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 (<152 thousand years ago (ka)), which has received little attention despite representing a critical period in the evolution of early Homo sapiens and its migration out of Africa. The Odele Member is 40–50 m thick and is dated using tephrochronology, radiometric, luminescence, and electron spin resonance techniques. The member spans 151 to 7 ka, defined at the base by the widespread Waidedo Vitric Tuff (WAVT, 151 ± 16 ka modeled age and 95.4% credible interval - 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 21.7 and 12.9 ka. The uppermost Odele Member also contains black, organic-rich mats, redox features, reed casts, and freshwater 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. These key findings suggest that periods of incision, aggregation, and landscape stability in the Odele Member have an orbital precession pacing. 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 Odele Member documents final Busidima half-graben infilling. • Changes in fluvial style occur on a precessional rhythm. • Fluvial incision and paleo-wetland deposits are associated with interglacials. [ABSTRACT FROM AUTHOR]

The study of buried soil archives in alluvial settings can be used to characterize floodplain and terrace habitats and complement other paleoenvironmental and archaeological archives. This study examines the early Holocene alluvial paleopedology along the Delaware River Valley in the northeastern USA. Chronological and alluvial stratigraphic data show that a widespread loam alluvial paleosol formed from 10.7 to 9.3 ka, herein referred to as the Jennings Lane Pedocomplex (JLP). The JLP at the Jennings Lane locality along the Delaware River, NJ, is a loam to silt loam soil that formed on mixed alluvium and windblown and/or reworked dust, likely sourced from nearby periglacial fines. This finding supports previous interpretations of late Quaternary dust deposition and reworking in the region. Fragic properties (e.g., firm peds with brittle manner of failure and rapid slaking in water) and cryogenic micromorphological features (lenticular pores, stone jacking and shattered grains) present within the JLP suggest cryoturbation. Soil mass-balance modeling suggests that the strain ( ɛ ) in immobile elements Ti and Zr show a dilation–collapse profile. These data indicate that organic matter and/or dust addition, and freeze-thaw created soil dilation at the JLP paleosurface. Mass-balance additions in Fe, K, and Mg are consistent with illuviated Fe-rich clay coatings present in thin section. The X-ray diffraction results show that these clays are illite, vermiculite, and kaolinite. Paleosol data presented here suggest that the JLP was overprinted by later pedogenic processes during the middle Holocene. τ Ti,Na shows minimal loss throughout much of the profile. Although rapid thaws may have promoted downward translocation of fines, the cold environment impeded base loss through either minimal release of cations from primary minerals or negligible water flux, as suggested by τ Ti,Na profiles and gains in τ Ti,K and τ Ti,Mg . This silt-rich JLP pedofacies experienced minimal chemical depletion over the course of ∼1200–3500 years. This cold-climate weathering coincides with rapid temperature fluctuations (10.5, 9.3, 8.2 ka) during the early Holocene in northeastern USA. This soil cryoturbation evidence has archaeological implications as well. The cryoturbation in this early Holocene pedocomplex suggests that vertical and lateral artifact redistribution is possible. Archaeologists should use caution when interpreting archaeological stratigraphy and intrasite artifact patterns at mid- to high-latitude archaeological sites. [ABSTRACT FROM AUTHOR]

Abstract: This study tests and refines the traditional floodplain development model for the partly confined middle Delaware River valley, which has shown that the main channel was relatively stable and flanked by a 6000–8000year old, vertically accreting alluvial terrace. The Holocene alluvial processes and history presented here in 6 fluvial phases were reconstructed using morphostratigraphy, 36 profile descriptions, 332 grain size analyses, and 82 14C ages from soil trenches, auger borings, and archaeological excavations. Fluvial phases I–III largely validate previous reconstructions showing a late Pleistocene (I: >10.7ka) braided stream transition into an early Holocene wandering stream with prolonged floodplain stability (II: 10.7–8ka), followed by early–middle Holocene erosion and then deposition (III: 8–5ka). The early and middle Holocene changes in alluvial stratigraphy and sedimentology broadly correlate with climatically derived Holocene subdivisions, suggesting climate change partly controls alluvial response along the middle Delaware River valley. This study documents for the first time a middle Holocene episode of channel incision occurring sometime between 6.0 and 5.0ka. Although the results reconfirm that the majority of alluvial landforms are composed of vertical accretion deposits, we present here new evidence of oblique, abandoned channel, and lateral accretion deposits inset to similar landforms with different formation histories (i.e., polycyclic terrace development), depicting a river valley that has experienced floodplain and terrace reworking. The majority of floodplain and terrace reworking occurs during the late–middle and late Holocene phases IV–VI (5.5–0ka), following the middle Holocene incision event. These phases demonstrate floodplain reworking processes in the form of channel abandonment, stripping, flood channeling, and convex bank erosion. The subsequent space filled rapidly with evidence of multistory soil formation, and eventually resulted in alluvial fill terraces with heights comparable to older surfaces. The most recent fluvial phase, VI (1.0–0ka), shows markedly faster sedimentation rates and coarser flood deposits associated with peak prehistoric population and the onset of Euroamerican settlement. The results presented here contribute newly documented evidence of erosion and depict a river valley that has undergone middle Holocene incision and floodplain and terrace reworking since the late–middle Holocene. Stream modelers, fluvial geomorphologists and geoarchaeologists working in the NE USA should consider floodplain reworking processes when examining alluvial history and predicting changes in stream dynamics. [Copyright &y& Elsevier]

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 SiO 2 , TiO 2 , Al 2 O 3 , 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 (MAT PPM ) and Mean Annual Precipitation (MAP PPM ) 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 p CO 2 is poorly constrained due to the uncertainty in duration of weathering and possibility of regolith removal by erosion, p CO 2 is calculated to be ≥4.8 × Pre-Industrial Atmospheric Level (280 ppm) for 100,000 years of weathering, which is consistent with the p CO 2 level at 1.7 Ga predicted by the weak sun model. [ABSTRACT FROM AUTHOR]

Accurate measurements of soil CO 2 concentrations (pCO 2 ) 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 pCO 2 , 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 pCO 2 distribution can predictably vary with topographic position. We measured soil pCO 2 at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), Pennsylvania, where controls on soil pCO 2 (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 pCO 2 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 pCO 2 because it controls soil depth, texture, porosity, and moisture, which govern soil CO 2 diffusive fluxes. The highest pCO 2 values were observed in the valley floor and swales where soils are deep (≥0.7 m) and wet, resulting in low CO 2 diffusion through soil profiles. In contrast, the ridge top and planar slope soils have lower pCO 2 because they are shallower (≤0.6 m) and drier, resulting in high CO 2 diffusion through soil profiles. Aspect was a minor predictor of soil pCO 2 : the north (i.e., south-facing) swale generally had lower soil moisture content and pCO 2 than its south (i.e., north-facing) counterpart. Seasonally, we observed that while the timing of peak soil pCO 2 was similar across the watershed, the amplitude of the pCO 2 peak was higher in the deep soils due to more variable moisture content. The high pCO 2 observed in the deeper, wetter topographic positions could lower soil porewater pH by up to 1 pH unit compared to porewaters equilibrated with atmospheric CO 2 alone. CO 2 is generally the dominant acid driving weathering in soils: based on our observations, models of chemical weathering and CO 2 dynamics would be improved by including landscape controls on soil pCO 2 . [ABSTRACT FROM AUTHOR]

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 Al 2 O 3 ) for the newly characterized Erra profile studied here showed loss of 25–50% SiO 2 and MgO , complete loss of CaO and Na 2 O, 25–50% loss of K 2 O, and 25–100% gain in Fe 2 O 3 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: PPM 1.0 ) were also estimated using bulk geochemical proxies. Calculations for p CO 2 follow an earlier thermodynamic method with evaluation of total mass flux for CaO, MgO, Na 2 O, and K 2 O CALC (corrected for K metasomatism) during weathering of each protolith material. When considering a range of acceptable values of paleoatmospheric p CO 2 based on earlier studies, a minimum of 1.88 × 10 5 and a maximum of 1.02 × 10 6  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. [ABSTRACT FROM AUTHOR]