Browsing by Subject "Isotope geochemistry."
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Item Climatic and human influences on Holocene alluvial history and paleoenvironment of the middle Delaware River Valley, USA.(2012-11-29) Stinchcomb, Gary E.; Driese, Steven G.; Nordt, Lee C.; Geology.; Baylor University. Dept. of Geology.The potential for future prolonged drought episodes in the Northeastern USA is alarming given that a humid climate currently provides water to +50 million people in the northeast, USA. Hydro-climatic projections are hampered by a lack of regionally-based paleoenvironmental reconstructions. The middle Delaware River Valley provides a unique opportunity to expand the Holocene alluvial history and paleoenvironment for the northeast, USA. Thirty-six soil profile descriptions, 332 grain size analyses, and 82 14C ages from trenches and auger borings show that similar alluvial landforms within the river valley have different formation histories and depict a valley that has experienced middle to late Holocene floodplain and terrace reworking. Despite erosion, secular changes in buried soil and sediment properties are closely associated with climate change and land-use. A Holocene time-series was constructed using 149 δ13Csom values from alluvial terrace profiles. There is good agreement between increasing δ13Csom and Panicoideae phytolith concentrations, suggesting that variations in C4 biomass are a contributor to changes in the soil δ13C. A measurement error deconvolution curve over time reveals two isotope stages (II and I), with nine sub-stages exhibiting variations in average δ13Csom (%C4). Stage II, ~10.7-4.3 ka, shows above average δ13Csom (increase %C4) values with evidence of an early Holocene warm/dry interval (sub-stage IIb, 9.8-8.3 ka) that coincides with rapid warming and cool/dry abrupt climate change. Sub-stage IId, 7.0-4.3 ka, is an above average δ13Csom (increase %C4) interval associated with the mid-Holocene warm/dry Hypsithermal. The Stage II-I shift at 4.3 ka documents a transition toward below average δ13Csom (decrease %C4) values, coinciding with decreasing insolation and moisture budget reorganization. Sub-stages Ib and Id (above average %C4) coincide with the first documented occurrence of maize in northeastern USA and population increase during the Late Woodland. These associations suggest that humans influenced δ13Csom during the late Holocene. The influence of land-use is further corroborated by a regionally extensive anthropogenic sedimentation event documented throughout eastern North America, pre-Colonial sediment (PCS) circa: A.D. 1,100–1,600. These data demonstrate that combined prehistoric land-use and climate change impacted eastern North American floodplains several hundred years prior to the onset of European Settlement.Item Field, micromorphologic and geochemical study of modern and ancient soils from Riesel, Texas and Rusinga Island, Lake Victoria, Kenya.(2014-06-11) Michel, Lauren Ashley.; Driese, Steven G.; Peppe, Daniel J.; Geology.; Baylor University. Dept. of Geology.Paleosol are utilized to reconstruct paleoclimate and paleoenvironment by paleopedologists. However, paleopedologists must constrain for all of the state factors that create a soil before paleoclimate or paleoenvironment can be assessed. Study of a modern catena in Riesel, TX shows that landscape position, parent material and time have important implications for pedogenesis. Soils with a limestone parent material component that form on ridge crests-side slopes may include marine bedrock within their pedogenic calcite nodules, because soil erosion rates exceed soil-production rates. Conversely, soils that form on toe slopes have a longer period of soil duration and formation, and more complete weathering of the parent material occurs. These soils have pedogenic carbonate nodules that reflect an admixture of CO₂ from the atmosphere and soil-derived CO₂. Since the initial discovery of Proconsul, Rusinga Island has been one of the most important Early Miocene fossil localities in East Africa, with vertebrate, invertebrate, and plant fossils recovered. Because environment is a catalyst for evolutionary adaptation, scientists have attempted to determine the paleoenvironment in which Proconsul lived. However, paleoenvironmental reconstructions have lead to conflicting results, with interpretations ranging from open grassland/parkland to closed-canopy rainforests. Recent revised stratigraphy, coupled with new paleomagnetic data and age dates, allows for a unique opportunity to reconstruct paleoenvironment for a time-slice of Miocene strata on Rusinga Island that contains both Pronconsul and the catarrhine Dendropithecus, among other vertebrate fossils. Paleopedological study has indicated the existence of a forest ecosystem developed on a landscape that received frequent inputs of sediment reworked from volcaniclastic materials. Global Positioning System mapping of fossil tree stump casts has allowed for modern forestry metrics to be applied whereas description of root cast density and structure, and paleosol descriptions have provided insight for below-ground processes. Although traditional paleosol-based proxies fail because of the lack of time for pedogenic development, paleobotanical proxies applied to fossil leaves in the overlying sandstone are able to constrain mean annual temperature and mean annual precipitation. Using this multi proxy approach, a higher-resolution picture has emerged that ties Proconsul definitively to a multistoried, closed-canopy, tropical seasonal forest that was probably relatively wet, and hot.