Late glacial and postglacial pollen and plant macrofossils from Lake West Okoboji, Northwestern Iowa

Pollen and plant macrofossils preserved in lake sediment from Lake West Okoboji, Dickinson County, Iowa, indicate how the vegetation of that area changed during the late glacial and postglacial. A closed coniferous forest, dominated by spruce and larch trees, produced the Picea-Larix pollen assemblage zone. Fir trees were a minor constituent of this forest; pine trees were probably absent. Black ash trees increased in abundance at Lake West Okoboji and by 13,500 yr ago were an important constituent of the forest. The sediment accumulation rate and the pollen influx were low throughout this time. Birch and alder pollen peaked in abundance approximately 11,800 yr ago. Pollen influx increased rapidly as birch and alder replaced coniferous trees on the uplands. A deciduous forest, containing abundant oak and elm trees, replaced the birch-alder-coniferous forest. This forest inhabited northwestern Iowa from approximately 11,000 to 9000 yr B.P. Nonarboreal species became prevalent between approximately 9000 and 7700 yr B.P. as prairie began to replace deciduous forest on the uplands. Charred remains of Amorpha canescens and other upland species attest to the presence of prairie fires as an aid in establishing prairie and destroying the forest. The pollen influx declined. The warmest, driest part of the postglacial occurred in northwestern Iowa from approximately 7700 to 3200 yr ago. Lake level fell 9 to 10 m, and prairie extended to the edge of the lake. Wet-ground weeds inhabited areas near lake level which were alternately flooded, then dry. Pollen influx was approximately 100 grains/cm²/yr during the driest time in this dry interval.Deciduous trees, particularly oaks, returned after approximately 3200 yr B.P. Prairie continued to occupy the uplands but trees were more common in the lowlying wet areas. Settlement by Europeans in northwestern Iowa about 1865 is marked by an increase in weed pollen. Macrofossil deposition changed in 1910 in response to the stabilization of lake level.     

Archean geotherms and supracrustal assemblages

Metamorphic mineral assemblages suggest the existence of variable geotherms and lithospheric thicknesses beneath late Archean continental crust. Archean granite-greenstone terranes reflect steep geotherms (50–70°C/km) while high-grade terranes reflect moderate geotherms similar to present continental crust with high heat flow (25–40°C/km). Corresponding lithosphere thicknesses for each terrane during the late Archean are 35–50 km and 50–75 km, respectively.Early Archean ( 3.0 b.y.) greenstones differ from late Archean ( 2.7 b.y.) greenstones by the rarity or absence of andesite and graywacke and the relative abundance of pelite, quartzite, and komatiite. Mature clastic sediments in early greenstones reflect shallow-water, stable-basin deposition. Such rocks, together with granite-bearing conglomerate and felsic volcanics imply the existence of still older granitic source terranes. The absence or rarity of andesite in early greenstones reflects the absence of tectonic conditions in which basaltic and tonalitic magmas are modified to produce andesite.A model is presented in which early Archean greenstones form at the interface between tonalite islands and oceanic lithosphere, over convective downcurrents; high-grade supracrustals form on stable continental edges or interiors; and late Archean greenstones form in intracontinental rifts over mantle plumes.     

Ionic and elemental composition of PM<Subscript>2.5</Subscript> aerosols over the Caribbean Sea in the Tropical Atlantic

To characterize atmospheric particulate matter equal or less than 2.5 μm in diameter (PM_2.5) over the Tropical Atlantic Ocean, aerosol sampling was carried out in Puerto Rico during August and September, 2006. Aerosols were analyzed by ion chromatography for water-soluble inorganic and organic ions (including Na⁺, NH₄ ⁺, Mg²⁺, Ca²⁺, K⁺, Cl^?, SO₄ ^2?, NH₄ ⁺, F^?, methanesulfonate (MSA), and oxalate), by inductive coupled plasma mass spectrometry (ICPMS) for trace elements (Al, Fe, Zn, Mn, Cu, Ni, V, Pb, Cr, Sb, Co, Sc, Cd), and by scanning electron microscopy for individual aerosol particle composition and morphology. The results show that the dominant cations in aerosols were Na⁺, (mean: 631 ng m^?3), accounting for 63.8 % of the total cation and NH₄ ⁺ (mean: 164 ng m^?3), accounting for 13.8 % of the total cation measured in this study. The main inorganic anions were Cl^? (576 ng m^?3, 54.1 %) and SO₄ ^2? (596 ng m^?3, 38.0 %). The main organic anion was oxalate (18 ng m^?3). Crustal enrichment factor calculations identified 62 % of the trace elements measured (Cu, Ni, V, Co, Al, Mn, Fe, Sc, and Cr) with crustal origin. Single particle analysis demonstrated that 40 % of the aerosol particles examined were Cl^? rich particles as sodium chloride from seawater and 34 % of the total particles were Si-rich particles, mainly in the form of aluminosilicates from dust material. Based on the combination of air-mass trajectories, cluster analysis and principal component analysis, the major sources of these PM_2.5 particles include marine, Saharan dust and biomass burning from West Africa; however, volcanic emissions from the Soufriere Hills in Montserrat had significant impact on aerosol composition in this region at the time of sample collection.     

Observations of Solitary Waves in the Stable Atmospheric Boundary Layer

Large amplitude, isolated, wave-like phenomena have been observed in the lowest 40 m of the strongly stably stratified atmospheric boundary layer overlying a coastal Antarctic ice shelf. The waves only occur when prevailing wind speeds are low. They always propagate from over the land, with phase speeds exceeding the local mean wind speed. They have wavelengths of the order of 200 m. Several examples are described and a summary of the statistical properties of these waves events is presented.     

Laboratory Studies of the Hydrogen Kinetic Isotope Effects (KIES) of the Reaction of Non-Methane Hydrocarbons with the OH Radical in the Gas Phase

The hydrogen kinetic isotope effects (KIEs) of the reactions of 15 non-methane hydrocarbons (NMHCs) with the OH radical were measured at 298 ± 2 K. The measurements were made using NMHCs without artificial isotopic labeling or enrichment. The following average hydrogen KIE values, in per mil (), were obtained: 29.8 ± 2.1 (toluene),51.6 ± 2.1 (n-butane), 97.3± 12.5 (i-butane), 63.2 ± 5.9 (cyclopentane), 10.5 (p-xylene), 26.8 ± 3.5 (ethylbenzene), 65.9± 7.0 (n-pentane), 79.5 ± 9.6 (cyclohexane), 52.8 ± 5.0(n-hexane), 38.9 ± 7.8 (n-heptane), 33.4 ± 3.1 (n-octane), 29.6 ± 1.6(n-nonane), and 29.0 ± 5.3 (n-decane). The KIEs for reactions of two alkenes (cyclohexene and 1-heptene) could not be determined accurately due to interference from reaction with ozone, but nevertheless the results clearly show that the KIEs for reaction of alkenes with OH are significantly lower than those for saturated hydrocarbons. The KIEs for reaction of alkanes are smaller than isotope effects reported in literature for the reactions of NMHCs artificially labeled with deuterium. The main reason for this difference is the reduced probability for reaction at a labeled site for compounds with close to natural deuterium abundance, although some impact of secondary isotope effects cannot be ruled out. Still, the KIEs for NMHCs with natural or close to natural abundance of deuterium are of sufficient magnitude to allow determination of the extent of chemical processing of hydrocarbons in the atmosphere using methods analogous to stable carbon KIE studies. Furthermore, it is shown that combining stable hydrogen and stable carbon isotope ratio data has the potential to also provide valuable information regarding the stable isotope ratios of emissions, and specifically to test one of the key assumptions of the stable isotope hydrocarbon clock, the absence of significant variations of the stable isotope ratio for the emitted NMHCs.     

Fine-Scale Structure Observed In A Stable Atmospheric Boundary Layer By Sodar And Kite-Borne Tethersonde

Co-located high resolution profiles of acoustic backscatter,wind vector and potential temperature are presented, measured within the stable atmosphericboundary layer over an Antarctic ice shelf. Acoustic profiles from a monostaticacoustic radar (Sodar) indicate complex structure within the boundary layer, whilstwind and temperature profiles from a tethersonde show corresponding bands of differingstability. Internal waves and fossil convection are shown to invalidate attemptsto compare backscatter measurements with theoretical estimates based on local wind and temperature gradients, but during ideal conditions, a qualitative agreementis observed.     

Avoiding Circular Logic in Climate Modeling

The current debate on climate change and its policy implications often hinges on the credibility of global climate models. In considering this topic, we point out the danger of circular logic if climate model inputs (specifically, climate forcings) are determined from the same temperature record against which climate model outputs are tested. We suggest specific guidelines for avoiding this danger.     

Carbon dioxide,water vapor and sensible heat exchanges of a grain sorghum canopy

With the aid of eddy correlation instrumentation, the components of the energy budget and CO₂ flux were measured over grain sorghum grown at Mead, Nebraska. Diurnal patterns of sensible heat, latent heat, CO₂ and momentum flux are examined for typical days. On a mostly clear day when the crop leaf area index was 3.7, net radiation reached a mid-day peak of 560 W m^-2, while sensible and latent heat fluxes peaked at 50 and 460 W m^-2, respectively. The peak CO₂ flux occurring just prior to solar noon was 1.5 mg m^-2(ground area) s^-1. CO₂ flux (respiration from plants, soil and roots) in the early evening was about -0.28 mg m^-2 (ground area) s^-1.A relationship between CO₂ flux and photosynthetically active radiation (PAR) was developed. Except during the late stage of growth (growth stage 8.5, toward the initiation of senescence), the crop showed no evidence of saturation up to PAR 1800 Ei m^-2s^-1. The light compensation point was found to be about 211 Ei m^-2s^-1. Examination of CO₂ flux-PAR relationships for selected days through the season indicated an aging effect in terms of a decrease in photosynthetic activity of the sorghum canopy. Measurements made on two consecutive days demonstrate the effects of weather conditions on CO₂ flux and carbon-water flux ratio (a measure of water use efficiency of the crop). The occurrence of regional sensible heat advection with concommitant high vapor pressure deficit and air temperature-limited CO₂ exchange reduced the carbon-water flux ratio.Published as Paper No. 7717, Journal Series, Nebraska Agricultural Research Division. The work was conducted under Regional Research Project 11-33 and Nebraska Agricultural Research Division Project 27-003.Formerly Post Doctoral Research Associate (now at the University of Connecticut Department of Renewable Natural Resources Storrs CT)     

Phase partitioning of aerosol particles in clouds at Kleiner Feldberg

The partitioning of aerosol particles between cloud droplets and interstitial air by number and volume was determined both in terms of an integral value and as a function of size for clouds on Mt. Kleiner Feldberg (825 m asl), in the Taunus Mountains north-west of Frankfurt am Main, Germany. Differences in the integral values and the size dependent partitioning between two periods during the campaign were observed. Higher number and volume concentrations of aerosol particles in the accumulation mode were observed during Period II compared to Period I. In Period I on average 87±11% (±one standard deviation) and 73±7% of the accumulation mode volume and number were incorporated into cloud droplets. For Period II the corresponding fractions were 42±6% and 12±2% in one cloud event and 64±4% and 18±2% in another cloud event. The size dependent partitioning as a function of time was studied in Period II and found to have little variation. The major processes influencing the partitioning were found to be nucleation scavenging and entrainment.     

Climatological aspects of convective parameters from the NCAR/NCEP reanalysis

Annual cycles of convectively important atmospheric parameters have been computed for a variety of from the National Center for Atmospheric Research (NCAR)/National Centers for Environmental Prediction (NCEP) global reanalysis, using 7 years of reanalysis data. Regions in the central United States show stronger seasonality in combinations of thermodynamic parameters than found elsewhere in North America or Europe. As a result, there is a period of time in spring and early summer when climatological mean conditions are supportive of severe thunderstorms.The annual cycles help in understanding the large-scale processes that lead to the combination of atmospheric ingredients necessary for strong convection. This, in turn, lays groundwork for possible changes in distribution of the environments associated with possible global climate change.