John R. Bowman

On the isotopic composition of carbon in soil carbon dioxide

Abstract In this study it is shown that the isotopic composition of carbon in soil CO2 differs from the isotopic composition of carbon in soil-respired CO2. Soil CO2 collected from a montane soil has an endmember δ13C value of −23.3%. whereas soil-respired CO2 in this system has a δ 13C value of −27.5%. This difference is very close to the theoretical difference of 4.4%. which is predicted by the difference in the diffusion coefficients for 12CO2 and 13CO2. Because of the measured and the theoretical difference between the isotopic composition of carbon in soil CO2 and soil-respired CO2 it is suggested that, in the future, a distinction should be made between them.

Systematic variations in the carbon and oxygen isotopic composition of pedogenic carbonate along elevation transects in the southern Great Basin, United States

Stable carbon- and oxygen-isotope variations in Holocene soil carbonates that formed in the unsaturated zone were examined along several elevation transects in the southern Great Basin, United States, a region with a semi-arid climate. Our intent was to study the relationship between the stable isotopic composition of pedogenic carbonates and climate, ecological variations, differences in parent material, and soil depth. δ 13 C of pedogenic carbonate in three different soil profiles from different elevations decreases with soil depth, indicating a decrease in the ratio of atmospheric to plantderived CO 2 downprofile. Pedogenic carbonate at the soil-air interface approaches a δ 13 C value in equilibrium with atmospheric CO 2 in all three soils. Observed δ 13 C profiles for pedogenic carbonate can be described using a one-dimensional model for 12 CO 2 and 13 CO 2 , assuming isotopic equilibrium between soil CO 2 and soil carbonate. The modeled best fit to observed isotopic profiles suggests that the profile differences in part result from differing soil-respiration rates at each site. δ 13 C in deep pedogenic carbonate (>50 cm) varies by about 12 per mil over a 2,440-m elevation range, being enriched in 13 C at the lowest elevations. The slope of δ 13 C for these carbonates versus elevation is very similar for soils developed on carbonate and on noncarbonate parent materials: depletion by 4.6 to 4.7 per mil per 1,000 m increase in altitude between 300 to 2,740 m above mean sea level for the localities studied. This concordance makes it likely that there has been complete isotopic exchange between HCO 3 - in solution and soil CO 2 prior to carbonate precipitation. Soil CO 2 and soil-respiration rates increase systematically with elevation. The plantderived component of soil CO 2 indicates that C 3 plants dominate the biomass at most measured sites, in agreement with plant surveys. Calculated equilibrium fractionation factors between soil CO 2 and soil carbonate are very similar to those observed, again indicating complete isotopic exchange between carbon species. In all, the soil CO 2 and soil-carbonate data suggest that the δ 13 C variation with elevation observed in the soil carbonates results from differing soil-respiration rates at each site, as well as from variations in the proportion of C 3 to C 4 and CAM plants in each site9s surface biomass. δ 18 O values in pedogenic carbonates are higher at lower elevations, due in part to the more positive δ 18 O values for meteoric waters at lower elevations. The average δ 18 O value of deep (>50 cm) pedogenic carbonate at all sites, however, is enriched 2.4 to 3.7 per mil with respect to values we predict should be in equilibrium with the isotopic composition of local meteoric waters. This suggests that evaporative isotopic enrichment of soil waters may have occurred at all elevations prior to precipitation of carbonate, or that seasonal differences in the isotopic composition of meteoric waters coupled with differential infiltration may be taking place. One or both of these processes also may explain the δ 18 O decrease in soil carbonate with depth observed in two of three soil profiles.

Diagenetic Hematite and Manganese Oxides and Fault-Related Fluid Flow in Jurassic Sandstones, Southeastern Utah

A variety of diagenetic hematite and manganese oxide deposits occur within well-exposed Jurassic eolian and related deposits of southeastern Utah. Hematite concretions (millimeters to tens of meters in size) and strata-bound layers occur in the permeable Navajo, Page, and Entrada sandstones. Localized manganese oxide deposits without significant iron oxide occur in the overlying rocks covering the Summerville-Tidwell interval. Field, lab, and numerical modeling studies indicate the diagenetic deposits are related to the Moab fault. Fluid inclusion studies show salinities of fault fluids range from 0 to 19.7 NaCl equivalent weight percent. The d18O (SMOW) and d13C (PDB) values of cements and veins range from 7 to 27o/oo and -12 to +5o/oo, respectively. The d87Sr (SMOW) values of these cements and veins range from 0.210 to 2.977o/oo, values substantially more radiogenic than Pennsylvanian seawater. Saline brines formed from solution of Pennsylvanian salts by meteoric water and are interpreted to have flowed up the Moab fault and outward into adjacent permeable rocks. These brines are reducing from interaction with hydrocarbon, methane, organic acids, or hydrogen sulfide, and thus remove iron, manganese, and 87Sr, and bleach the sandstones near the fault. The isotopic evidence suggests multiple episodes of fluid flow up the Moab fault system. When saline, reduced brines mixed with shallow oxygenated groundwater, iron and manganese oxides were precipitated as cements to form concretions and tabular deposits in the porous sandstones. Multiple episodes of iron oxide mineralization and concretionary geometries are evident and can be explained as the result of permeability heterogeneities in the host rock, presence of favorable nucleii for precipitation, a self-organization process, or the influence of microbes.

An isotopic study of a fluvial-lacustrine sequence: The Plio-Pleistocene koobi fora sequence, East Africa

Stable isotopic analyses of Plio-Pleistocene and modern sediments in the fluvial-lacustrine system occupying the Turkana Basin, East Africa provide constraints on the paleoenvironmental and diagenetic histories of the Pliocene through the Recent sediments in the basin. The δ13C values for carbonates in lacustrine sediments range from −15 to +22‰ relative to PDB, depending on the varying proportions of CO2 from the atmospheric reservoir and from various metabolic sources. The δ18O values of carbonates in lacustrine sediments indicate that the isotopic composition of paleolake water varied by over 10‰ from the Pliocene to the present. The δ13C values for pedogenic carbonates record paleoccologic variations and suggest that C4 plants did not become well established in the preserved depositional parts of the basin until about 1.8 myr ago. The δ18O values pedogenic carbonates suggest a range of over 10‰ for the isotopic composition of soil water during this interval. They also suggest a period of major climatic instability from about 3.4 to 3.1 myr and at about 1.8 myr. Together, the δ13C and δ18O values of pedogenic carbonates indicate that the present conditions are as arid and hot as any that had prevailed during deposition of these Plio-Pleistocene sediments.

Low-temperature alteration of volcanic glass: hydration, Na, K, 18O and Ar mobility

The low-temperature alteration of siliceous volcanic glass by meteoric water involves considerable hydrogen exchange for Na+ or K+ ions. This can occur with little alteration of the other major- or trace-element composition of the glass. As much as 40% of the alkali sites can be occupied by H+ ion. Most water in these glasses is present as molecular water rather than as hydroxyl groups. This hydration is accompanied by oxygen isotope exchange resulting in an oxygen isotopic enrichment to over 20‰ (relative to SMOW) for this suite of samples. The high degree of alkali mobility during this alteration process affects the KAr age dating method. In these glasses, Ar appears to be less mobile than alkali ions, resulting in discrepant KAr ages for glass with significant hydration and oxygen isotopic exchange.

Creation of a continent recorded in zircon zoning

We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from a lower crustal paragneiss in the Neoarchean Superior province. Detrital igneous zircon cores with δ 18 O values of 5.1‰–7.1‰ record creation of primitive to increasingly evolved crust from 2.85 ± 0.02 Ga to 2.67 ± 0.02 Ga. Sharp chemical unconformity between cores and higher δ 18 O (8.4‰–10.4‰) metamorphic overgrowths as old as 2.66 ± 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 ± 0.01 Ga included ∼80 m.y. of high-temperature (∼700–650 °C), nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512 ± 8 Ma (∼ 600 °C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted regional metamorphism, and the consequent advances now possible in reconstructing planetary dynamics with zircon zoning.

Zircon U-Pb isotope, δ18O and trace element response to 80 m.y. of high temperature metamorphism in the lower crust: Sluggish diffusion and new records of Archean craton formation

Coordinated cathodoluminescence (CL) imaging and ion microprobe (SHRIMP and CAMECA 1280) analysis document micron-scale U-Pb-O isotope and trace element zoning in zircons from deep crust exposed to 80 m.y. of high temperature and pressure metamorphism. Three, along-strike paragneiss samples across the amphibolite to granulite facies transition in the Kapuskasing Uplift crustal cross-section in the Archean Superior province yield detrital, originally igneous zircon cores overgrown by progressively larger volumes of metamorphic zircon with increasing grade. The cores generally retain primary age (2.85±0.03 to 2.67±0.02 Ga), oxygen isotope (5.1 to 7.0‰) and trace element compositions similar to those reported for magmatic arc sources. Dark CL, metamorphic zircon rims record nearly continuous overgrowth events for ∼80 m.y. from 2.66±0.01 to 2.58±0.01 Ga during uppermost amphibolite to granulite facies regional metamorphism. These rims have significantly higher δ18O values (8.4 to 10.4‰) and trace element compositions quite distinct from those of the cores; these differences indicate that their δ18O and trace element compositions were not inherited from the igneous cores, consistent with extensive textural evidence for rim formation as metamorphic overgrowths. Multi-spot traverses record steep oxygen isotope discontinuities (4‰ over <10 μm) at core-rim boundaries, confirming the extremely sluggish rates of volume diffusion of O in non-metamict zircon during extended (∼80 m.y.) granulite-grade metamorphism (peak T=750-800 °C) at substantial f(H2O) but water-undersaturated (fluid-absent) conditions. Likewise no evidence of significant diffusive exchange of δ18O could be detected along deformation microstructures such as annealed fractures in cores infilled with high δ18O zircon. Application of simple diffusion models to detailed δ18O profiles in a large number of zircon grains constrain maximum values of the diffusivity of oxygen in zircon (logDZrcox) to the range −27.5 to −26.4 m2/s. For the estimated 80 m.y. and 700 to 800 °C time-T window of rim formation, these maximum values are similar to or slower than values reported by Page and others (2007, 2010) and the experimentally-determined “dry” diffusivity of oxygen in zircon (Watson and Cherniak, 1997), but are markedly slower than the experimentally-determined “wet” diffusivity of oxygen in zircon (Watson and Cherniak, 1997). Fast diffusion of oxygen in zircon predicted by hydrothermal experiments may, in nature, require the presence of a hydrous fluid rather than a threshold value of f(H2O). Our test demonstrates that unrecrystallized metamorphosed igneous zircons and metamorphic zircons will retain the geochemical (U-Pb age, trace element and δ18O) record of their origin and evolution despite prolonged, high-grade metamorphism at significant f(H2O) but water under-saturated (fluid-absent) conditions. Such zircons, particularly those that exhibit δ18O zoning, are micron-scale records for the T-time-fluid interaction history of deep crustal rocks. Such records will not be preserved in less refractory phases and promise new insights into the processes of continent formation and evolution.

Petrogenesis of the Kirkpatrick Basalt, Solo Nunatak, northern Victoria Land, Antarctica, based on isotopic compositions of strontium, oxygen and sulfur

Chemical and isotopic compositions of Jurassic tholeiites of the Kirkpatrick Basalt Group from Solo Nunatak, northern Victoria Land, indicate that these rocks are contaminated with crustal material. The basalts are fine grained and contain phenocrysts of augite, pigeonite, hypersthene and plagioclase. The flows on Solo Nunatak are chemically more similar to average tholeiite than flows from Mt. Falla and Storm Peak in the Central Transantarctic Mountains (TAM) which appear to be more highly differentiated. Initial 87Sr/86Sr ratios of the flows on Solo Nunatak are high (>0.710) and are similar to those reported for the Kirkpatrick Basalt in the Central TAM. Whole-rock δ18O values are also high, ranging from +6.0 to +9.3‰ and correlate positively with initial 87Sr/86Sr ratios, similar to the Kirkpatrick Basalt in the Central TAM. The correlation between initial 87Sr/86Sr ratios and δ18O values is explained as the result of simultaneous fractional crystallization and assimilation of a crustal contaminant. Sulfur isotope compositions vary between limits of δ34S= -4.01 to +3.41‰ Variations in (δ34S probably resulted from outgassing of SO2 under varying oxygen fugacities.

Chemical and isotopic variations in an iron-rich lava flow from the Kirkpatrick Basalt, north Victoria Land, Antarctica: implications for low-temperature alteration

Chemical and isotopic (Sr, O, H) variations have been examined in an iron-rich lava flow of the Kirkpatrick Basalt from the Mesa Range in north Victoria Land, Antarctica. The flow is homogeneous with respect to the less mobile elements, whereas variations observed in K, Na, Si, Fe, and Rb result largely from alteration of glassy matrix material. Whole-rock Rb−Sr isotope data fall along a poorly-defined 103 Ma array attributed to secondary mobilization of Rb during the mid-Cretaceous. Alteration at that time is suggested by paleomagnetic data and would also account for discordant K−Ar dates. Whole-rock δ18O values vary from +5.8 to +8.2‰ and a plagioclase separate has a δ18O value of +5.6‰, reflecting the original composition of the magma. The range of δ18O values for the whole-rock samples results from low-temperature alteration occurring primarily in the Jurassic and/or mid-Cretaceous. Whole-rock δD values (-201 to -243‰) are markedly depleted, approaching equilibrium with modern meteoric water. In light of these data, variable Sr and O isotopic ratios in the underlying sequence of flows, previously interpreted in terms of an assimilation-fractionation model, may largely reflect post-magmatic alteration.

Iron isotopes constrain the pathways and formation mechanisms of terrestrial oxide concretions: A tool for tracing iron cycling on Mars?

New iron isotope data document open-system formation of terrestrial iron oxide concretions and the potentially important role of iron-reducing bacteria in mobilizing iron. These terrestrial insights can provide valuable models for understanding extraterrestrial hematite spherules and their diagenetic history at Meridiani Planum, Mars. Wholerock samples of Jurassic Navajo Sandstone host rock have δ 56 Fe values near 0 per mil (‰), whereas concretions typically have negative δ 56 Fe values. Negative δ 56 Fe values can be explained by complete oxidation and precipitation from aqueous fl uids that had δ 56 Fe values of −0.5‰ to −1.5‰. The low δ 56