The heterogeneous abyss

Abstract

The abyssal seafloor, that is, ocean depths of ∼3,000 to 6,000 m, is widely considered simply to be vast, featureless plains of sediment. For example, Wikipedia asserts that “abyssal plains cover more than 50% of the Earth’s surface” and “are among the flattest, smoothest, and least explored regions on Earth” (1). Featureless tracts of mud make intuitive sense since abyssal plains are formed by the deposition of thick blankets of fine-grained sediments sinking from the surface ocean or transported as turbidity currents down continental margins. In most of the abyss, especially on seafloors >10 My old, this sediment blanket is tens to thousands of meters thick (2) and seems likely to bury or smoothen most of the irregularities in seafloor crust (e.g., basalt pillows) formed at midocean spreading centers. Riehl et al. in PNAS (3) help to dispel this notion that the abyssal seafloor is featureless, providing evidence of extensive rocky habitats along transform faults in the abyss. Scientists studying the deep sea have long known that rocky or hard substrates do occur on abyssal plains. Perhaps the best known are polymetallic (“manganese”) nodules that were discovered during the Challenger expedition (1872 to 1876) (4); nodules are widespread in the Pacific abyss and, to a much lesser extent, the Indian and the Atlantic Oceans (5). In some Pacific regions, nodules may cover 10 to >50% of the seafloor area, providing abundant rocky habitat (6, 7). Seamounts also provide rocky substrates at abyssal depths where their slopes and outcrops are too steep to hold sediments (8). With as many as 200,000 seamounts in the world ocean, seamounts too provide significant habitat area for hard-bottom faunas, but once again, seamounts are concentrated in the Pacific Ocean (8). Surveys with lowered cameras, human-occupied vehicles, remotely operated vehicles, and autonomous underwater vehicles … [↵][1]1Email: craigsmi{at}hawaii.edu. [1]: #xref-corresp-1-1