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Published (U-Th)/He ages indicate that an oolitic Miocene channel iron deposit (CID) on Mesa J in the Hamersley province of Western Australia (∼22 °S latitude) contains three predominant generations of Fe (III) oxides – two generations of older hematite and one of younger goethite. Selective dissolution and sequential atom balance calculations were combined with chemical and oxygen isotope analyses to determine the δ18O values of hematite, goethite, and quartz in eight samples from a core drilled in that CID. For the quartz (Q), δ18OQ = +21.7(±0.9)‰. For the younger hematite (Hm1), δ18OHm1 ≥ +3.6(±0.6)‰, whereas for the older hematite (Hm2), δ18OHm2 ≤ +2.1(±0.8)‰. The oolitic CID goethite (G) crystallized in the late Miocene [7(±1) Ma] and is a solid solution represented as Fe(1−Y)AlY(CO3)XO(1−X)OH with values of δ18OG that range from −0.4‰ to +0.7‰. A mixing model for the solid solution expresses the value of δ18OG in terms of Y, X, relevant fractionation factors, and the δ18O of the endmember FeOOH (δ18Ogt). Mixing model-derived values of δ18Ogt range from −2.4‰ to −0.7‰. Paired values of δDgt and δ18Ogt in seven of eight late Miocene oolitic goethites from Mesa J indicate crystallization from meteoric waters at an average temperature of 20(±5) °C. In the late Miocene, Mesa J was in the subtropics at a paleolatitude of ∼29 °S, which invites comparison with the climate of the near-coastal community of Mingenew in Western Australia (presently at 29 °S latitude). The modern MAT at Mingenew is 20 °C – i.e., indistinguishable from the late Miocene Mesa J average of 20 °C. However, this similarity is not matched by the mean annual precipitation (MAP). The modern MAP at Mingenew is about 400 mm, but the lateritic weathering that produces abundant pedogenic Fe (III) oxides appears to require a seasonally contrasting climate with rainfall totals of at least 1300 mm/yr. In the late Miocene, southward-shifted summer season tropical cyclones might have delivered such large amounts of rain (with relatively low δD and δ18O values). If so, the δDgt and δ18Ogt of the oolitic goethites of Mesa J support the idea of a rainfall-driven, possibly microbially mediated, summer season bias in the timing of crystallization. Thermal buffering in the regolith can reconcile summer-dominated goethite crystallization with a temperature that would be analytically indistinguishable from the MAT. Comparison of the δ18Ogt values of the Mesa J goethites with published δ18O values of goethites of similar age from two other sites in the Hamersley province suggest a possible increase in δ18O of late Miocene rainfall with increasing distance from the coast.

The channel iron deposits (CID) of the Hamersley Province in Western Australia are dominated by pedogenic goethite/hematite-rich ooids and pisoids that were transported to, and deposited in, the meandering channels of Miocene rivers. Information about the Miocene weathering environments that produced the Fe(III) oxides is archived in the mole fraction (X) and δ13C of the Fe(CO3)OH component in solid solution in oolitic CID goethite (α-FeOOH). Values of X and δ13C were measured for 12 oolitic goethite samples from different depths in two cores drilled in CID of the Robe Formation of Mesa J. The weighted-average plateau values of X ranged from 0.0098 to 0.0334, which suggest ambient CO2 concentrations that ranged from ∼50,000 ppm V to perhaps as much as ∼200,000 ppm V at the time of goethite crystallization. In a vadose zone characterized by in situ production of CO2 with steady-state Fickian diffusive transport of the gas, such concentrations would correspond to modeled soil respiration rates (Q) ranging from about 10 to 30 mmol/m2/h. Values for Q of about 10 mmol/m2/h are reported for soils in modern tropical forests with MAP ≥ ∼2000 mm. However, model-derived values of Q that exceed 15 mmol/m2/h are larger than observed in modern systems. This could indicate that some of the CID goethites crystallized in conditions that were phreatic or near phreatic rather than vadose. The δ13C values of the Fe(CO3)OH component in these 12 CID samples ranged from −24.0‰ to −22.3‰, which are among the most negative measured to date. If they reflect steady-state diffusive transport of CO2 in vadose environments, the soil CO2 would have been derived from a source with δ13C values that ranged from ∼−31‰ to −29‰. If, on the other hand, the goethites crystallized in a nearly phreatic environment that was moderately acidic, the inferred δ13C of the ancient CO2 source would have been about −27.6‰ to −25.8‰. In either case, the δ13C values point to in situ oxidation of C3 organic matter as the predominant source of the ambient CO2. The Fe(III) oxides in the CID ooids suggest crystallization in aerobic environments. However, even in aerobic environments, many microbial species can reduce the Fe3+ in oxides to relatively soluble Fe2+ and may have facilitated progressive Fe enrichment during multiple cycles of Fe(III) oxide dissolution and recrystallization. At the same time, microbially mediated oxidation of organic matter could have produced the high concentrations of soil CO2 with the very negative δ13C values recorded in the Fe(CO3)OH component in oolitic goethite. More frequent summer storms in the Miocene, may have been a significant factor in forming and eroding these soil systems and in concentrating large volumes of oolitic Fe(III) oxides in the local river systems to form channel iron deposits. However, published (U-Th)/He ages indicate that the oolitic CID goethites of Mesa J became closed systems after ∼7 Ma, which suggests a change in local climate and/or conditions of burial at about that time in the Miocene.

Nineteen (U-Th)/He ages were determined for eight samples from a core drilled in an ore-grade channel iron deposit (CID) of the Robe Pisolite (Robe Formation) of Mesa J in Western Australia. With one exception, uncorrected ages of the analyzed aliquots range from 6.7(±0.4) Ma to 30.2(±3.1) Ma, while molar ratios of Th/U range from 0.42 to 5.06. The exception is an aliquot with an apparent age of 2.7 Ma and Th/U of 5.70. A three-component mixing model involving one generation of goethite and two generations of hematite suggests that the age of crystallization of the oolitic goethites is ∼7(±1) Ma. If so, the goethites have effectively been closed systems for ∼7 million years and should preserve a stable hydrogen isotope record of late Miocene rainfall in the vicinity of Mesa J. Cenozoic movement of the Australian continent had placed Mesa J and environs in the subtropics at a paleolatitude of about 29 °S during the late Miocene. Al-adjusted δD values of oolitic goethite in the eight CID samples range from −153‰ to −146‰ and imply that the δD of the late Miocene meteoric waters ranged from −61‰ to −53‰, with an average of −56‰. These relatively negative δD values might indicate that near-coastal, late Miocene rain was derived primarily from summer-season tropical cyclones with storm tracks that extended into the subtropics of western Australia. The postulated late Miocene tropical cyclones would have occurred more often and/or exhibited greater intensity at a paleolatitude of 29 °S than is the case for modern sites at approximately 30 °S on the west coast of Australia (e.g., Perth). Higher fluxes of meteoric water in the Miocene summers would have facilitated dissolution and removal of BIF-sourced silica with concomitant enrichment in oxidized Fe. Moreover, wetter late Miocene summers could have promoted multiple cycles of microbially mediated dissolution and recrystallization of Fe(III) oxides in the aerobic systems. The oolitic textures may be indicative of such recycling. However, the oolitic goethites of Mesa J were closed systems after ∼7 Ma. Therefore, the climate in the vicinity of Mesa J seems to have changed in the late Miocene to conditions that did not favor widespread recycling of Fe (III) oxides—perhaps changing from seasonally wet to the modern dry climate.

The initial ∼60% of an isothermal vacuum dehydration of goethite can commonly be approximated by first order kinetics. Also, natural goethites contain small amounts of an Fe(CO3)OH component in apparent solid solution. The 18O/16O of CO2 evolved from the Fe(CO3)OH during isothermal vacuum dehydrations is related to the 18O/16O of the goethite by an apparent fractionation factor (αapp) that is, in turn, correlated with a first order rate constant, |m|. A kinetic exchange model predicts that αapp should decrease as |m| increases for a range of |m| that corresponds to relatively slow rates of dehydration. This pattern has been observed in published results. In contrast, for rapid rates of dehydration, αapp is predicted to increase with increasing |m|. Isothermal vacuum dehydrations of two natural goethites had unusually large values of |m| and provided serendipitous tests of this rapid-rate prediction. For these experiments, the measured values of αapp were consistent with patterns of variation predicted by the model. This allowed an estimate of the activation energy (E2) of a model parameter, K2, which is the rate constant for oxygen isotope exchange between CO2 and H2O during the solid-state goethite to hematite phase transition. The estimated value of E2 is only ∼9 kJ/mol. Heterogeneous catalysis tends to decrease the activation energies of gas reactions. Consequently, the inferred value of E2 suggests that goethite and/or hematite catalyze oxygen isotope exchange between CO2 and H2O during the solid-state phase change. Yield, δ13C, and δ18O values are routinely measured for increments of CO2 evolved from the Fe(CO3)OH component during isothermal vacuum dehydration of goethite. Model-predicted values of αapp can be combined with plateau δ18O values of the evolved CO2 to estimate the δ18O of the goethite with a less than optimal, but potentially useful, precision of about ±0.8‰. Therefore, a single analytical procedure (incremental dehydration) applied to one aliquot of a sample could provide not only δ13C and mole fractions (X) of the Fe(CO3)OH component, but also hydrogen yield, δD, and the approximate δ18O value of the goethite. Recovery of multiple types of geochemical data from a single aliquot would be particularly useful if the amount of sample available for analysis were limited. Also, the method could be used to estimate the δ18O value of goethite in mixtures of minerals not amenable to selective dissolution – e.g., goethite admixed with hematite.

Measurements of the fractional abundance (m) of high temperature nonstoichiometric (HTN) hydrogen in goethite are shown to predict the reduction in O2 yield associated with removal of HTN oxygen by routine pre-treatment of a sample with BrF5 at 22 °C. The predicted relationship between smaller O2 yields and m is linear and defines an “oxygen yield limit” (OYL), which is introduced for use in oxygen isotope analyses of goethite. The OYL sets lower limits for O2 yields that would be consistent with: (1) removal of all HTN oxygen during pre-treatment and (2) complete recovery of the goethite structural oxygen during subsequent reaction with BrF5 at 450 °C. Measured O2 yields that are less than those predicted by the OYL are anomalously low and indicate probable partial loss of structural oxygen during pre-treatment at 22 °C. After such partial loss, measured δ18O values of the residue are more negative than the actual δ18O of the total structural oxygen. Therefore, the OYL can be used to distinguish between robust and suspect δ18O analyses of goethite. This OYL criterion was applied to some newly analyzed synthetic goethites in which Al substitutes for Fe up to mole fractions (XAl) of ∼0.13. As expected, the robust δ18O values indicate that 1000 ln 18α (where, 18α = [18O/16Omineral]/[18O/16Owater]) increases with increasing XAl, but comparison of these results with the predictions of a published solid solution model suggests that the relationship between 1000 ln 18α and XAl is sensitive to very high pH and possibly to the concentration of aqueous Cl− during synthesis of goethite. The cause of the apparent sensitivity to Cl− remains unknown. The various relationships are: equation(A) T=42°C,pH=1.5–2.5,[Cl-]≈0.16M:1000ln18α=18±5XAl+1.8±0.3 equation(B) low-pH model,[Cl-]=0.00M:1000ln18α=13(±2)XAl+4.1(±0.3) equation(C) T=22°C,pH=14,[Cl-]=0.00M:1000ln18α=5(±3)XAl+1.6(±0.3) equation(D) low-pH model,[Cl-]=0.00M:1000ln18α=13(±2)XAl+6.5(±0.3) However, for natural freshwater goethites, the low-pH, no-Cl, solid solution model (e.g., isothermal equations B and D) best predicts the oxygen isotope effects associated with substitution of Al for Fe. Thus, for goethites from freshwater systems with acidic to circum-neutral pH, the following expression can be used to calculate end member goethite (FeOOH) δ18O values from the measured values of δ18O and XAl: δ18OFeOOH=δ18Omeasured-13(±2)XAlδ18OFeOOH=δ18Omeasured-13(±2)XAl Because the slope of the preceding expression appears to be relatively insensitive to temperature, the equation should be applicable over the range of temperatures at which goethite commonly crystallizes in natural systems (0 °C ⩽ T ⩽ ∼70 °C). However, the value of δ18OFeOOH is sensitive to temperature via the temperature dependence of 1000 ln 18α for pure goethite at acidic to circum-neutral pH in freshwaters as follows (T in degrees Kelvin): 1000ln18α=1.66×106T2-12.6 The results of this study affirm the need to correct measured δ18O values of goethite for the effects of substituent Al to obtain useful paleoenvironmental information, and they support the existing model for doing so.

Goethite (Ax-2) from Axel Heiberg Island (∼80°N) on the margin of the Arctic Ocean is the dominant mineral in a sample of “petrified” Eocene wood, but U, Th, and He measurements suggest that the goethite (α-FeOOH) crystallized in the latest Miocene/Pliocene (ca. 5.5 to 2.8 Ma). Measured δD and δ18O values of Ax-2 are −221 (±6)‰ and −9.6 (±0.5)‰, respectively. The inferred δD and δ18O values of the ancient water were about −139‰ and −18.6‰, respectively, with a calculated temperature of crystallization of 3 (±5)°C, which compares with the modern summer (J-J-A) temperature of 3 °C and contrasts with a modern MAT of −19 °C. Published results from various biological proxies on nearby Ellesmere Island indicate a Pliocene (∼4 Ma) MAT of either −6 or −0.4 °C and corresponding seasonal amplitudes of about 18 or 13 °C. A conductive heat flow model suggests that a temperature of 3 °C could represent goethite crystallization at depths of ∼100–200 cm (for MAT = −6 °C) or ∼250–450 cm (for MAT = −0.4 °C) over seasonally restricted intervals of time. The δ18O value of the Ax-2 water (−18.6‰) is more positive than the modern J-J-A precipitation (−22‰). In combination, the paleotemperatures and δ18O values of ancient waters (from Ax-2 and published results from three Eocene or Pliocene proxy sites on Axel Heiberg and Ellesmere Islands) are consistent with a warm season bias in those isotopic proxies. The results are also consistent with higher proportions of J-J-A precipitation in the annual total. If so, this emphasizes the importance of seasonality at high latitudes even in times of warmer global climates, and suggests that the Arctic hydrologic cycle, as expressed in the seasonal distribution and isotopic composition of precipitation (perhaps modified by a warmer Arctic Ocean), differed from modern. The δ13C value of the Fe(CO3)OH component in the Ax-2 goethite is +6.6‰, which is much more positive than expected if crystallizing goethite incorporated CO2 derived primarily from oxidation of relict Eocene wood with δ13C values of about −24‰. This apparent paradox may be resolved if the goethite is a product of oxidation of 13C-rich siderite, which had previously replaced wood in an Eocene methanogenic burial environment. Thus, the goethite retains a carbon isotope “memory” of a diagenetic Eocene event, but a δD and δ18O record of the latest Miocene/Pliocene Arctic climate.

The isotopic composition of land snail shells was analyzed to investigate environmental changes in the eastern Canary Islands (28–29°N) over the last ~ 50 ka. Shell δ13C values range from −8.9‰ to 3.8‰. At various times during the glacial interval (~ 15 to ~ 50 ka), moving average shell δ13C values were 3‰ higher than today, suggesting a larger proportion of C4 plants at those periods. Shell δ18O values range from −1.9‰ to 4.5‰, with moving average δ18O values exhibiting a noisy but long-term increase from 0.1‰ at ~ 50 ka to 1.6–1.8‰ during the LGM (~ 15–22 ka). Subsequently, the moving average δ18O values range from 0.0‰ at ~ 12 ka to 0.9‰ at present. Calculations using a published snail flux balance model for δ18O, constrained by regional temperatures and ocean δ18O values, suggest that relative humidity at the times of snail activity fluctuated but exhibited a long-term decline over the last ~ 50 ka, eventually resulting in the current semiarid conditions of the eastern Canary Islands (consistent with the aridification process in the nearby Sahara). Thus, low-latitude oceanic island land snail shells may be isotopic archives of glacial to interglacial and tropical/subtropical environmental change.

From July to November 2009, concentrations of CO2 in 78 samples of ambient air collected in 18 different interior spaces on a university campus in Dallas, Texas (USA) ranged from 386 to 1980 ppm. Corresponding δ13C values varied from −8.9‰ to −19.4‰. The CO2 from 22 samples of outdoor air (also collected on campus) had a more limited range of concentrations from 385 to 447 ppm (avg. = 408 ppm), while δ13C values varied from −10.1‰ to −8.4‰ (avg.=-9.0‰). In contrast to ambient indoor and outdoor air, the concentrations of CO2 exhaled by 38 different individuals ranged from 38,300 to 76,200 ppm (avg. = 55,100 ppm), while δ13C values ranged from −24.8‰ to −17.7‰ (avg. = −21.8‰). The residence times of the total air in the interior spaces of this study appear to have been on the order of 10 min with relatively rapid approaches (∼30 min) to steady-state concentrations of ambient CO2 gas. Collectively, the δ13C values of the indoor CO2 samples were linearly correlated with the reciprocal of CO2 concentration, exhibiting an intercept of −21.8‰, with r2 = 0.99 and p The average concentration of outdoor Dallas atmospheric CO2 was ∼17 ppm higher than the average of CO2 concentrations measured on the same campus 10 a ago. In addition, Dallas outdoor CO2 concentrations at both times were higher than the contemporaneous global atmospheric CO2 concentrations. This observation, plus the fact that the increase of ∼17 ppm in the average concentration of Dallas outdoor CO2 was comparable to the global increase of ∼18 ppm over the same 10-a interval, is consistent with a significant role for urban CO2 “factories” in the global atmospheric CO2 budget.

Kaolinite, gibbsite and quartz are the dominant minerals in samples collected from two outcrops of a Cenomanian (∼95 Ma) laterite in southwestern Minnesota. A combination of measured yields and isotope ratios permitted mass balance calculations of the δ D and δ 18 O values of the kaolinite in these samples. These calculations yielded kaolinite δ D values of about −73‰ and δ 18 O values of about +18.7‰. The δ D and δ 18 O values appear to preserve information on the ancient weathering system. If formed in hydrogen and oxygen isotope equilibrium with water characterized by the global meteoric water line (GMWL), the kaolinite δ D and δ 18 O values indicate a crystallization temperature of 22 (±5) °C. A nominal paleotemperature of 22 °C implies a δ 18 O value for the corresponding water of −6.3‰. The combination of temperature and meteoric water δ 18 O values is consistent with relatively intense rainfall at that mid-paleolatitude location (∼40°N) on the eastern shore of the North American Western Interior Seaway. The inferred Cenomanian paleosol temperature of ∼22 °C is in general accord with published mid-Cretaceous continental mean annual temperatures (MAT) estimated from leaf margin analyses of fossil plants. When compared with results from a published GCM-based Cenomanian climate simulation which specifies a latitudinal sea surface temperature (SST) gradient that was either near modern or smaller-than-modern, the kaolinite paleotemperature of 22 °C is closer to the GCM-predicted MAT for a smaller equator-to-pole temperature difference in the mid-Cretaceous. Moreover, the warm, kaolinite-derived, mid-paleolatitude temperature of 22 °C is associated with proxy estimates of high concentrations of atmospheric CO 2 in the Cenomanian. The overall similarity of proxy and model results suggests that the general features of Cenomanian continental climate in that North American locale are probably being revealed.

An Early Eocene Oxisol in the Ione Formation of California formed in a coastal continental weathering environment at a paleolatitude of ∼38°N. The dominant minerals in the Oxisol are goethite, quartz, and kaolinite. Material balance calculations were applied to new measurements of chemical composition, D/H, and 18 O/ 16 O ratios of Oxisol samples to determine the δD (−150 ± 3‰) and δ 18 O (−2.4 ± 0.3‰) values of the goethite (α-FeOOH). These data, in combination with the global meteoric water line (MWL), yielded an isotopic temperature of 21(±4) °C. The nominal value of 21 °C contrasts with the modern mean annual temperature (MAT) of 16 °C in that area. The warmer temperature is consistent with formation of the goethite during the Early Eocene climatic optimum. The isotopic composition of the goethite and a temperature of 21 °C imply ancient water with a δD value of −61(±4)‰ and a δ 18 O value of −8.9(±0.5)‰. This Early Eocene δ 18 O (or δD) value is more negative than values in the range of isotopic scatter observed for modern global precipitation at sites with a MAT of 21 °C. At times of warm global climates, the location of a near-surface atmospheric isotherm would generally shift relative to its location under modern climatic conditions. A simple Rayleigh-type condensation model indicates that, if one “follows the isotherm”, the associated scatter in δD and δ 18 O of precipitation in very warm global climates should shift (for a given isotherm) to more negative values that may be detectable in proxy records. The isotopic results from the goethite of the Early Eocene Oxisol appear to add to evidence in support of this idea.

Twelve goethite samples with different degrees of substitution of Al for Fe were synthesized at 22°C to 48°C and pH values of 1.5 to 14 under closed system conditions and used to study the effects of Al substitution on the hydrogen isotopic fractionation between goethite and its ambient water. The syntheses followed two pathways: 1) Fe 3+ hydrolysis in high pH aqueous solutions; 2) oxidation of Fe 2+ to Fe 3+ in mid to low pH solutions. XRD and SEM analyses indicated that, irrespective of temperature and pH, goethite was the predominant product of the syntheses in all of the experiments (with degrees of Al substitution as high as ~13 mole percent). “High temperature nonstoichiometric” (HTN) water is present in all of the samples and rapidly exchanges D/H with ambient vapor at room temperature. Uncertainties in the value of the apparent D/H fractionation factor ( e-v) between HTN water and ambient exchange water at 22°C lead to significant uncertainties in determinations of the D values of structural hydrogen ( Ds) in goethites which contain high proportions of HTN water. As determined for the samples of this study, e-v has a nominal value of 0.942 (±0.02). Ds values determined using an e-v value of 0.942 indicate that Al substitution increases the D value of structural hydrogen in goethite by about 1.4 (±0.4) ‰ for each increase in Al of 1 mole %. This dependence on Al is of the same sign as, but somewhat larger in magnitude than, the effect of Al predicted by a published model (~0.7‰ per mole % Al). The overall uncertainties in the current results suggest that an increase of ~1‰ per mole % Al, as adopted by previous studies, may be a reasonable estimate with which to adjust Ds values of natural goethites to those of the pure FeOOH endmember and could be valid for degrees of Al substitution of up to at least 15 mole %. These synthesis experiments also yield a hydrogen isotopic fractionation factor ( D G-W) between pure goethite ( -FeOOH) and liquid water of 0.900 (±0.006), which is analytically indistinguishable from the published value of 0.905 (±0.004). Thus, use of an D G-W value

Goethite synthesis experiments indicate that, in addition to temperature, pH can affect the measured value of the 18O/16O fractionation factor between goethite and water (αG–W). A simple model was developed which expresses αG–W in terms of kinetic parameters associated with the growth of goethite from aqueous solution. The model predicts that, at a particular temperature, the range of pH over which αG–W changes as pH changes is expected to be comparatively small (∼3 pH “units”) relative to the range of pH values over which goethite can crystallize (pH from ∼1 to 14). Outside the range of sensitivity to pH, αG–W is predicted to be effectively constant (for constant temperature) at either a low-pH αG–W value or a high-pH αG–W value. It also indicates that the values of αG–W at high pH will be disequilibrium values. Values of αG–W for goethite crystallized at low pH may approach, but probably do not attain, equilibrium values. For goethite synthesized at values of pH from ∼1 to 2, data from two different laboratories define the following equation for the temperature dependence of 1000 ln αG–W (T in degrees Kelvin) equation(IV) 1000lnαG–W=1.66×106T2-12.6 Over the range of temperatures from 0 to 120°C, values of 1000 ln αG–W from Eq. (IV) differ by ⩽0.1‰from those of a published equation [Yapp C.J., 1990. Oxygen isotopes in iron (III) oxides. 1. Mineral–water fractionation factors. Chem. Geol.85, 329–335]. Therefore, interpretations of data from natural goethites using the older equation are not changed by use of Eq. (IV). Data from a synthetic goethite suggest that the temperature dependence of 1000 ln αG–W at low pH as expressed in Eq. (IV) may be valid for values of pH up to at least 6. This result and the model prediction of an insensitivity of αG–W to pH over a larger range of pH values could explain the observation that Eq. (IV) yields values of αG–W which mimic most 18O/16O fractionations measured to date in natural goethites.

A Late Paleocene (∼60 Ma BP) lateritic soil from Northern Ireland (the Antrim paleosol, herein referred to as Nire) contains coexisting goethite, gibbsite, phyllosilicate, and hematite. The Fe(III) oxides exhibit pisolitic and Liesegang-type morphologies that are mutually exclusive in hand specimens. X-ray diffraction (XRD) measurements of Al substituted for Fe in goethite indicate two populations: (1) low-Al, Liesegang-type goethites (∼0 mol% Al) and (2) high-Al, pisolitic goethites (∼9 to ∼24 mol% Al). Selective dissolution and incremental vacuum dehydration-decarbonation were used to determine the concentration and δ13C values of CO2 occluded in the respective structures of the goethites and gibbsites in this complex mixture of Nire lateritic minerals. The Fe(CO3)OH component in the high-Al goethites appears to retain a proxy carbon isotopic record of vadose zone CO2 in the ancient soil. The δ13C values of CO2 occluded in coexisting goethites and gibbsites indicate that these minerals did not form in equilibrium with the same environmental CO2. The measured mole fractions (X) of Fe(CO3)OH in the high-Al goethites range from 0.0059 (±0.0005) to 0.0077 (±0.0006) and correspond to soil CO2 concentrations of ∼28,000 to ∼37,000 ppmV. The average values of X and δ13C for the four high-Al goethites are 0.0067 ± 0.0007 and −20.1 ± 0.5‰, respectively. The δ13C value of the organic matter undergoing oxidation in this midlatitude (∼55°N) Late Paleocene soil appears to have been ∼ −28.2‰. Taken together, these data indicate an atmospheric CO2 concentration of ∼2400 ppmV (± ∼1200 ppmV) at ∼60 Ma BP. The inferred high concentration of atmospheric CO2 would have been coincident with the warm global climate of the Late Paleocene and is consistent with the idea that CO2 plays an important role in climate variation.

Incremental vacuum dehydration-decarbonation experiments were performed at 190°C on chemically “cleaned” aliquots of a gibbsite-dominated, Eocene-age bauxite sample with evolution of CO 2 and H 2 O. “Plateau” F (CO 2 /H 2 O ratios) and δ 13 C values of the CO 2 derived from gibbsite were attained over the dehydration interval, X v (H 2 ) = 0.16 to 0.67 (i.e., 16 to 67% breakdown of gibbsite). The plateau value of F for gibbsite was 0.0043 ± 0.0003, while the corresponding δ 13 C value of evolved CO 2 was −16.0‰±0.4‰. Additional experiments on chemically cleaned aliquots included (1) treatment with a solution of 0.3M Na-Citrate + 0.1M Na-Dithionite and (2) an exchange experiment with 0.1 bar of 13 C-depleted CO 2 (−46‰) at 105°C for 64.5 h. Neither of these additional treatments resulted in a measurable perturbation of plateau values of F or δ 13 C for CO 2 evolved from gibbsite during dehydroxylation. These results support published work on Holocene samples which suggested that CO 2 occluded in gibbsite may preserve information on δ 13 C values of CO 2 in ancient terrestrial systems. The plateau values of F observed in the Eocene gibbsite indicate that it may be possible to experimentally calibrate a relationship between the concentration of CO 2 occluded in gibbsite and CO 2 in the environment at the time of crystallization. Such a calibration would significantly enhance the value of gibbsite as a source of information on ancient oxidized carbon systems.

13C/12C and 18O/16O ratios of aragonite shells of modern land snails from the southern Great Plains of North America were measured for samples from twelve localities in a narrow east–west corridor that extended from the Flint Hills in North Central Oklahoma to the foothills of the Sangre de Cristo Mountains in Northern New Mexico, USA. Across the study area, shell δ18O values (PDB scale) ranged from −4.1‰ to 1.2‰, while δ13C values ranged from −13.2‰ to 0.0‰. δ18O values of the shell aragonite were predicted with a published, steady state, evaporative flux balance model. The predicted values differed (with one exception) by less than 1‰ from locality averages of measured δ18O values. This similarity suggests that relative humidity at the time of snail activity is an important control on the δ18O values of the aragonite and emphasizes the seasonal nature of the climatic information preserved in the shells. Correlated δ13C values of coexisting Vallonia and Gastrocopta suggest similar feeding habits and imply that these genera can provide information on variations in southern Great Plains plant ecology. Although there is considerable scatter, multispecies, transect average δ13C values of the modern aragonite shells are related to variations in the type of photosynthesis (i.e., C3, C4) in the local plant communities. The results of this study emphasize the desirability of obtaining isotope ratios representing averages of many shells in a locale to reduce possible biases associated with local variations among individuals, species, etc., and thus better represent the “neighborhood” scale temporal and/or spatial environmental variations of interest in studies of modern and ancient systems.

A simple flux balance model with a diffusive, evaporative boundary layer indicates that the time constant (characteristic time) for approach to oxygen isotope steady state in the body fluid of land snails is ∼19 min or less. These comparatively short times support an assumption that the snail’s aragonitic shell is commonly precipitated from a body fluid that is at, or near, isotopic steady state. The model indicates that the steady-state δ18O value of snail shell carbonate depends upon the temperature, relative humidity, δ18O of the input liquid water, and δ18O of ambient water vapor. Model shell δ18O values were calculated for the warm, wet months corresponding to times of snail activity at some European sites. Linear regression of these predicted values against published, measured values yielded the expression: δ18Ocalc = 0.93(±0.13) δ18Omeas −0.9(±0.2), with r2 = 0.65. As indicated by the value of r2, there is scatter in the relationship, but the slope and intercept are close to one and zero, respectively, which lends credence to the model. Therefore, temporal or spatial changes recorded in the δ18O values of land snail shells appear to be selectively seasonal—commonly the warm, wet months—and include the effects of relative humidity. For carbon, the time constant for approach to isotopic steady state in the bicarbonate dissolved in the body fluid of land snails is predicted to be ∼16 min or less. New and published δ13C measurements of aragonite shell and associated organic matter exhibit an overall correlation, but with considerable scatter. As noted by previous workers, 13C-rich dietary “limestone” may account for some of the scatter. Additional scatter, according to the model presented herein, could arise from changes in the proportion of total oxidized carbon that is expelled by the snail as bicarbonate dissolved in body fluid (i.e., effects of relative changes in metabolic rates). These results affirm the need for caution in the interpretation of δ13C values of land snail aragonite shells solely in terms of dietary proportions of C3 and C4 plants.

Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO2 components—atmospheric CO2 and CO2 from in situ oxidation of organic matter. The δ13C values measured for the Fe(CO3)OH component in solid solution in these Permian goethites are −13.5‰ for the Lower Leonardian (∼283 Ma BP) paleosol (MCGoeth) and −13.9‰ for the Upper Leonardian (∼270 Ma BP) paleosol (SAP). These goethites contain the most 13C-rich Fe(CO3)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO2 components. δ13C measured for 43 organic matter samples in the Lower Leonardian (Waggoner Ranch Fm.) has an average value of −20.3 ± 1.1‰ (1s). The average value yields a calculated Early Permian atmospheric P co 2 value of about 1 × PAL, but the scatter in the measured δ13C values of organic matter permits a calculated maximum P co 2 of 11 × PAL (PAL = present atmospheric level). Measured values of the mole fraction of Fe(CO3)OH in MCGoeth and SAP correspond to soil CO2 concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO2 concentrations are similar to modern soils in warm, wet environments. The average δ13C values of pedogenic calcite from 9 paleosol profiles stratigraphically associated with MCGoeth (Waggoner Ranch Fm.) range from −6.5‰ to −4.4‰, with a mean δ13C value for all profiles of −5.4‰. Thus, the value of Δ13C between the pedogenic calcite data set and MCGoeth is 8.1 (±0.9)‰, which is in reasonable accord with the value of 7.7‰ expected if atmospheric P co 2 and organic matter δ13C values were the same for both paleosol types. Furthermore, the atmospheric P co 2 calculated for the Early Permian from the average measured carbon isotopic compositions of the paleosol calcite and organic matter is also analytically indistinguishable from 1 × PAL, with a maximum calculated atmospheric P co 2 (permitted by one standard deviation of the organic matter δ13C value) of ∼5 × PAL. If, however, measured average δ13C values of the plant organic matter are more positive than the original soil organic matter as a result of diagenetic loss of 13C-depleted, labile organic compounds, calculated Permian atmospheric P co 2 using these 13C-enriched organic values would underestimate the actual atmospheric P co 2 using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO2 concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO2. The Fe(CO3)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in 13C relative to Fe(CO3)OH in goethites from soils in which there are mixtures of two isotopic CO2 components. Field-relationships and the δ13C value (−1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO2 components at the time of goethite crystallization. The three components were probably atmospheric CO2, CO2 from in situ oxidation of organic matter and CO2 from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO2 components, as recorded by Fe(CO3)OH in goethite, has been described in modern soil, this is the first example from a documented paleosol. Its preservation affirms the need for careful, case-by-case assessment of ancient paleosols to establish that goethite in any particular soil is likely to be a valid proxy of atmospheric P co 2.

Measured mole fractions (X) and δ13C values of the Fe(CO3)OH component in pedogenic goethite from a mid-latitude Oxisol of Early Eocene age (≈52 Ma B.P.) range from 0.0014 to 0.0064 and −20.1 to −15.4‰, respectively. These values of X imply that concentrations of CO2 gas in the paleosol were ≈7400 to ≈34,000 ppm. δ13C and 1/X are correlated and define a linear, soil-CO2 diffusive mixing line with a positive slope. Such positive slopes are characteristic of mixing of two isotopically distinct CO2 endmembers (atmospheric CO2 and CO2 from oxidation of soil organic matter). From the intercept of the mixing line, it is calculated that the δ 13C value of organic matter in the ancient soil was ≈−28.0‰. The magnitude of the slope implies an Early Eocene atmospheric CO2 concentration of ≈2700 ppm. A simple model for forest soils suggests that a “canopy effect” may cause atmospheric CO2 concentrations deduced from pedogenic minerals to underestimate the actual concentrations of atmospheric CO2. If a significant forest canopy were present at the time of formation of pedogenic goethite in the Ione Fm, the concentration of 2700 ppm calculated for atmospheric CO2 could be slightly low, but the underestimate is expected to be

“Plateau” δ 18 O values of CO 2 that evolved from the Fe(CO 3 )OH component during isothermal vacuum dehydrations (200–230 °C) of 18 natural goethites range from 8.2 to 28.1‰. In contrast, the measured δ 18 O values of the goethite structural oxygen range from −11.3 to 1.7‰. The results of this study indicate that the apparent oxygen isotope fractionation factor ( 18 α app ) between plateau CO 2 and initial goethite is systematically related to the rate of isothermal vacuum dehydration. The nonlinear correlation and the magnitudes of the 18 α app values are predicted by a relatively simple mass balance model with the following assumptions: (1) the rate of isothermal vacuum dehydration of goethite (for the interval from 0 to ∼60 to 80% loss of structural hydroxyl hydrogen) can be reasonably well represented by first-order kinetics and (2) isotopic exchange between evolving H 2 O vapor and solid occurs only in successive, local transition states. The generally good correspondence between the model predictions and the experimental data seems to validate these assumptions. Thus, the 18 O/ 16 O ratios of the evolved CO 2 can act as probes into the transient processes operating at the molecular level during the solid-state goethite-to-hematite phase transition. For example, the activation energy for the rate constant associated with the transition state, oxygen isotopic exchange between solid and H 2 O vapor, is tentatively estimated as 28 ± 11 KJ/mol. Such knowledge may be of consequence in understanding the significance of 18 O/ 16 O ratios in hematites from some natural environments (e.g., Mars?). Kinetic data and δ 18 O values of CO 2 are routinely obtained in the course of measurements of the abundance and δ 13 C values of the Fe(CO 3 )OH in goethite. The observed correlation between 18 α app and dehydration rates suggests that plateau δ 18 O values of evolved CO 2 may provide complementary estimates of the δ 18 O values of total goethite structural oxygen (O, OH, CO 2 ) with an overall precision of about ±1‰. However, because of isotopic exchange during the dehydration process, δ 18 O values of the evolved CO 2 do not reflect the original δ 18 O values of the CO 2 that was occluded as Fe(CO 3 )OH in goethite.

The concentrations and δ13C values of atmospheric CO2 were measured in ∼150 air samples collected at 8 sites in the Dallas metropolitan area over the period August 1998 to December 1999. Measured concentrations (C) of atmospheric CO2 ranged from 369 to 475 ppm, while the δ13C values ranged from –12.0 to –8.1‰. These values contrast with a “global” concentration at the time of this study of approximately 367 ppm and a corresponding δ13C value of about –8.0‰. δ13C was linearly correlated with 1/C for samples collected at heights of ∼2 m at 3 sites adjacent to streets with significant automobile traffic. Extrapolation of this two-component mixing line to 1/C=0 yielded a δ13C value of about –27‰ for the CO2 input—i.e., the same as that of gasoline. A simple box model, incorporating photosynthesis, respiration, and anthropogenic addition of CO2, indicates that differences between downwind and upwind concentration-weighted δ13C values ( Δ [ C ∗ δ13C]) of atmospheric CO2 may be linearly correlated with downwind and upwind differences in concentration ( C d ∗ − C u ∗ ), where C ∗ is reported as mol/m3. The model predicts that measurable effects of photosynthetic withdrawal of atmospheric CO2 are manifested by data arrays with slopes more positive than about –16. This effect of photosynthesis is evident in a linear array of “warm weather”, Dallas atmospheric CO2 data (slope of –12.7‰). Collectively, the data for all 8 sites exhibited considerable scatter about binary mixing lines that depict the addition of CO2 from combustion of natural gas and gasoline. However, when model slopes (m) were calculated for binary mixing between a “background” atmospheric CO2 and each individual sample, it was found that, in general, m increases with decreasing temperature. The effects of photosynthesis and respiration complicate this relationship, but the overall pattern suggests that, as temperature decreases, the proportion of anthropogenic CO2 derived from combustion of natural gas increases. This increase appears to reflect increased use of natural gas for home heating, etc., in cooler weather. Therefore, seasonally changing patterns of fossil fuel use are detectable in the atmospheric CO2 of this urban environment.

Paired D/H and 18O/16O analyses of natural goethites range from −224‰ to −93‰ for δD and from −15.5‰ to +2.8‰ for δ18O. The isotopic systematics of these goethites seem to be most closely mimicked by the goethite–water D/H and 18O/16O fractionation curves of Yapp 1987 , Yapp 1990 ). Most of these goethites are of unknown age, but the large range of their isotopic compositions illustrates the possible paleoenvironmental significance of “surface domains” calculated for hydroxyl minerals. A modern surface domain (MSD) for goethite in coordinates of δD and δ18O was calculated from the following information: (a) scatter in the modern relationship between surface temperatures and δ18O of meteoric water (Rozanski et al., 1993) ; (b) average annual surface air temperatures from 0°C to 30°C for the meteoric water sample sites; (c) the meteoric water line (MWL) of Craig (1961) ; and (d) the goethite–water isotopic fractionation factors of Yapp 1987 , Yapp 1990 ). Goethites that had formed in continental environments in approximate thermal equilibrium with the Earth’s modern atmosphere would be expected to lie within the MSD, and 16 of 31 goethites do so. The remaining 15 are “outliers.” Of the 16 MSD samples, 12 (including the three samples from active soil or bog environments) are “concordant.” The calculated isotopic temperatures of the concordant goethites range from 6°C to 27°C and are the same (within analytical uncertainty) as the modern average surface air temperatures at the respective sites. Calculated temperatures of the four “discordant” MSD samples range from 7°C to 25°C and are higher than corresponding modern surface temperatures. A “warm Earth surface domain” (WESD) was calculated with an assumed temperature of 35°C for the tropics. Eight of the 15 outlier samples plot within the portion of the WESD that does not overlap the MSD. Thus, these eight goethites may have formed at times when the Earth’s global climate was warmer than at present. Temperatures calculated from the eight WESD samples range from 20°C to 33°C and suggest lower latitudinal temperature gradients as predicted by climate models of a warmer Earth. Calculated temperatures of the seven samples that lie outside both the MSD and the WESD (non-sd samples) range from 19°C to 69°C. The non-sd status of these seven samples suggests that they formed in environments dominated by subsurface heat sources. Surface domains can be calculated for any hydroxyl mineral for which both the hydrogen and oxygen isotope mineral–water fractionation factors are known. The concept of surface domains provides a criterion for assessing the likelihood that a particular sample records information on ancient climatic temperature.

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.