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Abstract. High-quality ship soundings obtained since 1967 in French Polynesia are used to better define the pattern of depth anomalies in French Polynesia and to test the accuracy of previous results based on ETOPO5 gridded data. The modes of the differences between the ship data and ETOPO5 demonstrate that the latter database is accurate to _+250 m for nearly 70% of the observations in the region, although there is a tendency for ETOPO5 to be of the order of 35 m too shallow. The ship depth versus age distribution shows that the South Pacific's unusually shallow seafloor, termed the "Superswell," cannot simply be described as the cooling of a hotter or thinner thermal plate. Rather, the seafloor rises between the ages of -40 and 80 million years before resuming thermal subsidence. The Superswell appears to be bounded to the north by the Marquesas Fracture Zone, and the depth anomaly tapers down to smaller values south of the Austral Fracture Zone. Its modal-depth-versus-age relation cannot be mimicked easily by classical steady state thermal cooling models, leading us to prefer dynamic explanations for the depth anomaly. Modal depth analysis aids in the isolation of the individual swells superimposed on the Superswell. Whereas the Marquesas swell, just north of the Superswell, is similar to other swells worldwide in its width (> 1000 km) and amplitude (--1 km), both the Society and Austral swells are narrower and lower than global norms. We explain the small size of the Society and Austral swells as the reduced thermal buoyancy of these hotspots entrained in a broader mantle upwelling.•

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Since the first quantification 10 years ago of a large regional depth anomaly in French Polynesia, numerous studies have explored the origin and the geophysical and geochemical consequences of this “South Pacific Superswell.” These efforts would be widely viewed as a waste of time if this superswell were proven not to exist. Recently, Levitt and Sandwell (1996) have proposed just that, based on a modal analysis of multibeam bathymetric data collected on sea floor aged 15 to 35 Ma in eastern French Polynesia. Citing a discrepancy between the ETOPO5 gridded depths and ship data from 4 expeditions in their study area, they suggest that the entire Superswell could be an artifact of poor ETOPO5 sampling and gridding. Here we present a more comprehensive analysis of original ship soundings from 82 oceanographic expeditions with satellite navigation collected since 1967. The soundings span sea floor from less than 30 to more than 110 Ma. We confirm the conclusion of Levitt and Sandwell that depth anomalies calculated from ETOPO5 slightly overestimate the true values from original ship soundings on sea floor aged 30 to 35 million years, but a positive depth anomaly of 250 m exists nevertheless. On older sea floor, we find that ETOPO5 slightly underestimates the magnitude of the depth anomaly based on a modal analysis, but overall the agreement in the mode of the depth/age data between ETOPO5 and the original ship soundings is quite striking, especially given the fact that less than 20% of the ship data used in our analysis had been collected at the time that the global bathymetric grid was prepared. There indeed is a South Pacific Superswell, and the magnitude and age‐dependence of the depth anomaly is as originally calculated from the mode of gridded bathymetry.