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Description: Sediment gravity flows (SGFs) are the primary process by which sediment and organic carbon are transported from the continental margin to the deep ocean. Forty percent of the total marine organic carbon pool is represented by cohesive extracellular polymeric substances (EPS) produced by marine benthic and pelagic micro-organisms. EPS research to date has focussed on coastal environments, where EPS contribute to seabed stability by forming a cohesive matrix of bonds between sediment particles. The effects of this cohesive material on SGFs in the deep ocean have not been investigated, despite many decades of outcrop, subsurface, modern real-time observational, numerical, and experimental research. Here we present laboratory data that offer the first insights into the potential of biological cohesion for modulating muddy, physically cohesive, SGF dynamics. These data indicate that turbulence-modulated, high-density turbidity currents, mudflows and slides, are more susceptible to changes in flow properties than fully turbulent, low-density turbidity currents at concentrations of EPS encountered in the deep ocean. Even relatively low concentrations of EPS markedly decrease the head velocity and run-out distance of these high-density SGFs. These outcomes greatly improve our understanding of the natural distribution of SGF deposits, which form the world’s largest hydrocarbon reservoirs.