The Holocene Sedimentation Environment in a Seasonal Wetland in Eastern North Dakota

A Wyard soil (fine‐loamy, mixed, frigid Typic Haplaquoll), formed in Holocene materials in a seasonal recharge wetland on the glacial drift plain of North Dakota, contained strata with different kinds of sediments, pollen, and fossil fauna that reflect the prehistoric wetland environment. Branchiate molluscan fauna were present in the coniferous boreal forest that grew in the area as the Wisconsin glacier receded about 11,000 yr ago. Pollen data indicate that as the climate became warmer and drier, the coniferous forest was replaced by a deciduous forest which in turn was replaced by a prairie. In the most recent sediments, a significant increase in arboreal (coniferous and deciduous pollen was noted, indicating a return to wetter and colder conditions. The gill‐breathing molluscs were replaced by lung‐breathing (pulmonate) species as the soil and climate became drier and warmer. Lower‐gleyed soil horizons were formed in sediments deposited under reduced waterlogged conditions. Upper horizons have mollic colors due to deposition and humification of organic matter. The pollen above and below a buried argillic horizon present at the 145‐ to 152‐cm (57‐ to 60‐in.) depth indicate a stable environment during the period 4,000 to 6,000 yr before present (YBP). Increased precipitation and erosion has brought in more than 1‐m of new sediment during the past 4,000 yr. Inorganic C was absent in the top 198 cm, while Late Wisconsin and Early Holocene sediments contain 1.3% inorganic C. The Wyard soil sediments (Middle to Late Holocene) were finer textured and more uniform; with lower salt contents, pH, and bulk density values; and different sand mineralogies when compared to the unweathered glacial till. The sediments deposited during the coniferous boreal forest [198–236 cm (78–93 in.)] were different from the sediments above and below due to a wetter depositional environment.