Organic matter coating defines threshold of motion in natural sediments
Creators: name: Bruni, Elena
name: Blattmann, Thomas
name: Haghipour, Negar
name: Andersen, Thorbjørn J.
name: Eglinton, Timothy I.
Publisher: name: ETH Zurich
Year: 2022

description: The onset of sediment erosion in is governed by sediment properties such as grain size, density, and environmental controls such as current strength. Investigating the relevance of each of these parameters has been an ongoing part of experimental sedimentology, resulting in several empirical threshold of motion curves [1,2]. These are used for different geotechnical applications, but so far, none of these include the effects of organic matter (OM) on particle motion, a draw-back that has been identified early on and limits the applicability in natural systems [e.g., 3]. We perform sediment erosion experiments on natural, untreated OM-rich sediments to investigate the impact of high OM concentrations on the sediment threshold of motion. Six sediment cores from Swiss lakes were inserted to EROMES, a resuspension chamber using a propeller to produce and control shear stress [4,5]. By incrementally increasing propeller rotation rates, the first and second erosion thresholds were identified. Measuring OC (%) revealed the fluff eroded at er I has higher concentrations than the suspended particles at er II and consists of labile aggregates and free OM (e.g., leaves). Moreover, the presence of benthic organisms (tube worms) resulted in a measurable strengthening of the sediment surface. The measurements of er I and II are plotted against calibration measurements performed with standardized (quartz) grains of known grain size distribution, which reveals the lower threshold of motion of particles associated with OM. Consequently, we argue for the recalibration of threshold motion curves to include low-density, OM-rich particles and the stabilising effects of benthic organisms [6].
Key words: SLOB hypothesis, sediment threshold of motion, organic matter, EROMES erosion chamber

[1]	A. Shields, Anwendung der Ähnlichkeitsmechanik und der Turbulenzforschung auf die Geschiebebewegung, Technische Hochschule Berlin, 1936.
[2]	F. Hjulström, Studies of the Morphological Activity of Rivers as Illustrated by the River Fyris, Geogr. Ann. 18 (1936) 121. https://doi.org/10.2307/519824.
[3]	M.C. Miller, I.N. McCave, P.D. Komar, Threshold of sediment motion under unidirectional currents, Sedimentology. 24 (1977) 507–527. https://doi.org/10.1111/j.1365-3091.1977.tb00136.x.
[4]	T.J. Andersen, E.J. Houwing, M. Pejrup, On the erodibility of fine-grained sediments in an infilling freshwater system, in: Proc. Mar. Sci., Elsevier B.V., 2002: pp. 315–328. https://doi.org/10.1016/S1568-2692(02)80024-9.
[5]	T.J. Tolhurst, K.S. Black, D.M. Paterson, H.J. Mitchener, G.R. Termaat, S.A. Shayler, A comparison and measurement standardisation of four in situ devices for determining the erosion shear stress of intertidal sediments, Cont. Shelf Res. 20 (2000) 1397–1418. https://doi.org/10.1016/S0278-4343(00)00029-7.
[6]	E.T. Bruni, T.M. Blattmann, N. Haghipour, D. Louw, M. Lever, T.I. Eglinton, Sedimentary Hydrodynamic Processes Under Low-Oxygen Conditions: Implications for Past, Present, and Future Oceans, Front. Earth Sci. 10 (2022) 1–18. https://doi.org/10.3389/feart.2022.886395.