Gary E. Stinchcomb

Pre-colonial (A.D. 1100–1600) sedimentation related to prehistoric maize agriculture and climate change in eastern North America

Despite the importance of understanding the effect of land use on floodplains in eastern North America, few studies have directly addressed the possibility and extent of prehistoric indigenous land use on floodplain development. Here we report geoarchaeological evidence of increasing floodplain sedimentation and prehistoric land-use intensification in the Delaware River Valley (eastern United States) during the Medieval Climate Anomaly–Little Ice Age transition. The evidence of this anthropogenic sedimentation event, documented throughout eastern North America, is designated here as pre-colonial sediment (PCS), ca. A.D. 1100–1600. The data demonstrate that the combined effects of prehistoric land use and climate change affected eastern North American floodplain development several hundred years prior to the onset of major European settlement.

A data-driven spline model designed to predict paleoclimate using paleosol geochemistry

Paleosols (fossil soils) are abundant in the sedimentary record and reflect, at least in part, regional paleoclimate. Paleopedology thus offers a great potential for elucidating high resolution, deep-time paleoclimate records. However, many fossil soils did not equilibrate with climate prior to burial and instead dominantly express physical and chemical features reflective of other soil forming factors. Current models that use elemental oxides for climate reconstruction bypass the issue of soil-climate equilibration by restricting datasets to narrow ranges of soil properties, soil-forming environments and mean annual precipitation (MAP) and mean annual temperature (MAT). Here we evaluate a data-driven paleosol-paleoclimate model (PPM1.0) that uses subsoil geochemistry to test the ability of soils from wide-ranging environments to predict MAP and MAT as a joint response with few initial assumptions. The PPM1.0 was developed using a combined partial least squares regression (PLSR) and a nonlinear spline on 685 mineral soil B horizons currently forming under MAP ranging from 130 to 6900 mm and MAT ranging from 0 to 27 °C. The PLSR results on 11 major and minor oxides show that four linear combinations of these oxides (Regressors 1-4), akin to classic oxide ratios, have potential for predicting climate. Regressor 1 correlates with increasing MAP and MAT through Fe oxidation, desilication, base loss and residual enrichment. Regressor 2 correlates with MAT through temperature-dependent dissolution of Na- and K-bearing minerals. Regressor 3 correlates with increasing MAP through decalcification and retention of Si. Regressor 4 correlates with increasing MAP through Mg retention in mafic-rich parent material. The nonlinear spline model fit on Regressors 1 to 4 results in a Root Mean Squared Error (RMSEMAP) of 228 mm and RMSEMAT of 2.46 °C. PPM1.0 model simulations result in Root Mean Squared Predictive Error (RMSPEMAP) of 512 mm and RMSPEMAT of 3.98 °C. The RMSE values are lower than some preexisting MAT models and show that subsoil weathering processes operating under a wide range of soil forming factors possess climate prediction potential, which agrees with the state-factor model of soil formation. The nonlinear, multivariate model space of PPM1.0 more accurately reflects the complex and nonlinear nature of many weathering processes as climate varies. This approach is still limited as it was built using data primarily from the conterminous USA and does not account for effects of diagenesis. Yet, because it is calibrated over a broader range of climatic variable space than previous work, it should have the widest array of potential applications. Furthermore, because it is not dependent on properties that may be poorly preserved in buried paleosols, the PPM1.0 model is preferable for reconstructing deep time climate transitions. In fact, previous studies may have grossly underestimated paleo-MAP for some paleosols.

The influence of time on the magnetic properties of late Quaternary periglacial and alluvial surface and buried soils along the Delaware River, USA

Magnetic susceptibility of soils has been used as a proxy for rainfall, but other factors can contribute to magnetic enhancement in soils. Here we explore influence of century- to millennial-scale duration of soil formation on periglacial and alluvial soil magnetic properties by assessing three terraces with surface and buried soils ranging in exposure ages from <0.01 to ~16 kyrs along the Delaware River in northeastern USA. The A and B soil horizons have higher Xlf, Ms, and S-ratios compared to parent material, and these values increase in a non-linear fashion with increasing duration of soil formation. Magnetic remanence measurements show a mixed low- and high-coercivity mineral assemblage likely consisting of goethite, hematite and maghemite that contributes to the magnetic enhancement of the soil. Room-temperature and low-temperature field-cooled and zero field-cooled remanence curves confirm the presence of goethite and magnetite and show an increase in magnetization with increasing soil age. These data suggest that as the Delaware alluvial soils weather, the concentration of secondary ferrimagnetic minerals increase in the A and B soil horizons. We then compared the time-dependent Xlf from several age-constrained buried alluvial soils with known climate data for the region during the Quaternary. Contradictory to most studies that suggest a link between increases in magnetic susceptibility and high moisture, increased magnetic enhancement of Delaware alluvial soils coincides with dry climate intervals. Early Holocene enhanced soil Xlf (9.5 – 8.5 ka) corresponds with a well-documented cool-dry climate episode. This relationship is probably related to less frequent flooding during dry intervals allowing more time for low-coercive pedogenic magnetic minerals to form and accumulate, which resulted in increased Xlf. Middle Holocene enhanced Xlf (6.1 – 4.3 ka) corresponds with a transitional wet/dry phase and a previously documented incision event.......

Paleopedology as a Tool for Reconstructing Paleoenvironments and Paleoecology

Soils form as a product of physical, chemical, and biological activity at the outermost veneer of Earth’s surface. Once buried and incorporated into the sedimentary record, these soils, now paleosols, preserve archives of ancient climates, ecosystems, and sedimentary systems. Paleopedology, the study of paleosols, includes qualitative interpretation of physical characteristics and quantitative analysis of geochemical and mineralogical assays. In this chapter, the paleosol macroscopic, micromorphological, mineralogical, and geochemical indicators of paleoecology are discussed with emphasis on basic analytical and interpretative techniques. These data can reveal a breadth of site-specific interpretations of vegetation, sedimentary processes, climatic variables, and durations of landscape stability. The well-known soil-forming factors are presented as a theoretical framework for understanding landscape-scale soil evolution through time. Vertical and lateral patterns of stacked paleosols that appear in the rock record are discussed in order to address practical approaches to identifying and describing paleosols in the field. This chapter emphasizes a robust multi-proxy approach to paleopedology that combines soil stratigraphy, morphology, mineralogy, biology, and chemistry to provide an in-depth understanding of paleoecology.

Anatomy of a Sub-Cambrian Paleosol in Wisconsin: Mass Fluxes of Chemical Weathering and Climatic Conditions in North America during Formation of the Cambrian Great Unconformity

A paleosol beneath the Upper Cambrian Mount Simon Sandstone in Wisconsin provides an opportunity to evaluate the characteristics of Cambrian weathering in a subtropical climate, having been located at 20°S paleolatitude 500 My ago. The 285-cm-thick paleosol resulted from advanced chemical weathering of a gabbroic protolith, recording a total mass loss of 50%. Weathering of hornblende and plagioclase produced a pedogenic assemblage of quartz, chlorite, kaolinite, goethite, and, in the lowest part of the profile, siderite. Despite the paucity of quartz in the protolith and 40% removal of SiO2 from the profile, quartz constitutes 11%–23% of the pedogenic mineral assemblage. Like many other Precambrian and Cambrian paleosols in the Lake Superior region, the paleosol experienced potassium metasomatism, now containing 10%–25% mixed-layer illite-vermiculite and 5%–44% potassium feldspar. Estimates of mean annual precipitation and mean annual temperature are 1777 mm y−1 and 20.1°C, respectively, which are consistent with a paleolatitude of 20°S. For an atmospheric CO2 concentration of 4000–6000 ppm at 550–500 Ma, the duration of weathering is constrained to have been between 20,000 and 100,000 y. When the effects of erosion and influence of protolith composition are considered, the degree, or maturity, of weathering for the Wisconsin paleosol and four other sub-Cambrian paleosols is comparable to that for two modern soils in subtropical and temperate climates, despite the lack of land plants in Cambrian time. Such correspondent degrees of weathering likely result from the effects of elevated levels of atmospheric CO2 and microbial activity on weathering in Cambrian time.

Integrating Human Activities, Archeology, and the Paleo-Critical Zone Paradigm

Recent conceptual advances in the Earth sciences have led to an improved understanding of the dynamics governing the Critical Zone (CZ)—the interface where life meets rock and soil on land (Brantley et al., 2007; Nordt andDriese, 2013). Among the key insights is a renewed appreciation for the deeply intertwined and non-linear nature of the processes in play, where small changes in one or two variable values or in their interactions can have large and often non-intuitive consequences, including the emergence of complex phenomena (Brantley et al., 2017). This application of complex systems (CS) methods and theory to conceptualize the CZ, both in part and in whole, places a premium on valid model construction. The potential analytical consequences of inaccurately modeling variables, processes, or interactions here though is dramatic, especially in light of the non-linear nature and coupled dynamics of CSmodels. In this brief note, we offer our opinion to the CZ science community that CS modeling of Holocene CZ processes and records (i.e., the paleo-CZ [pCZ]; Beverly et al., 2017) may be further refined–and perhaps must be further refined–through increased integration of archeological data and theory.

Relating soil gas to weathering using rock and regolith geochemistry

Weathering-induced fracturing (WIF) has been posited to be a mechanism that develops secondary porosity when mineral reaction fronts separate over depth intervals in regolith, and, in particular, when oxidation (which can promote porosity development through volume expansion) occurs deeper than dissolution (which grows porosity through material removal). If this is true, then the protoliths capacity to reduce O2 [for example, the Fe(II) content] and O2 availability should affect WIF. This study explores the hypothesis that if the ratio of pO2 to pCO2, in soil water, R(aq), is greater than the ratio of the capacity of the protolith to consume O2 and CO2, R0, then WIF is more likely to occur, and regolith will become thicker. We evaluated this hypothesis by measuring the bulk geochemistry of regolith and rock and monitoring soil gas at three sites, encompassing a wide range of FeO concentrations and regolith thickness: a Pennsylvania (PA) diabase (10.15%; 3.8 m), a Virginia (VA) diabase (10.49%; 1.4 m), and a VA granite (1.45%; 20 m). We inferred soil water O2 concentrations from calculated equilibrium with the measured soil gas pO2. We observed WIF in the VA granite and PA diabase where R(aq) > R0, while at the site that lacked WIF – the VA diabase – R(aq) < R0, particularly during the wet season. In the VA diabase, the presence of swelling clays (smectite) limits the ability of the oxidant (O2) to diffuse deeper into the weathering profile during the wet season and microbially accelerated iron oxidation rapidly consumes O2, limiting O2 availability for WIF. Smectite has little to no observable effect on O2 consumption in the PA diabase because the PA diabase is more fractured. A compilation of dissolved soil gas oxidation ratios, the stoichiometric ratio of O2 consumed to CO2 produced, shows that for unsaturated conditions, the mean is −1.45 ± 0.88, which is consistent with aerobic root and microbial respiration and the oxidation of organic matter. For near-saturated conditions, the mean oxidation ratio of the compilation is −3.46 ± 1.79, which is consistent with Fe redox and microbial metabolism under reducing conditions. The consistency between the VA and PA data presented here and the compilation suggests that soil water surplus drives coupled Fe-redox reactions that may act as a negative feedback, limiting O2 supply and WIF under wetter soil moisture conditions. We defined Rz, the ratio of O2 consumption to CO2 consumption during weathering for each depth interval, z. For all profiles, R(aq) > Rz near the surface but R(aq) approaches Rz in the saprolite. We suggest that R(aq) > Rz in the soil reflects consumption of O2 and production of CO2 due to biotic processes whereas R(aq) approaching Rz suggests that low fluxes at depth are at least partly dictated by rock and regolith composition, notably tortuosity of pores. In the VA diabase, we observed R(aq) < Rz occasionally during the wet season in the lowermost soil and saprolite. Thus, at times the O2 availability may be less than the O2 consumption at that depth, consistent with Fe(II) loss and a lack of WIF. Mass-balance calculations show Fe loss in the VA diabase. The influence of rock composition on aqueous O2/CO2 concentrations in saprolite is consistent with the hypothesis that the protoliths capacity to consume O2 and CO2 has some effect on oxidation and acid consumption deep in the weathering profile.

Views on grand research challenges for Quaternary geology, geomorphology and environments

Citation: Forman SL and Stinchcomb GE (2015) Views on grand research challenges for Quaternary geology, geomorphology and environments.