How is Physical Depositional Setting Related to Silica Chemistry in the Platte River, USA?

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American Geophysical Union, Fall Meeting 2013, abstract #EP43A-0834

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Beginning in 2003, a non-native subspecies of Phragmites australis, a wetland grass, invaded the Platte River in Nebraska, USA. The plants' dense root and rhizome structures caused channel narrowing and increased deposition of fine sediment. We hypothesized that a significant proportion of the fine sediment was comprised of biogenic silica particles including terrestrial plant phytoliths. In this study, we determined a relationship between particle size and biogenic silica content in Platte River sediments to help characterize when and where silica is sequestered in the riparian areas of rivers. Historically a wide, braided, largely unvegetated sand-bed river, the Platte has undergone several major changes since the early 1900s. The main anthropogenic impact on the Platte has been a ~75 percent reduction in flow, leading to channel narrowing and more vegetation occupying riparian areas. Phragmites is particularly effective at building islands and extending river banks because its roots add cohesion to sediment. We suspect that the presence of Phragmites in the Platte River has resulted in a reduction of bioavailable silica (dissolved and particulate amorphous particles) being exported to the downstream receiving waters, ultimately including the Gulf of Mexico. We want to better understand silica sequestration in riverine environments, because silicon is often a limiting nutrient for some phytoplankton (e.g., diatoms and radiolaria) in coastal oceans. In the Platte, lower water levels and increased vegetation density cause reduced flow velocity, allowing more silica particles to settle out of suspension. We hypothesized that silica content in the riparian sediments of the Platte River negatively correlate with particle size, and that the non-native subspecies of Phragmites uses more silica than the native variety. In order to quantify the effect Phragmites is having on the Platte's silica load, plant and sediment samples were prepared using a timed NaOH digestion and silica was measured using inductively coupled plasma mass spectrometry (ICP-MS). Previously we observed an inverse trend between silica content and field observations of particle size. In this study we measured particle size using a laser diffractometer and imager to define that relationship.

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