Erosional processes on the slopes of Rennell Island and the Indispensable Reefs (Solomon Islands, Western Pacific)

Large erosional products such as scalloped scars, mass transport complexes and rockfalls are common features found on the steep slopes of isolated carbonate platforms. Factors and processes controlling these instabilities and their interactions are a subject of ongoing discussion. Here we are presenting novel multibeam data and seafloor imagery from the slopes of Rennell Island and the Indispensable Reefs, Solomon Islands.

Rennell Island and the Indispensable Reefs are located in the western Pacific Ocean, around 200 km south of the main Soloman Islands archipelago on top of the Louisiade Plateau. The plateau is separated by the San Cristóbal Trench from the Solomon Block, an elongate microplate underlying the main Solomon Islands and bounded by a dual subduction zone between the Australia and Pacific plates. While the Indispensable Reefs consist of three atolls reaching the modern sea surface, Rennel Island is an uplifted, dolomitized reef complex surrounded by modern coral reefs. These data were collected during several cruises, May to August 2025 onboard RV Hydra, and showcases spectacular slope morphologies and erosional features.

Seafloor imagery reveals calcareous algae (e.g., Halimeda, coralline algae) as major carbonate producer from the photic down to the mesophotic zone on the slopes of both Rennell Island and the Indispensable Reefs. The southwestern slope of Rennel Island is characterized by several scalloped scars along the upper slope ranging in width from 2.5 to 17.6 km. In combination with large reef blocks (> 6 km long) located on the lower slope, these scars document multiple historic catastrophic slope failures. In addition, the presence of numerous gullies, each hosting downslope wandering sandwaves, suggest regular export of skeletal carbonate sands. The three atolls of the Indispensable Reefs are separated from each other by two E/NE–W/SW striking channels ranging in width from 3.4 to 4.3 km. Reef blocks located in the central part of both channels indicate rockfalls derived from the adjacent atoll flanks. The southwestern slopes of all three atolls are characterized by several scalloped scars ranging in width from 4.6 to 10 km and dozens of u-shaped gullies, each 10s of meters wide. A prominent, 15 km long canyon deeply incises the central atoll. Its head has a width of 7.2 km and is sourced from several gullies connected via incised channels to the shallow water lagoon. Combined with a higher number of channels cutting through the western reef rim compared to the eastern rim, this canyon represents a main off-platform export path. However, the frequent presence of gullies along the northeastern mid-slope, followed towards the slope-foot by downslope wandering sandwaves, indicate an additional sediment export system.

The large number of prominent head scarps on the southwestern slopes suggests that the leeward sides of both platforms tend to be more easily destabilised, which might be caused by the tectonic regime. Platform-top morphologies indicate a strong influence of the North Vanuatu Jet on the westward-driven sediment transport, while the presence of gullies and sandwaves on the eastern slope of the Indispensable Reefs suggest a more complex sediment dynamics.