Groundwater-Fed Wetland Sediments and Paleosols: It’s All About Water Table
Gail M. Ashley, Daniel M. Deocampo, Julia Kahmann-Robinson, Steven G. Driese, 2013. "Groundwater-Fed Wetland Sediments and Paleosols: It’s All About Water Table", New Frontiers in Paleopedology and Terrestrial Paleoclimatology: Paleosols and Soil Surface Analog Systems, Steven G. Driese, Lee C. Nordt
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Wetlands are continental depositional environments and ecosystems that range between ephemerally wet to fully aquatic habitats, and, thus, the character of a wetland soil is directly related to the position of the water table over seasonal and longer timescales. The sediment and paleosol records of wetlands are products of a unique setting that can be both exposed to the atmosphere and water-saturated at the same time. Wetlands tend to occupy low-gradient portions of the landscape in places where the phreatic zone is at least ephemerally exposed at the surface, and hydrophytic vegetation has an opportunity to colonize. Groundwater-fed wetlands are an end member of a continuum of waterlogged environments and are associated with localized groundwater discharge (GWD); e.g., springs and seeps that can sustain permanent saturation. Research has tended to follow one of two parallel tracks: sedimentology or pedology. An objective of this paper is to bring these two separate lines of inquiry closer together. The signature of wetland pedogenesis includes redoximorphic features, enhanced hydrolytic alteration or dissolution of soluble phases, and preservation of biotic indicators of wetland habitats. Histosols (peats) and other hydric soils (indicated by gley color and reduced minerals like pyrite and siderite) are common in sites with a permanently high water table and anaerobic conditions. Illuvial clays, in contrast, record episodes in which wetlands dry out and drainage improves sufficiently for these features to form. A case study from Holocene-age Loboi Swamp, Kenya, illustrates the importance of integrating field observations and laboratory analyses. Wetland conditions were observed through thin section micromorphology, mineralogy, bulk geochemistry, and macro- and microfossils. The record of Loboi Swamp is characterized by the juxtaposition of features indicating episodes of soil saturation alternating with those indicating desiccation. In order to extract the most information recorded in groundwater-fed wetlands, soils and sediments should be studied as part of the larger spatial and climatic frameworks in which they occur.
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New Frontiers in Paleopedology and Terrestrial Paleoclimatology: Paleosols and Soil Surface Analog Systems
After initial breakthroughs in the discovery of fossil soils, or paleosols in the 1970s and early 1980s, the last several decades of intensified research have revealed the much greater role that these deposits can play in reconstructing ancient Earth surface systems. Research currently focuses on terrestrial paleoclimatology, in which climates of the past are reconstructed at temporal scales ranging from hundreds to millions of years, using paleosols as archives of that information. Such research requires interdisciplinary study of soils conducted in both modern and ancient environments. These issues and many others were discussed at the joint SEPM-NSF Workshop “Paleosols and Soil Surface Analog Systems”, held at Petrified Forest National Park.