Accumulation Of Organic-rich Sediments Research Articles

Assessing the geochemical correlation between petroleum source rocks in the Brazilian Equatorial margin basins and global oceanic anoxic events

This research aims to investigate the correlation between the Cretaceous Oceanic Anoxic Events (OAEs) and the source rocks of the Brazilian Equatorial Margin (BEM) basins. These source rocks, particularly from the Cenomanian-Turonian interval, are known to contain high levels of Total Organic Carbon (TOC) and exhibit significant hydrocarbon potential. The principal focus is on how global anoxic events, such as OAE-1b, OAE-1d, and OAE-2, influenced the depositional environments and organic matter types across the BEM basins, which include Foz do Amazonas, Pará-Maranhão, Barreirinhas, Ceará, and Potiguar. Key results indicate a strong correlation between periods of anoxia and the accumulation of organic-rich sediments in these basins. The Foz do Amazonas Basin features predominantly terrestrial-derived kerogen (Type III), while the Ceará and Potiguar basins show marine-derived kerogen (Type I and II), aligning with global productivity patterns during anoxic events. These variations in kerogen type reflect differences in depositional environments influenced by sea-level changes and nutrient influx. The research concludes that the OAEs significantly impacted organic matter preservation and hydrocarbon generation in the BEM basins. The study's findings contribute to the broader understanding of how Cretaceous OAEs shaped the geochemical and sedimentary processes in equatorial Atlantic margins, highlighting the potential for further oil and gas exploration in these regions.

Do beaver ponds increase methane emissions along Arctic tundra streams?

Beaver engineering in the Arctic tundra induces hydrologic and geomorphic changes that are favorable to methane (CH4) production. Beaver-mediated methane emissions are driven by inundation of existing vegetation, conversion from lotic to lentic systems, accumulation of organic rich sediments, elevated water tables, anaerobic conditions, and thawing permafrost. Ground-based measurements of CH4 emissions from beaver ponds in permafrost landscapes are scarce, but hyperspectral remote sensing data (AVIRIS-NG) permit mapping of ‘hotspots’ thought to represent locations of high CH4 emission. We surveyed a 429.5 km2 area in Northwestern Alaska using hyperspectral airborne imaging spectroscopy at ∼5 m pixel resolution (14.7 million observations) to examine spatial relationships between CH4 hotspots and 118 beaver ponds. AVIRIS-NG CH4 hotspots covered 0.539% (2.3 km2) of the study area, and were concentrated within 30 m of waterbodies. Comparing beaver ponds to all non-beaver waterbodies (including waterbodies >450 m from beaver-affected water), we found significantly greater CH4 hotspot occurrences around beaver ponds, extending to a distance of 60 m. We found a 51% greater CH4 hotspot occurrence ratio around beaver ponds relative to nearby non-beaver waterbodies. Dammed lake outlets showed no significant differences in CH4 hotspot ratios compared to non-beaver lakes, likely due to little change in inundation extent. The enhancement in AVIRIS-NG CH4 hotspots adjacent to beaver ponds is an example of a new disturbance regime, wrought by an ecosystem engineer, accelerating the effects of climate change in the Arctic. As beavers continue to expand into the Arctic and reshape lowland ecosystems, we expect continued wetland creation, permafrost thaw and alteration of the Arctic carbon cycle, as well as myriad physical and biological changes.

Accumulation of Organic-Rich Sediments Associated with Aptian–Albian Oceanic Anoxic Events in Central-Eastern Mexico

Accumulation of Organic-Rich Sediments Associated with Aptian–Albian Oceanic Anoxic Events in Central-Eastern Mexico

Multiple Controls on the Accumulation of Organic-Rich Sediments in the Besa River Formation of Liard Basin, British Columbia, Canada

The Late Devonian Besa River Formation is an organic-rich shale sequence in Liard Basin, northeastern British Columbia, Canada, with significant natural gas reserves. High-resolution elemental geochemistry of three long continuous cored intervals of the Besa River Formation was used to better understand the paleodepositional environment of organic-rich intervals in this thick marine shale. The studied core intervals were divided into five chemostratigraphic units based on organic and inorganic geochemical proxies. The highest total organic carbon (TOC) content (up to 13 wt.%) was identified in the upper part of the Patry member (Unit III) within the Liard Basin. During the deposition of Unit III, low clastic influx and euxinic bottom conditions mostly contributed to the high accumulation of organic carbon. Moreover, a high productivity and organic influx may have increased organic-rich basinal sediments, which further depleted the seawater column oxygen content in the presence of a large amount of organic matter. This took place within the oxygen-deficient bottom water from the Patry–Exshaw stratigraphic units. This high TOC interval was most likely deposited through abundant biogenic silica production by radiolarians, thereby utilizing the supply of nutrients from the upwelling. Sea level change was also an important factor that controlled organic matter accumulation in the Besa River Formation. The transgression in sea level changed the residence time of the organic matter in oxic zones within the water column, which limited its supply in deeper water; this decreased the TOC content in Unit IV. Before the deposition, silica production collapsed and was replaced by terrestrial sedimentation of clay minerals in the upper part of the Exshaw member, which caused organic matter dilutions in Unit V (under 5 wt.%). These results provide new insights into the effects of relative sea level changes on redox conditions, productivity, and detrital flux, which are related to organic matter enrichment patterns and their geographic variations. Unit III is characterized by an organic-rich interval as well as an abundance of biogenic silica that is closely related to fracturing. Thus, Unit III is expected to have the highest shale gas potential in the Devonian Besa River Formation. The high-resolution geochemical data integrated with well log and/or seismic data can be used to determine the distribution of the perspective interval for shale gas production in the Liard Basin.

Impact of natural re-oxygenation on the sediment dynamics of manganese, iron and phosphorus in a euxinic Baltic Sea basin

The Baltic Sea is characterized by the largest area of hypoxic (oxygen (O2) < 2 mg L−1) bottom waters in the world’s ocean induced by human activities. Natural ventilation of these O2-depleted waters largely depends on episodic Major Baltic Inflows from the adjacent North Sea. In 2014 and 2015, two such inflows led to a strong rise in O2 and decline in phosphate (HPO42−) in waters below 125 m depth in the Eastern Gotland Basin. This provided the opportunity to assess the impact of such re-oxygenation events on the cycles of manganese (Mn), iron (Fe) and phosphorus (P) in the sediment for the first time. We demonstrate that the re-oxygenation induced the activity of sulphur (S)-oxidising bacteria, known as Beggiatoaceae in the surface sediment where a thin oxic and suboxic layer developed. At the two deepest sites, strong enrichments of total Mn and to a lesser extent Fe oxides and P were observed in this surface layer. A combination of sequential sediment extractions and synchrotron-based X-ray spectroscopy revealed evidence for the abundant presence of P-bearing rhodochrosite and Mn(II) phosphates. In contrast to what is typically assumed, the formation of Fe oxides in the surface sediment was limited. We attribute this lack of Fe oxide formation to the high flux of reductants, such as sulphide, from deeper sediments which allows Fe(II) in the form of FeS to be preserved and restricts the penetration of O2 into the sediment. We estimate that enhanced P sequestration in surface sediments accounts for only ∼5% of water column HPO42− removal in the Eastern Gotland Basin linked to the recent inflows. The remaining HPO42− was transported to adjacent areas in the Baltic Sea. Our results highlight that the benthic O2 demand arising from the accumulation of organic-rich sediments over several decades, the legacy of hypoxia, has major implications for the biogeochemical response of euxinic basins to re-oxygenation. In particular, P sequestration in the sediment in association with Fe oxides is limited. This implies that artificial ventilation projects that aim at removing water column HPO42− and thereby improving water quality in the Baltic Sea will likely not have the desired effect.

Geochemical paleoredox indicators in organic-rich shales of the Irati Formation, Permian of the Paraná Basin, southern Brazil

ABSTRACT: Paleoredox indicators were used to assess the depositional environment of the Permian Irati Formation, and discuss the controls on the accumulation of organic-rich sediments. Geochemical data were measured for 43 drill-core samples in southern Paraná Basin. We infer that redox boundary was located at the sediment-water interface during the accumulation of these sediments. Trace-element data demonstrate that Irati shales were similar to average shales, except during deposition of two organic-rich beds in which U and Mo are enriched, suggesting fully anoxic conditions. The variable degree of oxygenation of bottom waters is also attested by DOPT, FeT/Al, and sedimentological indicators of episodic ventilation. DOPT and Fe and S correlation points to an iron-limited environment, where the main sulfur sink is organic matter rather than pyrite. Cu and Cu/Mo ratio suggests bottom-water anoxia driven by increased organic flux. The Irati Formation was probably deposited in a salinity-stratified body of water, with positive water balance. The main control on the accumulation of the organic-rich sediments was the high organic carbon flux resultant from high primary productivity in surface waters. Anoxia arose as a consequence of increased productivity, not as the primary driver in the formation of these organic-rich rocks.

The influence of regional factors in the expression of oceanic anoxic event 1a (OAE1a) in the semi-restricted Organyà Basin, south-central Pyrenees, Spain

The global occurrence of oxygen-deprived conditions in the ocean characterizes the early Aptian; hence the hallmark of that time interval is the accumulation of organic-rich sediments during an episode of severe oxygen deficiency known as oceanic anoxic event 1a (OAE1a). As seen elsewhere in the Tethyan domain, an overall change in facies marked by increased input of terrigenous material distinguishes the lower Aptian in the study area, but the nature of the lithologies associated with the organic carbon-rich deposits remain dominated by carbonate sediments and are expressed differently with respect to other areas where the occurrence of black shales characterizes OAE1a. This comparison indicates that significant spatial heterogeneity occurred in sub-Tethyan basins coeval with the global oceanic event.The present work provides a high-resolution, multi-proxy study (petrography, total inorganic carbon (TIC), total organic carbon TOC, carbon isotope [δ13Corg], clay mineralogy and biomarkers) of the upper 155m of the El Pui section, Organyà Basin, including the record of OAE1a. The results reveal that oxygenic conditions fluctuated throughout the studied interval, with episodes of oxygen depletion that concurred with TOC values >1%. However, petrographic observations, molecular biomarkers, and redox sensitive trace elements (RSTEs) indicate that full anoxia was not achieved.The results also provide the first high-resolution δ13Corg profile for the El Pui section that replicates in greater detail the carbon isotopic signature reported for the Tethyan domain. The upper 155m of the El Pui section include expanded and well-defined trends of the carbon isotope segments C2 (upper part), C3, C4, C5, and C6 (partial), which correlate with other Tethyan sections and allow for the recognition of the lower Aptian OAE1a in the uppermost 40m of the section. Such improved high-resolution chemostratigraphic curve could contribute to a more precise chronostratigraphic correlation of Aptian events in a broader scale.

オホーツク海および日本海に胚胎する表層型ガスハイドレート鉱床における間隙水のハロゲンと放射性ヨウ素同位体（&lt;sup&gt;129&lt;/sup&gt;I）の地球化学

Sulfate, halogen, and radioactive 129I concentrations were determined in pore waters associated with massive gas hydrate deposits in shallow sediments along the boundary between the Amurian and Okhotsk plates in the Okhotsk Sea and Japan Sea. Because of the strong biophilic behavior of iodine and weaker behavior of bromine, in contrast to conservative chlorine, in the marine system and the presence of a long-lived radioisotope of iodine (129I), these analyses are useful for determining the age and nature of source organic materials responsible for hydrocarbons, mostly methane, in gas hydrates. Rapid sulfate decreases with depth reflect active methane migration toward the seafloor, particularly around gas hydrate-bearing sites at both locations. While salt exclusion from gas hydrates during crystallization is likely to have resulted in the downward increase of Cl, and potentially Br and I concentrations, gas hydrate dissociation at depth caused a gentle dilution of pore waters. Biophilic Br and I concentrations rapidly increase with depth, reaching 1500 and 400 μM at the core bottom, respectively, indicating that upwelling fluids are enriched in Br and I derived from marine organic materials degraded in deep sediments. The 129I/I ratios thus reflect the potential ages of source formations of iodine and associated methane, providing ∼35 Ma northeast off Sakhalin Island in the Okhotsk Sea and ∼30 Ma in the Umitaka Spur-Joetsu Knoll region in the eastern margin of the Japan Sea. These ages correspond well with the initial activities of the Amurian and Okhotsk plates and the subsequent opening of the Japan Sea, which formed the present geological setting of the northeastern margin of the Eurasian continent. Active plate motions led to the rapid accumulation of organic-rich sediments that are responsible for iodine and methane in gas hydrate occurring along the plate boundary.

Sedimentology, geochemistry, and biota of Volgian carbonaceous sequences in the northern part of the central Russian Sea (Kostroma region)

Lithological, geochemical, stratigraphic, and paleoecological features of carbonaceous sediments in the Late Jurassic Volgian Basin of the East European Platform (Kostroma region) are considered. The shalebearing sequence studied is characterized by greater sedimentological completeness as compared with its stratotype sections in the Middle Volga region (Gorodishche, Kashpir). Stratigraphic position and stratigraphy of the shale-bearing sequence, as well as the distribution of biota in different sedimentation settings, are specified. It is shown that Volgian sediments show a distinct cyclic structure. The lower and upper elements of cyclites consist of high-carbonaceous shales and clayey-calcareous sediments, respectively, separated by transitional varieties. Bioturbation structures in different rocks are discussed. Microcomponent composition and pyrolytic parameters of organic matter, as well as distribution of chemical elements in the lithologically variable sediments are analyzed. Possible reasons responsible for the appearance of cyclicity and accumulation of organicrich sediments are discussed.

Accumulation of organic rich sediments in a dendritic fluvial/lacustrine mire system at Tasik Bera, Malaysia: implications for coal formation

The Tasik Bera mire system represents deposition of peat and peaty sediments within a dendritic fluvial drainage basin in the humid tropics. As such, it serves as an analogue for the earliest stages of coal deposition in low-relief topogenous coal swamps. Peaty sediments have been accumulating for at least 4500 years in the lowest part of the basin, but accumulation rates and hence the thickness and lateral extent of the mire underwent a rapid increase and expansion beginning at about 660 years B.P. The sediments are highly variable, both vertically and laterally, the variations principally controlled by the type of vegetation dominant. Vegetation in turn is related to the degree of wetness of the site. Three distinct environments of deposition contribute peat with physical characteristics which can be related to coals. The limnetic environment, dominated by algae and easily degraded aquatic macrophytes, contributes very fine hemic peaty sediment with high fine silt content and a large algal component. The littoral environment is dominated by sedges and the woody shrub Pandanus, both of which have a large subaqueous biomass, and are quite resistant to degradation. Sediment from this environment is woody, hemic to coarse hemic, with a moderate to high very fine silt content and a much smaller algal element. Forest swamps, which occupy most of the mire area, contribute woody, fibric to hemic peaty sediments with low to moderate mineral matter content in the form of clays and very fine silt. Succession from both limnetic to forest swamp, and the reverse, is recorded in cores from different sites. In coals these transitions would be manifest as both brightening-upward (algaedominated peat to woody peat) and dulling-upward (the reverse sequence) trends, the first in the topographically lower part of the basin, the second in the middle reaches. No part of the mire yet studied is approaching oligotrophic conditions. Allowing for a compression ratio of 5:1, thin, stony coals from the Tasik Bera mire would be laterally discontinuous and would vary from about 1 m in thickness at channel margins, tapering out to nothing as they onlap the interfluves.

Organic carbon flux and development of oxygen deficiency on the modern Benguela continential shelf south of 22°S: spatial and temporal variability

Abstract Anoxia in continental shelf waters has previously been ascribed to lack of circulation, advection from a remote source and in situ formation. It appears that in situ formation resulting from degradation of organic-rich material derived from phytoplankton blooms is generally the controlling factor in coastal upwelling ecosystems such as the Benguela system. Here, spatial variation in accumulation of organic-rich shelf sediments and associated anoxia is closely related to the degree to which primary production is enhanced at sites ‘downstream’ of coastal upwelling centres. Consequently, at sites where there is strong seasonal variation in the input of recently upwelled water, there is temporal variability in the incidence of oxygen-deficient waters overlying the shelf and anoxic sediments. The extent of phytoplankton blooms is also dependent on coupling between the benthic and pelagic zones and physical characteristics of the water column. This is supported by observations of phytoplankton abundance, sedimentation of particulate organic matter and benthic oxygen deficiency in the Benguela upwelling system, where the continental shelf is marked by oxygen-deficient bottom waters and anoxic sediments equatorward of three of the major upwelling centres where such coupling occurs. This work reaffirms the view that primary production rather than anoxia exerts direct control on the accumulation of organic-rich sediments.

Glacio-eustatic sea-level control on Red Sea salinity

Previous studies of the Red Sea demonstrated that glacial surface-and bottom-water salinities in the basin were significantly higher than at present. The very low abundance of planktonic foraminifera, the so-called 'aplanktonic zone', during the last glacial indicates that surface-water conditions approached or exceeded the tolerance limits of this plankton group1,2. Glacial sediments are also characterized by high concentrations of magnesian calcite3,4, dolomite3, inorganically precipitated aragonite4,5 and benthic foraminifera typical of hypersaline environments3,6. Additional evidence from oxygen isotope records of planktonic2,7 and benthic foraminifera8,9, as well as pteropods2, demonstrate that glacial–interglacial contrasts in the Red Sea have an amplitude much larger than typically observed in open ocean records. Here we use both oxygen isotope data and a 'fractional overmixing' model to estimate the impact of the most recent (18,000 yr BP) Pleistocene glacio-eustatic sea-level lowering on Red Sea salinity. We estimate that during the last glacial maximum, surface salinities in the central Red Sea were more than 10.0‰ higher than at present. Deep-water salinities were also higher during the last glaciation and remained unusually high through deglaciation. The combination of very high bottom salinities and the onset of pluvial conditions during deglaciation in the Red Sea region prevented ventilation of Red Sea bottom waters and resulted in the accumulation of organic-rich sediments.