Iron formations (IF) represent an iron-rich rock type that typifies many Archaean and Proterozoic supracrustal successions and are chemical archives of Precambrian seawater chemistry and ...post-depositional iron cycling. Given that IF accumulated on the seafloor for over two billion years of Earth's early history, changes in their chemical, mineralogical, and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving Earth. Perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere. Linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes, the supply of nutrients to the oceans, and, ultimately, diversification of the biosphere and complex life forms. Many of the key recent innovations in understanding IF genesis are linked to geobiology, since biologically assisted Fe(II) oxidation, either directly through photoferrotrophy, or indirectly through oxygenic photosynthesis, provides a process for IF deposition from mineral precursors. The abundance and isotope composition of Fe(II)-bearing minerals in IF additionally suggests microbial Fe(III) reduction, a metabolism that is deeply rooted in the Archaea and Bacteria. Linkages among geobiology, hydrothermal systems, and deposition of IF have been traditionally overlooked, but now form a coherent model for this unique rock type. This paper reviews the defining features of IF and their distribution through the Neoarchaean and Palaeoproterozoic. This paper is an update of previous reviews by Bekker et al. (2010, 2014) that will improve the quantitative framework we use to interpret IF deposition. In this work, we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evolution of the oceans.
The existence of stabilizing feedbacks within Earth's climate system is generally thought to be necessary for the persistence of liquid water and life. Over the course of Earth's history, Earth's ...atmospheric composition appears to have adjusted to the gradual increase in solar luminosity, resulting in persistently habitable surface temperatures. With limited exceptions, the Earth system has been observed to recover rapidly from pulsed climatic perturbations. Carbon dioxide (CO2) regulation via negative feedbacks within the coupled global carbon‐silica cycles are classically viewed as the main processes giving rise to climate stability on Earth. Here we review the long‐term global carbon cycle budget, and how the processes modulating Earth's climate system have evolved over time. Specifically, we focus on the relative roles that shifts in carbon sources and sinks have played in driving long‐term changes in atmospheric pCO2. We make the case that marine processes are an important component of the canonical silicate weathering feedback, and have played a much more important role in pCO2 regulation than traditionally imagined. Notably, geochemical evidence indicate that the weathering of marine sediments and off‐axis basalt alteration act as major carbon sinks. However, this sink was potentially dampened during Earth's early history when oceans had higher levels of dissolved silicon (Si), iron (Fe), and magnesium (Mg), and instead likely fostered more extensive carbon recycling within the ocean‐atmosphere system via reverse weathering—that in turn acted to elevate ocean‐atmosphere CO2 levels.
Key Points
Long term carbon sources and sinks are likely larger than traditional envisioned
There is significant silicate weathering in the marine as well as terrestrial settings
Ocean oxygenation and evolution of a biotic Si cycle forced a drop in reverse weathering rates and an increase in marine weathering rates
The ocean‐atmosphere system is typically envisioned to have gone through a unidirectional oxygenation with significant oxygen increases in the earliest (ca. 635 Ma), middle (ca. 580 Ma), or late (ca. ...560 Ma) Ediacaran Period. However, temporally discontinuous geochemical data and the patchy metazoan fossil record have been inadequate to chart the details of Ediacaran ocean oxygenation, raising fundamental debates about the timing of ocean oxygenation, its purported unidirectional rise, and its causal relationship, if any, with the evolution of early animal life. To better understand the Ediacaran ocean redox evolution, we have conducted a multi‐proxy paleoredox study of a relatively continuous, deep‐water section in South China that was paleogeographically connected with the open ocean. Iron speciation and pyrite morphology indicate locally euxinic (anoxic and sulfidic) environments throughout the Ediacaran in this section. In the same rocks, redox sensitive element enrichments and sulfur isotope data provide evidence for multiple oceanic oxygenation events (OOEs) in a predominantly anoxic global Ediacaran–early Cambrian ocean. This dynamic redox landscape contrasts with a recent view of a redox‐static Ediacaran ocean without significant change in oxygen content. The duration of the Ediacaran OOEs may be comparable to those of the oceanic anoxic events (OAEs) in otherwise well‐oxygenated Phanerozoic oceans. Anoxic events caused mass extinctions followed by fast recovery in biologically diversified Phanerozoic oceans. In contrast, oxygenation events in otherwise ecologically monotonous anoxic Ediacaran–early Cambrian oceans may have stimulated biotic innovations followed by prolonged evolutionary stasis.
A recent field-intensive program in Shark Bay, Western Australia provides new multi-scale perspectives on the world's most extensive modern stromatolite system. Mapping revealed a unique geographic ...distribution of morphologically distinct stromatolite structures, many of them previously undocumented. These distinctive structures combined with characteristic shelf physiography define eight 'Stromatolite Provinces'. Morphological and molecular studies of microbial mat composition resulted in a revised growth model where coccoid cyanobacteria predominate in mat communities forming lithified discrete stromatolite buildups. This contradicts traditional views that stromatolites with the best lamination in Hamelin Pool are formed by filamentous cyanobacterial mats. Finally, analysis of internal fabrics of stromatolites revealed pervasive precipitation of microcrystalline carbonate (i.e. micrite) in microbial mats forming framework and cement that may be analogous to the micritic microstructures typical of Precambrian stromatolites. These discoveries represent fundamental advances in our knowledge of the Shark Bay microbial system, laying a foundation for detailed studies of stromatolite morphogenesis that will advance our understanding of benthic ecosystems on the early Earth.
Phosphorus is an essential element for life, and the phosphorous cycle is widely believed to be a key factor limiting the extent of Earth's biosphere and its impact on remotely detectable features of ...Earth's atmospheric chemistry. Continental weathering is conventionally considered to be the only source of bioavailable phosphorus to the marine biosphere, with submarine hydrothermal processes acting as a phosphorus sink. Here, we use a novel 29Si tracer technique to demonstrate that alteration of submarine basalt under anoxic conditions leads to significant soluble phosphorus release, with an estimated ratio between phosphorus release and CO2 consumption (∑PO43−/∑CO2) of 3.99 ± 1.03 µmol mmol−1. This ratio is comparable to that of modern rivers, suggesting that submarine weathering under anoxic conditions is potentially a significant source of bioavailable phosphorus to planetary oceans and that volatile‐rich Earth‐like planets lacking exposed continents could develop robust biospheres capable of sustaining remotely detectable atmospheric biosignatures.
Plain Language Summary
It is conventionally thought that continents above sea level are required in order for habitable planets to support a robust biosphere. We use experimental geochemistry and a simple model of biological cycling to show that this is incorrect, significantly expanding the possible range of planets that may host surface biospheres that would be detectable through telescope observations.
Key Points
Continental weathering is conventionally considered to be the only phosphorus source to planetary biospheres
We experimentally demonstrate that weathering of ocean crust under anoxic conditions releases significant amounts of bioavailable phosphorus
Habitable planets (including the earliest Earth) without subaerial continents may support significant biogenic gas fluxes to the atmosphere
The concept of the Great Oxidation Event (GOE), during which atmospheric oxygen rose precipitously and perhaps to near-modern levels around 2.4–2.1billionyears ago (Ga), has become entrenched in our ...views on secular atmospheric evolution. Multiple proxies confirm a permanent shift towards more oxygenated conditions at some time near the Archean–Proterozoic boundary. However, it remains unclear precisely when this transition occurred, due in part to the likely temporal variability in those early levels and different sensitivities of the proxies utilized to track atmospheric oxygen partial pressures. Here, we provide a new look at the timing and magnitude of early atmospheric oxygenation through the record of uranium (U) concentrations in iron formations (IF). Just as IF are important archives of the redox state of seawater, concentrations of redox-sensitive U in IF are faithful proxies for oxidative continental weathering and associated delivery of dissolved U to seawater. Our dataset suggests that there was an increase in U redox cycling and transport at ca. 2.47Ga, just before the permanent loss of mass-independent sedimentary sulfur isotope anomalies traditionally used to define the onset of the GOE. Further, there is significant temporal variability in the IF U record that we propose reflects dynamic Precambrian redox conditions. We provide additional support for earlier suggestions that the GOE was a protracted event marked by vacillating oxygen levels.
•U in iron formations (IF) tracks oxidative U cycling.•There is an increase in the U content of IF at 2.47–2.43Ga.•We compare the IF record of seawater U with other proxies to examine GOE timing.•The GOE was a protracted process initiated near the Archean–Proterozoic boundary.
Redox-sensitive trace metals and their isotopes have emerged as important tools that are used to reconstruct the redox-evolution of the ocean-atmosphere system. However, reliability of such ...reconstructions ultimately depends on a solid understanding of the proxies in the present-day oceanic system and their archival potential in sediments. This study compares isotope fractionation of molybdenum (Mo) and uranium (U) during their removal from seawater and deposition into sediments by investigating sites at various depths of the presently two largest restricted anoxic oceanic basins: The Black Sea and the Cariaco Basin. In support of previous investigations, our data indicate that Mo scavenging and isotope fractionation are mainly controlled by water column sulfide levels. In contrast to Mo, U reduction and immobilization appears to occur mainly at the sediment-water interface and within the uppermost few cm of the sediment pile in both basins. In the Black Sea, decreasing δ238U of surface sediments with increasing water depth correlate with trends for water column δ238U, implying constant U isotope fractionation between water and sediment. However, increasing U concentrations and δ238U within the uppermost few cm of the sediment pile of both basins indicate additional U reduction with depth.
Despite the different mechanisms for Mo and U removal and associated isotope fractionations, a similar inverse correlation between δ98Mo and δ238U is observed for sediments of both basins, which translates in a positive correlation of Mo and U isotope fractionation between the sediments and open seawater. The correlation of δ98Mo and δ238U indicates a similar response of isotope fractionation to the efficiency of Mo and U removal that is mainly controlled by sulfate reduction rates. High dissolved sulfide concentrations and sulfate reduction rates are responsible for very effective Mo and U removal and corresponding minor Mo and U isotope fractionation relative to seawater. Further, high dissolved sulfide concentrations also correlate positively with deep water renewal times, resulting in an isotopically fractionated water column with low δ238U (and somewhat higher δ98Mo) in restricted basins with sluggish ventilations, such as the Black Sea. Both mechanisms result in negatively correlated δ98Mo and δ238U with high δ98Mo and low δ238U in sediments under strong euxinic conditions. The particularly strong correlation observed for Cariaco Basin sediments may indicate that its water column was variably stratified in the past.
The observed δ98Mo and δ238U correlation of both basins can be reproduced in a simple coupled water column and sediment reactive transport model. Different slopes in δ98Mo and δ238U trends can be linked to varying degree of basin restriction, sulfate reduction rates, and isotope compositions of the respective water columns. The offset towards lower δ98Mo (and δ238U), observed for Cariaco Basin sediments compared to those from the Black Sea, may be the result of inefficient Mo reduction with high Mo isotope fractionation or isotopically light Mo from a particulate Fe-Mn oxide shuttle. The results of this study will help to interpret sedimentary Mo and U isotope values, while showing that coupling of δ98Mo and δ238U in sedimentary archives may be useful for paleo-reconstruction work.
The atmosphere–ocean system experienced a progressive change from anoxic to more oxidizing conditions through time. This oxidation is traditionally envisaged to have occurred as two stepwise ...increases in atmospheric oxygen at the beginning and end of the Proterozoic Eon. Here, we present a study of the redox-sensitive element, uranium, in organic-rich shales to track the history of Earth's surface oxidation at an unprecedented temporal resolution. Fluctuations in the degree of uranium enrichment in organic-rich shales suggest that the initial rise of atmospheric oxygen ~2.4billionyr ago was followed by a decline to less oxidizing conditions during the mid-Proterozoic. This redox state persisted for almost 1billionyr, ending with a second oxygenation event in the latest Neoproterozoic. The U record tracks major fluctuations in surface oxygen level and challenges conventional models that suggest the Earth underwent a unidirectional rise in atmospheric oxygen during the Precambrian.
•The U of seawater has not been constant throughout geologic time.•The U of seawater is linked to changes in oceanic and atmospheric oxygen content.•Secular changes in U demonstrate a drop in O2 after the GOE.
Despite a surge of recent work, the evolution of mid‐Proterozoic oceanic–atmospheric redox remains heavily debated. Constraining the dynamics of Proterozoic redox evolution is essential to determine ...the role, if any, that anoxia played in protracting the development of eukaryotic diversity. We present a multiproxy suite of high‐resolution geochemical measurements from a drill core capturing the ~1.4 Ga Xiamaling Formation, North China Craton. Specifically, we analyzed major and trace element concentrations, sulfur and molybdenum isotopes, and iron speciation not only to better understand the local redox conditions but also to establish how relevant our data are to understanding the contemporaneous global ocean. Our results suggest that throughout deposition of the Xiamaling Formation, the basin experienced varying degrees of isolation from the global ocean. During deposition of the lower organic‐rich shales (130–85 m depth), the basin was extremely restricted, and the reservoirs of sulfate and trace metals were drawn down almost completely. Above a depth of 85 m, shales were deposited in dominantly euxinic waters that more closely resembled a marine system and thus potentially bear signatures of coeval seawater. In the most highly enriched sample from this upper interval, the concentration of molybdenum is 51 ppm with a δ98Mo value of +1.7‰. Concentrations of Mo and other redox‐sensitive elements in our samples are consistent with a deep ocean that was largely anoxic on a global scale. Our maximum δ98Mo value, in contrast, is high compared to published mid‐Proterozoic data. This high value raises the possibility that the Earth's surface environments were transiently more oxygenated at ~1.4 Ga compared to preceding or postdating times. More broadly, this study demonstrates the importance of integrating all available data when attempting to reconstruct surface O2 dynamics based on rocks of any age.
The diversification of macro‐organisms over the last 500 million years often coincided with the development of new environmental niches. Microbial diversification over the last 4 billion years likely ...followed similar patterns. However, linkages between environmental settings and microbial ecology have so far not been described from the ancient rock record. In this study, we investigated carbon, nitrogen, and molybdenum isotopes, and iron speciation in five non‐marine stratigraphic units of the Neoarchean Fortescue Group, Western Australia, that are similar in age (2.78–2.72 Ga) but differ in their hydro‐geologic setting. Our data suggest that the felsic‐dominated and hydrologically open lakes of the Bellary and Hardey formations were probably dominated by methanogenesis (δ13Corg = −38.7 ± 4.2‰) and biologic N2 fixation (δ15Nbulk =−0.6 ± 1.0‰), whereas the Mt. Roe, Tumbiana and Kylena Formations, with more mafic siliciclastic sediments, preserve evidence of methanotrophy (δ13Corg as low as −57.4‰, δ13Ccarb as low as −9.2‰) and NH3 loss under alkaline conditions. Evidence of oxygenic photosynthesis is recorded only in the closed evaporitic Tumbiana lakes marked by abundant stromatolites, limited evidence of Fe and S cycling, fractionated Mo isotopes (δ98/95Mo = +0.4 ± 0.4‰), and the widest range in δ13Corg (−57‰ to −15‰), suggesting oxidative processes and multiple carbon fixation pathways. Methanotrophy in the three mafic settings was probably coupled to a combination of oxidants, including O2 and SO42‐. Overall, our results may indicate that early microbial evolution on the Precambrian Earth was in part influenced by geological parameters. We speculate that expanding habitats, such as those linked to continental growth, may have been an important factor in the evolution of life.
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