in the presence of high manganese concentrations and may expand the upper bound for tolerable Fe²⁺ concentrations for cyanobacteria.

The redox structure of Ediacaran and early Cambrian oceans and its controls

The rapid diversification of early animals during the Ediacaran (635-541 Ma) and early Cambrian (ca. 541-509 Ma) has frequently been attributed to increasing oceanic oxygenation. However, the pattern of oceanic oxygenation and its relationship to early animal evolution remain in debate. In this review, we examine the redox structure of Ediacaran and early Cambrian oceans and its controls, offering new insights into contemporaneous oceanic oxygenation patterns and their role in the coevolution of environments and early animals. We review the development of marine redox models which, in combination with independent distal deep-ocean redox proxies, supports a highly redox-stratified shelf and an anoxia-dominated deep ocean during the Ediacaran and early Cambrian. Geochemical and modeling evidence indicates that the marine redox structure was likely controlled by low atmospheric O₂ levels and low seawater vertical mixing rates on shelves at that time. Furthermore, theoretical analysis and increasing geochemical evidence, particularly from South China, show that limited sulfate availability was a primary control on the attenuation of mid-depth euxinia offshore, in contrast to the existing paradigm invoking decreased organic carbon fluxes distally. In light of our review, we infer that if oceanic oxygenation indeed triggered the rise of early animals, it must have done so through a shelf oxygenation which was probably driven by elevated oxidant availability. Our review calls for further studies on Ediacaran-Cambrian marine redox structure and its controls, particularly from regions outside of South China, in order to better understand the coevolutionary relationship between oceanic redox and early animals.

Reconciling proxy records and models of Earth's oxygenation during the Neoproterozoic and Palaeozoic

A hypothesized rise in oxygen levels in the Neoproterozoic, dubbed the Neoproterozoic Oxygenation Event, has been repeatedly linked to the origin and rise of animal life. However, a new body of work has emerged over the past decade that questions this narrative. We explore available proxy records of atmospheric and marine oxygenation and, considering the unique systematics of each geochemical system, attempt to reconcile the data. We also present new results from a comprehensive COPSE biogeochemical model that combines several recent additions, to create a continuous model record from 850 to 250 Ma. We conclude that oxygen levels were intermediate across the Ediacaran and early Palaeozoic, and highly dynamic. Stable, modern-like conditions were not reached until the Late Palaeozoic. We therefore propose that the terms Neoproterozoic Oxygenation Window and Palaeozoic Oxygenation Event are more appropriate descriptors of the rise of oxygen in Earth's atmosphere and oceans.

Metals likely promoted protometabolism in early ocean alkaline hydrothermal systems

One of the most plausible scenarios of the origin of life assumes the preceding prebiotic autotrophic metabolism in sulfide-rich hydrothermal vent environments. However, geochemical mechanisms to harness the reductive power provided by hydrothermal systems remain to be elucidated. Here, we show that, under a geoelectrochemical condition realizable in the early ocean hydrothermal systems, several metal sulfides (FeS, Ag2S, CuS, and PbS) undergo hour- to day-scale conversion to the corresponding metals at ≤-0.7 V (versus the standard hydrogen electrode). The electrochemically produced FeS-Fe0 assemblage promoted various reactions including certain steps in the reductive tricarboxylic acid cycle with efficiencies far superior to those due to pure FeS. The threshold potential is readily generated in the H2-rich alkaline hydrothermal systems that were probably ubiquitous on the Hadean seafloor. Thus, widespread metal production and metal-sustained primordial metabolism were likely to occur as a natural consequence of the active hydrothermal processes on the Hadean Earth.

Heterogeneous and dynamic marine shelf oxygenation and coupled early animal evolution

It is generally agreed that early diversification of animals and significant rise of atmospheric and oceanic oxygen (O2) levels occurred in the Ediacaran (635–541 million years ago, Ma) and early Cambrian (ca. 541–509 Ma). The strength and nature of their relationship, however, remain unclear and debated. A recent wave of paleoredox research — with a particular focus on the fossiliferous sections in South China — demonstrates high spatial heterogeneity of oceanic O2 (redox) conditions and dynamic marine shelf oxygenation in a dominantly anoxic ocean during the Ediacaran and early Cambrian. This pattern shows a general spatiotemporal coupling to early animal evolution. We attribute dynamic shelf oxygenation to a complex interplay among the evolving atmosphere, continents, oceans, and biosphere during a critical period in Earth history. Our review supports the idea of a complex coevolution between increasing O2 levels and early diversification of animals, although additional work is required to fully delineate the timing and patterns of this coevolution and the mechanistic underpinnings.