Reconstructing the sediment concentration of a giant submarine gravity flow

Abstract

Submarine gravity flows are responsible for the largest sediment accumulations on the planet, but are notoriously difficult to measure in action. Giant flows transport 100s of km3 of sediment with run-out distances over 2000 km. Sediment concentration is a first order control on flow dynamics and deposit character. It has never been measured directly nor convincingly estimated in large submarine flows. Here we reconstruct the sediment concentration of a historic giant submarine flow, the 1929 “Grand Banks” event, using two independent approaches, each validated by estimates of flow speed from cable breaks. The calculated average bulk sediment concentration of the flow was 2.7–5.4% by volume. This is orders of magnitude higher than directly-measured smaller-volume flows in river deltas and submarine canyons. The new concentration estimate provides a test case for scaled experiments and numerical simulations, and a major step towards a quantitative understanding of these prodigious flows.

The influence of basin setting and turbidity current properties on the dimensions of submarine lobe elements

“…With the exception of the reconstructed velocities from the Pleistocene Fan of the Kutai Basin (1.75 to 9.1 m s À1 ) all of the calculated values reconstructed from lobe measurements are Table 4. Reconstructed turbidity current velocities from include the Amazon Fan (Jegou et al, 2008), Fan 3, Tanqua depocentre, Karoo Basin (Pr elat et al, 2009;Kane et al, 2017), the Golo Fan offshore Corsica (Deptuck et al, 2008;Hamilton et al, 2017) and the Pleistocene Fan, Kutai Basin, Indonesia (Saller et al, 2004(Saller et al, , 2008 deemed far too high (>10 m s À1 ) to be sensible in respect to other measured (0.4 to 3.5 m s À1 depth-average flow velocity; Khripounoff et al, 2003;Liu et al, 2012;Cooper et al, 2013;Xu et al, 2014) and estimated (3.8 m s À1 depth-average flow velocity; Stevenson et al, 2018) turbidity current velocities from natural systems that are not caused by major earthquakes (up to 20 m s À1 ; see Talling et al, 2013). However, using the lobe element dimension from Fan 3 of the Karoo Basin instead of lobe dimensions, a much more reasonable value of 4.2 m s À1 for the current velocity is estimated, although this is still somewhat high (Table 4).…”

Dispersion, Accumulation, and the Ultimate Fate of Microplastics in Deep-Marine Environments: A Review and Future Directions

“…Despite these limitations, our understanding of particulate density fractionation during transport and deposition provides a basis for an initial predictive assessment of their distribution within individual deposits. From the ancient record (e.g., Kneller and Branney, 1995;Talling et al, 2012b) and from observations in modern environments (e.g., Smith et al, 2007;Biscara et al, 2012;Clare et al, 2017;Hage et al, 2018;Mountjoy et al, 2018;Stevenson et al, 2018) we know that some individual event deposits (e.g., by gravity flows) can be many meters thick; hence, the assertion that plastics are only present in the first few centimeters of sediment below the seafloor is unlikely to hold (c.f. Martin et al, 2017).…”

The stratigraphic evolution of onlap in siliciclastic deep-water systems: Autogenic modulation of allogenic signals

“…6, 8). This parameter is further complicated by the possible presence of low-velocity dense basal layers within highly-concentrated stratified flows (e.g., Stevenson et al 2018). Turbulence is suppressed in these basal layers; this reduces velocity and run-up heights, resulting in the increased likelihood of an abrupt pinch-out of the sand-rich basal layer and bypass of the upper and low-concentration parts of the flow.…”