Submarine canyons are important geomorphic features on continental margins and act as major conduits for terrigenous material to the deep sea. Recent studies show that organic carbon preservation rates in submarine canyons can exceed 60%, with approximately 62–90 Mt of organic carbon buried in sediments annually via turbidity flows, representing an underestimated carbon sink. However, most well-investigated canyon systems are closely linked to river mouths or fed by longshore currents; such canyons account for only ~3.8% globally, and those supplied by cross-shelf transport represent merely ~30%. The remaining 66.2% have heads on the continental slope, where terrigenous input is limited during the present interglacial highstand, and their carbon sink dynamics remain poorly understood.
The Jiulong Submarine Canyon is located on the continental slope in the northeastern South China Sea. Its head lies on the upper slope and does not incise the shelf, disconnecting it from fluvial and longshore current pathways, which makes it an ideal site to study the sedimentary dynamics and carbon burial mechanisms of non-fluvially connected canyons. Meanwhile, the interaction between terrigenous clastics and biogenic carbonates in this area provides a unique perspective for sedimentary system research. In May 2018, a cruise funded by the South China Sea Deep Plan was conducted aboard the R/V Jiageng. Using an ROV (its 2067th dive, at water depths of 730–1421 m), ten push cores (30–45 cm long) were collected from a secondary branch valley of the Jiulong Canyon. This study selected representative cores for multi-proxy comprehensive analyses, systematically revealing the canyon’s sedimentary dynamic environment, the interaction processes between carbonates and terrigenous clastics, as well as the characteristics and mechanisms of organic carbon burial.
Comprehensive analyses indicate that under highstand conditions, the delivery of terrigenous organic carbon to the Jiulong Canyon is relatively limited. The sedimentary process is dominated by internal tides, marine productivity, and event-driven resuspension rather than continuous turbidity flows. Intense bottom-current reworking and a coarse-grained depositional environment significantly reduce organic carbon preservation efficiency, and the carbon burial capacity of surficial sediments is much lower than that of the underlying muddy strata. These findings reveal that non-fluvially connected canyons may not act as efficient deep-water carbon sink conduits during highstand periods. Global estimates of submarine canyon carbon burial should consider for their connection types, sedimentary dynamic evolutionary stages, and bottom-current reworking intensity to avoid overestimating their carbon sink potential.