Speleothems are valuable archives of climate change because of their extraordinary time resolution, which is unattainable in other terrestrial climate proxies. Analyses of 4796 ultraviolet fluorescent (UVf) layers observed in polished thin sections of a 15-cm-long speleothem collected from Raccoon Mountain Cave near Chattanooga, Tennessee, USA, as well as 200 δ 13 C and δ 18 O measurements and 11 high-precision U/Th dates permit refined interpretations of middle and late Holocene paleoclimate records in the southeastern United States. Speleothem UVf layers average 0.015 mm, identical to the average growth rate determined for the middle and late Holocene portions of the speleothem (ca. 7600–400 yr B.P.) based on the U/Th ages and interval thicknesses. UVf layer counts between paired U/Th ages are also consistent with determined ages and further support their interpretation as annual layers. The middle Holocene is typified by 100–400 yr intervals of higher rainfall characterized by thin UVf layers (0.003–0.010 mm) and more-negative δ 13 C values (−3‰ to −6‰ Peedee belemnite [PDB]), punctuated by shorter periods (5–20 yr, rarely 50–100 yr) of lower rainfall with thicker UVf layers (0.030–0.080 mm) and less-negative δ 13 C values (−1‰ to −3‰ PDB); “extreme drought” events are characterized by both the thickest UVf layers (0.150–0.170 mm) and the least-negative δ 13 C values (+0.05‰ to −1‰ PDB). The late Holocene, in comparison, is characterized by overall wetter conditions and more regular (sinusoidal curve) behavior, suggesting 50–100 yr cycles of higher and lower rainfall, with UVf layers ranging from 0.005 to 0.030 mm/yr. Statistical analyses of UVf layer thicknesses using order-two momentum threshold vector autoregressive models (MTVAR2) quantify the relationship between δ 13 C and δ 18 O, dependent upon the momentum in the climate. This study demonstrates that thickness of annual layers in speleothems can be used to resolve detailed paleorainfall records, provided there is preservation of organic matter sufficient to excite UVf response; however, relationships among changes in rainfall amounts, stable isotope values of speleothem calcite, and thicknesses of UVf annual layers (≈growth rates) are not straightforward.

Abstract The concept of paleosols dates back to the eighteenth century discovery of buried soils, geological unconformities, and fossil forests, but the term paleopedology was first coined by Boris B. Polynov in 1927. During the mid-twentieth century in the United States, paleopedology became mired in debates about recognition of Quaternary paleosols, and in controversy over the red-bed problem. By the 1980s, a new generation of researchers envisaged red beds as sequences of paleosols and as important archives of paleoenvironmental change. At about the same time, Precambrian geochemists began sophisticated analyses of paleosols at major unconformities as a guide to the long history of atmospheric oxidation. It is now widely acknowledged that evidence from paleosols can inform studies of stratigraphy, sedimentology, paleoclimate, paleoecology, global change, and astrobiology. For the future, there is much additional potential for what is here termed “nomopedology,” using pedotransfer functions derived from past behavior of soils to predict global and local change in the future. Past greenhouse crises have been of varied magnitude, and paleosols reveal both levels of atmospheric CO 2 and degree of concomitant paleoclimatic change. Another future development is “astropedology”, completing a history of soils on early Earth, on other planetary bodies such as the Moon and Mars, and within meteorites formed on planetismals during the origin of the solar system.

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 The paleosol–carbonate CO 2 barometer is widely accepted to be the most reliable method for reconstructing Earth’s atmospheric CO 2 concentrations in deep time. Currently, the largest source of error in atmospheric CO 2 calculated using the paleosol barometer originates from uncertainty in soil CO 2 concentrations during soil carbonate formation. Many of the paleosols used for CO 2 reconstruction were formed in clay-rich alluvium and have vertic properties, which may influence soil CO 2 .We hypothesized that the cracking during drying of shrink–swell clays results in rapid CO 2 escape and low soil CO 2 concentrations. We tested our hypothesis by monitoring soil cracking and the concentration of CO 2 in the pore spaces of surface Vertisols (the Houston Black and Heiden series fine, smectitic, thermic Udic Haplusterts). Crack porosity of soils was estimated by measuring soil subsidence, and CO 2 was measured in syringe samples collected from soil gas wells. During the period of study, pore-space CO 2 concentrations at ~1-m soil depth varied by two orders of magnitude, from 10% during water-saturated conditions to <0.1% during hot, dry episodes. These large seasonal variations likely result in significant calcite dissolution and reprecipitation during pedogenesis. Soil CO 2 concentrations decreased as soil-crack porosity increased. Moreover a compilation of published records of soil CO 2 variability indicates that the variability of CO 2 concentrations in Vertisols is significantly larger than the variability in other soil orders, regardless of climate or vegetation. These results suggest that soil cracking is a primary control on soil CO 2 in Vertisols and that a soil-order-specific calibration of the paleosol barometer should be developed for application to clay-rich paleosols with soil cracks and/or pedogenic slickensides.

Abstract Wetlands are continental depositional environments and ecosystems that range between ephemerally wet to fully aquatic habitats, and, thus, the character of a wetland soil is directly related to the position of the water table over seasonal and longer timescales. The sediment and paleosol records of wetlands are products of a unique setting that can be both exposed to the atmosphere and water-saturated at the same time. Wetlands tend to occupy low-gradient portions of the landscape in places where the phreatic zone is at least ephemerally exposed at the surface, and hydrophytic vegetation has an opportunity to colonize. Groundwater-fed wetlands are an end member of a continuum of waterlogged environments and are associated with localized groundwater discharge (GWD); e.g., springs and seeps that can sustain permanent saturation. Research has tended to follow one of two parallel tracks: sedimentology or pedology. An objective of this paper is to bring these two separate lines of inquiry closer together. The signature of wetland pedogenesis includes redoximorphic features, enhanced hydrolytic alteration or dissolution of soluble phases, and preservation of biotic indicators of wetland habitats. Histosols (peats) and other hydric soils (indicated by gley color and reduced minerals like pyrite and siderite) are common in sites with a permanently high water table and anaerobic conditions. Illuvial clays, in contrast, record episodes in which wetlands dry out and drainage improves sufficiently for these features to form. A case study from Holocene-age Loboi Swamp, Kenya, illustrates the importance of integrating field observations and laboratory analyses. Wetland conditions were observed through thin section micromorphology, mineralogy, bulk geochemistry, and macro- and microfossils. The record of Loboi Swamp is characterized by the juxtaposition of features indicating episodes of soil saturation alternating with those indicating desiccation. In order to extract the most information recorded in groundwater-fed wetlands, soils and sediments should be studied as part of the larger spatial and climatic frameworks in which they occur.

Abstract Paleosols are important sources of information about climate change. They carry a “memory” of past environments as features such as pedogenic carbonate, carbon isotopes, profile depth, and degree of chemical weathering. Certain features, such as soil organic matter, are more rapidly adjusting (i.e., sensitive) to climate change than are other features, such as mineralogy which are slowly adjusting (i.e., resistant) to climate change, but have a longer memory. In addition, the landscape itself carries a memory of climate change through features such as patterned ground, dune fields, glacial moraines, and lake shorelines. As is the case for soils, some landscapes are more sensitive to climate change than others, and provide better sedimentary and paleosol records. A semiarid grassland on a sand sheet, for example, is more sensitive to climate change and will produce a better paleosol record than a neighboring semiarid grassland on a low-gradient terrain of bedrock outcrop. Landscapes and soil profiles are connected to each other, to the aboveground ecosystem, and to climate as a complex adaptive system . A perturbation to the system can change vegetative cover, initiate erosion, and leave a record in paleosols as both “soil memory” and “lithomemory” (i.e., sedimentary deposits vertically separated by paleosols). A systematic examination of soil memory and lithomemory can be used as a prospecting tool for finding paleosols with high resolution paleoclimatic records. Some of the best paleosol records are in landscapes with erodible regolith and topographic relief, where soil memory develops during periods of landscape stability and lithomemory develops during intervening periods of landscape instability when erosion and sedimentation rates are highest.

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 Pedogenic siderite is a carbonate mineral that forms in the reducing groundwaters of poorly drained soils and paleosols in zonal climatic belts with strongly positive precipitation–evaporation balances. Microcrystalline and spherulitic forms of siderite are commonly recognized in micromorphologic studies of hydromorphic paleosols. Ancient paleosol sphaerosiderites commonly occur with diameters in excess of 1 mm, while modern pedogenic siderite crystal dimensions in excess of 100 µm are rare. Pedogenic siderites have been widely reported from Late Paleozoic, Mesozoic, and Cenozoic paleosols. The carbon and oxygen isotopic compositions of pedogenic siderites have been widely used as proxies for the oxygen isotopic composition of paleoprecipitation for their respective paleosols. Modern process studies of historic pedogenic siderites are yielding a more refined understanding of the stable isotopic systematics of low-temperature siderite. These works will lead to a future change in usage of published siderite–water 18 O fractionation equations.

Abstract Interpretations of critical zones, past and present, are dependent on the comprehensive characterization of morphological, physical, chemical, biological, and mineralogical properties of soils as the biogeochemical mediator of Earth’s surface processes. The traditional approach of studying fossil soils (paleosols), however, is modeled after methods developed during the advent of pedology in the early 20th century. Even though there have been remarkable advances in the development of analytical procedures for modern soils (pedology), advancements in paleopedology have not proceeded past whole-rock geochemical characterization. Here, we develop multianalytical strategies combining traditional and modern approaches to studying paleosols that include direct laboratory measurement, petrographic analysis, and pedotransfer functions. In addition to standardizing the characterization of paleosols, doing so will also contribute to more robust geoinformatic compilations and strengthen interpretations of soil processes, soil taxonomic classification, edaphic controls, and climate conditions in the past. We applied the multianalytical approach to a paleosol from the Late Triassic and demonstrate that it classifies as a Vertisol based on slickensides identified in the field, sepic fabric in thin section, and high values for variables such as total and fine clay content, coefficient of linear extensibility (COLE), smectite content, and available water capacity (AWC). Reconstructed cation exchange capacity (CEC), pH, and base saturation point to a plentiful supply of plant available nutrients. Most reconstructed properties appear to have been reasonably preserved because of shallow burial depths and the formation of slowly permeable claystones. Further testing of direct analytical techniques and the development of pedotransfer functions beyond Vertisols are needed to improve the characterization of the full range of properties expected in the fossil rock record of soils.

Abstract Modem sequence-stratigraphic theory has its foundation in the work of L.L. Sloss and W.C. Krumbein (1940s-1960s) and several Exxon researchers (1970s–1990s). This work largely focuses on the nature and origin of sedimentary cycles within marine stratal successions. More recently, sequence-stratigraphic concepts have evolved to include the analysis of terrestrial strata. Historically, the recognition of unconformity-bounded cyclic stratal units (such as sequences) has relied upon the geometric relationships of strata (i.e., onlap, toplap, truncation, and downlap) within two- and/or three-dimensional outcrop or subsurface successions. Oftentimes, however, outcrops or boreholes are isolated and do not preserve these diagnostic stratal relationships. In such instances, documentation of changes in the vertical, rather than lateral, succession of strata may allow reconstruction of the cyclic accommodation history and placement of associated bounding discontinuities. This technique, referred to as “stacking pattern” analysis, was originally developed for shallow-marine carbonate successions. More recently, the stacking pattern methodology has been similarly applied to alluvial successions and takes into account the unique processes of terrestrial deposition and pedogenesis. The most conspicuous and fundamental cyclic stratal units recognized within alluvial settings are fluvial aggradational cycles (FACs). Fluvial aggradational cycles are meter-scale, typically fining-upward successions that have a disconformable lower boundary and an upper boundary that either has a paleosol weathered into it or is disconformably overlain by the succeeding FAC without a paleosol. Fluvial aggradational cycles are thought to represent sediment accumulations during channel avulsion events that are subsequently weathered during the following period of channel stability. A thick succession of FACs indicates sediment accumulation during a prolonged episode of accommodation gain. Variations in the rate of accommodation gain (and loss) are interpreted to result in the organization of FACs into alluvial sequences and longer period composite sequences. Episodes of base-level rise result in relatively rapid rates of alluvial aggradation and less developed and more poorly drained paleosols. Associated FACs are thicker than average and transition from initially lower sinuosity, higher competence alluvial systems to comparably higher sinuosity, lower competence channel deposits. As base-level rise decelerates and initially falls, paleosols become increasingly well developed and better drained, and FACs are thinner than average and transition to even lower competence, higher sinuosity channel sandstones that are more extensive as a result of prolonged channel migration under low accommodation conditions. During base-level fall, the incisement of alluvial valleys produces sequence boundaries that are infrequently flooded across interfluve areas. Fluvial aggradational cycles across interfluve positions are much thinner than average and are characterized by the most well-developed and best-drained paleosols. Application of the alluvial stacking pattern methodology is demonstrated within three case studies. Case study 1, from Big Bend National Park, Texas, considers a latest Cretaceous to earliest Tertiary passive margin and coastal plain succession and correlates alluvial sequences and associated climate and ecosystem changes to eustatic sea-level oscillations. Case study 2, from northern and northeastern New Mexico, documents a Late Triassic foreland basin succession in which tectonically induced accommodation events are correlated between isolated outcrop successions that are located 200 km apart. Case study 3, from central New York, demonstrates how stacking pattern analysis allows correlation of a Middle Devonian alluvial composite sequence with equivalent regressive-transgressive marine strata along a convergent plate boundary.

Abstract Recent work indicates that most modern continental sedimentary basins are filled primarily by distributive fluvial systems (DFS). In this article we use depositional environment interpretations observed on Landsat imagery of DFS to infer the vertical succession of channel and overbank facies, including paleosols, from a hypothetical prograding DFS. We also present rock record examples that display successions that are consistent with this progradational model. Distal DFS facies commonly consist of wetland and hydromorphic floodplain deposits that encase single channels. Medial deposits show larger channel belt size and relatively well-drained soils, indicating a deeper water table. Proximal deposits of DFS display larger channel belts that are amalgamated with limited or no soil development across the apex of the DFS. The resulting vertical sedimentary succession from progradation will display a general coarsening-upward succession of facies. Depending on climate in the sedimentary basin, wetland and seasonally wet distal deposits may be overlain by well-drained medial DFS deposits, which in turn are overlain by amalgamated channel belt deposits. Channel belt size may increase upward in the section as the DFS fills its accommodation. Because the entry point of rivers into the sedimentary basin is relatively fixed as long as the sedimentary basin remains at a stable position, the facies tracts do not shift basinward wholesale. Instead, we hypothesize that as the DFS fills its accommodation, the accommodation/sediment supply (A/S) ratio decreases, resulting in coarser sediment upward in the section and a greater degree of channel belt amalgamation upward as a result of reworking of older deposits on the DFS. An exception to this succession may occur if the river incises into its DFS, where partial sediment bypass occurs with more proximal facies deposited basinward below an intersection point for some period of time. Three rock record examples appear to be consistent with the hypothesized prograding DFS signal. The Blue Mesa and Sonsela members of the Chinle Formation at Petrified Forest National Park, Arizona; the Tidwell and Salt Wash members of the Morrison Formation in southeastern Utah; and the Pennsylvanian-Permian Lodéve Basin deposits in southern France all display gleyed paleosols and wetland deposits covered by better-drained paleosols, ultimately capped by amalgamated channel belt sandstones. In the Morrison Formation succession, sediments that represent the medial deposits appear to have been partially reworked and removed by the amalgamated channel belts that show proximal facies, indicating that incomplete progradational successions may result from local A/S conditions. The prograding DFS succession provides an alternative hypothesis to climate change for the interpretation of paleosol distributions that show a drying upward succession.