Browsing by Subject "Post-settlement alluvium"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Hillslope redistribution of soil organic carbon in the depressional landscape in Minnesota
    (2014-07) Wu, An-Min
    Agricultural tillage has been estimated to cause a loss of 30-50% of the pre-settlement soil organic carbon (SOC) through enhanced decomposition and loss to the atmosphere or through erosion and subsequently loss to surface waters or burial in lower landscape positions. However, measures of whole landscape redistribution and fate of sediments and SOC are lacking. This research seeks to estimate change in SOC storage since agricultural settlement using soil-terrain modeling techniques in closed-depressional landscapes. The overall quantity of SOC in depressional landscapes may have not been lost to the atmosphere through enhanced decomposition but rather is redistributed downslope.I conducted field observations and soil sampling in hillslope transects in Lake Rebecca Park Reserve in East-Central Minnesota. The thickness of re-deposited sediments (termed post-settlement alluvium, or PSA) was identified by morphological indicators in the field. The spatial distribution of PSA presence and its thickness were modeled with local and regional terrain attributes using a two-stage regression approach. The current SOC inventory (1.119 Pg) in top 1-m soil at the Lake Rebecca site was estimated by spatial predictive models of SOC contents at four soil depths (0-10 cm, 10-30 cm, 30-60 cm, 60-100 cm). I estimated pre-settlement SOC inventory for erosional uplands with spatial predictive models for an uncultivated grassland in Morristown, Minnesota; for depositional lowlands, I calculated pre-settlement SOC inventory by applying models for soil profiles below the PSA depth at the study site. Erosional losses and depositional gains were determined by subtracting current SOC inventory from pre-settlement values.The results showed high SOC contents in surface soils at lower landscape positions, especially in wetlands near the surrounding marsh. Total SOC in the uppermost meter of this 6-ha study site was estimated as 1.528 Gg. The change in SOC density since European settlement was highly overestimated (36.7% increase). The prediction error is likely due to the lack of a mechanism to constrain the prediction of PSA under natural sedimentation patterns at the very bottom of the hillslope beyond the zone where PSA was observed. The model improvement is required to more accurately predict whole landscape SOC distribution and change over time.
  • Thumbnail Image
    Item
    Ravine alluvial fans as records of landscape change in the Le Sueur River Basin, southern Minnesota
    (2017-10) Treat, Ian
    Ravine alluvial fans in the Le Sueur River Basin (LSRB) of south-central Minnesota record post-glacial Holocene changes and modern anthropogenic disturbances to land cover and hydrology in high-latitude watersheds. Seventy meters of base-level drop at the end of the last glaciation initiated millennia of incision that continues on the LSRB today. Onto this template of on-going incision, Euro-American land clearing and drainage of previously stable upland prairie and wetlands in the mid-1800s further increased erosion rates in the basin. Ravines, first-order channels that link low-gradient uplands with the deeply-incised channel network, experienced changes in erosion rates over time from both impacts, with the erosional history preserved in alluvial fans at the mouths of ravines where they terminate on fluvial terraces. Establishing a post-settlement chronology is difficult in the highly erosive knickzone of the Le Sueur. We take advantage of six fan deposits spread throughout the LSRB to determine the fluvial response of upland agricultural land conversion on steep first-order drainages. Ravines respond quickly to sediment and hydrology fluxes in the basin that are reflected in their alluvial fans as packages of post-settlement alluvium (PSA) and incision through fan surfaces. Bulk soil samples collected at 10-, 20-, 40-, 100-, and 200-centimeter depths on the selected fans as well as samples from the incised channel were analyzed for fly ash, spherical silt-sized grains that are a byproduct of coal combustion. The presence of fly ash as an in-situ stratigraphic marker at depth was used to calculate conservative post-settlement deposition rates of 0.93 and 1.67 cm/yr using observation techniques from high-powered transmitted and reflected light microscopes as well as scanning electron microscopy, respectively. These rates are a three-fold increase over generous Holocene deposition rates of 0.27 cm/yr. Incision through fan surfaces also marks post-settlement changes. Trenching and tile drainage on the uplands allowed for greater transport of water down ravines and onto fans. These results confirm land use change triggered an increase in upstream erosion and fan deposition followed by incision on short time scales.