Calculation of simultaneous isotopic and trace element variations during water-rock interaction with applications to carbonate diagenesis

Biochemically induced diagenesis of Jurassic micrite: evidence from phase analysis, carbon, oxygen, and strontium isotopes (Franconian Alb, Germany)

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Full-text available

Jun 2024

Wolfgang Blendinger

The marine Upper Jurassic rocks of the Franconian Alb consist largely of micritic carbonate of partly dolomitized reef mounds and bedded basinal limestone. All carbonates were lithified in the shallow (centimeters, meters) subsurface and have a wide range of ∂ ¹³ C (≤ + 3‰ to − 10‰VPDB) but always negative ∂ ¹⁸ O (− 1 to − 6‰VPDB). Dolomite and reef limestone show the highest ∂ ¹⁸ O and ∂ ¹³ C values. The most negative ∂ ¹³ C (≥ − 10‰) occurs mainly as cement in dolomite of a basinal, partly dolomitic, biostrome interval. Basinal limestone shows intermediate ∂ ¹³ C values. Because freshwater diagenesis and elevated temperatures cannot explain the observed isotope values, pH is here considered a major factor influencing the isotope signal of micritic limestone. The bulk sediment isotope signal was reset to lower values, from an original lime mud with ∂ ¹³ C ≥ 3‰ and a ∂ ¹⁸ O of ≥ + 1‰, as a result of biochemically induced diagenesis. Carbonate, probably mostly aragonite but occasionally including dolomite, was dissolved in a zone where low pH developed as a result of organic matter degradation. Dissolved carbonate was translocated by diffusion and re-precipitated as cement (ca. 50vol%) in a zone with elevated pH where all in situ lime mud ∂ ¹⁸ O was reset. Imported cement carbonate precipitated in equilibrium with the pore fluid with negative isotope values, whereas ∂ ¹³ C of the in situ lime mud remained unmodified. The negative shift of the bulk ∂ ¹³ C and ∂ ¹⁸ O is variable and depends on pH and the contribution of ¹² C from anaerobic sulfate reduction in the zone of cement precipitation. This produced an ubiquitous covariance of ∂ ¹⁸ O and ∂ ¹³ C. Incorporation of seawater-derived Mg ²⁺ during recrystallization of carbonate can account for the local dolomitization. Elevated ⁸⁷ Sr/ ⁸⁶ Sr ratios are explained as a result of interaction of clay minerals with the stationary pore fluids. This study shows that the isotopic signal produced by biochemically induced shallow submarine subsurface carbonate diagenesis can be indistinguishable from freshwater diagenesis, that ∂ ¹⁸ O and ∂ ¹³ C of the bulk rock are always reset, and that carbonates can show, in the presence of clay minerals, elevated ⁸⁷ Sr/ ⁸⁶ Sr ratios even when the pore fluids were never exchanged. Graphical Abstract

IMPACTO DAS INTERAÇÕES DOS FLUIDOS PÓS-DEPOSICIONAIS NA EVOLUÇÃO DA CAPA CARBONÁTICA PUGA, SUL DO CRÁTON AMAZÔNICO

Conference Paper

Full-text available

Oct 2023

As sucessões de capas carbonáticas Marinoana (~635 Ma) registram a transição Criogeniano-Ediacarano e são marcadas por uma proeminente excursão negativa de isótopos de δ 13 C. Essa excursão é encontrada em sucessões de capas carbonática em muitos locais ao redor do mundo e geralmente é considerada como relacionada a uma perturbação global do ciclo do carbono após a glaciação. Os isótopos de oxigênio também exibem uma excursão negativa, no entanto, devido à maior suscetibilidade do δ 18 O à superimposição diagenética, esses dados geralmente não são utilizados em análises paleoambientais. O Sul do cráton amazônico registra uma dessas sucessões, a capa carbonática Puga, os registros δ 13 C desta sucessão tem sido comumente associada a um registro primário da água do mar Ediacarano. Aqui apresentamos novos dados de δ 13 C e δ 18 O e compilados, além de indicadores sedimentológicos e texturais para avaliar as in uências diagenéticas tardia na química de isótopos de carbonato no registro de capa carbonática. PALAVRAS-CHAVE: Dolomita; δ 13 C e δ 18 O; Diagênese precoce; Ediacarano.

Carbonate uranium isotopes across Cretaceous OAE 2 in southern Mexico: New constraints on the global spread of marine anoxia and organic carbon burial

Article

Jul 2023
PALAEOGEOGR PALAEOCL

Freshwater ecosystem collapse and mass mortalities at the Paleocene-Eocene thermal maximum

Article

Jun 2023
GLOBAL PLANET CHANGE

A Record of the Snowball Earth Events? The Parecis Basin May Encompass Three Major Neoproterozoic Glaciations on the Amazonian Craton

Article

May 2023
J S AM EARTH SCI

The Parecis Basin is an intracratonic basin located at the south-southeast portion of the Amazonian craton. Its stratigraphic units comprise siliciclastic and carbonate rocks that potentially record intense paleoenvironmental changes related to the Sturtian (~717–660 Ma), Marinoan (~657–635 Ma), and Gaskiers (~580 Ma) glaciations. We present novel X-ray fluorescence geochemistry, C, O and Sr isotopic data from carbonate rock samples retrieved from the stratigraphic well 2ANP 0006 MT. The results show two negative excursions of δ¹³C here correlated to post-Sturtian and Marinoan glaciations, recorded in the Salto Magessi and Mirassol d’Oeste car- bonate units, respectively. Both negative excursions were followed by δ¹³C positive incursions. At the base of the section, Salto Magessi carbonate presents unradiogenic Sr ratios (⁸⁷Sr/⁸⁶Sr from 0.70655 to 0.70776) typical of Cryogenian interglacial carbonate rocks found elsewhere. The negative δ¹³C excursion from Mirassol d’Oeste is associated with radiogenic ⁸⁷Sr/⁸⁶Sr ratios (up to ~0.7186), possibly related to continental influx from the intense erosion generated by the Marinoan deglaciation. Upward, the Nobres Formation displays preserved primary sulfate minerals (gypsum and anhydrite) and isotopically heavy oxygen compositions which point to an evaporative environment and support the scenario of enhanced oxidative weathering during early to middle Ediacaran. Finally, despite the absence of cap carbonate, diamictites related to the Serra Azul Formation occur overlying the Nobres Formation and are associated with the Gaskiers event. Therefore, the 2ANP 0006 MT may mising section for further studies ing chemostratigraphic and paleoenvironmental reconstructions during the Neoproterozoic Era.

Osteopontin stabilization and collagen containment slows amorphous calcium phosphate transformation during human aortic valve leaflet calcification

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May 2024

Calcification of aortic valve leaflets is a growing mortality threat for the 18 million human lives claimed globally each year by heart disease. Extensive research has focused on the cellular and molecular pathophysiology associated with calcification, yet the detailed composition, structure, distribution and etiological history of mineral deposition remains unknown. Here transdisciplinary geology, biology and medicine (GeoBioMed) approaches prove that leaflet calcification is driven by amorphous calcium phosphate (ACP), ACP at the threshold of transformation toward hydroxyapatite (HAP) and cholesterol biomineralization. A paragenetic sequence of events is observed that includes: (1) original formation of unaltered leaflet tissues: (2) individual and coalescing 100’s nm- to 1 μm-scale ACP spherules and cholesterol crystals biomineralizing collagen fibers and smooth muscle cell myofilaments; (3) osteopontin coatings that stabilize ACP and collagen containment of nodules preventing exposure to the solution chemistry and water content of pumping blood, which combine to slow transformation to HAP; (4) mm-scale nodule growth via ACP spherule coalescence, diagenetic incorporation of altered collagen and aggregation with other ACP nodules; and (5) leaflet diastole and systole flexure causing nodules to twist, fold their encasing collagen fibers and increase stiffness. These in vivo mechanisms combine to slow leaflet calcification and establish previously unexplored hypotheses for testing novel drug therapies and clinical interventions as viable alternatives to current reliance on surgical/percutaneous valve implants.

Nitrogen isotope gradient on continental margins during the late Paleoproterozoic

Article

Apr 2024
GEOCHIM COSMOCHIM AC

Marine anoxia events across the upper Cambrian (stage 10) of eastern Gondwana: Implications from paleoenvironmental geochemical proxies

Article

Full-text available

Mar 2024
MAR PETROL GEOL

A clumped isotope diagenetic framework for the Ediacaran dolomites: Insights to fabric-specific geochemical variabilities

Article

Oct 2023
SEDIMENTOLOGY

While marine dolomites formed under near surface conditions have been considered to be potentially reliable archives of past oceanic conditions, this interpretation comes with significant challenges because diagenetic alteration frequently produces diverse fabrics with large geochemical variability. It has been suggested that the Ediacaran dolomites in South China (Hamajing Member, Dengying Formation) recorded the oceanic conditions present at the time they formed, yet these dolomites are composed of five different fabrics (stromatolitic, micritic, oolitic, saddle dolomites and fibrous–radial dolomite cements) and show large variations in multiple geochemical isotope proxies (carbon, oxygen, clumped, magnesium and the sulphur of carbonate-associated sulphate). This study establishes a paragenetic sequence for these dolomites by combining the clumped and the oxygen isotopic compositions, thereby assessing whether they are geochemically representative of the original seawater. Using this diagenetic framework, the micritic and stromatolitic dolomites show a closed-system behaviour (low water–rock ratios; <0.3) and are largely resistant to the hydrothermal alteration during late diagenesis. In contrast, the ooid and cement fabrics have been affected by the hydrothermal fluid precipitating saddle dolomite in the open-system condition with the high stimulated water–rock ratios (>1). Furthermore, in a closed-system environment, the elevated δ24Mg and δ34S values in the stromatolitic dolomite reflect the isotopic Rayleigh fractionation that enriches the 26Mg and 34S through rock-buffered recrystallization, coupled with microbial sulphate reduction. These results demonstrate that the complex signals in early marine dolomite should be carefully evaluated when used as a palaeoproxy.

Expanded marine anoxia at the Cambrian-Ordovician transition: Evidence from lime mudstone I/Ca and δ238U signatures of the GSSP in western Newfoundland, Canada

Article

Jul 2023
MAR PETROL GEOL

The early Paleozoic witnessed two spectacular radiations of marine organisms—the Cambrian (Є) Explosion and the Great Ordovician (O) Biodiversification Event. However, the period between them was characterized by a marine biodiversity plateau attributed to recurring biocrises. In the current study, we present the I/Ca records from slope lime mudstones of the Green Point Formation in western Newfoundland, along with a three-sink U-isotope mass balance model and previously reported limestone δ238Ucarb signals of the same interval, to further explore oceanic redox conditions at the Є-O boundary. The I/Ca ratios of the lime mudstones, deposited along the eastern Laurentian (western Iapetan) continental slope, exhibit a narrow range between 0.02 and 0.33 μmol/mol. Poor correlations between the I/Ca values and their [Sr], δ18O, Mn/Sr, Fe/Sr, Mg/Ca, and δ13Corg counterparts, together with near-micritic textures of the limestones, argue against significant influences of postdepositional alterations on the I/Ca signatures. The iodine-depleted lime mudstones, with I/Ca values well below the Proterozoic Eon baseline I/Ca ratios (∼0.5–1 μmol/mol), suggest the presence of shallow marine oxic-anoxic interfaces along the regional continental margin. Substantially low limestone I/Ca ratios (<0.5 μmol/mol) and dysoxic to anoxic depositional conditions have also been reported from several other age-equivalent sections deposited along the shelf and slope of ancient Iapetan and Laurentian continental margins. As a result, seawaters surrounding the Iapetan and Laurentian continental margins at the Є-O transition might have been commonly poorly oxygenated with shallow oxyclines or expanded oxygen minimum zones in the shelf and/or slope areas. This interpretation is further supported by our three-sink U-isotope mass balance modeling, which predicts widespread marine anoxia at the Є-O boundary with anoxic to euxinic water covering 1.0–21.1% of the ocean floor, significantly higher than the modern day (∼0.2%). Furthermore, the wide range of the oceanic δ238UOC (−1.22 to −0.25‰) values, estimated from the limestone δ238Ucarb signals, might reflect oceanic redox oscillations during this period. However, the δ238UOC fluctuations could also be attributed to variable accumulations of the 238U-enriched authigenic U phases during early diagenesis. Overall, evidence from the lime mudstone I/Ca ratios and the estimated extent of marine anoxia at the Є-O boundary in this study aligns with earlier viewpoints that the slowness of marine biodiversity accumulation during the late Cambrian and the Early Ordovician was linked to widespread oceanic anoxia.

Early diagenetic mobilization of rare earth elements and implications for the Ce anomaly as a redox proxy

Article

Full-text available

Jun 2023
CHEM GEOL

Characterising the economic Proterozoic Glyde Package of the greater McArthur Basin, northern Australia

Article

Full-text available

May 2023
ORE GEOL REV

Detailed diagenetic investigation and their impact on the reservoir potential of Upper Cretaceous mixed carbonate-siliciclastic succession, Rakhi Nala Section, Eastern Sulaiman Range, Pakistan

Article

Full-text available

Apr 2023

The Upper Cretaceous mixed carbonate-siliciclastic succession of Southeastern Tethys in the Sulaiman Range, Lower Indus Basin in Rakhi Nala Section was investigated for understanding its diagenetic history and impact of diagenesis on the hydrocarbon reservoir potential. The studied succession is about 200 m thick and comprise thin-to-thick-bedded limestone with interbedded marls, shale, and sandstone. Based on field observations, detailed petrographic investigation of combined alizarin red-S and potassium ferricyanide stained thin sections, scanning electron microscopy, X-ray diffractometry, and geochemical analyses (stable C and O isotopes), a variety of diagenetic phases related to different stages has been observed. The normal paragenetic sequence including the three stages of diagenesis: eogenetic, mesogenetic, and telogenetic in succession are revealed. During the eogenetic stage, the diagenesis occurred in the meteoric and marine environments and is evident from the observed features such as burrowing, micritization, syndepositional syntaxial overgrowth, inversion neomorphism, internal geopetals, and drusy mosaic cement. The mesogenetic stage and the major phase of diagenesis in the studied strata encountered shallow to deep burial diagenetic environments and is characterized by dissolution, mechanical compaction (grains packing and micro-fractures of sub-millimeter scale), fractures of sub-centimeter scale, granular mosaic cement, later clear syntaxial overgrowth, blocky calcite cement (BC-3 to BC-5 in paragenetic sequence), chemical compaction and minor dolomitization. The teleogenetic or uplifting stage is evident from the cross-cutting relationship of sub-millimeter calcite-filled fractures post-dating late burial stage diagenetic features. The δ¹³C and δ¹⁸O values for whole rock (matrix and grains) are ranging from − 3.47 to 2.13 and − 3.07 to 0.3, respectively. Whereas the δ¹³C and δ¹⁸O values of micro-drilled samples from blocky calcite cements of undoubtedly burial stage range from − 1.58 to 3.05 and − 9.97 to − 3.68, respectively. The cross-plot of δ¹³C versus δ¹⁸O of whole rock samples mostly fall in the category of warm-water skeletons and reveals the marine diagenesis and suggests that the diagenetic fluids were buffered with the surrounding marine carbonates during their flow. The depleted δ¹⁸O values of the micro-drilled samples from veins reflect the precipitation of cements from fluids during deep burial diagenesis with elevated temperature. The studied carbonates display poor reservoir properties (Φ < 01% and K < 01 md). The detailed petrographic investigation and the observed paragenetic sequence show that the reservoir potential has been occluded by extensive precipitation of various cements in the different diagenetic environments.

Diagenetic effects on strontium isotope (87Sr/86Sr) and elemental (Sr, Mn, and Fe) signatures of Late Ordovician carbonates

Article

Jan 2023

Understanding the effect and extent of diagenesis on the isotopic compositions of Sr in marine carbonates is a critical prerequisite for their use to unravel past environments. Here, we explore the dominant controls on carbonate ⁸⁷Sr/⁸⁶Sr of a Late Ordovician section from the Monitor Range, USA. Our results reveal a distinct increase in ⁸⁷Sr/⁸⁶Sr from 0.70794 to 0.70830 in the mid-upper D. ornatus zone, which is markedly higher than the published datasets of contemporaneous samples with a relatively lower and stable ⁸⁷Sr/⁸⁶Sr ratio of ~0.7079. These elevated ⁸⁷Sr/⁸⁶Sr ratios suggest a local and post-depositional overprint and cannot be interpreted to reflect the ⁸⁷Sr/⁸⁶Sr of the coeval seawater. Furthermore, ⁸⁷Sr/⁸⁶Sr exhibits statistically significant positive correlations with geochemical indicators for diagenesis ([Mn], [Fe], Mn/Sr, Fe/Sr), indicating that diagenetic alteration is the principal control on the observed radiogenic ⁸⁷Sr/⁸⁶Sr values. Using a numerical model of marine diagenetic fluid-rock interaction, we demonstrate that the observed Sr isotopic and elemental data can be best explained by the chemical variations in bulk carbonates associated with diagenetic alteration. Our results highlight that diagenesis may significantly alter the pristine ⁸⁷Sr/⁸⁶Sr ratios of carbonates than previously thought, although the samples satisfy the stricter geochemical criteria of Sr isotope preservation ([Sr] > 300 ppm, [Mn] < 300 ppm, [Fe] < 1000 ppm, Mn/Sr < 0.2, Fe/Sr < 1.6), pointing to the need for more caution when using bulk carbonate ⁸⁷Sr/⁸⁶Sr as a tracer of paleoenvironmental changes.

C-Sr-Pb isotope systematics of the carbonate sequences of Kaladgi Supergroup, India: Implications for basin evolution and correlation with Proterozoic global events

Article

Mar 2023
PRECAMBRIAN RES

he Kaladgi Basin is one of the several Proterozoic sedimentary basins of India. This basin has a binary evolutionary history with an angular unconformity separating the older deformed and younger undeformed rock sequences of the Kaladgi Supergroup, named the Bagalkot and Badami Groups, respectively. The Bagalkot Group is known to be of late Paleoproterozoic-Mesoproterozoic age. However, the timings of deposition of the Badami Group sediments and the closure of the basin have been speculative. We carried out C-O-Sr isotope studies of the marine carbonate successions of this basin namely Chitrabhanukot, Chikshellikeri, Lakshanhatti (Bagalkot Group), and Konkankoppa (Badami Group). Our results indicate that δ13C of all the carbonate formations have preserved the primary marine signature. Restricted variation of δ13C in the carbonates of the Bagalkot Group (0 ± 2 ‰) suggests a steady state organic carbon burial; whereas much wider range of variation of δ13C in the Konkankoppa Limestone of Badami Group (-2.2 to + 3.5 ‰) hints at a dynamic organic carbon burial scenario. These signatures are consistent with the C-isotope stratigraphy of the Meso-Neoproterozoic times. The 206Pb-207Pb dating of the youngest unit of the Kaladgi Supergroup, the Konkankoppa Limestone yielded a depositional age of 604 ± 25 Ma. This age, considered together with the primary 87Sr/86Sr ratio of 0.70781, extends the active sedimentation of the Badami Group well into the Ediacaran Period. These data confirm the existence of a long duration depositional hiatus, of >500 million years, between the Bagalkot and the Badami Groups. Results of this study also refutes the claim that the sedimentation in most of the Proterozoic basins of peninsular India ended by 1000 Ma.

A 13 million-year record of Li isotope compositions in island carbonates: Constraints on bulk inorganic carbonate as a global seawater Li isotope archive

Article

Full-text available

Mar 2023
GEOCHIM COSMOCHIM AC

High resolution strontium isotope data from Nama Group, South Africa, constrain global stratigraphic relationships in the terminal Ediacaran

Article

May 2024
PRECAMBRIAN RES

The Ediacaran was a pivotal period, during which the first metazoans emerged and then experienced several biotic turnover events. At the same time, the oceans were undergoing large-scale geochemical changes, including oscillating redox conditions and nutrient fluxes. Understanding global patterns of biotic change during this interval, and any environmental drivers, is limited by our ability to correlate geochemical and palaeontological records between sections. The strontium isotope ratio of seawater (87Sr/86Sr) represents a powerful chemostratigraphic tool, because it is globally homogeneous, evolves on ∼ 1 myr timescales, and is well-preserved in a range of carbonate rocks. In addition, changes in 87Sr/86Sr track changes in continental weathering fluxes, which in turn can influence redox conditions and seawater chemistry. Recent age models for the Ediacaran have incorporated δ13C and 87Sr/86Sr data, but there is a dearth of high-resolution, age-constrained 87Sr/86Sr data from the critical Ediacaran-Cambrian Boundary interval. To address this, we collected a high-resolution stratigraphic transect through an expanded section of terminal Ediacaran carbonate rocks, from the Nama Group, South Africa and Namibia, deposited ∼550–538 Ma. The samples are interbedded with a series of closely-spaced, dated ash beds. To ensure that the 87Sr/86Sr ratios were reflective of primary seawater signals, we tested a sequential digestion procedure, already shown to be effective in Phanerozoic samples, on Ediacaran bulk rock carbonate powders. The successful implementation of the sequential digestion technique was verified through careful examination of samples for diagenetic alteration using petrography, trace element ratios and δ18O. This technique was then applied to 51 samples from a stratigraphic transect through the Kuibis Subgroup, collected at Zebra River, and the Schwarzrand Subgroup, collected at Orange River. The data through the Schwarzrand Subgroup are highly consistent, with an average of 0.708590 ± 0.000076. The isotopic signature is consistent with other terminal Ediacaran age rocks, with typically low, positive δ13C and relatively high 87Sr/86Sr ratios (∼0.7085). Based on correlation with other Ediacaran sections, our data reveal an 87Sr/86Sr peak of ∼0.7086 pre-550 Ma before a drop-off to ∼0.7084 immediately post-550 Ma. These data can help improve age models and provide important context for a critical period in metazoan evolution.

Geochemistry of Palaeoproterozoic Kajrahat Limestone, Vindhyan Supergroup, central India: insights into depositional conditions and sources of rare earth elements

Article

Full-text available

Mar 2024

The Lower Vindhyan (Semri) Group substantially consists of unmetamorphosed and undeformed sedimentary sequences of Palaeoproterozoic age. A detailed geochemical study has been carried out to infer the paleoenvironment and the source of REEs for the Palaeoproterozoic Kajrahat Limestone, Vindhyan Supergroup, Central India. This study investigates the geochemical composition of major, trace and rare earth elements (REEs). The studied limestones, have a high percentage of CaO (ranging from 30.87 to 48.59) implies that calcite was the primary mineral phase in these carbonates. SiO2 is second most abundant major oxide. Sr is dominant trace element showing a negative correlation with CaO. All trace elements exhibit depletion with respect to Post-Archean Australian Shale (PAAS). The PAAS-normalized REE pattern of studied limestone has a relatively uniform pattern, with slightly enriched LREE compared to HREE with negative Ce anomaly. Collectively, low concentration of U and U/Th, Ce/Ce* ratios clearly indicates an oxic depositional condition for the samples. The ΣREE shows a positive correlation with SiO2, TiO2 and Al2O3 (r² = 0.87, 0.84and 0.91 respectively) and a week positive correlation with CaO (r² = 0.12) suggested that siliciclastic sediments also serve as a source for the REEs beside seawater. This, in turn, indicates that the deposition of this limestone occurred in a coastal/shallow marine environment with some contribution from continental part.

Fluid inclusion calibration of clumped-isotope solid-state reordering in dolomite: Implication for thermal history reconstruction in deeply buried reservoirs

Article

Feb 2024
MAR PETROL GEOL

Upper Cambrian and lower Ordovician conodont and carbon isotope bio-chemostratigraphy of the eastern North China Platform: Implications for global correlation and palaeoenvironments

Article

Feb 2024
J ASIAN EARTH SCI

Carbonate Factory Response through the MECO (Middle Eocene climate Optimum) event: Insight from the Apulia Carbonate Platform, Gargano Promontory, Italy

Article

Full-text available

Mar 2024
SEDIMENT GEOL

During the Eocene, shallow-water carbonate systems were significantly impacted by climate fluctuations and hyperthermal events. Following the peak temperatures of the Early Eocene Climatic Optimum (EECO), a general cooling trend began, with short-lived (⁓200 kyr) warming events occurring alongside it. In the early Bartonian (around 40.1 Ma), a warming event known as the Middle Eocene Climatic Optimum (MECO) occurred, lasting approximately 500,000 years. In this scenario, the types and calcification rates ofmarine organisms such as corals and larger benthic foraminifera (LBF) were influenced by global CO2 and oceanographic changes, which had a major effect on photic carbonate factories. To better understand the effects of these factors on carbonate factories, a detailed study of shallow-water facies types, distributions, and evolution was conducted. The Middle Eocene Monte Saraceno sequence, located on the eastern margin of the Apulia Carbonate Platform (Gargano Promontory, southern Italy), was selected as a case study to investigate the relationships between carbonate fac- tory types and climatic changes around theMECOevent. This study identified twodistinct intervalswithdifferent modes of carbonate production, separated by a sharp boundary. The lower interval consists of clinostratified, thick beds of rudstone to floatstone, mostly made up of various large Nummulites tests, indicating an early Bartonian age (Shallow Benthic Zone 17). Instead, the upper interval consists of coral floatstone to rudstone with a packstone matrix, rich in branching corals in association with gastropods, bivalves, and rare small larger benthic foraminifera. The appearance of Heterostegina sp. and Glomalveolina ungaroi in this interval indicates a late Bartonian age (Shallow Benthic Zone 18). By integrating biostratigraphic and stable-isotope data, the lower interval, with abundant Nummulites, was linked to the MECO event, duringwhich higher sea-surface tem- peratures seem to enhance larger benthic foraminifera proliferation, as already occurred in the Early Eocene. However, in the late Bartonian, the sharp transition to a coral-dominated carbonate factory, with rare larger ben- thic foraminifera showing smaller sizes, could be attributed to a drop in temperature that created the conditions more favourable to corals. Overall, this study provides compelling evidence of how environmental changes can affect marine carbonate production, also highlighting the importance ofinvestigating these relationships, to bet- ter understand climate change in the past, present and near future.

Dolomitization of early-post rift Lower Jurassic carbonate platforms along the Moroccan Atlantic Margin: Origin and significance

Article

Full-text available

Mar 2024
J AFR EARTH SCI

Depositional condition of Paleoproterozoic Francevillian carbonate rocks revisited from rare earth element contents

Article

Full-text available

Dec 2023

First record of trace fossils in the Ediacaran–Cambrian transition on the northern Gondwana platform (Anti-Atlas, Morocco)

Article

Nov 2023
PRECAMBRIAN RES

The Anti-Atlas Mountains of Morocco preserve one of the most complete latest Neoproterozoic to Cambrian stratigraphic successions worldwide, with high-resolution chemostratigraphic δ13Ccarb coverage. However, the exact stratigraphic position of the Ediacaran–Cambrian boundary remains unresolved. Until now, no trace fossils or body fossils have been recorded from the terminal Ediacaran strata in the Anti-Atlas Mountains. The only exception is a discoidal body fossil-like structure from the shallow-marine sandstones of the Ediacaran Tabia Member, Adoudou Formation, in the Taroudant Group in the Igherm inlier. For the first time, we document the occurrence of abundant trace fossils from the base of the Adoudou Formation, including Treptichnids, Monomorphichnus isp., Helminthoidichnites isp., Gordia isp., Palaeophycus isp., Planolites isp., and Conichnus isp. The integration of both trace fossils and carbon isotope δ13Ccarb records suggests that the Ediacaran–Cambrian transition in the Sous Basin can be placed at the contact between the Tifnout and Tabia Members, which chemostratigraphically coincides with the large negative δ13Ccarb excursion that had been previously suggested for the Ediacaran–Cambrian boundary (BACE) in the western Anti-Atlas Mountains.

EVIDÊNCIAS PETROGRAFICAS E GEOQUÍMICAS DE ISÓTOPOS ESTÁVEIS (δ13C E δ18O) PARA A EVOLUÇÃO HIDROTERMAL DOS DEPÓSITOS DOMÍTICOS(EDIACARANO) DA BACIA ARARAS-ALTO PARAGUAI, SUDESTE DO CRÁTON AMAZÔNICO

Conference Paper

Full-text available

Oct 2023

As sequências carbonáticas localizadas no sudeste do Cráton Amazônico são geralmente superpostas por fases de migração de uidos e eventos tectônicos. Averiguar a natureza dos uidos dolomitizantes ajudam a entender a evolução sin e pós deposicional dessas rochas. Para isso, utilizou-se microscopia ótica associados com catodoluminescência (CL) para identi car as diferentes fases dolomíticas e isótopos de C e O para determinar a natureza genética das dolo-mitas. Foram diferenciados 3 tipos de assinaturas isotópicas com natureza distintas: assinatura marinha (eodiagenética) para os dolomicritos (Rd1); assinatura hidrotermal (δ18O abaixo de 8‰VPDB) para os cimentos e matriz substituída por dolomita espática; e valores de uidos meteóricos e/ou misturados para a primeira geração de cimentos de dolomita que preenche brechas cimentadas (Bc) e fraturas. Essas evidencias, sugerem múltiplos estágios de percolação de uidos, que atuaram na história diagenética/hidrotermal das rochas da Bacia Araras-Alta Paraguai (BAAP). PALAVRAS-CHAVE: Brecha cimentada; Fluidos marinhos dolomitizantes; Dolomitização hidrotermal; 1. INTRODUÇÃO Rochas carbonáticas são consideradas capazes de reter valores de δ 13 C sindeposicionais quando minimamente afetadas por eventos diagenéticos, no entanto, quando alteradas a correlação entre os valores de δ 13 C e δ 18 O são uteis para rastrear o efeito da alteração pós

Early Oligocene marine transgression and organic matter accumulation recorded in the upper Eocene to lower Oligocene Enping Formation of the Baiyun Sag, Pearl River Mouth Basin, South China Sea

Article

Nov 2023

Boron isotope constraints on the migration and accumulation of rare alkali metals in the geyserite cesium deposits in Southern Tibet

Article

Oct 2023
ORE GEOL REV

Tropical ocean temperatures and changes in terrigenous flux during the Paleocene-Eocene Thermal Maximum in southern Tibet

Article

Oct 2023
GLOBAL PLANET CHANGE

Lithium isotope systematics and water/rock interactions in a shallow-water hydrothermal system at Milos Island, Greece

Article

Oct 2023
MAR CHEM

Driver of eustatic change during the early Aptian Oceanic Anoxic Event 1a (~120 Ma)

Article

Oct 2023
GLOBAL PLANET CHANGE

Eustatic change across the Paleocene-Eocene Thermal Maximum in the epicontinental Tarim seaway

Article

Oct 2023
GLOBAL PLANET CHANGE

The Paleocene-Eocene Thermal Maximum (PETM) offers an excellent opportunity to study the ecological and environmental responses to global warming. Discussing the so far poorly constrained amplitude and mechanisms of sea-level changes during the PETM is the principal goal of the present study from the epicontinental Tarim seaway. A negative carbon isotope excursion precisely constrains the stratigraphic position of the PETM event within the Qimugen Formation. Microfacies data show that tidal and lagoonal carbonates or sandstone characterizing the Tarim seaway in the pre-PETM stage were gradually replaced by open-marine to middle-ramp marlstones at PETM onset, by outer-ramp mudrocks in the syn-PETM stage, and eventually by middle-ramp carbonates in the post-PETM stage. A deepening paleo-water depth trend documents a transgressive sequence leading to maximum flooding during PETM peak. The content of planktonic foraminifera in Qimugen Formation sediments deposited below storm wave base indicates a minimum paleo-water depth of 20 50 m. Regional and global comparisons of sea-level curves suggest that this sea level rise documented in the Tarim epicontinental seaway during the PETM is a largely eustatic consequence of global warming, inducing extensive melting of highmountain glaciers and thermal expansion of sea water.

Chemotrophy-based phosphatic microstromatolites from the Mississippian at Drewer, Rhenish Massif, Germany

Article

Full-text available

Aug 2023

The Drewer quarry located in the Rhenish Massif is a well-studied outcrop that comprises Upper Devonian (Famennian) to Lower Carboniferous (Viséan) strata. Within the Drewer deposits two black shale intervals have been described that are linked to two global oceanic anoxic events, the Hangenberg Event and the Lower Alum Shale Event. The black shales associated with the Middle Tournaisian Lower Alum Shale Event contain abundant phosphatic concretions, which were investigated using thin section petrography, powder X-ray diffraction, Fourier-transform infrared spectrometry and scanning electron microscopy. The concretions formed during several growth phases under anoxic and at least episodically sulphidic conditions within the sediment and served as a substrate for subsurface microbial mats that formed phosphatic microstromatolites. The microstromatolites occur either as partially branched columns of up to 600 µm in length attached to the phosphatic concretions or as smaller, bulbous aggregates surrounding the concretions. Element mapping identified the presence of pyrite and other metal sulphides within the phosphatic microstromatolites. The carbon and oxygen stable isotopic composition of phosphate-associated carbonate within the phosphatic microstromatolites suggests that the mat-forming microorganisms were probably anaerobic, chemotrophic microbial communities dwelling in the anoxic environment during the Lower Alum Shale Event. Such interpretation agrees with the deeper-water depositional setting of the Lower Alum Black Shale and its high content of organic matter, suggesting that chemotrophic microbial mats are potent agents of phosphogenesis in general, and of the formation of phosphatic stromatolites in particular.

Variations in extent, distribution and impact of dolomitization on reservoir quality of Upper Cretaceous foreland-basin carbonates, Abu Dhabi, United Arab Emirates

Article

Jul 2023
MAR PETROL GEOL

Abstract Petrography, petrophysics, and geochemistry of an Upper Cretaceous, foreland-basin carbonate reservoir, Abu Dhabi, United Arab Emirates, are used to constrain the spatio-temporal variations in the extent of dolomitization and its impact on reservoir quality. Dolomitization in highstand systems tracts (HST) is attributed to repeated tidal and evaporative pumping as well as seepage reflux of penesaline brines during restriction of the platform due to 4th and 5th order cycles of relative sea-level fall and particularly below parasequence boundaries. This interpretation is supported by the presence of rare poikilotopic gypsum cement and scattered laths and micro-nodules of calcitized gypsum in the dolomitized peritidal dolostones. Dolomitization along bioturbation sites, which is most common in the transgressive systems tracts (TST), is attributed to the development of suitable localized geochemical conditions (e.g., microbial sulfate reduction and related increase in carbonate alkalinity). Porosity and permeability of the dolostones are strongly controlled by depositional textures of precursor limestones and by subsequent diagenetic evolution. Dolomitization of mudstones, wackestones and matrix-rich packstones has resulted in the formation of micropore-dominated microcrystalline dolostones whereas dolomitization of the shoal grainstones resulted in the formation of coarse-crystalline dolostones with abundant well-connected intercrystalline and moldic macropores. Mesogenetic alterations of the dolostones, which are attributed to the flow of hot basinal brines along steep faults, include dolomite cementation and, subsequently, dissolution and calcitization of dolomite (dedolomitization) and calcite cementation. The lack of systematic differences in porosity and permeability of dolostones between the oil and water-saturated dolostones suggest that most diagenesis occurred prior to completion of oil emplacement and/or reflect the shallow maximum burial depths of the formation (around 1.3 km). This study demonstrates that variations in the distribution and extent of dolomitization within a sequence stratigraphic context across an oilfield should be considered as primary control on the spatio-temporal reservoir lithology and heterogeneity in carbonate successions.

Influence of turbidity deposition on biogeochemical cycling in sediments and oceanic redox stratification during the waning Sturtian glaciation

Article

Jul 2023
GLOBAL PLANET CHANGE

The Cryogenian Sturtian glaciation (717–660 Ma) may have completely frozen global oceans, which would have resulted in marine anoxia due to the oceans' prolonged separation from the atmosphere. Sedimentological evidence reveals that the ice sheets are dynamic throughout this glacial. The ice sheets' retreat increased the availability of nutrients and led to active marine biogeochemical cycles. However, direct geochemical constraints on the synglacial marine chemistry, such as data from carbonate rocks, are rare. We conducted detailed sedimentological and geochemical analyses on the Cryogenian Fulu formation in the Yangtze Block's southeastern margin. The Fulu Formation mainly consists of sandstones with rare carbonate rocks. Facies analysis shows that these sandstones were deposited in deep water environment during the waning Sturtian glaciation. Three facies were recognized, including the sand lobe, lobe fringe and basin plain facies. According to the geochemical results, the Fulu Formation has a large variation in organic carbon isotope (δ13Corg) and pyrite sulfur isotope (δ34Spy) (δ13Corg: −29.5‰ ~ −22.0‰, mean = −25.7‰, n = 143; δ34Spy: −19.4‰ ~ −29.0‰, mean = 1.7‰, n = 88), but shows low values of TOC (<0.1 wt%) and pyrite content (<0.4 wt%). Dolostone layers are mainly deposited in the basin environment and are characterized by: conspicuous Fe and Mn content (Fecarb: 15746–64,428 ppm, mean = 43,457 ppm; Mncarb: 4436–8576 ppm, mean = 6078 ppm; n = 13), stable inorganic carbon isotope (δ13Ccarb: −3.7‰ ~ −2.5‰, mean = −3.1‰, n = 23), and absence of Ce anomaly (0.93–1.07, mean = 0.99, n = 13). These stable δ13Ccarb, low TOC and pyrite content suggest that sustaining primary productivity and low seawater sulfate levels. Turbidity deposition, rather than marine chemistry controls the oscillations of δ13Corg and δ34Spy records under these conditions. The remineralization of Mn-oxide causes the coupling variation trend of REE and Mncarb content result from the Mn-oxide releases adsorbed REE. Mn-oxides in turbidity deposits from shallow water indicate that the shallow ocean is oxic/suboxic. The absence of the Ce anomaly, on the other hand, suggests that synglacial deep ocean remained anoxic. Thus, despite low primary productivity and seawater sulfate concentration, sustaining turbidity deposition maintained the ocean redox stratification during the waning Sturtian glaciation.

Magnesium–oxygen isotope constraints on the origin of rodingites in oceanic lithosphere

Article

Jun 2023
CHEM GEOL

Dynamic carbon and sulfur cycling in the aftermath of the Lomagundi-Jatuli Event: Evidence from the Paleoproterozoic Hutuo Supergroup, North China Craton

Article

Nov 2019

Carbonate factories and their critical control on the geometry of carbonate platforms (mid-Cretaceous, southern Iran)

Article

Jun 2023
PALAEOGEOGR PALAEOCL

Tropical rimmed platforms represent a much more efficient protection from coastal erosion than carbonate ramps. Understanding the controlling factors of platform geometry is therefore crucial to predict the fate of rimmed platforms under global-warming conditions. Here, we present detailed biostratigraphic, sedimentological, geochemical, and high-resolution carbon-isotope analyses of mid-Cretaceous (Albian-Cenomanian) carbonate rocks well exposed in the Khormoj section located in the Zagros Mountains of southern Iran. Sixteen microfacies were identified, allowing us to define the superposition of different platform geometries, a carbonate ramp in the Albian and a rimmed platform in the Cenomanian. Biostratigraphic and carbon-isotope analyses allowed us to identify the OAE1b, OAE1d, and Middle Cenomanian events in the Khormoj section. The paleodepth changes documented by carbonate microfacies testify to two transgressive events that can be correlated with long-term eustatic curves during the ramp stage. The Albian ramp was formed by orbitolinids under high siliciclastic input whereas the Cenomanian rimmed platform was built by rudists during clear water conditions. These two carbonate factories have drastic different carbonate sedimentation modes. The orbitolinid factory is characterized by a low carbonate production rate and fine carbonate grains resulting in ramp geometry, whereas the rudist factory is characterized by a high carbonate production rate and coarse carbonate grains leading to a rimmed platform geometry.

Variations in extent, distribution and impact of dolomitization on reservoir quality of Upper Cretaceous foreland-basin carbonates, Abu Dhabi, United Arab Emirates

Article

Jun 2023
MAR PETROL GEOL

Petrography, petrophysics, and geochemistry of an Upper Cretaceous, foreland-basin carbonate reservoir, Abu Dhabi, United Arab Emirates, are used to constrain the spatio-temporal variations in the extent of dolomitization and its impact on reservoir quality. Dolomitization in highstand systems tracts (HST) is attributed to repeated tidal and evaporative pumping as well as seepage reflux of penesaline brines during restriction of the platform due to 4th and 5th order cycles of relative sea-level fall and particularly below parasequence boundaries. This interpretation is supported by the presence of rare poikilotopic gypsum cement and scattered laths and micro-nodules of calcitized gypsum in the dolomitized peritidal dolostones. Dolomitization along bioturbation sites, which is most common in the transgressive systems tracts (TST), is attributed to the development of suitable localized geochemical conditions (e.g., microbial sulfate reduction and related increase in carbonate alkalinity). Porosity and permeability of the dolostones are strongly controlled by depositional textures of precursor lime-stones and by subsequent diagenetic evolution. Dolomitization of mudstones, wackestones and matrix-rich pack-stones has resulted in the formation of micropore-dominated microcrystalline dolostones whereas dolomitization of the shoal grainstones resulted in the formation of coarse-crystalline dolostones with abundant well-connected intercrystalline and moldic macropores. Mesogenetic alterations of the dolostones, which are attributed to the flow of hot basinal brines along steep faults, include dolomite cementation and, subsequently, dissolution and calcitization of dolomite (dedolomitization) and calcite cementation. The lack of systematic differences in poros-ity and permeability of dolostones between the oil and water-saturated dolostones suggest that most diagenesis occurred prior to completion of oil emplacement and/or reflect the shallow maximum burial depths of the formation (around 1.3 km). This study demonstrates that variations in the distribution and extent of dolomitization within a sequence stratigraphic context across an oilfield should be considered as primary control on the spatio-temporal reservoir lithology and heterogeneity in carbonate successions.

Genesis of island dolostones during rapid island subsidence: Example from the Xisha (Paracel) Islands, South China Sea

Article

Mar 2023

Miocene carbonate successions found on many isolated oceanic islands throughout south-east Asia commonly include unconformity capped ‘island dolostones’ units. On Shidao, located in the Xisha Islands in the South China Sea, well XK-1 (1268.2 m deep) penetrated a thick succession of carbonates before being terminated in the Jurassic metamorphic basement. The lower part of the succession is composed of the Sanya Formation (Early Miocene) and the overlying Meishan Formation (Middle Miocene), ca 700 m thick, that are formed of limestones and dolostones. An unconformity divides the Sanya Formation into a Lower Member formed largely of limestones, and an Upper Member formed of dolostones (DI-8). DI-8 is separated by a major unconformity into lower (DI-8L) and upper (DI-8U) units, that both include numerous subaerial unconformities. The overlying Meishan Formation is a limestone succession that includes three unconformity-capped dolostones intervals (DI-5, DI-6 and DI-7). Strata below the numerous exposure surfaces throughout the Sanya and Meishan formations are highlighted by Fe-staining, biomoulds, cavities, caves and solution breccia development. The weakly fabric-retentive dolostones in DI-5 to DI-8 are formed of dolomite crystals that have a dirty core encased by a clear rim, with δ¹⁸O values of +0.3 to +4.7‰ (average +2.0‰, n = 50), δ¹³C values of +2.2 to +3.2‰ (average +2.5‰, n = 50), Fe concentrations of 44 to 442 ppm (average 149 ppm, n = 50), Mn concentrations of 5 to 197 ppm (average 27 ppm, n = 50), Sr concentrations of 169 to 257 ppm (average 218 ppm, n = 50) and ⁸⁷Sr/⁸⁶Sr values of 0.70847 to 0.70900 (average 0.70879, n = 32). In all cases, dolomitization post-dated the subaerial exposure. The ⁸⁷Sr/⁸⁶Sr values suggest that the dolostones in DI-8L formed during the late Early Miocene, whereas the dolostones in DI-8U, DI-7, DI-6 and DI-5 probably formed simultaneously during the Late Miocene even though these units are stratigraphically hundreds of metres apart and were located well below sea-level. Dolomitization was preferentially focused in those strata with high porosities and permeabilities that allowed easy circulation of the seawater through the rock. This model suggests that dolomitization was not restricted to strata that were at or close to sea-level. Although dolomitization may have been related to the new oceanographic circulation regime established during that time, the exact properties of the seawater that promoted dolomitization are open to debate. This model of dolomitization may be applicable to many of the ‘island dolostones’ that are found in the oceans throughout the world.

The evolution of the marine carbonate factory

Article

Full-text available

Feb 2023
NATURE

Calcium carbonate formation is the primary pathway by which carbon is returned from the ocean–atmosphere system to the solid Earth1,2. The removal of dissolved inorganic carbon from seawater by precipitation of carbonate minerals—the marine carbonate factory—plays a critical role in shaping marine biogeochemical cycling1,2. A paucity of empirical constraints has led to widely divergent views on how the marine carbonate factory has changed over time3–5. Here we use geochemical insights from stable strontium isotopes to provide a new perspective on the evolution of the marine carbonate factory and carbonate mineral saturation states. Although the production of carbonates in the surface ocean and in shallow seafloor settings have been widely considered the predominant carbonate sinks for most of the history of the Earth⁶, we propose that alternative processes—such as porewater production of authigenic carbonates—may have represented a major carbonate sink throughout the Precambrian. Our results also suggest that the rise of the skeletal carbonate factory decreased seawater carbonate saturation states.

Late diagenetic alteration of carbonate evidenced by carbonate-deficient siliciclastic laminae

Article

Feb 2023
CHEM GEOL

The alteration of carbonate, caused by organic acid release during hydrocarbon generation, is usually neglected in carbonate chemistry, since the unaltered carbonate, which may reflect the (syn)depositional environments, can dilute and conceal the modification signals (e.g., δ13Ccarb and elemental compositions). In carbonate-deficient black shales, however, active hydrocarbon generation may provide sufficient organic acid to dissolve the original minerals, and the geochemical signals may remain evident in the altered carbonate. Moreover, laminae represent the smallest phase in sedimentary rock and can be distinguished by their chemical and mineral compositions, which reflect the alteration reactions and products. Thus, microscale investigation on laminae would help clarify alteration pathways during burial diagenesis and its geochemical effects. In order to understand the diagenetic reactions and organo-modifications of carbonate minerals, we examined both the mineral and geochemical characteristics of the carbonate-deficient, organic-rich, laminated black shales from the Yanchang Formation of the Ordos Basin, North China. Our study demonstrates that the carbonate content is negatively correlated with the ion concentrations within the carbonate lattice (e.g., Srcarb), providing evidence that carbonate phases have undergone considerable late diagenetic alteration during deep burial. This is consistent with the argument that stronger alteration indicators for carbonate should be found in rocks with lower carbonate content. In addition, the (re)precipitation of carbonate minerals, accompanied by corrosion of siliciclastic minerals, is also observed in thin sections, supporting the deep alteration scenario. By confirming the presence of deep diagenetic alteration of carbonate during deep burial and investigating this alteration in detail, this study provides new insights into interpretations of carbonate chemistry.

The source of Pleistocene carbonate below the CCD in the central basin of South China Sea: Evidences from coccolith and geochemistry

Article

Full-text available

Feb 2023
MAR GEOL

Stromatolitic carbonates from the Middle Miocene of the western Pannonian Basin reflect trace metal availability in microbial habitats during the Badenian Salinity Crisis

Article

Feb 2023
CHEM GEOL

Кристаллохимические, минералогические и геохимические особенности доломитов авзянской свиты среднего рифея (Южный Урал)

Article

Oct 2023

Введение: Определены кристаллохимические характеристики и причины их вариаций в слабо- и сильноизмененных эпигенетическими процессами доломитах авзянской свиты среднего рифея Башкирского мегантиклинория. Методы: Рентгенография, рентгенофлуоресцентный анализ, электронная микроскопия с энергодисперсионным микроанализом. Результаты и обсуждения: В доломитах авзянской свиты фиксируются повышенные параметры кристаллической решетки (аср=4.8105 Å, сср=16.0211 Å) и пониженные значения субструктурного рефлекса (Кср=0.66) по сравнению со стехиометрическими доломитами. Средние содержания Fe (II) равны 4545 г/т, Mn – 423 г/т, Sr – 124 г/т. Увеличение параметров кристаллической решетки доломитов авзянской свиты прямо коррелирует с ростом содержаний Fe и Mn, а значения субструктурного рефлекса имеют с ними обратную связь. В породах всех изученных разрезов обнаружено присутствие двух генераций доломита и кальцита. Образование кальцит-доломитовых прожилков происходило при температурах 260–530°С и давлениях 0.3–2.3 кбар. В доломитах разных подсвит средние значения параметров кристаллической решетки а и с уменьшаются от ранне- (катаскинская подсвита) к позднеавзянским (тюльменская подсвита). В этом же направлении увеличиваются средние значения коэффициента К упорядочения структуры доломитов. Заключение: Доломиты разных стратиграфических уровней авзянской свиты подвержены интенсивным эпигенетическим преобразованиям, обусловленным воздействием на породы Fe- и Mn-содержащих средне- и высокотемпературных флюидов. Изменение кристаллохимических характеристик доломитов авзянской свиты связаны не столько с процессами регионального метаморфизма, сколько с деятельностью постмагматических (разрезы у с. Веселовка и на р. Тюльмень) и катагенетических (разрезы у д. Исламбаево и на р. Б. Авзян) флюидов, циркуляция которых происходила по зонам разрывных нарушений.

A New Negative Carbon Isotope Interval Caused by Manganese Redox Cycling After the Shuram Excursion

Article

Full-text available

Mar 2024

Several negative C isotope excursions (CIEs) occurred at the end of the Neoproterozoic era which have been generally attributed to the oxidation of organic carbon using sulfate as the terminal electron acceptor and the subsequent release of ¹³C‐depleted dissolved inorganic C (DIC). Based on new analyses from the Doushantuo Formation in South China, we observe a negative C isotope excursion right after the well‐known Shuram excursion. This excursion is equivalent to the ∼550 Ma negative CIE which is globally expressed within several continental margins. However, the origins of this CIE in the termination of Ediacaran remain unresolved. Here, we hypothesize that this post‐Shuram negative CIE was caused by a localized manganese cycling that began with the oxidation of hydrothermal Mn(II) in a water column to insoluble Mn(IV)‐oxide, followed by accumulation of Mn(IV)‐oxide to the seafloor and its subsequent dissolution via Mn(IV) reduction leading to the release of dissolved Mn(II) and ¹³C‐depleted DIC into ambient seawaters. This ultimately led to the precipitation of particulate Mn(II)‐carbonate characterized by low δ¹³Ccarb values ranging from −11.1‰ to −2.8‰. The presence of microbial fabrics in association with the Mn(II)‐carbonate further suggests that Mn(II)‐carbonate precipitation took place at the seafloor in shallow sun‐lit waters rather than in the deeper sediment pile, which archived ambient seawater C isotopic signal. Although most Ediacaran negative CIEs were generally attributed to sulfate reduction, our findings suggest that at a local level, Mn cycling can also lead to negative CIE in the Neoproterozoic, and potentially at other times in Earth's history.

Authigenic Carbonate Burial Within the Late Devonian Western Canada Sedimentary Basin and Its Impact on the Global Carbon Cycle

Article

Full-text available

Mar 2024
GEOCHEM GEOPHY GEOSY

Stable carbon isotope ratios (δ¹³C values) of marine carbonates are widely used to infer the relative burial rates of organic carbon, a source of oxygen to the ocean‐atmosphere system. This inference, however, is based on the assumption that ocean‐atmospheric carbon is buried either as organic carbon or as marine carbonate minerals. The burial of authigenic carbonate minerals formed within sediments after deposition, with low δ¹³C values (i.e., similar to organic carbon), has been proposed to explain high δ¹³C values in marine carbonates without the need for high burial fluxes of organic carbon. To test this hypothesis, we focus on the Late Devonian, a time period with both pervasive ocean anoxia and a severe reduction in shallow‐water carbonate deposition—conditions hypothesized to promote authigenic carbonate formation. We present sedimentological and geochemical data from limestones and black shales of the Wabamun Group, Besa River and Exshaw formations of the Western Canada Sedimentary Basin. These data are compared to inorganic and organic weight percent measurements of North American shales acquired from the USGS National Geochemical Database (N = 4,437). Results show that basinal shale lack authigenic carbonate with low δ¹³C values and that the mean δ¹³C value of carbonate in these shales (−0.3‰) do not differ substantially from mean δ¹³C of carbonates in platform carbonates of a similar age (0.4‰). Furthermore, inorganic carbon content in Late Devonian shales (mean weight percent = 0.55%, N = 54) is lower than average Phanerozoic North American shale (mean of 1.95%, N = 4,055). Lastly, organic carbon‐to‐inorganic carbon ratios (OC:IC) of North American shales are well above 1 (mean = 3.72 for Late Devonian shales (N = 374), 2.25 for shales (N = 3,653) of all other ages). Therefore, even if the burial of fine‐grained siliciclastic formations carrying authigenic carbonates were to increase, the concomitant increase in organic carbon burial would be even larger. Together, data from this study do not provide evidence that the burial of authigenic carbonate would have a significant effect on global carbon isotope mass balance.

Geochemistry of Palaeo-Proterozoic Kajrahat Limestone, Vindhyan Supergroup, Central India: insights into Depositional Conditions and Sources of Rare Earth Elements

Preprint

Full-text available

Oct 2023

This study investigates the geochemical composition of major, trace and rare earth elements (REEs) in the Palaeo-Proterozoic Kajrahat Limestone, Vindhyan Supergroup, Central India to infer the depositional environment and source for the REEs. The studied limestones, have a high percentage of CaO (ranging from42.27–66.53, n = 17), followed by SiO 2 as the major oxides. Sr is dominant trace element showing a negative correlation with CaO. This indicates that either Sr precipitated its own minerals or it was contributed by the siliciclastic material. All trace elements exhibit depletion with respect to Post-Archean Australian Shale (PAAS) values. The PAAS-normalized REE pattern of studied limestone has a relatively uniform pattern, with slightly enriched LREE compared to HREE, negative Ce anomaly (ranging from 0.66–0.89), negative Eu anomaly (largely ranging from 0.84–1.43), and low U/TH ratio (ranging from 0.12–1.25).The studied limestones exhibit a negative correlation between ΣREE and Fe 2 O 3 , MnO, MgO, Sr but a positive correlation with other elements. These positive correlations suggest that, in addition to seawater, siliciclastic sediments also serve as a source for the REEs. The (La/Yb) N , La/Sc, La/Th, and Th/Sc ratios imply that the terrigenous admixture in studied limestone likely originated from felsic source rocks. The geochemical investigation of studied limestone indicates a low U content (0.3–1.3, n = 17) and U/Th (0.12–1.25, n = 17) ratios suggest an oxygen-rich environment. This, in turn, indicates that the deposition of this limestone occurred in a coastal/ shallow marine environment with some contribution from continental part.

Exploring Icehouse Cyclicity Pattern and Seawater Dynamics on an Ancient Carbonate Platform With Nd Isotopes (Carboniferous, Southern Kazakhstan)

Article

Full-text available

Mar 2023

Over the past decades, neodymium (Nd) isotopes have received considerable attention in paleoceanography as a tool for reconstructing past seawater circulation, local weathering inputs, and sea‐level change. In this study, we have investigated the Nd isotope composition of a shallow‐water Serpukhovian (Carboniferous) carbonate succession to explore icehouse cyclicity pattern and seawater dynamics on the Karatau carbonate platform in southern Kazakhstan. The cyclic succession formed in response to glacio‐eustasy and composed of subtidal and intertidal limestones displays a large variation in εNd(326 Ma) values from −1.6 to +4.3, corresponding to differences in the isotopic composition of two seawater masses present in the Uralian–Turkestan Ocean during the Serpukhovian: (a) highly radiogenic deep waters and (b) less radiogenic surface waters. The Nd isotope excursions within the icehouse cycles are more complex than simple transgressive‐regressive cycles. They probably reflect a temporal pattern of the sub‐Milankovitch climatic perturbations during Carboniferous interglacial intervals. The episodic appearance of rich brachiopod communities was forced by the inflow of highly radiogenic, nutrient‐enriched waters, presumably driven by upwelling. Nd isotope analyses of cyclic intertidal and subtidal carbonates have great potential to produce high‐resolution records of seawater dynamics on shallow‐water carbonate platforms.

Expansion of Reducing Marine Environments During the Ireviken Biogeochemical Event: Evidence From the Altajme Core, Gotland, Sweden

Article

Full-text available

Feb 2023

New δ³⁴Spy (pyrite) and δ³⁴SCAS (carbonate‐associated sulfate) across the Llandovery‐Wenlock boundary (∼432 Ma) provide evidence for the expansion of reduced marine environments during the Ireviken Biogeochemical Event. This event consists of a major positive carbon isotope excursion, increased biotic turnover, and other major perturbations and changes within biogeochemical cycles. This interval of time has been hypothesized to coincide with an expansion of reducing marine environments that caused increased organic carbon burial and led to the Ireviken positive carbon isotope excursion (ICIE). Previous high‐resolution carbon isotope work in the Altajme core from Gotland, Sweden provides the highest resolution record of the ICIE yet documented and provides an ideal expanded stratigraphic section to study this event. Local expansion of reduced marine environments within the deeper shelf setting of the Altajme core is indicated by a positive shift in δ³⁴Spy values and increase in pyrite sulfur concentrations at the onset of the ICIE. These data are indicative of increased microbial sulfate reduction within this portion of the Baltic Basin. Combined with new δ³⁴SCAS data from this core, as well as additional data from distant basins, the new data presented here suggest a global expansion of reduced environments led to an increase in organic carbon burial and the ICIE.

Fingerprinting Kinetic Isotope Effects and Diagenetic Exchange Reactions Using Fluid Inclusion and Dual‐Clumped Isotope Analysis

Article

Full-text available

Feb 2023
GEOCHEM GEOPHY GEOSY

Geochemical analyses of carbonate minerals yield multiple parameters which can be used to estimate the temperature and water composition at which they formed. Analysis of fluid trapped in minerals is a potentially powerful tool to reconstruct paleotemperatures as well as diagenetic and hydrothermal processes, as these could represent the parent fluid. Internal fluids play important roles during the alteration of carbonate fossils, lowering energetic barriers associated with resetting of clumped isotopes, as well as mediating the transport of elements during diagenesis. Here, we explore the behavior of the ∆47–∆48 “dual-clumped” isotope thermometer during fluid-carbonate interaction and demonstrate that it is highly sensitive to the water/carbonate ratio, behaving as a linear system in “rock buffered” alteration, and as a decoupled system in water-dominated systems due to non-linear mixing effects in ∆48. Dry heating experiments show that the extrapolated “heated” end-member is indistinguishable from the predicted ∆47 and ∆48 value expected for the experimental temperature. Furthermore, we evaluate two common laboratory sampling methods for their ability to thermally alter samples. We find that the temperature of the commonly used crushing cells used to vapourize water for fluid inclusion δ¹⁸O analyses is insufficient to cause fluid-carbonate oxygen isotope exchange, demonstrating its suitability for analyses of fluid inclusions in carbonates. We also find that belemnites sampled with a hand-drill yield significantly warmer paleotemperatures than those sampled with mortar and pestle. We conclude that thermally-driven internal fluid-carbonate exchange occurs indistinguishably from isotopic equilibrium, limited by the extent to which internal water and carbonate can react.

Calculation of simultaneous isotopic and trace element variations during water-rock interaction with applications to carbonate diagenesis

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