Erosional and depositional processes on the submarine flanks of Ontong Java and Nukumanu atolls, western equatorial Pacific Ocean

- First ever multibeam bathymetry around atolls on the Ontong Java Plateau
- Extensive submarine mass wasting of Ontong Java and Nukumanu atolls
- Base-of-atoll debris deposits indicate where atoll flanks have failed.
- ~ 1500 km3 of volcaniclastic material shed from the original edifices.
- Volcaniclastic and carbonate erosional products of atolls have fed Kroenke Canyon.

Ontong Java and Nukumanu atolls sit atop Earth's largest oceanic plateau - the Ontong Java Plateau - in the western equatorial Pacific Ocean. In 2014, scientists aboard the Schmidt Ocean Institute's RV Falkor mapped the seafloor surrounding Ontong Java Atoll (Solomon Islands) and Nukumanu Atoll (Papua New Guinea) for the first time using multibeam acoustic systems. These and pre-existing data help reveal the evolution of the atolls, which has involved volcanism, erosion, deposition, and carbonate growth. We use the new multibeam bathymetry and backscatter data, together with legacy seismic reflection data, to: (i) qualitatively characterise the submarine flanks of the atolls; (ii) identify broad and fine scale geomorphological features and classify the seafloor environment; (iii) investigate how submarine processes have influenced the morphological evolution of the atolls over geological timescales; and (iv) provide evidence that these atolls have fed sediment downslope to Kroenke Canyon that incises the NE flank of the Ontong Java Plateau.Submarine erosion is widespread on both atolls, and submarine landslides control, at least in part, the dynamic geomorphology and hence development of both atolls. Channels and gullies are abundant on the flanks of both atolls. Scarps are common on the upper flanks of both atolls, whereas undulating bedforms and crescent shaped bedforms are more common at the foot of the atolls' slopes. The Ontong Java Atoll coastline has widespread bights that appear to be linked to submarine debris deposits downslope. The overall shape of Ontong Java Atoll could result from one or more extensive edifice failure(s), or multiple smaller cones amalgamating to form one island. Active erosion of Nukumanu Atoll is manifested by crescent shaped bedforms, although compared to Ontong Java, Nukumanu has a much less sinuous coastline and more subdued mass wasting. We estimate ~ 1500 km3 of volcaniclastic sediment has been eroded and transported from Ontong Java and Nukumanu atolls, some of which has likely fed Kroenke Canyon downslope.

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