Saturday, 15 July 2006

Evidence of Sub-Tropical Humid Climate Paleosols from Paleocene-Eocene Sequences in the Williston Basin, North Dakota, USA.

Cynthia Stiles, D. Clay Kelly, Elizabeth Clechenko, Kathleen F. Bolger, and Eric D. Shullenberger. Univ of Wisconsin, Dept of Soil Science, 1525 Observatory Drive, Madison, WI 53706-1299

Paleosol characteristics preserve signatures of climatic inputs and can be used as stratigraphic indicators of significant environmental shifts in the rock record, especially if other common stratigraphic markers are obscured or absent. During the late Paleocene (55 Ma), a significant Carbon Isotope Excursion (CIE) and shift in geochemical composition of the marine sediment record occurred due to a possible large volume release of methane hydrates in the ocean. Following the CIE, sedimentation patterns and fossil flora distributions suggest that high latitude climates were considerably warmer and wetter than present, an interval referred to as the Paleocene-Eocene Thermal Maximum (PETM) and a scenario often projected to be inevitably repeated due to present human-induced greenhouse gas emissions. Such intense climatic shifts have a marked effect on regional soil-forming/weathering environments which are reflected in the physico-chemical properties of preserved paleosols. The Late Paleocene-Early Eocene Golden Valley Formation of the Williston Basin (WiB) of North Dakota, present as isolated mesa-like outcrops in the western part of the state, contains extensive paleosol sequences that developed in udic to perudic moisture regimes under dense vegetation with fluctuating seasonal water tables. These conditions produced paleosols with pronounced Fe- and Si-enriched placic horizons, fragipans with goethitic interstitial fillings in orthogonal crack patterns in the upper portion of the fragic horizons. The paleosols also contain thick kandic horizons that presumably evolved from weathering of pre-existing smectites during the PETM. Paleosols and sediments from many PETM age terrestrial basins around the world, as well as marine cores pulled from terrestrially influenced sediment plumes, also reflect marked increases in the occurrence of kaolinite relative to other clay minerals. The hypothesis has been that the abundance of kaolinite is due to increased weathering of surface soils at the time, followed by erosion into the sea. The WiB is relatively tectonically quiescence and received a continual fine-textured sediment influx from surrounding uplands during Golden Valley deposition. It is also some distance from direct marine basin outflow. The clay-rich sediments are intercalated with coal beds both below and above the paleosol sequences, but continuous stratigraphic correlations are difficult because of poor pollen preservation and lack of significant dispersed organic matter and/or pedogenic carbonates to evaluate either the characteristic paleobotanic assemblages or the stable isotopic signatures, making it problematic to date the exact onset of the PETM in this setting. However, the drastic shifts in clay mineral assemblages from chlorites and smectites, clay minerals which pedogenically persist in areas with seasonal wet-dry cycles and poor drainage conditions, to kaolinite, a mineral commonly associated with well-drained soils in relatively warm and humid climates with no seasonal moisture deficit may be a better indicator of the PETM. The change in clay mineral assemblages, paired with the presence of extensive placic and fragic horizons suggest that the water table dropped in the basin, allowing for better drainage and degradation of the 2:1 smectites/chlorites to 1:1 clay minerals, with a subsequent release of soluble cations. Stratigraphically separated silcretes and placic horizons suggest that weathering was intense enough to solubilize large quantities of phyllosilicates and reprecipitate silicic acid combined with Fe and Al in regionally extensive pans correspondent to a vadose water table that gradually dropped through time. Bulk geochemistry of these paleosols indicates that they are depleted in many major nutrient elements and enriched in phyllosilicate components. Although the paleosols themselves do not contain abundant trace fossils, their presence suggests an ecosystem where extensive forests cycled nutrients through the surface littering much like present day tropical to sub-tropical forests. The striking physical appearances of these paleo-Ultisols is enough to utilize them as markers of the PETM in the WiB and the properties of these paleosols still has marked effects on the present land arability and use in areas where they occur.

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