Flow‐process controls on grain type distribution in an experimental turbidity current deposit: Implications for detrital signal preservation and microplastic distribution in submarine fans

Abstract

Deep‐water depositional systems are the ultimate sink for vast quantities of terrigenous sediment, organic carbon and anthropogenic pollutants, forming valuable archives of environmental change. Our understanding of the distribution of these particles and the preservation of environmental signals, in deep‐water systems is limited due to the inaccessibility of modern systems, and the incomplete nature of ancient systems. Here, the deposit of a physically modelled turbidity current was sampled (n = 49) to determine how grain size and grain type vary spatially. The turbidity current had a sediment concentration of 17%. The sediment consisted of, by weight, 65% quartz sand (2.65 g/cm3), 17.5% silt (2.65 g/cm3), 7.5% clay (2.60 g/cm3) and 5% each of sand‐grade garnet (3.90 g/cm3) and microplastic fragments (1.50 g/cm3). The grain size and composition of each sample was determined using laser diffraction and density separation, respectively. The results show that: (a) bulk grain size coarsened axially downstream on the basin floor challenging the notion that basin floor deposits fine radially from an apex upon becoming unconfined; (b) no sample composition matched the input composition of the flow, indicating that allogenic signals can be autogenically shredded and spatially variable in sediment gravity flow deposits; and (c) microplastic fragments were concentrated in levee and lateral basin floor fringe positions; however, microplastic concentrations in these positions were lower than input, suggesting microplastics bypassed the sampled positions. These findings have implications for: (a) the development of ‘finger‐like’ geometries and facies distributions observed in modern and ancient systems; (b) interpreting environmental signals in the stratigraphic record; and (c) predicting the distribution of microplastics on the sea floor.

Blood, lead and spheres: A hindered settling equation for sedimentologists based on metadata analysis

“…Since the formulation of Stokes’ law for the settling velocity of particles with small Reynolds numbers in a viscous fluid (Stokes, 1851), the settling velocity has been used universally, either explicitly or implicitly, in sedimentological research. This includes numerical modelling of sediment transport rates in modern environments and as a proxy for flow energy, rate of deposition and deposit runout distance stored in sedimentary rocks (Bell et al, 2021; Spychala et al, 2020). These applications go beyond Stokes’ law, based on extensive research devoted to the settling velocity parameterisations that are valid outside the Stokes’ range, for which particle Reynolds numbers are larger than one (Dietrich, 1982; van Rijn, 1993; Soulsby, 1997; Wu & Wang, 2006).…”

The response of high density turbidity currents and their deposits to an abrupt channel termination at a slope break: Implications for channel–lobe transition zones

“…5A). In particular, the increase in velocity between UVP 2 and UVP 3 is attributed to a ‘velocity lag’ effect (Mulder & Alexander, 2001; Spychala et al ., 2020; Bell et al ., 2021), as the residual momentum of the turbidity currents gradually dissipates upon experiencing the reduction in slope angle occurring at the SB‐LOC. This velocity lag coincides with a significant reduction in the elevation of the maximum velocity and the overall HDTC layer height relative to the bed (Fig.…”

“…4.10–4.90 m downstream of the inlet), while another set of cores is taken at 0.10 m intervals in the lateral direction at the location of maximum deposit width [for Series S1, run R1.3 ( c 0 = 16%, 9° slope), Table 1]. Samples are prepared for size analysis by removing the upper 5 to 10 mm of the sediment sample to ensure the analysis is representative of the depositional conditions from the body of each HDTC (Bell et al ., 2021). Measurements of grain‐size distributions from core samples are obtained using the laser particle size analyser and GRADISTAT (Blott & Pye, 2001) to assess the spatial variation of grading with reference to the initial sediment size distribution.…”

“…due to suspended sediment particles combined with the interstitial fluid). These currents are capable of transporting large volumes of sediment (Stevenson et al ., 2018; Talling et al ., 2022), organic matter (Hage et al ., 2020) and anthropogenically‐derived particles, such as microplastics (Kane & Clare, 2019; Pohl et al ., 2020a; Bell et al ., 2021), into deep oceanic basins. When TCs occur on slopes that are sufficiently steep they begin to bypass their sediment load, and, if erosive, will incise channels into these slopes (Fildani et al ., 2013; de Leeuw et al ., 2016).…”

Plastic as a Sediment – A Universal and Objective practical solution to growing ambiguity in plastic litter classification schemes

“…Probably the most fundamental parameter to determine the transport behaviour of plastic is the settling velocity which can be determined experimentally (Kowalski et al, 2016;Khatmullina & Isachenko, 2017;Waldschläger & Schüttrumpf, 2019b;Waldschläger et al, 2020;Van Melkebeke et al, 2020;De Leo et al, 2021;Zhang & Choi, 2021;Francalanci et al, 2021;Khatmullina & Chubarenko, 2021;Choi et al, 2022;Kuizenga et al, 2022;Mendrik et al, 2023). Additionally, many laboratory experiments in flume tanks have been undertaken under different flow conditions (Waldschläger & Schüttrumpf, 2019a;Pohl et al, 2020;Bell et al, 2021;Russell et al, 2023). Such experiments find that whilst classic settling equations are able to accurately predict the settling velocity of simple microplastic shapes like spheres and cylinders, they do not provide an accurate prediction for more complex shapes such as fibres or films (Khatmullina & Isachenko, 2017;Waldschläger & Schüttrumpf, 2019b;Mendrik et al, 2023).…”