Genesis of Non-Allophanic E Horizons in Tephra-Influenced Spodosols
- P. A. McDaniel ,
- A. L. Falen and
- M. A. Fosberg
Holocene volcanic ash from the cataclysmic eruption of Mount Mazama in southwestern Oregon is an important component of many soils of the Pacific Northwest region. High-elevation forested soils of northern Idaho containing Mazama tephra have developed E-Bhs horizon sequences and are accordingly classified as Spodosols. In this study, we report on the formation and expression of E horizons in a developmental sequence of volcanic-ash-influenced Spodosols from this region. Results suggest that development of E horizons in these volcanic-ash-influenced soils proceeds systematically along an elevational gradient. The lightest colored, thickest E horizons are associated with the highest elevation soils of the subalpine fir [Abies lasiocarpa (Hook.) Nutt.] forest zone, and E horizons are more weakly expressed and then absent with decreasing elevation. Microprobe analysis of volcanic glass shows that E and Bhs horizons contain the same tephra, indicating that substantial pedogenesis has taken place in these horizons. Formation and eluviation of Al-humus complexes has given rise to E horizons whose mineralogy and chemistry are consistent with that of non-allophanic Andisols and differ markedly from underlying horizons. Beidellite, an Al-rich smectite, dominates the clay fraction of the well-developed, very strongly acid E horizons at the expense of allophanic clays and metal-humus complexes; other than beidellite, opaline SiO2 is the major secondary Si-bearing phase. Chemical and mineralogical properties of illuvial B horizons are consistent with those of allophanic soils; short-range-order aluminosilicates and metal-humus complexes dominate the clay fractions. Results indicate that coupled eluvial-illuvial processes associated with podzolization are responsible for the non-allophanic character of E horizons and the allophanic character of Bhs horizons in these forested, volcanic-ash-influenced ecosystems.
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