Stratigraphic Variations Research Articles

Geoenvironmental and geophysical methods applied to identify natural attenuation process on a complex hydrogeological environment contaminated by hydrocarbons

Environmental contamination found in urban areas generally has a chemical composition that favors the generation of a variety of physical, chemical and biological processes that differ from natural soil parameters. Thus, understanding natural attenuation processes in tropical environments is still a challenge, as is understanding how biogeochemical processes can influence and modify natural soil characteristics. The study aimed to apply methods that are not commonly used in investigations of contaminated areas, in order to assess the applicability of non-invasive methodology for detailed stratigraphic characterization and biogeochemical changes resulting from contamination by hydrocarbons in a complex hydrogeological environment site. To date, few studies have presented results obtained using non-invasive techniques to understand biochemical processes. The research was based on the interpretation of results obtained by high-resolution physical environment characterization techniques (such as gamma geophysical profiling and electrical conductivity surveys) and chemical analysis of groundwater. The data was interpreted to obtain a detailed characterization of the study area and evidence of the occurrence of natural attenuation of creosote contamination. The results indicated that the combination of geoenvironmental and geophysical methods is an interesting alternative for obtaining data and understanding the mineralogical and stratigraphic variation of the contaminated area. The physical parameters variations obtained by the methods suggested the presence of local geochemical alterations resulting from natural processes of attenuation of contamination by aliphatic and aromatic hydrocarbons in a tropical environment. The natural attenuation processes were confirmed through groundwater samples, biological analysis and metagenomic classification.

Testing the fidelity of zircon as a provenance indicator in fluvial‐fan successions: An example from the Palaeogene Colton Formation, Central Utah, USA

AbstractThe stratigraphic, spatial and temporal variability of detrital zircon age populations in continental sedimentary successions is a critical tool in understanding palaeodrainage networks and how these systems distributed detritus within sedimentary basins. However, multiple factors, such as variations in sediment‐transport processes, the scale of the depositional environment and the architecture of the sedimentary succession are often overlooked in detrital zircon studies. This article presents detrital zircon U–Pb geochronology from the fluvial‐dominated Colton Formation in the western Uinta Basin (Utah, USA) to assess the system's provenance and evolution. Significant differences in zircon age populations between the Colton Formation and the overlying Green River Formation suggest a reorganisation of the source‐to‐sink system during the transition between the two lithostratigraphic units. Notably, detrital zircon age spectra are not homogeneous across the Colton Formation, therefore physical morphometric parameters were used to verify the possible influence of selective bias during sediment transport. These data reveal that a relatively finer‐grained population of Precambrian, and to a less extent Mesozoic, zircon grains were affected by hydraulic sorting during transport, resulting in a greater relative abundance of older zircon grains in the distal reaches of the distributive fluvial system, whose basinward decrease in competence would have increased the relative proportion of finer zircon fractions in sandstones. Furthermore, there are different trends in the distributions of zircon age populations relative to their stratigraphic position, highlighting the complex architecture of the fluvial palaeo‐fan. The spatial and stratigraphic variability of provenance signals in fluvial‐fan successions must be carefully evaluated to improve the reliability of source‐to‐sink models and palaeodrainage reconstructions, as autogenically controlled noise can be generated during the dispersal of detrital zircon in fluvial sedimentary systems.

Depositional environments and thermal maturity of the hydrocarbon source rocks in the Devonian–Early carboniferous Ora Formation from palynological organic petrographic investigations in northern and western Iraq

The study investigates the palynofacies, organic matter character, and hydrocarbon generation potential of this rock unit, based on surface and subsurface samples from the Ora Formation type-section in extreme northern Iraq and the Akkas-3 well in western Iraq, respectively. Dark, mostly oxidized, gelified platy amorphous organic matter (AOM) dominates the palynological components in the surface section, in addition to a few rounded spores and structured phytoclasts. In contrast, palynomorphs, including Ambitisporites avitus, Aneurospora spp., Vallatisporites verrucosus, Acinosporites spp., Verrucosisporites nitidus and the algae Botryococcus, are predominant in studied samples from the Akkas-3 well. Statistical cluster analysis identified three palynofacies types in the surface section based on stratigraphic variations in the particulate organic matter. These vary from high amorphous organic matter (AOM), moderate phytoclast, and low palynomorph abundances that represent proximal suboxic–anoxic shelfal environments to moderate to good AOM and low to moderate palynomorph abundances that represent distal suboxic–anoxic or distal dysoxic–anoxic shelfal environments. The organic petrographic study of the outcrop section also revealed the strong effect of oxidation, where dispersed terrigenous and amorphous organic materials in the form of granular and gelified forms dominate and reflect a terrestrial origin of these components. In the subsurface section, a mixed terrestrial and less marine or lacustrine origin characterized the studied organic matter, where land plant spores (sporinite), in addition to vitrinite and inertinite, are dominant with a few scattered liptinite macerals. The difference in thermal maturity between the outcrop and subsurface samples is likely due to the higher tectonic burial of the outcrop samples that form part of the Northern Thrust Zone of Iraq. Nevertheless, higher abundances of AOM (oil-prone kerogen type II) accumulated in the northern outcrop type section. This might imply that the Ora Formation has a higher potential for hydrocarbon production north of the Akkas field.

Metagenomic insights into microbial community structure and metabolism in alpine permafrost on the Tibetan Plateau

Permafrost, characterized by its frozen soil, serves as a unique habitat for diverse microorganisms. Understanding these microbial communities is crucial for predicting the response of permafrost ecosystems to climate change. However, large-scale evidence regarding stratigraphic variations in microbial profiles remains limited. Here, we analyze microbial community structure and functional potential based on 16S rRNA gene amplicon sequencing and metagenomic data obtained from an ∼1000 km permafrost transect on the Tibetan Plateau. We find that microbial alpha diversity declines but beta diversity increases down the soil profile. Microbial assemblages are primarily governed by dispersal limitation and drift, with the importance of drift decreasing but that of dispersal limitation increasing with soil depth. Moreover, genes related to reduction reactions (e.g., ferric iron reduction, dissimilatory nitrate reduction, and denitrification) are enriched in the subsurface and permafrost layers. In addition, microbial groups involved in alternative electron accepting processes are more diverse and contribute highly to community-level metabolic profiles in the subsurface and permafrost layers, likely reflecting the lower redox potential and more complicated trophic strategies for microorganisms in deeper soils. Overall, these findings provide comprehensive insights into large-scale stratigraphic profiles of microbial community structure and functional potentials in permafrost regions.

Evolving fill‐and‐spill patterns across linked early post‐rift depocentres control lobe characteristics: Los Molles Formation, Argentina

ABSTRACTInherited rift topography controls the sediment routing, timing of sand supply, and sedimentary linkage of early post‐rift depocentres. Exhumed examples of early post‐rift turbidite systems are rare and previous studies have examined the evolution of individual depocentres; in contrast, the detailed evolution of early post‐rift turbidite systems across multiple depocentres has never been documented. Current fill‐and‐spill models do not detail the stratigraphic architecture and evolution of sedimentological characteristics of multiple intraslope fans developed across topography, including bed type distributions. Here, the evolution of three intraslope fans that developed across two early post‐rift depocentres is documented along an 18 km long transect in the southern Neuquén Basin, Argentina. The relative chronology of sand supply in depocentres is constrained with new U–Pb ages, and sediment source areas with provenance analysis. The early post‐rift intraslope fans record progradation of the system and progressive sedimentary linkage of post‐rift depocentres, transverse to local syn‐rift structures, with sediment routing subparallel to the cratonic basin margin. The large‐scale stratigraphic architecture of intraslope fans indicates an evolution as a fill‐and‐spill system, with initial confinement through flow stripping and overspill to spillover with erosion and bypass across a transverse topographic high separating the depocentres. Changes in early post‐rift intraslope fan characteristics, including thickness, sandstone content, lobe complex stacking patterns, stratal termination patterns and bed type distribution, record changing confinement through time within a depocentre, and spatially across depocentres. The strong spatial and vertical stratigraphic variability of transitional flow deposits and hybrid event beds reflects enhanced erosion, sediment bypass and flow transformation across transverse relief between the two depocentres during the spillover phase. These findings advance current understanding of early post‐rift turbidite systems and refine fill‐and‐spill models, which will help the prediction of spatial and vertical changes in rock quality and connectivity in subsurface hydrocarbon reservoirs and CO2 storage sites.

Provenance response to evolving palaeogeography recorded by Carboniferous sandstones in the northern Pennine Basin, UK

On the basis of a combination of heavy mineral data, provenance-sensitive heavy mineral indices, garnet major element chemistry, rutile trace element chemistry and zircon U–Pb geochronology, six major changes in sandstone provenance during Carboniferous deposition in the northern Pennine Basin, UK, have been recognised. These changes are a manifestation of both tectonic and climatic factors. The earliest Tournaisian sediment was supplied from the local Southern Uplands High, but the increasingly humid climate led to the establishment of the Pennine River system, which introduced northerly-derived sediment from farther afield in the mid Tournaisian. This system was operative until the mid Bolsovian, but shows stratigraphic variations due to changes in input from different parts of the Pennine River hinterland (East Greenland, northern Scotland, western Norway). These variations are believed to be at least partly related to tectonism, since the maximum supply from high-grade metamorphic sources in East Greenland in the Namurian was concurrent with a tectonically-driven change in drainage direction in this part of the northern sourcelands. In addition, at the base of the Visean, there was a temporary influx of mature sediment coincident with a climatically-driven regression and, at the base of the Duckmantian, sediment was briefly introduced from the west as a far-field manifestation of Alleghanian tectonism. Finally, continued uplift of the Variscan mountain belt to the south of the UK led to establishment of northward-directed transport systems that reached the northern Pennine Basin in the mid-Bolsovian, leading totermination of supply from the Pennine River.

Climate-induced shift of deep-sea benthic foraminifera at the onset of the mid-Brunhes dissolution interval in the northeast tropical Indian Ocean

The mid-Brunhes dissolution interval (MBDI; Marine Isotope Stage (MIS) 13 to 7; ~ 533–191 ka) is characterized by various paleoclimatic/paleoceanographic events in the world. We investigated fossil deep-sea benthic foraminifera and sediment geochemistry at the onset of the MBDI (~ 670–440 ka) using Ocean Drilling Program (ODP) Site 758 and core GPC03 in the northeast tropical Indian Ocean (TIO), primarily focusing on the relationship between the paleoceanographic conditions of the surface and deep oceans. Based on multi-dimensional scaling, MDS axis 1 is related to the specific depth habitats of benthic foraminiferal fauna, possibly at the trophic level. In MDS axis 1, the difference between the two core sites was smaller from ~ 610 to 560 ka, whereas it was larger from ~ 560 to 480 ka. In contrast, MDS axis 2 may be related to the low food supply at episodic food pulses/relatively stable and low food fluxes. MDS axis 2 showed generally similar stratigraphic variations between the two cores during ~ 610–560 ka, but was different during ~ 560–480 ka. The proportion of lithogenic matter to biogenic carbonate was relatively low from ~ 610 to 530 ka under the highstand when sediment transport to the study area was reduced. Thus, both the depth gradient in the distribution of benthic foraminiferal fauna and the lithogenic supply between the two cores changed coincidently across the MIS 15/14 (~ 570–540 ka) transition. Such paleoceanographic conditions across MIS 15/14 transition were attributed to the long-term weakening of the wind-driven mixing of surface waters, which might have been caused by the weakening of the Indian summer monsoon in the northeast TIO, possibly with the northward displacement of the InterTropical Convergence Zone in the Northern Hemisphere. In particular, the depth gradient in the distributions of benthic foraminiferal faunas represents the paleoceanographic linkage between the surface and deep oceans through particulate organic matter ballasting by calcareous plankton skeletons in addition to lithogenic matter, which changed transiently and significantly across MIS 15/14 transition close to the onset of the MBDI.

Enrichment mechanism of selenium in Lujiaping formation in central South Qinling: Controls from mixed sources, sedimentary environment, primary productivity and paleo-climate during Ediacaran to early Cambrian period

The Ediacaran to lower Cambrian Lujiaping Formation in South Qinling exhibited extremely high level of Se enrichment, which has provoked extensive concerns since the critical role of Se in paleo-biological evolution and diversification as well as its bidirectional effects on current ecological environment and human health. Because of complex geological setting within South Qinling during the Ediacaran–early Cambrian interval, distribution pattern and mechanism of Se enrichment in Lujiaping Formation have not been fully understood. In this study, therefore, stratigraphic variations of Se concentration and enrichment level together with enrichment mechanism in Lujiaping Formation in the central part of South Qinling were systematically explored. The result showed that, in the whole Lujiaping Formation, Se concentrations varied from 0.19 to 129 μg/g, with an average of 13.28 μg/g. According to the enrichment factor, most of samples can be classified as strong enrichment and anomaly levels (EF = 10–1000) which mainly occurred in organic matter-enriched siliciclatic rocks including mudstone, shale and stone coal seams. Meanwhile, Se enrichment was accompanied with enrichment of Ag, Mo, As, Cd and V in Lujiaping Formation. Various geochemical proxies were collectively employed to identify potential mechanism of Se enrichment. The results indicated that Se was derived from both terrestrial detrital materials and deep hydrothermal fluids. Strong Se anomaly was intimately linked to authigenic enrichment of U and Mo, manifesting advantageous role of euxinic benthic condition to Se accumulation. Besides, weak watermass restriction, intermittent upwelling, high primary productivity and warm and humid climate were also beneficial to ultimate Se enrichment in Lujiaping Formation. This study complemented fundamental data of Se concentration and enrichment degree in strata of the Ediacaran–early Cambrian period in central South Qinling. Meanwhile, preliminary insight of correlation among Se enrichment, sedimentary environment, paleo-climate and biological blooming during the Ediacaran to early Cambrian interval was obtained, which laid the foundation for further researches.

Immersive visualization of 3D subsurface ground model developed from sparse boreholes using virtual reality (VR)

Analytics and visualization of multi-dimensional and complex geo-data, such as three-dimensional (3D) subsurface ground models, is critical for development of underground space and design and construction of underground structures (e.g., tunnels, dams, and slopes) in engineering practices. Although complicated 3D subsurface ground models now can be developed from site investigation data (e.g., boreholes) which is often sparse in practice, it remains a great challenge to visualize a 3D subsurface ground model with sophisticated stratigraphic variations by conventional two-dimensional (2D) geological cross-sections. Virtual reality (VR) technology, which has an attractive capability of constructing a virtual environment that links to the physical world, has been rapidly developed and applied to visualization in various disciplines recently. Leveraging on the rapid development of VR, this study proposes a framework for immersive visualization of 3D subsurface ground models in geo-applications using VR technology. The 3D subsurface model is first developed from limited borehole data in a data-driven manner. Then, a VR system is developed using related software and hardware devices currently available in the markets for immersive visualization and interaction with the developed 3D subsurface ground model. The results demonstrate that VR visualization of the 3D subsurface ground model in an immersive environment has great potential in revolutionizing the geo-practices from 2D cross-sections to a 3D immersive virtual environment in digital era, particularly for the emerging digital twins.

Paleoceanographic importance of tri- and di-unsaturated alkenones through the early phase of Cretaceous Oceanic Anoxic Event 2 from southern high latitudes of the proto-Indian Ocean

Alkenones are biomarkers derived exclusively from species of haptophyte algae. The relative abundance of di- to tri-unsaturated C37 alkenones expressed as UK'37 is widely applied as a sea surface paleotemperature proxy for Cenozoic marine sediments. However, the absence of alkatrienones prior to the Eocene has precluded application of the UK'37 proxy for assessment of Cretaceous paleoclimates. Herein, we report a C40 alkatrienone (tetraconta-9E, 16E, 23E-trien-3-one; C40:3 Et) in deep-sea sediments from southern high latitudes (International Ocean Discovery Program: IODP site U1516). This discovery extends the geologic record of alkatrienones to the late Cenomanian, ∼70 million years earlier than previous reports. The parallel occurrence of higher abundances of a C40 alkadienone (tetraconta-16E, 23E-dien-3-one; C40:2 Et) allowed calculation of the UK’40 unsaturation index, comparable to UK’37. Stratigraphic variations in the δ13C of C40:2 Et revealed an elevated (∼1.5 ‰) positive carbon isotope excursion (CIE) relative to those observed in carbonate from other OAE2 sequences likely reflecting a decrease in global pCO2. The UK’40 profile suggests a concurrent drop in sea surface temperature associated with the decline in pCO2 during the early phase of OAE2. The timing of these environmental perturbations in the southern high latitude of the proto-Indian Ocean suggests they were triggered by volcanism associated with large igneous province (LIP) formation.

Microbial diversity and oil biodegradation potential of northern Barents Sea sediments

The Arctic, an essential ecosystem on Earth, is subject to pronounced anthropogenic pressures, most notable being the climate change and risks of crude oil pollution. As crucial elements of Arctic environments, benthic microbiomes are involved in climate-relevant biogeochemical cycles and hold the potential to remediate upcoming contamination. Yet, the Arctic benthic microbiomes are among the least explored biomes on the planet. Here we combined geochemical analyses, incubation experiments, and microbial community profiling to detail the biogeography and biodegradation potential of Arctic sedimentary microbiomes in the northern Barents Sea. The results revealed a predominance of bacterial and archaea phyla typically found in the deep marine biosphere, such as Chloroflexi, Atribacteria, and Bathyarcheaota. The topmost benthic communities were spatially structured by sedimentary organic carbon, lacking a clear distinction among geographic regions. With increasing sediment depth, the community structure exhibited stratigraphic variability that could be correlated to redox geochemistry of sediments. The benthic microbiomes harbored multiple taxa capable of oxidizing hydrocarbons using aerobic and anaerobic pathways. Incubation of surface sediments with crude oil led to proliferation of several genera from the so-called rare biosphere. These include Alkalimarinus and Halioglobus, previously unrecognized as hydrocarbon-degrading genera, both harboring the full genetic potential for aerobic alkane oxidation. These findings increase our understanding of the taxonomic inventory and functional potential of unstudied benthic microbiomes in the Arctic.

Stratigraphic distributions of biomarkers and carbon isotopes in coals constrain the Permo-Carboniferous climatic changes and floral turnovers in the north China block

The intricate relationship between climate and vegetation dynamics, particularly during the late Paleozoic ice age marked by multiple icehouse-greenhouse transitions, is a compelling aspect in Earth's historical narrative. In this study, we decipher the interactions between the Permo-Carboniferous climate fluctuations and floral turnovers within the North China Craton, based on the stratigraphic distributions of biomarkers and carbon isotopes from cored coals in Huainan coalfield, North China. Our findings reveal that the changes in n-alkane, terpane, sterane, and polycyclic aromatic hydrocarbon (PAH) distributions result from a confluence of maturation, depositional environment, and organofacies effects. Molecular maturity-sensitive ratios align with the maturity stage determined by Rock-Eval pyrolysis and petrography, categorizing the coals as high volatile bituminous A/B with a composite organic matter derived from terrestrial plants and marine algae. Notably, the 18α-22,29,30-trisnorneohopane (Ts) to 17α-trisnorhopane ratio (Ts/(Ts + Tm)), as well as C31–C34 22S/(22S + 22R) hopanes and C29 20S/(20S + 20R) steranes ratios exhibit moderate correlations with %Ro, indicative of epimerization reactions induced by maturation processes. The stratigraphic variations in pristane to phytane ratios (Pr/Ph), Pr/n-C17, Ph/n-C18, C30 αβ/(αβ+βα) hopanes, and organofacies sensitive PAH indices collectively capture a paleoenvironmental shift from reducing to oxic conditions attributed to marine regression. The systematic changes in PAH concentrations and their maturity indices reflect condensation and dealkylation reactions during the maturation process. The δ13C signature manifests a negative excursion in coals of the Artinskian Shanxi Formation and a subsequent positive excursion in the Capitanian Upper Shihezi Formation coals, respectively, reflecting changes in the δ13C values of contemporaneous atmospheric CO2 in response to regional aridity fluctuations.

Investigation of layered glazes coloured with carbon‐based pigments using micro spatially‐offset Raman spectroscopy

AbstractNon‐invasive and non‐destructive Raman spectroscopic techniques have been used to detect the stratigraphic variation of tints in semi‐translucent films, comprising stacked layers with varying low concentrations of carbon‐black pigment dispersed in a painting varnish imitating historical artist materials, as in the ‘Sfumato’ technique often associated with Leonardo da Vinci's works of art. Understanding the structures behind this effect could help to conserve such paintings. Micro‐Raman spectroscopy is one of the analytical techniques usually applied to painting materials that has proved suitable for tackling the challenging detection and characterization of carbon‐based pigments in organic‐rich paintings. Model transparent samples have been fabricated following a selected recipe from historical sources and analysed using Raman‐scattering‐based experimental techniques, including micro‐spatially offset Raman spectroscopy (micro‐SORS). Single glaze layers spanning a range of concentrations, and multilayer systems mimicking the resulting stratigraphy of the Sfumato glaze technique, have been studied. Raman‐spectroscopy performance to detect the pigment was assessed on the single layers; the spectral behaviour was characterized according to the concentration of pigment. The micro‐SORS technique was tested on the multilayer systems and appears to be able to distinguish between different stratigraphic samples, varying in concentration of the same carbon‐based pigment, and the order of layers. This proof‐of‐concept observation is promising. It calls for further studies to be undertaken to obtain comprehensive results about an increased number of model painting materials, especially for carbon‐based materials mixed with other pigments.

The record of geochemical changes in the biogenic-calcareous sediments of a kettle-hole in a young glacial landscape (western Kashubian Lakeland, North Poland)

The paper was focused on the reconstruction of past-environmental conditions dynamics based on the geochemical characteristics of sediments filling kettle-hole located in the western part of the Kashubian Lakeland, North Poland). Stratigraphic variability of lithogeochemical constituents and a set of 13 elements (TOC, N, P, Na, Ca, Mg, K, Al, Fe, Mn, Cu, Ni, Zn) were applied for Holocene reconstruction of certain processes and conditions in the studied kettle-hole. The detailed geochemical analysis allowed us to identify 6 phases in its development: Masz-1 stage covering sedimentation of sedge-moss peat over melting dead ice at the turn of the Preboreal and Boreal periods; Masz-2 stage of the initial phase of lake development with deep-water sedimentation; Late Boreal and Atlantic stage Masz-3 related to sedimentation of lacustrine chalk; Subboreal stage Masz-4 representing the beginning of lake terrestrialization; Subatlantic stage Masz-5 of lowland bog, and Masz-6 stage covering final phase of peatland evolution due to human activity. Principle component analysis highlighted the importance of two major factors controlling the geochemical variability of the studied sediments. These are the varied origin of supplying water reflected in the sedimentation of organic-calcareous sediments (PC1), and oxidative-reduction conditions determined by water level fluctuations (PC2).

Leachate Lithium Characteristics of Loess‐paleosol Sequences on the Chinese Loess Plateau and their Paleoclimatic Significance

AbstractThe geochemical components of the leachate from loess‐paleosol deposits can provide information about climate‐related post‐depositional processes. For example, leachate lithium ([Li]leachate) is a potential paleoclimate proxy because lithium is a typical lithophile element that is readily adsorbed by clay minerals during weathering and pedogenesis, and thus stratigraphic variations in [Li]leachate can reflect these processes. We investigated the [Li]leachate values of two loess‐paleosols profiles (the Luochuan and Weinan sections), on a north–south climatic gradient on the Chinese Loess Plateau. Independent paleoclimate information was provided by measurements of magnetic susceptibility, grain size, Rb/Sr ratios, and clay mineral content. During the last glacial‐interglacial period, [Li]leachate increased from 0.39 to 1.97 μg/g at Luochuan and from 0.67 to 2.45 μg/g at Weinan, mainly due to increasing pedogenesis. Based on these results we developed a conceptual model to explain the variations in [Li]leachate. Li+ within loess layers is mainly derived from dust input and the decomposition of primary minerals, influenced by the East Asian winter monsoon, while in paleosol layers Li+ is mainly derived from clay mineral adsorption during pedogenic processes, influenced by the East Asian summer monsoon.

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.

Detrital zircons of the vast Triassic Snadd and De Geerdalen formations, Barents Shelf, reveal temporal changes in sediment source

Studies of detrital-zircon age distributions from Triassic sedimentary rock successions on the Arctic Barents Shelf have resulted in different provenance interpretations. Here, we provide new U–Pb/Hf detrital-zircon data from eleven samples of the Snadd and De Geerdalen formations, covering the Barents Shelf in a north–south transect, that help refine the provenance of these sedimentary rocks and show how stratigraphic variations reflect changing source regions. The sample locations cover different stratigraphic levels and geographic locations within the formations. The detrital-zircon age spectra in the lower part (lowest Ladinian and lower Carnian) are dominated by two age peaks at 300 and 540 Ma, whereas the zircon distributions in the stratigraphically younger deposits (latest Carnian and early Norian) are dominated by a 235 Ma age peak and two minor peaks at 300 and 425 Ma. The apparent relative contribution of detritus of different ages changes gradually from lower to higher stratigraphic levels, marked by an increase in both the 235 and 425 Ma age peaks, a decrease in the 300 Ma component and a progressive loss of the 540 Ma age peak. The zircon Hf-isotopic compositions display predominantly juvenile zircon populations, except for a mixed juvenile and evolved signature in the youngest Permo-Triassic grains. A regional comparison of detrital-zircon age spectra in latest Carnian/early Norian deposits extends the depositional system into the Sverdrup Basin, supporting a much larger depositional system than hitherto known. The gradual change in detrital-zircon age spectra likely represents a gradual shift in zircon provenance. We interpret these variations to reflect an early source region in the north Urals, gradually changing to a dominant source region in the Taimyr Peninsula region.

Age variability and decadal time-averaging in oyster reef death assemblages

Abstract Using paleoecological data to inform resource management decisions is challenging without an understanding of the ages and degrees of time-averaging in molluscan death assemblage (DA) samples. We illustrate this challenge by documenting the spatial and stratigraphic variability in age and time-averaging of oyster reef DAs. By radiocarbon dating a total of 630 oyster shells from samples at two burial depths on 31 oyster reefs around Florida, southeastern United States, we found that (1) spatial and stratigraphic variability in DA sample ages and time-averaging is of similar magnitude, and (2) the shallow oyster reef DAs are among the youngest and highest-resolution molluscan DAs documented to date, with most having decadal-scale time-averaging estimates, and sometimes less. This information increases the potential utility of the DAs for habitat management because DA data can be placed in a more specific temporal context relative to real-time monitoring data. More broadly, the results highlight the potential to obtain decadal-scale resolution from oyster bioherms in the fossil record.

Data-driven multi-stage sampling strategy for a three-dimensional geological domain using weighted centroidal voronoi tessellation and IC-XGBoost3D

Conventional site investigation schemes typically use an empirical sampling strategy, which involves equal sampling spacing with regular grid patterns. However, this approach assigns equal importance to the entire site and does not account for subsurface stratigraphic variations and site constraints. In this study, a data-driven multi-stage sampling strategy is proposed to adaptively optimize the borehole locations for a three-dimensional (3D) geotechnical site, considering the subsurface stratigraphic uncertainties and irregular site geometries. The initial sampling plan is determined based on weighted centroidal Voronoi tessellation, which assigns various sampling densities to zones depending on their importance. Measurements obtained in the previous sampling stage are combined with prior geological knowledge to establish and update a 3D geological domain using a physics-informed geological modelling method. The next optimal location for sampling is adaptively determined with the objective of maximizing the reduction in the stratigraphic uncertainty. The proposed method represents the first data-driven 3D site planning method that explicitly considers 3D subsurface stratigraphic variations. The performance of the proposed multi-stage sampling strategy is illustrated using a simulation study. The results indicate that the proposed method efficiently identifies the optimal sampling locations while comprehensively considering the 3D subsurface geological uncertainties and irregular site geometries.

Bioturbated strata in the Upper Cretaceous Aruma Formation, central Saudi Arabia: An analog for aquifers and hydrocarbon reservoirs with large burrows

The upper Cretaceous Aruma Formation in Central Saudi Arabia exhibits Thalassinoides-bearing strata resembling a bioturbated carbonate reservoir with large burrows. These burrows possess distinct stratigraphic characteristics. Certain intervals within the formation feature open or partially filled Thalassinoides, creating three-dimensional permeability pathways, while others contain Thalassinoides filled with low-permeability sediments. This comprehensive study investigates the paleoenvironment, ichnological features, bulk permeability influence, and stratigraphic variations of these bioturbated intervals. Incorporating field observations, laboratory analyses (including petrography, geochemical assessments, petrophysical measurements, and computed tomography scanning [CT]), we present the division of the studied interval into three biogenically influenced fluid flow media (BIFM 1–3). BIFM 1 comprises solution-enlarged open Thalassinoides (average shaft diameter: 5 cm) hosted within a mud-dominated matrix. BIFM 2 consists of Thalassinoides with mud-dominated infills (average shaft diameter: 3 cm) embedded in a dolomite matrix. BIFM 3 exhibits compacted open Thalassinoides. These BIFM units demonstrate burrow percentages ranging from 10% to 45%, featuring interconnected networks. The identified units reflect diverse depositional settings, ranging from open marine to restricted lagoon, and distinct stratigraphic positions. By highlighting the sedimentological and stratigraphic controls on the petrophysical properties of carbonate strata containing Thalassinoides with large burrows, this research contributes to the prediction of super-permeability zones in analogous settings.