"What happens to a supercritical turbidity current when it travels onto a seafloor with increasingly gentle slope?" A question by Henry Pantin.

This question was posed to me on quite a few informal occasions by Henry Pantin. My intuitive answer was always that such flows decelerated, resulting in a drop of the Froude number. Depending on the rapidity of the slope change, this drop could be a hydraulic jump for strong slope changes, or a gradual decrease of the Froude number to 1, and possibly a weak, undular hydraulic jump, for very gradual slope changes.

Henry always laughed in his familiar way and told me that he was not so sure about this.

In this presentation I will show measurements from experimental turbidity currents that demonstrate that this intuition is flawed. The key misunderstanding arises from the fact that turbidity currents are not conservative flows. Decelerating turbidity currents deposit submarine fans on the sea floor. These consist entirely out of sediment that has been extracted from the turbidity current flow. This mass extraction may lead to diminishing excess density of the flow, which is a key control on supercriticality in turbidity currents: decreasing excess density increases the Froude number. As the flow decelerates, deposition thus generates a competing influence on the down-stream evolution of the Froude number.

We tested the compound effect of deceleration and deposition on the Froude number in experimental turbidity currents travelling down a slope onto more gently dipping basin floors. The results show that supercritical, decelerating turbidity currents may deposit their sediment so quickly that the Froude number is maintained, or even increased, in the downstream direction. Supercritical turbidity currents travelling onto more gentle sections of the sea floor may thus vanish into dilute, unstably stratified, gently mixing oceanic bottom layers. Extensive, thin silt deposits are formed from these expanding clouds as halos surrounding the sandy deposit that is often seen as the primary depositional element of turbidity current deposition.

Deepwater sedimentology and stratigraphy is a research area with many remaining unknown processes and controls. Posing simple questions and following through in the subsequent analyses, in the fashion of Henry Pantin, can still lead to surprising depths and counter intuitive insights.