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 Microbially induced sedimentary structures (MISS) are primarily known from transitional marine (tidal) and shallow-marine settings. New results from the ca. 1.1 Ga Midcontinent Rift System of North America extend their record into wholly terrestrial depositional settings, including capping paleosols, paludal environments, and alluvial sedimentary units. The MISS are present on both sides of present-day Lake Superior, at Good Harbor Bay, Minnesota, and at Copper Harbor, Michigan, indicating a regionally widespread biosphere. MISS in the Midcontinent Rift System include abraded Kinneyia, pustulose mound structures, multidirectional wave ripples, textured bedding plane surfaces, and stromatolites. Organic carbon is also preserved in MISS from both sides of the rift, and in finely laminated sedimentary strata, indicating that the biosphere extended also into the hinterland. Independent climatic evidence indicates a temperate setting; thus, the new MISS extend both the depositional and environmental niches where MISS are preserved.

The greenhouse-icehouse change across the Eocene-Oligocene transition and associated Oi-1 glaciation event is the most profound climatic change in Earth’s recent geological history. Marine reconstructions of the Oi-1 glaciation using foraminiferal δ 18 O isotopic compositions suggest that much of the change was associated with Antarctic ice growth rather than climatic change. Nonetheless, some cooling is expected to have occurred on land in addition to drier conditions associated with water tied up in the polar ice caps, and some recent results based on stable isotope analyses of bones support this viewpoint. Nonmarine paleoclimatic conditions (mean annual temperature, mean annual precipitation) may be quantitatively reconstructed using paleosols preserved in continental successions to test this general model. Results from Oregon and Nebraska suggest moderate drying and cooling, not as a stepwise change at the time of the Oi-1 glaciation, but as part of a long-term aridification and cooling event associated in part with emplacement of the Cascade Range. In contrast, intermontane Montana’s paleoprecipitation and paleotemperatures fluctuated on short-term (i.e., Milankovitch) time scales but on balance were both essentially unchanged by the Oi-1 glaciation. Results from Europe (UK, Spain) suggest a different pattern characterized by stable (i.e., unchanging) paleotemperatures in both localities and increasingly wet conditions in the UK. Taken together, these results indicate that (1) strongly regionalized climatic change was associated with the Oi-1 glaciation, (2) physiographic position with respect to orographic features played a key role in determining those regional climatic responses to the global event, and (3) there was little or no cooling on land associated with the Oi-1 glaciation.

Four different types of paleosols are recognized in the late Eocene–earliest Oligocene Solent Group (Isle of Wight, UK), representing a patchwork of ecosystems. Weakly developed marsh paleosols (Entisol-like; histic Inceptisol-like) are the most common, and there are relatively fewer, slightly elevated Inceptisol-like and Alfisol-like paleosols present as well. The more developed paleosols allow for a quantitative paleoclimatic reconstruction. The Eocene-Oligocene transition is associated globally with the Oi-1 glaciation event. Some nonmarine sequences show long-term cooling and aridification associated with the glaciation. Reconstructed paleoclimatic conditions using Solent Group paleosols do not; instead, they reflect steady mean annual temperatures and gradually increasing mean annual precipitation. This result is consistent with previous evaluations of floral assemblages, which indicate consistent vegetative covering and niche floral elements spanning the Eocene-Oligocene transition. In contrast, there is a significant change in the mammalian faunas found throughout western Europe (Grande Coupure). The evidence for relatively static climatic conditions is not consistent with the scenario of a climatically driven turnover event for the Grande Coupure, although the impact of increased seasonality cannot be ignored.