Chemical Geology: Isotope Geoscience section最新文献

The cross-sectional radius of a 3-m (diam.) brown coal gymnospermous log of Miocene age, previously analyzed for carbohydrate and lignin methoxyl content by solid-state¹³C nuclear magnetic resonance spectroscopy, was examined using stable carbon isotopic ratios in order to determine if the isotopic composition could be related to chemical changes or to radial position. This study found a possible relationship between δ¹³C-values and radial position; however, these changes cannot be linked to carbohydrate content and are probably attributable to changing growth conditions during the lifetime of the tree. An apparent linear relationship between the changes in carbohydrate content after sodium para-periodate treatment and corresponding changes in the δ¹³C-values indicates constant isotopic fractionation between lignin and carbohydrates along the cross-sectional radius. This result indicates that diagenesis has not produced any significant change in the lignin-carbohydrate carbon isotopic fractionation or, alternatively, that diagenesis has erased any fractionation pattern that once existed. A sample of fresh wood from another gymnospermous species was analyzed by the same methods and found to have lignin-carbohydrate carbon isotopic fractionation significantly different from that of the Miocene log section samples, suggesting that differences may be species-related or that the complex mixture of carbohydrates in the fresh wood was isotopically different from that of the degraded wood, and the whole Miocene log was uniformly altered.

Kinetic isotope effects associated with the hydrolysis of the dipeptide glycylglycine in unbuffered aqueous solution were investigated over a 60°C temperature range. The hydrolysis of the dipeptide resulted in the distinct fractionation of carbon and nitrogen isotopes. As the extent of the hydrolysis reaction increased, the residual peptide became increasingly enriched in¹³C and¹⁵N. The kinetic isotope effect for nitrogen ranged from 1.0025 to 1.0040 and in general increased with decreasing temperature. The experimental evidence indicates that isotope effects associated with peptide bond cleavage may complicate the interpretation of stable carbon and nitrogen isotope signatures of residual proteinaceous material preserved in fossils.

The stable carbon and nitrogen isotopic composition of bulk organic matter, lipids, chlorophylla, chlorophyllb and β,β-carotene were determined for selected photosynthetic plants. Lipids are depleted in¹³C (∼ − 6‰) compared to bulk organic matter. C₄- and C₃-type biosynthesis can be differentiated at the compound class and individual compound level by their stable carbon isotope ratios. The stable carbon isotopic ratios of chlorophylls isolated from C₃ and C₄ plants differ by ∼ 10ℵ.. The stable nitrogen isotope ratios of lipids vary over an ∼ 15‰ range, excluding one very positive sample. Various sources of carbon and nitrogen, differing assimilation mechanisms, and nutrient limitation contribute to the observed isotopic compositions. The stable isotopic composition of chlorophyllf and bulk organic matter are linearly correlated. Based on this relationship it is possible to resolve the assimilation of carbon and nitrogen by algae within complex mixture of detritus, bacteria and phytoplankton. The isotopic record of photosynthetic fixation of carbon and nitogren in chlorophylls suggests that geoporphyrins do retain information on paleo-productivity. The stable isotopic composition of chlorophylla, chlorophyllb and β,β-carotene, and their breakdown products provide unique insight into a wide range of biogeochemical processes.

Baffin Bay, Texas, a drowned Pleistocene river valley, is filled with up to 20 m of exceptionally well-preserved Holocene and recent sediments. Piston cores from the upper 4 m have been described and analyzed for elemental composition and δ¹³C of the total organic carbon and carbonate. Strong cyclic patterns were observed for all of these parameters. The total organic carbon (TOC) level varied between 1% and 6%. (C/N ratios varied between 10 and 14. δ¹³C showed a cyclicity with depth and a slight shift toward more positive values with depths. These variations are interpreted as being due to different relative inputs of seagrass (δ¹³C−≈ 10‰) and phytoplankton (δ¹³C−20‰.) to the sediment. The cores contained fine-grained carbonate and some massive dolomite. δ¹³C of the carbonate varied between −3% and + 1‰ except for two more positive values. The lack of a trend toward light carbonate with depth was taken to mean that little (< 5%) CO₂ from the oxidation of organic matter is present in the carbonates. Overall the changes in the relative intensity of these sources is thought to reflect regional climatic and weathering cycles.

A global shift in carbonate carbon δ¹³C-values from heavier values in the Maastrichtian to lighter values in the early Danian indicates recycling of isotopically light organic carbon to inorganic carbon reservoirs during a period of depressed marine productivity. Comparison of organic carbon δ¹³-values from globally dispersed K/T sections does not show a similar, globally well-developed pattern. Several factors evidently overwhelm the potential impact of an isotopically lighter inorganic carbon source on organic matter isotopic signatures: (1) species changes in biological assemblages may modify the averaged isotopic fractionation of organic matter; and (2) shifts in the proportion of land/marine organic matter contributions to coastal marine locations may overprint the isotopic record. Local phenomena evidently outweigh global change in determining the isotope signature of organic carbon deposited in K/T boundary sections.

Several hundred stable isotopic ratios (C, N and S) acquired over seven years of investigations at both seep and vent locations are compiled and interpreted. The stable isotopic compositions of tissues derived from the chemosynthetic fixation of carbon reflect a complex interaction between chemical and biological processes. The stable isotopic composition of bivalves that utilize reduced sulfur suggests that seawater-and/or vent water-dissolved inorganic carbon (DIC) is their primary source of carbon during chemosynthesis. All thiotrophic bivalves studied appear to possess a similar sulfide oxidation metabolism. The δ¹³C-values of tissues from methanotrophic mussels are close to the δ¹³C of the methane utilized. Apparently, little of the kinetic isotope fractionation associated with methanotrophy is expressed in the host's tissue. Vestimentiferan carbon isotopic composition reflects both carbon limitation and the isotopic composition of the substrate utilized. The δ¹³C-values of vent vestimentiferans tend to be affected by carbon limitation, whereas those of seep vestimentiferans reflect the variable isotopic composition of pore-water DIC. Stable nitrogen isotopic compositions are consistent with nitrogen (N₂) fixation, but the presence of the enzyme responsible for nitrogen fixation, nitrogenase, has not been conclusively demonstrated. A variety of nitrogen sources [N₂, NH₄⁺, PON (particulate organic nitrogen), DON (dissolved organic nitrogen) and NO₄⁻] may be utilized by vent and seep organisms. However, the δ¹⁵N data suggest that the mechanism of nitrogen metabolism is not a function of the species or the symbiont type. Sulfur is a key element in vent and seep environments and thiotrophy is the major chemosynthetic activity. The sources of sulfur are highly variable in quantity and isotopic composition but are almost always linked to bacterial activity,l either free-living and/or symbiont. Nitrogen and sulfur nutritional requirements appear to be derived from a wide variety of sources. The relative importance of nutrition derived from heterotrophy and chemoautotrophy depends on the chemical environment and animal physiology. Stable isotope compositions provide insight into these diverse metabolic strategies; however, a complete inventory of the concentration and isotopic composition of inorganic and organic substrates, as well as supporting biochemical, enzymatic and observational studies, are needed to resolve fundamental ecological questions.

The organic-rich, Middle Proterozoic Nonesuch Formation is part of a thick volcaniclastic rift-fill sequence (Mid-Continent Rift System, northern Wisconsin and Upper Peninsula Michigan) with a mild thermal history. Despite stratigraphic/sedimentologic similarities between the east (Michigan) and west (Wisconsin) study areas, distinctions in organic petrologic, elemental and stable isotopic parameters are noted. Geologic arguments and predictable relationships among organic carbon content, bulk and molecular pyrolysate composition and petrologic parameters are sufficient to invoke differential preservation of the same (or similar) primary producers as the principal cause of variability. Isotopic signals indicating precipitation of calcite via intense organic production (planktonic blooms) is evident among carbonate laminites. Incorporation or concentration of organic degradation by-products is evident, particularly in the west, from carbon and nitrogen stable isotope data. It is proposed that whereas preservation of organic productivity of a particular organism may be nearly complete in one organic facies in the east and west, organic remains in other intervals in the west were subject to extensive degradation and reconstitution to form protokerogens. Implications of the study include establishing parameters for recognizing pre-Devonian lacustrine vs. marine euxinic systems, clues to the complexity of Middle Proterozoic aquatic systems and in recognizing the input of hydrothermal fluids to water bodies and early sediments.

The condensation of amino acids and sugars (Maillard reaction) is one possible diagenetic pathway for the formation of humic materials in sediments. In this study, aqueous solutions of alanine and glucose were heated (100°C) for up to 40 days. The δ¹³C- and δ¹³N-values of reactants and products were monitored. With increased heating time, the stable carbon and nitrogen isotope compositions of the unreacted alanine in solution were enriched by up to 8.8‰ and 2.7‰, respectively, relative to their initial compositions. In contrast, the insoluble melanoidin product and alanine recovered by acid hydrolysis of the melanoidin were both depleted in¹³C and¹⁵N relative to the starting materials. The magnitude of this isotopic fractionation varied as a function of the relatieve concetration of alanine to glucose in the starting solution. The CO₂ that evolved during the reaction is depleted in¹³C relative to the initial δ¹³C composition of alanine and its car☐yl group, suggesting that¹³C-depleted amino acids initially condense with glucose to form insoluble melanoidins. Subsequent to melanoidin formation, a second isotope fractionation takes place whereby¹³C-depleted car☐yl groups are preferentially cleaved from the melanoidin. Assuming that humic substances may form in natural environments via condensation reactions like the Maillard reaction, it is hypothesized that the stable isotope fractionation that occurs during the transformation of organic matter to humic materials and kerogen might be at least partially explained by kinetic effects during condensation reactions rather than decar☐ylation of the primary amino acids.

Carbonate mineral phases were investigated with respect to distribution and isotopic composition from core holes in the Green River Formation, Wyoming, an ancient lake complex. Mineral distribution data suggest dolomite genesis on a carbonate mudflat surrounding a central lake facies with transportation and subsequent later alteration in the lacustrine facies. Calcite was the dominant carbonate mineral deposited in the lacustrine facies. Samples, investigated by stable isotopes, representing each member of the Green River Formation, displayed isotopic values for δ¹³C of from − 1.3 to + 7.5‰ and for δ¹⁸C of − 6.0 to −0.8‰. Isotopic values representing modern and paleolacustrine sequences were compared. δ¹³C-values in the Green River Formation sediments were below values that might be expected from sediments which formed in a stratified lake. Mineral distribution and stable isotope evaluations demonstrated evidence to support a playa-lake depositional environment.

The negative thermal ionisation mass spectrometry of boron, a powerful method for the determination of boron isotopic composition (¹¹B/¹⁰B) and concentration in geological materials, is described.

BO₂⁻ ions are formed in a single-filament ion source. With the addition of La(NO₃)₃ as an activator, a 100- to 1000-fold increase in sensitivity as compared to the classical sodium or cesium metaborate method is achieved. With this high sensitivity, as little as 0.1 ng B (0.1 ppb B with 1 g sample size) is needed for analysis. The precision is typically∼ ± 0.2%.

Boron concentrations are determined with a standard isotope dilution technique using a¹⁰B-enriched spike. Precisions are in the range of±0.01% (> 100ppm B)to± 4% (< 0.1ppm B).

For aqueous solutions, water- and HCl-soluble salts and minerals (e.g., borates, brines, seawater, groundwater, carbonates) no chemical treatment is required. The untreated solutions are directly loaded onto the filament.

The method has successfully been tested on a variety of hydrothermal waters and borate minerals from Larderello (Italy), Boron and the Death Valley (California, U.S.A.), and on seawater, tourmalines and carbonatites from different origins. Preliminary results on these investigations are presented.