Siderite Layers Research Articles

13C isotopic labeling to decipher the iron corrosion mechanisms in a carbonated anoxic environment

AbstractA two‐step corrosion experiment was performed on a ferritic steel (Armco) in a synthetic solution representing the Callovo–Oxfordian at 120°C. After the development of a carbonated corrosion product layer (CPL) during the first 15 days of the experimental step, corrosion front progression was investigated using 13C marked carbonate species during the second 15 days experimental step. CPL was characterized at each step, in terms of morphology (scanning electron microscopy), composition (energy‐dispersive spectroscopy), and structure (µ‐Raman). 13C corrosion product locations were analyzed by time‐of‐flight secondary ion mass spectrometry. Results evidenced that after a step of generalized corrosion, iron corrosion continues locally at the metal/CPL interface. These results suggest that although a protective siderite layer formed on the iron surface after 15 days, a local dissolution of the carbonate layer at the M/CPL interface occurred. A galvanic effect is developed between the bared surface (anode) and the covered one (cathode). This activates iron oxidation. The precipitation of carbonate corrosion products to the metal/CPL interface is possible by the diffusion of 13CO32− ions from the bulk through the siderite layer.

Localised corrosion of iron and steel in the Callovo-Oxfordian porewater after 3 months at 120 °C: Characterizations at micro and nanoscale and formation mechanisms

To investigate the fate of nuclear waste in geologic repositories, corrosion experiments were performed on a ferrito‐pearlitic (P285–NH) and a ferritic steel (Armco) in a synthetic solution representing the Callovo-Oxfordian porewater for 3-months at 120 °C. Corrosion product layers (CPL) were characterized from micro to nanoscale. Local corrosion areas of higher thicknesses were composed of siderite layers with different nano-morphologies, iron silicate, iron sulphate and magnetite. Their formation could be due to local pH variation, but also on metal microstructure (phases location and morphology). Differences in corrosion progress are suggested to be mainly related to differences in porewater chemistry and cathodic sites.

Hydrogen generation by soluble CO2 reaction with zero-valent iron or scrap iron and the role of weak acids for controlling FeCO3 formation

This study points out a new perspective on CO2 sequestration and H2 gas generation in a system of either zero-valent iron (Fe0) or scrap iron along with NaHCO3 and citric acid. Under a high bicarbonate environment, micro-size Fe0 (5–40 g/L) is oxidized at mild anaerobic conditions, generating hydrogen gas with a production rate in the range of 0.09–0.55 g(H2)/kg(Fe0)·h. At the same conditions without bicarbonate, the generation of H2 was negligible. At the end of the reaction, the pH was alkaline so it can be used to absorb CO2. Carbonate ions create a passivation layer on the Fe0 surface, called siderite (FeCO3), that entrapped Fe0 and hindered H2 production. By comparing citric, oxalic, and ascorbic acids for their ability to remove the siderite layer, citric acid was found to be the most effective. In a system of scrap iron-NaHCO3/citric acid, for seven consecutive cycles, it was found that by replacing the media solution with a new media after the end of each cycle and flushing with CO2 in the beginning of each cycle resulted in higher than 85 vol% H2 after one day of exposure and higher than 94 vol% H2 at the end of each cycle.

Evolution of the Mantle Fluids Composition in the Precambrian (on the example of the Ukrainian shield ferruginous formation)

The basic world ferruginous formations evolve from 3.5 to 1.9 billion years and have three main peaks of manifestation: 3.2—2.7, 2.6—2.5, and 2.0—1.9 billion years ago. Three ferruginous formations, which correspond to these periods, are highlighted at the Ukrainian shield: ferruginous-siliceous-volcanogenic (FSV), ferruginous-siliceous-shale (FSS), and ferruginous-siliceous-carbonate (FSC). The source of the material for ferruginous formations, associated primary volcanogenic and chemogenic-sedimentary formations are magmatic melts of the thermal asthenosphere. The FSV Archean formation occurs in greenstone structures of granite-greenstone areas. The ferruginous rocks are jespers, consisting of a mixture of quartz-magnetite, volcanogenic silicate and carbonate (siderite and breinerite) layers. They occur mainly among amphibolites and komatiites, and obligatory contain ferruginous amphiboles and pyroxenes. Similar melts and fluids form in the presence of oxidized fluids (О2–, СО2, Н2О, SiO2), at high activity of Cl–, which provides miscibility of melts and fluids. Ferruginous-siliceous-shale formation is generated at the boundary of Archean and Paleoproterozoic. Lower and upper parts of productive strata are folded by the iron carbonates, and in the center quartz-magnetite and quartz-hematite interlayers are separated, alternating with each other. Separation into an independent phase of a water-silicate fluid carrying iron chloride complexes indicates an increase in the alkalinity of fluids and melts of the feeding asthenosphere while maintaining a high oxidizing potential. An increase in alkalinity is indicated by the presence of graphite in the accompanying shale formations. FSC formation is territorially connected with mantle fault zones of deep laying. Associates with igneous rocks (serpentinites) and shale formations (high magnesian and high alumina shales with graphite) formed in an alkaline environment (with the participation of carbonate-fluoride-sodium fluids) at a depth of at least 250 km, with a high oxidizing potential. Ferro carbonate rocks, including olivine and hematite, are formed from a carbonate phase separating from similar depths. A directed change in the ferruginous formations’ composition reflects the deep fluids and melts composition evolution and is associated with the self-organization of the Earth’s shells.

Hexagonal magnetite in Algoma-type banded iron formations of the ca. 2.52 Ga Baizhiyan Formation, North China: Evidence for a green rust precursor?

Abstract Banded iron formations (BIFs) are iron-rich marine chemical sedimentary rocks, and their mineralogy and geochemistry can be used to gain insights into ancient ocean chemistry and biospheric evolution. Magnetite is the major iron-bearing mineral in many BIFs (particularly in the Archean) and is variably interpreted to be of primary, early diagenetic, or metamorphic origin. Different genetic interpretations for magnetite lead to divergent pictures of the Precambrian Earth system and its evolutionary models through time. The Baizhiyan Formation of the Neoarchean Wutai Group (Shanxi, North China) features magnetite-bearing, Algoma-type BIFs deposited ca. 2.52 Ga, in the lead-up to a major period of global iron formation deposition in the Paleoproterozoic. Abundant magnetite crystals found in the silica-rich bands of these BIFs show euhedral, hexagonal morphology. We suggest that this hexagonal magnetite likely represents pseudomorphs after green rust, a mixed-valence iron hydroxy-salt formed in the water column. The rare earth element composition of the BIFs shows negligible to slightly positive Ce anomalies (CeSN/CeSN* = 1.03 ± 0.07), which is characteristic of a dominantly anoxic water column. The presence of positive Eu anomalies (EuSN/EuSN* <3.9) suggests a substantial influence from proximal hydrothermal fluids. The co-occurrence of siderite layers associated with the magnetite-bearing strata may indicate iron cycling associated with ferruginous bottom seawater conditions. Geochemical signatures of the Baizhiyan BIFs are consistent with the interpretation that the magnetite was transformed from metastable green rust. This green rust could have formed via several processes, including the partial oxidation of Fe(II) by molecular oxygen/photoferrotrophs, the reaction of settling ferrihydrite with Fe(II)-rich hydrothermal fluids under anoxic conditions, or local dissimilatory iron reduction. In all cases, the contribution of primary green rust to BIF formation requires iron redox cycling, and similar pseudomorphs in the form of hexagonal magnetite may be more common in the geological record. Our findings support the models in which green rust was an important primary constituent of the Precambrian iron cycle, and the potential interactions of green rust with other elements (e.g., phosphorus) should be taken into consideration when reconstructing Precambrian biogeochemical cycles.

Corrosion of High-Strength Carbon Steels in Siderite Supersaturated Water at Near Neutral pH

When carbon steel corrodes in anaerobic carbonated water, and the steel surface area to liquid volume is high, the concentration of ferrous and bicarbonate ions increases rapidly even though the corrosion rate is low. Such solutions with high bicarbonate concentration and a near neutral pH are believed to induce stress corrosion cracking of high-strength carbon steels. This work was conducted to investigate the solid precipitation in siderite supersaturated solutions. It was also an objective to measure the corrosion rate of high-strength carbon steel in solutions with high bicarbonate concentration at pH close to neutral. Preloading the solutions with ferrous ions and bicarbonate made it possible to measure desupersaturation and corrosion rate as function of time. The initial siderite supersaturation was more than 1,000 in the desupersaturation experiments. Despite this, the nucleation and growth of siderite was so slow that the solutions remained supersaturated for 100 h to 500 h at 10°C to 25°C. The ferrous ion concentration decreased from 1,200 mg/kg to 100 mg/kg in less than 24 h at 40°C, but did not reach equilibrium within 250 h. The precipitate was siderite at 5°C to 25°C. At 40°C, the solid was a mixture of siderite and chuckanovite in low salinity water and siderite with dissolved Ca2+ in artificial seawater. The corrosion rate of carbon steel at pH 6.7 to 7.2 at 25°C decreased to less than 0.01 mm/y in 5 h. Siderite precipitated and grew to a thin protective layer at the steel surface. Even though the solutions were highly supersaturated with respect to siderite throughout the experiments, the carbonate layer at the steel surface did not grow to more 5 μm thickness during 250 h to 400 h. The investigated steels were armor wires for flexible pipes. They have an oxide layer at the surface which is an inherent result of the manufacturing process. The presence of these oxides did not impede the formation of protective siderite layer.

Anaerobic granular sludge and zero valent scrap iron (ZVSI) pre-treated with green tea as a sustainable system for conversion of CO2 to CH4

Global climate change is a worldwide concern that requires the dramatic reduction of greenhouse gases. Innovation in sustainable technologies for CO2 utilization to other high value products is therefore an emerging and rising field. It was found that CO2 as a sole carbon source can be converted to CH4 (higher than 97%) in a system consisted of anaerobic granular sludge, water and zero valent scrap iron. In this system (zero valent scrap iron, anaerobic granular sludge and CO2), pre-exposure of zero valent scrap iron to green tea resulted in a higher conversion rate and total CH4 production compared to bare zero valent scrap iron response. At the abiotic system (no anaerobic sludge) the zero valent scrap iron pre-treated with green tea also showed around 10% higher H2 production as well as higher final pH compared to zero valent scrap iron. The dominant crystalline product of the process both at the abiotic and at the biological system was siderite (FeCO3). It is likely that the formation of siderite on the zero valent scrap iron outer surface have reduced the amount of H2 release from zero valent scrap iron, shielding the substrate for methanogens. The presence of green tea compounds can form an iron-tannate complex with iron ions and this can possibly decrease or slow down the siderite layer, as a result more H2 was released when zero valent scrap iron was pre-exposed to green tea. Anaerobic spent filtered media was independently added to zero valent scrap iron to examine the effect of extracellular enzymes, however slight increase of the H2 production rate was found. The examined system combines potential solutions for two environmental problems; production of siderite derived from zero valent scrap iron for environmental applications and utilization of CH4 generated from CO2 for energy purposes.

Electrochemical and thermodynamic study on the corrosion performance of API X120 steel in 3.5% NaCl solution

The present work studied the effect of temperature on the corrosion behavior of API X120 steel in a saline solution saturated with CO2 in absence and presence of polyethyleneimine (PEI) as an environmentally safe green inhibitor. The effect of PEI on the corrosion behavior of API X120 steel was investigated using destructive and non-destructive electrochemical techniques. The overall results revealed that PEI significantly decreases the corrosion rate of API X120 steel with inhibition efficiency of 94% at a concentration of 100 μmol L−1. The adsorption isotherm, activation energy and the thermodynamic parameters were deduced from the electrochemical results. It is revealed that the adsorption of PEI on API X120 steel surface follows Langmuir adsorption isotherm adopting a Physi-chemisorption mechanism. Finally, the samples were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques to elucidate the effect of aggressiveness of corrosive media on the surface morphology and the corrosion performance of API X120 steel. The surface topography result indicates that the API X120 steel interface in PEI presence is smoother than CO2 with Cl− ions or Cl− ions only. This is attributed to the compact protective film limits the aggressive ions transfer towards the metallic surface and reduces the corrosion rate. Moreover, PEI inhibition mechanism is based on its CO2 capturing ability and the PEI adsorption on the steel surface beside the siderite layer which give the PEI molecules the ability to reduce the scale formation and increase the corrosion protection due to capturing the CO2 from the brine solution.

Cobalt enrichment in a paleo-karstic bauxite deposit at Yunfeng, Guizhou Province, SW China

Cobalt can be enriched in a variety of different geologic settings, while it is mainly produced as a by-product of sediment-hosted stratiform Cu deposits and magmatic Ni-Cu deposits associated with mafic and ultramafic rocks. In this study, we report elevated Co concentrations in a paleo-karstic bauxite deposit at Yunfeng, Guizhou Province. Whole-rock chemical analyses indicate that Co is concentrated in carbonaceous ferruginous clay layers (167–962 ppm, average 383 ppm, n = 5) and siderite layers (47–439 ppm, average 213 ppm, n = 7) within the bauxite deposit. The positive correlation between Co and S, together with petrographic and EPMA analyses, suggests that Co occurs primarily as Co-rich/bearing sulfides/sulfoarsenides rather than cobaltiferous oxides/hydroxides as in most lateritic Ni-Co deposits. Many of the Co concentrations measured in this study are higher than the minimum grade (300 ppm) or cut-off grade (200 ppm) for economic exploitation of Co from sulfides or arsenides. Basing on these results, we put forward the scientific significance of Co enrichment in paleo-karstic environments and the potential importance of Co resources (as by-products of aluminum production) in bauxite deposits.

Targeting Core Sampling with Machine Learning: Case Study from the Springbok Sandstone, Surat Basin

We show how clustering algorithms can ensure that the core intervals that are pertinent to specific objectives of a sampling campaign are actually sampled. We also show how clusters can be validated prior to sampling with auxiliary data not used for the cluster analysis. We chose to target our core sampling to ensure that both clay poor and clay rich intervals of the Springbok Sandstone are sampled. The clay phases in the Jurassic Springbok Sandstone generally do not exhibit a prominent gamma ray signature and are therefore poorly defined in wireline logs. Similar, hydrogeological properties of the Springbok Sandstone are not well defined through wireline logs. This introduces uncertainty to groundwater models of the Springbok Sandstone. Hence, a better understanding of the clay distribution is thought to be a key to improve the definition of the hydrogeological properties of the Springbok Sandstone. We applied our sample targeting approach to five study wells from the Surat Basin in Queensland. We tailored the application of the cluster analysis to our working hypothesis that the variability of hydrogeological properties of the Springbok Sandstone is controlled by the presence and type of clays, rather than compaction. This informed our choice of wireline logs to include in the clustering (nuclear logs) and of logs to be used for control purpose (resistivity logs, spontaneous potential). We show that identification of five clusters was the most useful number towards our sampling objectives. This allowed for example to exclude coal and siderite layers from sampling for clay analysis and to focus on the differentiation of the clastic sediments in the formation. Further, we show that certain clusters correlate with resistivity and spontaneous potential log signatures. The correlation between the categorical clusters based on nuclear logs and continuous wireline logs not used in the cluster analysis allowed us to interpret the meaning of the clusters in the context of our project and target our sampling to ensure that all clusters are represented in our sample set.

Using Pre-treated Carbon Steel for Post-combustion Carbon Capture Infrastructure

The reduction of capital cost in building post-combustion CO2 capture is necessary to realize its widespread use. Cheaper steels, e.g. carbon steel, are attractive but lack the necessary corrosion resistance. To protect from corrosion, one approach is to implement a protective coating on the surface. Particularly, a coating like siderite (FeCO3) is appealing because it can be formed by amines and therefore could be regenerated. This study investigates the formation, over seven days at 40 and 80°C, of such siderite layers formed in methyldiethanolamine (MDEA), 2-amino-2-methyl-1-propanol (AMP), and a K2CO3 solution. When tested against 2.5M monoethanolamine for 28 days, the coatings formed at 80°C in MDEA and AMP appear the most resilient. There is a substantial siderite crystal layer remaining on the surface and only minimal weight loss.

Siderite corrosion protection for carbon steel infrastructure in post-combustion capture plants

To mitigate CO2 release, large-scale post-combustion capture with amine solvents is essential. To achieve capital cost savings, carbon steel infrastructure can replace stainless steel if corrosion by CO2-loaded amine solvents is controlled. A coating, to protect the carbon steel, formed using an amine (or additive) is beneficial because it can be regenerated. Siderite has been shown to form a protective crystalline product layer, created when Fe oxidised at the surface reacts with carbonate ions. Tertiary or sterically-hindered CO2-loaded amine solutions can form this layer. Herein siderite was prepared on carbon steel substrates from 5M methyldiethanolamine (MDEA), 5M 2-amino-2-methyl-1-propanol (AMP), and 1MK2CO3 at 40 and 80°C. At 40°C, K2CO3 produced the most successful protective siderite layer; by contrast, the amine solutions developed layers with interlocking crystals at 80°C. After siderite formation, these substrates were tested in 2.5M MEA and AEPZ, both highly corrosive but with more desirable capture kinetics. At 80°C, substrates pre-treated with MDEA or AMP showed good resistance against the corrosive actions of MEA and AEPZ for four weeks. The siderite layer reduced Fe oxidation at the surface and ingress of solution species thereby ceasing contact and corrosion.

Siderite layers in the fresh-water Neogene sediments of Vietnam

Siderites forming beds and lenses in the Neogene lacustrine—swampy sediments of the Rinh Chua Formation (northern Vietnam) are considered in detail. Results of the mineralogical and chemical study of siderites and host terrigenous–clayey siltstones are reported. New analytical data characterize the composition and structure of microbiomorphic (bacterial) bodies in the siderites and terrigenous sediments. Microstructures (porosity) and compositional peculiarities (up to 18% P2O5) of individual horizons of the sediments testify to their lacustrine-swampy genesis. It is established that the siderites in association with the layer silicates were formed during the microbiochemogenic decomposition of terrigenous components, including quartz.

The effect of CO2-loaded amine solvents on the corrosion of a carbon steel stripper

The corrosive behaviour of loaded amine solvents was evaluated under stripper operating conditions, for post-combustion carbon capture, to determine the feasibility of using carbon steel in plant construction. In addition to monoethanolamine, three alternative amine solvents: methyldiethanolamine (MDEA), 2-amino-2-methyl-1-propanol (AMP), 1-(2-aminoethyl)piperazine (AEPZ), and the common additive K2CO3 were studied when in contact with carbon steel (C1018) over a 28-day period. Corrosive behaviour was evaluated using carbon steel coupons: gravimetric method for weight change, surface imaging (SEM) and analytical techniques (EDX and XRD), and Fe ion concentration in solution (ICP–OES). The results demonstrated that MDEA and AMP as well as K2CO3 develop a significant siderite (FeCO3) layer on the carbon steel surface. The presence of this layer is attributed to the preferred reaction pathway with CO2 for tertiary and sterically hindered amines. The FeCO3 layer formed in the case of MDEA provides superior protection from continued corrosion of the carbon steel. By contrast, MEA and AEPZ show significant corrosion to the carbon steel surface. In conclusion, MDEA, AMP, and K2CO3 can preferentially produce sufficient surface FeCO3 layers to reduce corrosion levels in carbon steels for use under stripper conditions in post-combustion carbon capture plants.

Impact of corrosion on sliding wear properties of low-alloyed carbon steel

Corrosion phenomena play an important role in limiting the lifetime of tribological systems. It is well known out of different studies that synergistic effects between mechanical sliding and corrosion often lead to increased volume losses when compared to their individual contributions. The aim of this work is to describe the tribological performance of low-alloyed C-steel under sliding/corrosion conditions at specific oxygen-free environments using CO2 atmosphere and brine. Low-alloyed C-steel was used as base material and was tribologically loaded in a novel close-to-reality test rig under reciprocating sliding contact against spray Ni-based coating counterpart. Additionally, pure sliding tests were carried out under N2 atmosphere with distilled water for comparison. Different microscopic methods were applied (e.g., SEM-FIB) for understanding the sliding/corrosion phenomena in addition to quantitative wear measurements by mass loss. The results show a significant influence of the microstructure on the tribological performance under sliding/corrosion conditions, where the formation of corrosion siderite layers (FeCO3) and their tribological properties plays a crucial role.

Corrosion and corrosion fatigue of AISI 420C (X46Cr13) at 60 °C in CO2-saturated artificial geothermal brine

The lifetime reduction of cyclically loaded AISI 420C (X46Cr13, 1.4034) constantly exposed to highly corrosive CO2-saturated hot thermal water is demonstrated in in situ-laboratory experiments (60°C, brine: Stuttgart Aquifer, flowing CO2: 30L/h). S–N plots, micrographic-, phase-, fractographic- and surface analysis were applied to obtain sustainable information on the corrosion and corrosion fatigue behavior. Maximum number of cycles (here 12.5×106cycles to failure) is reached at σa=173MPa. Hydroxide and siderite layers were found on pits and crack surfaces. No typical fatigue limit exists and pit corrosion prior to crack initiation may be identified as failure cause.

Understanding past climatic and hydrological variability in the Mediterranean from Lake Prespa sediment isotope and geochemical record over the Last Glacial cycle

Here we present stable isotope and geochemical data from Lake Prespa (Macedonia/Albania border) over the Last Glacial cycle (Marine Isotope Stages 5–1) and discuss past lake hydrology and climate (TIC, oxygen and carbon isotopes), as well as responses to climate of terrestrial and aquatic vegetation (TOC, Rock Eval pyrolysis, carbon isotopes, pollen). The Lake Prespa sediments broadly fall into 5 zones based on their sedimentology, geochemistry, palynology and the existing chronology. The Glacial sediments suggest low supply of carbon to the lake, but high summer productivity; intermittent siderite layers suggest that although the lake was likely to have mixed regularly leading to enhanced oxidation of organic matter, there must have been within sediment reducing conditions and methanogenesis. MIS 5 and 1 sediments suggest much more productivity, higher rates of organic material preservation possibly due to more limited mixing with longer periods of oxygen-depleted bottom waters. We also calculated lakewater δ18O from siderite (authigenic/Glacial) and calcite (endogenic/Holocene) and show much lower lakewater δ18O values in the Glacial when compared to the Holocene, suggesting the lake was less evaporative in the Glacial, probably as a consequence of cooler summers and longer winter ice cover. In the Holocene the oxygen isotope data suggests general humidity, with just 2 marked arid phases, features observed in other Eastern and Central Mediterranean lakes.

Structural Diagenesis of Siderite Layers in Black Shales (Albian Black Flysch, Northern Spain)

AbstractIn this study, we describe the structural diagenesis of diluted turbidite and abyssal lutite deposits of the late Albian Black Flysch (western Pyrenees, northern Spain). We analyze and interpret an association of brittle and ductile soft-sediment deformation structures. To our knowledge, some of them are described for the first time, although they are known in other tectonic settings. We reconstruct a diagenetic history that commenced with a generalized siderite cement precipitation after siliciclastic turbidite deposition. Siderite replaced the finer-grained, diluted uppermost turbidite parts. Siderite precipitation was followed by calcite cementation in turbidites and by soft-sediment brittle (thrust planes, shear veins, and microveins) and ductile (fault-bend folds and culminations developed in relation to thrust-ramp strain) deformation structures. This occurred before hosting lutite lithification by dewatering and compaction.

Erosion-Corrosion of Carbon Steels in a Laboratory: Three-Phase Flow

Abstract A new testing facility for a high-velocity, three-phase flow consisting of a gas flow loop and a jet impingement rig is described. Flow velocities between the nozzle and specimen have been determined through computational fluid dynamics (CFD) simulations and by particle image velocimetry. Tests were conducted on typical carbon steels (J55 and C95) that are used in tubings for the gas and oil industry. Flow conditions of a sweet gas condensate well have been applied. Mass-loss rates have been determined after testing times of between 4 h and 168 h using optical profilometry. Damaged surfaces were investigated using optical and scanning electron microscopy. The effects of material and flow velocity on the mass-loss rate have been investigated. Mass loss of specimens under given conditions is determined by siderite formation and increasing degradation of siderite layer by impacts of sand and fluid droplets. Degradation happens by erosion-enhanced corrosion. Normalized steel J55 behaves like a ductil...

Geological and trace element evidence for a marine sedimentary environment of deposition and biogenicity of 3.45 Ga stromatolitic carbonates in the Pilbara Craton, and support for a reducing Archaean ocean

ABSTRACTBedded carbonate rocks from the 3.45 Ga Warrawoona Group, Pilbara Craton, contain structures that have been regarded either as the oldest known stromatolites or as abiotic hydrothermal deposits. We present new field and petrological observations and high‐precision REE + Y data from the carbonates in order to test the origin of the deposits. Trace element geochemistry from a number of laminated stromatolitic dolomite samples of the c. 3.40 Ga Strelley Pool Chert conclusively shows that they precipitated from anoxic seawater, probably in a very shallow environment consistent with previous sedimentological observations. Edge‐wise conglomerates in troughs between stromatolites and widespread cross‐stratification provide additional evidence of stromatolite construction, at least partly, from layers of particulate sediment, rather than solely from rigid crusts. Accumulation of particulate sediment on steep stromatolite sides in a high‐energy environment suggests organic binding of the surface. Relative and absolute REE + Y contents are exactly comparable with Late Archaean microbial carbonates of widely agreed biological origin. Ankerite from a unit of bedded ankerite–chert couplets from near the top of the stratigraphically older (3.49 Ga) Dresser Formation, which immediately underlies wrinkly stromatolites with small, broad, low‐amplitude domes, also precipitated from anoxic seawater. The REE + Y data of carbonates from the Strelley Pool Chert and Dresser Formation contrast strongly with those from siderite layers in a jasper–siderite–Fe‐chlorite banded iron‐formation from the base of the Panorama Formation (3.45 Ga), which is clearly hydrothermal in origin. The geochemical results, together with sedimentological data, strongly support: (1) deposition of Dresser Formation and Strelley Pool Chert carbonates from Archaean seawater, in part as particulate carbonate sediment; (2) biogenicity of the stromatolitic carbonates; (3) a reducing Archaean atmosphere; (4) ongoing extensive terrestrial erosion prior to ∼3.45 Ga.