Using Soils to Reconstruct Mid-continental Climatic Change

Christoph E. Geiss, Trinity College

Collaborators and Students:C. William Zanner, Univ. of Nebraska, LincolnSubir K. Banerjee, Univ. of MinnesotaJames Bisbee, Daniel Scollan, Trinity CollegeJoanna 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 Münchenhttp://www.lrz-muenchen.de

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 profilemodified fromKukla et al, Geology, 16, 811-814, 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, 7843-7852 , 1982

Coercivity the Cheapo Way

• S-ratio: gives relative abundance of hard/soft minerals

• Hard IRM (HIRM): gives absolute abundance of hard/soft minerals

J300

Jsat

modified from:Butler, J. Geophys. Res., 87, 7843-7852 , 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

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