Goethite, calcite, and organic matter from Permian and Triassic soils: carbon isotopes and CO2 concentrations

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 Pco₂ value of about 1 × PAL, but the scatter in the measured δ¹³C values of organic matter permits a calculated maximum Pco₂ 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 Pco₂ and organic matter δ¹³C values were the same for both paleosol types. Furthermore, the atmospheric Pco₂ 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 Pco₂ (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 Pco₂ using these ¹³C-enriched organic values would underestimate the actual atmospheric Pco₂ 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 Pco₂.     

Stable isotope chemistry of fossil bone as a new paleoclimate indicator

During fossilization, bone is thought to recrystallize and alter chemically on timescales of kyr to a few tens of kyr, i.e., similar to the timescale for formation of soils. Therefore, C- and O-isotope compositions of bone apatite should correlate with trends in soil water composition and aridity, and serve as paleoclimate indicators. This hypothesis was tested by analyzing C- and O-isotope compositions of the CO₃ component of fossil bone apatite from mid-Oligocene through late Pleistocene units in Oregon and western Idaho, including the John Day (19.4-30.0 Ma), Mascall (15.2-15.8 Ma), and Rattlesnake (7.2-7.8 Ma) Formations, whose paleosol sequences have been studied in detail, and the Juntura (10-11 Ma), Hagerman (3.2 Ma), and Fossil Lake (<23-650 ka) fossil localities. Tooth enamel δ¹⁸O values provide a baseline of meteoric water compositions. Stable isotope compositions of bone CO₃ do change in response to broad climatic trends, but show poor correlation with compositions of corresponding paleosol CO₃ at specific horizons. Instead, compositional deviations between bone and paleosol CO₃ correlate with compositional deviations with the next higher paleosol; this suggests that the timescale for fossilization exceeds one paleosol cycle. Based on stratigraphic evidence and simple alteration models, fossilization timescales are estimated at 20-50 kyr, indicating that bone CO₃ will prove most useful for sequences spanning >100 kyr. C-isotopes show negative and strong positive deviations during wet and dry climates respectively, and short-term trends correspond well with changes in aridity within the Mascall and Rattlesnake Formations, as inferred from paleosols. A proposed correction to δ¹⁸O values based on δ¹³C anomalies implies a small, ∼1.5‰ increase in meteoric water δ¹⁸O during the late Oligocene global warming event, consistent with a minimum temperature increase of ∼4 °C. A strong inferred decrease in δ¹⁸O of 4-5‰ after 7 Ma closely parallels compositional changes in tooth enamel, and reflects a doubling in the height of the Cascade Range.     

Quantification of atmospheric oxygen levels during the Paleoproterozoic using paleosol compositions and iron oxidation kinetics

The increase in atmospheric oxygen during the Precambrian is a key to understand the co-evolution of life and environment and has remained as a debatable topic. Among various proxies for the estimation of atmospheric oxygen levels, paleosols, ancient weathering profiles, can provide a quantitative pattern of atmospheric oxygen increase during the Precambrian period of Earth history. We have re-evaluated the chemical compositions of paleosols, and presented a new method of applying Fe²⁺ oxidation kinetics to the Fe²⁺ and Fe³⁺ concentrations in paleosols to decipher the quantitative partial pressure of atmospheric oxygen (P_O2) between 2.5 and 2.0 Ga. We first estimated the compaction factor (CF, the fraction of original thickness) using the immobile elements such as Ti, Al and Zr on equal volume basis, which was then used to calculate retention fractions (M_R), a mass ratio of paleosol to parent rock, of redox-sensitive elements. The CF and Fe_R values were evaluated for factors such as homogeneity of immobile elements, erosion, and formation time of weathering. Fe_R increased gradually within the time window of ∼2.5-2.1 Ga and remained close to 1.0 since ∼2.1 Ga onwards. Mn_R also increased gradually similar to Fe_R but at a slower rate and near complete retention was observed ∼1.85 Ga, suggesting an almost continuous increase in the oxidation of Fe²⁺ and Mn²⁺ in paleosols ranging in age between ∼2.5 and 1.9 Ga.We have modeled P_O2 variations during the Paleoproterozoic by applying Fe²⁺ oxidation kinetics to the Fe²⁺ and Fe³⁺ concentrations in paleosols, which enabled us to derive an Fe²⁺ oxidation term referred to as ψ. Possible changes in temperature and P_CO2 during this time window and their effects on resulting models of P_O2 evolution have been also considered. We assumed four cases for the calculations of P_O2 variations between 2.5 and 2.0 Ga: no change in either temperature or P_CO2, long-term change in only P_CO2, long-term changes in both temperature and P_CO2, and short-term fluctuations of both temperature and P_CO2 during the possible, multiple global-scale glaciations. The calculations indicate that P_O2 increased gradually, linearly on the logarithmic scale, from <∼10⁻⁶ to >∼10⁻³ atm between 2.5 and 2.0 Ga. Our calculations show that the P_O2 levels would have fluctuated significantly, if intense, global glaciation(s) followed by period(s) of high temperature occurred during the Paleoproterozoic. This gradual rise model proposes a distinct, quantitative pattern for the first atmospheric oxygen rise with important implications for the evolution of life.     

Coexisting goethite and gibbsite from a high-paleolatitude (55°N) Late Paleocene laterite: Concentration and C/C ratios of occluded CO2 and associated organic matter

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.     

Lower Cretaceous paleosols and paleoclimate in Sichuan Basin,China

Abundant Lower Cretaceous (Berriasian–Hauterivian) paleosols have been recognized in the Sichuan Basin, along with the preserved pedogenetic features, e.g., soil horizons, soil structure, root traces and pedogenic nodules. Chemical, geochemical and mineralogical analyses were used to examine the paleosols. These paleosols were classified as Entisols, Inceptisols, Aridisols and Alfisols in terms of the modern soil taxonomic system. Early Cretaceous paleoprecipitation and paleotemperature in the Sichuan Basin were estimated from the degree of chemical weathering for non-calcareous paleosols, and from the depth to the calcic horizon and stable oxygen isotopic composition of pedogenic carbonates in calcareous paleosols, respectively. A temperate semi-arid climate generally prevailed in the Sichuan Basin as a part of the South China Block (SCB) and was controlled by subtropical high-pressure and a rain-shadow effect because the humid air masses from the Paleo-Pacific were impeded by the highlands of the South China Block. Further, several intervals of sub-humid paleoclimate occurred due to strengthened monsoonal circulation in the Early Cretaceous. Using the paleosol barometer, the paleoatmospheric CO₂ levels of the Early Cretaceous are estimated to range from ∼120 to ∼520 ppmv, with a mean of 305 ppmv. Regional temperature is generally coupled with atmospheric CO₂ concentration and is roughly consistent with the sea level fluctuation.     

REE chemistry and Sm-Nd systematics of late Archean weathering profiles in the Fortescue Group, Western Australia

Two weathering profiles, each consisting of an upper, sericite-rich zone and a lower, chlorite-rich zone, are preserved between flows of the Mt. Roe Basalt in the Fortescue Group, Hamersley Basin, Western Australia. REE concentrations in samples from these two profiles, which originally developed ca 2,760 Ma, show large variations depending on stratigraphic position. LREE abundances and (La/Yb)N are greatest at depths of 3-6 m below the paleosurface of the Mt. Roe #1 profile and are somewhat lower in samples above this level. The LREEs reach concentrations 6-9 times greater than in the underlying basalt, and thus appear to have been mobilized downward in the paleosol and concentrated in its middle part. LREE concentrations in the #2 profile show a similar distribution but with a sharp increase in all REE concentrations within 50 cm of the paleosurface. The distinction between the REE profiles in the two paleosols may be related to the difference in the overlying material. The #1 paleosol is overlain by a few meters of sediments and then by basalt, whereas the #2 paleosol is directly overlain by basalt. The LREEs appear to have been mobilized both during chemical weathering of the parental basalt and during later lower-greenschist-facies metamorphism and metasomatism of the paleosols. Remobilization of the REEs during the regional metamorphism of the Fortescue Group is confirmed by a whole-rock Sm-Nd reference isochron of Mt. Roe #1 samples with an age of 2,151 +/- 360 Ma. Variable initial 143Nd/144Nd values of unweathered basalt samples which may represent the paleosol protolith prevents a confident determination of the magnitude of LREE mobility. Both the initial mobilization of the REEs during weathering and the metasomatic remobilization appear to have taken place under redox conditions where Ce was present dominantly as Ce3+, because Ce anomalies are not developed within the sericite zone samples regardless of concentration. Europium anomalies in the paleoweathering profile are somewhat variable and were probably modified by mobilization of Eu2+ at metamorphic conditions. In all samples, the HREEs appear to have been relatively immobile and correlate with Al, Ti, Cr, V, Zr, and Nb. Sm-Nd systematics and REE patterns of four unweathered basalt samples indicate derivation of the Mt. Roe Basalts from a heterogeneous and enriched source having epsilon Nd between -4.0 and -7.4. Initial 143Nd/144Nd values of these basalts are even lower than those reported by NELSON et al. (1992) for Fortescue Group basalts and indicate a substantial crustal component in the generation of Mt. Roe Basalts.     

Chemical weathering, river geochemistry and atmospheric carbon fluxes from volcanic and ultramafic regions on Luzon Island, the Philippines

We investigated rates of chemical weathering of volcanic and ophiolitic rocks on Luzon Island, the Philippines. Luzon has a tropical climate and is volcanically and tectonically very active, all factors that should enhance chemical weathering. Seventy-five rivers and streams (10 draining ophiolites, 65 draining volcanic bedrock) and two volcanic hot springs were sampled and analyzed for major elements, alkalinity and ⁸⁷Sr/⁸⁶Sr. Cationic fluxes from the volcanic basins are dominated by Ca²⁺ and Mg²⁺ and dissolved silica concentrations are high (500-1900 μM). Silica concentrations in streams draining ophiolites are lower (400-900 μM), and the cationic charge is mostly Mg²⁺. The areally weighted average CO₂ export flux from our study area is 3.89 ± 0.21 × 10⁶ mol/km²/yr, or 5.99 ± 0.64 × 10⁶ mol/km²/yr from ophiolites and 3.58 ± 0.23 × 10⁶ mol/km²/yr from volcanic areas (uncertainty given as ±1 standard error, s.e.). This is ∼6-10 times higher than the current best estimate of areally averaged global CO₂ export by basalt chemical weathering and ∼2-3 times higher than the current best estimate of CO₂ export by basalt chemical weathering in the tropics. Extrapolating our findings to all tropical arcs, we estimate that around one tenth of all atmospheric carbon exported via silicate weathering to the oceans annually is processed in these environments, which amount to ∼1% of the global exorheic drainage area. Chemical weathering of volcanic terranes in the tropics appears to make a disproportionately large impact on the long-term carbon cycle.     

The CO2 system in rivers of the Australian Victorian Alps: CO2 evasion in relation to system metabolism and rock weathering on multi-annual time scales

The patterns of dissolved inorganic C (DIC) and aqueous CO₂ in rivers and estuaries sampled during summer and winter in the Australian Victorian Alps were examined. Together with historical (1978–1990) geochemical data, this study provides, for the first time, a multi-annual coverage of the linkage between CO₂ release via wetland evasion and CO₂ consumption via combined carbonate and aluminosilicate weathering. δ¹³C values imply that carbonate weathering contributes ∼36% of the DIC in the rivers although carbonates comprise less than 5% of the study area. Baseflow/interflow flushing of respired C3 plant detritus accounts for ∼50% and atmospheric precipitation accounts for ∼14% of the DIC. The influence of in river respiration and photosynthesis on the DIC concentrations is negligible. River waters are supersaturated with CO₂ and evade ∼27.7 × 10⁶ mol/km²/a to ∼70.9 × 10⁶ mol/km²/a CO₂ to the atmosphere with the highest values in the low runoff rivers. This is slightly higher than the global average reflecting higher gas transfer velocities due to high wind speeds. Evaded CO₂ is not balanced by CO₂ consumption via combined carbonate and aluminosilicate weathering which implies that chemical weathering does not significantly neutralize respiration derived H₂CO₃. The results of this study have implications for global assessments of chemical weathering yields in river systems draining passive margin terrains as high respiration derived DIC concentrations are not directly connected to high carbonate and aluminosilicate weathering rates.     

Origin of the Matauri Bay halloysite deposit, Northland, New Zealand

At the Matauri Bay halloysite deposit, economically valuable halloysite-rich clays are hosted by a sanidine rhyolite dome (Ar–Ar dated at 10.1?±?0.03?Ma). The rhyolite dome intrudes an older basalt and is overlain by alluvial sediments and a younger basalt (4.0?±?0.7?Ma). A blanket-like, halloysite-rich zone is restricted to depths of 10–30?m from the present day erosion surface. Primary sanidine and plagioclase phenocrysts in rhyolite are completely leached out in the halloysite-rich zone but are only partially leached out at greater depth. Halloysite was formed by hydrolysis and cation leaching of sanidine and plagioclase phenocrysts and groundmass glass in the rhyolite, resulting in loss of K, Ca, Na and Si and enrichment in OH (LOI 6–10%) and Al₂O₃ (20–30%) relative to least-altered rhyolite with 1.8% LOI and 14.5% Al₂O₃. Oxygen and hydrogen isotope data indicate the halloysite is supergene rather than hydrothermal in origin, which is consistent with the absence of pyrite, alunite and other acid-sulphate type hydrothermal minerals, and with the blanket-like alteration profile. The dominance of halloysite over kaolinite was favoured by water-saturated weathering conditions during the late Miocene-Pliocene subtropical weathering regime in Northland.     

O/O and D/H ratios of pedogenic kaolinite in a North American Cenomanian laterite: Paleoclimatic implications

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 δD and δ¹⁸O values of the kaolinite in these samples. These calculations yielded kaolinite δD values of about −73‰ and δ¹⁸O values of about +18.7‰. The δD and δ¹⁸O 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 δD and δ¹⁸O values indicate a crystallization temperature of 22 (±5) °C. A nominal paleotemperature of 22 °C implies a δ¹⁸O value for the corresponding water of −6.3‰. The combination of temperature and meteoric water δ¹⁸O 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 CO₂ 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.     

Early Cretaceous atmospheric pCO2 levels recorded from pedogenic carbonates in China

Pedogenic carbonates were collected from Early Cretaceous strata in Sichuan and Liaoning, China. These paleosol carbonates and calcareous paleosols were evaluated in order to reconstruct atmospheric CO₂ concentrations during the Early Cretaceous using a paleosol barometer. Using the isotopic ratios of pedogenic carbonates from Early Cretaceous (early-middle Berriasian, early Valanginian) strata in Sichuan Basin, averaged atmospheric pCO₂ is estimated to have been 360 ppmv in the early-middle Berriasian and a mean value of 241 ppmv in the early Valanginian. In the late Barremian in western Liaoning, however the average was 530 ppmv, with a range of 365 ppmv and 644 ppmv, lower than previous estimates of pCO₂ for these time periods, consistent with the suggestion of overall climate cooling and paleotemperature fluctuation during the Early Cretaceous. This indicates that not all of the Cretaceous was a high or continuous CO₂ greenhouse, especially during Early Cretaceous.     

Silicate weathering in anoxic marine sediments

Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances, and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration. Pore waters separated from the sediment cores are highly enriched in dissolved magnesium, total alkalinity, humic substances, and boron. The high contents of dissolved organic carbon in the deeper methanogenic sediment sections (50-150 mg dm⁻³) may promote the dissolution of silicate phases through complexation of Al³⁺ and other structure-building cations. A non-steady state transport-reaction model was developed and applied to evaluate the down-core trends observed in the solid and dissolved phases. Dissolved Mg and total alkalinity were used to track the in-situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces. The modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments. Depth-integrated rates of marine silicate weathering in methanogenic sediments derived from the model (81.4-99.2 mmol CO₂ m⁻² year⁻¹) are lower than the marine weathering rates calculated from the solid phase data (198-245 mmol CO₂ m⁻² year⁻¹) suggesting a decrease in marine weathering over time. The production of CO₂ through reverse weathering in surface sediments (4.22-15.0 mmol CO₂ m⁻² year⁻¹) is about one order of magnitude smaller than the weathering-induced CO₂ consumption in the underlying sediments. The evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments. The global rate of CO₂ consumption through marine silicate weathering estimated here as 5-20 Tmol CO₂ year⁻¹ is as high as the global rate of continental silicate weathering.     

Paleoenvironmental implications of concentration and C/C ratios of Fe(CO3)OH in goethite from a mid-latitude Cenomanian laterite in southwestern Minnesota

A mid-Cretaceous (∼95 Ma) laterite in southwestern Minnesota contains pisolites that consist primarily of gibbsite, quartz, and kaolinite with smaller amounts of goethite (α-FeOOH) and hematite. The presence of minor berthierine (an Fe(II) sheet silicate) suggests that this Cenomanian laterite experienced some degree of low temperature reductive diagenesis during its burial history. The prospects for extracting useful paleoenvironmental information from the pisolitic goethite were explored by studying measured mole fraction (X_m) and δ¹³C_m values of the Fe(CO₃)OH component in solid solution in the goethite using the method of incremental vacuum dehydration-decarbonation.Data arrays that occupy distinctly different domains in plots of δ¹³C_m vs. 1/X_m suggest the existence of two generations of goethite in the pisolites. The apparently younger generation of goethite (“generation 2”) evolves CO₂ at 170 °C, while the older generation (“generation 1”) evolves CO₂ at 220 °C. The distribution of the data suggests that generation 2 goethite is a proxy for mixing of CO₂ from three distinct CO₂ sources in an oxidative environment which post-dated the reductive diagenesis. The small amount of generation 1 goethite seems to have persisted through the reductive diagenesis, and nine of the generation 1 goethite data appear to define a proxy, two-endmember, soil CO₂ mixing line. Such two-component mixing is consistent with expectations for a highly evolved, carbonate-free laterite (i.e., the pre-diagenetic Cenomanian weathering system). The δ¹³C_m values of these nine data points range from −23.1‰ to −13.7‰, whereas X_m values range from 0.0007 to 0.0222. Linear regression of these nine data yields a slope of 0.0064, which corresponds to an ancient tropospheric CO₂ concentration of about 1900 ppmV.Isotopic data from pisolitic kaolinite indicate a paleotemperature of about 24 °C at a paleolatitude of ∼40°N. This is substantially warmer than modern continental temperatures at such latitudes and is consistent with published indications of a generally warmer mid-Cretaceous climate. Moreover, the correspondence of a warmer mid-Cretaceous climate with the inferred, relatively high concentration of Cenomanian tropospheric CO₂ (∼1900 ppmV) is consistent with the idea that variations of atmospheric CO₂ have a relation to climate change. The results of this study emphasize the importance of careful evaluation of incremental dehydration-decarbonation data from natural goethites to assess the possibility that more than one generation of goethite is present in a sample. However, the results also indicate that the carbon isotope information recorded in admixed goethite generations may be sorted out and used in paleoenvironmental interpretations.     

Fluxes of high- versus low-temperature water-rock interactions in aerial volcanic areas: Example from the Kamchatka Peninsula, Russia

Volcanic areas play a key role in the input of elements into the ocean and in the regulation of the geological carbon cycle. The aim of this study is to investigate the budget of silicate weathering in an active volcanic area. We compared the fluxes of the two major weathering regimes occurring at low temperature in soils and at high temperature in the active volcanic arc of Kamchatka, respectively. The volcanic activity, by inducing geothermal circulation and releasing gases to the surface, produces extreme conditions in which intense water-rock interactions occur and may have a strong impact on the weathering budgets. Our results show that the chemical composition of the Kamchatka river water is controlled by surface low-temperature weathering, atmospheric input and, in some limited cases, strongly imprinted by high-temperature water-rock reactions. We have determined the contribution of each source and calculated the rates of CO₂ consumption and chemical weathering resulting from low and high-temperature water/rock interactions. The weathering rates (between 7 and 13.7 t/km²/yr for cations only) and atmospheric CO₂ consumption rates (∼0.33-0.46 × 10⁶ mol/km²/yr for Kamchatka River) due to rock weathering in soils (low-temperature) are entirely consistent with the previously published global weathering laws relating weathering rates of basalts with runoff and temperature. In the Kamchatka River, CO₂ consumption derived from hydrothermal activity represents about 11% of the total HCO₃ flux exported by the river. The high-temperature weathering process explains 25% of the total cationic weathering rate in the Kamchatka River. Although in the rivers non-affected by hydrothermal activity, the main weathering agent is carbonic acid (reflected in the abundance of in rivers), in the region most impacted by hydrothermalism, the protons responsible for minerals dissolution are provided not only by carbonic acid, but also by sulphuric and hydrochloric acid. A clear increase of weathering rates in rivers impacted by sulphuric acid can be observed. In the Kamchatka River, 19% of cations are released by hydrothermal acids or the oxidative weathering of sulphur minerals.Our results emphasise the important impact of both low and high-temperature weathering of volcanic rocks on global weathering fluxes to the ocean. Our results also show that besides carbonic acid derived from atmospheric CO₂, hydrochloric acid and especially sulphuric acid are important weathering agents. Clearly, sulphuric acid, with hydrothermal activity, are key parameters that cause first-order increases of the chemical weathering rates in volcanic areas. In these areas, accurate determination of weathering budgets in volcanic area will require to better quantify sulphuric acid impact.     

Fe(CO3)OH in goethite from a mid-latitude North American Oxisol: estimate of atmospheric CO2 concentration in the Early Eocene “climatic optimum”

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 < ≈300 ppm (i.e., less than the analytical uncertainty). The relatively high concentration of 2700 ppm inferred for atmospheric CO₂ at ≈52 Ma B.P. would have been coincident with the Early Eocene climatic optimum. This result seems to support the case for an important role for variations of atmospheric CO₂ in the modification of global paleoclimate.     

Chemical and physical weathering in New Zealand’s Southern Alps monitored by bedload sediment major element composition

The Haast and Clutha rivers drain opposing flanks of New Zealand’s Southern Alps. Major element analyses of grain size fractions (2–1 mm, 1 mm–355 μm, 355–63 μm, and <63 μm) from bedload sediments collected throughout the reach of each river suggest that weathering is strongly partitioned between the chemical weathering of carbonates and the physical weathering of silicates. Sand size fractions from both rivers are depleted in CaO (∼0.2–2.1 wt%) relative to source schists (∼3 wt% CaO), while silt fraction CaO concentrations range from 2–5 wt%. The depletion of CaO in the sediments is interpreted to be due at least in part to removal of carbonate during chemical weathering of the schist protolith in the soil zone. The observed covariance of CaO and P₂O₅ concentrations in all river sediment suggests that most CaO is bound in a combination of phosphate-bearing minerals such as apatite along with other heavy mineral phases with similar hydrodynamic properties (e.g. epidote). Chemical index of alteration (CIA) values for grain size fractions from both rivers are similar (Haast: 54–63, Clutha: 49–61) and do not systematically vary with grain size or sample location. Al₂O₃–CaO^∗ + Na₂O–K₂O (A–CN–K) relationships suggest that CIA values are controlled by albite–muscovite mixing rather than feldspar weathering. Both A–CN–K relationships and modal mineralogical calculations from Clutha river samples indicate progressive downstream attrition of muscovite from coarser to finer grain size fractions. In contrast, Haast river sediments display less variable normative muscovite concentrations and no downstream enrichment/depletion trends.     

River dissolved and solid loads in the Lesser Antilles: New insight into basalt weathering processes

We present here the first available estimations of chemical weathering and associated atmospheric CO₂ consumption rates as well as mechanical erosion rate for the Lesser Antilles. The chemical weathering (100–120 t/km²/year) and CO₂ consumption (1.1–1.4 × 10⁶ mol/km²/year) rates are calculated after subtraction of the atmospheric and hydrothermal inputs in the chemical composition of the river dissolved loads. These rates thus reflect only the low-temperature basalt weathering. Mechanical erosion rates (approx. 800–4000 t/km²/year) are estimated by a geochemical mass balance between the dissolved and solid loads and mean unaltered rock. The calculated chemical weathering rates and associated atmospheric CO₂ consumption rates are among the highest values worldwide but are still lower than those of other tropical volcanic islands and do not fit with the HCO₃⁻ concentration vs. 1/T correlation proposed by Dessert et al. (2001). The thick soils and explosive volcanism context of the Lesser Antilles are the two possible keys to this different weathering behaviour; the development of thick soils limits the chemical weathering and the presence of very porous pyroclastic flows allows an important water infiltration and thus subsurface weathering mechanisms, which are less effective for atmospheric CO₂ consumption.     

The role of trace minerals in chemical weathering in a high-elevation granitic watershed (Estibère, France): chemical and mineralogical evidence

We investigated chemical weathering in a high elevation granitic environment in three selected watersheds located in the Pyrenees (France). The sites were located on glacial deposits derived from similar Hercynian (∼300 Ma) granites characterized by the occurrence of zoned plagioclases and trace calcic phases (epidote, prehnite, sphene, apatite). The surface waters at those sites show high Ca/Na molar ratios (>1) which could not be explained by the dissolution of the major plagioclase (oligoclase) present in the rocks. The coupled approach of investigating stream water chemistry and the mineralogy and chemistry of rocks and soils allowed us to explore the role of the weathering of trace calcic minerals in calcium export at the watershed scale. The weathering of the trace calcic minerals which represent ∼ 1% of the total rock volume are responsible for more than 90% of the calcium export at the sites. Annual cationic fluxes (∼ 23.10⁴ eq/km²/yr) calculated for the Estibère watershed are among the highest reported for high elevation systems draining granitic rocks and ∼ 80% of this annual cationic flux can be attributed to the weathering of trace calcic phases. Calculations based on isotopic values (⁸⁷Sr/⁸⁶Sr) go in the same direction. Except apatite, the trace calcic phases appear to be mainly silicates, thus the type of chemical weathering observed in the Estibère watershed may have an influence on atmospheric CO₂ consumption by granite weathering. However, comparison with other watersheds draining granitic environments worldwide, and with the two other sites in the Pyrenees, indicate that the role of trace calcic phases is important in most young environments exposed to chemical weathering (e.g., high elevation catchments on glacial deposits). Other factors such as the date of glacial retreat, the physical denudation rate, the hydrological functioning of the watershed and the nature and structure of the soil cover are also important.     

Carbon isotope evidence implying high O2/CO2 ratios in the Permo-Carboniferous atmosphere

Theoretical models predict a marked increase in atmospheric O₂ to ∼35% during the Permo-Carboniferous (∼300 Ma) occurring against a low (∼0.03%) CO₂ level. An upper O₂ value of 35%, however, remains disputed because ignition data indicate that excessive global forest fires would have ensued. This uncertainty limits interpretation of the role played by atmospheric oxygen in Late Paleozoic biotic evolution. Here, we describe new results from laboratory experiments with vascular land plants that establish that a rise in O₂ to 35% increases isotopic fractionation (Δ¹³C) during growth relative to control plants grown at 21% O₂. Despite some effect of the background atmospheric CO₂ level on the magnitude of the increase, we hypothesize that a substantial Permo-Carboniferous rise in O₂ could have imprinted a detectable geochemical signature in the plant fossil record. Over 50 carbon isotope measurements on intact carbon from four fossil plant clades with differing physiological ecologies and ranging in age from Devonian to Cretaceous reveal a substantial Δ¹³C anomaly (5‰) occurring between 300 and 250 Ma. The timing and direction of the Δ¹³C excursion is consistent with the effects of a high O₂ atmosphere on plants, as predicted from photosynthetic theory and observed in our experiments. Preliminary calibration of the fossil Δ¹³C record against experimental data yields a predicted O₂/CO₂ mixing ratio of the ancient atmosphere consistent with that calculated from long-term models of the global carbon and oxygen cycles. We conclude that further work on the effects of O₂ in the combustion of plant materials and the spread of wildfire is necessary before existing data can be used to reliably set the upper limit for paleo-O₂ levels.     

A pedogenic goethite record of soil CO2 variations as a response to soil moisture content

The terrestrial carbon cycle and the role of atmospheric CO₂ concentrations in controlling global temperatures can be inferred from the study of ancient soils (paleosols). Soil-formed goethite and calcite have been the primary minerals used as a geochemical proxy for reconstructing atmospheric pCO₂ from ancient terrestrial records. In the case of goethite, optimum sampling strategies for reconstructing pCO₂ focus on the portion of the soil profile that displays steep gradients in both soil CO₂ concentration and δ¹³C values of soil CO₂ such that a keeling plot can be developed for a given soil and atmospheric pCO₂ can be calculated from it. We report data from a Carboniferous paleosol that depart from the expected linear trends. The results indicate that pedogenic goethite is sensitive to variations in the isotopic composition of soil CO₂, over a range of timescales, and can record these variations in the carbon isotope composition and mole fraction of Fe(CO₃)OH in solid solution with goethite. We explore possible environmental conditions that can drive these changes as a function of either moisture controlled variations in soil respired CO₂ or in the residence time of carbon in soils. The implications of this result are overestimation of paleoatmospheric pCO₂ from pedogenic goethite.