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Does Organic Matter Have an Unacknowledged Influence on the Dynamics and Deposits of Fine-Grained Submarine Sediment Gravity Flows?


Fine-grained sediment gravity flows are common in the deep marine environment and an important process by which many desirable hydrocarbon source rocks and seals have and can be formed. Our understanding of the flow dynamics and depositional processes of these flows is limited. Clay particles can alter the flow rheology because of their ability to aggregate, forming floccules and more pervasive cohesive structures, called gels. These flocs and gels can enhance or dampen turbulent forces in sediment gravity flows, such that increasing the cohesive sediment content causes a transition from turbulent, Newtonian flow, via transient-turbulent flow, to laminar, non-Newtonian debris flow. Because the flow rheology controls the depositional style, a thorough understanding of how flow composition relates to flow rheology is essential for our interpretation of the architecture and the palaeo-environmental setting of deep-marine deposits. In the natural environment, clay-rich sediments are commonly associated with the presence of organic matter. There is a reasonable understanding of the effect that organic matter, in particular ‘sticky’ extracellular polymeric substances (EPS), has on clay flocculation and the stability of sedimentary deposits in shallow-marine environment, but it's impact on deep-marine sediment gravity flows has not been explored yet. Here, the influence of similar interactions for suspended clay and EPS within sediment gravity flows is presented. The above research gaps were addressed by means of flume experiments that recorded changes in dynamic behaviour and deposit run out distances of sediment gravity flows with variable amounts of biologically cohesive xanthan gum (a commonly used proxy for natural EPS) and physically cohesive kaolin clay (one of the most common clay minerals on Earth) and non-cohesive silica flour. Results indicate that very small quantities of EPS – three orders of magnitude smaller than the quantity of clay – are sufficient to enhance flocculation and alter the size distribution of clay flocs compared to a flow that lacks EPS. Ultrasonic Doppler Velocity data demonstrate changes in the internal turbulence behaviour of transitional flows as a result of adding EPS. These findings have the potential to change our understanding of sediment gravity flows in the natural environment, in particular those that result in the organic rich, fine-grained deposits regarded as potentially favourable source rocks.