Stable Water Column Stratification Research Articles

Source rock and climate model for the Cretaceous Jiufotang Formation, Kazuo Basin, China: Geochemistry and petrography

The Lower Cretaceous Jiufotang Formation, situated in western Liaoning Province, China, has attracted attention for abundant Jehol Biota fossils and the potential to improve understanding of the relationships among climate change, evolution of terrigenous organisms, and organic matter (OM) enrichment in lake systems during the Early Cretaceous. Based on organic petrology and biomarker analyses, this study investigated the depositional environment, OM sources, and enrichment mechanism of Jiufotang mudstone in the Kazuo Basin. Maceral composition shows that the OM source of Jiufotang mudstone shifted from mainly aquatic to terrigenous, and biomarkers such as abundant diterpenoids and retene indicate that terrigenous higher plants were mainly conifers during deposition of the Jiufotang Formation. Pr/Ph ratios (0.07–0.65) show that the studied section was deposited in anoxic bottom water that may be necessary for OM enrichment in lake systems. Gravity flows were accompanied by input of many clastic minerals and oxygenation of bottom waters, which is unfavorable for OM enrichment. Therefore, the best OM enrichment model for lake systems includes anoxic bottom water, stable water column stratification, low sedimentation rates, and high phytoplankton contributions. The climate changed from relatively dry to humid, which drove changes in the OM enrichment model in the lake. More importantly, the climate and terrigenous plant information obtained by biomarkers is useful to reconstruct the ecological environment of the Jehol Biota.

Characteristics and deposition models of the paleocene source rocks in the Lishui Sag, east China sea shelf basin: Evidences from organic and inorganic geochemistry

The Lishui Sag, situated at the East China Sea Shelf Basin, is regarded as one of the most prospecting oil exploration zones. Low exploration in this region requires detailed identification of source rocks to enhance further oil and gas exploration. Three potential source rocks, including the Paleocene Yueguifeng (E1y), Lingfeng (E1l) and Mingyuefeng (E1m) Formations, were investigated by integrating organic and inorganic geochemistry to estimate hydrocarbon generation potential, and to document original organic matter input and deposition environments. Two deposition models for source rocks were established considering tectonic subsidence and climate variation. The result shows that the E1y mudstones are dominated by high TOC content, oil/gas-prone kerogen at moderate maturity stage with relatively high hydrocarbon generation potential, whereas E1l and E1m mudstones are dominated by low TOC content, gas-prone kerogen at low to moderate maturity stage with low hydrocarbon generation potential, and coals with high hydrocarbon generation potential. The original organic matter input and deposition environment are obviously different among these three potential source rocks and are controlled by the coupling of tectonic subsidence and climate variation. Insufficient material supply lead mudstone deposition in a hemi-deep/deep lake during the E1y period, resulting in inefficient water mixing and stable water column stratification (high Gammacerane Indexs). The weak water inflow supplied minor oxygen and terrigenous organic matter to the lake due to arid-hot climate, as indicated by low Pr/Ph, C19 T/C23T, C20 T/C23T and C24Tet/C26T ratios, low Oleanane Indexs, and high C35 Homohopane Indexs and V/Ni ratios. Water algae, including pediastrums and chlorellas, which contributed to the development of high-quality source rocks, could be efficiently preserved in dysoxic condition. High subsidence rate together with sufficient sediment supply during the transgression in the E1l and E1m periods developed a coastal marine or paralic environment. The alternated climate brought unstable water inflow against stable water column stratification (low Gammacerane Indexs). Strong water inflow due to warm-humid climate provides abundant terrigenous organic matter and oxygen to the lake as indicated by high Pr/Ph, C19 T/C23T, C20 T/C23T and C24Tet/C26T ratios, high Oleanane Indexs, low C35 Homohopane Indexs and V/Ni ratios, and rich C29 steranes. Algal might be preferentially oxidized and degraded in aerobic conditions. Thus, mudstone is regarded as general-quality source rocks but coal is regarded as high-quality source rocks. Weak water inflow under arid-hot climate provided subtle aquatic and terrigenous organic matter to the lake, resulting in poor-quality source rocks.

Environmental reconstruction and formation mechanisms for lacustrine source rocks of the third member of the Shahejie Formation in the Raoyang Sag, Bohai Bay Basin, eastern China

The third (Es3) member of the Eocene Shahejie Formation is the main hydrocarbon source rock interval in the Raoyang Sag of the Bohai Bay Basin. The lower Es3 (Es3L) and upper Es3 (Es3U) submembers display clear differences in source rock quality. The formation mechanisms of the source rocks are investigated via geochemical methods using 60 samples from three wells to reconstruct the paleo-environment during deposition. The major element parameters exhibit changes in paleoclimate from humid to arid. Indicators such as Sr/Ba, B/Ga, the gammacerane index and isotopic data suggest fresh–brackish and hydrological open lakes with unstable water column stratification likely occurred during deposition of the Es3L submember and saline and hydrologically closed lakes with stable water column stratification likely during deposition of the Es3U submember. Carbon isotope values of organic matter, trace elements and biomarker parameters suggest that the Es3L submember had moderate productivity, with a significant contribution from terrigenous organic matter whereas the Es3U submember had slightly enhanced productivity, with no or minor contributions from terrigenous organic matter. Furthermore, the pristane/phytane ratio and the enrichment of Mo and U indicate that the euxinic bottom-water conditions (sulfidic) of the lakes during deposition of the Es3U submember were best for preserving organic matter. Comparison of the models of source rock deposition of the Es3L and Es3U submembers indicates that the redox conditions play an important role in the formation of organic-rich source rocks in the Raoyang Sag.

The geochemical and organic petrological characteristics of coal measures of the Xujiahe formation in the Sichuan Basin, China

Upper Triassic coaly and lacustrine source rocks complicate efforts to determine the source of hydrocarbons in Sichuan Basin. Total organic carbon analyses, pyrolysis experiments, petrological examinations, and gas chromatography and gas chromatography–mass spectrometry determinations were conducted on coals, carbonaceous mudstones and mudstones collected from two outcrop sections and cores of nine wells. Results revealed that the abundant organic carbon content will prolong the hydrocarbon generation cycle for coals and then the hydrocarbon generating capacity of coals will be enhanced by salinization, thereby contributing bacteria and algae microorganisms into humic coal of the Xujiahe Formation. Compared with mudstone, coal with the same maturity has a stronger adsorption effect on free hydrocarbons. When Ro is greater than 1.35, coal still has a strong hydrocarbon generation ability. The stable water column stratification and euxinic bottom water conditions are evidenced from the lower Pr/Ph and the higher gammacerane indices. The mixing of sea water has been proven by the existence of 4α,23,24,-trimethylcholestanes.

Palaeoenvironmental reconstruction of lacustrine source rocks in the lower 1st Member of the Shahejie Formation in the Raoyang Sag and the Baxian Sag, Bohai Bay Basin, eastern China

The lower 1st Member (Es1L) of the Oligocene Shahejie Formation is the most important source rock interval in the Raoyang Sag and the Baxian Sag. To reveal the controlling factors that affected the distribution of organic-rich source rocks, we analyzed the lower Es1 (Es1L) samples collected from the Raoyang Sag and the Baxian Sag using sedimentological and geochemical techniques to reconstruct the lacustrine palaeoenvironmental conditions. Most of the samples had chemical index of alteration (CIA⁎) values between 70 and 75, which reflect a stable and moderately weathered source under a transitional climate. The distribution of depocenters and palaeo-water depths indicated that the main catchment area was concentrated in the Raoyang Sag, with a maximum palaeo-water depth of 14m and a wave-base depth of 7.5m. Stable isotope data from carbonates (δ18Ocarb and δ13Ccarb) showed that the lakes with long residence times in the Raoyang Sag had a positive correlation and that the lakes with short residence times in the Baxian Sag had no isotopic covariance during the deposition of the lower Es1. The P/Al and P/Ti ratios, Ptotal and carbon isotope values of the organic matter (δ13Corg) in the two sags illustrated stable palaeoproductivity without much fluctuation. The enrichments of Mo, U and ratios of (C:P)org, B/Ga and Sr/Ba further indicated saline and anoxic bottom water conditions associated with stable water-column stratification in Well Xi63 and brackish and less anoxic bottom water conditions in Well ND1 during the deposition of the main source rock of the lower Es1. Moreover, the plots of Mo and U enrichment versus TOC and hydrogen index values versus Δδ13C (i.e., the difference between the δ13C values of carbonates and organic matter) corroborated the inference that the preservation of organic matter was the main factor controlling the formation of organic-rich source rocks in the Raoyang Sag and the Baxian Sag during the deposition of the lower Es1.

Effects of Water Depth, Seasonal Exposure, and Substrate Orientation on Microbial Bioerosion in the Ionian Sea (Eastern Mediterranean)

The effects of water depth, seasonal exposure, and substrate orientation on microbioerosion were studied by means of a settlement experiment deployed in 15, 50, 100, and 250 m water depth south-west of the Peloponnese Peninsula (Greece). At each depth, an experimental platform was exposed for a summer period, a winter period, and about an entire year. On the up- and down-facing side of each platform, substrates were fixed to document the succession of bioerosion traces, and to measure variations in bioerosion and accretion rates. In total, 29 different bioerosion traces were recorded revealing a dominance of microborings produced by phototrophic and organotrophic microendoliths, complemented by few macroborings, attachment scars, and grazing traces. The highest bioerosion activity was recorded in 15 m up-facing substrates in the shallow euphotic zone, largely driven by phototrophic cyanobacteria. Towards the chlorophyte-dominated deep euphotic to dysphotic zones and the organotroph-dominated aphotic zone the intensity of bioerosion and the diversity of bioerosion traces strongly decreased. During summer the activity of phototrophs was higher than during winter, which was likely stimulated by enhanced light availability due to more hours of daylight and increased irradiance angles. Stable water column stratification and a resulting nutrient depletion in shallow water led to lower turbidity levels and caused a shift in the photic zonation that was reflected by more phototrophs being active at greater depth. With respect to the subordinate bioerosion activity of organotrophs, fluctuations in temperature and the trophic regime were assumed to be the main seasonal controls. The observed patterns in overall bioeroder distribution and abundance were mirrored by the calculated carbonate budget with bioerosion rates exceeding carbonate accretion rates in shallow water and distinctly higher bioerosion rates at all depths during summer. These findings highlight the relevance of bioerosion and accretion for the carbonate budget of the Ionian Sea.

Deposition of coal and oil shale in NE China: The Eocene Huadian Basin compared to the coeval Fushun Basin

The Huadian and Fushun basins, located along the DunMi fault zone (NE China), are filled with non-marine coal- and oil shale-bearing sediments of Eocene age. Despite similar tectonic setting, the habitat of coal and oil shale differs significantly. During this study the depositional environment of organic-rich sediments in the Huadian Basin and factors controlling differences between both basins were investigated. Early in the history of the Huadian Basin, thin coaly and bituminous mudstones accumulated in shallow lake environments (Pyrite Member). Water depth increased during deposition of the Oil Shale Member, but did not exceed a few tens of meters preventing stable water column stratification. Moreover, deposition of fan delta sediments interrupted accumulation of fine-grained rocks. Nevertheless, algal blooms and oxygen-depleted conditions resulted in accumulation of 13 thin (<7 m) oil shale layers. Oil shale quality varies between different layers due to variable portions of terrestrial organic matter. Ash-rich coal layers mined in the Huadian Basin developed during the regressive late stage of basin evolution (Carbonaceous Shale Member). Biomarkers indicate a change from an angiosperm-to a gymnosperm-dominated vegetation. In the Fushun Basin, a single 120-m-thick coal seam formed during early basin subsidence. Subsequently, high subsidence rates established deep lacustrine conditions with photic zone anoxia and a water depth probably exceeding 150 m. Stable, strictly anoxic conditions allowed accumulation of a 300-m-thick oil shale layer. The main factor controlling differences between the Huadian and Fushun basins is tectonic subsidence.High subsidence rates in the Fushun Basin favored deposition of thick coal during the transgressive stage and of oil shale in a deep lake. In contrast minor subsidence in the Huadian Basin resulted in a lake with moderate water depth and deposition of several oil shale layers with varying quality. In this basin economic coal developed only during the regressive stage.

Bacteriohopanepolyols record stratification, nitrogen fixation and other biogeochemical perturbations in Holocene sediments of the central Baltic Sea

Abstract. The Baltic Sea, one of the world's largest brackish-marine basins, established after deglaciation of Scandinavia about 17 000 to 15 000 yr ago. In the changeable history of the Baltic Sea, the initial freshwater system was connected to the North Sea about 8000 yr ago and the modern brackish-marine setting (Littorina Sea) was established. Today, a relatively stable stratification has developed in the water column of the deep basins due to salinity differences. Stratification is only occasionally interrupted by mixing events, and it controls nutrient availability and growth of specifically adapted microorganisms and algae. We studied bacteriohopanepolyols (BHPs), lipids of specific bacterial groups, in a sediment core from the central Baltic Sea (Gotland Deep) and found considerable differences between the distinct stages of the Baltic Sea's history. Some individual BHP structures indicate contributions from as yet unknown redoxcline-specific bacteria (bacteriohopanetetrol isomer), methanotrophic bacteria (35-aminobacteriohopanetetrol), cyanobacteria (bacteriohopanetetrol cyclitol ether isomer) and from soil bacteria (adenosylhopane) through allochthonous input after the Littorina transgression, whereas the origin of other BHPs in the core has still to be identified. Notably high BHP abundances were observed in the deposits of the brackish-marine Littorina phase, particularly in laminated sediment layers. Because these sediments record periods of stable water column stratification, bacteria specifically adapted to these conditions may account for the high portions of BHPs. An additional and/or accompanying source may be nitrogen-fixing (cyano)bacteria, which is indicated by a positive correlation of BHP abundances with Corg and δ15N.

Hydrological changes in southern Africa over the last 200 Ka as recorded in lake sediments from the Tswaing impact crater

Sediments from Lake Tswaing (25°24′30″ S, 28°04′59″ E) document hydrological changes in southern Africa over the last 200 Ka. Using high-resolution XRF- scanning, basic geochemistry (TIC, TOC, TN), organic petrology and rock-eval pyrolysis, we identify intervals of decreased carbonate precipitation, increased detrital input, decreased salinity and decreased autochthonous (algal and bacterial) organic matter content that represent periods of less stable water column stratification and increased rainfall. Between 200 and 80 Ka BP, these intervals appear to be contemporaneous with local summer insolation maxima, indicating a strong influence of precessional variability (~23 Ka) on African subtropical climate. This influence weakens during the last glacial period (~80 to 10 Ka BP), when humid intervals at 73 to 68 Ka, 54 to 50 Ka, 37 to 35 Ka and 15 to 10 Ka BP are largely out of phase with insolation changes, and presumably reflect southward displacement of the ITCZ (Inter Tropical Convergence Zone) and/or changes in ocean circulation.

Intraspecific variation in the response of the cyanobacterium Nodularia spumigena to moderate UV-B radiation

Floating and nodularin-producing strains of Nodularia spumigena from the Baltic Sea are regarded as belonging to one species. However, intraspecific variation in the response of N. spumigena to environmental factors has been commonly overlooked. As blooms of N. spumigena occur in late summer, a period with strong light and stable water-column stratification, the cells can be expected to also be exposed to ultraviolet-B radiation (UV-B, 280–320 nm). The UV-B tolerance of four different strains of N. spumigena, isolated from the Baltic Sea, was investigated in the laboratory for 8 days, by measuring photosynthesis, growth and pigment composition. Variables included maximum quantum yield of photosynthesis ( F v/ F m, PAM fluorometry), growth rate (cell counts) and photosynthetic pigments, as well as mycosporine-like amino acids (HPLC). Intraspecific differences regardless of treatment were found for cell dimension, growth rate, F v/ F m and pigment concentrations. UV-B related effects differed between strains. By Day 8 one of the four strains showed a lower F v/ F m when treated with UV-B; in another strain the growth rate and cell numbers were lower. In three strains, UV-B exposure resulted in higher cell concentrations of carotenoids and chlorophyll a. In all strains, the concentrations of total mycosporine-like amino acids were 60–130% higher in the UV-B treated samples compared with samples shielded from UV-B. Although strain-specific differences in UV-B tolerance were observed, it is concluded that N. spumigena is a species that is not generally negatively affected by moderate levels of UV-B radiation.

Organic carbon burial rate and the molybdenum proxy: Theoretical framework and application to Cenomanian‐Turonian oceanic anoxic event 2

In this study the controls on organic carbon burial and molybdenum accumulation in ancient strata are investigated through an integration of biogeochemical modeling and data analysis. Critical aspects of the study are employment of a biogeochemical model for organic matter degradation to explore the controls on pore water sulfide generation and authigenic molybdenum accumulation and use of accumulation rate data for Mo, OC, CaCO3, Fe, and Ti to reconstruct organic matter remineralization processes. The model results form a conceptual framework for the interpretation of primary production estimates and geochemical burial fluxes (calculated on the basis of a high‐resolution orbital timescale) for the Cenomanian‐Turonian (C‐T) oceanic anoxic event 2 interval in the Western Interior basin. The results of this study suggest that the strong correlation between source rock development and intervals of transgression in the geologic record (such as the C‐T) could reflect a confluence of biogeochemical processes within sediments that obviates the need for large‐magnitude changes in primary production levels or oceanographic conditions (such as prolonged periods of stable water column stratification).

Organic and isotope geochemistry of the Early Cretaceous rift sequence in the Camamu Basin, Brazil: paleolimnological inferences and source rock models

The Camamu Basin, located at the eastern Brazilian continental margin, is characterized by a thick Early Cretaceous organic-rich rift sequence. The integration of bulk geochemical, isotopic and biomarker data with paleontological and geological information provided the basis for a better assessment of rift lake evolution (hydrological regime, redox potential, salinity, etc.) and its impact on source rock development. Pyrolysis and visual kerogen analyses reveal the dominance of lipid-rich algal (bacterial?) type I kerogen through the entire rift sequence. Paleolimnological reconstruction suggests that the lower rift sequence (Morro do Barro Formation, Neocomian) was deposited in a fault-bounded deep lake with fresh to brackish water, stable water column stratification and water bottom anoxia. High hydrogen indices (700–900 mg HC/gTOC) and 13C-depleted carbon isotopic composition of the organic matter (δ 13C values around −30‰) might reflect enhanced organic preservation, the recycling of light CH 4 generated by methanogenesis, and the incorporation of isotopically light lipid-rich bacterial biomass. On the other hand, the upper rift sequence (Rio de Contas Formation, Barremian) was probably deposited within a broader/shallower lake with a higher frequency of water-column overturn and a deeper thermocline. High organic carbon contents and strong 13C enrichment of the organic matter (δ 13C excursions of up to −23‰) are interpreted to have resulted from enhanced primary productivity triggered by an increased input/recycling of nutrients favored by lake morphology and humid climate.