Browsing by Author "Rutherfurd, I"

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    Engineers or opportunists? Examining the role that Australian native plants play in constructing fluvial landforms in subtropical river corridors
    (American Geophysical Union (AGU), 2019-12-14) Garber, J; Rutherfurd, I; Ghisalberti, M; Zawadzki, A; Hua, Q; Gadd, PS
    Sediment deposition around even-aged stands of riparian trees has been observed in many fluvial systems leading to the theory that most rheophytic species have biologically evolved to trap suspended sediment (SS), and construct fluvial landforms. That is, they are ecosystem engineers. However, few studies have attempted to show the causal relationship between vegetation by calculating the hydraulic impact of vegetation on flow and SS deposition. Furthermore, many of these examples of ecosystem engineering occur at locations in the river where SS deposition is already expected. Here we assess the question: does vegetation really influence deposition to a great extent, or is it more likely exploiting an area where deposition would have occurred anyway? We assess the effect of riparian vegetation on SS deposition in South East Queensland, Australia, where recent floods have scoured out much of the inner channel, and triggered recruitment of even aged cohorts of Casuarina cunninghamiana and Meleleuca bracteata. Using hydraulic theory from recent flume studies we explore whether these vegetation patches can induce deposition and creating new fine sediment deposits in the river corridor? We used Terrestrial LiDAR scanning (TLS) to estimate the physical effect of even-aged vegetation patches on SS deposition. In addition, we measured diameter at breast height and stem densities at even aged stands on the Brisbane, North Pine, and Mary River corridors in SEQld, while constraining patch ages via historic aerial imagery, and unconventional dendrochronology. By applying cutting edge hydraulic theory to our TLS data, we generate a process based estimate of the sediment trapping capability of these vegetation patches as they mature. We find that these patches will only trap fine sediment when channel velocities are less than 0.1 m/s, far lower velocities than experienced through any of the floods, limiting ecosystem engineering potential. These patches must first take advantage of the geomorphology in order to influence it. This methodology can be used elsewhere to determine where the “hotspots for ecosystem engineering” occur in a river corridor, and the relative effectiveness of different species of vegetation in constructing landforms and reducing SS yields. © 2019 American Geophysical Union
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    A multi-cosmogenic nuclide approach to assess sediment provenance and long-term denudation in the ancient Pilbara region, Western Australia
    (Australian Nuclear Science and Technology Organisation, 2021-11-17) Flatley, AJ; May, JH; Fujioka, T; Fink, D; Wilcken, KM; Rutherfurd, I
    The Pilbara region is characterized by extremely iron-rich Proterozoic bedrock geology which represents an enormous economic interest for Australia and the reason for extensive open-pit mining in the region. Investigating the role of terrigenous sediment supply is important to provide a baseline value of spatially averaged regional erosion rates across the landscape to help decipher in-channel responses of anthropogenically modified landscapes. In particular, quantifying long-term denudation rates within the region improves our understanding of sediment pathways, residence times and storage within the catchment setting. In this study, cosmogenic nuclides 26Al and 10Be from detrital samples are used to quantify basinwide erosion rates and sediment flux within the Upper Fortescue River and to understand sediment provenance and its transport process in the area where mining operations are active. Detailed analysis of catchment morphometric properties and lithology have been combined with multi cosmogenic nuclide (10Be and 26Al) measurements to provide much needed long-term denudation rates for the region, in addition to improving our understanding of first-order controls on catchment wide erosion. Our results indicate that erosion rates within the Upper Fortescue catchments are between 0.94-4.04 m/Ma. This value is similar to channel bedrock erosion rates (2.5 ± 0.8 m/Ma; n = 4), but somewhat higher than mesa summit or alluvial fan surfaces (0.8 ± 0.6 m/Ma; n = 13) in the Pilbara region, previously determined by cosmogenic 53Mn measurements (unpublished data). A two-isotope plot (10Be vs 26Al/10Be) shows that most of our sediment samples plot below the steady-state erosion-exposure island (26Al/10Be = 4.7 - 5.8), indicating a complex exposure history (or perhaps a reduced production rate) despite the shallow and transitory nature of Quaternary deposits in the generally bedrock-controlled landscape. The apparent offset between bedrock erosion rates (in-channel, mesa plateau) and basin-wide average denudation rates infers that vast areas of iron-rich outcropping rock surfaces in the region are not the major contributor of sediments to the system. We consider two scenarios - i) sediment samples, after an initial exposure on exposed parent rock, experienced episodes of deep burial for a minimum few hundred thousand years, and/or ii) sediments are derived from reworked river bank and floodplain deposits into the channel system following storage at shallow depth for a prolonged period of time. Most of our data can be explained by being sourced from the average depth of 0.5-2 m. Surprisingly, our cosmogenic nuclide derived erosion rates also show an apparent inverse relationship against average basin slopes. Hence, we explore various interpretations of our data in the context of specific morphometric, lithologic and environmental settings in the Pilbara region, and discuss the contribution of retreating vertical faces (e.g., gully, cliff) as a potential sediment source. Ultimately, this approach thereby contributes to the question whether foundational relationships between underlying morphometric conventions for catchment analyses are appropriate with these ancient, quiescent dryland landscapes. © The Authors