Search

The Busidima Formation in the Afar region, Ethiopia, spans the Quaternary and records the cultural evolution of the genus Homo. Yet, the Middle Pleistocene to Holocene fluvial environments in which early humans lived are undersampled in eastern Africa. This paper examines the stratigraphy, geochronology and paleoenvironments of the newly designated Odele Member of the uppermost Busidima Formation (Homo sapiens and its migration out of Africa. The Odele Member is 40–50 m thick and spans 151 to 7 ka, defined at the base by the widespread Waidedo Vitric Tuff (WAVT, 151 ± 16 ka modeled age and 95.4% C.I.). There are two prominent erosional unconformities in the Odele Member, a lower one after the WAVT deposition with a modeled 95.4% C.I. range of 124 − 97 ka; and an upper one involving widespread alluvial fan incision commencing between 13 and 10.6 ka. The uppermost Odele Member also contains black, organic-rich mats, redox features, reed casts, and semi-aquatic and aquatic gastropods marking wetter conditions during the terminal Pleistocene and Early Holocene. A black, fine-grained relict soil coeval with the Halalalee paleosol bounds the top of the Odele Member and has mollic and vertic properties, weathering since ~ 12 ka. These incision events and prominent paleosol development near/at the top of the Busidima Formation document Middle to Late Pleistocene Awash River incision to its present-day course. Paleo-rainfall estimates suggest that the Early Holocene-age Halalalee paleosol weathered under a climate with mean annual rainfall 10–15% higher than today. A compilation of radiocarbon ages from aquatic gastropods, carbonized wood and charcoal from the upper Odele Member shows wetter and possibly more vegetated conditions during late marine isotope stage (MIS) 3 and the African Humid Period (AHP) that are tightly coupled with precession-driven summer insolation maxima. The Odele Member revises upward the age of the Busidima Formation to 7 ka, showing that it spans into the Holocene and now includes Middle and Later Stone Age archaeological traditions.

The bulk elemental composition of soil subsurface (B) horizons is influenced by environmental, biological, geological, and climatic factors. Because fossil soils (paleosols) are common in the geologic record, quantitative models that link climate to paleosol geochemistry are highly desirable in the paleoclimate community. Error associated with these models is typically reported as the root mean square error (RMSE) of a regression analysis and reflects the variability imparted by non-climatic influences on soil formation and the uncertainty associated with model fitting. However, for prediction purposes, the RMSE is well known to underestimate model uncertainty. In this work we re-evaluate a widely used transfer function for mean annual precipitation (MAP) based on the chemical index of alteration minus potassium (CIA-K) using data science best practices on two continental-scale soil data sets. Data set inter-comparisons and cross-validation of exponential regression models indicate that the root mean square prediction error (RMSPE) between CIA-K and MAP for soils representative of climates across the continental United States is around 299 mm, significantly higher than the currently accepted 182 mm RMSE. Further, CIA-K is unable to predict perhumid (>2000 mm MAP) climate zones. We show that transitioning from a simple regression framework to one of recursive partitioning via random forests can significantly increase prediction accuracy while automating variable selection. We introduce two new, widely applicable random forest models for MAP (RF-MAP) that use 10 elemental oxides as input variables and were calibrated on the Baylor University Soil Informatics (BU-SI) data set. RF-MAP version 1.0 (RF-MAP1.0) was generated using the entire BU-SI data set (n = 685) and can predict MAP values up to 6865 mm with a RMSPE of 395 mm. RF-MAP version 2.0 (RF-MAP2.0) was generated using a modification of the BU-SI data set (n = 642) and can predict MAP values up to ∼1600 mm with a RMSPE of 209 mm. Pruned regression trees provide insight into the mechanisms driving the random forest models and demonstrate the first empirical confirmation of the sensitivity of soil elemental responses to global climate zones. The RF-MAP1.0 and RF-MAP2.0 models predict MAP values comparable to independent proxy estimates for a range of deep-time paleosols. We advocate for application of RF-MAP1.0 in settings where no a priori information on paleoclimate is available, and encourage the use of either RF-MAP1.0 or RF-MAP2.0 if users have independent constraints that paleo-MAP was below 1600 mm.

The capacity of terrestrial ecosystems to sequester carbon dioxide (CO2) from the atmosphere is expected to be altered by climate change and CO2 fertilization, but this projection is limited by our understanding of how the soil system interacts with plants. Understanding the soil–vegetation interactions is essential to assess the magnitude and response of terrestrial ecosystems to the changing climate. Here, we used soil profile and satellite data to explore the role that soil properties play in regulating water and carbon use by plants. Data obtained for 19 terrestrial ecosystem sites in a warm temperate and humid climate were used to investigate the relationship between remotely sensed data and soil physical and chemical properties. Classification and regression tree results showed that in situ soil carbon isotope (δ13C), and soil order were significant predictors (r2 = 0.39, mean absolute error (MAE) = 0 of 0.175 gC/KgH2O) of remotely sensed water use efficiency (WUE) based on the Moderate Resolution Imaging Spectroradiometer (MODIS). Soil extractable calcium (Ca), and land cover type were significant predictors of remotely sensed carbon use efficiency (CUE) based on MODIS and Landsat data-(r2 = 0.64–0.78, MAE = 0.04–0.06). We used gross primary productivity (GPP) derived from solar-induced fluorescence (SIF) data, based on the Orbiting Carbon Observatory-2 (OCO-2), to calculate WUE and CUE (referred to as WUESIF and CUESIF, respectively) for our study sites. The regression tree analysis revealed that soil organic matter and soil extractable magnesium (Mg), δ13C, and soil silt content were the important predictors of both WUESIF (r2 = 0.19, MAE = 0.64 gC/KgH2O) and CUESIF (r2 = 0.45, MAE = 0.1), respectively. Our results revealed the importance of soil extractable Ca, soil carbon (S13C is a facet of soil carbon content), and soil organic matter predicting CUE and WUE. Insights gained from this study highlighted the importance of biotic and abiotic factors regulating plant and soil interactions. These types of data are timely and critical for accurate predictions of how terrestrial ecosystems respond to climate change.

The authors of this chapter examine Middle Archaic human-environmental relationships and shifting resource procurement and settlement strategies. They argue that the warming and drying trends of the Middle Holocene resulted in local hunter-gatherers making frequent, short-term residential moves that sought to take advantage of increased upland nut mast and large deer populations. The seasonal rounds of Middle Archaic hunter and gatherers involved continued use of floodplains supplemented with increased use of upland caves and rockshelters. Sedimentation histories documented at both lowland and upland sites suggest that the drier conditions of the Middle Holocene resulted in significant upslope erosion and downhill accumulation. They also note that as groups settled into the region, projectile point styles such as Knob Creek Stemmed and Middle Archaic Corner Notched reflect a local social identity. These changes in settlement and mobility dynamics and projectile point styles are correlated with a shift from an emphasis on high quality Wyandotte chert, to poorer quality cherts located near upland camps.

The paleoregolith in this study was unconformably overlain by lower Cretaceous (Valanginian) marine deposits of the Asgard Formation and buried below two kilometers of sediments, on the Utsira High, Norwegian North Sea. The upwards intensification of chemical alteration, manifested in the replacement of feldspars by clays, leaching of base cations and reduced mechanical rock strength, in addition to the presence of terrestrial biotitic features, suggested weathering during subaerial exposure of the Utsira High during the Mesozoic. The paleoregolith can be divided into three weathering facies: altered coherent rock, saprock and saprolite, whereas the absence of true soil horizons suggested erosional truncation of the regolith before burial. Possible remnants of the eroded soil were identified as pedogenic mud aggregates in the overlying Asgard Formation, bearing similar compositional characteristics as the saprolite. Clay mineralogy, bulk geochemical composition, micromorphology and depth of biotic features were used to reconstruct the paleoenvironmental conditions that prevailed during the formation of the regolith in the Mesozoic North Sea. Our results point to a regolith formation under periodically water-restricted conditions, promoting the co-existence of interstratified illite-smectite (beidellite) and kaolinite, deep presence of endomycorrhizae (the fungal extension of the root structure) and precipitation of iron oxide nodules. Application of recently developed geochemical climofunctions indicated that the regolith formed in a mesic (10–12 °C) and subhumid (1157–1406 mm/yr) environment, and under strongly acidic soil conditions (pH 5.2–5.6).

Although stone tools generally co-occur with early members of the genus Homo, they are rarely found in direct association with hominins. We report that both Acheulian and Oldowan artifacts and Homo erectus crania were found in close association at 1.26 million years (Ma) ago at Busidima North (BSN12), and ca. 1.6 to 1.5 Ma ago at Dana Aoule North (DAN5) archaeological sites at Gona, Afar, Ethiopia. The BSN12 partial cranium is robust and large, while the DAN5 cranium is smaller and more gracile, suggesting that H. erectus was probably a sexually dimorphic species. The evidence from Gona shows behavioral diversity and flexibility with a lengthy and concurrent use of both stone technologies by H. erectus, confounding a simple "single species/single technology" view of early Homo.

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 protolith9s 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, R9(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 R9(aq) > R0, while at the site that lacked WIF – the VA diabase – R9(aq) We defined Rz, the ratio of O2 consumption to CO2 consumption during weathering for each depth interval, z. For all profiles, R9(aq) > Rz near the surface but R9(aq) approaches Rz in the saprolite. We suggest that R9(aq) > Rz in the soil reflects consumption of O2 and production of CO2 due to biotic processes whereas R9(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 R9(aq)

Soil pH is essential for understanding weathering, nutrient availability, and biological-edaphic relationships. However, standard pH measurement on soils requires friable material, thereby ...

A transient greenhouse warming event during the late Neoproterozoic has been invoked to explain the incongruence of cooler temperatures and intense weathering at high latitudes. We explore this further by examining three deep weathering profiles of the Neoproterozoic Baltic paleosol (600–560 Ma). These profiles have an Oxisolic horizon dominated by quartz, kaolinite, phlogopite/vermiculite, hematite and goethite. Mass-balance calculations (assuming immobile Al2O3) for the newly characterized Erra profile studied here showed loss of 25–50% SiO2 and MgO, complete loss of CaO and Na2O, 25–50% loss of K2O, and 25–100% gain in Fe2O3 in the upper Oxisolic horizon. pH was estimated as moderately acid to very acidic (4.5–6) during weathering using a new paleo-pH proxy. MAP (1500–1600 mm yr−1: CIA-K) and MAT (13–15(+) °C: PPM1.0) were also estimated using bulk geochemical proxies. Calculations for pCO2 follow an earlier thermodynamic method with evaluation of total mass flux for CaO, MgO, Na2O, and K2OCALC (corrected for K metasomatism) during weathering of each protolith material. When considering a range of acceptable values of paleoatmospheric pCO2 based on earlier studies, a minimum of 1.88 × 105 and a maximum of 1.02 × 106 yr weathering duration is indicated, consistent with previous interpretations that the Baltic paleosol is a deep paleo-Oxisol. This long weathering duration estimate does not require a “transient Neoproterozoic greenhouse event” to explain the depth (to 30 m) and intensity of weathering, especially given newer paleolatitudinal reconstructions.

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...

The Paleoproterozoic Baraboo paleosol is the most chemically mature Precambrian paleosol in the Lake Superior region, having formed under the influence of a warm, humid climate in a region of subdued topography. We have determined the chemical variability among six samples of granitic protolith and found that such variability is inherited in the weathering profile. To minimize the effect of sample heterogeneity on various geochemical calculations, transformed compositions of saprolite and regolith were generated by obtaining linear least squares fits to the concentrations of SiO2, TiO2, Al2O3, and Zr through the weathering profile and summing the totals to 100%. Fortunately, use of measured or transformed compositions makes little difference in calculation of mass fluxes. However, transformed compositions allow for clearer comparisons among paleosols and are essential for determining certain paleosol parameters, such as Chemical Depletion Fraction. Calculations of Total and Reduced Mass Fluxes, Chemical Index of Alteration, Feldspar Index of Weathering, and Chemical Depletion Fraction demonstrate that the Baraboo paleosol is chemically more mature than the Paleoproterozoic Pronto and Cambrian Timna paleosols, but less mature than modern subtropical and tropical soils. Estimates of Mean Annual Temperature (MATPPM) and Mean Annual Precipitation (MAPPPM) for Baraboo are 14.3 19.2 9.4 °C and 1177 1695 658 mm yr−1, confirming the previous interpretation of a warm, humid climate. Although the level of atmospheric pCO2 is poorly constrained due to the uncertainty in duration of weathering and possibility of regolith removal by erosion, pCO2 is calculated to be ≥4.8 × Pre-Industrial Atmospheric Level (280 ppm) for 100,000 years of weathering, which is consistent with the pCO2 level at 1.7 Ga predicted by the weak sun model.

Many areas in the world are characterized by shallow soils underlain by weathered bedrock, but root-rock interactions and their implications for regolith weathering are poorly understood. To test the role of tree roots in weathering bedrock, we excavated four pits along a catena in a shale-dominated catchment at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) in central Pennsylvania. We measured a variety of biological, physical, and chemical properties including: 1) root density, distribution, and respiration, 2) soil gas, and 3) elemental compositions, mineralogy, and morphology of soil, rock, and rock fracture fill at ridge top, mid-slope, toe-slope, and valley floor sites. As expected, root density declined rapidly with depth; nevertheless, fine roots were present in rock fractures even in the deepest, least weathered shale sampled (~ 180 cm below the land surface). Root densities in shale fractures were comparable between the ridge top and mid-slope pits. However, they were significantly lower in the toe-slope, despite increasing rock fracture densities, which is likely due to a shallower water table depth at the downslope site. Average root respiration (per mass of dry root tissue) in rock fractures was comparable to rates in the soil. Thus, the total flux of CO2 from root respiration tracked root densities, decreasing with depth. Potential microbial respiration, estimated as the laboratory C mineralization potential, was about an order of magnitude lower than measured root respiration in both the soil and shale fractures. Roots were only observed in large aperture (> 50 μm) shale fractures that were filled with particulate material. The fill in these fractures was mineralogically and geochemically similar to the lowest soil horizons with respect to clay composition, element mobility, extractable dissolved organic C (DOC), inorganic N-species, and potentially mineralizable C and N, while total C and total N values for the fracture fill were similar to the shale bedrock. In the bulk soil, depletion profiles (Al, Fe, K, Mg, and Si) relative to unweathered shale reflected characteristic weathering of illite and vermiculized chlorite to kaolinite and are similar between soils and fracture fill. Such similarities indicate that the fracture coatings are likely the result of pedogenic processes that occur at depth in the fractures rather than translocation of soil particles downward into the fractures. Overall, our data suggest that roots and fill in shale fractures down to ~ 180 cm are qualitatively similar to those in surface soil horizons. Thus, the deepest manifestation of the chemical depletion profiles observed in the pits consists of the rock fracture fill, and this fill is present at low concentrations with similarly low concentrations of fine roots.

O2 and CO2, the two essential reactants in weathering along with water and minerals, are important in deep regolith development because they diffuse to weathering fronts at depth. We monitored the dynamics of these gas concentrations in the hand-augerable zone on three ridgetops—one on granite and two on diabase—in Virginia (VA) and Pennsylvania (PA), U.S.A. and related the gas chemistry to regolith development. The VA granite and the PA diabase protoliths were more deeply weathered than the VA diabase. We attribute this to high protolith fracture density. The pO2 and pCO2 measurements of these more fractured sites displayed the characteristics of aerobic respiration year round. In contrast, the relation of pO2 versus pCO2 on the more massive VA diabase is consistent with seasonal changes in the dominant electron acceptor from O2 to Fe(III), likely regulated by the expansion/contraction of nontronite in the soil BC horizon. These observations suggest that the fracture density is a first order control on deep regolith gas chemistry. However, fractures can be present in protolith but also can be caused by oxidation of ferrous minerals. We propose that subsurface pO2 and weathering-induced fracturing can create positive feedbacks in some lithologies that cause regolith to thicken while nonetheless maintaining aerobic respiration at depth. In contrast, in the absence of weathering-induced fracturing and depletion of pO2, a negative feedback that may be modulated by soil micro-biota ultimately results in thin regolith. These feedbacks may have been important in weathering systems over much of earth's history.

Both vertical and lateral flows of rock and water occur within eroding hills. Specifically, when considered over geological timeframes, rock advects vertically upward under hilltops in landscapes experiencing uplift and erosion. Once rock particles reach the land surface, they move laterally and down the hillslope because of erosion. At much shorter timescales, meteoric water moves vertically downward until it reaches the regional water table and then moves laterally as groundwater flow. Water can also flow laterally in the shallow subsurface as interflow in zones of permeability contrast. Interflow can be perched or can occur during periods of a high regional water table. The depths of these deep and shallow water tables in hills fluctuate over time. The fluctuations drive biogeochemical reactions between water, CO 2 , O 2 , and minerals and these in turn drive fracturing. The depth intervals of water table fluctuation for interflow and groundwater flow are thus reaction fronts characterized by changes in composition, fracture density, porosity, and permeability. The shallow and deep reaction zones can separate over meters in felsic rocks. The zones act like valves that reorient downward unsaturated water flow into lateral saturated flow. The valves also reorient the upward advection of rock into lateral flow through solubilization. In particular, groundwater removes highly soluble, and interflow removes moderately soluble minerals. As rock and water moves through the system, hills may evolve toward a condition where the weathering advance rate, W , approaches the erosion rate, E . If W = E , the slopes of the deep and shallow reaction zones and the hillsides must allow removal of the most soluble, moderately soluble, and least soluble minerals respectively. A permeability architecture thus emerges to partition each evolving hill into dissolved and particulate material fluxes as it approaches steady state.

Homo erectus was anatomically variable and behaviorally flexible using both Oldowan and Acheulian artifacts., Although stone tools generally co-occur with early members of the genus Homo, they are rarely found in direct association with hominins. We report that both Acheulian and Oldowan artifacts and Homo erectus crania were found in close association at 1.26 million years (Ma) ago at Busidima North (BSN12), and ca. 1.6 to 1.5 Ma ago at Dana Aoule North (DAN5) archaeological sites at Gona, Afar, Ethiopia. The BSN12 partial cranium is robust and large, while the DAN5 cranium is smaller and more gracile, suggesting that H. erectus was probably a sexually dimorphic species. The evidence from Gona shows behavioral diversity and flexibility with a lengthy and concurrent use of both stone technologies by H. erectus, confounding a simple “single species/single technology” view of early Homo.

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.

At Earth's surface, bedrock transforms to regolith in a process that has many implications for water storage, soil formation, and nutrient availability to ecosystems. To understand deep regolith formation, we investigate three zones demarcating changes in mineralogy, chemistry and Fe isotopic composition in a 4-m thick regolith profile developed at a ridge top in Pennsylvania (U.S.A.) that is underlain entirely by diabase. Relative to the unweathered diabase, Zone 1 at the bottom is characterized by major element depletion, abrupt oxidation of Fe(II), a rapid decrease in sulfur concentrations and the presence of short-range ordered (SRO) Fe with depleted δ56Fe values relative to unweathered diabase. We attribute many of the observations at this depth (which we refer to as the lithogenic zone) to the effects of spheroidal weathering, which may be releasing a flux of Fe(II) with low δ56Fe values as the mineral pyroxene is weathered. This Fe(II) is oxidized due to the presence of oxygen at the bottom of the profile and is likely precipitated as isotopically light SRO Fe phases, which then recrystallize with time. Zone 2 is interpreted as a region of crystallization in which minimal bulk chemical changes occur. Finally, SRO Fe isotopic compositions become lighter again near the land surface in Zone 3, the zone of most intense biogenic weathering. Biological Fe weathering likely involves enhanced microbial activity, exudation of protons and organic acids by plant roots and fungi, and plant uptake. This pattern of SRO Fe isotopic variability (with isotopically depleted values both at depth and near the surface) has been observed in another regolith profile on spheroidally weathering bedrock in Puerto Rico. We further used pore-water concentrations and soil chemistry to calculate rates of diabase weathering. While pore-water concentrations yield modern rates, soil chemistry concentrations indicate longer-term rates. The similarity in the diabase dissolution rates using modern pore fluids and soil chemistry is consistent with steady-state weathering. Similar approaches can be applied to other Appalachian soil profiles in the future.

Archived soil samples hold important information for reconstructing ancient environments and can provide data on prehistoric land use, manipulation and changes over time. Archeological investigations at the Abbott Farm National Historic Landmark (NHL) located near Trenton, New Jersey recovered occupations from Paleoindian to Late Woodland periods. Soil samples collected during the 1930s WPA excavations have never been analyzed until now. This paper discusses the results of a carbon isotope analysis of soil organic carbon, δ 13 C soc , performed on 54 samples from a number of archeological contexts from the Abbott Farm NHL. Data from Early and Middle Woodland pits and associated soils exhibit average δ 13 C soc values of − 25.0 ± 1.0‰ suggesting that the suite of samples have an isotopic signature that primarily reflects aboveground C 3 vegetation. These values agree with current interpretations of an Early and Middle Woodland that had no maize and a gradual Late Woodland introduction of maize (C 4 plant) into the Lower Delaware River Valley. Assuming these samples have not experienced significant organic matter oxidation in their ~ 80 years of storage, the Abbott Farm results suggest that preserved archeological feature and soil samples in other collections that have been desiccated and housed in climate controlled facilities serve as an untapped resource for paleoenvironmental reconstruction for sites that either no longer exist or where access is limited. In the case of Abbott Farm NHL, the archeological feature and soil samples provide information on prehistoric site vegetation and paleoenvironment before the beginnings of agriculture in the Delaware Valley.

Accurate measurements of soil CO2 concentrations (pCO2) are important for understanding carbonic acid reaction pathways for continental weathering and the global carbon (C) cycle. While there have been many studies of soil pCO2, most sample or model only one, or at most a few, landscape positions and therefore do not account for complex topography. Here, we test the hypothesis that soil pCO2 distribution can predictably vary with topographic position. We measured soil pCO2 at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), Pennsylvania, where controls on soil pCO2 (e.g., depth, texture, porosity, and moisture) vary from ridge tops down to the valley floor, between planar slopes and slopes with convergent flow (i.e., swales), and between north and south-facing aspects. We quantified pCO2 generally at 0.1–0.2 m depth intervals down to bedrock from 2008 to 2010 and in 2013. Of the variables tested, topographic position along catenas was the best predictor of soil pCO2 because it controls soil depth, texture, porosity, and moisture, which govern soil CO2 diffusive fluxes. The highest pCO2 values were observed in the valley floor and swales where soils are deep (≥0.7 m) and wet, resulting in low CO2 diffusion through soil profiles. In contrast, the ridge top and planar slope soils have lower pCO2 because they are shallower (≤0.6 m) and drier, resulting in high CO2 diffusion through soil profiles. Aspect was a minor predictor of soil pCO2: the north (i.e., south-facing) swale generally had lower soil moisture content and pCO2 than its south (i.e., north-facing) counterpart. Seasonally, we observed that while the timing of peak soil pCO2 was similar across the watershed, the amplitude of the pCO2 peak was higher in the deep soils due to more variable moisture content. The high pCO2 observed in the deeper, wetter topographic positions could lower soil porewater pH by up to 1 pH unit compared to porewaters equilibrated with atmospheric CO2 alone. CO2 is generally the dominant acid driving weathering in soils: based on our observations, models of chemical weathering and CO2 dynamics would be improved by including landscape controls on soil pCO2.

The Archean atmosphere is thought to have been devoid of oxygen but, instead, containing high concentrations of greenhouse gases, such as CO2 and possibly CH4, that were required to keep the Earth surface environments warm enough for maintenance of liquid water under faint-young-Sun conditions. Earlier studies have suggested that the CO2 concentrations must have been as high as over 1000 times present atmospheric levels (PAL) to overcome the effects of weaker solar luminosity. However, more recent studies of Precambrian paleosols imply existence of at least tenfold-lower, but still high pCO2 levels. The appearance of minute amounts of atmospheric O2 at the transition of Archean and Proterozoic is well documented, however, the bulk composition of the atmosphere and its dynamics at this crucial time is still widely debated. Different paleoclimate proxies of the ~2.45 Ga Kuksha paleoweathering crust suggest its formation in a cool, temperate climate. Paleoatmospheric pCO2 estimates by geochemical and isotopic mass-balance models suggest remarkably low paleoatmospheric CO2 levels, with the best guess estimates of atmospheric CO2 between 1 and 10 PAL. We speculate that the Kuksha paleoweathering crust either predates, or partly overlaps with, the onset of Huronian glaciation and corresponds to a time interval of low CO2 level during the first recorded large glaciation in Earth history.

Soil organic carbon (SOC) storage in alluvial depositional zones is important as it serves as a centennial to millennial timescale sink of C. This study examines SOC variability in alluvial bars, floodplains and terraces in a forested, humid-subtropical setting along the Clarks River in the Mississippi River basin (MRB) using replicated soil sampling, characterization and modeling. Soil and depositional profiles range in age from a minimum of 8000 years ago to modern. Normalized (by depth) mean SOC stocks (kg m−2) of the surface soil and buried layers, i.e., buried soils and layers underlying lithologic discontinuities, are highest in the terraces. Normalized mean surface SOC stocks decrease from terraces to floodplains and then to bars; whereas normalized mean buried layer SOC stocks decrease from terraces to bars, and then floodplains. The depth distribution of stocks showed that buried layers accounted for >50% of the total SOC pool, regardless of landform type. A regression tree analysis shows that soil horizon, pH, landform, and magnetic susceptibility were significant predictors of mean SOC content. Notably, the regression tree shows that alkaline pH (>7.9), observed only in buried layers in the terrace, is an important predictor in higher mean SOC. These alkaline soil pH values were associated with paleosol calcite, where the δ13C and 14C from the calcite suggests it formed under C3 vegetation during the drier than modern mid-Holocene hypsithermal, ~8000 years ago. Buried soil and sediment contain the majority of the SOC in this humid-subtropical valley bottom, while landform position and past buried soil-forming environments (paleosols) played an important role in the storage of that SOC. This work reconfirms that there is a long record of a passive, buried SOC pool found deep in our river valley bottoms that holds clues to the past biogeochemical cycling of C on land prior to intensive agriculture and industrialization. The legacy of past Holocene climates and paleosols are directly linked to our current understanding of long-term storage of SOC.

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.

Ponencia presentada en: Geological Society of America 130th Annual Meeting: Indianapolis, Indiana, USA, 4-7 november, 2018

The impact of changing environments on the evolution and dispersal of Homo sapiens is highly debated, but few data are available from equatorial Africa. Lake Victoria is the largest freshwater lake in the tropics and is currently a biogeographic barrier between the eastern and western branches of the East African Rift. The lake has previously desiccated at ~17 ka and again at ~15 ka, but little is known from this region prior to the Last Glacial Maximum. The Pleistocene terrestrial deposits on the northeast coast of Lake Victoria (94–36 ka) are ideal for paleoenvironmental reconstructions where volcaniclastic deposits (tuffs), fluvial deposits, tufa, and paleosols are exposed, which can be used to reconstruct Critical Zones (CZ) of the past (paleo-CZs). The paleo-CZ is a holistic concept that reconstructs the entire landscape using geologic records of the atmosphere, hydrosphere, lithosphere, biosphere, and pedosphere (the focus of this study). New paleosol-based mean annual precipitation (MAP) proxies from Karungu, Rusinga Island, and Mfangano Island indicate an average MAP of 750 ± 108 mm year−1 (CALMAG), 800 ± 182 mm year−1 (CIA-K), and 1,010 ± 228 mm year−1 (PPM1.0) with no statistical difference throughout the 11 m thick sequence. This corresponds to between 54 and 72% of modern precipitation. Tephras bracketing these paleosols have been correlated across seven sites, and sample a regional paleo-CZ across a ~55 km transect along the eastern shoreline of the modern lake. Given the sensitivity of Lake Victoria to precipitation, it is likely that the lake was significantly smaller than modern between 94 and 36 ka. This would have removed a major barrier for the movement of fauna (including early modern humans) and provided a dispersal corridor across the equator and between the rifts. It is also consistent with the associated fossil faunal assemblage indicative of semi-arid grasslands. During the Late Pleistocene, the combined geologic and paleontological evidence suggests a seasonally dry, open grassland environment for the Lake Victoria region that is significantly drier than today, which may have facilitated human and faunal dispersals across equatorial East Africa.