EROSION OF A STABLE DENSITY GRADIENT BY SEDIMENTATION-DRIVEN CONVECTION

STABLE density gradients are common in the world's oceans and lakes, and play a crucial part in their physical, chemical and biological evolution. It has recently been suggested 1 that turbidity currents provide an indirect mechanism for eroding this stratification. In these currents, light ambient fluid that has been mixed by cyclones with dense suspended sediment is transported to greater depths, where it convectively erodes the gradient as the sediment settles. Here I present experimental results which show that in these circumstances the density gradient is eroded in a number of cycles of decreasing intensity and increasing duration. In each cycle some of the sediment settles to the base of a stagnant region of unimpeded sedimentation, while the rest is mixed into an overlying region that is vigorously convecting. My experimental observations agree with a simple physical model that predicts the rate and extent of vertical mixing as well as the variation of particle sizes in the resulting turbidite.