Direct Monitoring Reveals Initiation of Turbidity Currents From Extremely Dilute River Plumes.

Sophie Hage ,Cooper Stacey ,William Symons ,Joshua R Allin ,Age Vellinga ,J A Gales ,Alessandro Mozzato ,Peter J Talling ,J L Hizzett ,John E Hughes Clarke ,Mark E Vardy ,S Simmons ,Esther J Sumner ,Gwyn Lintern ,Matthieu Cartigny ,María Azpiroz-Zabala ,Ricardo Silva Jacinto ,D Parsons ,James E Hunt ,Ed Pope ,Michael Clare ,C Watts

doi:10.1029/2019gl084526

Abstract

Rivers (on land) and turbidity currents (in the ocean) are the most important sediment transport processes on Earth. Yet how rivers generate turbidity currents as they enter the coastal ocean remains poorly understood. The current paradigm, based on laboratory experiments, is that turbidity currents are triggered when river plumes exceed a threshold sediment concentration of ~1 kg/m3. Here we present direct observations of an exceptionally dilute river plume, with sediment concentrations 1 order of magnitude below this threshold (0.07 kg/m3), which generated a fast (1.5 m/s), erosive, short‐lived (6 min) turbidity current. However, no turbidity current occurred during subsequent river plumes. We infer that turbidity currents are generated when fine sediment, accumulating in a tidal turbidity maximum, is released during spring tide. This means that very dilute river plumes can generate turbidity currents more frequently and in a wider range of locations than previously thought.

Highlights

Turbidity currents are seafloor hugging flows that are driven by their suspended sediment (Daly, 1936, Middleton and Hampton, 1973)
Experiments suggest that turbidity currents are generated by dilute river plumes with sediment concentrations as low as 1 kg.m-3 (Fig. 1c; Parsons et al, 2001) if the plume locally becomes denser than ambient seawater
6 Conclusion It was previously thought that rivers needed to exceed a sediment concentration threshold to generate turbidity currents offshore river mouths (e.g. 40 kg.m-3, Mulder and Syvitski, 1995; 1 kg.m-3, Parsons et al 2001)

Summary

Introduction

Turbidity currents are seafloor hugging flows that are driven by their suspended sediment (Daly, 1936, Middleton and Hampton, 1973). Experiments suggest that turbidity currents are generated by dilute river plumes with sediment concentrations as low as 1 kg.m-3 (Fig. 1c; Parsons et al, 2001) if the plume locally becomes denser than ambient seawater (by double diffusion or settling-driven convection; Hoyal et al, 1999a,b; Jazi and Wells, 2016; Parsons et al, 2001; Sutherland et al, 2018) This 1 kg.m-3 threshold implies that 61 of the 150 studied rivers studied by Mulder and Syvitski (1995) can generate turbidity currents

Methods

Results

Discussion

Conclusion