857 / 2024-09-19 16:47:56

A heterogeneous sediment dynamics caused by multiple triggering factors in the Diamantina Fracture Zone (SE Indian Ocean)

Deep-sea sediments,Grain size,Organic geochemical analyses,Event deposition,Triggering factors

Diamantina Fracture Zone in the SE Indian Ocean is one of the less unexplored hadal zones (>6000 m) of our planet, which develops the second deepest point (Dordrecht Deep, ca. 7019 m depth) in the Indian Ocean. Humans did not visit its ocean floor until the Chinese submersible Fendouzhe conducted 27 scientific dives in January 2023. This expedition collected push cores of shallow sediments, high-resolution submarine videos, and multibeam bathymetry data at a wide range of water depth and geomorphology. This study incorporates these materials to fully assess the sediment dynamics of the Diamantina Fracture Zone and their major causative factors.

The western section with water depth <6,000 m and the Broken Ridge to the north develops foraminifera and calcareous nannofossil oozes and brownish pelagic sediments, whereas the eastern hadal section consists of brownish pelagic sediments with sponge spicule and Fe-Mn nodules.

Grain size analyses show the sediments contain two background peaks at 2 and 9-11 μm. In the western to middle section of the fracture zone, angular volcanic-rich sediments with an additional peak at 200-300 μm cover the underlying fine pelagic sediments and calcareous oozes. The upper layer of coarse grains is considered to be gravity flow deposits, which are likely triggered by the enhanced contour current, rather than earthquakes. The current intensity increases due to the topographical constraints, which also explains the large-scale sediment ripples in the western section. Moreover, an increasing sediment contribution with peaks at 9-11 and 40-50 μm to the east among the four cores in the most eastern section indicates the Australian aeolian input, probably during the cold periods.

The TOC/TN ratio (4.12-8.85) and δ¹³Corg (-27‰ to -21‰) of the sediments (n=169) confirmed a primary marine source. Importantly, the organic carbon content shows the highest abundance (0.30%-0.88%) at the surface of most sediment cores, rapidly decreases with increasing core depth, and remains relatively stable at 0.20% below 10 cm depth. The relatively high TOC at the seafloor is related to the high primary productivity rate in the mid-latitude Indian Ocean. The profound degradation of organic matter is probably associated with microbial activities.

This research provides the first knowledge of the highly spatial heterogeneity of sediment dynamics and their various triggering factors in the Diamantina Fracture Zone. The ongoing ¹⁴C datings will further aid in reconstructing the paleoenvironmental changes and the potential responses of climatic events to the deep ocean processes.