Discovery of widely available abyssal rock patches reveals overlooked habitat type and prompts rethinking deep-sea biodiversity

Abstract

<jats:p>Habitat heterogeneity and species diversity are often linked. On the deep seafloor, sediment variability and hard-substrate availability influence geographic patterns of species richness and turnover. The assumption of a generally homogeneous, sedimented abyssal seafloor is at odds with the fact that the faunal diversity in some abyssal regions exceeds that of shallow-water environments. Here we show, using a ground-truthed analysis of multibeam sonar data, that the deep seafloor may be much rockier than previously assumed. A combination of bathymetry data, ruggedness, and backscatter from a trans-Atlantic corridor along the Vema Fracture Zone, covering crustal ages from 0 to 100 Ma, show rock exposures occurring at all crustal ages. Extrapolating to the whole Atlantic, over 260,000 km<jats:sup>2</jats:sup>of rock habitats potentially occur along Atlantic fracture zones alone, significantly increasing our knowledge about abyssal habitat heterogeneity. This implies that sampling campaigns need to be considerably more sophisticated than at present to capture the full deep-sea habitat heterogeneity and biodiversity.</jats:p>

The heterogeneous abyss

“…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.…”

“…To begin to fill this abyssal gap, Riehl et al ( 3 ) used high-resolution multibeam sonar to survey a 2,700-km-long swathe of abyssal seafloor along the Vema Fracture Zone, a transform fault crossing the Mid-Atlantic Ridge between the Caribbean Sea and the bulge of Africa. The total area surveyed was ∼94,000 km 2 and spans crustal ages of 0 to 100 Ma.…”

“…The total area surveyed was ∼94,000 km 2 and spans crustal ages of 0 to 100 Ma. Riehl et al ( 3 ) then combined analyses of seafloor relief (using a ruggedness index) with acoustic backscatter strength to apportion the abyssal seafloor into habitat types; both high ruggedness and high acoustic backscatter are correlated with the occurrence of hard surfaces at the seafloor. Riehl et al ( 3 ) then classified their survey area into three general habitat types: “Sediment” with low hard-rock exposure potential, “Transitional” with moderate hard-rock exposure potential, and “Hard Rock” characterized by high hard-rock exposure potential.…”

“…Riehl et al ( 3 ) then combined analyses of seafloor relief (using a ruggedness index) with acoustic backscatter strength to apportion the abyssal seafloor into habitat types; both high ruggedness and high acoustic backscatter are correlated with the occurrence of hard surfaces at the seafloor. Riehl et al ( 3 ) then classified their survey area into three general habitat types: “Sediment” with low hard-rock exposure potential, “Transitional” with moderate hard-rock exposure potential, and “Hard Rock” characterized by high hard-rock exposure potential. They then used bottom dredges, grab samplers, gravity corers, a towed camera sledge, and an autonomous underwater vehicle to ground-truth their habitat classification, verifying that rocky substrates such as outcrops and polymetallic nodules predictably occurred as indicated by ruggedness and backscatter indices.…”

Seabed mining could come at a high price for a unique fauna

“…The vast deep seafloor is not a homogeneous featureless expanse (Riehl et al., 2020). Thousands of seamounts (>1,000 m high) with exposed rocks are interspersed across the seabed, and large areas of abyssal plains are covered with mineral deposits (“polymetallic nodules”; Figure 1a).…”