Spatial heterogeneity of sediments on a topographically diverse descending plate at convergent margins: An example from the outer slope of the Mariana Trench

Unraveling deep ocean sediment composition and its controlling mechanisms is crucial to understanding the interplay between the Earth's surface and the deep Earth. Convergent margins are generally featured by various seafloor landforms, but how submarine topography affects sediment accumulation processes remains poorly understood. Here we focus on submarine surface sediments from the outer slope of the Mariana Trench (covering water depths of 1218–6310 m) and combine microscopic observation, clay mineralogy, and elemental geochemistry to characterize sediment composition and to reveal physical, chemical, and biological sedimentary processes at topographically diverse convergent margins. Smear slide observations indicate that foraminifera shells dominate in shallow areas, while pelagic clays and diatom oozes prevail in trench and slope areas. Clay minerals primarily consist of illite (26–98 %) with low crystallinity index values and diverse chemical index values, followed by chlorite (0–45 %) and kaolinite (0–29 %). Smectite (0–61 %, 46 of 70 samples have smectite contents <5 %) of the samples from the outer slope area is low but appears dominant close to the trench. Bulk element compositions are mainly affected by the relative calcareous, siliceous, and lithogenous components that dissolved or accumulated inconsistently as submarine topographic feature changes. Our results imply that submarine topography and sediment provenance lead to significant heterogeneity of sediment composition at topographically diverse convergent margins. The submarine topographic relief, together with associated bathymetry and deep-sea circulation primarily controls carbonate deposition on the Caroline Plateau, and influences the sinking particles (including diatom shells and arc volcanic-sourced materials) accumulation in the trench basin. Continent-derived aeolian dust introduces illite-rich detritus, while localized volcanism potentially contributes to smectite-rich sediment in certain topographic provinces. We propose that the deep sea sedimentation processes in the Mariana convergent system are more diverse and complex than those at the margins with relatively plain topographic characteristics (e.g., the Japan Trench). The significant heterogeneity on descending plates along convergent margins may lead to various subduction behaviors. These findings contribute to a better understanding of deep ocean sediment source-to-sink systems and have implications for Earth's material cycle and multi-sphere interaction studies.