The heterogeneous abyss Proceedings of the National Academy of Sciences

The abyssal seafloor, that is, ocean depths of∼3,000 to 6,000 m, is widely considered simply to bevast, featureless plains of sediment. For example,Wikipedia asserts that“abyssal plains cover more than50% of the Earth’s surface”and“are among the flat-test, smoothest, and least explored regions on Earth”(1). Featureless tracts of mud make intuitive sensesince abyssal plains are formed by the deposition ofthick blankets of fine-grained sediments sinking fromthe surface ocean or transported as turbidity currentsdown continental margins. In most of the abyss, espe-cially on seafloors>10 My old, this sediment blanket istens to thousands of meters thick (2) and seems likelyto bury or smoothen most of the irregularities in sea-floor crust (e.g., basalt pillows) formed at midoceanspreading centers. Riehl et al. in PNAS (3) help todispel this notion that the abyssal seafloor is feature-less, providing evidence of extensive rocky habitatsalong transform faults in the abyss.