Bottom Water Environment Research Articles

Precipitation mechanism of Mn ore deposits in the Datangpo Formation, Nanhua Basin, South China

During the interglacial period of the Cryogenian glaciation, the extensive “Datangpo-type” manganese ores were deposited in the Nanhua Basin. However, there has been significant controversy over the precipitation mechanism of manganese for decades. Based on iron and nitrogen isotope data from the manganese ores of the Datangpo Formation in Chongqing (South China), combined with previous research, we propose that the manganese ores of the Datangpo Formation precipitated directly in an anoxic bottom-water environment in the form of rhodochrosite. The δ56Fe values in the Bijiashan manganese ore (δ56Fe = −0.19 ± 0.13 ‰) are close to those of modern marine hydrothermal iron, and there is a strong positive correlation between the δ56Fe value and iron concentration (R2 = 0.81), indicating that the precipitation of manganese ore could be mainly affected by hydrothermal input. After comprehensively comparing the δ56Fe values of the whole rock and pyrite in the manganese ore of the Datangpo Formation, we believe that iron from hydrothermal sources directly and quantitatively combined with sulfides to form pyrite. Additionally, the bottom seawater of the basin was mainly anoxic. The nitrogen isotope composition indicates the presence of a stable nitrate reservoir in the ocean at that time, and manganese ore deposition may have been influenced by the input of high-salinity water from the open ocean. Due to the supply of NO3- from the open ocean to the graben basin, the δ15N in manganese ore is significantly higher than that in black shale. The input from the open ocean led to an increase in alkalinity in the bottom water of the basin, which favored the precipitation of rhodochrosite. Considering that the oxidation potential required for manganese oxidation precipitation is higher than that required for iron oxidation precipitation, it is most reasonable to conclude that under alkaline anoxic conditions in the bottom water layer, Mn2+ directly precipitated in the form of rhodochrosite.

Links between marine incursions, lacustrine anoxia and organic matter enrichment in the Upper Cretaceous Qingshankou Formation, Songliao Basin, China

There is a general relationship between the formation of terrestrial hydrocarbon source rocks and marine incursions during the Mesozoic–Cenozoic in China. To better understand this scientific issue, this study focused on the little-studied Upper Cretaceous Qingshankou Formation (early Late Cretaceous) in the Songliao Basin, China, which was investigated using organic petrological and organic and inorganic geochemical methods. The mudstone and shale in the Qingshankou Formation were deposited in an anoxic, reducing, slightly brackish water environment and contain benthic macroalgae, which is a typical marine fossil in Qingshankou Member I. The δ34S values are generally similar to those of Late Cretaceous marine sulfate, and the total N contents and isotopic compositions (δ15N) are both high, suggesting that the Qingshankou Formation was affected by marine incursions that triggered lake anoxia. The total organic carbon (TOC) contents of the mudstones and shales in the Qingshankou Formation are 0.21–3.86 wt%, with an average value of 1.91 wt%, which meet the criterion of a very good–excellent source rock. The organic matter varies from type I to III kerogen but is mainly types I and II, which generate oil. The hydrocarbon-generating bio-precursors are mainly planktonic and benthic algae. The hydrocarbon generation potential of the Qingshankou Member I is high, mainly due to the high productivity during its deposition. Increased productivity caused by marine intrusions, water stratification caused by high salinity, and reducing bottom-water environment provided favorable conditions for organic matter enrichment. The marine incursions during deposition of the Qingshankou Formation (especially Member I) improved the quality of the source rocks by enhancing the primary productivity, which might be a common feature of terrestrial source rocks affected by such incursions in general.

Hydrocarbon charging process of Ordovician strike‐slip fault zones in the Shunbei area, Tarim Basin, NW China

Ultra‐deep marine carbonate reservoirs is an important field for oil and gas exploration, and it is also a hot research topic in recent years. However, their hydrocarbon episodic charging model is still under debate up to now. A great breakthrough has been made in the newly discovered ultra‐deep fault‐karst oil reservoir in the Shunbei area of Tarim Basin. Based on quantitative gas chromatogram‐mass spectrometry, stable carbon isotopes of reservoir oils, microthermometry and confocal laser scanning analyses of fluid inclusions, the reservoir oil origin and hydrocarbon charging history in Shunbei No. 1, No. 5 and No. 7 strike‐slip fault zones are expounded. The results showed that the Ordovician reservoir oil derives from the source rocks within the Cambrian Yuertusi Formation deposited in a highly anoxic bottom water environment. There were four stages of hydrocarbon charging in the study area, in the late Caledonian, late Hercynian–Indosinian, middle Yanshanian and late Himalayan. During this period, the force‐field strength of the charged oil gradually decreased, and the early‐stage charged crude oil with low maturity and high force‐field strength preferentially accumulated in the Yijianfang Formation, while the late‐stage charged crude oil with high maturity and low force‐field strength preferentially accumulated in the Yingshan Formation. This study reveals the history and differences of petroleum accumulation in the Shunbei fault zones, which is significance for understanding the hydrocarbon distribution in ultra‐deep marine carbonate reservoirs.

Formation mechanism of organic-rich mixed sedimentary rocks in saline lacustrine basin, Permian Lucaogou Formation, Jimsar Sag, Junggar Basin, Northwest China

The organic-rich mixed sedimentary rocks deposited in a saline lacustrine environment are widespread in the Lucaogou Formation (P2l), Jimsar Sag, Junggar Basin. These rocks have complex lithology (mainly including mudstone, dolomitic mudstone and argillaceous dolomite) and significant changes in geochemical characteristics. Presently, the formation mechanism of organic-rich mixed sedimentary rocks in the P2l of the Jimsar Sag is obscure. To address these issues, the formation mechanism of mudstone, dolomitic mudstone and argillaceous dolomite is revealed by evaluating their sedimentary environment, primary productivity and sources of organic matter using organic petrology, organic geochemistry and inorganic geochemistry. The results show that the mixed sedimentary rocks of the P2l exhibit excellent hydrocarbon generation potential. The mixed sedimentary strata were mainly deposited in saline and suboxic to anoxic water column, and the main source of organic matter was aquatic organisms. During the changes from mudstone, dolomite mudstone to argillaceous dolomite deposition, the paleoclimatic condition varied from warm-humid to arid-hot with more frequent hydrothermal activity, which enhanced the salinity of the water column, leading to a higher proportion of halotolerant algae (cyanobacteria and green algae). The improvement in the paleoproductivity caused by volcanic activity was the most critical factor to promote the enrichment of organic matter in the mixed sedimentary rocks. In addition, the decomposition of algae and stratified water column were conducive to the formation of a stable and suboxic to anoxic bottom water environment, which improves the preservation conditions of organic matter, and also favored the organic matter enrichment in mixed sedimentary rocks. Therefore, the frequent changes in paleoclimatic conditions and periodic hydrothermal activity controlled the formation of different lithologies in the P2l. High primary productivity and good preservation conditions promoted the accumulation of organic matter in these mixed sedimentary rocks.

Organic matter accumulation mechanisms in the Wufeng-Longmaxi shales in western Hubei Province, China and paleogeographic implications for the uplift of the Hunan-Hubei Submarine high

Organic matter accumulation exhibits different distribution patterns in time and space and tends to be controlled by many factors. This study investigated the mechanisms that control the accumulation of organic matter in the Xianfeng area of western Hubei Province, South China through sedimentological and geochemical analyses of wellbore samples. The results showed that the total organic carbon (TOC) content, mineral content, and elemental geochemical data of the Wufeng-Longmaxi Formations varied significantly with depth. Based on the sharp changes in the vertical profiles of the TOC content, Gamma-ray, and numerous geochemical parameters, as well as regional stratigraphy, this study divided the section from the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation into six units (W1, W2, GYQ, L1, L2, and L3). The TOC content was positively correlated with redox proxies (DOPT, VEF, and MoEF) and primary productivity proxies (CuEF and NiEF), and negatively correlated with terrigenous input proxies (Al, K, and Ti). The correlation coefficients suggest that the primary factors affecting organic matter accumulation are redox conditions and paleoproductivity, while terrigenous input is a secondary controlling factor. This study proposed an organic matter accumulation model based on the organic matter characteristics, lithofacies, sedimentary environment, and geological setting of the study area. During the deposition of Unit W1, organic matter was well-preserved owing to the anoxic bottom water environment resulting from strong water mass restriction. During the deposition of Unit W2, the rising sea level led to the development of anoxic-euxinic bottom water conditions, and the joint influence of nutrients from upwelling in the open sea and volcanic eruptions enhanced productivity, resulting in the most intense accumulation of organic matter. Unit GYQ had low TOC content due to low productivity despite anoxic conditions. During the deposition of Unit L1, the uplift of the Hunan-Hubei Submarine High caused the depocenter to shift westward and the sea level in this area to drop, resulting in suboxic bottom water and relatively low productivity in the surface water layer. During the deposition of Unit L2, the regional sea level drop resulted in increased input and dilution of terrigenous clasts, thus leading to low TOC content. During the deposition of Unit L3, continuously enhanced terrigenous input due to climate warming and sea level drop became the major factor controlling the organic matter accumulation. In summary, the main factors controlling the organic matter accumulation differed significantly at different evolutionary stages. Compared with the Jiaoshiba area, the Xianfeng area in western Hubei experienced continuously increased terrigenous input, a less reductive water mass, a significant decrease TOC content, and a westward shift of the depocenter since the early stage of the Longmaxi Formation deposition. These changes may be related to the intense uplift of the Hunan-Hubei Submarine High triggered by the Kwangsian Orogeny. These results provide important implications for the understanding of the accumulation of organic matter in the Wufeng-Longmaxi Formations and paleogeographic changes.

Pleistocene Paleoceanographic Conditions in the Ioffe Drift Area (South Atlantic) Based on Benthic Foraminiferal Assemblages

The Ioffe Drift located in the western South Atlantic to the north-east to the Vema Channel was discovered in the 32nd cruise of the R/V “Akademik Ioffe” in 2010. The analysis of the seismoacoustic, biostratigraphic, lithological, grain-size, geochemical and X-ray fluorescence data allowed to estimate the Pleistocene age of the upper part of the drift as well as to confirm contourite genesis of the sediments. Quantitative analyses of benthic foraminiferal assemblages are carried out in the sediment core AI-2436 (26°51.6′ S, 34°01.40′ W, 3800 m water depth) collected near the drift summit. The obtained results allowed reconstructing relative changes of the organic matter flux to the seafloor, its periodicity and bottom current intensity as well as suggesting interplay of the deep-water masses during the Pleistocene. Visual evaluation of benthic foraminiferal tests allowed distinguishing three groups of damages of presumably variable genesis. First group includes damages possibly resulted from impacts of particles transporting by bottom currents. Second group consists of damages that appeared due to movement of the tests by strong bottom currents. Third group contains deformations caused by dissolution in carbonate-aggressive bottom-water environment. Thus, specific features of the tests in combination with traditional methods may serve as a supporting technique for diagnostics of contourites. However, this method demands a development of application criteria and should be verified on reliable material.

Deep-sea bottom-water environment change caused by sediment resuspension on the continental slope off Sanriku, Japan, before and after the 2011 Tohoku Earthquake

The 2011 off the Pacific coast of Tohoku Earthquake occurred off southern Sanriku, Japan, on March 11 and generated strong shaking and huge tsunami along the entire eastern coast of Tohoku. The mainshock and numerous large aftershocks caused mass-wasting sedimentation events on the continental shelf, slope, and trench bottom. To investigate the impacts of the 2011 Tohoku Earthquake on the deep-sea bottom-water environment off Sanriku, we conducted shipboard surveys up to ~ 2000 dbar during 2011–2018 and long-term monitoring of the seafloor on the continental slope using a deep-sea station (~ 1000 dbar) off Otsuchi Bay during 2012–2018. The high turbidity (maximum ~ 6%) was observed for the bottom water deeper than 500 dbar on the continental slope of the entire area off Sanriku during 2012–2018. This high turbidity was caused by sporadic sediment resuspension induced by frequent large aftershocks. Furthermore, dissolved oxygen concentrations in the bottom layer from 1000 to 1500 dbar dropped significantly by about 10% after the earthquake, while nutrients and dissolved inorganic carbon showed no significant changes but exhibited wide variations. The high turbidity was associated with the increase in the concentrations of phosphate, dissolved inorganic carbon, and methane, as well as the decrease in those of dissolved oxygen and nitrate. This suggests that remineralization of suspended organic matter resulting from the respiration and denitrification of microbial communities after the earthquake caused the chemical properties of the deep-sea bottom-water. The deep-sea bottom-water environment change was maintained by sporadic sediment resuspension due to continued large aftershocks and was likely caused by variations in dissolved inorganic carbon and phosphate. There are two peaks in the concentration and carbon isotope ratio of methane on the deeper slope from 1000 to 2000 dbar near the hypocenter, which were advected along isopycnal surfaces of 27.38σθ (1000 dbar) and 27.56σθ (1500 dbar). The source of the shallower peak of chemical input is considered to be the sediment resuspension from the shallow sediment on the continental slope induced by the mainshock and large aftershocks.Graphical abstract

Quantifying Iron Oxide Mineral Contents in Miocene Oceanic Red Beds for the Deep-Sea Oxidation Evolution in the South China Sea

The International Ocean Discovery Program (IODP) Expedition 349 recovered Miocene oceanic red beds overlying the basaltic basement in the South China Sea. The occurrence of oceanic red beds provides an opportunity to understand the deep-sea redox conditions when the South China Sea was open to the western Pacific during the Miocene. Here, we investigated iron oxide mineral contents along with major element compositions of the oceanic red beds at Site U1433 to reveal the Miocene deep-sea oxidation environment of the South China Sea and its interaction with the western Pacific. The results show that these samples contain 0.20–1.48% hematite (average 0.50%) and 0.30–2.98% goethite (average 1.20%). Their contents have good linear correlations with color reflectance a* (red) and b* (yellow), respectively, implying that the reddish-brown color of the Miocene oceanic red beds resulted from a mixture of hematite and goethite. Compared to other oceanic red beds worldwide, the occurrence of hematite and goethite in the South China Sea is considered to form under an oxic bottom water environment with an extremely low sedimentation rate. The (hematite + goethite)/(100%—Al2O3) ratio is adopted to reconstruct the evolution of bottom water oxidation during the Early–Middle Miocene. A continuously decreased oxidation trend from 18.4 to 11.6 Ma, along with two strengthened oxidation events occurring at around 15 Ma and 14 Ma, is observed to dominate the environment evolution of the abyssal South China Sea. We infer that this long-term decreased oxidation trend was caused by the gradual blocking of oxygen-rich bottom water from the western Pacific since the Early Miocene, while the two oxidation events were likely attributed to the rapid thermal subsidence of the South China Sea and the global cooling during the Middle Miocene climate transition, respectively.

Intermediate water variability of the subtropical Northeastern Atlantic during 490–424 ka (late MIS 13 and MIS 12)

High-resolution foraminiferal stable isotopes and benthic foraminiferal faunal records of IODP Site U1391 drilled off the western Iberian margin were adopted to reconstruct intermediate water variability of the subtropical Northeastern Atlantic during the late Marine Isotope Stage (MIS) 13 and MIS 12. Five faunal turnovers were recognized based on multivariate statistical analyses of benthic foraminiferal census data from the size fraction >125 μm. The dominance of Uvigerina peregrina parva and Melonis barleeanum coincides with high benthic foraminiferal accumulation rates (BFAR), high benthic δ13C and the presence of dark-colour sediments during the final stage of MIS 13, also accompanied by frequent occurrences of Planulina ariminensis, an indicator of Mediterranean Outflow Water (MOW), which indicates MOW-related high oxic and mesotrophic to slightly eutrophic bottom water environments. MIS 12c and MIS 12b are characterized by Bulimina mexicana assemblage, together with low BFAR, high benthic δ13C and the presence of light-colour sediments, revealing mesotrophic and well‑oxygenated seafloor conditions associated with the possible advection of Glacial North Atlantic Intermediate Water (GNAIW) to the studied site. A prominent increase in organic matter supply and a slight decrease in dissolved oxygen concentration during MIS 12a were reflected by more abundant Bulimina aculeata, higher BFAR, lower benthic δ13C and the darker-colour sediments relative to MIS 12c-b. A shift in the dominant species and significantly decreased benthic δ13C, suggest an increased influence of southern-sourced waters (SSW) and a decreased influence of GNAIW during MIS 12a. During the early Termination V (TV), infaunal taxa mainly composed of B. aculeata, Bulimina exilis, Nonionella turgida, Brizalina sp. and Uvigerina proboscidea dominate the benthic foraminiferal population, which may be attributed to eutrophic and poorly‑oxygenated bottom water environment strongly influenced by SSW. During the late TV, N. turgida rapidly became the predominant taxa, and its predominance was probably the result of further reduction in dissolved oxygen concentration.

Iron speciation in organic-rich shales from the Upper Triassic Yanchang Formation, Ordos Basin, Northern China: Implications for depositional environment

The iron (Fe) cycle is one of the most important marine and lacustrine geochemical cycles and is closely related to environmental redox conditions, which directly affects carbon, sulfur, phosphorus cycling. Analysis of Fe speciation (including Feox, Femag, Fecarb, and Fepy) of organic-rich shale samples provides new insights into the controversial redox conditions during the Chang 7 sedimentary period. Our results show that Fecarb and Fepy of the Chang 7 organic-rich shale are the main forms of reactive Fe pools, indicating that reactive Fe mainly existed in the form of Fe2+ in sediments. With a few exceptions, most samples fall into the ferruginous region in crossplots of Fepy/FeHR versus FeHR/FeT, indicating that pore water environments during early diagenesis were ferruginous accompanied by intermittent euxinic. Previous works examining the size of framboidal pyrite, the molar ratio of organic carbon and total phosphorus (P), and nitrogen isotope values suggest the bottom water environments conditions were oxic-suboxic accompanied by intermittent anoxic. We argue these patterns may be explained by the proximity of the redox boundary of depositional environment to the sediment–water interface during most of the Chang 7 period, with short-term fluctuations. Euxinic pore water conditions correspond to anoxic bottom water conditions and elevated total organic carbon (TOC) content, indicating that these environmental conditions would increase the flux of P diffused from sediments. As a result, lacustrine primary productivity would be elevated, creating conditions conducive to the accumulation of organic matter that is characteristic of the Chang 7 sedimentary period. This work will further our understanding of the accumulation mechanism of organic matter in the Chang 7 shale, the principal source rock for the Mesozoic oil-bearing system in the Ordos Basin.

Mineralogy and geochemistry of Lower Silurian black shales from the Yangtze platform, South China

Early Silurian black shales are widely deposited over the entire Yangtze platform and their origin are highly controversial. In order to identify the controlling factors of these black shales formation, the mineralogy and geochemistry of the Early Silurian organic-rich shales from the Yangtze Platform, South China, were investigated by using an optical microscopy, X-ray diffraction analysis, X-ray fluorescence spectroscopy, inductively coupled plasma mass spectrometry and field emission scanning electron microscopy. These black shales are characterized by high total organic carbon (0.37%–10.47%, avg. 3.86%) and total sulphur (0.02%–1.68%, avg. 0.67%) content as well as enrichment of some trace elements, including V (70.66 ppm–1740.52 ppm, avg. 312.75 ppm), Cr (68.72 ppm–301.94 ppm, avg. 117.97 ppm), U (3.90 ppm–52.49 ppm, avg. 16.58 ppm), Ba (316.69 ppm–2985.82 ppm, avg. 1523.38 ppm) and rare earth elements (94.26 ppm–340.50 ppm, avg. 202.46 ppm). The minerals mainly consist of quartz and clay mineral with average content of 54.8% and 29.3%, respectively. K-feldspar, dolomite, calcite and pyrite are next in abundance, followed by trace amounts of apatite, barite and Cr-bearing minerals. The dissolution of biogenic silica and recrystallization during burial is an important source of quartz cement. In addition to biological sources, terrestrial, volcanic and authigenic sources are responsible for these mineralogical and geochemical compositions. The Al2O3/TiO2, Zr/Sc, Th/Sc, LREE/HREE and Eu anomalies indicate that the sediment-source region is derived from a mixture of felsic and intermediate rocks of the old upper continental crust. The Th/U and V/(V + Ni) ratios, Ce anomalies and authigenic U suggest that these black shales are deposited under an anoxic bottom water environment. The elevated concentrations of U-V-Ni-Cu-Zn-Cr-Ba and REE in these shales are predominated by the marine environment during deposition and influenced by biologic production in water columns and volcanic ash fall. High marine productivity resulting from volcanic nutrients promote the anoxic/sulfidic water column conditions of the restricted Yangtze basin, further resulting in organic matter enrichment. Elemental enrichment processes usually result in a valence change under anoxic environment, which is associated either with the clay minerals or direct precipitation from sea water. Understanding the coenrichments of organic carbon and trace elements in these black shales is important for studying the formation mechanism and resource potential of hydrocarbon and critical elements.

Why does Clark I remain in the marine environment for a long time?

After World War II, based on decisions made at the Potsdam Conference in 1945, over 15,000 tons of chemical warfare agents (CWAs) were dumped in the Baltic Sea. These chemicals are still a threat to people and the maritime economy, but most of all to marine ecosystems. It is important to study the mechanisms of CWA reactions that could take place in the marine environment to evaluate the environmental risk. These studies are focused on the transformations of Clark I (diphenylchloroarsine, DA) in simulated Baltic Sea bottom sediments and water conditions. GC and LC methods for the analysis of Clark I and its degradation products were developed and applied. Clark I reactions in the bottom sediments and water environment were monitored systematically. This enabled the kinetics curves of Clark I degradation in various environments (matrix, temperature 4–70 °C, pH) to be determined and degradation products to be identified. Based on the kinetics curve reaction rates, the half-life period, reaction rate constants, and temperature coefficient (Q10) for water and sediment were established. These data allowed the activation energy for the hydrolysis reaction of DA in an aqueous environment to be ascertained, which was 69.2 kJ/mol. Clark I half-times in the conditions present in the Baltic Sea were about eight days for water and 18 days for sediment.

Oxygen minimum zone variability during the last 700 years in a coastal upwelling area of the Humboldt system (Mejillones, 23° S, Chile). A new approach from geochemical signature

Mejillones bay presents a permanent and shallow OMZ, whose upper boundary fluctuates between 30 m and 60 m water depth during a normal year. Under this condition, the bottom sediments preserve a geochemical signature of the past oceanic variability. Particularly, biological productivity and bottom dissolved oxygen can be reconstructed using 13C, 15N and trace elements metals measured along sediment cores.Trace elements like V, Ni, Mo and U preserved in surface sediment of Mejillones bay as a consequence of poorly oxygenated bottom waters, were used to develop a statistical model that allowed the reconstruction of the bottom water dissolved oxygen variability during the last 700 years.During these seven centuries, the marine sediments of Mejillones recorded three ocean-climatic periods. From 1350 to 1500 CE the bay was dominated by a strong and shallower OMZ, intense denitrification, but a low productivity and flux of biodetritus. Bottom waters recorded oxygen values around a mean of 3.52 µmol L−1, during this period. During the Little Ice Age (1500 to 1850) the OMZ was less intense, and bottom waters recorded mean dissolved oxygen of 5.35 µmol L−1. Finally, during the Current Warm Period (1900 to date) the OMZ has strengthened and is located at shallower depth, generating a bottom water environment with values of dissolved oxygen slightly lower than in the previous period, around 4.51 µmol L−1. Other studies show that during these same climatic periods the Sea Surface Temperatures changed from warm waters before the LIA, a succession of cold and warm periods during the LIA, and warm waters at the beginning of the CWP, but a prevalence of cold waters since the 1940s decade until the present.The sedimentological approach to reconstruct the paleoxygenation of bottom environments proposed for Mejillones bay must be considered as preliminary and of local-use potential, thus being necessary to explore similar methodologies in other environments in order to support and improve this approach.

Signals of combined chromium–cadmium isotopes in basin waters of the Early Cambrian – Results from the Maoshi and Zhijin sections, Yangtze Platform, South China

The combination of authigenic chromium and cadmium stable isotopes in marine sedimentary archives enables reconstruction of the redox conditions and levels of primary productivity in past oceans and basins. The Ediacaran to Cambrian Nanhua Basin (Yangtze Platform, South China) and its sedimentary sequences have been studied intensively in the past with respect to investigating the effects of post-Snowball Earth glaciations on the oxygenation of Earth's atmosphere, and intimately related, on the consequences of climate changes on the evolution of the ocean environment and of life during the Precambrian-Cambrian transition. Following recent propositions of using cadmium isotopes as a novel proxy for reconstructing the extent of primary productivity in paleooceans, we here present results from a combined cadmium (Cd)‑chromium (Cr) isotope study on metalliferous black shale, black shale and phosphorite sediments deposited during the Early Cambrian Niutitang Formation at the Maoshi and Zhijin sections. Results point toward strongly oxidized and highly productive surface water layers in the Nanhua Basin during this period. The samples have strongly positively fractionated δ114Cd and δ53Cr values up to +0.6‰ and + 1.4‰, respectively, similar to such values from carbonaceous sediments in modern oceans. The association of Cd with sulfides in the sediments, together with high Cd/TOC ratios particularly in the metalliferous black shales, implies a sulfide-controlled enrichment of Cd under restricted euxinic conditions. The lower Cd/TOC values in phosphorites and barren black shales and negatively fractionated δ114Cd values down to −0.33‰ point to less effective precipitation of Cd under more suboxic conditions as likely prevailing in an open shallow shelf. The Cr content in the sediments is mostly close to average shale, and the δ53Cr values are little fractionated in the 0–0.2‰ range, corroborated by flat shale-like rare earth element + yttrium (REE + Y) patterns. However, some metal-rich shale samples have clearly authigenic Cr isotope signatures with up to +1.4‰ δ53Cr, and the authigenic Cr component in the phosphorite samples is around 1‰ δ53Cr, indicating oxic surface waters. The latter sediments also depict seawater-like, middle REE enriched REE + Y patterns with distinct negative Ce and positive Y anomalies indicative of an oxygenated surface water layer, and with positive Eu anomalies which they likely received during phosphatogenesis in the euxinic bottom water environment. In accord with previous studies reporting on extreme trace metal enrichments (Mo, Ni, V, Hg, etc.) in these Early Cambrian sediments, our Cd and Cr isotope signatures favor a scenario of a strongly stratified basin in which high primary productivity thrived in an oxidized surface water layer, potentially as a result of increased nutrient supply from a strongly weathering continental hinterland in the aftermath of the last Snowball Earth glaciations in the Ediacaran.

Enrichment Mechanism of the Upper Carboniferous-Lower Permian Transitional Shale in the East Margin of the Ordos Basin, China: Evidence from Geochemical Proxies

The organic-rich shale of the Upper Carboniferous-Lower Permian transition period in the eastern margin of the Ordos Basin, China, was formed in a marine-continental facies sedimentary environment. With a high content of total organic carbon (TOC) and a large cumulative thickness, it is considered a good source rock for shale gas development. The sedimentary environment of marine-continental transitional shale is obviously different from that of marine shale, which leads to different enrichment characteristics of organic matter. In this paper, shale samples were collected from XX# well of the Taiyuan and Shanxi Formations across the Upper Carboniferous-Lower Permian, which is typical marine-continental transitional shale. The TOC, major elements, and trace elements were measured, and the formation and preservation conditions were investigated using multiple geochemical proxies, including paleoclimate, redox parameters, paleoproductivity, and controls on the accumulation of organic matter. The TOC of Shanxi Formation is higher than that of Taiyuan Formation. In the Taiyuan Formation, TOC is positively related to the redox index (V, U, and V/Cr), indicating that the dysoxic bottom water environment is the key factor controlling organic matter accumulation. For Shanxi Formation, there is a positive correlation between TOC and paleoclimate, which indicates that the enrichment of organic matter is affected by warm and humid paleoclimate and oxic environment. In addition, the paleoproductivity is lower with a positive correlation with TOC for the marine-continental transitional organic-rich shale, suggesting that it was inferior to the gathering of organic matter.

The Campanian paleoenvironment: Inferences based on benthic foraminifera from northeastern Iran

The benthic foraminifera from the Campanian rocks (Globotruncanita elevata to Globotruncana aegyptiaca zones) exposed in the Kopeh-Dagh Basin (northeast Iran) are studied quantitatively to reconstruct the Campanian paleoenvironment. Based on unconstrained heriarchial clustering, Nonmetric Multidimensional Scaling (NMDS), and analysis of similarity (ANOSIM), three statistically significant benthic foraminiferal assemblages are noted, Paralabamina lunata (lower and upper), Haplophragmoides spp., and Gyroidinoides nitida. Based on benthic foraminiferal marker species, the following paleodepths are assigned - lower P. lunata: upper bathyal; upper P. lunata: upper-middle bathyal boundary; Haplophragmoides spp. and G. nitida: outer neritic. The P. lunata Assemblage can be further subdivided into a lower Coryphostoma incrassata-dominated interval, whereas the upper interval is P. lunata-dominated, and reflects a change in the quality of food; the upper experienced higher input of fresh phytodetritus. The overall P. lunata Assemblage reflects a stable bottom water environment. The Haplophragmoides spp. Assemblage is characterized by reduced paleoproductivity and environmental instability, whereas the G. nitida Assemblage is marked by increasing oligotrophy within a well-oxygenated bottom water and ventilated basinal conditions. The Partial Least Squares regression yielded significant correlations with % Planktic (positive; proxy for sea level) and % agglutinated species (negative; proxy for shallow depths and low organic-flux); the Reduced Major Axis regression models also yielded significant negative correlations with % agglutinated and % infaunal species (nutrient availability), suggesting that these factors were responsible for the inferred benthic foraminiferal distribution patterns. Two globally comparable shallowing trends are also noted - one in the middle of the R. calcarata Zone (= Haplophragmoides spp. Assemblage) and the other in the G. aegyptiaca Zone (= Gyroidinoides nitida Assemblage) that coincide with KCa6 and KCa7 sequence boundaries and lowstands, respectively. For the KCa6 event, the occurrence of Spiroplectammina (= Haplophragmoides spp. Assemblage) represents a shallow-water outer neritic fauna invasion event within a deeper upper bathyal setting (= Paralabamina lunata Assemblage) and for the KCa7 event, a similar shallowing from upper-middle bathyal boundary to outer neritic depths is noted (= G. nitida Assemblage), and attributed to eustasy and basinal paleohighs and lows.

Geochemical characteristics of organic-rich shale, Upper Yangtze Basin: Implications for the Late Ordovician–Early Silurian orogeny in South China

The tectonic–sedimentary environment of the Upper Yangtze Basin in China underwent significant changes from the Late Ordovician to Early Silurian. The widely distributed shale formed during this interval has recorded key information about paleo-ocean conditions. Analysis of major, trace and rare earth elements was conducted on 135 shale samples collected from five wells in the study area. The covariation of the MoU enrichment factors indicates that the Upper Yangtze Basin in the Wufeng stage was characterized by sedimentary differentiation. The southern basin was dominated by a reducing bottom water environment under euxinic conditions. The northern and central basins were dominated by a suboxic–anoxic environment. Up to the early Longmaxi stage, the Upper Yangtze Basin formed a relatively uniform sedimentary environment with gradually increasing oxygen content in the bottom water. The paleoproductivity index indicates that marine organisms flourished in the Wufeng to early Longmaxi stages, making an important contribution to the siliceous content in organic-rich shale; subsequently, terrestrial siliciclastics gradually dominated. Analysis of paleogeography, rare earth elements, and weathering indicates that Chuanzhong and Qianzhong uplifts in the Wufeng stage were the provenance area of the Upper Yangtze Basin, with a relatively high weathering degree. Weathering decreased in the beginning of the Longmaxi stage and then gradually increased, with the Xuefeng Uplift becoming the main provenance areas after the early Longmaxi stage. Controlled by stress transfer from the continuous syn–Caledonian Kwangsian Orogeny in eastern South China, the Upper Yangtze Basin and Xiangzhong Basin were squeezed and pushed up successively. The uplift separated the open shallow sea and formed a relatively limited deep-water shelf environment, resulting in a relatively euxinic bottom water environment, which was favorable for organic matter accumulation. The tectonic–sedimentary evolution of the Upper Yangtze Basin in the Late Ordovician to Early Silurian proposed in this study indicates that the formation of organic-enriched shale was not only related to glacio-eustatic change but also strongly controlled by tectonic activities in South China.

Study on Corrosion Law of Large Crude Oil Storage Tank Floor and Risk-Based Inspection and Maintenance Technology

In this paper, the author''s team has carried out a comparative experimental study on the corrosion characteristics of Q235 steel commonly used in large-scale storage tanks under the specific bottom water environment found with Russian and Daqing crude oil. It was found that there is a certain degree of uniform or local corrosion on the tank floor depending on the kind of bottom water. The bottom water corrosion of Daqing crude oil is a uniform corrosion caused by carbon dioxide. While the Russian crude oil bottom water corrosion is clearly local corrosion caused by co-corrosion of carbon dioxide and hydrogen sulfide, here the corrosion rate is obviously higher than that caused by Daqing crude oil. There are two modes of storage tank inspection and maintenance that have been currently adopted by Chinese refining and chemical enterprises: a regular inspection mode and a API581-2016 risk-based detection mode. These modes have been effectively combined to form an intelligent tank inspection and maintenance mode, software tools to support this intelligent inspection and maintenance management have been developed.

Diagenesis and iron paleo-redox proxies: New perspectives from magnetic and iron speciation analyses in the Santa Barbara Basin

Sedimentary redox proxies are usually employed to reconstruct the paleo-redox conditions of bottom water environments, assuming that porewater and bottom water dissolved oxygen concentrations are similar. Using a combination of geochemical and magnetic techniques, we investigate the relationship between iron speciation and mineralogy in recent (~1760–2009 CE) sediments retrieved from the Santa Barbara Basin (SBB) – a modern silled basin with low-oxygen (dissolved O2 < 10 μmol/kg) and sporadically anoxic (no O2 detected) bottom waters. Magnetic analyses reveal that biogenic magnetite is preserved in SBB sulfidic porewaters on at least decadal to centennial time scales. Highly-reactive Fe (oxyhydr)oxides remain preserved despite observed sulfidic porewaters, indicating incomplete pyrite conversion, and producing low Fepy [pyrite Fe]/FeHR ratios. We attribute this observation to restricted porewater reaction kinetics under high sedimentation rates. Our results also reveal non-steady state diagenesis caused by instantaneous depositional events (e.g., turbidites and flood layers). The most reducing water column suggested by Fe speciation coincided with the Macoma layer, where in situ colonization of hypoxia-intolerant bivalve shells argues for the most oxygenated bottom water in the 250 year record. A turbidite potentially introduced fresh unsulfidized FeHR that buffered upward-diffusing sulfide from underlying sediments. Subsequent pyrite precipitation following re-establishment of sulfidic porewaters could have facilitated a “false positive” interpretation. In comparison, redox-sensitive metal enrichments (MoEF, UEF, and ReEF) were not obscured by post-depositional diagenesis and appear to accurately record redox geochemistry at the sediment-water interface.

An integrated approach to understanding the depositional environment and organic matter enrichment factor in Carboniferous source rocks, Junggar Basin, NW China

Multiple indicators (trace fossils, trace metals, and δ13Corg) are used to assess the depositional environments and controls on organic carbon accumulation in the Lower Carboniferous Nanmingshui Formation, Junggar Basin, NW China. The majority of trace fossils belong to the Nereites ichnofacies, representing low energy, oxygen‐poor, and relatively quiet bottom water environments. However, the shallow water trace fossil Planolites also appears in the strata, possibly reflecting variations in the bottom water oxygen contents of the turbidite succession. Redox proxies Th/U, V/Sc, and V/(V + Ni) reveal variable redox conditions; most of the samples are in the range from suboxic to anoxic depositional conditions. Ni and Cu show a positive correlation with TOC, but U and V have no correlation with TOC, also suggesting the redox conditions were mainly suboxic. The P/Al and Cu/Al ratios of all samples are higher than those of Post‐Archean Australian Shales (PAAS), and about two‐thirds of the samples' Ba/Al and Ni/Al ratios are close to or higher than PAAS, suggesting the Early Carboniferous Junggar Basin had relatively high palaeo‐productivity levels. δ13Corg values average − 23.75‰, reflecting a mixture of terrestrial organic and marine organic matter. The indexes of Ba/Al, Cu/Al, Ni/Al, and δ13Corg show good positive correlations with TOC, indicating the palaeo‐productivity played an important role for the organic matter accumulation. On the other hand, the TOC correlates inversely with redox proxies (Th/U, V/Sc, and V/(V + Ni)), suggesting the preservation conditions (anoxia) are not the control factor of organic matters accumulation. Using these multiple approaches, our results indicate that productivity was the primary factor controlling organic matter accumulation.