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 ¹⁸O fractionation equations. After initial breakthroughs in the discovery of fossil soils, or paleosols in the 1970s and early 1980s, the last several decades of intensified research have revealed the much greater role that these deposits can play in reconstructing ancient Earth surface systems. Research currently focuses on terrestrial paleoclimatology, in which climates of the past are reconstructed at temporal scales ranging from hundreds to millions of years, using paleosols as archives of that information. Such research requires interdisciplinary study of soils conducted in both modern and ancient environments. These issues and many others were discussed at the joint SEPM-NSF Workshop “Paleosols and Soil Surface Analog Systems”, held at Petrified Forest National Park.

The terrestrial Lower Cretaceous Cedar Mountain Formation, Utah, is a critical archive of paleoclimate, tectonics, and vertebrate ecology and evolution. Early Cretaceous carbon cycle perturbations associated with ocean anoxia have been interpreted from this succession, as expressed in stable carbon isotopes. However, refining the timing of the observed stable isotope excursions remains a key challenge in understanding how marine anoxia affects the Earth system, and is ultimately recorded in the terrestrial realm. The geochronology and geochemistry of a terrestrial carbonate near the base of this succession, which potentially records the Ap7 global carbon isotope excursion, is studied here. Petrographic and geochemical analyses are used to test plausible mechanisms for U incorporation into the calcite lattice in this sample. Using these methods, the hypothesis that the incorporation of U was at or close to the timing of carbonate precipitation is evaluated. U–Pb geochronology of calcite indicates a plausible Early Cretaceous age. However, comparison of the new U–Pb ages of calcite with detrital zircon maximum depositional ages immediately beneath the studied sample indicates a disparity in the apparent sedimentation rates if both types of geochronologic information are interpreted as reflecting the timing of sediment deposition. The totality of data supports an early, and high-temperature, diagenetic timing of U incorporation, with potential for minor leaching of U in subsequent fluid–rock interaction. The most likely mechanism for U transport and immobilization in these samples is hydrothermal fluid–rock interaction. Therefore, the radiometric ages, and corresponding stable isotope composition of U-bearing carbonate domains in this sample, indicate early subsurface fluid–rock interactions and not a record of atmosphere–soil geochemical reactions.

We report estimated stable isotope compositions of depositional waters and paleoprecipitation from the Cretaceous Arctic to further elucidate the role of the global hydrologic cycle in sustaining polar warmth during that period. Estimates are based on new hydrogen isotopic analyses of n-alkane biomarkers extracted from Late Cretaceous and mid-Cretaceous terrestrial deposits in northern Alaska and the Canadian High Arctic. We integrate these new results with earlier published work on oxygen isotopic analyses of pedogenic siderites, dinosaurian tooth enamel phosphates, and pedogenic clay minerals from the same field areas. Average Late Cretaceous δD values of −143‰ VSMOW corresponded with average δ¹⁸O values of −24.1‰ VSMOW, and average mid-Cretaceous δD values of −106‰ VSMOW corresponded with average δ¹⁸O values of −22.1‰ VSMOW. The distributions of water isotope δD and δ¹⁸O values from Cretaceous Arctic deposits do not intersect with the Global Meteoric Water Line, suggesting an apparent deuterium excess ranging from about 40 to 60 per mil. We considered several possible explanations for these Cretaceous results including (1) mass-balance changes in zonal patterns of evaporation and precipitation at lower latitudes, (2) concentration of ²H in leaf tissue waters from continuous transpiration by coniferous paleofloras during the Arctic growing season, and (3) concentration of ²H in the groundwaters of methane-emitting Arctic soils.

Stable isotope, trace element, and cathodoluminescence analyses were performed on gastropods and matrix from rock cores of the Middle to Upper McMurray Formation (Lower Cretaceous/Aptian, Northwestern Alberta, Canada) to characterize the extent to which pristine material remained from which depositional environmental conditions could be estimated. Aragonitic gastropod shells retain their original depositional chemistry, and along with other closely associated authigenic carbonate components, indicate that the Middle to Upper McMurray Formation was deposited in a freshwater environment that was part of a continent-scale river system.

The highly biodiverse Iberian Peninsula has winter‐dominated precipitation and summer drought. The onset of this modern climate regime, termed a Mediterranean‐Type Climate, began during the Late Pliocene–Early Pleistocene, concurrent with global cooling. To advance our understanding of regional hydroclimate changes in relation to past global changes, we present a stable oxygen isotope record measured on paleosol carbonates (δ ¹⁸ O _c ) from Teruel, Spain spanning 4.1–2.4 Ma, when the regional ecosystems shifted from subtropical requiring high humidity to dominantly drought‐tolerant. The δ ¹⁸ O _c values show a negative excursion during Marine Isotope Stage M2 (∼3.3 Ma) and a subsequent decrease during the onset and intensification of the Northern Hemisphere Glaciation (iNHG: ∼2.73 Ma). While the δ ¹⁸ O _c decrease during global cooling could have resulted from less high‐δ ¹⁸ O summer rainfall and/or intensified low‐δ ¹⁸ O winter rainfall, comparison with previously published proxy records point toward drying in the Mediterranean Basin during the Late Pliocene. We therefore propose that global cooling at this time resulted in lower Atlantic and Mediterranean sea surface temperatures during the iNHG which decreased moisture availability to the Mediterranean Basin, and together with enhanced and equatorward shifted Hadley circulation, reduced summer rainfall in the Mediterranean Basin when compared to the conditions of the Early Pliocene.

Previously published anomalous whole-rock stable isotopic values from the Poison Strip Sandstone Member of the Cretaceous Cedar Mountain Formation (CMF) of eastern Utah are of uncertain origins. This study investigated the diagenetic history and the processes responsible for these anomalous data. Accordingly, we integrated photomicroscopic techniques including polarized light microscopy, epifluorescence and cathodoluminescence (CL) imaging, micromilling of stable isotope samples, and fluid-inclusion heating and freezing measurements to this end. The key observations involved the microscopic mapping of calcite cement stratigraphy using CL imaging to permit the analysis of stable isotopes of calcite cements that crystallized during early and late diagenesis. The mapping of calcite cement zones of sufficient submillimeter size to mill out and isolate microgram-sized stable isotope samples enabled this discrimination. Early diagenetic calcite cements have the most positive δ18O values (-10 to -8.5‰ Vienna Pee Dee Belemnite [VPDB]) of all components. The pattern of δ13C and δ18O variation in this early diagenetic cement indicates affinities with early meteoric diagenesis previously documented in published literature on the CFM. The late diagenetic calcite cements yield the most negative δ18O values (-18 to -16‰ VPDB). We interpret the late diagenetic cements to be responsible for the anomalously low whole-rock δ18O values previously reported from the Poison Strip Sandstone Member. Our discoveries of bitumen in late-stage pore fillings and liquid petroleum in the fluid inclusions of late diagenetic calcite cements of the Poison Strip Sandstone Member explain the lower whole-rock organic matter δ13C values and anomalous Δ13C values reported from the unit. Comparatively lower carbonate δ18O and organic δ13C values originally derived from whole-rock analyses of samples from the Poison Strip Sandstone Member resulted from high-temperature basinal diagenesis (hydrothermal circulation and/or petroleum migration), rather than the alternative interpretation of early diagenesis related to a Cretaceous paleoclimatic perturbation. Our results are illustrative of methods to resolve the long-standing geologic problem of discriminating and characterizing products of early vs. late diagenesis in terrigenous clastic sedimentary strata.