MAGNETIC SUSCEPTIBILITY OF EARLY PALEOZOIC AND PRECAMBRIAN PALEOSOLS

Abstract

Paleosols formed by weathering and plant colonization during breaks in eolian deposition of Quaternary loesses are marked by enhanced magnetic susceptibility. These paleosols can be recognized in the field from their soil structure and soil horizons, but the most obvious and diagnostic feature of paleosols are root traces. For paleosols of early Paleozoic and Precambrian age, pre-dating the evolution of rooted land plants, root traces cannot be used as a diagnostic field criterion for recognizing paleosols. Surficial enhancement of magnetic susceptibility can be useful confirmation that some Precambrian and early Paleozoic rocks were paleosols. Unfortunately, magnetic susceptibility enhancement of some paleosol profiles is compromised by burial gleization, which reduces overall susceptibility values, but especially susceptibility at the surface of the profile, and produces a marked covariance of magnetic susceptibility with total iron content. Groundwater and surface water gleization during soil formation also depletes susceptibility over a wide range of iron content to give relatively flat depth functions of susceptibility. In contrast, well-drained soils and paleosols have pronounced surficial susceptibility enhancement over a narrow range of iron content. Magnetic susceptibility peaks are useful supporting evidence for paleosols entirely red and oxidized, but not for paleosols with common green-gray reduction spots, abundant organic matter, iron–manganese nodules or other evidence of gleization. All known Paleozoic and Precambrian paleosols have susceptibility an order of magnitude less than in comparable Quaternary paleosols and soils, so that Quaternary magnetic susceptibility climofunctions and chronofunctions cannot yet be applied to Paleozoic and Precambrian paleosols. Although their susceptibility values are comparable with those of modern gleyed soils, Paleozoic and Precambrian paleosols maintain a depth function of surficial enhancement of susceptibility, as well as red color, deeply penetrating cracks and other features of well-drained soils. Burial recrystallization of fine-grained magnetite and maghemite may have compromised the magnitude of their susceptibility peaks. Alternatively, they may not have been as biologically active in susceptibility-enhancing microbes as modern soils.