Hamelin Pool in Shark Bay, Western Australia, is a natural laboratory to study in situ stromatolitic carbonate formation in a hypersaline lagoon. Stromatolitic carbonates sampled at three different tidal environments (supratidal, intertidal and subtidal; cf. Martin et al., 2023) show a gradual increase in Ba isotopic compositions (d138Ba) from -0.12 ‰ to modern open ocean values (up to 0.6 ‰) with decreasing Ba concentrations following classic Rayleigh pattern. Declining Ba/Ca ratios (0.93 to 0.32) follow conservative mixing trends with increasing Co, Li, Sr and Ni concentrations from the shore to subtidal environments. Due to the lack of riverine influx into Shark Bay, groundwater discharge is the likeliest source for two end-members mixing with seawater. We observe fingerprints of the groundwater end-member in a supratidal sample showing particular low, stable Ba isotope values (-0.12 ‰), which further corresponds with elevated Mn/Sr ratios and the lowest O isotope compositions (2.9 ‰) as an indicator for a meteoric origin. This end-member likely reflects carbonate precipitation near shore under the influence of groundwater discharge and reaction with high alkalinity fluids derived from the local Tamala Limestone aquifer (d138Ba = 0.09 to 0.24 ‰ at 0.3 to 19 PSU; cf., Mayfield et al., 2021). In contrast, the stromatolitic carbonates that form in equilibrium with a saline end-member show heavy Ba isotope compositions (0.57‰).Injection of alkaline groundwaters into the hypersaline waters of Hamelin Pool likely contributed to an enhanced rate of carbonate precipitation, possibly catalysed by nucleation within or onto extra-polymeric substances of diverse microbial mats. Interaction of benthic microbes, especially cyanobacteria, in alkaline waters may offer promising carbon sequestration pathways in the modern high pCO2 atmosphere and small-scale mitigation to the harmful impacts of the current climate crisis.References: Martin et al., 2023, GCA; Mayfield et al., 2021, Nature Communications

Ferromanganese (Fe-Mn) nodules are marine chemical sediments that represent unique archives for the reconstruction of ambient seawater conditions. This record is limited from modern to Cenozoic times due to the instability of Fe-Mn oxides during burial and subduction of the oceanic crust and overlying sediments. However, almost uncharacterized Fe-Mn nodules exist in Jurassic sedimentary strata throughout the ancient Tethyan region approximately 100 Ma older than the oldest yet-investigated Cenozoic nodules. Their reliability as geochemical archives for the reconstruction of ancient seawater is, however, poorly understood. In this study, FeMn nodules from the Pyhrntal area (Austria) are geochemically and mineralogically characterized and subdivided into four types with variable amounts of carbonates (calcite, rhodochrosite), todorokite, and hematite as major phases. Shale-normalized rare earth element and yttrium systematics of all types indicate a hydrogenetic origin with characteristic positive Ce anomalies and negative Y anomalies. In-situ Fe isotope measurements of the Fe-Mn nodules display a delta 56/54Fe range between-0.32 and-0.02 %o for the Jurassic Tethyan Ocean, similar to values from modern Atlantic nodules. Stable Mo (delta 98/95Mo =-0.97 to-0.56 %o) and U (delta 238/235U =-0.75 to-0.47 %o) isotope compositions resemble those of modern and Cenozoic Fe-Mn nodules, suggesting that Middle Jurassic oceans were similarly well-oxygenated as modern oceans. Our results demonstrate the reliability of fossil Fe-Mn nodules in the Pyhrntal as geochemical archives for the composition of paleo-seawater, encouraging the investigation of other ancient Fe-Mn deposits which may significantly improve and complement the picture of the redox evolution of Phanerozoic oceans.

Redox- and bio-productivity changes in the Trans-Saharan epicontinental seaway during and after the Cenomanian-Turonian anoxic event (OAE2): insights from stable isotopes and trace metalsUsman Abubakar 1,2*, Simon V. Hohl1, Sebastian Viehmann3, Stefan Weyer3, Musa Bappah Usman21State Key Laboratory of Marine Geology, Tongji University, Shanghai, P.R China2Department of Geology, Gombe State University, Gombe, Nigeria3Department of Mineralogy, Leibniz University Hannover, Hannover, Germany*uabubakar2002@tongji.edu.cnThe Cenomanian-Turonian boundary (~ 94 Ma) marked a presumably global ocean anoxic event (OAE2), resulting in the widespread deposition of black shales, a positive carbon isotope excursion, biotic turnover, and significant changes in global climate. While increased volcanic activity is often linked to enhanced nutrient supply into the ocean and increasing primary productivity and O2 consumption during their decomposition, recent studies have revealed contradictory redox conditions ranging from anoxic to oxic from the open ocean to epicontinental seas. We present the first integrated geochemical data from the Ashaka quarry in one of the basins flooded by the Trans-Saharan epicontinental seaway: the Upper Benue Trough, Nigeria. The data include δ13Corg and δ238U, total organic carbon (TOC), and redox-sensitive and bio-essential trace metal concentrations. We aim to determine the possible location of OAE2 within this strata and reconstruct local variations in redox and bio-productivity systematics. δ13Corg displays a positive excursion of ~2 ‰ (-25.5 ‰ to -23.5 ‰) at the base of the section, indicating the occurrence of an OAE. However, this event coincides with relatively low TOC values (0.3-1.2 wt.%), showing regionally low burial rates at a potentially increasing influx of terrigenous organic matter, as evidenced by increasing C/N ratios from 4.4 to 10.3. Enrichment factors of redox-sensitive trace metals (UEF and MoEF) exhibit depletion, enrichment, and subsequent depletion at the beginning, middle, and end of the OAE2 within the stratigraphy, respectively. In contrast, enrichment factors of bio-essential trace metals (CdEF and ZnEF) consistently show a depletion throughout the event and display a low ratio of micronutrients to macronutrients (Cd/P and Zn/P). These patterns correspond with δ238U (-0.46 to -0.32 ‰) varying around the value of modern seawater (-0.4 ‰), suggesting fluctuations from oxic to sub-oxic redox conditions and a reduced element shuttle at possibly suppressed paleo-productivity. After the OAE2, the middle part of the Ashaka section records primarily oxic conditions supported by very low TOC and δ238U values similar to the modern ocean. The top of the section exhibits highly depleted redox-sensitive metals and high enrichment of bio-essential metals, indicating a rebound to fully open marine conditions characterized by high productivity, upwelling, and well-oxygenation concurrent with a slight positive shift in δ238U (-0.37 to -0.29).This study demonstrates partly oxygenated conditions during the OAE2 in the epicontinental Trans-Sahara Seaway, correlating the Ashaka quarry section with strata deposited in the epicontinental Western Interior Seaway and several shallow marine environments of the Tethys Sea, bringing this new OAE2 interval into the global context for the first time.

Modern to Cenozoic hydrogenetic FeMn nodules and crusts are reliable geochemical archives of past seawater chemistry. In this study, we report the first petrographic and geochemical data of Jurassic FeMn nodules and crusts from the Calcaereous Alps of the Pyhrntal area (Austria) that were formed ca. 170 million years ago and, thus, ~ 10 million years after the Toarcian extinction event. The combined approach of petrographic data, including XRD and SEM+BSE, with major and trace element signatures and stable U-Mo isotopes of individual FeMn nodule and crust layers obtained by tandem ICP-MS and MC-ICP-MS, respectively, is used to extend the geochemical record of marine FeMn deposits roughly 100 million years back in time and evaluate their reliability as archives for Jurassic seawater. Trace elements and redox-sensitive U-Mo isotopes aid in reconstructing the origin of the FeMn nodules and redox conditions of Tethian seawater in the aftermath of the Toarcian extinction event.The FeMn deposits of the Pyhrntal area can be subdivided into four types: Type I nodules rich in carbonates (< 90wt %; calcite, rhodochrosite) with minor Fe-oxides (10 wt%; hematite, goethite) and clays (< 20 wt %). Manganese-rich type II nodules (< 75 %; todorokite, ranceite) contain fewer carbonates (< 47 wt %), Fe oxides (

Hydrothermal systems have been invoked as a major driver for the evolution of life, but the impact of hydrothermal fluids on Earth's ancient oceans and their habitats remains ambiguous. Europium (Eu) enrichments trace high temperature hydrothermal fluids in rock archives and may serve as proxy for hydrothermal input into ancient oceans. Here, we provide Eu abundances from stromatolites and iron formations between 3.8 and 0.542 billion years (Ga) ago and reconstruct the impact of hydro-thermal systems on shallow and deeper marine environments. Our results document a continuous decrease in positive Eu anomalies until 2.5 Ga ago, followed by almost complete disappearance, suggesting a decreasing impact of submarine hydrothermal systems on ancient oceans. Exceptional positive Eu excursions between 2.8 and 2.6 Ga, and potentially also at 3.5 and 2.2 Ga, are only preserved in deep marine settings and reflect magmatic pulses triggered by elevated upper mantle temperatures. Our results demonstrate the significance of high temperature hydrothermal systems on Archean seawater chemistry with implications for the supply of bio-essential elements. However, life in shallow marine environments was likely supported by fluxes from emerging continents, at the least from the Neoarchean onwards.

Activity of methanogenic archaea on early Earth is mainly deduced based on modern processes and supported by the carbon isotope record (S13Ccarb). Recently, Ni isotopes preserved in authigenic carbonate have been proposed to provide a further signature to trace methanogenesis in ancient environments because methanogens require nickel (Ni) as a metal co-factor for key enzymes to produce CH4. Further, uptake in cultured methanogens has been shown to fractionate stable Ni isotopes (S60Ni) from their aqueous medium, a process that could potentially be recorded in authigenic carbonates. Therefore, Ni isotopes in microbialites could provide valuable insights into whether methanogenesis was active in ancient microbial habitats. Here, we explore this idea using combined C-Ni isotope analyses on individual layers of microbial carbonates from the Lagoa Salgada, a Holocene ephemeral lake in Brazil where abundant microbialites formed in the presence of methanogens. Microbial carbonates show distinct positive S13Ccarb (up to +20 %o) at negative S60Nicarb values (down to -1.36 %o) that can be linked to methanogenic metabolism-in contrast to higher S60Ni of ambient gastropod shells (+1.72 %o) and authigenic non-lithified sediments from the lagoon (+1.1 to +1.58 %o). Our results show that Ni isotopes in microbial carbonates are a promising novel isotope proxy for methanogenic Ni isotope fractionation. We anticipate this study as a starting point for future research on combined metallome and microbiome evolution reflected in microbialites through time on Earth and beyond.

The magnesium (Mg) isotopic compositions of Tibetan Plateau rivers have been recently used to quantify silicate weathering fluxes and associated CO2 consumption. In this work, we present a systematic study on the element and Mg isotope geochemistry of rivers and lakes in the interior of the Tibetan Plateau, to improve our understanding of the magnesium-carbon (C) cycle. delta 26Mg values of river waters range from -2.00 %o to -0.60 %o, with an average of -1.12 f 0.60 %o. The observed delta 26Mg variability of the rivers reflects overall changes in the proportion of silicate over dolostone weathering fluxes, while the effects of geothermal fluid and rainfall are rendered insignificant. The restricted lakes fed by these rivers have low Mg/Na, Al/Na and Si/Na ratios but are considerably enriched in 26Mg, with an average delta 26Mg value of -0.61 f 0.65 %o, presumably controlled by the in-situ formation of authigenic Mg-bearing minerals. Element and isotope mass balance modeling indicates that nearly 90 % of the riverine Mg2+ is consumed by the formation of authigenic Mg-bearing minerals in the lakes, and the Mg isotope fractionation factor between the minerals and water (Delta 26Mgmineral-water) is -0.09 %o to -0.06 %o. The best explanation to the contrast between the large Mg sink and a small Delta 26Mgmineral-water value is the coprecipitation of Mg-bearing carbonate minerals and phyllosilicates in the lakes. The microscopic observation and EDS analyses of paleo-lake sediments shows that the laminae of carbonate and authigenic Mg-rich clay minerals alternatively overlapped, and elemental and delta 26Mg values also suggested that the mixture of Mg-bearing carbonate and authigenic phyllosilicates dominated the sediment geochemistry. An estimation yields that the precipitation of authigenic phyllosilicates due to reverse weathering accounts for nearly 80 % of the total Mg sink in these restricted lakes, from which a considerable amount of CO2 had been released into the atmosphere. Our results provide new insights into the long-term Mg-C cycle on the Tibetan Plateau.

Over the past two decades, a large number of zircon U‐Pb ages from the Yangtze and Yellow River Basins have been published, yet distinguishing the sources of sediment between these regions remains challenging. Issues related to sampling, analytical methods, and biases complicate the interpretation of detrital zircon geochronology. In this study, we leveraged machine learning techniques to analyze a data set of over 33,000 zircon U‐Pb ages, refining the data to 28,082 ages for our analysis. We employed two characterization strategies: tectonic classification and kernel density estimation, and optimized our models through hyperparameter tuning. Our results demonstrated that the machine learning algorithm, eXtreme Gradient Boosting (XGBoost), significantly improved the accuracy of predicting sediment sources when compared to conventional methods (e.g., multidimensional scaling diagram). Additionally, we found that the most informative age populations were associated with the orogenic events (e.g., Jinning, 800–1,000 Ma, Tianshan, 260–394 Ma, and Nanhua, 680–800 Ma) rather than the movements of Lvliang (1,800–2,500 Ma) and Wutai (2,500–2,800 Ma), as suggested in previous studies. Finally, we tested the optimized models on several case studies, illustrating the effectiveness in identifying provenance signals for modern and quaternary sediments in East China Seas and the Yangtze Delta. While this machine learning approach shows great potential for improving sediment provenance analysis in these case studies, it is still limited by the availability and quality of detrital zircon age data for more detailed provenance analysis on sub‐basin scales.

The Cenomanian-Turonian boundary marks a global ocean anoxic event (OAE2), leading to the widespread deposition of black-shales due to enhanced primary productivity and O2 consumption. However, recent studies predict contradictory redox conditions from the open ocean to the epicontinental seas. This study combined existing C and U isotopic compositions of marine OAE2-bearing sediments to understand global redox-variations better and introduce the previously overlooked Trans-Saharan Epicontinental Seaway. We examined the first integrated geochemical dataset from the Ashaka section, Nigeria, including S13Corg, S238U, TOC, redox-sensitive and bio-essential trace metal concentrations in authigenic sediments. We propose the potential location of the OAE2 and reconstruct local variations in redox and bio-productivity in the Trans-Saharan Seaway. The chemostratigraphic onset of the OAE2 is marked by a globally occurring positive S13Corg excursion (-25.5 to -23.5%o). However, unlike many OAE2 sections (e.g., Tarfaya Basin and Demerara Rise in the N.-Atlantic), exhibiting high TOC, the Trans-Saharan Seaway records low TOC, comparable to the Western Interior Seaway and the PaleoPacific Ocean. These differences indicate poor regional preservation, contrasting with organic-rich, marinedominated preservation in the deeper Tarfaya Basin and N. Atlantic. Micro-to-macronutrient ratios in the TransSaharan Seaway were low during the OAE2, suggesting suppressed productivity akin to the Western Interior Seaway and differing from the high-productivity regimes in the Tarfaya Basin and Tethys Sea. Furthermore, redox conditions highlight regional contrasts: predominantly oxic-suboxic conditions in the Trans-Saharan and Western Interior Seaway and Paleo-Pacific Ocean vs. anoxic in the Gubbio section and Demerara Rise. Despite partially oxygenated conditions, a negative S238Usw shift in the Trans-Saharan Seaway (-1.6%o) mirrors the Demerara Rise, Eastbourne, Western Interior Seaway, and Morelos Formation. However, the magnitude varies globally, with epicontinental seaways recording the largest shifts. These inconsistencies suggest that the extent of ocean anoxia undulates across basins calling for a cautious interpretation of U isotopes as a global redox proxy.

The geochemical signatures in cap carbonate successions are critical records of paleoceanographic conditions following Snowball Earth events. These deposits offer insight into the shifts in redox conditions and the reestablishment of biogeochemical cycles during postglacial periods, providing a window into the evolving marine environments and potential drivers of early oxygenation. To track redox changes during this transition, we present improved high-resolution analyses of redox proxies across Puga cap carbonate (-635 Ma) on the Southern Amazon Craton, Brazil, allowing for the identification of temporal redox transitions during the postMarinoan transgression. The depletion of trace elements, particularly redox-sensitive elements (RSEs), such as Mo, U, and V in microbialites formed in basal cap dolostone, was deposited under oxic conditions. Following the initial melting of Marinoan glaciers, microbial mats flourished and grew in a semi-restricted shallow marine environment in the coastal paleoenvironment along the Amazon cratonic margin, where nutrient-rich surface waters fueled primary productivity. In contrast, the increase in RSEs in upper wave-dominated dolostone facies indicates predominantly dysoxic conditions in continuous sea level rise, resulting in the drowning of these early microbial environments and the precipitation of cap limestones. The sequential oxic-dysoxic redox marks the transition from shallow sea to deepening CaCO3-oversaturated platform conditions. These results demonstrate an unequivocal synchronous relationship between the initial oxygenation of the Amazon margin and the local microbial mat flourishment shortly after the Marinoan glaciation.

In the search for reliable archives to reconstruct Earth's paleo-surface ocean chemistry, stromatolites have gained much attention in the past decade. Stromatolitic carbonates, i.e., lithified microbial mats, form in shallow neritic environments and have the unique potential to record the ambient water chemistry of the photic zone. Their wide occurrence in shallow marine sedimentary successions from at least ca. 3.5 billion years ago until today highlight these bio-sedimentary archives as excellent recorders of elemental fluctuations in microbial habitats of the near-shore surface ocean, embayed basins, and lacustrine systems. In this study, we test the application of Ba concentrations and its isotopes in Paleoarchean to Holocene stromatolites as a potential deep-time proxy for biogeochemical element cycling in ancient microbial habitats of different depositional environments. Barium systematics in stromatolites reflect biogeochemical Ba cycling in local, aqueous (micro)environments of photic zones under variable oxygen and sulphate saturation conditions. We obtained significant differences in authigenic Ba concentrations and isotopic compositions between open ocean and restricted depositional settings: Microbial communities in restricted embayment or lacustrine environments show distinct negative δ138Baauth values inversely correlated with Ba concentrations due to Ba (re)cycling. In contrast, stromatolites from non-restricted, marine depositional settings have the potential to record ambient seawater Ba isotopic compositions with the heaviest obtained δ138Basw = 0.45 ‰ in the Archean, 0.61 ‰ in the Proterozoic and 0.57 ‰ in the Phanerozoic at a precision of ±0.04 ‰ (2SD).Our study suggests that variabilities in δ138Ba of stromatolitic carbonates can directly be linked to environmental restriction and associated biogeochemical Ba cycling, probably related to micro-barite formation on microbial biofilms, with subsequent recycling in restricted environments leading to lighter Ba isotopic values in the ambient waters and carbonates. However, stromatolites of open marine settings record local surface ocean δ138Ba compositions and may be used as unique archives in future studies to investigate the evolution of primary productivity in the photic zone through deep-time.

Nontraditional stable isotopes of bioactive metals emerged as novel proxies for reconstructing the biogeochemical cycling of metals, which serve as cofactors in major metabolic pathways. The fractionation of metal isotopes between ambient fluid and microorganisms is ultimately recorded in authigenic minerals, such as carbonates, which makes them potentially more reliable than standard biomarkers in organic matter. Stromatolitic carbonates are geochemical archives that allow for the study of the long-term interplay of the biosphere, atmosphere, and hydrosphere through deep time, with the unique potential to investigate early life environments and the evolution of the metallome. The present study uses stromatolites from the similar to 2.95-billion-year-old Pongola Supergroup (S. Africa) as a field laboratory for combined in situ trace metal mapping and layer-specific, novel stable metal isotope compositions to infer early Earth microbial metal cycling via phototrophic and chemo-litho-autotrophic metabolisms. Quantitative in situ trace element maps reveal intrinsic biosedimentary enrichments of nickel (Ni), cadmium (Cd), phosphorus (P), iron (Fe), and manganese (Mn) in stromatolitic laminae. In contrast, barium (Ba) shows a more homogeneous distribution. Authigenic carbonates from pristine stromatolite laminae show distinct delta 138Ba and delta 112Cd fractionation above detrital background and bulk silicate Earth values, but opposing correlation with trace metal concentrations. Authigenic delta 60Ni values overlap with Mesoarchean diamictite compositions. Nickel isotopic compositions in authigenic stromatolitic carbonates, potentially a new proxy for methanogenic metal uptake, do not show any proof of the presence of this metabolism in the samples of this study. Meanwhile, Cd isotopic compositions in authigenic carbonates follow typical Rayleigh-type isotope fractionation; that is, the isotopic composition of Cd evolves to heavy values close to modern surface compositions. Correlations of delta 112Cd with the micronutrients copper (Cu), molybdenum (Mo), and P, at positively fractionated carbon (C) isotopes (delta 13C similar to+2 parts per thousand), argue for active photosynthesis in the Pongola microbial habitat. We show that Ba isotopes can be used to infer carbonate precipitation rates similar to modern microbial carbonates. Thus, the combination of Cd and Ni isotopes has the unique potential as novel isotope biomarkers for the biochemical sedimentary record of early Earth where traditional lipid biomarkers are not applicable due to the incomplete preservation of organic matter. Key Words: Early life-Stromatolites-Novel stable isotopes-Cadmium isotopes-Nickel isotopes-Barium isotopes-Trace element mapping

Evaporites have recently been suggested as a potential archive for recording the Mg isotope compositions (S26Mg) S 26 Mg) of coeval seawater. However, episodic dolomitization during the deposition of massive evaporites could cause considerable Mg removal and isotopic fractionation. To constrain the hydrological changes and influence of dolomitization on ambient brine S 26 Mg, we present petrographic and mineralogical features, as well as Mg-CO-Sr isotope data extracted from carbonate phases of a middle Triassic (ca. 247 Myr) marine anhydrite-dolostone sequence from a drill core in eastern China. The drilled lithologies are characterized by massive dolostone layers in the lower part, followed by an upward decline in dolomite contents accompanied by a rise in anhydrite. Multiple lines of evidence consistently point to a syn-depositional origin for the dolostones in a marginal basin of the Tethys Ocean and a lack of diagenetic alteration since deposition. We reconstructed the dynamic changes of S 26 Mg values of basin waters based on Mg isotope compositions in dolomite leachates. According to our findings, the S 26 Mg values of the basin waters were remarkably high (about 0.38 +/- 0.05%o) at the onset of evaporation, indicating a significant Mg sink of the massive dolomitization. The S 26 Mg of brine in the basin then changed towards the value of coeval seawater (about-0.32 +/- 0.05%o) starting with the deposition of evaporites. Concurrently, 87 Sr/ 86 Sr ratios of dolomites shift from radiogenic values towards contemporaneous seawater composition. Our results demonstrate that the evaporite basin was not strictly restricted, and water exchange with the open ocean never ceased. Modeling calculation reveals that, even when the seawater exchange rate is far below the average ocean circulation, S 26 Mg of brine in the basin will reach the value of open ocean within 1 Myr, completely removing the influence of early dolomitization. We suggested that massive marine evaporite sequences have the potential to record the long-term evolution of seawater Mg isotopes.

Chloride is the most important anion in ore-forming hydrothermal fluids, and chlorine isotopes are therefore potentially sensitive tracers of the origin and evolution of ore-forming fluids. However, they remain a relatively under-utilized tool in ore deposit geochemistry due to the lack of knowledge of chlorine isotope fractionation during ore-forming processes. Using first-principles density functional theory, this study estimates chlorine isotope fractionation factors for various ore-forming processes. All metal-Cl complexes in hydrothermal fluids are enriched in Cl-37 compared to chloride ions, but the change in delta Cl-37 of the fluid trapped in ore minerals is small after the destabilization of metal-chloride complexes during ore mineral deposition. In the precipitation of evaporite minerals from brine, the sequence of enrichment of the heavy Cl isotope (Cl-37) is halite > carnallite > aqueous chloride > kainite > sylvite > bischofite. The results of this study agree with the experimental observations that progressive precipitation of halite from brine lowers the delta Cl-37 value of the residual fluid until the formation of K-Mg chlorides. In low-temperature deposits, delta Cl-37 values for fluid inclusions and minerals reflect those of the hydrothermal fluid provenance and the mixing of this fluids with saltwater or basinal brines. In high-temperature magmatic-hydrothermal ore deposits that undergo liquid-vapour phase separation, chlorine isotopes fractionate among phases of silicate melt, vapour and chloride-rich liquid. The considerable range in delta Cl-37 in fluid inclusions may also reflect fluid mixing and hydrothermal alteration. At ambient temperature, the delta Cl-37 values may reflect evaporative processes and, in the case of chemical weathering of metallic mineral deposits, may record the supergene enrichment of the metals. This study highlights the use of chlorine isotopes as a new tool for interpreting ore-forming processes.

Newly analysed major and trace element compositions of the Kanawa shales deposited in the Pindiga Formation reveal the provenance history and depositional conditions during the late Cenomanian - early Turonian time in the Gongola Sub-Basin of the Upper Benue Trough. We conclude that the source of the siliciclastic detritus in the Kanawa shales of the Pindiga Formation are igneous rocks of predominantly intermediate chemistry with a temporal trend towards more felsic compositions. The source rocks presumably underwent intense chemical weathering with CIA, PIA and CIW values up to 89.51, 97.30, and 97.56, respectively, which led to enhanced shale deposition in the Gongola Sub-basin. Obtained relatively high Fe/Mn and Zr/Rb ratios in the Kanawa shales suggest deposition under relatively strong hydrodynamic conditions in presumably shallow water. Additionally, integrated bivariate plots of Ga/Rb vs. K2O/Al2O3, SiO2 vs. Al2O3+K2O + Na2O with the ratios of Sr/Ba, Rb/K, MgO/Al2O3x100, V/Cr, V/(V + Ni) V/(V + Cr), P/Ti and Ba/Ti indicate deposition under a warm, humid climate in dominantly anoxic and brackish water conditions under relatively low primary productivity conditions. This interpretation aligns with the global climate and redox conditions during the late Cenomanian - early Turonian times. The tectonic setting bivariate diagrams of K2O/Na2O vs. SiO2 and SiO2/Al2O3 vs. K2O/Na2O, as well as the high and low quartz multidimensional diagrams, reveal a rift basin in a passive margin depositional environment, confirming earlier works on the evolution of the Benue Trough.

Abstract Modern Fe‐Si oxyhydroxide deposits occur in global marine hydrothermal vent sites. Despite their role as biogenic substrates and potential ore resources, much remains unknown about their formation processes. Here, we apply analyses of major and trace elements as well as Sr‐Nd‐Pb‐Fe isotopes combined with 238U‐230Th dating to Fe‐Si oxyhydroxides obtained from several hydrothermal fields along the Southwest Indian Ridge. These mineralized oxyhydroxides primarily consist of poorly crystalline two‐line ferrihydrite and amorphous opal‐A, with lesser amounts of nontronite and birnessite. The ubiquitous and characteristic Fe‐rich ultrastructures in the oxyhydroxides directly indicate microbial activity. The 238U‐230Th dating constrains their crystallization ages from ca. 11,873 to 384 years old. The seawater‐like 87Sr/86Sr and varying 143Nd/144Nd ratios underline a high proportion of seawater mixed with hydrothermal fluids. The radiogenic Pb isotopic patterns suggest a primary derivation of Pb leached from substrate basalts and to a lesser extent Pb from seawater. Stable iron isotopic compositions for different oxyhydroxides display a remarkable range between −1.47 and 0.82‰, which were interpreted as reflecting the fractionation processes during the formation of the deposits under evolving depositional redox conditions. The partial oxidation of Fe(II) and the subsurface removal of isotopically heavy Fe oxyhydroxides are suggested to play a vital role in shifting the Fe isotopic signature toward more negative values. Given that these Fe‐Si oxyhydroxide deposits exhibit features similar to certain ancient iron formations (IFs), Fe isotope systematics of these deposits may hold significant potential for fingerprinting the biological Fe oxidation processes that drove IF deposition on early Earth.

Some of the earliest bio-sedimentary records of life on Earth are represented by microbial carbonates, which are also critical geochemical archives of ancient seawater chemistry and the environmental circumstances in which they precipitated. Reconstructing paleo-microbial environments on Earth and potentially other planets requires precise determination of the depositional ages of these materials. The (abiogenic) carbonate geochemistry communities can now use developments in in-situ laser ablation U-Pb dating using inductively coupled plasma mass spectrometry (LA-ICP-MS). Due to the effects of impurity mixing and diagenesis, microbial carbonates have received little geochronological study despite their broad relevance for understanding ancient seawater's environmental conditions and geochemical compositions. This study demonstrates using time-of-flight mass spectrometry (TOF-MS) to perform quick, quantitative elemental mapping before U-Pb spot dating to improve experiment success rates and data reliability and offers four practical application examples.

Stromatolitic carbonates of the Middle Miocene Oberpullendorf Basin (Austria) provide a great opportunity to study the evolution of microbial habitats under extreme environmental changes during the Badenian (Langhian and early Serravallian) Salinity Crisis. We here present the first geochemical data for Badenian stromatolites and show in a combined approach using major, trace element, and C - O isotope compositions obtained in individual stromatolitic carbonate laminae that short-term variations of palaeo-environmental conditions within the Oberpullendorf Basin coincide with individual microbialite morphologies. The studied carbonates were affected by both detrital contamination and post-depositional alteration pro-cesses to different degrees. While fluid-mobile elements show clear evidence for post-depositional alteration processes, the rare earth element and yttrium (REY) as well as bio-essential element (Fe, Mn, Co, Zn, Mo, W) compositions of the carbonates remained unaffected. Stromatolitic carbonates that are devoid of detrital contamination (< 300 ppm Al) show typical shale-normalized seawater-like rare earth element and yttrium (REYSN) patterns with positive LaSN, GdSN anomalies, super-chondritic Y/Ho ratios, and heavy over light REYSN enrichments in the lower stromatolite units. These features suggest an open ocean seawater influenced deposi-tional setting at the north-western margin of the Paratethys Sea. Stratigraphically upwards, pure stromatolitic carbonates show suppressed seawater-like REYSN signatures that argue for the development of a (semi)closed lagoon with restricted access to the open sea. Seawater-like REYSN patterns in the uppermost part resemble a reappearance of open marine environmental conditions. Interestingly, geochemical data of the upper part of the section contradict the ambient fossil record, showing the urge for future interdisciplinary approaches targeting the understanding and interplay of geochemistry, palaeontology, and geomicrobiology in modern and ancient microbial habitats. Enrichment factors of bio-essential trace elements that are either used as co-factors in met-alloenzymes or metal-activated enzymes in biochemical reactions can be directly linked to the reconstructed environmental conditions: Sufficient element availability is ensured during marine conditions in the lower and uppermost stratigraphic sections; in contrast, continuous decreasing element availability of these elements is directly related to the temporary development of a (semi)closed lagoon.