Abstract Several years of weekly sampling of waters from the Shinfa River watershed in the lowlands of northwestern Ethiopia yielded 275 samples with δ D vsmow and δ 18 O vsmow values ranging from c. −10 to +100‰ and from c. −2 to +20‰, respectively. Wet season (summertime) Shinfa River water stable hydrogen and oxygen isotope values are among the lowest reported in this study, whereas the dry season (winter/spring) usually records a progressive trend towards +100 and +20‰, respectively. Overlapping with this interval of Shinfa River water sampling, air temperatures ( n = 155) also were recorded at the same time; temperatures range from c. 18 to 47°C. The coolest temperatures occur during the summer wet season, associated with the arrival of the Kiremt rains in the region, whereas the warmest temperatures occur towards the end of the dry season. In order to evaluate the extent to which this rather extreme isotope hydrology is recorded in the sediments and biota of the Shinfa River system, both hardwater calcareous deposits precipitated on basalt cobbles by evaporation in the Shinfa River channel during the dry season and aragonite from three different modern bivalve mollusc species were collected and analysed for their stable oxygen and carbon isotope compositions. Hardwater calcareous deposit δ 18 O vpdb and δ 13 C vpdb values range from c. −2 to +5‰ and c. −9 to +7‰, respectively, and preserve a trend towards progressively more positive δ 18 O vpdb and δ 13 C vpdb values through the course of the dry season. Shinfa River mollusc aragonite powders ( n = 51) were serially sampled from cf. Coelutura aegyptica , cf. Chambardia rubens and Etheria elliptica species. All species record oxygen and carbon isotopes between c. −2 and +7‰ and between c. −18 and −8‰, and each species records coherent trends between those extremes as well as a positive parametric correlation between measured oxygen and carbon isotope values. However, there does appear to be some variability of measured isotope values by species, suggesting that species-specific metabolic differences may impact the resulting range of aragonite stable carbon and oxygen values. Based upon the measured Shinfa River water δ 18 O vsmow and corresponding water temperatures at the time of sampling, a possible range of Shinfa River calcite and aragonite δ 18 O vpdb values were calculated in conjunction with well-established calcite–water and aragonite–water oxygen isotope fractionation equations. These ‘fictive’ calcite and aragonite δ 18 O vpdb values range from c. −5 to +15‰, which is a much larger range than previously documented from analyses of the hardwater calcareous deposits and mollusc aragonite samples. The narrower range of values in the natural calcite and aragonite samples may be attributed to several mechanisms, including time averaging and environmental stress. Nevertheless, the stable oxygen isotopic compositions of these natural samples offer a minimum assessment of the environmental extremes which occur in this region today, and provide a model for reconstructing the environments of the past.

Abstract Fourteen soil profiles from California were collected in order to measure the δ 13 C of coexisting soil calcite and organic matter. Thirteen of the profiles contained a measurable amount of calcite ranging from 0.04 to 54.6 wt %. Soil calcite δ 13 C PDB (δ 13 C value vs. the calcite standard Peedee Belemnite) values range from −14.4 to 1.3‰, whereas organic matter δ 13 C PDB values range from −24.0 to −27.7‰. The hydrology of these profiles is divided into two broad groups: (1) soils characterized by gravity-driven, piston-type vertical flow through the profile and (2) soils affected by groundwater within the profile at depths where calcite is present. The difference between soil calcite and organic matter δ 13 C PDB values, Δ 13 C cc _ om , is smaller in profiles affected by groundwater saturation as well as most Vertisols and may be a product of waterlogging. The larger Δ 13 C cc-0 m values in soils with gravity-driven flow are consistent with open-system mixing of tropospheric CO2 and CO2 derived from in situ oxidation of soil organic matter with mean soil PCO2 values potentially in excess of ~20,000 ppmV at the time of calcite crystallization. There is a correlation between estimates of soil PCO2 and a value termed “E ppT.U ” (kJm 2 /yr) among the soil profiles characterized by gravity-driven flow. E ppT.U is the energy flux through the soil during periods of soil moisture utilization, and it is the product of water mass and temperature in the profile during the growing season. Thus, soils with high water-holding capacity/storage and/or low/high growing season temperature may form soil calcite in the presence of high soil PCO 2 , and vice versa. The results of this research have important implications for reconstructions of paleoclimate from stable carbon isotopes of calcareous paleosol profiles.

Abstract It has long been understood that the primary control on atmospheric carbon dioxide levels over geologic time (10 6 −10 7 years) is silicate weathering. Schematically, this relationship is given by the “Urey Equation,” such as CaSiO 3 + CO 2 = CaCO 3 + SiO 2 , where the equation represents weathering going from left to right and metamorphism going from right to left. The logic of the Urey Equation can be inverted to look instead at the consumption (and therefore burial) of carbon due to weathering because, for example, it requires 2 moles of CO 2 from all sources (diffusion, rainfall, in situ productivity) to weather 1 mole of silicate Ca 2+ . Thus, by characterizing chemical losses during pedogenesis, it is possible to determine the total CO 2 that was consumed during pedogenesis. With reasonable estimates of formation time, the gross consumption can be reconfigured as a rate of carbon consumption. This theoretical framework is applied to basalt-parented paleosols from the Picture Gorge Subgroup (Oregon) that span the middle Miocene climatic optimum. The calculations indicate that CO 2 consumption is not simply a function of soil formation time and that it is instead controlled by the atmospheric CO 2 level. Benthic foraminiferal δ 13 C values also indicate a carbon burial event at this time that is consistent with the paleosol carbon sequestration estimates. As atmospheric CO 2 declined toward the end of the middle Miocene climatic optimum by a factor of three, CO 2 consumption by silicate weathering dropped by at least 50%, indicating a strong relationship between the two, even on relatively short timescales (10 4 –10 5 years).

Abstract Patterns of plant distribution by palaeoenvironment were examined across the Pennsylvanian–Permian transition in North–Central Texas. Stratigraphically recurrent packages of distinct lithofacies, representing different habitats, contain qualitatively and quantitatively different macrofloras and microfloras. The species pools demonstrate niche conservatism, remaining closely tied to specific habitats, during both short-term cyclic environmental change and a long-term trend of increasing aridity. The deposits examined principally comprise the terrestrial Markley and its approximate marine equivalent, the Harpersville Formation and parts of lower Archer City Formation. Fossiliferous deposits are lens-like, likely representing fill sequences of channels formed during abandonment phases. Palaeosols, represented by blocky mudstones, comprise a large fraction of the deposits. They suggest progressive climate change from minimally seasonal humid to seasonal subhumid to seasonal dry subhumid. Five lithofacies yielded plants: kaolinite-dominated siltstone, organic shale, mudstone beds within organic shale, coarsening upward mudstone–sandstone interbeds and channel sandstone. Both macro- and microflora were examined. Lithofacies proved compositionally distinct, with different patterns of dominance diversity. Organic shales (swamp deposits), mudstone partings (swamp drainages) and coarsening upward mudstone–sandstone interbeds (floodplains) typically contain Pennsylvanian wetland vegetation. Kaolinite-dominated siltstones and (to the extent known) sandstones contain taxa indicative of seasonally dry substrates. Some kaolinite-dominated siltstones and organic shales/coals yielded palynomorphs. Microfloras are more diverse, with greater wetland–dryland overlap than macrofloras. It appears that these two floras were coexistent at times on the regional landscape.

The stratigraphic and regional distributions of paleosol morphology in latest Pennsylvanian through Early Permian strata in Colorado, Utah, Arizona, New Mexico, Texas, and Oklahoma are presented in this paper. This regional extent corresponds to a paleolatitudinal gradient spanning ~5°S to 10°N. Morphological trends from this region delineate significant and systematic temporal and spatial changes in Permian-Carboniferous paleoenvironment and paleoclimate. The inferred latest Pennsylvanian (Virgilian) through early Early Permian environmental pattern is complex, but it indicates persistently dry, semiarid to arid conditions in Colorado, Utah, and Arizona, at paleolatitudes north of ~2°N, whereas lower paleolatitude (~2°S to 2°N) tropical regions in New Mexico exhibit a stepwise shift from subhumid to semiarid and variably seasonal conditions throughout late Pennsylvanian and the first half of Early Permian (Virgilian through Wolfcampian) time, followed by a subsequent shift to more arid conditions during the latter part of the Early Permian (Leonardian). Notably, strata from the southernmost paleosites, in Texas and Oklahoma, exhibit the most significant and abrupt climate changes through this period; they show a rapid transition from nearly ever-wet latest Pennsylvanian climate (at ~5°S) to drier and seasonal climate across the Permian-Carboniferous system boundary, and finally to arid and seasonal climate by Leonardian time (at ~2–4°N). The inferred climate patterns show no robust long-term correlation with the high-latitude Gondwanan records of glaciation. Rather, the long-term record of Permian-Pennsylvanian climate indicators from the southwestern United States is most simply explained by an ~8° northward tectonic drift through (essentially) static climate zones over western tropical Pangea during the interval of study. However, the relatively rapid perturbations to climate recorded by these pedogenic archives appear to be too rapid for tectonic forces and might correspond to changes in climate drivers, such as atmospheric p CO 2 , atmospheric circulation, and glacial-interglacial cycles.

Diverse wetland vegetation flourished at the margins of the Midland Basin in north-central Texas during the Pennsylvanian Period. Extensive coastal swamps and an ever-wet, tropical climate supported lush growth of pteridosperm, marattialean fern, lycopsid, and calamite trees, and a wide array of ground cover and vines. As the Pennsylvanian passed into the Permian, the climate of the area became drier and more seasonal, the great swamps disappeared regionally, and aridity spread. The climatic inferences are based on changes in sedimentary patterns and paleosols as well as the general paleobotanical trends. The lithological patterns include a change from a diverse array of paleosols, including Histosols (ever-wet waterlogged soils), in the late Pennsylvanian to greatly diminished paleosol diversity with poorly developed Vertisols by the Early–Middle Permian transition. In addition, coal seams were present with wide areal distribution in the late Pennsylvanian whereas beds of evaporates were common by the end of the Early Permian. During this climatic transition, wetland plants were confined to shrinking “wet spots” found along permanent streams where the vegetation they constituted remained distinct if increasingly depauperate in terms of species richness. By Leonardian (late Early Permian) time, most of the landscape was dominated by plants adapted to seasonal drought and a deep water table. Wetland elements were reduced to scattered pockets, dominated primarily by weedy forms and riparian specialists tolerant of flooding and burial. By the Middle Permian, even these small wetland pockets had disappeared from the region.

The thicknesses of stratigraphic sections of the Late Triassic (Carnian) Ischigualasto Formation change significantly, from ∼300 to 700 m, along a 15 km transect in the Ischigualasto Provincial Park, San Juan, NW Argentina. Channel sandstone deposits dominate the thickest section, whereas pedogenically altered layers dominate the thinnest stratigraphic section. Eight paleosol types have been recognized in the study area, and they are unevenly distributed across the basin. In particular, paleosol B horizons are thinner and redoximorphic soil morphologies dominate in the thickest, whereas B horizons are thickest and argillic and calcic morphologies dominate in the thinnest stratigraphic section. These observations suggest that the geomorphic evolution of the Ischigualasto basin exerted the primary control on sediment distribution, depositional rate, soil drainage, and depth of the groundwater table through most of Late Triassic time in the Ischigualasto basin. In addition, δ 18 O values of paleosol calcite nodules are similar to modern soil calcites that form in frigid to cool climates between ∼0 °C and 10 °C. Considering both lateral and stratigraphic distribution of paleosol morphological variability, there appears to be three different general modes of climate recorded throughout deposition of the Ischigualasto Formation: (1) Humid conditions recorded by Argillisols, Gleysols, and Vertisols in the lower quarter of the formation; (2) relatively dry conditions recorded by Calcisols, calcic Argillisols, and calcic Vertisols in the middle half of the formation; and (3) generally more humid conditions in the upper quarter of the formation recorded by Argillisols, Gleysols, and Vertisols. La Formación Ischigualasto del Triásico Superior (Carniense) presenta cambios importantes de espesor (de 300 a 700 m), a lo largo de una transversal de 15 km dentro del Parque Provincial de Ischigualasto, San Juan, en el noroeste de Argentina. En las zonas en las que la Formación es más potente dominan los canales de areniscas, mientras que en las zonas en las que el espesor de la Formación es menor dominan los niveles edáficos. Se han reconocido ocho tipos distintos de paleosuelos, que se distribuyen de forma desigual a lo largo de la cuenca. En particular, los horizontes B de los paleosuelos son menos potentes y presentan morfologías redoximórficas en las secciones estratigrá-ficas de mayor espesor; por el contrario, en las secciones estratigráficas de menor espesor los horizontes B son más potentes y argílicos y en ellos son frecuentes los rasgos calcáreos. De forma conjunta, la distribución de los depósitos canalizados de areniscas y la morfología de los paleosuelos a lo largo de la Formación Ischigualasto indican que la evolución geomorfológica de la cuenca fue el principal factor de control sobre la distribución de los sedimentos, la tasa de sedimentación, el drenaje de los suelos y la profundidad del nivel freático durante la mayor parte del Triásico.

A paleokarst fill deposit from the Wichita Mountains, south-central Oklahoma, United States, consists primarily of sparry calcite, Fe-sulfides, and goethite. Previous cement-stratigraphic studies and paleontological finds suggest that calcite mineralization was initiated during Permian time, whereas goethite and other oxides apparently formed from oxidation of preexisting Fe-sulfides during Pleistocene time. Therefore, these deposits have the potential to offer insight into surficial hydrology and paleoenvironment in an upland setting from two time periods at a single site. δ 13 C PDB and δ 18 O SMOW measurements of 17 samples from growth bands in a single karst-fill calcite crystal range from −10.7‰ to −6.6‰ (mean = −8.6‰) and 27.1‰ to 28.3‰ (mean = 28‰), respectively. Large oscillations in the δ 13 C values through the growth series may originate from seasonal changes in the magnitude of biological productivity during Permian time. These δ 13 C oscillations contrast with the relative stability of the δ 18 O values, which are more positive than would be expected for isotopic equilibrium with local modern waters. The δ 18 O values of the calcite may reflect the δ 18 O values of ambient meteoric groundwaters in the Permian that were isotopically similar to waters in modern, seasonally dry, low-latitude coastal regions. Goethites are not in equilibrium with modern waters or coexisting calcites in the fissure-fill deposit as determined from δ 18 O and δD values of the goethites. Furthermore, the combined δ 18 O and δD values of the goethites are indicative of formation from meteoric waters at a temperature of ∼9 °C ± 3 °C. This inferred temperature is 7 °C ± 3 °C cooler than local modern mean annual temperature and corresponds well with independent studies that propose temperatures ∼6 °C cooler in this region during Pleistocene time. The mole fraction and δ 13 C values of the Fe(CO 3 )OH component in solid solution in the goethite sample are 0.0103 and −10.1‰, respectively. In combination, these values suggest that goethite formed in an environment characterized by mixing of three isotopically distinct CO 2 components: (1) oxidized biological carbon, (2) atmospheric CO 2 , and (3) CO 2 from dissolution of carbonate in the karst system. Oxidized biological carbon may have originated either from flora characterized by C3 or mixed C3:C4 photosynthesizers. Mass balance calculations between these three CO 2 end members correspond to an inferred soil CO 2 concentration [CO 2 contributed from (1) and (2) above] ranging from ∼8,000 ppmV to ∼16,000 ppmV for a local ecosystem dominated by C 3 flora. This inferred range of soil CO 2 concentrations is typical of grasslands characterized by relatively high biological productivity. If C4 flora were a significant source of oxidizing carbon, the higher calculated ambient CO 2 concentration at the time of goethite crystallization in the cave (∼20,000 ppmV) might be interpreted to correspond to an unusually productive C4 soil present at a time of generally cooler and drier conditions across the southern Great Plains of North America.