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

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)

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

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)

Sampling Sites

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

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

image courtesy of Leibnitz Rechenzentrum München

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, , 1988

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?

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 ?

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

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

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

Magnetic Enhancement of 4G-99A

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

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)

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)

Grainsize Variations in 4G-99A

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

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, , 1982

Coercivity the Cheapo Way S-ratio: gives relative abundance of hard/soft minerals Hard IRM (HIRM): gives absolute abundance of hard/soft minerals J 300 J sat modified from: Butler, J. Geophys. Res., 87, , 1982

4G-99A HIRM measurements

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

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?

Climate Dependence of Magnetic Enhancement

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 Some Preliminary Conclusions