Ferromanganese nodules fill the field of view. The center of each nodule may host a basaltic rock, sediment, or even a fossil.

Ferromanganese nodules fill the field of view. The center of each nodule may host a basaltic rock, sediment, or even a fossil. Image courtesy of the NOAA Office of Ocean Exploration and Research, 2017 American Samoa. Download larger version (jpg, 2.5 MB).

Dive 03: Rose Atoll
February 18, 2017
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Dive 3: Siphonophore

This potentially new species of dandelion siphonophore, from the family Rhodalidae, was imaged using its tentacles to attach to the iron-manganese encrusted rocks in the deep slopes of Rose Atoll. Video courtesy of the NOAA Office of Ocean Exploration and Research, 2017 American Samoa. Download (mp4, 18.4 MB)

The dive began at 2,538 meters on a large ridge extending away from the east side of Rose Atoll and moved north to the ridge crest, revealing abundant individual ferromanganese nodules with no noticeable sessile (attached) fauna. The narrow spaces between nodules were filled with light-colored, sandy-textured sediment – possibly carbonate from the reef. A ferromanganese-encrusted rock was collected while a sea cucumber swam in the distance. Moving uphill, Deep Discoverer (D2) observed several ophiuroid brittle stars, two different kinds of crinoids, a dandelion siphonophore, two types of sea cucumbers, a possibly new species of sea star, carnivorous and other glass sponges, corals, and several large, old manganese-encrusted sponge skeletons. When D2 arrived on top of what appeared to be a ferromanganese-encrusted lava flow, the geology changed to more boulders. A final geologic sample, roughly twice the size of the first boulder, was collected. Continuing along the dive track, the abundance and diversity of corals increased. As the terrain varied with rougher flow textures, which may have been talus deposits cemented with ferromanganese deposits, there were beautiful pillow structures. The current increased and the slope became steeper; it was entirely composed of pillow basalts that gave way to sparsely sedimented ropy lava textures. The benthic community remained relatively constant – bamboos, chrysogorgiids with associates, and black corals. Around 2,415 meters, several dead sponge stalks and a large branching bamboo colony with a predatory sea star were observed. The topography became highly varied, containing five to 10-meter vertical walls. A large block of pillow flows, which appeared unattached to the flows below, was observed and may have been the result of a large landslide. The flow edge terminated and boulders with large quantities of sediment dominated the area; many overhanging ledges had large dislodged blocks of basalt. Loose boulders, reduced abundance of animals, and manganese-encrusted sponge skeletons were seen at about 2,360 meters. The top of one of these sponges containing other sponges, barnacles and hydroids was collected. We will continue seamount sampling on our next dive!