Cerium Anomalies Research Articles

Evidence for benthic oxygen production in Neoarchean lacustrine stromatolites

Abstract The evolution of oxygenic photosynthesis fundamentally altered the global environment, but the history of this metabolism prior to the Great Oxidation Event (GOE) at ca. 2.4 Ga remains unclear. Increasing evidence suggests that non-marine microbial mats served as localized “oxygen oases” for hundreds of millions of years before the GOE, though direct examination of redox proxies in Archean lacustrine microbial deposits remains relatively limited. We report spatially distinct patterns of positive and negative cerium (Ce) anomalies in lacustrine stromatolites from the 2.74 Ga Ventersdorp Supergroup (Hartbeesfontein Basin, South Africa), which indicate that dynamic redox conditions within ancient microbial communities were driven by oxygenic photosynthesis. Petrographic analyses and rare earth element signatures support a primary origin for Ce anomalies in stromatolite oxides. Oxides surrounding former bubbles entrained in mats (preserved as fenestrae) exhibit positive Ce anomalies, while oxides in stromatolite laminae typically contain strong negative Ce anomalies. The spatial patterns of Ce anomalies in Ventersdorp stromatolites are most parsimoniously explained by localized Ce oxidation and scavenging around oxygen bubbles produced by photosynthesis in microbial mats. Our new data from Ventersdorp stromatolites supports the presence of oxygenic photosynthesis ~300 m.y. before the GOE, and add to the growing evidence for early oxygen oases in Archean non-marine deposits.

Rare Earth Element Geochemistry of Late Cenozoic Island Carbonates in the South China Sea

Marine carbonates, precipitating from seawater through inorganic geochemical and biogeochemical processes, are considered to have recorded the seawater geochemical signatures reflecting the marine environmental conditions during their formation. However, they are susceptible to post-depositional diagenetic alteration. The redox conditions and chemical composition of the diagenetic fluid may be different from those of the overlying seawater. Therefore, assessing whether carbonate rocks that have experienced variable diagenesis could still preserve primary seawater geochemistry is a prerequisite before inferring ancient marine environments using geochemical tracers such as the cerium anomaly (Ce/Ce*). Here, we investigate rare earth elements plus yttrium (REY) geochemical features of reefal carbonates from the XK-1 core in the Xisha Islands of the South China Sea. We aim to evaluate whether island carbonates have the potential to preserve reliable primary seawater REY geochemical characteristics after experiencing meteoric diagenesis, marine burial diagenesis, or dolomitization. The results show that even after variable diagenetic alteration, all carbonate samples exhibit seawater-like REY patterns, which are characterized by negative Ce anomalies (Ce/Ce* < 1), distinctively high Y/Ho ratios (>44), and uniform depletion of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) ((Pr/Yb)N < 1). This suggests that the original seawater REY signatures are retained, regardless of varying degrees of changes in the mineralogical composition, diagenetic fluid composition, and redox state. The unmodifiable REY characteristics in carbonates during diagenesis can be attributed to three aspects: (1) during meteoric diagenesis, the low REY content of meteoric fluids and the short-term reactions between fluids and carbonates make it difficult to significantly alter the REY composition of carbonates; (2) during marine burial diagenesis, the ubiquitous cementation creates a relatively closed environment that facilitates the inheritance of REY signatures from primary carbonates; (3) during dolomitization, the dolomitizing fluids derived from penecontemporaneous seawater would not destroy but rather promote the preservation of the original seawater REY signatures in dolostones. The Ce/Ce* variations indicate that the Xisha carbonates have been deposited under constantly oxic conditions since the Neogene, consistent with paleontological and redox-sensitive element geochemical evidence.

Early Diagenetic Dolomite as a Potential Archive of Paleo‐redox Fluctuations in an Early Tonian Marine Basin?

AbstractThe Tonian Period witnessed important environmental changes and critical evolutionary innovations. Published iron speciation data suggest a global redox transition of mid‐depth seawaters from euxinic to ferruginous in early Tonian, but details of this transition remain unknown. This study explores Tonian stromatolitic carbonates as a possible archive of paleoenvironmental changes, through the investigation of dolomitic limestones and dolostones associated with stromatolites of the Weiji Formation in the Huaibei region of North China. Three types of dolomitization are recognized on the basis of petrographic and geochemical data. Type I and II dolomitization resulted in dolomitic limestones characterized by LREE depletions, MREE enrichments, positive yttrium anomalies, and a lack of europium anomalies, indicating early diagenetic dolomitization, possibly in the iron reduction zone and under the influence of bottom seawater. The lack of cerium anomalies in these carbonates suggests anoxia in shallow marine environments. The coexistence of ferroan/non‐ferroan dolomite crystals and overgrowth bands is interpreted as possible evidence for rapid fluctuations between iron‐rich and iron‐depleted conditions in pore‐waters or seawaters. In contrast, type III dolomitization resulted in pervasively dolomitized stromatolitic carbonates and likely represents late diagenetic processes. This study highlights the potential of early diagenetic dolomite as an archive for paleoenvironmental investigations.

Rare Earth Elements in Soils of the Central Forest State Nature Biosphere Reserve

The total content of rare earth elements (REEs) and the content of their oxalate-soluble fraction were studied in four soil profiles (Gleyic Umbrisols, Albic Gleyic Folic Retisols (Raptic), Dystric Albic Retisol) of the Central Forest Nature Reserve. The REE content was normalized to a local standard (the Russian Platform clay), and the cerium and europium anomalies were quantified [8]. In addition to the initial redistribution of REEs during the formation of rocks, REEs in the studied soils undergo geochemical fractionation, which results in the depletion of soil horizons of heavy lanthanides and europium. The negative europium anomaly is maximal in eluvial horizons and is weaker or absent in organic horizons. The oxalate-soluble REE fraction affects the redistribution of heavy lanthanides and cerium in the studied soils, and also results in the development of a positive cerium anomaly in some horizons.

Regolith-hosted rare earth exploration in the Chilean Coastal Range of the Central Andes

Regolith-hosted rare earth element deposits (RH-REE) are well-known in Asia, especially China, and are becoming increasingly important with the growing global demand for such metals. Ore formation controls of these deposits in Asia, which include sub-tropical to temperate climate, and mainly calc-alkaline granites with variable HREE and LREE bearing-minerals, are used to search for compatible exploration targets. However, the poor understanding of parameters controlling the formation of RH-REE mineralization outside China, e.g., Chile, might be hindering exploration efforts world-wide. This work addresses RH-REE mineralization in the Nahuelbuta Range, the local name of the Chilean Coastal Range between the Biobío and Araucanía regions. The studied prospect is associated with late Carboniferous–early Permian I-type biotite tonalite and amphibole-biotite tonalite. Mineralogical analysis (TIMA-X) indicates that allanite and monazite are the main REE-bearing minerals in the host rocks. The regolith in the study area is up to 60 m deep and contains an exchangeable-REE concentration up to 2000 ppm, as determined by portable X-ray fluorescence (pXRF) and ICP-MS. The mineralization is divided into two types: 1) Preserved regoliths (e.g. drillhole RLS-09) where mineralization is HREE-dominant (56% of total REE, (La/Yb)N ~2) and is covered by a leached upper pedolith; and 2) partially eroded regoliths (e.g. drillhole RLS-07) where mineralization is LREE-dominant (58% of total REE, (La/Yb)N ~4) and exposed. A morphometric analysis (SCM and swath profiles) and two 3D models (geomorphic and geochemical) indicate the current preservation of pediments and ore. Analyses of the exchangeable REE-fraction show that pedogenesis, leading to the deposit, is associated with REE-fractionation processes, most notably: a positive cerium anomaly (Ce/Ce*) in the upper pedolith and preferential adsorption of LREE at shallower depths, revealed by decreasing (La/Yb)N ratio with depth. We propose a mineralization model where the advance of the pedolith/saprolith horizon during progressive weathering and the breakdown of allanite and monazite was responsible for the neoformation of RH-REE clays at the base of the pedolith. Additionally, the landscape evolution of the Nahuelbuta Range controlled the preservation of ore bodies, which are either cropping out or hidden underneath the upper pedolith. A good understanding of the interplay between tonalite regolith development controls and regolith preservation during landscape evolution provide a strategy for REE exploration in the region. Moreover, the search for pediplains on suitable lithologies, mapping of regolith profiles, and their analysis via pXRF in the field by quantifying yttrium, appears to be a suitable strategy for early exploration of RH-REE deposits in the Chilean Coastal Range.

Controlling factors and impacts of river-borne neodymium isotope signatures and rare earth element concentrations supplied to the Canadian Arctic Archipelago

Determining the factors controlling the neodymium (Nd) isotopic compositions (expressed as εNd) and rare earth element (REE) concentrations of rivers is imperative to improve our understanding of the distribution of these water mass tracers in the ocean. Here we present the first measurements of <0.45 μm-filtrate REE concentrations (i.e., the concentration of truly dissolved, nano-particulate and colloidal REEs passing through a 0.45 μm filter, hereafter referred to as dissolved) and εNd in rivers draining into the Canadian Arctic Archipelago (CAA). Results show a large variation in both REE concentrations (Nd=[8;37,260] pmol/kg) and εNd ([−32.1;−15.1]) in the thirteen rivers sampled. Dissolved REE concentrations increase with increasing fractions of metamorphic rocks and ice cover in the rivers' watersheds, while εNd and PAAS-normalized REE patterns are inherited from the watershed's bedrock lithology. The four rivers draining watersheds composed of Precambrian metamorphic rocks exhibit the highest REE concentrations, the least radiogenic εNd, and shale-normalized patterns enriched in light REEs. The remaining nine rivers drain mainly or exclusively sedimentary bedrock and have lower REE concentrations, more radiogenic εNd and generally show heavy REE enrichment, with variable negative cerium (Ce) anomalies. The presence of ice sheets in the drainage area, and the ionic strength, pH, and dissolved organic carbon concentration of river water are the main factors determining riverine dissolved REE concentrations by controlling the formation and stabilization of colloids. We estimate a flow of dissolved Nd transported by Arctic rivers discharging into the CAA equivalent to the Nd flux of seawater entering the CAA. Depending on the percentage of Nd removal during mixing with seawater, the impact of river water on the εNd of seawater exiting the CAA could be modest (if 90% removal) or more significant (if ≪90% removal). This riverine contribution could potentially impart a climate-sensitive εNd signature to the seawater reaching Baffin Bay, with possible implications for the use of εNd in paleoceanographic reconstructions of water mass distribution.

Diagenetic Barite-Pyrite-Wurtzite Formation and Redox Signatures in Triassic Mudstone, Brooks Range, Northern Alaska

Mineralogical and geochemical studies of interbedded black and gray mudstones in the Triassic part of the Triassic-Jurassic Otuk Formation (northern Alaska) document locally abundant barite and pyrite plus diverse redox signatures. These strata, deposited in an outer shelf setting at paleolatitudes of ~45 to 60°N, show widespread sedimentological evidence for bioturbation. Barite occurs preferentially in black mudstones (TOC = 0.93–6.46 wt%), forming displacive euhedral crystals with pyrite inclusions and rims, and late albite inclusions or intergrowths. Pyrite also occurs as small (3–20 μm) framboids, discontinuous laminae, euhedral and anhedral crystals, and replacements of barite and fossils (mainly radiolarians). Paragenetically early wurtzite is present as clusters of very small (1–3 μm) aggregates of radiating crystals 0.5 to 1.0 μm long with cores of organic matter that overgrow framboidal pyrite; later wurtzite forms 10- to 30-μm bladed crystals. Equant grains (3–30 μm) and small (20 μm) angular clusters of zinc sulfide that include <1-μm-long, comb-like structures are sphalerite or wurtzite, or both. Minor siderite forms euhedral crystals intergrown with albite that enclose wurtzite and barite. Illite shows intergrowths with sphalerite; rare K-feldspar is intergrown with barite. Formation of these minerals and assemblages is attributed to early diagenetic processes.Whole-rock geochemical data for 15 samples show large ranges in redox proxies including Post Archean Average Shale (PAAS)-normalized enrichment factors (EFs) for V, U, Mo, and Re, and Al-normalized ratios for V, U, and Mo. Results for most black mudstones, with or without abundant barite and/or pyrite, suggest deposition within an oxygen minimum zone. Cerium anomalies, PAAS-normalized and calculated on a detrital-free basis, range widely from 0.49 to 0.96 and may reflect diagenetic overprinting by Ce-depleted fluids. Considering data for both black and gray mudstones, the overall geochemical pattern together with evidence from pyrite framboid sizes suggest that redox conditions fluctuated greatly from euxinic to oxic, like the redox profiles reported for modern shelf sediments offshore Peru and Namibia. The euxinic redox signatures in some Otuk black mudstones may correlate with widespread Early to Middle Triassic ocean anoxic events proposed for other regions.Calculations of median EFs for trace elements in Otuk black mudstones reveal both enrichments and depletions. Normalizations to the median composition of the three least-mineralized black mudstones show that barite- and/or pyrite-rich samples display large (>50%) positive changes for Li (+80.4%), V (+75.6%), Sr (+75.9%), Ba (+790%), Cu (+92.1%), Ni (+169%), Ag (+156%), Au (+3091%), As (+109%), Sb (+476%), and Se (+205%); Zn shows a moderate positive change of +42.1%. Moderate negative changes are evident only for Ge (−47.2%) and W (−30.6%). The local enrichments may reflect one or more factors including redox variations in bottom waters and pore fluids, element mobility during diagenesis, and selective fractionation into minerals such as barite, pyrite, and wurtzite. Anomalously low U/Al and UEF values, compared to those for other modern and ancient organic-rich sediments and sedimentary rocks, are attributed to increased solubility and loss of U during bioturbation-related oxygenation in the subsurface.Physicochemical constraints on barite, pyrite, and wurtzite formation are informed by use of a pH-fO2 plot constructed at 10 °C. Based on paragenetic evidence for multistage deposition of these three minerals, together with the presence of illite intergrown with ZnS and K-feldspar with barite, proposed diagenetic trends involve an increase in pH and fO2 related to the ingress of sulfate-rich pore fluids during bioturbation, followed by a return to lower then higher pH and fO2 conditions linked to carbon, sulfur, barium, and iron cycling during diagenesis. Labile Ba of marine pelagic origin was mobilized from organic-rich sediment upward to the sulfate-methane transition zone where barite precipitated during the interaction of reduced Ba- and CH4-rich fluids with sulfate-bearing pore fluids. The formation of paragenetically early wurtzite (ZnS) crystals, as well as locally high EF values for Cu, Ni, Ag, and Au, is attributed to metal enrichment of pore fluids, with sources being derived in part from water-column deposition from hydrothermal plumes related to coeval Triassic seafloor vent systems including a volcanogenic massive sulfide deposit in British Columbia and the Wrangellia Large Igneous Province in Alaska.

Analyses of depositional environments of the Marcellus formation in New York using biomarker and trace metal proxies

The purpose of this study was to observe how the composition of organic matter (OM) and the extent of anoxia during deposition within the Marcellus Formation in New York varied by distance from the sediment source in eastern New York. Lipid biomarkers (n-alkanes and fatty acids) in the extractable organic component (bitumen) of the shale samples were analyzed, and proxies such as the average chain length (ACL), aquatic to terrestrial ratio (ATR) and carbon preference index (CPI) of n-alkanes were calculated. Fatty acids were relatively non-abundant due to the age of the shale bed, but n-alkane distributions revealed that the primary component of the OM was terrigenous plants. The presence of shorter n-alkane chain lengths in the samples indicated that there was also a minor component of phytoplankton and algal (marine) sourced OM. Whole rock analyses were also conducted, and cerium anomalies were calculated as a proxy for anoxia. All samples had a negative anomaly value, indicating anoxic conditions during deposition. Two samples, however, contained values close to zero and thus were determined to have suboxic conditions. Anoxia and total organic matter (TOM) did not show any spatial trends across the basin, which may be caused by varying depths within the basin during deposition. A correlation between nickel concentrations and TOM was observed and indicates that algae was the primary source of the marine OM, which supports the lipid biomarker analysis. It was determined that the kerogen type of the Marcellus Formation in New York State is type III, consistent with a methane-forming shale bed.

Rare Earth Elements and Yttrium (REE+Y) patterns in recent Anadara brasiliana shells from Playa Norte, Barra de Cazones (Veracruz, Mexico): Evidence of anthropogenic contamination linked to river output?

We measured and interpreted Rare Earth Element and Yttrium concentrations of ten recent Anadara brasiliana (Lamarck) shells from Playa Norte (Barra de Cazones, Veracruz, Mexico). We aimed to identify the source of these elements, along with addressing some environmental conditions attributable to them. REE + Y patterns show modern seawater signals, particularly positive La/La* and Y/Y* anomalies as well as marked depletions of light REEs and middle REEs in comparison to heavy REEs) (x̄ PrSN/YbSN = 0.127, x̄ NdSN/YbSN = 0.16 and x̄ DySN/YbSN = 0.444, respectively). The cause for La/La* anomalies can also be explained by means of anthropogenic contamination, yet this question remains unresolved. Pronounced negative cerium anomalies were also recognised [(Ce/Ce*) = 0.262], which point to oxidising conditions. Y/Ho ratios (ranging from 35.85 to 64.91; average = 44.68) are in part superchondritic (range 44–74). Values lower than these (35.85–39.83) may account for the closeness to the nearby-flowing Cazones River, which runs through many important cities and populations in Veracruz, such as Poza Rica, Coatzintla and Barra de Cazones.In addition to the referred REE anomalies, we report notable positive Sm/Sm* and Gd/Gd* anomalies. The former may be an evidence of contamination caused by petroleum industry in this region, which is well known for its hydrocarbon production. For its part, the latter may mirror river contamination by means of contrast reagents poured into freshwater and ultimately into the ocean.This study shows the first results of a long-term project, which aims to elucidate the REE + Y dynamics in the Gulf of Mexico coastal system at the state of Veracruz, eastern Mexico.

Anoxygenic photosynthesis linked to Neoarchean iron formations in Carajás (Brazil).

Microbial activity is often invoked as a direct or indirect contributor to the precipitation of ancient chemical sedimentary rocks such as Precambrian iron formations (IFs). Determining a specific metabolic pathway from the geological record remains a challenge, however, due to a lack of constraints on the initial conditions and microbially induced redox reactions involved in the formation of iron oxides. Thus, there is ongoing debate concerning the role of photoferrotrophy, that is the process by which inorganic carbon is fixed into organic matter using light as an energy source and Fe(II) as an electron donor, in the deposition of IFs. Here, we examine ~2.74-Ga-old Neoarchean IFs and associated carbonates from the Carajás Mineral Province, Brazil, to reconstruct redox conditions and to infer the oxidizing mechanism that allowed one of the world's largest iron deposits to form. The absence of cerium (Ce) anomalies reveals that conditions were pervasively anoxic during IF deposition, while unprecedented europium (Eu) anomalies imply that Fe was supplied by intense hydrothermal activity. A positive and homogeneous Fe isotopic signal in space and time in these IFs indicates a low degree of partial oxidation of Fe(II), which, combined with the presence of 13 C-depleted organic matter, points to a photoautotrophic metabolic driver. Collectively, our results argue in favor of reducing conditions during IF deposition and suggest anoxygenic photosynthesis as the most plausible mechanism responsible for Fe oxidation in the Carajás Basin.

Regional fault-controlled shallow dolomitization of the Middle Cambrian Cathedral Formation by hydrothermal fluids fluxed through a basal clastic aquifer

AbstractThis study evaluates examples of hydrothermal dolomitization in the Middle Cambrian Cathedral Formation of the Western Canadian Sedimentary Basin. Kilometer-scale dolomite bodies within the Cathedral Formation carbonate platform are composed of replacement dolomite (RD), with saddle dolomite-cemented (SDC) breccias occurring along faults. These are overlain by the Stephen Formation (Burgess Shale equivalent) shale. RD is crosscut by low-amplitude stylolites cemented by SDC, indicating that dolomitization occurred at very shallow depths (&amp;lt;1 km) during the Middle Cambrian. Clumped isotope data from RD and SDC indicate that dolomitizing fluid temperatures were &amp;gt;230 °C, which demonstrates that dolomitization occurred from hydrothermal fluids. Assuming a geothermal gradient of 40 °C/km, due to rift-related basin extension, fluids likely convected along faults that extended to ∼6 km depth. The negative cerium anomalies of RD indicate that seawater was involved in the earliest phases of replacement dolomitization. 84Kr/36Ar and 132Xe/36Ar data are consistent with serpentinite-derived fluids, which became more dominant during later phases of replacement dolomitization/SDC precipitation. The elevated 87Sr/86Sr of dolomite phases, and its co-occurrence with authigenic quartz and albite, likely reflects fluid interaction with K-feldspar in the underlying Gog Group before ascending faults to regionally dolomitize the Cathedral Formation. In summary, these results demonstrate the important role of a basal clastic aquifer in regional-scale fluid circulation during hydrothermal dolomitization. Furthermore, the presence of the Stephen Formation shale above the platform facilitated the build-up of fluid pressure during the final phase of dolomitization, leading to the formation of saddle dolomite-cemented breccias at much shallower depths than previously realized.

Dramatic changes in the carbonate-hosted barium isotopic compositions in the Ediacaran Yangtze Platform

Barium (Ba) isotopic compositions (δ138Ba) have been advocated as a novel paleo-productivity proxy, because of the strong control of biological productivity on the distribution of the Ba isotopic composition in the modern oxic oceans. However, the ocean was dominantly anoxic throughout the majority of the Earth’s history, and the biogeochemical cycle of Ba in anoxic oceans may have been different from the case in oxic ones. It is not clear whether δ138Ba can be used to trace biological productivity in ancient oceans. The Ediacaran Period was marked by a dramatic evolution in the climate, atmospheric and oceanic oxygen level, marine carbon cycle, and biosphere, which have been documented in multiple successions of the Ediacaran sedimentary rocks. This provides an opportunity to investigate variations in biogeochemical Ba cycle during the crucial period. In this study, we find large δ138Ba variations in carbonate rocks from the Ediacaran Doushantuo and Dengying Formations in the Yangtze Gorges area, South China. We suggest that detrital contamination and potentially post-depositional alteration cannot explain the δ138Ba variations. The post-Marinoan carbonate rocks record generally negative carbonate-hosted δ138Ba (δ138Bacarb) around −0.3‰, reflecting the existence of a replete and homogeneous reservoir of light Ba isotopes in the postglacial ocean. Although the Shuram/Wonoka correlative carbonate rocks have documented a prominent negative δ13Ccarb excursion, the δ138Bacarb merely shows minor fluctuations between 0.00‰ and 0.35‰, possibly due to local anoxia/euxinia inhibiting the biogeochemical cycle of Ba. The late-Ediacaran carbonate rocks show a gradual increase in δ138Bacarb systematically with enhanced negative cerium anomalies, suggesting the establishment of the modern-ocean-like cycle of Ba in shallow waters of the Yangtze Platform, which may result from the replacement of cyanobacteria by eukaryotic macroalgae as the dominant primary producer. We suggest that the biogeochemical Ba cycle in ancient marine system is strongly controlled by local redox conditions, and thus Ba isotopic compositions could be utilized to trace variations in paleo-productivity, especially in oxic environments.

Magmatic volatiles episodically flush oceanic hydrothermal systems as recorded by zoned epidote

Circulation of seawater at oceanic spreading centers extracts heat, drives rock alteration, and transports leached metals to shallower levels of the crust, where they may precipitate and form ore deposits. Crystallization of the lower crust, may exsolve and introduce magmatic volatiles into the seawater-dominant system. However, the role of magmatic volatiles added to the hydrothermal system, including pathways of these fluids are lesser known. Here we present coupled in-situ strontium isotope and rare earth element data of distinct domains in epidote, a common hydrothermal mineral throughout the Troodos ophiolite, to track magmatic fluid input and flow. Epidote crystal domains characterize three distinct strontium isotope-rare earth element signatures—suggesting sequential growth from magma-derived fluids (0.704, negative europium anomalies), rock-buffered fluids (0.7055, positive europium anomalies) and seawater-derived fluids (0.7065, negative cerium anomalies). Epidote records episodic fluxing of magmatic fluids from plagiogranites, through epidosites in the upflow zone and into metal ore deposits.

REE+Y patterns and positive europium anomalies recorded in limestones from zone XX of the Tlayúa Quarry Konservat-Lagerstätte (Tlayúa Formation, Lower Cretaceous; Tepexi de Rodríguez, Mexico)

Major and trace element concentrations of thirteen limestone samples from zone XX of the Tlayua Quarry Fossillagerstatte were analysed with the aim of elucidating the source of rare earth elements plus yttrium (REE+Y) as well as some palaeoenvironmental conditions attributable to them. The concentrations of cortex-derived elements/element oxides (SiO2, Al2O3, Zr), as well as of phosphorus, show no significant contamination by means of terrigenous material and/or phosphates, which means that the REE+Y concentrations can be interpretable. REE+Y patterns display partly typical modern seawater signals, particularly positive La and Y anomalies, mostly superchondritic Y/Ho ratios (between 42.34 and 56.38) and moderate depletions of light rare earth elements (LREE) and middle rare earth elements (MREE) in comparison to heavy rare earth elements (HREE) (x PrSN/YbSN = 0.12, x NdSN/YbSN = 0.14 and x DySN/YbSN = 0.42, respectively). Negative cerium anomalies could also be identified [(Ce/Ce*SN) = 0.73], which indicate oxidising conditions. In addition, positive Eu anomalies [(Eu/Eu*SN) = 2.53] were detected. Since no convincing evidence for diagenesis, hydrothermal or aeolian input exists, the origin of these anomalies can, to some extent, be attributable to the influence of the dissolution of plagioclases in the water column, which can be inferred from the bivariate correlation of (Eu/Eu*)SN and Na/Al. Despite depicting modern seawater-like signals, an important difference between the REE+Y patterns from zone XX of the Tlayua Quarry and modern seawater REE+Y patterns can be determined. For instance, modern seawater does not display significant europium anomalies. Nevertheless, REE+Y patterns from zone XX of the Tlayua Formation can altogether be reliable palaeoenvironmental proxies for the Cretaceous. The inferred palaeoenvironment for zone XX from these proxies is represented by an open-marine setting, showing oxidising conditions, evidence of plagioclase dissolution and overall negligible terrigenous influence. The palaeoecological interpretation from these patterns does not exclude, however, the probability of monsoonal or tidally influenced sedimentation in other sediments from zone XX or from other zones in Tlayua, as previous works suggest.

Weathering, alteration and reconstructing Earth's oxygenation.

Deciphering the role-if any-that free oxygen levels played in controlling the timing and tempo of the radiation of complex life is one of the most fundamental questions in Earth and life sciences. Accurately reconstructing Earth's redox history is an essential part of tackling this question. Over the past few decades, there has been a proliferation of research employing geochemical redox proxies in an effort to tell the story of Earth's oxygenation. However, many of these studies, even those considering the same geochemical proxy systems, have led to conflicting interpretations of the timing and intensity of oxygenation events. There are two potential explanations for conflicting redox reconstructions: (i) that free oxygen levels were incredibly dynamic in both time and space or (ii) that collectively, as a community-including the authors of this article-we have frequently studied rocks affected by secondary weathering and alteration (particularly secondary oxidation) while neglecting to address the impact of this alteration on the generated data. There are now multiple case studies that have documented previously overlooked secondary alteration, resolving some of the conflicting constrains regarding redox evolution. Here, an analysis of a large shale geochemistry database reveals significant differences in cerium (Ce) anomalies, a common palaeoredox proxy, between outcrop and drill core samples. This inconsistency provides support for the idea that geochemical data from altered samples are frequently published in the peer-reviewed literature. As individuals and a geochemical community, most of us have been slow to appreciate how pervasive the problem is but there are examples of other communities that have faced and met the challenges raised by such quality control crises. Further evidence of the high potential for alteration of deep-time geochemical samples, and recognition of the manner in which this may lead to spurious results and palaeoenvironmental interpretations, indicate that sample archiving, in publicly accessible collections needs to become a prerequisite for publication of new palaeoredox data. Finally, the geochemical community need to think about ways to implement additional quality control measures to increase the fidelity of palaeoredox proxy work.

Rare earth element cycling and reaction path modeling across the chemocline of the Pettaquamscutt River estuary, Rhode Island

Rare earth elements (REEs) were measured in the water column of the highly stratified and euxinic Upper Basin of the Pettaquamscutt River estuary (Rhode Island) to investigate their cycling and evolution across the chemocline. The euxinic bottom waters of the estuary exhibit total dissolved sulfide (i.e., ΣS-II) concentrations up to 4 mmol kg−1, which exceeds values of the Black Sea by nearly 10-fold and makes this system ideal for study of redox-sensitive elements. The REE concentrations decrease with depth in the oxic surface waters, before substantially increasing across the chemocline, corresponding to redox cycling of manganese (Mn) and iron (Fe). For example, the mean ± 1σ neodymium (Nd) concentration in the oxic surface waters is 511 ± 184 pmol kg−1, but increases by approximately 5-fold across the chemocline reaching 2476 pmol kg−1 at 7.4 m depth. Concurrently, Mn and Fe concentrations increase by 5- and 11-fold, respectively, within the chemocline relative to their oxic surface water values. The pore water within the Upper Basin sediments exhibits REE concentrations that are on average 130-fold greater than their values in the overlying waters, driving an upward diffusive REE flux into the euxinic bottom waters. An outstanding feature of the REEs in this study is the preservation of negative cerium (Ce) anomalies across the chemocline into the highly euxinic bottom waters. Likewise, our reaction path model shows that the mass balance of the local water sources is the major driver of the REE compositions of the water column. Primarily, the negative Ce anomaly in the euxinic bottom waters highlights the influence of the Ce depleted sediment pore water on the REE composition and fractionation patterns of the euxinic bottom waters. This study underscores the importance of local sources and indicates that negative Ce anomalies can persist in highly euxinic waters that evolve from Ce depleted sources, further demonstrating that negative Ce anomalies are not necessarily a good proxy of the redox conditions of aqueous environments.

Anomalous phase association of REE in ferromanganese crusts from Indian mid-oceanic ridges: Evidence for large scale dispersion of hydrothermal iron

Ferromanganese crusts collected at selected locations along the ~7000 km long stretch of Carlsberg, Central Indian and Southwest Indian ridges are studied for bulk geochemical composition and distribution of rare earths and yttrium in major geochemical phases; exchangeable/carbonate, Mn-oxide, Fe-oxyhydroxide and residual silicate. The crusts are dominated by vernadite with traces of quartz and calcite. A high concentrations of major (Fe, Mn), trace (Co, Ni, Cu & Zn) and rare earth elements indicates that these crusts are hydrogenous in nature. However, the negative or no cerium anomalies in the shale normalized REE pattern indicates that some of these crusts are of mixed type (hydrogenous and hydrothermal). Irrespective of their nature (mixed or hydrogenous), the phase distribution of REE-Y shows 70–83% of total REE-Y are associated with Fe-oxyhydroxide phase, whereas only about 17–30% are associated with Mn-oxide phase. Such phase-distribution is expected in mixed type crusts because it is a combination of hydrothermal (where 90% of the total REE-Y is associated with only Fe-oxyhydroxide phase) and hydrogenous processes (where 50% of the total REE-Y is associated with only Fe-oxyhydroxide phase). Conversely, the hydrogenous crusts also shows similar phase-distributions (70–83% association with Fe-oxyhydroxide phase) which is certainly different from typical hydrogenous crusts where both Fe-oxyhydroxide and Mn-oxide phases host equal amounts (50% each). The large scale dispersion of hydrothermal particulate iron along the Indian Ocean ridges is likely to contribute an additional Fe-bound REE-Y pool to these crusts. The positive yttrium anomaly in Mn-oxide phase of all these crusts substantiates the hydrothermal contribution.

Geology and Geochemistry of Sediments from Lewumeji and Idogun Wells, Eastern Dahomey Basin Southwestern Nigeria

Selected composited samples from Lewumeji (0-111m) and Idogun (0- 54m) Abeokuta Group, Eastern Dahomey Basin, were subjected to detailed lithologic and geochemical studies. The studies aimed at determining the lithological sequence, paleoenvironments of deposition, tectonic settings, provenance, weathering history and the classification of sediments. The core samples were subjected to lithological description and geochemical analysis were done following standard procedure through the use of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) through lithium metaborate fusion method.The lithologies from both wells consist of reddish to brown colour, rounded to sub rounded sandstone, brownish clay, and dark grey shale denoting possibly fluvial, brackish, lagoonal and marine environments. The sandstone revealed dominance of quartz which were classified as quartz arenites to sublitharenite and subarkoses, this suggests a recycled orogen source for the sandstones. Shale in the study area shows patches of CaCO 3 indicating the presence of biogenic materials. And the reddish brown colouration of clay suggests ferruginization. The geochemistry revealed low values for K 2 O/Al 2 O 3 ratio coupled with high average values of Plagioclase Index of Alteration (81.64% and 73.80%), Chemical Index of Alteration (79.22% and 71.52%) and Chemical Index of Weathering (82.41 % and 75.03%) for Idogun and Lewumeji wells respectively. This values indicate intense weathering condition. The plots of Log (Fe 2 O 3 /K 2 O) against Log (SiO 2 /Al 2 O 3 ) revealed sediments dominated by Fe - sand and Fe - shale. Also, Al 2 O 3 -(K 2 O+CaO+MgO)-(Fe 2 O 3 +MgO), (AKF) ternary plots reveals that the sediments are derived from continental environment. The Log (K 2 O/Na 2 O) against SiO 2 and the discriminant function plots indicate sediments deposited within the passive margin. Also, the Trace elements ternary plots of Th-Sc-Zr/10 and Th-Co-Zr/10 reveal deposition within Continental Island Arc and Passive Margin. The discriminant function plots characterized the sediments as Mafic Igneous rock and quartoze sedimentary provenances with minor input from Intermediate and felsic igneous provenances. The chondrite normalized REE plots show enrichment of Light REE over Heavy REE in the study area with negative Europium and Cerium anomalies greater than 1. This indicated an oxidising and a shallow marine environments. The REE pattern is similar to those for the Upper Continental Crust sediments (UCC). Conclusively, the study shows that the sediments in the study area have multiple provenances subjected to high weathering conditions and were deposited within an oxidizing and continental to shallow marine settings. Keywords: Dahomey Basin, lithostratigraphy, paleoenvironment, Provenance, Tectonic setting. DOI: 10.7176/JEES/10-5-03 Publication date: May 31 st 2020

Positive cerium anomalies imply pre-GOE redox stratification and manganese oxidation in Paleoproterozoic shallow marine environments

The Paleoproterozoic Koegas Subgroup (Transvaal Supergroup, South Africa) was deposited in the immediate prelude to the Great Oxidation Event (GOE), and can therefore shed light on oceanic paleoredox conditions just before atmospheric oxidation. Manganese enrichments of ~16 wt% in diagenetic kutnahorite horizons suggest that Mn2+ oxidation occurred, either by free O2 or by an ancient photosystem. Iron and molybdenum isotope trends also support the existence of a Mn4+ oxide sediment flux, suggesting that the Koegas basin may have been redox stratified. Evidence from detrital and authigenic pyrite with mass-independently fractionated sulfur isotopes, however, suggests that the atmosphere was devoid of oxygen. To resolve this contradiction, this paper presents new constraints on pathways of Mn2+ oxidation from field, petrographic, stable isotope, and rare earth element and yttrium (REYSN) analysis of stromatolitic carbonates from the upper Koegas Subgroup. Ferroan dolostones and limestones preserve marine REYSN arrays with positive CeSN anomalies. These differences are explained by a redox stratified basin, whereby Mn2+ and Ce3+ are oxidized at a redoxcline and Ce is adsorped onto sinking Mn oxide particles. Mn oxide particles and a negative Ce anomaly from the oxidized upper water column are transferred into carbonates accumulating above the redoxcline. Diagenetic fluids later reduce the Mn oxides to kutnahorite. Below the redoxcline, reduction of Mn oxides enriches carbonates in Mn and a positive Ce anomaly. This contribution adds evidence for the development of oxygen oases and redox-stratified basins before the GOE. Redox stratification was best developed during transgressions. During regressions, a deltaic system prograded into the Koegas Basin. High sedimentation rates likely allowed for preservation of detrital pyrite only in the deltaic sandstones, thus explaining the contradictory geochemical evidence. No previously unknown ancient photosystem of Mn oxidation is required to explain Mn oxidation.

A Re‐evaluation of the Plenus Cold Event, and the Links Between CO2, Temperature, and Seawater Chemistry During OAE 2

AbstractThe greenhouse world of the mid‐Cretaceous (~94 Ma) was punctuated by an episode of abrupt climatic upheaval: Oceanic Anoxic Event 2. High‐resolution climate records reveal considerable changes in temperature, carbon cycling, and ocean chemistry during this climatic perturbation. In particular, an interval of cooling has been detected in the English Chalk on the basis of an invasive boreal fauna and bulk oxygen‐isotope excursions registered during the early stages of Oceanic Anoxic Event 2—a phenomenon known as the Plenus Cold Event, which has tentatively been correlated with climatic shifts worldwide. Here we present new high‐resolution neodymium‐, carbon‐, and oxygen‐isotope data, as well as elemental chromium concentrations and cerium anomalies, from the English Chalk exposed at Dover, UK, which we evaluate in the context of &gt;400 records from across the globe. A negative carbon‐isotope excursion that correlates with the original “Plenus Cold Event” is consistently expressed worldwide, and CO2 proxy records, where available, indicate a rise and subsequent fall in CO2 over the Plenus interval. However, variability in the timing and expression of cooling at different sites suggests that, although sea‐surface paleotemperatures may reflect a response to global CO2 change, local processes likely played a dominant role at many sites. Variability in the timing and expression of changes in water mass character, and problems in determining the driver of observed proxy changes, suggest that no single simple mechanism can link the carbon cycle to oceanography during the Plenus interval and other factors including upwelling and circulation patterns were locally important. As such, it is proposed that the Plenus carbon‐isotope event is a more reliable stratigraphic marker to identify the Plenus interval, rather than any climatic shifts that may have been overprinted by local effects.