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How supercontinents and
superoceans affect seafloor roughness
  
Left: The seafloor
of the Tasman Sea and SE Australia. Large
fracture zones are visible south of Australia as
well as the relatively smooth Tasman Sea ocean
floor with its extinct spreading ridge to the
east. Centre: The rough seafloor of the Indian
Ocean - looking north towards Madagascar we see
the extremely rough ocean floor, dissected by
giant fracture zones across the Southwest Indian
Ridge. Right: The smooth seafloor of the Pacific
Ocean - looking westward across the Pacific from
Ecuador. In the foreground the Cocos-Nazca
mid-ocean ridge intersects with the Pacific
mid-ocean ridge and in the background the smooth
Pacific Plate stretches out to the horizon.
Seafloor roughness
varies considerably across the world's ocean
basins and is fundamental to controlling the
circulation and mixing of heat in the ocean and
dissipating eddy kinetic energy. Models derived
from analyses of active mid-ocean ridges suggest
that ocean floor roughness depends on seafloor
spreading rates, with rougher basement forming
below a half-spreading rate threshold of 30-35
mm/yr, as well as on the local interaction of
mid-ocean ridges with mantle plumes or
cold-spots. Here we present a global analysis of
marine gravity-derived roughness, sediment
thickness, seafloor isochrons and
palaeospreading rates of Cretaceous to Cenozoic
ridge flanks. Our analysis reveals that, after
eliminating effects related to spreading rate
and sediment thickness, residual roughness
anomalies of 5-20 mGal remain over large swaths
of ocean floor. We found that the roughness as a
function of palaeospreading directions and
isochron orientations indicates that most of the
observed excess roughness is not related to
spreading obliquity, as this effect is
restricted to relatively rare occurrences of
very high obliquity angles (> 45 degrees).
Cretaceous Atlantic ocean floor, formed over
mantle previously overlain by the Pangaea
supercontinent, displays anomalously low
roughness away from mantle plumes and
independent of spreading rates. We attribute
this observation to a sub-Pangaean
supercontinental mantle temperature anomaly
leading to slightly thicker than normal Late
Jurassic and Cretaceous Atlantic crust, reduced
brittle fracturing and smoother basement relief.
In contrast, ocean crust formed above Pacific
superswells, probably reflecting metasomatized
lithosphere underlain by mantle at only slightly
elevated temperatures, is not associated with
basement roughness anomalies. These results
highlight a fundamental difference in the nature
of large-scale mantle upwellings below
supercontinents and superoceans, and their
impact on oceanic crustal accretion.
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Commons Attribution 3.0 Unported License.
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Press Release
Ocean floors - the
rough and the smooth of it
Dr Joanne Whittaker and
Professor Dietmar Müller, from the School of
Geosciences, and colleagues in France and USA,
have found a previously unknown connection between
the break-up of the ancient supercontinent Pangaea
and the topography of the deep ocean floor. The
research, published in the journal Nature on 18
December 2008, reveals for the first time how
smooth flat expanses and rough hilly areas of
ocean floor form. Ocean floors have remarkably
contrasting topography: ship and satellite
geophysical data reveal steep cliffs and valleys
over vast areas, sometimes with elevations of over
three kilometres, while other parts of the ocean
floor are incredibly flat. "Seafloor roughness is
really important in ocean systems as it influences
the circulation and mixing of heat in the water,"
said Dr Joanne Whittaker, who completed the
research as part of her PhD with Professor Dietmar
Müller as her supervisor.
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