Abstract
Magnetic susceptibility of soils has been used as a proxy for rainfall, but other factors can contribute to magnetic enhancement in soils. Here we explore influence of century- to millennial-scale duration of soil formation on periglacial and alluvial soil magnetic properties by assessing three terraces with surface and buried soils ranging in exposure ages from <0.01 to ~16 kyrs along the Delaware River in northeastern USA. The A and B soil horizons have higher Xlf, Ms, and S-ratios compared to parent material, and these values increase in a non-linear fashion with increasing duration of soil formation. Magnetic remanence measurements show a mixed low- and high-coercivity mineral assemblage likely consisting of goethite, hematite and maghemite that contributes to the magnetic enhancement of the soil. Room-temperature and low-temperature field-cooled and zero field-cooled remanence curves confirm the presence of goethite and magnetite and show an increase in magnetization with increasing soil age. These data suggest that as the Delaware alluvial soils weather, the concentration of secondary ferrimagnetic minerals increase in the A and B soil horizons. We then compared the time-dependent Xlf from several age-constrained buried alluvial soils with known climate data for the region during the Quaternary. Contradictory to most studies that suggest a link between increases in magnetic susceptibility and high moisture, increased magnetic enhancement of Delaware alluvial soils coincides with dry climate intervals. Early Holocene enhanced soil Xlf (9.5 – 8.5 ka) corresponds with a well-documented cool-dry climate episode. This relationship is probably related to less frequent flooding during dry intervals allowing more time for low-coercive pedogenic magnetic minerals to form and accumulate, which resulted in increased Xlf. Middle Holocene enhanced Xlf (6.1 – 4.3 ka) corresponds with a transitional wet/dry phase and a previously documented incision event.......
Highlights
Increased magnetic susceptibility within soil horizons is a widespread observable phenomenon that has been a longstanding subject of pedologic, rock magnetic, and paleoenvironmental research (e.g., Le Borgne, 1955; Mullins, 1977; Maher, 1986; Geiss and Zanner, 2006)
The weathering profile along the T3 surface is mapped by the United State Department of Agriculture (USDA) as the Hoosic series, a Sandy-skeletal, mixed, mesic, Typic Dystrudept formed on glacial outwash
SOIL MICROMORPHOLOGY Results from the T2 soil micromorphology show a parent material composed of a mixed mineralogy that is dominated by quartz (Figure 4A)
Summary
Increased magnetic susceptibility within soil horizons is a widespread observable phenomenon that has been a longstanding subject of pedologic, rock magnetic, and paleoenvironmental research (e.g., Le Borgne, 1955; Mullins, 1977; Maher, 1986; Geiss and Zanner, 2006). The origins of the increased soil magnetic susceptibility have often been attributed to the biotic or abiotic weathering and alteration of hydrous ferric oxides into finegrained magnetite and maghemite (e.g., Dearing et al, 1996b; Singer et al, 1996; Boyle et al, 2010), or generation of magnetite or maghemite from burning (e.g., Le Borgne, 1960; Thompson and Oldfield, 1986; Kletetschka and Banerjee, 1995; Oldfield and Crowther, 2007). Despite advances in paleoenvironmental and paleoclimatic reconstruction, much of the magnetic research has been performed on successions of buried soils forming on loess parent materials, possibly limiting its application to a specific depositional environment
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