Topographic controls on gravity flow behaviour, and microplastic transfer in the deep sea

Gravity currents are sensitive to interactions with seafloor topography and changes to the degree of flow confinement: both factors impart first-order controls on flow behaviour and sedimentation patterns. The form of seafloor topography and range of sediment gravity flow types varies markedly across different geomorphic elements of the deep sea. The resulting flow- topography interactions control the routeing and burial of sediment and microplastics in the deep sea. However, the present understanding of flow-topography interactions and microplastic transfer processes in all deep sea environments remain poorly understood. This thesis presents a new process-product model for flow interactions with complex canyon-margin topography, a new mechanistic model from physical experiments of flow-topography interactions in unconfined settings, and uses modern seafloor sediment cores from the Whittard Canyon to assess the role canyons play in transferring microplastics into the deep sea. The localised heterogeneity in the deposit type and architecture related to mass-wasting of a canyon-margin documented in the Rosario Formation, Mexico, is poorly accounted for in existing canyon-fill models and highlights the role of transient storage of particulate matter in canyon overbanks. The new model for combined flow generation and mechanics for onlap styles documented experimentally in unconfined settings shows that sediment gravity flow behaviour and sedimentation patterns can be used to support interpretations of palaeogeographic reconstructions and sediment pathways in the deep sea. The pervasiveness of microplastic pollution throughout the Whittard Canyon and down to 10 cm sediment depth highlights how anthropogenic activity and subsurface burial processes add noise to the signal in microplastic source-to-sink models. Linking the findings from three different methods helps develop a comprehensive understanding of the role seafloor topography imparts on the delivery, transfer and burial of particulate matter in the deep sea. This work presents new insights into sediment gravity flow behaviour, and microplastic transfer, which provide criteria to support palaeogeographic reconstructions, assessments of sediment routeing patterns and microplastic flux calculations in deep-water sedimentary systems.