Fractionation of carbon and hydrogen isotopes by methane-oxidizing bacteria

Carbon isotopic analysis of methane has become a popular technique in the exploration for oil and gas because it can be used to differentiate between thermogenic and microbial gas and can sometimes be used for gas-source rock correlations. Methane-oxidizing bacteria, however, can significantly change the carbon isotopic composition of methane; the origin of gas that has been partially oxidized by these bacteria could therefore be misinterpreted.We cultured methane-oxidizing bacteria at two different temperatures and monitored the carbon and hydrogen isotopic compositions of the residual methane. The residual methane was enriched in both ¹³C and D. For both isotopic species, the enrichment at equivalent levels of conversion was greater at 26°C than at 11.5°C. The change in δD relative to the change in δ¹³C was independent of temperature within the range studied. One culture exhibited a change in the fractionation pattern for carbon (but not for hydrogen) midway through the experiment, suggesting that bacterial oxidation of methane may occur via more than one pathway.The change in the δD value for the residual methane was from 8 to 14 times greater than the change in the δ¹³C value, indicating that combined carbon and hydrogen isotopic analysis may be an effective way of identifying methane which has been subjected to partial oxidation by bacteria.     

Fractionation of oxygen and hydrogen isotopes in evaporating water

Variations in oxygen and hydrogen isotope ratios of water and ice are powerful tools in hydrology and ice core studies. These variations are controlled by both equilibrium and kinetic isotope effects during evaporation and precipitation, and for quantitative interpretation it is necessary to understand how these processes affect the isotopic composition of water and ice. Whereas the equilibrium isotope effects are reasonably well understood, there is controversy on the magnitude of the kinetic isotope effects of both oxygen and hydrogen and the ratio between them. In order to resolve this disagreement, we performed evaporation experiments into air, argon and helium over the temperature range from 10 to 70 °C. From these measurements we derived the isotope effects for vapor diffusion in gas phase (ε_{diff(HD¹⁶O)} for D/H and ε_{diff(H2¹⁸O)} for ¹⁸O/¹⁶O). For air, the ratio ε_{diff(HD¹⁶O)}/ε_{diff(H2¹⁸O)} at 20 °C is 0.84, in very good agreement with Merlivat (1978) (0.88), but in considerable inconsistency with Cappa et al. (2003) (0.52). Our results support Merlivat’s conclusion that measured ε_{diff(HD¹⁶O)}/ε_{diff(H2¹⁸O)} ratios are significantly different than ratios calculated from simplified kinetic theory of gas diffusion. On the other hand, our experiments with helium and argon suggest that this discrepancy is not due to isotope effects of molecular collision diameters. We also found, for the first time, that the ε_{diff(HD¹⁶O)}/ε_{diff(H2¹⁸O)} ratio tends to increase with cooling. This new finding may have important implications to interpretations of deuterium excess (d-excess = δD − 8δ¹⁸O) in ice core records, because as we show, the effect of temperature on d-excess is of similar magnitude to glacial interglacial variations in the cores.     

Fractionation of silicon isotopes during biogenic silica dissolution

Silicon isotopes have been investigated for their potential to reveal both past and present patterns of silicic acid utilization, primarily by diatoms, in surface waters of the ocean. Interpretation of this proxy has thus far relied on characteristic trends in the isotope composition of the dissolved and particulate silicon pools in the upper ocean, as driven by biological fractionation during the production of biogenic silica (bSiO₂, or opal) by diatoms. However, other factors which may influence the silicon isotope composition of diatom opal, particularly post-formational aging and maturation processes, remain largely uninvestigated. Here, we report a consistent fractionation of silicon isotopes during the physicochemical dissolution of diatom bSiO₂ suspended in seawater under closed conditions. This fractionation acts counter to that occurring during bSiO₂ production and at about half its absolute magnitude, with dissolution discriminating against the release of the heavier isotopes of silicon at an enrichment factor ε_DSi–BSi of −0.55‰, corresponding to a fractionation factor α_30/28 of 0.99945. The enrichment factor did not vary with source material, indicating the lack of a significant species effect, or with temperature from 3 to 20 °C. Thus, the dissolution of bSiO₂ produces dissolved silicon with a δ³⁰Si value that is 0.55‰ more negative than its parent bSiO₂, an effect that must be accounted for when interpreting oceanic δ³⁰Si distributions. The δ³⁰Si values of both the dissolved and particulate silicon pools increased linearly as dissolution progressed, implying a measurable (±0.1‰) change in the relative δ³⁰Si of opal samples whenever the difference in preservation efficiency between them is >20%. This effect could account for 10–30% of the difference in diatom δ³⁰Si values observed between glacial and interglacial conditions. It is unlikely, however, that the inferred maximum possible change in δb³⁰SiO₂ of +0.55‰ would be manifested in situ, as a high mean percentage of dissolution would include complete loss of the more soluble members of the diatom assemblage.     

Fractionation change of hydrogen isotopes in trees due to atmospheric pollutants

Atmospheric pollution effects on hydrogen fractionation in trees are demonstrated for the first time in this study. The δ²H ring-cellulose series obtained for black spruce trees at a site near a SO₂-emitting smelter display short-term fluctuations superimposed on a first order −11‰ shift coincident with the onset of smelter operations. The isotopic depletion in trees exposed to various levels of SO₂ diminish with increasing distance relative to the location of the smelter, and it is not detected at the most distant selected stand, 116 km downwind from the point source. Both the spatial and temporal isotopic trends likely result from the combined effects of leaf transpiration, biochemical processes and water uptake by the root system. The spatial and temporal trends of δ²H values are the reverse of the δ¹³C trends previously obtained for the same tree ring series. These coupled isotopic fractionations underline an important response of trees to pollution stresses likely generated by ambient atmospheric SO₂ concentrations. The degradation of global air quality during the last 150 yr likely generated large scale modifications in the way terrestrial vegetation grows. In this respect, hydrogen dendrogeochemistry combined with other tracers such as C isotope ratios constitutes a new tool to evaluate the past behavior of forest ecosystems in terms of C uptake and acclimation to various types of atmospheric pollution.     

Fractionation of sulfur isotopes during laboratory synthesis of pyrite at low temperatures

The reaction products and the accompanying sulfur isotope fractionations during the reaction of H₂S with goethite in aqueous media at 22–24°C for periods from 0.5 hr to 65 days were studied. Fine-grained pyrite formed within two days and was isotopically 0.8‰ lighter than the H₂S source. After 65 days reaction time the pyrite had nearly the same isotopic value as the H₂S. Aqueous precipitation of pyrite from H₂S and goethite at room temperature involved three major steps, namely: (1) the rapid oxidation of H₂S and reduction of Fe³⁺ during which elemental S is formed; (2) the formation of acid-volatile sulfides and the disappearance of elemental S; and (3) the formation of pyrite at the expense of acid-volatile sulfides.     

Fractionation of oxygen isotopes in phosphate during its interactions with iron oxides

Iron (III) oxides are ubiquitous in near-surface soils and sediments and interact strongly with dissolved phosphates via sorption, co-precipitation, mineral transformation and redox-cycling reactions. Iron oxide phases are thus, an important reservoir for dissolved phosphate, and phosphate bound to iron oxides may reflect dissolved phosphate sources as well as carry a history of the biogeochemical cycling of phosphorus (P). It has recently been demonstrated that dissolved inorganic phosphate (DIP) in rivers, lakes, estuaries and the open ocean can be used to distinguish different P sources and biological reaction pathways in the ratio of ¹⁸O/¹⁶O (δ¹⁸O_P) in PO₄³⁻. Here we present results of experimental studies aimed at determining whether non-biological interactions between dissolved inorganic phosphate and solid iron oxides involve fractionation of oxygen isotopes in PO₄. Determination of such fractionations is critical to any interpretation of δ¹⁸O_P values of modern (e.g., hydrothermal iron oxide deposits, marine sediments, soils, groundwater systems) to ancient and extraterrestrial samples (e.g., BIF’s, Martian soils). Batch sorption experiments were performed using varied concentrations of synthetic ferrihydrite and isotopically-labeled dissolved ortho-phosphate at temperatures ranging from 4 to 95 °C. Mineral transformations and morphological changes were determined by X-Ray, Mössbauer spectroscopy and SEM image analyses.Our results show that isotopic fractionation between sorbed and aqueous phosphate occurs during the early phase of sorption with isotopically-light phosphate (P¹⁶O₄) preferentially incorporated into sorbed/solid phases. This fractionation showed negligible temperature-dependence and gradually decreased as a result of O-isotope exchange between sorbed and aqueous-phase phosphate, to become insignificant at greater than ∼100 h of reaction. In high-temperature experiments, this exchange was very rapid resulting in negligible fractionation between sorbed and aqueous-phase phosphate at much shorter reaction times. Mineral transformation resulted in initial preferential desorption/loss of light phosphate (P¹⁶O₄) to solution. However, the continual exchange between sorbed and aqueous PO₄, concomitant with this mineralogical transformation resulted again in negligible fractionation between aqueous and sorbed PO₄ at long reaction times (>2000 h). This finding is consistent with results obtained from natural marine samples. Therefore, ¹⁸O values of dissolved phosphate (DIP) in sea water may be preserved during its sorption to iron-oxide minerals such as hydrothermal plume particles, making marine iron oxides a potential new proxy for dissolved phosphate in the oceans.     

Fractionation of hydrogen and oxygen isotopes of gypsum hydration water and assessment of its geochemical indications

Fundamental knowledge of the isotopic fractionation between the hydration water and the mother solution and whether the primary information recorded in hydration water can be preserved or not in deposits or mines have long been unclear. In order to calculate the accurate hydrogen and oxygen isotopic fractionation factors between gypsum hydration water and its mother solution with new methods, to understand the mechanism of fractionation and synthetically assess the record-keeping abilities of the isotopic composition of hydration water during the process of diagenesis after deposition, experiments on the hydrogen and oxygen isotopic compositions of gypsum hydration water and its mother solution at different isothermal temperatures from 5 to 50°C were systematically conducted. In addition, samples from two typical gypsum deposits formed in different environmental conditions were also determined. Results show that during gypsum crystallisation, both hydrogen and oxygen isotopes show significant fractionation between the hydration water and the mother solution. The calculated hydrogen isotopic fractionation factors are <1, while the oxygen isotopic fractionation factors are >1 at temperatures from 5 to 50°C. The fractionation factors show no functional relationships with temperature. Isotopic compositions of gypsum hydration water in arid lake sediments can be used to trace the source of water and primary deposit environmental information. However, the isotopic composition of the gypsum hydration water can easily be altered by dissolution and secondary precipitation of gypsum during later diagenesis, particularly in areas with humid climate and abundant groundwater. A very careful assessment on record-keeping abilities of the primary isotopic composition of hydration water in gypsum during later diagenesis must be considered before application.     

Fractionation of hydrogen,oxygen and carbon isotopes in n-alkanes and cellulose of three Sphagnum species

Compound-specific isotope measurements of organic compounds are increasingly important in palaeoclimate reconstruction. Searching for more accurate peat-based palaeoenvironmental proxies, compound-specific fractionation of stable C, H and O isotopes of organic compounds synthesized by Sphagnum were determined in a greenhouse study. Three Sphagnum species were grown under controlled climate conditions. Stable isotope ratios of cellulose, bulk organic matter (OM) and C₂₁–C₂₅ n-alkanes were measured to explore whether fractionation in Sphagnum is species-specific, as a result of either environmental conditions or genetic variation. The oxygen isotopic composition (δ¹⁸O) of cellulose was equal for all species and all treatments. The hydrogen isotopic composition (δD) of the n-alkanes displayed an unexpected variation among the species, with values between −154‰ for Sphagnum rubellum and −184‰ for Sphagnum fallax for the C₂₃ n-alkane, irrespective of groundwater level. The stable carbon isotopic composition (δ¹³C) of the latter also showed a species-specific pattern. The pattern was similar for the carbon isotope fractionation of bulk OM, although the C₂₃ n-alkane was >10‰ more depleted than the bulk OM. The variation in H fractionation may originate in the lipid biosynthesis, whereas C fractionation is also related to humidity conditions. Our findings clearly emphasize the importance of species identification in palaeoclimate studies based on stable isotopes from peat cores.     

Fractionation of stable carbon isotopes by marine phytoplankton

Stable carbon isotope fractionation by seventeen species of marine phytoplankton, representing the classes of Bacillariophyceae, Chlorophyceae, Prasinophyceae, Chrysophyceae, Haptophyceae and Dinophyceae have been determined in laboratory culture experiments using bicarbonate enriched artificial sea water. The values (Δ_HCO3^? = δ¹³C of algae vs HCO₃^?) range from ?22.1 to ?35.5%. Nitzschia closterium shows the smallest fractionation of ? 22.1% and Isochrysis galbana, the greatest of ?35.5%,. Since these algae were cultured under identical laboratory conditions, the wide range of Δ_HCO3^? values is seemingly due to the presence of different metabolic pathways within these organisms.A temperature dependent fractionation of 0.36% per °C with decreasing temperatures was measured for Skeletonema costatum whereas, smaller temperature dependencies of ?0.13, +0.15 and ?0.07%. per °C were observed for Dunaliella sp., Monochrysis lutheri and Glenodinium foliaceum, respectively.The consistency of values of Skeletonema costatum, Dunaliella sp. and Monochrysis lutheri grown at salinities of 22, 26, 32 and 36% indicates that natural salinity variations have negligible effects on the isotopic composition of marine phytoplankton.     

Fractionation of carbon isotopes in biosynthesis of fatty acids by a piezophilic bacterium Moritella japonica strain DSK1

We examined stable carbon isotope fractionation in biosynthesis of fatty acids of a piezophilic bacterium Moritella japonica strain DSK1. The bacterium was grown to stationary phase at pressures of 0.1, 10, 20, and 50 MPa in media prepared using sterile-filtered natural seawater supplied with glucose as the sole carbon source. Strain DSK1 synthesized typical bacterial fatty acids (C_14-19 saturated, monounsaturated, and cyclopropane fatty acids) as well as long-chain polyunsaturated fatty acids (PUFA) (20:6ω3). Bacterial cell biomass and individual fatty acids exhibited consistent pressure-dependent carbon isotope fractionations relative to glucose. The observed Δδ_FA-glucose (−1.0‰ to −11.9‰) at 0.1 MPa was comparable to or slightly higher than fractionations reported in surface bacteria. However, bulk biomass and fatty acids became more depleted in ¹³C with pressure. Average carbon isotope fractionation (Δδ_FA-glucose) at high pressures was much higher than that for surface bacteria: −15.7‰, −15.3‰, and −18.3‰ at 10, 20, and 50 MPa, respectively. PUFA were more ¹³C depleted than saturated and monounsaturated fatty acids at all pressures. The observed isotope effects may be ascribed to the kinetics of enzymatic reactions that are affected by hydrostatic pressure and to biosynthetic pathways that are different for short-chain and long-chain fatty acids. A simple quantitative calculation suggests that in situ piezophilic bacterial contribution of polyunsaturated fatty acids to marine sediments is nearly two orders of magnitude higher than that of marine phytoplankton and that the carbon isotope imprint of piezophilic bacteria can override that of surface phytoplankton. Our results have important implications for marine biogeochemistry. Depleted fatty acids reported in marine sediments and the water column may be derived simply from piezophilic bacteria resynthesis of organic matter, not from bacterial utilization of a ¹³C-depleted carbon source (i.e., methane). The interpretation of carbon isotope signatures of marine lipids must be based on principles derived from piezophilic bacteria.     

Fractionation of sulfur isotopes by the yeast Saccharomyces cerevisiae

Growing and resting cell suspensions of Baker's yeast (Saccharomyces cerevisiae) were permitted to metabolize sulfite and sulfate under a variety of environmental conditions. Enrichment of S³² in the H₂S released during reduction of sulfate under growing conditions was consistently lower (δS³⁴ < ?25%.) than enrichment during reduction of sulfite under similar conditions (δS³⁴ ~ ?30 to ?50%.). Yeast cells harvested from a sulfite medium released small quantities of H₂S when suspended in glucose-sulfate solution; cells from a sulfate growth medium did not. The enrichment of H₂S in S³² by sulfite-grown resting cells suspended in either SO^2?₄ or SO^2?₃ solutions was about 10%.in less than the maximum obtained during growth on sulfite. Isotopic fractionation was always significantly less in assimilated, than in dissimilated sulfur.     

Fractionation of stable carbon isotopes during anaerobic production and degradation of propionate in defined microbial cultures

In many anoxic environments propionate is, after acetate, the second most important fermentation product, being degraded further to finally result in CH₄ production. In principle, isotope discrimination can be used to assess the path of organic matter degradation to acetate, CO₂ and CH₄. However, nothing is known about the isotope fractionation in primary and secondary fermentation steps involving propionate, although it is an important precursor of acetate. We therefore studied the degradation of propionate with a syntrophic coculture of Syntrophobacter fumaroxidans and Methanobacterium formicicum. The isotope enrichment factor for propionate degradation to acetate, CO₂ and CH₄ was almost negligible (ε_prop 0.9‰). The fermentative production of propionate was studied in cultures with Opitutus terrae growing on pectin, xylan and starch. These polysaccharides were fermented to acetate, succinate, propionate, H₂ and CO₂. While the δ¹³C value of the initially produced propionate was similar to that of the organic substrates (ca. −28 to −25‰), the δ¹³C value of the other fermentation products was higher. The δ¹³C values of all products generally decreased during the course of fermentation. Finally, a small depletion in ¹³C (ca. 6‰) with respect to the organic substrate was observed for propionate, while the other fermentation products where slightly enriched. Overall, stable carbon isotope discrimination was small during both fermentative production and consumption of propionate in the anaerobic microbial cultures, so that propionate turnover probably only marginally affects isotope fractionation during anaerobic degradation of organic matter.     

Biogeochemistry of the stable hydrogen isotopes

The fractionation of H isotopes between the water in the growth medium and the organically bonded H from microalgae cultured under conditions, where light intensity and wavelength, temperature, nutrient availability, and the H isotope ratio of the water were controlled, is reproducible and light dependant. All studies were based either on the H isotope ratios of the total organic H or on the lipids, where most of the H is firmly bonded to C. H bonded into other macromolecules, proteins, carbohydrates and nucleic acids, does not exchange with water, when algae are incubated in water enriched with deuterium. Only after the destruction of quaternary H bonds are labile hydrogens in macromolecules free to exchange with water. By growing algae (18 strains), including blue-green algae, green algae and diatoms, in continuous light, the isotope fractionations in photosynthesis were reproducibly ?93 to ?178 %. depending on the organism tested. This fractionation was not temperature dependent. Microalgae grown in total darkness with an organic substrate did not show the isotope fractionation seen in cells grown in light. In both light- and dark-grown algae, however, additional depletion of deuterium (?30 to ?60%.) in cellular organic matter occurs during the metabolism of carbohydrates to form lipids. Plants from several natural populations also fractionated isotopes during photosynthesis by an average of ?90 to ?110%. In addition, the organically bonded H in nonsaponifiable lipids was further fractionated by ?80%. from that in saponifiable lipids, isolated from two geographically distinct populations of marsh plants. This difference between H isotope ratios of these two groups of lipids provides an endogenous isotopic marker.     

Fractionation of multiple sulfur isotopes during phototrophic oxidation of sulfide and elemental sulfur by a green sulfur bacterium

We present multiple sulfur isotope measurements of sulfur compounds associated with the oxidation of H₂S and S⁰ by the anoxygenic phototrophic S-oxidizing bacterium Chlorobium tepidum. Discrimination between ³⁴S and ³²S was +1.8 ± 0.5‰ during the oxidation of H₂S to S⁰, and −1.9 ± 0.8‰ during the oxidation of S⁰ to , consistent with previous studies. The accompanying Δ³³S and Δ³⁶S values of sulfide, elemental sulfur, and sulfate formed during these experiments were very small, less than 0.1‰ for Δ³³S and 0.9‰ for Δ³⁶S, supporting mass conservation principles. Examination of these isotope effects within a framework of the metabolic pathways for S oxidation suggests that the observed effects are due to the flow of sulfur through the metabolisms, rather than abiotic equilibrium isotope exchange alone, as previously suggested. The metabolic network comparison also indicates that these metabolisms work to express some isotope effects (between sulfide, polysulfides, and elemental sulfur in the periplasm) and suppress others (kinetic isotope effects related to pathways for oxidation of sulfide to sulfate via the same enzymes involved in sulfate reduction acting in reverse). Additionally, utilizing fractionation factors for phototrophic S oxidation calculated from our experiments and for other oxidation processes calculated from the literature (chemotrophic and inorganic S oxidation), we constructed a set of ecosystem-scale sulfur isotope box models to examine the isotopic consequences of including sulfide oxidation pathways in a model system. These models demonstrate how the small δ³⁴S effects associated with S oxidation combined with large δ³⁴S effects associated with sulfate reduction (by SRP) and sulfur disproportionation (by SDP) can produce large (and measurable) effects in the Δ³³S of sulfur reservoirs. Specifically, redistribution of material along the pathways for sulfide oxidation diminishes the net isotope effect of SRP and SDP, and can mask the isotopic signal for sulfur disproportionation if significant recycling of S intermediates occurs. We show that the different sulfide oxidation processes produce different isotopic fields for identical proportions of oxidation, and discuss the ecological implications of these results to interpreting minor S isotope patterns in modern systems and in the geologic record.     

Fractionation of oxygen isotopes between mammalian bone-phosphate and environmental drinking water

The δ¹⁸O of mammalian bone-phosphate varies linearly with δ¹⁸O of environmental water, but is not in isotopic equilibrium with that water. This situation is explained by a model of δ¹⁸O in body water in which the important fluxes of exchangeable oxygen through the body are taken into account. Fractionation of oxygen isotopes between body and environmental drinking water is dependent on the rates of drinking and respiration. Isotopic fractionation can be estimated from physiological data and the estimates correlate very well with observed fractionation. Species whose water consumption is large relatively to its energy expenditure is sensitive to isotopic ratio changes in environmental water.     

Fractionation of carbon isotopes by phytoplankton and estimates of ancient CO2 levels

Reports of the 13C content of marine particulate organic carbon are compiled and on the basis of GEOSECS data and temperatures, concentrations, and isotopic compositions of dissolved CO2 in the waters in which the related phytoplankton grew are estimated. In this way, the fractionation of carbon isotopes during photosynthetic fixation of CO2 is found to be significantly correlated with concentrations of dissolved CO2. Because ancient carbon isotopic fractionations have been determined from analyses of sedimentary porphyrins [Popp et al., 1989], the relationship between isotopic fractionation and concentrations of dissolved CO2 developed here can be employed to estimate concentrations of CO2 dissolved in ancient oceans and, in turn, partial pressures of CO2 in ancient atmospheres. The calculations take into account the temperature dependence of chemical and isotopic equilibria in the dissolved-inorganic-carbon system and of air-sea equilibria. Paleoenvironmental temperatures for each sample are estimated from reconstructions of paleogeography, latitudinal temperature gradients, and secular changes in low-latitude sea surface temperature. It is estimated that atmospheric partial pressures of CO2 were over 1000 micro atm 160 - 100 Ma ago, then declined to values near 300 micro atm during the next 100 Ma. Analysis of a high-resolution record of carbon isotopic fractionation at the Cenomanian-Turonian boundary suggests that the partial pressure of CO2 in the atmosphere was drawn down from values near 840 micro atm to values near 700 micro atm during the anoxic event.     

板块俯冲过程中的Mg-Li-Fe-Cr同位素分馏

金属稳定同位素体系是示踪板块俯冲对壳幔物质再循环影响的全新工具,因此其在俯冲带的地球化学行为备受关注.Mg同位素在俯冲过程中不发生显著分馏,但大陆玄武岩具有低于洋中脊玄武岩的Mg同位素,这可能是碳酸岩的俯冲再循环导致的.与角闪岩继承原岩的Li同位素组成不同,榴辉岩具有轻于原岩的Li同位素组成,可归因于俯冲折返过程中的动力学扩散、脱水反应或低Li同位素的流体交代.作为变价元素,Fe和Cr的同位素在榴辉岩的形成过程中均不发生显著分馏,但是蛇纹岩的Fe同位素和Cr同位素与氧逸度指标具有相关性,指示氧化还原条件变化时脱水过程或流体交代会导致同位素分馏.      

Fractionation of azaarenes during oil migration

A comparative study of alkylbenzoquinolines in crude oils and rock extracts (bitumens) from Japan and Sumatra shows that the ratios of nitrogen-masked isomers (NMIs) to nitrogen-exposed isomers (NEIs), and the ratio of higher homologs to lower homologs is higher in crude oils than in corresponding bitumens. Also, a regular increase in the ratios of NMIs to NEIs with increasing migration distance is observed for a series of crude oils from the Sarukawa Oil Field, northeastern Japan. These results are attributed to the preferential migration of NMIs caused by their weak adsorption on clay minerals and/or their low solubility in interstitial water, and the selective removal of lower honologs caused by their irreversible adsorption onto clays during oil migration, which are regarded as geochromatographic phenomena. It is suggested that those ratios can be used to estimate the degree of fractionation of oils during primary and secondary migration.