Exploring South Bank

by Cristi Richards

Exploratory map of South Bank, American Samoa
(ARC GIS map created by Tomoko Acoba)

South Bank is a sub-surface rise in the ocean floor, or seamount, located approximately 37 miles south of the island of Tutuila. Until recently, there has been little scientific knowledge about the depths, habitats, or living communities of South Bank. Reported minimum depths varied widely and proposed minimum depths from 10 meters (30 feet) to 30 fathoms. Fisherman have known about and frequently visit South Bank in search of wahoo, tuna, and other pelagic fish that are attracted to the shallower depths. However, it appears that South Bank has only very rarely been observed underwater.

South Bank is probably not part of the Samoan chain, in geologic terms, due to its location and age. The Samoan chain has been building from the Pacific plate moving over a hotspot, creating islands in a similar fashion to the Hawaiian chain. The older islands are found to the west with the youngest islands in the east. However, South Bank may be greater than 10 million years old, much older than the other Samoan islands in the area. This is similar to how Swains and Rose Atolls are geologically not part of the Samoan chain either, despite their proximity.

Heliopora coerulea, Blue Coral at South Bank
(Photograph by Cristi Richards)

In an effort to understand more about this area, we spent several nights last week mapping South Bank using the multibeam sonar installed in the hull of the NOAA Ship Hi’ialakai. For the first time, we found that not only is South Bank a shallow spot in the ocean floor, but it is a former coral atoll which drowned at some point. We can tell that it is a former atoll by the submerged barrier reef, a ring of shallower depths, surrounding a deeper lagoon with a minimum depth of approximately 25 meters (85 feet). There was only one previous known dive to the area that reported the presence of a rubble flat and high currents. Based on this, we planned a series of reconnaissance dives with members from the fish, benthic, oceanography and towed-diver survey teams aboard the Hi’ialakai. Our survey techniques had to be modified to accommodate the deeper habitat and reduced dive times. We were able to complete a total of 36 person dives and approximately 5 km of towed-diver surveys along the raised rim, encircling the lagoon. We encountered mostly rubble flats with a high abundance of macroalgae and low coral cover and diversity. The area seemed highly scoured and although we experienced only moderate currents, it is probable that the area is subjected to high currents.

South Bank appears to be a reef that has not kept up with sea-level rise, the sinking of the atoll due to the weight of the original island at its center and the sinking of the Pacific plate. It is unclear what the original reef ecosystem was like and it is a mystery why this reef wasn’t able to keep up with these processes. Rose and Swains Atolls experience similar conditions, yet continue to have thriving reef ecosystems. South Bank is an area that will require more investigation to fully understand the history and processes of this submerged atoll. It is exciting that the new investigational maps and surveys may provide more information to aid in future explorations.