Crayton J. Yapp

CARBON ISOTOPES IN CONTINENTAL WEATHERING ENVIRONMENTS AND VARIATIONS IN ANCIENT ATMOSPHERIC CO2 PRESSURE

Abstract Abundance and carbon isotope data from an Fe(CO3)OH component in apparent solid solution in oolitic goethites have been used to infer ancient atmospheric CO2 pressures. A test of the validity of these estimates might be comparisons of the carbon isotope compositions of Fe(CO3)OH in oolitic goethites with time-equivalent pedogenic calcites. Temporal trends of the oolitic goethite and pedogenic calcite δ13C values are generally similar, but time-equivalent samples from each of these two groups are not common in the existing data. To facilitate discussion of the concept, comparisons were made of available goethite and calcite samples even though ages of the compared samples in each pair were not identical. In four out of the five comparisons, Fe(CO3)OH abundance and δ13C data were combined with pedogenic calcite δ13C data to calculate physically reasonable soil CO2 concentrations for the ancient calcitic soids. This suggests that the compared oolitic goethite and pedogenic calcite systems were responding to the same global scale phenomenon (i.e., atmospheric CO2). Atmospheric PCO2 as determined from the goethites in these four “well-behaved” cases ranged from values indistinguishable from modern (within analytical uncertainty) to values up to approximately 16 time modern (modern atmospheric PCO2 was taken to be 10−3.5 atm). One interpretation of the fifth, “anomalous”, comparison is that atmospheric CO2 levels increased from about 3 times modern to about 18 times modern from the Triassic into the Early Jurassic. This inferred value for the PCO2 of the Early Jurassic atmosphere is not uniquely constrained by the existing data and needs to be substantiated. However, even considerably lower Early Jurassic atmospheric PCO2 values of 6 to 9 times modern (i.e., 1/3 to 1/2 of the estimated value of 18 times modern) would still indicate significant differences between the global carbon cycles then and now. These results highlight the need for more research on the behavior of the atmosphere during and after the Triassic-Jurassic transition.

Oxygen and hydrogen isotope variations among goethites (α-FeOOH) and the determination of paleotemperatures

Abstract A plot of δD vs.δ18O values from a diverse group of natural goethites defines a data array which is approximately parallel to the meteoric water line. The δ18O values range from −9.5 to +4.4 per mil, while the corresponding δD values vary from −202 to −94 per mil. There is considerable scatter in the δD vs.δ18O array, which can be attributed primarily to different temperatures of mineral formation. As determined previously (YAPP and Pedley, 1985), and confirmed in this study, the goethite-water D H fractionation factor is essentially independent of temperature over the range from 25°C to 145°C. However, the goethitewater 18 O 16 O fractionation factor does vary with temperature, as indicated by initial goethite synthesis experiments conducted at 25°C, 44°C and 62°C. The synthetic goethite 18αG–w values suggest that the natural goethites in this study formed at temperatures that may range from about 8°C to 66°C. Support for the validity of these approximate, calculated temperatures is provided by a natural goethite sample from the Atlantis II Deep in the Red Sea. These results suggest that goethite can be used as a single-mineral, low temperature geothermometer. Perhaps more importantly, goethite can be used in low temperature, oxygen isotope mineral-pair geothermometers such as chert-goethite, calcite-goethite, phosphate-goethite, etc.

Oxygen isotopes in iron (III) oxides: 1. Mineral-water fractionation factors

Experimental data obtained in this laboratory indicate that goethite and hematite may be isotopically indistinguishable at equilibrium. The hematite (goethite)-water oxygen isotope fractionation based on synthesis experiments at five temperatures over the range from 25° to 120°C is represented by the expression: 1000 ln 18αmin-H2O=1.63·106T−2−12.3 This equation predicts a “crossover” at 91°C. The hematite (goethite)-water curve of the current work is approximately parallel to the magnetite-water curve of Blattner et al. at temperatures of ≲ 200°C. Furthermore, the hematite (goethite)-water curve suggests that hematite should be enriched in 18O by ∼ 2.5−3‰ relative to coprecipitated magnetite defined by the curve of Blattner et al. This predicted temperature insensitivity of the hematite-magnetite oxygen isotope fractionation and the relative enrichment of 18O in hematite are generally consistent with the characteristics of hematite-magnetite oxygen isotope fractionation noted previously by Clayton and Epstein for natural samples.

Research paperOxygen isotopes in iron (III) oxides: 1. Mineral-water fractionation factors

Experimental data obtained in this laboratory indicate that goethite and hematite may be isotopically indistinguishable at equilibrium. The hematite (goethite)-water oxygen isotope fractionation based on synthesis experiments at five temperatures over the range from 25° to 120°C is represented by the expression: 1000 ln 18αmin-H2O=1.63·106T−2−12.3 This equation predicts a “crossover” at 91°C. The hematite (goethite)-water curve of the current work is approximately parallel to the magnetite-water curve of Blattner et al. at temperatures of ≲ 200°C. Furthermore, the hematite (goethite)-water curve suggests that hematite should be enriched in 18O by ∼ 2.5−3‰ relative to coprecipitated magnetite defined by the curve of Blattner et al. This predicted temperature insensitivity of the hematite-magnetite oxygen isotope fractionation and the relative enrichment of 18O in hematite are generally consistent with the characteristics of hematite-magnetite oxygen isotope fractionation noted previously by Clayton and Epstein for natural samples.

Stable hydrogen isotopes in iron oxides—II. DH variations among natural goethites

Abstract Goethite samples analyzed for this study have a δD range from −202 to −98 per mil with a corresponding δD range of associated waters from about −110 to +7 per mil. Goethites with the most positive δD values are from marine environments. Goethite-water equilibrium D H fractionation factors measured in this laboratory at 100°C and 145°C and estimated for sedimentary temperatures from data on natural samples have values of about 0.900 at all three temperatures. These data suggest that goethite δD values may be a direct, temperature-independent measure of the δD values of the waters with which the goethite last equilibrated. Most of the goethite δD values in this study reflect the δD values of the modern waters in the locales of origin. Substitution of Mn and/or Al for Fe in the goethite structure may affect the mineral-water fractionation factor. Manganese appears to decrease the value of α. The effect of Al substitution on α has not yet been measured, but preliminary arguments suggest that increasing Al content could increase α values.

The carbon isotope geochemistry of goethite (α-FeOOH) in ironstone of the Upper Ordovician Neda Formation, Wisconsin, USA: Implications for early Paleozoic continental environments

Carbon isotope exchange experiments and data from natural samples in a state of isotopic disequilibrium indicate that the apparent Fe(CO3)OH component in natural goethites is a closed system. These results support the solid solution model for Fe(CO3)OH in goethite. The carbon isotope geochemistry of the oolitic Neda Formation ironstone at occurrences in Wisconsin and Iowa is consistent with goethite formation in a Late Ordovician subaerial weathering environment. δ13C values of the Fe(CO3)OH component in Neda Formation goethite indicate that organic matter was being oxidized to produce CO2 in the ancient weathering profile. The δ13C value of this organic matter was about −27‰. At depths greater than about 20 cm, the partial pressure of CO2 in the Late Ordovician weathering profile was 5.6 times larger than the PCO2 of the Earths atmosphere at that time. This high “soil” CO2 partial pressure and its origin in the oxidation of organic matter suggest that there was substantial biological activity on continental land surfaces prior to the widespread colonization by vascular plants. It indicates a possible role for biological activity in the chemical weathering of continents in the early Paleozoic.

Stable carbon isotope budget of CO2 in a wet, modern soil as inferred from Fe(CO3)OH in pedogenic goethite: possible role of calcite dissolution

Abstract δ13C values of the Fe(CO3)OH component in pedogenic goethites from a young soil developed on the Eocene Weches formation in east Texas increase from approximately −13 or −14‰ at depths of 31–64 cm to values around −6 to −4‰ at depths greater than 122 cm. This spatial distribution of δ13C values suggests that dissolution of precursor fossiliferous marine calcite (still present at deeper levels in the soil) has contributed significantly to the isotopic budget of CO2 in this soil. A local isotopic material balance was calculated for the soil CO2 at each sample depth using a calcite δ13C value of −1‰, an organic matter δ13C value of −25‰, and the measured δ13C value of the Fe(CO3)OH component in each sample. Although there is presently no calcite in the upper 120 cm of the profile, the calculated apparent contribution of CO2 from a calcite source ranges from 16% to 45% at these shallow depths. Below 120 cm, dissolution of calcite appears to contribute more than 50% of the CO2 in the soil gas. Similar results might be expected in other wet, goethite-bearing soils that contain relict calcite and thus have not achieved the highly leached characteristics of laterites (such as those in the Amazon basin). Models of the soil CO2 budget in such systems may need to consider both oxidation of organic matter and dissolution of carbonate minerals as in situ sources of carbon isotope variation in CO2. The variation with depth of the δ13C values of the Fe(CO3)OH component suggests an ongoing process of goethite dissolution and reprecipitation in the active, aerobic soil zone. If so, the extremely low solubility of goethite in oxidizing environments suggests that this dissolution process is probably biologically mediated. Dissolution and reprecipitation of goethite in an aerobic soil would favor the recording of steady-state soil CO2 δ13C patterns. Preservation of such information in ancient soils would probably depend upon burial and consequent removal from the biologically active soil zone.

An assessment of isotopic equilibrium in goethites from a bog iron deposit and a lateritic regolith

Abstract Hydrogen and oxygen isotope ratios have been measured in goethites from a lateritic regolith in the eastern Amazon Basin of Brazil and a young bog iron deposit in New Jersey, USA. The presence of exchangeable hydrogen and admixed minerals required the use of material-balance calculations to determine relevant δD and δ18O values. For the Brazilian goethite, δD is −121‰ and δ18O of −1.2‰ (adjusted for Al content). These values yield a calculated temperature of formation of 24 ± 3°C, which is in good agreement with the modern average annual temperature in that locale. Goethite from the New Jersey bog has a δD of −134‰ and δ18O of −0.9‰. Its calculated temperature of formation (23 ± 3°C) resembles summer temperatures in southeastern New Jersey. Bacterially mediated rates of precipitation of ferric hydroxide appear to be higher in the bog during the summer months (Crerar et al., 1979). Thus, the temperature calculated for the New Jersey goethite could indicate that the conversion of ferric hydroxide to goethite also occurs predominantly in the summer. δ13C values of the Fe(CO3)OH component in goethite (−21.7‰ for Brazil and −19.3‰ for New Jersey) reflect the δ13C values of the associated organic carbon (−28.4‰ for Brazil and −26.3‰ for the New Jersey). Values of PCO2 in the regolith (0.063 bar) and bog (0.039 bar) were calculated from the measured abundances of the Fe(CO3)OH component. The values of δ13C and PCO2 obtained from the goethites suggest that the ambient CO2 in these systems originated primarily from oxidized organic matter. In the low-pH, high-PCO2 waters characteristic of bogs and laterites, isotopic equilibrium may be closely approached during the formation of goethite from ferric hydroxide (ferrihydrite), even if the initial precipitation of the ferric hydroxide precursor is bacterially mediated. Therefore, meaningful paleotemperatures might commonly be calculated from stable isotope ratios of nonexchangeable oxygen and hydrogen in geothites from such environments.

A possible goethite-iron (III) carbonate solid solution and the determination of CO2 partial pressures in low-temperature geologic systems

Dehydration-decarbonation, isotopic and mass-balance data suggest that a solid solution between goethite and iron (III) carbonate may exist in natural samples. The following reaction is suggested as a possibly suitable representation of equilibrium in such a system: FeOOH + CO2⇌Fe(CO3)OH The mole fractions of this proposed iron(III) carbonate in the goethites studied thus far range from ∼0.00066 to ∼0.013. With such low concentrations it has been assumed, as a first approximation, that the proposed solid solutions can be considered ideal. A tentative calibration of CO2 isobars for the above reaction has been attempted using measured trapped CO2 and isotopic temperature data for a natural goethite from a documented marine environment, as well as information obtained from the literature. The calibration indicates a range of CO2 partial pressures of ∼0.003-0.1 bar for the natural non-marine goethites of this study. This range of PCO2 for the goethites compares to a calculated range of PCO2 of ∼0.00032-0.08 bar in modern groundwaters in limestones. Thus, although the calibration of the proposed goethite-iron(III) carbonate PCO2 indicator is tentative, it appears to be yielding results which are geologically reasonable. Much more work (including long-term laboratory syntheses of goethite under controlled PCO2 conditions) needs to be done to substantiate the solid-solution model proposed here. However, the preliminary ideas and results presented in this paper suggest that the iron oxyhydroxide, goethite, may be a source of quantitative information on CO2 partial pressures in low-temperature geologic systems.

Carbon in natural goethites

Abstract Carbon, as a possibly “indigenous” minor element impurity in goethites, appears to exist in two principal forms: 1. (1) CO2 trapped in the mineral structure and 2. (2) organic matter. With one exception the total carbon content of the samples in this study ranges between 0.2 and 2.0 weight percent (reported as CO2). Total carbon δ13C values have a range from −27.2 to −8.1 per mil. Values of the mole fraction of trapped CO2 in the goethite total carbon as determined for seven samples vary from 0.11 to 0.50. Much of the variation of total carbon δ13C values in these samples can be attributed to different trapped CO2/organic matter ratios in the different goethites. By analogy with speleothems and soil carbonates, δ13C values of trapped CO2 could indicate whether the environment of goethite formation was relatively open or closed to mixing with CO2 from the Earths atmosphere. In goethite samples subjected to H2O2 treatment, calculated organic matter δ13C values range from about −35 to −24 per mil. These values probably reflect a predominance of organic matter derived from C3 plants in this particular sample population. 13 C 12 C ratios in organic matter associated with goethite could be a source of information on the Earths ancient biosphere.