Studies of ancient drylands can enhance our understanding of ecosystem responses to changing climate, but reconstructing deep-time dryland environments using paleosols is challenging because these fossil soils are often weakly developed and many proxies are not equilibrated to past climate. Paleosols from the uppermost Honaker Trail Formation and lower Cutler beds (Rico and Halgaito Formations) span the Pennsylvanian-Permian transition within the Paradox Basin of southeast Utah (USA) and are an ideal setting to explore dryland paleoenvironments. Integration of bulk oxide, micro-XRF, and stable isotopic data allows for assessments of paleoclimate, provenance, and pedogenic processes. Ti/Al ratios track a broadly consistent provenance while Ti/ Zr trends follow allochthonous dust contributions, possibly indicating a shift in paleowinds near the Rico-Halgaito boundary that is supported by Ti/Zr and Zr/Hf ratios from within-zircon analysis. Carbon isotopic data suggest pedogenic carbonate formation under conditions of low soil productivity while modeled MAP estimates reveal an average of 409 mm/yr +/- 209. Evaluation of MAT and MAP relationships suggests that Inceptisols identified within this study could represent ancient Aridisols. We conclude that dry climatic conditions promoted an unstable landscape for the region encompassing the study interval while also precluding hydrolysis and leaching, resulting in weakly developed paleosols with abundant carbonate features and red coloration. These findings are consistent with other work from the region, pointing to dry conditions and shifting paleowinds within paleoequatorial Pangea during the Pennsylvanian-Permian transition.

Fluvially derived tuffaceous Chinle sandstones from Petrified Forest National Park provide a well‐preserved Late Triassic archive of climate information. Petrographic analysis of 38 Chinle sandstones provides new insight into the depositional history and evolution of palaeoclimate during Chinle deposition. This study focuses on the relationship between climate and meteoric diagenesis as a guide for constraining climate change in western equatorial Pangea during the Late Triassic. Petrographic analysis of Chinle sandstones reveals their wide range of textural attributes, as well as pedogenic and shallow burial diagenetic features that occurred during the Late Triassic. These diverse petrological characteristics are indicative of the evolving Late Triassic climate, when placed into a well‐constrained stratigraphic and geochronological framework. The stratigraphic succession is characterized by variations in the abundance of framework grains, detrital matrix, weathering intensity of feldspar and volcanic rock fragments, and the mineralogy of clay cements. Climate records from Chinle palaeosol geochemistry indicate a progression from wet to dry conditions. This trend is also reflected in the meteoric diagenetic features of Chinle sandstones. During deposition of the lower Chinle, elevated rainfall promoted the weathering of labile volcanic detritus to kaolinite, whereas feldspars (especially plagioclase) were partially or completely dissolved. In the upper Chinle, a trend towards drier conditions favoured the formation of smectite and less feldspar dissolution resulting in a higher abundance of well‐preserved plagioclase grains. Shallow burial meteoric weathering reactions in Chinle sandstones reflect the evolving climate during the Late Triassic.

The burial of organic matter (OM) in marine sediments is an important control on global climate and petroleum formation on geological timescales. While the oxygen exposure time of sediment has been shown to affect the efficiency of organic carbon burial, and influence OM properties in recent sediments, the implications for the chemical composition of kerogen have not been tested. This study tests the hypothesis that oceanic redox conditions are an important control on the molecular structure and composition of kerogen. Samples from a number of outcropping Late Cretaceous mudstones of the Eagle Ford Formation were chosen, spanning a gradient of paleo-redox depositional conditions (oxic/sub-oxic to anoxic) and the bulk molecular composition of the kerogen was determined using solid state C-13 nuclear magnetic resonance (NMR) spectroscopy. The NMR spectral properties of kerogen covaried with redoxsensitive trace metal abundance, whereas the thermal maturity and OM source biomarkers were relatively invariant. Anoxic environments favor the preservation of alkyl C moieties, while kerogen formed in oxic depositional environments was enriched in aromatic C. This study is the first to demonstrate that diagenetic consequences of bottom water redox conditions are retained in the molecular structure of the kerogen macromolecule on geological timescales. (C) 2017 Elsevier Ltd. All rights reserved.

Discharge in mountain streams may be a mixture of snowmelt, water from surface runoff, and deep return flow through valley bottom alluvia. We used O-18 and H-2, solute concentrations, and Rn-222 to determine water sources of a headwater stream located at the McDonald Creek watershed, Glacier National Park, USA, during summer recession flow period. We analysed minimal water isotope ranges of -17.6 parts per thousand to -16.5 parts per thousand and -133 parts per thousand to -121 parts per thousand for O-18 and H-2, respectively, potentially due to dominance of snow-derived water in the stream. Likewise, solute concentrations measured in the stream through the watershed showed minimal variation with little indication of subsurface water input into the stream. However, we observed Rn-222 activities in the stream that ranged from 39 to 2646Bq/m(3) with the highest value measured in middle of the watershed associated with channel constriction corresponding to changes in local orientation of underlying rocks. Downstream from this point, Rn-222 activity decreased from 581 to 117Bq/m(3) in a series of punctuated steps associated with small rapids and waterfalls that we hypothesized to cause radon degassing with a maximum predicted loss of 427Bq/m(3) along a 400m distance. Based on mass balance calculations using Rn-222 activity values, streamflow, and channel characteristics, we estimated that groundwater contributed between 0.3% and 29% of total flow. Overall, we estimated a 5.9% of groundwater contribution integrated for stream reach measured at McDonald Creek during recession flow period. Finally, a lower mean hyporheic flux of 14m(3)/day was estimated compared to the groundwater flux of 70710m(3)/day. These assessments highlight the potential for radon as a conservative tracer that can be used to estimate subsurface water contribution in mountain streams within a complex geologic setting. Copyright (c) 2015 John Wiley & Sons, Ltd.

This study improves upon previously identified correlations between the chemical structure of kerogen and potential hydrocarbon (oil and gas) yields assayed by Rock-Eval pyrolysis. We propose a quantitative structure-catagenesis relationship that predicts the hydrocarbon generation potential of source rocks and of lacustrine, marine, and terrestrial origin (types I, II, and humic coals). We used one-dimensional solid-state C-13 Nuclear Magnetic Resonance (C-13 NMR) spectroscopy with H-1 spectral editing to determine the abundance of carbon functional groups, including non-protonated and mobile groups. An NMR-based van Krevelen analysis readily separated the kerogen types. Single regression matrices of NMR-based structure parameters against Rock-Eval hydrocarbon yield revealed distinct dynamics of the kerogen types upon pyrolysis. Multiple regression showed that alkyl, oxygen-substituted alkyl, and carbonyl groups were strong contributors to hydrocarbon production, while oxygen-substituted aromatic carbons were strongly counterproductive. Catagenetic relationships established for kerogen provide insight into kerogen structure evolution upon pyrolysis, and can more closely constrain the mechanisms of hydrocarbon generation for use in sedimentary basin modeling. (C) 2016 Elsevier Ltd. All rights reserved.

During the Late Triassic, western equatorial Pangea, in the present-day American Southwest, was unusually humid, as indicated by sedimentological evidence and inferred from general circulation models. These studies show that once equatorial Pangea was assembled, cross-equatorial summer air flow penetrated into western equatorial Pangea, bringing abundant megamonsoon rainfall and warm temperatures. However, many of these investigations indicate that gradual aridification began sometime during the Late Triassic to Jurassic. We show from field properties, geochemical transfer functions, and isotopic analysis of paleosols in the Chinle Formation of Petrified Forest National Park, Arizona, that the western equatorial Pangea megamonsoon collapsed by 214.7 Ma during the "middle Norian climate shift." Paleosols include Entisols, Inceptisols, Vertisols, Aridisols, and Alfisols, with transfer functions for mean annual precipitation (chemical index of alteration minus potassium [CIA-K]) and mean annual temperature (NAK) (salinization) developed from analogous modern data sets. The most notable shift is the appearance of carbonate-enriched paleo-Inceptisols and paleo-Vertisols after 214.7 Ma and paleoAridi- sols after 210 Ma.Prior to the middle Norian climate shift, the region is classified as humid (humidity province) based on rainfall estimates. Initiation of the middle Norian climate shift was characterized by an increase in paleosol carbonate content and rapidly declining rainfall as the region shifted to subhumid, and eventually to semiarid and arid after 210 Ma. Paleosol-derived temperatures are indicative of warm temperate to subtropical ecozones, whereas general circulation models show higher temperatures and tropical conditions, perhaps because boundary conditions were set to atmospheric pCO 2 4-5x present. Elevations of >1.5-2 km adjacent to the Cordilleran magmatic arc complex located similar to 400 km west of the study area may account for nontropical temperatures. After 210 Ma, however, temperatures began to increase, possibly as a result of escalating atmospheric pCO(2) from shale oxidation during marine regression and from coal and paleosol organic matter oxidation during prolonged aridification.Paleomagnetic studies have suggested northward continental drift of western equatorial Pangea to outside of the Intertropical Convergence Zone as the cause of monsoonal collapse during the Late Triassic to Early Jurassic. The formation of a rain shadow as a result of the evolving Cordilleran magmatic arc as the cause of aridification is supported by recent magnetostratigraphic work substantiating that the region remained in the tropics through the Triassic. According to our age model, the middle Norian climate shift is dated to near the same time as the Manicouagan impact crater, but there is no evidence of such an event in the study area, either geochemically or sedimentologically at our scale of observation. However, a regionally defined faunal turnover may have been a response to rapidly changing climates in the region.

The Upper Cretaceous Eagle Ford Formation is an organic-rich mudrock of economic significance for oil and gas exploration. In order to facilitate a better understanding of paleoceanographic conditions during Eagle Ford deposition, this study integrates the isotope chemistry of bulk organic matter with inorganic geochemical data. Measurements of total organic carbon (TOC), total N, [Formula: see text], [Formula: see text] and inorganic major, and trace elements were taken from 166 Eagle Ford and Pepper Formation outcrop samples from McLennan County, central Texas. These data reveal the chemostratigraphic character and the evolution of Cretaceous seawater chemistry on the Texas shelf and allowed the identification of six distinct chemofacies that are useful for correlation purposes. Based on these data, changing paleoredox conditions were documented ranging from normal marine (oxic) conditions associated with the Pepper Formation, anoxic conditions associated with the Lower Eagle Ford Formation, suboxic conditions associated with most of the upper Eagle Ford, and then a return to normal marine conditions at the top of the Eagle Ford Formation. The high TOC content of the Lower Eagle Ford was most likely caused by high productivity that in turn drove conditions to anoxia. Geochemical data that correlate well with TOC were used to identify intervals of potential organic enrichment.

Although pedogenic barite has been documented in many modern soils and palaeosols, no actualistic studies on its formation have been reported. Because barite is stable over the entire range of pressure and temperature of the Earth's crust, it preserves reliable data about the original environment in which it formed. Pedogenic barite and barite‐bearing soils have been used as indicators of landscape stability, environmental conditions, climate and microbial acti‐vity. This study compares field data, micromorphology and stable isotope geochemistry of a barite‐bearing palaeosol from the Morrison Formation (Jurassic) and a modern analogue soil in south‐central Texas, USA. Morrison barite‐bearing palaeosols are over‐thickened cumulic palaeosols that developed in subaerially exposed lacustrine sediments during an extended lake contraction event. Lateral facies relationships document changes in hydrology and duration of episaturated conditions (perched water table above the Btg horizons) that correspond to differences in barite nodule morphology and abundance. Barite precipitation occurred at a redox boundary higher on the landscape after organic matter was completely oxidized. Sulphur isotope data indicate that the initial source of sulphur was soil organic matter. Meteoric water is the likely source of oxygen for the sulphate. Barium sourced from weathering feldspars and clays. The modern analogue displays similar catenary relationships, redox features and micromorphological characteristics compared to the Morrison palaeosols, suggesting that similar pedogenic processes led to barite precipitation. Synthesized data suggest that conditions favourable to barite‐bearing soil formation are low‐gradient basins that have received feldspar‐rich sediments (i.e. volcanically influenced basins), soils that developed near salt domes, soils that developed in exposed wetland or lacustrine sediments and coastal plain deposits. When studied in a well‐documented palaeogeographic context, barite‐bearing soils are valuable to palaeoclimate, palaeoenvironmental and palaeohydrological studies. Combined with regional interfluve palaeosols, barite‐bearing palaeosols may document temporal changes in drainage, surface stability, and accommodation consistent with sequence boundaries/maximum flooding surfaces and climate changes.

Small-basin floodplain buried soils formed in association with low-order tributary streams are valuable archives of past climates, but have not been studied extensively in central Texas, USA. Four buried soils exposed along Owl Creek, within the larger Brazos River drainage basin, were examined using soil morphology and micromorphology, optically stimulated luminescence (OSL) dating, soil characterization, whole-soil geochemical and stable isotope analyses of soil organic matter and pedogenic carbonate. These buried soils provide a record of changes in paleoecological and paleo-alluvial conditions spanning ~14ky. Morphological and geochemical differences between buried soils reflect changes in landscape attributable to climate, with a distinct 5‰ increase in δ13C values of soil organic matter corresponding to the Holocene onset and drier conditions. Paleoecological reconstructions coupled with depth to Bk suggest possible amounts of erosion of ~1m for each of the buried soils. Compilation of the proxies presented shows evidence for a cooler and wetter late Pleistocene climate, followed by a warmer and drier climate dominating during the Holocene.

Carbon and oxygen isotope compositions of pedogenic carbonate preserved in paleosols have been used extensively to reconstruct ancient environmental conditions. One concern is that pedogenic carbonate precipitated in association with a limestone parent material may include a contribution from inherited detrital (lithogenic) marine calcite, thus compromising interpretations of stable-isotope compositions. To investigate the impact of lithogenic calcite on environmental interpretations using paleosols, we measured stable-isotope ratios in pedogenic carbonates and coexisting soil organic matter (SOM) in modern Vertisols (fine, smectitic, thermic, Udic Haplusterts) from Riesel, Texas (Heiden series) and Zabcikville, Texas (Houston Black series) forming on Cretaceous chalk deposits. The pedogenic and lithogenic components in the Heiden series soil at Riesel were identified using micromorphology and cathodoluminescence (CL), which showed incorporation of luminescent marine allochems (i.e., foraminifera, mollusks) into hard nodules and soft powdery pedogenic masses. Additionally, micromorphology showed evidence of meteoric recrystallization of the allochems.Despite the presence of these inherited carbonates into hard nodules and soft powdery masses, the stable-isotope compositions recorded a partial pedogenic signal. Recrystallization of the marine allochems resulted in overprinting of primary marine isotope ratios with ratios that are more similar to those expected for pedogenic carbonates in these soils. This study demonstrates that incorporation of parent material into pedogenic carbonates does not necessarily compromise the pedogenic signal that is useful for paleoenvironmental reconstructions. However, overprinting of marine allochems, if present, can be important and needs to be identified using micromorphology and cathodoluminescence, before using stable-isotope ratios of pedogenic carbonates for paleoenvironmental reconstructions.

The Upper Triassic Sonsela Member of the Chinle Formation is an alluvial succession containing interbedded sandstone and pedogenically modified mudstone. Despite preservation of silicified logs within channel sandstone beds, the Sonsela plant ecosystem is less understood than other intervals due to decreased preservation of nonconifer plant taxa. Sonsela paleosols and rhizoliths are evaluated using macromorphology, micromorphology, and geochemistry to determine the spatial distribution of paleosol characteristics and plant sizes and densities across the study area. Three pedotypes identified within the Sonsela are classified as Inceptisols and Vertisols that exhibit fining of matrix textures (from clayey siltstone to claystone) and reduced drainage with distance from the paleochannel. Overall, Sonsela paleosols are immature, suggesting that the Sonsela fluvial system experienced high rates of lateral migration and cannibalization of overbank sediments in a low-subsidence regime. Rhizohalos within the Sonsela Member are likely diagenetic and commonly include silicified roots (silica root petrifactions). Silicified roots provide information on root size and density that is not commonly afforded by other rhizolith types. Diagenetic rhizohalo diameters may be controlled by paleosol matrix textures within the Sonsela Member. Rhizolith characteristics suggest that channel-proximal paleosols contained only small-stature plants while distal floodplain paleosols may have hosted both small-stature and arborescent plants. Paleosols within the Sonsela Member do not contain rhizoliths whose size or abundance are reflective of a dense coniferous forest. Floodplain plants were commonly small of stature and immature, unable to evolve into more mature communities due to high rates of floodplain cannibalization during fluvial migration.

High-precision geochronology provides unprecedented insights into the depositional history of the Upper Triassic Chinle Formation of the Colorado Plateau, as well as its paleoenvironmental and paleobiological records. The Chinle succession exposed in the Petrified Forest National Park (PEFO) and vicinity, Arizona, includes two large-scale alluvial composite sequences. Although each composite sequence fines upward, the upper composite sequence is more dominated by coarser-grained deposits. Petrographic analysis of sandstone lithic content indicates an upward decrease in the proportion of volcanic rock fragments in each composite sequence. Paleocurrent indicators in the lower composite sequence suggest a variable paleoflow direction, whereas northward paleoflow dominated the upper composite sequence. The change in paleoflow appears to coincide with a reorganization of alluvial depositional processes and associated source terranes, and precedes a rapid acceleration in basin subsidence.Climate proxy records from paleosol geochemistry indicate a gradual shift from humid to dry conditions across the transition between the lower and upper composite sequences and the Adamanian-Revueltian biotic turnover. Composite-sequence depositional reorganization, climatic shift and biologic turnover, in turn, appear to coincide with episodes of magmatism recorded in Triassic granitoid plutons presently exposed in southern California. Taken collectively, these observations suggest that the Late Triassic depositional, climatic, and ecologic history at PEFO may be related to emergence of the incipient Cordilleran magmatic arc along the convergent western margin of Pangea. A new U-Pb date for the lower part of the Chinle Formation suggests that most or all of the formation was deposited in the Norian Stage.