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

• Published in: The depositional record 2021-09, Vol.7 (3), p.392-415
• Main Authors: Bell, Daniel, Soutter, Euan L., Cumberpatch, Zoë A., Ferguson, Ross A., Spychala, Yvonne T., Kane, Ian A., Eggenhuisen, Joris T.
• Format: Article
• Language: English
• Subjects: Anthropogenic factors; Basins; Carbon; Environmental changes; environmental signal; Experiments; Grain size; Gravity; Gravity flow; heavy mineral; microplastic; Microplastics; Minerals; Ocean floor; Organic carbon; Particle size; Plastic pollution; Pollutants; Process controls; Sand; Sediment concentration; Sediment pollution; Sediments; signal preservation; Soil erosion; submarine lobe; Turbidity; turbidity current
• ISSN: 2055-4877; 2055-4877
• DOI: 10.1002/dep2.153

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. We investigate the distribution of grain size, heavy minerals and microplastic fragments in an experimental turbidity current. Heavy minerals were strongly concentrated in axial positions, and no sampled position was fully representative of initial flow composition, suggesting well‐constrained depositional context is essential to making informed interpretations in natural systems. Microplastic fragments were most abundant in levees and fringe positions, but were depleted with respect to initial flow composition, suggesting microplastic fragments may bypass the sandy parts of submarine fans and be spread over vast areas of the sea fl