Ediacara-style fossils represent Earth's earliest ecosystems of complex macroscopic organisms, preserved as three-dimensional casts and molds within sandstone beds. Differences in Ge/Si ratios between grains and cements in fossiliferous sandstones from the Ediacara Member of South Australia have been used to support early silica cementation as a key factor in moldic preservation. However, experimental studies tracking Ge/Si partitioning during decay-associated silicification are lacking. Here, we employ an experimental framework to further investigate the utility of Ge/Si ratios as a fossilization proxy. We conducted fossilization experiments using marine invertebrate animals, aquatic plants, and silica-rich artificial seawater to track the fate of Ge throughout the early stages of tissue silicification. We found that experimentally precipitated amorphous silica associated with decaying organisms is characterized by Ge/Si values of 0.25-2.07 mu mol mol- 1, and displayed an overall increase during each experiment, concurrent with Ge incorporation into silica precipitating onto the experimental carcasses. These results help build a framework for using Ge/Si ratios to track fossilization processes and highlight the role of early silica precipitation in preserving soft tissues. Comparison of our experimental data with Ge/Si signatures from Ediacara fossiliferous sandstones improves our understanding of early diagenetic silicification and enables more robust reconstructions of the mechanisms responsible for the preservation of Earth's earliest animal communities.

The Sn and Zn stable isotope compositions of cassiterite and sphalerite have been widely used to understand the formation of magmatic-hydrothermal Sn and Zn mineral deposits. Here, we report Sn isotopic compositions of cassiterite and Zn isotopic compositions of paragenetically later, lower-temperature sphalerite from ten Sn and Zn-bearing polymetallic deposits in the Bolivian tin belt. These deposits represent distinct mineralization styles and metal transport pathways. The delta 124Sn values for cassiterite vary from -0.56 to 1.64%o (n = 33) and delta 66Zn values for sphalerite vary from -0.22 to 0.92%o (n = 20), encompassing much of the global variance observed for each isotopic system. The Llallagua and Huanuni deposits show significantly heavier Sn isotope compositions (average = 1.24 +/- 0.24%o delta 124Sn; n = 10) compared to all other deposits (average = 0.56 +/- 0.55%o; n = 23) and contain several distinguishing geological attributes including: genetic association with porphyry phase stocks and/or dikes, vapor-rich and moderately to highly saline fluid inclusions, vertically restricted and/or discontinuous high-grade veins, and extensive tourmaline-rich magmatic-hydrothermal breccias. Together, these features are consistent with Rayleigh fractionation and suggest that orthomagmatic fluid separation, fluid boiling, and oxidation contributed to cassiterite precipitation and caused Sn isotope fractionation. The residual fluids after cassiterite precipitation were depleted in heavy Sn isotopes and propagated along district-scale fluid pathways. The Sn isotopic ranges for cassiterite from the Sayaquira (- 0.13-1.45%o), Colquiri (0.18-1.44%o), Colcha (0.07-1.47%o), and Monserrat (- 0.56-0.93%o) deposits may be a consequence of cassiterite precipitation over longer mineralization length scales with increased meteoric water influx, where distal cassiterite precipitated from hydrothermal ore fluid that was depleted in heavy 124Sn. The wide and heavy range of Zn isotopic compositions of sphalerite suggests that early orthomagmatic fluid separation and boiling may have caused significant isotopic fractionation during early fluid transport prior to sphalerite precipitation. Subsequent processes including dilution by Sn- and Zn-poor meteoric water and the concomitant change in fluid pH and increase in oxygen fugacity plausibly contributed to the precipitation of isotopically lighter sphalerite at the Monserrat, El Salvador, and Huari Huari deposits.

The influences of bedrock lithological composition on weathering fluxes, including dissolved lithium isotopes (delta Li-7(diss)), in rivers remain unclear. To assess this, we present new elemental, mineralogical, and lithium isotopic data from river localities in Hong Kong that drain either purely silicic volcanic or siliciclastic sedimentary bedrock that were sampled during both the wet (summer) and dry (winter) seasons. Our data show marked geochemical and mineralogical differences in relation to bedrock composition. The silicic volcanic-draining rivers exhibit delta Li-7(diss) values ranging from 8.6 parts per thousand to 20.9 parts per thousand, elevated dissolved alkali metal concentrations, and relatively higher kaolinite abundances, whereas siliciclastic sedimentary-draining rivers yield delta Li-7(diss) values ranging from 3.2 parts per thousand to 8.6 parts per thousand, higher dissolved alkaline-earth metal concentrations, and relatively greater illite abundances. Collectively, the volcanic-draining rivers have higher average delta Li-7(diss) of similar to 12.5 parts per thousand and higher bedload kaolinite/illite average ratios of 2.4, compared to the siliciclastic sedimentary-draining bedrock rivers that yield average delta Li-7(diss) of 6.0 parts per thousand and kaolinite/illite of 0.5. Mechanistically, this could have been driven by the lower abundance of reactive minerals in the siliciclastic bedrock and/or a higher degree of fracturing in the volcanic rocks, which can both increase water-rock interaction times and secondary clay formation, driving higher delta Li-7(diss) values. Seasonal variations were also observed in the Li data, with higher Li concentrations and dissolved delta Li-7 values (up to 8.0 parts per thousand heavier) measured during the dry winter season. This is attributed to lower river discharges in the dry season, enabling longer residence times for clay formation, and higher riverine delta Li-7 values. Higher major, alkali, and alkaline-earth concentrations were also measured during the dry season, with Group 3, transition metals, and rare earth elements more concentrated in the wet season, reflecting the impact of elemental mobility. Overall, our study highlights the influence of both bedrock lithology and regional weather patterns on riverine geochemical signatures within the same tectono-climatic setting. These findings help enhance understanding of the Li cycle, wherein riverine delta Li-7 fluxes to the ocean reflect a combination of lithology, hydrology, and weathering regime, further refining the utility of lithium isotopes for assessing modern and ancient silicate weathering processes on Earth's surface.

The terrestrial dissolved riverine lithium isotope (delta Li-7(diss)) flux can provide valuable information on silicate weathering processes, which are directly tied to the long-term carbon cycle, at various spatiotemporal scales. However, a limited understanding of how bedrock lithological composition impacts delta Li-7(diss) values hinders the utility of the Li proxy for deep-time weathering reconstructions. Accordingly, we investigated tropical rivers in the Philippines and Mariana Islands that drain either distinctly ultramafic-to-mafic or silicic bedrock. We find that mafic lithologies produce markedly heavier delta Li-7(diss) than those draining silicic lithologies, potentially due to preferential formation of Mg-rich clays during incongruent weathering. Furthermore, we observe that known tectonically driven changes in Earth's global ophiolitic-collisional suture length correlate with the marine delta Li-7 record over the past similar to 60 million years. This observation, supported by mass-balance modelling, indicates that variations in mafic bedrock weathering could have been an important influence on Cenozoic seawater chemistry.

Lacking substantial local tin deposits after 2000 BC, the Eastern Mediterranean depended on the importation of large quantities of tin from extraneous sources. Situated roughly equidistant (ca. 3000 km) between the large tin deposits of Western Europe and Central Asia, both localities have been proposed as potential sources of tin metal used in Eastern Mediterranean bronze production. However, such speculation must be substantiated. Herein, we assert that the application of a central tendency-based approach to the comparison of tin isotope analyses of artifact assemblages with those of tin ores is an effective approach to differentiating between Central Asian and European tin sources in both pure tin metal and tin alloys. We apply the TIA approach to the reinterpretation of existing tin isotope analyses of ingots and introduce a large body of new evidence drawn from tin isotopes analysis of ancient (2000-900 BC) bronze artifacts from the Black Sea coast of Bulgaria, Tell Atchana in the Amuq Valley of Turkey, the eastern part of the northern Adriatic/Istria and Kvarner (Croatia), Egypt and Peloponnese, Greece. A general chronological trend from high delta Sn in artifacts dating to 2000-1600 BC to moderate values in the Early Iron Age (1100-900 BC) indicate a gradual shift from dominated by Central Asian imports to tin derived from European deposits. These changes in the movement of tin are mirrored in other traceable commodities such as Baltic amber and glass.

Lithium (Li) enrichment in the formational brines of deep sedimentary basins has emerged as a crucial component of global Li inventories. However, the processes driving the formation of Li brines remain poorly understood. Here we use lithofacies analysis and Li isotope geochemistry to investigate the sources and emplacement mechanisms within weathered subcropping units and overlying detrital sediments of the Peace River Arch (PRA) in the Western Canada Sedimentary Basin (WCSB). We analyze data from three drill cores that traverse Precambrian basement and five of its overlying siliciclastic and carbonate units. These cores reside both within and outside of the fault zone proposed as a migration pathway for hydrothermal emplacement. Lithofacies analysis revealed that these sediments were weathered directly from crystalline basement of the cratonic uplift and transported via a fluvial-deltaic system into the surrounding shallow marine basin. Like modern weathering regimes, we find Li concentrations are strongly lithofacies dependent, ranging from 0.4 ppm to 167.3 ppm, with δ7Li values ranging from 1.5‰ to 23.5‰. Our results show that superficially weathered, coarse-grained lithologies and carbonate facies are Li-depleted and δ7Li-enriched, whereas fine-grained facies characterized by the formation of secondary clay minerals are δ7Li-depleted and exhibit the highest Li concentrations. Contrary to the prevailing model of hydrothermal emplacement, we find no visual, mineralogical, or geochemical evidence of hydrothermal alteration. Instead, Li enrichment is attributed to weathering of the crystalline basement and syndepositional emplacement during basin evolution. Sedimentation continued throughout the overall transgression of the Devonian, resulting in the interfingering of these clastics with every onlapping unit until the PRA was buried at the end of the Devonian. This study is the first to directly trace Li from source to sink in an ancient sedimentary basin, and we show that the modern distribution of Li brine concentrations can be explained by their proximity and intercalation with weathered subcropping units. Moreover, our results provide a source and mechanism of transporting dissolved Li into the restricted basin, supporting previous suggestions that Li brines toward the southeastern portion of the WCSB are the result of basin scale evaporation-concentration of paleoseawater. Our results underscore the link between the nature and distribution of basin fill sediments and the formation of Li-enriched brines. As formational brines gain prominence as future Li resources, the methodology presented here establishes a framework for characterizing Li genesis, with applications for sedimentary basins worldwide.

Lithium isotopes (d7Li) are a useful proxy to track silicate weathering, the fundamental process in which carbon is removed from Earth’s surface. Here we present d7Li and elemental data from 14 riverine localities in the Zambales region, Philippines. The warm, humid climate coupled with monolithic rivers draining ophiolitic massifs and volcanic deposits (from the major 1991 Pinatubo eruption) allows for the comparison of silicate weathering and riverine geochemistry across different lithologies. The most striking part of our dataset is that all rivers draining ophiolitic terranes have heavier d7Lidiss values (range from 22.8 to 37.1‰) than those draining Pinatubo volcanic deposits (range from 8.9 to 18.4‰). As all rivers feature similar topographic relief and hydrological conditions, this suggests a strong lithological influence on d7Lidiss values despite both bedrock lithologies being highly weatherable. We postulate that the mafic and ultramafic composition of the ophiolite terrain significantly enhances incongruent weathering and clay mineral formation, increasing Li fractionation, and leading to the heavier d7Lidiss values. Conversely, the lighter d7Lidiss values for the Pinatubo rivers could be explained by the more felsic composition and unconsolidated nature of the volcanic deposits leading to increased congruent weathering and low clay formation, and thus low Li fractionation. Notable differences in major element concentrations are also observed. The Mg2+ is the dominant cation in ophiolitic-draining rivers reflecting the weathering of Mg-rich mafic and ultramafic minerals. Meanwhile, Na+ and Ca2+ dominate in rivers flowing off the Pinatubo volcanic deposits. The ophiolitic-draining rivers also have total major cation concentrations ([Na+] + [Mg2+] + [K+] + [Ca2+]) almost 3.5 times lower than those draining the volcanic deposits (~1600 vs ~5600 µM). Despite the composition of the ophiolites consisting of highly weatherable mafic and ultramafic minerals, the volcanic deposits are seemingly even more favorably weathered. This is interpreted to be caused by the freshness of the material deposited from the recent eruption. Overall, our study shows that despite all rivers in this study draining highly weatherable lithologies, their intrinsic lithological differences can lead to significantly contrasting d7Lidiss and major cation signatures. New clay mineralogical data from riverine sediments will further inform us on silicate weathering processes in the region.

The Kupferschiefer ore bodies display horizontal and vertical metal zonation in a sequence of Fe3+(Au, PGE) - Cu(Ag) - Pb-Zn-Fe2+ in distinct solubility and redox controlled large-scale zones outward from hematitic Rote Faule altered areas. In the Lubin-Sieroszowice Copper District, delta Cu-65 values from 40 samples range from -2.73 to 0.65 parts per thousand (avg. -0.77 parts per thousand), increasing eastward from the Rote Faule zone (-0.9 parts per thousand) through the Cu zone (-0.71 parts per thousand) to the Pb-Zn zone (-0.31 parts per thousand). This trend reflects heavier Cu isotopes with distance from oxidized areas, tracing fluid flow pathways. This delta Cu-65 isotopic zonation is further confirmed, starting at -1.00 parts per thousand in the Gaworzyce mine area, then increasing to -0.81% in the high-grade Cu-Ag deposits (Polkowice, Sieroszowice, and Rudna), and reaching -0.66 parts per thousand in the peripheral Lubin-Malomice mine area. This pattern indicates a flow of mineralizing fluids from west to east. The main causes of broad isotope fractionation and zoning seem to be the oxidative alteration of reduced rocks and large-scale lateral fluid circulation from the feeder area to peripheral parts of the ore system.

The silicon (Si) isotopic composition (delta Si-30) of siliceous biomineralizers has been proposed to provide a key record of changes in biological Si utilization over the past 500 million years of Earth's history. In particular, diatoms and sponges are known to fractionate Si isotopes during uptake from seawater to form their siliceous (opal) frustules and spicules, respectively. Previous work has demonstrated significant spread in delta Si-30 values in these materials, suggesting that the delta Si-30 signatures of fossilized biogenic silica (BSi) could yield insights into past seawater dissolved silica concentrations (DSi) as well as the size and environmental distribution of skeletal silica production. However, the extent to which the delta Si-30 signatures of marine BSi archives are resistant to various stages of diagenetic alteration remains poorly constrained. Here, we describe an experimental approach we employed to measure the magnitude of isotope exchange during early diagenesis of BSi from two modern silica-biomineralizing taxa. We incubated BSi (spicules and frustules, respectively) from the demosponge Cinachyra sp. and diatom Chaetoceros gracilis in 29Si-spiked seawater in batch-reactors at a constant temperature (25 degrees C) and near-natural seawater pH (7.8). The dissolved 29Si tracer technique allowed us to calculate BSi dissolution rates and to assess the extent of isotope exchange, shedding light on the impact of early diagenetic processes on the preservational fidelity of BSi silicon isotope signatures. Four different sets of experiments were carried out over a continuous duration of 919 days to explore variations in the diagenetic alteration of BSi from each taxon, both in the presence and absence of associated organic matter. The resulting time series data indicate that all systems experienced BSi dissolution at the onset of each experiment, followed by relatively rapid Si isotope exchange (within similar to 400 days), with the greatest extent of isotope exchange observed in experiments with diatoms, which we attribute to higher reactive surface areas. We found that the rate and degree of Si isotope equilibration, a previously underexplored process, could, in some cases, play an important role in controlling the Si isotope composition of natural BSi samples during early diagenesis. Our results suggest that caution should be exercised in attempting to reconstruct primary paleoseawater conditions and silica cycle evolution from BSi delta Si-30 values. We additionally outline several environmental and evolutionary factors that may have shaped both diagenetic style and delta Si-30 fidelity of biosiliceous archives across the Neoproterozoic-Paleozoic transition in conjunction with the emergence and radiation of silica-biomineralizers.

The Zambezian Biome of southern-central Africa was an important environmental setting for ancient human interactions and biogeographic dispersals. Here, we assess herbivore migratory behavior and habitat selection with a similar to 2,500 km(2) isoscape of bioavailable strontium (Sr-87/Sr-86) for the Kasitu Valley of northern Malawi, and a probabilistic spatial assignment of fossil herbivore enamel from four sites dated to the last 25,000 calibrated years BP. The results show a very large range in bioavailable Sr-87/Sr-86 values in the study area (0.7248-0.8521) and reveal that foragers hunted small game locally and procured most large prey in the Afromontane grasslands to the southeast of the study area and along narrow riparian corridors. We found no evidence of migratory behavior in species that exhibit this behavior today. Ancient foragers likely hunted prey that were available year-round, consistent with zooarchaeological and genetic evidence for reduced mobility at the end of the Pleistocene.

The weathering of silicate minerals regulates climate on million-year timescales. Some silicate bedrock, particularly ophiolites, are more susceptible to enhanced weathering than other lithologies. Lithium isotopes (δ7Li) are a proxy that can be used to help track weathering processes due to the fractionation of Li during secondary clay mineral formation. Here we present data collected from tributaries that source the Nagaland-Manipur Ophiolite Complexes in northeastern India, which ultimately flow into the Irrawaddy River in Myanmar, to test the hypothesis that the weathering of ultramafic terrains generated by arc-continent collisions can drive rapid atmospheric carbon drawdown. Major cations, anions, trace elements, δ7Li, and clay mineralogical compositions were measured from river water, bedload, and suspended sediment to gain insight into silicate weathering processes in the region. Our results show streams and rivers in this region contain δ7Li values that range from 20.6 to 31.2‰, with the ophiolitic-sourced tributaries being heavier than the global riverine average of ~23‰. This indicates that rivers draining ultramafic lithologies in warm humid climates are experiencing higher degrees of weathering intensity than other drainages comprised of more felsic lithologies. Additionally, XRD results show that most river bedload contains smectite clays, which may help promote organic carbon burial. These data combined highlight new pathways toward understanding silicate weathering as it relates to atmospheric CO2 drawdown.

Isotope ratio analyses of trace elements are applied to tooth enamel, ostrich eggshell, and other archaeological hard tissues to infer mobility and other aspects of hominin and animal paleoecology. It has been assumed that these highly mineralized tissues are resistant to diagenetic alteration, but this is seldom tested and some studies document diagenetic alteration over brief time spans. Here, we build on existing research on Maximum Threshold Concentrations (MTCs) to develop screening tools for diagenesis that can inform heavy isotopic analyses. The premise of the MTC approach is that archaeological tissues are likely contaminated and unsuitable for isotope ratio analysis when they exceed characteristic modern concentration ranges of trace elements. Furthermore, we propose a new metric called the Maximum Threshold Ratio (MTR) of 85Rb/88Sr or whole element Rb/Sr, which can be measured simultaneously with 87Sr/86Sr during laser ablation (LA) MC-ICP-MS or applied during post hoc screening of specimens. We analyzed 56 enamel samples from modern Kenyan mammals and 34 modern ostrich eggshells from South Africa, Namibia, and the United States by solution ICP-MS, as well as a subset of shells using LA-MC-ICP-MS. Our results indicate that thresholds are consistent across taxa at a single location, but likely vary across locations. Therefore, MTCs and MTRs need to be tissue and locality specific, but not necessarily taxon-specific. Other important differences are observed between the inner and outer surfaces of the eggshells and between LA and solution ICP-MS. This exploratory study provides guidelines for building reference thresholds to screen enamel and eggshell for diagenesis potentially impacting biogenic isotope ratios.

Stibnite precipitates in the form of massive boulders at two active hydrothermal mounds in the submarine Wakamiko Crater (Ryukyu Volcanic Arc) as opposed to commonly observed accessory stibnite in the seafloor hydrothermal deposits. The stibnite dimorph, metastibnite, found here for the first time on the seafloor, appears to always form whenever stibnite is precipitated under submarine hydrothermal conditions. Our study shows that hydrothermal conditions of low temperatures (<50 degrees C) and low values of pH (<6) are favorable for the precipitation of stibnite on the seafloor. The stibnite probably does not precipitate at the measured vent fluid temperatures (i.e., 177.6-187.0 degrees C) along the chimney conduits, but rather at temperatures <50 degrees C and at slightly reduced to slightly oxic conditions (Eh = -0.5 to +0.5 V) within the chimney walls and hydrothermal mounds. Metastibnite deposition appears to be the result of rapid quenching of hot hydrothermal fluid when mixed with cold seawater and rapid precipitation at the interface between stibnite and vent fluid. The low concentrations (usually below detection limits) of the trace elements (Cd, Co, Cr, Cu, Li, Mn, Mo, Ni, P, Pb, Sr, V, Zn) in the stibnite deposits from Wakamiko Crater are likely a result of the decreased metal-transporting capacity of the precipitating vent fluid due to its low chlorinity. Low-chlorinity venting implies sub-seafloor boiling and phase separation of the hydrothermal fluid. Sluggish hydrothermal fluid/seawater mixing within the walls of the chimneys and mounds favors the reduction of sulfate dissolved in the hydrothermal fluids and results in a heavy S isotope composition of the sulfate in the vent fluids. Sulfate reduction and disproportionation of magmatic SO2, both leading to heavy S isotope composition of sulfate in the vent fluids, seem to be common processes in volcanic arc/back-arc submarine hydrothermal settings.

The abundance and fractionation of the stable strontium (Sr) isotope system are being increasingly utilized to move forward our understanding in geological and cosmological processes. Two analytical techniques are commonly used to measure stable Sr isotopes: 1) double-spike thermal ionization mass spectrometry (DS-TIMS) and 2) Zr-doped sample-standard bracketing multi-collector inductively coupled plasma mass spectrometry (Zr-doped SSB via MC-ICP-MS). Relative to DS-TIMS, Zr-doped SSB via MC-ICP-MS allows simultaneous determinations of both 87Sr/86Sr and 88Sr/86Sr ratios, increasing measurement efficiency and sample throughput. However, this technique is currently associated with greater uncertainties in measurement precision and accuracy. In this study, we evaluated potential factors that can affect the quality of Sr isotope measurements during Zr-doped SSB. Our tests show that incomplete Sr recovery during chromatographic separation, mismatches of Sr and Zr concentrations and acid molarity between sample and bracketing standard, and cation contamination could all affect the precision and accuracy of Sr isotope measurements. We present evidence that, with updated preparation procedures and diligent concentration checks, a long-term reproducibility (2σSD: 87Sr/86Sr = +/-0.000015 and δ88/86Sr = +/-0.03 permil) comparable to that of DS-TIMS is achievable when using the Zr-doped SSB method via MC-ICP-MC.

A seafloor hydrothermal system located at the Iheya Ridge (Okinawa Trough), named CLAM, deposits Mn‑carbonate chimneys that have no analogue found so far on the seafloor. The chimneys are composed of Mn-calcite and Ca-rhodochrosite. The crystallographic differences between these carbonates appear to control the rare earth elements (REE) partitioning between them that results in enrichment of the Ca-rhodochrosite in middle and heavy REE, and enrichment of the Mn-calcite in light REE. Chemistry of the CLAM hydrothermal fluids suggests: (1) low water/rock ratio of the hydrothermal system; (2) phase separation and dominance of low-chlorinity vapor phase; (3) sub-seafloor formation of Na-rich alteration minerals during fluid/rock reactions; (4) removal of some elements from the seawater to the host rocks during the seawater/rock interaction; (5) high Mn/Ca ratio of the basement rocks is responsible for the high Mn concentration in the hydrothermal fluids. C-O-isotope compositions of the CLAM Mn‑carbonates suggest they precipitated through binary mixing of end-member hydrothermal fluid and seawater accompanied by progressive degassing and cooling of the fluid. Mn-calcite precipitated from almost pure end-member hydrothermal fluid, whereas Ca-rhodochrosite precipitated from seawater-dominated vent fluid. Mg-isotope fractionation during Mn‑carbonate precipitation is assumed to depend on carbonate growth conditions and resulting carbonate mineralogy. S-isotope composition of the CLAM Mn‑carbonates suggests that the Ca-rhodochrosite precipitated in oxic conditions through rapid mixing of hydrothermal fluid and seawater, whereas the Mn-calcite precipitated in reduced conditions (thermochemical or microbial sulfate reduction) through slow mixing of hydrothermal fluid and seawater. Sr-isotope composition of the CLAM hydrothermal fluids is close to that of Okinawa Trough deep seawater. In contrast, Sr-isotopes in the CLAM Mn‑carbonates are more variable, indicating that Sr was derived from seawater, local lavas and sediments. Nd-isotope composition of the Mn‑carbonates indicates that Nd was derived from the local lavas and sediments. Pb in the majority of the CLAM Mn‑carbonates is of sedimentary origin (Pb isotope data), but involvement of anthropogenic Pb in the hydrothermal system is inferred for some Mn-calcite samples. Stability phase diagram modeling coupled with C-O-S-Sr-isotope data suggest that in the CLAM vent fluid the rhodochrosite is stable in a narrow Eh-pH range (6 < pH < 10; Eh > 0) and in a wide range of [Mn] and [Ca] activities, whereas calcite precipitates from a close to the end-member hydrothermal fluid in reduced conditions (Eh < 0).

Enhanced rock weathering (ERW) is a promising scalable and cost-effective carbon dioxide removal (CDR) strategy with significant environmental and agronomic co-benefits. A major barrier to large-scale implementation of ERW is a robust monitoring, reporting, and verification (MRV) framework. To successfully quantify the amount of carbon dioxide removed by ERW, MRV must be accurate, precise, and cost-effective. Here, we outline a mass-balance-based method in which analysis of the chemical composition of soil samples is used to track in situ silicate rock weathering. We show that signal-to-noise issues of in situ soil analysis can be mitigated by using isotope-dilution mass spectrometry to reduce analytical error. We implement a proof-of-concept experiment demonstrating the method in controlled mesocosms. In our experiment, a basalt rock feedstock is added to soil columns containing the cereal crop Sorghum bicolor at a rate equivalent to 50 t ha-1. Using our approach, we calculate rock weathering corresponding to an average initial CDR value of 1.44 ± 0.27 tCO2eq ha-1 from our experiments after 235 days, within error of an independent estimate calculated using conventional elemental budgeting of reaction products. Our method provides a robust time-integrated estimate of initial CDR, to feed into models that track and validate large-scale carbon removal through ERW.

It is generally accepted that photosynthetic marine planktonic bacteria were responsible for the oxidation of dissolved ferrous iron (Fe(II)) and the subsequent deposition of iron formations (IFs) throughout the Archean and early Paleoproterozoic. However, the relative roles of the different biological Fe oxidation mechanisms in driving IF deposition-such as anoxygenic photosynthesis (photoferrotrophs) and oxy-genic photosynthesis (cyanobacteria)-remain poorly resolved. Here, we present coupled bulk-rock Fe isotope and manganese (Mn) versus Fe ratios from Archean to early Paleoproterozoic IFs in order to pro-vide a new perspective on Earth's early redox history and processes leading to IF deposition. Based on this updated IF geochemical record, we bolster the case that the partial oxidation of Fe(II) to Fe(III) was central to IF genesis, arguing against extensive water column Fe(II) silicate formation as the main process driving IF deposition. The geochemistry of IFs deposited prior to the Great Oxidation Event (GOE) shows that par-tial Fe(II) oxidation was a common feature in either anoxic or low oxygen (O2) conditions, where meta-bolic Fe(II) oxidation by photoferrotrophs is likely to have prevailed over ambient Fe(II) oxidation by O2 produced by cyanobacteria. Assuming that cyanobacteria evolved in the Archean, the presence of partial Fe(II) oxidation suggests that O2 production was relatively muted during this time. This points to a model for Archean surface redox conditions, whereby oxygen oases were relatively limited in extent, likely due to low primary productivity of cyanobacteria and high Fe fluxes. We further demonstrate a gradual dis-placement of metabolic Fe(II) oxidation in the Archean by quantitative O2-driven Fe(II) oxidation during the GOE by ca. 2.31 Ga.(c) 2022 Elsevier Ltd. All rights reserved.