Monday, 10 July 2006 - 11:05 AM

Post-Active Acid Sulfate Soils.

Delvin S. Fanning1, Daniel Wagner2, Philip Zurheide1, Martin C. Rabenhorst1, and J. Patrick Megonigal3. (1) Univ of Maryland, Dept of Natural Resource Sciences and Landscape Architecture, H. J. Patterson Hall, College Park, MD 20742-4452, (2) Geo-Sci Consultants, Inc., 4410 Van Buren St., University Park, MD 20782, (3) Smithsonian Environmental Research Center, PO Box 28, Edgewater, MD 21037-0028

Post-active acid sulfate soils are geographically extensive on Tertiary and Cretaceous geologic sediments of the upper Coastal Plain of the U.S. Mid-Atlantic States and in many other parts of the world as well--although such soils commonly are not recognized as having acid sulfate origins. Such a soil in Northern Germany was called a Fossil Acid Sulfate Soil by Buurman and others in the 1973 Proceedings of the 1st International Acid Sulfate Soils Symposium. In another paper in the same Proceedings, Brinkman and Pons called such soils Pseudo Acid Sulfate Soils. They represent a post-active stage of sulfuricization, the gross “soil-forming” process of active acid sulfate soils. They are soils in which sulfide minerals, primarily pyrite, are inferred to have once been present to or close to the soil surface, but lost by pedogenic processes from the oxidized zone of the soil-geologic column, presently often several meters thick. Two post-active acid sulfate soils are described in this paper. They both formed on glauconitic marine sediments, one in MD (Maryland) and the other in NJ (New Jersey). The sites where they occur will be visited on WCSS field trips. One, a Typic Hapludult, will be visited at SERC (Smithsonian Environmental Research Center) in Anne Arundel Co., MD on July 7 on the pre-congress acid sulfate soils field trip (WCSS tour 7). The other, an Aeric Endoaquult in Burlington Co., NJ, will be visited on July 12 on the mid-week field trip to the New Jersey pine-barrens (tour 26). Both soils are acid, with pH's (measured in water ≈ 1:1 by weight) close to 4.0 throughout most of their profiles down to underlying sulfidic materials, which occur at a depth of 4.5 meters in the MD soil and at 6 meters in the NJ soil. Both soils have common to many jarosite concentrations (mottles) throughout the bulk of the oxidized zone from near the base of an argillic horizon (at about 1.6m depth in NJ and about 0.6 m in MD) to the underlying sulfidic materials at depth. The soils also have (probably mainly goethitic) iron (hydr)oxide concentrations, often closely associated with the jarosite, throughout much of the oxidized zone. The NJ soil has been more thoroughly characterized than the MD soil. It occurs on relatively flat terrain (2% slope), is very rich (>70% by lab analysis) in highly glauconitic clay in most of its thick argillic horizon and some slickensides in the lower part of this horizon. This horizon has high levels of extractable acidity (>20 cmol/kg) and KCl extractable Al (9 cmol/kg) and is remarkably green in color (greener than 5G4/2). The MD soil occurs near the base of a steep (about 25%) slope, has a thin (30cm) sandy clay loam argillic horizon, but otherwise has fine sandy loam to loamy fine sand textures of which the sand is almost entirely quartz and glauconite pellets. We think that the jarosite in the upper parts of these deep profiles formed many millennia ago. We are exploring the possibility of dating it by argon isotope methods. We find zones in other associated post-active acid sulfate soils with silica and iron (hydr)oxide cementation features (e.g. iron stone) presumably formed by sulfuricization. Post-active acid sulfate soils are easy to recognize when jarosite is present in the oxidized zone of the soils, however, many other upland soils without jarosite are presumably post-active acid sulfate soils as well. We need other indicators beyond the presence of jarosite. It is important to recognize post-active acid sulfate soils from a practical viewpoint. They are usually underlain at some depth (within 20 meters of the soil surface) by sulfidic materials that must be considered to avoid acid sulfate problems in human earth-moving activities such as highway construction. The soils are also important to consider in soil genesis modeling schemes. A “big bang” in the pace of soil genesis typically takes place when sulfidic materials are abruptly exposed to aerobic conditions. It leaves its mark on many characteristics of a soil far down the time-road. We find that sulfuricization has a major soil loosening effect as upland sulfidic materials oxidize. There commonly is perched free water at the base of the oxidized zone on top of dense, typically dark gray, sulfidic materials. The chemistry of this water (e.g. high soluble Fe) may be strongly affected by continued oxidation of the underlying sulfidic materials. Pumping of the water may accelerate the oxidation rate.

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