Abstract

Submarine mass failures triggered by energetic forcing events such as hurricanes and earthquakes are relatively well studied due to the potential for infrastructure damage and tsunami generation; such failures are common on heavily sedimented margins where underconsolidated deposits are preconditioned to fail. However, studies of seafloor sediment movement between large events remain scarce. Using repeat bathymetric surveys of the Mississippi River Delta Front (MRDF), we document substantial seafloor movement in absence of major hurricanes. About 1 m/yr of deepening was observed within preexisting failures, with downslope sediment transport on the order of 105 m3/yr. Outside failure features, seafloor depths remained stable or showed minor (<20 cm/yr) accretion. MRDF volumetric sediment flux during hurricane-driven mass failures is an order of magnitude greater than the annual flux during a quiescent interval. When normalized by time, however, sediment flux during the quiescent interval (5.5 × 105 m3/yr) was half that of hurricane-driven mass failures (1.1 × 106 m3/yr). These observations corroborate our wave modeling results, which infer that even waves of 1 yr recurrence interval can generate differential seafloor pressures sufficient to trigger submarine landslides; this does not exclude the possibility of river floods also being agents of failure. These findings indicate that sub-decadal submarine landslides are important to MRDF dynamics, comparable to the role of major hurricanes, and observation during seemingly quiescent periods is necessary to holistically assess sediment flux. The periodicity and prevalence of moderate-scale mass transport documented here corroborates similar recent studies offshore other deltas globally, indicating that highstand mass and time budgets of shelf to deep-sea sediment flux, in addition to organic carbon and bioreactive particles, may need to be revised.

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