Sedimentation on mid-ocean ridges

The sediments accumulating on and around mid-ocean ridges are mostly formed from the calcareous and siliceous tests of pelagic organisms. This research is concerned with understanding how the rate of sediment supply varies from place to place due to varied productivity of pelagic organisms, how the sediments accumulate on the complex topography of a mid-ocean ridge, and with using the sediments to study mid-ocean ridge processes such as faulting and volcanism.

Sediment transport and accumulation

When pelagic materials reach the seafloor, they are redistributed by bottom currents and by sedimentary flows. This work studied the form of the accumulation using sediment profiler records collected with a Deep Tow system from the Scripps Institution of Oceanography deployed over the Mid-Atlantic Ridge in the early 1970s. The records showed that both sets of transport processes are important. The shapes of deposits were studied to see to what extent they conform to the diffusion transport model - many deposits have parabolic surfaces, which are the steady state forms expected from the diffusion transport model under boundary conditions of constant input or output flux to basins. (Abstract and full article (PDF)*.)

In a second study, profiler records from the Galapagos spreading centre were used to assess sediment transport rates. The method essentially involved quantifying the amount of material eroded from (or not deposited on) the crests of fault scarps, and dividing this value by the age of the scarp which is well known from the seafloor spreading history. Due to the method used, the result was an upper bound on rates of surficial sediment movement. (Abstract and full article (PDF)*.)

Accumulation rates

Sediment on ridge flanks commonly thicken with distance from the spreading axes, reflecting the increasing age of the volcanic seafloor. Complications to this simple picture occur where there is substantial sediment transport or varied dissolution of carbonate. High resolution sediment profiler records collected on the Mid-Atlantic Ridge in 1996 (CD99) show that this simple picture is not applicable there, which has implications for attempting to use the thickness of the sediment cover as a dating tool on slow-spreading ridges (abstract.). However, sediments on the Galapagos spreading centre (figure right) do show a simple systematic trend, partly because the ridge lies beneath the equatorial zone of high pelagic productivity causing very high accumulation rates relative to rates of sediment redistribution and dissolution. Accumulation rates are higher on the south flank, closest to the centre of the high productivity zone. Abstract.

Mapping lava flows

Due to the high accumulation rates on the Galapagos spreading centre relative to sediment redistribution rates, the thickness of sediment is a useful proxy for seafloor age. The ridge flanks have areas of near uniform sediment thickness which correspond to flat basement topography and are bounded by fault scarps. These areas are highlighted by horizontal bars in the figure (left) and were interpreted as regions of uniform age due to large lava flows. As a consequence of the flows, the surficial oceanic crust is slightly younger than might be predicted by simple seafloor spreading and distance of the seafloor from the ridge axis. Hence the sediment does not thicken monotonically away from the ridge axis but begins to thicken from a point away from it (e.g., as shown in the figure (above), from around 5 km). Using these sediment thickness data, the crustal age anomaly was estimated to be approximately 120-150 ky for the Galapagos spreading centre. Abstract

See also work on sonar penetration into marine sediment and the possible use of variations in backscatter to infer variations in the sediment cover over young seafloor.

Relevant publications

Funding for the above work was provided by Research Fellowships from the Royal Society and the NERC. This research was primarily based on data collected with the Deep Tow system of Scripps Institution of Oceanography.

*The American Geophysical Union owns the copyright to these documents. Further reproduction or electronic distribution of them is not permitted.

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