Salt leaching by freshwater and its impact on seafloor stability: an experimental investigation

Abstract

Offshore freshened groundwater (OFG) has been documented in many continental margins worldwide. OFG systems are dynamic, expanding and contracting with falling and rise sea-levels. OFG has long been thought to be an important geomorphic agent in continental margins, either via active discharge at the seafloor, which can erode depressions, or the generation of excess pore pressure, which can deform sediments and cause slope failure. It has also been proposed that OFG flow can drive the loss of sediment shear strength via salt leaching, when seawater in pores is replaced by freshwater. Here, we measure changes in the geotechnical properties of seafloor clayey silt due to salt leaching using flushing experiments, and assess the implications of these changes on the stability of siliciclastic continental margins with 2D limit equilibrium modelling. We document a ∼ 50% decrease in undrained cohesive strength of seafloor sediment after flushing, as well as a decrease in its shear strength, bulk density, and moisture content, which is similar to that reported for subaerial quick clays undergoing salt leaching. When applied to a theoretical submarine domain 300 m wide by 100 m high, we estimate that salt leaching can trigger slope failure when the thickness of the flushed layer is >3.5 m or when the slope gradient is >3°. Such conditions are primarily satisfied on the continental slope or the shallow seafloor close to the shoreline. Salt leaching by OFG flow merits consideration as a potential mechanism destablising submarine sedimentary slopes.