George E. Claypool

The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation

Abstract Three hundred new samples of marine evaporite sulfate, of world-wide distribution, were analyzed for δ34S, and 60 of these also for δ18O in the sulfate ion. Detailed δ34S age curves for Tertiary—Cretaceous, Permian—Pennsylvanian, Devonian, Cambrian and Proterozoic times document large variations in δ34S. A summary curve for δ18O also shows definite variations, some at different times than δ34S, and always smaller. The measured δ34S and δ18O correspond to variations in these isotopes in sulfate of the world ocean surface. The variations of δ18O are controlled by input and output fluxes of sulfur in the ocean, three of which are the same that control δ34S: deposition and erosion of sulfate, and deposition of sulfide. Erosion of sulfide differs in its effect on the S and O systems. δ18O in the sulfate does not seem to be measurably affected by equilibration with either seawater or with subsurface waters after crystallization. In principle, the simultaneous application of both δ34S and δ18O age curves should help reduce the number of assumptions in calculations of the cycles of sulfur and oxygen through geological time, and a new model involving symmetrical fluxes is introduced here to take advantage of the oxygen data. However, all previously published models as well as this one lead to anomalies, such as unreasonable calcium or oxygen depletions in the ocean—atmosphere system. In addition, most models are incapable of reproducing the sharp rises of the δ34S curve in the late Proterozoic, the Devonian and the Triassic which would be the result of unreasonably fast net sulfide deposition. This fast depletion could result from an ocean that has not always been mixed (as previously assumed in all model calculations).

Generation, Accumulation, and Resource Potential of Biogenic Gas

Biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms. More than 20% of the worlds discovered gas reserves are of biogenic origin. A higher percentage of gases of predominantly biogenic origin will be discovered in the future. Biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread, large quantities at shallow depths. In rapidly accumulating marine sediments, a succession of microbial ecosystems leads to the generation of biogenic gas. After oxygen is consumed by aerobic respiration, sulfate reduction becomes the dominant form of respiration. Methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted. The most important mechanism of methane generation in marine sediments is the reduction of CO2 by hydrogen (electrons) produced by the anaerobic oxidation of organic matter. CO2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures. The factors that control the level of methane production after sediment burial are anoxic environment, sulfate-deficient environment, low temperatu e, availability of organic matter, and sufficient space. The timing of these factors is such that most biogenic gas is generated prior to burial depths of 1,000 m. In marine sediments, most of the biogenic gas formed can be retained in solution in the interstitial (pore) waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column. Under certain conditions of high pressures and (or) low temperatures, biogenic methane combines with water to form gas hydrates. Biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses. The hydrocarbon fraction of biogenic gas consists predominantly of methane. The presence of as much as 2% of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low-temperature degradation of organic matter. The amounts of hydrocarbon components other than methane generally are proportional to temperature, age, and organic-matter content of the sediments. Biogenic methane is enriched in the light isotope 12C (^dgr13C1 lighter than -55 ppt) owing to kinetic isotope fractionation by methanogens. The variations in isotopic composition of biogenic methane are controlled primarily by ^dgr13C of the original CO2 substrate, which reflects the net isotopic effect of both addition and removal of CO2. The methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas. The possible complicating factors require that geologic, chemical, and isotopic evidence be considered in attempts to interpret the origin of gas accumulations. Accumulations of biogenic gas have been discovered in Canada, Germany, Italy, Japan, Trinidad, the United States, and USSR in Cretaceous and younger rocks, at less than 3,350 m of burial, and in marine and nonmarine rocks. Other gas accumulations of biogenic origin have undoubtedly been discovered; however, data that permit their recognition are not available.

Depletion of 13C in Cretaceous marine organic matter: Source, diagenetic, or environmental sigal?

Geochemical studies of Cretaceous strata rich in organic carbon (OC) from Deep Sea Drilling Project (DSDP) sites and several land sections reveal several consistent relationships among amount of OC, hydrocarbon generating potential of kerogen (measured by pyrolysis as the hydrogen index, HI), and the isotopic composition of the OC. First, there is a positive correlation between HI and OC in strata that contain more than about 1% OC. Second, percent OC and HI often are negatively correlated with carbon isotopic composition (σ 13C) of kerogen. The relationship between HI and OC indicates that as the amount of organic matter increases, this organic matter tends to be more lipid rich reflecting the marine source of the organic matter. Cretaceous samples that contain predominantly marine organic matter tend to be isotopically lighter than those that contain predominantly terrestrial organic matter. Average σ 13C values for organic matter from most Cretaceous sites are between −26 and −28‰, and values heavier than about −25‰ occur at very few sites. Most of the σ 13C values of Miocene to Holocene OC-rich strata and modern marine plankton are between −16 to −23‰. Values of σ13C of modern terrestrial organic matter are mostly between −23 and −33‰. The depletion of terrestial OC in 13C relative to marine planktonic OC is the basis for numerous statements in the literature that isotopically light Cretaceous organic matter is of terrestrial origin, even though other organic geochemical and(or) optical indicators show that the organic matter is mainly of marine origin. A difference of about 5‰ in σ 13C between modern and Cretaceous OC-rich marine strata suggests either that Cretaceous marine planktonic organic matter had the same isotopic signature as modern marine plankton and that signature has been changed by diagenesis, or that OC derived from Cretaceous marine plankton was isotopically lighter by about 5‰ relative to modern plankton OC. Diagenesis does not produce a significant shift in σ 13C in Miocene to Holocene sediments, and therefore probably did not produce the isotopically light Cretaceous OC. This means that Cretaceous marine plankton must have had σ 13C values that were about 5‰ lighter than modern marine plankton, and at least several per mil lighter than Cretaceous terrestrial vegetation. The reason for these lighter values, however, is not obvious. It has been proposed that concentrations of CO2 were higher during the middle Cretaceous, and this more available CO2 may be responsible for the lighter σ 13C values of Cretaceous marine organic matter.

In situ methane concentrations at Hydrate Ridge, offshore Oregon: New constraints on the global gas hydrate inventory from an active margin

The widespread presence of bottom-simulating reflectors (BSRs) on continental margins has bolstered suggestions that gas hydrates and free gas constitute a large dynamic reservoir of CH4 carbon and a vast potential source of energy. However, only a few hydrate-bearing areas have been drilled, and of these, the amount of CH4 has only been directly quantified in 18 discrete samples from 3 holes on Blake Ridge, east of Georgia. Here we report and discuss 30 direct measurements of CH4 concentration in sediments above and below the BSR at Hydrate Ridge on a tectonically active margin offshore Oregon. High CH4 concentrations (71–3127 m M ) support abundant gas hydrate (occupying an average of ∼11% of porosity) and free gas (occupying ∼4% of porosity in 1 sample) in a restricted area where hydrocarbon gases migrate from the deep accretionary complex to the seafloor. In a larger area lacking this hydrocarbon supply, lower CH4 concentrations (10–893 m M ) indicate less gas hydrate (average ∼1% of porosity) and little or no free gas. Overall, the amount of CH4 at Hydrate Ridge is significantly less than that at Blake Ridge. These results challenge certain interpretations, including the global volume of hydrate-bound CH4, which though large, may be four to seven times less than widely cited estimates. Speculations on the distribution and role of gas hydrate and free gas need revision.

Geochemical Relationships of Petroleum in Mesozoic Reservoirs to Carbonate Source Rocks of Jurassic Smackover Formation, Southwestern Alabama

Algal carbonate mudstones of the Jurassic Smackover Formation are the main source rocks for oil and condensate in Mesozoic reservoir rocks in southwestern Alabama. This interpretation is based on geochemical analyses of oils, condensates, and organic matter in selected samples of shale (Norphlet Formation, Haynesville Formation, Trinity Group, Tuscaloosa Group) and carbonate (Smackover Formation) rocks. Potential and probable oil source rocks are present in the Tuscaloosa Group and Smackover Formation, respectively. Extractable organic matter from Smackover carbonates has molecular and isotopic similarities to Jurassic oil. Although the Jurassic oils and condensates in southwestern Alabama have genetic similarities, they show significant compositional variations due to differences in thermal maturity and organic facies/lithofacies. Organic facies reflect different depositional conditions for source rocks in the various basins. The Mississippi Interior Salt basin was characterized by more continuous marine to hypersaline conditions, whereas the Manila and Conecuh embayments periodically had lower salinity and greater input of clastic debris and terrestrial organic matter. Petroleum and organic matter in Jurassic rocks of southwestern Alabama show a range of thermal transformations. The gas content of hydrocarbons in reservoirs increases with increasing depth and temperature. In some reservoirs where the temperature is above 266°F (130°C), gas-condensate is enriched in isotopically heavy sulfur, apparently derived from thermochemical reduction of Jurassic evaporite sulfate. This process also results in increased H2S and CO2 in the gas, and depletion of saturated hydrocarbons in the condensate liquids. Thermochemical sulfate reduction probably depends on the mineralogic composition of the reservoir rock as well as temperature, because some deep (18,000 ft or 5.5 km) and hot (320°F or 160°C) Smackover and Norphlet reservoirs contain low-sulfur petroleum.

Offscraping and underthrusting of sediment at the deformation front of the Barbados Ridge: Deep Sea Drilling Project Leg 78A

On Leg 78A we drilled Sites 541 and 542 into the seaward edge of the Barbados Ridge complex, and Site 543 into the adjacent oceanic crust. The calcareous ooze, marls, and muds at Sites 541 and 542 are lithologically and paleontologically similar to the upper strata at Site 543 and are apparently offscraped from the down-going plate. A repetition of Miocene over Pliocene sediments at Site 541 documents major thrust or reverse faulting during offscraping. The hemipelagic to pelagic deposits offscraped in the Leg 78A area include no terrigenous sand beds, but they contain numerous Neogene ash layers derived from the Lesser Antilles Arc. Hence, this sequence is quite unlike the siliciclastic-dominated terranes on land that are inferred to be accretionary complexes. The structural fabric of the offscraped deposits at Sites 541 and 542 is disharmonic, probably along a decollement, with an underlying acoustically layered sequence, suggesting selective underthrusting of the latter. The acoustically layered sequence correlates seismically with pelagic strata cored at Site 543 on the incoming oceanic plate. Cores recovered from the possible decollement surface at both Sites 541 and 542 show scaly foliation and stratal disruption. Approximately lithostatic fluid pressure measured in the possible decollement zone probably facilitates the underthrusting of the pelagic sediments beneath the offscraped deposits. In the incoming section, a transition from smectitic to radiolarian mud with associated increases in density and strength probably controls the structural break between offscraped and underthrust strata. In the Leg 78A area, the underthrust pelagic section can be traced seismically at least 30 km arcward of the deformation front beneath the Barbados Ridge complex.

Reaction rates and δO18 variation for the carbonate-phosphoric acid preparation method

Reliable analyses of coexisting calcite and dolomite for δO18 pose a major problem in sample preparation when the phases cannot be physically separated and complicate attempts to interpret the geologic significance of the two carbonates. When mixtures of calcite and dolomite are reacted with phosphoric acid, CO2 is evolved from both phases simultaneously. An investigation of the reaction rates of pure calcite and pure dolomite with 100% phosphoric acid, indicates that for mixtures the particle size range should be limited to minimize the cross-contamination of evolved CO2 which is later analyzed for δO18. For crushed samples of most carbonate rocks 5–44 μ is a satisfactory size range. Prolonged fine-grinding of calcite and dolomite mixtures results in more cross-contamination than with ordinary crushing. Tests with pure calcite and pure dolomite samples reacted at 25°C with phosphoric acid show that the O18O16 ratio in the CO2 evolved increases during the course of a reaction. We found that CO2 from the first-reacting carbonate had δO18 as much as 3 per mil lighter than CO2 from the last reacting carbonate, for either calcite or dolomite. This change in δO18 during reaction is related in part to particle size of the reacting carbonate (Fritz and Fontes, 1966) and perhaps in part also to particle surface strain caused by crushing and grinding. However, there is also an additional kinetic isotope fractionation which may be associated with diffusion of the reaction products CO2 (gas), CO2 (dissolved) and H2O (?) away from the reacting particle surfaces. The variation of reaction rates for calcite and dolomite as a function of particle size and the change in δO18 as a function of reaction completion are especially important for carbonates having very large or very small ratios of calcite to dolomite; for these the δO18 determination of the minor component is very uncertain using traditional methods.

Carbon Isotope Composition of Marine Crude Oils (1)

A histogram of (isotope){13}C values of 621 post-Ordovician marine oils shows a trimodal distribution. Within this distribution, four groups of oils can be recognized on the basis of (isotope){13}C values, in conjunction with pristane/phytane ratios and sulfur contents. Oils with (isotope){13}C values of -32.0 to -28.0 o/oo are mainly marine shale oils older than Oligocene; oils with (isotope){13}C values of -28.0 to -23.5 o/oo are mainly deltaic oils of varying geologic age or Mesozoic carbonate oils; and oils with (isotope){13}C values heavier than -23.5 o/oo are mainly marine shale oils of Miocene age. The (isotope){13}C variation among oil groups is explained primarily by different factors that control the fractionation of carbon isotopes during primary production of rganic carbon. Miocene and younger marine shale oils are isotopically heavier than older marine shale oils because of decreased atmospheric CO[2] concentration since 25 Ma, which has resulted in decreased isotope fractionation by marine plankton during photosynthesis. Deltaic oils are mostly derived from terrestrial organic matter, and their (isotope){13}C values reflect the time-invariant average (isotope){13}C value of land plant carbon at about -25 o/oo. Mesozoic carbonate oils are isotopically heavy due mainly to salinity and temperature effects during primary production in restricted environments from which terrestrial organic matter is excluded. Processes that occur after oil generation generally have smaller effects on (isotope){13}C values of oils. As a result, (isotope){13}C val es of oils are useful for determining oil-oil and oil-source rock relationships and, in conjunction with other geochemical properties, can indicate the possible age and depositional environment of source rocks.

Petroleum Associated with Polymetallic Sulfide in Sediment from Gorda Ridge

A sediment sample, impregnated with asphaltic petroleum and polymetallic sulfide, was dredged from the southern end of Gorda Ridge (the Escanaba Trough) off northern California, within the offshore Exclusive Economic Zone of the United States. The molecular distributions of hydrocarbons in this petroleum show that it was probably derived from terrestrial organic matter in turbidite sediment filling the Escanaba Trough. Hydrothermal activity at the Gorda Ridge spreading center provided the heat for petroleum formation and was the source of fluids for sulfide mineralization.

Geochemistry of a naturally occurring massive marine gas hydrate

During Deep Sea Drilling Project (DSDP) Leg 84 a core 1 m long and 6 cm in diameter of massive gas hydrate was unexpectedly recovered at Site 570 in upper slope sediment of the Middle America Trench offshore of Guatemala. This core contained only 5–7% sediment, the remainder being the solid hydrate composed of gas and water. Samples of the gas hydrate were decomposed under controlled conditions in a closed container maintained at 4°C. Gas pressure increased and asymptotically approached the equilibrium decomposition pressure for an ideal methane hydrate, CH4.5-3/4H2O, of 3930 kPa and approached to this pressure after each time gas was released, until the gas hydrate was completely decomposed. The gas evolved during hydrate decomposition was 99.4% methane, ∼0.2% ethane, and ∼0.4% CO2. Hydrocarbons from propane to heptane were also present, but in concentrations of less than 100 p.p.m. The carbon-isotopic composition of methane was −41 to −44 permil((000), relative to PDB standard. The observed volumetric methane/water ratio was 64 or 67, which indicates that before it was stored and analyzed, the gas hydrate probably had lost methane. The sample material used in the experiments was likely a mixture of methane hydrate and water ice. Formation of this massive gas hydrate probably involved the following processes: (i) upward migration of gas and its accumulation in a zone where conditions favored the growth of gas hydrates, (ii) continued, unusually rapid biological generation of methane, and (iii) release of gas from water solution as pressure decreased due to sea level lowering and tectonic uplift.