Modal depths from shipboard bathymetry: There is a south pacific superswell

Abstract

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.

“Plate‐like” subsidence of the East Pacific Rise–South Pacific superswell system

“…The preprocessed data was then displayed as density contours, in a similar way to McNutt and Fischer [1987]. McNutt and Sichoix [1996] reassert that 30-35 Ma seafloor is typically 250 m shallower than expected, the depth anomaly increases to !1300 m on older seafloor (Figure 2), and that the superswell does indeed exist.…”

“…Dubbed a ''superswell'' by McNutt and Fischer [1987], the depth anomaly reportedly has a maximum amplitude of !1 km [McNutt and Sichoix, 1996;Sichoix et al, 1998], a width of approximately 3000 km [McNutt, 1998], and covers some 15 million km 2 of the seafloor [McNutt and Fischer, 1987].…”

“…[9] McNutt and Sichoix [1996] question, however, how representative an analysis is that is based on 5 ship tracks and only overlaps the eastern extremity of the superswell (137°W -130°W). They therefore used a more extensive data set of original, quality controlled, single beam, bathymetry data.…”

Hotspot Concept: The French Polynesia Complexity

“…They propose that the Superswell could be an artifact of the poor sampling and gridding of ETOPO5, which in oceanic regions corresponds to the SYNBAPS database. Subsequent studies using original ship depth soundings (McNutt et al, 1996;Sichoix et al, 1998) confirm that the seafloor is indeed 1 km shallower. The precise description of the location and extent of the superswell remained approximate for a while.…”

Seamount detection and isolation with a modified wavelet transform

“…median and mode) were therefore utilized [e.g. McNutt & Fischer 1987;Smith, 1990;McNutt & Sichoix, 1996;Levitt & Sandwell, 1996;Sichoix, 1998]. These significantly mitigate this problem, but still have some tendency to pass through, rather than beneath, seamounts where many seamounts co-exist [Hillier & Watts, 2004] or occur down trenches (Figure 12).…”