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Using Soils to Reconstruct Mid-continental Climatic Change

Using Soils to Reconstruct Mid-continental Climatic Change. Christoph E. Geiss, Trinity College Collaborators and Students: C. William Zanner, Univ. of Nebraska, Lincoln Subir K. Banerjee, Univ. of Minnesota James Bisbee, Daniel Scollan, Trinity College Joanna Minott, Mt. Holyoke College.

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Using Soils to Reconstruct Mid-continental Climatic Change

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  1. Using Soils to Reconstruct Mid-continental Climatic Change Christoph E. Geiss, Trinity College Collaborators and Students: C. William Zanner, Univ. of Nebraska, Lincoln Subir K. Banerjee, Univ. of Minnesota James Bisbee, Daniel Scollan, Trinity College Joanna Minott, Mt. Holyoke College

  2. Great Plains Region • intensively farmed agricultural region • western part dependent on irrigation • very few good records of paleoclimatic change • large parts covered by wind-blown dust (loess)

  3. Long-Term Plan • establish transfer function between modern climate and soil properties • invert transfer function and apply to buried soils (paleosols) • reconstruct paleoclimate for certain time slices over last 130,000 years • low temporal, but high spatial resolution

  4. Site Selection • modern soils • loessic substrate • stable upland positions • transsect to capture climate gradient most soil forming factors are held constant, except for climate and biota (which are assumed to be controlled by climate)

  5. Sampling Sites

  6. Site Selection • initial sites: public lands • second fieldseason: cemetaries (undisturbed by agriculture, set aside often prior to settlement)

  7. Useful Parameters • In-situ profile description • Color (Munsell and spectrophotometer) • Chemistry (org. matter, carbonates, Fe, Mn) • Magnetic enhancement of upper soil horizons

  8. image courtesy of Leibnitz Rechenzentrum München http://www.lrz-muenchen.de

  9. Chinese Loess Plateau • Modern soil and paleosols are more magnetic than loess • Magnetic enhancement of modern soils reflects modern precipitation gradient • Paleoprecipitation proxy? Xifeng loess – paleosol profile modified from Kukla et al, Geology, 16, 811-814, 1988

  10. Questions • What causes magnetic enhancement ? • Is magnetic signal preserved after burial? • Is magnetic enhancement a universal proxy ? • Can we use it to reconstruct paleoclimate for central United States?

  11. Some Potential Processes of Magnetic Enhancement • Depletion of non-magnetic particles (lessivage) • Reduction of weakly magnetic minerals to magnetite / maghemite • Neoformation of Fe-oxides / Fe-oxyhydroxides • Systematic changes in parent material ?

  12. Magnetic Methods Want to characterize: • Abundance • Particle-size distribution • Mineralogy indirect (magnetic) methods: fast, (mostly) sensitive, cost-effective

  13. Concentration of Ferrimagnetic Minerals • Magnetic susceptibility • Isothermal Remanent Magnetization (IRM) • Anhysteretic Remanent Magnetization (ARM) • and a few others

  14. Example: Site 4G-99A • Located in NE Nebraska • Sampled in 1999 using Giddings corer • Subsampled into plastic boxes in 2000 • Analyzed in 2000 (REU project) and 2003

  15. Magnetic Enhancement of 4G-99A

  16. Characterization of Magnetic Grain-size • grainsize characterized by domain state • multi domain MD (< 10 μm) • single domain SD (0.01 – 0.1 μm) • superparamagnetic SP ( < 0.01 μm) domain state affects magnetic behavior of mineral grains

  17. Grain-size Dependent Parameters • many parameters concentration and grainsize dependent • normalized parameters • ARM / IRM : fine SD particles • susceptibility / IRM : super fine SP particles • Frequency dependent susceptibility (SP)

  18. Normalized Parameters • IRM, ARM both concentration and grain-size dependent • Ratio of ARM/IRM (concentration indep.) mostly proxy for small single-domain (SD) grains, (d ≈ 0.01 – 0.1 µm)

  19. Grainsize Variations in 4G-99A

  20. Magnetic Mineralogy • magnetic minerals occur • in low concentrations (< 1 %) • in poorly crystalline states → hard to characterize using XRD, Mössbauer etc. • magnetic ordering and phase transitions • magnetic coercivity measurements but: magnetization of magnetite >> magnetization of goethite, hematite

  21. IRM-Acquisition Curves • describes how easy mineral is to magnetize • magnetite = magnetically soft, saturates in low fields • hematite, goethite = magnetically hard, probably impossible to saturate after Butler, J. Geophys. Res., 87, 7843-7852 , 1982

  22. Coercivity the Cheapo Way • S-ratio: gives relative abundance of hard/soft minerals • Hard IRM (HIRM): gives absolute abundance of hard/soft minerals Jsat J300 modified from: Butler, J. Geophys. Res., 87, 7843-7852 , 1982

  23. 4G-99A HIRM measurements

  24. 4G-99A magnetic summary • upper soil horizons are enhanced in magnetic minerals • concentration increases • grainsize decreases • pedogenic component is mixture of magnetite and goethite / hematite

  25. Cause for Magnetic Enhancement • concentration of Fe slightly decreases in enhanced horizons • weathering of Fe-bearing minerals and neoformation of poorly crystalline magnetite and goethite/hematite • microbially mediated?

  26. Climate Dependence of Magnetic Enhancement

  27. Climate Dependence of Magnetic Enhancement

  28. Some Preliminary Conclusions • Midwestern modern soils are magnetically enhanced • Climatic influence seen best in parameters that are biased towards small particles • Magnetic enhancement due to neoformation of magnetite and magnetically hard minerals such as goethite or hematite • Neoformation likely aided by microbial activity

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