The influence of time on the magnetic properties of late Quaternary periglacial and alluvial surface and buried soils along the Delaware River, USA

Abstract

Magnetic susceptibility of soils is a common proxy for rainfall, but other factors can contribute to magnetic enhancement in soils. Here we explore influence of centuryto 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 X lf , 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 and/or magnetite that contributes to the magnetic enhancement of the soil. Room-temperature and lowtemperature field-cooled and zero field-cooled remanence curves confirm the presence of goethite and magnetite and/or maghemite 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 X lf 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 X lf (9.5-8.5 ka) corresponds with a welldocumented 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 X lf . Middle Holocene enhanced X lf (6.1-4.3 ka) corresponds with a wet to dry transitional phase and a previously documented incision event along the valley bottom. In this case the incision and terrace development resulted in prolonged surface exposure and more time for the accumulation of secondary ferrimagnetic minerals, enhancing X lf . The results of this study agree with previous modeling efforts, and show that in Quaternary (and possibly pre-Quaternary) periglacial and alluvial soils and paleosols that weathered for 10 1 -10 4 years, duration of pedogenesis, rather than climate, is an important control on magnetic enhancement.

Linking anthropogenic burning activities to magnetic susceptibility: Studies at Brazilian archaeological sites

“…Reference soil samples CX1 and CX2 were associated with clusters “c” and “e” and were collected in the A and B horizons from a Haplic Cambisol in a dry forest environment. A dry forest is a vegetation community that hosts frequent natural fires, and the data prove the capacity of wildfire to produce magnetic signatures in the natural soil, as previously shown by Ketterings et al (2000) and Stinchcomb and Peppe (2014).…”