Lacustrine Environments Research Articles

Transport and depositional processes of mud in lacustrine environments：From sediment plumes to hyperpycnal flows

Lacustrine mudstones are usually considered as deposited dominantly through vertical suspension fallout in low-energy environments influenced by only occasional fine-grained turbulent flows, while the deposition of their marine counterpart have been increasingly documented to be subject to more complex and dynamic processes. To provide insights into the dynamic processes that transport and deposit mud in lacustrine settings, this study presents an integrated petrographic, sedimentological, geochemical, and sequence-stratigraphic analysis of an ancient lacustrine mudstone-dominated succession using outcrops of the Lucaogou Formation in the Junggar Basin. Detailed facies characterization has revealed seven facies associations, which show spatial and temporal variations within a stratigraphic framework and stack to form eight repetitive parasequence styles. Process-based sedimentological analysis allows the identification of multiple transport mechanisms in different depositional settings, which strongly suggests that mud in lake systems is commonly subject to multiple dynamic transport processes (i.e., sediment plumes, turbidity flows, hyperpycnal flows, and wind-driven bottom currents), rather than suspension fallout under overall quiet conditions. The differences in the expression of parasequence styles can be attributed to the combination of allogenic and autogenic processes, which systematically change with distances from the shoreline. It is important to note that river-floods can discharge large amounts of mud into the deep lake setting. The strong correlation between geochemical data and the distribution of facies generated by riverine processes indicates that in saltwater lakes terrigenous mud is mainly transported to the deep water by sediment plumes and turbidity flows, whereas in freshwater lakes it is transported primarily by hyperpycnal flows. This study demonstrates that the transport and depositional processes of lake mud are diverse and vary systematically in response to lake conditions and environmental changes. High-resolution petrographic and sedimentologic analysis is essential to identify these dynamic processes, which allows more critical and accurate interpretations of depositional conditions and long-term depositional trends in lake systems.

Sedimentary factors controlling the organic matter enrichment in oil shale, Seyitömer Lacustrine Basin, Western Anatolia: New implications from organic and inorganic geochemistry

AbstractWestern Anatolia, an important segment of the Alp‐Himalayan Belt, hosts numerous Neogene lacustrine basins with potential oil shale reserves surpassing 1.6 billion tonnes. Among these basins, the Seyitömer Basin (Kütahya) stands out, containing oil shales that are intercalated with claystone, marl, limestone and coal layers. This study presents a comprehensive dataset of organic and inorganic chemistry to gain insight into the palaeoclimate and palaeoenvironmental factors that control the enrichment of organic matter. The studied samples exhibit high total organic carbon contents, averaging 12.85% (ranging from 2.22 to 36.21%), high hydrogen indices (486–812 mgHC/g rock) and low oxygen indices (33–70 mgCO2/g total organic carbon), indicating their substantial hydrocarbon‐source potential. These characteristics indicate predominantly ‘excellent’ to occasionally ‘very good’ source rock qualities and very high oil potential. Their pyrolysis, gas chromatography and gas chromatography–mass spectrometer parameters indicate an immature‐early mature characteristic for Seyitömer oil shale samples. They comprise dominantly Type‐I kerogen with minor Type‐II kerogen and lacustrine algal organic matter. Their sedimentological characteristics, along with various geochemical values, such as total organic carbon versus S, B versus Ga and dibenzothiophene/phenanthrene, reveal a moderately deep fresh/brackish to saline lacustrine environment. The Ga/Rb, K/Al and Sr/Cu ratios suggest dominantly humid and warm climate conditions, occasionally interrupted by periods of less humidity. The prevalence of warm and humid climate conditions leads to intense chemical weathering processes, supported by high Chemical Index of Alteration and low Rb/Sr ratios in the associated oil shale samples. Intense chemical weathering and high runoff resulted in dissolved nutrient enrichment, promoting ecological dynamics favourable to increased productivity. Their low Pr/Ph and Mo/total organic carbon ratios, high Ni/Co ratio and, relatively low MoEF/UEF ratio, along with well‐developed lamination of the oil shales, indicate the presence of anoxic conditions in the bottom water. These anoxic conditions would have facilitated the preservation potential of organic matter in the samples. Thus, the palaeoclimate conditions integrated with sedimentary factors have an important role in the ecological dynamic and physical–chemical environmental conditions, ultimately contributing to the organic matter enrichment of the studied samples. This work provides a case study to better understand the sedimentary factors controlling organic matter enrichment in the lacustrine basins of the Alp‐orogenic belt.

Site characterization of the Endurance CO 2 store, Southern North Sea, UK

The Endurance saline aquifer CO 2 store in the UK North Sea is the planned storage site for CO 2 emissions that will be captured from the East Coast Cluster. The site has been characterized to assess its suitability for injectivity, containment and capacity. The injection target is the Early Triassic Bunter Sandstone Formation, consisting of fine-grained sandstones deposited in fluvial, lacustrine and playa lake environments that were subject to repeated aeolian and fluvial reworking. The sandstones have excellent reservoir quality and are likely to be well connected due to the lack of preserved heterolithic layers. The trap is a large, four-way dip-closed anticline, sealed by mudstones and evaporites of the Middle Triassic Dowsing Formation. The top seal sediments are laterally extensive and geomechanically strong, and faulting observed in the overburden does not appear to connect to the reservoir, providing confidence that CO 2 containment will be secure. The risk of leakage through existing legacy wells is assessed as low. The maximum effective CO 2 storage capacity is c . 450 Mt: Phase 1 development of 100 Mt is equivalent to a storage efficiency factor of 3%, which is achievable without the need for brine production. First injection is planned from 2027.

Reconstructing early sedimentation patterns of the lower Codó Formation (Early Cretaceous, NE Brazil): Evidence of marine influence and the onset of an alkaline hypersaline lake

The upper Aptian lower Codó Formation is an attractive stratigraphic unit within the Parnaíba and São Luís basins, northeastern Brazil, formed during the break-up of Western Gondwana. Located in a large sag basin, the formation records a dynamic depositional history of deltaic, lacustrine, and sabkha environments. This sedimentary evolution is reflected in a diverse lithological succession characterized by intermittent terrestrial input, episodic marine incursions, and restricted lacustrine phases culminating in sulfate evaporite precipitation. While the later stages of the formation have been extensively studied, the initial sedimentary processes and environmental conditions remain poorly understood. This research aims to reconstruct the depositional paleoenvironments during the initial stages of Codó sedimentation, elucidating the onset of the alkaline hypersaline lake environment and the earlier marine incursions in the study region. To achieve this, it was conducted a high-resolution facies analysis using core descriptions, trace fossil analysis, petrography, various types of X-ray fluorescence, scanning electron microscopy, X-ray diffractometry, and isotope analyses (δ1³C, δ1⁸O, and ⁸⁷Sr/⁸⁶Sr) on a ∼63-m-thick succession from borehole 2-TV-1-MA. Thirteen facies were recognized and grouped into brackish lake, prodelta, delta front, and alkaline hypersaline lake facies successions. The lower Codó Formation exhibits cyclic sedimentation, indicating fluctuating water dynamics associated with shifts between open and closed drainage systems controlled by paleoclimate. Tectonic subsidence drove the basin's evolution from a brackish lacustrine-deltaic environment with an incipient marine influence (balanced-fill basin stage) to a restricted system with the establishment of an alkaline hypersaline lake (underfilled basin stage).

Vertebrate taphonomy and taphofacies of the Capianga Member, Aliança Formation (Jurassic), Jatobá Basin, Brazil

In addition to the study of sedimentary facies, the reconstruction of paleoenvironments depends on a detailed understanding of the taphonomy of a fossil accumulation. The Capianga Member of the Aliança Formation (Middle to Late Jurassic of the Jatobá Basin, northeastern Brazil) reveals vertebrate fossil accumulations composed predominantly of disarticulated bone and osteoderms, scales, spines and isolated teeth from a lacustrine paleoenvironment. However, the taphonomic history of the accumulation has not yet been studied extensively. The present work aims to interpret taphonomic facies based on lithological data and the systematic collection of fossils from the Capianga Member of the Aliança Formation, located in the northeastern portion of the Jatobá Basin, mainly in the municipality of Ibimirim. Six lithofacies were identified: Fm – massive claystones, Fl – Shale laminated Lt – calcilutites, Lc – calcarenites, Gf – fibrous gypsum and Scl – calciferous sandstones. Two primary taphonomic classes were recognized, consisting essentially of disarticulated elements, including (1) incomplete, semi-complete, or rarely complete bioclasts (such as osteoderms and fish scales) and (2) small-sized, occasionally complete bioclasts (osteoderms and scales) that sometimes compose bonebeds. According to the vertebrate taphonomy characteristics, three taphofacies were identified, with the Taphofacies A and B occurring in calcarenites and the Taphofacies C in calcilutites. The interpretation of these taphofacies reveals a multi-episodic history of the lacustrine environment. The integrated model suggests that the genesis of the identified taphofacies is linked to the action of lowering the water level of the paleolake, as well as the agents responsible for transport and disarticulation. These agents are related to pre-burial sub-aerial exposure, the action of unidirectional turbidity currents in shallow waters, and the likely influence of storms.

Redox conditions influence the chemical composition of iron-associated organic carbon in boreal lake sediments: A synchrotron-based NEXAFS study

The global carbon and iron cycles are intimately linked as redox-sensitive iron oxides readily bind organic carbon in a variety of environmental settings, including marine and lacustrine sediments. While these iron-organic carbon complexes sequester vast quantities of organic carbon, the composition of the organic matter within them remains unknown for lacustrine environments. Here we present C K1s and Fe L3,2 edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of surface sediments and authigenic iron complexes from adjacent basins of a pristine boreal lake located in Québec, Canada, with contrasting oxygen exposure regimes. We demonstrate differences in organic carbon speciation in sediments from both basins, as well as co-localization of organic carbon and iron on a sub-micron scale in 100 nm thick samples. Differences in redox cycling across these two basins allow for a direct comparison of the effect of oscillating redox conditions on the composition of organic carbon sequestered by iron. Our results suggest that reactive organic molecules, which may be polysaccharides, were found preferentially associated with iron in the perennially oxic sediments compared to more phenol rich organics in the seasonally anoxic sediments, highlighting the importance of iron oxides in the protection and preservation of labile organic compounds. Traces of aliphatic carbon were observed in sediments from the anoxic basin, alongside carboxyl and aromatic functionalities. This carboxyl-rich aliphatic material could possibly interact with the sediment mineral matrix either through a ligand exchange mechanism between the mineral phases and the carboxyl functionalities, or via non-specific hydrophobic interactions involving the aliphatic moieties. Finally, our work also shows that OC:Fe ratios should be used with caution when inferring a binding mechanism between OC and iron oxides.

Aliphatic and polycyclic aromatic compounds in coal and coal-based solid wastes: Relationship with coal-forming paleoenvironment and implications for environmental pollution

In this study, coal and coal-based solid wastes (coal gangue, fly ash, bottom ash, desulfurized gypsum and tar residue) were collected from major coal mines, power plants and coking plants in Lianghuai mining area (LH), China, and were analyzed for 76 polycyclic aromatic compounds (PACs), 27 n-alkanes and 2 isoprenoids (phytane and pristane). The total n-alkanes concentrations and ∑76 PACs in raw coals (640 ± 600 and 180 ± 87 μg/g) were higher than those in coal-based solid wastes (47 ± 40 and 24 ± 25 μg/g), but were lower than those in tar residue (3700 and 63,000 μg/g). It was discovered that the depositional paleoenvironment in LH was mostly a lacustrine and freshwater environment with oxidizing conditions and mixed organic matter input, but the Huainan coalfield had stronger oxidizing conditions and more input of terrestrial organic matter than that of the Huaibei coalfield. Alkylated PACs made up 56 ± 12 % of the ∑76PACs in raw coals, whereas solid wastes mainly consisted of 16 EPA PAHs (66 ± 16 %). Coal combustion and gangue weathering altered the structural properties of n-alkanes and PACs, resulting in a significant loss of n-alkanes and PACs, a higher proportion of parent PACs, and an increased abundance of short n-alkanes in the products (No apparent change of n-alkanes composition was observed through gangue weathering). The toxicity of PACs in raw coal and its solid wastes in LH from high to low was tar residue, raw coal, coal gangue, and coal-fired products. This investigation further confirmed that traditional diagnostic ratios may distort source information, and that they should not be used to assess PACs sources from raw coal particles or coal gangues, but rather to identify combustion sources near the point source. In addition, Retene/(Retene + Chrysene) < 0.03 may indicate direct contamination of raw coal particles.

Evaluating evaporites as potential archives of lake and seawater lithium isotopic ratios: An experimental study using various natural saline fluids

Reconstructing the lithium isotope composition (δ7Li) of ancient seawater and lacustrine environment is crucial for tracing silicate weathering processes that regulate Earth's carbon cycle and climate. This study examines the reliability of evaporite minerals in preserving the Li isotope signature of the original brines from which they precipitate. We conducted evaporation experiments on modern natural waters from Qinghai Lake, the Yellow Sea, and Chaka Salt Lake, each with unique salinity and chemical composition, to examine the behavior of lithium isotope ratios in the evolving brine and precipitated evaporite minerals.Our results show that initial hydrochemical conditions of the lake and ocean are the primary control on the evolution of major and trace elements and the mineral precipitation sequences during evaporative concentration. Aragonite, calcite, gypsum, and/or nesquehonite precipitated prior to halite. Li isotope fractionation of these non-halite precipitates varied significantly between 4‰ and 17‰. However, the δ7Li values of halite closely mirror those of the initial lake and seawater, indicating negligible Li isotope fractionation (<1‰) during halite precipitation. The δ7Li values of the evolving brines remain unchanged throughout the evaporation experiment. Solid phases precipitating before halite constitute <1–2% of the total moles of salt precipitated from the complete evaporation of the initial waters, thus, having little to no influence on the δ7Li values of evolving brines. Although halite comprises >77% of the total moles of salt, it does not fractionate Li isotopes. Therefore, halite more accurately reflects the δ7Li values of the co-evolving brine as well as those of the initial, unevaporated lake and seawater. These results underscore the potential of late stage evaporites, particularly halite, to serve as high-fidelity archives of ancient lacustrine and seawater δ7Li values. Such records are instrumental in reconstructing past silicate weathering and climatic conditions.

Outcrop spectral gamma-ray signals of distal arid continental basins: Facies identification and controls

Spectral gamma ray (SGR) studies can help link outcrop exposures to downhole data and provide useful case studies to reduce uncertainty in interpreting geophysical logs where core and other geological data is unavailable. However, this technique is underused in continental outcrop studies due to difficulties in determining facies comprising similarly sourced sediment, despite the fact that these deposits form prolific reservoirs for hydrocarbon and geothermal exploration, as well as carbon capture and hydrogen and storage. This work demonstrates the usefulness of spectral gamma ray measurements as an outcrop logging technique in determining quantifiable facies from the dryland continental sediments of the Cedar Mesa Sandstone Formation, Utah, USA. Complex sedimentary interactions between the aeolian, fluvial and lacustrine depositional environments are present, resulting in shared similar sedimentary characteristics within the preserved strata, such as mineralogy, grain size, sorting and provenance signals. Consequently, the geophysical signatures observed within the spectral gamma ray logs, have been suppressed, making it difficult to distinguish sedimentary facies from solely the spectral gamma ray responses. However, by combining K:Th cross-plot analysis and gamma ray log motif interpretation, this work demonstrates that individual facies within arid continental settings can be distinguished based on SGR outcrop logging, and unique spectral trends and values can be quantified and used to correlate facies across the depositional basin.

Holocene paleolimnological changes in rundvågshetta lakes in the Soya Coast region and their paleoenvironmental significance with glacial isostatic adjustment in East Antarctica

This study investigated Holocene paleolimnological changes inferred from a multi-proxy dataset of C, N, and S elements, biomarkers, and microscopic observations of microalgae and cyanobacteria in sediment cores from two Rundvågshetta lakes (Maruwanminami-ike and Maruwan-oike) in the Soya Coast region of Antarctica, along with sedimentary facies and accelerator mass spectrometry 14C dating. We discuss biological production and sources, the transition from marine to brackish and lacustrine environments in the lakes, paleoenvironmental changes, transition ages, and relative sea level (RSL) falls (or subsidence) from glacial isostatic adjustment (GIA) in the Soya Coast region. The ages of the Maruwanminami-ike (MwS4C-01, length 147 cm) and Maruwan-oike sediment cores (Mw4C-01, length 226 cm) were 1.4 (5.8 cm core depth)–4.9 and 2.3–5.7 cal ker BP, respectively. The local reservoir effects of the Rundvågshetta lakes were very high, possibly because of the influence of dead carbon in the glacially eroded marine sediments and bed rock. The coastal marine environments of Lakes Maruwanminami-ike (4.9–2.6 cal ker BP) and Maruwan-oike (5.7–3.3 cal ker BP) were characterized by low biological production. A transition period marked by a stratified brackish lake environment in Lakes Maruwanminami-ike (2.6–2.4 cal ker BP) and Maruwan-oike (3.3–2.9 cal ker BP) was characterized by stratified conditions with an anoxic layer at the bottom of the photic zone. The freshwater lake environment of Lake Maruwanminami-ike [(65.6–0 cm, 2.4–1.4 (5.8 cm core depth) cal kyr BP)] was characterized by high biological (cyanobacteria and green algae) production, while that of Lake Maruwan-oike (2.9–2.3 cal ker BP) was characterized by low biological production due to the influence of glacial clay. Around 7.2–3.0 cal ka BP in the Holocene was probably a warm period, and progressive glacial retreat with RSL fall occurred during the warm periods in the Soya Coast region. The RSL falls of all raised beaches and isolated basins in the Soya Coast region ranged from 0.36 ± 0.03–4.4 ± 0.5 mm/yr with an average 2.1 ± 0.2 mm/yr (standard deviation, SD). They were similar to GNSS–based GIA–induced uplift with an average of 2.4 ± 1.5 mm/yr (SD) at Soya Coast region.

Chronology of Sedimentation and Landscape Evolution in the Okavango Rift Zone, a Developing Young Rift in Southern Africa

AbstractThe Kalahari Basin in southern Africa, shaped by subsidence and epeirogeny, features the Okavango Rift Zone (ORZ) as a significant structural element characterized by diffused extensional deformation forming a prominent depocenter. This study elucidates the Pleistocene landscape evolution of the ORZ by examining the chronology of sediment formation and filling this incipient rift and its surroundings. Modeling of cosmogenic nuclide concentrations in surficial eolian sand from distinct structural blocks around the ORZ provides insights into sand's residence time on the surface. Sand formation occurred from ∼2.2 to 1.1 Ma, coinciding with regional tectonic events. Notably, provenance analyses of sand within ORZ's lowermost block where large alluvial fans are found indicate different source rocks and depositional environments than those of the eolian sands found at a higher elevation. This suggests that the major phase of rift subsidence and the following incision of alluvial systems into the rift occurred after eolian dune formation. Luminescence dating reveals that deposition in alluvial fan settings in the incised landscape began not later than ∼250 ka, and that a lacustrine environment existed since at least ∼140 ka. The established chronological framework constrains the geomorphological effects of the different tectono‐climatic forces that shaped this nascent rifting area. It highlights two pronounced stages of landscape development, with the most recent major deformation event in the evolving rift probably occurring during the middle Pleistocene transition (1.2–0.75 Ma). This event is reflected as a striking change in the depositional environments due to the configurational changes accompanying rift progression.

Facies Analysis and Paleoenvironmental Reconstruction of Ghar Formation in Al-Muthanna Governorate, Southern Iraq

The Oligocene-Miocene succession in Al-Muthanna Governorate of southern Iraq shows significant mixed siliciclastic-carbonate formation. Fourteen samples are collected from two wells and four outcrops in Al-Muthanna area for studying the facies and paleoenvironments of the Ghar Formation. It is mainly composed of sandstone, calcareous sandstone, and carbonate rocks. Six facies associations are identified in the mixed siliciclastic-carbonate deposit: the carbonate type, including sabkha, lagoon, intertidal-subtidal, and lacustrine; the mixed type, including shoreface; and the siliciclastic type, including distributary channels. The major minerals in this formation are quartz, calcite, and dolomite, whereas feldspar and gypsum are the minor minerals. Clay minerals such as Palygorskite and Montmorillonite were also identified. The carbonate rocks of the Ghar Formation reflected deposition in the evaporitic platform (Sabkha), shallow restricted (lagoon), tidal and lacustrine environments. Ghar formation is considered transitional in conditions between a very restricted shallow marine and near-shore environment, whereas the clastic facies in Ghar Formation indicated deposition in the delta front channel.

Mid to Late-Holocene environmental dynamics recorded in Lake Pup Lagoon, East Antarctica: Insights from environmental magnetism and biogeochemical proxies

Paleoenvironmental archives in East Antarctica have revealed significant changes during the Holocene, marked by ice sheet retreat leading to the isolation of submarine basins. These basins offer valuable insights into past climate, glaciology, and oceanography shifts that impact sedimentary processes. In this study, environmental magnetism and biogeochemical proxies to investigate the Mid-to-Late-Holocene transitions in Pup Lagoon, a coastal isolation basin is presented. Our findings reveal distinct stratigraphic zones reflecting shifts from marine to lacustrine environments. The results reveal predominant mechanical weathering in the Stornes region, producing coarse-grained “soft” ferrimagnetic minerals. Notably, a period of warm oceanographic conditions between 6000 and 4722 cal. yr BP was characterized by mixed magnetic grain sizes and ultrafine superparamagnetic grains, indicating relatively oxic open waters in the basin. Subsequent shifts to reducing conditions coincide with persistent marine sea ice cover from 4722 to 2634 cal. yr BP, favoring the retention of coarse-grained ferrimagnets. Finer magnetic grain sizes between 2634 and 2185 cal. yr BP was attributed to the increased freshwater inputs associated with the Mid-Holocene Hypsithermal. Further, diagenetic changes under persistent sea ice cover between 2185 and 1970 cal. yr BP led to the selective dissolution of fine-grained ferrimagnets. Transitioning to freshwater isolated basin conditions between 1970 and 588 cal. yr BP, fine ferrimagnet precipitation indicate oxic to suboxic conditions alongside drier conditions. Biogenic productivity increased post-isolation, which was reflected in increased (Total Organic Carbon) TOC and (Total Nitrogen) TN percentages. Additionally, the presence of greigite in the isolated phase sediment indicates reducing conditions owing to organic matter decomposition. Notably, χfd% exhibits an inverse trend to sea ice concentration, potentially indicating anoxic-dysoxic conditions due to the presence of sea ice. These observations align with broader regional sea ice concentration changes, emphasizing the interconnected behavior of local and regional factors shaping Antarctic coastal environments.

Swimming, confusion, and plenty of brews: Negotiating ambivalence within Windermere's fragile waters

Windermere is a complex and contested freshwater site which encounters fluctuating social and environmental pressures. Swimmers at Windermere regularly practice across all four seasons while negotiating social concerns such as access, conflicting user groups, public health communications, and swim safety, alongside environmental complications including extreme weather, wastewater, run-off, plastic pollution, algal blooms, biosecurity, and climate change. Simultaneously, these entangled pressures generate ongoing adaptation, ambivalence, and avoidance within the swim communities. Furthermore, they disrupt individualised and inwardly focused understandings of ‘healthy’ outdoor swimming practices. In contribution to the special issue (on outdoor swimming), this article reflects on how outdoor swimming researchers may methodologically attend to these social and environmental complexities within contested lacustrine environments through an immersive 12-month wet ethnographic approach, combining ‘lake-hangouts’ and ‘swim-along interviews’ with different swimmers at Windermere. The article discusses how these relational in-situ approaches can continue to broaden inwardly focused understandings of ‘healthy’ outdoor swimming practices towards the wider social and environmental relations for both the participants and researcher. The article also highlights senses of ambivalence and ethical tension while negotiating conflicting concerns of ill-health, in and out of Windermere's fragile waters.

Investigating the impact of paleoclimatic conditions and diagenesis on the genesis of Permian Continental Red Beds: A case study from the Bohemian Massif, Czechia

The processes responsible for reddening of Continental Red Beds (CRBs) and the relationship between color variation and paleoenvironmental conditions are presented focusing on a comprehensive multi-proxy study of Permian sediments in the Bohemian Massif, Czechia. The investigation incorporates facies analysis, quantitative color assessment using diffuse Vis-spectral reflectance (DRS), optical and electron microprobe microscopy, bulk-rock (XRF and XRD), and in-situ geochemistry (laser-ablation ICP-MS). Results indicate a progressive drying trend from the Cisuralian to Guadalupian series in studied continental red sediments. Different facies indicate the change of the sedimentary environment from a deep lacustrine environment (lower part of Rudník Member, Cisuralian) to a fluvial floodplain and eolian environment (Trutnov Formation, Guadalupian). Examination of the three major categories (white, gray–green and red sediments) identified in the studied continental red beds indicates that diagenetic alteration of clay minerals and biotite was the main source of iron fueling the growth of hematite responsible for their red color. Early diagenetic processes and paleoenvironmental conditions, particularly the oxidizing or reducing conditions play a key role in the red sediment formation. It is suggested that later diagenetic stages are incapable of coloring non-red, iron-rich sediments formed in deep anoxic lacustrine environments. Microbial activities and reducing fluids have been identified as the main factors in the formation of gray–green sediments forming distinct reduction zones. The reduction spots formed during the early stages of diagenesis (eodiagenesis), and they were likely never red. In contrast, reduction strips, initially exhibiting a red hue, underwent a color change during more advanced stages of diagenesis (mesodiagenesis).

Comparative study on pore-connectivity and wettability characteristics of the fresh-water and saline lacustrine tuffaceous shales: triggering mechanisms and multi-scale models for differential reservoir-forming patterns

Although particular attention has been paid to responses of hydrocarbon storage and percolation capacity to the devitrification concerning lacustrine tuffaceous shale reservoirs in recent years, there is still a lack of systematical and comparative investigation on differential patterns and potential triggering mechanisms concerning development of the pore-microfracture systems and characteristics of surface wettability between the fresh-water and saline lacustrine settings, which is of considerable importance in fully understanding of genesis and spatial distribution of dessert reservoir intervals of tuffaceous shale reservoirs, and to provide further conceptual basis for deciphering shale-oil movability of saline lacustrine fine-grained mixed sedimentary sequences. In this study, tuffaceous shales from both the Upper Triassic Yanchang Formation in the Ordos Basin and Middle Permian Jingjingzigou Formation in the Junggar Basin are targeted to unravel the differential behavior of tuff devitrification and potential impacts on reservoir wettability and pore connectivity concerning fresh-water and saline lacustrine settings, and we present new results here from integrated analyses and combined interpretation of FE-SEM, Image Pro Plus (IPP) software image processing, contact angle and spontaneous imbibition experiments. In view of comparative analysis from representative samples, the tuffaceous shales from saline lacustrine environments are characterized by well-developed intergranular-intercrystalline and dissolution pores, and inorganic microfractures, generally yield a higher plane porosity of representative pore-fracture spaces and spontaneous imbibition slopes, a relatively lower average of n-decane contact angles and corresponding wettability parameters. The saline lacustrine tuffaceous shales are thus suspected to have undergone more intense devitrification resulting in a higher amount of devitrification and associated dissolution pores, and a relatively better connectivity between isolated micropore systems with adjacent microfractures. This would significantly facilitate the interface wettability reversal and occurrence of movable hydrocarbon fluid in microscopic reservoir spaces. Finally, a comprehensive and conceptual model is established illustrating the effects of differential devitrification on reservoir-forming patterns concerning tuffaceous shales developed in the fresh-water and saline lacustrine settings, respectively. These findings are of great theoretical and practical significance to enrich theory of high-quality reservoir formation and shale-oil accumulation in saline lacustrine tuffaceous shale reservoirs, and lay the foundation for guiding efficient exploration of continental fine-grained mixed sedimentary sequences.

First lacustrine application of the diatom‐bound nitrogen isotope paleo‐proxy reveals coupling of denitrification and N2 fixation in a hyper‐eutrophic lake

AbstractPast changes in the input/output, and internal cycling, of bioavailable nitrogen (N) in marine and lacustrine environments can be reconstructed by analyzing the N isotopic composition (δ15N) of organic matter in the sedimentary record. To verify, and eliminate, potential biases of bulk sedimentary δ15N (δ15Nbulk) signatures by diagenetic alteration and external N inputs, we applied, for the first time, the diatom‐bound N isotope (δ15Ndb) paleo‐proxy to lake sediments. By comparing δ15Nbulk and δ15Ndb in a sedimentary record from eutrophic Lake Lugano (Switzerland), we demonstrate that changing redox conditions influence the degree of N‐isotopic alteration of the bulk sediment, emphasizing the need for caution when interpreting δ15Nbulk in paleolimnological studies. Furthermore, in combining δ15Ndb measurements with X‐ray fluorescence scanning and state‐of‐the‐art molecular biomarker analyses, we reconstruct nutrient cycling and paleoenvironmental conditions in the lake over the past ~ 125 yr. Coeval with the period of severe eutrophication in Lake Lugano in the 1960s, our proxy data indicate that export production, δ15Ndb, and the concentration of heterocyst glycolipids (a biomarker for N2‐fixing cyanobacteria) increased simultaneously. Together, these data suggest that the rise in δ15Ndb is likely the result of enhanced water‐column denitrification in response to increased phytoplankton productivity. We hypothesize that greater export production during eutrophication led to anoxic conditions in the hypolimnion as a result of enhanced organic matter remineralization, raising water‐column denitrification. Enhanced N loss and remobilization of phosphorous (P) from the sediments under anoxic conditions lowered the N : P ratio in the lake, fostering cyanobacterial N2 fixation in surface waters.

Paleolimnological Investigations in Coastal Sarıkum Lagoon, Sinop, Turkey

Lagoons are dynamic systems, making ecosystem management difficult. Paleolimnological approach is the only way to track past long term environmental changes and background conditions that are essential to build environmental management plans. Here, we investigated a sediment record from Sarıkum Lagoon, on the Black Sea coastal plain in North Anatolia, through a multiproxy paleolimnological approach to reveal long term environmental change and background conditions in the lake. For which, dry weight, organic and carbonate carbon, chlorophyll a (including its main diagenetic products), magnetic susceptibility and ostracod analysis were undertaken. The core was dated via radionuclide technique. At around 30 cm of the sediment record, there are abrupt transitions in all proxies, indicating a massive sediment input, a transition from transitional water to lacustrine environment, and lake’s biota also changed. Biological proxies preserved in the sediments suggest that the latest zone, after this event, is represented by lacustrine and hydrologically more isolated conditions from the Black Sea. Whatever the reason of that event (e.g. earthquake, flood), Sarıkum Lagoon has had lacustrine conditions during the last ~ 2 and half centuries following that abrupt event. This long term environmental change information should be considered while making future ecosystem management and conservation plans for the lake. (e.g. keep the lake connected with the Black Sea).

Hydrocarbon-generating potential of the Middle Permian Lucaogou source rock in the eastern Junggar Basin, China

The Middle Permian Lucaogou Formation is the most significant source rock in the eastern Junggar Basin. Previous studies have confirmed its excellent hydrocarbon-generating potential in the Jimsar Sag. However, its potential in other areas of the eastern Junggar Basin remains uncertain. Based on total organic carbon and pyrolysis, organic petrology, hydrocarbon simulation experiments, basin simulation, and combined well-seismic coupling interpretation, this study systematically compares the hydrocarbon-generating potential of the Lucaogou source rock in the Jimsar Sag with other areas of the eastern Junggar Basin. It discusses the sedimentary environment of high-quality source rocks and depicts the distribution of practical source kitchens. The Lucaogou source rocks in the eastern Junggar Basin are oil-prone, dominated by type I–II kerogen, and generally classified as good to excellent source rocks. Nowadays, the area of the Lucaogou source rocks that have entered the main oil-generating window is approximately 11 × 103 km2. Except for the bulge area, the Lucaogou source rocks in the eastern Junggar Basin successively entered the hydrocarbon-generating threshold during the Jurassic and the main oil-generating window in the Cretaceous. The Lucaogou source rocks in the Jimsar Sag and other parts of the eastern Junggar Basin share similar biomarker fingerprints, characterized by relatively low ratios of Pr/Ph, Pr/n-C17, Tm/C30 hopane, C19/C20 tricyclic terpene, and C24 tetracyclic terpene/C26 tricyclic terpene, and high β-carotene content, gammacerane index, and Ts/Tm ratios. These characteristics reflect deposition in a strongly reducing brackish lacustrine environment with parental sources dominated by lower organisms such as algae and bacteria. Generally, the Lucaogou source rocks in the eastern Junggar Basin have an oil-generating intensity of more than 3 × 106 t/km2. Several oil-generating centers with an intensity of more than 5 × 106 t/km2 have developed in the front of the Bogda Mountain, Jimsar Sag, Dongdaohaizi Sag, Wucaiwan Sag, and Shazhang Fault Zone, covering a total area of approximately 12,500 km2. These characteristics of the Lucaogou source rocks promise favorable potential for forming large and medium oil fields. The results further consolidated the oil and gas resources in the eastern Junggar Basin and provided valuable references for exploring future oil and gas fields.

Statistical analysis of the Hurst index indicating sedimentary processes' evolution trend in the deep lacustrine mudrock sequences: A case study of the Jiyang Depression, Bohai Bay Basin, eastern China

Quantitative identification of sedimentary processes' evolution trend has substantially influenced the exploration of lacustrine mudrocks. To explore the indicative applicability of the Hurst index (HI) for analyzing the sedimentary processes' evolution trend in a deep lacustrine sequence, lacustrine mudrocks from wells L69, FY1, and NY1 in the Jiyang Depression, eastern China are selected. The comparable fourth-order cycle is divided through lithofacies observation, total organic carbon (TOC), and natural gamma ray (GR) logs combined with previous studies. In addition, the variation of accommodation space is selected to represent the sedimentary processes' evolution trend in the lacustrine environment, characterized by the frequency of oscillation (FST) and directionality (D). Further, HI is introduced to identify the long-term persistence of the sedimentary processes' evolution trend in the lacustrine environment. Through the criteria for the series selections, the TOC series and GR logs passed the Hurst phenomenon test, and the computed HI of those two series was selected to evaluate its effectiveness toward the lacustrine sedimentary processes' evolution trend. The HI computed from the TOC series demonstrates strong negative linear relationships with FST and D, with R2 in 0.833 and 0.814, respectively. Besides, the HI from the more accessible GR logs demonstrates robust negative linear relationships with FST and D, with R2 in 0.981 and 0.976, respectively. The statistical analysis indicates that the content of deep lake subfacies, shale, and mudstone could affect the long-term persistence of the lacustrine mudrock sequence. Thus, the HI could be an alternative indicator to identify the imperceptible sedimentary processes' evolution trend in the deep lacustrine environment.