Topic
Delta
About: Delta is a research topic. Over the lifetime, 6931 publications have been published within this topic receiving 151725 citations.
Papers published on a yearly basis
1 / 3
Papers
TL;DR: The effects of the chemistry of ore-forming fluids on the sulfur and carbon isotopic compositions of hydrothermal minerals are quantitatively evaluated from available thermochemical data and isotopic fractionation factors as discussed by the authors.
Abstract: The effects of the chemistry of ore-forming fluids on the sulfur and carbon isotopic compositions of hydrothermal minerals are quantitatively evaluated from available thermochemical data and isotopic fractionation factors.The isotopic composition of both sulfur and carbon in hydrothermal minerals is strongly controlled by the f (sub O 2 ) and pH values of hydrothermal fluids as well as by the temperature and the isotopic composition of sulfur and carbon in the fluids (delta S 34 (sub Sigma S) and delta C 13 (sub Sigma C) values). For example, at 250 degrees C and within geologically important f (sub O 2 ) -pH regions, an increase in f (sub O 2 ) value by 1 log unit or in pH by 1 unit can cause a decrease in delta S 34 values of sulfur-bearing minerals by as much as 20 per mil. An increase in f (sub O 2 ) by 1 log unit or in pH by 2 units can cause a decrease of about 30 per mil in delta C 13 values of carbon-bearing minerals. Large variation in the delta S 34 values or in the delta C 13 values of hydrothermal minerals, which often have been interpreted as an indication of biogenic sulfur or carbon, could also be caused by slight variation in the f (sub O 2 ) and/or pH of ore-forming fluids during ore deposition.The concentrations in an ore solution of sulfur (or f (sub S 2 ) ) and of carbon (or f (sub CO 2 ) ) place limits on possible delta S 34 and delta C 13 values for hydrothermal minerals. Sulfur-bearing minerals and carbon-bearing minerals precipitating from sulfur- and carbon-rich solutions can have wider ranges of delta S 34 and delta C 13 values than those minerals precipitating from sulfur- and carbon-poor solutions.Sulfide minerals which precipitated in equilibrium with magnetite, hematite, or sulfate minerals, and carbonate minerals which precipitated in equilibrium with graphite, could exhibit isotopic compositions markedly different from those of the depositing fluids. Therefore, sulfides with delta S 34 values near zero per mil or carbonates with delta C 13 values near -6 per mil do not necessarily indicate a magmatic origin for the sulfur or the carbon.The mode of variation on the delta S 34 values of sulfide minerals and in the delta C 13 values of carbonate minerals in a given deposit may indicate the relative oxidation states of ore-forming fluids: variable delta S 34 + uniform delta C 13 values may suggest that the minerals were precipitated under relatively high f (sub O 2 ) conditions; uniform delta S 34 + uniform delta C 13 values, under intermediate f (sub O 2 ) conditions; and uniform delta S 34 + variable delta C 31 values suggesting deposition under relatively low f (sub O 2 ) conditions.Sulfur and carbon isotopic data, combined with geological and mineralogical data of ore deposits, may define the physico-chemical parameters (T, f (sub O 2 ) , f (sub S 2 ) , f (sub CO 2 ) , m (sub Sigma S) , m (sub Sigma C) ) and the origin (delta S 34 (sub Sigma S) and delta C 13 (sub Sigma C) values) of ore-forming fluids as well as the mechanisms of ore deposition.
1,418 citations
TL;DR: In this paper, a new threefold lithostratigraphic subdivision for the Niger delta subsurface is introduced, comprising an upper sandy Benin Formation, an intervening unit of alternating sandstone and shale named the Agbada Formation, and a lower shaly Akata Formation.
Abstract: The coastal sedimentary basin of Nigeria has been the scene of three depositional cycles. The first began with a marine incursion in the middle Cretaceous and was terminated by a mild folding phase in Santonian time. The second included the growth of a proto-Niger delta during the Late Cretaceous and ended in a major Paleocene marine transgression. The third cycle, from Eocene to Recent, marked the continuous growth of the main Niger delta. A new threefold lithostratigraphic subdivision is introduced for the Niger delta subsurface, comprising an upper sandy Benin Formation, an intervening unit of alternating sandstone and shale named the Agbada Formation, and a lower shaly Akata Formation. These three units extend across the whole delta and each ranges in age from early T rtiary to Recent. They are related to the present outcrops and environments of deposition. A separate member of the Benin Formation is recognized in the Port Harcourt area. This is the Afam Clay Member, which is interpreted to be an ancient valley fill formed in Miocene sediments. Subsurface structures are described as resulting from movement under the influence of gravity and their distribution is related to growth stages of the delta. Rollover anticlines in front of growth faults form the main objectives of oil exploration, the hydrocarbons being found in sandstone reservoirs of the Agbada Formation.
1,380 citations
TL;DR: The results suggest that the delta(15)N signatures of nitrogen at the base of the food chain will provide a useful tool in the assessment of anthropogenic nutrient inputs.
Abstract: Recent studies have shown the utility of delta(15)N to model trophic structure and contaminant bioaccumulation in aquatic food webs. However, cross-system comparisons in delta(15)N can be complicated by differences in delta(15)N at the base of the food chain. Such baseline variation in delta(15)N is difficult to resolve using plankton because of the large temporal variability in the delta(15)N of small organisms that have fast nitrogen turnover. Comparisons using large primary consumers, which have stable tissue isotopic signatures because of their slower nitrogen turnover, show that delta(15)N increases markedly with the human population density in the lake watershed. This shift in delta(15)N likely reflects the high delta(15)N of human sewage. Correcting for this baseline variation in delta(15)N, we report that, contrary to expectations based on previous food-web analysis, the food chains leading up to fish varied by about only one trophic level among the 40 lakes studied. Our results also suggest that the delta(15)N signatures of nitrogen at the base of the food chain will provide a useful tool in the assessment of anthropogenic nutrient inputs.
1,124 citations
TL;DR: This article showed that 10,000-13,500 km2 of the Mississippi Delta could be submerged by AD 2100 due to global sea-level rise, reduced sediment supply and subsidence.
Abstract: Global sea-level rise, reduced sediment supply and subsidence threaten the stability of the Mississippi Delta. Calculations of riverine sediment load and storage indicate that 10,000–13,500 km2 of the delta could be submerged by AD 2100.
1,028 citations
TL;DR: The Tertiary Niger delta is composed of an overall regressive clastic sequence which reaches a maximum thickness of 30,000 to 40,000 ft (9, 000 to 12,000 m).
Abstract: The Tertiary Niger delta covers an area of about 75,000 sq km and is composed of an overall regressive clastic sequence which reaches a maximum thickness of 30,000 to 40,000 ft (9,000 to 12,000 m). The development of the delta has been dependent on the balance between the rate of sedimentation and the rate of subsidence. This balance and the resulting sedimentary patterns appear to have been influenced by the structural configuration and tectonics of the basement. Structural analysis of the Tertiary overburden shows that individual fault blocks can be grouped into macrostructural and eventually megastructural units. Such megaunits are separate provinces with regard to time-stratigraphy, sedimentation, deformation, generation and migration of hydrocarbons, and hydrocarbon distribution. A recurrent pattern emerges in the distribution both of absolute volumes of hydrocarbons and the ratio of volume of gas-bearing reservoir rocks to the volume of oil-bearing reservoir rocks within megaunits and macrounits. The maturity of potential source rock in a given fault trend was achieved when sedimentation had almost reached the present surface, and when the active depocenter had been advanced seaward by several trends. Thus, migration started when deposition, together with the intrinsically synsedimentary structural deformation, had almost come to a halt in that particular trend. The source rocks of the Niger delta yield a light waxy paraffinic oil, which is transformed bacterially to a heavier nonwaxy crude at temperatures below 150 to 180°F (65 to 80°C). The coincidence of the boundary between transformed and unaltered oils, within a rather narrow temperature range on a delta-wide basis, implies that little or no subsidence with concomitant increase in geotemperature of the oil-bearing reservoirs has occurred after migration. The conclusion that migration took place after the structural geometry of a given trend had been determined originates from several independent lines of evidence. The observed uneven distribution of oil and gas in the delta therefore cannot be explained in terms of the passage of the source rocks through the oil-generating zone into the gas-generating zone (oil and gas "kitchens," respectively), with early structures receiving mainly oil and late traps receiving mainly gas. Rather, the hydrocarbon distribution probably is the result of original heterogeneity of the source rock and of segregation during migration and remigration.
788 citations
Related Topics (5)
Performance Metrics
| Year | Papers |
|---|---|
| 2026 | 10 |
| 2025 | 1,283 |
| 2024 | 2,117 |
| 2023 | 1,742 |
| 2022 | 2,491 |
| 2021 | 543 |