Seawater-like Rare Earth Element Research Articles

Rare earth element geochemistry of reef carbonates in the South China Sea since the Miocene: Insights into paleoclimatic significance

Seawater chemistry critically affects the functioning of ocean ecosystems. However, the evolution of the seawater chemistry in the South China Sea (SCS) since the Miocene is not fully resolved, and its links to tectonic and climate changes remain unclear. In-situ, shallow marine carbonates can provide insights into long-term changes in seawater chemistry and associated environmental processes. In this study, we conducted the rare earth element (REE) analysis of the NK-1 core in the southern SCS since the Miocene. The results reveal poor relationships between siliciclastic/diagenetic indicators (mineralogical, isotopic, and elemental data) and REE proxies, which demonstrate that the REE compositions are insensitive to non-carbonate contaminations and diagenesis. The results exhibit seawater-like REE patterns with LREE depletion, positive La anomaly, superchondritic Y/Ho ratio, and negative Ce anomaly, which demonstrate that this core has preserved a signal of the primary seawater REE composition. The Y/Ho ratios range from 45 to 80 and La anomalies range from 1.0 to 2.3 during ∼23–13 Ma, and they then decrease after ∼13 Ma, with the Y/Ho ratios ranging from 30 to 52 and La anomalies ranging from 0.6 to 1.5. Considering that seawater REE components are mainly sourced from terrigenous sources which exhibit lower La anomalies and Y/Ho ratios, our results indicate an enhanced terrigenous input since ∼13 Ma. We consider that the climate cooling and aridity since ∼13 Ma might be responsible for the phenomenon.

Major, trace element, and Nd isotopic compositions of banded iron formation and shales from the Sirsi shelf, Dharwar Craton, India: Implications for paleo-seawater chemistry, post-depositional alteration, and provenance

We report geochemical and Nd isotopic data for ca. 2.5 Ga old Banded Iron Formation (BIF) and associated clastic rocks which include red- and purple-coloured shales, mudstone, and sandstone from the Sirsi shelf of the Dharwar Craton. Based on lanthanide and non-lanthanide (yttrium) rare earth element (REE) concentration patterns, the Sirsi BIF samples are sub-divided into two groups. The group-1 BIF samples show low shale-normalized (SN) ratios of (La/Yb)SN (0.37–0.53) and (Pr/Yb)SN (0.29–0.34), and superchondritic Y/Ho (41–52); these samples also show insignificant negative Ce anomaly ((Ce/Ce*)SN = 0.83–1.22) compared to modern seawater and seawater-derived precipitates. In contrast, the group-2 BIF samples show strong light REE (LREE) enrichment with high (La/Yb)SN (1.73–18.48) and (Pr/Yb)SN (1.50–7.58), negative Ce anomaly ((Ce/Ce*)SN = 0.02–0.16), and mostly subchondritic Y/Ho (19–39). The group-1 BIF displays modern-day seawater-like REE patterns and their initial εNd at 2.5 Ga overlaps with contemporaneous global BIF, suggesting their precipitation from Archean seawater. The REE compositions of the group-2 Sirsi BIF cannot be explained by contribution of detritus and are best explained by alteration by a LREE-enriched and Ce-depleted fluid. The Sm–Nd isotopic compositions suggest that this alteration event took place approximately 0.5 Ga ago. The timing of the alteration event coincides with the Pan-African orogeny, which had regionally affected the Greater Dharwar Craton. Among the associated clastic rocks, red and purple shales show group-2-BIF-like REE signatures with negative Ce anomaly, and display high Chemical Index of Alteration (CIA) values (78–98), which suggest high degree of chemical alteration. The enrichment factors (calculated as [(Xelement/XTi)sample/(Xelement/XTi)reference-1] relative to the Archean Upper Continental Crust) of La, Pr, and Ce, in the Sirsi red shales further confirm post-depositional modifications by a LREE-enriched and Ce-depleted fluid, and indicate that negative Ce anomaly in ancient sedimentary rocks could be generated by post-depositional fluid overprint. Geochemical compositions of the Sirsi clastic rocks suggest a felsic provenance, while Nd isotopic compositions of selected shale samples indicate that these sediments were most likely derived from granitoids in the Goa-Dharwar Sector.

Elemental and isotopic geochemistry of carbonate rocks from the Pila Spi Formation (Middle–Late Eocene), Kurdistan Region, Northern Iraq: implication for depositional environment

Major, trace, rare earth elements (REE), and stable isotopic compositions in carbonate rocks of Pila Spi Formation were measured to constrain the source of rare earth elements and depositional environment. The low ΣREE content (average = 6.53 ppm, n = 14) indicates that the carbonates seawater-like REE patterns with (1) LREE depletion (average (Nd/Yb)n = 0.81), (2) HREE enrichment, (3) negative Ce anomaly (average = 0.91), and (4) superchondritic Y/Ho ratio (average = 30.07); slightly lower average value of Y/Ho ratio than the typical seawater value (~ 44–74) suggests modification of the seawater by input of fresh water. The (Nd/Yb)n and (Dy/Yb)n ratios of the Pila Spi carbonates have similarity with that of the Arabian sea carbonate. The low concentrations of U (0.35–1.0 ppm) suggest the deposition under oxygen-rich environment. This study indicates that the carbonates still preserve their original seawater-like REE pattern, provided that the contamination with terrigenous materials was small, and they serve as a seawater proxy. The Pila Spi carbonates have δ13C values ranged from − 6.17 to 0.76‰ PDB, and the δ18O values range between − 3.85 and 0.46‰ PDB. The positive correlation between δ13C and δ18O (r = 0.949, n = 14) values indicates the alteration of original isotopic compositions due to diagenesis. Finally, it is conceived that the Pila Spi carbonate was deposited in the lagoon environment with seawater invading and mixing of the continental (terrigenous) materials to the basin is viable.

Recognition of diagenetic contribution to the formation of limestone-marl alternations: A case study from Permian of South China

Abstract Limestone-marl alternations (LMAs) have been extensively used in studies of sedimentology, cyclostratigraphy, and paleoclimatology. However, it has long been debated whether LMAs are derived from primary sedimentation or from diagenetic alteration, and the markers to distinguish one source from the other remain ambiguous. In this study, we investigate widespread Permian LMAs from various depositional settings in South China using traditional petrographic, mineralogical, and major element (Al2O3 vs.TiO2) methods, as well as emerging trace and rare earth element (REE) analyses performed using solution-based and in situ techniques. The original depositional differences of the limestone and its coupled marl beds can be identified based on (1) the high volumes of siliciclastic minerals in the marls, (2) unremarkable differential compaction, (3) the recognizable signatures of the detrital admixture (i.e., the Al, Zr, and Th characteristics), and (4) flattened shale-normalized REE patterns. Diagenetic LMAs are characterized by the compaction of fossils, rare aragonite skeletons, and abundant diagenetic clays (sepiolite and talc) preserved in the marl beds. We also identified fully diagenetic LMAs originating from primary carbonate sedimentation based on two almost completely consistent seawater-like REE patterns with diagnostic positive La and negative Ce anomalies ((La/La*)SN = 1.64 ± 0.17, (Ce/Ce*)SN = 0.73 ± 0.05), superchondritic Y/Ho (47 ± 4), and depleted light REEs relative to high REEs ((Pr/Yb)SN = 0.39 ± 0.09). Diagenesis also dominates the formation of LMAs with limited terrigenous input in shallow-water carbonate depositional environments. It is likely that both the active metabolic processes in early diagenesis (especially aerobic sulfide oxidation) and the temporal palaeoceanographic conditions, e.g., high levels of dissolved oxygen and sulfate, play crucial roles in the genesis of the diagenetic LMAs in the Permian strata of South China.

Modern carbonate ooids preserve ambient aqueous REE signatures

Skeletal and non-skeletal components of marine sedimentary rocks have been analyzed for the purpose of reconstructing the rare earth element (REE) and yttrium (Y) compositions of paleo-seawater, but skeletal carbonates frequently have proven to be unreliable recorders of seawater chemistry. Here, we present a systematic multi-technique assessment of rare earth and other trace elements in ooid sands from the modern Great Bahama Bank (GBB–marine) and Great Salt Lake (GSL–continental) based on strong-acid (hydrofluoric and nitric) and weak-acid (acetic) digestions, as well as laser ablation (LA) of ooid cortices and nuclei. The results show that Bahamian ooid cortices possess shale-normalized REE + Y features nearly identical to those of shallow seawater, including limited contamination from siliciclastic REEs. An admixture of even 0.2% of detritally sourced material can modify the primary marine REE + Y patterns by, for example, increasing the light REE (LREE) content. Mean values of LA data for Bahamian ooid cortices exhibit similar REE + Y signatures to those produced by acetic acid digestion, but LA data are generally noisier, primarily as a result of low REE concentrations and the small volume of carbonate ablated in analysis. Screening out samples with ΣREE <0.9 ppm reveals more uniform, seawater-like REE + Y patterns in individual ooid cortices as well as high, uniform REE distribution coefficients (116 ± 21, from La to Lu) with respect to ambient seawater. Ooid laminae show no distinct alternation between oxic and anoxic pore fluid characteristics, suggesting that growing ooids primarily formed under oxic conditions in carbonate shoal settings, even when buried during their resting phase. Unlike Bahamian ooids, shale-normalized REE + Y patterns of GSL ooids digested in weak acid show slightly depleted LREE compositions that may deviate from ambient fluids owing to release of REEs from an included clay fraction based on comparison between digestion and LA methods. Multiple ablation spots within the same cortex allowed direct comparison of more and less contaminated portions of a single ooid. Nearly uniform positive Ce anomalies may be related to strongly alkaline water conditions. Individual ablation spots that are partially overprinted by siliciclastic-derived REEs contain variable Zr contents (from 0.096 to ~4 ppm) and flattened shale-normalized LREE patterns. The least-contaminated GSL ooid cortex yields an LREE-depleted REE + Y distribution. Normalization of the REE + Y distributions of least-contaminated GSL ooids using reliable GBB ooid distributions (i.e., with ΣREE > 0.9 ppm) returns a flat pattern, suggesting similar degrees of LREE depletion controlled by carbonate complexation under similar aqueous alkalinity conditions in Great Salt Lake and Bahamian waters. In summary, ooids can be a reliable proxy for REE + Y characteristics of ambient surficial waters when adopting suitable analytical methods, including laser ablation, that allow the identification and isolation of a contamination signal from siliciclastic detritus.

Diagenetic evolution of the upper Devonian phosphorites, Alborz Mountain Range, northern Iran

The upper Devonian, mixed siliciclastic‑carbonate stratigraphic succession of the Geirud Formation in the Alborz Mountain Range, north of Iran, hosts numerous phosphorites and phosphatic rocks. These phosphorites, which represent one of the most globally important deposits in the Devonian, have gained little attention regarding their genesis and diagenetic evolution. This study focuses mainly on the post-depositional changes that occurred during shallow to deep burial diagenetic evolution and subaerial weathering of the Geirud phosphorites. The phosphorites consist of phosphatic massive mudclasts, detritus-rich lithoclasts, bioclasts and bones formed by several stages of phosphogenesis and erosion, reworking, winnowing and condensation associated with Famennian sea level changes. The phosphatic clasts consist of early diagenetic, poorly crystalline, carbonate fluorapatite (CFA) and pyrite cementing detrital quartz, illite, muscovite, rutile and zircon. Phosphatic clasts are intensely compacted with sutured grain contacts, along which dissolution and reprecipitation of chemically-refined, well crystalline CFA with a bright blue cathodoluminescence and illite occurred during deep burial diagenesis. Similarly, quartz shows deformation planar features; overgrowth cement was developed as a result of pressure dissolution in phosphatic sandstones, followed by precipitation of ferroan dolomite, ankerite and chlorite in secondary porosity. Subaerial weathering led to oxidation of pyrite and generation of acidic pH condition, under which Ca and P from phosphates, Fe and S from pyrite, Fe, Ca, Mg and Mn from ferroan carbonates and Al and Si from aluminosilicates were released to form goethite, psilomelane, calcite, gypsum and crandallite.The PAAS-normalized rare earth elements (REEs) show bell-shaped patterns with middle (REEs) enrichment, positive Eu anomaly and a weak negative Ce anomaly. This implies recrystallization of CFA in phosphorites and phosphatic rocks during deep burial diagenesis, deviating from the modern seawater-like REE patterns. In this regard, deep burial diagenesis affecting the upper Devonian Geirud phosphorites is possibly a common process in the Proterozoic-Paleozoic phosphorites in northern Iran and elsewhere in the world.

Continental shelves as potential resource of rare earth elements

The results of this study allow the reassessment of the rare earth elements (REE) external cycle. Indeed, the river input to the oceans has relatively flat REE patterns without cerium (Ce) anomalies, whereas oceanic REE patterns exhibit strong negative Ce anomalies and heavy REE enrichment. Indeed, the processes at the origin of seawater REE patterns are commonly thought to occur within the ocean masses themselves. However, the results from the present study illustrate that seawater-like REE patterns already occur in the truly dissolved pool of river input. This leads us to favor a partial or complete removal of the colloidal REE pool during estuarine mixing by coagulation, as previously shown for dissolved humic acids and iron. In this latter case, REE fractionation occurs because colloidal and truly dissolved pools have different REE patterns. Thus, the REE patterns of seawater could be the combination of both intra-oceanic and riverine processes. In this study, we show that the Atlantic continental shelves could be considered potential REE traps, suggesting further that shelf sediments could potentially become a resource for REE, similar to metalliferous deep sea sediments.

Iron Oxide Mineralization at the Contact Zone Between Phyllite and Itabirite of the Pau Branco Deposit, Quadrilátero Ferrífero, Brazil—Implications for Fluid-Rock Interaction During Iron Ore Formation

This study examines the whole-rock geochemistry and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) iron oxide chemistry of itabirite and related iron ore, as well as underlying phyllitic wall rock, of the Pau Branco iron ore deposit, Quadrilatero Ferrifero, Brazil. The phyllite and iron ore of Pau Branco extend from hypogene deep-seated into supergene weathered zones. The phyllitic sequence that underlies the itabirite-hosted iron ore along a sheared contact includes (from the bottom to top) carbonaceous and carbonate-, chlorite-, and Fe-rich zones, which likely reflect distal to proximal alteration halos. The Fe-rich phyllite and iron ore comprise a complex iron oxide mineralogy: (1) magnetite-martite is hosted in itabirite and related medium-grade iron ore and present as disseminated minerals in phyllite; (2) microplaty hematite replaces chlorite and carbonate in the phyllite; (3) granoblastic hematite is a recrystallization/precipitation product and the dominant iron oxide of the hard iron ore; (4) schistose specular hematite overgrows earlier iron oxides; and (5) goethite replaces earlier hematite and amphibole within the weathering horizon. Whole-rock geochemistry and related mass balance calculations reveal that, unlike the itabirite-hosted iron ore, interpreted to be the result of a solely residual iron enrichment via depletion of most major oxides and trace elements, the iron enrichment of the phyllite is caused by addition of Fe 2 O 3 . The Fe 2 O 3 is likely sourced from the overlying itabirite/iron ore and is mineralogically reflected in the replacement of carbonate and chlorite by hematite. The rare earth elements (REE) abundances of the different ore and phyllite zones reveal seawater-like REE signatures and Y/Ho ratios of the itabirite, hypogene and supergene iron ore, shale-atypical REE patterns in the unaltered phyllite, and Eu fractionation trends. This suggests a distinct input of hydrothermal fluids during the Fe enrichment of the phyllite. Locally, restricted Ce fractionation is attributed to distinct (e.g., Mn-rich) lithostratigraphic intercalations within the itabirite. Laser ablation-ICP-MS mineral chemistry of phyllite-hosted martite and microplaty hematite suggests a strong chemical inheritance by the host rock and precursor mineral. Positive Ce anomalies are restricted to phyllite- and itabirite-hosted martite only and, therefore, may reflect the highly oxidative conditions during martitization. At Pau Branco, the fluid-rock interaction affected not only the itabirite, but also the underlying phyllite, resulting in distinct alteration halos that extend beyond the itabirite toward the footwall. The significant chemical influence and importance of country-rock lithologies as a possible elemental source, for example, must be taken into account when interpreting whole-rock geochemical data and investigating an itabirite-/banded iron formation-hosted iron ore system.

Neodymium isotopes and concentrations in aragonitic scleractinian cold-water coral skeletons - Modern calibration and evaluation of palaeo-applications

Cold-water corals (CWCs) are unique archives of mid-depth ocean chemistry and have been used successfully to reconstruct the neodymium (Nd) isotopic composition of seawater from a number of species. High and variable Nd concentrations in fossil corals however pose the question as to how Nd is incorporated into their skeletons.We here present new results on modern specimens of Desmophyllum dianthus, Balanophyllia malouinensis, and Flabellum curvatum, collected from the Drake Passage, and Madrepora oculata, collected from the North Atlantic. All modern individuals were either collected alive or uranium-series dated to be <500years old for comparison with local surface sediments and seawater profiles. Modern coral Nd isotopic compositions generally agree with ambient seawater values, which in turn are consistent with previously published seawater analyses, supporting small vertical and lateral Nd isotope gradients in modern Drake Passage waters. Two Balanophyllia malouinensis specimens collected live however deviate by up to 0.6 epsilon units from ambient seawater. We therefore recommend that this species should be treated with caution for the reconstruction of past seawater Nd isotopic compositions.Seventy fossil Drake Passage CWCs were furthermore analysed for their Nd concentrations, revealing a large range from 7.3 to 964.5ng/g. Samples of the species D. dianthus and Caryophyllia spp. show minor covariation of Nd with 232Th content, utilised to monitor contaminant phases in cleaned coral aragonite. Strong covariations between Nd and Th concentrations are however observed in the species B. malouinensis and G. antarctica. In order to better constrain the source and nature of Nd in the cleaned aragonitic skeletons, a subset of sixteen corals was investigated for its rare earth element (REE) content, as well as major and trace element geochemistry. Our new data provide supporting evidence that the applied cleaning protocol efficiently removes contaminant lithogenic and ferromanganese oxyhydroxide phases. Mass balance calculations and seawater-like REE patterns rule out lithogenic and ferromanganese oxyhydroxide phases as a major contributor to elevated Nd concentrations in coral aragonite. Based on mass balance considerations, geochemical evidence, and previously published independent work by solid-state nuclear magnetic resonance (NMR) spectroscopy, we suggest authigenic phosphate phases as a significant carrier of skeletal Nd. Such a carrier phase could explain sporadic appearance of high Nd concentrations in corals and would be coupled with seawater-derived Nd isotopic compositions, lending further confidence to the application of Nd isotopes as a water mass proxy in CWCs.

Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures.

Biological activity is a major factor in Earth's chemical cycles, including facilitating CO2 sequestration and providing climate feedbacks. Thus a key question in Earth's evolution is when did life arise and impact hydrosphere-atmosphere-lithosphere chemical cycles? Until now, evidence for the oldest life on Earth focused on debated stable isotopic signatures of 3,800-3,700 million year (Myr)-old metamorphosed sedimentary rocks and minerals from the Isua supracrustal belt (ISB), southwest Greenland. Here we report evidence for ancient life from a newly exposed outcrop of 3,700-Myr-old metacarbonate rocks in the ISB that contain 1-4-cm-high stromatolites-macroscopically layered structures produced by microbial communities. The ISB stromatolites grew in a shallow marine environment, as indicated by seawater-like rare-earth element plus yttrium trace element signatures of the metacarbonates, and by interlayered detrital sedimentary rocks with cross-lamination and storm-wave generated breccias. The ISB stromatolites predate by 220 Myr the previous most convincing and generally accepted multidisciplinary evidence for oldest life remains in the 3,480-Myr-old Dresser Formation of the Pilbara Craton, Australia. The presence of the ISB stromatolites demonstrates the establishment of shallow marine carbonate production with biotic CO2 sequestration by 3,700 million years ago (Ma), near the start of Earth's sedimentary record. A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life's origin in the Hadean eon (>4,000 Ma).

Rare earth element and Sr–Nd isotope geochemistry of phosphatic rocks in Neoproterozoic Ediacaran Doushantuo Formation in Zhangcunping section from western Hubei Province, South China

Phosphatic rocks are widely distributed in Neoproterozoic Ediacaran Doushantuo Formation in Yangtze Platform, South China. In this study, we examined samples from the Zhangcunping section in western Hubei Province with major element, rare earth element (REE) and Sr–Nd isotopic analyses on the phosphorites and the host dolostone rocks. We employed an acid-leaching method to separate the carbonate (acetic acid leachable) and phosphate (hydrochloric acid leachable) fractions in all the samples. The geochemical differences between the different fractions can be recognized based on the P2O5, REE contents and Sr–Nd isotopic characteristics. The carbonate fractions in the rocks display low total REE concentrations with seawater-like REE patterns, super-chondritic Y/Ho ratios, moderate negative Ce anomalies and positive Eu anomalies, indicating these carbonates have retained the signatures of paleo-seawater. On the contrary, the phosphate fractions display high total REE concentrations with hat-shaped REE patterns, generally lower Y/Ho ratios, weak negative Ce anomalies and positive Eu anomalies, which may have recorded the features of diagenetic fluids. Negative Ce anomalies and positive Eu anomalies displayed in the carbonate fractions of the studied samples may also indicate the anoxic environments of ocean basins in South China during the deposition of the Neoproterozoic Ediacaran Doushantuo Formation. A geochemical comparison between Lower Phosphorite Layer and Upper Phosphorite Layer indicates a greater degree of diagenesis occurred in the Lower Phosphorite Layer than the Upper Phosphorite Layer. The negative εNd(t) values of the samples suggest continental crustal sources for the REEs, and the lower values in the phosphorite samples (−6.9 to −6.0) than the dolostone samples (−5.5 to −3.8) may suggest different processes for them, and it is likely that the REEs in the phosphorites have been modified by diagenetic fluids with REE contributions of pore fluids derived from the clastic silicates in the strata which generally have lower εNd(t) values, whereas the dolostone took the REEs from the ambient seawater mainly.

Rare earth elements and stable isotope geochemistry of carbonates from the mélange zone of Manipur ophiolitic Complex, Indo-Myanmar Orogenic Belt, Northeast India

Carbonates that occur in the melange zone of Manipur ophiolitic complex (MOC), Indo-Myanmar Orogenic Belt, Northeast India contain diverse fauna with dominance of foraminifera assemblages (planktonic and benthic) and calcareous nanofossils. Rare earth element (REE) contents (~51 ppm average value) in the MOC carbonates are high compared with the average value of typical marine carbonate (~28 ppm). The Post-Archean Australian Shale (PAAS)-normalized REE patterns of these carbonates exhibit seawater-like REE patterns with light rare earth elements (LREE) depletion and relative heavy rare earth elements (HREE) enrichment with negative Ce anomalies (Ce/Ce* = 0.28–0.69) and positive Eu anomalies (Eu/Eu* = 1.07–2.08). The carbonates show depleted values of δ13C ‰ (PDB) (0.89–2.74 ‰) which characterize marine precipitates while depletion in δ18O ‰ (PDB), reaching minimum values (−6.29 to −11.40 %). The negative anomalies of Ce in these carbonates suggest that they were deposited under oxygenated environment. The observed shale-normalized positive Eu anomalies and spread in negative δ18O values to a lesser extent of δ13C ‰ (PDB) values of these carbonates suggest their formation were altered by diagenesis in shallow marine environment. Field, petrographical studies in conjunction with REE and stable isotope characteristics suggest that these carbonates form part of the ophiolitic melange zone that emplaced during subduction and obduction processes of the Indian plate and Myanmar plate collision.

Isotopic and REE evidence for the paleoenvironmental evolution of the late Ediacaran Dengying Section, Ningqiang of Shaanxi Province, China

Weakly mineralized taxa are most likely the ancestors of higher organisms that existed during the Cambrian explosion, thereby representing an important yet least understood evolutionary link between simple and complex life. An investigation of the seawater temperature and the carbon cycle during the late Ediacaran, in the time span of the emergence of weakly mineralized taxa, contributes to the deeper understanding of the environmental conditions existing during the early stages of animal skeleton mineralization. Towards this end, we conducted the analyses on the carbon and oxygen isotope composition, petrography, major elements, and trace elements, involving the rare earth elements (REEs) variations in the carbonate rocks of Ningqiang County in Shaanxi Province of China. The samples from the Dengying Formation (Fm.) include the Gaojiashan and Beiwan Members, in ascending order. In addition, the Gaojiashan Member hosts one of the most diverse collections of biota with weakly mineralized skeletons discovered to date. Our data shows that a positive carbon excursion, high δ18O values, positive correlation between δ13C and δ18O values, high REE concentrations, a distinctive MREE-enriched pattern, high Fe concentrations and slightly negative Ce anomalies in the Gaojiashan Member. These evidences indicate a restricted warm sea environment with an influx of river water or groundwater. Conversely, the low carbon isotope values, high δ18O values and its heterogeneity, the absence of correlation between carbon and oxygen, combined with a seawater-like REE distribution pattern in the Beiwan Member suggested that a restrict, shallow, slightly cool and/or arid sea environment condition that was subject to periodic perturbations in the hydrologic balance. Therefore, we surmised that the environmental differences between the Gaojiashan and the Beiwan Members probably resulted in the preservation of a number of animal skeleton fossils in the middle section of the Dengying Fm.

Trace and rare earth element geochemistry of phosphate nodules from the lower Cambrian black shale sequence in the Mufu Mountain of Nanjing, Jiangsu province, China

Phosphate nodules are abundant in the lowermost section of the Lower Cambrian black shale sequence along a NE-trend belt in the Yangtze Platform, south China. In this study, we examined the Neoproterozoic-Cambrian Mufu Mountain section near Nanjing of Jiangsu province, and conducted a detailed trace and rare earth element (REE) study on the phosphate nodules. We employed a step-leaching acid dissolution protocol of 1 M HAc and 1 M HCl, respectively, to separate carbonate from phosphate mineral contributions in the nodules. Both the HAc-leached and HCl-leached fractions of the phosphate nodules show similar symmetrical variations in trace and REE concentrations and ratios. The total REEs and a number of redox-sensitive trace metals such as V, Cr, Mo, Ni abundances show a systematic increase from the core to rim, whereas the U and Mn abundances decrease from the core to rim in most of the nodules. Similar trends are also observed for the Rb/Sr, U/Th, Co/Ni, and V/Ni ratios, but these trends in the HAc-leached fractions are less pronounced than the HCl-leached fractions. All of the HCl-leached fractions for the nodule cores show seawater-like shale-normalized REE patterns, but the rims of the nodules display slightly MREE-enriched and HREE-depleted patterns. We suggest that the phosphate nodules may have formed in a basinal setting beneath a stratified water column during Early Cambrian, and the compositional variations of REEs and redox-sensitive trace elements from the core to rim may record a changing redox condition and fluid mixing during different stages of nodule growth. The cores of the nodules preserve more pronounced negative Ce anomalies and progressively HREE-enriched patterns, which may retain primary Early Cambrian seawater chemistry, whereas the nodule rims record chemistry consistent with changed redox conditions and/or influences from pore fluids such as generated from degradation of organic matter.

Effect of diagenesis on trace element partitioning in shales

In order to investigate the partitioning of trace elements in mineral phases of black shales and diagenetic remobilization of trace elements within these phases, we isolated major black shale components (sulfide, carbonate, silicate and organic) and analyzed the isolated fractions as well as corresponding whole rock for their trace element composition by DRC ICP-MS. This study focused on black shales of different grades of thermal maturity from the Ordovician Utica Shale Magnafacies of Quebec (USM) (immature: T max 20–50 °C, R o < 0.5), Ontario (mature: T max 50–140 °C, R o 0.5–0.8), and New York (post mature: T max > 140 °C, R o > 1). Analysis of the isolated rock fractions revealed that the carbonate fraction preserves a sea water-like rare earth elements (REE) abundance pattern with enrichment of the heavy rare earth elements (HREE–Dy–Lu) relative to light rare earth (LREE–La–Sm). The sulfide and organic fractions show a slight enrichment in the LREE. Mass balance calculations showed that the organic fraction contains up to about 20% of total trace element content of the whole rock with the sulfide and carbonate fractions each containing less than 10%. The partitioning and fractionation of trace elements between different phases may have important implications for paleoenvironmental reconstructions and provenance studies. Our data suggest that the whole rock signature may be less representative of source area or depositional environment but rather reflects the diagenetic history. This is especially the case in organic rich sediments such as black shales where the organic fraction hosts up to 20% of the trace element content of the whole rock.

Do Archean chemical sediments record ancient seawater rare earth element patterns?

Rare earth elements (REE) concentrations of Archean and Proterozoic chemical sediments are commonly used as proxies to study secular trends in the geochemistry of Precambrian seawater. In addition, similarities in the REE signatures of Archean chemical sediments and modern seawater have led researchers to argue that some Archean rocks originated as biochemical precipitates (i.e., microbial carbonates) in shallow marine (e.g., peritidal) environments. However, terrestrial waters, including river water and groundwater, also commonly exhibit REE fractionation patterns that resemble modern seawater. Here, we present the seawater-like REE data for groundwaters from central México as additional evidence that these patterns are not unique to the marine environment. The shale-normalized REE patterns of the groundwaters are compared to those of modern seawater (open ocean and nearshore), Holocene reefal microbial carbonates and corals, and Archean chemical sediments using statistical means (i.e., ANOVA and Wilcoxon analyses) in order to quantify the similarities and/or differences in the REE patterns. Shale-normalized (SN) Ce anomalies and measures of REE fractionation [i.e., (La/Yb) SN, (Pr/Yb) SN, (Nd/Yb) SN, and (Gd/Yb) SN] of the central México groundwater samples are statistically indistinguishable from those of modern seawater. Moreover, except for differences in the Ce anomalies, which are lacking in Archean chemical sediments, the REE patterns of the central México groundwaters are also statistically similar to REE patterns of Archean chemical sediments, especially those of the 3.45 Ga Strelley Pool Chert. Consequently, we suggest that without additional information, it may be premature to unequivocally conclude that Archean chemical sediments record REE signatures of an Archean ocean.