Cores 11 through 26 by Gary B. Dr - University of Illinois at Urbana ...

The average silica (SiO 2) content of samples from core 12 was 70.43%, the second lowest average SiO 2content of the sixteen cores. This core was the only one of the sixteen that was collected from ancientlacustrine (lake bottom) sediments. Lacustrine sediments generally contain more clay-size particles thanother types of sediments and less sand. The samples from core 16 appeared similar to those from core 12in terms of average SiO 2, sand, and clay-size contents, but core 16 was collected from an upland location.The other upland soil samples contained from 73 to 78% SiO 2.Five cores (11, 17, 22, 25, and 26) were collected from alluvial (floodplain) locations. The SiO 2content ofcores 11, 17, 22, and 25 did not vary appreciably with depth. In core 26, the SiO 2content was relativelyconstant throughout the four upper samples, at about 83%, but decreased sharply in the bottom twosamples to 75% and 60%, respectively.The SiO 2content in core 25 also decreased with depth, but not as drastically as in core 26. We note that thebottom two samples from both cores were from ancient buried soils (paleosols) in which the clay contentwas also higher than in the upper four samples from each of the cores. The SiO 2content in the upper foursamples was also relatively constant.The relatively constant SiO 2content with depth is not surprising, because all source materials in awatershed are blended in the formation of alluvium, and alluvial soils generally are not well developed.Periodic flooding provides a rapid addition of fresh sediment to the soil surface. In cores 25 and 26 theburied soil apparently had time to develop prior to being covered by the surficial alluvium, thus theobserved differences between the upper four samples and the lower two in these two cores.Aluminum The aluminum content of soils is mostly inherited from the parent materials (Kabata-Pendias,2001). The concentration of Al in the cores generally increased with depth, although in some cores theconcentration passed through a maximum at some depth. These trends reflected the trends in clay contentof the subsamples.The profile of Al 2O 3content versus depth was generally a mirror image of the SiO 2profile. The aluminumcontent also was nearly constant in cores from the alluvial soils (cores 11, 17, 22, 25, and 26). The Al 2O 3in cores 25 and 26 increased in the two bottom samples where the cores penetrated paleosols.Iron Iron in soils occurs principally as oxyhydroxides, most commonly as goethite (FeOOH) in soilsof temperate, humid regions. Iron minerals commonly occur as coatings on clay minerals, silt, and sandparticles, and cements in concretions. The iron oxyhydroxides typically are very fine-grained, possesslarge surface areas, and are active adsorbers of other cations, particularly metals such as copper, nickel,vanadium, and zinc (Kabata-Pendias, 2001).Iron has an affinity for organic molecules and forms complex compounds with them. These organo-ironcomplexes may be largely responsible for the migration of iron through the soil profile (Kabata-Pendias,2001). The Fe 2O 3content in these cores tended to pass through a maximum with increase in depth, anindication of downward migration.The iron content of samples from the upland forest soils (cores 14-16, 20, 21, 23, and 24) typically washigher than in other soil environments, although the lacustrine soil, core 12, had the highest average Fe 2O 3of all 16 cores.18