Multiple Geochemical Proxies Research Articles

How does a soft collision orogen uplift and collapse? Insight from the eastern Central Asian Orogenic Belt

The mechanism of uplift and collapse is critical for understanding orogenic evolution within the Wilson cycle. The Central Asian Orogenic Belt (CAOB) represents one of the largest Phanerozoic accretionary orogens on Earth, experiencing terminal soft collision following the closure of the Paleo-Asian Ocean. However, the timing and mechanism of crustal thickening and thinning in the eastern CAOB remain unclear. Here, we present geochronological, mineralogical, geochemical, and Sr-Nd-Hf isotopic data of the newly identified Late Triassic bimodal dike associations in the easternmost CAOB. The ca. 236−230 Ma mafic dikes can be divided into two groups based on petrographic and geochemical characteristics. Major element modeling using the MELTS software indicates that they evolved via independent differentiation paths. Trace element and isotope simulations reveal that the ca. 236−230 Ma mafic dikes originated from the 4%−10% partial melting of spinel- to garnet-lherzolite lithospheric mantle sources over a range of depths, with varying inputs of asthenospheric mantle materials. Coeval ca. 233 Ma felsic dikes exhibit adakitic geochemical characteristics and strong imprints of crust-mantle interaction, suggesting derivation from melting of a heated juvenile mafic lower crust as a result of the upwelling of asthenospheric mantle. The formation of bimodal dike associations records the transition from lithospheric mantle thinning to delamination. Integrating a large dataset and employing multiple geochemical proxies, our results reveal that the crust of the easternmost CAOB reached a thickness of 54 ± 3 km at ca. 280−255 Ma, likely resulting from magmatic underplating due to rollback of the subducting Paleo-Asian Oceanic slab. This region underwent a further slight increase in crustal thickness to 61 ± 2 km at ca. 254−237 Ma in response to limited tectonic shortening associated with soft collision orogeny before it thinned to 45 ± 13 km at ca. 236−210 Ma due to lithospheric delamination during post-collisional extension. Our findings reveal that the uplift of the eastern CAOB was primarily driven by magmatic underplating, with minimal contribution from tectonic shortening. Lithospheric delamination emerged as an important factor leading to the eventual collapse of the eastern CAOB. Compared to typical hard collisional orogens (e.g., the Himalaya-Tibet orogen), the CAOB experienced significantly weaker tectonic shortening followed by similar lithospheric delamination during post-collisional extension. This study highlights the importance of integrating geochemical and isotopic data in quantifying the complex evolutionary histories of ancient collisional orogenic belts.

Depositional characteristics in the Lower Congo Basin during the Cenomanian-Turonian stage: Insights from fine-grained sedimentary rocks

The Cenomanian-Turonian (C-T) stage, marked by extensive black shales and Oceanic Anoxic Event 2 (OAE 2), is pivotal for deciphering paleoceanographic conditions and the main drivers of hydrocarbon formation. Despite its significant oil and gas potential, the Lower Congo Basin (LCB) in central West Africa, particularly during the C-T stage (Likouala Formation), has been severely understudied, primarily due to limited exploration and a historical preference for other basins. Therefore, understanding the depositional characteristics of the C-T stage in the LCB is essential for elucidating paleoceanographic conditions, evaluating hydrocarbon potential and interpreting environmental changes. Due to the limited availability of published data and samples, we employed a targeted and integrated geological analysis to gain comprehensive insights, encompassing logging, lithofacies identification, thin-section observations, grain size measurements, and geochemical analyses (major, trace, and rare earth elements), to provide a cohesive and confirmatory assessment. Utilizing relative sea-level curves and geochemical indicators, and by comparing with other relevant basins, this study preliminarily identified OAE 2 stratigraphic intervals and attempted to discuss the impact of OAE 2 on organic matter enrichment in the LCB. The results indicate that, during the C-T stage, the LCB featured a restricted sea bay environment with limited terrestrial input and anoxic waters. Multiple geochemical proxies suggest that relative sea-level changes significantly affected paleoproductivity, thereby influencing organic matter enrichment. During the depositional period of Lower Likouala Formation (LLF), a relative sea-level drop resulted in weakly reducing conditions in a marginal restricted sea bay environment, leading to low primary productivity. Conversely, during the depositional period of Upper Likouala Formation (ULF), enhanced aeolian input and sea-level rise increased nutrient supply from the open ocean. This led to elevated surface marine productivity and intensified reducing conditions in the water column, forming a marginal semi-restricted sea bay environment. Enhanced reducing conditions within the lower stratigraphic intervals of the ULF are likely consequences of OAE 2. However, the marginal restricted sea bay environment and limited water exchange may have constrained the enhancement of paleoproductivity by OAE 2. These insights not only significantly enhance our understanding of the organic-rich fine-grained sedimentary rocks in the LCB during the C-T stage, but also contribute to a broader comprehension of the South Atlantic's paleoceanographic conditions and the interplay between regional geological processes and global climate events during the C-T stage.

Mapping paleoelevations along active continental margins with igneous geochemistry: A case study from South America

Mountains and mountain ranges, often situated at convergent plate margins, play a pivotal role in many fields of the Earth, climate, and biological sciences. Reconstructing past episodes of mountain building from the geological rock record is one of the main challenges for unravelling the ancient physical geography of Earth’s surface. Established methods for quantifying past elevations traditionally relied on sedimentary rocks, but in recent years, alternative approaches have emerged on the basis that geochemical signatures of magmatic rocks formed in convergent settings correlate with crustal thickness or elevation. Consequently, methods based on igneous samples have the potential to allow quantitative mapping of past topographic change for large spans of space and time where existing maps are largely based on a qualitative approach. Here, we investigate the application of paleoelevation estimates derived from geochemistry using the western margin of South America as a case study. We investigate their consistency with independent indicators of past elevations such as stratigraphy, stable isotopes, fossils etc. for Cenozoic samples along the Andean margin. For older times, we compare the estimated paleoelevations with more general aspects of the geological record, as well as paleoelevations from global paleogeography models widely used in climate modelling studies, to evaluate the extent to which these models are consistent with the igneous geochemical proxies. We find that estimates based on multiple geochemical proxies, and which account for variations in elevation as a function of MgO, are preferable to those based on individual proxies. The multi-mohometer estimates yield estimates of elevation that better match both present-day topography and Cenozoic paleoelevations, and retain more samples during the data filtering stage. In deeper time, we show that igneous geochemistry quantifies changes in elevation related to documented phases of crustal thickening and thinning, and is thus likely to allow improvements to existing maps of paleotopography.

Dual pathways of aragonite-to-calcite transformation in stalagmites: Implications for paleoclimate reconstructions

Multiple geochemical proxies (e.g., trace elements, δ18O and δ13C) and precise 230Th ages in speleothems are crucial for conducting robust paleoclimatic reconstructions, but the impact of diagenesis on these proxies and 230Th ages is not well-established. In this study, we investigate the petrography and geochemical composition of six aragonite-calcite stalagmites from Shennong Cave in southeastern China to analyze the transformation from aragonite to calcite (primary versus secondary) and its effects on geochemical proxies and 230Th ages. No aragonite relics are observed in columnar calcite in four aragonite-calcite stalagmites (SN55, SN49, SN104 and SNB1), although traces of aragonite exist between the calcite crystals or grow parallel at the aragonite-calcite borders. This is different from the obvious aragonite relics preserved in secondary mosaic calcite in the other two aragonite-calcite stalagmites (SN3 and SN15). Both mosaic and columnar calcite show various changes in geochemical proxies and 230Th ages compared with primary aragonite. The trend in the geochemical proxies analyzed (trace elements, δ18O and δ13C) of columnar calcite remains consistent with those of aragonite, despite significant differences in their absolute values within the overlapping periods. These differences are due to the different fractionation coefficients of isotopes and trace elements in aragonite and calcite formed under the same solution conditions rather than late diagenesis. The 230Th ages of columnar calcite are similar to those of primary aragonite, and the values of subsamples from the same calcite layer are consistent. In contrast, the trace elements and δ13C values of mosaic calcite are similar to those of primary aragonite, while its 230Th age significantly differs from that of primary aragonite, exhibiting no consistent patterns. Therefore, these geochemical characteristics of columnar calcite are significantly different from secondary mosaic calcite, indicating that the columnar calcite in these four aragonite-calcite stalagmites is primary. A comparison of trace elements and δ13C records between aragonite-calcite and pure aragonite stalagmites during the overlapping period further confirms that these columnar calcites are primary. Their geochemical proxies can be used for paleoclimate reconstruction after careful evaluation, and the transformation from aragonite to columnar calcite is caused by increased precipitation. However, due to age and geochemical proxies change, secondary mosaic calcite is unsuitable for paleoclimate reconstruction, unlike columnar calcite. This study provides competent evidence to identify whether calcite in aragonite-calcite stalagmites is primary versus secondary and whether it can be used for paleoclimatic reconstruction.

SISALv3: a global speleothem stable isotope and trace element database

Abstract. Palaeoclimate information on multiple climate variables at different spatiotemporal scales is becoming increasingly important to understand environmental and societal responses to climate change. A lack of high-quality reconstructions of past hydroclimate has recently been identified as a critical research gap. Speleothems, with their precise chronologies, widespread distribution, and ability to record changes in local to regional hydroclimate variability, are an ideal source of such information. Here, we present a new version of the Speleothem Isotopes Synthesis and AnaLysis database (SISALv3), which has been expanded to include trace element ratios and Sr isotopes as additional, hydroclimate-sensitive geochemical proxies. The oxygen and carbon isotope data included in previous versions of the database have been substantially expanded. SISALv3 contains speleothem data from 365 sites from across the globe, including 95 Mg/Ca, 85 Sr/Ca, 52 Ba/Ca, 25 U/Ca, 29 P/Ca, and 14 Sr-isotope records. The database also has increased spatiotemporal coverage for stable oxygen (892) and carbon (620) isotope records compared with SISALv2 (which consists of 673 and 430 stable oxygen and carbon records, respectively). Additional meta information has been added to improve the machine-readability and filtering of data. Standardized chronologies are included for all new entities along with the originally published chronologies. Thus, the SISALv3 database constitutes a unique resource of speleothem palaeoclimate information that allows regional to global palaeoclimate analyses based on multiple geochemical proxies, permitting more robust interpretations of past hydroclimate and comparisons with isotope-enabled climate models and other Earth system and hydrological models. The database can be accessed at https://doi.org/10.5287/ora-2nanwp4rk (Kaushal et al., 2024).

Lacustrine shale lithofacies and depositional environment in the paleocene second member of the funing formation, subei basin, China: Insights into shale oil development prospects

Lacustrine fine-grained sedimentary rocks play a crucial role in the petroleum industry, particularly in the exploration of unconventional oil and gas resources. This study focuses on a petroliferous fine-grained succession deposited prior to the Paleocene–Eocene Thermal Maximum (PETM), specifically, the Paleocene second member of the Funing Formation. This formation represents the primary target for shale oil exploration in the Qintong Depression of the Subei Basin. Employing detailed sedimentological descriptions and utilizing multiple geochemical proxies, our investigation aimed to investigate lithofacies distribution and reconstruct the paleoenvironmental conditions prevalent in the Qintong Depression. Through the integration of lithological observation and well log data, the second member of the Funing Formation was categorized into three distinct system tracts: the early expanding system tract (EEST), the late expanding system tract (LEST), and the highstand system tract (HST). Within the established sequence stratigraphy framework, six lithofacies were identified, displaying comparable lateral and planar distributions. Results indicate that lithofacies exhibit greater thickness in the Deep Sag zone compared to the Slope and Fault Terrace. The EEST is characterized by laminated calcareous mudstone (LCM), laminated dolomitic mudstone (LDM), and massive argillaceous dolomite (MAD). Whereas the LEST predominantly comprises LCM and massive calcareous mudstone (MCM). These lithofacies were deposited under relatively arid climatic conditions, characterized by high salinity and strong terrigenous input, with a moderate abundance of organic matter consisting of type Ⅱ and type Ⅲ kerogen. In contrast, the HST is dominated by MCM with stable thickness, indicative of deposition under relatively humid climatic conditions with low salinity and terrigenous input. These conditions fostered an environment conducive to plankton and algae, resulting in a high abundance of organic matter composed of type Ⅰ kerogen. These new characterizations have significant implications for considering favorable lithofacies regarding the vertical orientation and target zones regarding the planarity of shale oil resources in the Qintong Depression of the Subei Basin.

Origin and Formation Mechanism of the Late Permian Black Siliceous Rocks in the Lower Yangtze Region.

The Late Permian witnessed a Permian Chert Event (PCE) and distinctive oceanic geochemical fluctuations, such as an ocean acidification event, large-scale volcanic eruption, and rapid global warming. However, the links between siliceous rock formation mechanism, ocean, and climate changes are rarely discussed. In this article, two well-preserved deeper-water sections of the Dalong Formation from the Lower Yangtze region in southeast China are selected for analysis. To document the coeval oceanic changes, we present thin section authentication, scanning electron microscope (SEM) observation, and multiple geochemical proxies, including total organic carbon contents (TOC), major element contents, trace element contents, and rare earth element contents (REEs). The results show that siliceous rocks are mainly of biological origin in this region. The low content of MgO (0.10-0.94%, mean = 0.36%) in the Fantiansi section of Tongling area indicates that it is affected by regional hydrothermal fluids. The correlation between Al2O3/TiO2, Al2O3 versus SiO2 /Al2O3, and TiO2/ΣREEs indicates that the Late Permian was greatly influenced by the continuous input of terrigenous materials. The correlations of MnO/TiO2, LaN/CeN, Ce/Ce*, and LREE/HREE imply that the Dalong Formation siliceous rocks were deposited in a continental margin setting. Redox geochemical data (EFU, EFMo, and EFV) imply the water column experienced widespread anoxic/euxinic during the Late Permian, which aided in the preservation of organic matter following biological decay. The accumulation of siliceous rocks is related to South China experiencing a hot tropical climate, coastal upwelling and continental weathering-enriched marine nutrients, fostering high primary productivity, and benefiting abundant siliceous zooplankton. Volcanic and regional hydrothermal activity further enhanced nutrient flux, and the simultaneous ocean acidification event provided favorable chemical conditions for the preservation of silica, leading to the formation of siliceous rocks in the Dalong Formation.

A dolomite-based record of seawater calcium isotope composition over the Neogene

The calcium isotope composition of seawater (δ44/40Casw) records important information on the evolution of the calcium and carbon cycles over geologic time. Over the past two decades, many attempts have been made to reconstruct the Neogene δ44/40Casw variation using various calcium isotope records of seawater or seawater-like pore fluids. Nonetheless, large discrepancies still exist among these Neogene δ44/40Casw records. Here we report δ44/40Ca values in a suite of Neogene island dolomites (20.8–3.4 Ma, n = 18) from the South China Sea. Combining multiple geochemical proxies (δ44/40Ca, δ13C, and δ18O, this study; Sr/Ca, ∑REY, δ7Li, δ26Mg, 87Sr/86Sr, and Δ47, previous studies) and evidence from petrography, fluid inclusions, and magnetostratigraphy, we suggest that the dolomitization for these dolomites was buffered by seawater-like fluids in the shallow marine burial domain or even marine diagenetic domain (<180 m) over a considerable period of time (<2 Myr). Then, we apply the equilibrium calcium isotope fractionation factor between dolomite and seawater of −0.52 ± 0.20 ‰ (2SD, n = 13) estimated from the literature to these fluid-buffered dolomites and reconstruct a new Neogene δ44/40Casw record. After using the Locally Weighted Regression (LOWESS) curve fitting method, our dolomite-based δ44/40Casw record is consistent with the reconciled seawater record from other calcium isotope archives, including shark teeth, corals, foraminifera, brachiopods, barites, phosphates, and bulk pelagic carbonates. Our results support the use of fabric-retentive early marine diagenetic dolomites to trace the evolution of δ44/40Casw through Earth’s history.

The covariation between Mg[sbnd]C isotopes and trace elements in dolostones: Implications for reconstructing the seawater chemistry

Magnesium isotopes of marine dolomites have the potential to reconstruct the Mg isotopic signals of coeval seawater. However, a massive dolostone sequence can form via the coupling effects of multiple dolomitization events, which can potentially reset the Mg isotopic compositions of dolomites during each migration of the dolomitization fluid. How to distinguish changes of seawater chemistry and early diagenetic effects which have been recorded in dolomites is crucial for the reconstruction of seawater Mg isotopic signals. Here we studied the Mg-Sr-C-O isotope systematics and trace elements of a dolostone sequence from the Qiangtang Basin in the hinterland of Tibet. Multiple lines of evidence consistently indicate the lack of deep-burial diagenesis and high-temperature alteration in the carbonates. The stratigraphic variations of isotopic and elemental compositions of the dolostone profile are generally characterized by the co-variation between MgC isotopes and trace element contents. Our modeling results indicated that a single downward dolomitization can cause the large Mg isotopic variability of dolomites, ca. 1 ‰, within a dolomitization front. However, when the limestones in the profile were completely dolomitized, the coupling effects of multiple downward dolomitizations will homogenize the δ26Mg values of dolostones. Therefore, we attribute the observed co-variation in isotopic and elemental signals in the dolostone sequence to the changes in the chemistry of evolved seawater within the platform. The reconstructed seawater δ26Mg values from the Carnian to Early Jurassic are lower than previous model-based estimations. This result implies an overestimation of the production of shallow-water carbonate in literature during Late Triassic to Early Jurassic. Our study highlights the importance of utilizing multiple geochemical proxies to identify the controlling factors driving Mg isotope variability in dolomites.

Petrographic and geochemical characteristics of the Middle Devonian Eifelian Nabiao Formation in the Youjiang Basin, South China: Implications for paleoenvironment and organic matter accumulation mechanism

The Nabiao Formation of the Middle Devonian Eifelian period has been identified as a promising site for shale gas exploration in the Youjiang Basin, South China. And yet, little is known about the accumulation mechanism of organic matter under transgression in the archipelago paleogeography. In this study, the core samples were analyzed by thin section microscopy, field emission-scanning electron microscopy, pyrite morphology, X-ray diffraction, and geochemical analysis (total organic carbon (TOC) content, sulfur content, organic petrography, inorganic carbon isotope and major and trace elements) to understand changes in and controls on the mineralogy, lithofacies, depositional environments, as well as the organic matter accumulation mechanism of the Nabiao Formation. The study indicates that organic-rich sediments were deposited during the two transgressions of the Chotec Event and the Kacak Event. The deposition of carbonaceous mudstone occurred in water columns with weak upwelling, while siliceous mudstone and siliceous shale were developed in a strong upwelling water column. The correlations between multiple geochemical proxies (paleoredox, paleoproductivity, and terrestrial detrital influx) and TOC content indicate that the Chotec Event is more conducive to organic matter enrichment of the Nabiao Formation than the Kacak Event. In stage 1, TOC content is low due to oxic water columns and low productivity. In stage 2, the Chotec Event causes the relative sea-level of Member 2 to rise rapidly and the basin bottom water to quickly change to dysoxic conditions. This is the key to organic matter enrichment. In stage 3, the oceanic upwelling is crucial in organic matter enrichment by increasing paleoproductivity. Under the protection of dysoxic condition, organic matter is accumulated in stage 4. Organic matter of the Nabiao Formation is not significantly affected by the terrigenous detrital flux. By taking into account the changes in paleoproductivity, redox conditions, upwelling events, and terrigenous detrital flux, an evolution model is proposed in this paper to elucidate the paleoenvironmental change and the organic matter enrichment under transgression in the archipelago paleogeography.

Effects of Emeishan Large Igneous Province on organic matter accumulation, Sichuan Basin, China

Volcanic activity is often associated with the formation of organic-rich sedimentary deposits. However, the relationships between volcanism and depositional events of hydrocarbon source rock during geological history remain unclear. To fill the knowledge gap, we investigated the relationships based on a case study of the Emeishan Large Igneous Province (ELIP) during the late Guadalupian. We carried out a detailed petrological and geochemical investigation of the upper Maokou Formation (i.e., the Mao III Member) in the Shangsi section, Sichuan Province, China. The Mao III Member has a relatively high abundance of organic matter (OM), good OM type, and appropriate thermal maturity that is conducive to hydrocarbon generation. Paleoenvironmental reconstructions based on multiple geochemical proxies indicate that the Mao III Member records relatively high primary productivity, enhanced anoxic conditions, intense upwelling but weak water mass restriction, decreased detrital input and sedimentation rate, and fluctuations in volcanic and hydrothermal activity. The marked positive correlations between primary productivity, redox conditions, and OM abundance suggest that the former two factors controlled OM accumulation. In addition, the detrital input has a slight dilution effect on the OM abundance. No direct relationship was observed between hydrothermal activity and OM abundance. In contrast, volcanic activity related to ELIP had a major role in enhancing the primary productivity and more reducing conditions, which is confirmed by the marked correlations between Hg/TOC, Cuauth, and Vauth values. This suggests that ELIP volcanism had a significant positive effect on OM enrichment in the Mao III Member, which may be a common feature associated with large volcanic events at other times in Earth's history.

Contributions of the Yangtze River and Yellow River to terrestrial organic carbon deposition in the Yangtze River Estuary during the last 700 years

Terrestrial organic carbon (OC) could be transported to marginal oceans by large rivers and deposited under different hydrological conditions. A mixture of the fresh & lithospheric OC, superimposed by the OC from different rivers, makes it challenging to distinguish sources and contributions of terrestrial OC. Here, we employed multiple geochemical proxies, including the hydrogen isotopic composition of leaf wax (δDwax), the carbon isotopic composition of total organic carbon (δ13Corg), TOC/TN ratio, and major element distributions, to investigate the variability in sources of terrestrial OC in the Yangtze River Estuary (YRE) since 1270 CE. Our results revealed that the terrestrial OC deposition in the YRE was lower before 1950 CE and considerably increased after 1950 CE. The δDwax record in core A3 indicates that the OC deposition in the YRE was primarily modulated by the variability of El Niño-Southern Oscillation (ENSO) and East Asia Summer Monsoon. The strengthened El Niño-like state since 1600 CE resulted in increased precipitation in the middle and lower reaches of the Yangtze River basin and in turn, affected the terrestrial OC deposition in the YRE. Several episodes with lower carbon preference index (CPI) values of n-alkanes (CPI < 3) were identified. These episodes were concurrent with the streamflow shift of the ancient Yellow River during the transitions from wet to dry conditions, highlighting the potential contribution of the ancient Yellow River to the aged OC deposition in the YRE.

Organic matter accumulation mechanisms and source rock potential of the Mesoproterozoic Xiamaling Formation, Yanliao Basin, North China

The Mesoproterozoic Xiamaling Formation (∼1.4 Ga) is considered to be a set of promising strata for shale oil and gas exploration and production in the Yanliao Basin, North China Craton. This study aimed to investigate the organic matter accumulation mechanism, and hydrocarbon generation and expulsion characteristics of the Xiamaling Formation, using various methods, including scanning electron microscopy, rock-eval pyrolysis, and elemental geochemistry. Multiple geochemical proxies indicate that Unit 3—Unit 1 of the Xiamaling Formation was deposited in a climate that alternated between highly humid and semiarid conditions, with the strong-to-weak terrigenous influx. The redox conditions of bottom water were undulatory oxic in Unit 3 and the upper part of Unit 1, but anoxic in Unit 2 and the lower part of Unit 1. Sulfurization of organic matter was the main reason for the high TOC content in Unit 3. However, the high TOC content in Unit 2 and the lower part of Unit 1 was caused by an oxygen-deficient water column. Unit 2 and the lower part of Unit 1 exhibit a significant shale oil potential owing to low maturity, high hydrogen index (HI) values, and prevalence of type Ⅰ, II1, and II2 kerogens.

Assessment of Sediment Quality Through Multiple Geochemical Proxies in a Heavily Industrialized Harbor Area in South America, Santos Estuarine Channel, São Paulo, Brazil

ABSTRACT The aim of the present study was to evaluate bottom sediments quality in 32 stations at Santos Estuarine Channel, home to one of the main international ports in South America, and to the Cubatão industrial complex, the most important petrochemical and metallurgical area in Brazil. Sediment quality was approached through contamination (mCd), ecological (PEI, SQG-Q, PLI) and geoaccumulation (Igeo) indices. Phosphorus Enrichment Index (PEI) revealed phosphorus enrichment, with potential risk to biota regarding eutrophication effects. Index of geoaccumulation (Igeo) revealed accumulation of Pb, Cu, Zn, Ni, Cr, Cd and As, all reaching class 6 of contamination, pointing to extreme contamination. Contamination index revealed null to moderate contamination for most stations. Pollution Load Index (PLI) revealed no contamination, and Sediment Quality Guideline (SQG-Q) pointed to moderate impact, suggesting risk to benthic biota. The mud content of the samples did not correlate with arsenic, phosphorus and trace metals. Organic matter seemed to be the main carrier of these elements in the system. Results revealed accumulation of trace metals and As in bottom sediments of Santos estuarine channel, which can be used to develop guidelines for monitoring these pollutants in anthropogenic activities along the harbor area.

Geochemical characteristics of shales from Upper Carboniferous Yanghugou formation in Weiningbeishan area, China: Implication for provenance, source weathering and tectonic setting

The Upper Carboniferous paralic succession in Weiningbeishan area has been a preliminary target for shale gas exploration in a new round of strategic mineral prospecting operations (2021–2035). However, the provenance, source weathering, paleoclimate conditions and tectonic setting of this succession have not yet been studied. The mineral compositions and geochemical characteristics of shale samples from ZK03 well in the east of the area are investigated to constrain the setting of the region and to establish the tectonic and sedimentary evolution model for this period. Yanghugou shales are a clay dominated lithotype, but an advantageous condition of interbedded sandstone and coal seams enables an improvement in the hydraulic fracturing capacity of these shales. The shales are relatively enriched in Al, Fe, Ti, Sc, V, Co, Ga, Nb, Cs, Th, U, Zr and Hf relative to UCC, but depleted in Si, Na, Cr, Ni and Rb. The rare earth elements (REEs) display an enrichment of light rare earth elements (LREEs) and a depletion of heavy rare earth elements (HREEs) relative to chondrite; a distinct negative Eu anomaly is also present. The plot of Zr/Sc versus Th/Sc and the index of compositional variability (ICV) values suggest that the shales are deposited in the first sedimentary cycle and are not the results of sedimentary reworking. The Al2O3–CaO*+Na2O–K2O (A–CN–K) ternary diagram and the chemical index of alteration (CIA) values combined with the cross plots of C-value versus CIA and K2O/Al2O3 versus Ga/Rb imply that the parent rocks of shales underwent a strong degree of weathering under a humid and warm paleoclimate. Multiple geochemical proxies indicate that Yanghugou shales are mainly originated from diorite and granite (a felsic-intermediate igneous source), formed in a continental island arc environment that were mainly prevalent in the Beidasha, Nuoergong, Yamatu and Langshang regions in Alxa block. Under the influence of transgression of paleo-tethys ocean and warm and humid late Carboniferous climate, high quality source rocks (TOC>1.5%) are developed in Weiningbeishan area.

High productivity promoted exceptional fossil preservation of the early Middle Triassic Luoping biota of Yunnan Province, China

The early Middle Triassic (Anisian) Luoping biota is representative of marine ecosystems following their full recovery after the Permian-Triassic mass extinction; however, its exceptional preservation remains poorly understood. In this paper, we report multiple geochemical proxies (TOC, TN, P/Al, Cuxs, Nixs, Baxs, δ13Ccarb, and δ13Corg) from Member II of the Middle Triassic Guanling Formation at the Xiangdongpo section in Luoping County, Yunnan Province, China, to assess the relationship between primary productivity and exceptional preservation of the Luoping biota. Variations in TOC, TN, P/Al, Cuxs, and Nixs values show that exceptional preservation coincides with two intervals of high productivity. Relatively low C/N ratios and distributed Δ13Ccarb-org values indicate that primary productivity was dominated by eukaryotic algae and prokaryotic microbes. Based on geochemical analyses and regional correlations, we conclude that, following sea level rise, nutrients were supplied to the Luoping area by upwelling from the open ocean and facilitated the blooming of marine communities in an open platform setting. In the intra-platform depression, increased nutrients supply also fueled high primary productivity in surface waters and oxygen consumption in the water column, causing bottom water anoxia. The lack of oxygen in the bottom water reduced the rate of biomass degradation and bioturbation, promoting growth of microbial mats, leading to the exceptional preservation of macrofauna. This study highlights the important role of elevated primary productivity in exceptional fossil preservation by triggering anoxia of bottom waters. Our findings confirm the widespread development of anoxic conditions during the middle Anisian (Pelsonian), from the eastern Tethys to the Panthalassa, and also reveal the interactions between organisms and the environment after Permian-Triassic mass extinction.

Editorial preface to special issue: Extreme environments and biotic responses during the Neoproterozoic-Phanerozoic transition

This special issue deals with Extreme environments and biotic responses during the Neoproterozoic-Phanerozoic transition and comprises 23 papers. In this preface, we summarise the major findings and place them in a wider context. Some key discoveries are as follows: (1) After the Marinoan global glaciation (end-Cryogenian), a rise in seawater carbon isotopic composition of dissolved inorganic carbon (δ13CDIC), together with various biomarkers, indicate an increase in marine primary productivity, and imply a postglacial increase in atmospheric O2 level by 3.813.4% of present atmospheric levels. (2) The earliest Cambrian sulfidic ocean was characterized by widespread but low levels of microbial sulfate reduction (MSR) in seawater with active microbial iron reduction in sediment porewaters; this state differs from present day sulfidic water masses, such as those in the Black Sea, which show predominantly MSR in the sediment. Multiple geochemical proxies demonstrate that a dynamic ‘euxinic wedge’ may have existed at mid-water depths, and its expansion/contraction controlled redox variations during the early Cambrian. (3) Studies of Hg concentration and isotope analyses shed new light on the Late Ordovician mass extinction event, and reveal that volatile volcanism probably acted as a trigger for several catastrophes, involving climate change, ocean acidification, and anoxia. (4) Other findings show that primitive foraminiferans may have occurred already prior to the Cambrian Explosion, brachiopod-dominated shell beds were already present in the early Cambrian, and reef-builders diversified during the Middle Ordovician, altering our understanding of Cambrian-Ordovician evolution. The papers in this special issue therefore improve knowledge of the Neoproterozoic-Phanerozoic transition and will be of interest to a broad readership of palaeontologists and earth system scientists.

Marine redox evolution and organic accumulation in an intrashelf basin, NE Sichuan Basin during the Late Permian

The Late Permian oceanic redox conditions are suggested to have had a causal relationship with the latest Permian mass extinction (LPME); however, the tempo-spatial variations of marine redox conditions which could have controlled environmental context of the LPME and organic accumulation remain loosely constrained. Here, we selected an organic-rich Lopingian succession deposited in an intrashelf basin, NE Sichuan Basin, SW China. To document the coeval oceanic changes, we present multiple geochemical proxies, including iron speciation, (major and trace) element contents, mercury (Hg) contents, total organic carbon contents (TOC) and carbonate carbon isotopic composition. Four intervals (I-IV) of discrete redox conditions were identified based on Fe–Mo–U–V datasets, revealing the evolutions from oxic to euxinic, and further to ferruginous conditions. Meanwhile, the primary productivity increased, then decreased in these intervals. The euxinic water wedge appeared in parallel with the high primary productivity and enrichment of organic matter, and advanced upslope and retreated downslope dynamically in the pace of sea-level rise and fall. In this case, marine redox variations were related to interactions of eustatic changes, organic decomposition, paleogeographic setting, upwelling currents, terrigenous input and volcanic activities. Compared with previous studies from other continents, it seems that the Paleo-Tethys was more anoxic and stagnant than Neo-Tethys and Panthalassa during the Late Permian, due to paleogeographic contexts and the more restricted oceanic circulation. In general, the prominent marine anoxia prevailed in the moderate water depth, and predated the major pulse of the LPME in several sections, which implies that anoxia may have not acted as the direct “killing mechanism” of this event.

Organic Matter Accumulation in the Youganwo Formation (Middle Eocene), Maoming Basin, South China: Constraints from Multiple Geochemical Proxies and Organic Petrology

Organic Matter Accumulation in the Youganwo Formation (Middle Eocene), Maoming Basin, South China: Constraints from Multiple Geochemical Proxies and Organic Petrology

Influence of a Paleosedimentary Environment on Shale Oil Enrichment: A Case Study on the Shahejie Formation of Raoyang Sag, Bohai Bay Basin, China

The characteristics of paleosedimentary environments are of great significance for the enrichment of organic matter (OM) and hydrocarbons in lacustrine shale. This study analyzed mineralogy, well logging data, organic geochemical parameters (total organic carbon and pyrolyzed hydrocarbon), inorganic geochemical parameters (major and trace elements), and multiple geochemical proxies based on inorganic geochemical parameters. These were used to reconstruct the paleosedimentary environment of the lower 1st Member of the Shahejie Formation (Es1L) to reveal OM and shale oil enrichment mechanisms and establish a shale oil enrichment model. The (Fe2O3+Al2O3)/(CaO + MgO), Sr/Ba, Rb/Sr, Cu/Al, and Th/U parameters indicate that the Es1L in Raoyang Sag was deposited in a paleoenvironment dominated by arid paleoclimate, reducing conditions, and saltwater. Paleoclimate, clastic influx intensity, preservation conditions, paleoproductivity, and paleosalinity all affect OM abundance. The OM accumulation in the shale of Es1L was mainly controlled by the high primary productivity of surface water due to algal blooms and moderate salinities, which was achieved using stratified water columns with low oxygen conditions in bottom water. As the main valuable sites for shale oil storage, carbonate mineral depositions are of great significance for oil enrichment. As the dominant lithofacies for oil enrichment, carbonate-rich shale and calcareous shale lithofacies were deposited under a drier paleoclimate, low clastic influx intensity, strong reducing conditions, high paleoproductivity, and moderate salinity paleoenvironment. Additionally, the profile of the shale oil sweet spot was determined through the combination of lithofacies, logging, and paleosedimentary environment data.