Can Offshore Meteoric Groundwater Generate Mechanical Instabilities in Passive Continental Margins?

Offshore meteoric groundwater (OMG) has long been hypothesized to be a driver of seafloor geomorphic processes in continental margins worldwide. Testing this hypothesis has been challenging because of our limited understanding of the distribution and rate of OMG flow and seepage, and their efficacy as erosive/destabilizing agents. Here we carry out numerical simulations of groundwater flow and slope stability using conceptual models and evolving stratigraphy—for passive siliciclastic and carbonate margin cases—to assess whether OMG and its evolution during a late Quaternary glacial cycle can generate the pore pressures required to trigger mechanical instabilities on the seafloor. Conceptual model results show that mechanical instabilities using OMG flow are most likely to occur in the outer shelf to upper slope, at or shortly before the Last Glacial Maximum sea‐level lowstand. Models with evolving stratigraphy show that OMG flow is a key driver of pore pressure development and instability in the carbonate margin case. In the siliciclastic margin case, OMG flow plays a secondary role in preconditioning the slope to failure. The higher degree of spatial/stratigraphic heterogeneity of carbonate margins, lower shear strengths of their sediments, and limited generation of overpressures by sediment loading may explain the higher susceptibility of carbonate margins, in comparison to siliciclastic margins, to mechanical instability by OMG flow. OMG likely played a more significant role in carbonate margin geomorphology (e.g., Bahamas, Maldives) than currently thought.

Plain Language Summary: The flow of fresh to brackish groundwater has been proposed as an important process shaping the seafloor. However, we still have a poor understanding of how groundwater behaves in the sub‐seafloor and whether it can erode seafloor sediments. In this study, we test this hypothesis by using conceptual and realistic numerical models of two common types of seafloor margins—siliciclastic and carbonate—to assess the role of groundwater in making the seafloor susceptible to erosion. We show that the flow of groundwater offshore could have driven seafloor erosion close to the shelf break during the Last Ice Age, when sea level was lower than at present. Carbonate margins are more susceptible to this type of failure than siliciclastic margins. This may be explained by the higher variability in sediment properties across carbonate margins as well as the lower strength of their sediments. Groundwater has likely played an important role in shaping the seafloor in carbonate margins, and it may be responsible for landforms such as canyons, scars, and depressions in the Bahamas and the Maldives.

Key Points:

Offshore meteoric groundwater (OMG) flow can drive mechanical instabilities in the outer shelf to upper slope.

Such instabilities occur at, or shortly after, the Last Glacial Maximum sea‐level lowstand.

Carbonate margins are more susceptible to mechanical instability by OMG than siliciclastic margins.